cascade, CO2, sublimation, R23
Trifluoromethane (R-23) is currently the most widely used non-flammable refrigerant for applications down to about -80°C (193K). However, R-23 has an enormous global warming potential (GWP100=14800) and recent price increases lead to a loss of attractiveness in industry. As an alternative, some studies in recent years indicated the possibility of extending the application temperature of the natural refrigerant carbon dioxide (CO2) beneath its triple conditions in a refrigeration cycle. Thus there would be a great potential for CO2 to replace R-23. In present paper, the design of a cascade refrigeration machine using CO2-sublimation is introduced. R-452A is used as refrigerant for the upper stage. The lower stage can be switched between the CO2-sublimation cycle, featuring a two-stage compression with intercooling as well as an internal heat exchanger, and a conventional R-23 cycle. Both the CO2-sublimation as well as the R-23 lower stage cycles can be compared directly under the same ambient conditions. Stationary process simulations were made in order to compare the CO2 and R-23 cycles under various boundary conditions. It turned out that the overall system efficiency of the CO2-sublimation cycle could exceed the conventional R-23 cycle in many operation points. The design of this cascade refrigeration machine is currently used for building-up a demonstrator to prove the concept experimentally.