The “TES Last-Mile Delivery” demonstrator aims to improve the environmental sustainability of last-mile deliveries through the use of a thermal energy storage (TES) inside isothermal boxes used for the transport of frozen products. The TES is cooled before departure for delivery missions by a stationary cooling unit that uses natural refrigerants (propane), installed outside the vehicle used for deliveries, and uses its latent/sensible heat to maintain the correct temperature range inside the isothermal boxes. This solution avoids the presence of an active refrigeration unit installed directly on the vehicle, and is therefore compatible with the use of non-specialized vehicles, allowing greater flexibility in fleet and logistics management. This approach also allows for the easy simultaneous transport of refrigerated and non-refrigerated products. Since the refrigeration unit is powered by the electrical network, this solution guarantees lower greenhouse gas (GHG) emissions compared to traditional fossil-fuel engine powered active refrigeration units installed on the vehicle, reducing localized emissions and noise pollution in urban areas or Zero Emission Zones (ZEZ) during last mile delivery activities.
Figure 1. Simplified schematic of the demonstrator system.
The thermal energy storage is achieved by means of a eutectic solution of inorganic salts, characterized by a nominal phase change temperature of -33°C, contained in eutectic plates fixed to the internal walls of the isothermal box. Thanks to this solution, the eutectic box is characterized by an autonomy of more than 12 hours: with an ambient temperature of 30°C, the internal temperature is constantly kept below the threshold of -20°C without the need for an active refrigeration system, therefore without the supply of electrical energy from outside.
Figure 2. Test case transportable insulated box with eutectic plates (TES).
A dynamic numerical model of the baseline system currently employed was built and validated against experimental data. The same numerical approach was then employed to evaluate the performance of the novel natural refrigerant unit to assess its performance improvements. Numerical simulations highlighted that the propane refrigeration unit, compared to the synthetic refrigerant baseline solution, can achieve a -7.1% reduction in energy consumption and a +12.6% increase in the pulldown COP during 24 hours of operation in regulated ATP test conditions (constant ambient temperature equal to 30°C). The use of the natural refrigerant also allows to reduce the direct emissions related to possible refrigerant leakages to negligible values. Furthermore, numerical simulations suggest that the refrigeration unit can operate with a quantity of propane lower than 150 g, limiting the need for risk assessment or specific precautionary measures related to the flammability of the refrigerant.
The propane refrigeration unit is currently being built by the industrial supporter company Eutectic System Srl. Experimental tests will be carried out to evaluate the performance of the new system and compare it to that of the baseline system under regular last mile delivery operations of frozen products.
