TEWI Analysis of a Hybrid Ejector CO2 Refrigerated Transport System

Mankind has developed the cold chain to store and preserve food and to transport it from the farm or the ocean to the plate. However, energy is required to remove the heat to freeze the products and maintain them at a given temperature. The vast majority of refrigeration systems use working fluids that when released into the atmosphere contribute to a build up of gases that are damaging to the environment. The indirect impact of the energy consumed and the direct impact due to leakage contributes to environmental degradation including an anthropogenic warming effect. It has been proposed that the environmental impact of refrigerated transport systems could be substantially reduced by using a CO2 refrigeration system that utilises waste heat from an independent engine for improved efficiency and energy consumption. The direct and indirect effects can be accounted for by using the concept of total equivalent warming impact (TEWI). TEWI for refrigeration systems is defined as the global warming impact based on the total related emissions of greenhouse gases during operation and disposal of the fluids at end of life, and includes three main components, operational leakage, end of life leakage and energy consumption, as shown below. A TEWI analysis has been carried out to compare a standard refrigerated transport system operating with R404a and a hybrid CO2 transport refrigeration system. By replacing traditional fluorocarbon refrigerant such as R404a with CO2 the TEWI is reduced by half. Subcooling by 10K reduces TEWI by 60% and subcooling by 25K reduces it by 65%. This shows that the environmental benefits of using CO2 as a refrigerant are substantial and subcooling enhances its environmental benefits.