Two-stage cascade configurations based on ejectors for ultra-low temperature refrigeration with natural refrigerants

Research in ultra-low temperature refrigeration applications has intensified in recent years after the appearance of vaccines in response to the COVID-19 pandemic. There are few current technologies for this low-temperature range, with reduced energy performance and high global warming potential refrigerants. This work analyses the introduction of the ejector in two-stage cascade cycles for ultra-low temperature refrigeration. The proposal includes the assessment of the behaviour of the ejector while implementing it in a single stage or simultaneously in both stages. The study is carried out with refrigerants R-290 in the high-temperature stage and R-170 in the low-temperature stage since these are natural refrigerants with very low global warming potential. The results show that the ejector is a component that causes improvements in the cycle when placed in the high-temperature and low-temperature stages. On the other hand, changing evaporation and condensation temperatures, the evaporation temperature is more critical regarding cycle energy performance. With the results obtained, a cascade cycle with an ejector in both stages is proposed, obtaining a 21% higher coefficient of performance than the standard cascade cycle. Also, the cycle with the ejector in both stages causes an improvement of 13.6% compared to the previous generation's refrigerants (R-23 and R-507A) in the same cycle. The carbon footprint analysis shows that this cycle emits less than half of the equivalent CO(2) than actual cycles for ultra-low temperatures, also with a new refrigerant like R-472A.