Investigation of the optimal intermediate water temperature in a combined r134a and transcritical CO₂ heat pump for space heating

In this paper, the effect of intermediate water temperature on the performance of a combined R134a and transcritical CO₂ heat pump was studied theoretically and experimentally. The mathematical model was first validated using experimental data and then applied to analyze the performance of the combined system. The results show that there exists an optimal intermediate water temperature (water inlet temperature at the gas cooler) at which the combined system has the highest COP. This optimal intermediate water temperature varies with the ambient air temperature. Furthermore, the effect of intermediate water temperature on individual R134a and transcritical CO₂ subsystems was investigated. The results show that both heating capacity and power consumption in the R134a subsystem increase as the intermediate water temperature increases. However, power consumption in the CO₂ system drops slightly, and heating capacity reaches an optimal value as the intermediate water temperature increases from 15 to 32 °C under ambient air temperatures ranging from −20 to 7 °C.