ON THE EFFICIENCIES OF ABSORPTION HEAT TRANSFORMERS

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 transformers. 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 pair H2O-NaOH is used in two heat transformer systems. The given examples clearly show that the COP can only be used to compare different heat transformers 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 transformer operated at a fixed circulation ratio. The thermodynamic efficiency, E(th), is shown to be a more logical measure of the heat transformer efficiency, since it takes into account both heat losses, heat exchange and the temperature lift. The exergetic efficiency, E(ex), is an alternative to E(th) since both are mathematically compatible. The possibility of taking into consideration the temperature level, at which heat energy may be considered economically worthless, is demonstrated to be a major advantage of the exergetic efficiency. However, its numerical value does not provide a clear interpretation of the importance of exergetic losses in the system. The exergetic index, I(ex), is directly related to the exergetic efficiency but its numerical value is more significant for evaluating the performance of the heat transformer system.