Investigation of a cascaded CO2 refrigeration system using phase change materials for energy-saving potentials

A high-pressure lift often triggers an increased power input to the vapor compression systems. The increased power consumption becomes a bottleneck in R-744 refrigeration systems for freezing and refrigeration applications. Meanwhile, phase change materials (PCM) offer operation flexibility in the form of the compressor runtime from the energy storage potential. In this article, the energy-saving potential of the PCMs on a cascade refrigeration system using CO2 is investigated focusing on the impacts of charge amounts and the thermal resistance of the PCM. The validated dynamic model in Simscape (TM)/MATLAB for an R-744 vapor compression system is adopted for a cascade refrigeration system together with the validated PCM model. In the studied system, the PCM is installed in the storage compartment as a thermal buffer. The comprehensive model employed an acausal, object-oriented, and equation-based paradigm adopting detailed heat transfer characteristics. The effect of PCM on the compressor running time was investigated under the cyclic steady-state operating conditions. The results showed that the compressor "On-time" ratio decreases when using the PCM; subsequently, the power reduction. The system consumes about 6.76 kWh (without PCM) and 5.93 kWh with PCM; thus, the power consumption decreases by 12.3%. The threshold PCM charge ratio is observed to be 1. Increasing the PCM charge value above this threshold does not trigger a significant decrease in power reduction. The increase in the overall thermal resistance of PCM has a negative impact on the "On-time" ratio and power consumption. The benefit of PCM is insignificant for thermal resistance above 0.02 K W-1. Despite the shortcomings of several assumptions involved, the present results clearly highlight the positive impacts of the PCM in terms of power savings for low-temperature refrigeration applications using R-744.