Optimal intermediate temperature of two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling

Abstract

This paper presents a model for finding the optimal intermediate temperature to achieve the highest coefficient of performance of the standard two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling. The optimal intermediate temperature remains unaffected by the pinch point temperature at the cascade heat exchanger, though this temperature does impact heat pump performance. The optimal intermediate temperature rises with increasing high-side condensing and low-side evaporating temperatures and the value is deviating from the average temperature between the condensing and evaporating temperatures of the high and low-temperature sides of the cascade heat pump, respectively by a correction factor. The factor is found to be dependent on the critical temperature ratio of the working fluids in the low-temperature side and high-temperature side including the condenser temperature of the high-temperature side of the cascade heat pump cycle. A developed model consisting of the average temperature with the correction factor for estimating the optimal intermediate temperatures were examined with a set of non-azeotropic refrigerants with different compositions at different high-side condensing (65–95 °C) and low-side evaporating (5–15 °C) temperatures and pinch point (2–11 °C) temperatures. The results on the optimal intermediate temperatures and the maximum COPs agreed very well with those calculated from the enthalpy method within ±7 % and ±3 % deviations, respectively.