Theoretical evaluation of minimum superheat, energy, and exergy in a high-temperature heat pump system operating with low GWP refrigerants

Suitable low global warming potential (GWP) refrigerants that conform to F-gas regulations are needed to extend the operational envelope of high temperature heat pumps (HTHPs) used for industrial processes and waste heat recovery. The thermophysical properties and characteristics of these alternative working fluids need to be assessed to provide a comprehensive understanding of operational effectiveness in HTHP applications. This paper presents the results of a theoretical simulation to investigate a range of low-GWP refrigerants and their suitability to supersede refrigerants HFC-245fa and HFC-365mfc. A steady-state thermodynamic model of a single stage HTHP with internal heat exchanger (IHX) was developed to assess system cycle characteristics at temperature ranges between 60 to 80 °C heat source and 90 to 140 °C heat sink. A practical approach to maximise the operational efficiency was examined to determine the effects of regulating minimum superheat within the process and subsequent influence on energetic and exergetic efficiencies. A comprehensive map of minimum superheat across the HTHP operating variables was used to assess specific tipping points in performance at 30 and 70 K temperature lifts. Based on initial results the refrigerants HCFO-1233zd(E) and HFO-1336mzz(Z) were found to be closely aligned matches to replace refrigerants HFC-245fa and HFC-365mfc. The overall results show effective performance for HCFO-1233zd(E) occurs between 5-7 K minimum superheat, and HFO-1336mzz(Z) between 18-21 K dependant on temperature lift. This work provides a method to optimise refrigerant selection based on operational indicators to maximise overall HTHPs system performance.