2016 Volume 3 Issue 6 Pages 16-00169
From environmental point of view, natural refrigerants, whose Global Warming Potential (hereinafter GWP) is lower than commonly used refrigerants, have been considered in several industrial sectors. Refrigerated display cabinet that are installed in supermarkets and other small food stores such as Convenience Stores (hereinafter CVS) are commonly equipped with direct expansion refrigeration system using high GWP refrigerants such as HFC404A and HFC410A. Leakage rate and market stock of such high GWP refrigerants are the cause of global warming as a direct impact. Thus, natural refrigerants are considered as sustainable alternative refrigerants. Carbon dioxide (hereinafter CO2) is the most likely alternative refrigerant in the retail refrigeration sector because of non-toxicity and non-flammability. Furthermore, the efficiency level of refrigeration systems contribute to global warming as indirect impact. In these days, several refrigeration systems using CO2 refrigerant have been developed and installed in supermarket and CVS. New refrigeration systems using binary (cascade) cycle of CO2-CO2 refrigerant is developed for the refrigerated cabinets. In the early development, HFC404A refrigerant single cycle system and HFC-CO2 refrigerants cascade cycle system were usually used because the COP of CO2 refrigerant was low at high ambient temperature. Nevertheless, relatively high COP system could be constructed by optimizing the binary cycle using CO2-CO2 refrigerant. In this study, details of the system design and performance optimizations of the CO2-CO2 binary cycle will be discussed. Data of developed binary CO2-CO2 cycle system was compared with the data of current single HFC cycle and single CO2 cycle systems. As a result, it was confirmed that the energy efficiency of binary CO2-CO2 cycle became competitive to the current single HFC cycle. Moreover, some technical issues for enhancement of COP of CO2-CO2 binary cycle will be discussed in the literature.