Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling

• Dedicated mechanical subcooling is analysed in CO2 transcritical refrigeration cycles.
• Modifications of optimum CO2 cycle conditions are evaluated.
• Evaluation covers evaporating levels from 5 to −30 °C and environment from 20 to 35 °C.
• Dedicated mechanical subcooling can improve a maximum of 20% of COP.
• It is more convenient for environment temperatures above 25 °C.

In this work the possibilities of enhancing the energy performance of CO2 transcritical refrigeration systems using a dedicated mechanical subcooling cycle are analysed theoretically. Using simplified models of the cycles, the modification of the optimum operating conditions of the CO2 transcritical cycle by the use of the mechanical subcooling are analysed and discussed. Next, for the optimum conditions, the possibilities of improving the energy performance of the transcritical cycle with the mechanical subcooling are evaluated for three evaporating levels (5, −5 and −30 °C) for environment temperatures from 20 to 35 °C using propane as refrigerant for the subcooling cycle. It has been observed that the cycle combination will allow increasing the COP up to a maximum of 20% and the cooling capacity up to a maximum of 28.8%, being both increments higher at high evaporating levels. Furthermore, the results indicate that this cycle is more convenient for environment temperatures above 25 °C. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed.