Experimental evaluation of zeotropic refrigerants in a dedicated mechanical subcooling system in a CO2 cycle

Use of zeotropic blends in the dedicated mechanical subcooling system of a CO2 refrigeration system was suggested as a possible improvement due to matching of evaporating temperature with CO2 temperature profile during subcooling. This work has verified this possibility and has determined theoretically the best performing compositions of R-600, R-32 and CO2 with the base fluid R-152a. Then, the mixtures have been tested experimentally in a lab-test bench for constant heat load temperature for three heat rejection temperatures (25.1, 30.3 and 35.1°C). Optimum conditions are measured (subcooling degree and heat rejection) and a COP increase of 1.4% has been obtained. The work, for the optimum conditions, analyses the operating parameters of the cycles and focus specially on the thermal parameters of the subcooler. It has been verified that the use of zeotropic mixtures allows to reduce irreversibilities in the cycle, as pointed out theoretically by Dai et al. (2018).