Refrigerated transport: innovations and challenges for sustainability
Review of the most promising technologies for refrigerated transport: vapour compression systems with natural refrigerants, photovoltaic systems with electrical energy storage, cryogenic systems, absorption-adsorption systems, PCMs, fuel cells.
It is estimated that around 15% of global fossil fuel energy is used in the refrigerated transport sector. In the case of vapour compression refrigeration (VCR) units – which are currently the most widely used systems in road refrigerated transport – the greenhouse gas emissions due to the operation of the refrigeration system, including both direct and indirect emissions, can reach 40% of the total vehicle’s engine emissions. The power supply by internal combustion engines, mainly diesel powered, and the use of refrigerants characterized by high GWP values explain these high emissions. (1)
In a recent review paper, Italian researchers present the currently used refrigerating systems and their possible improvements as well as alternative technologies that could reduce the environmental impacts of road refrigerated transport. Their main conclusions are the following:
- The use of electrical energy storage allows a strong reduction in consumption and emissions due to the operation of a VCR system, especially if coupled with a photovoltaic system. However, PV systems, even if coupled with a battery pack, could not be sufficient to overcome the cooling demand, so a backup system (internal combustion engine) is still needed.
- The application of phase-change materials is currently limited by the stability problems between PCMs and the containers, as well as the limiting operating conditions imposed (operating temperature, duration of the refrigeration effect, no optimization of the shape of the containers). However, the potential of PCMs for reducing energy consumption and emissions is far from negligible.
- Cryogenic systems represent a valid alternative to traditional VCR systems and allow to obtain lower pull-down phase durations. However, the high energy consumption linked to the liquefaction processes of cryogenic gases does not allow for an effective reduction in emissions related to refrigeration. Consequently, the use of these systems is still limited.
- Air-based and CO2-based refrigeration systems respond to the demand for more environmentally friendly refrigerants than currently used HFCs, allowing a reduction in direct emissions. The use of air systems is however limited by the lack of standard components for application in refrigerated transport. In recent years, the use of R290 (propane) in VCR systems has been considered in refrigerated transport. However, to date, there are still few studies on its performance.
- Absorption/adsorption systems allow the recovery of residual thermal energy contained in vehicle exhaust gases, but their operation is compromised at low engine loads (exhaust gas temperature below 200 °C). This problem can be solved using several methods, but they greatly increase the complexity of the system. In general, these systems are still studied as prototypes. An optimisation of the design of the components, in terms of sizing, is necessary to include these systems within the physical envelope of a refrigerated vehicle.
In general, the alternative refrigeration technologies currently under study for application in refrigerated transport are not competitive in terms of performance. Systems powered by recovering the residual thermal energy from the exhaust gases of the vehicle (i.e., absorption/adsorption systems, Brayton cycle for the supply of refrigeration systems) are valid solutions for the reduction of energy consumption related to refrigeration. Surely the most interesting alternative in regard to reducing emissions is the use of fuel cells for the production of thermal and electrical energy, since they are able to reduce the load of the engine and guarantee the supply of the refrigeration system.
Regarding the possible improvement of current VCR systems, the authors put forward a solution that could reduce both direct and indirect emissions: the use of a PV/battery system to power the VCR unit, the addition of PCM to reduce the thermal load and/or increasing the OFF time of the compressor, and the use of a natural refrigerant, such as R290.
(1) Maiorino A, Petruzziello F, Aprea C. Refrigerated Transport: State of the Art, Technical Issues, Innovations and Challenges for Sustainability. Energies. 2021; 14(21):7237. https://doi.org/10.3390/en14217237