June 11, 2016 | Author: Sammuel Go Go | Category: N/A
Natural Ventilation in Double-Skin Façade Design for Office Buildings in Hot and Humid Climate
Pow Chew Wong
A thesis submitted in fulfillment of the requirement for the degree of Doctor of Philosophy
at The University of New South Wales Australia
December, 2008
I
Acknowledgements I would like to thank my Supervisors, Professor Deo Prasad and Professor Masud Behnia, and my co-Supervisor Mr. Steve King, for their unfailing guidance and patient for me throughout my quest in fulfilling one of the hardest tasks in my life yet.
My thanks and appreciation also go to those who gave up their valuable time and effort to guide, advise and teach me, especially to Dr. Nathan Groenhout, Dr. Graeme S. Wood, Mr. KW Ng, Mr. Sherman Heng and Ms. Gabi Duigu.
My greatest love and appreciation would go to my wife, Joyce, who has been supporting me day and night, giving me un-surpassing encouragement and love to help me to complete this Thesis. My love also goes to my three children; Jonathan, Janice and Jessica, for their understanding in lighten my load during the hectic period of completing my work.
Lastly, I would like to thank my family overseas for their great patient and support, which has made my study somewhat much more enjoyable and a great sense of fulfillment.
II
Abstracts
The specific climatic conditions and to certain extent the preferred living style in the hot and humid climate of Singapore, most of the electricity consumed in buildings goes towards air-conditioning and refrigeration, especially in work places like commercial and institutional buildings, which are mostly designed to be fully air-conditioned. It is argued that by combining appropriate natural ventilation strategies with advanced technologies in building façade design will be able to reduce energy consumption in high-rise office buildings in the tropics.
This research seeks to find a design solution for reducing the energy usage in highrise office buildings in Singapore. There are numerous methods and techniques that could be employed to achieve the purpose of designing energy efficient buildings. The Thesis explores the viability of double-skin façades (DSF) to provide natural ventilation as an energy efficient solution for office buildings in hot and humid environment by using computational fluid dynamic (CFD) simulations and case study methodologies.
CFD simulations were used to examine various types of DSF used in office buildings and the behaviour of airflow and thermal transfer through the DSF; the internal thermal comfort levels of each office spaces were analyzed and compared; and an optimization methodology was developed to explore the best DSF configuration to be used in high-rise office buildings in the tropics. The correlation between the façade configurations, the thermal comfort parameters, and the internal office space energy consumption through the DSF is studied and presented.
The research outcome of the Thesis has found that significant energy saving is possible if natural ventilation strategies could be exploited with the use of DSF. A prototype DSF configuration which will be best suited for the tropical environment
III
in terms of its energy efficiency through cross ventilation strategy is proposed in this Thesis. A series of comprehensive and user-friendly nomograms for design optimization in selecting the most appropriate double-skin façade configurations with considerations of various orientations for the use in high-rise office buildings in the tropics were also presented.
IV
Table of Contents
Acknowledgements Abstract
I II
Table of Contents
IV
List of Figures
XII
List of Tables
XVII
List of Graphs
XIX
List of Acronyms
XXI
Chapter 1:
Introduction
1
1.1
Introduction
1
1.2
Sustainable development
2
1.3
Energy consumption in commercial buildings
4
1.4
Air conditioning in office buildings and human comfort
5
1.5
Façade design for office buildings
6
1.6
Energy consumption for office buildings in Singapore
7
1.7
Natural ventilation and indoor environment
9
1.8
Thermal comfort standards
10
1.9
Adaptive thermal comfort model and natural ventilation in buildings
11
1.10
Thermal comfort analysis and double-skin façades
12
1.11
Research questions
13
1.12
Scope of research
14
1.13
Methodology
15
1.14
Structure of Thesis
15
V
Chapter 2:
Thermal Comfort in Hot and Humid Climates
17
2.1
Evolvement of thermal comfort studies
17
2.2
Definitions of thermal comfort
18
2.3
Important parameters in thermal comfort
19
2.4
Measurement of thermal comfort
20
2.4.1
Psychrometric chart
22
2.4.2
Thermal comfort indices
23
2.4.3
Thermal comfort studies
26
2.5
Questions of adaptability and human comfort
27
2.6
Adaptive thermal comfort model
29
2.7
The ASHRAE Standard 55-2004
31
2.8
Passive solar design in a hot and humid climate
34
Chapter 3:
Natural Ventilation Designs in a Hot and Humid Climate
38
3.1
Introduction
38
3.2
The choice of natural ventilation
39
3.3
Natural ventilation and indoor air quality
40
3.4
Development of sustainable designs in buildings
42
3.5
Natural ventilation designs
43
3.5.1 Natural ventilation strategies and techniques
45
3.5.2 Natural ventilation designs in the tropics
48
3.5.3
52
3.6
3.7
3.8
Natural ventilation research
Natural ventilation and office buildings
54
3.6.1
55
Natural ventilation and bio-climatic office building designs
Energy consumption for office buildings in the tropics
56
3.7.1 The tropical climate of Singapore
57
3.7.2 Energy usage for office buildings in Singapore
59
Natural ventilation and double-skin façades
60
VI
Chapter 4:
Double-Skin Façades and Natural Ventilation
61
4.1
Intelligent façades
61
4.2
Double-skin façades (DSF)
62
4.2.1 Introduction
62
4.2.2 Classification of double-skin façades
64
4.2.3 Thermal transfer through double-skin façades
70
4.2.4 Design considerations for double-skin façades
71
4.3
Natural ventilation in double-skin façades
73
4.4
Implementation of double-skin façades in office buildings
74
4.4.1 Examples of double-skin façade buildings
74
4.4.2 Fire protection in double-skin façades
78
4.4.3 Sunshading in double-skin façades
79
4.4.4 Effect of nigh-time ventilation in double-skin façades
80
4.4.5 Condensation in double-skin façades
80
4.4.6 Review of the limitations of double-skin facades
81
Case study for double-skin façade buildings
82
4.5.1 Stadttor (City Gate) at Düsseldorf, Germany
82
4.5
4.5.2
4.5.1.1
The façade system (double-skin corridor façade)
83
4.5.1.2
Natural ventilation
84
4.5.1.3
Conclusion
84
Occidental Chemical Center at Niagara Falls, New York, USA
4.5.3
85
4.5.2.1
The façade system (double-skin multi-storey façade) 86
4.5.2.2
Ventilation systems
86
4.5.2.3
Conclusion
87
Super Energy Conservation Building, Kiyose City, Tokyo, Japan
87
4.5.3.1
The façade system (double-skin multi-storey façade) 88
4.5.3.2
Ventilation systems
88
4.5.3.3
Conclusion
89
VII
4.6
Concluding remarks
Chapter 5:
5.1
5.2
5.3
5.4
5.5
5.6
Computational Fluid Dynamics
89
90
Simulating naturally ventilated double-skin façade
90
5.1.1 Building simulation programs
90
5.1.2 Simulating buildings with double-skin façades
92
5.1.3 Coupling CFD and building energy simulations
93
5.1.4 The choice of using CFD program
94
CFD software
95
5.2.1 Theoretical background for CFD software
95
5.2.2 Grid resolution in CFD software
96
5.2.3 Verification and validation in CFD
97
5.2.4 Constraints for CFD simulation
97
Research into CFD simulation for building design
100
5.3.1 CFD simulation in building design
100
5.3.2 CFD approaches in indoor environment simulation
105
A case study of a CFD simulation for double-skin façade
107
5.4.1 Thermal considerations
108
5.4.2 Fluid dynamics considerations
108
5.4.3 Modelling of the façade
109
5.4.4 Findings
109
Review of several building simulation software packages
110
5.5.1 Apache software
110
5.5.2 Flovent software
111
5.5.3 Microflo software
111
5.5.4 Phoenics software
112
5.5.5 Airpak software
112
5.5.6 Conclusion
113
Airpak CFD software
114
VIII
5.7
5.6.1 The Airpak CFD software
114
5.6.2 Buoyancy-driven flows and natual convection in Airpak
116
5.6.3 Radiation simulation in Airpak
117
5.6.4 Solution procedures in Airpak
117
5.6.5 The validation of Airpak software
119
Conclusion
122
Chapter 6:
Research Methodology
123
6.1
Introduction
123
6.2
Research strategies in architectural research
124
6.2.1 Literature review
125
6.2.2 Research approach: Qualitative versus Quantitative
127
6.3
The knowledge gap and research questions
130
6.4
Research methods for a tropical double-skin façade
132
6.4.1 Building simulation methodology
132
6.4.2 Computational Fluid Dynamic and Airpak software
133
6.4.3 The CFD Modelling
134
6.4.3.1
Stage 1 – Single office
134
6.4.3.2
Stage 2 – Office blocks
136
6.4.3.3
Stage 3 – Optimization
137
6.4.3.4
Stage 4 – Nomograms
137
6.5
Goals for the research
137
6.6
Limitations of the research
138
Chapter 7:
7.1
Preliminary Modelling
140
Preliminary modeling
140
7.1.1 The geometry of the CFD model
140
IX
7.1.2 The construction materials used for the model
140
7.1.3
The heat sources in the model
141
7.1.4
The boundary conditions of the model
141
7.2
Strategies in modelling
143
7.3
Analysis of preliminary models
144
7.4
Discussion
149
7.5
Initial findings
151
7.6
Progressive modelling
154
7.6.1
Comparison of results for single-skin and double-skin façades
7.7
Conclusion
Chapter 8:
8.1
Multi-Storey Building Modelling
157 161
162
Stage 1 of the multi-storey modeling
162
8.1.1 Simulation results for South facing DSF system
165
8.1.2 Simulation results for North facing DSF system
169
8.1.3 Analysis of results and findings for Stage 1
170
Stage 2 of the multi-storey modelling
172
8.2.1 Simulation results for South facing DSF system
175
8.2.2 Simulation results for North facing DSF system
177
8.2.3 Analysis of results and findings for Stage 2
178
Stage 3 of the multi-storey modelling
180
8.3.1 Simulation results for South facing DSF system
183
8.3.2 Simulation results for North facing DSF system
185
8.3.3 Analysis of results and findings for Stage 3
186
8.4
Comparison results for different orientations
188
8.5
The complete 18-storey office building
190
8.6
Conclusion
191
8.2
8.3
X
Chapter 9:
9.1
Parametric Studies of Optimization
193
Strategies for natural ventilation optimization
193
9.1.1
Investigation of different opening locations on the outer pane of DSF system
9.1.2
Investigation of different opening sizes on the outer pane of DSF system
9.1.3
194
198
A new type of DSF configuration for hot and humid climate
9.1.4 Investigation of different shaft heights of DSF system
199 200
9.1.5 Investigation of different air gap sizes with optimum shaft height 9.1.6 Comparison of ‘Fan’ and ‘Shaft’ ventilation methods 9.1.6.1
Analysis of ‘Fan’ and ‘Shaft’ ventilation methods
206 209 214
9.1.7 Investigation of sun shading devices to the DSF system
214
9.1.8 Summarizing of results and findings for optimization
217
9.2
An improved DSF system for the tropics
218
9.3
Limitations of the research
220
9.4
Nomograms for natural ventilation designs with DSF system
221
9.4.1
Formulation of the nomograms
221
9.4.2
The application of the nomograms
225
9.4.3
The limitations of the nomograms
231
9.5
Conclusion
232
Chapter 10: Contributions and Future Works
234
10.1
Contributions of the research work
234
10.2
Viability of natural ventilation for office buildings in the tropics
235
10.3
Conclusion
236
XI
10.4
Recommendations for future works
236
10.5
Final note
238
References
240
Appendix A
257
Selected benchmarking simulation results.
Appendix B
272
Selected optimization simulation results.
Appendix C Selected referred papers submitted to International Conferences and International Journals.
276
XII
List of Figures Chapter 1 Figure 1.1
Energy consumption by sectors
Figure 1.2
Electricity consumption by sectors
Chapter 2 Figure 2.1
Thermal exchanges between the human body and its environment
Figure 2.2
Representation of graphical comfort scale
Figure 2.3
Olgyay’s bio-climatic chart in metrics, modified for warm climates
Figure 2.4
Psychrometric Chart
Figure 2.5
The PPD as a function of PMV
Figure 2.6
The psycho-physiological model of thermal perception: the adaptive model
Figure 2.7
Neutralities predicted and compared with results of field experiments
Figure 2.8
Acceptable ranges of operative temperature and humidity
Figure 2.9
Air speed required to offset increased temperature
Figure 2.10
Acceptable operative temperature ranges for naturally conditioned spaces
Figure 2.11
Cross ventilation
Chapter 3 Figure 3.1
Natural ventilation through buildings
Figure 3.2
Ventilation rate for good indoor air quality
Figure 3.3
Relation between airflow rate, pollution level and energy demand
Figure 3.4
Wind velocity gradients for urban spaces: (a) wooded, (b) countryside and (c) open country
XIII
Figure 3.5
Wind patterns around buildings
Figure 3.6
Airflow patterns through rooms for various sizes and positions of openings
Figure 3.7
Velocity of airflow is increased outside the room if the inlet is larger than the outlet (a); velocity of airflow is increased inside the room if the inlet is smaller than the outlet (b); internal partition positions will affect the airflow patterns (c & d)
Figure 3.8
Wind patterns altered by different layouts of groups of buildings
Figure 3.9
Louvers can deflect the airflow upwards or downwards
Figure 3.10
A canopy over a window tends to direct airflow upwards (d); a gap between the canopy and the wall will create a downward pressure (e); airflow within a room will improve if a louvered sunshade is used
Figure 3.11
The inlet of the wind tower can usually be closed to keep out dust or cold air
Figure 3.12
Wind catchers in the oriental courtyard houses of Iraq
Figure 3.13
Wind towers in the Bastakia district of Dubai
Figure 3.14
Map of Singapore
Figure 3.15
Electricity consumption among different sectors in Singapore
Chapter 4 Figure 4.1
Typical double-skin façade construction
Figure 4.2
Plan and Section of box window façade
Figure 4.3
Examples of double-skin façades
Figure 4.4
Section through the multi-storey façade of the Victoria Ensemble in Cologne
Figure 4.5
Consolidated classification tree diagrams
Figure 4.6
Heat transfer through double-skin façade
XIV
Figure 4.7
Exterior views and the cavity space within the double-skin façade
Figure 4.8
Exterior view and façade details for One Peking Road
Figure 4.9
Exterior view of Jiu Shi Tower
Figure 4.10
City Gate at Düsseldorf, Germany
Figure 4.11
The corridor façade system showing the inner vertically pivoted windows and the façade cavity
Figure 4.12
The Occidental Chemical Center at Niagara Falls
Figure 4.13
The Super Energy Conservation Building at Kiyose City, Tokyo
Chapter 5 Figure 5.1
Isometric view of the office room
Figure 5.2
Overview of the solution method
Figure 5.3
Simulation model used for the validation constructed in Airpak
Chapter 6 Figure 6.1
Section through the model (with external space at the left)
Figure 6.2
Rear elevation of the model
Chapter 7 Figure 7.1
Sectional elevation of the single office module
Figure 7.2
Longitudinal section of the single office module
Figure 7.3
Isometric view of the CFD model
Figure 7.4
The single office module with studied openings A, B, C, D&E
Figure 7.5
Location points for taking the simulation results (section of model)
Figure 7.6
Example of velocity vectors generated through simulation
Figure 7.7
Example of temperature contours generated through simulation
XV
Figure 7.8
Thermal Environment Conditions for Human Occupancy, ANSI/ASHRAE Standard 55-2204
Figure 7.9
Ventilation Comfort Chart of Singapore
Figure 7.10
Standard curtain walling model
Figure 7.11
Double-skin façade model
Figure 7.12
Nomogram showing the acceptable thermal comfort conditions (shaded area) for standard curtain wall system
Figure 7.13
Nomogram showing the acceptable thermal comfort conditions (shaded area) for double-skin façade system
Chapter 8 Figure 8.1
Model geometry of Stage 1 of the 6-storey building
Figure 8.2
Boundary conditions and ranges of parameters used in the CFD simulations
Figure 8.3
Location points for monitoring the simulation results (Stage 1)
Figure 8.4
Thermal Environment Conditions for Human Occupancy from ANSI/ASHRAE Standard 55-2004
Figure 8.5
Location points for recording the simulation results (Stage 2)
Figure 8.6
Model geometry of Stage 2 of the 18-storey office building
Figure 8.7
Location points for monitoring the simulation results (Stage 3)
Figure 8.8
Model geometry of Stage 3 of the 18-storey office building
Figure 8.9
The model of the complete 18-storey office building
Chapter 9 Figure 9.1
Investigation of different opening locations (L1) for the outer pane of DSF system
Figure 9.2
Schematic drawing showing selected points for monitoring simulation results
Figure 9.3
Investigation of different opening sizes for the outer pane of DSF system
XVI
Figure 9.4
A new type of double-skin façade model for hot and humid climate
Figure 9.5
Model configurations for simulations
Figure 9.6
Location points for monitoring the simulation results (for the extended shaft model)
Figure 9.7
Isometric view – 3.6m shaft with openings at outer pane of DSF
Figure 9.8
Velocity vectors – study of air velocity magnitudes and its moving directions
Figure 9.9
Temperature contours – study of temperature distribution within the office spaces
Figure 9.10
Velocity particle traces – study of air flow patterns within the office spaces
Figure 9.11
Pressure contours – study of external and internal pressures acted onto the building
Figure 9.12
Configurations of the model for simulations
Figure 9.13
CFD models for ‘Shaft’ and ‘Fan’ configurations
Figure 9.14
Locations of record for thermal comfort parameters (example for the mechanical fan at the top of the double-skin façade)
Figure 9.15
Study of the effects of sun shading device within the DSF sir gap
Figure 9.16
An improved new type of double-skin façade model for hot and humid climate
Figure 9.17
Three ‘Axis’ of the nomogram
Figure 9.18
‘Limits’ and ‘Linear Spacing’ of the nomogram
Figure 9.19
‘Non-comfort Zone’ of the nomogram
Figure 9.20
‘Comfort Zone’ of the nomogrma
Figure 9.21
Nomograms for the DSF design in the tropics
XVII
List of Tables Chapter 4 Table 4.1
Double-skin façade buildings with various ventilation types and façade systems
Chapter 5 Table 5.1
Comparison of typical functions of ES and CFD programs for building performance studies
Chapter 6 Table 6.1
Reproduced from Glesne, C., & Peshkin, A. (1992): Becoming qualitative researchers: An introduction
Chapter 7 Table 7.1
Simulation results A
Table 7.2
Simulation results B
Table 7.3
Simulation results C
Chapter 8 Tables 8.1
Simulation results for P1, P6 and P11 of different boundary conditions (Stage 1 - South facing DSF system)
Tables 8.2
Simulation results for P1, P6 and P11 of different boundary conditions (Stage 1 - North facing DSF system)
Tables 8.3
Simulation results for P1, P6 and P11 of different boundary conditions (Stage 2 - South facing DSF system)
Tables 8.4
Simulation results for P1, P6 and P11 of different boundary conditions (Stage 2 - North facing DSF system)
Tables 8.5
Simulation results for P1, P6 and P11 of different boundary conditions (Stage 3 - South facing DSF system)
XVIII
Tables 8.6
Simulation results for P1, P6 and P11 of different boundary conditions (Stage 3 - North facing DSF system)
Tables 8.7
Comparison of selected simulation results for different orientations of DSF system
Chapter 9 Table 9.1
Table showing sample of simulation results for different opening locations (L1)
Table 9.2
Simulation results at locations P1a, P6a and P11a (various air gap sizes)
Table 9.3
Simulation results at locations P2a, P7a and P21a (various air gap sizes)
XIX
List of Graphs Chapter 5 Graph 5.1
Measured hourly outdoor temperatures
Graph 5.2
Measured results (Series 1) vs. Airpak simulation results (Series 2)
Chapter 8 Graph 8.1
Comparison of Operative Temperatures for South facing DSF (Stage 1)
Graph 8.2
Comparison of Operative Temperatures for North facing DSF (Stage 1)
Graph 8.3
Comparison of Operative Temperatures for South facing DSF (Stage 2)
Graph 8.4
Comparison of Operative Temperatures for North facing DSF (Stage 2)
Graph 8.5
Comparison of Operative Temperatures for South facing DSF (Stage 3)
Graph 8.6
Comparison of Operative Temperatures for North facing DSF (Stage 3)
Graph 8.7
Comparison of Operative Temperatures for four major orientations of DSF system
Chapter 9 Graph 9.1
Graph showing the temperatures (oC) comparison for different opening locations
Graph 9.2
Graph showing the air velocity (m/s) comparison for different opening locations
Graph 9.3
Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (various shaft heights)
XX
Graph 9.4
Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (various shaft heights)
Graph 9.5
Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (various air gap sizes)
Graph 9.6
Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (various air gap sizes)
Graph 9.7
Comparison of ‘Fan’ and ‘Shaft’ configurations in relation to thermal comfort parameters
Graph 9.8
Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (1.5m shaft)
Graph 9.9
Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (2.5m shaft)
Graph 9.10
Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (1.5m shaft)
Graph 9.11
Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (2.5m shaft)
Graph 9.12
Comparison of indoor Operative Temperatures (oC) for different DSF systems
XXI
List of Acronyms ACH
Air change hour
AHU
Air handling unit
ASEAN
Association of Southeast Asian Nations
ASHRAE
American Society of Heating, Refrigerating, and Air-Conditioning Engineers
ASHVE
American Society of Heating and Ventilation Engineers
BMS
Building management system
BRE
Building Research Establishment
CFD
Computational fluid dynamic
DBT
Dry-bulb temperature
DOE
Department of Energy
DSF
Double-skin façade
EREN
Energy Efficiency and Renewable Energy Network
ES
Energy simulation
ET
Effective temperature
GDP
Gross domestic product
HVAC
Heating, ventilation and air-conditioning
IAQ
Indoor air quality
LES
Large Eddy Simulation
MRT
Mean radiant temperature
NV
Natural ventilation
OT
Operative temperature
PD
Percentage dissatisfied
PLEA
Passive and Low Energy Architecture
PMV
Predicted mean vote
PPD
Predicted percentage dissatisfied
RANS
Reynolds Average Navier-Stokes
RH
Relative humidity
XXII
RNG
Random number generator
SET
Standard effective temperature
VAV
Variable air volume
VOC
Volatile-Organic-Compound
WEF
World Economic Forum
CHAPTER 1
INTRODUCTION
This Chapter provides an overview of the backgrounds for energy consumption, façade design, natural ventilation and thermal comfort issues in high-rise commercial buildings in the tropics, and set out the research questions, scope and methodology of the research and the structure of the Thesis.
1.1
Introduction
The amount of energy used and spent in the modern world has been escalating in an alarming way. Air-conditioning accounts for the major portion of the total energy consumption used for the operation of most of the present high-rise buildings. The situation is even more alarming in the case of high-rise buildings in a hot and humid climate where greater energy consumption is desired to provide comfort in the man-made environment. However, the call for energy efficient building design is growing and the situation is particularly critical for the design of office buildings, because the energy consumed by this building type constitutes the most energy usage intensive built environment within the building industry sectors.
This thesis seeks to find a design solution for reducing the energy usage in office buildings, in particular those in the tropics. There are numerous methods and techniques that can be employed to achieve the purpose of designing an energy efficient building and the latest development in façade technology of double-skin façade system claims to be able to reduce energy usage substantially by allowing natural ventilation especially for commercial buildings. With that in mind, the thesis explores the viability of double-skin façades in providing natural ventilation as an energy efficient solution for office buildings in a hot and humid environment.
Double-skin façades (DSF) are multiple layer skin constructions, with an external skin, an intermediate space and an inner skin. The external and 1
internal skins can be either single glaze or double glazed glass panes of float glass or safety glass. An adjustable sun-shading device is usually installed in the intermediate space for thermal controls. This research has involved the study of various types of DSF used in office buildings, and the behaviour of airflow and thermal transfer through the DSF, and the internal thermal comfort levels are analyzed through the use of computational fluid dynamic (CFD) simulations.
1.2
Sustainable development
Environmental damage and current climate change concerns are directly linked to human activity. The economic blueprint for industrialised societies was first publicly questioned in 1968 by the newly founded international think-tank, the Club of Rome. In 1972 members of this group published the now-famous report, “The Limits to Growth”, putting forward the idea that economic development must be combined with environmental protection. In 1984, the United Nations Assembly gave the then Prime Minister of Norway, Gro Harlem Brundtland, the mandate to form and preside over the World Commission on Environment and Development, also known as the Brundtland Commission. The work of the Commission led to the release in 1987 of the report entitled Our Common Future, also called the Brundtland Report, which popularized the term ‘sustainable development’ and its definition as ‘ meets the needs of the present without compromising the ability of future generations to meet their own needs’. Today the Commission’s work has been recognized for having promoted the values and principles of sustainable development.
At the 1992 Rio Earth Summit, heads of states committed their nations to exploring ways of achieving “development which fulfils current needs without compromising the capacity of future generations to fulfils theirs”. The notion of sustainable development was based on an awareness of environmental risk. It was also seen as a social project that seeks to reconcile ecological, economic
2
and social factors. This concept of sustainable development is based on three principles: - Consideration of the “whole life cycle” of materials - Development of the use of natural raw materials and renewable energy sources - Reduction in the materials and energy used in raw material extraction, product use and the destruction or recycling of waste
The Kyoto Summit in 1996 was designed to achieve more concrete measures after the Rio Summit’s emphasis on social and cultural factors. Under the Kyoto Protocol, participating nations pledged to bring average greenhouse gas emissions over the period 2008 to 2012 back to 1990 levels. To keep to this agreement, the industrialised countries need to make progress in three areas: - Reductions in energy consumption - Replacement of energy from fossil reserves by energy from renewable sources - Carbon storing
The principles of the Rio Declaration are connected with the formulation of a development plan for the 21st century, known as Agenda 21. The recommendations in Agenda 21 are: - protection of the earth’s atmosphere - integrated land-use planning and management - combating deforestation - preservation of fragile ecosystems - promotion of sustainable development in a rural and agricultural context - maintenance of biodiversity - an environmentally rational approach to biotechnology - protection of the oceans and coastlines - protection of water supplies and quality - environmentally acceptable treatment of waste, including toxic chemicals, radioactive and other dangerous waste, solid waste and waste water 3
In 2002, the World Summit on Sustainable Development, or commonly called Earth Summit 2002, was help in Johannesburg, South Africa. The participating nations had renewed their commitment to the principles defined in the Rio Declaration and the Agenda 21 objectives. They pledged to develop national sustainable development strategies to be implemented before 2005.
Implementation of the measures agreed at Kyoto has wide-ranging implications in terms of land use, urban planning and architecture. The attempt to reduce the consumption of energy and natural resources, bring down greenhouse gas emissions and produce less waste will have a particularly significant impact on the building and civil engineering sectors.
The application of sustainable development principles to building is one of the most efficient responses we have to the need to reduce greenhouse effect and the destruction of our environment. Such a response is based on three complementary, closely linked tenets: - Social equity - Environmental caution - Economic efficiency
1.3
Energy consumption in commercial buildings
Global consumption of primary energy to provide heating, cooling, lighting and other building related energy services grew from 86 exajoules in 1971 to 165 exajoules in 2002. This is an everage annual growth rate of 2.2% per year (Price et al., 2006). Energy demand for commercial buildings grew about 50% faster than for residential buildings during the same period.
Energy demand in buildings is driven by population growth, the addition of new energy-demand equipment, building and appliance characteristics, climatic
4
conditions, and behavioural factors. The rapid urbanization that is occurring in many developing countries has important implications for energy consumption in the building sector.
The two most important sources of energy demand in the U.S. commercial buildings are space heating, ventilation, and air conditioning (HVAC) systems, which accounted for 31% of total building primary energy use; and lighting, which accounts for 24% of total building primary energy use (USDOE, 2005). The results for large commercial buildings in many other countries are thought to be similar to those for the United States, although no such statistical breakdown is available for other IEA member nations or for the developing world.
The above statistic has called for a great attention to reduce energy usage for commercial buildings to in turn reducing the emission of Green House Gases. This is also the thought behind the aims of this Thesis to focus unto proposing an effective way to reduce energy consumption of office buildings in the tropics to give a ‘little’ contribution to the building sector.
1.4
Air conditioning in office buildings and human comfort
The Larkin Building built in 1960 at Buffalo, USA by Frank Lloyd Wright is thought to have been the first air-conditioned building in the world where cooled air was pumped into the building via specially designed air-ducts. The popularity of the International Style that followed saw the upsurge of buildings with strong geometric forms, with an emphasis on large windows and a curtain wall system.
This preferred style at that time, and the advances in technology caused the dissociation of the buildings’ indoor environment from their surrounding climate. An office building which is not constrained by daylight, ceiling height
5
and plan depth could have a deeper plan, lower ceiling height and greater floorto-envelope ratio, which has a great impact on the occupants and the effect on human comfort is tremendous.
1.5
Façade design for office buildings
The late 19th century saw a time of accelerated economic growth that led to a global building boom. Real estate values skyrocketed, especially in the city center areas and together with the advancement of building technology like steel skeleton construction and the invention of elevators, this led to the creation of the first high-rise building. Skidmore, Owings and Merrill (SOM) in New York achieved the ‘true’ curtain wall system for office buildings during the middle of the 20th century. Since then, glass curtain wall buildings have appeared everywhere, under the influence of the so call International Style, until in the late 20th century office buildings with glass façades had become a normal feature in all the cities around the world and the building facades for our offices had degenerated into monotonous surfaces.
Since the awareness of the need for energy efficiency increased dramatically in the wake of the oil crisis of the 1970s, the design of smooth glass containers for most of our office buildings, which rely heavily on artificial means to provide an acceptable internal environment, has come under intense scrutiny. The building façade has become even more increasingly important in recent years in the areas of research and development as a result of growing awareness of the importance of sustainable living. High-rise buildings are within the critical category, as more than 80% of the façade for these types of building are constructed using some sort of glazing for their envelopes. The urgency to improve the energy usage of our ‘glass-box’ office buildings with original curtain wall design has thus led to the development of ‘intelligent skin’ to ‘clothe’ these ever more demanding functional spaces created by mankind. The term ‘breathing skin’ was also developed for façade systems that allow the
6
required external energy to enter the indoor environment but at the same time to expel any unwanted built-up heat within it. The present technologies have allowed very complex façade systems to be developed and to function according to the clients’ requirements so as to dramatically reduce the energy usage of large buildings.
The availability of technologies, the desire for an all-glass facade and the commitment to improve the energy usage of large buildings had lead to the development of double-skin façade, which originally is a European Union architectural phenomenon believe to be able to improve indoor air quality through natural ventilation without the acoustic and security constrains of naturally-ventilated single-skin facades.
1.6
Energy consumption for office buildings in Singapore
The 2000 World Competitiveness Yearbook, complied by the World Economic Forum (WEF), ranked Singapore 25th out of 45 countries in terms of energy intensity or the amount of commercial energy consumed per dollar of GDP. This could be due to the fact that Singapore depends heavily on airconditioning to cool its buildings all year round. The hike in recent oil prices and the global decline in the supply of fossil resources, together with the growing international concern about carbon dioxide emissions and greenhouse effects, have all resulted in a call for an effective use of energy resources.
The 1998 Kyoto Protocol of the United Nations Framework Convention on Climate Change, to which Singapore is a signatory, established a legally binding obligation on the developed countries to reduce their emissions of carbon dioxide and other greenhouse gases by an average of 5.2% below 1990 levels by the years 2008-2012. Singapore’s energy consumption growth over the period 1980-1995 was 11.9%. The average annual growth in GDP over the same period was around 7.6%. All this means that Singapore will come under
7
increasing international pressure to reduce its CO2 emissions and to directly lower its energy consumption.
Energy consumption in Singapore can be attributed to the three main sectors of industry, residential and commercial buildings, and transport (Figure 1.1). In the hot and humid climate of Singapore, most of the electricity consumed in buildings goes towards air-conditioning and refrigeration, especially in work places like commercial and institutional buildings, which are mostly designed to be fully air-conditioned. Figure 1.2 shows the distribution of electricity consumption in the building sector, with commercial and industrial buildings constituting close to two-thirds of the total electrical energy consumed.
Energy Consumption in Singapore by Sectors Transport 34%
37%
Industries
Buildings (Residential/ Commercial) 29%
Figure 1.1 Energy consumption by sectors (Source: Power Supply Pty Ltd)
8
Electricity Consumption in the Building Sector
25% Public Residential Buildings Private Residential Buildings Commercial/Industrial Buildings
57% 18%
Figure 1.2 Electricity consumption by sectors (Source: Power Supply Pty Ltd)
1.7
Natural ventilation and indoor environment
Most vernacular buildings in the world were naturally ventilated designed, even though some of the buildings have been compromised by the additions of internal walls and mechanical systems. Natural ventilation has become an increasingly attractive method for reducing energy use and costs, and for providing acceptable indoor air quality in order to maintain a healthy, comfortable and productive indoor climate. In favorable climates and building types, natural ventilation can be used as an alternative to air conditioning plants with savings of 10%-30% of total energy consumption.
However, using natural ventilation to prevent overheating within a building presents a great challenge to maintaining acceptable indoor air quality (IAQ) standards. Controlling indoor air quality appears to be more of a concern during winter periods when interior spaces need to be heated to provide
9
acceptable thermal comfort and most of the windows in a building may be closed. The control of airflow rates then becomes the ultimate consideration.
For summertime cooling, important considerations are internal heat loads, external solar gains, building characteristics such as thermal mass and insulation levels, the overall building floor area, and site layout. Controlling airflow rates is not as much of a concern here, as long as the occupants are comfortable. The higher the airflow rate the greater the cooling effect.
1.8
Thermal comfort standards
There are a number of international thermal comfort standards that have make substantial contribution to the knowledge of thermal comfort. The main thermal comfort standard is ISO 7730, which is based upon the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) thermal comfort indices (Fanger, 1970). The standard also provides methods for assessing the local discomfort caused by draughts, asymmetric radiation and temperature gradients. Other thermal comfort standards include ISO 8996, which describes six methods for estimating the metabolic heat production and it is an important requirement in the use of ISO 7730 and the assessment of thermal comfort. ISO 9920 provides a database of the thermal properties of clothing and garments that based upon measurements on heated manikins, and ISO 7726 supports thermal comfort assessment with measuring instruments.
Thermal comfort research carried out in Europe and the USA during the mid20th century was mainly concentrated on using climate chamber studies. The thermal comfort standards of ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, were first established in 1966. Since then, there had been a number of revisions to incorporate the latest understanding and findings of thermal comfort. The ASHRAE Standard 55 derived its results from laboratory experiments using a thermal-balance model of the human
10
body. Six key variables were identified as affecting the perception of thermal comfort, namely air temperature, radiation, relative humidity, air movement, clothing and metabolic rate. The standard attempted to provide an objective criterion for thermal comfort by specifying personal and environmental factors that will produce acceptable interior thermal environment for at least 80% of a building’s occupants. The standard defined thermal comfort as ‘the condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation’ (p. 2). It also defines thermal sensation as a conscious feeling, commonly graded into categories of cold to neutral to hot.
The ASHRAE Standard 55 was originally developed to provide guidelines for centrally controlled HVAC (Heating, Ventilation and Air-Conditioning) systems. The general application of the standard has limited the efforts to develop more person-centered strategies for thermal control in naturally ventilated or mixed-mode buildings. Such strategies may provide important social and environmental benefits through energy consumption reduction and increase occupant satisfaction and work efficiency, especially in office buildings.
1.9
Adaptive thermal comfort model and natural ventilation in buildings
The primary limitation of the original ASHRAE Standard 55 is its “one-sizefits-all” approach where clothing and activity are the only modifications one can make to reflect seasonal differences in occupant requirements. The standard has allowed important cultural, social and contextual factors to be ignored which lead to an exaggeration of the “need” for air conditioning in indoor environment. In view of the standard limitation, many researchers argued that the level of occupant satisfaction with indoor environment and the energy consumption of buildings could be reduced if we allowing people greater control of their indoor environments. This has lead to the development 11
of adaptive thermal comfort model with consideration for naturally ventilated buildings.
The latest ASHRAE Standard 55 -2004 has incorporated the adaptive thermal comfort model with an analytical method based on the PMV-PPD indices and the introduction of the concept of adaptation with a separate method for naturally conditioned buildings. The standard is intended for use in design, commissioning and testing of new or existing buildings and other occupied spaces (residential or commercial) and their HVAC systems.
One important criterion in applying an adaptive model for thermal comfort like ASHRAE Standard 55-2004 is the possibility of individual control. Occupants of naturally ventilated buildings have possibilities for changing the air velocity in the indoor environment by operating the windows and can often create an acceptable environment even with a relatively high indoor temperature. Psychological adaptation also plays an important part in naturally ventilated buildings because the occupants have a more direct contact with the external weather, and higher temperatures are expected for the indoor environment.
1.10 Thermal comfort analysis and double-skin façades A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin facades. Even though most of the research has been done in temperate conditions, it has revealed a close link between natural ventilation design and the function of a double-skin façade.
Grabe et al. (2001) developed a simulation algorithm to investigate the temperature behaviour and the flow characteristics of double façades with natural convection through solar radiation. Ziskind et al. (2002, 2003), Bansal et al. (1994), Hamdy and Fikry (1998), and Priyadarsini et al. (2003) reported
12
similar natural convection ventilation studies. Most of these have used the concept of stack effect or the solar chimney and found that passive ventilation in summer is possible even for multi-storey buildings. In particular Priyadarsini et al. (2003) have established the energy efficiency of a stack system used in residential buildings in a hot and humid climate region. Li and Delsante (2001) went a step further to investigate the effects of natural ventilation caused by wind and thermal forces in a single zone building with two openings. Ventilation graphs are plotted using the air change parameters (thermal air change, wind air change and the heat loss air change) for design purposes.
Gratia and Herde (Gratia and Herde 2004) also attempted to look at the impact of double-skin façade facing a southern direction in a temperate climatic condition. Thermal analysis using simulation software for the different seasons of a year was done for a low-rise office building with and without double-skin façade. It was found that significant energy saving is possible if natural ventilation could be exploited through the use of a double-skin façade.
1.11 Research questions Natural ventilation strategies and double-skin construction are not new concepts and much research and development has been done to improve on those ideas. In fact most of the vernacular architecture in the tropics uses much natural ventilation concepts in ventilating the indoor environment. Double-skin curtain walls were also first used at the Steiff Factory in Giengen, Germany during the early twentieth century.
Although attempts have been made in recent years to use double-skin façades to introduce natural ventilation into high-rise buildings in Europe, China and Hong Kong, most of these buildings are located in the temperate countries. There are still questions remains to be answered that formed the basic research questions for this thesis as follow:
13
•
Whether double-skin façades are able to provide acceptable indoor conditions for the high-rise buildings occupants under natural ventilation strategies in hot and humid climate regions.
•
If that is possible, the question will be what is the ‘opening window’ regime during the day would need to be, so that natural ventilation could be introduced.
•
This will also lead to the question of the possibility for formulating some useful guidelines for designing double-skin façade for high-rise buildings in the tropics.
The above are the main questions raised in this Thesis that hoped to answer satisfactory and it was discussed in greater length in Chapter 6.
1.12 Scope of research This research seeks to find a design solution for reducing the energy usage in high-rise office buildings in the tropics, and more specifically in the tropical island of Singapore. There are numerous methods and techniques that could be employed to achieve the purpose of designing energy efficient buildings. The thesis explores the viability of double-skin façades to provide natural ventilation as an energy efficient solution for office buildings in a hot and humid environment by using computational fluid dynamic simulations and a case study methodology.
14
1.13 Methodology Computational Fluid Dynamic (CFD) has become a useful tool for designers in the study of indoor and outdoor environmental conditions in building design especially for the close observations of thermal and energy transfer between different environments. Furthermore, the parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment. CFD techniques have been applied with considerable success in building design and their advantages in analyzing ventilation performance have been reported by Murakami (1992) and Liddamant (1992). Papakonstantinou et al. (2000) have demonstrated that numerical solutions for ventilation problems can be obtained quickly and in good agreement with experimental measurements.
CFD simulations for a series of modular office spaces will be studied and analyzed and the emphasis will be on the use of natural ventilation strategies in providing acceptable indoor environment conditions in a tropical environment like Singapore. The results will be compared against the findings from the case studies for double-skin façade buildings completed in recent years, to study the differences and to learn of any constructive lessons.
A comprehensive methodology in simulating the high-rise office buildings is proposed and a new type of double-skin façade configuration for the use in the tropics is recommended. A series of initial design rule of thumb in the form of nomograms are proposed at the end for designers who wanted to design highrise office buildings using double-skin façade in the hot and humid climate.
1.14 Structure of Thesis The first three Chapters of the Thesis will look at the research and findings of thermal comfort studies and standards formulated in the world, the various natural ventilation 15
strategies and their positive impact on high-rise office building design in reducing energy consumption, the use of double-skin façade technologies in achieving energy efficient building designs, and how all these could help in providing an option for the future of high-rise office building design in the tropic. These Chapters will explain the gap in the research and the supportive arguments for carrying out the whole painstaking work for this thesis.
Chapters 4, 5 & 6 will explain the methods used to achieve the goals of formulating some design guidelines for using double-skin façades for high-rise office buildings in a hot and humid climate.
Chapters 7, 8 & 9 will present the results and the findings of the research and propose some options for naturally ventilating high-rise buildings in the tropic.
Chapter 10 will list the achievements of the research work and the limitations and recommend future follow-up to the work that has been done.
16
Chapter 2
Thermal Comfort In Hot and Humid Climates
This Chapter provides a general overview of what are thermal comfort and the development of adaptation models for supporting the introduction of natural ventilation strategies to be used in double-skin facades system.
2.1
Evolvement of thermal comfort studies
In the early 1920s Houghten and Yagloglo (1923) attempted to define the ‘comfort zone’ at the ASHVE (American Society of Heating and Ventilation Engineers) laboratories. In England, Vernon and Warner (1932) and later Bedford (1936) carried out empirical studies among factory workers in relation to industrial hygiene. However, the studies of thermal comfort got their real momentum during and after World War II , involving not only fields like engineering, but also the areas of physiology, medicine, geography and climatology. In architecture, Victor Olgyay (1963) was the first to bring together findings of the various disciplines and interpret these for architectural purposes.
Although thermal comfort studies began more than a century ago, more significant research was carried out by Fanger in 1970, explaining that thermal comfort is an influential factor in human performance and that man’s intellectual, manual and perceptual performance is at the highest when he is experiencing thermal comfort. An improvement in environmental conditions occurred as a result of people spending most of their lives in an artificial climate. The aim of creating artificial climates was to adaptat the thermal environment so that every individual is in a state of thermal comfort. Ruck (1989) went as far as to say that the human factor is the principal concern in the design of buildings, where human well-being and performance should be considered as much as the human need for a suitable and stimulating environment.
17
2.2
Definitions of thermal comfort
The definitions for thermal comfort are manifold and it is a difficult task to pinpoint, which is most accurate, and which best explains the state of human response. The following are some acceptable definitions of thermal comfort:
Fanger (1970, p13) defined thermal comfort for a person as the condition of mind that expresses satisfaction with the thermal environment. Due to the biological variations in people, the aim is to create optimal thermal comfort in such a way as to provide that the highest possible percentage of a group’s feels thermal comfort.
Givoni (1976, p3) defined thermal comfort as the absence of irritation and discomfort due to heat or cold, and as a state involving pleasantness.
O’ Callaghan (1978, p43) defined thermal comfort as the study of the effects of climatic impact on human response.
The ASHRAE (2004, p.2) definition of comfort is ‘the condition of mind that expresses satisfaction with the thermal environment; it requires subjective evaluation’. This clearly embraces factors beyond the physical or physiological. Figure 2.1 shows schematically the thermal exchange between the human body and its environment through radiation, evaporation and convection processes.
18
Figure 2.1 Thermal exchanges between the human body and its environment (Source: Design Primer for Hot Climate, Konya, 1980, p.26)
2.3
Important parameters in thermal comfort
The variables that affect thermal comfort can be grouped in to three categories, namely environmental, personal and contributing factors. Environmental factors include air temperature, air movement, humidity and radiation. Personal factors means metabolic rate or activity and clothing. Contributing factors include food and drink, acclimatization, body shape, subcutaneous fat, age and gender.
Air temperature is the most important environmental factor and is measured by the dry bulb temperature (DBT). This will determine the convective heat dissipation with any air movement. Air movement is measured in m/s (velocity, v) and it affects the evaporation of moisture from the skin and thus gives an evaporative cooling effect. Humidity in the air will affect the
19
evaporation rate and is expressed by relative humidity (RH, %), absolute humidity or moisture content (AH, g/kg), or vapour pressure (p, kPa). Radiation exchange will depend on the mean temperature of the surrounding surfaces and is referred to as the mean radiant temperature (MRT). The mean radiant temperature cannot be measured directly but can be approximated by globe temperature measurements.
The metabolic rate may be influenced by food and drink, and the state of acclimatization. Clothing is one of the dominant factors affecting heat dissipation. The unit for the thermal comfort measurement of the clothing effect is clo. This corresponds to an insulation cover over the whole body of a transmittance (U-value) of 6.45 W/m2K (a resistance of 0.155 m2K/W). 1 clo is the insulating value of a normal business suit with cotton underwear. Shorts with short-sleeved shirts would be about 0.25 clo, heavy winter suit with overcoat will give around 2 clo and the heaviest arctic clothing is around 4.5 clo (Szokolay, 1997, p.9).
2.4
Measurement of thermal comfort
The ASHRAE Scale used in laboratories and the Bedford Scale used in field studies are the most frequently applied scales, producing similar results for human comfort experiments. They use a seven-point scale, where 3 means hot and –3 means cold. Table 2.1 below shows the comparison of the two scales.
In laboratory studies factors influencing thermal sensation, especially clothing, are reduced to a minimum, and an independent environmental variable is manipulated, while the dependent variable, comfort level are isolated from external influences (de Dear, Leow and Foo, 1991). In field studies, personal factors are left uncontrolled, so the results are more representative of real-life conditions (Ruck, 1989).
20
Table 2.1 Comparison of thermal comfort scales (Source: PLEA Notes: Thermal Comfort, 1997, p.15)
The original ASHRAE scale used numbers from 1 to 7 where 1 meant cold and 7 meant hot. The ‘graphic scale’ used by Woolard in a Solomon Islands study shown in Figure 2.2 below used a scale from 1 to 7.
Figure 2.2 Representation of graphical comfort scale (Source: PLEA Notes: Thermal Comfort, 1997, p.15)
Olgyay (1953) in his bio-climatic chart (Figure 2.3) has the dry-bulb temperature (DBT) on the Y-axis and the relative humidity (RH) on the X-axis. The aerofoil shape in the middle is the comfort zone. Curves above the aerofoil 21
show how air movement can extend the upper limits of thermal comfort and lines below the aerofoil show how radiation could extend the lower limits of the comfort zone. According to Olgyay, cool-humid conditions are often referred to as dank and hot-dry as torrid or scorching.
Figure 2.3 Olgyay’s bio-climatic chart in metrics, modified for warm climates (Source: Introduction to Architectural Science, Szokolay, 2004, p. 21)
2.4.1 Psychrometric chart
The atmosphere is a mixture of air and water vapour. The science dealing with this mixture is called psychrometry and the graphic representation of various attributes of this mixture is the psychrometric chart (Figure 2.4). The attributes represented in the chart are dry bulb temperature (oC), absolute humidity (g/kg), saturation humidity, relative humidity lines (%), wet bulb temperature lines (oC), specific volume lines (m3/kg), and enthalpy lines (kJ/kg).
Psychrometric process can be traced on the chart by locating the status point on the chart of known quantities and all other quantities can then be read from the chart. At the center of the chart, at DBT 25oC and RH 50%, a small circle marks a reference point, to be used in conjunction with the uppermost scale. This is often used in air conditioning calculations. By locating the status point of outdoor air and projecting a line from the reference point through this status 22
point to the uppermost scale, it will give the sensible heat/total heat (HS/H) ratio.
Figure 2.4 Psychromertic Chart (Source: PLEA Notes: Thermal Comfort, 1997, p. 13)
2.4.2
Thermal comfort indices
Most of the indices of warmth were developed within the first 50 years of the 20th century, both empirically and analytically, and were established from controlled chamber studies with fit young Americans and Europeans. Inevitably they specify an optimum value that has been assumed to apply equally to all people.
Empirical indices and analytical indices were developed mainly for defining limits of comfort, setting exposure thresholds, and determining the optimum control measures for thermal comfort. Examples of some of the empirical indices are: 23
•
Effective temperature (Houghten and Yagloglou, 1923),
•
Operative temperature (Winslow, Herrington & Gagge, 1937),
•
Wet bulb globe temperature (Yaglou and Minard, 1957),
•
Equivalent temperature (Dufton, 1932 & 1933),
•
Equatorial comfort index (Webb, 1960),
and some of the examples of the analytical indices are: •
Predicated 4-hour sweat rate (McArdle and collaborators, 1947),
•
Index of thermal stress (Givoni, 1963),
•
Predicted mean vote (PMV),
•
Standard effective temperature (Nishi and Gagge, 1977),
•
Index of thermal sensation (Gagge).
Macpherson (1962) suggested that there were many factors not recognised by the various indices and that the most important of these was acclimatization. The static models like the PMV approach denies the role of acclimatization. O’ Callaghan (1978) developed models for thermal comfort in three areas of human response, namely physical, physiological and sociological. The physical model defined the body as a thermal system, in which heat exchange between the body and the environment through the skin and clothing occurs. The physiological model explained the subjective responses to the thermal environment and the involuntary actions that occur when the body is outside the neutral state, like sweating and shivering. The sociological model denotes the factors that mostly prevent the application of the accepted human comfort criteria to the environmental conditioning of interiors.
Fanger’s (1970) comfort equation is probably the most meticulous and detailed analysis of human thermal relationships with the proximal environment. He stated that the thermal balance of the body is influenced by air temperature, mean radiant temperature, relative air velocity and relative humidity, and by personal parameters called activity level or metabolic rate and clothing thermal
24
resistance. The dependence of these parameters on each other in providing thermal comfort was discussed by Kut (1970).
The PMV (predicted mean vote) and the PPD (predicted percentage dissatisfied) form the basis of the formulation of ISO 7730:1994, Determination of the PMV and PPD indices and specification of conditions for thermal comfort. According to the standard, the PMV equation can be written as: PMV = (0.303 x e-0.036xM + 0.028) x {(M-W) – -3.05 x 10-3 x [5733-6.99(M-W)-pa] – -0.42 x [(M-W)-5815] – -1.7 x 10-5 M(5867-pa) – -0.0014 x M(34-ta) – 3.96 x 10-8 x fcl x [(tcl+273)4 – (tr+273)4] – -fcl x hc x (tcl - ta) } where: fcl is the ratio of man’s surface area while clothed to man’s surface area while nude; ta is the air temperature oC; tr is the mean temperature oC; pa is the partial water vapor pressure Pa; hc is the convective heat transfer coefficient W/(m2K); tcl is the surface temperature of clothing oC. The PMV equation above can be calculated for different combinations of metabolic rate, clothing, air temperature, mean radiant temperature, air velocity and air humidity. The tcl and hc can be solved by iteration. The PMV index predicts the mean vote of the votes of a large group of persons on the following 7-point thermal sensation scale:
+3=hot, +2=warm, +1=slightly warm, 0=neutral, -1=slightly cool, -2=cool, -3=cold
The predicted percentage dissatisfied (PPD) is found as a function of the predicted mean vote (PMV) from the equation below and can be represented as the graph in Figure 2.5.
25
PPD = 100 – 95 x e-(0.03353 x PMV4 + 0.2179 x PMV2)
Figure 2.5 The PPD as a function of PMV (Source: ISO 7730:1994, Determination of the PMV and PPD indices and specification of conditions for thermal comfort, p.3)
Auliciems (1981) went on to propose a psycho-physiological model of thermal comfort, which is also the basis of his adaptation hypothesis. Later on, the twonode model of the JB Pierce laboratories and the ET* (and SET) indices derived from this form the basis of ASHRAE Standard 55-1992: Thermal environmental conditions for human occupancy.
2.4.3 Thermal comfort studies
Dry bulb temperature is the most useful measure for the specification of comfort. For the measurement of the magnitude of discomfort or stress, the other environmental factors like humidity, radiation and air movement should be taken into consideration. Most of the thermal comfort models used the DBT (dry bulb temperature) as an index of thermal comfort or neutrality. Drysdale (1950) had demonstrated that at or near comfort level the best measure of thermal conditions is the dry bulb temperature. Macpherson (1962) found that the simpler the index chosen, the more likely it is to prove satisfactory and the 26
simplest index of all is the DBT; and also under ordinary conditions in still air the DBT in itself is a better index of warmth than is effective temperature and any other composite index.
In recent years, there have been further findings in the effectiveness of the thermal indices developed to represent thermal comfort in specific climatic zones. Williamson et al. (1995) found that the PMV overestimates warm discomfort, especially in warm climates. Karyono (1996) found that people in South East Asia (hot and humid climate) prefer up to 6K higher temperature than suggested by Fanger, and this is explained as the result of adaptation of people to higher outdoor temperatures.
De Dear, Leow and Foo (1991) carried out a study on both air-conditioning and naturally ventilated buildings in Singapore and found that air-conditioned buildings showed similar results but naturally ventilated buildings showed 3K warmer than Fanger’s values. De Dear, Leow and Ameen (1991) had also carried out climate chamber experiments on thermal acceptability in Singapore. They found that the upper limit of the acceptable comfort zone at 70% RH was 27.6oC and at 35% RH was 27.9oC. The results were in line with the predictions of the current international comfort standard, ISO 1984, despite the fact that the empirical bases of the standard were subjects from much colder climates in northern Europe and the US.
2.5
Questions of adaptability and human comfort
Physiological neutrality or thermal equilibrium does not necessarily mean comfort but other factors such as past experiences, socio-cultural factors, habits and expectations will influence perceptions of thermal comfort. The original ASHRAE standard 55 was developed through laboratory tests of perceived thermal comfort with the limited intent of establishing optimum HVAC levels for fully climate-controlled buildings. Therefore the standard was initially
27
applied universally across all building types, climates and populations. As a result, even in relatively mild climatic zones it was hard to meet the standard’s requirement of thermal comfort without mechanical systems. Many argued that the standard ignored the importance of cultural, social and contextual factors. It was argued that giving people greater control of indoor environments and allowing temperatures to more closely track patterns in the outdoor climate could improve levels of occupant satisfaction with indoor environments and reduce energy consumption. This argument was supported by the research done by de Dear and Brager (1998), which they found that when occupants have control over operable windows and are accustomed to conditions that are more connected to the natural swings of the outdoor climate, the subjective notion of comfort and preferred temperatures changes as a result of the availability of control, of different thermal experience and of resulting shifts in occupant perceptions or expectations. Such issues are particularly relevant with regard to naturally ventilated buildings.
By responding to the above questions, an alternative thermal comfort standard based on field measurements could account for contextual and perceptual factors absent in a laboratory setting. Research focusing on three primary modes of adaptation, namely physiological, behavioral and psychological, emerged to deal with the issues. Physiological adaptation (also known as acclimatization) refers to biological responses that result from prolonged exposure to characteristic and relatively extreme thermal conditions. Behavioral adaptation refers to any conscious or unconscious action a person might make to alter their body’s thermal balance. The psychological adaptation refers to an altered perception of and reaction to physical conditions due to past experience and expectations. This research work led to the formation of the latest ASHRAE Standard 55 in 2004.
28
2.6
Adaptive thermal comfort model
Auliciems (1981) formulated an adaptive model of thermoregulation within which thermal preference was seen as the result of both physiological responses to immediate indoor parameters (those measured by the indices) and expectations based on ‘climato-cultural’ determinants (past experiences). Figure 2.6 below shows the adaptive model by Auliciums.
Figure 2.6 The psycho-physiological model of thermal perception: the adaptive model (after Auliciems, 1981)
The adaptive model subsequently was investigated and verified by de Dear in Darwin (1985), Schiller and Auliciems in San Francisco Bay Area (1988), Busch in Bangkok (1990), de Dear, Leow and Ameen in Singapore (1991) and de Dear and Fountain in Townsville (1994). Much more discussion on the matter was carried out and it became evident that the notion of a constant or static optimum was no longer an acceptable hypothesis. In a major report to 29
ASHRAE, de Dear, Brager and Cooper (1997) exhaustively analysed all research reports from both naturally ventilated and mechanically controlled buildings, and concluded that while a mechanistic model of heat transfer may well describe the responses of people within closely controlled thermal environments like air-conditioned space, it is “… inapplicable to naturally ventilated premises because it only partially accounts for processes of thermal adaptation to indoor climate.”
Figure 2.7 shows the comparison of responses by people at the same location in a different environment, i.e. air conditioned buildings and naturally ventilated buildings. The observed results are even higher than the adaptive model predictions.
Figure 2.7 Neutralities predicted and compared with results of field experiments (Source: PLEA Notes: Thermal Comfort, 1997, p.47)
30
2.7
The ASHRAE Standard 55-2004
The ANSI/ASHRAE Standard 55-2004, Thermal Environment Conditions for Human Occupancy, deals with thermal comfort in the indoor environment. It may be used for residential or commercial buildings or for new or existing buildings. It can apply to occupied spaces such as means of transportation like cars, trains, planes and ships.
The standard defines an acceptable thermal environment as one in which there is 80% overall acceptability, based on 10% dissatisfaction criteria for general thermal comfort plus an additional 10% dissatisfaction that may occur on average from local thermal discomfort. The standard also specifies separate percentages of acceptability levels for the various physical variables that may cause local discomfort, with ranges from 5% to 20%.
The standard does not cover hot or cold stress in thermally extreme environments or comfort in outdoor spaces. It also does not address nonthermal environmental conditions like air quality or acoustics or the effect of any environmental factors on non-thermal human responses like the effect of humidity on health.
The standard includes requirements for providing thermal comfort using the PMV-PPD method for determining acceptable operative temperatures for general thermal comfort followed by additional requirements for humidity, air speed, local discomfort and temperature variations with time.
The PMV-PPD method of calculation to determine the comfort zone is incorporated into Standard 55, which is more consistent with ISO 7730 that will provide a more accurate estimation of the acceptable range of the thermal conditions for a particular situation. Using the PMV-PPD model, the acceptable range of operative temperatures is shown in a psychrometric chart for people wearing two different levels of clothing: 0.5 clo (typical for summer or cooling season) and 1.0 clo (typical for winter or heating season). The
31
graphic comfort zones (Figure 2.8) correspond to a PPD of 10% (thermal discomfort). The graphic zones are only applicable for metabolic rates between 1.0 to 1.3 met and air speed of less than 0.20 m/s.
Figure 2.8 Acceptable ranges of operative temperature and humidity (Source: ANSI/ASHRAE Standard 55-2004, Section 5.2.1)
The operative temperature limits in the standard are based on a limit of air speed of less than 0.20 m/s, but higher levels of air movement can be beneficial for improving comfort at higher temperatures. Figure 2.9 below shows the relationship between elevated air speed and a rise in temperature above the upper limit of the comfort zone. This graph is especially important for commercial buildings that are primarily in cooling mode in order to reduce energy use while improving comfort.
32
Figure 2.9 Air speed required offsetting increased temperature (Source: ANSI/ASHRAE Standard 55-2004, Section 5.2.3)
Section 5.3 of the standard is a new method for determining acceptable thermal conditions in naturally conditioned spaces. It is applicable to spaces where the thermal conditions are regulated primarily by the occupants through opening and closing of windows, with no mechanical cooling and with metabolic rates ranging from 1.0 to 1.3 met. Figure 2.10 below shows the graph for the range of acceptable operative temperatures as a function of outdoor temperature. The model has accounted for people’s clothing adaptation and local thermal discomfort and no humidity limits and air speed limits are required for using this method.
33
Figure 2.10 Acceptable operative temperature ranges for naturally conditioned spaces (Source: ANSI/ASHRAE Standard 55-2004, Section 5.3)
Building systems should be designed so that at design conditions the thermal conditions of the spaces can be maintained within the specifications and also within the expected extreme conditions. While the ASHRAE standard specifies conditions that will satisfy 80% of the occupants, that still leaves 20% dissatisfied. The best way to improve upon this level of acceptability is to provide occupants with personal control of their thermal environment, enabling them to compensate for inter- and intra-individual differences in preference.
2.8
Passive solar design in a hot and humid climate
The first step in the bioclimatic design approach is to examine the climatic conditions of a building’s location and to establish the nature of the climatic problem in order to relate the climate to human requirements. One good way is to plot out the comfort zone onto the psychrometric chart. The next step would be the choice of a passive control strategy.
34
The four design variables that have the greatest influence on thermal performance for buildings are shape, fabric, fenestration and ventilation. In most countries there are regulatory requirements for the insulation of envelope elements, walls, roof and windows. These may stipulate a maximum U-value or a minimum R-value that must be achieved by the construction. A quick look at any table of U-values would let one know that the weakest point of any building envelope is the window, which depends on elements like glazing, frame and exposure. A window with a sealed double glaze unit would have a U-value of 2.7 to 4.5 W/m2K, depending on the type of frame. A good window must perform five functions, namely to provide a view; to admit daylight; reduce heat loss; admit solar heat during cold condition and allow controllable ventilation. Passive solar heating in its simplest form requires no more than a good window facing the equator.
A sensible air velocity can be relied on to provide physiological cooling. The critical point is to ensure an air velocity at the body surface of the occupants. This may be provided by cross-ventilation by means of the wind effect. A stack-effect alone, which relies on the rise of warm air, cannot be relied on for this purpose.
Cross ventilation demands that there should be both an inlet and an outlet opening. The difference between positive pressure on the windward side and negative pressure on the leeward side provides the driving force (Figure 2.11). The inlet opening should face within 45o of the wind direction dominant during the most overheated periods. To produce the maximum total airflow through a space, both inlet and outlet openings should be as large as possible. The inlet opening will define the direction of the air stream entering. To get the maximum localized air velocity, the inlet opening should be much smaller than the outlet. The positioning of the inlet opening, louvers or other shading devices as well as the aerodynamic effects outside will determine the direction of the indoor air stream.
35
Figure 2.11 Cross ventilation (Source: Introduction to Architectural Science, Szokolay, 2004, p.16)
In a hot and humid climate the diurnal variation is very small (often less than 5K); thus the mass effect in providing thermal comfort cannot be fully relied upon. As the humidity is high, evaporation from the skin is restricted and evaporative cooling might not be effective or desirable. The designer must ensure that the interior does not become warmer than the outside by providing adequate ventilation to remove any excess heat input. Beyond the prevention of heat gains the only passive cooling strategy possible is the physiological cooling effect of air movement through cross ventilation. The solar orientation should be the dominant consideration, as we cannot influence the solar incidence. North and South walls could have large openings and rooms could be arranged in one row, to allow both inlet and outlet openings for each room for ventilation purposes.
Natural ventilation has served as an effective passive cooling design strategy to reduce energy usage in the tropical regions. The vernacular architecture in those regions has long been using this strategy to cool their buildings and the results are promising. But lives in the past two to three decades have changed dramatically. People tend to be more accustomed to air-conditioned environments, especially in the urban areas. These dramatic needs for large amounts of electricity and fuel have alarmed many due to the depreciation of natural resources and negative environmental impacts on the whole world. The development of ‘green building’ designs has captured the attentions of many and ways have been fought to introduce passive designs to modern buildings especially to high-rise buildings in urban areas, as these huge urban built environments usually constitute more than one-third of the energy usage in 36
cities. Therefore passive design strategies and new construction technologies have in recent times been introduced to design ‘environmentally friendly’ highrise buildings in various parts of the world in the past ten to fifteen years.
In the next two Chapters natural ventilation designs in tropical regions will be discussed in depth and the focus will be on how natural ventilation strategies could be applied to high-rise office buildings with the use of the newly developed double-skin façade systems.
37
Chapter 3
Natural Ventilation Design In a Hot and Humid Climate
This Chapter provides the important design issues and strategies for natural ventilation in high-rise office buildings in particular for the hot and humid climatic conditions.
3.1
Introduction
Natural ventilation can supply fresh air for interior spaces, the cooling of the interior by convection and the cooling of the inhabitants under certain circumstances. The forces producing natural ventilation in buildings result from air changes caused by differences in temperature, the so-called ‘stack effect’, and by air movement or flow produced by pressure differences. Even though the movement of air at a relatively slow pace resulting from the stack effect may be adequate to supply fresh air and produce convection cooling, these forces are rarely sufficient to create the required air movement for thermal comfort in certain hot zones of a living space. The only natural force one can rely on for this purpose is the dynamic effect of wind, and great effort must be made to capture this force.
Figure 3.1 Natural ventilation through buildings (Source: Design Primer for Hot Climate, Konya, 1980, p.52)
38
3.2
The choice of natural ventilation
A good design for energy efficient buildings should include, but not be limited to, the following criteria:
A good performance of the building envelope An appropriate selection of heating and cooling systems and daylight An acceptable quality of indoor climate in terms of thermal comfort, ventilation effectiveness and indoor air quality
The conscious effort to reduce energy consumption after the 1973 oil crisis saw the reduction of energy usage in the building sector, but it also resulted in the emergence of sick building syndrome and building related sickness among building occupants. This was mainly due to the effort to increase the insulation levels of building envelopes and to reduce air infiltration by sealing the building envelope in order to reduce energy losses from the building. This led to a rethink of the original function of the built environment, which was to protect the occupants against harsh outdoor climates and at the same time to provide a comfortable and healthy indoor environment. The conservation of energy became the new concern. This set in train the new era of ‘energy efficient design’ in the 1990s; of design integrating the passive concepts of heating, cooling and indoor climate conditioning.
In relation to the criterion of good energy efficient design and the issue of unhealthy built environments, natural ventilation appears to be a very attractive solution to ensure both good indoor air quality and acceptable comfort conditions in many regions. Natural ventilation seems to provide an answer to many complaints about the use of mechanical ventilation systems, and it can provide a more energy efficient, healthier and more comfortable environment if integrated properly.
However ‘natural’ also means that behavior will be random, and efficient control of the building will be difficult. Furthermore, in many urban
39
environments outdoor air conditions and acoustics may not be acceptable because of air and noise pollution. Natural ventilation in this kind of situation will need special design features in order to avoid a direct link between indoor and outdoor environments. In order to be effective, natural ventilation also requires a high degree of permeability within the building and this can cause security risks and conflicts with fire or safety regulations. In the case of deepplan design, fresh air delivery or a good mixture of air may not be possible without special design considerations.
Natural ventilation design therefore certainly requires careful design considerations at a very early stage of the design process and needs to involve all professional expertise to produce an energy efficient and healthy building.
3.3
Natural ventilation and indoor air quality
Optimum indoor air quality may be defined as air that is free of pollutants that cause irritation, discomfort or ill health in the occupants. A poor environment can manifest itself as a sick building, in which occupants experience symptoms of illness during the period of occupation.
Natural ventilation as a strategy for achieving acceptable indoor air quality is essentially based on the supply of fresh air to a space, and the dilution of indoor pollution concentrations (Liddament et al., 1990). The effectiveness of the ventilation needed to ensure acceptable indoor air quality depends on the amount and the nature of the dominant pollutant source in a space, and quality of the incoming air. If the emission characteristics are known, it is possible to calculate the ventilation rate necessary to prevent the concentration from exceeding a pre-defined threshold concentration. Wouters et al. (1996) developed a simple graph to demonstrate the ventilation rate required to achieve acceptable indoor air quality with natural ventilation design (Figure 3.2). The graph shows that the pollution level decreases exponentially with the
40
airflow rate and if we know the recommended pollution level we can easily define the required airflow rate. In practice, if a sufficient ventilation rate is achieved to control the dominant pollutant, it will be sufficient to maintain the remaining pollutants below their respective threshold concentrations.
Figure 3.2 Ventilation rate for good indoor air quality (Source: Natural Ventilation in Buildings, Allard F., 1998, p.3)
For a naturally ventilated building the energy demand will increase directly in proportion to the ventilation rate, will vary as a function of time and depend on the wind characteristics and the thermal state of the building. Occupant behavior such as opening or closing windows and doors will have a substantial impact on the total energy consumption of a building. Figure 3.3 shows the effect of airflow rate as a function of both the pollution level and the energy demand.
41
Figure 3.3 Relation between airflow rate, pollution level and energy demand (Source: Natural Ventilation in Buildings, Allard F., 1998, p.4)
3.4
Development of sustainable designs in buildings
The natural ventilation concept works well in temperate countries, especially in Europe, not only because of a culture that favors generous ventilation to provide air freshness, and the commitment to reduce energy usage, but also because it is strongly associated with people’s love of the outdoor environment. This has produced a wider tolerance for temperature fluctuations than in the US culture, where a constant temperature environment at a low set point of around 68 oF (20 oC) is preferred during the summer. In the United Kingdom, ‘green’ building practices are also considered progressive, socially responsible and correct. As European economies have not enjoyed the same amount of success as the United States economy, every construction project that is funded is highly visible to the public and must demonstrate long-term sustainability. To this end, in the UK the Building Research Establishment (BRE) has created a rating system (Environmental Assessment Method) that gives an indication of the environmental-friendliness of a building. This system is an effective way of encouraging green building design, and a building that is considered ‘green’ becomes not only a good reflection on the designers but also on those that work in the building.
42
The US Department of Energy (DOE) is also very much aware of the need for sustainability and its Center of Excellence for Sustainable Development as well as the Energy Efficiency and Renewable Energy Network (EREN) encourages green buildings that are designed using an integral approach. These energy efficient buildings should be designed to promote conservation of energy resources, use renewable energy, conserve water, consider environmental impacts and waste management, reduce operation and maintenance costs, etc. A green building is ideally designed with its entire life cycle in mind.
3.5
Natural ventilation designs
Natural ventilation is thought of as a low energy cooling strategy which can provide year round comfort, with the availability of user control, and at a low capital and maintenance cost. A key consideration in adopting natural ventilation is climate. But climate is not necessarily the primary barrier to the use of natural ventilation in building design. The main problem could be a lack of design tools and of an understanding of the principles of natural ventilation.
The use of natural ventilation is not a new concept by any means, and civilizations have used a great deal of creativity to maintain thermal comfort for their built environments. This has been expressed particularly in vernacular architectures. Air-conditioning was only extensively used in building design from the early twentieth century, for many reasons, one of which was the ‘abundant’ availability of ‘cheap energy’ during that time. This gave the designers the ‘freedom’ to use air conditioning.
The general design guidelines and criteria for natural ventilation design involve consideration of: •
Site design – location, orientation and layout of buildings on site including landscaping;
43
•
Design program – indoor air quality requirements, ventilation cooling requirements;
•
Building design – building form, distribution of internal spaces, location and sizing of openings;
•
Opening design – selection of types of opening and their operational features.
Physical features such as neighboring building walls, trees etc., which may influence air movement, must be taken onto account in natural ventilation design as shown in Figure 3.4. There is a difference between the shelter provided by windbreaks composed of plants, and that provided by buildings, as the extent of shelter depends not only on height but also on the degree of permeability. Figure 3.4 Wind velocity gradients for urban areas: (a) wooded, (b) countryside, and (c) open country (Source: Design Primer for Hot Climate, Konya, 1980, p.36)
Air movement or winds are equally affected by buildings, whose length, height and roof pitch all influence the wind patterns and in so doing have a distinct impact on the surrounding microclimate. The same is true of groups of buildings. Great care must be taken in the layout to minimize any channeling or funneling effects, which can more than double the wind velocity and cause strong turbulence. Figure 3.5 shows how a vortex is formed in front of a building facing the wind and results in an unpleasant wind effect at ground level. This undesired effect could be reduced by introducing a canopy near the ground level. The problem increases if a low building is located in front of a tall building. The wind velocity at ground level between low buildings, however, is usually less than the prevailing wind velocity.
44
Figure 3.5 Wind patterns around buildings (Source: Design Primer for Hot Climate, Konya, 1980, p.37)
3.5.1
Natural ventilation strategies and techniques
Natural ventilation is in effect a form of passive design and can be categorized as cross ventilation, stack ventilation and the use of night cooling or thermal mass. The concept of cross ventilation is simple and has to do with pressure differentiation between the outdoor and indoor environment. When wind hits one side of a building (the windward side), the air will speed up in order to flow around the building to the opposite side of the building (the leeward side). This creates a positive pressure on the windward side and a negative pressure on the leeward side. If windows are open in a building, air will be forced to enter from the windward side and will leave at the leeward side, which creates a force for air crossing through the building. As long as the outdoor temperature is lower than the indoor temperature cross ventilation can be very effective in cooling down the indoor spaces. The effectiveness of cross ventilation design will depend on factors like the size and distance of adjacent buildings, predominant wind directions, the interior layout of the building concerned, sizes and locations of windows, and climatic conditions.
45
The larger the windows, the higher will be the indoor air speeds, provided both the inlet and outlet openings are increased at the same time. When a room has unequal openings and the outlet is larger, higher interior maximum velocities and average speeds are obtained (Figure 3.6). Care must be taken not to impede cross ventilation with incorrectly designed interior partitions and to ensure effective air movement by the correct positioning and sizes of openings. Satisfactory ventilation is possible in buildings when air has to pass from one room to another as long as the connection between the spaces remains open for the required ventilation (Figure 3.7).
Figure 3.6 Airflow patterns through rooms for various sizes and positions of openings (Source: Design Primer for Hot Climate, Konya, 1980, p. 53)
46
Figure 3.7 Velocity of airflow is increased outside the room if the inlet is larger than the outlet (a); velocity of airflow is increased inside the room if the inlet is smaller than the outlet (b); internal partition positions will affect the airflow patterns (c & d) (Source: Design Primer for Hot Climate, Konya, 1980, p.53)
The concept of the stack effect has to do with temperature differentiation between the indoor and outdoor environment. Hot air will rise within a building and escape through an opening at the roof and pull cooler air from outside into the building. This will create a cross airflow within the interior space and create ventilation for the building. A stack will increase this effect, and the longer the stack the greater the airflow obtained. Stack ventilation will work irrespective of whether there is any prevailing wind available.
The concept of night cooling rests on the fact that outdoor temperatures are usually lower at night than during the day. Cooler night air is brought into the interior space to ‘flush’ out warm stale air that has accumulated during the day. The night cooling concept is relatively simple to implement but consideration should be given to security risks if leaving windows open during the night, to the uncomfortable effect of over-cooling, and condensation issues on the inside face of windows if air-conditioning is used the next day.
Thermal mass is incorporated into a building structure to absorb heat during the daytime hours in order to keep the interior space cool. Cooler outside air can be brought in to bring the temperature of the thermal mass back down to preoccupancy levels at night. Typically this mass is incorporated into ceiling spaces and walls in the form of masonry construction. This is an effective method of providing ventilation to buildings. 47
3.5.2
Natural ventilation designs in the tropics
In hot regions airflow is encouraged in order to promote cooling by evaporation and a feeling of comfort from the air movement if the outdoor air temperature is below the temperature in the building. In warm-wet regions the floors of the traditional airy pavilion-like houses are sometimes raised on stilts for better exposure to prevailing breezes, which tend to be damped by surrounding vegetation. This method of construction also enables cooling of the floor from below especially during nighttime. The hot and dry regions have also developed good strategies for controlling daylight and shading, natural ventilation and heat storage.
Natural ventilation in combination with an optimization of the microclimate has always been the main feature of vernacular architecture in hot and humid climates and most of the countries in this region have well developed sustainable buildings. Air movement is known to significantly improve the thermal comfort conditions. In a warm climate as in the tropics provision for air movement must be a major consideration in deciding on the layout of groups or clusters of buildings. The effect of tall buildings must be analysed and it must be kept in mind that if a low building is situated in the wind shadow of a high block the air flowing through the low building could be in a direction opposite to that of the wind (Konya, 1980). Figure 3.8 shows a checkerboard layout with buildings staggered rather than lying in a rigid row would give a more uniform airflow within a cluster of buildings.
48
Figure 3.8 Wind patterns altered by different layouts of groups of buildings (Source: Design Primer for Hot Climate, Konya, 1980, p.55)
In the humid tropics it is important also to ensure that air flows into a room at a level that will suit its function. Louvers can deflect the airflow upwards or downwards as shown in Figure 3.9. A canopy over a window tends to direct airflow upward and a gap between a canopy and the wall ensures a downward pressure that could be further improved by introducing a louvered sunshade (Figure 3.10).
Figure 3.9 Louvers can deflect the airflow upwards or downwards (Source: Design Primer for Hot Climate, Konya, 1980, p.54)
49
Figure 3.10 A canopy over a window tends to direct airflow upwards (d); a gap between the canopy and the wall will create a downward pressure (e); airflow within a room will improve if a louvered sunshade is used (f). (Source: Design Primer for Hot Climate, Konya, 1980, p.54)
Wind catchers and wind towers (Figure 3.11) can be found in hot climate areas ranging from Pakistan through the Gulf States to Egypt and North Africa. Although the form and details may vary from region to region, the basic principle of catching unobstructed higher-level breezes remains the same. In some places the catchers are unidirectional and orientated to catch favorable breezes, while in other places pivoted scoops and multi-directional towers utilize winds from any directions. In the oriental courtyard houses of Iraq and in Dubai, as shown in Figures 3.12 and 3.13, a series of wind catchers on the roof provides natural ventilation for a basement room where the residents normally take their summer afternoon siesta. Each catcher is connected to the basement by a duct contained between the two skins of a party wall, which is cooled during the night by natural ventilation. The surfaces of the internal party wall remain at a lower temperature than the rest of the interior space throughout the day because it does not receive any direct solar radiation. The incoming air is cooled by conduction when it comes into contact with the cold inner surfaces of the duct walls. After passing through the basement the air flows into the courtyard, helping to ventilate this area during the daytime.
50
Figure 3.11 The inlet of the wind tower can usually be closed to keep out dust or cold air. (Source: Design Primer for Hot Climate, Konya, 1980, p.56)
Figure 3.12 Wind catchers in the oriental
Figure 3.13 Wind towers in
courtyard houses of Iraq
the Bastakia district of Dubai
(Sources: Design Primer for Hot Climate, Konya, 1980, p.56)
51
3.5.3
Natural ventilation research
Much research has been devoted to natural ventilation designs in the past two to three decades due to the increasing awareness of greenhouse gas emissions and the need for energy efficient buildings. Researchers mainly concentrated on natural ventilation strategies like cross ventilation or stack effect and natural ventilation devices like solar chimney or wind tower to investigate their effects on ventilation rate in buildings. External wind influences and the effects of thermal convection onto natural ventilation in buildings are also being studied.
Gan (1998) had investigated the air movement in a naturally ventilated room induced through the use of a Trombe wall during the summer cooling of buildings. He found that the ventilation rate induced by the buoyancy effect of the Trombe wall increases with the wall temperature, solar heat gain, wall height and thickness. The ventilation rate will also increases with the increase in distance between the inlet and outlet openings and the width of the channel of the Trombe wall. To maximize the ventilation rate for summer cooling, the interior surface of the storage wall should be insulated.
Solar chimney utilizes solar radiation energy to build up stack pressure and driving airflow through the chimney channel. It is used for ventilation, passive solar heating and cooling of buildings. Trombe wall, in which the sun-facing wall of a channel is glazed, could be considered as a special type of solar chimney. Barozzi et al. (1992) conducted experimental tests on a 1:12 scale model of solar chimney and the results were then used to validate a 2-D laminar flow computational fluid dynamics simulation model. Bansal et al. (1994) used a solar chimney coupled with a wind tower to induce natural ventilation. It was found that the effect of a solar chimney was relatively much higher for lower wind speeds. Bouchair (1994) studied the performance of a typical cavity used as a solar chimney in inducing ventilation into a house. It was observed that the properly designed solar chimneys can be used for daytime ventilation as well as night cooling in hot climates by driving cooler outdoor air into buildings using the thermal energy stored during the daytime. Chen et al. (2003) conducted experimental tests on solar chimney model with 52
uniform wall heat flux and different chimney inclination angles. It was found that a maximum airflow rate was achieved at an inclination angle around 45o for a 200mm gap and 1.5m high chimney. The airflow rate is about 45% higher than that for a vertical chimney of similar conditions. Mathur et al. (2006) conducted similar investigation as Chen et al. (2003) using mathematical and experimental models to look at inclined roof solar chimney summer performance for natural ventilation and found that optimum inclination at any place varies from 40o to 60o depending upon latitude. More recent study carried out by Gan (2006) used solar heated open cavities like solar chimney and double facades in enhancing natural ventilation of buildings. It was found that the optimum cavity width for maximizing the buoyancy-induced flow rate was between 0.55m and 0.6m for a solar chimney of 6m high but the increase of the ventilation rate was small when the width was larger than 0.7m. The ventilation rate in a double façade of four-storey high generally increased with cavity width but decreased with floor level from bottom to top. These research findings had shown that ventilation rate in a building can be improved dramatically by introducing natural draft device such as solar chimney or Trombe wall, which uses solar energy to build up stack effect.
Li et al. (2001) presented analytical solutions derived for natural ventilation in a single-zone building with two openings. Natural ventilation induced by combined wind and thermal forces were studied and they found that external wind can either assist the buoyancy force or oppose the airflow. For assisting winds, the flow is always upwards and for opposing winds, the flow can either be upwards or downwards depending on the relative strength of the two forces. This study gives a better understanding of airflow within a building with natural ventilation. Moeseke et al. (2005) carried out studies on the wind pressure induced natural ventilation on multi-storey office building with focus on wind incidence and large-scale environment density influences. They found that wind incidence influences air movements qualitatively while environment density influences air change hour’s (ach) levels within the building. They showed that wind driven natural ventilation is possible even in urban area.
53
Ziskind et al. (Ziskind et al. 2002, 2003) and Letan et al. (2003) carried out studies on passive ventilation by natural convection of a one-storey detached building and a multi-storey building using heated vertical ducts to enhance stack effect. They found that passive ventilation in summer and passive heating in winter are feasible for both building types using similar method.
Priyadarsini et al. (2004) and Wong et al. (2004) conducted experimental studies on stack effect in enhancing natural ventilation in high-rise residential buildings in Singapore. The study shows that the external wind effect is the most important factor that determines the natural ventilation performance of the building. The passive stack effect does not enhance air velocity within the building however the use of active stack strategy could significantly increase the average air velocity within the building by 47% and within the particular rooms where the stack was located by 54%.
The use of natural ventilation strategies and devices are feasible solution to ventilate not only single storey building but also for multi-storey building. Even though there are many variables in making a naturally ventilated building a success, but the effort is worthwhile in consideration of a bigger picture of environmental benefits in doing so.
3.6
Natural ventilation and office buildings
Conventional office buildings are typically conditioned with mechanical heating, ventilating and air-conditioning (HVAC) systems. These mechanical HVAC systems can maintain fairly constant internal thermal conditions and can be applied in any geographical location. These systems use a great amount of energy and the concept of integrating passive natural ventilation into conventional office buildings has received great attention in recent years. The occupants are also likely to wish to be involved in ways that can improve indoor air quality through the introduction of fresh air through building
54
windows. This is in part as a reaction to the problems that result from poorly maintained conventional HVAV systems that have resulted in sick building syndrome, Legionnaire’s disease, etc. Recently ASHRAE has incorporated a new adaptive model for naturally conditioned spaces in ANSI/ASHRAE Standard 55-2004, which has already been discussed in detail in Chapter 2.
The use of natural ventilation in European commercial office buildings has received much attention in recent years and some of the important criteria to be considered in order to achieve good natural ventilation design for those buildings are (Whole Building Design Guide, www.wbdg.org/design):
a) Orientation b) Solar radiation: direct and diffuse radiation c) Optimum room size for natural ventilation d) Opening sizes and locations for natural ventilation e) Façade materials used f) Opening vents, cross vents, stack effect, cooling pond, etc g) Wind direction
3.6.1
Natural ventilation and bio-climatic office building designs
During the 1940s and 50s schools of architecture in Canada and the USA were teaching architectural responses to climate in their professional courses. The bio-climatic design principles, the principles of designing with climate, were relatively advanced for low and medium rise buildings up to 1960s. By the early 60s, cheap oil prices had enabled designers to negate the environmental factors of a place, which led to a proliferation of the internalized environment and architecture with high levels of energy consumption.
The bio-climatic approach has offered the designer a solution in office building designs by focusing on the relationship between the architectural form and its environmental performance in relation to the climate of the place. The resulting built form then illustrates how an understanding of the environmental aspects 55
of design that already influence the culture and life of that locality can contribute to architectural expression. The approach also helps minimize dependence on non-renewable energy sources. A bio-climatic tall building should be ‘unique’ and ‘responsive’ to the particular environment in which it is to be built. The building’s particular design approach could be regarded as a subset of ecological design.
Ken Yeang in his book entitled “The Skyscraper: Bio-climatically Considered”, defined a bio-climatic skyscraper as ‘a tall building whose built form is configured by design, using passive low-energy techniques to relate to the site’s climate and meteorological data, resulting in a tall building that is environmentally interactive, low-energy in embodiment and operations, and high quality in performance’ (1996, p.18). He believed/s the bio-climatic approach is applicable to all climatic zones and can be applied to high quality buildings also.
The most obvious justification for the bio-climatic design approach to the skyscraper is the lowering of life-cycle financial and energy costs which arise from lowering the energy consumption in the operation of the building. Another factor is that the climatically responsive building enhances its users’ well being by providing a more human high-rise environment, i.e. better natural ventilation to the internal spaces, increases in overall productivity, etc. A further justification is the ecological consideration, that is, designing with climate results in the reduction of the overall energy consumption of the building through the use of passive devices and strategies.
3.7
Energy consumption for office buildings in the tropics
Architecture and urban design have an important impact on the energy efficiency and sustainability of society. Building design lost its sustainability with the introduction of air conditioning systems. Their direct effects on energy
56
consumption can be observed all over the world. For example, in Florida, 47% of the total energy consumption is used in buildings as compared to 35% used in transport. More than 90% of energy used is electric energy (FSEC, 2002). In Brazil, also, 42% of the electricity energy is consumed in buildings (Lamberts et al., 1997). The average energy consumption of office buildings is particularly high. In Rio de Janeiro it is around 340 kWh/m2 (Lamberts et al., 1997) and the average consumption in ASEAN countries ranges from 200 to 300 kWh/m2 (Levine et al., 1992).
The first attempt at designing an office building for natural ventilation purposes in the tropical region is the Menara UMNO building in Malaysia by Ken Yeang. The building uses wing walls to capture and redirect wind into the internal spaces. It was claimed that the building has helped to reduce energy usage dramatically but the initial design intent of using natural ventilation strategy to ventilate the building has failed due to the occupants’ behaviour and preferences in the space usage and the lack of understanding how a naturally ventilated building should work (Kishnani, 2002).
It is the scope and intend of this Thesis work to focus on the sustainable design of office buildings in the tropics and in more specific in the country of Singapore. Therefore it is crucial first to understanding the context of Singapore in terms of its climatic conditions and energy consumption of office buildings in that country.
3.7.1
The tropical climate of Singapore
Singapore lies north of the Equator near lat 1.5 oN and long 104 oE. Its climate is characterized by uniform temperature and pressure, high humidity and abundant rainfall. The climate of Singapore can be divided into two main seasons, namely the Northeast Monsoon (from December to early March) and the Southwest Monsoon (from June to September), separated by two relatively short inter-monsoon periods (late March to May and October to November). There is no distinct wet or dry season and maximum rainfall occurs in 57
December and April. The drier months are usually February and July. During the Northeast Monsoon period, the northeast winds prevail, with wind speeds reaching 8 to 11 m/s in the months of January and February. Southeast or southwest winds prevail during the Southwest Monsoon period with wind speeds reaching 6m/s. The sky is mostly cloudy, with frequent afternoon showers during these times. Light and variable winds occur during the two inter-monsoon periods.
The temperature differences in Singapore are not distinct, with the minimum diurnal temperature ranging from 23oC to 26oC and the maximum diurnal temperature from 31oC to 34oC. The diurnal pressure shows no significant variation, with the maximum pressure usually occurring at 11a.m. and 12 midnights, and the minimum pressure occurring at 5 a.m. and 5 p.m..
The diurnal relative humidity ranges in the high 90s in the early morning to around 60% in the mid-afternoon. The mean relative humidity value is 84% and it often reaches 100% during prolonged heavy rain. Figure 3.8 shows the map of Singapore with indicated directions for Northeast and Southwest monsoons.
Northeast Monsoon
Southwest Monsoon
Figure 3.14 Map of Singapore
58
3.7.2
Energy usage for office buildings in Singapore
An office building can be defined as a building in which spaces are used, or intended to be used, for rendering services such as agency, commission, banking, administrative, legal, architecture, engineering and other professional services. According to the Urban Redevelopment Authority of Singapore’s statistics, in 2001 there were around 6 million square meters of office space in the country.
Figure 3.15 below shows the electricity consumption distribution in Singapore for the year 2001. The consumption of office buildings has been included in the non-manufacturing sector. It can be seen that 61% of electricity consumption was in buildings, excluding the industry sector. The domestic sector alone was responsible for 20%. The consumption of office buildings, part of the nonmanufacturing sector, accounted for 18% of the overall non-manufacturing sector’s consumption. It can therefore be seen that offices are indeed a major energy user among the various building types. Energy Consunption (Gwh/year)
35
Total
30 25
National Total
20
Domestic Non-Manufacturing
15 10 5
Non-Manu.
Manu.
Office Manufacturing
Dom. Office
0 Building Types
Figure 3.15 Electricity consumption among different sectors in Singapore (Source: Singapore Power Annual Report 2001, p.64)
Siew Eang Lee (2002) reported that the energy consumption of office buildings in Singapore was directly proportional to the gross floor area. While the 59
principal concepts in sustainable design are quite similar for both residential and commercial buildings, the approaches are different. A low-tech approach will prevail for most residential buildings due to the cost structure and more active user behavior. For commercial buildings, a more technical approach will be more successful, taking into consideration a higher initial investment and more passive user behavior.
