Introducing full air conditioning into a design can often add around 50% to the eventual running costs of the building and should therefore be avoided where possible. Although full air conditioning involves humidity control, in the UK the term is commonly used for the provision of mechanical cooling to the conditioned space. In the UK, there is a limited set of circumstances in which a building will require air conditioning. However, it can be necessary in certain circumstances due to pollution, external noise and high heat gains.
Table 7.2 Basic fan capacity benchmarks Building Fan capacity
/ (litre·s–1)·m–3
of ventilated space)
Offices 1.4
Retail stores, halls and theatres 2.1 Restaurants 3.5
Table 7.3 Advantages and disadvantages of balanced ventilation Advantages Disadvantages
Contaminants removed at source. Building must be effectively sealed to prevent air ingress by
infiltration.
Incoming air can be conditioned Expensive, requiring two complete and cleaned. duct work systems.
Potential for heat recovery from Regular cleaning and maintenance exhaust air is necessary.
Weather independent, provided Electrical energy consumed in fans structure is moderately airtight
7-4 Part A: Designing the building
When designing mixed-mode or mechanically ventilated buildings, a balance should be sought between moving small amounts of cool air rather than large amounts of tempered or ambient air.
Figure 7.2 provides useful guidance in assessing the need for full air conditioning, comfort cooling or mechanical ventilation(23).
7.2.3.1 Avoiding mechanical cooling
Avoiding the need for mechanical cooling is a function of integrated service and fabric design as discussed in section 4(8). There are several steps to be taken to minimise the energy need for air conditioning:
— Minimise the heat gains to the space by careful design of the building envelope.
— Minimise internal heat gains from lighting, office equipment etc.
— Wherever possible, meet the cooling requirements with free or passive cooling sources (see below). — Ensure the systems and controls are able to match
the cooling requirements efficiently.
— When mechanical cooling is required, limit where it is employed and ensure that the cooling plant operates as efficiently as possible.
In general, the liberation of more than about 40 W·m–2 from lighting and other sources may be regarded as
excessive and will constitute grounds for reassessing the design to minimise these heat gains.
The main problem in avoiding mechanical cooling is that the period when heat gains are greatest usually coincides with periods when outside air temperatures are at a maximum, thus reducing the cooling potential. It is necessary, therefore, to consider ways in which stored ‘coldness’ can be exploited.
Using night air to cool the building fabric can avoid the need for mechanical cooling. The building fabric can be used as a thermal store acting as a heat sink during the occupied hours by absorbing the incidental gains.
The thermal mass of the building then needs to be directly available for heat exchange. Exposed concrete ceilings are widely used and can achieve an additional 2–3 °C reduc- tion of the daytime internal temperature. The reduced surface temperature of the exposed concrete also influences the radiant thermal environment and enhances the effect (see 4.2.5).
7.2.3.2 Need for humidity control
Full air conditioning with humidity control is even more energy intensive than systems providing heating and cooling only. It is essential, therefore, to question the requirements, tolerances and need for humidity control(1,2,8). For most human comfort applications, relative humidity can drift between 30% and 70%(6).
Does the application have to account for consistently high heat loads (eg main frame computer or crowds of people)?
Does the application require close control of humidity?
Will it be acceptable for the office space to exceed 28°C for a few hours each year? Does the building include large open plan offices?
Does the application require very large volumes of fresh air (eg hospitals, laboratories, etc)?
Any local system
(small buildings) constantCentral volume or dual duct system Any centralised/ partially centralised system Small buildings Large buildings Air conditioning is required
Any centralised/ partially centralised
system (large buildings) This may be natural ventilation,
local ventilation fans, ventilation via atria, or a centralised ventilation system
Ventilation only is required
i ii iv v v iii Yes Yes No No No Yes No Yes No Yes No Yes
Ventilation and air conditioning design 7-5
Where humidity needs to be controlled within much tighter bands, both capital and running costs will be increased. In general, the closer the design humidity tolerances, the greater the energy consumption (see Figure 7.3).
The BCO’s Best practice in the specification for offices(10) suggests that humidification is rarely needed for general office use and that humidity control should not be installed within the base office scheme unless a particularly high rate of fresh air is required. In which case it should be controlled to give a minimum of 35–40%.
Close control of humidity is sometimes required for specialist applications such as the protection of exhibits in a museum. Designers should carefully evaluate whether both humidification and de-humidification are required to maintain conditions within acceptable limits(24).
