AN_Ventilation and Air-conditioning Guide

Power Quality and Utilisation Guide

Energy Efficiency Ventilation and air-conditioning Bohdan Soroka 66%

4000 Pa 3000

10.0 8.0 6.0 5.0 Pw(Pa) 4.0 3.0 2.8

2000

75% 80%

Laborelec September 2007

81% 80% nmax 70% 60% 50%

2.0 40%

1000 800

1.0 0.8 0.6

600 500

25%

400

3760 tr/min 3200 2800

300

2400 200

2100 1950 1800 1600

100 80

1400

60 50

1200 1000

40 2000

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10000 15000 20000 30000 8000 m3/h

Air blown

Air expelled

Fresh air

Energy Efficiency

Air extracted

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Introduction - the importance of an organised ventilation system ... or the reasons for creating air vents after insulating a building. It is sometimes difficult to understand why buildings need to be insulated… only for new openings to be made to let in fresh air. Rational use of energy (RUE) aims to ensure that occupants are comfortable and energy consumption is controlled. It is difficult to apply these two principles in older buildings. Flows of air from the outside are entirely uncontrollable (in terms of quantity, temperature, direction and duration) and vary widely depending on atmospheric conditions: ventilation in some rooms is insufficient whilst in others there is too much ventilation. By contrast, organised ventilation (i.e. intentional ventilation as opposed to natural inflows of air) provides the right quantity of air required by occupants, thus limiting energy consumption to the minimum required whilst also guaranteeing air quality. Yet, the better insulated buildings are, the greater the proportion of heat lost through ventilation in relation to the total consumption. Ventilation, therefore, becomes a major item to be controlled if the overall consumption of the building is to be brought under control.

Assessing the energy efficiency of the existing ventilation system Air quality must be guaranteed with minimum energy consumption, both in terms of heating and in terms of electricity. Consumption is dependent on several parameters, illustrated by the following two formulas: Pheat = 0.34 × qv × ΔTmean × t / η heat Pelec = (qv / 3600 )× Δp × t / η vent where: Pheat = energy consumed to heat fresh air Pelec = electricity consumed by ventilators In following paragraphs, several examples will be given to clarify the different parameters and their units of the formulas above. In a mechanical ventilation system, consumption for heating air generally accounts for 80-90% of total consumption, compared with 10-20% for ventilator consumption. By contrast, in terms of costs, this relation balances out or reverses because electricity is significantly more expensive than fuel. The formulas clearly show the factors that need to be acted upon to limit consumption: 1. Reduce new air flows, qv, to the minimum level required: eliminate uncontrollable infiltrations, guide the flow of fresh air taking into account actual occupation (detection of presence, CO2 sensor etc.), reduce the generation of pollutants as much as possible (e.g. VOCs), distribute ventilation circuits standard occupation areas, do not make equipment too large, prevent all leaks in the air distribution systems and so forth. 2. Minimise Δp charge losses: operate at low speed in the shortest and least turbulent networks possible, choose suitable filters and maintain them regularly etc. 3. Minimise operating time t: adapt ventilation periods to the periods during which the building is occupied. 4. Use ventilators and motors that have a high ηvent output in the operating area that is used the most.

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Improving an existing ventilation system For reasons of comfort or energy efficiency, an existing ventilation system- may not be satisfactory. Several solutions are possible as an alternative to designing a new installation.

Reduce flows Existing ventilators are often larger than they need to be, often because safety margins were included when calculating charge losses. These ventilators therefore carry flows that are too great during their entire lifetime. At the end of their lifetime, they are replaced with an identical ventilator and the problem is perpetuated. An example is given in table.

Speed (rpm) Airflow (m3/h) Charge losses (kPa) Power absorbed by the ventilator (kW)

Initial situation 2 0 00 21 600 1.4 12.2

Reviewed situation 1 000 (1 000 / 2 000) x 21 600 = 10 800 (1 000 / 2 000)2 x 1.4 = 0.35 (1 000 / 2 000)3 x 12.2 = 1.52

Table 1: Advantages of a new design

In other words, an 88% saving in electricity consumed! How can this be achieved? There are several possibilities: •

by regulating the pre-rotary vane assemblies or variable angle blades if used;

•

by changing the diameter of the pulleys (belt drive);

•

by reducing the speed of the multi-speed motors.

