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 ranscritical 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.