Numerical modelling of a single-compression multi-temperature ejector-supported R744 refrigeration unit for last mile delivery

A novel R744 vapor-compression refrigeration system has been developed to meet the cooling needs of a medium-size refrigerated truck at two temperature levels: 4-5 kW for medium-temperature (MT) refrigeration at 0°C and 1-2 kW for low-temperature (LT) refrigeration at -20°C. This system is designed for transporting chilled and frozen goods efficiently during last-mile deliveries in urban areas. The key innovation of this system lies in its single compression stage with two different evaporation levels. Firstly, the unit incorporates an MT ejector to enhance energy efficiency by reducing the compressor pressure ratio. Furthermore, an LT ejector is employed to allow providing LT cooling with a single-stage compressor by pre-compressing vapor before it enters the compressor, avoiding excessive compression ratios. This extends the LT operational range with a single stage of compression from 34°C to 40°C ambient temperature. When the ambient temperature is not sufficient to sustain the ejector cycle, the system can switch to a back-pressure cycle. A numerical model of this refrigeration system has been developed to evaluate its steady-state and dynamic performance in both back-pressure and ejector configurations. For a specific cooling effect production (3.8 kW in MT, 1.1 kW in LT operation), the ejector cycle increases the COP by 25.8% for MT and 42.0% for LT operation compared to the back-pressure cycle. However, the back-pressure cycle offers greater flexibility in cooling power production, ranging from -32.8% to +8.2% for MT and -8.9% to +82.5% for LT, making it useful for pulldown and part-load operations.