Energy Efficiency Building Design Guidelines for Botswana

Energy Efficiency Building Design Guidelines for Botswana

Department of Energy Ministry of Minerals, Energy and Water Resources

Developing Energy Efficiency and Energy Conservation in the Building Sector, Botswana Project Funded by Danida

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA September 2007

Developing Energy Efficiency & Energy Conservation in the Building Sector, Botswana Funded By Danida

Department of Energy Ministry of Minerals, Energy and Water Resources

Author: Andreas Groth Acknowledgments: Jacob Knight of Arup Botswana wrote most of Section 8, Mechanical Systems, and made helpful comments on the other sections. Jesper Vauvert of Danish Energy Management A/S was the team leader for the project. He guided the preparation of the Guidelines throughout and reviewed the document. A Task Force representing interested stakeholders reviewed the various drafts of the Guidelines as it developed and helped to guide the process. The following were members of this Task Force: o o o o o o o o o o o o o o o o

Mr. J. McCrory (Architects Association of Botswana) Mr. A. Ntlhaile (Botswana Bureau of Standards) Mr. M. Tafila (Association of Citizen Development Consultants) Mr. T. Morewagae (Association of Consulting Engineers, Botswana) Mr. N.Ofetotse (Botswana Housing Corporation) Mr. E. Mazhani (Botswana Institute of Development Professions) Mr. H.T. Tumisang (Botswana Technology Centre) Mr. H.B. Brown (Department of Building and Engineering Services) Mr. B. Kgaimena (Department of Energy) Mr. G. Kumar (Department of Energy) Mr. A. Groth (Department of Energy) Mr. J. Vauvert (Department of Energy) Mr. A. Sebinyane (Department of Housing) Dr. Sajja (Department of Local Government and Development) Mr. R.F. Rankhuna (Department of Town and Regional Planning) Mr. F. Masuku (Gaborone City Council)

The project team for the project: Developing Energy Efficiency & Energy Conservation in the Building Sector, Botswana, and the staff of the Department of Energy, Ministry of Minerals, Energy and Water Resources, Government of Botswana all gave their full support and encouragement in the preparation of this document. Danida funded the work (contract no.: 104 Botswana. 1.MFS.15).

Layout: The Guidelines has been formatted in landscape orientation in order to make it easy to read on screen as a pdf file. The font size and scale for images have been chosen to allow it to be read at a scale that shows one page at a time. In print format the Guidelines is intended to be printed on both sides and bound on the left side of the odd pages. Comments and recommendations: Comments and recommendations for revisions should be sent to: Ministry of Minerals, Energy and Water Resources, Department of Energy, Private Bag 00378, Gaborone, Botswana Tel: +267 3914221, Fax: +267 3914201, email: [email protected], website:www.energyaffairs.bw or the author: Andreas Groth, Motheo (Pty) Ltd., P.O. Box 2224, Gaborone, Botswana, Tel: +267 3923462, Fax: +267 3923632, email: [email protected]

Published by Department of Energy © Department of Energy, Danish Energy Management A/S, and Motheo Pty. Ltd. All rights reserved, 2007 Printed in Gaborone, Botswana

REVISION TABLE Revison No. 0

Date issued: July 2007

Sections Revised: All

1

September 2007

1, 2, 3, 4, 7, 8, 9, 10, 13

Comments: Original document, based on revisions to Draft No. 3 as presented at a Workshop in Gaborone, Botswana on 7 March 2007. Amendments based on comments by Jacob Knight. Additional properties of materials and elements.

SECTION 1

INTRODUCTION

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 1.

INTRODUCTION

4

1.1. Background

4

1.2. Overview 1.2.1. Overall aim. 1.2.2. Classes of building. 1.2.3. Codes and Regulations.

4 4 5 5

1.3. Structure of the Guidelines 1.3.1. The Design Brief. 1.3.2. Technical Sections.

5 5 5

1.4. Who is the Guidelines intended for?

8

Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

Page 3

1.

INTRODUCTION

1.1.

Background

developments in the knowledge base and the regulatory environment of the building sector. The Guidelines and any subsequent revisions will be available as ‘pdf’ files on the website of the Department of Energy and the project website at http://www.eecob.com/.

The project ‘Developing Energy Efficiency and Energy Conservation in the Building Sector, Botswana’ was established in the Ministry of Minerals, Energy and Water Resources in 2005 to address the Government policy as stated in NDP 9: “… Improving energy efficiency and conservation is cost effective, offers a chance to defer new investment and helps reduce energy related pollution. During NDP 9, Government will continue to support and encourage improved energy efficiency and conservation in all sectors of the economy. Planned measures to achieve the policy objectives are: • Carrying out information and educational campaigns. • Conducting energy audits of energy intensive industries and Government institutions • Promoting energy efficient design and operation of buildings. • Developing and implementing a national energy management plan.” One activity of the project was to develop guidelines for the design of energy efficient buildings. This was done through a process of consultation with interested parties through a Task Force that has been established for this purpose. It is expected that this document will need to be regularly revised over the coming years to keep it up to date with Page 4

1.2.

Overview

1.2.1.

Overall aim. The Guidelines is intended to be a resource that will help in achieving the overall aim to improve energy efficiency and energy conservation in the building sector. To achieve this, energy efficiency should be considered from the beginning of the lifecycle of a building. This is typically the stage when the initial Design Brief is prepared For this reason the Design Brief has been chosen as the core document around which these Guidelines are structured. Energy efficiency needs to be considered at every stage of the lifecycle of a building. An optimum level of energy efficiency can be achieved when all aspects of the building design, construction and operation are integrated with each other in a coordinated manner to take full advantage of the opportunities that such synergies offer. The Guidelines can assist in this by providing relevant information and guidance on key issues related to the various stages in the life of a building from inception, procurement, design, construction, commissioning, operation, and ultimately decommissioning and demolition.

Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

This will hopefully facilitate timely incorporation and consideration of those aspects early in the design process. 1.2.2.

Classes of building. Requirements and opportunities for energy efficiency differ in certain ways for different types of buildings. The first edition of the Guidelines is specifically directed at the following broad classes of building: o Office buildings. o Public facilities, such as Police Stations. o Health facilities, e.g. hospitals and clinics. o Schools. o Residential houses.

1.2.3.

Codes and Regulations. At present the Codes and Regulations relating to buildings in Botswana make little or no reference to energy efficiency. In the absence of a specific Botswana code for energy efficiency in buildings, building developers may wish to use the Guidelines as a tool to achieve energy efficiency in new buildings. This may be done by encouraging consultants to work in accordance with the recommendations of the Guidelines throughout the design and construction process.

1.3.

Structure of the Guidelines

1.3.1.

The Design Brief. When the need for a building has been established, it is good practice to prepare a Design Brief for the building. This should define all the requirements of the building, including the overall objectives that the building is intended to meet, the specific spaces that it will provide, their characteristics and relationships to each other, how the building will respond to its environment, constraints imposed by the site, the budget, the programme, and many other issues relating to the project. A well-prepared Design Brief should guide the project throughout the design and construction process. The client and the design team can use the Design Brief as a tool for monitoring the development of the project, to ensure that the original objectives and requirements are being achieved. These Guidelines have been structured around the Design Brief. The core document is Section 2, Design Brief. This sets out a suggested format for the Design Brief, and gives guidance for the preparation of each section of this suggesting how it can assist to enhance energy efficiency.

1.3.2.

It is the intention that the information and recommendations contained in the Guidelines will be helpful in the development of an Energy Efficiency Code for buildings if and when this happens.

Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

Technical Sections. The core document has deliberately been kept short and simple, so that it can be useful to a wide variety of people. The more detailed and technical content relating to specific aspects of building design are included in Technical

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Sections that are referred to in the relevant parts of Section 2, Design Brief. The Technical Sections themselves also refer to other reference material including Standards, Codes of Practice, books, papers, websites, etc. where relevant information may be found. Section 13, Appendices provides data on the thermal properties of materials and construction details, and other relevant information.

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Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

Technical Sections:

Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

3.

Climate.

4.

Indoor Environment.

5.

Design and construction process.

6.

Planning.

7.

Building envelope.

8.

Mechanical Systems.

9.

Lighting - artificial and daylighting.

10.

Operation and Maintenance & Building Management Systems.

11.

Simulation.

12.

Life-Cycle Cost Analysis.

13.

Appendices.

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1.4.

However it is specifically intended to be used by those involved in preparing and implementing the Design Brief. This includes the ‘client’ and the consultant team responsible for the design, construction and commissioning of the building.

Who is the Guidelines intended for? It is hoped that the Guidelines will be of interest to all people involved in the process of procurement, design and operation of buildings. This includes the following groups of people: Owners and developers. o Building owners. o Developers. o Company employees with responsibility for property development. o Government employees with responsibility for property development. Planners and design consultants. o Town Planners. o Landscape architects. o Architects. o Civil Engineers. o Structural Engineers. o Electrical Engineers. o Mechanical Engineers. o Quantity Surveyors. People responsible for operation and maintenance of buildings. o Facility Managers. o Property Managers. o Owners.

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Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

SECTION 2

DESIGN BRIEF

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 0

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 2.

DESIGN BRIEF.

5

2.1. Project Objectives.

6

2.2. Project Requirements. 2.2.1. Schedule of accommodation. 2.2.2. Indoor environment specifications. 2.2.3. Lighting requirements. 2.2.4. Aesthetic considerations.

7 7 7 8 9

2.3. Opportunities and Constraints. 2.3.1. Siting. 2.3.2. Climate. 2.3.3. Budget. 2.3.4. Time.

10 10 10 11 11

2.4. Performance Targets. 2.4.1. Financial performance targets. 2.4.2. Energy performance targets.

12 12 12

2.5. Environmental Rating Schemes

13

2.6. Design Approach. 2.6.1. Procurement Strategy. 2.6.2. Integrated design approach. 2.6.3. Planning and landscape. 2.6.4. Envelope and structural design. 2.6.5. Lighting and electrical design. 2.6.6. HVAC design.

14 14 14 15 16 19 19

Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

Page 3

2.7. Operation and maintenance.

20

2.8. Resource Material 2.8.1. Books and reports. 2.8.2. Web resources

21 21 21

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Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

2.

DESIGN BRIEF. The Design Brief for a building project is essentially a Terms of Reference for the project consultants, setting out the client’s objectives, requirements, constraints, targets and the design approach to be implemented in addressing these.

In the following sections, some key elements of a design brief are considered with an emphasis on energy efficiency considerations. A typical structure for a Design Brief is shown in the table below.

Typically the amount of thought and effort that is applied to preparing the Design Brief varies enormously from one project to another, as does the amount of attention that is later paid to the document during the implementation of the project. A well-prepared, accurate and comprehensive Design Brief can make an important difference to the quality of the final building, and can also be a focus for ensuring that issues are raised and resolved before they become problems. A well-prepared Design Brief can be used throughout the project as a reference to ensure that the original objectives are achieved. It should be revised as necessary to reflect any changes that are agreed with the client. The more competitive the procurement process for consultants is, the more the pressure on them to reduce costs, and hence the need to verify performance against an agreed scope of work. A well-prepared, detailed design brief is a valuable component of the contract between a client and the consultants. Energy Efficiency Building Design Guidelines - Section 2. Design Brief

Page 5

DESIGN BRIEF - STRUCTURE Project Objectives. Project Requirements. • Schedule of accommodation. • Indoor Environment requirements. • Aesthetic considerations. Opportunities and Constraints. • Siting. • Climate. • Financial. • Time. Performance Targets. • Financial • Energy Design and Construction Approach. • Procurement strategy. • Integrated design approach • Planning and landscape. • Envelope and structural design. • Lighting and electrical design. • HVAC design. • Operation and maintenance considerations.

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2.1.

Project Objectives. A description of the background to the project will be followed by general statements of project objectives to indicate what is required from the project, and what is important to the client. This will include the ‘direct’ objectives that have motivated the client to initiate the project, e.g. the need for additional office space, a building that is needed to implement a business plan, or a new art department for a school. It can also include indirect or secondary objectives that relate to the client’s overall philosophy, or mission statement. These could include an emphasis on environmental sustainability, a desire to promote the local economy, or the wish to communicate a particular corporate identity. A general statement could be included here relating to energy efficiency, such as: The development shall be designed to achieve an appropriate level of energy efficiency, taking into account life cycle costs and having due consideration for the likely increase in energy costs relative to other costs over the design life of the building.

Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

Similar statements may be included for other environmental considerations, such as water management, waste management, etc.

2.2.

Project Requirements. This section will contain the specifications for the building and other developments. The actual structure and content will vary depending on the type of development that is required.

2.2.1.

Schedule of accommodation. The schedule of accommodation will indicate the main types of space that are required, how large they need to be, and any particular requirements related to the use of each space. It is also helpful define as far as possible the way in which different spaces should relate to each other.

2.2.2.

Indoor environment specifications. The primary requirement is likely to relate to the comfort of the building’s occupants. Specifications for comfort are considered in more detail in Section 4, Indoor Environment. They should include consideration of the types of activity for which the building is intended. Comfort conditions are affected by the overall approach to air conditioning. Different specifications may apply to buildings that are mechanically air conditioned than to those that are naturally ventilated. Initially both specifications could be included, so that the decision on air conditioning approach may be delayed.

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

TYPICAL INDOOR ENVIRONMENT SPECIFICATIONS Fresh air to achieve required air quality: o Air volume 8 – 12 litres/second/person o Air changes Min. 0.5ACH Temperature: Air conditioned buildings o Summer 23-27°C o Winter 20°C min Naturally ventilated / evaporatively cooled buildings o Summer 22-29 °C o Winter 19-26 °C Relative Humidity: o Maximum

80%

With regard to energy efficiency, it is important that the specifications are appropriate to the actual needs of the building. An unduly restrictive specification may result in higher capital and recurrent cost as well as increased energy consumption. The specification may also indicate the period of time for which the specifications could be exceeded. If the indoor temperature exceeded the limit for say one week of the Page 7

year, this may not cause great problems, but could result in a substantially smaller capacity HVAC system, with savings in energy consumption as well as capital and recurrent cost Requirements for air quality should also be considered, as these will affect the need for ventilation. Again, unnecessarily demanding specifications will lead to increased cost and energy consumption. 2.2.3.

TYPICAL INDOOR LIGHTING REQUIREMENTS Public spaces – no visual tasks Background lighting, offices Task lighting, office work Task lighting, detailed work Task lighting, very fine work

50 lux 150 lux 300 lux 750 lux 3,000 lux

Lighting requirements. Lighting requirements should be specified in the Design Brief, as well as some indications of the approach to be taken in the design of lighting. Lighting levels required in different areas or rooms should relate to the intended use of these spaces. Section 9, Lighting – artificial and daylighting gives indications of typical specifications for light level for different activities, as well as references to various standards and codes that provide more detailed information.

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Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

2.2.4.

Aesthetic considerations. One of the greatest challenges in improving energy efficiency in public and commercial buildings is to develop an architecture that is both aesthetically satisfying, and meets the technical requirements determined by the local climate and available material options. It is important to set clear objectives regarding how the buildings should look, and to understand the implications on energy performance, initial cost and life cycle cost. If it is regarded as an important objective for the building that it makes a particular architectural statement, then the cost, energy and other implications should be clearly stated and agreed to.

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

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TEMP DATA MONTHLY GABORONE 2000-2002 35.0

30.0

2.3.

Opportunities and Constraints.

2.3.1.

Siting. If the client already has a site for the project, then an assessment should be made of opportunities and constraints of the site that are relevant to the project. These are discussed in more detail in Section 6, Planning.

25.0

20.0 DEG C 15.0

10.0

5.0

Energy considerations will include the orientation of the site in relation to the sunpath and typical wind directions, shading features such as trees, hills, other buildings, and other factors affecting the local climate such as vegetation, ponds / rivers, wind breaks, etc. 2.3.2.

MIN MAX AVG MAXDIFF

0.0 JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

MONTH

Fig. 2.1 DB Temperature in Gaborone, by month.

Climate. The energy performance of a building is largely determined by how well the design is adapted to the local climate.

RH DATA MONTHLY GABORONE 2000-2002 100.0 90.0

It is therefore important that the design team has a clear understanding of the local climate with its daily and seasonal variations.

70.0 60.0 RH %

During the course of a year the climate changes with the seasons, and there are also variations in climate from one year to the next. It is therefore necessary to define the climate for a typical year for use in building design.

80.0

MIN MAX AVG

50.0 40.0 30.0 20.0 10.0 0.0 JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

MONTH

Fig. 2.2 Relative Humidity in Gaborone, by month.

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Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

Project Cost and Energy Efficiency in Section 5, Design and Construction Process is relevant here. An assessment of the variation in climate for different locations in Botswana suggests that for purposes of building design at least two climatic zones should be considered. The northern zone includes Chobe District and Ngamiland District. The southern zone includes all the remainder of the country.

Possible trade-offs between initial cost and life-cycle costs may affect the way the project is financed. If access to capital finance is restricted this may reduce the scope for investment choices that will reduce life cycle cost. Section 12, Life-Cycle Cost Analysis gives a background to the methods that can be used to analyse various options and determine the most cost effective solution based on assumptions regarding future energy costs, maintenance interventions and other relevant parameters

Generally the winters in the northern zone are sufficiently mild that there is little or no requirement for heating in buildings. In the southern zone heating in the winter is generally required, and may require more energy than summer cooling, depending on the building design and the amount of heat generated by activities in the building. Maun has been taken as a typical location in the northern zone, and Gaborone as a typical location in the southern zone. For each of these locations, the most relevant climate parameters have been determined for each hour of a typical year. Any particular features of the local climate should be noted, such as dominant wind direction, shading effect of any tall trees, hills or buildings, etc. Further details are included in Section 3. Climate. 2.3.3.

Budget. Opportunities and constraints regarding the financing of the project should be considered at this stage. The chapter on

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

It may be cost effective to invest in additional work and cost in the design stage in order to optimise the energy performance of the building. The anticipated costs and benefits should be carefully considered. 2.3.4.

Time. The client’s particular requirements regarding the project programme should be defined. This may then be subdivided into pre-contract and post-contract programmes, to determine the amount of time available for the design process. Detailed analysis of different approaches with regard to energy efficiency takes time to carry out. The costs, both in consultant fees and project timing need to be considered and evaluated in relation to the opportunity to achieve a more cost effective and better quality project.

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conditions of supply and demand, as well as the policies of the authorities that set the price.

2.4.

Performance Targets.

2.4.1.

Financial performance targets. A building represents a substantial, long-term investment by the owner. In many cases an important objective in making this investment is to obtain a financial return, either in the form of rental income, or saving of rental expense that would otherwise be incurred in the case of owneroccupied buildings. It is therefore helpful to establish financial performance targets for the building against which actual performance can later be assessed. This will also inform decisions made during the design phase of the project, and guide the Quantity Surveyor in making recommendations regarding the cost of different components. The financial performance targets should be broken down into capital costs, recurrent costs and recurrent income. These figures may if appropriate be developed into a lifecycle performance model to show the long term return on investment and predicted cash flows for the project.

2.4.2.

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In many countries codes and standards for energy performance of buildings have been introduced. In some cases these are voluntary and give guidance to investors regarding what can be achieved. They can be used as specifications that the design team is expected to achieve, with or without financial incentives (see Section 5, Design and Construction Process ) Information regarding the actual energy performance of different types of building in Botswana is becoming available through the work of the EECOB project in the Department of Energy, both through audits of existing buildings and simulations of typical ‘generic’ building types. The following figures for specific energy consumption (energy consumption per unit area) may be used as targets in the interim until actual energy performance standards become available.

