Second-Law Analysis of Vapor Compression Heat Pumps With Solution Circuit

A second-law analysis is conducted on both the single-stage vapor compression heat pump with solution circuit (VCHSC) and its modified version, the cycle with a preheater and additional desorber. The results are compared to a conventional heat pump cycle operating with pure ammonia. The location and magnitude of the irreversibilities of the individual components constituting the cycles are determined. The entropic average temperature is used in computing the irreversibilities. The total work input to the heat pumps is then conveniently decomposed into two parts: the minimum work input or the work of a reversible cycle operating between the desorber and absorber entropic average temperatures, plus an additional input of work caused by the irreversibilities of the different processes of the cycles. The analysis reveals that the compressor is the most inefficient component of the heat pumps with losses accounting for about one fourth of the work input. The irreversibilities in the desorber and absorber are found to be minimum when there is a good match in both the solution and heat transfer fluid temperature glides. By adding a preheater and an additional desorber, the irreversibilities in the single-stage VCHSC are considerably reduced. However, it is shown that it is the preheater and not the additional desorber that has by far the most significant impact on the heat pump’s efficiency improvements. Compared to a conventional ammonia vapor compression cycle, the modified VCHSC, which has twice as many sources of irreversibility, shows nevertheless a maximum improvement of 56.1 percent in second-law efficiency.