Field Evaluation for Air-source Transcritical CO2 Heat Pump Water Heater with Optimal Pressure Control

Several optimal discharge pressure correlations for transcritical CO2 heat pump in literature have been discussed in this paper. Most of them are related to the ambient temperature, the evaporation temperature and the gas cooler outlet temperature. A revised optimal discharge pressure model as the function of ambient temperature and water outlet temperature is developed based on earlier experimental results. To validate the model developed and control effect in practical applications, field tests are conducted to evaluate the performance of an air-source transcritical CO2 heat pump water heater in practical application. The results show that the coefficient of performance (COP) can achieve 3.76 in the nominal test condition with 15°C water inlet temperature and 80°C hot water supply temperature. Even when the hot water temperature is higher than 90°C, the COP remains at 3.21 with 20°C dry-bulb temperature and 15°C wet-bulb temperature. Under -15°C low ambient air temperature condition, the COP was 2.19 with the hot-water supply temperature of 60°C. Comparison between the field test results and the model predictions show that the maximum relative error of discharge pressure control was 3.2% in the high temperature of water outlet condition. The overall ATHP system performance based on the revised model and PLC control strategy can meet the requirements in practical applications.