De-humidification is generally only required in summer. The most common method of removing moisture is by cooling the air to its dew-point using a chilled water cooling coil so that moisture condenses out. This com- monly occurs when air is being cooled for comfort conditioning. For close control of relative humidity the air may be cooled further to the required moisture content, then re-heated to bring it back to the required temperature. Cooling then re-heating makes de- humidification an energy intensive process that should be avoided where possible, unless heat rejected from the cooling process can be used for re-heat(2).
Desiccants offer an alternative method to de-humidify air. Exposure of the air to chemicals such as calcium chloride or silica gel will remove moisture. In order for the desiccant to be re-used, it has to be regenerated by heating to drive off the moisture. Often much more air is dehumidified and reheated than is necessary, avoidable by using a by-pass plant. The cost of the regeneration plant and the cost of providing heat for the regeneration process have so far limited the take-up of this technology. However, it may become more attractive if a local source of waste heat exists for regeneration. It is also particularly appropriate if there is a need for de-humidification without the need for significant sensible cooling, e.g. clean rooms.
Humidification is mainly required in winter and can be achieved using a range of equipment including steam injection, water spray and ultra sound humidifiers. Spray humidifiers are now less common due to health concerns. The characteristics of different types of humidification equipment are discussed in CIBSE Guide B2(2) and in BSRIA AG10/94.1(25).
Steam injection systems, whether direct from a steam distribution system or from local electrode boilers, give rise to very significant energy consumption. Where steam is available on site, it is sensible to make use of it and directly inject steam to humidify. However, this will add significantly to the winter demand on the boiler plant. More commonly, local electrode humidification units are used to inject steam into the air stream. These units generally have independent controls and can often result in excessive electricity consumption if not maintained. Humidification can be a significant energy user in air con- ditioned offices. Typical and good practice performance indicators are shown in Table 7.4(20). The ‘energy use indicator’ (EUI) is the product of(26):
(a) the installed capacity in W·m–2of treated floor area. This typically varies between about 15 and 25 but should be kept to a minimum once the need for humidification has been established
(b) the annual running hours
(c) the average percentage utilisation of the plant expressed as a decimal fraction.
The result is then divided by 1000 to obtain the EUIin (kW·h)·m–2per year.
Energy use can be minimised by close control of the hours of operation and ensuring that the humidity set point is kept as low as possible.
Ultrasonic humidification systems are now becoming more common, using up to 90% less energy than electrode systems and, hence, provide a cost-effective alternative. Lower electrical consumption results in reduced electrical wiring costs, although a water purification plant is required to operate the system. Ultrasonic humidifiers require little energy themselves, although the air temper- ature will decrease due to the evaporative cooling effect. Re-heating is then necessary to bring the air up to the required supply temperature.
Boiler Chiller 30% – 70% r.h. 40% – 60% r.h. 35% – 65% r.h. 45% – 55% r.h. Steam 8 x 103 6 x 103 4 x 103 2 x 103 0
Energy consumption of plant / kW
. h
Control plant
Figure 7.3 Effect of humidity control on energy use
Table 7.4 Humidification benchmarks for air conditioned offices Parameter Benchmark for stated office type
Air conditioned Air conditioned standard office prestige office (Type 3) (Type 4)
Good Typical Good Typical practice practice
Installed capacity (W·m–2) 15 20 20 25
Running hours (h/yr) 2750 3500 3000 3700 Utilisation (%) 20 25 20 25 Energy use indicator 8 18 12 23 (EUI) ((kW·h)·m–2)/yr
Note: factors for converting treated floor area to nett and gross are given in Table 20.1
7-6 Part A: Designing the building
7.3
Efficient air conditioning
systems
There is a wide range of air conditioning plant, including some that offer greater scope for energy efficiency. For a more detailed description of the systems, see CIBSE Guide B2(2)and BSRIA TN15/92(27). Good Practice Guide GPG 71(23)gives a useful summary of the relative advan- tages and disadvantages of the various systems. Figure 7.4(23)and Table 7.5(23)list the systems and indicate the capital and running costs of some of options, based on gross floor area and 1992 prices.
GPG 257(3)provides a useful comparison of capital and running costs for high, medium and low velocity systems.
This shows that running costs are reduced for low velocity systems and that some components become more expen- sive, others become cheaper. Systems velocity is therefore a key factor in the design of an energy efficient system.