Latent thermal storage for air conditioning

Abstract

The advantages of thermal storage as part of an installation that provides intermittent temperature conditioning are obvious: the peak power requirement is reduced; this peak can be shifted to a lower tariff time of day; chilling plant can operate at close to 100% capacity, i.e. maximum efficiency; and the maximum electrical demand can be reduced significantly, further reducing running costs. Unfortunately the space required, using sensible thermal storage, is usually too great to provide anything more than a buffer to prevent chillers starting too frequently. To overcome this space problem system designers are now resorting to latent thermal storage. Commercially available equipment can be divided into that based on ice and that based on the use of encapsulated eutectic mixtures (phase change materials). For the air conditioning designer the systems can be further subdivided into pressurised and atmospheric storage. Both types rely on the latent heat of fusion to reduce to a third or a quarter the storage volume necessary, but there are important differences in the satisfactory application of the various methods. The note examines and discusses the basic principles relevant to system design, describes the range of ice-based equipment available including ice builders, ice tanks and ice harvesters. The range of phase change materials and associated equipment is then discussed and analysed in the context of suitable air conditioning applications. It is concluded that both ice systems and phase change materials can offer advantages and savings but that they must be carefully matched to the air conditioning system design if their full benefits are to be realised.