Optimal refrigerant selection and parameter analysis of a wastewater heat pump using CO2 zeotropic mixtures

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Heat pump technology can effectively recover waste heat from low-grade wastewater, which is crucial for energy conservation and carbon emission reduction in buildings. In this study, zeotropic mixtures were used to recover heat from low-grade wastewater and produce high-temperature water in a heat pump. The deep recovery of waste heat was realized by using a CO2 zeotropic mixture with a large glide temperature. Furthermore, this method ensures an excellent temperature matching between the heat transfer fluids and effectively reduces irreversible losses. To obtain an optimal zeotropic mixture, a method based on heuristic screening of the optimal CO2 zeotropic mixture was proposed. Furthermore, a model of a CO2 zeotropic mixture heat pump was established and experimentally validated. Based on the validated model, a sensitivity analysis of the various operating parameters of the heat pump was conducted. The results show that the zeotropic mixture CO2/R1234yf with a mass fraction of 0.17/0.83 is selected as the optimal refrigerant for the heat pump with the coefficient of performance, exergy efficiency and volumetric heating capacity of 7.46, 0.37, and 7978 kJ⋅m−3, respectively. The sensitivity analysis of operating parameters shows that the inlet temperature of hot water has more crucial influence on the coefficient of performance and exergy efficiency than various operating parameters, with sensitivity coefficients of −0.3868 and 0.2157, respectively. The volumetric heating capacity and system total exergy destruction per heating capacity unit are most sensitive to the flow rate of wastewater and hot water, with sensitivity coefficients of 0.1121 and −0.4985, respectively. These results provide a foundation for the design and optimization of a CO2 zeotropic mixture heat pump system.