Investigation on thermodynamic characteristics of refrigerants for high temperature heat pumps based on the principle of corresponding states

High-temperature heat pumps (HTHPs) provide a powerful solution to access carbon-free thermal energy from the renewables. Refrigerants have a central place in HTHP design. This study explored the thermodynamic characteristics of ideal refrigerants that produce the ultimate performance benchmarks defined by fluid universal behaviors quantitatively and qualitatively. A systematic analytic framework is constructed based on the principle of corresponding states. Through the parametric characterization of refrigerants, the corresponding ideal refrigerants are identified from the comprehensive refrigerant space by using a heuristic algorithm. Various operating conditions and scenarios are incorporated with two competing indicators, namely, coefficient of performance (COP) and volumetric heating capacity (VHC), employed separately to determine the ideal refrigerants. A holistic sensitivity analysis of the characteristic parameters is performed. Moreover, the macro behaviors and qualitative features of the ideal refrigerants are also analyzed. Results show that in addition to evaluation criteria, subcooling has a clear influence on the ideal refrigerant characteristics. Under COP, the ideal refrigerants in small subcooling exhibit a nearly isentropic fluid characteristic; while those in large subcooling manifest a dry fluid characteristic. Apart from the critical temperature, the contrasting preference in the ideal gas heat capacity is responsible for the inherent trade-off between COP and VHC in large subcooling.