The “Fresh and Green Delivery” demonstrator (read more here) lead by CNR is integrating a cooling unit based on natural refrigerants (CO2) and solar panels to minimise GHGs emissions of a medium-sized van dedicated to short distance delivery of fresh food products. The use of a natural refrigerant prevents the emissions related to the lifecycle of synthetic refrigerants. The CO2 refrigeration unit includes advanced solutions, such as a specifically developed ejector, and optimized components to increase the overall performance of the system, lowering the energy request to preserve the transported food. Integration of rooftop photovoltaic panels further reduces the draw of primary energy of the system thanks to the produced electricity during the day.
The demonstrator system with main energy flows
The prototypes of the MT and LT ejectors, previously designed by project partner SUT (Silesian University of Technology) according to the nominal cooling power of the multi-temperature CO2 unit, were commissioned to a manufacturer, built and installed at SUT laboratories for experimental tests. An experimental campaign was conducted on the ejectors to assess their performance in different operating conditions, including two pressure levels at the motive nozzle, three pressure levels at the suction nozzle and various pressure lifts. Experiments proved the ability of the ejectors to operate in the desired range of operating conditions.
To comply with the operating constraints of the components available on the market and to simplify the unit control logic, an optimization of the cooling unit schematic was realized, resulting in the final design of a light-weight and efficient single-temperature unit (for fresh products transportation only). A light-weight variable-speed scroll compressor was identified in the market, allowing to achieve high-pressure control of the unit acting on the compressor speed under all configurations and operating conditions. This reduced the weight about 60 kg compared to the previously considered compressor and in the switch from an expensive electronic expansion valve to a simple fixed expansion valve.
Numerical simulations were performed to assess the performance of this unit under different ambient temperature conditions. The new and optimized schematic presents a coefficient of performance (COP) in the simple expansion valve configuration ranging from 2.8 to 1.0, for ambient temperature varying between 10-40°C, respectively, and in ejector configuration ranging from 2.4 to 1.3 for ambient temperature ranging between 20-40°C, respectively. The COP of the new schematic is increased between 6.2-22.2%, depending on ambient temperature, compared to the initial unit schematic. Numerical simulations assessed that the COP increase, combined with the weight reduction of the new schematic, can lead to a total emissions reduction during yearly operation between –30.0% in cold climate (Helsinki) and –15.5% in hash climate (Phoenix).
The final schematic cooling unit is currently under construction by project partner Enex, while the monitoring tool and data acquisition software is developed by project partner Eletica. The electric vehicle which will be used for the final installation of the cooling unit is identified.
The 1st generation unit (At CNR’s) and the ejectors.
