Investigation on multi-stage compression high-temperature heat pump system based on R718 and other low GWP refrigerants

Under the dual-carbon strategy, electrifying industrial heating is a key trend. This study develops three types of thermodynamic cycle models for compression high-temperature heat pump (HTHP) systems. The effects of evaporation temperature on the compression characteristics of R718, R245fa, R1234ze(Z), and R1336mzz(Z) are investigated, along with an analysis of the available energy variation within the system. Results show that R718 exhibits superior thermodynamic properties compared to other refrigerants at high evaporation temperatures, with its coefficient of performance (COP) increasing from 4.70 to 5.34. However, the high pressure ratio and excessive superheat (up to 150 °C at 90 °C evaporation temperature) limits its practical application. When the lifting temperature is set at 40 °C, the three-stage compression (THC) system achieves an optimal performance value of 5.47, with a pressure ratio of 1.58 and exhaust superheat of 35.38 °C. The exergy efficiency of the three-stage compression (TWC) and THC systems using R718 is 39.6 % and 56.1 % higher than that of the single-stage compression (SC) system at a waste heat source temperature of 95 °C, respectively. These findings emphasize that multi-stage compression significantly enhances energy efficiency. Furthermore, this study provides valuable guidance for practical engineering applications.