Experimental investigation of a novel cascade refrigeration system with a CO2 sublimation cycle as the lower stage

Abstract

• A cascade system with a CO 2 sublimation cycle in the lower stage is experimentally investigated. • Continuous operation below the triple pressure of CO 2 has been realized. • The transient behaviors and blocking issues as an example case are discussed. • In addition, HFE 7500 is used as carrier-fluid to reduce the blockages. • The performance of the system both with and without the carrier-fluid is investigated at different sublimation temperatures. • Optimization potentials of the system are analyzed. Some studies in recent years have shown the possibility of extending the application temperature of the natural refrigerant carbon dioxide (CO 2 ) below its triple point. Thus, CO 2 could be a potential alternative refrigerant for R-23 in ultralow-temperature (ULT) refrigeration systems. This paper introduces a special cascade system, whose lower stage can be switched between a CO 2 sublimation cycle and a conventional R-23 cycle, so that both cycles can be compared directly under the same ambient conditions. The sublimation system features a unique sublimation heat exchanger, which allows the operation with an extra carrier fluid in order to improve the heat transfer and reduce the blockage by solid CO 2 . In this study, experiments were carried out using HFE-7500 as the carrier fluid (“wet sublimation”) as well as without the carrier fluid (“dry” sublimation). The system was able to be operated continuously in both operating mode. A refrigerating capacity of 600 W was achieved at a sublimation temperature of -79°C. The transient behavior and blocking issues of the system are discussed. The efficiency of the sublimation system during stable operations are compared with the result of the reference R-23 system. It shows that there is still large potential for improvement on the system performance of the CO 2 sublimation system.