CH-24-C097 - Conversion of a Transcritical CO2 Heat Pump Water Heater from an Air-Source Configuration to a Water-Source Configuration

The impending phase-out of commonly used heat pump refrigerants has accelerated the need to develop and deploy refrigerants that have low global warming potential (GWP). Carbon dioxide (CO2) as a refrigerant has a suitably low GWP, is naturally occurring in the environment, and is promising for heat pumps in cold climates. The objective of this study was to convert an air-source heat pump water heater (HPWH) to a water-source HPWH for use in a solar-assisted configuration and ensure the performance was comparable. This study is the first stage in a multistage project that will assess a solar-assisted CO2 (HPWH) for space and domestic hot water (DHW) heating. This paper covers the first stage, in which a commercially available, CO2 air-source HPWH was converted to a water-source HPWH. Furthermore, the effect of refrigerant charge and different operating parameters on the performance of the water-source HPWH were studied. The conversion of the CO2 HPWH can be divided into three phases: finding a suitable replacement evaporator and installing it, optimizing the refrigerant charge for the new configuration, and experimental testing to evaluate the performance of the converted unit. The water-source unit was tested for source temperatures ranging from -20 oC to 45 oC to create a performance map for simulation use. The results show the COP of the converted water-source unit is comparable to the original air-source unit: at -19 oC source temperature, the COP was 2.1 and 2.0 for the air-source and water-source unit, respectively. At a source temperature of 26.5 oC, the COP was 4.4 and 5.2 for the air-source and water-source unit, respectively. Initial performance is promising for use in a solar-assisted configuration for providing space and DHW heating.