Experimental investigation of a carbon dioxide refrigeration unit designed for medium size refrigerated trucks

1 EXPERIMENTAL INVESTIGATION OF A CARBON DIOXIDE REFRIGERATION UNIT DESIGNED FOR MEDIUM SIZE REFRIGERATED TRUCKS Wasim Shah(1)*, Francesco Fabris(2), Silvia Minetto(2), Antonio Rossetti(2), Sergio Marinetti(2) (1) University of Padova, Department of Industrial Engineering, Via Venezia 1, 35131 Padova, Italy (2) National Research Council, Construction Technologies Institute, Corso Stati Uniti 4, 35127 Padova (Italy) *Corresponding author: [email protected] Abstract The EU project ENOUGH is focusing on strengthening the sustainability of EU's food supply chain by offering technological, financial, and political resources to lower greenhouse gas emissions by 2030 and attain carbon neutrality by 2050 within the food sector. As the project is approaching its conclusion in September 2025, several demonstrators across Europe are collecting results to demonstrate feasible solutions to decarbonize the EU food industry. In particular, to help reducing the carbon footprint of transport operations within the cold chain, a new CO₂ refrigerating unit for medium-sized refrigerated trucks has been developed and designed at CNR (Padova), and an experimental campaign to evaluate the performance of such system has been conducted on a stationary prototype installed in the lab. The steady-state performance of the unit has been experimentally assessed for different values of the refrigerated space temperature (-5 ºC ,0 ºC and 5 ºC) and ambient temperature (varying between 20 and 40 ºC, with a step of 5 °C). Results demonstrate good energy performance of the developed CO2 unit compared to baseline synthetic refrigerants-based solutions currently employed in the market. INTRODUCTION Approximately 70% of food intended for human consumption requires refrigeration at some stages to maintain freshness and safety (IIR, 2021). The cold chain is essential for keeping and delivering perishable food by maintaining temperature, hence slowing biological decay processes. An estimated 14% of all food produced globally for human use is lost, while 17% is wasted (UNEP and FAO, 2022). The absence of efficient refrigeration significantly contributes to this issue, resulting in a loss of 12% of worldwide produced food only in 2017 (IIR, 2021). Improving cold chain is then essential to support UN Sustainable Development Goals (United Nations, 2024), which means extending the cold chain in a sustainable way, as the current cold chain is already responsible for 4% of the total greenhouse gases emissions (UNEP and FAO, 2022), related to energy consumption and refrigerant leakages. IIR (2021) states that an improved global cold chain would allow a reduction of almost 50% of the CO2 emissions of the current cold chain. 2 Under Horizon 2020, the European Union-funded ENOUGH project aims at supporting the EU Farm to Fork strategy (European Commission, 2020) by proposing strategies and solutions to reach carbon neutrality in the food sector by 2025. While ENOUGH adopts an holistic approach, analyzing also financial and political boundaries, from a technical point of view it proposes technologies which optimize energy use, increase renewables, promote electrification, improve processing and preservation conditions and use only natural refrigerants, to improve the overall sustainability of food systems. From harvesting to consumption, at several European sites, demonstrators for different product categories and for different applications within the food chain have been developed employing innovative technology solutions like high temperature heat pumps and refrigeration systems using natural refrigerants across important parts of the food chain for reducing emissions. Moreover, ENOUGH also aims to create smart data systems, digital tools, and strategic roadmaps to maximize operations and reduce emissions. Transport refrigeration is one of the essential parts in cold chain for preserving the quality and safety of perishable goods and food throughout their supply chain. It ensures that these goods remain within specific temperature ranges to prevent spoilage and degradation. However, transport refrigeration faces several constraints compared to stationary refrigeration such as limited space and weight, limited power source, vibrations and shocks during transportation, temperature and environmental fluctuations and maintenance and serviceability issues (Minetto et al., 2023). Addressing these constraints requires specialized design, technology, and operational considerations. Besides these constraints, refrigerant leakage represents a major challenge in the current transport refrigeration sector, as it is still mainly based on the employment of synthetic refrigerants (HFCs, HFOs), characterized by a high Global Warming Potential (GWP) and by negative environmental impacts. However, these issues can be addressed by using natural refrigerants with optimized components and control techniques. In addition, new policies and regulations have been introduced in recent years to reduce emissions, ensure safety, and transition towards more sustainable and environmentally friendly substances in transport refrigeration systems. To face the above-mentioned challenges of this critical stage of the food supply chain, one of the demonstrators developed within the EU ENOUGH project consists of a new refrigerating unit employing a natural refrigerant (CO2) as the operating fluid, specifically developed and designed at CNR in Padova, Italy, for medium-sized refrigerated trucks. To enhance system performance in different climate conditions, this novel CO₂ cooling unit is able to operate in various configurations by including or excluding a two-phase ejector and an auxiliary evaporator, in addition to a simple back-pressure configuration which is further described in the following section. This study describes the unit design and layout and presents the results of an experimental campaign conducted on a stationary prototype installed in the lab. Experimental results are useful to determine the performance of such a system before components and layout optimization and before manufacturing of the final mobile unit. THE REFRIGERATION UNIT This work presents a novel R744 (CO2) cooling unit layout designed to offer medium temperature (MT) refrigeration in road temperature-regulated transportation applications. 3 This unit is able to operate in both subcritical and transcritical mode depending on the outside ambient conditions. The simplified schematic of the cooling unit is shown in Fig.1. Figure 1. Simplified schematic of the R744 cooling unit. The schematic presented in Fig. 1 allows to achieve operation in different configurations. In back-pressure configuration, a simple high-pressure valve (HPV) is used to expand the refrigerant and optimize the pressure at the gas cooler outlet. After expansion through the HPV, the refrigerant flows to the main evaporator, where it delivers the required cooling effect. From the evaporator output, the refrigerant goes to the liquid separator, from which the vapor phase is fed to the suction of compressor via an internal heat exchanger (IHX) to provide the required superheat to avoid liquid refrigerant at the compressor suction. Depending on external ambient conditions, the refrigerating system schematic allows modifying the cycle to use the high-pressure valve in parallel with a fixed geometry twophase ejector, thus operating in an ejector transcritical cycle. In such configuration, the expansion work of the refrigerant at the gas cooler outlet can be partially exploited to provide a pressure lift to the refrigerant mass flow rate from evaporation pressure to the liquid separator pressure, thus reducing the compression work. An auxiliary evaporator between the liquid separator and the ejector exit can provide further modification to the ejector cycle, allowing to operate with ejector configuration towards lower ambient temperatures. A detailed description of the refrigeration unit concept can be found in Artuso et al. (2020). A stationary refrigeration unit based on the schematic presented in Fig.1 was commissioned and installed in the CNR laboratories located in Padova, Italy. The objective of this preliminary lab installation was to allow experimental assessment of the performance of such cooling unit concept in different configurations and under different operating conditions before defining the final schematic, with optimized components, 4 encumbrance and controls, for the mobile unit to be engineered and installed on a refrigerated vehicle. Fig. 2 illustrates the stationary experimental facility implemented in the CNR laboratories. The gas cooler and the evaporators of the refrigeration unit are placed in two separate insulated chambers, in which the air temperature can be controlled to experimentally test the unit under the desired operating conditions: different ambient temperature conditions can be achieved by controlling the air temperature at the gas cooler (in the hot room) and different temperatures for perishable goods preservation can be achieved by controlling the air temperature at the evaporators (in the cold room). Figure 2. Experimental facility (CNR Padova): (a) Refrigeration unit; (b) Hot room; (c) Cold room; (d) PC for data acquisition. A list of the main components integrated into the experimental refrigeration facility is shown in Table 1, along with their corresponding main specifications. It should be specified that the gas cooler installed in the experimental facility is oversized compared to the design cooling capacity of approximately 6 kW in nominal conditions (𝑇𝑎𝑚𝑏 = 30 °C, 𝑇𝑖𝑛𝑡 = 0 °C) determined by the compressor size. Therefore, the speed of the gas cooler fans has been reduced in order to realize the design thermodynamic cycle with a temperature approach of approximately 2 K between ambient air and refrigerant at the gas cooler outlet. Table 1. Components and their main specifications. Components Specifications Compressor Displacement volume: 2.39 m3 /h @50 Hz Gas cooler External Heat transfer Area: 78 m2 Main Evaporator External Heat transfer Area: 39.4 m2 Auxiliary Evaporator External Heat transfer Area: 19.7 m2 Liquid Receiver Volume: 40 L Internal Heat Exchanger Number of plates: 26 Two-phase ejector Throat diameter: 1 mm 5 THE EXPERIMENTAL SETUP The experimental facility's data acquisition system integrates high-precision sensors to monitor thermal and electrical parameters required to assess the system performance. All measurements are recorded using National Instruments (NI) data acquisition hardware, which is interfaced with a LabVIEW-based monitoring program. T-type thermocouples, calibrated against a reference Pt100 RTD (thermo-resistance detector), are mounted at different points of the facility to measure the temperature of the refrigerant along its thermodynamic cycle and of the air in the insulated chambers and at inlet/outlet of every air heat exchanger. Pressure transducers are placed at compressor suction and discharge, at gas cooler outlet, at liquid separator and at ejector suction and discharge ports. Mass flow rate of refrigerant is monitored through Coriolis mass flow meters installed at exit of the gas cooler and at ejector motive and suction ports. Current and voltage of each of the three phases of the unit power supply line are monitored and then used to assess the refrigeration system electrical power consumption. Table 2 presents the measuring equipment installed in the experimental facility together with the corresponding accuracy. Table 2. Data acquisition equipment with corresponding accuracy. Instruments Accuracy Thermocouples ±0.15°C Pressure Sensors ±0.1% of set span Mass flow meters ±0.50% of reading Electrical Current ±0.2% of set span Electrical Voltage ± (0.2% of reading + 0.005% of set span) All the monitored parameters are measured with a sampling frequency of 1 Hz, with the exception of voltage and current values. Specific enthalpies, necessary for the calculation of the unit energy performance, are evaluated from the measured values of pressure and temperature through the REFPROP 9.1 database. For each operating condition considered in the experimental campaign, data are measured for 15 minutes (960 samples) and then averaged to determine the steady-state performance in those specific conditions. RESULTS AND DISCUSSION As previously stated, the refrigeration unit is capable of operating in various configurations; however, experimental results in back-pressure configuration operation will be presented in this study. A simplified schematic of the experimental facility for backpressure is shown in Fig 3. To experimentally evaluate the steady state performance of the refrigeration unit in backpressure configuration at conditions relevant to the selected application, experiments were performed for different values of the refrigerated space temperature 𝑇𝑖𝑛𝑡 (-5 ºC ,0 ºC and 5 ºC) and ambient temperature 𝑇𝑎𝑚𝑏 (varying between 20 °C and 40 ºC, with a step of 5 °C). 6 Figure 3. Back-pressure configuration. The desired setpoint temperatures of 𝑇𝑖𝑛𝑡 and 𝑇𝑎𝑚𝑏 were maintained by controlling the air temperature in hot and cold rooms. The effect of ambient and internal temperatures on the Coefficient of Performance (COP) and on the cooling capacity of the refrigeration unit is shown in Fig 4. Considering the design conditions of 𝑇𝑖𝑛𝑡 = 0 °C, the experimental COP is in the range between 2.8 (for 𝑇𝑎𝑚𝑏 = 20°C) and 1.3 (for 𝑇𝑎𝑚𝑏 = 40°C). To compare the performance of the R744 unit presented in this study to the performance of baseline transport refrigeration units currently available in the market, data from prior studies available in literature, referred to a R404A unit (Colbourne et al., 2017) and to a R410A unit (Wu et al., 2013), have been compared in Fig.5 to the experimental results of the R744 unit. At 20°C, the R744 system demonstrates significant performance improvement of 42.5% while at 30°C and 40°C the improvements are approximately 30.8% and 19.5%, respectively. The employment of ejector configurations, not considered in this study, could result in further performance improvement at high ambient temperatures. It should be pointed out that, besides the above discussed significant performance improvement, the proposed unit represents a viable solution to fulfill the EU goals of environmental sustainability of the food chain also thanks to the replacement of synthetic refrigerants with the natural refrigerant R744 (GWP = 1). (a) (b) Figure 4. Effect of operating conditions on (a) COP ;(b) Cooling capacity of the refrigerating unit. 7 Figure 5. Comparison of current study with previous baseline studies. CONCLUSIONS This study presented the steady state performance of the R744 cooling unit in back pressure configuration designed for medium sized refrigerated trucks. An experimental campaign was conducted for three refrigerated space temperature 𝑇𝑖𝑛𝑡 (-5 ºC ,0 ºC and 5 ºC) and ambient temperature 𝑇𝑎𝑚𝑏 was varied between 20 °C and 40 ºC, with a step of 5 °C. Experimental results demonstrated the good performance of the proposed unit, assessing a COP in design conditions (𝑇𝑖𝑛𝑡 = 0 °C) ranging between 2.8 and 1.3 for 𝑇𝑎𝑚𝑏 varying between 20 °C and 40°C, respectively, and highlighting a significant COP improvement (between +42.5% and +19.5% for 𝑇𝑎𝑚𝑏 varying between 20 °C and 40 °C) compared to synthetic refrigerant baseline solutions reported in literature. ACKNOWLEDGEMENTS The activity described in this manuscript has been performed within the project ENOUGH. ENOUGH has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101036588. REFERENCES Artuso, P., Marinetti, S., Minetto, S., Col, D.Del, Rossetti, A, 2020. Modelling the performance of a new cooling unit for refrigerated transport using carbon dioxide as the refrigerant. Int. J. 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