Performance optimization of CO2 heat pump water heater

A preliminary analysis was conducted to analyze the performance of a heat pump water heater (HPWH) that uses CO2 as the refrigerant. A model to predict the performance was developed and calibrated based on the experimental data for an existing HPWH using a CO2 refrigerant. The calibrated model was then used to run a parametric analysis in which factors such as water supply temperature, water circulation rate, tank stratification, and condenser configuration were considered. The performance of a commercial CO2 system was compared with that of a similar system using R-134a as the refrigerant. It was found that CO2 HPWH performance was comparable to that of an R-134a HPWH, more so for a separated gas cooler configuration. For comparable performance, the compressor size and the tube-in-tube heat exchanger (condenser/gas cooler) size were compared for CO2- and R-134a-based systems. The impact of the water circulation rate on the water temperature stratification in the tank, an essential requirement for higher performance for CO2 HPWH systems was also investigated.