Comparison of CO2 heat pump water heater performance with baseline cycle and two high COP cycles

A CO2 heat pump water heater breadboard facility was built and a performance evaluation was conducted to investigate the effect of ambient temperature and hot water temperature on the overall coefficient of performance (COP) during full tank heating tests in three scenarios typical of residential water heating. Results show that the overall COP was maximized at higher ambient temperatures and at lower hot water temperatures. The overall COP was 30% higher for heating a full tank of cold water than reheating a warm tank water after standby losses. Additionally, potential COP enhancements of two cycle options were investigated. Performance enhancement of the two-stage cycle with internal heat exchanger was found only for standby loss reheating at low ambient temperatures and was 7.5%, while that of the suction line heat exchanger cycle was up to 7.9% for initial tank water heating and around 3.4% for reheating a warm tank water.