A novel propane – CO2 refrigeration system for mobile insulated boxes in last mile delivery

A novel propane – CO2 refrigeration system for mobile insulated boxes in last mile delivery Francesco Fabris(a), Wasim Shah(b), Sergio Marinetti(a), Silvia Minetto(a), Antonio Rossetti(a) (a) Consiglio Nazionale delle Ricerche, Istituto per le Tecnologie della Costruzione, Corso Stati Uniti, 435127 Padova – Italy (b) Università degli Studi di Padova, Doctoral Program in Industrial Engineering, Via Venezia, 1 – 35131 Padova - Italy Abstract The transport refrigeration industry predominantly relies on mechanical vapor compression systems, which still use refrigerants with high environmental impact, for both long and short distance transportation. In short distance application, the integration of thermal energy storage (TES) and the use of natural refrigerants represent a promising path in reducing the environmental impact of the sector. This paper presents an innovative indirect expansion refrigeration system utilizing natural refrigerants (R290 and R744) to freeze eutectic plates, located inside an insulated box, designed for last-mile delivery of frozen food. A numerical model is developed and first validated against experimental data available for the baseline R452A solution, achieving a -1.2% error on the estimated energy consumption over the entire pulldown. When considering the new system, numerical simulations of a 36-hours pulldown show that, despite delivering the same cooling energy to the eutectic plates (-0.6 % compared to the baseline), the R290-R744 system can achieve a better distribution of the cooling effect in the eutectic plates, avoiding subcooling. During a 36-hours pulldown, the R290-R744 can perform with the same overall COP (0.79). However, on an annual basis, the use of natural refrigerants instead of synthetics allows achieving a reduction of the system overall carbon footprint equal to -69.6%. Keywords: Refrigerated transport, Last-mile delivery, Thermal energy storage, Eutectic plates, Pulldown COP 1. Introduction The cold chain is essential for preserving and transporting perishable products, ensuring they remain at the proper temperature to slow down biological decay. However, around 14% of the world's food produced is lost annually due to inadequate cold chain infrastructure. This food is enough to feed approximately one billion people (UNEP and FAO, 2022). On the other hand, while being essential for reducing food loss and guaranteeing safe food to increasing population, the cold chain is responsible for significant energy consumption, accounting for almost 4% of global greenhouse gas emissions (UNEP and FAO, 2022). For this reason and in view of the necessity of increasing the cold chain all over the world (IIR, 2021), targeting energy efficiency and overall environmental sustainability is mandatory. Transport refrigeration is vital in the cold chain, as it not only regulates temperature to ensure the quality and safety of perishable goods during transportation operations but also influences energy and fuel consumption, as well as pollutant emissions from the transport vehicle (Fabris et al., 2024). Current refrigeration methods in cold chain logistics predominantly rely on vapor compression systems powered by diesel engines, either using the vehicle main engine power or a separate engine for the compressor. These engines are known for their high energy consumption, noise, and relatively expensive equipment (Horuz et al., 1999). Moreover, the need for lightweight cooling appliances and space constraints in transport equipment often leads to inefficient layouts, where the evaporator is positioned too close to the condenser and in some cases to the diesel engine, increasing heat losses (Chatzidakis and Chatzidakis, 2004). In many different refrigeration applications, the adoption of thermal energy storage (TES) technology has been proven effective to reduce energy consumption, and it has been suggested that such technology can lead to the achievement of the objectives outlined in the European Community "Energy Roadmap 2050" (Jouhara et al., 2020). The use TES systems has been considered in many recent research studies to improve the energy efficiency of refrigeration systems. Rocha et al. (2023) provided a detailed study on the application of phase change materials (PCMs) in small-scale refrigeration systems and thermal energy storage, particularly focusing on their potential to enhance energy efficiency, reduce compressor working time and on temperatures