Dynamic numerical modelling of a single stage dual temperature R744 ejector-supported refrigeration unit for last mile delivery

A novel design of a R744 vapor-compression refrigeration unit isproposed and developed to provide cooling capacity at two differenttemperature levels, in the range of 4-5 kW of MT refrigeration at 0degrees C and 1-2 kW of LT refrigeration at -20 degrees C, inside theinsulated box mounted on a typical medium-size refrigerated truck. Sucha system can be employed for multi-temperature (chilled and frozen)goods transport to enhance the range of ambient conditions served withhighly efficient operation during last-mile deliveries in urbanenvironment.The main innovation of the proposed cycle is related to the presence ofa single compression stage with two different evaporation levels.Firstly, the cooling system design is based on the implementation of aMT ejector, with the purpose of energy efficiency improvement of thesystem due to the consequent reduction of the compressor pressure ratio.Secondly, an implemented LT ejector allows for the production of LTcooling effect with a single stage compressor. The system can switch toa back-pressure cycle once the ambient temperature is not sufficient tosustain the ejector cycle. On the other hand, the use of an ejector forthe LT cycle allows to extend the application field of the single stagecompressor to high ambient temperatures.A dynamic numerical model of the refrigerating system has been developedto assess the system performance in ejector and back pressure mode. Thenumerical model was also used to characterize the system pull-down fromambient temperature.