Optimization of two-stages CO2 transcritical cycles for low temperature water-cooled cabinets application.

Nowadays the use of carbon dioxide (CO2) as a working fluid is widespread in commercial refrigeration, especially for remote packs and cabinets. Regarding plug-in and self-contained water cooled cabinets, innovative solutions are available, but further research and development are required. This paper deals with the optimization of self-contained transcritical cycles for low temperature applications, where the use of a two-stages cycle is mandatory to contain the CO2 compressor discharge temperature. A simulation model for the liquid injection cycle (LIC) and split cycle (SC) was developed, highlighting the variables that mostly influence the system, in terms of optimal operation. The theoretical results show that an optimal gas cooler pressure still exists in transcritical regime, but can significantly differs from the one given by single stage simple correlations (e.g. Liao et al., 2000). Moreover, variables such as intermediate pressure and/or intermediate superheat have a non-negligible impact on the efficiency, and must be taken into account during the design and optimization process. Finally, new correlations are presented, so that they can be easily implemented on case controllers.