3.8
Natural ventilation and double-skin façades
Great interest has been focused on double-skin façades due to the advantages claimed for this new technology in terms of energy saving, protection from external noise, admitting large amounts of daylight, and their high-tech image. Double-skin façades can provide natural ventilation in buildings by the stack effect and that this is possible even in conditions of high outdoor noise levels and high wind speeds. These advantages of the use of double-skin façade technology in commercial buildings for natural ventilation purposes will be discussed in depth in the following Chapter 4.
60
Chapter 4
Double-Skin Façades and Natural Ventilation
This Chapter provides the classification, deign and implementation issues and selected case studies for double-skin façade systems.
4.1
Intelligent façades
For many people the idea of an intelligent building means the use of information technology and control systems to make the functioning of the building more useful to its occupants, in relation to its management or in respect of the building’s operational purposes. The term ‘intelligent building’ has been so widely used and so diversified that needs further clarification. Some of the definitions for intelligent buildings can be summarized as follows:
- Any building that provides a productive and cost-effective environment through optimization of its four basic elements, i.e. structure, systems, services and management and the interrelationships between them (So, T.P. et al).
- A building that creates an environment which maximizes the effectiveness of the building’s occupants while at the same time enabling efficient management of resources with minimum life-time costs of hardware and facilities (So, T.P. et al).
Atkin (1988, p.1) claimed that intelligent buildings should possess three important attributes, namely: The buildings should ‘know’ what is happening inside and immediately outside. The buildings should ‘decide’ the most efficient way of providing a convenient, comfortable and productive environment for the occupants. The buildings should ‘respond’ quickly to occupants’ requests
61
The above definitions seem to suggest that an intelligent building should have certain functions similar to the intelligence faculties of living beings. A truly intelligent building should be closely related to the realms of both artificial and natural intelligence, with the ability to respond and react to external stimuli in a predictable manner.
The façade of a building can account for between 15% and 40% of the total building cost and it may be a significant contributor to up to 40% more of the cost through its impact on the cost of building services. The intelligent façade is an integral part of the intelligent building, performing the main function of protecting the occupants inside the building. Such a façade will allow energy flows through the building envelope in both directions that will be automatically controlled for maximum gain and minimum reliance on imported energy. The façade system is connected to other systems in the building by a central building management system.
4.2
Double-skin façades (DSF)
4.2.1
Introduction
The concept of a double-skin façade is not a new one as it was introduced centuries ago, and the first double-skin curtain wall appeared in 1903, in the Steiff Factory in Giengen, Germany (Internet page of BuildingEnvelopes.org, History of Double-skin Façades, http://envelopes.cdi.harvard.edu/envelopes/web_pages/home/home.cfm, last visited on 28-7-2007). Double-skin façades have been developed to improve air quality, occupants’ visual and thermal comfort, acoustic performance and energy use for modern buildings. The use of double-skin façades has became popular for many high-rise buildings in Europe and the technology also has been demonstrated in the Armoury Tower in Shanghai, China (Yeang, 1996), No.1 Peking Road and the Dragon Air Office which are both located in Hong
62
Kong (Haase, Wong and Amato 2004). These buildings are some of the major high-rise office buildings that fitted with double-skin façades in the Asia Pacific.
Double-skin façades are constructions with multiple layers of skins with an external skin, an intermediate space and an inner skin. The external and internal skins could be of either single glazed or double glazed float or safety glass panes. An adjustable sun-shading device is usually installed in the intermediate space for thermal controls.
The performance of the double-skin façade depends closely on the chosen means of ventilation within its intermediate space. The modes of ventilation could be natural (buoyancy driven), forced (mechanically driven) or mixed (both natural and forced). Since the temperature difference between the outside air and the heated air within the intermediate space must be significant enough for natural ventilation to work, the buoyancy driven system alone is not suitable for use in hot climates. Both the forced (e.g. active wall) and mixed (e.g. interactive wall) systems could be used in hot climate conditions, but the latter has the advantage of introducing natural ventilation even for high-rise buildings.
Figure 4.1 Typical double-skin façade construction (Source: Double-Skin Façades – Integrated Planning, Oesterle et al., 2001, p.134)
63
4.2.2
Classification of double-skin façades
Since their first use, there have been some inconsistencies and ambiguities in their classifications. Oesterle et al. (2001) attempted to classify double-skin constructions into four different types, namely box window façades, shaft-box façades, corridor façades and multi-story façades.
A box window façade (Figure 4.2) consists of a frame with inward-opening casements. The external pane has openings that allow the ingress and egress of air, which helps to ventilate both the intermediate and the internal space. The air cavity of the façade is divided horizontally on a room-to-room basis and vertically at storey height. It is commonly used where there are high external noise levels or where special sound insulation between adjoining rooms is required. A recent built example is the 90-meter-high office building at Potsdamer Platz 1 by Hans Kollhoff built in Berlin in 2000.
Figure 4.2 Plan and Section of box window façade (Source: Double-Skin Façades – Integrated Planning, Oesterle et al., 2001, p.13)
A shaft-box façade (Figure 4.3) consists of a system of box windows with continuous vertical shafts that extend over a number of stories to create a shaft 64
effect. The vertical shafts are linked with the adjoining box windows by means of a bypass opening and the stack effect draws the air from the box windows into the shafts and from there up to the top of the shafts where it is emitted. The façade system has fewer external openings and it helps to provide stronger thermal uplift within the stack and to insulate against external noise. The 120meter-high ARAG 2000 office tower built in 2000 in Düsseldorf is a modern high-rise building using this particular façade system.
A corridor façade (Figure 4.3) is formed with the intermediate space between the two skins of the façade closed at the level of each floor, and vertical divisions for acoustic and fire protection or ventilation. The intake and extract openings in the external skin are situated near the floor and the ceiling and are laid out in staggered form from bay to bay to prevent contamination of supply and exhaust air. Special attention must be given to sound transmission between rooms when this façade system is used. The 80-meter-high Düsseldorf City Gate built in 1998 uses a façade corridor that is divided into 20-meter-long sections for ventilation purposes.
A multi-storey façade (Figure 4.3) has its intermediate space between the two skins adjoined vertically and horizontally by a number of rooms. The air intake and extraction of the immediate space occurs via large openings near the ground floor and the roof and the façade is used as an air duct. It is suitable for use where external noise levels are very high, as the external layer does not require any openings over its height. An example of this type of façade construction is shown in Figure 4.4.
65
Shaft-box Façade
Corridor Façade
Figure 4.3 Examples of double-skin façades (According to Oesterle et al., 2001, pp.16, 20, 23)
66
Multi-storey Façade
Figure 4.3 Examples of double-skin façades (cont.) (According to Oesterle et al., 2001, pp.16, 20, 23)
67
Figure 4.4 Section through the multi-storey façade of the Victoria Ensemble in Cologne (Source: Double-Skin Façades – Integrated Planning, Oesterle et al., 2001, p. 24)
Parkin (2003) has reviewed the classifications in depth and proposes a ‘consolidated classification’ through the provision of tree diagrams to explain all the types and subtypes in the classification (Figure 4.5); illustrations of all main types through 3-D CAD models; and additional examples of 3-D descriptions of one specific branch (corridor façade) of the consolidated classification. Poirazis (2004) has also done somewhat similar work in classification of the different double-skin façades for office buildings and pointed out that a proper classification is important because it will influence the design stage of a building that lead to more precise predictions of the performance of the façade system.
68
Parkin has defined a ventilated double-skin façade as “a façade system containing an additional layer of predominantly glass, positioned on the outside of an external wall and/or window, separated by a cavity through which air flows and usually containing a shading device” (2003, p. 232). He has identified five characteristics that are significant in the formulation and description of the new consolidated classification. These five characteristics are:
a) Characteristic 1 – the formal arrangement: a description of the arrangement of the air cavity between the exterior and interior skins, and the extent of the horizontal and vertical partitioning within the cavity and its interconnectivity as a system. b) Characteristic 2 – the ventilation driving force: the nature of the ventilation within the façade system being driven by predominantly natural or mechanical forces. c) Characteristic 3 – the airflow concepts: the type of ventilation airflow concepts within the cavity and the ability to open and close the external and internal skins. These include supply, exhaust, return, exterior and interior air curtains and static air buffer modes. d) Characteristic 4 – the subtype description: meaningful descriptions that readily identify the sub-types within the classification. e) Characteristic 5 – illustrations of the classification: the means of depicting the classification through drawings and the like.
The first three characteristics are the major identifiers for the new consolidated classification. The last two characteristics are there to enhance the description of the classification.
The clear classification of the façade system could help a great deal in future research and it could be used as a useful design tool with appropriate performance information about each façade type and subtype, to assist in decision making for selection in particular climates.
69
Figure 4.5 Consolidated classification tree diagrams (Source: according to Parkin, 2003, p.233)
4.2.3
Thermal transfer through double-skin façades
The issue of thermal transfer through an active wall like the double-skin façade is a complex one. The heat transfer occurs simultaneously for all the component layers of the double-skin façade under the influence of the surrounding environmental conditions, the properties of the layers of the façade and the ventilation system introduced into the double-skin façade. The overheating of the air gap between the double-skins of the façade is more evident during high ambient temperatures and it can be reduced by manipulating the openings of the glazing façade, a well positioned shading device and the optimization of the width of the air gap between the glazing panes (Oesterle et al., 2001, p.75). Figure 4.6 below shows an example of heat transfer through a double-skin façade. 70
Indoor Outdoor
Figure 4.6 Heat transfer through double-skin façade (Source: Internet page of Whole Building Design Guide, Natural Ventilation, http://www.wbdg.org/design/resource.php)
4.2.4
Design considerations for double-skin façades
The design parameters that have the main influence on the air mass flow and the temperatures within the double-skin façades are (Gratia and Herde 2007a, 2007b, 2007c, 2007d; Oesterle et al., 2001):
a) The size of the upper and the lower vent of the façade; b) The depth of the façade and the position of the shading device, especially the absorption coefficient; c) The size of the vents of the shading device; d) The quality of the outer and the inner pane, especially the solar transmission factor, but also the U-value and the absorption coefficient.
71
Oesterle et al. (2001, pp.30-32) have recommended ten steps for designers in planning various types of double-skin façades. They are:
1) Checking the constraints – suitability of window ventilation for the available façade systems, economic viability of the systems. 2) Determining the type of construction – establishing the matrix of requirements for determining the appropriate form of double-skin façade construction. 3) Ensuring a good fresh-air supply – planning the dimensions of the openings and airflow routes into the rooms. 4) Avoiding overheating in the intermediate façade space – airflow is designed in such a way that heat gain does not increase upwards from floor to floor. 5) Optimising the flow of air – size and position of the openings, adding extra propulsion where required. 6) Planning the conditions for operation – varying the size of openings for thermal and acoustic insulation requirements. 7) Exploiting the construction to the full – collaboration and coordination with the façade planner and other consultants during the early stage of the design. 8) Putting the dimensions to the test – integrating the building’s physics and the ventilation technology of the façade system with the mechanical ventilation concept. 9) Integrating clients and users within the planning process –the overall optimisation of the systems will require all parties’ involvement in the planning stage to assume a share of responsibility. 10) Taking the control mechanisms into operation – coordinating the adjustment of the façade functions and the mechanical ventilations and allowing time for the tuning process before users move in.
As the success of the façade system depends a great deal on the understanding of the functionality of the façade technologies, the above are just general 72
guidelines for designers who wish to consider implementing double-skin façade construction in their buildings.
4.3
Natural ventilation in double-skin façades
A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin façades. Even though most of the research has been done in temperate climate conditions, the studies have revealed a close link between natural ventilation design and the function of the double-skin façade. It was found that significant energy saving is possible if natural ventilation could be exploited through the use of double-skin façade.
For example Grabe et al. (2001) developed a simulation algorithm to investigate the temperature behaviour and the flow characteristics of double façades with natural convection through solar radiation. It was found that the air temperature increased greater near the heat sources that are near the panes of the window and the shading device. Gratia and Herde (2004a, 2004b, 2004c, 2004d, 2004e, 2007a, 2007b, 2007c, 2007d) attempted to look at natural ventilation strategies, greenhouse effects and the optimum position of sun shading devices for double-skin façades facing in a southern direction in a temperate climatic in the northern hemisphere. They found that sufficient day or night ventilation rate can be reached by window opening, even if wind characteristics are unfavourable. If natural cooling strategies are used with double facades, greenhouse effect is favourable if the façade is facing south. Thermal analysis using simulation software of different seasons of a year was done for a low-rise office building with and without double-skin façade. They further provided some general guidelines in improving natural daytime ventilation in office building with a double-skin façade and demonstrated that efficient natural cross ventilation is possible in climatic conditions in Belgium.
73
4.4
Implementation of double-skin façades in office buildings
The double-skin façade is a system that can create opportunities for maximizing daylight and improving energy performance. There are many issues to be considered in the development of appropriate façade systems for an office building. A natural stack effect often develops in the cavity and the façade can reduce solar gains as the heat load against the internal skin can be reduced through the ventilated cavity. The relatively new double-skin façade technology requires greater care in implementation, especially for high-rise buildings.
The effects of wind and strong thermal uplift are two of the more important issues that need to be dealt with in design. A precise survey of wind loads acting on buildings can be obtained through measurements in a wind tunnel or by using appropriate simulation software. Intelligent control mechanisms have been used in most double-skin façade buildings to regulate the admittance of air into the cavity automatically and also closing it up to create a thermal buffer. Further details of important issues in designing double-skin façades for high-rise office buildings are discussed below.
4.4.1
Examples of double-skin façade buildings
The following are some examples of existing double-skin façade buildings constructed in Asia and in Europe. They are examples of the implementation of both naturally and mechanically ventilated double-skin façade systems. Even though the information for the summer months’ performance for One Peking Road and Jin Shi Tower are not available, but these buildings were chosen due to their climatic locations which will give the ‘next best’ examples for the review.
74
(a) Debis Headquaters, Berlin, Germany - 1997 Architects: Renzo Piano Building Workshop and Christoph Kohlbecker
This 21-storey office building uses a corridor façade system with an exterior skin consisting of automated, pivoting, 12-mm thick laminated glass louvers. Minimal air exchange occurs through these louvers when closed. The interior skin consists of two bottom-hung double-pane operable windows. The upper window is electrically operated. On the interior of the internal windows are Venetian blinds. Walkway grills occur at every floor within the 70-cm wide interstitial spaces and are covered with glass to prevent vertical smoke from spreading between floors (Figure 4.7). During the summer the exterior glass louvers are tilted to allow for outside air exchange. The users can open the interior windows for natural ventilation. Nighttime cooling of the building' s thermal mass is automated during the winter when the exterior louvers are closed. The users can open the internal windows to admit warm air on sufficiently sunny days. The building is mechanically ventilated during peak winter and summer periods.
Figure 4.7 Exterior views and the cavity space within the double-skin façade (Source: Website of High-Performance Commercial Building Façades, http://gaia.lbl.gov/hpbf/casest_b.htm)
75
(b) One Peking Road, Hong Kong, China - 2003 Architects: Rocco Design Limited
The 30-storey high-rise office building is the first skyscraper in Hong Kong and one of the first in the world to power the major part of the building with solar energy. It has won the top architectural award from the Hong Kong Institute of Architects for its aesthetics and its environmentally friendly design. It uses a mechanically ventilated corridor façade double-skin system as an energy efficient strategy (Figure 4.8).
Figure 4.8 Exterior view and façade details for One Peking Road (Source: Website of Emporis Buildings, www.emporis.com)
76
(c) Jiu Shi Tower, Shanghai, China - 2000 Architects: Foster and Partners
The 39-storey office building located between the river and two traditional neighborhoods of Shanghai uses a mechanical ventilated corridor façade double-skin system as the ventilation strategy (Figure 4.9).
Figure 4.9 Exterior view of Jiu Shi Tower (Source: Website of Emporis Buildings, www.emporis.com)
77
Table 4.1 below lists 86 double-skin façade buildings constructed in 15 countries, mainly in Europe, Asia and Northern America for the past 20 years. The main ventilation mode chosen for those buildings (about 40% of the built examples) is natural ventilation using a multi-storey façade system.
Ventilation Type
DSF Type
Number of building
Corridor façade
20
Multi-storey façade
8
Box window façade
2
Corridor façade
7
Multi-storey façade
34
Shaft box façade
1
Box window façade
6
Corridor façade
4
Multi-storey façade
4
A) Mechanical ventilation
B) Natural ventilation
C) Hybrid ventilation
Table 4.1 Double-skin façade buildings with various ventilation types and façade systems (Source: Ventilated Double Facades, Belgian Building Research Institute, 2004, p.3)
4.4.2
Fire protection in double-skin façades
A basic treatment for assessing the risks associated with double-skin façades and for providing protection in the event of fire is discussed by Oesterle et al. (2001, pp. 83-85) as follows:
78
Fire protection risks: a) Localization of the fire space – difficult to localize the fire space visually from the outside; difficult to break the toughened glass in the outer layer; difficult access from outside; sound contact between inside and outside is difficult. b) Smoke in the façade intermediate space – the air-intake and extract openings in the outer façade may not provide adequate means of removing smoke from the intermediate space. c) Fire may spread – from inner façade into the intermediate space.
Fire protection measures: a) Automatic early fire-warning systems in the rooms and the façade intermediate space. b) Automatic activation of the smoke-extraction system for the façade’s intermediate space. c) Automatic fire-fighting systems in the rooms and/or the façade’s intermediate space.
4.4.3
Sunshading in double-skin façades
Sunshading in a double-skin façade plays a crucial role in absorbing heat from sunlight and liberating heat within the intermediate space. The sunshading surfaces will absorb around one third of the heat passing through the façade and the heat will be transmitted to the surrounding air through means of radiation and convection. This helps to block any substantial heat gain in the interior space.
The position of the sunshading within the air gap of the façade plays a major role in the distribution of heat gains in the intermediate space. The smaller space will heat up to a greater extent than the larger space. If the shading is located close to the inner pane of the façade it will considerably heat the air in front of the window, and this is undesirable. Therefore sunshading should be positioned at roughly a third of the depth of the façade cavity, with good 79
ventilation to the outer space above and below the sunshading. A minimum of 15cm between the sunshading and the external skin of the façade is recommended to give an acceptable ventilation rate within the air gap (Gratia et al., 2007b).
4.4.4
Effect of nighttime ventilation on double-skin façades
During hot summer days the interior of buildings will absorb a great amount of heat and this trapped internal heat will still be present throughout the night and it will be perceived as too warm if the excess heat is not expelled before the next morning. Nighttime ventilation that permits a natural exchange of air and heat during summer nights through a controlled opening of windows or flaps will help to cool down the internal space of the building during the night. A double-skin façade provides the opportunity for nighttime ventilation and at the same time gives the security required by just having the internal windows opened for that purpose.
4.4.5
Condensation in double-skin façades
Wong et al. (2004) carried out studies on the condensation issue in double-skin façades in a hot and humid climate country, Singapore. TAS and CFD simulation software was used to determine the energy consumption, thermal comfort and condensation in a 6-storey building. The differences between the ambient temperature and that of the glass surface of the façade were found to be the major cause of condensation during nights of high humidity. The use of mechanical fans was recommended to remove condensation from the façade system in hot and humid conditions. The researchers found that east and west orientations produced the most condensation on the façade and that the south façade had the least problem. It was also found that on the lower the floor the condensation rate was lower.
80
4.4.6
Review of the limitations of double-skin facades
There are not many critical review could be found of double-skin facades and Dr. Karl Gertis of the Fraunhofer Institute of Building Physics in Stuttgart, Germany had given a relatively comprehensive review on the limitations of the façade system (Lee E. et al, 2002). A number of the limitations are listed below which will help to set out the requirement for computer simulations and drawing conclusions for the research:
a) One cannot achieve a comfortable indoor climate with natural ventilation alone during most period of the year without active cooling b) The boundary conditions of the simulation model are often not exactly stated so the results are not useful because critical interpretation cannot be made c) The actual airflow patterns within the air gap of the façade system are complex because there is airflow exchange on the leeward and windward sides of the building and within the air gap itself. The airspeed in the gap gets smaller with increased of exterior wind speed due to the air resistance within the façade. d) The Venetian blinds positioned in the air gap should be reflective to prevent temperature increase in the air gap. e) The air temperature in the air gap can create significant thermal discomfort and forced closure of internal windows designed to allow natural ventilation.
81
4.5
Case study for double-skin façade buildings
4.5.1
Stadttor (City Gate) at Düsseldorf, Germany Architect: K.H. Petzinka
51.29oN Latitude
This 80-meter-high office building was completed in 1997 and is composed of two16-storey towers connected at the top with three bridging levels (Figure 4.10). The entire building is enclosed in a glass skin with a huge 50m high atrium void at the centre, creating a ‘gateway’ effect. A double skin cavity up to 1.4m in depth provides an enclosed balcony for all offices. The façade corridor is divided into 20-meter-long sections by an escape staircase, the atrium and divisions at the corners of the building.
The building is predominantly naturally ventilated, controlled by a building management system (BMS) that automatically determines natural ventilation or mechanical ventilation modes. Natural ventilation is achieved through computer control of ventilation flaps within the building envelope, which run in horizontal bands at each floor level. The BMS has sensors for wind, temperature, rain and sun to provide optimum control strategies for heating, cooling and fresh air supply. Venetian blinds within the façade cavity are lowered and raised automatically according to light levels and the need for nighttime insulation. The ventilated double skin limits the required cooling loads by ventilating away the solar heat built up in the cavity.
Figure 4.10 City Gate at Düsseldorf, Germany (Source: Website of High-Performance Commercial Building Façades, http://gaia.lbl.gov/hpbf/casest_d.htm)
82
4.5.1.1 The façade system (double-skin corridor façade)
The double-skin façade enveloped three sides of the office floors with the cavity varying between 0.9 and 1.4m in depth. The outer skin is 15mm toughened planar glazing of low-iron ‘opti-white’ glass for maximum transparency. The inner skin is made of vertically pivoted high performance timber windows. The full-height double-glazing has a low-E coating. The Uvalues of the double skin are 1.2 W/m2K and 1.0 W/m2K when the vents are opened and closed respectively. The vertical atrium walls are single glazed with planar glass. Light transmission through the envelope is 68%. The overall energy transmission equates to 50%, without the blinds, and 10% with the blinds lowered.
The venetian blinds of the façade system are located 200mm behind the outer pane of the system. The blinds are automatically lowered in response to photocell detectors on each façade, which indicate if the sun is shining on a particular building façade. Once they are lowered they will tilt to 45o, which will help to reduce glare but still allow daylight into the building. If the sun is not directly shining on a particular façade, then the blinds are raised. The users have the facility to override whether the blinds are up or down through a simple ‘light switch’.
Figure 4.11 The corridor façade system showing the inner vertically pivoted windows and the façade cavity. (Source: Website of High-Performance Commercial Building Façades, http://gaia.lbl.gov/hpbf/casest_d.htm) 83
4.5.1.2 Natural ventilation
Natural ventilation is predicted to be achievable for 70% of the year for temperatures between 5oC and 22oC. Pre-heated mechanical ventilation will be used for 25% of the year when the temperatures are below 5oC, and the remaining 5% pre-cooled mechanical ventilation is used when temperatures are above 22oC.
For natural ventilation to be operative the users will need to open the inner windows manually. The ventilation flaps in the outer façade, which admit air into the cavity, are automatically controlled by the BMS. The atrium is naturally ventilated. Four areas of glass louvers provide openings in the two end walls within the atrium. The glass louvers are controlled according to wind speed and direction. In windy conditions one side can be closed to avoid wind traveling through the building. Outer offices are side ventilated from the double-skin cavity and inner offices from the atrium.
4.5.1.3 Conclusion
The performance of the façade system is being monitored and tests have shown that air leaving the cavity is 6oC hotter than incoming air, suggesting that the system is performing a useful cooling effect. Air coming into the offices is only 1 or 2 degrees hotter than the outside air.
Delivered energy consumption figures for the building are not available, but during the design phase heating was simulated at 30kWh/m2 per year. This is considered a very energy efficient high-rise office building in the region.
84
4.5.2
Occidental Chemical Center at Niagara Falls, New York, USA Architects: Canon Design Architects
43.00oN Latitude
The 9-storey plus basement office building was completed in 1981 and is located diagonally on an axis with the Rainbow Bridge that links America with Canada over the Niagara River Gorge (Figure 4.13). The square plan of the building provides a column-free office space around a central building core. The building overlooks the Niagara Falls and is located in a cold and cloudy area, experiencing many of the secondary climatic effects from the colder northern areas of Canada.
The building uses a building management system (BMS) to control its facilities such as security, alarm and fire alarm systems, as well as energy management of the HVAC system. The perimeter lighting system and louvers are controlled by the BMS to respond to ambient daylight levels. When heat is not required in the double-skin cavity, sensors will operate venting dampers at the top and bottom of the cavity to release the warm air at the top of the cavity. The system is fully equipped with data collection capabilities to provide information on energy usage patterns throughout the building.
Figure 4.12 The Occidental Chemical Center at Niagara Falls (Source: U.S. Website of Environmental Protection Agency, www.epa.gov)
85
4.5.2.1 The façade system (double-skin multi-storey façade)
The double-skin façade system wrapped around the building with a 1500mm cavity acts as a thermal buffer in winter and it can automatically open in summer to vent away convective warm air. The outer skin of the façade is a white aluminium curtain walling system with blue-green tinted insulating glass. It transmits up to 80% of visible light. The inner skin consists of clear single glazing. The U-value of the double-skin is 1.54 W/m2K.
Louvers within the cavity of the double façade are adjustable for solar control and can be closed at night to increase insulation. They provide an effective balance between solar shielding and thermal conditioning demands. In summer and winter the operable louvers in the glazed cavity are automatically adjusted by means of a photocell controller to prevent direct solar radiation from entering the building. At night and during other unoccupied hours the louvers are closed for increased insulation, retaining the conditioned air from daytime operation. Users can locally override the position of the louvers by wallmounted switches within each of the corner offices if required.
4.5.2.2 Ventilation systems
The heating, ventilating and air conditioning needs are met by two lowpressure variable air volume (VAV) air-handling units. The double-skin serves to reduce the impact of severe outside temperatures by limiting the effect of infiltration on the conditioned interior to an acceptable level. The skin serves as a thermal buffer in winter and vents out the warm air at the top and bottom to increase airflow for reducing solar build-up within the cavity during summer. It was estimated that the building consumes less than one-third of the energy required for a conventionally designed office building of a similar type.
86
4.5.2.3 Conclusion
The building was found to maintain unique energy demand stability when compared to a conventional office design. This was in agreement with the results obtained through extensive computer based simulations done for energy usage prediction for the building during the design stage. It was believed to be the most energy-efficient building in its particular climatic zone at that time.
4.5.3
Super Energy Conservation Building, Kiyose City, Tokyo, Japan Architects: Ohbayashi-Gumi
35.80 oN Latitude
The 3-storey plus basement office building was built in 1982 and was claimed to be the ‘most energy-efficient building in the world’ at that time (Figure 4.14). The rectangular plan building has an inclined glass wall on its south façade with service rooms located at the east and west ends. A rooftop plant room serves as a buffer to the indoor conditioned spaces below. On the south side the ground has been excavated to form a sunken garden that allows windows to the library area at the basement. Earth berming up to the ground floor window sill level was used on the east, west and north sides of the building.
A building management system (BMS) is used to operate the heating, cooling and ventilating plant according to the weather and usage conditions of the building to optimize energy consumption.
87
Figure 4.13 The Super Energy Conservation Building at Kiyose City, Tokyo (Source: Website of Obayashi Corporation, www.obayashi.co.jp/english/ir)
4.5.3.1 The façade system (double-skin multi-storey façade)
The double-skin glass wall is only located on the south façade of the building and is used to preheat incoming air for the air handling units. In summer vents at the top and bottom of the double-skin are opened to create natural ventilation and reduce the cooling loads for the building. The main office spaces are protected from direct solar gain by louvered blinds at the outside of the office windows within the double-skin cavity.
4.5.3.2 Ventilation systems
The building is fully air conditioned with a variable air volume (VAV) system. Opening vents in the double-skin façade ensure adequate ventilation of the cavity in summer. In winter they are opened to admit fresh air, which is preheated prior to passing through the air conditioning plant. Fresh air inlets on the north side of the building feed the roof-mounted AHUs in summer.
88
4.5.3.3 Conclusion The building is very energy efficient. Its annual energy usage of 112 kWh/m2 is only about 25% of that of conventional Japanese office buildings’ annual usage of 442 kWh/m2 during the eighties.
4.6
Concluding remarks
Double-skin façade systems provide greater controllability for the occupants over the thermal exchange between the perimeter zone of buildings and the outside environment. Outdoor fresh air can be brought into the interior space of the built environment through natural ventilation strategies (e.g. stack effect, displacement ventilation, etc) and unwanted radiant heat can be expelled out into the external environment.
Even though more research needs to be done in order to explore the full extent of the capabilities of double-skin façade systems in reducing energy usage, there have been positive and encouraging results from both the research and industry fields. In particular from the findings of the case studies in Section 4.5 there is great possibility for the multi-storey façade system to reduce energy usage in high-rise buildings through the use of natural ventilation strategies.
89
Chapter 5
Computational Fluid Dynamics
This Chapter provides the supportive materials for the used of computational fluid dynamic simulation in the research and its creditability and accuracy in modelling complex conditions for double-skin façade buildings.
5.1
Simulating a naturally ventilated double-skin façade
5.1.1
Building simulation programs
During the last three or four decades computer simulations of physical processes have been used in scientific research and in the analysis and design of engineered systems. Computer simulations are used for environmental predictions, in the analysis of surface-water quality, in the risk assessment of underground nuclear-waste repositories, etc. These kinds of predictions are beneficial in the development of public policy, in the preparation of safety procedures, and in the determination of legal liability.
In recent years energy simulation (ES) and computational fluid dynamics (CFD) programs have been used to provide important information about building performance in building design. ES programs such as EnergyPlus (Crawley et al. 2000) provide energy analysis for a whole building and the heating, ventilation and air conditioning (HVAC) systems used. These programs produced acceptable convection heat results from enclosures and can provide reasonably accurate estimations of the building energy consumption and dynamic thermal behaviors of building envelopes. Information on spaceaveraged indoor environmental conditions, cooling/heating loads, coil loads and energy consumption can be obtained on an hourly or sub-hourly basis for a period of time ranging from a design day to a reference year. However, most ES programs assume that the air in an indoor space is well mixed, and cannot accurately predict building energy consumption for buildings with non-uniform air temperature distributions in an indoor space such as those with displacement ventilation systems. Furthermore, the spatially averaged comfort 90
information generated by the single node model of ES cannot satisfy advanced design requirements. Most ES programs cannot determine accurate airflow entering a building by natural ventilation, while room air temperature and heating/cooling load depend on the airflow within the space.
CFD programs such as Fluent (Fluent 6.2, 2001) require more precise and realtime thermal boundary conditions and can provide detailed predictions of thermal comfort and indoor air quality, such as the distributions of air velocity, temperature, relative humidity and contaminant concentrations. The distributions can be used further to determine thermal comfort and air quality indices such as the predicted mean vote (PMV), the percentage of people dissatisfied (PPD) due to discomfort, the percentage dissatisfied (PD) due to draft, ventilation effectiveness and the mean age of air. These programs can determine the temperature distribution and convective heat transfer coefficients and can accurately calculate the natural ventilation rate driven by wind effect, stack effect, or both. Table 5.1 below shows the comparison between the typical functions of CFD and ES programs for building performance studies.
Building performance study
ES
CFD
a) Weather and solar impact
Yes
No
b) Enclosed thermal behaviors
Yes
No
c) HVAC system capacity
Yes
No
d) Energy consumption
Yes
No
e) Thermal comfort (air temperature, air velocity, relative
No
Yes
f) Indoor air quality (contaminant concentrations)
No
Yes
g) Air distribution
No
Yes
humidity, airflow turbulence, etc)
Table 5.1 Comparison of typical functions of ES and CFD programs for building performance studies
91
5.1.2
Simulating buildings with double-skin façades
The ‘ideal’ simulation software for modeling buildings with double-skin façades should be able to: a) correctly model the outdoor climate b) correctly model the double-skin façade, i.e. the glass skins, shading devices, natural or mechanical ventilation, etc c) correctly model the building, i.e. the ‘connection’ between the façade and the building d) correctly model the control systems that influence the performance of the building
Of course such ‘ideal’ software does not exist. Each of the simulation software has its own strengths and limitations. The choice of the most appropriate software depends on the main objective of the simulation. For example, whether the goal is to design a new concept for a double-skin façade, or to determine the indoor thermal comfort of the building, or its yearly energy consumption, or to study the condensation risk, or to determine the maximum indoor temperatures during summertime, etc. would have a bearing on whether to select an ES program or a CFD program. In the case of a naturally ventilated double-skin façade, wind speed and wind direction are two important sets of data required for correctly predicting the ventilation around and within the building. The solar performance of the façade system is also critical and therefore detailed data of the solar radiation is required. The simulation software package chosen to model this special façade construction needs to be able to model the outdoor climate conditions precisely.
A further difficulty is the prediction of the airflow rate in the cavity of the façade because of the combined buoyancy (stack effect) and wind effects in natural convection. The airflow rate is unknown and depends on the temperature profiles. This combined effect of wind and temperature differences creates a pressure difference between the inlet and outlet of the façade, which 92
determines the airflow rate in the façade cavity. When wind velocities are low the stack-effect will be dominant, and if the wind velocities are high the windeffect will dominate. There is a ‘transition’ regime between both effects that may assist or counteract the overall natural convection conditions.
As in the case of naturally ventilated double-skin façade the airflow rate and the temperature profiles are mutually dependent, the thermal system must be solved iteratively. There are two choices of method to achieve the goal of accurately simulating the combined effect of heat and mass flow, i.e. to use a CFD program, or to use an ES program coupled with a CFD program (Hensen et al., 2002).
5.1.3
Coupling CFD and building energy simulations
Building energy simulation (ES) and computational fluid dynamics (CFD) programs provide essential information about building thermal performance such as space cooling and heating loads, distributions of indoor air velocity, temperature and humidity, contaminant concentrations, etc. This information is important for assessing thermal comfort, indoor air quality and the energy consumption of a building. In recent years attempts have been made to couple these two programs in order to provide more accurate predictions of building behavior (Zhai et al., 2002, 2006). Zhai and Chen (2002, 2003, 2005, 2006) explored the principles, methodologies, strategies, implementation and performance of the ES-CFD thermal coupling, and later Wang and Wong (2006) experimented with the coupling as a natural ventilation strategy in tropical conditions. Their studies showed that a unique coupled solution may exist in theory, but different coupling methods can lead to different performance solutions in terms of computing accuracy, stability and speed.
Further research on the subject has verified that the data coupling method, which transfers interior surface temperatures of enclosures from ES to CFD, and returns convective heat transfer coefficients and indoor air temperature gradients from CFD to ES, is the most reliable and efficient coupling method 93
(Zhai and Chen, 2006). Zhai and Chen also proposed a staged coupling strategy that could reduce the total computing time of a coupling simulation, and developed a prototype of an integrated ES-CFD building simulation tool to further examine the accuracy of the results. While the concept and coupling methodology of the combined ES and CFD programs are still in the process of maturation, designers are left with the option of using either the ES program or the CFD program in simulating specific domains for building performance studies.
5.1.4
The choice of using a CFD program
Since this research is concerned with finding out whether a double-skin façade is viable for providing energy saving through natural ventilation to high-rise buildings in the tropics, and more specifically in the context of Singapore, the conditions that constitute acceptable indoor thermal comfort for high-rise buildings in the studied context need to be determined with the use of doubleskin façade through natural ventilation strategies. In order to achieve this it requires numerous simulation runs with different combinations of façade configurations and outdoor conditions, and certainly with a high degree of accuracy in the results produced.
In view of the capabilities of ES programs and of CFD programs, as discussed above and compared in Table 5.1, and of the goal of the research, it was found that using a CFD program had the potential to achieve the main objective of the simulation and to produce acceptably accurate results. More importantly, a CFD program can accurately simulate a naturally ventilated double-skin façade building.
Therefore, on the basis of the functionalities and the abilities of both types of programs, the specific goals and requirements of this study determined the selection of a CFD rather than an ES program, to formulate the numerical models needed to investigate the issues at hand.
94
5.2
CFD software
5.2.1
Theoretical background for CFD software
Computational Fluid Dynamics (CFD) uses three fundamental conservation principles involving mass, momentum and energy for simulation. The simplified governing equations are given below (Airpak User’s Manual):
a) Conservation of mass (continuity equation)
∂ρ ∂ (ρu i ) = 0, + ∂t ∂xi
(5.1.1)
where ρ is the density of fluid, t is the time, ui is the velocity vector component (u, v, w) and xi is the Cartesian coordinate axis of x, y and z representing space/volume.
b) Conservation of momentum
∂ (ρui ) + ∂ (ρuiuj ) = - ∂p + ∂ µ ∂ui + ∂uj ∂t ∂xj ∂xi ∂xj ∂xj ∂xi
+ ρfi ,
(5.1.2)
where p represents pressure, µ indicates kinetic viscosity and fi is the body force per unit mass acting on the fluid particles/elements in x, y or z direction.
c) Conservation of energy
∂ ∂ ∂ K ∂H ( ρ H) + ( ρ uiH) = ∂t ∂xi ∂xi cp ∂xi
+ SH ,
(5.1.3)
95
where H is the enthalpy, K is the thermal conductivity of the element, cp is the specific heat and S is a source term. The above Navier-Stokes and energy equations are applicable to viscous flow where the transport phenomena of friction, thermal conduction and/or mass diffusion are concerned. The Reynolds Averaged Navier-Stokes method (RANS) is then introduced to model turbulence properties by using mean turbulent flow to average the flow equations over a time scale much larger than the turbulent motion. Many turbulence models have been developed based on RANS, namely one equation model and two equation models such as standard k- and RNG k- , and the Reynolds-stress model.
5.2.2
Grid resolution in CFD software
Numerical procedure is important for achieving accurate results. In most cases one would demand a grid-independent solution. Chen and Zhai (2003) have used Fisher’s (1995) measured data to investigate the difference that grid resolution makes to CFD results. They used four sets of grids to simulate the indoor airflow, namely a coarse grid (22 x17 x 15 = 5,610 cells), a moderate grid (44 x 34 x 30 = 44,880 cells), a fine grid (66 x 51 x 45 = 151,470 cells) and a locally refined coarse grid (27 x 19 x 17 = 8,721 cells) that has the same resolution in the near-wall regions as the fine grid.
When comparing the predicted temperature gradient along the vertical central line of the room, a course grid resolution was found not to produce a satisfactory result. The moderate and fine grid distributions produced quite similar temperature profiles and could be considered as grid independent. They found that instead of using a global refined grid that may need long computational time, a locally refined course grid could effectively predict airflow and heat transfer for indoor airflow investigations.
96
5.2.3
Verification and validation in CFD
Verification and validation are the primary means to assess accuracy and reliability in computational simulations. Verification is the assessment of the accuracy of the solution of a computational model by comparison with known solutions. Validation is the assessment of the accuracy of a computational simulation by comparison with experimental data. In validation the relationship between computation and the real world, i.e. the experimental data, is important, whilst in verification the relationship of simulation to the real world is not critical. Therefore verification is primarily a mathematical issue and validation is primarily a physics issue.
Oberkampf and Trucano (2002) extensively addressed the fundamental issues in verification and validation, such as code verification versus solution verification, model validation versus solution validation, the distinction between error and uncertainty, conceptual sources of error and uncertainty, and the relationship between validation and prediction. They found that the fundamental strategy of verification is the identification and quantification of errors in the computational model and its solution, and the fundamental strategy of validation is to assess how accurately the computational results compare with the experimental data, with quantified error and uncertainty estimates for both. In verification activities the accuracy of a computational solution is primarily measured relative to two types of highly accurate solutions: analytical solutions and numerical solutions. In validation strategy a hierarchical methodology that segregates and simplifies the physical and coupling phenomena involved in the complex engineering system of interest is employed.
5.2.4
Constraints for CFD simulation
CFD simulation has become less expensive and results have been able to be obtained faster in the past few of years because of developments in computing power and capacity. CFD used to be applied to test flow and heat transfer 97
conditions where experimental testing could prove to be very difficult and expensive. However the CFD results could not always be trusted because of the assumptions used in the modeling, and the approximations used in simulation to simplify the complex real problem of an indoor environment. Although a CFD simulation can always give a result, it may not necessarily be the correct result.
In order to illustrate some of the practical problems encountered in using CFD for modeling an indoor environment, Figure 5.1 gives the example of the initial research work of this thesis in modeling a simple small office room with displacement ventilation. The room was 3.5m wide, 5.5m deep and 2.7m high with cold air supply through 2 diffusers at the lower part of the rear wall of the office. The basic heat sources in the room were computers, occupants and lighting. Warm air was exhausted naturally through the buoyancy effect from the front window and expelled through the double-skin façade through the stack effect. Air temperature, air velocity and relative humidity were the main parameters used and measured. The study used the standard k- model of RANS. Some of the more important problems found in this initial simulation task were as follows:
difficulty in selecting an appropriate turbulence model for the study difficulty in setting correct boundary conditions selection of an appropriate grid resolution estimating the convective heat from the heat sources using the correct relaxation factors and internal iteration numbers for simulation
98
Figure 5.1 Isometric view of the office room
Using CFD for simulating the specific problem of an indoor environment is not an easy task and it requires much understanding of the tool itself. An example of this encountered in simulating the above task was how to model the supply air diffusers appropriately and to estimate the appropriate convective heat to the heat sources in the room. A commercial software package like Airpak (Airpak, 2003) from Fluent has a library of diffusers that can be selected to simulate an array of complex diffusers and a whole library range of heat sources for accurate computation. Without such a library one would find that only experienced CFD users would know how to model those items properly.
Most CFD programs are generalized and designed to solve flow and heat and mass transfer, not just to simulate indoor environments. Therefore a user can fine-tune the parameters and select the many options provided by the program 99
to obtain a result. With different tuning values the CFD results are often not the same. ASHRAE has developed a guide for using CFD to simulate indoor environments. The guide helps to establish a CFD model that can simulate a specific problem and that should be used for verification, validation and reporting of CFD indoor environment analysis (Chen and Srebric, 2002).
5.3
Research into CFD simulation for building design
5.3.1
CFD simulation in building design
Computational Fluid Dynamics (CFD) has become a useful tool for designers in the study of indoor and outdoor environment conditions in building designs. It is an effective research facility providing large quantities of data that is complementary to experimental measurements (Fletcher et. al., 2001). CFD technique has been applied with considerable success in building design and the advantages in analysing ventilation performance have been reported by Murakami and Liddament (1992) as the parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment. Stankovic (Internet page of Technology, Environmental Engineering in The Tropics, http://www.sia.org.sg) found that the accuracy of
CFD results is critically dependent on boundary conditions. The highest accuracy of the boundary conditions is normally achieved by combining the measured specific site data with dynamic thermal simulations. Papakonstantinou et al. (2000) has demonstrated that numerical solutions for ventilation problems can be obtained quickly and in good agreement with the experimental measurements. Computational results are realistic and in good agreement with the experimental measurements and that computer simulations are capable of assisting the designer to optimize in building design.
Jaros et al. (2002) established that the characteristics of CFD solved problems could be grouped under the following:
100
with problems of interest that are transient (unsteady) with timevariable boundary conditions (e.g. the direction and intensity of solar radiation, the outdoor temperature, etc) where fluid flow is predominantly driven by buoyancy forces (natural or mixed convection) where heat transfer arrives simultaneously by conduction (in solids), convection and thermal/solar radiation (in fluid) where temperature fields have to be solved simultaneously in solids and surrounding fluids (i.e. conjugate heat transfer problem)
They also found that solar radiation (short wavelength) and thermal radiation (long wavelength) should be handled separately in CFD simulation. Diffusive solar irradiation is significant on cloudy days because on clear days the diffusive intensity of solar radiation accounts for less than 12% of the direct radiation. The correct choice of the type of fluid flow is a very important aspect of the CFD simulation too. There are usually regions with and without turbulence in the same space. The model of turbulence must be able to deal with laminar and transitional flow at the same time and the RNG k- model of turbulence appears to be the most suitable choice.
Gan (1995a, 1995b) has carried out detailed research into the effects of displacement ventilation in building deign using CFD simulation. He used the standard k-e turbulence model for the prediction of indoor air flow patterns, temperature and moisture distributions, and taking account of heat transfer by conduction, convection and radiation. The thermal comfort level and draught risk are predicted by incorporating Fanger’s comfort equations in the airflow model. He found that common complaints of local thermal discomfort in offices with low turbulent air flow such as displacement ventilation often result from unsatisfactory thermal sensation rather than draught itself or alone. Decreasing supply air velocity or increasing supply air temperature reduces the potential cold thermal discomfort. Optimal supply air conditions of a displacement system vary with the distance between the occupant and air diffuser besides cooling load and load distribution. Thermal discomfort in the 101
displacement-ventilated offices can be avoided by optimizing the supply air velocity and temperatures.
In evaluating room air distribution systems Gan (1995b) uses air-flow model based on the continuity equation, Navier-Stokes equation, thermal energy equation and concentration equation together with the k-e turbulence model equations. He found that the most effective air distribution system for heating does not coincide with that for cooling. An air supply system that results in upward displacement flow performs better than conventional air supply systems such as ceiling diffuser or sidewall slot jet. The supply air velocity for displacement ventilation should be appropriate so that fresh air can reach occupants without causing draught. The performance of displacement ventilation system can be further enhanced when used with a chilled ceiling, especially when heat gains in the room are large.
