General method for the thermodynamic evaluation of heat pump working fluids

Abstract

Worldwide experience with Rankine cycle prime movers demonstrates that organic fluids have the potential of meeting future needs of high temperature heat pumps. Many promising candidate fluids exhibit a complex molecular structure, which imparts a peculiar character to the thermodynamics of the heat pump cycle. Since reliable tables of thermodynamic properties are unavailable for most of the fluids of potential interest, a general method was worked out which requires a small amount of data to evaluate the energy performance of different fluid classes. Cycle quality, defined as the ratio of actual to ideal COP, is recognized to be a function of the following main parameters: complexity of the fluid molecular structure, reduced temperature at which evaporation is performed; fractional temperature lift, ΔT/ T C. For fluids having rather complex molecules both technical feasibility (owing to condensation during compression) and energy performance are unsatisfactory unless regenerative precooling of the liquid prior to expansion is applied. If this is done, the same good performance can be obtained from all fluids. In any case the reduced evaporation temperature should be moderate, preferably below 0.7. Knowledge of the aforesaid main parameters, together with the set of graphs provided, allows the energy performance of heat pump cycles to be estimated.