stability. Similarly, Sekhar et al. (2024) explored the integration of PCMs in solar based refrigeration systems, with special focus on their application in household refrigerators and freezers and highlights the development of zero-carbon food preservation technologies that utilize PCM to enhance energy efficiency and thermal performance, especially in remote areas. Selvnes et al. (2021) provided an overview of the integration and applications of PCMs in cold thermal energy storage systems, applied to such areas as refrigeration for supermarkets, food transportation, and biomedical products. They discussed the benefits of using PCMs for enhancing energy efficiency, reducing peak load consumption, and maintaining temperature stability during transport. TES has been used in transport refrigeration for very long time, starting from ice as PCM, still used for fish, to eutectic plates. However, based on the encouraging outcomes seen in the use of TES in such diverse refrigeration application fields, researchers are showing renovated interest also for transport refrigeration. Comprehensive assessments on the integration of phase change materials (PCMs) for temperature-regulated transportation and distribution applications across the cold chain are available in Calati et al. (2022a) and Umate and Sawarkar (2024), while Ben Taher et al. (2022) conducted a study on computational and experimental studies on refrigerated vehicles and trailers, including also TES solutions. The first use of PCMs in refrigerated transportation is by incorporating them into the insulated walls of refrigerated containers. This can be done by dispersing micro-encapsulated PCMs into traditional insulation materials or adding PCM layers directly to the walls both in conjunction with a refrigeration unit (Zdun and Uhl, 2022; Chandran et al., 2022) to reduce its energy consumption, or as standalone improvement to the insulated body, to stabilize the internal temperature of the box (Ahmed et al., 2010; Calati et al., 2022b). A second approach for TES in transport refrigeration is making it an integral part of the refrigeration unit, mainly to reduce internal temperature fluctuations (Principi et al., 2017; Kang et al., 2024). The third and common approach to incorporate TES in transport refrigeration is represented by eutectic plates or other in-kind solutions. In this case, the thermal energy is stored in the TES system before the transport missions starts by a vapour compression unit where the TES component is used as evaporator. The eutectic plates systems are an established technology and represent the most mature TES based solution in cold chain transport applications, as the ATP agreement by the United Nations (2024) regulates their use in the industry. However, despite being identified as a valid alternative to traditional vapor compression units, especially for short distance or when a daily use is planned, eutectic plates systems are still underutilized. A study by Fertel et al. (2023) revealed that only 2.7% of refrigerated vehicles in France employed eutectic plates, while 95.3% still relied on vapor compression systems. Recent studies have been focused on the optimization of the PCM choice and of eutectic plates placement in refrigerated vehicles. For instance, Mousazade et al. (2020) examined the thermal characteristics of three low temperature PCMs and assessed their behaviour under different truck speeds. Radebe et al. (2021) numerically investigated eutectic plates fitted in a medium-size refrigerated truck in order to determine the optimum configuration for transporting agricultural items and found that eutectic plates placed at top and sides of the truck showed better performance. Jeong et al. (2022) numerically studied the effects of different configuration of eutectic plates on door opening in frozen food transport system. The PCM melting phase drives the design of eutectic systems, as it represents the most critical phase when dealing with refrigerated transport (Calati et al., 2022a). However, solidification and melting phases should be equally considered for optimization, as the overall environmental sustainability of this technology significantly depends also on the reduction of the carbon emissions related to the process of PCM freezing. In particular, the effect of direct emissions, linked to the refrigerant employed in the cooling unit used to charge the PCM, should not be neglected. To the Authors best knowledge, no research work addressing the employment of natural refrigerants in place of HFC/HFO solutions