Experimental Investigation of Heat Pump Water Heater Using CO₂ Zeotropic Mixture with Large Temperature Glide for Waste Heat Recovery

Heat pump technology can be used to upgrade and enhance the quality of low-grade waste heat in production and daily life. Zeotropic mixtures can achieve temperature matching during the heat exchange process of heat pump cycles, which realizes in-depth utilization of waste heat resources. This study conducts an experimental analysis of CO₂/R1234yf, CO₂/R290, CO₂/R600a, and CO₂/R32 zeotropic mixtures using a water-source heat pump experimental system. The performances of heat pumps with evaporators and condensers that simultaneously satisfy high-temperature differences were investigated using the zeotropic mixture. Results show that the amount of charge in the zeotropic mixture primarily affects the degree of cycle subcooling, thereby influencing the coefficient of performance (COP) of the system. When the mass composition of CO₂/R1234yf is 15%/85%, an optimal refrigerant charge, which corresponds to a COP of 7.66 under the experimental condition (heat source inlet water temperature at 25 ℃, outlet at 5 ℃; heat sink inlet water temperature at 15 ℃, outlet at 45 ℃) is obtained. Furthermore, compared with pure R290, the CO₂/R290 working pair exhibits an enhancement of 75.2% in the optimal COP, with a corresponding increase of 107.7% in the volumetric heating capacity (q_v). Similarly, the CO₂/R1234yf working pair demonstrates an improvement of 27.7% in the optimal COP and an increase of 92.0% in q_v. The CO₂/R32 working pair exhibits a 15.0% increase in the optimal COP. However, the CO₂/R600a working pair exhibited only a 1.7% enhancement in the optimal COP, indicating a relatively low application potential.