Stop ventilation outside occupation periods Controlling operating time is one of the easiest solutions for operators to implement themselves. The intervention required is simple, huge energy savings can be made and the equipment does not suffer as much wear and tear. Therefore, the question has to be asked whether the present ventilation period is necessary. In order to maximise the gain of this management method, certain effort needs to be made to: •

Adjust the operating hours when requirements and room usage change.

•

Regularly check the clock programming (hang a sign close by with the valid schedule).

•

Modify the schedule depending on the season if necessary.

The payback time for this type of operation is generally less than a year.

Example: A ventilator extracts 1 000 m3/h from an office building that is occupied from 8 a.m. to 6 p.m. Compared with continuous operation, adapting the operating hours for the ventilator to the hours during which the building is occupied could generate the following savings: Electricity: 0.25 [W/(m3/h)] x 1 000 [m3/h] x 4 130 [h/year] /1 000 = 1 032 [kWh/year]

[2]

where: •

0.25 W/(m3/h) is the power absorbed for a single extraction. For a double-flow installation, the power absorbed by the ventilators varies between 0.25 (high-performance installation) and 0.75 W (average performance installation) per m3/h of air transported.

•

4 130 h/year is the number of hours during which the offices are not occupied during the season when heating is required (35 weeks/year).

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Heating: 0.34 [W/m3.K] x 1 000 [m3/h] x (16 [°C]-5 [°C]) x 4 130 [h/year]/0.7/1 000 = 22 066 [kWh/year] or 2 229 litres of oil or 2 140 Nm3 of (Slochteren) gas per year

[1]

where: •

16° is the heating set temperature when the offices are not occupied and 5° is the assumed local average external night-time temperature during the season when heating is required

•

0.7 is the efficiency of the boiler

This represents a total financial saving of €735 per year (at €0.3/litre of oil and €0.065/kWh(e) during off-peak periods). A programmable clock only costs a few tens of euros. However, it should be noted that switching off the heating over night means that an extra heating demand will be added in the mornings. In practise, an optimal starting hour needs to be found.

Improve the efficiency of the ventilator and its drive system More than one third of the electricity consumed by a mechanical ventilation system is used to compensate for losses. New developments in ventilation technology have made it possible to significantly improve the efficiency of ventilators. Consequently, it is a good idea to find out whether or not it is possible to replace existing ventilators with ventilators that do not consume as much energy.

Example: A 400-mm ventilator has a flow of 8 000 m3/h at 1 000 Pa for 2 500 h/year. The electrical horsepower measured on the supply for its motor is 4 kW. This ventilator could be replaced by a more efficient ventilator, offering the same flow with an efficiency of 81% and a shaft power of 2.8 kW, by turning at a speed of 1 950 rpm. 66%

4000 Pa 3000

10.0 8.0 6.0 5.0 Pw(Pa) 4.0 3.0 2.8

2000

75% 80%

81% 80% nmax 70% 60% 50%

2.0 40%

1000 800

1.0 0.8 0.6

600 500

25%

400

3760 tr/min 3200 2800

300

2400 200

2100 1950 1800 1600

100 80

1400

60 50

1200 1000

40 2000

3000 4000

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10000 15000 20000 30000 8000 m3/h

Figure 1: Set point of a ventilator 4

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If the motor’s efficiency is estimated at 86% and that of the belt drive is 94%, the power that will be absorbed by the new installation can be estimated at: 2.8 kW / 0.94 / 0.86 = 3.5 kW The potential saving is therefore: (4 kW - 3.5 kW) x 2 500 h/year x €0.065/kWh(e) = €81/year for an investment of approximately €500, representing a payback time of less than four years. This type of saving is only possible if the operating point is located in the maximum efficiency area for the new ventilator, which could be difficult if the size of the ventilator is to be maintained.

