Experimental improvement of a CO2 transcritical cycle using parallel compressor and integrated subcooler

Due to the low critical temperature of Carbon Dioxide (CO₂), the simplest CO₂ transcritical refrigeration cycle is usually upgraded with different configurations to enhance the performance, increase the cooling capacity, and reduce the flash-gas generated during the expansion process. Considering this, different arrangements were proposed over time with excellent results but rarely compared in a unique refrigeration plant. The present work experimentally analyses and compares three-cycle arrangements for CO₂ transcritical cycles, adding an internal heat exchanger (IHX), a parallel compressor, and an integrated subcooler.

Taking the IHX arrangement as a reference, the parallel compression and the integrated subcooler are compared at the heat rejection temperatures of 20, 27, and 34°C for commercial refrigeration purposes. The combination of parallel compressor and IHX provides maximum coefficient of performance (COP) and cooling capacity improvements of 3.6 % and 18.5 %, respectively, while the combination of integrated subcooler and IHX boosts the COP and the cooling capacity by up to 11.0 % and 19.5 %, respectively. The results demonstrate the convenience of using these configurations, fairly comparing arrangements with the same refrigerating plant.