Thermodynamic performance assessment of carbon dioxide blends with low-global warming potential (GWP) working fluids for a heat pump water heater

• CO2 blends with low-GWP refrigerants used for heat pump water heater are evaluated.
• A maximum COP is obtained at a corresponding optimal high pressure for each component ratio.
• Addition of low-GWP working fluids to CO2 can help to reduce operating pressure and improve system COP.
• Temperature match in the evaporator plays a leading role for the whole system performance.
• CO2 mixtures are recommended on the base of the working condition of chilled water temperature glide.

Blends of CO2 with ten low-global warming potential (GWP) working fluids are evaluated for use in a heat pump water heater. The effects that the discharge pressure, component ratio, hot-water outlet temperature and chilled water inlet temperature have on the coefficient of performance (COP) of heat pump are analyzed when the pinch point of the heat exchange is considered. It is found that temperature glide of zeotropic mixture has a good thermal match with the temperature change of water as two pinch points appear in the gas cooler/condenser or evaporator. The good thermal match in the heat exchangers promotes the system COP. Addition of low-GWP working fluids to pure CO2 can reduce the high-side pressure. The results show that CO2/R41 and CO2/R32 are suitable candidates for heat pump water heaters because of their high COP and low high-side pressure in comparison with those of a pure CO2 cycle.