Comparative Selection of Pure and Zeotropic Refrigerants in Cascade Heat Pumps: Effects on System Performance and Optimal Intermediate Temperature

This study develops a model to determine the optimal intermediate temperature (Tm,opt) that maximises the coefficient of performance (COP) in a typical two-stage cascade heat pump. Focusing on applications requiring simultaneous heating and cooling, the research compares the performance of zeotropic and pure refrigerants in both the high-stage (HS) and low-stage (LS) cycles. It also explores a hybrid configuration where zeotropic refrigerants are used in one stage and pure refrigerants in the other, offering insights into practical design flexibility. A correction factor based on the critical temperature ratio of the working fluids and the condenser temperature on the high-temperature side, the ideal intermediate temperature is found to differ from the mean of the condensing and evaporating temperatures. The model incorporates this adjustment factor to estimate Tm,opt for various refrigerant combinations. The model evaluates performance across a range of operating conditions, with LS evaporating temperatures between 5°C and 15°C and HS condensing temperatures between 65°C and 95°C. Quantitative results demonstrate that the model achieves deviations of less than ±7% for Tm,opt, ±3% for COPs, and under 5.5% for the extended Figure of Merit (FOM) compared with the enthalpy method, confirming its accuracy. The study's novelty lies in integrating correction factors for zeotropic refrigerants and extending the FOM model to hybrid two-stage configurations, confirming its accuracy. The study provides a foundation for improving the energy efficiency and adaptability of cascade heat pump systems, suitable for diverse industrial and residential applications requiring simultaneous heating and cooling.