Experimental study and optimal control strategy of a transcritical CO₂ heat pump water heater

In this study, a transcritical CO₂ heat pump water heater prototype was experimentally investigated in order to clarify the effects of the opening of electronic expansion valve (EEV), research the optimal discharge pressure and obtain an optimal control strategy for performance optimization. The tests were carried out by regulating the opening of EEV in multiple working conditions. The results showed that the decrease of opening led to reduction in evaporator inlet pressure and gas cooler outlet temperature, augment in discharge temperature, and decrease first then increase in discharge pressure. The optimal opening to reach maximum coefficient of performance (COP) always existed in all working conditions, however, there were two different situations where the corresponding optimal discharge pressure was near or away from the minimal value of the pressure curve. The reason for this phenomenon was the difference in water inlet temperature. Moreover, a correlation for the optimal discharge pressure was established with ambient temperature, water inlet and outlet temperature as independent variables. Furthermore, an optimal control strategy, in which the control of discharge pressure and superheat degree was considered, was proposed based on the correlation. Testing results proved that the optimization of performance could be achieved by using the strategy and the deviations between the COP under the control strategy and the optimal COP measured by experiments were within 5%.