to charge eutectic plate systems for refrigerated transport is available, with the exception of the study by Vaitkus et al. (2024), where five systems using R452A, two employing R290, and two using R1270 are compared and experimentally tested, discussing the challenges related to flammability which need research and risk assessments. The Authors considered a PCM characterized by a very low phase change temperature, as the end of crystallization is at -41°C, and further supercooling of the refrigerated goods stored into the insulated box. With such characteristics, the evaporation temperature in the last portion of the pulldown period can be as low as -60°C, which would correspond to operation below atmospheric pressure, making R290 unfeasible for the specific application. R1270 still does not reach such low temperatures, but allows complete crystallization of the eutectic plates while maintaining its evaporation pressure above atmospheric pressure. Following these results, the same Authors built and experimentally tested a second generation unit prototype operating with R1270 refrigerant (Vaitkus and Prakopavicius, 2024). Experimental tests confirmed that with R1270 the eutectic plates mostly finish crystallizing when the evaporation pressure does not yet go to vacuum. Moreover, the new evaporator configuration proposed by the Authors, with eutectic plates connected in parallel and fed through a distributor, ensures sufficiently uniform freezing of the eutectic plates. However, it needs to be specified that the vacuum related challenges highlighted by the aforementioned studies derive from the extremely low operating temperatures determined by the specific PCM characteristics. In this study a novel stationary unit to freeze the eutectic plates is proposed. It employs only naturals fluids, while addressing at the same time the safety issue related to hydrocarbons flammability: the system consists of an indirect expansion unit based on a R290 vapor compression cycle and a secondary R744 loop which freezes the eutectic plates. The simultaneous use of passive refrigeration by eutectic plates during deliveries, and natural fluids in the cooling unit represent an environmentally sustainable solution for last mile delivery of frozen goods. In this study, a numerical approach, validated on experimental data of a baseline R452A system, is used to assess the performance of the novel R290R744 system and to estimate its improvement, in terms of both energy efficiency and cooling effect management, compared to the baseline solution. 2. The refrigeration systems This study focuses on a transportable insulated box, equipped with eutectic plates for last mile delivery of frozen food. The plates are frozen by a stationary refrigeration system. In this solution, when the pulldown process starts, the PCM stored inside the plates solidifies at the selected temperature level, thanks to the connection to the refrigerating unit. This step is usually referred as the PCM “charging” phase. The unit is then disconnected from the insulated box, which is ready to be loaded with frozen food to be delivered. During the delivery, which corresponds to the “discharging” phase of the PCM, the required temperature for preservation is maintained thanks to the latent heat of the eutectic material that grant a passive refrigeration effect inside the insulated box. In this study, an insulated box with external dimensions 1.70 m × 1.00 m × 1.75 m and internal volume equal to 1.86 m3 is considered, as depicted in Figure 1. Each of the box walls is made of three layers, corresponding to a 94 mm polyurethane foam layer sandwiched between two 3 mm glassfiber reinforced plastic (GRP) layers. Wood and metallic structural elements are used as needed to increase the sturdiness. The box front has two doors equipped with rubber sealings. The global heat transfer coefficient of the insulated box, evaluated through steady state experimental tests carried out according to Annex 1 of the ATP agreement (United Nations, 2024), is equal to 𝐾 = 0.31 W m−2 K−1. Three eutectic plates are mounted inside the box and connected in series. Two eutectic plates with external dimensions 985 mm × 690 mm × 53 mm and one eutectic plate with external dimensions 795 mm × 485 mm × 54 mm are installed on the ceiling, on the upper part of the side wall and on the lower part of the side wall, respectively. The two biggest plates are charged with 28.48 kg of PCM and the smallest plate is charged with 15.13 kg of PCM. The PCM, an eutectic salt-water solution, is