Energy performance targets. An important element in the financial performance of a building is energy cost. This requires estimates of the energy consumption for different purposes, as well as estimates of the price of different energy supplies. The major energy source for the types of building under consideration in these Guidelines is electricity. The cost of electricity is subject to change based on the changing

Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

Building type Office School Residential (high cost, air conditioned)

Total 150 40 89

Specific annual energy consumption [kWhr/m2.yr] Lighting HVAC Office Equipment 34.5 63 43.5 14.8 3.6 2.8 21.4 23.1 0

Other 10.5 18.8 44.5

Table 2.1. Specific energy consumption targets by building type.

2.5.

Environmental Rating Schemes In many countries, environmental ratings are being adopted by private clients and governments as a way of demonstrating that they are environmentally responsible. Well known rating systems include BREEAM (Building Research Establishment Environmental Assessment Method) and LEED (Leadership in Energy and Environmental Design). A client may make it a part of their brief that their building should achieve a BREEAM "excellent" rating, or a company may make it part of its sustainability policy that any new buildings which it procures will be constructed to achieve a BREEAM "very good" rating. For example, in 2003, the UK government made it a condition that all government departments when undertaking new or refurbishment projects carry out an environmental assessment, and that all new build projects must achieve a BREEAM "excellent" and refurbishment projects a "very good" rating.

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

These ratings consider a wide range of factors and compare them against a local benchmark of "typical" construction practice. They can therefore be adapted to any country, although there are initial costs involved in establishing suitable local benchmarks, particularly if there are no regulatory requirements as in Botswana. The BREEAM rating assesses the following: o management: overall management policy, commissioning, site management and procedural issues such as controlling noise, dust and waste materials on the construction site. o energy use: operational energy and carbon dioxide (CO2 ) issues such as the predicted energy use of air conditioning systems. o health and well-being: indoor and external issues affecting health and well-being such as provision of

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o o

o

o o o

adequate daylight, artificial lighting and good air quality. pollution: air and water pollution issues, such as use of refrigerants, pollution from coal fires etc. transport: transport-related CO2 and locationrelated factors, such as availability of public transport links to the building and whether occupants are encouraged to use alternative forms of transport to private cars. land use: greenfield and brownfield sites, whether the project is a refurbishment or built on a site already developed, or whether it is built on virgin ground. ecology: ecological value conservation and enhancement of the site, including landscaping etc. materials: environmental implication of building materials, including life-cycle impacts. water: consumption and water efficiency.

Credits are available under each of these headings, and the total number of credits obtained determines the final score achieved (from Pass to Excellent).

2.6.

Design Approach.

2.6.1.

Procurement Strategy. There are a number of different procurement strategies that can be used for the appointment of the professional team and the contractors for a building project.

These have implications for the energy performance of the building, which are discussed in more detail in Section 5, Design and Construction Process. The most appropriate approach for a particular project should be determined based on the priorities and resources of the owner. 2.6.2.

Integrated design approach. A simple building such as a low cost residential house can be fully designed by a competent, experienced designer such that all aspects of the building work well together in a coordinated, sensible way. The designer takes into consideration decisions that relate to one aspect of the building when making decisions on other aspects. Since the same person is making all the design decisions, she or he can easily consider the implications of a decision about say the location of windows on the planning of the rooms and the switching of lights. Larger, more complex buildings require a team of specialised designers, each working on different aspects of the overall design. Often they work for different firms located in different places, with limited communication. They will be coordinated by a team leader, often the architect or project manager, who is responsible for ensuring that the different elements of the building work in relation to each other. Typically energy efficiency has not been a key consideration in building design, and as a result the added

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Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

requirement of ensuring that the different design aspects work together to achieve optimum energy efficiency has tended to be overlooked.

made as early in the design of the building as possible, since the time and work required in making changes increases rapidly as the design becomes more detailed.

By deliberately adopting an integrated approach to energy efficient design, the design team can be encouraged to take advantage of opportunities to achieve improved energy performance.

It is helpful therefore to have a systematic approach to the coordination of these approaches, and the Design Brief is a good opportunity for providing this. It is suggested that some initial indications are included in the Design Brief at the project inception stage, and that the consultants amend these as the design develops.

This is discussed in more detail in Section 5. Design and Construction Process. The integration of the different design aspects almost always requires that changes in approach be made in each aspect to accommodate the others. Such changes should be Energy Efficiency Building Design Guidelines - Section 2. Design Brief

2.6.3.

Planning and landscape. The planning of the building on the site provides many opportunities for improving energy performance. The overall approach to energy performance should be considered, e.g. whether the building will require

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mechanical systems to control the indoor climate, or whether passive heating and cooling approaches will be used. In practice a combination of these may often be appropriate. Different solutions may be needed for different types of building. For buildings such as residential houses and classrooms it should be possible to achieve the required comfort levels with little or no mechanical equipment. In office buildings comfort conditions can be achieved with passive methods for much of the year, but some form of mechanical cooling may be required to deal with summer conditions. The overall shape of the building is important to achieving energy efficiency. It has been found that the walls perform an important role in removing heat from a building, suggesting that a high surface area to volume ratio is useful. This also allows for maximum use of daylight, reducing the energy needed for lighting, and indirectly helping to keep the building cool as well, since artificial lighting also generates heat. Plants can be used very effectively to amend the local climate on site, e.g. using trees for shade and wind breaks, ground cover to reduce reflected heat, climbing plants on frames to provide shade and evaporative cooling, etc. Planning and landscaping are discussed further in Section 6, Planning.

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2.6.4.

Envelope and structural design. The building envelope consists of all the different elements that make up the fabric of the building, such as the floor, walls, windows and roof. Most of the design decisions relating to the building envelope are the responsibility of the architect and structural engineer. They have a large impact on the thermal performance of the building, and it is therefore essential that the performance of the envelope is coordinated with the design of the HVAC system. This is the area that offers most opportunities for improved building performance through an integrated design approach. Energy codes and standards for buildings typically specify the performance requirements for the building envelope in terms of an ‘overall thermal transfer coefficient’ (OTTC), which gives an indication of the amount of heat that will flow between the building and the environment. In some cases the standard defines the requirements for the thermal properties of different building elements. Typical values for the total thermal resistance of the walls and roof proposed for the South African Standard SANS 283 and 204, The Energy Efficiency Standards are given in Table 2.2.

Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

Building Element Wall

Total ‘R’ value [m2.K/W]

Total ‘U’ value [W/m2.K]

Typical construction to achieve this value:

Min. 1.4

Max. 0.71

Roof and ceiling

Min. 2.7

Max 0.37

Sand-cement brick cavity wall with 25mm insulation in cavity plastered both sides. Galvanised roof sheets, 100mm insulation and 6mm ceiling.

Table 2.2. Thermal properties of building envelope elements (draft SANS 204) Source: TIASA. Frequently energy codes offer an alternative method of demonstrating compliance based on a computer simulation of the proposed building using approved methods to verify whether it achieves the required minimum standard of performance. Details regarding computer simulation of building energy performance are provided in Section 11, Simulation. Windows and other glazing elements are frequently responsible for more heat gain and loss than any other building element. Assuming that the roof is insulated to the level recommended in Table 2.2, the greatest source of solar heat gain in most buildings will be glazing. Glazing however also provides the opportunity to admit natural daylight into the building, reducing the energy consumption for artificial lighting. It is therefore important to achieve an optimum balance whereby the opportunity for effective daylight is achieved with minimal unwanted solar heat gain.

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

2.6.4.1. Summary of simulation results. Simulations of three building types; Classroom, Residential and Office have been carried out for the Gaborone climate to quantify the effect on energy consumption of various alternative envelope and operational parameters. Some of the key results are summarised below. In each case references to changes in energy cost refer to total heating and cooling energy, not total building energy. Full tables of results are included in the EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’. Orientation: Orientation in the N-S direction resulted in a 6% increase in energy consumption over an E-W orientation for the classroom type of building. For the office it was only 0.8% and for the residential house it was 1.8%. This suggests that orientation is less significant than expected. However local effects within the building and impact on quality of daylight are also important considerations that are strongly related to orientation. It is therefore recommended that an Page 17

A 500mm wide mass wall with insulation on the inside gave similar results, with energy cost increased by 10% for the classroom and 7% for the office. In the residential house the energy saving increased to 30% compared with the insulated cavity wall.

E-W orientation be used whenever possible, particularly where daylight is an important consideration as in offices and classrooms. Roof: In the classroom building a white roof reduced energy by 45% compared to a galvanised roof with no ceiling insulation in both cases. In the residential building, with ceiling insulation the white metal roof is comparable in performance to a concrete tiled roof also with insulation. In the three storey office building, a white metal roof reduced energy consumption by 5% compared to a green coloured metal roof. The addition of 100mm insulation on the ceiling reduced energy consumption by 43% in the classroom (galvanised roof), 2.7% in the residential house (tiled roof), and had no effect in the office building (green metal roof). Wall. An insulated cavity wall in place of a standard 220mm solid wall increased energy consumption by 8% for the classroom and by 5% for the office. However it reduced energy consumption by 27% in the residential building. This energy saving was due to reduced heating cost. The insulated cavity wall was almost three times as effective as an uninsulated cavity wall, so the small extra cost of providing insulation in the cavity is well rewarded.

Page 18

The simulation showed that the walls provide some cooling during the day when they absorb radiant heat from the ceiling. A width of 220mm seems to be about optimum; 115mm walls are worse, as are wider walls. The simulation confirmed that different solutions are appropriate for different types of building. Solid 220mm walls are best for classrooms and offices that are primarily occupied during the day, and insulated cavity walls or mass walls are effective for residential houses that are occupied more during the night. Floor. The ground floor is also an important cooling element in all buildings in summer, and also in winter for office buildings that require cooling all year. For the classroom, providing floor insulation resulted in a 23% increase in annual energy cost. In residential buildings there is some unwanted heat loss to the ground floor in winter, but this is marginal compared to the benefit in summer. Windows. Heat flow through the windows from direct and indirect solar radiation is in many cases the largest source of heat gain to the building. The easiest way to reduce this is to reduce the size of windows to the minimum required to

Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

The design of artificial lighting should aim to provide an adequate level of background illumination for general purposes, with higher levels of task lighting in the specific areas where more light is needed. This results in energy savings and also allows for more flexibility should the use of spaces change in future.

provide daylight and views. It was found that a glazing ratio of 20% (window to wall area) provided more than enough daylight in the classroom. Ventilation. The use of ventilation to control indoor temperature was found to be highly effective in the office building, resulting in a 28% energy saving. It was less effective in the residential house (11% saving) and in the classroom (2% saving). This should be considered as an option for office buildings, and would need to be included in the HVAC design approach, as it may require increased duct sizes, larger fans, and different control systems. It appears that the substantial savings that can be achieved would justify this extra expense. Further suggestions for appropriate design approaches for different building envelope elements are described in Section 7, Building Envelope. 2.6.5.

Lighting and electrical design. Optimal use of daylight can result in reduced energy consumption, and also has other benefits. Studies have shown that people perform better under daylight than artificial light. Views of the world outside the building are also important for the well-being of the occupants, and have been found to improve performance and productivity. There are a number of opportunities to improve the effectiveness of daylight without excess heat gain, including use of light shelves, light tubes and skylights.

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

Control of lighting should be designed to ensure that lights are only on when and where they are needed. This is discussed in more detail in Section 9, Lighting – artificial and daylighting. 2.6.6.

HVAC design. The mechanical systems or HVAC (heating, ventilation and air conditioning) are designed to amend the indoor climate of a building to achieve the requirements of the particular application in buildings for which this cannot be achieved using natural ventilation alone. The first decision that is required is therefore whether such systems are required or not. This depends on how stringent the indoor environment requirements are, the internal loads from occupants and equipment, the local climatic conditions and the design of the building envelope. If an HVAC system is required, the design approach should be coordinated with the envelope design to ensure that the building requirements are achieved with an optimal energy performance.

Page 19

It has been found that HVAC systems designed using hourly computer simulation are more accurately matched to the needs of a particular building in relation to the local climate than those designed using steady state methods.

The Design Brief should specify the requirement for the design team to prepare a draft Operations and Maintenance Manual as one of their tasks. This should be developed as an ongoing process during the design, to ensure that the O&M implications are given consideration

It is recommended that for all projects that will have a centralised HVAC system installed, computer simulation be used to determine the system capacity that is required. Use of ventilation to control indoor temperatures offers substantial energy savings, particularly in buildings with high heat gains from occupants, equipment and lighting. It is recommended that centralised HVAC systems be designed to use ventilation for this purpose as well as providing adequate indoor air quality. In many situations it may be possible to achieve the required comfort conditions using evaporative coolers in place of air conditioning systems, with far lower recurrent cost and energy consumption. Further information is included in Section 8, Mechanical Systems.

2.7.

In particular, the human resource requirements for the operation and maintenance of the building should be considered.

The draft O&M manual will then be revised and finalised during and following the commissioning of the building. Possibly the greatest opportunity for reducing energy consumption in buildings, and certainly the cheapest and quickest to implement is to encourage occupants to turn off lights and other equipment when these are not needed. The simulation of the office building indicated that 39% of total energy use could be saved through such behaviour change. Operation and Maintenance considerations are discussed in more detail in Section 10, Operation & Maintenance and Building Management Systems that also includes a suggested format for an O&M Manual.

Operation and maintenance. The decisions that are made during the design phase of a building have implications for how it will be operated and maintained. The overall approach to operation and maintenance should be specified in the Design Brief, so that this can guide the decisions taken in the design process.

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Energy Efficiency Building Design Guidelines for Botswana – Section 2. Design Brief

2.8.

Resource Material

2.8.1. Books and reports. TIASA, The Thermal Insulation Guide for Energy Efficiency in Buildings. Thermal Insulation Association of Southern Africa. January 2006. EECOB Report: ‘Energy Efficiency and Energy Conservation in the Building Sector, Botswana, Report on Baseline Energy Surveys’, Department of Energy, Government of Botswana, July 2005. Bauer, C. and Groth, A. EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’. Department of Energy, Government of Botswana, January 2007. 2.8.2. Web resources BREEAM Building Research Establishment Environmental Assessment Method http://www.breeam.org LEED Leadership in Energy and Environmental Design. http://www.usgbc.org/leed

Energy Efficiency Building Design Guidelines - Section 2. Design Brief

Page 21

SECTION 3

CLIMATE

HOURLY AVERAGE TEMPS GABORONE 2000-2002

HOURLY AVERAGE RH GABORONE 2000-2002

35.0

100.0 90.0

30.0

80.0

DEG C

20.0

15.0

10.0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

70.0 60.0 RH %

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

25.0

50.0 40.0 30.0 20.0

5.0

10.0

00 :0 0 01 :0 0 02 :0 0 03 :0 0 04 :0 0 05 :0 0 06 :0 0 07 :0 0 08 :0 0 09 :0 0 10 :0 0 11 :0 0 12 :0 0 13 :0 0 14 :0 0 15 :0 0 16 :0 0 17 :0 0 18 :0 0 19 :0 0 20 :0 0 21 :0 0 22 :0 0 23 :0 0

00 :0 0 01 :0 0 02 :0 0 03 :0 0 04 :0 0 05 :0 0 06 :0 0 07 :0 0 08 :0 0 09 :0 0 10 :0 0 11 :0 0 12 :0 0 13 :0 0 14 :0 0 15 :0 0 16 :0 0 17 :0 0 18 :0 0 19 :0 0 20 :0 0 21 :0 0 22 :0 0 23 :0 0

0.0

0.0

TIME

TIME

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS

3.

CLIMATE

5

3.1. Overview 3.1.1. Climate of Botswana. 3.1.2. Elements of climate. 3.1.3. Climatic zones. 3.1.4. Climate patterns. 3.1.5. Climate and simulation.

5 5 5 5 5 5

3.2. Climate of Botswana 3.2.1. Classification. 3.2.2. Cycles of climate and global warming.

6 6 8

3.3. Elements of Climate

9

3.3. Elements of Climate 3.3.1. Meteorological data 3.3.2. Temperature 3.3.3. Design Day Conditions 3.3.4. Humidity 3.3.5. Radiation 3.3.6. Wind 3.3.7. Rainfall

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

10 10 10 12 13 13 14 15

Page 3

3.4. Climatic Zones.

15

3.5. Climate Patterns.

18

3.6. Resource Material 3.6.1. Books and papers 3.6.2. Web resources

19 19 20

Page 4

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

3.

CLIMATE

3.1.

Overview

3.1.4.

Climate patterns. In addition to the geographical variations in climate, there are also patterns of climate within one locality. In considering the impact of climate on building energy performance, it is important to consider the different patterns that occur, and differentiate these from the average characteristics, which may never be experienced.

3.1.5.

Climate and simulation. Building energy performance may be predicted using software that simulates the interaction of the building with the climate.

This Section addresses the subject of climate and its impact on building energy performance in Botswana. The topics that will be covered are briefly outlined below. 3.1.1.

Climate of Botswana. The section begins with an overview of the climate of Botswana in a global context. The classification of the climate is considered, and various cycles in the climate are identified.

3.1.2.

Elements of climate. This section includes a general discussion of the principal elements of climate and how they relate to building energy performance. The ways in which data are collected and made available are also considered.

3.1.3.

Typical meteorological year data has been prepared for Gaborone and Maun, which have been taken as typical of the Northern and Southern climate zones. This data is available in a format that may be used for computer simulation of building energy performance.

Climatic zones. The variation in climate with location is considered, with particular reference to the implications for building energy performance. It is recommended that the country be divided into two climatic zones for the purposes of these Guidelines.

Energy Efficiency Building Design Guidelines – Section 3. Climate

Page 5

3.2.

Climate of Botswana

3.2.1.

Classification. In the Köppen Climate Classification System, the climate of most of Botswana falls in the classification ‘BSh: semiarid steppe, hot’. The exception is the extreme north of the country, which is classified, as ‘Aw: tropical wet-dry (low sun dry) – savanna’. Approximately two thirds of the area of the country is within the tropics. The Tropic of Capricorn crosses the Jwaneng - Ghanzi road just north of Kang, runs through the middle of Khutse game reserve, and crosses the Gaborone - Francistown road just north of Dibete. Generally Botswana experiences a very high proportion of clear, sunny days, with little cloud cover or rain. The summers are warm to hot in the day and cool at night, particularly in the southwest of the country. Rainfall generally occurs in the time between October and April, which coincides with the summer months. Winters are warm in the day and cool at night, with minimum temperatures lower in the south, and increasing as one moves further north.

and evaporative cooling from the moisture in the soil. In years of drought, and in regions that receive less rain the maximum temperatures continue to rise until January or February. Botswana is completely landlocked, and is located in the centre of the southern African plateau. The country is approximately equidistant from the Atlantic Ocean coast, 1,000km to the west, and the Indian Ocean coast about 960km to the east (measured to the middle of the country). The country is relatively flat, at an average elevation of approximately 1000m above sea level. As a result moist air from the oceans seldom reaches Botswana without having first shed its moisture on the escarpments between. The distance from the ocean together with the relatively high altitude result in low, intermittent and unreliable rainfall. The rain that does occur is a result of localised regions of low pressure that draw in moist air from the coast. Not only is the average rainfall in Botswana low, it is also very variable, both within a particular year, and from one year to the next. There is a trend for average rainfall to reduce and variability to increase from north to south, and from east to west.

Summer maximum daytime temperatures are closely related to rainfall, rising rapidly in times of drought. In years of reasonable rainfall, the highest average maximum temperature often occurs in October, before the rain begins, after which temperatures drop due to increased cloud cover

Page 6

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

Fig. 3.1 Koppen climate classification.

