Experimental study of optimum heat rejection pressure for a CO2 heat pump water heater

As an environmentally friendly refrigerant, CO₂ is seen as one of the best fluids for a heat pump water heater application system. To investigate the relationships between optimum heat rejection pressure and other related operating parameters, an experimental research has been conducted for a transcritical CO₂ heat pump water heater with two throttling valves. It proved that it is relatively reliable to control the heat rejection pressure of the CO₂ system with two throttling valves; there exists an optimal heat rejection pressure, under which the system can reach the maximum heating coefficient of performance. Meanwhile, the research also reveals that the optimal heat rejection pressure chiefly depends on the outlet refrigerant temperature of the gas cooler, and in the next place, the evaporating temperature; on the other hand, the superheat degree of CO₂ at the inlet of the compressor was found to have little to do with the optimum heat rejection pressure.