Sustainability in road transport refrigeration: reduction of direct and indirect emissions through electrification and natural refrigerants

The increasing urbanization and globalization phenomena, together with the COVID-19 outbreak which characterized the last years, caused an overall growth of the home-delivered food consumption, of the transport and preservation of pharmaceutical products and of the online grocery shopping, leading to a significant rise of the importance of refrigerated transport. Within the temperature-controlled transport sector, road transport through the employment of refrigerated vehicles represents the primary transportation option. The operation and the control of a transport refrigeration unit is generally more challenging compared to a stationary cooling unit, due to specific issues as initial temperature conditions of the cargo, strong variability of external ambient conditions, frequent door openings with consequent infiltration of external air, highly variable range of thermal loads and accelerations and vibrations linked to the vehicle motion. At the same time, climate change issues need to be carefully considered and faced, to control and limit Green-House Gases emissions in the atmosphere to prevent the harmful effect of climate change on humans and environment. In such a scenario, the refrigerated road transport sector needs to develop and implement strategies and solutions to guarantee the correct preservation of the transported products quality along their Cold Chain, but at the same time to minimize its negative impacts on the environment. Therefore, a reduction of both the indirect emissions, linked to the system operation and efficiency, and the direct emissions, linked to refrigerant leakage in the atmosphere, needs to be achieved. The present thesis contains the results of the studies carried out during three years of PhD activity, conducted at the Construction Technology Institute (ITC) of the National Research Council (CNR) in Padova, Italy, which has focused on the development of possible solutions to reduce the indirect and direct environmental impact of the road refrigerated transport sector. Firstly, a baseline numerical assessment of the environmental impact of traditionally employed HFC refrigeration systems will be carried out. Results will highlight that the use of an energy storage system can mitigate the increase of fuel consumption and the pollutant emissions due to the cooling unit presence and that the insertion of photovoltaic panels can lead to an additional reduction of the power drawn from the vehicle engine. As a sustainable alternative to commonly used HFC refrigerants, R744 (CO2) has then been selected as a long-term future proof refrigerant, reducing to zero the direct refrigerant emissions. Therefore, a novel R744 cooling unit design developed for road transport refrigeration applications will be proposed and its performance will be numerically evaluated. Supported by the positive numerical results, the installation, setup and first startup tests of the experimental setup of a R744 refrigeration unit will be described. Moreover, the performance of MT single-temperature R744 cooling units and MT-LT multi-temperature R744 cooling units will be numerically assessed, suggesting optimal control strategies to maximize the system performance, assessing the crucial importance of the refrigeration cycle design, including specific components such as ejector to increase the unit efficiency, and highlighting the crucial issues linked to a non-stationary application as road transport refrigeration. The results of this thesis will be useful to highlight that sustainable alternatives to replace the traditionally used HFC/HFO transport refrigeration systems are possible and under practical development, both in terms of improvement of the refrigerating system performance and efficiency, through innovative solutions and electrification, and in terms of development of highly efficient units employing natural refrigerants such as R744.