Energy Efficiency Building Design Guidelines – Section 3. Climate

Page 7

3.2.2.

Cycles of climate and global warming. A number of different climatic cycles have been observed, including a short-term cycle of about 6-10 years during which a few years of good rain are followed by years of below average rain or drought. This takes place within the framework of a longer cycle spanning several centuries, and another even longer cycle of several thousands of years. Although the Kalahari has generally been a semi-arid area for millions of years, during that time there have been periods of sufficient rainfall to maintain large inland seas and perennial rivers that now remain as fossil river valleys.

in temperature will be experienced throughout this century, together with increased energy cost, both in economic and environmental terms. This makes it even more urgent that buildings are designed and built to achieve human comfort with minimal energy consumption.

Over the past century the natural long-term climatic cycles of the earth have been subject to increasing influences from human activity, particularly the enormous increase in energy consumption from fossil fuels and resulting emissions of carbon dioxide. This has resulted in increased concentrations of greenhouse gasses in the atmosphere. These act as a radiation filter surrounding the earth, which allows solar radiant heat to pass through, but reflects thermal radiant heat back to the earth, as does the glass in a greenhouse. The consensus view of the Intergovernmental Panel on Climate Change (IPCC), the world authority on global warming, is that this could result in an increase in average temperatures over southern Africa of between 25°C over the coming century. The following excerpt from an article by Mike Davis in The Science News suggests that this may be a highly optimistic view. The actual rate of change of climate may not be accurately predictable, but there seems to be little doubt that increases

Page 8

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

The Science News Scientific discussions of environmental change and global warming have long been haunted by the specter of nonlinearity. Climate models, like econometric models, are easiest to build and understand when they are simple linear extrapolations of well-quantified past behavior: when causes maintain a consistent proportionality to their effects. But all the major components of global climate - air, water, ice and vegetation - are actually nonlinear: at certain thresholds they switch from one state of organization to another, with catastrophic consequences for species too finelytuned to the old norms. Until the early 1990s, however, it was generally believed that these major climate transitions took centuries if not millennia to accomplish. Now, thanks to the decoding of subtle signatures in ice cores and sea-bottom sediments, we know that global temperature and ocean circulation can change abruptly - in a decade or even less. The paradigmatic example is the so-called 'Younger Dryas' event, 12,800 years ago, when an ice dam collapsed, releasing an immense volume of meltwater from the shrinking Laurentian ice-sheet into the Atlantic Ocean via the instantly-created St. Lawrence River. The freshening of the North Atlantic suppressed the northward conveyance of warm water by the Gulf Current and plunged Europe back into a thousand-year ice age. Abrupt switching mechanisms in the climate system, like relatively small changes in ocean salinity, are augmented by causal loops that act as amplifiers. Perhaps the most famous example is sea-ice albedo: the white, frozen Arctic Ocean reflects heat back into space, thus providing positive feedback to cooling trends; alternatively, shrinking sea-ice increases heat absorption and accelerates its own melting and planetary warming. Thresholds, switches, amplifiers, chaos - contemporary geophysics assumes that earth history is inherently revolutionary. This is why many prominent researchers - especially those who study topics like ice sheet stability and North Atlantic circulation - have always had qualms with the consensus projections of the Intergovernmental Panel on Climate Change (IPCC), the world authority on global warming. by Mike Davis; October 05, 2005

Energy Efficiency Building Design Guidelines – Section 3. Climate

Page 9

3.3.

Elements of Climate

3.3.1.

Meteorological data In Botswana the responsibility for the collection, processing, storage and dissemination of meteorological data rests with the Department of Meteorological Services (DMS) in the Ministry of Environment, Wildlife and Tourism. The DMS maintains synoptic weather stations at the following locations around Botswana: o o o o o o o o o o o o o o

Francistown Ghanzi Jwaneng Kasane Letlhakane Mahalapye Maun Pandamatenga Selebi-Phikwe Sir Seretse Khama Airport Shakawe Sua Pan Tshabong Tshane

A wide range of variables are measured, including the following: o Dry Bulb Temperature o Humidity o Wind Speed o Wind Direction o Rainfall

o o o o

Sunshine hours Evaporation Air pressure Soil Temperature

In addition, rainfall and temperature are measured at a large number of other locations by volunteers who regularly submit their data to DMS. 3.3.2.

Temperature Air temperature (Dry Bulb temperature) is the characteristic of climate that most directly affects comfort. It determines the rate of heat transfer by conduction and convection. Assuming that there are no significant sources of radiant heat transfers, DB temperature is the main determinant of human comfort, and therefore the most significant variable to be specified when defining indoor climate requirements. Heating and cooling equipment is generally controlled by thermostats that are set to a particular target temperature or temperature range. Dry bulb temperatures in Gaborone vary throughout the year, between an average daily maximum temperature of 32°C in October, and an average daily minimum temperature of 4°C in July. [Bauer Consult]. The maximum daily temperature in summer typically occurs at about 3.00pm, and the minimum daily temperature in winter typically occurs at 7.30am.

35.0

30.0

25.0

DEG C

20.0

Page 10

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

15.0

10.0

HOURLY AVERAGE TEMPS GABORONE 2000-2002 TEMP DATA MONTHLY GABORONE 2000-2002

35.0

35.0

30.0 30.0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

25.0 25.0

DEG C

20.0 20.0 DEG C

MIN MAX AVG MAXDIFF

15.0

15.0

10.0 10.0

5.0 5.0

0.0 JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

00 :0 0 01 :0 0 02 :0 0 03 :0 0 04 :0 0 05 :0 0 06 :0 0 07 :0 0 08 :0 0 09 :0 0 10 :0 0 11 :0 0 12 :0 0 13 :0 0 14 :0 0 15 :0 0 16 :0 0 17 :0 0 18 :0 0 19 :0 0 20 :0 0 21 :0 0 22 :0 0 23 :0 0

0.0 DEC

TIME

MONTH

Fig. 3.2 Temperatures in Gaborone, by month.

Fig. 3.3 Temperatures in Gaborone, by hour. HOURLY AVERAGE RH GABORONE 2000-2002

RH DATA MONTHLY GABORONE 2000-2002 100.0 100.0

90.0 90.0

80.0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

80.0

70.0 70.0

60.0

RH %

60.0

50.0 MIN MAX 40.0 AVG

50.0 40.0

30.0 30.0

20.0 20.0

10.0 10.0

Fig. 3.4 Relative Humidity in Gaborone, by month. Energy Efficiency Building Design Guidelines – Section 3. Climate

:0 0

:0 0

:0 0

:0 0

:0 0 23

22

21

20

19

:0 0

:0 0 18

:0 0 16

17

:0 0

:0 0

:0 0 15

14

:0 0

:0 0

:0 0

:0 0

:0 0

:0 0

:0 0

:0 0

:0 0

:0 0

:0 0

13

12

11

DEC

10

NOV

09

OCT

08

SEP

07

AUG

06

JUL

MONTH

05

JUN

04

MAY

03

APR

02

:0 0 MAR

01

FEB

00

JAN

:0 0

0.0 0.0

TIM E

Fig. 3.5 Relative Humidity in Gaborone, by hour. Page 11

ASHRAE design temperature Gaborone Airport (Jan) based on 0.4% chance of exceedance (derived using IES software) CIBSE A guide (5th Ed) design temperature Maun (October) CIBSE A guide (5th Ed) design temperatures Maun (Jan) CIBSE A guide (5th Ed) design temperatures Ghanzi (Nov) CIBSE A guide (5th Ed) design temperatures Ghanzi (Jan) Standard design conditions in common usage (Gaborone) More extreme design conditions (Gaborone) Based on the Typical Meterological Year (TMY) for Gaborone and Maun generated by Meteonorm: Heating Dry Bulb temperature (99% chance of no lower temperature, Gaborone) Cooling Dry Bulb temperature (1% chance of higher temperature, Gaborone) Heating Dry Bulb temperature (99% chance of no lower temperature, Maun) Cooling Dry Bulb temperature (1% chance of higher temperature, Gaborone)

dry bulb °C 37.7

wet bulb °C 19.9

relative humidity 20%

39 37 38 37 38 40

22 25 23 24 25 27

24% 39% 29% 36% 36% 38%

25.6

61%

22.4

45%

2.5 34.1 6.3 39.1

Table 3.1 Design Day Conditions for Gaborone, Maun, and Ghanzi

3.3.3. Design Day Conditions Although it is recommended that buildings are simulated using real weather data (see section @@) some buildings may continue to be designed using “design day” methods. Typical design temperatures for both cooling and heating design are provided in Table 3.1 above. The choice of design day temperatures is something that the client must sign off, since it involves a choice about how often the building is likely to overheat, versus the risk of oversizing plant. Generally, for an energy efficient building

Page 12

it is desirable to use lower design temperatures and allow the building to overheat occasionally. One of the reasons that more extreme design conditions are used is to give a design margin and effectively to give the client future flexibility for increased heat loads or for variations/defects in the construction of the building post design stage. However, this should be avoided as it is likely to result in over sizing of plant.

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

3.3.4.

Humidity Humidity is a measure of the moisture content of the air. It is generally measured as relative humidity, which indicates the percentage saturation of the air. Relative humidity (RH) is an important characteristic of climate with regard to building design for the following reasons: o It is a determinant of the comfort zone temperatures. o It determines the effectiveness of evaporative cooling. Generally RH varies inversely with temperature through the day. It is higher in the summer months when rain occurs than in the dry months of winter. For Gaborone the highest hourly average RH is 90% and occursin June. The lowest hourly average RH is 28% and occurs in September. [Bauer Consult]. Maximum RH typically occurs at 7.00am, while minimum RH typically occurs at 5.00pm.

3.3.5.

Radiation Radiation is a critically important characteristic of climate, both at a macro, outdoor level and in relation to indoor climate. Heat transfer by radiation is proportional to the difference in temperature of the surfaces raised to the fourth power. It is therefore a minor component of total heat flow between surfaces where the temperature difference is small, and rapidly becomes the major component of heat flow when

Energy Efficiency Building Design Guidelines – Section 3. Climate

temperature difference increases. It is also affected by other characteristics of the surfaces, including colour and texture, as well as the translucence of the intervening space. During the day radiant heat transfer between a building and its surroundings is primarily in the form of solar heat gain, and includes direct, diffuse and reflected radiation. During the night, radiant heat loss to the night sky occurs from any surface in view of the sky. Total solar radiation received on a horizontal surface has been recorded at Sebele since 1977. For other locations it has been calculated from recorded measurements of bright sunshine duration using the Angstrom formula. The annual average daily total radiation on a horizontal surface varies between 19.6 MJ/m2.day in Sebele, to 22.0 MJ/m2.day in Tsabong. [Bhalotra] The monthly average daily total radiation on a horizontal surface for Gaborone varies from 14.6 MJ/m2.day in June, to 26.2 MJ/m2.day in December. [Bhalotra] The indoor radiant environment is often underestimated as a factor in determining comfort. A space may feel uncomfortably hot even when the air temperature is several degrees below the minimum comfort level, if there is a hot surface in view (such as the sun, seen through a window, or even a warm wall). Likewise, a space with an air temperature higher than the maximum comfort level may feel cold if there is a view to a cold body such as the night sky. (See Section 4, Indoor Environment.)

Page 13

3.3.6.

Wind Wind is significant in energy efficient building design as a driving force for ventilation, which is of benefit in the following ways: o Natural ventilation to improve air quality. o Natural ventilation to provide cooling air movement. o Wind driven evaporative cooling. Wind driven infiltration is a problem in the following ways: o Heat loss through infiltration. o Heat gain through infiltration. o Excessive air speeds due to infiltration in high winds. o Entry of dust or other contaminants due to infiltration. Wind direction for most of Botswana is predominantly from the East, with a significant component from the south to southwest in the extreme southwest of the country. There are extensive periods of calm, e.g. 37.7% for Gaborone. It would be important to analyse wind data to determine whether there is a difference between the dominant wind direction for light winds and for strong winds.

Page 14

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

3.3.7.

Rainfall Rainfall has limited direct effect on building energy performance, but is important since it is closely linked to other climate variables. For example, in a year of good rainfall, the hottest month of the year is frequently October, which in such years is generally still dry with little cloud cover. Rainfall during the months of December and January helps to reduce temperatures through evaporation and reduced sunshine hours. In years of drought, the reverse is the case, with temperatures in December and January exceeding those of October.

The variations in climate across the country are such that they need to be taken into consideration in building design for comfort and energy efficiency. Fig. 3.7 shows the monthly mean maximum and minimum temperatures for various locations around Botswana.

Rainfall must be taken into consideration in designing the landscape around a building. Plants that require much irrigation should be avoided, since water is a scarce resource in Botswana. Opportunities for using greywater should be considered in any building project. The website at www.oasisdesign.net has useful information on practical greywater design solutions.

3.4.

Climatic Zones. Botswana extends from latitude 17°,50’ at Kasane in the north, to latitude 26, 59’ at Bokspits in the south. The western border with Namibia runs along longitude 20, 0’ E, while the confluence of the Limpopo and Shashe rivers in the east is located at longitude 29°, 30’E. The country spans approximately 1,100 km from north to south, and 965km from west to east. Fig.3.6 Map of Botswana (source: US – CIA)

Energy Efficiency Building Design Guidelines – Section 3. Climate

Page 15

40.0

35.0

F'town Max F'town Min Gab Max

30.0

Gab Min Ghanzi Max Ghanzi Min

25.0

Kasane Max Kasane Min M'h'pye Max

20.0

M'h'pye Min Maun Max Maun Min

15.0

Shakawe Max Shakawe Min Tsabong Max

10.0

Tsabong Min Tshane Max 5.0

Tshane Min

0.0 JUL

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUN

Fig. 3.7 Temperatures in different locations. (1961-1990)

Page 16

Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

90.0 F'town 0800 80.0

F'town 1400 Gab 0800 Gab 1400

70.0

Ghanzi 0800 Ghanzi 1400

60.0

Kasane 0800 Kasane 1400

50.0

M'h'pye 0800 M'h'pye 1400

40.0

Maun 0800 Maun 1400

30.0

Shakawe 0800 Shakawe 1400 Tsabong 0800

20.0

Tsabong 1400 Tshane 0800

10.0

Tshane 1400 0.0 JUL

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUN

Fig. 3.8 Relative Humidity in different locations. (1961-1990)

Energy Efficiency Building Design Guidelines – Section 3. Climate

Page 17

There is considerable variation in temperature in different areas of Botswana. Generally winter minimum temperatures are higher the further north you go, with average minimum temperatures in July of 1°C for Tsabong, compared to 11°C in Kasane. Extreme minimum temperatures vary much more, with the coldest monthly mean temperature in Tsabong being –9.5°C compared to 3°C in Kasane. Maximum summer temperatures show less variation, with the mean maximum temperature for January of 35.1°C in Tsabong, compared to the mean maximum temperature for October in Kasane of 33.9°C. The highest monthly mean temperature in Kasane was 41.5°C compared to 42.1°C in Tsabong. In the north of the country there is little or no need for winter heating, whereas this is required in the south, and particularly southwest. It is recommended that for building energy purposes the Ngamiland District and Chobe District which include Maun, Shakawe and Kasane should be regarded as the Northern Climate Zone, and the remainder of the country be regarded as the Southern Climate Zone.

3.5.

Climate Patterns. In addition to the geographical variations in climate, there are also patterns of climate within one locality. In considering the impact of climate on building energy performance, it is important to consider the different patterns that occur, and differentiate these from the average characteristics, which may never actually be experienced. During the winter there tend to be a succession of cold fronts that move across southern Africa from the south to the north. These are experienced in Botswana as a period of time, ranging from a few days to about two weeks with low temperatures, cold southerly winds, and clear skies. In low lying areas between hills, these are the times when frost is experienced. Typically these cold spells occur in the month of June, or Seetebosigo (don’t visit at night). In between these cold fronts, the winter weather may be relatively warm during the day, when the sun warms the still air, and cool at night with the minimum temperature experienced at dawn when the earth has had a full 9-10 hours of radiation to the clear night sky. The difference may be as much as 8°C in minimum temperature within a week. If the average hourly temperature were to be taken for design purposes, the actual conditions would never be reflected. In the summer there is perhaps even more variety in climatic patterns, with some years being generally dry years of drought, some years wet, with ‘good’ rains, and many years falling somewhere in between. During years of good

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Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

rain, a daily cycle may occur for several weeks at a time, with clear skies until mid afternoon, when thunderclouds roll in from the southwest, breaking into a violent thunderstorm in the late afternoon. When this has exhausted its load of moisture onto the earth, the clouds simply disappear, leaving a clear sky at or just after sunset, and throughout the night. Following a cycle of such daily storms, there may be a period of dry weather with not a cloud to be seen for days or even weeks at a time. Again, the average data for a month that includes both types of weather pattern would provide a weather picture that may never actually occur in reality.

3.6.

Resource Material

3.6.1. Books and papers Anderson, R. 1970. ‘Climatic Factors in Botswana’. Botswana Notes and Records Volume 2 pp. 75-78. The Botswana Society. Bauer Consult. Gaborone climatic data based on hourly data for years 2000-2002, provided by Department of Meteorological Services. Bhalotra, Y.P.R. 1987. Climate of Botswana Part II: Elements of Climate. Department of Meterorological Services. 1. Rainfall. 2. Sunshine and Solar Radiation & Evaporation. 3. Temperatures & Humidity of the Air. 4. Surface Winds & Atmospheric Pressure. Bhalotra, Y.P.R. 1985. Rainfall maps of Botswana. Department of Meterorological Services. van Deventer, E.N. 1971. “Climatic and other Design Data for Evaluating Heating and Cooling Requirements of Buildings” CSIR Research Report 300. Reprinted as CSIR Report Number: BOU/R9704, June 1997. Green Building Guidelines: Meeting the Demand for Low-energy Resource-Efficient Homes, 2004. Sustainable Buildings Industry Council. Hamilton, L.B., et. al. 1984. Passive Solar Design Workbook. BRET. Botswana.

Energy Efficiency Building Design Guidelines – Section 3. Climate

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Lechner, N. 1990. Heating, Cooling, Lighting – Design Methods for Architects. USA. John Wiley & Sons. 3.6.2. Web resources ASHRAE American Society of Heating, Refrigerating and Airconditioning Engineers. http://www.ashrae.org/ CIBSE Chartered Institute for Building Services Engineers http://cibse.org/

South African Weather Service http://www.weathersa.co.za/ SQUARE ONE environmental design, software, architecture, sustainability. http://www.squ1.com/site.html U.S. DOE Energy Efficiency and Renewable Energy (EERE) Home Page http://www.eere.energy.gov/ WBDG - Whole Building Design Guide http://www.wbdg.org/

Department of Meteorological Services, Botswana Government. http://www.weather.info.bw/ EDR. Energy Design Resources http://www.energydesignresources.com/ EERE Building Technologies Program Home Page http://www.eere.energy.gov/buildings/ Intergovernmental Panel on Climate Change http://www.ipcc.ch/ Oasis Design http://www.oasisdesign.net/ SBIC. Sustainable Buildings Industry Council. http://www.sbicouncil.org/

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Energy Efficiency Building Design Guidelines for Botswana – Section 3. Climate

SECTION 4

INDOOR ENVIRONMENT COMFORT TEMPERATURE GABORONE 2000-2002 (based on Tc=13.5+0.54To) 35.0

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ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 4.

INDOOR ENVIRONMENT

5

4.1. Overview

5

4.2. Elements of Indoor Environment.

5

4.3. Climatic aspects of the indoor environment. 4.3.1. Temperature and humidity. 4.3.2. Mean radiant temperature. 4.3.3. Air velocity.

