Improving efficiency of transcritical CO2 cycles through a magnetic refrigeration subcooling system

Subcooling methods for transcritical CO2 plants are being studied in order to improve the behaviour of these systems in hot climates, where basic configurations are not competitive enough. To achieve important improvements in the transcritical CO2 performance, it is necessary to perform the subcooling with a refrigeration cycle working with a Coefficient of Performance higher than that of the CO2 system without subcooling. Magnetic refrigeration devices can achieve high Coefficient of Performance values when the temperature difference between the hot sink and cold source is small, and therefore they meet the requirements to be applied as a CO2 subcooling method. This work presents the coupling of two refrigeration technologies: vapour compression and magnetocaloric refrigeration, which has not yet been presented in the literature. The magnetic refrigeration system is, based on the experimental results of the existing prototype, analysed semi-empirically and further evaluated, as a subcooling method for a transcritical CO2 cycle in a wide range of ambient conditions. Gas-cooler pressure, subcooling degree and operating parameters of the magnetic refrigerator were optimized for each condition to obtain the maximum Coefficient of Performance. We show that subcooling with the existing prototype of magnetic refrigeration system can enhance the overall Coefficient of Performance of transcritical CO2 cycle by up to 9%.