Theoretical Evaluation of Energy, Exergy, and Minimum Superheat in a High-Temperature Heat Pump with Low GWP Refrigerants

Suitable low global warming potential (GWP) refrigerants that conform to F-gas regulations are fundamental to the operation and future development of high-temperature heat pumps (HTHPs) used for industrial processes and waste heat recovery. 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 an internal heat exchanger (IHX) was developed to assess system cycle characteristics and performance at temperature setpoints at 60 and 70°C heat source, 90 and 140°C heat sink, at 30 and 70 K lift. This study focuses on energetic and exergetic efficiencies within the system and the impact of regulating superheat to optimise performance. Based on energetic and exergetic theoretical results, a trade-off between COP, VHC, and exergetic efficiency indicates HCFO-1233zd(E) and HFO-1336mzz(Z) as the most likely replacements for HFC-245fa and HFC-365mfc respectively. The refrigerant HC-601, followed by HFO-1336mzz(Z) and HCFO-1233zd(E), exhibited the lowest exergetic destruction within test conditions. Mapping the minimum superheat indicated optimum performance for HCFO-1233zd(E) between 5 to 8 K and HFO-1336mzz(Z) between 17 to 22 K, depending on temperature lift. Validation of the theoretical results with experimental data indicates that simulated COP closely matches empirical values. This work provides a method to optimise refrigerant selection in HTHPs based on operational indicators to maximise overall system performance.