Book - School of Science and Technology

544 Refrigeration: water chillers and heat pumps100Zero storage Approx. 66% storage 100% storageChilleroperationLoad (per cent)5000 12 24 0 12 24 0 12 24Time of day(a) To match instantaneousloadFigure 19.11 Chilled water storage options(b) Chiller operationcontinuous(c) Off-peakarise since the specific mass of water at that temperature is critical and stratification isunstable.The use of alternative media such as ice and phase-change chemicals must be consideredif storage volumes are to be of practical size. As a result, there has been anincreasing use of ice storage systems over the past few years, particularly large city centreoffice developments where roof space is at a premium and quite often there is a surplus oflower value basement space. Another advantage to the office occupier is that becausethere is a significant, sometimes by 50%, reduction in the refrigeration capacity requiredthe use of dry air coolers in lieu of wet cooling towers becomes more practical. Thesystems also have inherent stored cooling capacity but care must be taken where there is a24 hour cooling requirement, for example computer equipment rooms.CIBSE Technical Memorandum TM18: 1994 Ice storage provides comprehensive informationon the use of ice for cool thermal storage. Whilst there are various systemsavailable in commercial office schemes it is more usual to incorporate indirect ice-on-coilsystems with a control strategy providing chiller priority. In this case, the cooling load forthe building is met initially by the chillers with ice store topping up any shortfall incapacity, then, if advantageous from an energy tariff point of view, the chillers will beturned off at a time that allows the ice store to meet the cooling load and be depleted bythe end of the day. The advantages of this arrangement are that the chillers will beoperating more often at their maximum capacity and hence more efficiently and the sizeof the ice store is usually less.Free coolingUse may be made of the principle of evaporative cooling in order to produce water at atemperature low enough to be suitable for cooling in an air handling plant. A coolingtower, provided to reject condenser heat in summer, may be usefully employed to providea source of cooling at times when the outside wet bulb temperature is low enough. A basicarrangement is shown in Figure 19.12 where a closed circuit water cooler is piped into thechilled water circuit: an alternative arrangement would be to use an open circuit towerwith a heat exchanger interposed between the open circuit and the closed chilled watercircuit, in order to prevent fouling in the closed circuit.