characterized by a latent heat of fusion equal to 242.64 kJ/kg and the nominal phase change temperature declared by the manufacturer is equal to -33 °C (FIC, 2023). The resulting latent energy inside the three eutectic plates is 17.5 MJ. Figure 1. Insulated box equipped with eutectic plates considered for this study. The baseline stationary cooling unit currently employed consists of a R452A direct expansion vapor compression unit, whose simplified schematic is presented in Figure 2. The connection points between the stationary unit and the box are highlighted in yellow in the figure. In this case, the pipe inside the eutectic plates acts as the evaporator of the cooling unit. The unit operates with fixed compressor speed. A thermostatic expansion valve controls the superheat (5 K) at the evaporator exit. The baseline cooling unit is charged with 2.5 kg of R452A and has 560 W nominal cooling capacity at 𝑇𝑎𝑚𝑏 = 30 °C and heat source at -33 °C. The system proposed to enhance the sustainability of the concept is described in Figure 3. The new system consists of two separate circuits: a primary circuit in which propane (R290) realizes a traditional vapor compression cycle, and a secondary loop with low-pressure CO2 (R744) as secondary two-phase heat transfer fluid. Liquid R744 at low pressure (and consequently at low temperature) is taken from a liquid separator and pumped into the eutectic plates, where it partially evaporates removing heat from the PCM, progressively freezing the plates. After heat exchange in the plates, the two-phase R744 is directly sent to a plate heat exchanger, where it rejects heat to the R290 in the primary circuit and fully condensates again, at almost constant operating pressure. Such a solution has been developed with the objective of avoiding any flow of the flammable refrigerant R290 inside the confined space of the box and eutectic plates (where only non-flammable R744 circulates) and, at the same time, of minimizing the overall R290 charge thanks to the compactness and reduced volume of the R290 primary circuit. Moreover, the extremely low temperature level at which the PCM changes its phase allows operating with low-pressure two-phase R744 in the secondary loop, which displays extremely good heat transfer characteristic and very low viscosity. The unit operates with fixed speed for both the R290 compressor and the R744 pump, and a thermostatic expansion valve enforces a 5 K superheat at the R290 compressor suction. The proposed indirect cooling unit is able to deliver a cooling effect equal to 630 W during steady-state operation in nominal conditions (𝑇𝑎𝑚𝑏 = 30 °C and heat source at -33 °C). The charge of the system is exstimated in 0.13 kg of R290 and 5.5 kg of R744. Figure 2. Direct expansion refrigeration system schematic. Figure 3. Indirect expansion refrigeration system schematic. 3. Numerical model description The refrigeration system is dynamically modelled employing the multi-physics commercial software Simcenter Amesim (Siemens, 2025). The model is based on a lumped parameters approach, where real components are discretized into elements that are connected to represent the entire system. Each element is governed by nonlinear time-dependent differential equations that involve the state variables. These equations are assembled into a system of differential equations, according to the connection of the elements. The system of differential equations is then integrated over time to obtain the model dynamic. This numerical approach has been extensively described in previous publications by the same authors (Artuso et al., 2020; Fabris et al., 2021; Fabris et al., 2023). However, a brief description of the system components considered in the model, of the numerical discretization and assumptions, as well as of the empirical correlations employed to evaluate the heat transfer processes, are here provided for the sake of completeness. A fixed displacement compressor model is adopted, where the volumetric and overall compression efficiencies are interpolated from the manufacturer data, specific for the R452A and the R290 compressors, as functions of the pressure ratio and the suction pressure. The fin-and-tube condenser is discretized into four lumped volumes. Each discretized volume is subdivided in three nodes, one referring to the refrigerant flow, one to the state of tube wall and fins and one referring to the state of the air. The geometric characteristics of the heat exchanger, such as surface and mass, are equally distributed in each lumped element. Internal