Efficiencies of subcritical and transcritical CO2 inverse cycles with and without an internal heat exchanger

An internal heat exchanger (IHX) is often used to improve the coefficient of performance (COP) of CO2 inverse cycles. This paper presents a detailed analysis of the IHX’s effect in CO2 inverse cycles and finds suitable operating conditions for the IHX from a thermodynamic performance point of view. The results indicate that the COP is slightly reduced by an IHX in a CO2 subcritical inverse cycle, so an IHX is not justified. However, for transcritical CO2 inverse cycles, the compressor discharge pressures and CO2 gas cooler outlet temperatures both have significant impacts on system performance. The analysis results for transcritical CO2 inverse cycles show that a transition discharge pressure and a transition CO2 gas cooler outlet temperature are objective existence above which the IHX improves the cycle performance.

Research highlights
► Find suitable operating conditions for the IHX.
► Above transition CO2 gas cooler outlet temperature IHX improves cycle performance.
► The IHX is not very useful for optimized space heating and refrigerating cycles.