On the efficiencies of absorption heat pumps

Abstract

Although commonly used, the coefficient of performance COP is not always an adequate measure to describe the effectiveness of a sorption heat pump. Equations for four different efficiencies are derived, discussed and compared for absorption heat pumps (AHP). A flow-sheeting computer program, developed for both design and evaluation simulations of arbitrarily complex absorption cycles, is used to exemplify the derived equations. The working fluid pair H 2OLiBr has been used in two different AHP configurations. The examples given clearly show that the COP can only be used to compare different AHPs operated at the same circulation ratio. The COP can be considered as an indicator of the effectiveness of heat exchange within and thermal insulation of a heat pump operated at a fixed circulation ratio. Nevertheless, it is an insufficient measure to compare different AHPs, even when they are operated at the same circulation ratio. On the other hand, the coefficient of performance for cooling Q e/ Q g is better in this respect since it takes into account the real heat flow to the generator. The Carnot efficiency COP rev takes into consideration both the real heat outputs from the absorber and condenser, and the temperature of heat sources and heat sinks. The thermodynamic efficiency E th is shown to be a more logical measure of the heat pump efficiency, since it takes into account the real heat input the generator. The exergetic efficiency E ex can be considered as an alternative to the thermodyanamic efficiency E th but it offers a possibility to take into account any temperature level where heat energy may be considered worthless. However, both E ex and E th are not conventionally used since their numerical values are always less than 1.0. On the other hand, the exergetic index I ex is directly related to E ex but its numerical value shoul be considered as a more significant measure for evaluating the performance of AHP systems, since it properly takes into account the exergy losses which inevitably occur in the system. It may however be stressed that exergy analysis should be used as a compliment to the First Law analysis.