Experimental evaluation of a dedicated mechanical subcooling system in a CO2 transcritical refrigeration cycle.

Pure CO2 transcritical refrigeration cycles do not match the performance of other refrigeration solutions at high environment temperatures, being it necessary to improve their operation cycle to be applicable. Initial theoretical studies have demonstrated that the combination of a transcritical cycle with a dedicated mechanical subcooling cycle improves the capacity and the overall COP; however no experimental validation has been presented up to now. In this study, the energy improvements, regarding capacity and COP, of using a dedicated mechanical subcooling cycle in combination with a CO2 transcritical refrigeration cycle are evaluated experimentally. Specifically, the experimental evaluation has tested the combination of a single-stage refrigeration cycle with a semihermetic compressor using R1234yf for the dedicated mechanical subcooling cycle, and a single-stage CO2 transcritical refrigeration plant with a semihermetic compressor and a double-stage expansion system. The combination has been evaluated at one evaporating level of the CO2 cycle and several heat rejection temperatures. The study analyses the modification of the optimum operating conditions of the combination and the performance improvement regarding capacity and COP.