Gan (2001) in his research on thermal transmission through the double-skin facades used CFD for predicting the convective heat transfer coefficient, thermal resistance and thermal transmittance for a double-glazing unit. The unit was an unventilated enclosure and the flow within it would be buoyancyinduced natural convection. A numerical method for predicting the thermal transmittance of multiple glazing systems under both laminar and turbulent regimes is presented. The findings are:
o The accuracy of the numerical prediction was found to be influenced by the turbulence models employed. o The convective heat transfer coefficient, thermal resistance and thermal transmittance vary with the air space width between glazing panes up to about 25mm. After which, the convective heat transfer coefficient increases slightly with air space. o Both the convective heat transfer coefficient and thermal transmittance increase linearly with the temperature difference between the hot & cold panes of glass. o The effect of the temperature difference across an air space on the convective heat transfer coefficient is significant. 102
o For moderate climate conditions, the effect of the temperature difference on the thermal transmittance may be considered negligible.
MIT (Chen and Srebric, 2000) has developed several Reynolds Averaged Navier-Stokes (RANS) equation models and Large Eddy Simulation (LES) models to enhance the capabilities of CFD for use in indoor and outdoor environment design. The new models have been used to assess building shape design, to evaluate the effectiveness of natural ventilation in buildings, to model Volatile-Organic-Compound (VOC) emissions from building materials, and to calculate indoor environment parameters. A two-layer turbulence model, a single k-equation turbulence model for near-wall flow and the standard kmodel for flow in the outer-wall region, could accurately predict heat transfer on a wall. The computing time needed is slightly higher than the standard k-e model but much lower than a low-Reynolds number k- model. A zeroequation model (a single algebraic function) is used to simulate transient flow that significantly reduces computing costs. The coupling with an energy simulation program gives more accurate results for building energy analysis and indoor environment design. A new dynamic sub grid-scale model is used to predict indoor airflow without a homogenous flow direction. The model uses two different filters to obtain the model coefficient as a function of space and time. The model can accurately predict flow in a room with a heated floor and in an office with displacement ventilation.
Manz (2003a, 2003b) used overall convective heat transfer in an air layer within a rectangular cavity was calculated using CFD code (Flovent ver. 3.1 with revised k- turbulence model with logarithmic wall functions) and compared with correlations based mainly on experimental results. The calculated Nusselt numbers do not deviate more than 20% from analytical correlations. A grid independence analysis showed that variations in grid spacing had only a very minor impact on the calculated Nusselt numbers. Flovent is currently being applied to double-skin facades with ventilated or unventilated cavities that include a shading device. The work involves combining an optical model for determining absorbed solar radiation in layers 103
of façade elements such as glass panes, roller blinds, etc with CFD modeling with the objective of increasing the reliability of predictions of these elements’ thermal transmission and total solar energy transmission.
Zhai et al. (2002, 2003, 2005, 2006) used the techniques of coupling energy simulation (ES) programs with CFD simulations (an integrated program E+MIT-CFD which developed by coupling an ES program E+ with CFD solver MIT-CFD) to provide energy analysis for a whole building and the heating, ventilating and air conditioning (HVAC) systems used. Spaceaveraged indoor environmental conditions, cooling/heating loads, coil loads, and energy consumption can be obtained on an hourly or sub-hourly basis for a period of time ranging from a design day to a reference year.
They found that:
CFD programs make detailed predictions of thermal comfort and indoor air quality, such as the distributions of air velocity, temperature, relative humidity and contaminant concentrations. The programs could determine thermal comfort and air quality indices such as the predicted mean vote (PMV), the percentage of people dissatisfied (PPD) which due to discomfort, the percentage dissatisfied (PD) which due to draft, ventilation effectiveness and the mean age of air. CFD can determine the temperature distribution and convective heat transfer coefficients. It can accurately calculate natural ventilation rate driven by wind effect, stack effect, or both. CFD needs information such as heating/cooling load and wall surface temperatures from ES as inputs. CFD applies numerical techniques to solve the Navier-Stokes equations for fluid flow. It also solves the conservation equation of mass for the contaminant species and the conservation equation of energy for building thermal comfort and indoor air quality analysis. For buoyancy-driven flows, the Buossinesq approximation, which ignores the effect of pressure changes on density, is usually employed.
104
The buoyancy-driven force is treated as a source term in the momentum equations. CFD solves the equations by discretizing the equations with the finitevolume method. The spatial continuum is divided into a finite number of discrete cells, and finite time-steps are used for dynamic problems. The discrete equations can be solved together with the corresponding boundary conditions. Iteration is necessary to achieve a converged solution. The accuracy of CFD prediction is highly sensitive to the boundary conditions supplied (assumed) by the user. The boundary conditions for CFD simulation of indoor air flows relate to the inlet (supply), outlet (exhaust), enclosure surfaces, and internal objects. The temperature, velocity and turbulence of the air entering from diffusers or windows determine the inlet conditions, while the interior surface convective heat transfers in terms of surface temperatures or heat fluxes are for the enclosures. These boundary conditions are crucial for the accuracy of the CFD results. Room air has a characteristic time of a few seconds while building envelope has a few hours. CFD simulation must be performed over a long period for the thermal performance of the building envelope, but it must use a small time-step to account for the room air characteristics.
5.3.2
CFD approaches in indoor environment simulation
Indoor environments consist of four major components, namely the thermal environment, indoor air quality, acoustics and the lighting environment. Building thermal environment and indoor air quality include parameters like air temperature, air velocity, relative humidity, environmental temperature, contaminant and particulate concentrations, etc. CFD programs can be used particularly to deal with problems associated with thermal environment, indoor air quality and building safety because the programs solve these important parameters.
105
The applications of CFD in indoor environments are very wide and there are a number of recent examples of its application for natural ventilation design (Carrilho da Grace et al., 2002), the study of building material emissions for indoor air quality assessment (Topp et al., 2001; Murakami et al., 2003), building elements design (Manz, 2003) and for building energy and thermal comfort simulations (Bartak et al., 2002; Beausoleil-Morrison, 2002; Zhai and Chen, 2003). Jiang and Chen (2002) found out that the outdoor environment has a significant impact on the indoor environment, especially in buildings with natural ventilation. They recommended that both of the indoor and outdoor environments together with the combined indoor and outdoor airflow needed to be studied together.
Almost all the flows in indoor environments are turbulent. CFD can analyse these turbulent flows by different means, namely direct numerical simulation, large eddy simulation (LES) and the Reynolds averaged Navier-Stokes equations with turbulence models (RANS).
Direct numerical simulation computes turbulent flow by solving the highly reliable Navier-Stokes equation without approximations. It requires a very fine grid resolution to capture the smallest eddies in the turbulent flow in very small time steps, even for a steady-state flow. Direct numerical simulation would therefore require a very fast computer and even then it would take years of computing time to predict an indoor environment.
Large eddy simulation (LES) separates turbulent motion into large eddies and small eddies (Deardorff, 1970). The method computes the large eddies in a three-dimensional and time dependent way, while estimating the small eddies with a sub grid-scale model. When the grid size is sufficiently small, the impact of the sub grid-scale model on the flow motion is negligible. Sub grid-scale models tend to be universal because turbulent flow on a very small scale seems to be isotropic. Since the flow information obtained from sub grid-scale may not be as important as that from large-scale grids, LES can be used as a general and accurate tool to study engineering flows. LES has been successfully applied to study airflow in and around buildings (Emmerich and McGrattan, 106
1998; Jiang and Chen, 2002; Kato et al., 2003), and is much faster than direct numerical simulation, but it requires a large computer capacity and a long computing time for predicting indoor environments.
The Reynolds averaged Navier-Stokes equations (RANS) with turbulence models solve the statically averaged Navier-Stokes equations by using turbulence transport models to simplify the calculation of the turbulence effect. The use of turbulence models can significantly reduce the requirements for computer memory and speed and problems can be solved in a few hours of computing time with a modern PC. The RANS modeling provides detailed information on indoor environments. The method has been successfully applied to the analysis of indoor airflow and thermal comfort and of indoor air quality (Ladeinde and Nearon, 1997). Whether it is LES or RANS modeling, boundary conditions must be specified in order for the highly nonlinear and interrelated equations to be used to solve a specific problem of the indoor environment.
5.4
A case study of a CFD simulation for double-skin façade
Grabe et al. (2001) carried out a detailed study of the effects of ventilation with double-skin façades which provides important guidelines in using CFD simulation for double-skin façade investigation. They determined that the simulation of a double-skin façade must yield the following information:
a) The air mass flow through the façade gap to control the possibility of natural ventilation of the room behind. b) The temperature of the façade air related to the height of the façade, which determines the temperature of the supply air in the case of natural ventilation. It also helps in estimating the cooling load required in the case of conditioning. c) The temperature of the façade perimeter to predict possible deformations of the materials due to thermal elongation.
107
5.4.1
Thermal considerations
In their study Grabe et al. determined that the heat flow was one dimension and the air mass flow was one directional in either the positive or negative ydirection. There was no consideration of any diagonal flow or any local, secondary and reverse currents. The temperature function for the shaft air over the height of the system was based on the energy transport equation. Only steady state conditions were considered and single heat sources did not occur. The molecular heat transport within the air was not considered, only the net heat flow into and out of the gap in a positive or negative x-direction. The convective transport was taken as the net heat flow in the main y-direction.
The heat transfer depended on the Rayleigh number, which was determined by thermal diffusivity, temperature differences and the height of the surface. The radiative heat exchange between the surfaces and the heat exchange between the system and the ambient climate was also considered.
The thermal driving force in all buildings constructed with double-skin facades is always the sun’s radiation that is absorbed by the surfaces of the double façade. These surfaces are mainly the shading device, the inner pane and the outer pane. The absorbed energy, determined by the solar intensity and the absorption coefficient of the material, leads to an energy flow to and from the element to its surrounding (either shaft air by convection, the other planes by radiation or to the external/internal climate by both). The shaft air temperatures and the constant solar energy together with the temperature of the internal and external climate determine the temperatures of the surfaces.
5.4.2
Fluid dynamics considerations
In this study only buoyancy forces were considered for the motion of the air. The sum of energy per mass (Nm/kg) according to the Bernoulli equation remains the same between point 1 and point 2 of a streamline and may only 108
change its character between static, dynamic, potential and dissipated energy. The equation is only applicable for systems with constant density p. If a system is divided into a number of finite subsystems and the fluid properties are regarded as constant for each subsystem, the pressure might be related to the respective density. Only 2 subsystems were chosen – the external air and the façade air and only the pressure difference between inside and outside was considered. With buoyancy driven natural ventilation it is very common to determine the dissipated energy in a similar way to the determination of the turbulence losses of pipes.
The temperature distribution over the height of a double-skin façade is dependent on the mass flow through it. On the other hand the air velocity is dependent on the density of the air, which is determined by the temperature of the air. Therefore, the problem has to be solved by an iteration process.
5.4.3
Modelling of the façade
The physical model was closed to the external climate and opened to the internal climate. It was still fully exposed to the external temperature and to solar radiation but ventilated with internal air. This also avoided the effect of wind pressure on the ventilation of the façade. The total height of the façade was 2.05m, the breadth 0.95m and the total depth (both shafts) was 0.24m.
The monitored results were the flow resistance, especially at the inlet and outlet of the double-skin façade. The best predicted result observed was to model both the inlet and outlet as a flange with an abrupt enlargement to the duct diameter and a diffuser with a preceding abrupt contraction.
5.4.4
Findings
Assuming the same flow conditions for natural ventilation as those used for mechanical ventilation caused the main problem found. The driving force for 109
natural ventilation is the reduction of the density due to the increase in air temperature. This increase is greater near the heat sources and thus near the panes and shading device. The ventilation could be non-symmetrical because of different magnitudes of the heat sources.
The flow conditions in the double-skin façade were found to be turbulent and with increasing turbulence the velocity profile became more similar to the turbulent profile of the pipe flow. This could lead to a better prediction since resistance factors are usually determined under turbulent conditions. The researchers found that when using the resistance factor for analysing the flow characteristics of buoyancy driven ventilation one runs the risk of ending up with the wrong results.
5.5
Review of several building simulation software packages
5.5.1
Apache software
Apache is a component of the IES Virtual Environment software that is capable of performing dynamic thermal simulation using hourly weather data. Its application includes thermal design (heating, cooling and latent load calculations), equipment sizing, codes and standards checks, dynamic building thermal performance analysis, systems and controls performance, and energy use.
The software has a range of building analysis functions including HVAC systems designs with a strong links with CAD and provides rigorous analysis and visualization of shading and solar penetration of building designs. Geometrical building data may be imported from a range of CAD systems via customized links or DXF files. Building and climatic information can be input via graphical interfaces and is supported by extensive databases. The output results are presented in tabular and graphical form and can be exported in a
110
variety of common formats. The software will run on a standard PC with a minimum 100MB of Ram and it only takes a few days to learn the basic modules to start any meaningful simulation work.
5.5.2
Flovent software
Flovent software uses CFD techniques to resolve issues in the design and optimization of ventilation systems. It can calculate and predict airflow, heat transfer and contamination distribution for built environments. Users will input building geometry, ambient temperature, the flow rate for air supplies, the thermal conductivity of solid items, etc. for simulation runs and the output results are presented in 3-D visualizations of predicted air velocity, temperature profile streamlines and contaminant concentrations for the built environments. The software runs on either the Windows NT or UNIX platform and it requires some degree of knowledge of ventilation systems to master the program.
5.5.3
Microflo software
Microflo is part of the IES Virtual Environment software. It uses CFD simulation techniques coupled dynamically with thermal simulation for a full building analysis. It can assess building airflow, air quality and thermal performance, solving the 3D non-isothermal continuity, momentum, energy and species conservation equations, and incorporating the k-e turbulence model using the finite difference method.
The software provides a high level graphical user interface for pre-processing, including creating 3D models, mesh generation, defining boundary conditions, run monitoring, etc. It has extensive graphical post-processing tools which provide coloured cut-planes for any selected variable, 3D arrows with variable coloring, animated airflow streamlines, 3D animation, flying through, etc. Currently only orthogonal objects can be used due to the restriction of the Cartesian co-ordinate system of the program. Non-orthogonal objects like 111
polygons and cylinders will automatically be reformatted into orthogonal prisms during the mesh generation.
It requires a large amount of memory and of hard disk space to run, and knowledge of CFD, and an understanding of the environmental physics of buildings are desirable for using the program.
5.5.4
Phoenics software
Phoenics is a general purpose CFD program that can simulate fluid movement and heat transfer for a wide variety of applications. It can predict smoke spread and ventilation in buildings, fire modeling in and around buildings, internal airflows in ventilated spaces, thermal comfort, contaminant spread and deposition of airborne sediment, etc. Flair is a reduced version of Phoenics for use in analyzing airflows in air conditioning and ventilation systems and fire or smoke spread in buildings.
The software requires very detailed input of the CFD model, for example details of each inlets and outlets including their attributes, and details of the domain grid and initialization for the simulation. The program solves for pressure, temperature, velocity, contamination or smoke and any fluid property desired to be determined, and display the outputs through 3D visualization of the domain and plots isosurfaces and streamlines.
It runs on standard PCs but the software requires considerable experience and knowledge of CFD to be able to obtain accurate results.
5.5.5
Airpak software
Airpak is an easy-to-use design tool for the design and analysis of ventilation systems that are required to provide acceptable thermal comfort and indoor air quality solutions. It can accurately model airflow, heat transfer, contaminant 112
transport, and thermal comfort in any ventilation system, as well as external building flows. Computer models can easily be built for the required application and quickly tested for a variety of design options to find the best solution. The software can reduce risks for new designs and improve the efficiency of current designs, and eliminates the guesswork in designing ventilation systems for non-standard and one-of-a-kind facilities.
Simulation models can be built with Airpak’s object-oriented model building tools or imported from CAD. Users can specify the ventilation system design, including types, flow rates, temperatures, and locations of air inlet diffusers and exhausts; define the thermal boundary conditions that represent heating loads of occupants, lighting, equipment and external conditions including solar loads; and define humidity and contaminant boundary conditions. The output capabilities of the software include full-colour animation, pictures, and plots of ventilation airflows showing airflow patterns, air turbulence, room air distribution, temperature distribution, thermal comfort conditions, and contaminant distribution as well as the ability to automatically generate detailed quantitative reports specified by the user.
It runs on either Windows, Unix or Linux platforms and users can start using the software after one day of training.
5.5.6
Conclusion
Besides the reasons given in Section 5.1 for using CFD software instead of ES software for the purpose of this study, there are still questions regarding which commercially available CFD software is the most appropriate to be used for the very specific simulation work of the research.
As the study had to consider the constraints of time, the availability of suitable software and hardware, and limitations on funding, the criteria for the selection included the following:
113
Ease of use in terms of user input and learning curve for the software The ability of the software to quickly test different design options for the best solution The software had to allow for user determination of the required ventilation system design and various boundary conditions that relate to indoor thermal comfort The software had to be able to produce reliable results for airflow patterns, temperature distributions and thermal comfort conditions The software could be supported by any standard PC in terms of required RAM memory and hard disk space The software could be obtained and maintained within the research budget
In assessing several of the available commercial CFD software packages described above, Airpak software was found to have obvious advantages over the others in terms of the listed criteria for the purpose of this research, which was to find out the viability of double-skin façades in a hot and humid climate to reduce energy usage in high-rise buildings.
5.6
Airpak CFD software
5.6.1
The Airpak CFD software
Airpak uses object-based model building tools and libraries coupled with automatic unstructured meshing that enables complex model of building. It uses the FLUENT CFD solver engine for thermal and fluid-flow calculations. Its post-processing features also allow results to be tabulated easily for any ventilation problems at hand. (Airpak User’s Manual, 2003)
114
In conducting solid regions, Airpak solves a simple conduction equation that includes the heat flux due to conduction and volumetric heat sources within the solid as shown in the following equation:
∂ ( ρ H) = ∇ ⋅ [K∇T ]+ SH ∂t
(5.1.4)
where ρ is density, K is conductivity, T is temperature and SH is the volumetric heat source.
Equation 5.1.4 is solved simultaneously with the energy transport equation 5.1.3 in the flow regions (see Section 5.2), to yield a fully coupled conduction/convection heat transfer prediction.
Airpak predicts the local mass fraction of each species, Yi, through the solution of a convection-diffusion equation for the ith species. This conservation equation for species takes the following general form:
∂ ( ρ Yi) + ∇ ⋅ (ρν Yi )= - ∇ ⋅ Ji + Si ∂t
(5.1.5)
where Si is the rate of creation by addition from user-defined sources. An equation in this form will be solved for N-1 species where N is the total number of fluid phase species present in the system.
There are four turbulence models available in Airpak, namely the mixinglength zero-equation model, the indoor zero-equation model, the two-equation (standard k-e) model, and the RNG k-e model. In turbulent flows Airpak computes the mass diffusion in the following form:
Ji = - ρDi , m +
µ
t
Sct
∇ Yi
(5.1.6)
115
where Sct is the turbulent Schmidt number,
µ (with a default ρD t
t
setting of 0.7).
5.6.2
Buoyancy-driven flows and natural convection in Airpak
Airpak uses either the Boussinesq model or the ideal gas law in the calculation of natural-convection flows.
The importance of buoyancy forces in a mixed convection flow can be measured by the ratio of the Grashof and Reynolds numbers as follow:
Gr gβ ∆TL = 2 v2 Re
(5.1.7)
When this number approaches or exceeds unity, a strong buoyancy contribution to the flow is expected. Conversely, if it is very small, buoyancy forces may be ignored in the simulation.
In pure natural ventilation, the Rayleigh number measures the strength of the buoyancy-induced flow as follows:
Ra =
gβ ∆TL3 ρ
(5.1.8)
µα
where β is the thermal expansion coefficient of β = -
α is the thermal diffusivity of α =
1 ∂ρ ρ ∂T
and p
k . ρcp
Rayleigh numbers less than 108 indicate a buoyancy-induced laminar flow, with transition to turbulence occurring over the range of 108 < Ra < 1010.
116
5.6.3
Radiation simulation in Airpak
The terms radioactive heat transfer and thermal radiation are commonly used to describe heat transfer caused by electromagnetic waves. All materials continually emit and absorb electromagnetic waves or photons. The strength and wavelength of emissions depends on the temperature of the emitting material. At absolute zero K no radiation is emitted from a surface. Airpak only consider the wavelengths in the infrared spectrum for heat transfer applications in its simulation and it provides two models for radiation heat transfer simulations, namely the surface-to-surface (S2S) radiation model and the discrete ordinates (DO) radiation model.
5.6.4
Solution procedures in Airpak
Airpak solves the governing integral equations for mass and momentum, and when appropriate for energy, species transport, and other scalars such as turbulence. A control-volume-based technique is used with the procedures as follows:
a) Division of the domain into discrete control volumes using a computational grid. b) Integration of the governing equations in the individual control volumes to construct algebraic equations for the discrete dependent variables such as velocities, pressure, temperature and conserved scalars. c) Linearization of the discretized equations and solution of the resultant linear equation system to yield updated values of the dependent variables.
The governing equations are solved sequentially, and because the equations are not linear several iterations of the solution loop must be performed before a converged solution is obtained. Each iteration consists of the steps outlined below and illustrated in Figure 5.2: 117
a) Fluid properties are updated based on the current solution. If the calculation has just begun the fluid properties will be updated based on the initialized solution. b) The u, v and w momentum equations are each solved in turn using current values for pressure and face mass fluxes, in order to update the velocity field. c) Since the velocity obtained in Step (b) may not satisfy the continuity equation locally, a “Poisson-type” equation for the pressure correction is derived from the continuity equation and the linearized momentum equations. This pressure correction equation is then solved to obtain the necessary corrections to the pressure and velocity fields and the face mass fluxes such that continuity is satisfied. d) Where appropriate, equations for scalars such as turbulence, energy, species and radiation are solved using the previously updated values of the other variables. e) A check for convergence of the equation set is made. f) The above steps are continued until the convergence criteria are met.
118
Update properties
Solve momentum equations
Solve pressure-correction (continuity) equation Update pressure, face mass flow rate
Solve energy, species, turbulence and other scalar equations
Converged?
STOP
Figure 5.2 Overview of the solution method
5.6.5
The validation of Airpak software
A traditional approach for examining whether a CFD result is correct is by comparing the CFD result with corresponding experimental data. The validation of the Airpak software was carried out by comparing the simulation results from Airpak with the experimental and simulation results from another commercial simulation software called FloVent, which was carried out by Manz (2003). The measured hourly outdoor air temperatures shown in Graph 5.1 were used for piecewise linear interpolation for the transient simulations. 119
The simulation model for the validation is shown in Figure 5.3 and one of the comparison results is shown in Graph 5.2 below. Series 1 are the measured surface temperatures for the inner pane in the experimental results at Measurement Point 2, and Series 2 are the simulation results from Airpak. Both results were analyzed and compared and it was found that the variation was within 5% of the acceptable error tolerance.
Inner pane measurement point 2
Figure 5.3 Simulation model used for the validation constructed in Airpak (Source: according to Manz, 2003)
120
40
Air Temp (C)
35 30 25 20
Outdoor Temp
15 10 5 0 1
4
7
10 13 16 19 22 25 Tim e (h)
Graph 5.1 Measured hourly outdoor temperatures (Source: “Numerical simulation of heat transfer by natural convection in cavities of façade elements” by Manz., 2003)
Inner Pane Surface Temp (C)
30 25 20 Series1
15
Series2
10 5 0 1
3
5
7
9
11 13 15 17 19 21 23 Time (h)
Graph 5.2 Measured results (Series 1) vs. Airpak simulation results (Series 2) (Source: Manz., 2003 and Wong, 2008)
121
The software is selected to be used in this research to model the complex energy transfer of the double-skin façade in view of the capabilities and good interface of the Airpak software discussed above. The software is used to optimise the appropriate opening sizes on the glazing, the width of the intermediate space and the ventilation rate through the internal office space.
5.7
Conclusion
This Chapter has given a detailed analysis of the background to the choice of appropriate software to be used for this research, and has demonstrated why Airpak CFD software was chosen. The next chapter will describe the thinking underlying the process of selection for a suitable methodology for this research.
122
Chapter 6
Research Methodology
This Chapter provides the research methodology for the modelling of the highrise double-skin façade building using computational fluid dynamic simulation and set out the knowledge gap and goals for the research.
6.1
Introduction
Architectural research has been conducted throughout the history of architecture and the development of particular structural forms and building materials is the outcome of systematic experimentation and observation, and the application of such building principles to the building projects. On the other hand, recent development in architectural research has seen its domain spanning more broadly across a range of topic areas such as design methods, climate and structural studies, and energy conservation.
Given the breadth and complexity of architectural research, James Snyder in his edited book titled Architectural Research defined research as “systematic inquiry directed toward the creation of knowledge” (Snyder, 1984). It suggested that the inquiry is systematic in some way and there is a conscious separation of particular information from our daily experience, and how the information is categorised, analysed and presented. The creation of knowledge is not only referred to the grand theories of sciences but new knowledge can also be attained through different means like testing materials from a number of building projects; and evaluating different window configurations for better thermal and wind resistance.
Architectural research is vitally important because of the ever-increasing proportion of architectural practice involves into unfamiliar domains beyond the expertise of individual practitioners and beyond the conventional understanding of the profession as a whole. For example if a project is for a particular user group with brain disorder whose specific requirements for the physical environment are not well documented. In this instance, an in-depth 123
research into the matter will certainly helps to develop a satisfactory and functional built environment for the users. Therefore, architectural research also important in the success and viability of the profession, in that sense, and should be pursued continuously for the common good.
6.2
Research strategies in architectural research
In their book titled Architectural Research Methods, Groat and Wang (2002) had identified seven research strategies in architectural research. They are interpretive-historical research, qualitative research, correlational research, experimental research, simulation research, logical argumentation research, and case studies and mixed-method research.
The interpretive-historical research strategy draws upon evidence derived from archival or artifactual sources as the research question usually focuses on a setting or circumstance from the past. The qualitative research strategy seeks to understand settings and phenomena in a holistic way by focusing on contemporary social and cultural circumstances. Some of the data collection tactics used in qualitative research is interviews, focus groups, surveys and observation.
The important characteristic of the correlational research strategy is the discovery of patterns or relationships among specified variables of interest in a particular setting or circumstance. Surveys, observation, mapping and sorting are some of the data collection tactics used in correlational research. Many viewed the experimental research strategy as the preeminent standard for empirical research. It emphasized on the careful manipulation of variables with the goal of attributing causality.
The simulation research strategy recreated some aspect of the physical environment in one of a variety mode, from a highly abstract computer
124
simulation to full-scale real life mock-up. In scholarly research, simulation is increasingly used as an alternative to lengthy and costly physical experiments. The logical argumentation research strategy uses sequence of logical steps within a closed system and in architecture it can be used in a philosophical treatise on architectural aesthetics. Finally, in case study research strategy a particular setting or circumstance is investigated holistically using a variety of data collection and analysis tactics.
Increasingly, researchers in many fields, including architecture, are advocating a more integrative approach to research whereby multiple research strategies are incorporated in one study, for example combining interpretive-historical research strategy with qualitative research strategy. They believe these combining methods provides checks against the weak points in each strategies and enabling the benefits to complement each other. However, difficulties might arise when different researchers advocate the combining of research strategies at different levels of the research process including topic areas; research design; and tactics in data collection.
Groat and Wang (2002) attempted to propose three combined research strategies namely two-phase research strategy, dominant-less dominant research strategy, and mixed-methodology research strategy. The mixedmethodology research strategy is the most complete level of integration among two or more research strategies. In this model, the researcher conducts aspects of both strategies in roughly comparable sequences and with approximately equals degrees of emphasis. While there is much to be gained in integrating different research strategies, the researcher may also find that combining strategies may require a higher level of sophistication in research methodology than would be expected if a more conventional approach were to be used.
6.2.1
Literature review
A literature review is defined as “the totality of activities the researcher undertakes to use that body of information in such a way that a topic of inquiry 125
can be competently defined and addressed” (Groat and Wang, p.46). Thus, a literature review exists only after the general material has been arranged into a coherent system, one that has been customized to fit the research question.
A literature review is often confused with an annotated bibliography. An annotated bibliography is an intermediate point toward the literature review. The aim of an annotated bibliography is to respond to each reference cited with a descriptive paragraph of the work’s goal, its theoretical stance, and most importantly, its relevance for the investigation. This process helps focus the emerging research question.
The literature review will make use of the references in the annotated bibliography and go beyond it to include the following information:
a) An introductory statement of the general intent of the literature exploration that will include the direction of the proposed research to come. b) A summary of the lines of existing research that will provide background for the proposed research. c) Observations on the state of the literature in terms of how it can be expanded by the proposed research. The reviewer needs to identify specific areas that have not been covered by the extant literature, arguments that the reviewer wishes to challenge, or subject of study that can be reconfigured by a new conceptual framework.
The use of literature review in research could be identified as follow:
a) Literature review is used to identify the research question, as topics of inquiry can emerge from analyzing, critiquing and suggesting improvement to an extant work. Research questions can emerge from a comparison of representative works in the literature. b) Literature review is used to focus the topic of inquiry, as a topic of inquiry should not be too general. An indicator that a topic of inquiry 126
may be either too broad or too restrictive is the inability to clearly and simply identify a body of literature to which the topical question can be referred. c) Literature review is used to understand the makeup of the research question. d) Literature review is used to understand an idea’s genetic roots and the current conceptual landscape of the topic.
Barzun and Graff (1985, p.44-45) emphasised that the researcher must have a ‘love of order’. He or she must have a system that allows for any piece of information to be retrieved. That means during the initial work on the literature search, whether on the Internet or in a library, the researcher would need to drop down information clearly and chronologically for easy retrieve in the future.
6.2.2
Research approach: Qualitative versus Quantitative
Qualitative research is an enquiry into an identified problem, based on testing a theory, measured with numbers and analyzed using statistical techniques. The goal of qualitative methods is to determine whether the predictive generalizations of a theory hold true.
Quantitative research approach has the goal of understanding a social or human problem from multiple perspectives. This type of research is conducted in a natural setting and involves a process of building a complex and holistic picture of the phenomenon of interest.
The selection of which research approach is appropriate in a given study should be based upon the problem of interest, resources available, the skills and training of the researcher, and the audience for the research. Although some research may incorporate both quantitative and qualitative methodologies, in their ‘pure’ form there are significant differences in the assumptions underlying 127
these approaches, as well as in the data collection and analysis procedures used.
It is important to be able to identify and understand the research approach underlying any given study because the selection of a research approach influences the questions asked, the methods chosen, the statistical analyses used, the interface made and the ultimate goal of the research.
There are three general types of quantitative methods:
a) Experiments – they are characterized by random assignment of subjects to experimental conditions and the use of experimental controls. b) Quasi-experiments – the studies share almost all the features of experimental design except that they involve non-randomized assignment of subjects to experimental conditions. c) Surveys – they include cross-sectional and longitudinal studies using questionnaires or interviews for data collection with the intent of estimating the characteristics of a large population of interest based on a smaller sample from that population.
The three general types of qualitative methods are:
a) Case studies – single entity or phenomenon (‘case’) bounded by time and activity and collects detailed information through a variety of data collection procedures over a sustained period of time. The case study is a descriptive record of an individual’s experiences and/or behaviors kept by an outside observer. b) Ethnographic studies – the researcher studies an intact cultural group in a natural setting over a specific period of time. A cultural group can be any group of individuals who share a common social experience, location, or other social characteristic of interest.
128
c) Phenomenological studies – human experiences are examined through the detailed description of the people being studied. The goal is to understand the ‘lived experience’ of the individuals being studied. This approach involves researching a small group of people intensely over a long period of time.
Quantitative Mode
Qualitative Mode
Assumptions
Assumptions
•
Social facts have an objective reality
•
Reality is socially constructed
•
Primacy of method
•
Primacy of subject matter
•
Variables can be identified and
•
Variables are complex, interwoven,
relationships measured •
Etic (outside' s point of view)
and difficult to measure •
Emic (insider' s point of view)
Purpose
Purpose
•
Generalizability
•
Contextualization
•
Prediction
•
Interpretation
•
Causal explanations
•
Understanding actors'perspectives
Approach
Approach •
Ends with hypotheses and grounded
•
Begins with hypotheses and theories
•
Manipulation and control
•
Uses formal instruments
•
Emergence and portrayal
•
Experimentation
•
Researcher as instrument
•
Deductive
•
Naturalistic
•
Component analysis
•
Inductive
•
Seeks consensus, the norm
•
Searches for patterns
•
Reduces data to numerical indices
•
Seeks pluralism, complexity
•
Abstract language in write-up
•
Makes minor use of numerical
theory
indices 129
•
Descriptive write-up
Researcher Role
Researcher Role •
Detachment and impartiality
•
Personal involvement and partiality
•
Objective portrayal
•
Empathic understanding
Table 6.1 Reproduced from Glesne, C., & Peshkin, A. (1992): Becoming qualitative researchers: An introduction.
Table 6.1 above shown the difference between qualitative and quantitative research methodologies and what the two research approaches encompassed.
6.3
The knowledge gap and research questions
The research questions arise from the commitment in contributing in reducing global warming due to the harmful by-products of human built environment. In search of fulfillment to the course of reducing energy usage, the new technology of using double-skin façade in commercial buildings seem giving some sort of positive direction to achieve that goal. Literature review is used to narrow down the scope of the research field, further streamlining the ‘right’ research questions to be asked, and in identifying the knowledge gap of the interest research topic.
This research is carried out using the combined research strategies mentioned in Section 6.2 in this Chapter by integrating simulation research strategy with case study research strategy. It is mainly based on the quantitative research approach by identifying an issue at hand and proposing a theory, then tests it out vigorously using the most appropriate research strategies at hand. Results
130
are analysed and conclusions established to prove the viability of the theory proposed.
The research attempts to bridge the gap of looking into the possibilities of natural ventilation in office buildings specifically in the hot and humid climate region with the use of double-skin façade. The research questions are:
a) Is the technology of double-skin façade viable in the hot and humid climatic environment?
b) Could naturally ventilating a commercial building using the doubleskin façade technology possible at all?
c) How well does double-skin façade technology in providing natural ventilation to the high-rise building in the hot and humid climatic environment?
d) What will be the window periods for such advance system in order for it to work in the hot and humid climate?
The unique double-skin façade construction is thought to be able to act as a stack in providing required ventilation for the internal space. It is the intent of the research to analyse the airflow patterns induced by the wind & thermal forces through the double-skin façade into the interior office space and their effects onto the thermal comfort within the space. Computer simulation is used to analyse the results obtained through the different opening sizes of the glass panel and the size of the intermediate space of the double-skin façade with variation of vent sizes at the rear of the office space to generate an acceptable cross ventilation rate within the internal space.
131
6.4
Research methods for a tropical double-skin façade
6.4.1
Building simulation methodology
The scope of building simulation is wide and it includes the early studies of the energy and mass flow process in the built environment. Building simulation tools had been used in the energy performance field since in the 60s and extended into other fields like lighting, heating ventilation and air-conditioning (HVAC), airflow, and more recently into areas like heat transfer, acoustics, control systems, urban and micro climate simulations.
Simulation involves the ‘creation’ of behavioral models of a building for a given stage of its development. The development stage can range from ‘asdesigned’ to ‘as-built’ to ‘as-operated’. The distinction is important as correctness, depth, completeness and certainty of the available building information varies over different life cycle stages. The actual simulation involves executing a model that is deduced from the available information on a computer. Models are developed by reducing real world physical entities and phenomena to an idealized form at some level of abstraction. From this abstraction, a mathematical model is constructed by applying physical conservation laws.
The modelling and simulation of complex systems requires the development of a hierarchy of models, which represent the real system at differing levels of abstraction. The selection of a particular modeling approach is based on a number of criteria, including the level of detail needed, the objective of the simulation, available knowledge resources, etc.
Simulation is credited with speeding up the design process, increasing efficiency and enabling the comparison of broader range of design variants. Simulation provides a better understanding of the consequences of design decisions, which increases the effectiveness of the design process as a whole.
132
Simulation has not only increasingly being used during the early design stages; it is also being applied during the commissioning and operational facility management phases of a project.
6.4.2
Computational Fluid Dynamic and Airpak software
Computational Fluid Dynamic (CFD) has become a useful tool for designers in the study of indoor and outdoor environment conditions in building designs. The parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment.
The complex issues of the need in analysing a number of parameters of thermal comfort (e.g. humidity, air velocity, etc) at the same time together with the related changes of different building envelope (double-skin façade) opening sizes, with acceptable accuracy, the capability of CFD has provided the appropriate option to be used to test out the theory for the research.
In view of the capabilities and good interface of the Airpak software as discussed in details in Section 5.5.5, it is selected to be used in this research to model the complex energy transfer through the component layers of the multilayer façade of the double-skin through the optimisation of the appropriate opening sizes on the glazing, the width of the intermediate space and the ventilation rate through the internal office space. The validation of the software (Section 5.6.5) has been carried out by comparing the experimental and simulation results from another commercial simulation software called FloVent which carried out by Manz H (2003). Both of the results are compared and analyzed and it was found that the variation is within 5% of the acceptable error tolerance. A detail analysis of the use and capability of CFD in relation to the selection of this particular software for this research has been discussed in Chapter 5.
133
6.4.3
The CFD Modelling
6.4.3.1 Stage 1 – Single office
Literature review and case studies had been used to establish the domain and boundary conditions for the CFD modeling and simulation in this research. The final goal of this research is to look into the possibilities of natural ventilation in a high-rise office building in a hot and humid climate condition using double-skin façade. In order to realise this complex problem, several stages of different levels of complexity modelling are introduced. Airflow effects induced by wind and thermal forces onto a single storey office model constructed are to be observed for the first stage before a complex multi-storey office with all the thermal comfort parameters included are to be analysed.
In recognition of the complexity of the problem at hand, the modeling of the computer model has been broken down into several ‘levels’. The initial simulations were concentrated onto a single office space within a high-rise office building. The single office module in 3D is constructed using Airpak CFD software with the geometrical dimensions of 3.5m x 5.0m x 2.6m height. Numerous of simulation runs have been carried out for the benchmarking purposes in which a typical curtain walling office module was observed and simplified ‘nomograms’ had been established to define the initial parameters for thermal comfort in the tropic region. These results are compared with the simulation runs from the office module with double-skin façade construction.
The simplified double-skin façade of the office module has openings on each of the external and internal panes with 6mm thick glass used at the external pane and 6/12/6mm double glazed used for the inner pane (Figures 6.1 and 6.2). Internal heat sources of two computers, four ceiling lights and two persons are introduced in the office space for thermal comfort analysis. The office module has two vents at the rear wall to introduce cross ventilation from the internal a/c space across the internal office space. For this first stage of the analysis, combination of different opening sizes and its locations of the openings together with the different sizes of the vents are looked at and their 134
effects onto the airflow patterns within the double-skin and the internal office space are observed and analysed.
W
3500
H3
H1 350
7000
EXTERNAL
OFFICE
Figure 6.1 Section through the model (with external space at the left)
3500
H3
H1
3500
V1
H2
V2
Figure 6.2 Rear elevation of the model
The simulations are performed under steady state condition using k-epsilon equation turbulent model. The simulated wind speeds of 1.5m/s and 3.0m/s are used to model expected ground level wind velocities with ambient temperature of 30 degree C. The external temperature at the rear wall is set at 23 oC to simulate an internal air-conditioning space like internal corridor. Only wind 135
V1
WIND DIRECTION
350 H2
3500
V
direction which perpendicular to the double-skin façade has been looked at. The upwind distance from the outer pane of the double-skin façade is set at 7m to simulate half the distance between office buildings at the city centre. The results of the airflow velocities, temperatures and the airflow patterns are recorded and observed with different combinations of glass opening sizes of the double-skin façade and the vents.
6.4.3.2 Stage 2 – Office blocks
The results and findings in Stage 1 are very positive as presented in Chapter 7. The next step is to model the multi-storey office building within Airpak environment. Instead of using the software to model the whole high-rise building in one complete computer model, the office building has been ‘divided’ into several ‘blocks’ vertically. This is done to reduce the simulation time needed for each simulation run and any mistake or problem in the modeling process or domain settings will easier be identified and rectified. An 18-storey high building is believed to be a well representation of a high-rise office building and generate sufficient parameters to be explored within the scope of the research topic.
The high-rise office building is ‘divided vertically’ into three office blocks of 6-storey each. Each ‘office block’ of 6-storey will be modeled with the similar boundary conditions and simulations were runs to study the thermal comfort of the internal spaces created. Results were analysed and details of the step-tostep modeling process were documented in Chapter 8. At this stage, a new type of double-skin façade configuration to be used in hot and humid climate is presented.
6.4.3.3 Stage 3 – Optimisation
Stage 3 is the optimisation of the configuration of the new type of double-skin façade presented at the end of Chapter 8. The parameters used for the 136
optimisation are sizes and locations of the openings of the double-skin façade, the height of the ventilation shaft of the double-skin system, different air gap sizes, different ventilation modes for the façade system, and the sun shading location within the façade system. These parameters are found to be most important in affecting the efficiency of the facade system in providing optimum internal comfort for the office spaces. The parametric studies of the optimisation process are presented in Chapter 9.
6.4.3.4 Stage 4 – Nomograms
A new ‘revised’ double-skin façade configuration was presented from the optimisation studies and a series of nomograms are generated to be used to help in the initial design process of double-skin façade in hot and humid climate. The nomograms presented are by no means exhaustive but it will serve as a general rule of thumb for the design of this very specialised façade system before serious investment is made.
6.5
Goals for the research
In attempting to answer the research questions raised in the first section of this Chapter, an extensive research into the relevant subjects was carried out. Topics like natural ventilation techniques, human thermal comfort requirement and responses, double-skin façade technologies, building simulation tools, high-rise building designs in the urban context, and others built environment criteria that related to the research topic had been studied and critically reviewed.
The goals of the research are aim to contribute to the reduction of energy usage and the reduction of CO2 emission by high-rise office buildings in the tropics. The research is attempting to use passive solar design concept, in this case
137
natural ventilation techniques, combining with the newly developed doubleskin facades technologies, and applying them onto high-rise office buildings in the tropics. A series of nomograms for double-skin façade design on high-rise office buildings in the hot and humid climate are proposed in the hope to help designers to have a better understanding of the functions of double-skin façade in providing natural ventilation to high-rise office buildings in the tropics. The nomograms will also help the designers in decision making and appropriate selection of design criteria for double-skin façade design in terms of openings’ sizes and locations, air gap width, height of stack, etc. These will certainly help in time saving and shorten the design process.
6.6
Limitations of the research
Due to the complexity of the research topic, certain careful control and defining the scope of the research is needed. Firstly the research is only deal with the issue at hand in the hot and humid climatic conditions. This is seen as a positive approach because there isn’t much research being done in that particular region which related to the topic. Further more it is hoped that the finding of the research will contribute to the knowledge in that particular area. Secondly a more specific ‘domain’ is needed to test the theory of the research and to apply any useful finding onto it in order to realize the impact of the research work. Singapore is chosen as the ‘domain’ and it is seen as an appropriate choice because it is a developed country of resource dependent and with high-energy usage per capital. The country is also located in the tropic.
Thirdly the research is only looking into certain building type, namely the office building. The office building type is chosen because in Singapore it is one of the highest energy consumption sectors. It will be of greater impact in the reduction of energy usage if the proposed theory will work for this sector.
138
Fourthly the economical impact of the energy saving and the cost in using the double-skin façade are not cover in this research. Besides they are not in the scope of the research, others had covered these topics quite comprehensively in different published books and journals.
Fifthly, as the research area is new therefore there isn’t much existing information in terms of measured data, etc and not many constructed similar buildings directly related to the field. In fact, there isn’t any naturally ventilated cavity double-skin façade building in Singapore during this research is being carried out. Therefore the research has to rely heavily on external sources and data and the testing of the built environment has to be carried out using modeling techniques.
Notwithstanding constrains of the research mentioned above, it is a constructive challenge throughout and the research will provide a specific answer to a specific issue and filling up a gap within the knowledge in the mentioned field.
139
Chapter 7
7.1
Preliminary Modelling
Preliminary modelling
For this first stage of the analysis, combination of different opening sizes and its locations of the openings for both the outer and inner panes of the doubleskin façade, the different sizes of the vents at the rear wall of the office module, with different external wind velocities are studied. The effects of these variations on the airflow patterns, temperatures, relative humidity within the double-skin façade air gap and the internal office space were observed, and the thermal comfort for the office space is analysed.
7.1.1
The geometry of the CFD model
The first stage of the whole complex modelling process is to construct a single storey office module with the geometrical dimensions of 3.5m wide, 5.0m deep and 3.5m in height with a ceiling depth of 0.9m. Therefore the effective height of the internal space of the office module is 2.6m in height and has a volume of 45.5m3. This size is a typical small office module found in an office building in Singapore (Lee, 2002). The double-skin façade in front of the office has a dimension of 3.5m wide and 3.5m high. The simplified double-skin façade construction has two openings at the outer pane and one opening at the inner pane with the top ventilated. The office space also has two vents at the rear wall to introduce cross ventilation, if any, to the internal space. The model is constructed in 3D in Airpak as shown in Figures 7.1, 7.2 and 7.3.
