A novel R744 multi-temperature cycle for refrigerated transport applications with low-temperature ejector: Experimental ejector characterization and thermodynamic cycle assessment

A novel vapor-compression system concept employing carbon dioxide as the refrigerant is proposed to serve the needs of a typical medium-size refrigerated truck used for multi-temperature (MT and LT) goods delivery. The system design is based on the implementation of an ejector as the only component increasing the refrigerant pressure from the LT evaporation pressure to the MT evaporation pressure, thus providing cooling effect at two different temperature levels with only one stage of compression. The ejector was experimentally tested and its ability to effectively entrain mass flow rate from low pressure suction conditions (corresponding to a LT evaporation temperature of −25 °C) was assessed. Lower external ambient temperatures and consequent lower expansion energy available at the ejector motive nozzle leads to a reduction of the maximum achievable pressure lift. Moreover, a significant degradation of the ejector performance towards the highest pressure lifts is experienced. Based on the ejector experimental data, a numerical evaluation of the proposed cooling unit performance has been performed, highlighting that in design conditions (LT evaporation at −25 °C) the cooling unit provides a LT freezing power ranging between 1.1 kW and 2.3 kW and a corresponding minimum MT cooling power ranging between 5.1 kW and 3.8 kW, depending on the chosen ejector lift. The MT cooling power can be further increased by increasing the compressor mass flow rate. The system COP is maximized at the maximum available lift provided by the ejector.