6 6 6 7

4.4. Non-climatic aspects of the indoor environment. 4.4.1. Air quality. 4.4.2. The aesthetic environment. 4.4.3. Lighting levels and daylighting. 4.4.4. Static electricity 4.4.5. Ionising radiation

7 7 8 9 9 9

4.5. Human comfort. 4.5.1. Mechanisms of heat exchange. 4.5.2. Evaporation. 4.5.3. Convection. 4.5.4. Radiation. 4.5.5. Conduction.

9 10 10 11 11 11

4.6. Factors affecting human comfort. 4.6.1. Clothing. 4.6.2. Activity. 4.6.3. The comfort zone - the psychometric chart.

12 12 13 13

Energy Efficiency Building Design Guidelines for Botswana – Section 1. Introduction

Page 3

4.7. Specification of the indoor thermal environment. 4.7.1. ASHRAE Standard 55-2004. 4.7.2. Adaptive comfort. 4.7.3. Adaptive comfort in conditioned buildings. 4.7.4. Adaptive comfort applied to Botswana climate.

14 15 17 19 19

4.8. Resource material 4.8.1. Books and papers 4.8.2. Codes and Standards. 4.8.3. Websites.

21 21 21 21

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.

INDOOR ENVIRONMENT

4.1.

Overview

climatic comfort will be discussed, as well as the factors that affect this. Standards are available that attempt to define indoor conditions that will be experienced as ‘comfortable’. These are briefly considered, as well as some recent developments in our understanding of how to define standards for comfort, particularly with regard to improving energy efficiency in buildings.

This Section addresses the subject of indoor environment and its impact on building energy performance in Botswana. The topics that will be covered are briefly outlined below. A building may be defined as: A structure that provides spaces having an environment that is amended from that of its surroundings to suit particular purposes.

4.2.

The definition of the indoor environment that will be suitable for a particular purpose is therefore very important, as this is a key component of the specification for the building. Indoor environment has a strong relation to energy performance in most buildings, since a large proportion of the building’s energy consumption is used to amend the indoor environment particularly the climate and lighting. The paper will consider the elements that make up the indoor environment, which include both climatic and nonclimatic aspects. Human comfort is often the main requirement of the indoor environment. The processes that the body uses to achieve

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

Elements of Indoor Environment. The concept ‘indoor environment’ includes all aspects of the relationship between the occupants and contents of a building and their surroundings within the building. This may be considered in terms of climatic and non-climatic aspects, which are defined by the following principle parameters: Climatic: o Dry Bulb temperature. o Relative Humidity. o Mean radiant temperature. o Air velocity. Other parameters: o Air quality. o Aesthetic environment including Spatial geometry Colour. Views. o Lighting levels and daylighting.

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o o o o o

4.3.2.

Acoustic environment. Vibration. Static electricity Ionizing radiation Occupancy density

These are discussed in more detail in the following sections, with emphasis on parameters relating to energy consumption.

4.3.

Climatic aspects of the indoor environment.

4.3.1.

Temperature and humidity. Temperature and humidity are the most important aspects of the indoor climate. They largely determine human comfort; due to the impact they have on several of the body’s heat transfer mechanisms (see below). Storage of sensitive materials such as books, paper, food, medicines etc. and specifications for machines and equipment may dictate particular requirements for temperature and relative humidity other than for human comfort. Relative humidity needs to be controlled both for comfort, but also to prevent algae, moulds, fungi etc from forming. Condensation at cold surfaces can also cause problems if humidity is too high. Part of the ventilation requirement comes from the fact that humans emit humidity into the air.

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Mean radiant temperature. The radiant environment may be as important a criterion for comfort as temperature and humidity. The extent of radiant heat transfer between the body and the environment is mainly dependant on the following: Geometric arrangement of the radiating surfaces. Surface characteristics of opaque surfaces (wall, ceiling, floor): o Surface colour and texture (emissivity). o Surface temperature. Characteristics of translucent surfaces (window): o Transmissivity o Temperature / surface characteristics of bodies beyond the translucent surface (e.g. sun or night sky) Human body: o Surface area exposed o Colour / texture of clothing. Radiant heat transfer will be particularly significant in spaces in which people are exposed to large surfaces that are at a temperature that is different from the ambient temperature. This may be the case in buildings with large areas of glazing. If these are orientated to admit direct sun, this can be a source of heat gain. Thermal mass walls, floors and ceilings may be used for radiant cooling if the surface temperature is lower than ambient. However it is generally recommended (e.g. by the Danish Building Institute) not to have a ΔT > 5-10° C to avoid

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

difference) will generally cause discomfort due to draught (typically at air velocities > 0.2 m/s – especially if the air temperature is significantly different from the comfort temperature (most people will have experienced discomfort from e.g. sitting in the cold air stream of an air conditioner, or the relief a fan can provide in an otherwise stifling heat).

compromising human comfort in locations with stationary workplaces. The radiant environment at any particular location is defined by the Mean Radiant Temperature, which is defined as: “the uniform surface temperature of a black enclosure with which an individual exchanges the same heat by radiation as the actual environment considered”. The weighted average of the Mean Radiant Temperature and the Dry Bulb temperature is termed the ‘Operative Temperature’ and is the temperature that is generally used in standards for human comfort (e.g. the ASHRAE Standard 55-2004). Buildings with heavyweight ceilings and floors tend to have a lower Mean Radiant Temperature than those with lightweight partitioning elements due to the thermal capacity of these elements. As a result they have a lower Operative Temperature in the summer, even when the air temperature is the same resulting in a more comfortable indoor environment. 4.3.3.

4.4.

Non-climatic aspects of the indoor environment.

4.4.1.

Air quality. Air quality is an important aspect of the indoor environment that is often neglected in naturally ventilated buildings. It may also conflict with other strategies for energy efficiency.

Air velocity. Air movement affects both convection and evaporation, which are important methods of heat loss from the body. The comfort temperature is highly dependant on air velocity, particularly if light clothing is worn. Control of air movement with fans is an important opportunity to give individuals control over their climatic environment. Using air movement to control comfort is a delicate balance since too high an air velocity (or too large a temperature

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

Reduction of infiltration is an important strategy to reduce energy consumption. This however has the effect of reducing natural ventilation, which allows the build up of indoor air contaminants. Indoor air quality is determined by many factors, including: o Equipment and appliances used in the building. o Occupant activity (e.g. smoking). o Building materials. o Outdoor air quality. Typical contaminants that affect air quality include gasses, particularly Carbon Dioxide, vapours and odours, fungi, moulds, dust particles that may be biological or mineral in origin.

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Indoor air quality may be controlled by adopting standards for ventilation such as ASHRAE Standard 62-1989 or the proposed CEN standard “Ventilation for Buildings. Design Criteria for the indoor environment.” CEN/CR 1752: 199812; CEN; Bruxelles 1998. The ASHRAE standard is currently under revision, and sets requirements for outdoor air ventilation for different purposes (typically 2.5l/s per person for office spaces). Danish guidelines recommend at least 7 l/s per person in offices (4 l/s per person is the minimum), where smoking is not permitted. If smoking is permitted 10 l/s pr person is the minimum and 20 l/s pr. person is recommended. Danish Building regulations require a minimum 0.5 ACH (air changes per hour). Alternatively performance criteria may be adopted, specifying target concentrations of contaminants. An example of such criteria is the National Ambient Air Quality Standards that are defined by the EPA as a requirement of the Clean Air Act (USA). Such standards are difficult to implement, due to the problem of measuring a large number of different potential contaminants. 4.4.2.

The aesthetic environment. The aesthetic environment is an important aspect of the indoor environment.

of spaciousness, but can also be intimidating, whereas low ceilings can make a room feel more intimate. Spatial geometry also affects the air temperature and air movement in a room. High ceilings can allow stratification of air, so that the warmer air rises above the inhabited zone, which will be cooler. Colour is a very important aspect of the indoor environment. People respond to colours with their emotions and feelings, and colour can be used to change the perception of space, e.g. a dark colour on the ceiling makes it appear lower. Colours also impact on illumination contrasts which if too high may cause discomfort. Colour is an integral aspect of lighting design; light colours reflect light, and can reduce the number and power of light sources required to achieve a particular lighting level. White ceilings combined with light shelves can allow daylight to penetrate deeper into a building. Views from windows change the way people respond to indoor environments. The opportunity to see outdoors can make people feel less enclosed, which can affect their work performance positively. Views to green areas, vegetation and water are generally considered to positively affect the perceived comfort of indoor environments. Excessive distraction can also reduce performance, especially in classrooms, where views may need to be limited to avoid this.

The geometry of the spaces in the building affects how people respond to the rooms. High ceilings create a feeling

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.4.3.

Lighting levels and daylighting. The light characteristics of the indoor environment have a major impact on almost all activities that take place there, and also on the energy performance of the building.

It is not especially relevant in relation to energy efficiency. 4.4.5.

Ionising radiation Increasing concern is being focussed on sources of ionising radiation, as for example from leakage of the gas Radon from the ground. This is a very localised phenomenon that appears not to have been sufficiently researched in Botswana to determine the extent to which it may be a problem.

4.5.

Human comfort.

The details of what these characteristics should be, and how they may be achieved in an energy efficient manner are discussed in detail in Section 9, Lighting, Artificial and Daylighting. Specifications of lighting levels required for different tasks have been defined in various standards, codes and guidelines. Recent research has focussed on the impact of the quality of lighting as well as the quantity. For many years the importance of factors such as colour response of different light sources have been studied. Many studies have been conducted on the impact of lighting levels on behaviour, including productivity, retail sales, absenteeism, etc. These have demonstrated a strong correlation between behaviour and lighting levels including for example, significant increases in retail sales with higher light levels. 4.4.4.

Static electricity Static electricity tends to be an important criterion for comfort in environments with low air humidity; frequently the case in Botswana. It is more serious where insulating floor finishes are used, and is a particular problem in environments in which sensitive electronic equipment is used, manufactured or repaired.

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

Human comfort is related to the individual’s perception of the quality of the environment in which he or she is situated. People experience the environment differently, so that one person may feel uncomfortably hot and another too cold in the same place. Designing for human comfort is therefore always a compromise, the aim being to provide an indoor climate that is experienced as adequately comfortable by a large majority of people. A common unit for the measurement of human comfort is the PMV. This is the ‘predicted mean vote’, and indicates the percentage of people who are predicted to feel comfortable in any given set of conditions. In practice it is generally difficult to get acceptance ratings much over 80-90%, which are therefore generally used as normal design values. The physiology of human beings as warm-blooded mammals requires the internal body temperature to be maintained within very close limits (between 36°C and 38°C, the normal temperature being 37°C). If it falls below 30°C or rises above 41°C, death is imminent. Considering that humans live in environments where the external

Page 9

temperature varies between -40°C to over 50°C, this is quite a demanding requirement. The body has a number of mechanisms that it uses to achieve this. Heat is released into the body by all metabolical processes, including eating, respiration, movement, etc. In order to maintain a balanced temperature, the body must therefore find ways to lose heat at the same rate at which it is being produced by these processes. 4.5.1.

The rate of heat transfer by evaporation is determined by the rate at which moisture can be removed by the air. This in turn is dependant on both the capacity of the air to absorb moisture, and the rate of movement of the air. The capacity of the air to absorb moisture is dependant on its relative humidity, which is a function of temperature and absolute moisture content.

Mechanisms of heat exchange. There are essentially four basic mechanisms by which the body exchanges heat with its environment. o Evaporation. o Convection. o Radiation. o Conduction. Evaporation and convection are mechanisms of heat loss for the body. Radiation and conduction can result in either heat gain or heat loss depending on the temperature of body relative to its surroundings.

4.5.2.

Evaporation. Evaporation takes place during respiration, whereby fluid from the body enters the air that we breath in the lungs and the respiratory duct and is evaporated, absorbing the latent heat of evaporation from the surfaces of these organs. Evaporation also takes place at the skin as a result of perspiration.

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.5.3.

Convection. Convective heat transfer occurs where the skin is in contact with a fluid at a different temperature, such as air. The rate of heat transfer by convection is determined by the difference in temperature, and the flow rate of the fluid (air speed), as well as the geometry of the surface-flow interface (e.g. the exposed surface area).

4.5.4.

Radiation. Radiant heat transfer takes place between any two bodies that are in sight of each other and at different temperatures. The rate of heat transfer by radiation is determined by the relative areas of the two surfaces, their surface temperatures and their emmittance / absorbtance properties at the respective wavelengths relating to these temperatures.

4.5.5.

Fig 4.1 Heat exchange between the human body and its surroundings.

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

Conduction. Heat transfer by conduction occurs where the skin is in direct contact with another surface, such as the floor, and there is a difference in temperature between the surfaces. The rate of heat transfer by conduction is determined by the conductivity of the two surfaces, their heat capacity and the difference in temperatures.

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summer, as a way to reduce energy consumption in office buildings.

4.6.

Factors affecting human comfort.

4.6.1.

Clothing. All the four mechanisms of heat transfer are greatly influenced by clothing. This can provide insulation to reduce mainly convective and radiant heat transfer (but also conductive – e.g. wearing gloves and protective clothing when hot or very cold surfaces/objects are handled. Shoes reduce heat loss/gain from the floor). Clothing can prevent air movement at the skin, which almost eliminates convective and evaporative heat transfer from the skin. The effect of clothing on evaporative heat transfer is dependant on the type of material. Some materials, such as cotton and wool allow moisture to pass through, and therefore do not inhibit evaporative heat loss as much as non-porous materials. Clothing can also reduce radiant heat transfer, as the thermal resistance of the clothing will reduce the flow of heat from the body. By selecting appropriate clothing for a particular climate and activity, the range of indoor climate that is experienced as comfortable can be considerably extended, both to lower temperatures, if insulating clothing is worn, or to higher temperatures with clothing that allows free flow of air to a larger area of the body.

A measure of the thermal resistance of clothing has been developed, called the ‘clo-value’. This is a measure of the ratio of thermal resistance of clothing to a standard value of 0.155m2K/W, which is typical of a business suit. Typical clo values are as given in Table 4.1 Clo – value 0 0.5 1.0 2.0

Example Naked, swimwear Light trousers + shirt, light dress + blouse Business suit, dress + jumper Heavy suit, overcoat, gloves and hat

Table 4. 1.Typical clo-values. Source: www.esru.strath.ac.uk/Courseware/Class-16293/6Comfort.pdf

Cultural aspects can have an important influence on what is acceptable attire for particular activities. Recently the Japanese government has introduced a policy called ‘Cool Biz’ to discourage the wearing of jackets and ties in the

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.6.2.

Activity. As stated earlier, the body is continuously producing heat as a result of metabolic processes. The rate of heat production varies greatly depending on the activity that one is engaged in. Typical rates of heat output are presented in Table 4.2. Activity Sleeping Seated Light work Medium work Heavy work

Heat output (male): Watts 70 115 150 265 440

Heat output (female): Watts 60 98 128 225 374

Table 4.2. Heat output for different activities. 4.6.3.

The comfort zone - the psychometric chart. Human comfort in the indoor environment is related to the interaction of a large number of variables in addition to temperature.

Fig 4.2 Psychometric chart (source: The Psych Tool, Square One Research Ltd.).

These can be illustrated by means of a psychometric chart, which shows the interaction of temperature and humidity. The combination of these parameters that is experienced as comfortable can be shown for different levels of clothing and air speed. This is illustrated in Fig. 4.2.

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

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4.7.

o there is no significant difference due to ethnic origin.

Specification of the indoor thermal environment. The specification of the indoor climate is an important component of an effective design brief for any building, particularly in relation to energy performance. The preceding discussion on the impact of climate on performance shows how comfort is influenced by factors such as activity and clothing. Other factors also influence comfort, including expectations based on recent weather. There is also considerable variation between individuals in their perception of comfort. As a result it is impossible to satisfy all the occupants of an indoor space.

Based on these studies, empirical formulae have been prepared that predict the degree of comfort that will be reported by a certain proportion of occupants under particular conditions.

Many studies have been conducted to determine the conditions for human comfort. These included surveys of large numbers of people who were asked to indicate their level of comfort on a scale from say, -3 (cold) to +3 (hot). This is known as the PMV scale (Predicted Mean Vote). Tests were carried out on large groups of individuals by Fanger in Denmark and by others in many other countries. Fanger concluded that: o there is no significant difference in comfort perceptions due to geographical location or season (including tropical regions); o there is no significant difference due to age (e.g. because older people have lower metabolic rate counteracted by lower perspiration rates); o there is no significant difference due to sex; o there is no significant difference due to body build;

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.7.1.

ASHRAE Standard 55-2004. The most commonly accepted standard specifying the thermal indoor environment for comfort is the ASHRAE Standard 55-2004 - Thermal Environmental Conditions for Human Occupancy (ANSI Approved). This is similar to the CIBSE Standard 55-1992 - Thermal Environmental Conditions. This standard specifies the combinations of indoor space environment and personal factors that will produce thermal environmental conditions acceptable to 80% or more of the occupants within a space. The environmental factors addressed are temperature, thermal radiation, humidity, and air speed; the personal factors are those of activity and clothing.

evaporative cooling in situations where it may in fact be effective.

The ASHRAE standard has an upper limit for humidity ratio of 0.012. This translates approximately to a relative humidity of 75% at a dry bulb temperature of 21°C and 53% at 27°C. In Botswana RH is often above the minimum level, particularly in the mornings in summer. There is some doubt as to whether this requirement is fully justified. The characteristic that is defined is humidity ratio, whereas it may be more appropriate to define relative humidity. It may also be that the boundary is lower than necessary in terms of people’s perception of comfort. The specification to be adopted for maximum humidity has a big impact in determining the conditions under which evaporative cooling is effective. An unnecessarily low ceiling for humidity would therefore restrict the use of

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

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Fig 4.3 Simplified graphs indicating winter and summer comfort zones based on ASHREA 55-2004 (Source: Energy Plus Reference Manual)

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.7.2.

Adaptive comfort. The standard acknowledges the concept of adaptive comfort. It has been found that people experience thermal comfort quite differently in buildings that are naturally ventilated without mechanical cooling than in buildings that are mechanically cooled. The standard specifies a far more relaxed set of comfort conditions for such buildings, with the requirement that people should have the opportunity to open and close windows, and freedom to adapt their clothing to achieve comfort. The comfort zone is then related to mean outdoor air temperature, and for typical January conditions in Gaborone (To=25°C) would be between 22-29°C. The acceptable temperature range for air-conditioned buildings in the same situation is between 25-28°C for light clothing (0.5clo) or 19-25°C with more formal clothing (1.0clo). The specification for air-conditioned buildings also requires that the variation in temperature during any 15min period is no more that 1.1°C. This typically determines the cycling band for the control system. No such requirement is made for the adaptive comfort specification, since it is assumed that people will respond to any variations within the comfort zone by making adjustments to their clothing, or ventilation.

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

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Fig. 4.4 Adaptive comfort temperatures (Source: ASHRAE Standard 55-2004)

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.7.4. 4.7.3.

Adaptive comfort in conditioned buildings. It appears that there is considerable scope for energy savings if the concepts of adaptive comfort could also be applied to conditioned buildings. This would require an approach that allowed some user adaptation within the overall framework of a controlled mechanically conditioned building. A simple example of such an approach would be the use of individually controlled fans and radiant heaters to allow individuals more control of their immediate surroundings. Encouraging the use of thermally appropriate clothing would further relax the demands on the mechanical equipment. Research summarised by Nicol, J.F. and Humphreys, M.A. in their paper “Adaptive thermal comfort and sustainable thermal standards for buildings.” [3] suggests that the monthly mean temperature may not be the most appropriate for determining the comfort zone in an adaptive comfort model, and suggest that a method that accounts for the temperature variation of the previous few days would provide a more accurate model. An algorithm for determining this is proposed, which could also be used control temperature in conditioned buildings, resulting in substantial energy savings.

Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

Adaptive comfort applied to Botswana climate. The relationship between indoor comfort temperature and outdoor mean temperature has been consistently found to be close to: Tc = 13.5 + 0.54To Where

Tc = Thermal comfort temperature To = Monthly mean outdoor temperature

When this is applied to the temperature data for Gaborone, the thermal comfort temperature is as shown in Fig.4.5. This indicates that at all times of the year the comfort temperature is above the mean monthly temperature. This suggests that for buildings for which the envelope loads dominate, thermal comfort should be achievable with no mechanical equipment, although other aspects of indoor environment may still require this, e.g. mechanical ventilation to achieve air quality standards. Even buildings with more substantial internal loads could be comfortable with minimal energy use if this is acknowledged as an important design criterion, and the knowledge and tools to achieve it are available.

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COMFORT TEMPERATURE GABORONE 2000-2002 (based on Tc=13.5+0.54To) 35.0

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Fig. 4.5 Comfort temperature for Gaborone.

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

4.8.

Conference Paper. Presented at the American Solar Energy Society Conference Madison, Wisconsin June 16, 2000

Resource material

4.8.1. Books and papers Energy Plus. “Input Output Reference - The Encyclopedic Reference to EnergyPlus Input and Output” December 2005. Green Building Guidelines: Meeting the Demand for Low-energy Resource-Efficient Homes, 2004. Sustainable Buildings Industry Council. Hamilton, L.B., et. al. 1984. Passive Solar Design Workbook. BRET. Botswana. Hunn, B.D. (ed) 1996. “Fundamentals of Building Energy Dynamics.” Massachusetts Institute of Technology. Koch-Nielsen, H. 2002 Stay Cool - A design Guide for the Built Environment in Hot Climates. London: James & James (Science Publishers) Ltd. Lechner, N. 1990. Heating, Cooling, Lighting – Design Methods for Architects. USA. John Wiley & Sons. Nicol, J.F. and Humphreys, M.A. “Adaptive thermal comfort and sustainable thermal standards for buildings.” Oxford Centre for Sustainable Development, School of Architecture, Oxford Brookes University. Plympton, P. et. al. “Daylighting in Schools: Improving Student Performance and Health at a Price Schools Can Afford.”

Tutt, P. and Adler, D. (Ed.). 1979. New Metric Handbook – Planning and Design Data. Oxford: ButterworthHeinemann Ltd. University of Strathclyde. Unit 6 Thermal Comfort. Course 16293: “Environmental Engineering Science 1.” Course material for Energy Systems Research Unit, (ESRU), University of Strathclyde. 4.8.2. Codes and Standards. ASHRAE Standard 55-2004. Thermal Environmental Conditions for Human Occupancy ASHRAE Standard 62.2-2004 – Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings (ANSI Approved) CEN Standard: “Ventilation for Buildings. Design Criteria for the indoor environment. CEN/CR 1752: 1998-12; CEN; Bruxelles 1998 4.8.3. Websites. ASHRAE American Society of Heating, Refrigerating and Airconditioning Engineers. http://www.ashrae.org/ CIBSE Chartered Institute for Building Services Engineers http://cibse.org/

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EERE Building Technologies Program Home Page http://www.eere.energy.gov/buildings/ EDR. Energy Design Resources http://www.energydesignresources.com/ SBIC. Sustainable Buildings Industry Council. http://www.sbicouncil.org/ SQUARE ONE environmental design, software, architecture, sustainability. http://www.squ1.com/site.html WBDG - Whole Building Design Guide http://www.wbdg.org/

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Energy Efficiency Building Design Guidelines for Botswana – Section 4. Indoor Environment

SECTION 5

DESIGN & CONSTRUCTION PROCESS

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 5.

DESIGN & CONSTRUCTION PROCESS

5

5.

DESIGN & CONSTRUCTION PROCESS

5

5.1. Overview 5.1.1. Project cost and energy efficiency 5.1.2. Procurement systems 5.1.3. Integrated Design Methods. 5.1.4. Construction and Commissioning.

5 5 5 5 5

5.2. Project Cost and Energy Efficiency

5

5.3. Procurement Systems. 5.3.1. Conventional appointment of consultants. 5.3.2. Competitive tendering. 5.3.3. Turnkey development. 5.3.4. Public, Private Partnership. 5.3.5. Fee incentives for energy efficiency.

7 7 8 8 9 9

5.4. Integrated Design Methods. 5.4.1. Integrated design coordinator. 5.4.2. Structured methodology. 5.4.3. Incentives. 5.4.4. Timing of design decisions. 5.4.5. Construction.

12 13 13 14 15 16

5.5. Commissioning.

16

5.6. Resource Material

17

Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

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5.6.1. 5.6.2.

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Books and Papers Web resources

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Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

5.

DESIGN & CONSTRUCTION PROCESS

5.1.

Overview This section addresses the subject of the design and construction process and its impact on building energy performance in Botswana.

5.1.3.

Integrated Design Methods. Substantial improvements in energy efficiency have been achieved through the development and implementation of what is known as ‘integrated design’. This is essentially a holistic approach to the design, construction, operation and demolition of a building.

5.1.4.

Construction and Commissioning. The effort that has gone into achieving an energy efficient building design can easily be compromised in the construction process if adequate supervision and coordination is not provided to ensure that the critical aspects of the building meet the design requirements.

The following topics are covered in this section: o Project cost and energy efficiency. o Procurement systems and their implications for energy performance. o Integrated design methods. o Construction and Commissioning. 5.1.1.

Project cost and energy efficiency The relationship that exists between project cost (capital and recurrent) and energy efficiency is described.

5.1.2.

Procurement systems The process and methodology by which the design, construction, operation and demolition of buildings is implemented has gone through dramatic changes over past 30 years in many countries of the world. A number of different approaches to procurement of design services are now implemented, including competitive tendering, turnkey development and Public / Private Partnership. The implications of these approaches with regard to improving energy efficiency are considered, as well as the relationship between initial cost, life-cycle cost and energy efficiency.

The systematic application of commissioning to both new and existing buildings has been found to be a highly cost effective means to ensure that the building and all its systems are functioning as intended. It can lead to dramatic improvements in energy efficiency, and overall environmental performance.

5.2.

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

Project Cost and Energy Efficiency Over the past two or three decades there has been an increasing concern for energy efficiency generally, including in the building sector. This has been driven by the increasing cost of conventional sources of energy, as reserves of fossil fuel are becoming more scarce, as well as the impact of our rapidly increasing energy consumption on the local and global environment in the form of pollution and climate change.

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This has resulted in a change in the way in which building costs are viewed and assessed. Previously the main concern was with the initial construction cost of a building, and in many cases this is still the only cost that is taken into consideration in the design stages of a building project. The project manager is asked to prepare project budgets, and decisions are based largely on an assessment of whether the client can afford particular features or finishes in the building. There is a growing awareness that the initial construction cost is only one aspect of the overall building cost, and that future costs of operation, maintenance and ultimately demolition may be as important or even more so over the total life of the building. There are many choices of material, design, equipment or finishes that influence ‘lifecycle’ cost in different ways. Some choices may lead to reduced life-cycle cost and save on the construction cost as well. Others may reduce life-cycle costs and have no influence on construction cost, and many interventions may require a trade-off between increased construction cost resulting in reduced life-cycle cost. Life cycle cost is defined more fully in Section 12, LifeCycle Cost Analysis. This also gives a brief introduction to various methods of calculating LCC, as well as references to more detailed information.

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Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

5.3.

Procurement Systems. When a client needs a construction project to be implemented, the first requirement is usually for some professional advice to assist with preparing the design brief and getting started on the process of design and construction. The conventional approach to this has often been to employ a project manager who becomes the client’s agent and manages the project. The project manager then engages an architect to lead the design phase of the project. In many cases the client may employ an architect directly, who then also takes on the functions of project manager. Until recently this was also the most common approach taken by the Botswana Government. Recently however a number of other procurement options have been tried, including: o Competitive tendering for consultancy services. o Turnkey development. o Public, Private Partnership (PPP). These different procurement methods have considerable implications on the financial and other motivations that influence the work of the consultants. These are discussed in the following sections with particular reference to energy efficiency and energy conservation.

5.3.1.

Conventional appointment of consultants. In this case the choice of consultant is based on their reputation for capability, professional integrity, and

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

capacity to carry out the work for a reasonable fee. Fees may be negotiated, but are usually based on agreed standard rates that are set by professional institutes. The initial stages of a project may be paid on an hourly or lump sum basis, but the major portion of fees is generally calculated as a percentage of the contract sum related to that consultant’s scope of work. The consultant therefore does not have a financial incentive to reduce contract cost. He or she does have an incentive to reduce the work required of them in completing the project. Such a consultant has no particular motivation to reduce life-cycle cost, except in so far as this is included as a concern in the design brief. Some may also see it as a fundamental objective in their work, and seek to achieve this as a matter of course. Choices that reduce construction cost will result in reduced fees, and those that increase construction cost will lead to increased fees. The arrangement is based on an assumption of professional integrity, which should ensure that these financial motivations do not affect the consultant’s work in any way. This is to some extent reinforced by the codes of conduct that Professional Institutions require their members to adhere to. In practice it is perhaps rather naïve to assume that all consultants have the integrity to totally disregard the financial implications to themselves of decisions that are made in the design and project management process.

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5.3.2.

Competitive tendering. Recently the Botswana Government changed the standard method of procurement for consultants to a competitive tendering process. A ‘terms of reference’ (ToR) is prepared and advertised. Consultants prepare tenders that are submitted through the Public Procurement and Asset Disposal Board. Typically the ‘two envelope’ system is used, whereby the technical and financial proposals are submitted in separate envelopes. The technical proposals are first evaluated against a set of criteria. The financial proposals of those tenders that score higher than a certain minimum on the technical evaluation are then opened, and the best value tender is selected. Generally competitive tendering has resulted in greatly reduced fees compared to the use of standard fee scales. This is a benefit to the client in that it reduces the portion of project budgets that is spent on fees. It also means that consultants are required to carry out the same amount of work for a lower fee. They are therefore under considerable pressure to minimise their costs in terms of hours spent and the cost of their professional staff (which is generally related to the level of qualification and experience). This may make them reluctant to spend additional time investigating the life-cycle cost implications of different strategies to reduce operating costs generally and energy consumption in particular. The consultant’s terms of reference (or design brief) therefore becomes an even more important document and it is essential that environmental considerations and energy performance requirements in particular are clearly defined.

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There is also now more of a need to verify that the consultant is actually addressing the requirements of the ToR. On large projects it may be advisable to hire an independent consultant to confirm this. The commissioning procedure (see below) can also help to verify performance against targets, but at that stage it may be too late to correct fundamental design issues. If the tender is based on percentage rates, then the financial motivation regarding changes in construction cost versus life-cycle cost will be similar to those for the directly appointed consultant. If the tender is based on a lump sum fee, then there will be neither a fee incentive to increase the construction cost, nor a fee penalty if it is reduced. 5.3.3.

Turnkey development. The turnkey procurement method is a radical departure from the traditional relationship between client, consultant and contractor. The design consultants now become part of the same team as the contractor, and tender for a project as a joint venture. The division of the payment between contractor and consultant is decided between them and does not concern the client. The challenge in this system is to ensure that the client’s requirements in terms of function, performance and quality are achieved. For larger projects this will often require the client to hire an independent consultant to supervise the project and provide expert advice throughout.

Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

The concessionaire therefore has no direct incentive to design and operate the building in such a way as to minimise energy or water use, since the client covers the cost of these.

With this system the interests of the contractor and the design consultants are aligned, and they have a financial motivation to reduce costs once a contract has been signed, in order to maximise their profit. This could result in decisions that result in increased life-cycle costs to achieve reduced construction cost, since the turnkey developer has no further involvement in the project once the contract is completed.

The Request for Proposals (RFP) may however include requirements relating to environmental considerations, energy efficiency, and life cycle costing. The extent to which each proposal addresses these will then be considered in the evaluation of the proposals, and will be one of many criteria used for selecting the successful proposal.

As with the competitive tender procedure, it becomes more important to have a watertight design brief, and a means to verify compliance with the brief. 5.3.4.

Public, Private Partnership. Public, private partnership is a relatively new concept in procurement that is rapidly gaining popularity for medium to large-scale public infrastructure projects, including public buildings. Essentially it takes the turnkey concept further, such that the contractor’s team (the concessionaire) not only designs and builds the project, but also arranges finance, and manages the project for its entire life (or at least a substantial portion thereof). The client in this case pays for the project through ‘unitary’ payments that include for maintenance, building staff, rental, finance, etc. These are calculated as annual payments but are usually paid in monthly instalments rather like a lease charge.

The PPP process includes a procedure to verify that the completed project meets the targets and requirements of the RFP. This is implemented by the concessionaire under the supervision of a client’s representative, and stringent penalties are charged for any failures to comply. At this stage however it is of course too late to rectify any fundamental design faults. 5.3.5.

Utility costs such as electricity and water are treated as ‘through costs’ that are paid by the concessionaire and then charged to the client, with an agreed mark-up for profit.

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

Fee incentives for energy efficiency. In some countries including the USA a system of fee incentives and penalties has been introduced for certain projects, to provide a direct financial incentive to consultants to achieve energy efficiency and other objectives. In this case a certain portion of the fees is retained by the client until the initial commissioning process has been completed, during which the performance of the building is monitored. If it is found that the building achieves or exceeds the performance targets, the consultants are rewarded with a bonus. If performance falls

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short of the targets, the consultants are penalised. In some cases there may even be ongoing rewards for achieving operation and maintenance cost targets. An example of a performance based design contract is provided on the following page.

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Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

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5.4.

Integrated Design Methods. There are substantial opportunities for improving the environmental performance of buildings through what has become known as ‘integrated building design’ (sometimes also known as ‘integrated energy design’). The concept requires a re-thinking of the approach to building design from the one that is traditionally used. Traditionally there has been a tendency to separate out different systems of a building, with each consultant solving the problems that relate to their expertise in relative isolation. Of course from time to time they come together to look at the implications of each other’s work on the building as a whole, and to coordinate the ‘points of contact’. Usually the design process begins with the architect who develops an overall design concept, including the aesthetic and spatial layouts for the building. A structural engineer takes the concept, ensures that it is structurally feasible, and works out the structural system that can support it. A mechanical and electrical engineers then design the HVAC, lighting and other services systems, trying to fit these into the building as efficiently as possible.

The integrated design approach, in contrast, views the building and its surroundings as a whole, comprised of all the different systems interacting with each other to achieve the optimum performance in every respect. There is a deliberate process of looking for opportunities that can arise from these interactions to achieve improved energy efficiency, comfort, quality, beauty, etc.

A number of tools have been developed that can help to achieve a successful integrated design process, some of which are briefly described below.

INTEGRATED BUILDING DESIGN The complete building design concept integrates the different system design concepts. o o o o o

Landscape / environmental design concept. Architectural design concept. Thermal design concept. Structural design concept. Mechanical and Electrical design concept.

If one is included in the team, then the landscape architect will be required to create a suitable surrounding for the building.

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Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

5.4.1.

Integrated design coordinator. From the beginning of the project, a specialised energy consultant is appointed by the client to act as integrated design coordinator. This person is responsible for ensuring that the different members of the design team take into consideration the opportunities that arise in the work of other members, and facilitates the creative interaction between them. He or she is responsible for assessing the life cycle cost implications of different alternative approaches that may be suggested by the team, and coordinates the process of selecting the most appropriate combination of design decisions.

5.4.2.

Communication channels and media should be agreed on at the beginning, with the integrated design consultant acting as the link between other consultants, to ensure that each has the information that they need at each stage. Communication with the client, contractor and users must also be effectively managed, so that they are included in decisions where appropriate, have the information that they need, but are not overloaded with unnecessary information. Key requirements for integrated building design to be successful: o The client is convinced of the benefits of this approach and is willing to invest time and money to achieve these.

Structured methodology. The integrated design approach requires a greater amount of interaction between the consultants, and a more creative and less formal relationship in the stages where the different design concepts are integrated. However, because much of the design work is carried out concurrently, it is essential that the interaction is facilitated by effective structures for the technical communication.

o

Energy efficiency is included as an important objective in the design brief.

o

The work of the design consultants is coordinated towards achieving the agreed objectives.

o

The construction process is monitored and managed effectively.

Details regarding CAD draughting protocols such as layer names and colours, pensize tables, drawing file names, revision numbering, etc. can make an important difference to the effectiveness of communication between consultants.

o

The end users and building operators are trained in the operation and maintenance of the building.

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

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5.4.3.

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Incentives. As can be expected, the integrated design process is not easy, and cannot be achieved without some cost. It will not therefore be generally adopted by choice by consultants unless there is a clear incentive to do so. Much of the potential benefit is only enjoyed by the building owner and / or users during the building’s life time, in the form of reduced energy and other operating costs, better comfort, and a higher quality environment generally. The improvement in these areas can be quite dramatic, with up to 60-70% reduction in energy cost being achieved in certain projects compared to similar, conventionally designed buildings. There is therefore a need to develop an incentive package to compensate the design team for the additional work that is required. This can be done in two distinct ways. One option is to simply pay increased fees up front for the increased service. The alternative is to link the fee to the performance of the building, so that the consultants receive a bonus and / or pay a penalty based on the actual performance of the completed building.

Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

5.4.4.

Timing of design decisions. The timing of design decisions is critical to the success of integrated design. The cost of making changes increases exponentially with time as the design becomes more detailed, whereas the opportunity to achieve energy savings declines. This is illustrated in the graph in Fig. 5.1. Options become more and more limited, and aspects of the design get “locked in”. This implies that if a change has to be made that does not fit in with decisions already taken, it will be more expensive since it requires numerous changes to other aspects of the design that have proceeded on the assumption of the original decision – effectively turning back the clock and starting over in many aspects of the design. It is therefore worth taking the time to carefully consider the options relating to all the design concepts and how they interact with each other early on in the process, to avoid the need to discard large amounts of detail design work later when a more effective solution is suddenly identified. Fig. 5.1 Cost / benefit of design change with regard to energy savings. (Source: ENSAR Group)

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

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5.4.5.

Construction. Design of an energy efficient building is only the first stage. The benefits will only be realised if the construction of the building is carried out in accordance with the design. In practice the quality of work varies greatly from one site to another, and is influenced by many factors, including the quality of the design drawings and specifications, skills of the artisans, contractor’s quality control systems, supervision by the consultants, etc. The contractor should understand the concepts behind the design, so that he / she is aware of the purpose for particular specifications and details. The work on site needs to be regularly inspected and checked to ensure that details that are particularly relevant to energy performance are properly constructed. This requires appropriate training for the people involved whether this is the resident engineer, clerk of works, architect, or others.

5.5.

Commissioning. “Commissioning is a systematic process of ensuring that all building systems perform interactively according to the contract documents, the design intent and the owner’s operational needs.” (The Building Commissioning Guidelines, EDR) The importance of the commissioning process for a building has recently been recognised, particularly as a means to reduce operating costs in general and energy costs in particular. It has been found that a carefully managed, comprehensive commissioning procedure for new buildings can greatly reduce the number of problems that are experienced with building systems in the initial period of occupation, and also improve energy performance. For existing buildings it can be an effective way to identify systems that are not functioning optimally, and to rehabilitate a building to a state where it is functioning optimally resulting in reduced operating and energy costs.

