Experimental Investigation on the Performance and Environmen
Experimental Investigation on the Performance and Environmental Impact of a CO2 Cascade Refrigeration System (2017) - This study evaluates the performance of cascade R744/R134 and R744/R290 systems, with CO2 in subcritical operation, by means of a drop-in operation. The experimental apparatus consists of two cycles. The low temperature cycle (LT) is formed by an R744 reciprocating variable speed compressor, an electronic expansion valve and a direct expansion air-source CO2 evaporator placed inside the cold room (2.3m x 2.6m x 2.5m). The high temperature cycle (HT) consists of a reciprocating compressor for R134a, an electronic expansion valve, and an air-cooled condenser. To connect both sides (cascadecondenser) a plate heat exchanger was used, acting, at the same time, as the condenser for the R744 and evaporator for the R134a. The degree of superheat for the R744 was varied from 5 to 15 K and the compressor operating frequency from 40 to 65 Hz. The drop-in operation was performed at the HT cycle, where the refrigerant R134a was replaced by R290, resulting in a charge reduction of 50%. The highest COP for the R744/R134a pair was 2.09 and the lowest, 1.81. Cooling capacity ranged between 4.30 and 5.58 kW and the air temperature inside the cold room varied from -17.7 °C to -0.8 °C. Finally, it was found that the results obtained for the R744/R290 system attended the air temperature conditions inside the cold room with similar COP values. However, the R744/R290 pair presented higher cooling capacities, from 5.20 to 5.88 kW. As for the environmental impact, based on the TEWI index, the best results were also obtained with the hydrocarbon option, with a reduction of approximately 10%.
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