Energetic analysis and performance improvement algorithm of transcritical CO2 heat pump water heater system

Abstract

Transcritical CO2 heat pump water heaters have been widely studied by researchers for its environmentally friendly nature and energy efficiency. To analyze the impact of various operating parameters on the heating performance of transcritical CO2 heat pump water heater systems, simulation models were developed and their accuracy was verified through experiments. Results indicated that the discharge pressure and CO2 temperature at the gas cooler outlet (Tgc,out) had a significant effect on the system performance, the coefficient of performance (COP) increased notably with higher evaporation temperatures, and the optimal discharge pressure varied depending on the evaporation temperature. The system equipped with intermediate heat exchanger demonstrated a lower optimal discharge pressure and a higher COP compared to the basic system. The inclusion of an intermediate heat exchanger in a system, the maximum COP is 2.99 % to 3.83 % higher than the basic system under the same conditions. It has been observed that the introduction of intermediate heat exchanger significantly improved the system performance when Tgc,out were 35 °C, 40 °C and 45 °C, corresponding to discharge pressures lower than 10.75 MPa, 11.167 MPa and 11.625 MPa. According to the research results, the optimal discharge pressure equation was fitted and an improved algorithm was proposed. Compared to the experimental values, the improved algorithm exhibits a maximum error of 8.81 % and 7.55 % for determining the optimal discharge pressure and temperature, and the improved algorithm is effective in engineering applications. Consequently, the simulation data holds great importance in guiding the design of transcritical CO2 heat pump water heater systems.