convection between the refrigerant and the internal wall, conduction through wall and fins and external convection between the fins and the outside air are considered. The plate heat exchanger is modelled in a similar way: it is divided into four volumes, each one composed of three nodes (R290 flow, tube wall, R744 flow). In this case, convection between the operating fluid and the tube wall (for both R290 and R744 sides) and conduction through the tube wall are solved. Eutectic plates are discretized into four lumped volumes of equal size each. Internal convection between refrigerant and internal tube wall, conduction through the tube wall, conduction through PCM, conduction through plate wall, external convection between plate wall and air inside the insulated box, and radiation between plate wall and box internal walls are considered. Each discretized volume considers heat transfer in all four directions around the refrigerant tube, according to the discretization schematic presented in Figure 4. The PCM behaviour is described by providing its main thermophysical properties (density and specific heat of liquid and solid phases, thermal conductivity, latent heat of fusion and temperatures of phase change) as an input of the model. Nominal data declared by the manufacturer has been used for these parameters, with the exception of the temperature of solidification and the thermal conductivity. This will be explained in detail in section 4.2. Figure 4. Discretization schematic of each eutectic plate lumped volume. R: refrigerant; TW: tube wall; PCM: phase change material; PW: plate wall; A: insulated box air; BW: insulated box internal wall. The convective heat transfer coefficients are evaluated through empirical correlations, available in the literature. For the refrigerant elements (R452A, R290, R744), the used correlations are the following: Gnielinski (1976) for single-phase; Shah (1979) for two-phase condensation; and VDI for horizontal tubes (1992) for two-phase boiling. Colburn j-factor correlation (McQuiston, 1978) is used in the airside convective elements of the condenser. Flat plate correlations (Churchill and Chu, 1975, for vertical plate and Lloyd and Moran, 1974, for horizontal plate) are used on the box and eutectic plates surfaces. Radiation is modelled considering the eutectic plates external surface and the internal walls of the insulated box as two gray surfaces forming an enclosure, exchanging radiative heat flow only with each other. Each of the box side is modelled as a 1D series of 7 capacities and 6 thermal resistances. The thermal capacities are estimated according to the box geometrical data and materials. The thermal resistances were defined to reproduce the experimental global heat transfer coefficient of the box. 4. Results and discussion 4.1 Experimental setup of baseline system The baseline R452A system has been experimentally tested to assess its thermodynamic and energy performance under nominal operating conditions. In particular, the experimental test followed the ATP test procedure on eutectic plates equipment, as defined in Annex 1 of the ATP itself (United Nations, 2024). This test consists of a first part defined as pulldown, in which the cooling unit is operated to lower the temperature inside the insulated box from initial equilibrium with the environment to a value at which the perishable goods to be transported can be correctly preserved, while at the same time storing latent energy in the TES, and of a second part in which the cooling unit is turned off and the maintainment of the temperature below a temperature threshold is verified, to simulate delivery mission conditions. The test is conducted at 𝑇𝑎𝑚𝑏 approximately constant and equal to 28.9 °C during the first 24 hours pulldown, when the refrigeration system is on, with standard deviation equal to 𝜎 = 0.9 °C. The small variations of environmental temperature over the test duration are caused by the operation of the auxiliary system employed to thermostat the lab chamber in which the insulated box is placed for the test. Once the mean inside temperature of the insulated box and the temperature of the eutectic plates are in equilibrium with the environmental temperature, the doors are closed and the cooling unit is turned on. The insulated box is kept closed with the cooling unit in operation for 24 hours, to freeze the PCM contained inside the eutectic plates. After 24 hours, the cooling unit is turned off. The ATP test is deemed satisfactory if the mean temperature measured inside the