Experimental evaluation of transcritical CO₂ refrigeration with mechanical subcooling

Abstract

Transcritical CO₂ booster refrigeration systems have seen a growing interest in recent years as they can provide two-stage refrigeration with R744, which has a global warming potential of 1.0, as the working fluid. However, the efficiency of these systems, particularly in transcritical operation, is often not as good as the efficiency of comparable conventional equipment. One possible way to improve performance in hot climates is through the use of a mechanical subcooler. This study describes laboratory testing of a lab-scale transcritical booster system, with and without mechanical subcooling and in transcritical and subcritical operating modes. In addition, a new steady-state model for generalized vapor compression systems was developed and is presented here. The model is used to simulate the system tested in the current article. The testing showed a significant improvement in both capacity and efficiency at all operating conditions with the subcooler. The solver was able to match capacity, power, and efficiency to within 3% for all test conditions. The model was used to perform a parametric study of subcooler capacity on the cycle; the results show that added subcooling substantially improves overall coefficient of performance and reduces the need for bypass flow, but the overall coefficient of performance benefit diminishes as the efficiency of the main cycle increases.

Acknowledgments

The laboratory portion of this effort was performed by the Electric Power Research Institute with the support of Southern California Edison and funded by the utility ratepayers of California. The modeling work was supported by the Modeling and Optimization Consortium (MOC) at the University of Maryland.