Book - School of Science and Technology

560 Refrigeration: water chillers and heat pumpsTable 19.2 Typical temperature ranges ofambient energy sourcesSource Temperature ( C)Outside air 10 to 20Ground water 8 to 12Surface water 2 to 6Ground coils 6 to 12If the heat source originates from a natural ambient supply, the temperature will vary:typical ranges are shown in Table 19.2.Compressor driveWhilst it is common practice to consider the heat pump as being electrically driven, this isnot necessarily the case except where small production units are concerned. If a broadapproximation of 25% be considered as representative of the conversion of primaryenergy to electrical power, it is obvious that a heat pump so driven, having a practicalcoefficient of performance of, say, 2.5, will equate as follows to, say, a boiler fired bynatural gas:Electrical heat pumpCOP (primary energy) ˆ 0:25 2:5 ˆ 0:625Gas fired boilerCOP approx: (primary energy) ˆ 0:62In consequence, it is proper to consider the case of a heat pump driven by a prime moversuch as a gas or a diesel engine in circumstances where the waste heat from the powersource may be recovered. Commonly quoted data for prime movers are, at full load:Shaft power 33%Waste heat to oil and jacket coolers 30%Waste heat to exhaust 32%If a recovery potential of 50% were to be applied to the sources of waste heat and anaverage 5% energy overhead considered for oil or gas supply, then the following equationresults:Motive power 0:33 2:5 ˆ 0:825Waste heat 0:62 0:5 0:95 ˆ 0:295Combined COP ˆ 1:12In consequence, a clear case exists for further examination of the use of heat pumps drivenby other than electricity, the ratio of advantage being of the order of 1.8:1 in favour of thisapproach. A further advantage of an engine driven heat pump is that both low and highgrade heat are available; the former from the heat pump condenser and the latter fromcooling of the engine jacket. Notwithstanding these apparent advantages, electricallydriven machines have become more popular in commercial applications due mainly to