7.1.2
The construction materials used for the model
The external walls of the office consist of solid brick with density of 1970 kg/m3 and thermal conductivity of 0.7 W/m-K. The walls are painted with
140
white acrylic paint with emissivity value of 0.9, reflective index of 1.5 and shading coefficient of 1.
The outer pane of the double-skin façade consists of 6mm clear heat-resistant glass of 2230 kg/m3 density and thermal conductivity of 1 W/m-K. The inner pane is a double glazing of 6/8/6 mm clear heat-resistant glass with similar density and thermal conductivity as the outer pane. Sun shading located within the double-skin façade was introduced at the later stage to investigate its effect on the thermal comfort of the internal space.
7.1.3
The heat sources in the model
There are three main heat sources in this CFD model with two occupants, two computers and four ceiling lights. They are introduced in the office space for thermal comfort analysis. Each human model is assigned with 75 W/m2 of heat generation based on ASHRAE standards for sedentary office activities with clothing value (clo) of 1.0 and metabolic rate (met) of 1.2. Heat generated for the computers are assumed to be 108 W/m2 and 173 W/m2 and are assigned evenly to all the surfaces of the models. Energy saving lighting fixture is used and heat flux is assumed to be 38 W/m2.
7.1.4
The boundary conditions of the model
The simulations are performed under steady state condition using k-epsilon equation turbulence model. The simulated wind speeds of 1.5m/s and 3.0m/s are used to model expected ground level wind velocities with ambient temperature of 30 oC. The external temperature at the rear wall is set at 23 oC to simulate an internal air-conditioning space like internal corridor. Only wind direction that is perpendicular to the double-skin façade has been looked at. The upwind distance from the outer pane of the double-skin façade is set at 7m to simulate half the distance between office buildings at the city centre. The illumination parameters were set as following: 141
Spectral fraction:
0.5
Diffuse entry angle:
60o
Refractive index of air:
1.0
Scattering fraction:
1.3
Diffusive solar intensity:
500 W/m2
The location time and position of the model are set at latitude 1oN and longitude 104oE (Singapore location), in 1 July at 10 a.m. (which is usually the hottest month in Singapore).
Figure 7.1 Sectional elevation of the single office module
Figure 7.2 Longitudinal section of the single office module
142
Figure 7.3 Isometric view of the CFD model
7.2
Strategies in modelling
For the first stage of the analysis, combination of different glass opening sizes [A, B and C] and its locations together with the different sizes of the intermediate space [D] and the vents [E=E1xE2] at the rear are looked at (Figure 7.4) and their effects on the airflow patterns and temperature within the double-skin and the internal office space are observed and analysed to determine the level of thermal comfort within the space. Two scenarios had been looked at for this initial stage of the modelling, i.e. one with airconditioning space and one with ambient temperature conditions at the rear of the building behind air vent E (Figure 7.4). Solar loading that is similar to morning sun in the middle of the year in a tropical country is modelled to observe the impact on the thermal comfort for the internal space. Seven points/locations of the office module were selected to monitor the air velocities and temperature values for those simulations (Figure 7.5) and some of the more 143
critical results are listed in Tables 7.1, 7.2 and 7.3. The results obtained from the benchmarking simulations, which is a typical curtain walling system façade, are compared to the results from the proposed prototype double-skin façade. Some selected benchmarking results for double-skin façade are presented in Appendix A. Analysis and findings for the comparison of the two façade systems are discussed in Sections 7.3, 7.4 and 7.5.
D
H1
B
C
H
W IND DIRECTION
A/C SPACE
INTERNAL SPACE
A
E
Figure 7.4 The single office module with studied openings A, B, C, D & E
7.3
Analysis of preliminary models
A large numbers of simulations were generated with different combinations of wind velocities (V), glass opening sizes (H1 and H2 are the top and bottom openings of the outer pane, H3 is the opening of the inner pane), width of air gaps (W) and vent sizes (V1 & V2) as indicated in Tables 7.1, 7.2 and 7.3. Twenty (20) of such simulation results and their variables of the parameters are indicated in Table 1, Table 2 and Table 3. The difference between the simulations generated in Table 7.1 and Table 7.2 is that the vents’ area. The area of the vent has increased 100% for the models in Table 7.2. Simulations 15 and 16 are generated with a narrower air gap of 300mm to investigate if there is any influence on the indoor comfort level. Simulations 17 to 20 are computed with two openings at the outer pane of the double-skin façade with 300mm air gap. 144
P2
P1 P3
P6 P4 P5
P7
Figure 7.5 Location points for taking the simulation results (section of model)
Simulation
1
2
3
4
5
6
V (m/s)
1.5
1.5
1.5
3.0
3.0
3.0
H1 (mm)
0
0
300
0
0
300
H2 (mm)
200
300
0
200
300
0
H3 (mm)
200
300
300
200
300
300
W (mm)
450
450
450
450
450
450
V1 (mm)
300
300
300
300
300
300
V2 (mm)
600
600
600
600
600
600
P1 (Vel, m/s)
0.83
0.86
0.97
1.55
1.63
1.88
(Temp, oC)
30
30
30.02
30
30
29.99
P2 (Vel, m/s)
0.25
0.41
0.51
0.39
0.71
0.97
(Temp, oC)
30.12
30.08
30.05
30.02
30.02
29.97
P3 (Vel, m/s)
0.12
0.22
0.08
0.21
0.40
0.15
(Temp, C)
30.58
30.40
30.37
30.10
30.09
29.69
P4 (Vel, m/s)
0.05
0.05
0.05
0.01
0.01
0.02
(Temp, oC)
31.70
31.74
31.11
30.75
30.86
30.13
P5 (Vel, m/s)
0.11
0.11
0.17
0.03
0.03
0.27
(Temp, oC)
24.19
24.21
23.82
24.08
24.16
23.66
P6 (Vel, m/s)
0.22
0.40
0.11
0.40
0.75
0.22
30
30
30.12
30
30
29.92
o
o
(Temp, C)
Table 7.1 Simulation results A 145
Simulation
7
8
9
10
11
12
13
14
V (m/s)
1.5
1.5
1.5
1.5
3.0
3.0
3.0
3.0
H1 (mm)
0
200
0
300
0
200
0
300
H2 (mm)
200
0
300
0
200
0
300
0
H3 (mm)
200
200
300
300
200
200
300
300
W (mm)
450
450
450
450
450
450
450
450
V1 (mm)
300
300
300
300
300
300
300
300
V2 (mm)
1200
1200
1200
1200
1200
1200
1200
1200
P1 (Vel, m/s)
0.86
0.94
0.90
0.97
1.59
1.83
1.68
1.88
(Temp, oC)
30
30
30
30
30
29.90
30
29.96
P2 (Vel, m/s)
0.24
0.39
0.40
0.54
0.39
0.75
0.71
1.05
(Temp, C)
30.03
29.97
30.03
30.01
29.99
29.72
30
29.90
P3 (Vel, m/s)
0.14
0.08
0.24
0.07
0.25
0.18
0.43
0.15
(Temp, oC)
30.21
29.81
30.14
30.01
29.92
28.70
29.97
29.22
P4 (Vel, m/s)
0.05
0.05
0.05
0.05
0.01
0.01
0.01
0.02
(Temp, oC)
31.17
30.50
31.19
30.66
30.33
29.08
30.46
29.48
P5 (Vel, m/s)
0.04
0.09
0.04
0.07
0.02
0.19
0.03
0.13
(Temp, C)
24.36
23.88
24.36
23.95
25.15
23.55
26.06
23.68
P6 (Vel, m/s)
0.20
0.05
0.36
0.12
0.36
0.07
0.68
0.22
(Temp, oC)
30
29.96
30
30.02
30
29.59
30
29.80
o
o
Table 7.2 Simulation results B
Simulation
15
16
17
18
19
20
V (m/s)
1.5
1.5
1.5
1.5
3.0
3.0
H1 (mm)
0
200
200
300
200
300
H2 (mm)
200
0
200
300
200
300
H3 (mm)
200
200
200
300
200
300
W (mm)
300
300
300
300
300
300
V1 (mm)
300
300
300
300
300
300
V2 (mm)
600
600
600
600
600
600
P1 (Vel, m/s)
0.78
0.48
0.90
0.93
1.77
1.83
(Temp, C)
30
30
30
30
30
30
P2 (Vel, m/s)
0.36
0.47
0.74
0.67
1.40
1.28
(Temp, oC)
30.04
30
30
30
30
30
o
146
0.21
0.08
0.73
0.90
1.35
1.74
(Temp, C)
30.27
29.96
30.07
30.03
30.01
30.01
P4 (Vel, m/s)
0.05
0.05
0.20
0.29
0.41
0.54
(Temp, oC)
31.35
30.63
30.77
30.22
30.18
30.08
P5 (Vel, m/s)
0.07
0.14
0.06
0.06
0.01
0.01
(Temp, C)
24.32
23.87
32.48
32.14
31.35
31.18
P6 (Vel, m/s)
0.34
0.07
0.06
0.29
0.29
0.71
(Temp, C)
30
30
29.97
30.86
30.24
30.18
P7 (Vel, m/s)
-
-
0.31
0.42
0.58
0.80
(Temp, oC)
-
-
30
30
30
30
P3 (Vel, m/s) o
o
o
Table 7.3 Simulation results C
It was observed that by just changing the glass opening sizes of the double-skin façade with similar external wind velocity would not contribute much to the indoor thermal quality of the office. This was due to the indoor airflow velocities are almost similar for each case. (e.g. Simulations 7 & 9)
The locations of the glass opening on the outer pane of the double-skin façade will have effect onto the indoor thermal and airflow velocity. It was found that the higher the opening is located from the floor level it will generate a stronger stack effect within the air gap which in turn will pull more air out from the office space through the vents at the rear wall. The temperature generated within the office space is much desirable and closer to human comfort requirement. The airflow pattern created will be a good cross ventilation effect with cool air coming into the office space from the vents and right across and above the internal space and discharge out through the high level opening at the inner pane. (e.g. Simulations 7-14)
A narrower air gap between the double-skin façade constructions will provide a more desirable indoor thermal level as it generates stronger stack effect which pulls more air out from the internal office space. (e.g. Simulations 1 and 15)
147
There is not much of an advantage to provide larger vents at the rear of the space in order to provide cross ventilation to the internal space. The resultant air movement and temperature of the internal space are not much better as compared to smaller vent sizes. This might give a slightly better condition if the external wind velocity is stronger but it will not be able to justify the cost in providing a larger vent opening. It might also not be feasible for some construction constrains with big vents. (e.g. Tables 7.1 & 7.2)
Simulations 17-20 show that the indoor airflow velocities are the most desirable with 2 openings on the outer pane of the double-skin façade. The indoor temperatures are also lower as compared to only one opening at the outer pane. The internal airflow pattern is different from the outer pane with 1 opening on the façade. The warm air from the air gap is passing through the opening of the inner pane right across the office space and exit through the rear vents. This will have an undesirable mixing of warm air to the internal cool air at the rear of the office.
Figure 7.6 shows an example of the velocity vectors generated through one of the simulations with particular boundary conditions specified. The ‘red colour’ shows the highest velocity and the ‘blue colour’ shows the lowest velocity for that particular simulated result.
Figure 7.6 Example of velocity vectors generated through simulation 148
Figure 7.7 below shows an example of the temperature contours generated through one of the simulations with boundary conditions specified as in Figure 7.6. The ‘green colour’ shows the highest temperature within the model and the ‘blue colour’ shows the lowest temperature for that particular simulated result.
Figure 7.7 Example of temperature contours generated through simulation
7.4
Discussion
All simulations, be it the benchmarking cases or the double-skin scenarios, generated cross ventilation effects from the internal a/c space across the office space and discharged out through the internal pane opening into the intermediate space of the double façade. The strength of the cross ventilation will mainly depend on the airflow resistances within the intermediate space and the internal office space, together with the pressure differences between them. The magnitude of the internal ventilation will depend on the summation of the airflow resistances and in turn control by the smallest cross section area of the opening within the space. 149
The first group of simulations has the opening areas comparison of D>A/B/C>E (Figure 7.4). When the areas of A/B/C increased in the same proportion by as much as 50%, the airflow velocities at P2, P3 and P6 have increased almost 100% but at P4 remained the same. The temperatures observed are almost the same at all points or locations.
The airflow velocities within and around the double façade have doubled with the wind velocity increased by 100%. But the airflow velocities within the internal space, P4 and P5 have reduced 5 and 4 times respectively.
It could be concluded from the above results that by just changing the glass opening sizes of the external and internal panes of the double-skin façade it will not contribute much to the indoor thermal quality of the office space. This was due to the indoor airflow velocities being almost similar for each case.
The location of the glass openings on the outer pane of the double-skin façade in relation to the inner pane will have effect onto the indoor thermal and airflow velocity. It was found that the higher the opening is located from the floor level it will generate a stronger stack effect within the air gap which in turn will pull more air out from the office space through the vents at the rear wall. The temperature generated within the office space is much desirable and closer to human comfort requirement. The airflow pattern created will be a good cross ventilation effect with cool air coming into the office space from the vents and right across and above the internal space and discharged out through the high level opening at the inner pane. The opening areas comparison for these findings are D>B=C>E.
The combination of larger rear vents and a high level external opening in the opening areas comparison of D>B=C=E gives the most desirable results. The internal temperature at P4 has reduced by 0.45K to 0.65K, depending on the wind velocities. A narrower air gap between the double-skin façade will provide a more desirable indoor thermal level also as it generates stronger stack- effect which extract more air out from the internal office space. 150
The second group of simulations with two external openings produced slightly better results as compared to only one external opening. The indoor temperatures are lower but the airflow velocities increased tremendously. The internal airflow pattern is different from the single external opening on the façade. The warm air from the air gap is passing through the opening of the inner pane right across the office space and exit through the rear vents. This will have an undesirable mixing of warm air to the internal office space and the a/c space at the rear of the office.
Solar loading has been introduced to the third simulations group with highlevel external opening. The area of the external opening is smaller by 2.5-4 times as compared to the internal opening. This was done to control the amount of external hot airflow from entering the intermediate space but allowing as much internal room air as possible to be discharged into the air gap of the double-skin façade. This will have to rely upon sufficient stack effect to be generated. The area of opening for the intermediate space is also smaller than the internal pane opening. These groups of simulation by far gave the lowest internal temperature at P4 but the internal airflow velocities are low. The stackeffect combined with the thermal buoyancy effect within the intermediate space of the double-skin façade has produced a stronger suction for cross ventilating the internal office space. The opening areas comparison for these findings are C>D>B=E.
7.5
Initial findings
Thermal sensation plays a major role in the perception of comfort and the comfort parameters are highly subjective. Some of these parameters are air temperature, the relative humidity of the air, the local air velocity and human activity. A comprehensive explanation of thermal comfort is listed in Chapter 8 of ASHRAE Fundamentals. (ASHRAE Fundamentals, 1993) and thermal
151
comfort in the hot and humid climate has been discussed in Chapter 2. Figure 7.8 below shows the operative temperature ranges for naturally ventilated spaces with 90% and 80% acceptable limits indicated. This graph shows that there is a broader acceptable temperature ranges for naturally ventilated spaces. For air velocity lower than 1m/s and temperatures difference between radiant and ambient of lower than 4K, the operative temperature (Top) would need to be adjusted according to the formula Top=Ata + (1-A)Tr, where ta is the ambient air temperature and Tr is the radiant temperature.
Air movements determine the convective heat and mass exchange of the human body with the surrounding air. In hot and humid climate, high air velocities will increase the evaporation rate at the skin surface and results in cooling sensation. The recommended upper limit of indoor air movement is usually 0.8m/s for human comfort and such air velocity permits the interior space to be 1-2 degree higher than the human comfort temperature to maintain desirable comfort level (Hien and Tanamas, 2002).
Air speed < 0.2m/s Difference between radiant & air temp < 4C Top = Ata + (1-A)Tr V A
Steady State Condition > Solved With Radiation, Species, IAQ/Comfort and Solar Loading > k- Turbulence Model > Wind Perpendicular To Building Façade > Wind Speed = 0m/s – 3m/s > Ambient Temperature = 22oC – 30oC > Relative Humidity = 70% – 100%
Concealed Ground Floor Space
Figure 7.10 Standard curtain walling model
156
Concealed Ground Floor Space
Figure 7.11 Double-skin façade model
7.6.1
Comparison of results for single-skin and double-skin façades
The results obtained from the benchmarking simulations, which is a typical curtain walling system façade, are compared to the results from the proposed prototype double-skin façade. The results of the benchmarking are first presented in the form of a nomogram here for the research. There are many ways in representing huge amount of information generated through simulations, which could be in the form of graphs, tables, charts (e.g. Pychrometric Chart), spreadsheet, customised computer programs, etc. These methods of representation are valid but some could be very time consuming to formulate and others could be too complicated to be use effectively by designers. After much consideration in finding a way to formulate the results into useful information that could be used by others in design purposes, nomogram has been chosen. Even though nomogram traditionally has been used to represent mathematical relationships or laws in graphical form, but it 157
provides an easy way to quickly and accurately finding an answer to a rather complex problem. Nomogram is also be able to comprehensively represent a complex correlation between different variables to generate desirable and acceptable conditions or results, if it is constructed appropriately. The detail construction, formulation and usage of the nomogram used in this research are presented in Chapter 9, Section 9.4.
The nomograms are formed by three axes, which represent the three most important parameters in thermal comfort analysis. The parameters are air temperature, air velocity and relative humidity. Boundaries of thermal comfort are plotted onto the nomograms from the analysis of the simulation results and they are compared to see whether there are any advantages for using doubleskin construction for an office building in the tropical climate. The area bounded by the acceptable thermal comfort limits is ‘shaded’, showing the ‘extent’ of acceptable indoor conditions. Figures 7.12 & 7.13 have shown that there are positive points in using the double-skin construction, as the ‘shaded area’ for the double-skin façade is larger than the normal curtain walling construction, even though this finding is only representing the low level results for the high-rise office building in study. These findings are encouraging as the double-skin façade construction does improve the internal thermal comfort for a naturally ventilated office space by as much as 10%, as compared to conventional curtain wall system.
158
Figure 7.12 Nomogram showing the acceptable thermal comfort conditions (shaded area) for standard curtain wall system
159
A
B
Figure 7.13 Nomogram showing the acceptable thermal comfort conditions (shaded area) for double-skin façade system
An example of an acceptable internal thermal comfort condition is shown at Point B (which could have air temperature=25oC, relative humidity=70% and air velocity=0.5m/s) in the nomogram in Figure 7.13 above. Point A is not an acceptable thermal comfort condition, which could have the values of air temperature=22oC, air velocity=0m/s, and relative humidity=60%.
160
7.7
Conclusion
This initial stage of the research work has shown that double-skin façade has a possibility to provide acceptable internal thermal comfort for office space through natural ventilation strategy in the hot and humid climate. These results have helped to answer the first question set out in the Research Questions section mentioned in Chapter 1.
A specific research methodology has then been developed following this finding to systematically explore the use of double-skin façade on high-rise office building in the tropical climate and to find out the possible ‘opening window’ in terms of periods of time of the day that double-skin façade could provide acceptable indoor thermal environment. This will help to answer the second research questions that set out in Section 1.11. The methodology and findings of the investigation are presented in Chapters 8 and 9.
161
Chapter 8
8.1
Multi-Storey Building Modelling
Stage 1 of the multi-storey modelling
The initial model was constructed as a ‘single-room’ condition to test out whether double-skin façade is able to provide a better indoor environment under the hot and humid climate as compared to typical single-skin curtain wall. In the earlier Chapter it was found that double-skin façade does perform better compared to single-skin curtain wall system.
The positive finding has confirmed the possibility of introducing double-skin façade to the hot and humid climatic condition in a country like Singapore. This will bring us to the next stage of the modelling by extending the numerical model to a ‘six-storey’ building. The choice of implementing a six-storey high building for the next stage for analysis is because the height of the overall building will give a sufficient complexity to study the problem at hand and the height is also being defined as ‘high-rise’ building in the context of Singapore situation.
The first block (Stage 1) of the six-storey building (Figure 8.1) consists of a ground floor (which is not served by the double-skin façade, as this will be the typical design for any high-rise building) and 5 stories of office spaces above. The double-skin façade is a ventilated-shaft with a design of 2.8m from ground level. In earlier findings it is a practical and economical option to introduce a shaft to improve the stack effect of the natural ventilation and in turn will improve the airflow rates required to reach thermal comfort level within the interior office space.
The parameters and boundary conditions for this stage of the modelling are listed below and shown in Figure 8.2:
Multistory Space (6-storey) DSF façade system is orientated towards South and North 162
Simulations run for 2 periods of time => morning (10 am) and afternoon (2 pm) Wind direction => Perpendicular to the wall system Wind speed => 0.5m/s to 3m/s External temperature => 26oC to 30oC Relative humidity => 70% to 100% DSF opening size for inner pane => 300mm Air gap size => 300mm to 1200mm Vent size => 300mm x 600mm
3500
3500
WIND
5th FLOOR
3500
VENT
4th FLOOR
3500
WIND
DSF 3rd FLOOR
3500
WIND
21500
VENT
3500
2nd FLOOR
1st FLOOR WIND
GROUND FLOOR
4000
2800
VENT
Figure 8.1 Model geometry of Stage 1 of the 6-storey building
163
The heat sources for this CFD model will only be introduced at alternate floor, starting from 1st-storey. This was done to reduce the complexity of the model and computing time, but at the same time will be able to give a comprehensive view of the indoor thermal comfort of the office spaces. Each alternate floor will have two occupants, two computers and four ceiling lights, which are the same as the initial single office model discussed in Chapter 7. Each human model is assigned with 75 W/m2 of heat generation with clothing value (clo) of 1.0 and metabolic rate (met) of 1.2 for sedentary office activities. Heat generated for the computers are 108 W/m2 and 173 W/m2 respectively and the heat flux of the lighting fixture is assumed to be 38 W/m2 each.
External Temperature - 26C to 30C
300
DSF OPENING
AIR GAP SIZE 300 - 1200
Relative Humidity - 70% to 100%
OFFICE SPACE
300
DSF OPENING
WIND - 0m/s to 3m/s
VENT SIZE 300 x 600
DSF FACING SOUTH / NORTH
Figure 8.2 Boundary conditions and ranges of parameters used in the CFD simulations
Boundary conditions for wind velocity, external temperature and relative humidity were set to the ranges similar to the climatic conditions for Singapore. The ambient temperature in Singapore is hot with high humidity 164
and relatively low wind velocity throughout most of the year. Only the optimum opening sizes on the inner pane and the air gap sizes of the doubleskin façade (DSF) are being considered (as shown in Figure 8.2 above) for this stage of simulations, based on the findings from the preliminary modelling in Chapter 7. The optimum vent size at the rear wall was found to be 300mm by 600mm from previous findings and the condition behind the vent is an external space with ambient temperature conditions (26oC – 30oC). The scope of the problem in investigation has been ‘narrowed down’ and carefully ‘controlled’ to find the ‘optimum DSF configuration’ for use in Singapore climatic conditions.
8.1.1
Simulation results for South facing DSF system
The first group of simulations is generated with the DSF system constructed at the south facing façade of the building only. The simulation periods are at 10 a.m. or 2 p.m. on either 15 January or 1 July of the month with wind direction perpendicular to the DSF wall and with wind velocities of 0.5 m/s, 1.5 m/s and 3 m/s. The external ambient temperatures were set from 26oC to 30oC with relative humidity ranging from 70% to 100%. The opening size for the inner pane of the DSF system used is 300mm. The air gap sizes used for the DSF are 300mm, 600mm, 900mm and 1200mm. The air vent size at the rear office wall is fixed at 300mm x 600mm.
There are a total of 18 location points being identified to record the simulation results on thermal comfort parameters. Figure 8.3 shows six of those location points that are 0.8m above the office floor level and 0.2m away from the two human figures. These six points are chosen to monitor the thermal comfort conditions experienced by the occupants. Tables 8.1 show some of the comparative results for the simulation with different parameters used for the boundary conditions and DSF configurations taken at P1, P6 and P11 (Figure 8.3). The indoor Operative Temperature (OT) calculated in the above table was using the formula stated in Figure 8.4 and the value was used to identify
165
acceptable thermal comfort for naturally ventilated spaces in hot and humid climate using the graph given in the same figure.
5th Floor
P11 P12
3rd Floor P6 P7
1st Floor
P1 P2
Figure 8.3 Location points for monitoring the simulation results (Stage 1)
166
Simulation
Orientation
Date
Time
Air Temp. o C
Wind Vel. m/s
Air RH %
Air Gap Size mm
S1-1
South
15 Jan
2pm
28
1.5
80
300
S1-2
South
15 Jan
2pm
28
1.5
80
600
S1-3
South
15 Jan
2pm
28
1.5
80
900
S1-4
South
15 Jan
2pm
26
1.5
80
300
S1-5
South
15 Jan
10am
26
1.5
80
300
S1-6
South
15 Jan
10am
28
1.5
80
300
Simulation
S1-1
S1-2
S1-3
S1-4
Floor Level
Temp. o C
Air Vel. m/s
Radiant Temp. o C
RH %
PMV
OT o C
1
28.5
0.04
29.93
70.49
1.99
29.22
3
29.08
0.01
29.52
77.3
1.8
29.30
5
29.47
0.01
29.64
75.59
1.85
29.56
1
29.95
0.02
32.96
70.69
2.41
31.46
3
30.67
0.04
31.50
76.60
1.97
31.09
5
30.72
0.03
31.19
75.42
2.13
30.96
1
29.99
0.03
32.48
70.61
2.38
30.59
3
30.47
0.06
32.25
77.40
2.04
31.36
5
30.43
0.04
31.98
76.25
1.9
31.20
1
27.49
0.04
28.98
70.54
1.8
28.24
3
28.15
0.05
29.45
76.39
1.66
28.80
5
27.40
0.03
28.67
75.44
1.6
28.04
167
S1-5
S1-6
1
27.38
0.02
28.74
70.22
1.97
28.06
3
28.07
0.04
29.59
76.13
1.62
28.83
5
27.61
0.03
28.81
74.85
1.59
28.21
1
28.41
0.03
29.73
70.17
1.92
29.07
3
29
0.01
29.38
77.04
1.78
29.19
5
29.26
0.01
29.5
75
1.84
29.38
(Note: Shaded results are acceptable thermal comfort conditions) Tables 8.1 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 1 – South facing DSF system)
29.5
26
Air speed < 0.2m/s Difference between radiant & air temp < 4C Top = Ata + (1-A)Tr V A
morning (10 am) and afternoon (2 pm) Wind direction => Perpendicular to the wall system Wind speed => 0.5m/s to 3m/s External temperature => 26oC to 30oC Relative humidity => 70% to 100% DSF opening size for inner pane => 300mm Air gap size => 300mm to 1200mm Vent size => 300mm x 600mm
The heat sources for this stage of CFD model will only be introduced at alternate floors also, namely at 7th, 9th and 11th. Each alternate floor will have two occupants, two computers and four ceiling lights, which are the same as the initial single office model discussed in Chapter 7. Each human model is assigned with 75 W/m2 of heat generation with clothing value (clo) of 1.0 and metabolic rate (met) of 1.2 for sedentary office activities. Heat generated for the computers are 108 W/m2 and 173 W/m2 respectively and the heat flux of the lighting fixture is assumed to be 38 W/m2 each. Figure 8.5 below shows the location points selected for obtaining thermal comfort parameters for analysis.
173
11th Floor P11 P12
9th Floor P6 P7
7th Floor P1 P2
Figure 8.5 Location points for recording the simulation results (Stage 2)
174
3500
WIND
11th FLOOR VENT
3500
3500 10th FLOOR
3500
WIND
21000
9th FLOOR VENT
3500
DSF
8th FLOOR
3500
WIND
7th FLOOR VENT
3500
WIND
6th FLOOR
FAN
Figure 8.6 Model geometry of Stage 2 of the 18-storey office building
8.2.1
Simulation results for South facing DSF system
The simulation period are at 10 a.m. or 2 p.m. on either 15 January or 1 July of the month with wind direction perpendicular to the DSF wall and with wind velocities of 0.5 m/s, 1.5 m/s and 3 m/s. The external ambient temperatures were set from 26oC to 30oC with relative humidity ranging from 70% to 100%. The opening size on the DSF system used is 300mm. The air gap sizes used for the DSF are 300mm, 600mm, 900mm and 1200mm. The air vent size at the rear office wall is fixed at 300mm x 600mm. 175
Simulation
Orientation
Date
Time
Air Temp. o C
Wind Vel. m/s
Air RH %
Air Gap Size mm
S2-1
South
15 Jan
2pm
28
1.5
80
300
S2-2
South
15 Jan
2pm
28
1.5
80
600
S2-3
South
15 Jan
2pm
28
1.5
80
900
S2-4
South
15 Jan
2pm
26
1.5
80
300
S2-5
South
15 Jan
10am
26
1.5
80
300
S2-6
South
15 Jan
10am
28
1.5
80
300
Simulation
S2-1
S2-2
S2-3
S2-4
Floor Level
Temp. o C
Air Vel. m/s
Radiant Temp. o C
RH %
PMV
OT o C
7
28.81
0.06
31.11
74.12
1.91
29.96
9
28.87
0.03
30.52
74.47
1.89
29.70
11
30.18
0.08
32.08
72.56
2.07
31.13
7
30.24
0.04
33.98
74.29
2.37
32.11
9
30.46
0.05
32.43
73.88
1.87
31.44
11
31.42
0.08
33.65
72.72
1.91
32.54
7
30.29
0.05
33.58
74.21
2.28
31.94
9
30.23
0.07
33.23
74.59
1.96
31.73
11
31.14
0.10
34.38
73.69
2.10
32.76
7
27.79
0.06
30.09
74.14
1.71
28.94
9
27.91
0.06
30.45
73.62
1.76
29.18
11
28.11
0.09
31.13
72.78
1.82
29.62
7
27.66
0.06
29.83
73.80
1.86
28.75
176
S2-5
S2-6
9
27.85
0.05
30.58
73.92
1.72
29.22
11
28.32
0.09
31.22
72.23
1.81
29.77
7
28.72
0.05
30.83
73.77
1.87
29.78
9
28.88
0.02
30.38
76.35
1.88
29.63
11
29.97
0.07
31.72
72.66
1.64
30.85
(Note: Shaded results are acceptable thermal comfort conditions) Tables 8.3 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 2 – South facing DSF system)
8.2.2
Simulation results for North facing DSF system
This group of simulations is generated with the DSF system constructed at the north facing façade of the building with similar boundary conditions and DSF system configurations as the south facing façade in section 8.2.1.
Simulation
Orientation
Date
Time
Air Temp. o C
Wind Vel. m/s
Air RH %
Air Gap Size mm
S2-1a
North
15 Jan
2pm
28
1.5
80
300
S2-2a
North
15 Jan
2pm
28
1.5
80
600
S2-3a
North
15 Jan
2pm
28
1.5
80
900
S2-4a
North
15 Jan
2pm
26
1.5
80
300
S2-5a
North
15 Jan
10am
26
1.5
80
300
S2-6a
North
15 Jan
10am
28
1.5
80
300
177
Simulation
S2-1a
S2-2a
S2-3a
S2-4a
S2-5a
S2-6a
Floor Level
Temp. o C
Air Vel. m/s
Radiant Temp. o C
RH %
PMV
OT o C
7
28.58
0.06
37.26
70.08
2.39
32.92
9
30.07
0.05
38.66
61.95
2.83
34.36
11
29.86
0.10
38.98
69.70
2.71
34.42
7
30.03
0.06
39.21
70.48
2.26
34.62
9
31.65
0.05
38.19
68.72
2.17
34.92
11
31.43
0.09
39.88
69.54
2.39
35.66
7
29.82
0.07
39.77
70.38
2.27
34.80
9
31.46
0.06
38.86
63.23
3.01
35.16
11
31.27
0.11
38.76
70.43
2.78
35.02
7
27.45
0.05
35.31
71.14
2.25
31.38
9
29.16
0.06
36.69
65.39
2.46
32.93
11
28.11
0.08
38.46
72.76
2.44
33.28
7
27.36
0.06
36.29
70.55
2.24
31.82
9
29.05
0.05
37.73
63.18
2.40
33.39
11
28.34
0.09
39.39
71.89
2.37
33.86
7
28.59
0.06
37.24
70.42
2.17
32.92
9
29.99
0.06
37.45
64.23
2.74
33.72
11
29.95
0.08
39.68
72.96
2.63
34.82
Tables 8.4 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 2 – North facing DSF system)
8.2.3
Analysis of results and findings for Stage 2
The selected results for both South and North facing DSF configurations are having similar parameters as Stage 1 for direct comparison. The external wind velocity is 1.5m/s and air humidity is 80% respectively and the variable parameters in consideration for this Stage are external air temperature, the DSF air gap size and the time of the day, as tabulated in Tables 8.3 and Tables 8.4. Results for S2-1, S2-2 and S2-3 (South facing DSF) as shown in Tables 8.3 and 178
Graph 8.3 indicated that the DSF air gap size of 300mm gives the best result for the particular conditions in a natural ventilated space, as in Stage 1 also. The findings are the same for the Northern orientation façade as presented in Tables 8.4 and Graph 8.4. In most cases the lower floor of the office space would generate the lowest operative temperature due to the ‘stack effect’ provided by the DSF configuration.
There is not much of a difference in terms of the internal thermal comfort conditions for either period of time in a given day (morning or afternoon) as seen in the results for S2-4 and S2-5 for South facing DSF but for North facing DSF morning period has a higher operative temperatures compare to afternoon period (as indicated in S2-4a and S2-5a). This could be due to the higher internal radiant temperatures generated during this particular period of time (Tables 8.4).
The South facing DSF configuration has produced an 80% Acceptability Limit for the 300mm air gap size for external temperature of 26oC up to 9th Floor as indicated in Tables 8.3. The North facing DSF configuration did not produce any acceptable internal thermal comfort condition for the office space (Tables 8.4 and Graph 8.4) as the operative temperatures for all the floors are above 31oC.
34 33 32 31
S2-1 S2-2 S2-3 S2-4 S2-5 S2-6
30 29 28 27 26 7th Floor
9th Floor
11th Floor
Graph 8.3 Comparison of Operative Temperatures (oC) for South facing DSF (Stage 2) 179
36 35 34 S2-1a
33
S2-2a
32
S2-3a
31
S2-4a S2-5a
30
S2-6a
29 7th Floor
9th Floor
11th Floor
Graph 8.4 Comparison of Operative Temperatures (oC) for North facing DSF (Stage 2)
8.3
Stage 3 of the multi-storey modelling
The Stage 3 of the simulation model (Figure 8.8) consists of a 6-storey office spaces. A fan is introduced at the bottom portion of the ventilated-shaft design of the DSF to simulate the ‘continuity’ of the airflow from the stage 2 model. The flow rate of the fan is 17.7m3/s and it was calculated from Stage 2 simulations. There is a 1m high parapet at the rooftop of the office building model.
The parameters and boundary conditions for this stage of the modelling are listed below:
Multistorey Space (6-storey) DSF façade system is orientated towards South and North Simulations run for 2 periods of time => morning (10 am) and afternoon (2 pm) Wind direction => Perpendicular to the wall system Wind speed => 0.5m/s to 3m/s 180
External temperature => 26oC to 30oC Relative humidity => 70% to 100% DSF opening size for inner pane => 300mm Air gap size => 300mm to 1200mm Vent size => 300mm x 600mm
The heat sources for this stage of CFD model will only be introduced at alternate floors also, namely at 13th, 15th and 17th. Each alternate floor will have two occupants, two computers and four ceiling lights, which are the same as the initial single office model discussed in Chapter 7. Each human model is assigned with 75 W/m2 of heat generation with clothing value (clo) of 1.0 and metabolic rate (met) of 1.2 for sedentary office activities. Heat generated for the computers are 108 W/m2 and 173 W/m2 respectively and the heat flux of the lighting fixture is assumed to be 38 W/m2 each. Figure 8.7 below shows the location points selected for obtaining thermal comfort parameters for analysis.
181
17th Floor P11 P12
15th Floor P6 P7
13th Floor P1 P2
Figure 8.7 Location points for monitoring the simulation results (Stage 3)
182
1000 3500
WIND
17th FLOOR VENT
3500
3500 16th FLOOR
3500
WIND
21000
15th FLOOR VENT
3500
DSF
14th FLOOR
3500
WIND
13th FLOOR VENT
3500
WIND
12th FLOOR
FAN
Figure 8.8 Model geometry of Stage 3 of the 18-storey office building
8.3.1
Simulation results for South facing DSF system
The simulation periods are at 10 a.m. or 2 p.m. on either 15 January or 1 July of the month with wind direction perpendicular to the DSF wall and with wind velocities of 0.5 m/s, 1.5 m/s and 3 m/s. The external ambient temperatures were set from 26oC to 30oC with relative humidity ranging from 70% to 100%. The opening size on the DSF system used is 300mm. The air gap sizes used for the DSF are 300mm, 600mm, 900mm and 1200mm. The air vent size at the rear office wall is fixed at 300mm x 600mm. 183
Simulation
Orientation
Date
Time
Air Temp. o C
Wind Vel. m/s
Air RH %
Air Gap Size mm
S3-1
South
15 Jan
2pm
28
1.5
80
300
S3-2
South
15 Jan
2pm
28
1.5
80
600
S3-3
South
15 Jan
2pm
28
1.5
80
900
S3-4
South
15 Jan
2pm
26
1.5
80
300
S3-5
South
15 Jan
10am
26
1.5
80
300
S3-6
South
15 Jan
10am
28
1.5
80
300
Simulation
S3-1
S3-2
S3-3
S3-4
S3-5
Floor Level
Temp. o C
Air Vel. m/s
Radiant Temp. o C
RH %
PMV
OT o C
13
28.11
0.2
28.28
77.35
1.33
28.2
15
28.16
0.15
27.48
77.6
1.35
27.82
17
29.57
0.09
28.55
75.09
1.62
29.06
13
29.51
0.21
31.04
77.5
1.8
30.28
15
29.72
0.17
29.35
77.01
1.36
29.54
17
30.84
0.09
30.1
75.38
1.42
30.47
13
29.53
0.21
30.67
77.44
1.75
30.1
15
29.52
0.19
30.15
77.77
1.44
29.84
17
30.51
0.11
30.83
76.31
1.62
30.67
13
27.06
0.22
27.16
77.37
1.15
27.11
15
27.2
0.18
27.38
76.76
1.22
27.29
17
27.5
0.1
27.58
75.41
1.34
27.54
13
26.93
0.22
26.96
77.05
1.3
26.94
15
27.12
0.17
27.5
77.09
1.21
27.31
17
27.73
0.1
27.68
74.83
1.33
27.7
184
S3-6
13
28.02
0.19
27.93
77
1.32
27.98
15
28.18
0.15
27.34
79.52
1.34
27.76
17
29.33
0.08
28.27
75.29
1.13
28.8
(Note: Shaded results are acceptable thermal comfort conditions) Tables 8.5 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 3 – South facing DSF system)
8.3.2
Simulation results for North facing DSF system
This group of simulations is generated with the DSF system constructed at the north facing façade of the building with similar boundary conditions and DSF system configurations as the south facing façade in section 8.3.1.
Simulation
Orientation
Date
Time
Air Temp. o C
Wind Vel. m/s
Air RH %
Air Gap Size mm
S3-1a
North
15 Jan
2pm
28
1.5
80
300
S3-2a
North
15 Jan
2pm
28
1.5
80
600
S3-3a
North
15 Jan
2pm
28
1.5
80
900
S3-4a
North
15 Jan
2pm
26
1.5
80
300
S3-5a
North
15 Jan
10am
26
1.5
80
300
S3-6a
North
15 Jan
10am
28
1.5
80
300
185
Simulation
S3-1a
S3-2a
S3-3a
S3-4a
S3-5a
S3-6a
Floor Level
Temp. o C
Air Vel. m/s
Radiant Temp. o C
RH %
PMV
OT o C
13
27.88
0.18
33.5
74.38
1.87
30.69
15
29.14
0.15
33.72
69.58
2.05
31.43
17
29.62
0.08
35.84
75.04
2.24
32.73
13
29.28
0.2
36.22
74.53
2.33
32.75
15
30.96
0.17
35.55
68.79
2.06
33.26
17
30.88
0.08
37.38
75.34
2.04
34.13
13
29.19
0.21
35.89
74.45
2.28
32.54
15
30.75
0.18
36.33
69.59
2.14
33.54
17
30.55
0.1
38.11
76.28
2.22
34.33
13
26.93
0.2
32.34
74.47
1.66
29.64
15
28.57
0.17
33.58
68.68
1.92
31.08
17
27.55
0.08
34.87
75.34
1.96
31.21
13
26.59
0.21
32.17
74
1.8
29.38
15
28.43
0.18
33.75
68.97
1.89
31.09
17
27.77
0.1
35.01
74.81
1.92
31.39
13
27.69
0.18
33.2
73.89
1.85
30.44
15
29.45
0.16
33.58
71.22
2.02
31.52
17
29.4
0.07
35.6
75.23
1.75
32.5
(Note: Shaded result is acceptable thermal comfort condition) Tables 8.6 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 3 – North facing DSF system)
8.3.3
Analysis of results and findings for Stage 3
The selected results for both South and North facing DSF configurations are having similar parameters as Stages 1 and 2 for direct comparison. The external wind velocity is 1.5m/s and air humidity is 80% respectively and the variable parameters in consideration for this Stage are also external air temperature, the DSF air gap size and the time of the day, as tabulated in Tables 8.5 and Tables 186
8.6. Results for S3-1, S3-2 and S3-3 (South facing DSF) as shown in Tables 8.5 and Graph 8.5 indicated that the DSF air gap size of 300mm gives the best result for the particular conditions in a natural ventilated space, as in Stages 1 and 2. The findings are the same for the Northern orientation façade as presented in Tables 8.6 and Graph 8.6. In most cases the lower floor of the office space would generate the lowest operative temperature due to the ‘stack effect’ provided by the DSF configuration, as in Stages 1 and 2 also.
There is not much of a difference in terms of the internal thermal comfort conditions for either period of time in a given day (morning or afternoon) as seen in the results for S3-4 and S3-5 for South facing DSF, and results for S34a and S3-5a for North facing DSF.
The South facing DSF configuration has produced an 80% Acceptability Limit for the 300mm air gap size for external temperatures between 26oC and 28oC as indicated in Tables 8.5 and Graph 8.5. The North facing DSF configuration did not produce any acceptable internal thermal comfort condition for the office space except for the lower floor for 300mm air gap configuration with external air temperature of 26oC during morning period (Tables 8.6 and Graph 8.6).
31 30 29 S3-1
28
S3-2 S3-3
27
S3-4 S3-5
26
S3-6
25 13th Floor
15th Floor
17th Floor
Graph 8.5 Comparison of Operative Temperatures (oC) for South facing DSF (Stage 3)
187
35 34 33 S3-1a
32
S3-2a
31
S3-3a S3-4a
30
S3-5a S3-6a
29 28 13th Floor
15th Floor
17th Floor
Graph 8.6 Comparison of Operative Temperatures (oC) for North facing DSF (Stage 3)
8.4
Comparison results for different orientations
The simulation results for the three stages of the modelling have shown that the South-facing orientation provide a better outcome compared to the Northfacing direction. The optimum air gap size for the double-skin façade construction is found to be 300mm and the best results were obtained during the morning period.
Tables 8.7 below recorded the comparison of selective results between the four major orientations for a double-skin façade installation for a typical high-rise office building. The results show that the South-facing façade has the best outcome followed by the East-facing façade during the morning period in the month of January. The North-facing and the West-facing façades do not provide an acceptable indoor thermal comfort for the purposes of office function in a high-rise building. Graph 8.7 shows the comparison of operative temperatures for the four orientations, which has further reinforced the simulation results, that the best orientation for installing double-skin façade for high-rise office building in the tropics is the South-facing orientation.
188
Simulation
Orientation
Date
Time
Air Temp. o C
Wind Vel. m/s
Air RH %
Air Gap Size mm
S3-5
South
15 Jan
10am
26
1.5
80
300
S3-5a
North
15 Jan
10am
26
1.5
80
300
S4-5
East
15 Jan
10am
26
1.5
80
300
S4-5a
West
15 Jan
10am
26
1.5
80
300
Simulation
Floor Level
Temp. o C
Air Vel. m/s
Radiant Temp. o C
RH %
OT o C
13
26.93
0.22
26.96
77.05
26.94
S3-5
15
27.12
0.17
27.5
77.09
27.31
(South)
17
27.73
0.1
27.68
74.83
27.7
13
26.59
0.21
32.17
74
29.38
S3-5a
15
28.43
0.18
33.75
68.97
31.09
(North)
17
27.77
0.1
35.01
74.81
31.39
13
27.16
0.16
27.61
76.93
27.38
S4-5
15
27.28
0.14
27.77
76.37
27.52
(East)
17
27.61
0.13
28.06
74.93
27.84
13
26.88
0.15
32.33
77.23
29.6
S4-5a
15
27.09
0.17
33.54
76.83
30.32
(West)
17
27.3
0.11
34.97
75.28
31.14
(Note: Shaded results are acceptable thermal comfort conditions) Tables 8.7 Comparison of selected simulation results for different orientations of DSF system
189
32 31 30 South
29
North
28
East West
27 26 13th Floor
15th Floor
17th Floor
Graph 8.7 Comparison of Operative Temperatures (OT) for four major façade orientations of DSF system
8.5
The complete 18-storey office building
With the completion of the three stages of simulations, numerous simulation runs had been carried out with various ambient temperatures, different external air velocities, different orientations of the double-skin façade, different periods of time during the day, etc in order to find out the appropriate window periods for acceptable indoor conditions for office workers in the Singapore context. These findings will be of outmost important as an indication whether doubleskin façade is really possible to be used as a mean to introduce natural ventilation to the high-rise buildings in the tropics. The results and findings will also bear an important decision in how to carry out the optimization of the façade system for the whole high-rise office building, which will be discussed in detailed in Chapter 9.
