“…Against this background, in recent years, the importance of using natural refrigerants, which do not affect ozone layer depletion and have a small impact on global warming compared to HFC refrigerants, has been increasing [5,6]. Among natural refrigerants, CO 2 is one of the promising environmentally friendly refrigerants because of its superior properties such as low global warming potential, zeroozone depletion potential, non-toxicity, non-flammability, and inertness [7][8][9][10]. In addition, CO 2 has excellent heat transfer properties with a volumetric capacity of three to four times greater than other refrigerants used in the market [11], such as chlorofluorocarbon and HFC.…”