Cycle analysis of a single-stage transcritical R744 system in supermarkets: The impact of isentropic efficiency and oil

Carbon dioxide is increasingly present in modern refrigeration systems, finding significant application in commercial cooling, especially in supermarkets. However, its application encounters many challenges due to its unfavorable thermodynamic properties. This serves as a starting point for numerous engineering solutions, as well as for many theoretical and experimental analyses of various systems utilizing R744. Some analyses often overlook the fact that, in addition to the refrigerant, oil also passes through all the system components in a certain quantity, which subsequently impacts heat transfer as well as the cycle itself. This paper presents a cycle analysis of a typical single-stage transcritical CO2 system used for medium-temperature (MT) cooling applications in a supermarket. The analysis included the impact of oil, considering several realistic steady-state scenarios with predicted OCR (Oil Circulation Ratio) values. In addition to the effect of oil, the study was expanded by examining available equations for the isentropic efficiency of the compressor. The results showed that considering a realistic compression process leads to an efficiency reduction of 15.4% to 39.9%, while the presence of oil decreases it by 5.7% to 27.8% under varying conditions.