Stop leaks In a building, measurements showed that to offer a flow of 650 m3/h in workrooms, the ventilator must propel 1 300 m3/h, i.e. double the flow required. Half of the air propelled by the ventilator therefore never reaches its destination and is lost through leaks in the building. This is a common situation in systems designed using rectangular ducts. As a result, the ventilator clearly consumes more energy than is necessary. Leak flow at 100 Pa(m3/h) 700 600 500 400 300 200 Class A

100

Class B Class C

0 1

2

3

4

5

6

Figure 2: Air tightness of ventilation ducts in a building: 1. Initial situation (rectangular ducts) 2 - 5. Successive air tightening with adhesive tapes 6. Replacement of rectangular ducts with round ducts with two joints or connectors.

However, there is no miracle solution because it is extremely difficult to ensure that an existing system is airtight. The only really effective solution is to replace rectangular ducts with round ducts with joints.

Balance flows Balancing a ventilation system means guaranteeing the exact flow in each room so as to ensure the atmosphere is comfortable and air quality is guaranteed. The better the flow balancing, the easier the task of controlling and optimising the heating system.

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Fit a heat exchanger

Air extracted

Air expelled

Air blown

Fresh air

Figure 3: Placement of a heat exchanger The fresh ventilation air, after having been brought up to the comfort level demanded inside the building, is expelled outside with an energy level higher than that of the external air being introduced. In this case, its enthalpy (or heat content) is greater than the external air. The concept involves transferring this heat from the extracted air to the fresh air. This will recover almost 50% of the budget for heating ventilation air. The profitability of the heat exchanger is the result of the comparison between the ‘profit’, i.e. the cost of the energy recovered, and the ‘expenses’, i.e.: •

Capital required to purchase and fit heat exchangers, pumps, ventilator etc. and adaptation of existing equipment.

•

Operating costs for energy consumed by pumps, ventilators and accessories.

•

Equipment maintenance costs.

In practice, the profitability is strongly dependent on the cost price of the thermal kWh. As far as technically possible, adapting a heat recovery system for the extracted air is particularly attractive: •

for high flows (over 10 000 m3/h);

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when the ventilation facility is in constant use;

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thermal requirements are high (e.g. swimming pools);

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when a ventilator and its motor are larger than required in the first instance which can avoid the need to make provision for replacing the pulleys and the motor to maintain the flows required.

Example: Calculate the energy contained in 1 m3 of air expelled outside. Given that air with a temperature of 22°C is expelled outside the building where air temperature is 6°C. The quantity of heat, Q, contained in this 1 m3 of expelled air is equal to the product of the volume of air multiplied by the volumetric heat capacity of the air (0.34 Wh/m3°C) and by the difference in temperature between the air expelled and the air outside (DeltaT). Q = 0.34 [Wh/m3.°C] x 1 [m3] x (22 [°C] - 6 [°C]) = 5.4 Wh In fact, the energy lost is proportional to the temperature difference and the humidity rate: •

the hotter the expelled air is (sensible heat loss);

•

the more humid the expelled air is (latent heat loss), 6

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•

the lower the outside temperature is;

•

the higher the energy content of the expelled air will be.

Calculate the energy expelled per hour by a ventilation system with a flow of 10 000 m3/h. Suppose that this ventilation air simply needs to be heated and that humidity does not need to be controlled. This group will then potentially expel the following quantity of energy each hour: Energy expelled per hour: 5.4 [W/m3] x 10 000 [m3/h] = 54 [kWh] If the air is heated using an oil-fired installation, with an efficiency of 70% (average efficiency installation), representing a fuel equivalent of: 54 [kWh] / (0.7 x 10 [kWh/litre]) = 7.7 [litre] A heat exchanger on the extracted air can recover around 50% of this consumption, i.e. the equivalent of 3.6 litres or €1.08 (at €0.3/litre) per hour in operation (some exchangers can even recover 75-90% of the energy consumed). 13.6 KWh 1.8 l heating oil