Figure 8.9 shows the complete 18-storey office building with typical multistorey double-skin faade configuration. The proposed DSF starts from 1st storey at 2.8 meters from ground level up to the 17th storey with 1-meter parapet above the roof level. The office spaces are assumed to be divided into a number of small office usages and are tenanted out to various occupants. All
190
office spaces are assumed to face the DSF at the front and facing open corridor
1000
at the rear.
3500
3500
WIND
17th FLOOR
3500
VENT
16th FLOOR WIND
DSF
3500
WIND
63500
VENT
3500
2nd FLOOR
1st FLOOR WIND
GROUND FLOOR
4000
2800
VENT
Figure 8.9 The model of the complete 18-storey office building
8.6
Conclusion
The completion of this stage of research has found a new type of double-skin façade configuration for use in the tropic and the optimum orientation for the 191
façade is South-facing. The findings helped to answer the second research questions set out in Section 1.11 that the acceptable ‘opening window’ for introducing natural ventilation into the office space using double-skin façade system are between 10am in the morning and 2pm in the afternoon for a hot and humid climate condition. These findings also confirmed the review done by Dr. Karl Gertis (Section 4.4.6) that double-skin façade cannot provide acceptable indoor climate condition with natural ventilation alone for most period of the year.
It is also the intent of this research to find an optimum façade configuration for the high-rise office building in the tropic. The next Chapter has been devoted to present the optimisation of this façade system and to propose a ‘refined’ façade configuration and to present a series of nomograms to help designers in their design process for double-skin façade for high-rise office building in hot and humid countries.
192
Chapter 9
Parametric Studies of Optimization
Strategies for natural ventilation optimization
9.1
The completion of the 18-storey building analysis in previous Chapter had revealed that only the South facing DSF system is viable in terms of providing acceptable internal thermal comfort through natural ventilation in hot and humid climatic conditions. The analysis is confined to using a multi-storey faade DSF system which could provide the maximum extraction force required to ventilate the internal office spaces through combined wind and stack effects. The following strategies are designed to further investigate possibilities to improve the ventilation rates within the office spaces by modifying the configurations of the DSF system. The strategies are:
a)
Modify the ventilated shaft by introducing openings onto the outer pane of the DSF system. Different sizes and locations for the openings will be investigated and their effects onto internal thermal comfort for the office spaces are observed.
b)
Extending the ventilation shaft of the DSF system above the roof level to find out their effects onto the ventilation rates within the internal office spaces.
c)
Compare the results of the extended shaft design with the installation of mechanical fan at the top part of the DSF system. This is the comparison between natural ventilation and mixed mode ventilation for thermal comfort using DSF in the hot and humid climate.
d)
Investigating the effect of sun shading device to the DSF system.
e)
Computation of graphs and nomograms to help in the design process of using DSF system in the hot and humid climate. 193
9.1.1
Investigation of different opening locations on the outer pane of DSF system
In this investigation openings are introduced on the outer pane of the DSF system. The optimum opening location will be investigated for providing the lowest indoor temperature and relative humidity and at the same time giving acceptable indoor air velocities for carrying out normal office tasks. Figure 9.1 shows the various locations (L1) of the opening under investigation. The 1st location (Opening A) selected is the midway between the ceiling and the ground level of the next floor. The 2nd location (Opening B) is at the ceiling level. The 3rd location (Opening C) is at the same level as the opening of the inner pane. The 4th location (Opening D) chosen is at the mid height of the room and the 5th location (Opening E) is 300mm above the floor level of the room. The outer pane opening size used for this investigation is 200mm.
The boundary conditions used for the simulation are: -
Wind speeds of 0.5m/s, 1.5m/s and 3m/s
-
External temperatures between 26oC to 30oC
-
Relative humidity between 70% to 100%
-
DSF air gap size of 300mm
-
Inner pane opening size is 400mm
-
Rear wall vent size is 300mm x 600mm
L1 L1
(i) Opening A
(ii) Opening B 194
L1 L1
(iii) Opening C
(iv) Opening D
L1
(v) Opening E Figure 9.1 Investigation of different opening locations (L1) for the outer pane of DSF system
Measurements of indoor conditions for air temperature, air velocity and relative humidity are investigated at 1.2m above floor level and at the center of the office space. There are four measurement points (C1, C2, C3 and C4) selected across the internal room space as indicated in Figure 9.2. Table 9.1 shows a sample of the measurements taken from the simulations with external temperature of 30oC, wind velocity of 1.5m/s and relative humidity of 80%.
195
500
OUTDOOR
1000
500
C2
C3
500
C4
1200
C1
1000
OFFICE
Figure 9.2 Schematic drawing showing selected points for monitoring simulation results
Opening A
Opening B
Opening C
Opening D
Opening E
Temp (oC) Velocity (m/s) Relative Humidity (%) Temp (oC) Velocity (m/s) Relative Humidity (%) Temp (oC) Velocity (m/s) Relative Humidity (%) Temp (oC) Velocity (m/s) Relative Humidity (%) Temp (oC) Velocity (m/s) Relative Humidity (%)
C1
C2
C3
C4
30.08 0.11 79.74
30.12 0.04 79.61
30.16 0.02 79.51
30.14 0.02 79.55
30.1 0.09 79.69
30.18 0.03 79.45
30.2 0.02 79.4
30.19 0.03 79.42
30.09 0.11 79.72
30.13 0.1 79.6
30.17 0.09 79.49
30.10 0.04 79.71
30.22 0.07 79.36
30.21 0.05 79.38
30.22 0.04 79.35
30.21 0.04 79.39
30.23 0.05 79.33
30.23 0.02 79.33
30.2 0.03 79.42
30.21 0.03 79.39
Table 9.1 Table showing sample of simulation results for different opening locations (L1) 196
30.25 30.2
Opening A Opening B
30.15
Opening C 30.1
Opening D Opening E
30.05 30 C1
C2
C3
C4
Graph 9.1 Graph showing the temperatures (oC) comparison for different opening locations
0.12 0.1
Opening A Opening B Opening C Opening D Opening E
0.08 0.06 0.04 0.02 0 C1
C2
C3
C4
Graph 9.2 Graph showing the air velocity (m/s) comparison for different opening locations
Graphs 9.1 and 9.2 had shown the temperature and air velocity comparison for the four measuring points of the different opening locations. Opening A has the lowest indoor temperature and Openings D & E having the highest temperatures. Even though Opening C has the highest internal air velocities compared to the rest, three out of four of the velocity values are higher than 0.08m/s, which is the maximum acceptable indoor air velocity for operation of normal office tasks. Opening D would provide the best indoor air velocity required. Opening A gives the second best solutions as far as air velocity is
197
concerned. After analyzing all the simulation results and taking all the thermal comfort parameters into consideration, Opening A would give the optimum result as compared to the rest of the options.
9.1.2
Investigation of different opening sizes on the outer pane of DSF
system
Following from the above findings, different opening sizes are used to investigate the optimum size for the 1st location. The opening sizes used are 150mm, 300mm and 450mm. Figure 9.3 below shows the configuration for the model used in the simulations. The boundary conditions for this stage of the simulations are similar to the investigation for the optimum opening locations discussed in section 9.1.1. The findings show that opening size of 300mm provides the optimum ventilation rates for the internal office space and gives the most desirable thermal comfort conditions.
External Temperature - 26C to 30C
300
DSF OPENING
DSF OPENING
AIR GAP SIZE 300
Relative Humidity - 70% to 100%
OFFICE SPACE
300
DSF OPENING
DSF OPENING
WIND - 0m/s to 3m/s
VENT SIZE 300 x 600
DSF FACING SOUTH / NORTH
Figure 9.3 Investigation of different opening sizes for the outer pane of DSF system 198
9.1.3
A new type of DSF configuration for hot and humid climate
From the above investigations, a new type of DSF system (Figure 9.4) has emerged for the use in the hot and humid climate. The new DSF faade system is a combination of Multi-storey faade configuration with specific openings located at the outer pane of the system. The openings at the outer pane are located above the ceiling level and usually there will be spandrel panels behind at these portions of the building construction. In view of that, this new faade configuration will not have negative impact onto the external aesthetic of the high-rise building design. D
3500
3500
C 17
WIND
17th FLOOR V 17 E 16
3500
C 16
16th FLOOR V 16
WIND E 15
DSF
C 15
63500
V3 E2 C2
3500
WIND
2nd FLOOR V2 E1
3500
C1
1st FLOOR WIND
GROUND FLOOR
4000
2800
V1
Figure 9.4 A new type of double-skin façade model for hot and humid climate 199
9.1.4
Investigation of different shaft heights of DSF system
With the optimum openings location and size obtained for the outer pane of the DSF system, the next step is to investigate the different heights of the DSF system above the roof level. The shaft heights being selected are 1.5m, 2.5m and 3.6m and Figure 9.5 below shows the parameters for the model used in simulations. The optimum opening size for the outer pane is 300mm and the air gap size is set at 300mm also for this investigation.
There are 18 points being carefully selected for the CFD model to record the parametric results in order to compute the thermal comfort indices generated. Thermal comfort parameters (e.g. air temperature, air velocity, relative humidity, air pressure, radiant temperature, etc) are recorded for 1st, 3rd and 5th floors of the internal office spaces and three points at the double-skin façade itself, namely at the bottom, middle and top parts. Figure 9.6 shows all the location points selected for the simulation results. AIR GAP SIZE 300
External Temperature - 26C to 30C SHAFT HEIGHT 1.5m, 2.5m or 3.6m
Relative Humidity - 70% to 100%
WIND - 0m/s to 3m/s
300 300
DSF OPENING
DSF OPENING
OFFICE SPACE
VENT SIZE 300 x 600
WIND - 0m/s to 3m/s
DSF OPENING
DSF OPENING
DSF FACING SOUTH / NORTH
Figure 9.5 Model configurations for simulations 200
P18a
P15a P11a
P12a P13a
P14a
P17a
P10a P6a
P7a P8a
P9a
P5a P1a
P2a P3a
P4a P16a
Figure 9.6 Location points for monitoring the simulation results (for the extended shaft model)
201
Figure 9.7 Isometric view – 3.6m shaft with openings at outer pane of DSF
Figure 9.7 showing the Airpak modelling screen plot of the isometric view for the model under investigation. The model is constructed with a 6-storey block office building with extended DSF shaft above the roof level.
Figures 9.8 to 9.11 are the screen plots of one of the simulation results obtained. From the velocity vectors plot (Figure 9.8) one could observe that the airflow patterns within the office spaces for all the floors are similar. It is flowing in a clock-wise direction from the top of the room to the bottom part with a strong velocity at the high level. The flow does not create a cross ventilation effect within the room space.
202
Figure 9.8 Velocity vectors – study of air velocity magnitudes and its moving directions
Figure 9.9 Temperature contours – study of temperature distribution within the office spaces
203
Figure 9.10 Velocity particle traces– study of air flow patterns within the office spaces
Figure 9.11 Pressure contours – study of external and internal pressures acted onto the building
204
Temperature contour study from Figure 9.9 has shown that the lower floor of the office space has lower internal temperatures compared to higher floor. Top floor would be the hottest and it might require some sort of mechanical means to provide a better thermal comfort environment. It is interesting to find out that the back portion of the office for all the floors are having higher temperatures compare to the front part. This could be due to the warm air has been brought to the back part of the office space caused by the airflow pattern generated.
The velocity particle traces plot in Figure 9.10 shows that the wind velocity has been slowed down when it approaches the façade system of the building block. This has created a strong upturn force when the wind hitting the wall surface and created a drastic and irregular turbulence patterns over and above the extended shaft of the DSF system.
The pressure contour plot in Figure 9.11 showing higher pressure is inserted at the façade system at the lower floor of the building. It created a ‘cone’ shape pattern in front of the façade. The pressure inside the air gap of the DSF system is also higher at lower level and gradually decreases while it goes higher. This is also true for the pressure within the office space for different floors.
30.5 30 29.5 1.5m Shaft 2.5m Shaft 3.6m Shaft 4m Shaft
29 28.5 28 27.5 27 26.5 Level 1 Level 3 Level 5
Graph 9.3 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (various shaft heights)
205
30 29.5 29
1.5m Shaft 2.5m Shaft 3.6m Shaft 4m Shaft
28.5 28 27.5 27 26.5 Level 1 Level 3 Level 5
Graph 9.4 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (various shaft heights)
Both of the graphs 9.3 and 9.4 show that the 3.6m-shaft is the optimum solution and gives better indoor operative temperatures for offices at all the floors. The height of the shaft is about the same as one-storey high and this will be a better solution in terms of construction cost and architectural aesthetic of the building design. Some of the selected shaft heights investigation results are presented in Appendix B with comparison of thermal comfort profiles for two selected simulations that show the 3.6m-shaft is the better solution.
9.1.5
Investigation of different air gap sizes with optimum shaft height
After establishing the optimum shaft height for the new type of DSF system, the next parameter to look into is the effect of different air gap sizes on the indoor thermal comfort. Different air gap sizes of 300mm, 600mm, 900mm and 1200mm will be introduced to the DSF system with the optimum outer skin opening size of 300mm, inner pane opening size of 300mm, and the shaft height of 3.6m. Figure 9.12 shows the configurations of the CFD model for simulations with specific conditions.
206
Tables 9.2 and 9.3 show the simulation results for one particular case with external wind velocity of 3m/s, relative humidity of 80% and external temperature of 26oC. The results were taken at 10am in July for South facing DSF system at locations P1a, P6a, P11a, P2a, P7a and P12a (Figure 9.6). The first group of results (P1a, P6a and P11a) is taken next to the occupant located near to the DSF system and the second group of results (P2a, P7a and P12a) is taken next to the other occupant in the office away from the façade system. The results show that the optimum air gap sizes are between 300mm and 900mm as presented in Graphs 9.5 and 9.6. These findings are agreeing with the results found by Gan (2006) in the investigation of buoyancy-induced flow in open cavities for natural ventilation. The research found that the optimum cavity width for maximizing the buoyancy-induced flow rate was between 0.55m and 0.6m for a solar chimney of 6m high. It also found that the ventilation rate in a double-skin façade of four-storey high building increased with the cavity width but the increase was small when the width was larger than 0.7m.
AIR GAP SIZE 300, 600, 900, 1200
SHAFT HEIGHT
Relative Humidity - 70% to 100%
WIND - 0m/s to 3m/s
3.6m
External Temperature - 26C to 30C
300 300
DSF OPENING
DSF OPENING
OFFICE SPACE
VENT SIZE 300 x 600
WIND - 0m/s to 3m/s
DSF OPENING
DSF OPENING
DSF FACING SOUTH / NORTH
Figure 9.12 Configurations of the model for simulations 207
Air Gap Size
Floor Level
Temp (oC)
Air Vel (m/s)
RH (%)
PMV
OT (oC)
0.09 0.08 0.08
Radiant Temp (oC) 29.04 29.2 29.34
300mm
Level 1 Level 3 Level 5
26.74 26.79 26.94
76.56 76.33 75.68
1.42 1.46 1.48
27.89 28 28.14
600mm
Level 1 Level 3 Level 5
26.76 26.79 27.01
0.1 0.09 0.07
28.98 29.05 29.55
76.48 76.32 75.35
1.41 1.42 1.52
27.87 27.92 28.28
900mm
Level 1 Level 3 Level 5
26.78 26.87 27.07
0.1 0.09 0.07
29 28.89 29.33
76.38 75.98 75.1
1.41 1.41 1.51
27.89 27.88 28.2
1200mm
Level 1 Level 3 Level 5
26.72 26.9 27.12
0.1 0.1 0.07
28.95 29.16 29.8
76.67 75.86 74.87
1.4 1.44 1.56
27.84 28.03 28.46
Table 9.2 Simulation results at locations P1a, P6a and P11a (various air gap sizes)
RH (%)
PMV
OT (oC)
0.3 0.3 0.3
Radiant Temp (oC) 28.69 28.92 29.1
76.94 76.18 75.58
1.17 1.21 1.25
27.67 27.87 28.03
Level 3 Level 5
26.67 26.76 27
0.33 0.32 0.27
28.61 28.76 29.29
76.87 76.45 75.37
1.14 1.17 1.29
27.64 27.76 28.15
900mm
Level 1 Level 3 Level 5
26.69 26.74 27.13
0.33 0.3 0.27
28.61 29.28 29.27
76.79 76.56 74.83
1.15 1.24 1.31
27.65 28.01 28.2
1200mm
Level 1 Level 3 Level 5
26.72 26.74 27.15
0.33 0.3 0.26
28.61 29.45 29.64
76.65 76.57 74.73
1.15 1.24 1.35
27.67 28.1 28.4
Air Gap Size
Floor Level
Temp (oC)
Air Vel (m/s)
300mm
Level 1
26.65 26.82 26.96
Level 3 Level 5 600mm
Level 1
Table 9.3 Simulation results at locations P2a, P7a and P12a (various air gap sizes)
208
28.5 300mm Air Gap
28.25
600mm Air Gap 28
900mm Air Gap 1200mm Air Gap
27.75
27.5 Level 1
Level 3
Level 5
Graph 9.5 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (various air gap sizes)
28.5 300mm Air Gap
28.25
600mm Air Gap
28
900mm Air Gap
27.75
1200mm Air Gap
27.5 Level 1
Level 3
Level 5
Graph 9.6 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (various air gap sizes)
9.1.6
Comparison of ‘Fan’ and ‘Shaft’ ventilation methods
The ‘Fan’ ventilation configuration (Figure 9.13) uses a fan to extract the air within the air gap of the double-skin façade in order to improve the ventilation rates of the internal office space of different levels. The extraction flow rate of the fan used is 3.5m3/s with a wind velocity of 3m/s. 209
The ‘Shaft’ ventilation configuration (Figure 9.13) uses an extended shaft of 3.5m above the roof level of the building concerned. This method is purely using the stack effect to extract the air out of the air gap of the double-skin façade.
Both of the configurations are having double-skin façade of 300mm wide air gap with openings to the outer skin of the façade. Figure 9.14 shows the locations (L1, L2, L3 and L4) selected for tabulating the thermal comfort parameters for the simulations.
Figure 9.13 CFD models for ‘Shaft’ and ‘Fan’ configurations
210
L1
L2
L3
L4
L1
L2
L3
L4
L1
L2
L3
L4
Figure 9.14 Locations of record for thermal comfort parameters (example for the mechanical fan at the top of the double-skin façade)
The comparisons for both ‘Fan’ and ‘Shaft’ configurations in relation to thermal comfort parameters are presented in Graph 9.7 below:
211
Air Temperature Profile for Different Levels
Air Temperature Profile for Different Levels
29.00
29.00
Level 1 Level 3
28.00
Level 5
27.50 27.00 26.50 26.00
Level 1 Level 3 Level 5
28.50 Temperature (C)
Temperature (C)
28.50
28.00 27.50 27.00 26.50 26.00 25.50
25.50
25.00
25.00 1
2
3
1
4
‘Fan’ ventilation method
3
Velocity (m/s)
Velocity (m/s)
Level 5
2
1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Level 1 Level 3 Level 5
1
4
Locations of Measurement
2
3
4
Locations of Measurement
‘Fan’ ventilation method
‘Shaft’ ventilation method
Radiant Temperature Profile for Different Levels
Radiant Temperature Profile for Different Levels
38
38
Level 1
36
Level 3
34
Level 5
32 30 28
Temperature (C)
Temperature (C)
4
Air Velocity Profile for Different Levels Level 1 Level 3
1
3
‘Shaft’ ventilation method
Air Velocity Profile for Different Levels 1.10 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00
2
Locations of Measurement
Locations of Measurement
36
Level 1 Level 3
34
Level 5
32 30 28 26
26 1
2
3
4
Locations of Measurement
‘Fan’ ventilation method
1
2
3
4
Locations of Measurement
‘Shaft’ ventilation method
212
Relative Humidity Profile for Different Levels
79
Level 1 Level 3
79
77
Level 5
77
75
%
%
Relative Humidity Profile for Different Levels
75
73
73
71
71 69
69 1
2
3
1
4
‘Fan’ ventilation method
3
4
‘Shaft’ ventilation method
PMV Profile for Different Levels
PMV Profile for Different Levels 2.5
2.5
Level 1 Level 3 Level 5
2 1.5 1
Level 1 Level 3
2
Level 5 PMV
PMV
2
Locations of Measurement
Locations of Measurement
1.5 1 0.5
0.5
0
0 1
2
3
1
4
2
3
4
Locations of Measurement
Locations of Measurement
‘Fan’ ventilation method
‘Shaft’ ventilation method
Operative Temperature Profile for Different Levels
Operative Temperature Profile for Different Levels
33 Level 1 Level 3 Level 5
32.00 31.00 30.00 29.00 28.00
Level 1 Level 3 Level 5
32 Temperature (C)
33.00
Temperature (C)
Level 1 Level 3 Level 5
31 30 29 28 27
27.00
26
26.00 1
2
3
1
4
2
3
4
Locations of Measurement
Locations of Measurement
‘Fan’ ventilation method
‘Shaft’ ventilation method
Graph 9.7 Comparison of ‘Fan’ and ‘Shaft’ configurations in relation to thermal comfort parameters 213
9.1.6.1 Analysis of ‘Fan’ and ‘Shaft’ ventilation methods
In comparison of values of air temperature, air velocity, radiant temperature, relative humidity, PMV and operative temperature profiles shown in the graphs above, the ‘Shaft’ ventilation method gave a better result as compared to the mechanical fan method and the configuration of the double-skin façade is much energy efficient as it doesn’t use any mechanical means for the improvement of the internal ventilation rate of the office spaces.
9.1.7
Investigation of sun shading device to the DSF system
In this investigation, sun shading devices are placed at the centre within the air gap of the DSF system in front of the office space as shown in Figure 9.15. This is the most efficient position for the shading devices to be located within a DSF system as found by Gratia and Herde (2007). In their research they found that shading device that placed at the middle of the DSF cavity uses the least cooling load (-20%), followed by shading device placed against the windows of the outer pane (-6%), then shading device that placed against the windows of the inner pane of the DSF system. These results applied to either the DSF system is closed or opened.
The results of different DSF system shaft heights are compared with and without the installation of sun shading devices as shown in Graphs 9.8, 9.9, 9.10 and 9.11 in order to investigate the effects of sun shading on the indoor comfort with the façade system. The results had shown that the installation of sun shading devices might not help in the indoor thermal comfort for a naturally ventilated building using the DSF system. The sun shading devices absorbed the heat from the sun and caused the temperature within the air gap to rise even further as compared to the DSF system without the sun shading being installed. The increased temperature within the air gap has further heated up the interior spaces of the office and the effect is quite substantial for lower ventilating shaft (the 1.5m shaft) as seen in Graphs 9.8 and 9.10. 214
P11a
P12a
P6a
P7a
P1a
P2a
Figure 9.15 Study of the effects of sun shading device within the DSF air gap
31.5 31 30.5
1.5m Shaft No Sun Shading
30 29.5
1.5m Shaft Sun Shading
29 28.5 28 27.5 Level 1
Level 3
Level 5
Graph 9.8 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (1.5m shaft)
215
30.5 30
2.5m Shaft - No Sun Shading
29.5 29
2.5m Shaft - Sun Shading
28.5 28 27.5 Level 1 Level 3 Level 5
Graph 9.9 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (2.5 shaft)
31.5 31 30.5
1.5m Shaft - No Sun Shading
30 29.5
1.5m Shaft - Sun Shading
29 28.5 28 27.5 Level 1
Level 3
Level 5
Graph 9.10 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (1.5 m shaft)
30.5 30
2.5m Shaft - No Sun Shading
29.5 29
2.5m Shaft - Sun Shading
28.5 28 27.5 Level 1
Level 3
Level 5
Graph 9.11 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (2.5m shaft)
216
9.1.8
Summarizing of results and findings for optimization
After numerous attempts and comparisons in optimizing the DSF configurations in order to find out the window periods for acceptable indoor thermal comfort conditions for office workers in the Singapore context, there are a numbers of positive findings observed. These results and findings will be of outmost important as an indication whether double-skin façade is possible to be used as a mean to introduce natural ventilation to the high-rise buildings in the tropics.
The findings for the optimization are summarized below:
a) The optimum location for the opening at the outer pane of the DSF system is midway between the ceiling and the ground level of the next floor.
b) The optimum opening size for the outer pane of the DSF system is 300mm.
c) The optimum shaft height above the roof level for the DSF system is 3.6m.
d) The optimum air gap sizes for the DSF system with the optimum shaft height of 3.6m are in the range between 300mm and 900mm. e) The use of the DSF system as a ventilated shaft will provide a better result and is more energy efficient as compared to merely using a mechanical fan for air extraction purposes. The optimum shaft height of 3.6m has improved the thermal comfort of the high floor levels to meet the 80% thermal comfort acceptability limits.
217
f) The introducing of sun shading devices within the air gap of the DSF system do not seems to improve the indoor thermal comfort of the working spaces.
9.2
An improved DSF system for the tropics
The parametric studies in optimizing the configurations of the DSF system have lead to the construct of an improved DSF system for use in the tropics. This improved system (Figure 9.16) is also able to produce a better result as compared to the typical DSF system with no opening at the outer pane, as indicated in Graph 9.12. Some of the important findings are tabulated below:
a) DSF system with no openings at external fenestration Introduction of shaft at the top of the system does not improve much the indoor thermal comfort conditions. Installation of fan at the top of the system has improved the comfort conditions at the lower and middle floors to comfort level (80% acceptability limits). Combination of shaft and sun shading device has only improved the thermal comfort slightly at lower floor.
b) DSF system with openings at external fenestration - Fan with openings vs. shaft with openings Both have improved the indoor thermal comfort for lower and middle floors compared to DSF system with no external openings (90% acceptability limits). Shaft with openings combination has improved the high floors thermal comfort to 80% acceptability limits.
218
32.5 32 31.5 31 30.5 30 29.5 29 28.5 28 27.5
DSF - No opening DSF Opening DSF Opening with 3.6m Shaft
Level 1
Level 3
Level 5
Graph 9.12 Comparison of indoor Operative Temperatures (oC) for different DSF systems
WIND
E 17
3500
3500
C 17
WIND
17th FLOOR V 17 E 16
3500
C 16
16th FLOOR V 16
WIND E 15
DSF
C 15
63500
V3 E2 C2
3500
WIND
2nd FLOOR V2 E1
3500
C1
1st FLOOR WIND
GROUND FLOOR
4000
2800
V1
Figure 9.16 An improved new type of double-skin façade model for hot and humid climate
219
9.3
Limitations of the research
The research work has covered extensively a number of the important aspects of the DSF system that will affect the thermal comfort of the internal spaces of an office building in the tropical conditions. In view of the complexity of the issues, there are a few assumptions made and limitations are acknowledged before the problem could be effectively investigate into without losing the focus of the whole research work. Firstly is the lack of existing built examples of DSF high-rise buildings in the tropical region. This has made it quite difficult to validate the results and findings from the research directly with any known field experimental results. This shortcoming could be overcame to certain extend by installing full size experimental model in a tropical region and monitor the experimental results for a sufficient period of time. This will certainly need enough funding, sufficient technical knowledge for constructing the system and selection of a suitable site for erecting the experimental model. In view of the time and cost limitation for this research, the next optimum choice of using simulations technology to investigate the issues for the research has been chosen together with using the summer period experimental results from DSF system in temperate countries for validation purposes.
Secondly is the complexity of thermal comfort issues associated with the mere scale of the high-rise office buildings. The research attempting to introduce natural ventilation concepts onto high-rise buildings has further diversify the problem even though it is a challenging and important issue that one needs to face if reduction of energy usage for built environment in the city is to be addressed. Therefore this research work has to define a manageable scope and yet produce a conclusive outcome in order to have satisfactory contribution to the knowledge. The scope of the research is then defined to investigate a highrise building up to 18-storey in height, which is a manageable scale both in terms of its complexity and the capability of computer facilities available at the time. The internal office size under investigation is controlled to small office area with a shallow office plan. This will avoid much more complex issues
220
associated with the effects internal partitions may have on the internal airflow patterns for large office plan.
The research has looked into all the parameters that affected the human thermal comfort like air temperature, air velocity, radiant temperature, relative humidity, PMV vote, etc and by investigating various parametric variables of the DSF system to achieve the required indoor conditions. There are others issue associated with the technical part of the DSF system, the day lighting and condensation issues of the façade system and the building regulations required for the construction of the façade system in a particular country are not within the scope of this research.
This research work has achieved to propose a new configuration of DSF system for the used in tropical countries and developed a series of nomograms as guideline for used in the initial design of DSF system in those region.
9.4
Nomograms for natural ventilation designs with DSF system
9.4.1
Formulation of the nomograms
The simplest form of nomogram is a scale such as a Fahrenheit vs. Celsius scale seen on an analog thermometer or a conversion chart. Nomograms could be designed from straight scales with a range of interesting formations through the analyzing of their geometric properties and these seem to be the most common types. The nomogram design for this research has been formulated with these advantages in mind.
The reasons and a simple way of reading off the nomograms formed in this research to help designer in selecting the appropriate DSF system during the
221
design stage of projects have been presented in Chapter 7, Section 7.6.1. The formulation of the nomogram will be presented in the few simple steps below:
a)
Formulation of the ‘Axis’ of the nomogram
There are three main thermal comfort parameters that determine the human comfort in the environment, namely the air temperature, air velocity and relative humidity. These three parameters will also determine the required Operative Temperature (OT) needed for human comfort according to the latest ASHREA Standard (2004). Therefore these three parameters will form the main Axis of the nomogram.
Air Temp. ( C)
Air Velocity (m/s)
Relative Humidity (%)
Axis
Figure 9.17 Three ‘Axis’ of the nomogram
222
b)
Formulation of the ‘Limits’ and ‘Linear Spacing’ on the straight scales of the nomogram
The ‘Limits’ on the straight scales of the nomogram are determined from the simulation results that provide the extent of thermal comfort obtained with various boundary conditions and DSF system configurations. The determination of the ‘Linear Spacing’ on the nomogram scales will then be only a matter of simple division into appropriate increment.
Air Temp. ( C) 30
25
20 0 0.5
60 70
1.0 1.5
80 90
2.0
100
Air Velocity (m/s)
Relative Humidity (%)
Limits / Linear Spacing
Figure 9.18 ‘Limits’ and ‘Linear Spacing’ of the nomogram
c)
Determination of the ‘Non-comfort Zone’
The ‘Non-comfort Zone’ is determined from simulation results with Operative Temperature (OT) that is not acceptable to bring about thermal comfort to the occupant. The boundary limits of the ‘Non-
223
comfort Zone’ are formed by just simply joining the three parameters’ limits obtained (through simulation results) by means of projection.
Air Temp. ( C) 30
25
20 0
60
0.5
70
1.0
80
1.5
90
2.0
100
Air Velocity (m/s)
Relative Humidity (%)
Non-comfort Zone
Figure 9.19 ‘Non-comfort Zone’ of the nomogram
d)
Determination of the ‘Comfort Zone’
The ‘Comfort Zone’ is determined from simulation results with Operative Temperature (OT) that is acceptable to bring about thermal comfort to the occupant. The boundary limits are formed similar to the method used in forming the ‘Non-comfort Zone’.
224
Air Temp. ( C) 30
25
20 0
60
0.5
70
1.0
80
1.5
90
2.0
100
Air Velocity (m/s)
Relative Humidity (%)
Comfort Zone
Figure 9.20 ‘Comfort Zone’ of the nomogram
9.4.2
The application of the nomograms
A set of 9 nomograms has been developed below in Figure 9.21 for the new type of DSF system to be used in the tropics. They are coded for easy reference as Nomogram-S-300, Nomogram-S-600, Nomogram-S-900, Nomogram-N300, Nomogram-N-600, Nomogram-N-900, Nomogram-EW-300, NomogramEW-600 and Nomogram-EW-900 respectively. Each of the nomogram consists of three axes with the most important parameters that affect the human thermal comfort, namely air temperature, air velocity and relative humidity. Two triangles are constructed using these three axes, which indicate the acceptable Operative Temperature of 80% acceptability for human comfort in a natural ventilated space. The table used to calculate the acceptable Operative Temperature (OT) and how to use the nomogram have been discussed in depth in Sections 7.5 and 7.6.1.
225
There are three nomograms constructed for each orientation of the DSF system, namely the South, North and East/West facing. Each orientation is further indicated with three different air gap sizes (300mm, 600mm and 900mm) of the façade system in which will produce different indoor thermal comfort with various values of the comfort parameters. Earlier finding in the optimization of the parametric studies show that the air gap size of 1200mm is not an optimum configuration for the façade system to be used.
Only shaft height of 3.6m is being considered because the shaft heights of 1.5m and 2.5m will not be able to provide satisfactory indoor thermal conditions. In fact both of the 1.5m and 2.5m shaft heights gave a higher Operative Temperature value of 12% and 10% respectively as compared to the optimum 3.6m shaft height.
The shaded area between the two triangles is the acceptable comfort condition with 80% acceptability. Any combination of the different values of air temperature, air velocity and relative humidity that fall within the shaded area will indicate that the condition is acceptable for human comfort. Take an example if a designer wants to design a South-facing DSF system with 300mm air gap and 3.6m height shaft, the designer would refer to the Nomogram-S300 for an initial idea whether the façade system will give an acceptable indoor condition in relation to the known air temperature, air velocity and relative humidity of where the building to be designed is located. Nomogram-S-300 Application of nomogram Outdoor thermal conditions: External air temp. = 25oC External air velocity = 2m/s Relative humidity = 69% Intersection point (outdoor thermal conditions) shown on the nomogram at the left hand side will provide an acceptable indoor thermal condition, with a South-facing DSF system of 300mm air gap and a 3.6m shaft height.
226
Nomogram-S-300
Nomogram-S-600
227
Nomogram-S-900
Nomogram-N-300 228
Nomogram-N-600
Nomogram-N-900 229
Nomogram-EW-300
Nomogram-EW-600
230
Nomogram-EW-900
Figure 9.21 Nomograms for DSF design in the tropics
9.4.3
The limitations of the nomograms
All design guidelines and indicators had their limitations and most of them are limited to the domain and boundary conditions that one has set out during the initial formulation of those guidelines. Similarly the nomograms that presented in this research have their limitations. Firstly, the extent of the application of the nomograms is very much limited by the scope of the research discussed in Section 9.3. The user would need to find out the three thermal comfort parameters, namely external air temperature, air velocity and relative humidity, of where the building is to be designed in order to use the nomograms for design guides. The nomograms also only applied to external thermal comfort conditions falling within 20oC to 30oC of external temperature, 0 to 4 m/s of external wind velocity and 60 to 100% of relative humidity even though these
231
ranges are believed to produce most acceptable thermal comfort conditions in a hot and humid region.
Secondly, the nomograms only cover naturally ventilated double-skin façade systems with 300mm, 600mm and 900mm air gap constructed with a 3.6m high shaft. The selection of these particular air gap sizes and the height of the shaft are the results of the optimisation studies carried out in the research, which will provide optimum internal thermal comfort conditions for the designed building in the tropic region.
Thirdly, there are only nine nomograms formulated covering four major orientations. The user will need to make certain assumptions for building façade that is not facing exactly the major orientations of North, East, South or West in selecting which nomogram to use for design purposes. These will certainly has effect on the thermal comfort conditions expected as indicated by the nomogram when the actual double-skin façade system is constructed.
Even though the nomograms formulated present certain limitations to the user and they only cover buildings up to 18-storey high, but they still provide a very useful tool for designers who needed a quick reference and guide to whether the double-skin façade system that chosen for the new building will be able to provide satisfactory indoor thermal conditions. These guides also encourage the designers to first look at the possibilities of using natural ventilation for their buildings and changing the mind set that only mechanical means are possible for high-rise buildings ventilation purposes.
9.5
Conclusion
A ‘revised’ and optimised double-skin façade configuration has been presented and a careful formulated set of nomograms is proposed to be used for designing double-skin façade in the tropic. The formulation of the nomograms helped to
232
answer the third research questions set out in Section 1.11 for providing some useful guidelines for designing high-rise buildings with double-skin facade in the tropics. These nomograms covered all four orientations of building façade and applied to optimum air gap sizes of 300mm, 600mm and 900mm with a roof shaft height of 3.6m. These would certainly help the designers to reduce design time by just referring to the easy understandable nomograms and provide an ‘encouragement’ to those who would like to consider in reducing energy usage for high-rise office buildings in the tropic.
233
Chapter 10
Contributions and Future Works
10.1 Contributions of the research work The research work has bridged the gap of investigating in substantial details the possibility of using the double-skin façade system as a mean to provide natural ventilation to the high-rise office building in the tropical region. As there are not much comprehensive experimental works done on the subject and the lack of existing double-skin façade buildings built in the tropical region, this research work will help to provide an overall insight to the use of natural ventilation strategies together with current building technology of double-skin façade in the possibility of reducing energy usage for high energy consumption human built environment like the cities.
The research work found that the double-skin façade system has an advantage over typical curtain wall system used in high-rise buildings in terms of reducing cooling loads by allowing passive solar design to be introduced to the building design. The use of the double-skin façade as a ‘ventilating stack’ system has further enhanced the passive design of natural ventilating the internal spaces of the high-rise building. During the process in attempting to find out the possibilities and limitations of using the double-skin façade technology in the tropics, a new type of double-skin façade configuration was developed which will better respond to the hot and humid climatic conditions. This new type of façade system is a combination of typical multi-storey façade, as described in Chapter 4, with certain opening sizes and positioned at specific locations at the outer and inner panes of the façade systems. The findings have concluded that this new type of façade system provides a better result as compared to the typical system by as much as 10% for every storey of the highrise building in terms of energy reduction. The results are further improved by another 5% per storey when an extended shaft of 3.6m is introduced to the new system. This has constituted a substantial energy saving for a high-rise building. 234
A set of nine nomograms has been developed to help the designers who wish to use double-skin façades in their building design in the tropics. The designers would decide which elevation of their building would be installed with doubleskin façade system and by referring to the nomograms the designers could obtain the period of time natural ventilation might be introduced to their building and when secondary mean of ventilation mode like mechanical ventilation would require. The designers could quickly work out roughly the viability of using double-skin façade system for that particular project and advise the client accordingly before any detail planning and commitment are being made. This is a very handy tool for any designers in saving time and money before the design is being realized into construction.
10.2 Viability of natural ventilation for office buildings in the tropics From the studies of the proposed nomograms constructed from this research, the opening window period for possible natural ventilation is from 8am to 10am in the morning and from 4pm to 6pm in the evening. A window period of total four hours out of a ten hours working period (with 1 hour break in between), which is a quite common daily working hours for office workers in major tropical city like Singapore. This is a 40% per working hour/day opportunity for using passive cooling for the building and could reduce enormous energy usage each day.
The research work has shown that there are great opportunities and possibilities for introducing passive design to high-rise buildings in the tropics, especially combining the passive design strategies with cutting edge building technologies available. Others researcher like Gan (2006) has found also that it is possible to use a double façade not only for ventilation cooling of the façade cavity but also enhancing natural ventilation of the building it is incorporated 235
with. Gratia and Herde (2007) further confirmed that double-skin façade could be considered for building design with the application of natural cooling strategies. Even though expert understanding of each of the fields mentioned is important to make things work and the well integration of the two are not easy, this should not prevent us from venture into helping to reduce the usage of natural resources of the world.
10.3 Conclusion The research has achieved the objectives set out in the beginning of this long process of mentally challenging work. The research question of the viability of double-skin façade to be used in high-rise buildings in the tropics has been answered. Natural ventilation strategies could be used together with this unique façade system to provide satisfactory indoor thermal conditions for high-rise building in particular periods during a typical hot and humid climatic condition.
CFD is a feasible tool to be used in early design stage in testing out uncertainty in building design and it is an economical way to study possibility of solutions and options for a particular problem. With continuous research works being carried out in the field of CFD, there will be further cost and time saving in using this technology for built environment designs.
10.4 Recommendations for future works This research work has attempted to cover an appropriate scope to produce a beneficial outcome for the research questions that were asked in the earlier chapters. The areas of natural ventilation, double-skin façade and high-rise
236
buildings combined are very complex issues that some important parts of the problem are worth further investigations.
Following are some of the areas of research that could further contribute to the understanding of the used of the double-skin façade system in the tropics and helping in reducing energy usage by high-rise buildings:
a) Large office configurations
Large office configurations would have very different effect onto natural ventilation strategies especially due to the large volume of space and different internal partitions’ positions. The internal airflow patterns would be diversified in which would affect the human thermal sensation that caused satisfactory in thermal comfort conditions.
b) Mixed mode ventilation strategies
Natural ventilated design building required careful planning at the early stage of the design process and it will need to incorporate and integrate with the building structures design and other building services design. Sometime this optimum design process is not available. Therefore, a mixed mode ventilation strategy is always good to be put in place in designing a natural ventilated building.
Mixed mode ventilation strategies would have very different criteria as to a pure natural ventilated building. Much research have been carried out in investigating the mixed mode ventilation strategies in buildings with double-skin façade system in the temperate countries but it is only at the early stage for research in high-rise buildings in the tropical region.
237
c) Integration of PV with DSF system
PV technologies are maturing and this could be incorporated with double-skin technologies to further enhanced energy saving. There are some positive initial research findings that could support this argument. Gan (2006) found that integration of photovoltaics (PV) into a doubleskin façade could further enhance natural ventilation of the building and meanwhile reduce the variation of the flow rate with floor level. Using indoor air with a lower temperature than that of outdoor air in summer to ventilate the PV façade would be beneficial to the electrical performance of PV resulting from the reduced cell temperature and increased electricity conversion efficiency. This research was carried out with low-rise building and further works need to be done for highrise buildings especially in the tropics.
d) Night ventilation
Night ventilation is a compliment to the natural ventilation strategies and double-skin façade system has provided the opportunity for it to be implemented. Night ventilation strategy could further reduce the indoor temperature over night and help to extend the window period for natural ventilation.
10.5 Final note Many new technologies are invented through times and they are continuous to help in improving the quality of human lives. Wise and appropriate mastering of these technologies will benefit and prosper the human race. Technology and architecture are like ‘brother and sister’ and together they could face the future of the world. I would like to finish with a quote from the Master Architect of
238
all time, Le Corbusier, whom have acknowledged the importance of the used of technology in creating ‘beautiful’ architecture!
“ You employ stone, wood and concrete, and with these materials you build houses and palaces; that is construction. Ingenuity is at work. But suddenly you touch my heart, you do me good, I am happy and I say: ‘This is beautiful’. That is architecture. Art enters in.” -
Le Corbusier, 1927
239
References
Airpak User’s Manual. 2003. Fluent Inc. and ICEM-CFD Engineering.
Allard, Francis (ed). 1998. Natural Ventilation In Buildings: A Design Handbook, James & James (Science Publishers) Ltd, London, UK.
Anderson, J.D. 1995. Computational fluid dynamic: the basics with applications, McGraw-Hill, New York.
Argiriou, A. 1997. Determination of the impact of shading devices in singlesided ventilation buildings. In: Proceedings of the Technical Report Presented
at Contractors Meeting of the Project JOR3-CT96-00113, EU Report.
ASHRAE Fundamentals. 1993. American Society of Heating, Refrigeration and Air Conditioning, Atlanta.
ASHRAE. 2001. ANSI/ASHRAE Standard 562-2001, Ventilation for Acceptable Indoor Air Quality, Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
ASHRAE. 2004. ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
Atkin, B. (ed). 1988. Intelligent Buildings, John Wiley & Sons, New York.
Balocco, C. and Colombari, M. 2006. Thermal behavior of interactive mechanically ventilated double glazed façade: Non-dimensional analysis,
Journal on Energy and Buildings, 38, pp 1-7.
240
Bansal, NK, Mathur, R. and Bhandari, MS. 1994. A study of solar chimney assisted wind tower system for natural ventilation in buildings, Building and
Environment, 29(4), pp 495-500.
Barozzi, GS., Imbabi, MSE., Nobile, E. and Sousa, ACM. 1992. Physical and numerical modeling of a solar chimney-based ventilation system for buildings,
Building and Environment, 27(4), pp 433-445.
