Simulation study on R1234ze (E)/CO2 dual-evaporator cascade heat pump with ejector

In order to achieve more efficient, clean and reliable heating in severe cold regions, this paper designs a new type of R1234ze(E)/ CO2 cascade air source heat pump (CASHP) system. A modified dual-evaporator ejector expansion cycle is established in the low-temperature cycle (LTC) loop in which the ejector is used as the expansion device to recover expansion work, and an internal heat exchange is added to improve the system performance. The new CASHP system simulation model was built on the Dymola simulation platform, the model was tested and analyzed, and the effects of several major parameters on the system performance are studied, including the intermediate temperature, the ambient temperature, the output temperature and the ejector throat area. The simulation results show that the coefficient of performance (COP) of the new CASHP system is improved by about 15.3-19.5% compared with the conventional cascade system with ejector under output temperature of 60 ℃ and ambient temperature of -30-0 ℃. There exists an optimal intermediate temperature which makes the system energy consumption minimum and the COP of the system maximum, at the same time, the mass flow rate in high-temperature cycle (HTC) and LTC increase with the rise of the intermediate temperature while the mass flow ratio of HTC and LTC is opposite. The optimal intermediate temperature and the maximum COP of the system increase continuously under output temperature of 60 ℃ and ambient temperature of -30-0℃, and both of them decrease under ambient temperature of -15 ℃ and output temperature of 50-75 ℃. And there is an optimal ejector throat area in the new system that makes the system performance best.