54.4 KWh 7.3 l mazout

22°C

10 000 m3/h

22°C 6°C

26°C

10 000 m3/h

66 KWh 9.1 l heating oil

13.6 KWh 1.8 l heating oil

27.5 KWh 3.6 l heating oil

14°C

22°C 27.5 KWh 3.6 l heating oil

6°C

10 000 m3/h

22°C

14°C

26°C

10 000 m3/h

40.8 KWh 5.5 l heating oil

Figure 4: Advantage due to the placement of a heat recuperator

Improve maintenance Maintenance of ventilation systems is a cost that is often forgotten. It is not uncommon to come across installations with filters or batteries that are so clogged up that the air can barely pass through them or heat economisers that are bypassed because someone ‘forgot’ to maintain them. Maintenance also plays a major role in energy efficiency and in the general efficiency of the ventilation installation. Belt drives deteriorate, leading to a significant drop in efficiency and the dust that collects reduces the efficiency of ventilators and increases charge losses in the filters and batteries. For example, it can be calculated that a ventilation system operating in an urban environment and propelling 54 000 m3/h or fresh air, will draw 43 kg of dust per year into the installation if there are no filters!

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Improving an existing air-conditioning installation Limit equipment consumption levels The best saving is, firstly, to limit the installation requirements: •

By reducing lighting, the savings are twofold in an air conditioned room: a saving in terms of the energy consumed directly by the lighting installation and a saving in terms of heat emitted by the installation (1 kWh of light = 1 kWh of heat in the room).

•

The same applies to office equipment: e.g. computers, photocopiers or printers.

As an initial approximation, lighting and office equipment accounts for more than 50% of the building’s energy bill!

Limit solar gains by using blinds Outside blinds are the most simple type of highly effective solar protection. Management of blinds will be closely examined and discussed with the occupants. If the rooms are unoccupied, the blinds should be left closed, this applies to the weekends in particular. Close blinds on the eastern side from the morning onwards before heat enters the rooms.

Adapt temperature settings It is important that a neutral area is created between heating and cooling: heating is cut off above 21°C and cooling starts from 24°C. If this does not happen, a surge will occur between the two systems with one cancelling out the effect of the other.

Stop humidification during mid-season Humidification is necessary during the middle of the winter because the heated air is too dry at that time. By contrast, humidification during mid-season is expensive and there is the risk of problems with bacteria. The humidification process can be stopped depending on the outside temperature. When the system is stopped, the installation needs to be emptied to avoid contamination caused by stagnant water.

Check the regulation settings It is easy to check if the regulating system is set correctly: enter the building at the time when the installation should be off! Go in the evening to see if the bathroom extractor has stopped, visit during the weekend to see if the air conditioning system has stopped, or check the programme entered into the regulator.

Opt for night-time cooling Consideration should be given to the extent to which it might actually be beneficial to partially cool the building at night, either using ventilation alone (but requiring a Delta T° of at least 8°C), or accompanied by the cooling unit.

Improve the efficiency of the ventilator and its drive system More than one third of the electricity consumed by a mechanical ventilation system is used to compensate for losses from the entire motor-transmission-ventilator system. Moreover, developments in ventilation technology have made it possible to significantly improve the efficiency of ventilators. Consequently, it is a good idea to find out whether or not it is possible to replace existing ventilators with ventilators that do not consume as much energy.

Analyse the specific possibilities offered by each air-conditioning system or associated cooling unit The information provided above will help you make the most significant energy savings. However, there are many ways to improve operation of existing appliances. This will require a specific analysis of the air condition system in the building.

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Conclusions Two situations are possible in an existing building: •

Either the building does not have a special ventilation system. In this case, it is a matter of finding out if uncomfortable situations (draughts, noise, air quality) would not justify such an installation.

•

Or the building already has a ventilation system. In this case, it is a matter of finding out if this system offers the level of comfort expected and if, at an equal comfort level, it would be possible to reduce the amount of energy consumed by the installation.

References [1] CD-ROM Energie+ (version5), Architecture et climat, UCL, 2006 [2] HVAC Systems and equipment, ASHRAE Handboek, Atlanta, 2004

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