Energy and environment analysis of R1234yf/R245fa cascade air source heat pump system with double ejectors

For the purpose of improving the performance of the cascade heat pump system (CHPS) at low ambient temperature and promoting the process of global industrial decarbonization, single ejector cascade heat pump system (SECHPS) and double ejectors cascade heat pump system (DECHPS) are proposed in this research. The performance of nine refrigerant pairs (R1234yf/R245fa, R1234yf/R134a, R1234yf/R152a, R1234yf/R236ea, R1234yf/R1234ze, R32/R245fa, R125/R245fa, R152a/R245fa, R134a/R124fa) consisting of eight refrigerants (R1234yf, R125, R152a, R32, R134a, R245fa, R236ea, R1234ze) in the low temperature cycle (LTC) and high temperature cycle (HTC) in the three systems are compared initially. From the perspective of coefficient of performance (COP), safety and environmental characteristics, R1234yf is selected in the LTC, and R245fa is chosen in the HTC. The thermodynamic performance and economic efficiency are analyzed through thermodynamic and economic modeling, especially compared with the operating cost of the natural gas (NG) heating operation. Therefore, the COP of R1234yf/R245fa in the three systems is compared, and the results show that when evaporation temperature is -25 degrees C and the condensation temperature is 80 degrees C-100 degrees C, the COPs of SECHPS and DECHPS are increased by 3.87 %-4.68 % and 10.12 %-11.29 %, respectively. The pressure ratios of DECHPS in the LTC and HTC are decreased by 18.59 % and 17.23 %, respectively. The compressor power consumption of the SECHPS and DECHPS is lower than CHPS. Reducing the condensation temperature or increasing the evaporation temperature can reduce the amount of circulating working fluid charge and increase the COP of the system, and the ejection rate of LTC in the DECHPS is always higher than that of HTC. Further, when the heat supply is constant, the ejector system can reduce the electricity consumption of the system and the annual operating cost. Compared to the CHPS, the annual electricity consumption of SECHPS and DECHPS is reduced by 6.03 %-7.36 % and 13.34 %-15.83 %, respectively. The annual operating cost of DECHPS is reduced by 0.12 %-44.77 %. Moreover, the total equivalent warming impact (TEWI) of SECHPS decreased by 5.99 %- 7.30 %, and that of DECHPS decreased by 13.25 %-15.70 %, which can effectively reduce the impact on environmental emissions. The systems proposed in this paper have general applicability, and have more advantages in areas with hot summer and warm winter, high gas price and low or moderate electricity price, providing a theoretical reference for improving the energy efficiency of the HPS and realizing the goals of carbon peaking and carbon neutrality.