Bartak, M., Beausoleil-Morrison, I., Clarke, J.A., Denev, J., Drkal, F., Lain, M., Macdonald, I.A., Melikov, A., Popiolek, Z. and Stankov, P. 2002. Integrating CFD and building simulation, Building and Environment, 37(8), pp
865-871.
Barzun, J and Graff, H. 1985. The Modern Researcher, Harcourt Brace Janovich, Fort Worth, Texas, USA.
Beausoleil-Morrison, I. 2002. The adaptive conflation of computational fluid dynamics with whole-building thermal simulation, Journal on Energy and
Buildings, 34(9), pp 857-871.
Bouchair, A. 1989. Passive solar induced ventilation. In: Proceedings of the Miami International Conference on Alternative Energy Sources, Miami Beach, FL, 1987, Alternative Energy Sources, vol. 1: Part VIII, Solar Energy Fundamentals and Applications, Hemisphere Publishing Corporation, New York, USA.
Bouchair, A. 1994. Solar chimney for promoting cooling ventilation in southern Algeria, Building Service Engineering, Research and Technology,
15(2), pp 81-93.
Brager, G.S. and de Dear, RJ. 1998. Thermal adaptation in the built environment: a literature review, Journal on Energy and Buildings, 27(1), pp
83-96.
241
Brager , G.S. and de Dear, RJ. 2000. A standard for natural ventilation, ASHRAE Journal, October 2000, 42, 10, pp 21-28.
Carrilho da Grace, G., Chen, Q., Glicksman, L.R. and Norford, L.K. 2002. Simulation of wind-driven ventilative cooling systems for an apartment building in Beijing and Shanghai, Journal on Energy and Buildings, 34(1), pp
1-11.
Charles, K. E. 2003. Fanger’s Thermal Comfort and Draught Models, IRC Research Report RR-162, Institute for Research in Construction, National Research Council of Canada, Ottawa, Canada.
Chen, Q. and Srebric, J. 2000. Application of CFD tools for indoor and outdoor environment design, International Journal on Architectural Science,
Vol. 1, No. 1, pp 14-29.
Chen, Q. and Srebric, J. 2002. A procedure for verification, validation, and reporting of indoor environment CFD analyses, International Journal of HVAC
& R Research, 8(2), pp 201-216.
Chen, ZD., Bandopadhayay, P., Halldorsson, J., Byrjalsen, C., Heiselberg, P. and Li, Y. 2003. An experimental investigation of a solar chimney model with uniform wall heat flux, Building and Environment, 38, pp 893-906.
Chou, S.K. 2001. Building Energy Standard (BEST) – An Engineering Tool For Compliance Of Energy Performance Standards, Department Of
Mechanical Engineering, National University Of Singapore.
Colomban, Massimo. 1994. History and Technical Development of Curtain Wall, Permasteelisa Group President from Italy.
de Dear, RJ., Leow, KG. and Foo, SC. 1991. Thermal comfort in the humid tropics, International Journal of Biometeorology, 34, pp 259-265.
242
de Dear, RJ and Brager, G.S. 1998. Developing an adaptive model of thermal comfort and preference, ASHRAE Transaction 104(1a), pp 145-167.
Deardorff, J.W. 1970. A numerical study of three-dimensional turbulent channel flow at large Reynolds numbers, Journal of Fluid Mechanics, Vol. 41,
pp 453-480.
Ding, W., Hasemi, Y. and Yamada, T. 2005. Natural ventilation performance of a double-skin façade with a solar chimney, Journal on Energy and buildings,
37, pp 411-418.
Druckman, S., Moshka, E., Dubovsky, V., Letan, R., Weiss, Y. and Ziskind, G. 2000. Visualization of natural convection inside a ventilated enclosure. In: Proceedings of the Ninth International Symposium on Flow Visualization, Edinburgh, p. 83.
Eicker, U., Huber, M., Seeberger, P. and Vorschulze, C. 2006. Limits and potentials of office building climatisation with ambient air, Journal on Energy
and Buildings, 38, pp 574-581.
Emmerich, S.J. and McGrattan, K.B. 1998. Application of a large eddy simulation model to study room airflow, ASHRAE Transactions, Vol. 104, pp
1128-1140.
Evola, G. and Popov, V. 2006. Computational analysis of wind driven natural ventilation in buildings, Journal on Energy and Buildings, 38, pp 491-501.
Fanger, P.O. 1970. Thermal Comfort. Copenhagen: Danish Technical Press.
Feriadi, H. and Wong, NH. 2004. Thermal comfort for naturally ventilated houses in Indonesia, Journal on Energy and Buildings, 36, pp 614-626.
Flamant, G., Loncour, X., Wouters, P. and L’Heureux, D. 2004. Ventilated Double Façades, Ministry of Economic Affairs Project, Belgian Building
243
Research Institute, Department Building Physics, Indoor Climate and Building Services.
Fletcher, C.A.J., Mayer, I.F., Eghlimi, A. and Wee, K.H.A. 2001. CFD as a building services engineering tool, International Journal On Architectural
Science, Vol. 2, No. 3.
Fluent 6.2 User’s Guide. 2001. Fluent Inc.
Gan, Guohui. 1995. Numerical investigation of local thermal discomfort in offices with displacement ventilation, Journal on Energy and Buildings, 23, pp
73-81.
Gan, Guohui. 1995. Evaluation of room air distribution systems using CFD, Journal on Energy and Buildings, 23, pp 83-93.
Gan, Guohui. 1998. A parametric study of Trombe walls for passive cooling of buildings, Journal on Energy and Buildings, 27, pp 37-43.
Gan, Guohui. 2001. Thermal transmittance of multiple glazing: computational fluid dynamics prediction, Applied Thermal Engineering, 21, pp 1583-1592.
Gan, Guohui. 2006. Simulation of buoyancy-induced flow in open cavities for natural ventilation, Journal on Energy and Buildings, 38, pp 410-420.
Georgakis, C. and Santamouris, M. 2006. Experimental investigation of airflow and temperature distribution in deep urban canyons for natural ventilation purposes, Journal on Energy and Buildings, 38, pp 367-376.
Givoni, B. 1998. Climate Considerations in Building and Urban Design, Van Nostrand Reinhold, New York, USA.
Glesne, C and Peshkin, A. 1992. Becoming qualitative researchers: an introduction, White Plains, New York: Longman.
244
Goh, Thomas. 2001. Enhancing the Energy Standards, Building Management Department, Building and Construction Authority, Singapore.
Goto, T., Mitamura, T., Yoshino, H., Tamura, A. and Inomata, E. 2007. Longterm field survey on thermal adaptation in office buildings in Japan, Building
and Environment, 42, pp 3944-3954.
Grabe, J, Lorenz, R. and Croxford, B. 2001. Ventilation of double façades, Building Simulation, pp 229-236.
Gratia, E. and Herde, A. 2004a. Optimal operation of a south double-skin façade, Journal on Energy and Buildings, 36, pp 41-60.
Gratia, E. and Herde, A. 2004b. Natural ventilation in a double-skin façade, Journal on Energy and Buildings, 36, pp 137-146.
Gratia, E. and Herde, A. 2004c. Natural cooling strategies efficiency in an office building with a double-skin façade, Journal on Energy and Buildings, 36,
pp 1139-1152.
Gratia, E. and Herde, A. 2004d. Is day natural ventilation still possible in office buildings with a double-skin façade?, Journal on Energy and Buildings, 39, pp
399-409.
Gratia, E., Bruyere, I. and Herde, A. 2004e. How to use natural ventilation to cool narrow office buildings, Building and Environment, 39, pp 1157-1170.
Gratia, E. and Herde, A. 2007a. Greenhouse effect in double-skin façade, Journal on Energy and Buildings, 39, pp 199-211.
Gratia, E. and Herde, A. 2007b. The most efficient position of shading devices in a double-skin façade, Journal on Energy and Buildings, 39, pp 364-373.
245
Gratia, E. and Herde, A. 2007c. Are energy consumptions decreased with the addition of a double-skin?, Journal on Energy and Buildings, 39, pp 605-619.
Gratia, E. and Herde, A. 2007d. Guidelines for improving natural daytime ventilation in an office with a double-skin façade, Journal on Solar Energy, 81,
pp 435-448.
Groat, L and Wang, D. 2002. Architectural Research Methods, john Wiley & Sons, Inc., New York.
Haase, M, Wong, F, and Amato, A. 2004. Double-skin façades for Hong Kong, Proceedings of International Conference on Building Envelope Systems and Technology, pp 243-250.
Haase, M. and Amato, A. 2006. Double-skin façades and thermal comfort, In: Proceedings of Healthy Building Hong Kong Conference, Construction Industry Institute iV Hong Kong (CII-HK), Hong Kong.
Hadlock, C., Collins, M. and Athienitis, A. 2006. Numerical model development of a PV-integrated double façade. In: Proceedings of the
Canadian Solar Buildings Conference, Concordia University, Montreal, Quebec.
Hamdy, IF. and Fikry, MA. 1998. Passive solar ventilation, Renewable Energy, 14(1-4), pp 381-386.
Hamza, N. 2008. Double versus single skin façades in hot arid area, Journal on Energy and Buildings, 40, pp 240-248.
Harris, R.M. and Ledbetter, S.R. 1994. Assessing The Thermal Performance Of Curtain Walling, Centre for Window and Cladding Technology, University of
Bath, Claverton Down, Bath, England.
246
Hensen, J., Bartak, M. and Drkal, F. 2002. Modeling and simulation of a double-skin façade system, ASHRAE journal.
HØseggen, R., Wachenfeldt, B.J. and Hanssen, S.O. 2008. Building simulation as an assisting tool in decision making, Case study: With or without a doubleskin façade?, Journal on Energy and Buildings, 40, pp 821-827.
Hyde, R. 2000. Climate Response Design: A study of buildings in moderate and hot humid climates, E & FN Spon, London.
Internet page of Building Envelopes Organization, History of Double-skin Façades,
http://envelopes.cdi.harvard.edu/envelopes/web_pages/home/home.cfm
Internet page of Building Envelopes Organization, Building Envelopes and Mechanical Systems,
http://envelopes.cdi.harvard.edu/envelopes/web_pages/home/home.cfm
Internet page of Whole Building Design Guide, Natural Ventilation, http://www.wbdg.org/design/resource.php
ISO EN 7730, 1994. Moderate thermal environments – Determination of the PMV and PPD indices and specification of the conditions for thermal comfort,
International Standard Organization, Geneva.
Jaros, M., Charvat, P., Svorcik, P. and Gorny, R.I. 2002. Possibilities of CFD-Simulation of Solar Heated Spaces, Brno University of Technology,
Faculty of Mechanical Engineering, Institute of Power Engineering, Dept of Thermodynamics and Environmental Engineering.
Jiang, Y. and Chen, Q. 2002. Effect of fluctuating wind direction on cross natural ventilation in building from large eddy simulation, Building and
Environment, 37(4), pp 379-386.
247
Jiru, T.E. and Haghighat, F. 2008. Modeling ventilated double skin façade – A zonal approach, Journal on Energy and Buildings, 40, pp 1567-1576.
Jones, J. and West, A.W. 2001. Natural ventilation and collaborative design, ASHRAE Journal, 43(11), pp 46-51.
Karyono, T.H. 1996. Thermal comfort in the tropical South East Asia region, Architectural Science Review, 39(3), pp 135-139.
Kato, S., Ito, K. and Murakami, S. 2003. Analysis of visitation frequency through particle tracking method based on LES and model experiment, Indoor
Air, 13(2), pp 182-193.
Khan, N., Su, Y. and Riffat, S.B. 2008. A review on wind driven ventilation techniques, Journal on Energy and Buildings, 40, pp 1586-1604.
Kishnani, N. 2002. Climate, buildings and occupant expectations: A comfortbased model for the design and operation of office buildings in hot humid conditions, PhD thesis, Curtin University of Technology, Australia.
Koinakis, C.J. 2005. Combined thermal and natural ventilation modeling for long-term energy assessment: validation with experimental measurements,
Journal on Energy and Buildings, 37, pp 311-323.
Konya, Allan. 1980. Design Primer for Hot Climates, The Architectural Press Ltd, London.
Laar, M. and Grimme, FW. 2002. Sustainable Buildings in the Tropics, RIO 02 – World Climate and Energy Event.
Ladeinde, F. and Nearon, M. 1997. CFD applications in the HVAC & R industry, ASHRAE Journal, 39(1), p. 44.
248
Le Corbusier. 1923. Towards a new architecture, trans. F. Etchells, London, 1946, p. 187.
Lee, E., Selkowitz, S., Bazjanac, V., Inkarojrit, V. and Kohler, C. 2002. Highperformance commercial building facades, LBNL Report 50502,
http://gaia.lbl.gov/hpbf/main.html.
Lee, Siew Eang. 2002. Energy Efficiency Of Office Buildings In Singapore, Department of Building, School of Design and Environment, National University Of Singapore.
Letan, R., Dubovsky, V. and Ziskind, G. 2003. Passive ventilation and heating by natural convection in a multi-storey building. Building and Environment,
38, pp 197-208.
Li, Y. and Delsante, A. 2001. Natural ventilation induced by combined wind and thermal forces. Building and Environment, 36, pp 59-71.
Liddament, MW. 1992. The role and application of ventilation effectiveness in design. In: Proceedings of International Symposium on Room Air Convection
and Ventilation Effectiveness, University of Tokyo, pp 59-75.
Linden, A.C., Boerstra, A.C., Raue, A.K., Kurvers, S.R. and de Dear, R.J. 2006. Adaptive temperature limits: A new guideline in the Netherlands. A new approach for the assessment of building performance with respect to thermal indoor climate, Journal on Energy and Buildings, 38, pp 8-17.
Manz, H. 2003. Numerical simulation of heat transfer by natural convection in cavities of façade elements, Journal on Energy and Buildings, 35(3), pp 305-
311.
Manz, H. 2003. Total solar energy transmittance of glass double façades with free convection, Journal on Energy and Buildings, 36, pp 127-136.
249
Manz, H., Schaelin, A. and Simmler, H. 2004. Airflow patterns and thermal behavior of mechanically ventilated glass double façades, Building and
Environment, 39, pp 1023-1033.
Manz, H. and Frank, Th. 2005. Thermal simulation of buildings with doubleskin façades, Journal on Energy and Buildings, 37, pp 1114-1121.
Marques da Silva, F. and Glória Gomes, M. 2008. Gap inner pressures in multi-storey double skin façades, Journal on Energy and Buildings, 40, pp
1553-1559.
Mathur, J., Mathur, S. and Anupma. 2006. Summer-performance of inclined roof solar chimney for natural ventilation, Journal on Energy and Buildings,
38, pp 1156-1163.
Moeseke, G., Gratia, E., Reiter, S. and Herde, A. 2005. Wind pressure distribution influence on natural ventilation for different incidences and environment densities, Journal on Energy and Buildings, 37, pp 878-889.
Murakami, S. 1992. New scales for ventilation efficiency and their application based on numerical simulation of room airflow, In: Proceedings of
International Symposium on Room Air Convection and Ventilation Effectiveness, University of Tokyo, pp 22-38.
Nicol, J.F. and Humphreys, M.A. 2002. Adaptive thermal comfort and sustainable thermal standarda for buildings, Journal on Energy and Buildings,
34, pp 563-572.
Nicol, J.F. 2004. Adaptive thermal comfort standards in the hot-humid tropics, Journal on Energy and Buildings, 36, pp 628-637.
Oberkampf, W.L. and Trucano, T.G. 2002. Verification and validation in computational fluid dynamics, Progress in Aerospace Sciences, 38, pp 209-
272. 250
Oesterle, Lieb, Lutz and Heusler. 2001. Double-Skin Façades – Integrated Planning, Prestel Verlag, Munich, Germany.
Olesen, B.W. and Parsons, K.C. 2002. Introduction to thermal comfort standards and to the proposed new version of EN ISO 7730, Journal on Energy
and Buildings, 34, pp 537-548.
Olesen, B.W. and Brager, G.S. 2004. A better way to predict comfort, ASHRAE Journal, 46(8), pp 20-28.
Papakonstantinou, KA., Kiranoudis, CT. and Markatos, NC. 2000. Numerical simulation of airflow field in single-sided ventilated buildings, Journal on
Energy and Buildings, 33, pp 41-48. Pappas, A. and Zhai, Z. 2008. Numerical investigation on thermal performance and correlations of double skin façade with buoyancy-driven airflow, Journal
on Energy and Buildings, 40, pp 466-475.
Parkin, S. 2003. Development of a contemporary classification for ventilated double-skin façades, PLEA 2003 – The 20th Conference on Passive and Low
Energy Architecture, Santiago, Chile.
Parsons, K.C. 2003. Human Thermal Environments: The effects of hot, moderate, and cold environments on human health, comfort and performance,
Taylor & Francis, London.
Pasquay, T. 2001. Natural ventilation in high-rise buildings with double façades, saving or waste of energy. In: Proceedings of the 18th International
Conference on Passive and Low Energy Architecture, Brazil.
Phillips, E.M. and Pugh, D.S. 1987. How to get a PhD, Milton Keynes, Open University Press.
251
Poirazis, H. 2004. Double Skin Façades for Office Buildings: Literature review, Report EBD-R-04/3, Department of Construction and Architecture,
Lund University.
Powell, Robert. 1999. Rethinking The Skyscraper: The complete architecture of Ken Yeang, Thames & Hudson, London.
Priyadarsini, R., Cheong, K.W., and Wong, N.H. 2004. Enhancement of natural ventilation in high-rise buildings using stack system, Journal on Energy
and Buildings, 36(1), pp 61-71.
Priyadarsini, R., Wong, N.H. and Cheong, K.W. 2006. Modeling of the urban thermal environment of Singapore to mitigate urban heat island, iNTA
Conference.
Raja, I.A., Nicol, J.F., McCartney, K.J. and Humphreys, M.A. 2001. Thermal comfort: use of controls in naturally ventilated buildings, Journal on Energy
and Buildings, 33, pp 235-244.
Rajapaksha, I., Nagai, H. and Okumiya, M. 2002. Indoor airflow behaviour for thermal comfort in a courtyard house in warm humid tropics, In: Proceedings
Indoor Air 2002, Monterey, USA, pp 1072-1077.
Ruck, NC. 1989. Building Design and Human Performance. New York: Van Nostrand Reinhold.
Saelens, D., Roels, S. and Hens, H. 2004. The inlet temperature as a boundary condition for multiple-skin façade modeling, Journal on Energy and Buildings,
36, pp 825-835.
Saelens, D., Roels, S. and Hens, H. 2006. Strategies to improve the energy performance of multiple-skin façades, Building and Environment. In Press:
available online 4 December 2006.
252
Santamouris, M. (ed). 2007. Advances in Passive Cooling, Earthscan, London.
Schittich, C. (ed). 2001. Building Skins: Concepts Layers Materials, Birkhauser, Basel.
Schwede, D.A. 2007. Interpreting Fanger’s comfort equation within the adaptive paradigm. In: Proceedings of the 24th International Conference on
Passive and Low Energy Architecture, Singapore.
Sekhar, S.C. and Willem, H.C. 2004. Impact of airflow profile on indoor air quality – a tropical study, Building and Environment, 39, pp 255-266.
Shahid, H. and Naylor, D. 2005. Energy performance assessment of a window with a horizontal Venetian blind, Journal on Energy and Buildings, 37, pp 836-
843. So, T.P., Wong, C.W. and Wong, K-c. 1999. A new definition of intelligent buildings for Asia, Facilities, 17(12/13), pp 485-491.
Snyder, J. (ed). 1984. Architectural Research, Van Nostrand, New York.
Srebric, J., Vukovic, V., He, G. and Yang, X. 2008. CFD coundary conditions for contaminant dispersion, heat transfer and airflow simulations around human occupants in indoor environments, Journal on Energy and Buildings,
43, pp 294-303.
Stankovic, S. 2004. Internet page of Technology, Environmental Engineering in the Tropics, http://www.sia.org.sg.
Stavrakakis, G.M., Koukou, M.K., Vrachopoulos, M.Gr. and Markatos, N.C. 2008. Natural cross-ventilation in buildings: Building-scale experiments, numerical simulation and thermal comfort evaluation, Journal on Energy and
Buildings, 40, pp 1666-1681.
253
Stec, W.J., Paassen, A.H.C. and Maziarz, A. 2005. Modeling the double-skin façade with plants, Journal on Energy and Buildings, 37, pp 461-469.
Tan, B.K., CR, Uma M. and Hong, S. 2007. Naturally ventilated office building in the tropics – Learning from Bawa. In: Proceedings of the 24th
International Conference on Passive and Low Energy Architecture, Singapore.
Tan, G. and Glicksman, L.R. 2005. Application of integrating multi-zone model with CFD simulation to natural ventilation prediction, Journal on Energy and
Buildings, 37, pp 1049-1057.
Tenhunen, O., Lintula, K., Lehtinen, T., Lehtovaara, J., Viljanen, M., Kesti, J. and Makelainen, P. 2001. Double Skin Façades – Structures And Building Physics, Laboratory of Steel Structures, Building Technology, Department of
Architecture, Laboratory of Structural Engineering and Building Physics, Lighting Laboratory, Helsinki University of Technology, Finland.
Topp, C., Nielsen, P.V. and Heiselberg, P. 2001. Influence of local airflow on the pollutant emission from indoor building surfaces, Indoor Air, 11(3), pp
162-170.
Williams, D. 1996. Research proposals, In Greenfield, T (ed).
Wang, L. and Wong, N.H. 2006. Coupling between the CFD simulation and building simulation for better prediction of natural ventilation, iNTA
Conference.
Wong, N.H. and Tanamas, J. 2002. The effect of wind on thermal comfort in naturally ventilated environment in the tropics, In: Proceedings of International
Symposium on Building Research and the Sustainability of the Built Environment in the Tropics, edited by Tri H Karyono, Fergus Nicol and Susan Roaf. Jakarta, Indonesia, pp 192-206.
254
Wong, N.H. and Heryanto, S. 2004. The study of active stack effect to enhance natural ventilation using wind tunnel and computational fluid dynamics simulations, Journal on Energy and Buildings, 36, pp 668-678.
Wong, N.H., Wang, L, Aida, NC. 2005. Effects of double glazed façade on energy consumption, thermal comfort and condensation for a typical office building in Singapore, Journal on Energy and Buildings, 37, pp 563-572.
Wong, Yew Wah. 2000. Energy Audit for Buildings, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore.
Xu, X-l. and Yang, Z. 2008. Natural ventilation in double skin façade with venetian blind, Journal on Energy and Buildings, 40, pp 1498-1504.
Yeang, Ken. 1996. The Skyscraper Bio-climatically Considered: A design primer, Academy Editions, London.
Yeang, Ken. 2002. Reinventing The Skyscraper: A vertical theory of urban design, Wiley-Academy, London.
Yun, G. Y. and Steemers, K. 2008. Time-dependent occupant behaviour models of window control in summer, Building and Environment, 43, pp 1471-
1482.
Zhai, Z., Chen, Q., Haves, P. and Klems, J.H. 2002. On approaches to couple energy simulation and computational fluid dynamics programs, Building and
Environment, 37, pp 857-864.
Zhai, Z. and Chen, Q. 2003. Solution characters of iterative coupling between energy simulation and CFD programs, Journal on Energy and Buildings,
35(5), pp 493-505.
Zhai, Z. and Chen, Q. 2005. Performance of coupled building energy and CFD simulations, Journal on Energy and Buildings, 37(4), pp 333-344.
255
Zhai, Z. and Chen, Q. 2006. Sensitivity analysis and application guides for integrated building energy and CFD simulations, Journal on Energy and
Buildings, 38, pp 1060-1068.
Zhai, X.Q., Dai, Y.J. and Wang, R.Z. 2005. Comparison of heating and natural ventilation in a solar house induced by two roof solar collectors, Journal on
Applied Thermal Engineering, 25, pp 741-757.
Ziskind, G, Dubovsky, V. and Letan, R. 2002. Ventilation by convection of a one-storey building, Journal on Energy and Buildings, 34, pp 91-102.
Zollner, A., Winter, E.R.F. and Viskanta, R. 2002. Experimental studies of combined heat transfer in turbulent mixed convection fluid flows in double-skin façades, International Journal of Heat and Mass Transfer, 45, pp 4401-4408.
256
Appendix A Selected benchmarking simulation results.
General parameters Time variation = Steady Solar loading = 10am, 1 July. Diffuse solar intensity = 500 W/m2 Flow regime = Turbulent Two Equation Room size = 3.6m x 5m x 2.6m (46.8m3) Opening area = 1.08 m2 Vent area = 0.36 m2 DSF Glazing = 6mm (outer pane) and 6mm x 8mm x 6mm (inner pane)
Abbreviation used
Temp = Temperature Vel = Velocity RT = Radiant temperature RH = Relative humidity PMV = Predicted mean vote OT = Operative temperature TC = Thermal comfort Std Dev = Standard deviation N = Thermal comfort condition not acceptable Y = Thermal comfort condition acceptable
257
Simulation Cases Table
Curt ain Wall Opening (High Level)
Vent Size (Low Level)
Double Skin Façade – Width of air gap
Double Skin Façade – Inlet opening size
Double Skin Façade – Outlet opening size
Cases
Wind Velocity
Relative Humidity
Temperature
Benchmarking
0m/s 1.5m/s
60% 100%
22 0C-300C
3.6mx0.3m = 1.08m 2
2x(0.6mx0.3m) = 0.36m 2
-
-
-
3.6mx5mx2.6m
Double -Skin
0m/s 1.5m/s
60% 100%
22 0C-300C
3.6mx0.3m = 1.08m 2
2x(0.6mx0.3m) = 0.36m 2
0.3m
0.3mx3.6m = 1.08m 2
0.3mx3.6m = 1.08m 2
3.6mx5mx2.6m
The following selected simulation results (ss-1 to ss-27) are for double-skin system. If there is an ‘N’ indicated under the TC (thermal comfort) column means the indoor condition is not acceptable, where else if there is a ‘Y’ indicated that’s mean the indoor condition is within the acceptable limit according to the ASHREA Standard 55-2004.
ss-1
Temperature = 30oC Velocity = 0 m/s Solutions = Converged
P1 P2
Temp o C 31.45 31.38
Vel. m/s 0.05 0.05
RT o C 33.85 34.97
RH = 85%
RH % 77.32 77.59
PMV 2.55 2.68
OT o C 32.65 33.18
TC N N
Notes: PMV – Min=1.85715, Max=3, Mean=2.05961, Std Dev=0.165451
Temp at back wall = 30oC
258
Room Size
ss-2
Temperature = 30oC Velocity = 0.5 m/s Solutions = Converged
P1 P2
Temp o C 32.83 32.11
Vel. m/s 0.07 0.05
RT o C 34.48 35.62
RH = 85%
RH % 71.57 74.49
PMV 2.74 2.83
OT o C 33.66 33.86
TC N N
Notes: PMV – Min=1.80065, Max=3, Mean=2.0209, Std Dev=0.218831
Temp at back wall = 30oC
ss-3
Temperature = 30oC Velocity = 1.0m/s Solutions = Converged
P1 P2
Temp o C 31.78 32.17
Vel m/s 0.01 0.04
RT o C 32.72 33.41
RH = 85%
RH % 75.85 74.19
PMV 2.47 2.58
OT o C 32.25 32.79
TC N N
Notes: PMV – Min=1.76256, Max=3, Mean=1.90133, Std Dev=0.148305
Temp at back wall = 30oC
259
ss-4
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.61 31.49
Vel. m/s 0.04 0.02
RT o C 32.64 32.77
RH = 85%
RH % 76.61 77.13
PMV 2.44 2.45
OT o C 32.12 32.13
TC N N
Notes: PMV – Min=1.72711, Max=3, Mean=1.8748, Std Dev=0.149356
Temp at back wall = 30oC
ss-5
Temperature = 30oC Velocity = 1.0m/s Solutions = Not Converged
P1 P2
Temp o C 31.72 31.55
Vel. m/s 0.11 0.15
RT o C 33.58 33.05
RH = 85%
RH % 76.12 76.85
PMV 2.51 2.42
OT o C 32.65 32.30
TC N N
Notes: PMV – Min=1.72691, Max=3, Mean=1.87011, Std Dev=0.147357 Normal under-relaxation used
Temp at back wall = 30oC
260
ss-6
Temperature = 34oC Velocity = 0 m/s Solutions = Converged
P1 P2
Temp o C 34.18 33.79
Vel. m/s 0.10 0.04
RT o C 35.90 36.82
RH = 85%
RH % 70.10 71.15
PMV 2.95 3.00
OT o C 35.04 35.30
TC
OT o C 32.14 32.14
TC
N N
Notes: PMV – Min=2.09, Max=3, Mean=2.7278, Std Dev=0.26179
Temp at back wall = 30oC
ss-7
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.62 31.50
Vel. m/s 0.04 0.02
RT o C 32.65 32.79
RH = 60%
RH % 76.55 77.05
PMV 2.44 2.45
N N
Notes: PMV – Min=1.72706, Max=3, Mean=1.82436, Std Dev=0.11536
Temp at back wall = 30oC
261
ss-8
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.62 31.50
Vel. m/s 0.04 0.02
RT o C 32.66 32.79
RH = 65%
RH % 76.53 77.06
PMV 2.44 2.45
OT o C 32.14 32.14
TC N N
Notes: PMV – Min=1.72698, Max=3, Mean=1.83482, Std Dev=0.120114
Temp at back wall = 30oC
ss-9
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.62 31.50
Vel. m/s 0.04 0.02
RT o C 32.66 32.79
RH = 70%
RH % 76.54 77.07
PMV 2.44 2.45
OT o C 32.14 32.14
TC N N
Notes: PMV – Min=1.727, Max=3, Mean=1.84533, Std Dev=0.126258
Temp at back wall = 30oC
262
ss-10
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.62 31.50
Vel. m/s 0.04 0.02
RT o C 32.66 32.79
RH = 75%
RH % 76.55 77.09
PMV 2.44 2.45
OT o C 32.14 32.14
TC N N
Notes: PMV – Min=1.727, Max=3, Mean=1.85584, Std Dev=0.133617
Temp at back wall = 30oC
ss-11
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.59 31.47
Vel. m/s 0.04 0.02
RT o C 32.63 32.76
RH = 100%
RH % 76.66 77.19
PMV 2.44 2.44
OT o C 32.11 32.12
TC N N
Notes: PMV – Min=1.72689, Max=3, Mean=1.90866, Std Dev=0.183096
Temp at back wall = 30oC
263
ss-12
Temperature = 34oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 34.69 34.54
Vel. m/s 0.01 0.02
RT o C 35.17 35.01
RH = 100%
RH % 80.80 81.46
PMV 3.00 3.00
OT o C 34.96 34.78
TC N N
Notes: PMV – Min=2.2445, Max=3, Mean=2.69621, Std Dev=0.318578
Temp at back wall = 30oC
ss-13
Temperature = 22oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 24.84 24.19
Vel. m/s 0.10 0.18
RT o C 28.14 28.16
RH = 60%
RH % 70.76 73.56
PMV 1.06 0.85
OT o C 26.49 26.18
TC Y Y
Notes: PMV – Min=-0.656184, Max=3, Mean=0.360306, Std Dev=1.06217 No solar loading – night time
Temp at back wall = 30oC
264
ss-14
Temperature = 22oC Velocity = 0 m/s Solutions = Not Converged
P1 P2
Temp o C 30.61 30.54
Vel. m/s 0.08 0.08
RT o C 32.93 32.81
RH = 100%
RH % 81.12 81.41
PMV 2.36 2.33
OT o C 31.77 31.68
TC N N
Notes: PMV – Min=-0.541333, Max=3, Mean=0.683009, Std Dev=1.08609 No solar loading – night time
Temp at back wall = 30oC
ss-15
Temperature = 24oC Velocity = 0 m/s Solutions = Not Converged
P1 P2
Temp o C 30.59 30.40
Vel. m/s 0.08 0.06
RT o C 33.16 33.06
RH = 100%
RH % 81.19 82.04
PMV 2.38 2.37
OT o C 31.84 31.73
TC N N
Notes: PMV – Min=0.0379054, Max=3, Mean=1.05121, Std Dev=0.90066 Temp at back wall = 30oC
265
ss-16
Temperature = 22oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 24.94 24.36
Vel. m/s 0.09 0.19
RT o C 27.99 28.48
RH = 100%
RH % 70.33 72.82
PMV 1.07 0.89
OT o C 26.46 26.42
TC Y Y
Notes: PMV – Min=0.660857, Max=3, Mean=0.413299, Std Dev=1.12843 Temp at back wall = 30oC
ss-17
Temperature = 24oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 26.78 26.25
Vel. m/s 0.08 0.08
RT o C 29.24 29.42
RH = 100%
RH % 71.18 73.44
PMV 1.44 1.42
OT o C 28.01 27.84
TC Y Y
Notes: PMV – Min=-0.0686469, Max=3, Mean=0.777555, Std Dev=0.888892 Temp at back wall = 30oC
266
ss-18
Temperature = 26oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 28.27 28.17
Vel. m/s 0.07 0.03
RT o C 30.18 30.54
RH = 100%
RH % 73.48 73.93
PMV 1.75 1.78
OT o C 29.22 29.36
TC Y Y
Notes: PMV – Min=0.526193, Max=3, Mean=1.1483, Std Dev=0.651454 Temp at back wall = 30oC
ss-19
Temperature = 28oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 29.94 29.89
Vel. m/s 0.04 0.03
RT o C 31.29 31.63
RH = 100%
RH % 75.05 75.26
PMV 2.09 2.12
OT o C 30.62 30.76
TC N N
Notes: PMV – Min=1.02475, Max=3, Mean=1.5258, Std Dev=0.414738 Temp at back wall = 30oC
267
ss-20
Temperature = 30oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 31.15 31.00
Vel. m/s 0.05 0.01
RT o C 31.88 31.86
RH = 100%
RH % 78.60 79.27
PMV 2.31 2.30
OT o C 31.52 31.43
TC N N
Notes: PMV – Min=1.02475, Max=3, Mean=1.5258, Std Dev=0.414738 Temp at back wall = 28oC
ss-21
Temperature = 30oC Velocity = 0 m/s Solutions = Not Converged
P1 P2
Temp o C 30.85 30.73
Vel. m/s 0.05 0.03
RT o C 32.79 34.11
RH = 100%
RH % 80.02 80.53
PMV 2.38 2.53
OT o C 31.82 32.42
TC N N
Notes: PMV – Min=1.69504, Max=3, Mean=1.89521, Std Dev=0.132062 Temp at back wall = 28oC
268
ss-22
Temperature = 30oC Velocity = 0.5 m/s Solutions = Converged
P1 P2
Temp o C 31.97 31.97
Vel. m/s 0.05 0.05
RT o C 32.77 32.95
RH = 100%
RH % 75.03 75.04
PMV 2.48 2.50
OT o C 32.37 32.46
TC N N
Notes: PMV – Min=1.6891, Max=3, Mean=1.86642, Std Dev=0.105374 Temp at back wall = 28oC
ss-23
Temperature = 30oC Velocity = 1.0 m/s Solutions = Converged
P1 P2
Temp o C 31.47 31.47
Vel. m/s 0.03 0.04
RT o C 31.98 32.36
RH = 100%
RH % 77.18 77.18
PMV 2.36 2.39
OT o C 31.72 31.92
TC N N
Notes: PMV – Min=1.66715, Max=3, Mean=1.78357, Std Dev=0.105544 Temp at back wall = 28oC
269
ss-24
Temperature = 28oC Velocity = 0 m/s Solutions = Not Converged
P1 P2
Temp o C 30.34 30.42
Vel. m/s 0.08 0.11
RT o C 31.33 32.04
RH = 100%
RH % 82.15 81.92
PMV 2.15 2.21
OT o C 30.84 31.23
TC N N
Notes: PMV – Min=1.22895, Max=3, Mean=1.40816, Std Dev=0.171451 Temp at back wall = 26oC
ss-25
Temperature = 28oC Velocity = 0.5 m/s Solutions = Converged
P1 P2
Temp o C 30.47 30.36
Vel. m/s 0.06 0.06
RT o C 30.95 31.25
RH = 100%
RH % 79.33 80.65
PMV 2.12 2.16
OT o C 30.71 30.80
TC N N
Notes: PMV – Min=1.24848, Max=3, Mean=1.36056, Std Dev=0.143109 Temp at back wall = 26oC
270
ss-26
Temperature = 28oC Velocity = 1 m/s Solutions = Converged
P1 P2
Temp o C 29.46 29.46
Vel. m/s 0.03 0.04
RT o C 29.96 30.33
RH = 100%
RH % 77.15 77.14
PMV 1.89 1.93
OT o C 29.71 29.90
TC N N
Notes: PMV – Min=1.13598, Max=3, Mean=1.25979, Std Dev=0.118216 Temp at back wall = 26oC
ss-27
Temperature = 28oC Velocity = 1.5 m/s Solutions = Converged
P1 P2
Temp o C 29.26 29.01
Vel. m/s 0.05 0.01
RT o C 29.77 29.86
RH = 100%
RH % 78.04 79.19
PMV 1.85 1.84
OT o C 29.52 29.44
TC N N
Notes: PMV – Min=1.06875, Max=3, Mean=1.22331, Std Dev=0.112814 Temp at back wall = 26oC
271
Appendix B Selected optimization simulation results for different shaft heights.
Table B-1 Boundary conditions and thermal comfort parameters 272
Table B-2 Simulation results for locations P1 and P2
Table B-1 and Table B-2 above show some of the selected simulation results for different shaft heights. The shaded areas represent acceptable indoor thermal comfort conditions for that particular DSF system configuration, which the OT (Operative Temperature) is within the acceptable ranges for naturally conditioned spaces according to ASHRAE Standard 55-2004. 273
Graphs B-1 Thermal comfort profiles for simulation results dsf-1n
Graphs B-1 above shows the thermal comfort parameters’ profiles for dsf-1n simulation with Operative Temperatures for Levels 1, 3 and 5 which are within the acceptable ranges of indoor thermal comfort conditions, except for Level 5.
274
Graphs B-2 Thermal comfort profiles for simulation results dsf-1q
Graphs B-2 above shows the thermal comfort parameters’ profiles for dsf-1q simulation with Operative Temperatures for Levels 1, 3 and 5 which are all within the acceptable ranges of indoor thermal comfort conditions.
275
Appendix C Selected referred papers submitted to International Conferences and International Journals.
A)
Energy efficiency in double-skin facade design for high-rise buildings of glass-metal facade systems in the tropics - published in the proceedings of the 1st International Tropical Architecture (iNTA), organised by the National University of Singapore, Singapore, February 2004.
Energy Efficiency In Double-Skin Façade Design For High-Rise Buildings Of Glass-Metal Façade Systems In The Tropics Pow Chew WONG1, Deo PRASAD2 and Masud BEHNIA3 RC 1019 Postgraduate Research Centre, Faculty of Built Environment, University of New South Wales, Australia 2 Centre for Sustainable Built Environment (Solarch), Faculty of Built Environment, University of New South Wales, Australia 3 The University of Sydney, Australia 1
Keywords:
Tropical, high-rise office buildings, double-skin facade, natural ventilation, heat transfer, computational fluid dynamic simulation, sustainability
Abstract:
The natural resources of the world have been reduced tremendously for the past half a century since the close of the WWII. The energy used and spent in the modern world has been escalating in an alarming way. The call for energy efficient building design is increasing and the situation is even more critical for designing high-rise buildings because the energy consumed by those building type is constituted for the major part of all the energy used in building industry. The viability of double-skin façade is studied to provide natural ventilation as an energy efficient solution for the high-rise office buildings in a hot and humid environment. The behaviour of airflow and thermal transfer through the double-skin façade and the internal thermal comfort are analysed through the use of computational fluid dynamic simulations.
1
INTRODUCTION
Double-skin façade is not a new concept as it started centuries ago and the first double-skin curtain wall appears in 1903, in the Steiff Factory in Giengen, Germany. (Internet page of BuildingEnvelopes.org, History of Double-skin Facades) Until recently the use of double-skin facades had became more popular in many high-rise buildings in Europe and most recently the technology has been demonstrated in the Armoury Tower in Shanghai, China. (Yeang 1996) Double-skin facades are multiple layer skins construction with an external skin, an intermediate space and an inner skin. The external and internal skins could be of either single glaze or double glazed glass panes of float glass or safety glass. An adjustable sun-shading device is usually installed at the intermediate space for thermal controls. Types of double-skin constructions include Box Window façade, Shaft-box façade, Corridor façade and Multi-story
276
façade. (Oesterle et al. 2001) The performance of the double-skin façade depends closely on the chosen ventilation means within its intermediate space. The modes of ventilation could be natural (buoyancy driven), forced (mechanically driven) or mixed (both natural and forced). Since the temperature difference between outside air and the heated air within the intermediate space must be significant enough for the natural ventilation wall to work, this façade system is not suitable to be used in the hot climates. Both the forced (e.g. active wall) and mixed (e.g. interactive wall) systems could be used in the hot climate condition but the latter has the advantage of introducing natural ventilation even for the high-rise buildings. The issue of thermal transfer through an active wall like the double-skin façade is a complex one. The heat transfer occurred simultaneously for all the component layers of the double-skin façade with the influence from the surrounding environmental conditions, the properties of the layers of the façade and the ventilation system introduced to the double-skin façade. The overheating of the air gap between the double-skin façade is more evidence during high ambient temperature and it could be reduced by manipulating the openings of the glazing façade, a well positioned shading device and the optimisation of the width of the air gap between the glazing panes. (Oesterle et al. 2001) Figure 1 below shows an example of heat transfer through double-skin façade.
Outside Inside
Figure 1 Heat Transfer Through Double-Skin Façade
A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin facades. Even though most of the researches are done mainly in temperate countries conditions but they have revealed close link between natural ventilation design and the function of double-skin façade. Grabe et al. (Grabe et al. 2001) developed a simulation algorithm to investigate the temperature behaviour and the flow characteristics of double facades with natural convection through solar radiation. Similar works on natural convection ventilation also reported by Ziskind et al. (Ziskind et al. 2002, 2003), Bansal et al. (Bansal et al. 1994), Hamdy and Fikry (Hamdy and Fikry 1998), and Priyadarsini et al. (Priyadarsini et al. 2003). Most of them are using the idea of stack effect or the solar chimney concept and found that passive ventilation in summer is possible even for multi-storey buildings. In particular Priyadarsini et al. (Priyadarsini et al. 2003) have concluded the energy efficiency of stack system used in residential of a hot and humid climate region. Yuguo Li and Delsante (Li and Delsante 2001) went a step further to investigate the effects of natural ventilation caused by wind and thermal forces in a single zone building with two openings. Ventilation graphs are plotted using the air change parameters (thermal air change, wind air change and the heat loss air change) for design purposes. Gratia and Herde (Gratia and Herde 2004) attempted to look at the impact of double-skin façade facing southern direction in a temperate climatic condition. Thermal analysis using simulation software of different seasons of a year was done for a low-rise office building with and without double-skin façade. It was found that significant energy saving is possible if natural ventilation could be exploited through the use of double-skin façade. This paper attempts to bridge the gap of looking into the possibilities of natural ventilation in high-rise office buildings specifically in the hot and humid climate region with the use of double-skin façade. The unique façade construction is thought to be able to act as a stack in providing required ventilation for the internal space. It is the intent of the research to analyse
277
the airflow patterns induced by the wind & thermal forces through the double-skin façade into the interior office space and their effects onto the thermal comfort within the space. Computer simulation is used to analyse the results obtained through the different opening sizes of the glazing and the size of the air gap of the double-skin façade with variation of vent sizes to generate an acceptable cross ventilation rate within the office space.
2
METHOD
2.1
Computational Fluid Dynamic Simulation
Computational Fluid Dynamic (CFD) has become a useful tool for designers in the study of indoor and outdoor environment conditions in building designs. The parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment. CFD technique has been applied with considerable success in building design and the advantages in analysing ventilation performance have been reported by Murakami (Murakami 1992) and Liddament (Liddament 1992). Papakonstantinou et al. (Papakonstantinou et al. 2000) has demonstrated that numerical solutions for ventilation problems can be obtained quickly and in good agreement with the experimental measurements.
2.2
The Airpak CFD Software
Airpak is an easy-to-use design tool for the design and analysis of ventilation systems which are required to provide acceptable thermal comfort and indoor air quality solutions. It is a virtual prototyping software that allows for accurate modelling of airflow, heat transfer, contaminant transport and thermal comfort. Computer models could be easily built and tested for variety of design options to find the best solution. Full colour animations, pictures and plots help to effectively analyse the results obtained. Airpak uses object-based model building and libraries coupled with automatic unstructured meshing that enables complex models building. It uses the FLUENT CFD solver engine for thermal and fluid-flow calculations which provides robust and quick calculations. It postprocessing features also allow fast and comprehensive results for the ventilation problems at hand. (Airpak User’s Manual) In view of the capabilities and good interface of the Airpak software, it is selected to be used in this research to model the complex energy transfer through the component layers of the multiplayer façade through the optimisation of the appropriate opening sizes on the glazing, the width of the intermediate space and the ventilation rate through the internal office space.
