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

• 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. [ABSTRACT FROM AUTHOR]