Performance investigation of a two-phase transcritical CO2 ejector heat pump system

Abstract

This study of the thermodynamic performance of a transcritical CO2 ejector heat pump cycle presents a numerical analysis under the constraint of constant total heat transfer area. An experimentally validated model of a CO2 heat pump system and a model of a two-phase ejector are combined to perform the study. The effects of chosen parameters on system performance are investigated: operating conditions as determined by the gas cooler and evaporator pressures; ejector design parameters including the primary throat diameter, effective area ratio, and diffuser outlet diameter; system design as defined by the heat transfer area ratio. The results of this analysis allow specifying the optimal range for the design parameters in order to maximize the COP and the heating capacity of the heat pump. The heat transfer area ratio is shown to have important effects on the COP and heating capacity as well as the optimal gas cooler pressure. As the heat transfer area ratio increases, COP and heating capacity increase as well, while the optimum gas cooler pressure decreases. At given ejector geometries and operating conditions, this optimization can increase the COP and heating capacity of the ejector heat pump by approximately 17% and 20% respectively by increasing the heat transfer area ratio.