insulated box is maintained below the limit temperature of 𝑇𝑖,𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑 = -20 °C (class C equipment) for 12 hours after switching off the cooling unit. Several thermocouples are placed in key points of the refrigeration system, in order to assess the refrigerant thermodynamic cycle and the air and plates temperatures. Temperatures are logged through an Agilent 34970A Data Acquisition Unit equipped with two Agilent 34901A multiplexer modules. The main electrical parameters (power consumption, voltage, current, frequency and power factor) are logged through a high-precision multimeter. The list of the equipment used for data acquisition, their accuracy and the sampling rate is reported in Table 1. Table 1. List of the equipment used for data acquisition and their accuracy. Type Instrument Placement Accuracy Sampling rate Temperature sensors T-type thermocouples Compressor suction and discharge, condenser outlet, inlet and surface of each plate, plates outlet, internal and external air ± 1.0 °C 1/60 Hz Electrical parameters HT Instruments PQA 824 System electrical utilities (compressor + condenser fan) ± 0.5 % (Voltage) ± 0.5 % (Current) ± 1.0 % (Power) 1/120 Hz The evolution of the ambient temperature (𝑇𝑎𝑚𝑏), of the air temperature inside the insulated box (𝑇𝑖) and of the refrigerant evaporation temperature (𝑇𝑒𝑣) over the 36 hours of the complete experimental test is presented in Figure 5. The test proves that the energy stored in the eutectic plates during the 24 hours pulldown is sufficient to guarantee a 12 hours autonomy below the internal air temperature setpoint of -20 °C with fixed external temperature conditions, ensuring a correct preservation of perishable goods during delivery missions and compliance with ATP requirements. Figure 5. Experimental test on baseline R452A system. 4.2 PCM characterization and numerical model validation A numerical model of the baseline R452A system has been developed following the approach presented in Section 3, with the objective of comparing the simulated cooling system performance to the experimental performance. In the simulations, the first 24 hours of the test are considered, corresponding to the system pulldown and to the refrigeration unit operation. The ambient temperature profile registrered during the experimental test presented in Section 4.1, including fluctuations, is used as the input of the numerical simulation, in order to evaluate the unit under the same operating conditions occurred during the experiment. As previously described in Section 3, the thermophysical properties of the PCM represent an input of the model. However, the information declared by the eutectic plates manufacturer related to the PCM properties (FIC, 2023) is limited due to industrial secrecy and company patents on the product. The density and specific heat of liquid and solid phases were derived from datasheets of a similar PCM available on the market (Rubitherm, 2023). The values of thermal conductivity and temperature of phase change are instead defined through best fit of the numerical model on the experimental results. The 24 hours pulldown simulation were performed for thermal conductivity (𝜆) values between 0.5 W (m K)-1 and 0.65 W (m K) -1 and the phase change temperature (𝑇𝑠𝑜𝑙) between -35 °C and -39 °C. The root mean square error (RMSE) on the internal air temperature (𝑇𝑖), normalized on the experimentally measured range of variation of 𝑇𝑖, was calculated as defined in Eq. (1), where 𝑡0 and 𝑡𝐸𝑁𝐷 represent the start and ending time of the test. Similarly, the RMSE on the electrical power consumption (𝑃), normalized on the mean value of the experimentally measured 𝑃, was calculated as defined in Eq. (2). The overall accuracy of the model was then assessed through an overall normalized RMSE, calculated as described in Eq. (3). 𝑅𝑀𝑆𝐸𝑇𝑖 =√1 (𝑡𝐸𝑁𝐷 −𝑡0) ∫(𝑇𝑖,𝑒𝑥𝑝 −𝑇𝑖,𝑠𝑖𝑚)2dt tEND t0 (𝑇𝑖,𝑒𝑥𝑝,𝑚𝑎𝑥 −𝑇𝑖,𝑒𝑥𝑝,𝑚𝑖𝑛) (1) 𝑅𝑀𝑆𝐸𝑃=√1 (𝑡𝐸𝑁𝐷 −𝑡0) ∫(𝑃𝑒𝑥𝑝 −𝑃𝑠𝑖𝑚)2dt tEND t0 Pexp,max (2) 𝑅𝑀𝑆𝐸𝑇𝑂𝑇 =𝑅𝑀𝑆𝐸𝑇𝑖 +𝑅𝑀𝑆𝐸𝑃 (3) The variation of the overall normalized RMSE as a function of the thermal conductivity and of the phase change temperature of the PCM is presented in Figure 6. The sensitivity analysis results show that the best fit of the numerical model on experimental data was achieved with 𝜆 = 0.55 W (m K) -1 and 𝑇𝑠𝑜𝑙 = - 38 °C. This is in line with the nominal phase change of -33 °C (FIC, 2023) that can be considered as an average between the solidification and melting temperatures. 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