Energy Conversion and Management: X (Oct 2023)
Thermodynamic evaluation of CO2 for ultra-low temperature refrigeration
Abstract
Carbon dioxide (CO2, R744) is the only refrigerant in the safest class by the ASHRAE 34 Standard in the group of natural refrigerants, with zero ozone depletion potential and a global warming potential of 1. It has been recently proposed for commercial refrigeration and heat pumps. Ultra-low temperature (ULT) refrigeration considers two-stage cascades with hydrofluorocarbon synthetic refrigerants (R404A/R23 high and low-temperature stages, HTS and LTS, respectively) and, lately, hydrocarbons (R290/R170). This paper examines the potential of R744 in ULT refrigeration cascade configurations in combination with other promising refrigerants. R744 is proposed in the medium temperature stage (MTS) of a three-stage cascade and the HTS of a two-stage transcritical operation (subcritical and transcritical with and without ejector, respectively). The operational and energy performance are compared with standard two- and three-stage ULT refrigeration cascades. Also, the cycles have been optimized, changing the main parameters as cascade heat changer temperatures or gas cooler pressure to maximize COP. This optimization and all the models have been made with Python, extracting the thermodynamic properties of REFPROP. The results show that in the HTS, the coefficient of performance (COP) is 39 % lower than the same two-stage cascade cycle with R290. In the MTS of a three-stage cascade, COP is 10 % lower than the same three-stage cascade cycle with R290. The ejector increases the COP by 38 % in the transcritical HTS, but remains below the hydrocarbon two-stage cascade. The choice of alternative refrigerants in the other stages does not significantly vary the COP results. Technological advancements in single subcritical and transcritical R744 configurations should be transferred to ULT refrigeration cascades to increase competitiveness and take advantage of its environmental and safety characteristics.
