Theoretical and experimental studies on optimum heat rejection pressure for a CO2 heat pump system

The experimental and simulation researches have been conducted to investigate the relationships between optimum heat rejection pressure and other related operating parameters for a transcritical CO2 heat pump system with two throttle valves. It proved that it is relatively reliable to control the heat rejection pressure of the CO2 system with two expansion valves in series. The experimental results also show similar trends with those from simulation, under widely different operating conditions. Thus both the simulation and experimental results meet here: for a transcritical CO2 cycle, there exists an optimal heat rejection pressure, under which the system can reach the maximum heating coefficient of performance (COP). Furthermore, the research also reveals that the optimal heat rejection pressure mainly depends on the refrigerant outlet temperature of gas cooler whereas the evaporating temperature and the performance of the given compressor have smaller effect on the optimum heat rejection pressure. Based on the experimental data, a correlation of the optimal heat rejection pressure with respect to mainly involved parameters is obtained for specific conditions.