Comprehensive evaluation of energy, exergy and economic performance of transcritical CO2/R41 air-source heat pump water heater based on entropy weight method

This study proposes CO2/R41 azeotropic mixture to address the dual challenges of low coefficient of heating performance (COPheat) and high discharge pressure in pure CO2 air-source heat pump water heater (ASHPWH). Energy, exergy, and economic performance models are established to evaluate the application potential of four transcritical cycles: transcritical CO2 single-stage compression cycle (CO2+TSCC), transcritical CO2 two-stage compression cycle (CO2+TTCC), transcritical CO2/R41 single-stage compression cycle (CO2/R41+TSCC), and transcritical CO2/R41 two-stage compression cycle (CO2/R41+TTCC). Through the established thermodynamic model, the synergistic effects on system performance of four cycles under varying evaporation temperature (Teva) and gas cooler outlet temperature (Tgc, out) are systematically evaluated. These cycles are also assessed for their economic performance in four typical Chinese cities: Beijing, Xi'an, Shanghai, and Nanchang. Furthermore, an innovative multi-criteria evaluation framework incorporating entropy weight method is developed, integrating five critical indicators: discharge temperature, optimal high pressure, COPheat, exergy efficiency, and annualized cost. The validity of the indicator system is rigorously verified. The results demonstrate that compared with pure CO2, the CO2/R41 azeotropic mixture displays an improvement in COPheat. The CO2/R41+TTCC obtains the highest exergy efficiency and achieves the lowest life cycle cost, showing reductions of 68.61 %, 69.69 %, 72.41 %, and 72.83 % compared with direct electric heater in the four typical cities, respectively. Moreover, CO2/R41+TTCC obtains the highest annual comprehensive performance scores of 10.8925, 10.8929, 11.1215, and 11.0286 in Beijing, Xi'an, Shanghai, and Nanchang, respectively. These scientific results provide favorable insights for advancing the development and implementation of transcritical heat pump water heater systems.