Energy, exergy, economic and environmental analysis of refrigerant charge in air source transcritical carbon dioxide heat pump water heater

Currently, the air source transcritical carbon dioxide heat pump is extensively used for water heating. The refrigerant charge plays a significant role in optimizing system performance. However, most previous studies only focus on energy analysis. In this paper, an experiment was conducted and the effect of refrigerant charge was evaluated with energy, exergy, economic and environmental analyses. Results showed that the optimal normalized refrigerant charge was 0.214 in the comprehensive analysis. In the most important energy analysis, the peak coefficient of performance was increased by up to 16.27% with optimal charge over other charge amounts, corresponding to the maximum heating capacity and superheat of 5.13 °C. The maximum exergy efficiency was 34.17%, and the largest exergy destruction was observed in the evaporator, accounting for 44.04–49.22% of the total. Compared with the direct electric heater, the annual operating cost was decreased by 184.13 thousand Chinese yuan at the optimum charge. Moreover, the payback period was lower than 1.0 years and most sensitive to operating time as shown by the sensitivity analysis. The reduction of carbon dioxide emissions reached the peak value of 378.01 tons at the optimal charge, which was profitable in reducing the greenhouse effect. Also, considering the various operating conditions and domestic hot water load, the energy, economic as well as environmental benefits and the applicability of the evaluated optimal charge were presented.