Details that are of particular relevance to energy performance include: o o o o o

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Proper installation of damp proofing membranes. Avoidance of thermal bridges, e.g. in cavity walls. Installation of insulation according to specifications. Seals in ductwork and fittings to avoid leaks. Duct insulation.

Energy Efficiency Building Design Guidelines for Botswana – Section 5. Design and Construction Process

5.6.

Resource Material

5.6.1. Books and Papers Energy Design Resources. Design Brief. Performance Based Compensation. http://www.energydesignresources.com/resource/33/ Energy Design Resources. “The Building Commissioning Guidelines”. http://www.energydesignresources.com/resource/37/ 5.6.2. Web resources EDR. Energy Design Resources http://www.energydesignresources.com/ EERE Building Technologies Program Home Page http://www.eere.energy.gov/buildings/ SBIC. Sustainable Buildings Industry Council. http://www.sbicouncil.org U.S. DOE Energy Efficiency and Renewable Energy (EERE) http://www.eere.energy.gov/ WBDG - Whole Building Design Guide http://www.wbdg.org/

Energy Efficiency Building Design Guidelines - Section 5. Design and Construction Process

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SECTION 6

PLANNING

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 6.

PLANNING

4

6.1. Site Planning

4

6.2. Location.

4

6.3. Orientation.

4

6.4. Surfaces and vegetation. 6.4.1. Ground surfaces. 6.4.2. Trees and shrubs. 6.4.3. Climbers.

5 5 5 6

6.5. Resource Material 6.5.1. Books and Papers 6.5.2. Web resources

7 7 7

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6.

PLANNING The building in relation to its environment / modifying the local climate. Included in this section are the location of the building, the general shape and orientation, and the planning of the immediate surroundings.

6.1.

Site Planning Issues relating to climate and energy efficiency need to be considered from the earliest stages of site planning. Some of the important considerations are as follows: o Location o Orientation o Surfaces and vegetation

6.2.

Location. The location of buildings on a site is largely determined by considerations such as access, planning requirements for site boundary set-backs, views and other constraints. Energy considerations that should be considered include making use of shading from features on or around the plot, such as other buildings or established trees.

6.3.

Orientation. The east and west elevations of a building present most problems related to heating from the sun, since the sun hits these directly in the early morning and late afternoon throughout the year. For this reason buildings should generally present their smaller elevations to the east and west.

Fig 6.1 The sunpath in Gaborone in summer and winter. As plots sizes are reduced to increase plot density, and the spaces around buildings reduce, it becomes more important that the plots are optimally orientated to allow for the houses on the plot to be aligned with the long axis running east-west. The opportunity to save energy by correct orientation is increased if buildings are rectangular, with a high ratio of length to breadth.

6.4.

Surfaces and vegetation.

6.4.1.

Ground surfaces. The surfaces around a building influence the amount of reflected direct and indirect radiant heat that falls on the walls. Using surfaces that absorb solar radiation can help to keep buildings cool in summer. Plants are perhaps the most effective, in that they are good at absorbing solar heat, and also cool the air through evapo-transpiration. There are several species of ground cover plants that can survive long periods with very little water, and are therefore suitable for locations with restricted water availability. Suitable species include members of the following families: Crassula Mesembryanthemaceae

Fig 6.2 Trees as shade for buildings. 6.4.2.

Trees and shrubs. Plants may also be used to good effect for shading and windbreaks. The shape and characteristics of the fully grown plant should be considered in selecting species for a particular location. In some cases it may be helpful to use deciduous trees that provide shade in summer but lose their leaves in winter allowing the sun to provide some warmth when it is needed.

Energy Efficiency Building Design Guidelines for Botswana – Section 6 Planning

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Trees that grow well in Botswana include: Fruit trees: o Paw paw o Banana o Avacodo o Citrus (Lemon, orange, grapefruit) o Guava o Peach o Mulberry

Suitable species of climbers include the following: o Grapes o Jasmine o Morning Glory o Virginia creeper o Honeysuckle o Bougainvillea

Exotic shade trees: o Brazilian Pepper tree o Weeping willow o Jacaranda o Neem o Flamboyant Indigenous trees: o Morula o Terminalia (Mogonono) o Combretum o Acacia (various species) 6.4.3.

Climbers. Climbing plants can be trained over vertical and horizontal structures to provide both shade and windbreaks.

Fig 6.3 Climbers as shade for buildings.

6.5.

Resource Material

6.5.1. Books and Papers Hamilton, L.B., et. al. 1984. Passive Solar Design Workbook. BRET. Botswana. Koch-Nielsen, H. 2002 Stay Cool - A Design Guide for the Built Environment in Hot Climates. London: James & James (Science Publishers) Ltd. Lechner, N. 1990. Heating, Cooling, Lighting – Design Methods for Architects. USA. John Wiley & Sons. Tutt, P. and Adler, D. (Ed.). 1979. New Metric Handbook – Planning and Design Data. Oxford: ButterworthHeinemann Ltd. Ward, Sarah. 2002. The Energy Book for urban development in South Africa. Sustainable Energy Africa. (www.sustainable.org.za) 6.5.2. Web resources Sustainable Energy Africa. http://www.sustainable.org.za

Energy Efficiency Building Design Guidelines for Botswana – Section 6 Planning

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SECTION 7

BUILDING ENVELOPE

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 7.

BUILDING ENVELOPE

5

7.1. Overview 7.1.1. Definition. 7.1.2. Building envelope and energy performance. 7.1.3. Building envelope energy performance in the climate of Gaborone.

5 5 5 6

7.2. Thermal properties of building materials 7.2.1. Material properties. 7.2.2. Construction properties

7 7 8

7.3. Orientation. 7.3.1. Reducing solar heat gain in summer. 7.3.2. Allowing solar heat gain in winter. 7.3.3. Quantifying the effect of orientation.

10 10 10 12

7.4. Characteristics of envelope elements 7.4.1. Ground Floor. 7.4.2. Roof. 7.4.3. Walls. 7.4.4. Fenestration. 7.4.5. Ventilation.

12 12 13 16 18 21

7.5. Codes and Standards

22

7.6. Resource Material 7.6.1. Books and reports. 7.6.2. Codes and Standards. 7.6.3. Web sites.

23 23 23 23

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

7.

BUILDING ENVELOPE

7.1.

Overview

7.1.1.

Definition. The building envelope is defined in this context as those elements of the building that form the boundary between the indoor environment of a building and the external environment in which it is located for example, the floor, walls, roof, windows, etc.

7.1.2.

Building envelope and energy performance. The building envelope is in a sense a filter between the internal and external environments. It serves to protect the indoor spaces from undesirable impacts such as excessive cold, heat, radiation, and wind, while allowing desirable impacts to pass through such as cool breezes on a hot day, warmth from the sun on a cold day, daylight, etc. The building envelope directly influences the energy performance of a building in the following ways: o o o o o o o

Resisting undesirable heat transfer. Allowing desirable heat transfer. Providing heat storage (delayed heat transfer). Allowing daylight penetration. Preventing undesirable light penetration (glare). Allowing desirable ventilation. Preventing undesirable ventilation.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

These are discussed in more detail in the following sections. Essentially there are two different approaches to envelope design in relation to building energy performance, which are described in more detail in the Section 6, Planning. One approach seeks to isolate the interior of the building as much as possible from the external environment. Insulation is used extensively in all the envelope elements to reduce heat transfer as far as possible. Such buildings rely entirely on air conditioning systems to provide heating or cooling to maintain comfort conditions. This is often referred to as an ‘active’ approach to building energy design. Another approach to building energy design is referred to as “passive” design. This seeks to encourage beneficial interactions between the building and the outside environment, while reducing as far as possible the undesirable interactions. In climates such as that of Botswana, where the average daily temperature is generally close to indoor comfort conditions, this approach tends to make use of thermal mass to reduce the extremes of day and night temperature. Careful use of both insulating and conductive materials as appropriate for different elements of the building prevent or encourage heat transfer when it is useful, and controlled ventilation allows air movement through the building to provide fresh air and help to keep the temperature in the comfort zone. When successful, this approach can allow the external environment to address some or all of the internal loads, reducing the energy required by mechanical systems.

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Cooling of the building takes place when heavy elements such as walls absorb heat from the building during the day and release it to outside at night. Ventilation of the building when the outdoor air is cool can also help to cool the building. In winter heat from the sun can be stored in the walls and released into the building at night when heating is needed. When it fails, this approach can lead to high energy consumption if mechanical systems are required to pump heat into or out of thermal mass elements that conflict with the desired internal temperature. Generally buildings such as residential houses with relatively low levels of internal heat gain from occupants, lights and equipment, can be designed using passive principles to achieve comfort conditions for most of the year with little or no mechanical heating or cooling. Buildings with high levels of internal heat gain such as office blocks will generally require mechanical systems to maintain comfort conditions, but there are significant opportunities to reduce the energy consumption with careful design. In the design of energy efficient buildings dominated by internal heat gains particular attention should be given to matching the mechanical systems to the internal loads, and to ensure that control systems are designed and operated to avoid conflict between the mechanical systems and the thermal mass elements of the envelope and internal structure.

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7.1.3.

Building envelope energy performance in the climate of Gaborone. Much information is available on how to design energy efficient buildings in climates similar to that of Botswana. In most cases there are however no figures to show the actual impact of such recommendations on building energy consumption, or indoor temperature. Recently an exercise was carried out to quantify the impact of different strategies for improving building performance, using computer simulations of three ‘generic’ building types to determine the effect on comfort conditions and energy consumption of changing various building envelope or operational parameters. The results are summarised in the more detailed discussion below, and indicate that the local climate is such that some of the ‘conventional wisdom’ relating to energy efficient building design needs to be re-considered. For buildings with little internal heat gain from occupants and equipment, it was found that the energy needed for heating in winter is similar to or even greater than the energy needed for cooling in summer. Even in summer, there is opportunity for buildings to loose heat to the environment through the walls and roof at night, and through the floor at all times of day. The result is that the optimal interaction between building and environment is quite complex, and certainly not as simple as providing maximum insulation all round, as may be the case in climates that are generally either too cold or too hot.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

7.2.

Thermal properties of building materials Thermal properties of building materials include those that are related to a particular material irrespective of its dimensions and location, and properties that relate to the material or groups of materials in a particular configuration as it is used in a building. The first group are described below under ‘material properties’. The second are described under ‘construction properties’.

7.2.1.

Material properties. Building materials can conveniently be considered in two categories; opaque and translucent. Opaque materials are those that do not allow transmission of light or thermal radiation. They include typical wall materials such as bricks, concrete, timber, metals and fibre insulation. The properties of opaque building materials that are most relevant to the thermal performance include the following: • Thermal conductivity. • Thermal resistivity. • Specific heat capacity. • Density.

The thermal properties of a number of common building materials are given in Appendix 1. 7.2.1.1. Thermal conductivity. Thermal conductivity is a measure of the ability of a material to transfer heat by conduction. It is measured in units of [W/m.K]. 7.2.1.2. Thermal resistivity. Thermal resistivity is a measure of the ability of a material to resist heat transfer by conduction. It is the inverse of thermal conductivity, and is measured in units of [m.K/W]. 7.2.1.3. Specific Heat Capacity. Specific Heat Capacity measures the ability of a material to store heat energy. It is measured in units of [kJ/kg.K] 7.2.1.4. Density. Density measures the mass of a unit volume of material. It is useful to allow the calculation of heat capacity by volume. It is measured in units of [kg/m3].

Translucent materials are those that allow the transmission of light or thermal radiation. They include materials such as glass and plastics used in windows, curtain walling and skylights. In addition to the properties of opaque materials, it is important to know the transmissivity of translucent materials.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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7.2.2.

Outside Solar Radiation

Inside Conduction through a wall A b s o r b e d s o la r r a d ia tio n

Convection Transmitted Solar Radiation

Long wave radiation

Long wave radiation

Construction properties A material or group of materials forming a construction element of a building has properties that are determined partly by the thermal properties of the materials themselves, and partly by the surface characteristics and geometry of the construction. The properties of construction elements that are most relevant to thermal performance are as follows: o Overall heat transfer coefficient (U-value). o Overall thermal resistance (R-value). o Heat capacity. o Emissivity (= absorptivity). o Relectivity o Transmissivity. The thermal properties of a number of common building construction elements are given in Appendix 1.

Fig. 7.1 Mechanisms of Heat Transfer through the Building Envelope.

7.2.2.1. Overall heat transfer coefficient (‘U’ value). The overall heat transfer coefficient is an approximate measure that simplifies the calculation of heat transfer through walls, floors and roofs. It combines the heat transfer coefficients for convective and radiative heat transfer from both surfaces with the conductive heat transfer to provide a single overall heat transfer coefficient for the surface. It is somewhat approximate, since the surface heat transfer coefficients for both convective and radiant heat transfer are dependant on the surface temperatures. It is measured in units of [W/m2.K].

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Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

7.2.2.2. Overall thermal resistance (‘R’ value). Overall thermal resistance is the inverse of overall heat transfer coefficient, and is a measure of the resistance to heat transfer of a building element such as a wall, floor or roof. It is found by adding the individual thermal resistances of each layer of the element, including the surface resistances of the inner and outer surfaces. It is measured in units of [m2.K/W]. 7.2.2.3. Heat Capacity. Heat capacity is a measure of the ability of a building element to retain heat. Specific Heat Capacity measures the ability of a material to store heat energy. It is measured in units of [kJ/m3]. 7.2.2.4. Combined Heat Capacity and Resistance. It has been suggested that the combined effect of heat capacity and resistance may be an important criterion for the effectiveness of wall materials. This is defined as the product CR (heat capacity multiplied by overall thermal resistance). It is suggested by Hamilton et. al that a figure of CR=93 [x103sec] may be optimum for the climate in Botswana.

7.2.2.5. Emissivity. Emissivity is a measure of the ability of a surface to emit radiant heat energy, relative to that of a black surface at the same temperature. It is a dimensionless ratio between 0 and 1. It is equal to absorptivity, which is a measure of the ability of a surface to absorb radiant heat energy. 7.2.2.6. Reflectivity. Reflectivity is a measure of the ability of a surface to reflect radiant energy. It is a ratio between 0 and 1. 7.2.2.7. Transmissivity. Transmissivity is a measure of the ability of a material to transmit radiant energy through it. It only applies to translucent materials such as glass. It is a ratio between 0 and 1.

It has units of seconds.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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7.3.

Orientation.

7.3.1.

Reducing solar heat gain in summer. The main factor that determines the optimal orientation for a building is the daily path of the sun through the sky, and the pattern by which this changes through the year. The main aim is to minimise solar gain on vertical surfaces in summer. The east and west walls are exposed to the sun in the mornings and afternoons respectively, and the area of these walls should be reduced as far as possible. The optimum orientation is therefore with the longer axis of the building running east - west.

7.3.2.

Allowing solar heat gain in winter. In buildings that require heating in winter, a further consideration is to achieve solar heat gain in the winter. The north wall may be designed to receive sunshine in winter, but not in the summer, by arranging shading devices (which may include the roof overhang), that expose the wall to the low winter sun, but shade it from the sun in the spring and autumn when heating is not needed.

Fig. 7.2 Sunpath in Gaborone in summer and winter.

Buildings with high internal heat gains such as offices have very little need for heating even in winter, and in this case solar heat gain should be avoided at all times.

Fig. 7.3 Using the roof overhang to shade the north wall in summer.

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Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

Indicator Orientation Summer (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over E-W

Classroom E-W (base) N-S

Residential E-W (base) N-S

Office E-W (base) N-S

0 47.3 47.3

0 52.2 52.2 10.4%

0.3 15.2 15.5

0.2 15 15.2 -1.9%

0 99 99

0 104 104 5.1%

Winter (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over E-W

3.6 14.2 17.8

3.5 13.5 17 -4.5%

12.6 1.7 14.3

13.4 1.7 15.1 5.6%

1 53 54

1 49 50 -7.4%

Annual (12 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over E-W

1.8 30.8 32.6

1.7 32.9 34.6 6.1%

6.4 8.5 14.9

6.8 8.4 15.2 2.0%

1 76 77

1 76 77 0.0%

Table 7.1 Effect of orientation on energy consumption for three types of building in Gaborone (EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’)

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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7.3.3.

By considering the optimal characteristics of each element of the building envelope, the most appropriate combination of elements can be achieved. This requires the coordinated input of different specialists to ensure that all the disciplines involved in the building are considered.

Quantifying the effect of orientation. The effect of changing orientation from E-W to N-S was simulated for three building types. The effect on energy performance in summer, winter and the full year is summarised in table 7.1. The overall effect on energy performance is significant for the classroom building (6.1% increase in annual energy consumption for heating and cooling). It was less for the residential building (2% increase in annual energy consumption for heating and cooling). It had no effect at all for the office building; a winter saving of 8.4% cancelled a summer additional cost of 5.9%. Total energy consumption is however not the only important criterion. Windows that admit direct sunshine result in internal areas that are too hot and subject to glare (see Section 9. Lighting). An E-W orientation allows for the larger elevations of a building to face north and south. The north elevation can be more easily protected from the sun than the east and west elevations, and the south elevation is not a problem in this regard.

7.4.

Characteristics of envelope elements The design team must find the combination of characteristics for each building element that best achieves the requirements of the design brief. This requires consideration of the particular conditions that each element is exposed to, in terms of the opportunities and threats that these offer the building.

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Fig. 7.4 Envelope heat flows. 7.4.1.

Ground Floor. The average monthly temperature in Gaborone ranges between 25°C in January and 12°C in July. The ground temperature at a depth of 500mm below natural ground level is approximately equal to the average monthly temperature. Buildings generally benefit from ground floors that are in good thermal contact with the ground, by losing heat to the ground.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

ceiling fixed under or over the structure depending on whether this is designed to be seen as part of the indoor spaces.

The simulation for a classroom building indicated an increase in annual heating and cooling energy of 22.9% when 50mm of insulation was provided between the floor and the ground, compared with no insulation. 7.4.2.

If the ceiling is suspended under the structure, there is typically a roof void that may or may not be ventilated to the outside of the building.

Roof. Of all the building elements, the roof is most exposed to climatic sources of heat gain and heat loss. Throughout the day the roof is exposed to direct solar radiation, which is potentially the most significant source of heat gain. During the night the roof radiates to the night sky, and also loses heat by convection to the cool night air.

Generally the roof should be light coloured to reflect solar radiation, and well insulated to prevent heat gain in the summer and heat loss in the winter. The most cost effective improvement that can be made to a building with a galvanised roof is to paint it white.

