Book - School of Science and Technology

Chapter 14Air-conditioningThe science of air-conditioning may be defined as that of providing and maintaining adesirable internal atmospheric environment irrespective of external conditions. As a rule`ventilation' involves the delivery of air which may be warmed, while `air-conditioning'involves delivery of air which may be warmed or cooled and have the moisture content(humidity) raised or lowered.National and international concern directed at the global environmental effects arisingfrom release of refrigerants into the atmosphere, as described in Chapter 19, and theenergy used in mechanical cooling systems have, together, opened the door to a period offundamental change in attitudes towards the use of air-conditioning in certain buildings.As mentioned in Chapter 3, for the first time the 2001 edition of Part L of the buildingregulations impose requirements on air-conditioning and mechanical ventilation systems,in that if they are installed, they should be `reasonably efficient'. Also, it should be notedthat the requirement to control solar overheating does not impose any restriction on theuse of air-conditioning. What it does do is to make the building capable of being keptreasonably comfortable either in the absence of air conditioning (provided internal gainsare modest) or at reduced air-conditioning loads.The difficulty, as far as the efficiency of the system is concerned, is that the energyconsumed by an air-conditioning system is dependent on a number of different aspects ofthe building design:. the form and fabric of the envelope, especially in relation to the solar gains to the space. the efficiency of the cooling plant (chiller CoP). the method of `coolth' distribution (air, water or refrigerant). the design of the distribution network (high velocity/low velocity), and the efficiency ofthe prime movers (fans, pumps). the control system (use of free cooling etc.).In order to provide a simple but robust mechanism for assessing the diversity ofdesigns, the estimated annual carbon emissions from an air-conditioning system has beenrelated to the installed capacities of the primary plant (chillers, fans and pumps), sincethese act as a proxy for the combined effect of most of the above items. The installedcapacity terms are then modified by factors that depend on the degree and extent ofcontrol and monitoring provisions included in the design. This simple model has thenbeen calibrated against the ECON19 dataset of energy consumption in office buildings toarrive at a target design figure for carbon emissions for the system. The targets in the 2001edition of Approved Document L are not particularly demanding, but it is consideredlikely that these targets will be made much tighter at subsequent revisions, once industryhas become familiar with the requirements.