A Numerical and Experimental Analysis of a Transcritical, CO2 Heat Pump Water Heater

Due to the environmental concerns regarding traditional refrigerants, carbon dioxide (CO2) is being studied as an alternative refrigerant in HPWHs. In this study, a commercially available, transcritical CO2 HPWH was numerically and experimentally assessed. This included developing a model of the CO2 HPWH and calibrating it with experimental data. The model was theoretically verified and utilized to predict the cycle pressures and compressor isentropic efficiency. While the predicted discharge pressures were within the expected range, the significant biases indicate that further work is required to increase the model accuracy. Additionally, study of the control system indicated that the water outlet temperature controls the water pump, the ambient temperature controls the compressor and evaporator fan, and that the expansion valve controls the high side pressure. Lastly, the performance data shows that the HPWH achieves the best performance at ambient air temperatures above 0°C and at water inlet temperatures below 30°C.