Book - School of Science and Technology

C.O.PHeat pumps 5595435ºC Flow3245551–15 –10 –5 0 5 10 15Outside air temperature (ºC)Figure 19.24 Variation of COP with source temperaturesmaller the temperature difference, the better the performance characteristic. In applicationto practical problems, it must be remembered that the theoretical performance shownin Figure 19.2 is distorted in practice by superheat and sub-cooling effects: these affectpredicted performance which may well be only two-thirds of the theoretical.In consequence of the requirement that the temperature difference between thecondensing and evaporating temperatures (and hence between heat utilisation mediumand heat sink) be minimised, applications to space heating are restricted. In winter,where heat output is required to be at maximum, heat sources such as outside air, riverwater or the earth's crust are likely to be at their annual minimum temperature. Thisis illustrated in Figure 19.24 which gives the likely order of variation in COP for anair source heat pump. Industrial effluents and power station waste are, however,relatively constant as to temperature throughout the year and it is to these thatattention should be directed as well as to any advantage that may arise from collectionof solar energy.The reduction in performance at low source temperature may lead to the need for plantof high capacity, and therefore cost, to meet the heating demand under extreme conditions,in consequence of which, a heat pump may be selected to match only a part ofthe full design load, say 70%, the remainder being met by a secondary heating source.Such a system is termed bivalent and is referred to in Chapter 6. A further considerationwith air-source plant is the requirement to defrost the evaporator coil during coldweather, which may account for an overall loss of heat output of up to 10%.In selecting the alternative energy sources for a bivalent system, it is necessary toconsider the flexibility of a combination of a direct supply (electricity or gas) with astorage fuel (oil or coal) with respect to the future reliability of supply of either category ofenergy source.There are four basic types of heat pump suitable for use in heating and air-conditioningwhich may be defined according to the heat sources and method of transporting heat topoint of use, namely:. air-to-air (air source to air system). air-to-water. water-to-water. water-to-air.