The most important strategy is to manage the transfer of heat through the roof structure. For most of the year this is achieved by reducing heat transfer as much as possible. The most effective strategy is to use a reflective surface for the roof finish, such as white painted galvanised steel. This reduces the amount of heat that passes into the roof space in the first place. A similar advantage may be achieved by using a reflective underlay (such as Sisalation) under concrete tiles. Ventilation of the roof space can help to reduce the temperature further, and insulation laid over the ceiling can help to reduce the transfer of heat from the roof space into the occupied rooms below. 7.4.2.1. Lightweight roofs. Lightweight roofs generally consist of an outer weatherproof layer, typically sheet metal or concrete tiles. This is supported on either steel or timber trusses, with a

Fig 7.5. Roof details.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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7.4.2.2. Heavyweight roofs. Heavy roofs are generally constructed as concrete slabs, having significant thermal mass and a degree of insulation within one element. Additional insulation is needed to avoid excessive heat gain to the building in summer and heat loss in winter. The positioning and capacity of this insulation is important, and again there are a number of options. Insulation may be added above the slab. This will reduce heat exchange between the slab and the outdoor environment, and maximise the effectiveness of the slab as a heat capacitor for the building, reducing fluctuations. As with all thermal mass elements, there is the danger that this may be in conflict with the mechanical heating and cooling system leading to excessive energy consumption. In a building with no air-conditioning system this may be beneficial in reducing temperature fluctuations, and moderating the indoor temperature to a reasonably comfortable average of outdoor maximum and minimum temperatures. This depends on many other factors, including internal loads and the performance of other building elements. Insulation may be added below the slab.

daytime radiant heat gain and night time radiant cooling, the underside of the slab may be close to the indoor comfort temperature for much of the time. This may be a good solution for buildings that are airconditioned, since the heat transfer through the insulating element will be further reduced by the smaller temperature difference across this element. 7.4.2.3. Simulation of roof interventions. The three storey commercial building showed an annual energy saving of 4.8% with a white roof compared to a galvanised roof. In the classroom building, the equivalent saving was 46%. The simulation for a double storey residential house showed that increasing the ceiling insulation from 50mm to 150mm lead to an energy saving of 2.7%. However this was fitted with a concrete tile roof finish with Sisalation underlay which is reasonably efficient to begin with. For the classroom building the saving by providing 100mm insulation was 43.6%. An insulated concrete roof resulted in an energy saving of 52.9% in the classroom building, compared with the galvanised steel roof with no ceiling insulation. In the residential building the concrete roof resulted in a 3.6% increase in energy cost, and similarly in the office building annual energy cost was increase by 5%.

Providing insulation below the slab will moderate the effect of the thermal mass. Depending on the relative impact of Page 14

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

Indicator Roof

Classroom Residential Galv White 100mm Conc. tiles White uninsulat'd metal insulation (base) metal (base)

+100mm Insul'n

Green metal (base)

Office White metal

+100mm Insul'n

Summer (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over base

0 47.3 47.3

0.1 24.3 24.4 -48.4%

0 27 27 -42.9%

0.3 15.2 15.5

0.4 14.1 14.5 -6.5%

0.3 15 15.3 -1.3%

0 99 99

0 94 94 -5.1%

0 97 97 -2.0%

Winter (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over base

3.6 14.2 17.8

5.7 5.4 11.1 -37.6%

3.1 6.6 9.7 -45.5%

12.6 1.7 14.3

13.1 1.5 14.6 2.1%

12.1 1.7 13.8 -3.5%

1 53 54

1 50 51 -5.6%

1 54 55 1.9%

Annual (12 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over base

1.8 30.8 32.6

2.9 14.8 17.7 -45.7%

1.6 16.8 18.4 -43.6%

6.4 8.5 14.9

7.2 7.8 15 0.7%

6.2 8.3 14.5 -2.7%

1 76 77

1 72 73 -5.2%

1 76 77 0.0%

Table 7.2 Effect of roof colour and insulation on energy consumption of three building types in Gaborone (EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’)

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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7.4.3.

Walls. In designing the walls consideration should be given to the different conditions that they will be exposed at each time of day and season depending on their orientation. In some cases there are conflicting opportunities or constraints at different times of year, e.g. a west facing wall may benefit from the heat of the sun in winter, but suffer in the summer. It appears that different solutions are appropriate for different types of building.

7.4.3.1. East and West Elevations. Walls that face the east and west should generally be as well insulated as possible, to prevent summer heat gain from the low morning and evening sun. These elevations can benefit from shading from trees, shrubs or climbing plants. If these are deciduous, the building can benefit from morning and afternoon heat gain in the winter months while being protected in the summer. For buildings with large internal loads that require cooling in winter, evergreen trees or climbers would be more appropriate. (See Section 6. Planning). 7.4.3.2. North Elevation. The north elevation receives sunshine during the winter months, with the sun at an average midday altitude of 42° in June. In midwinter the sun rises in the northeast and sets in the northwest. During this time, the north elevation is therefore exposed to quite large amounts of direct solar radiation that can provide some useful heat gain in this cold period for buildings such as residential houses that require heating. Buildings such as offices or classrooms that have

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Fig 7.6. Sunpath in Gaborone in summer and winter. very little need for heating should have the north walls protected from the sun if possible. The heating season typically begins in April and ends in August. At these times of year the midday sun reaches an altitude of about 55°. It now rises and sets about 15° north of the east – west axis so that it sees little of the north wall in the early morning and late afternoon. Shading of windows on this elevation should therefore be designed to protect the windows when the sun is above an altitude of about 60°.

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

Indicator Wall insulation

Classroom 220mm ins. cavity base

ins. mass

Residential 220mm ins. cavity base

ins. mass

Office 220mm ins. cavity base

ins. mass

Summer (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over 220mm

0 47.3 47.3

0 51.8 51.8 9.5%

0 52.5 52.5 11.0%

0.3 15.2 15.5

0.1 15.1 15.2 -1.9%

0.1 15.1 15.2 -1.9%

0 99 99

0 101 101 2.0%

0 102 102 3.0%

Winter (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over 220mm

3.6 14.2 17.8

2.6 16 18.6 4.5%

2.5 16.3 18.8 5.6%

12.6 1.7 14.3

4.7 1.8 6.5 -54.5%

3.8 1.9 5.7 -60.1%

1 53 54

0 60 60 11.1%

0 62 62 14.8%

Annual (12 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over 220mm

1.8 30.8 32.6

1.3 33.9 35.2 8.0%

1.3 34.4 35.7 9.5%

6.4 8.5 14.9

2.4 8.5 10.9 -26.8%

2 8.5 10.5 -29.5%

1 76 77

0 81 81 5.2%

0 82 82 6.5%

Table 7.3 Effect of wall insulation on energy consumption for three building types in Gaborone (EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’)

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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7.4.3.3. South Elevation. The south elevation receives only a glancing blow from the sun in the early morning and late afternoon in mid summer. By midday the sun is almost directly overhead. The south wall may therefore be a good opportunity to introduce thermal mass to increase the thermal capacitance of the building. This is the best elevation on which to locate windows for daylighting, since these receive little or no direct sunlight. 7.4.3.4. Simulation of wall interventions. The simulation showed that for the residential building type substantial energy savings can be achieved by using insulated cavity walls, or insulated mass walls in place of standard 220mm walls. Increased mass walls with insulation are marginally better than insulated cavity walls, but the improvement was marginal.. The benefit was almost entirely in reduced heating energy in winter. When the building requires cooling, the walls actually help by absorbing heat from the inside during the day and transferring it to the outside at night. As a result, insulated walls resulted in increased energy consumption for both the classroom and office building types, where cooling energy greatly exceeds heating energy. (See Table 7.3)

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7.4.4.

Fenestration. The primary objective in designing the fenestration for a building should be to maximise the benefits, namely: o Daylighting. o Views. o Ventilation. while minimising the negative qualities: o Glare. o Radiant heat gain. o Conductive and convective heat gain and heat loss.

7.4.4.1. Daylighting. The subject of daylighting is covered in more detail in Section 8. Lighting. The objective of window design with respect to lighting should be to provide as much of the indoor lighting requirement with daylighting as is possible without compromising other energy efficiency considerations. In particular this will require consideration of the heat transfer properties of the glazing. This is an element that may justify some cost analysis, as there is a clear relation between cost and thermal effectiveness. Improved insulation can be achieved using various configurations of multiple glazing, and selective coatings, the cost of which is generally more the greater the effectiveness of the product. Selective coatings may also reduce the light penetration, so that in some cases the same quantity of daylight may be achieved with smaller clear windows as

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

with larger coated windows, with lower cost and overall heat loss. Typical properties of different types of glass available in Southern Africa are given in Appendix x. The shading coefficient is the ratio of Total Solar Energy Transmission of a glass compared to the Total Solar Energy Transmission for ordinary 3mm glass. The ratio of Total Visible Light Transmission compared to Total Solar Energy Transmission has been included to give a comparison of which glass is most effective at transmitting maximum light with minimum energy. 7.4.4.2. Views. Views of the outdoor environment have an important impact on the quality of the indoor environment for a variety of occupations, and can significantly improve people’s productivity. 7.4.4.3. Simulation of window interventions. Various interventions were simulated on the three types of buildings, including shading of north facing windows, double glazing, increasing the glazing ratio, and using specialised glass.

North window shading has some benefit for classrooms and office buildings, but not for the residential building, with annual energy savings as follows: o Residential -0.3% (increase in energy) o Classroom 4.8% o Office 6.0% Increasing the glazing area from 20% to 40% of external wall area resulted in substantial increases in energy consumption for all building types. This was somewhat mitigated by using ‘Coolvue’ glass with a selective coating, but overall energy consumption was still between 10% and 30% higher than in the base case. (See Table 7.4.) It is recommended that glazing areas are generally kept to no more than about 30% of external wall area. Higher glazing levels in air conditioned buildings will lead to excessive consumption of energy unless sophisticated design measures such as ventilated double facades or solar control glass with external shading are employed. In buildings that are not airconditioned, large amounts of glazing will result in high indoor temperatures and uncomfortable buildings.

Double glazing was found to have very little effect on energy consumption, with an annual savings as follows: o Residential 3.6% o Classroom 0.3% o Office 0.0%

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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Indicator Window glazing

Classroom 20% clear 40% clear (base)

Residential 40% 10% clear 40% clear Coolvue (base)

Office 40% 20% clear 40% clear Coolvue (base)

40% Coolvue

Summer (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over base

0 47.3 47.3

0 53.3 53.3 12.7%

0 52.6 52.6 11.2%

0.3 15.2 15.5

0.3 21.4 21.7 40.0%

0.3 19.9 20.2 30.3%

0 99 99

0 106 106 7.1%

0 102 102 3.0%

Winter (6 months) Heating energy [kWh/m2.yr] Cooling energy [kWh/m2.yr] Total [kWh/m2.yr] % increase over base

3.6 14.2 17.8

3.7 18.5 22.2 24.7%

3.6 18.3 21.9 23.0%

12.6 1.7 14.3

10.5 4 14.5 1.4%

11.6 3.6 15.2 6.3%

1 53 54

1 66 67 24.1%

1 60 61 13.0%

Annual (12 months) Heating energy [kWh/m2.yr] 1.8 1.9 1.8 6.4 5.4 5.9 1 1 Cooling energy [kWh/m2.yr] 30.8 35.9 35.5 8.5 12.7 11.8 76 86 Total [kWh/m2.yr] 32.6 37.8 37.3 14.9 18.1 17.7 77 87 % increase over base 16.0% 14.4% 21.5% 18.8% 13.0% Table 7.4 Effect of glazing interventions on energy consumption for three building types in Gaborone (EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’)

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1 81 82 6.5%

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

Ventilation is the primary means of improving air quality, by removing polluted air and replacing this with better quality outdoor air. This of course assumes that the outdoor air is of an adequate quality to achieve this. Where this is not the case, filtration may be required to achieve an acceptable level of indoor air quality (see Section 4. Indoor Environment).

Even stimulating views -- widely argued to be distracting for students and workers alike -- seem to have a positive effect on performance. A 2003 energy commission study of the Fresno School District found that complex window views -- with greenery or people and distant landscapes -- supported better learning results. Similarly, a study of the effects of views at the Sacramento Municipal Utility District's customer service call center found that better views were consistently associated with better performance. Workers enjoying the best possible views processed calls 7 to 12 percent faster than those with no views. Better views have also been associated with better health conditions. In one study, computer programmers with views spent 15 percent more time on their primary task, while those without views spent 15 percent more time chatting on the phone or to one another.

Ventilation can also be used as a means of improving indoor climate under favourable conditions. Ventilation may be achieved by openings in the walls or roof that are controlled by the occupants, such as doors and windows, or by mechanical means using fans and ducting. Design of ventilation systems must take into consideration the potential problem of drafts which can cause localised discomfort as well as creating problems such as paper blowing around.

Healthy buildings pay for themselves Daylight, views are more than mere amenities. Carol Lloyd Sunday, July 23, 2006 San Francisco Chronicle

7.4.5.

Ventilation. Ventilation in buildings serves two main purposes; improvement of air quality, and improvement of indoor climate.

The issue of security also needs to be considered if windows are to be left open at night to take advantage of cool night air. 7.4.5.1. Simulation of ventilation interventions. Simulations were carried out to quantify the effect of using ventilation to modify indoor temperature when the outdoor air temperature is beneficial. The annual energy savings for the three types of building were as follows: o Residential 27.0% o Classroom 5.1% o Office 27.8%

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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For the office building type, ventilation has the greatest potential to reduce energy cost of all the interventions to the building envelope that were simulated. Some practical problems do need to be addressed in order to achieve this level of ventilation in a controlled manner, without causing problems with draft.

7.5.

Codes and Standards Codes have been adopted in a number of countries that define minimum energy performance standards for different classes of buildings. These typically include specific requirements for building envelope elements. An example of such a code is the ASHRAE Standard 90.1 2001: Energy Standard for Buildings except Low Rise Residential Buildings. This specifies maximum permissible U-values and minimum R-values for various envelope components, based on the climate in which the building is situated. Climate is defined in terms of heating and cooling degree days. Appendix 3 gives details of some of the requirements of this standard for Pretoria, together with the relevant climatic data.

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Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

7.6.

Resource Material

7.6.1. Books and reports. ASHRAE Standard 90.1-2001. Energy Standard for Buildings except Low Rise Residential Buildings. Energy Code for New Federal Commercial and Multi-Family High Rise Residential Buildings; Final Rule, October 2000. Department of Energy, Office of Energy Efficiency and Renewable Energy, US Government. EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’. Department of Energy, Government of Botswana, January 2007. Hamilton, L.B., et. al. 1984. Passive Solar Design Workbook. BRET. Botswana. Hunn, B.D. (ed) 1996. “Fundamentals of Building Energy Dynamics.” Massachusetts Institute of Technology. Koch-Nielsen, H. 2002 Stay Cool - A design Guide for the Built Environment in Hot Climates. London: James & James (Science Publishers) Ltd. Lechner, N. 1990. Heating, Cooling, Lighting – Design Methods for Architects. USA. John Wiley & Sons. Rogers, G.F.C., Mayhew, Y.R. 1967. “Engineering Thermodynamics Work and Heat Transfer” Longman.

Tutt, P. and Adler, D. (Ed.). 1979. New Metric Handbook – Planning and Design Data. Oxford: ButterworthHeinemann Ltd. 7.6.2. Codes and Standards. ASHRAE Standard 90.1-2001. Energy Standard for Buildings except Low Rise Residential Buildings. Energy Code for New Federal Commercial and Multi-Family High Rise Residential Buildings; Final Rule, October 2000. Department of Energy, Office of Energy Efficiency and Renewable Energy, US Government. Malaysian Standard MS1525: 2001. Code of Practice on Energy Efficiency and Use of Renewable Energy for NonResidential Buildings. Department of Standards, Malaysia. Guam Energy Code, American Samoa and Guam Energy Code Development Project. January 25, 2000 7.6.3. Web sites. Soil Temperature Variations With Time and Depth http://soilphysics.okstate.edu/toolkit/temperature/index0.html PG Glass http://www.smartglass.co.za ASHRAE American Society of Heating, Refrigerating and Airconditioning Engineers. http://www.ashrae.org/

Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

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CIBSE Chartered Institute for Building Services Engineers http://cibse.org EDR. Energy Design Resources http://www.energydesignresources.com/ EERE Building Technologies Program Home Page http://www.eere.energy.gov/buildings/ SBIC. Sustainable Buildings Industry Council. http://www.sbicouncil.org SQUARE ONE environmental design, software, architecture, sustainability. http://squ1.org/wiki/Concepts WBDG - Whole Building Design Guide http://www.wbdg.org/

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Energy Efficiency Building Design Guidelines for Botswana – Section 7. Building Envelope

SECTION 8

MECHANICAL SYSTEMS

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Revision 1

September 2007

ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA

Sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Introduction. Design Brief. Climate. Indoor Environment. Design and construction process. Planning. Building envelope. Mechanical Systems. Lighting - artificial and day lighting. Operation & Maintenance and Building Management Systems. Simulation. Life-Cycle Cost Analysis. Appendices.

CONTENTS 8.

MECHANICAL SYSTEMS (HVAC)

5

8.1. Overview

5

8.2. Building heating & cooling loads 8.2.1. Fabric heat gains 8.2.2. Internal heat gains 8.2.3. Passive Cooling

5 5 6 6

8.3. HVAC system design. 8.3.1. Design Brief. 8.3.2. Zoning. 8.3.3. Selecting the type of system. 8.3.4. Sizing the system.

7 7 7 7 7

8.4. Heating, ventilation and air conditioning systems (HVAC) 8.4.1. Naturally Ventilated Buildings 8.4.2. Semi-passive buildings 8.4.3. Mixed-mode Buildings 8.4.4. Comfort Cooling / Unitary Systems 8.4.5. Air Conditioning / Centralised Systems

8 9 9 10 10 10

8.5. Ventilation & Cooling Systems 8.5.1. Evaporative Cooling 8.5.2. Local Comfort Cooling 8.5.3. Constant Volume Ventilation systems 8.5.4. Variable Air Volume Ventilation systems 8.5.5. Chilled Beams/Chilled Slabs 8.5.6. Fan Coil Units

11 11 12 13 14 14 14

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

Page 3

8.5.7. 8.5.8.

Desiccant Cooling Groundwater cooling / Ground source heat pumps

15 15

8.6. Ventilation Equipment 8.6.1. Fans and AHUs 8.6.2. Performance Standards - Ventilation

16 16 17

8.7. Refrigeration and Cooling Equipment 8.7.1. Cooling Towers/Water Cooled Chillers 8.7.2. Absorption Chillers 8.7.3. Air Cooled Chillers 8.7.4. Thermal Storage 8.7.5. Performance Standards - Cooling

17 17 18 18 18 19

8.8. Heating Equipment

19

8.9. Controls

19

8.10. Commissioning & Handover

20

8.11. Maintenance & Replacement

20

8.12. Resource material 8.12.1. Books and papers 8.12.2. Codes and Standards. 8.12.3. Websites.

21 21 21 21

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Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

8.

MECHANICAL SYSTEMS (HVAC)

8.1.

Overview

The cooling load of a building in summer will depend on: a) the desired internal temperature (cooling a building to 20°C will use a lot more energy than cooling a building to 24°C) – see design criteria in Section 4, Indoor Environment. b) the amount of fresh air ventilation delivered to the occupants – see design criteria in Section 4, Indoor Environment. c) fabric heat gains - the amount of heat entering the building from outside (through windows, walls, roof and by uncontrolled air infiltration) d) internal heat gains – the amount of heat generated inside the building by the occupants, equipment such as computers, lighting etc. e) amount of passive cooling available (from thermal mass etc)

This section addresses the subject of mechanical systems in buildings and the heating and cooling loads that they are designed for. It begins by discussing the loads, which include both external loads relating to the influence of the external climate through the building envelope as well as internal loads generated by the users of the building, their lighting, and other equipment. It is not the intention to present comprehensive design guidelines, but rather to highlight opportunities for increased energy efficiency related to the classes of building covered by these Guidelines. The section is structured with chapters 8.2 and 8.3 providing background information on thermal loads and design of HVAC systems. Chapter 8.4 discusses HVAC system selection, and Chapter 8.5 outlines a range of different HVAC systems available. The following chapters 8.6, 8.7 and 8.8 include detailed advice on ventilation, cooling and heating systems respectively. Chapters 8.9 and 8.10 address controls, commissioning and maintenance of systems.

8.2.

8.2.1.

Building heating & cooling loads In Botswana in most large buildings such as offices there is very little requirement for heating even in the winter period.

