Theoretical and experimental investigation for optimal discharge pressure of air-source trans-critical CO₂ heat pump

To reveal the main factors of the optimal discharge pressure in a trans-critical CO₂ heat pump system, the heating performance is tested in enthalpy difference laboratory. The results show that the evaporation pressure and temperature in gas-cooler outlet decline with increasing discharge pressure; the superheat increases with discharge pressure; the heating capacity and the coefficient of performance (COP) increase firstly and then drop when discharge pressure rises, indicating that an optimal value exists. According to the test data, it is found that the optimal discharge pressure drops with declining environment temperature, inlet water temperature and outlet water temperature. At unchanged inlet water temperature (ambient water temperature), an experimental correlation taking environment temperature and outlet water temperature as independent variables is constructed by data fitting to predict the optimal discharge pressure. The experiments indicate that the COP difference between predicted optimal discharge pressure and actual optimal discharge pressure is less than 1.3%.