Effect of an IHX on energy and exergy performance of a modified double-evaporator transcritical CO2 heat pump with ejector
At present, some studies have analyzed the effect of internal heat exchanger (IHX) on the performance of transcritical CO2 heat pump system with ejector (TCHSE). However, the effect of IHX on betterperforming double-evaporator transcritical CO2 heat pump system with ejector (DE-TCHSE) have not yet been studied. In this paper, a modified DE-TCHSE simulation model based on variable efficiency ejector was established by Modelica, the model was tested and analyzed, and the energy and exergy performance of DE-TCHSE by IHX was conducted under different working conditions. The simulation results show there exists an optimal discharge pressure which make the coefficient of performance (COP) of TCHSE and DE-TCHSE maximum, and the COP of the latter was up to 21.5% higher than that of the former under ambient temperature of 5~20?. The addition of an IHX results in lower optimal discharge pressure and higher COP, and optimal discharge pressure decreased by up to12.3% while the COP increased by up to 12.9%. With the larger ejector throat area and higher ambient temperature, the IHX becomes more effective in improving system COP and lowering the optimal discharge pressure, at the same time, the reduction of optimal discharge pressure also could be amplified by the increase of water outlet temperature and the decrease of water inlet temperature. In addition, the ejector efficiency decreased by up to 8.5% due to the supercooling effect of the IHX, and increasing the IHX area can weaken this effect. From exergy analysis point of view, the exergy destruction percentages of ejector significantly decreased with the application of IHX. Furthermore, the product exergy and the total exergy destruction decreased, especially the exergy efficiency of DE-TCHSE increased in all working conditions.