Approaching optimum COP by refrigerant charge management in transcritical CO2 heat pump water heater

Abstract

• Transcritical CO 2 HPWH can approach optimum COP without high pressure control. • Charge management method is developed to replace high pressure control. • Average COP loss of the redesigned system without high pressure control is −0.2%. Optimal high pressure is crucial to the performance of transcritical CO 2 heat pump water heater (HPWH). From the perspective of refrigerant charge management, this work aims to approach optimum COP in CO 2 HPWH without high pressure control. By modeling a running CO 2 HPWH, we in-depth investigated the relation between optimal high pressure and refrigerant charge. It was found that the essence of optimal high pressure control is to manage optimum refrigerant charge of the system. If the optimum charge of the system could remain constant under any working conditions, the system would operate automatically at optimum COP with fixed refrigerant charge. Then influences of water inlet temperature, water outlet temperature and ambient temperature on optimum refrigerant charge were numerically analyzed. Finally, system design methods to approach optimum COP by minimizing the variation of optimum charge under variable operating conditions were developed. As a result, the average COP loss of fixed-charge system after resizing heat exchangers is only −0.8%. By comparison, with a high-pressure reservoir at the outlet of gas cooler, the average COP loss can be as low as −0.2%.