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

Fabric heat gains There are three main sources of heat gain through the building envelope a) Solar gain through windows – solar radiation passing through windows will cause very high heat gains if windows are too large and without external shading. Gain can be reduced using solar coated glazing (see Section 7, Building Envelope). In a well designed building, it should be possible to limit gains to around 25W/m² (floor area) or less. In a poorly designed building with extensive glazing, solar gains may easily be over 100W/m². b) Conduction gain through windows, walls and roof – when it is warmer outside than inside, heat is conducted through the building fabric. The amount of heat

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conducted depends on the insulation properties of the construction. Insulation is particularly important in the roof. With roof insulation and a ventilated cavity it should be possible to reduce conduction gains to around 5 W/m² or less. c) Uncontrolled air infiltration – warm outside air entering the building through gaps in the envelope, cracks, open windows, open doors etc will heat up the internal spaces. Any building with cooling should always be constructed so as to be as airtight as possible (allowing the amount of fresh air to be controlled). With poor construction techniques, infiltration can lead to gains of well over 50 W/m². With good airtight construction, it should be possible to limit these to 5 W/m². 8.2.2.

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Internal heat gains Sources of internal heat gains will depend on the use of the building, but typically include a) Occupants – people generate heat from their bodies See Section 4, Indoor Environment for further details. The greater the occupant density (ie people/m²) the greater this heat gain will be. Typical values for an occupant density of 1 person/12m² would be 8W/m² b) Lighting – fluorescent lighting generates much less heat than intumescent lighting and will therefore reduce cooling loads. Typical values are 12W/m² for an efficient fluorescent scheme, 50W/m² or more for a scheme with extensive dichroic and other decorative lights. c) Office equipment – computers, photocopiers etc all give off heat. Modern LCD flat screens give off much less heat than CRT monitors and should therefore be

encouraged. Typical values for an office would be 1015 W/m². Note that the amount of heat given off by a piece of equipment is usually much less than the rated power requirement (which is the peak power requirement). d) Catering equipment. e) HVAC equipment – equipment such as fan coil units located inside the building give off some heat. 8.2.3.

Passive Cooling Heavyweight buildings with exposed high density finishes such as concrete ceilings or tiled floors absorb heat during the day and radiate it out during the night. This has the effect of stabilising internal temperatures and means that less active cooling is required to maintain comfortable conditions. To analyse the extent of cooling achieved normally requires a computer simulation, but can be equivalent to 15-20 W/m² in a well designed building.

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

8.3.

HVAC system design. There are substantial opportunities for reducing energy consumption in buildings in Botswana by optimising HVAC system design for energy efficiency. This will require a move away from current designs which are directed more towards ensuring that there is always excess capacity.

8.3.1.

8.3.3.

Design Brief. It is vital that a clear brief is agreed between the designers and client. This will include: - required internal temperatures and whether occasional periods outside these conditions are acceptable. - occupant numbers or density (m²/person). - hours of occupation. - equipment to be used in the space. The designer must then agree the appropriate external design conditions (either a design summer day or preferably a weather file to be used for thermal simulation) (see Section 3, Climate).

8.3.2.

perimeter zones experience the combination of internal loads and envelope loads. These may vary through the day, with the impact of solar radiation on the different elevations of the building. At certain times of year it may be that one zone requires heating while another requires cooling, which offers the opportunity to transfer heat from one zone to another in the same building. Selecting the type of system. Some advice on selection of systems is given in the following section. Many factors need to be considered, including the following: o Energy efficiency. o Resilience in the event of breakdowns. o Maintenance and repair capabilities. Clearly in a financial data centre or food processing factory, the implications of failure of the systems and overheating are likely to be much more severe than in say a college, and the design will have to reflect this. 8.3.4.

Zoning. Effective zoning of the building for HVAC design is critical to achieving energy efficiency. Generally the zoning should match the thermal performance of different parts of the building. Large buildings are often divided into an internal zone, and a number of perimeter zones for each floor. The internal zone will tend to be dominated by internal loads, while the

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

Sizing the system. Accurate sizing of the HVAC system can lead to substantial savings both in initial capital cost and operating cost. Design Day Calculations – here a design day is chosen (typically the hottest day in a “typical” year), and the various heat gains into the space calculated at each hour of the day. The cooling system is then designed to meet the highest heat gain. Such methods tend to lead to oversizing of equipment, but are widespread not least since occupants

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will rarely complain if a system is oversized but will complain if the building overheats. A study in Ireland found that a large commercial building in Dublin had been designed with between 28% - 90% higher capacity systems using static methods than was required when dynamic simulation was used to size the system. Computer simulations – here the building is modelled typically every 10-60 minutes and heat flows in and out of the building analysed. Either one design day is taken (which is repeated several times to establish constant results) or else a whole summer period of typical weather data is used. This type of modelling allows the passive cooling effects of the building to be taken into account, and allows much more accurate sizing. Computer simulation is essential to predict the performance of low energy, passive or semi-passive buildings. Refer Section 11, Simulation

8.4.

Heating, ventilation and air conditioning systems (HVAC) The mechanical systems in a building provide ventilation, cooling and heating to maintain comfortable conditions. To achieve good energy efficiency, the selection of the type of mechanical systems in a building has to be decided at the very early concept stages of the design (See Section 2, Design Brief and Section 5, Design and Construction Process). “Low Energy” cooling systems typically have limited capacity for cooling, and will therefore only be successful if the building envelope, building structure and indeed the use of the building is controlled within strict limits. It is essential that these limitations are understood by the client and the rest of the design team. The usual way to represent this is in terms of the system’s ability to provide cooling in Watts/m². Note that this is approximate only.

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Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

and/or heavyweight floor finishes. For this to work the building must be ventilated at night so that heat absorbed during the day is released. Concrete ceilings must have insulation above them so that they are not heated up by the sun. Windows must be openable and controllable such that they can be opened a small amount without causing large drafts. Ceiling fans can be used to provide air movement on hot days which improves comfort conditions, at the expense of increased energy use. For a naturally ventilated building to be successful in Botswana, windows should be designed to avoid all direct sunlight into the building during summer months, and internal heat gains from lighting and equipment must also be kept very low. Fig. 8.1 Selection of cooling systems (Source: ARUP) 8.4.1.

8.4.2.

Naturally Ventilated Buildings There are many examples of naturally ventilated buildings in Botswana such as houses, schools, offices, etc. many of which remain reasonably comfortable throughout the year. Such buildings use much less energy than air conditioned buildings, and the design of such buildings is therefore to be encouraged. To remain comfortable in summer, naturally ventilated buildings rely on passive cooling from the use of thermal mass. Typically this involves exposed concrete ceilings

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

Semi-passive buildings It is possible to use fans to assist the ventilation process and increase the amount of cooling available from the structure. Typical systems include blowing air through the centre of extruded concrete beams (e.g. Termodek system), basement thermal labyrinths or using chambers filled with rocks as a thermal store. By using a fan it is easier to guarantee night time ventilation, and it is also easier to control the volumes of fresh air being delivered during the day, reducing the risk of temperatures being too cold early in the morning. For

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these buildings to be successful, it is essential that they are made airtight. 8.4.3.

Mixed-mode Buildings Here buildings are provided with cooling systems, but are designed such that this can be switched off and the building operated as a naturally ventilated building when outside conditions allow. In Botswana there is a large part of the year when buildings could be operated like this, thus saving considerable amounts of money. However, to be successful, buildings have to be carefully designed with all of the features of a naturally ventilated building outlined above. Unless the system is simple to operate and the occupants understand it, there is a risk that the cooling systems will be used all year round.

8.4.4.

Comfort Cooling / Unitary Systems It may be that only certain of the rooms within a building require cooling (for example computer rooms, meeting rooms etc). Unitary systems (such as dx split units) may be suitable where only a few rooms need to be conditioned.

8.4.5.

Air Conditioning / Centralised Systems Generally centralised air conditioning systems are likely to be suitable for buildings that require conditioning throughout most of the building, and have relatively high internal loads, such as multi-storey office buildings which have been designed with large areas of glazing (gains of 75W/m² and over). Different types of centralised systems are described in the following section.

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Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

8.5.

Ventilation & Cooling Systems The choice of ventilation & cooling system will depend on many factors, including: o Capital cost o Running and replacement costs o Energy use o Comfort level requirements o Maintenance requirements o Adaptability and flexibility o Space requirements (for plant rooms and risers) The following gives some guidance on what systems could be considered.

8.5.1.

Evaporative Cooling The principle behind evaporative cooling is that when water evaporates, it takes heat from the surrounding air. This makes the air cooler, but also more humid. Evaporative cooling systems may be suitable for rooms with lower heat gains and/or where internal design conditions are less stringent, for example classrooms, clinic waiting areas, shops and residential houses. They may in some cases be combined with a few unitary systems, e.g. in a clinic it may be appropriate to use evaporative cooling for general waiting areas and corridors, and have a split units in the consulting rooms.

Fig 8.2 Downflow evaporative cooling unit a) Local Downflow units These units are typically roof mounted, and are widely used in retail spaces in Botswana. Air is drawn downwards by a fan over pads which are continually wetted with water. The cool, moist air is then supplied to the space. b) Evaporative Coolers incorporated within AHUs i) Direct

Various types of evaporative cooling can be used:

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

The units are designed to fit within air handling units (typically after the supply fan), and humidify the air either using wetted pads or spraying a fine mist of water into the air. Typically water treatment is required to ensure that the sprays do not clog and that health risks are minimised. This is because it is likely that building occupants will inhale the water droplets. As with downflow units, it is essential that the units are maintained and cleaned properly.

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ii) Indirect In these units the humidification occurs on the extract air. A thermal wheel in the AHU then transfers some cooling from the cooled extract air to the supply air. The advantage of this arrangement is that the supply air is cooled but not humidified. Disadvantages are the increased complexity and decreased efficiency.

c) Other ways of taking advantage of evaporative cooling include fountains, water features etc. However, these are usually less controllable and effective than purpose designed equipment. 8.5.2.

Local Comfort Cooling

iii) Two stage A number of variations of basic evaporative coolers exist. In one, the water is first circulated in a dry coil in front of the incoming air, to cool the air without humidification. The slightly warmer water is then trickled over a core and evaporated into the incoming air.

Fig. 8.4

VRV outdoor unit and indoor concealed unit

a) Variable Refrigerant Volume (VRF/VRV) These systems comprise several indoor units (cassettes or concealed ducted units) connected to a large condenser unit(s) typically mounted on the roof. The units are linked with refrigerant pipework. Care should be taken when routing refrigerant pipework through spaces where people may be sleeping, and depending on the volume of refrigerant, leak detection may be required in such spaces. Fig. 8.3

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Two stage evaporative cooler AHU

Refrigerants with a zero ozone depletion potential and low global warming potential should be used and R22 should be avoided.

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

8.5.3.

Fig. 8.5Split system outdoor unit and indoor cassette unit b) Direct Expansion (DX) split units (heat pumps) These systems should be used with a separate system supplying fresh air (typically into the back of the unit). Opening doors and windows to provide fresh air will waste significant energy. Again, refrigerants with a zero ozone depletion potential and low global warming potential should be used and R22 should be avoided. Although relatively easy to install and maintain, these units have significantly higher energy use than other options for cooling.

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

Constant Volume Ventilation systems In these systems, a central air handling unit provides all the air required for cooling as well as for fresh air. This means the AHU air volumes will typically be in the order of 50100 litres/sec/person. The AHU is equipped with filters and a cooling coil (either using chilled water or refrigerant) and serves a single zone of the building, so that all areas in that zone have similar cooling/heating loads and hours of operation. When the air temperature outside is hot, the majority of air in the AHU is recirculated, with only a % of fresh air (typically 10 litres/sec/person) introduced. This minimises the energy used cooling the fresh air. However, when outside temperatures are below say 20°C, the AHU can use 100% of outside air, and this provides “free cooling” reducing the number of hours that chillers are required to run. Reducing the pressure drop through ducts and grilles will greatly reduce the energy used by the fan. A typical standard to aim for is a specific fan performance of 3 W/l/s. One way of achieving very low pressure drops is to use a raised floor to supply the air. Air is blown from the AHU in to the raised floor (typically at least 300mm high) and allowed out via diffusers in the floor.

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8.5.4.

8.5.5.

Variable Air Volume Ventilation systems These systems are similar to constant volume system, but are able to vary the volume of air supplied to each room. This requires more complex controls and automated dampers (VAV boxes) on each zone. The advantage of such a system is that it can cope with some rooms with higher heat gains than others, and the air-conditioning can be turned off in a room if it is not in use. Chilled Beams/Chilled Slabs These systems are used in conjunction with an AHU which supplies the minimum fresh air requirement. They do not use fans to distribute cooling, and so are potentially more efficient than fan coil solutions.

chilled beams are combined with the air supply system to increase the cooling available. Care must be taken to avoid condensation forming on the plates, which limits how cold the surfaces can be made, unless the supply air is carefully controlled and dehumidified. Chilled slabs are case with plastic pipework in situ, and chilled/cool water circulated through the slab. Such a system might be appropriate for a building which is naturally ventilated to improve comfort at peak summer conditions. However, control of the system is not straightforward. 8.5.6.

Fan Coil Units Fan coil units are local fans with a filter and cooling coil which recirculate and cool/heat air locally. They are typically mounted either on the perimeter (under windows) or in a false ceiling. They provide very accurate local control of temperature, but are not very energy efficient due to the energy required to run the fan, pump the chilled water etc. Fresh air is typically supplied by an AHU.

Fig 8.6 Chilled ceiling installation and active chilled beam Chilled beams consist of metal plates attached to the ceiling through which chilled water is passed. The surface then provides cooling both by convection and radiation. Active Page 14

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

8.5.8.

Groundwater cooling / Ground source heat pumps The ground and groundwater remain at a relatively cool temperature throughout the year. If groundwater is available it can be used as a cooling medium (and the warmed water then used for other purposes). Ground source heat pumps operate by pumping water through a loop of pipe which is buried in a borehole or similar.

Fig 8.7 Ceiling concealed fan coil unit and typical perimeter fan coil Recent improvements in fan coils include the use of EC (Electronically commutated) motors which use significantly less energy than conventional AC motors. 8.5.7.

Because the ground stays at a constant temperature all year round, it can be used for cooling in summer and heating in winter. These systems are becoming more common in Europe but are relatively complex and have a high initial capital cost for the installation of boreholes etc.

Desiccant Cooling A variation on evaporative cooling is desiccant cooling. A thermal wheel in an AHU is coated with desiccant (a material which absorbs moisture). The supply air is sprayed with water (cooling it and making it more humid) and then passes through the desiccant wheel which reduces its humidity. Depending on the resulting humidity, it may be possible to lower the air temperature further by spraying with water again. The extract air is heated before it passes through the desiccant wheel in order to recharge (dry out) the desiccant.

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

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8.6.

Ventilation Equipment For an energy efficient ventilation system, consideration is needed of: o Evaporative cooling where appropriate. o Variable speed motors for fans. o Variable air volume systems. o High efficiency motors for fans. o Insulation to air supply ducts. o Optimised AHU and duct sizes and design of fittings to reduce friction losses. o Optimised zoning and controls o Design details and construction supervision to avoid leaks and flow restrictions. o metering of electrical consumption of major plant

8.6.1.

Fans and AHUs

Fig 8.8 Guide to system selection (BSRIA AG15/2002 Illustrated Guide to Building Services) Fig. 8.9 Typical AHU EC and DC motors use less energy than standard AC motors, and are of particular benefit in smaller fans (eg

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Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

extract fans, fan coil units etc). Generously sized AHU (air handling units) will use less energy due to lower air velocities. Typically a cooling coil should be selected at air velocity < 2.5m/s and large ducts used (velocity in ducts typically < 8m/s) to minimise fan power. Filters should also be regularly cleaned and replaced since the pressure drop increases as the filters become blocked. 8.6.2.

8.7.

For an energy efficient cooling system, consideration is needed of: o o o o o o o o

Performance Standards - Ventilation The Specific Fan Power takes account of • the efficiency of the fan and • the efficiency of the motor driving the fan. • the pressure drop through the entire ventilation system. It is calculated by taking the motor input power (in Watts) and dividing by the fan flow rate (in litres/sec) A typical target is to achieve 2.5 - 3 W/l/s specific fan power (that is 3 Watts of electrical power supplied to the fan motor to move each litre/second of air through the building).

Refrigeration and Cooling Equipment

8.7.1.

Efficient equipment. Thermal storage if appropriate. Variable speed pumping systems Variable refrigerant volume systems. High efficiency motors for fans and pumps. Insulation of pipes. Optimised zoning and controls metering of electrical consumption of chillers and pumps

Cooling Towers/Water Cooled Chillers

Fig 8.10

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

Cooling Tower diagram

Page 17

For large buildings, cooling towers offer much more efficient cooling than standard air-cooled chillers. However, the water quality must be very carefully controlled to minimise the risk of legionella and other bacteria. In a cooling tower, warm water is sprayed from the top of the tower typically over a metal honeycomb, with air blown upwards to cool the water. The cool water is collected in a pond at the bottom of the unit. Typically cooling tower might cool water from 35°C to 20°C. The cooling towers are then linked to water cooled chillers which provide chilled water at 6°C or as required.

There are examples of solar powered absorption chillers in use, though they are not common, due to the high temperatures required to operate the absorption chillers efficiently. Some chillers are claimed to work with water temperatures as low as 80-95°C, which potentially makes them viable for use with evacuated tube solar heaters, but there are as yet few examples of such installations. 8.7.3.

Regular maintenance and water treatment is essential to minimise the risk of legionella infections. Typical COP is 6-8. 8.7.2.

Absorption Chillers Most absorption chillers use a material lithium bromide which produces a cooling effect when water is absorbed into it. Heat is required to regenerate the absorber. Typically absorption chillers are most effective where there is a source of waste heat in the form of very high temperature water or steam (>100°C). Even then, the machines can be unreliable and unable to cope with varying chilled water demands. Typical COP is 0.7, so the chillers are usually only a good solution if a low cost source of heat is available.

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Air Cooled Chillers The most common form of chillers are air cooled. Typical COPs are in the range 3-5. Refrigerant R22 should not be used since this is an HCFC which damages the ozone layer. Alternatives such as R134a and R410a are now commonly used which, while less damaging to the ozone layer than R22, still have a significant global warming potential if released into the atmosphere. Designing a system to operate at higher chilled water temperatures (such as 8°C flow 14°C return) significantly reduces energy use since the chillers operate with a higher COP.

8.7.4.

Thermal Storage Ice or phase change materials (PCM) can be used to store coolth and reduce the size of installed equipment. They also allow the chillers to be run more efficiently at night, when outside temperatures are lower and cheaper electricity may be available. However, due to losses from the storage vessel, they have limited effect on the overall energy use of a building.

Energy Efficiency Building Design Guidelines for Botswana – Section 8 Mechanical Systems

8.7.5.

Performance Standards - Cooling The energy efficiency of cooling systems is measured either by the Coefficient of Performance (COP), the Energy Efficiency Ratio (EER) or the Seasonal Energy Efficiency Ratio (SEER). All these measure the useful energy output (heating or cooling), in relation to the energy input (usually electrical power but sometimes heat).

therefore more efficient to generate heat locally using fuel sourced locally. The ideal would be to use unwanted materials (refuse, agricultural waste) or renewable fuels (biogas, wood) which are produced and managed sustainably. However, even using fossil fuels such as coal and oil is more efficient than using electricity.

8.9.

Many standards for energy efficiency specify minimum requirements for COP for different conditions. Table 8.1 gives the recommended minimum efficiency of various cooling systems: Minimum Size category (cooling Efficiency [COP] capacity) [kW] Air cooled split 19, 40,