Dynamic modelling of a CO2 transport refrigeration unit with multiple configurations

The paper presents the dynamic characterization of a novel CO2 system designed for the refrigerated transport sector, when equipped to a medium-size refrigerated truck, during a daily typical long-distance delivery mission. The system takes advantage of a two-phase ejector to improve its performances and an auxiliary evaporator to extend the ejector operating range towards mild ambient temperatures. The system is designed allowing the possibility to switch between different configurations during operation, in order to maximize the COP or the refrigerating capacity, as a function of internal air and external environmental conditions. A numerical model of the system and its control strategy is developed using a 0-D dynamic commercial software, and coupled with a validated model of a refrigerated body. Simulation results show an average COP equal to 1.60 and an overall Duty Cycle of 10% for the delivery mission. The 96.6% of the total cooling energy is provided exploiting both the main and the auxiliary evaporator; however, dynamic simulations allowed highlighting that using the configuration exploiting both the evaporators during the hottest moments of the day can become counterproductive. Moreover, the behavior of the two-phase ejector under different environmental air temperature conditions is investigated. The maximum values of the average ejector efficiency (10.1%) and of the average pressure lift (1.59 bar) are reached for the highest external ambient temperatures (36.0 degrees C). Finally, to avoid the temperature and pressure drift inside the low pressure side of the refrigerating system during a long time of inactivity, an innovative safety control system based on the definition of a maximum allowed pressure level inside the low pressure side of the cooling unit is proposed.