2.3
The Model
The final goal of this research is to look into the possibilities of natural ventilation in a highrise office building in a hot and humid climate condition using double-skin façade. In order to realise this complex problem, several stages of different levels of complexity modelling are introduced. Airflow effects induced by wind and thermal forces onto a single storey office model constructed are to be observed for the first stage before a complex multi-storey office with all the thermal comfort parameters included are to be analysed. Therefore it is the intention of this paper to report on the findings of the first stage of the problem at hand. The single storey office module in 3D is constructed in Airpak and the geometrical dimensions of the office are shown in Figs. 2 and 3. The simplified double-skin façade of the model has openings on each of the external and internal panes. Heat sources of two computers, four ceiling lights and two persons are introduced in the office space for future thermal comfort analysis. The office space also has two vents at the rear wall to introduce cross ventilation to the space. For the first stage of the analysis which this paper is going to report on the findings, combination of different opening sizes and its locations of the openings together with the different sizes of the vents are looked at and their effects onto the airflow patterns within the double-skin and the internal office space are observed and analysed.
278
7000
3500
H3
H1 350
W
EXTERNAL
OFFICE
V1
WIND DIRECTION
350 H2
3500
V
Figure 2 Section Through The Model (With External Space)
3500
H3
H1
3500
V1
H2
V2
Figure 3 Rear Elevation Of The Model The simulations are performed under steady state condition using k-epsilon equation turbulent model. The simulated wind speeds of 1.5m/s and 3.0m/s are used to model expected ground level wind velocities with ambient temperature of 30 degree C. The external temperature at the rear wall is set at 23 degree C to simulate an internal air-conditioning space like internal corridor. Only wind direction which perpendicular to the double-skin façade has been looked at. The upwind distance from the outer pane of the double-skin façade is set at 7m to simulate half the distance between office buildings at the city centre. The results of the airflow velocities, temperatures and the airflow patterns are recorded and observed with different combinations of glass opening sizes of the double-skin façade and the vents in Tables 1 to 3.
3
SIMULATION RESULTS
There are total 20 simulations generated with different combinations of wind velocities (V), glass opening sizes (H1, H2, H3), width of air gaps (W) and vent sizes (V1 & V2). The variables of the parameters are indicated in Figs 2 & 3. The difference between the simulations generated in Table 1 and Table 2 is the vents’ area has increased 100% for the models in Table 2. Simulations 15 and 16 are generated with a narrower air gap of 300mm to investigate if there is any influence to the indoor comfort level. Simulations 17 to 20 are computed with two openings at the outer pane of the double-skin façade with 300mm air gap. P2 P1 P3 P6 P4 P7
P5
Figure 4 Location Points For Taking The Simulation Results (Section of Model)
279
Table 1 Simulation Results - A Simulation
1
2
3
4
5
6
V (m/s)
1.5
1.5
1.5
3.0
3.0
3.0
H1 (mm)
0
0
300
0
0
300
H2 (mm)
200
300
0
200
300
0
H3 (mm)
200
300
300
200
300
300
W (mm)
450
450
450
450
450
450
V1 (mm)
300
300
300
300
300
300
V2 (mm)
600
600
600
600
600
600
P1 (Vel, m/s)
0.83
0.86
0.97
1.55
1.63
1.88
(Temp, C)
30
30
30.02
30
30
29.99
P2 (Vel, m/s)
0.25
0.41
0.51
0.39
0.71
0.97
(Temp, C)
30.12
30.08
30.05
30.02
30.02
29.97
P3 (Vel, m/s)
0.12
0.22
0.08
0.21
0.40
0.15
(Temp, C)
30.58
30.40
30.37
30.10
30.09
29.69
P4 (Vel, m/s)
0.05
0.05
0.05
0.01
0.01
0.02
(Temp, C)
31.70
31.74
31.11
30.75
30.86
30.13
P5 (Vel, m/s)
0.11
0.11
0.17
0.03
0.03
0.27
(Temp, C)
24.19
24.21
23.82
24.08
24.16
23.66
P6 (Vel, m/s)
0.22
0.40
0.11
0.40
0.75
0.22
(Temp, C)
30
30
30.12
30
30
29.92
Table 2 Simulation Results - B Simulation
7
8
9
10
11
12
13
14
V (m/s)
1.5
1.5
1.5
1.5
3.0
3.0
3.0
3.0
H1 (mm)
0
200
0
300
0
200
0
300
H2 (mm)
200
0
300
0
200
0
300
0
H3 (mm)
200
200
300
300
200
200
300
300
W (mm)
450
450
450
450
450
450
450
450
V1 (mm)
300
300
300
300
300
300
300
300
V2 (mm)
1200
1200
1200
1200
1200
1200
1200
1200
P1 (Vel, m/s)
0.86
0.94
0.90
0.97
1.59
1.83
1.68
1.88
(Temp, C)
30
30
30
30
30
29.90
30
29.96
P2 (Vel, m/s)
0.24
0.39
0.40
0.54
0.39
0.75
0.71
1.05
(Temp, C)
30.03
29.97
30.03
30.01
29.99
29.72
30
29.90
P3 (Vel, m/s)
0.14
0.08
0.24
0.07
0.25
0.18
0.43
0.15
(Temp, C)
30.21
29.81
30.14
30.01
29.92
28.70
29.97
29.22
P4 (Vel, m/s)
0.05
0.05
0.05
0.05
0.01
0.01
0.01
0.02
(Temp, C)
31.17
30.50
31.19
30.66
30.33
29.08
30.46
29.48
P5 (Vel, m/s)
0.04
0.09
0.04
0.07
0.02
0.19
0.03
0.13
(Temp, C)
24.36
23.88
24.36
23.95
25.15
23.55
26.06
23.68
P6 (Vel, m/s)
0.20
0.05
0.36
0.12
0.36
0.07
0.68
0.22
(Temp, C)
30
29.96
30
30.02
30
29.59
30
29.80
280
Table 3 Simulation Results - C Simulation
15
16
17
18
19
20
V (m/s)
1.5
1.5
1.5
1.5
3.0
3.0
H1 (mm)
0
200
200
300
200
300
H2 (mm)
200
0
200
300
200
300
H3 (mm)
200
200
200
300
200
300
W (mm)
300
300
300
300
300
300
V1 (mm)
300
300
300
300
300
300
V2 (mm)
600
600
600
600
600
600
P1 (Vel, m/s)
0.78
0.48
0.90
0.93
1.77
1.83
(Temp, C)
30
30
30
30
30
30
P2 (Vel, m/s)
0.36
0.47
0.74
0.67
1.40
1.28
(Temp, C)
30.04
30
30
30
30
30
P3 (Vel, m/s)
0.21
0.08
0.73
0.90
1.35
1.74
(Temp, C)
30.27
29.96
30.07
30.03
30.01
30.01
P4 (Vel, m/s)
0.05
0.05
0.20
0.29
0.41
0.54
(Temp, C)
31.35
30.63
30.77
30.22
30.18
30.08
P5 (Vel, m/s)
0.07
0.14
0.06
0.06
0.01
0.01
(Temp, C)
24.32
23.87
32.48
32.14
31.35
31.18
P6 (Vel, m/s)
0.34
0.07
0.06
0.29
0.29
0.71
(Temp, C)
30
30
29.97
30.86
30.24
30.18
P7 (Vel, m/s)
-
-
0.31
0.42
0.58
0.80
(Temp, C)
-
-
30
30
30
30
4
DISCUSSIONS
It was observed that by just changing the glass opening sizes of the double-skin façade with similar external wind velocity would not contribute much to the indoor thermal quality of the office. This could due to the indoor airflow velocities are almost similar for each case. (e.g. Simulations 7 & 9) The locations of the glass opening on the outer pane of the double-skin façade will have effect onto the indoor thermal and airflow velocity. It was found that the higher the opening is located from the floor level it will generate a stronger stack effect within the air gap which in turn will pull more air out from the office space through the vents at the rear wall. The temperature generated within the office space is much desirable and closer to human comfort requirement. The airflow pattern created will be a good cross ventilation effect with cool air coming into the office space from the vents and right across and above the internal space and discharge out through the high level opening at the inner pane. (e.g. Simulations 7-14) A narrower air gap between the double-skin façade construction will provide a more desirable indoor thermal level as it generates stronger stack effect which pull more air out from the internal office space. (e.g. Simulations 1 and 15) There is not much of an advantage to provide larger vents at the rear of the space in order to provide cross ventilation to the internal space. The resultant air movement and temperature of the internal space are not much better as compare to smaller vent sizes. This might give a slightly better condition if the external wind velocity is stronger but it will not be able to justify the cost in providing a larger vent opening. It might also not be feasible for some construction constrains with big vents. (e.g. Tables 1 & 2) Simulations 17-20 shown that the indoor airflow velocities are the most desirable with 2
281
openings on the outer pane of the double-skin façade. The indoor temperatures are also lower as compared to only one opening at the outer pane. The internal airflow pattern is different from the outer pane with 1 opening on the façade. The warm air from the air gap is passing through the opening of the inner pane right across the office space and exit through the rear vents. This will have an undesirable mixing of warm air to the internal cool air at the rear of the office.
5
CONCLUSIONS
Thermal sensation plays a major role in the perception of comfort and the comfort parameters are highly subjective. Some of such parameters are air temperature, the relative humidity of the air, the local air velocity and human activity. A comprehensive explanation of thermal comfort is listed in Chapter 8 of ASHRAE Fundamentals. (ASHRAE Fundamentals 1993) Air movements determine the convective heat and mass exchange of the human body with the surrounding air. In hot and humid climate, high air velocities will increase the evaporation rate at the skin surface and results in cooling sensation. The recommended upper limit of indoor air movement is usually 0.8m/s for human comfort and such air velocity permits the interior space to be 1-2 degree higher than the human comfort temperature to maintain desirable comfort level. (Hien and Tanamas 2002) This paper has found that a high level single opening at the outer pane of the double-skin façade will create a desirable cross ventilation airflow pattern at the internal office space. The cross ventilation effect will bring the cool air from the internal air-conditioned space across the internal space and prevent the warm air from the air gap entering the office space. The internal temperatures are still considered a bit high (as the model constructed for this paper is only considering a one storey space) but the situation will be expected to improve when multi-storey spaces are linked together in a high-rise building when the stack effect of the air gap will increase tremendously. The other option could be using wind turbine to increase the air velocity at the air gap to give effective airflow speed within the internal space. This will be looked at and analysed further in the coming development of the research. The design of two openings (one at high and one at low levels) on the outer pane of the façade by far is the best solution as found by this paper. The internal air velocities between 0.2-0.5 m/s observed could provide more than 80% of human satisfaction for comfort, as shown in Figure 5. Due care should be given to deal with the warm air coming into the internal space through the air gap especially for higher wind velocity experienced at the high level of a high-rise building. The testing and finding of more and better solutions are not of the scope of this paper but it is the goal of the research to find out the viability of double-skin façade in providing natural ventilation as an energy efficient solution for the high-rise office buildings in a hot and humid environment.
Figure 5 Ventilation Comfort Chart of Singapore
282
REFERENCES Airpak User’s Manual. 2002. Fluent Inc. and ICEM-CFD Engineering. ASHRAE Fundamentals. 1993. American Society of Heating, Refrigeration and Air Conditioning, Atlanta. Bansal NK, Mathur R, and Bhandari MS. 1994. A Study Of Solar Chimney Assisted Wind Tower System For Natural Ventilation In Buildings. Building and Environment, 29(4):495-500. Grabe J, Lorenz R, and Croxford, B. 2001. Ventilation Of Double Facades. Building Simulation, 229-236. Gratia E and Herde A. 2004. Optimal Operation Of A South Double-Skin Façade. Energy and Buildings, 36:41-60. Hamdy IF and Fikry MA. 1998. Passive Solar Ventilation. Renewable Energy, 14(1-4):381386. Hien WN and Tanamas J. 2002. The Effect Of Wind On Thermal Comfort In The Tropical Environment. Proceedings of the International Symposium on Building Research and the Sustainability of Built Environment in the Tropics, Jakarta, Indonesia. Internet page of BuildingEnvelopes.org, History of Double-skin Facades, http://envelopes.cdi.harvard.edu/envelopes/web_pages/home/home.cfm Letan R, Dubovsky V, and Ziskind G. 2003. Passive Ventilation and Heating By Natural Convection In A Multi-Storey Building. Building and Environment, 38:197-208. Li Y and Delsante A. 2001. Natural Ventilation Induced By Combined Wind and Thermal Forces. Building and Environment, 36:59-71. Liddament MW 1992. The Role and Application Of Ventilation Effectiveness In Design. Proceedings of International Symposium on Room Air Convection and Ventilation Effectiveness, University of Tokyo, 59-75. Murakami S. 1992. New Scales For Ventilation Efficiency and Their Application Based On Numerical Simulation Of Room Airflow. Proceedings of International Symposium on Room Air Convection and Ventilation Effectiveness, University of Tokyo, 22-38. Oesterle, Lieb, Lutz, and Heusler. 2001. Double-Skin Facades – Integrated Planning. Germany: Prestel Verlag. Papakonstantinou KA, Kiranoudis CT, and Markatos NC. 2000. Numerical Simulation Of Airflow Field In Single-Sided Ventilated Buildings. Energy and Buildings, 33:41-48. Priyadarsini R, Cheong KW, and Wong NH. 2004. Enhancement Of Natural Ventilation In High-Rise Buildings Using Stack System. Energy and Buildings, 36(1):61-71. Yeang, Ken. 1996. The Skyscraper, Bio-climatically considered; A design primer. London: Academy Editions. Ziskind G, Dubovsky V, and Letan R. 2002. Ventilation By Convection Of A One-Storey Building. Energy and Buildings, 34:91-102.
283
B)
Methodology for natural ventilation design for high-rise buildings in hot and humid climate - published in the proceedings of The 2005 World Sustainable Building Conference (SB05) in Tokyo, Japan, September 2005.
METHODOLOGY FOR NATURAL VENTILATION DESIGN FOR HIGH-RISE BUILDINGS IN HOT AND HUMID CLIMATE P C Wong1 D Prasad2 M Behnia3 1
Faculty of Built Environment, University of New South Wales, Australia,
[email protected] 2 Centre for Sustainable Built Environment (CSBE), Faculty of Built Environment, University of New South Wales, Australia 3 The University of Sydney, Australia
Keywords: thermal comfort, double-skin façade, computational fluid dynamic, natural ventilation
Summary The research attempts to look into the viability of double-skin façade in providing natural ventilation for the high-rise office buildings in hot and humid environment. The behaviour of airflow patterns induced by wind and thermal forces through the double-skin façade into the interior office space and their effects onto the thermal comfort within the space are analysed with the use of computational fluid dynamic simulations and to identify the possible window periods for natural ventilation to be introduced to the office space.
1.
Introduction
Extensive research has been carried in defining what is thermal comfort and the parameters that affecting it. All those findings had confirmed the importance of human factors and human influence towards the creation of a thermally comfortable indoor environment (Fanger 1970 and Ruck 1989). In more recent experimental studies concerning the effects of some human factors on the comfort conditions in particular geographical location, Dear, Leow and Foo (1991) found that people working in naturally ventilated buildings in hot and humid country could accept a temperature value of up to 30C warmer than Fanger’s values. This with other similar findings especially the newly published ASHRAE standard 55-2004 (2004) have given the opportunity in 284
introducing natural ventilation for commercial buildings in the hot and humid region.
1.1
Double-skin Façade and Thermal Comfort
A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin facades. Even though most of the researches are done mainly in temperate countries conditions but they have revealed close link between natural ventilation design and the function of double-skin façade. Grabe et al. (2001) developed a simulation algorithm to investigate the temperature behaviour and the flow characteristics of double facades with natural convection through solar radiation. Similar works on natural convection ventilation also reported by Ziskind et al. (2002, 2003), Bansal et al. (1994), Hamdy and Fikry (1998), and Priyadarsini et al. (2003). Most of them are using the idea of stack effect or the solar chimney concept and found that passive ventilation in summer is possible even for multi-storey buildings. In particular Priyadarsini et al. (2003) have concluded the energy efficiency of stack system used in residential of a hot and humid climate region. Li Y and Delsante (2001) went a step further to investigate the effects of natural ventilation caused by wind and thermal forces in a single zone building with two openings. Ventilation graphs are plotted using the air change parameters (thermal air change, wind air change and the heat loss air change) for design purposes. Gratia and Herde (2004) attempted to look at the impact of double-skin façade facing southern direction in a temperate climatic condition. Thermal analysis using simulation software of different seasons of a year was done for a low-rise office building with and without double-skin façade. It was found that significant energy saving is possible if natural ventilation could be exploited through the use of double-skin façade. This paper attempts to bridge the gap of looking into the possibilities of natural ventilation in high-rise office buildings specifically in the hot and humid climate region with the use of double-skin façade. The unique façade construction is thought to be able to act as a stack in providing required ventilation for the thermal comfort of the internal space. Airflow effects induced by wind and thermal forces onto a single office module constructed are to be observed for the first stage before a complex multi-storey office with all the thermal comfort parameters included are to be analysed. Therefore it is the intention of this paper to report on the findings of the first stage of the problem at hand.
2.
Methodology
2.1
Computational Fluid Dynamic Simulation
Computational Fluid Dynamic (CFD) has become a useful tool for designers in the study of indoor and outdoor environment conditions in building designs. 285
The parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment. CFD technique has been applied with considerable success in building design and the advantages in analysing ventilation performance have been reported by Murakami (1992). Papakonstantinou et al. (2000) has demonstrated that numerical solutions for ventilation problems can be obtained quickly and in good agreement with the experimental measurements.
2.2
Validation of the CFD Software
A virtual prototyping simulation software called Airpak (2002) is used in this research to model the complex energy transfer through the component layers of the multiplayer façade through the optimisation of the appropriate opening sizes on the glazing, the width of the intermediate space and the ventilation rate through the internal office space. The validation of the software has been carried out by comparing the experimental and simulation results from another commercial simulation software called FloVent which was carried out by Manz H (2003). The measured hourly outdoor air temperature shown in Figure 1 are used for piecewise linear interpolation for the transient simulations. The simulation model for the validation is shown in Figure 2 and one of the comparison results are shown in Figure 3 below. Series 1 are the measured surface temperatures for the inner pane in the experimental results and Series 2 are the simulation results from Airpak. Both of the results are compared and analyzed and it was found that the variation is within 5% of the acceptable error tolerance.
40
Air Temp (C)
35 30 25 20
Outdoor Temp
15 10 5 0 1
4
7
10 13 16 19 22 25 Tim e (h)
Figure 1 Measured hourly outdoor air temperature
Figure 2 Simulation model for the validation 286
Inner Pane Surface Temp (C)
30 25 20 Series1
15
Series2
10 5 0 1
3
5
7
9
11 13 15 17 19 21 23 Time (h)
Figure 3 Comparison between the measured and CFD results
2.3
The CFD Models
The single office module in 3D is constructed with the geometrical dimensions of 3.5m x 5.0m x 2.6m height (Figure 4). Numerous of simulation runs have been carried out for the benchmarking purposes in which a typical curtain walling office module was observed and a simplified ‘nomogram’ has been established to define the initial parameters for thermal comfort in the tropic region. These results are compared with the simulation runs from the office module with double-skin façade construction. The simplified double-skin façade of the office module has openings on each of the external and internal panes with 6mm thick glass used at the external pane and 6/12/6mm double glazed used for the inner pane (Figure 5). Internal heat sources of two computers, four ceiling lights and two persons are introduced in the office space for thermal comfort analysis. The office module has two vents at the rear wall to introduce cross ventilation from the internal a/c space across the internal office space.
287
Figure 4 Standard curtain walling for office module
Figure 5 Double-skin façade for office module 288
2.3
The CFD Simulation
In view of the complexity of the problem at hand, the modeling of the computer model has been broken down into different ‘levels’. The initial simulation was concentrated onto a single office space within a high-rise office building. The commercial office spaces could be grouped under three different sizes, namely small (~20m2), medium (~50m2) and large (>100m2). This paper is focusing on the first office group, which is the small office space. The simulations are performed under steady state condition using k-epsilon turbulent model. The simulated wind speeds of 0m/s to 3.0m/s are used to model expected ground level wind velocities with ambient temperature of 30 0 C and relative humidity of 60% to 100%. The external temperature at the rear wall is set at 30 0C to simulate a corridor area open to the external space. Only wind direction which perpendicular to the double-skin façade has been looked at for this stage. The upwind distance from the outer pane of the double-skin façade is set at 3 times the length of the office module.
3.
Discussion
3.1
The Analysis of the Simulation Results
For the first stage of the analysis which this paper is going to report on, all simulations, be it the benchmarking cases or the double-skin scenarios, generated a cross ventilation effects from the internal naturally ventilated space across the office and discharged out through the internal pane opening into the intermediate space of the double façade. The strength of the cross ventilation will mainly depends on the airflow resistances within the intermediate space and the internal office space, together with the pressure differences between them. The magnitude of the internal ventilation will depend on the summation of the airflow resistances and in turn control by the smallest cross section area of the opening within the space. The locations of the glass openings on the outer pane of the double-skin façade in relation to the inner pane will have effect onto the indoor thermal and airflow velocity. It was found that the higher the opening is located from the floor level it will generate a stronger stack effect within the air gap which in turn will pull more air out from the office space through the vents at the rear wall. The temperature generated within the office space is much desirable and closer to human comfort requirement. The airflow pattern created will be a good cross ventilation effect with cool air coming into the office space from the vents and right across and above the internal space and discharged out through the high level opening at the inner pane. This has lead to the selection of specific type of the double-skin construction, namely the Multi-storey Façade, which will create the strongest stack effect to pull maximum amount of air from the internal office space (Oesterle 2001).
289
3.2
Formulation of the Nomogram
The results obtained from the benchmarking simulations, which is a typical curtain walling system façade, are compared to the results from the proposed prototype double-skin façade. The nomogram is formed by three axis which represent the three important parameters in thermal comfort analysis, temperature, air velocity and relative humidity. Boundaries of thermal comfort are plotted onto the nomograms from the analysis of the simulation results and they are compared to see whether there are any advantages for using doubleskin construction for an office building in the tropical climate. Figure 6 has shown that there are positive points in using the double-skin construction, as the ‘shaded area’ for the double-skin façade is larger than the normal curtain walling construction (left side nomogram), even though this finding is only represent the low level results for the high-rise office building in study.
Figure 6 Nomograms for benchmarking (left side) and double-skin facade
4.
Conclusion
This paper has found that a high level single opening at the outer pane of the double-skin façade will create a desirable cross ventilation airflow pattern at the internal office space. It was also found that double-skin façade did improve the thermal comfort of an internal office space by reducing the temperature from 1.00C to 1.50C, with the external wind velocity to be around 1.5m/s. The internal temperatures are still considered a bit high (as the model constructed for this paper is only considering the low level of a high-rise office building) but the situation will be expected to improve when multi-storey spaces are linked together in a high-rise building when the stack effect of the air gap will increase tremendously. The results could be improved by using wind turbine at the top of the façade to increase the airflow velocity at the intermediate space to give effective airflow speed within the internal space. This will be looked at and analysed further in the coming development of the research.
290
References Airpak User’s Manual. 2002, Fluent Inc. and ICEM-CFD Engineering. ASHRAE. 2004, ANSI/ASHRAE Standard 55-2004. Thermal Environmental Conditions for Human Occupancy, Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Bansal NK, Mathur R, and Bhandari MS. 1994, A Study Of Solar Chimney Assisted Wind Tower System For Natural Ventilation In Buildings. Building and Environment 29(4): 495-500. Dear RJ de, Leow KG and Foo SC. 1991, Thermal Comfort in the Humid Tropics. International Journal of Biometeorology 34: 259-265. Fanger P.O. 1970, Thermal Comfort. Copenhagen: Danish Technical Press. Grabe J, Lorenz R, and Croxford, B. 2001, Ventilation Of Double Facades. Building Simulation, pp 229-236. Gratia E and Herde A. 2004, Optimal Operation Of A South Double-Skin Façade. Energy and Buildings 36:41-60. Haase M, Wong F, and Amato A. 2004, Double-Skin Facades For Hong Kong. Proceedings of International Conference on Building Envelope Systems and Technology, pp 243-250. Hamdy IF and Fikry MA. 1998, Passive Solar Ventilation. Renewable Energy 14(1-4):381-386. Li Y and Delsante A. 2001, Natural Ventilation Induced By Combined Wind and Thermal Forces. Building and Environment 36:59-71. Manz H. 2003, Total Solar Energy Transmittance of Glass Double Facades With Free Convection. Energy and Buildings 36: 127-136. Murakami S. 1992, New Scales For Ventilation Efficiency and Their Application Based On Numerical Simulation Of Room Airflow. Proceedings of International Symposium on Room Air Convection and Ventilation Effectiveness, University of Tokyo, pp 22-38. Oesterle, Lieb, Lutz, and Heusler. 2001, Double-Skin Facades – Integrated Planning. Germany: Prestel Verlag. Papakonstantinou KA, Kiranoudis CT, and Markatos NC. 2000, Numerical Simulation Of Airflow Field In Single-Sided Ventilated Buildings. Energy and Buildings 33:41-48.
291
Priyadarsini R, Cheong KW, and Wong NH. 2004, Enhancement Of Natural Ventilation In High-Rise Buildings Using Stack System. Energy and Buildings 36(1):61-71. Ruck NC. 1989, Building Design and Human Performance. New York: Van Nostrand Reinhold. Yeang, Ken. 1996, The Skyscraper, Bio-climatically considered; A design primer. London: Academy Editions. Ziskind G, Dubovsky V, and Letan R. 2002, Ventilation By Convection Of A One-Storey Building. Energy and Buildings 34:91-102.
292
C)
Simulation methodology for high-rise office buildings with doubleskin façade in the hot and humid climate – published in the proceedings of The 2008 World Sustainable Building Conference (SB08) in Melbourne, Australia, September 2008.
SIMULATION METHODOLOGY FOR HIGH-RISE OFFICE BUILDINGS WITH DOUBLE-SKIN FAÇADE IN THE HOT AND HUMID CLIMATE Pow Chew WONG Ph.D1 Deo PRASAD Dr. Arch.2 Masud BEHNIA Dr. Eng.3 1
2
Faculty of Built Environment, University of New South Wales, Sydney, Australia,
[email protected] Faculty of Built Environment, University of New South Wales, Sydney, Australia,
[email protected] 3 The University of Sydney, Sydney, Australia,
[email protected]
Keywords: simulation methodology, high-rise office building, double-skin façade, computational fluid dynamic
Summary A number of recent investigations and findings are reported in the literature pertaining to the used of double-skin façade for passive ventilation in buildings and the researches have revealed close link between natural ventilation design and the design of double-skin façade. It was found that significant energy saving is possible if natural ventilation strategy could be exploited with the use of double-skin façade. In this research, CFD was used to analyse the correlation between thermal comfort parameters and different double-skin façade orientations to be used in high-rise office buildings in hot and humid climate. A comprehensive methodology is proposed and results were presented.
1. Introduction Extensive research has been carried in defining what is thermal comfort and the parameters that affecting it. All those findings had confirmed the importance of human factors and human influence towards the creation of a thermally comfortable indoor environment (Fanger 1970 and Ruck 1989). In more recent experimental studies concerning the effects of some human factors on the comfort conditions in particular geographical location, Dear, Leow and Foo (1991) found that people working in naturally ventilated buildings in hot and humid country could accept a temperature value of up to 3oC warmer than Fanger’s values. This with other similar findings especially the newly published ASHRAE standard 55-2004 (2004) have given the opportunity in 293
introducing natural ventilation for commercial buildings in the hot and humid region. A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin facades. Even though most of the researches are done mainly in temperate countries conditions but they have revealed close link between natural ventilation design and the function of double-skin façade. Grabe et al. (2001) developed a simulation algorithm to investigate the temperature behaviour and the flow characteristics of double facades with natural convection through solar radiation. Similar works on natural convection ventilation also reported by Ziskind et al. (2002, 2003), Bansal et al. (1994), Hamdy and Fikry (1998), and Priyadarsini et al. (2003). Most of them are using the idea of stack effect or the solar chimney concept and found that passive ventilation in summer is possible even for multi-storey buildings. In particular Priyadarsini et al. (2003) have concluded the energy efficiency of stack system used in residential of a hot and humid climate region. Li Y and Delsante (2001) went a step further to investigate the effects of natural ventilation caused by wind and thermal forces in a single zone building with two openings. Ventilation graphs are plotted using the air change parameters (thermal air change, wind air change and the heat loss air change) for design purposes. Gratia and Herde (2004) attempted to look at the impact of double-skin façade facing southern direction in a temperate climatic condition. Thermal analysis using simulation software of different seasons of a year was done for a low-rise office building with and without double-skin façade. It was found that significant energy saving is possible if natural ventilation could be exploited through the use of double-skin façade. This paper attempts to bridge the gap of looking into the possibilities of natural ventilation in high-rise office buildings specifically in the hot and humid climate region with the use of double-skin façade. The unique façade construction is thought to be able to act as a stack in providing required ventilation for the thermal comfort of the internal space. Airflow effects induced by wind and thermal forces on a high-rise office building are observed with all the thermal comfort parameters included are analysed.
2. Research Methodology A virtual prototyping computational fluid dynamic (CFD) simulation software called Airpak (2003) is used in this research to model the complex energy transfer through the component layers of the multiplayer façade through the optimisation of the appropriate opening sizes on the glazing, the width of the intermediate space and the ventilation rate through the internal office space. The validation of the software has been carried out by comparing the experimental and simulation results from another commercial simulation software called FloVent which carried out by Manz H (2003). Both of the
294
results are compared and analysed and it was found that the variation is within 5% of the acceptable error tolerance. In view of the complexity of the problem at hand, the modelling of the computer model has been broken down into different ‘levels’. The initial single office module in 3D is constructed with the geometrical dimensions of 3.5m x 5.0m x 3.5m in height. Numerous simulation runs have been carried out for the benchmarking purposes in which a typical curtain walling office module was observed and a simplified ‘nomogram’ has been established to define the initial parameters for thermal comfort in the tropical region. These results are compared with the simulation runs from the office module with double-skin façade construction. Following-up with the simulations analysis of a single office module discussed above, the computer model is ‘extended vertically’ to incorporate a ‘concealed ground floor space’ which usually used as shop front space for most high-rise office buildings. The office space is only starting at 1st level. Two different groups of modelling are constructed at this stage. First group is the benchmarking model with standard curtain walling system generally used in most modern high-rise office buildings (Figure 1). The other group is replaced with a standard vertically vented double-skin façade construction (Figure 2). The strategy is to ‘break-down’ a very complex problem of simulating a multistorey high-rise building into a ‘6-storey building block’. Simulations will be run for the 1st building block of 6-storey for the modelling of the office building from ground floor to 6-storey. Subsequently another 6-storey building block of the model will be constructed for modelling of the office building from 7-storey to 12-storey. The last building block will be the modelling of the office building from 13-storey to 18-storey. The building height of 18-storey or about 60m high will constitute the majority of the office buildings height in a medium to medium-dense modern city. This will give a good representation for investigating the problem at hand.
Concealed Ground Floor Space
Figure 1 Standard curtain walling model.
Concealed Ground Floor Space
Figure 2 Double-skin façade model. 295
3. CFD Modelling The first block (Stage 1) of the six-storey building (Figure 3) consists of a ground floor (which is not served by the double-skin façade, as this will be the typical design for any high-rise building) and 5 stories of office spaces above. The double-skin façade is a ventilated-shaft design that is 2.8m from ground level. In earlier findings it is a practical and economical option to introduce a shaft to improve the stack effect of the natural ventilation and in turn will improve the airflow rates required to reach thermal comfort level within the interior office space. The heat sources for the CFD model will only be introduced at alternate floor, starting from 1st-storey. This was done to reduce the complexity of the model and computing time, but at the same time will be able to give a comprehensive view of the indoor thermal comfort of the office spaces. Each alternate floor will have two occupants, two computers and four ceiling lights, which are the same as the initial single office model. Each human model is assigned with 75 W/m2 of heat generation with clothing value (clo) of 1.0 and metabolic rate (met) of 1.2 for sedentary office activities. Heat generated for the computers are 108 W/m2 and 173 W/m2 respectively and the heat flux of the lighting fixture is assumed to be 38 W/m2 each. Boundary conditions for wind velocity, external temperature and relative humidity were set to the ranges similar to the climatic conditions for Singapore. The ambient temperature in Singapore is hot with high humidity and relatively low wind velocity throughout most of the year. Only the optimum opening sizes on the inner pane and the air gap sizes of the doubleskin façade (DSF) are being considered (as shown in Figure 4) for this stage of simulations, based on the findings from the preliminary modelling. The optimum vent size at the rear wall was found to be 300mm by 600mm from previous findings. The scope of the problem in investigation has been ‘narrowed down’ and carefully ‘controlled’ to find the ‘optimum DSF configuration’ for use in Singapore climatic conditions.
296
3500
3500
WIND
5th FLOOR
3500
VENT
4th FLOOR
3500
WIND
DSF 3rd FLOOR
3500
WIND
21500
VENT
3500
2nd FLOOR
1st FLOOR WIND
Figure 3
GROUND FLOOR
4000
2800
VENT
Model geometry of Stage 1 of the 6-storey office building.
4. Results 4.1 Comparison of results for single-skin and double-skin facades
The results obtained from the benchmarking simulations, which is a typical curtain walling system façade, are compared to the results from the proposed prototype double-skin façade.
Figure 5
Nomogram showing the acceptable thermal comfort conditions (shaded area) for standard curtain wall system. 297
Figure 6
Nomogram showing the acceptable thermal comfort conditions (shaded area) for double-skin facade system.
The nomograms are formed by three axes, which represent the three important parameters in thermal comfort analysis, temperature, air velocity and relative humidity. Boundaries of thermal comfort are plotted onto the nomograms from the analysis of the simulation results and they are compared to see whether there are any advantages for using double-skin construction for an office building in the tropical climate. Figures 5 & 6 above have shown that there are positive points in using the double-skin construction, as the ‘shaded area’ for the double-skin façade is larger than the normal curtain walling construction, even though this finding is only representing the low level results for the high-rise office building in study. The findings are encouraging as the double-skin façade construction does improve the internal thermal comfort for a naturally ventilated office space by as much as 10%, as compared to conventional curtain wall system.
4.2 Simulation results for South facing DSF system (Stage 1)
The first group of simulations is generated with the DSF system constructed at the south facing façade of the building only. The simulation periods are at 10 a.m. or 2 p.m. on either 15 January or 1 July of the month with wind direction perpendicular to the DSF wall and with wind velocities of 0.5 m/s, 1.5 m/s and 3 m/s. The external ambient temperatures were set from 26oC to 30oC with relative humidity ranging from 70% to 100%. The opening size for the inner pane of the DSF system used is 300mm. The air gap sizes used for the DSF are 300mm, 600mm, 900mm and 1200mm. The air vent size at the rear office wall is fixed at 300mm x 600mm. 298
There are a total of 18 location points being identified to record the simulation results on thermal comfort parameters. Six of those location points which are 0.8m above the office floor level and 0.2m away from the two human figures. These six points are chosen to monitor the thermal comfort conditions experienced by the occupants. Table 1 shows some of the comparative results for the simulation with different parameters used for the boundary conditions and DSF configurations taken at strategic locations. The indoor Operative Temperature (OT) calculated in the above table was using the formula stated in Figure 7 and the value was used to identify acceptable thermal comfort for naturally ventilated spaces in hot and humid climate using the graph given in the same figure.
Table 1 Simulation results for different boundary conditions (Note: Shaded results are acceptable thermal comfort conditions) Simulation
Orientation
Date
Time
Air Temp.
Wind Vel.
Air RH
Air Gap Size
m/s
%
mm
o
C
S1-1
South
15 Jan
2pm
28
1.5
80
300
S1-2
South
15 Jan
2pm
28
1.5
80
600
S1-3
South
15 Jan
2pm
28
1.5
80
900
S1-4
South
15 Jan
2pm
26
1.5
80
300
S1-5
South
15 Jan
10am
26
1.5
80
300
S1-6
South
15 Jan
10am
28
1.5
80
300
Simulation
Floor Level
Temp. 0
C
Air Vel.
Radiant
RH
m/s
Temp.
%
0
S1-1
S1-2
S1-3
PMV
OT 0
C
C
1
28
0.04
30
70
1.99
29
3
29
0.01
30
77
1.8
29
5
29
0.01
30
76
1.85
30
1
30
0.02
33
71
2.41
31
3
31
0.04
32
77
1.97
31
5
31
0.03
31
75
2.13
31
1
30
0.03
32
71
2.38
31
3
30
0.06
32
77
2.04
31
5
30
0.04
32
76
1.9
31
299
S1-4
S1-5
S1-6
1
27
0.04
29
71
1.8
28
3
28
0.05
29
76
1.66
29
5
27
0.03
29
75
1.6
28
1
27
0.02
29
70
1.97
28
3
28
0.04
30
76
1.62
29
5
28
0.03
29
75
1.59
28
1
28
0.03
30
70
1.92
29
3
29
0.01
29
77
1.78
29
5
29
0.01
30
75
1.84
29
Air speed < 0.2m/s Difference between radiant & air temp < 4C Top = Ata + (1-A)Tr V A
Figure 7
morning (10 am) and afternoon (2 pm) Wind direction => Perpendicular to the wall system Wind speed => 0.5m/s to 3m/s
External temperature => 26C to 30C Relative humidity => 70% to 100% DSF opening size for inner pane => 300mm Air gap size => 300mm to 1200mm Vent size => 300mm x 600mm 5. Simulation Results Simulations were carried out for all four orientations of north, south, east and west and the first group of simulations is generated with the DSF system constructed at the south facing façade of the building. The simulation period are at 10 a.m. or 2 p.m. on either 15 January or 1 July of the month with wind direction perpendicular to the DSF wall and with wind velocities of 0.5 m/s, 1.5 m/s and 3 m/s. The external ambient temperatures were set from 260C to 300C with relative humidity ranging from 70% to 100%. The opening size for the inner pane of the DSF system used is 300mm. The air gap sizes used for the DSF are 300mm, 600mm, 900mm and 1200mm. The air vent size at the rear office wall is fixed at 300mm x 600mm. There are a total of 18 location points being identified to record the simulation results on thermal comfort parameters. Six of those location points are positioned at 0.8m above the office floor level and 0.2m away from the two human figures. These six points are chosen to monitor the thermal comfort conditions experienced by the occupants. Table 1 below shows some of the comparative results for the simulation with different parameters used for the boundary conditions and different DSF configurations. The indoor Operative Temperature (OT) calculated in Table 1 was using the formula stated in Fig. 3 [17] and the value was used to identify acceptable thermal comfort for naturally ventilated spaces in hot and humid climate using the graph given in the same figure. 6. Discussion Selected results for South facing DSF with external wind velocity of 1.5m/s and air humidity of 80% respectively are tabulated in Table 1. The variable parameters in consideration for this instance are external air temperature, the DSF air gap size and the time of the day. Results for S1-1, S1-2 and S1-3 (South facing DSF) as shown in Table 1 and Fig. 4 indicated that the DSF air gap size of 300mm gives the best result for the particular conditions in a natural ventilated space. These findings are the same for the Northern orientation façade. In most cases the lower floor of the office space would generate the
308
lowest operative temperature due to the ‘stack effect’ provided by the DSF configuration. This has enhanced the natural ventilation strategy to provide better internal thermal comfort condition for the office spaces. There is not much of a different in terms of the internal thermal comfort conditions for either period of time in a given day (morning or afternoon) as seen in the results for S1-4 and S1-5 for South facing DSF and also for North facing DSF. There is an internal temperature difference of 0.50C for the mid-floor of North facing DSF and this could be due to the slower internal air velocity generated. The South facing DSF configuration has produced an 80% Acceptability Limit for the 300mm air gap size for external temperatures between 260C and 280C, according to the Thermal Environment Conditions for Human Occupancy from ANSI/ASHRAE Standard 552004 [17] as indicated in Fig. 3. This is tabulated in accordance with the context that the office building is located in the Northern Hemisphere of tropical climate like the country of Singapore. The North facing DSF configuration did not produce any acceptable internal thermal comfort condition for the office space as the operative temperatures for all the floors are above 300C. This has again confirmed that the southern orientation is the best facing for buildings in the Northern Hemisphere. For Stage 2 simulation results, there is not much of a different in terms of the internal thermal comfort conditions for either period of time in a given day (morning or afternoon) for South facing DSF but for North facing DSF morning period has a higher operative temperatures compare to afternoon period. This could be due to the higher internal radiant temperatures generated during this particular period of time. The South facing DSF configuration has produced an 80% Acceptability Limit for the 300mm air gap size for external temperature of 260C up to 9th Floor. The North facing DSF configuration did not produce any acceptable internal thermal comfort condition for the office space as the operative temperatures for all the floors are above 310C. In most cases the lower floor of the office space would generate the lowest operative temperature due to the ‘stack effect’ provided by the DSF configuration. Stage 3 results are very similar to Stage 2 mentioned above. The South facing DSF configuration has produced an 80% Acceptability Limit for the 300mm air gap size for external temperatures between 260C and 280C. The North facing DSF configuration did not produce any acceptable internal thermal
comfort condition for the office space except for the lower floor for 300mm air gap configuration with external air temperature of 260C during morning period. In most cases the lower floor of the office space would generate the lowest operative temperature due to the ‘stack effect’ provided by the DSF configuration, as in Stages 1 and 2 also. The results show that the South-facing façade has the best outcome following by the East-facing façade during the morning period in the month of January. The North-facing and the West-facing façades do not provide an acceptable indoor thermal comfort for the purposes of office function in a high-rise building. The optimum air gap size for the double-skin façade construction is found to be 300mm and the best results were obtained during the morning period. 7. Conclusion Fig.5 below shows the complete 18-storey office building with typical multi-storey double-skin façade configuration. The proposed DSF starts from 1st storey at 2.8 meters from ground level up to the 17th storey with 1-meter parapet above the roof level. The office spaces are assumed to be divided into a number of small office usages and are tenanted out to various occupants. All office spaces are assumed to face the DSF at the front and facing open corridor at the rear. With the completion of the three stages of simulations, numerous simulation runs had been carried out with various ambient temperatures, different external air velocities, different orientations of the double-skin façade, different periods of time during the day, etc in order to find out the appropriate window periods for acceptable indoor conditions for office workers in the Singapore context. These findings will be of outmost important as an indication whether double-skin façade is really possible to be used as a mean to introduce natural ventilation to the high-rise buildings in the tropics. References [1] Oesterle, Lieb, Lutz, and Heusler, DoubleSkin Facades – Integrated Planning, Germany: Prestel Verlag, 2001. [2] Internet page of BuildingEnvelopes.org, History of Double-skin Facades, http://envelopes.cdi.harvard.edu/envelopes/we b_pages/home/home.cfm, accessed on 28 April 2006. [3] Yeang, Ken, The Skyscraper, Bioclimatically considered: A design primer,
309
London: Academy Editions, 1996. [4] Haase M, Wong F, and Amato A., DoubleSkin Facades For Hong Kong, Proceedings Of International Conference on Building Envelope Systems and Technology, pp 243-250, 2004. [5] Grabe J, Lorenz R, and Croxford, B., Ventilation Of Double Facades, Building Simulation, 229-236, 2001.
[6] Ziskind G, Dubovsky V, and Letan R., Ventilation By Convection Of A OneStorey Building, Energy and Buildings, 34:91-102, 2002. [7] Letan R, Dubovsky V, and Ziskind G., Passive Ventilation and Heating By Natural Convection In A Multi-Storey Building, Building and Environment, 38:197-208, 2003. [8] Bansal NK, Mathur R, and Bhandari MS., A Study Of Solar Chimney Assisted Wind Tower System For Natural Ventilation In Buildings, Building and Environment, 29(4):495-500, 1994. [9] Hamdy IF and Fikry MA., Passive Solar Ventilation, Renewable Energy, 14(14):381-386, 1998. [10] Priyadarsini R, Cheong KW, and Wong NH., Enhancement Of Natural Ventilation In High-Rise Buildings Using Stack System, Energy and Buildings, 36(1):6171, 2004. [11] Li Y and Delsante A., Natural Ventilation Induced By Combined Wind and Thermal Forces, Building and Environment, 36:5971, 2001. [12] Gratia E and Herde A., Optimal Operation Of A South Double-Skin Façade, Energy and Buildings, 36:41-60, 2004. [13] Murakami S., New Scales For Ventilation Efficiency and Their Application Based On Numerical Simulation Of Room Airflow, Proceedings of International Symposium on Room Air Convection and Ventilation Effectiveness, University of Tokyo, 22-38, 1992. [14] Liddament MW., The Role and Application Of Ventilation Effectiveness In Design, Proceedings of International Symposium on Room Air Convection and Ventilation Effectiveness, University of Tokyo, 59-75, 1992. [15] Papakonstantinou KA, Kiranoudis CT, and Markatos NC., Numerical Simulation Of Airflow Field In Single-Sided Ventilated Buildings, Energy and Buildings, 33:41-48, 2000. [16] Airpak User’s Manual, Fluent Inc. and ICEM-CFD Engineering, 2002.
[17] ASHRAE, ANSI/ASHRAE Standard 552004. Thermal Environmental Conditions for Human Occupancy, Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 2004.
Note: Figures and Table for the article had been submitted as a different file. Full complete article could be accessed from the journal’s homepage on www.elsevier.com/locate/enbuild
310
311
