Researchers from Chiang Mai University in Thailand have proposed a new approach to find the optimal temperature to increase the coefficient of performance (COP) of two-stage cascade heat pumps using non-azeotropic refrigerants.
Despite their limited success in the industry, non-azeotropic refrigerant mixtures are considered promising in heat pump applications as they offer improved efficiency both in terms of coefficient of performance and compressor power.
Two-stage cascade heat pump systems work on standard high-temperature side (HS) cycles and low-temperature side (LS) cycles that are interconnected via a heat exchanger. Each cycle consists of a condenser, an expansion valve, an evaporator, and a compressor, with the overall system including a heat exchanger and a heat sink and with all components being under steady flow, steady-state condition.
During the LS cycle, the refrigerant is heated and vaporized at the evaporator to emerge then as a saturated vapor. This pressure is elevated by the LS compressor under an isentropic process and the vapor is transformed into a saturated liquid, which goes through irreversible adiabatic expansion at the expansion valve and then is vaporized again through heat absorption at the low-temperature evaporator.
During the HS cycle, the working fluid arrives at the heat exchanger as a saturated vapor and then moves into the HS compressor under an isentropic process. After condensation in the condenser, the working fluid transforms into a saturated liquid, which is then reduced in temperature and pressure to re-enter the HS evaporator.
The academics explained that identifying the intermediate temperature of these heat pump systems, as well as the condensing and evaporating temperatures of their HS and LS, has always been a difficult exercise. “In the case of non-azeotropic refrigerants, so far, there is no general approach that can be employed to indicate the optimal for a two-stage cascade heat pump in order to achieve the highest COP,” they stated.
“For specified values of HS condensing and LS evaporating temperatures, an increase in the intermediate temperature leads to a corresponding decrease in HS compression work and an increase in LS compression work. Conversely, a decrease in the intermediate temperature results in the opposite effect. Then, the overall COP for simultaneous heating and cooling are varying with the intermediate temperature,” they added.
Through their analysis, the scientists found that the COP for simultaneous heating and cooling is affected by the pinch point temperature at the cascade heat exchanger, with lower temperature levels being responsible for a higher COP. They also found the intermediate temperature increases with both the HS condensing temperature and the LS evaporating temperature. “Most of the optimal temperatures were found to be less than the average value of the HS condensing temperature and LS evaporating temperature,” they also explained.
They also concluded that the optimal temperature can be identified by considering the correction factor correlation in conjunction with the average temperature between HS condensing and LS evaporating temperatures as a reference. “The results from the newly developed model for optimal intermediate temperatures and maximum COPs showed very good agreement with those calculated using the enthalpy method, with deviations within ±7 % and ±3 %, respectively,” they emphasized.
The proposed approach was described in the study “Optimal intermediate temperature of two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling,” published in Results in Engineering. Looking forward, the group said it intends to test the method in multi-stage heat pumps designed for very high-temperature heating and low-temperature cooling.
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