Thermal performance analysis of PCM in refrigerated container envelopes in the Italian context – Numerical modeling and validation
Introduction
The cold chain is believed to be responsible for approximately 2.5% of global greenhouse gas emissions considering both direct and indirect effects [1], [2]. Refrigerated containers are widely used for transporting and preserving perishable food by rail, road and ship including interchanges between these forms of transport [3]. One study reported by Gac [4] shows that at least 1 million refrigerated road vehicles and 400,000 refrigerated containers are used worldwide. This represents an important issue in the global cold food chain which accounts for up to 31% of the world’s food supply and has been estimated to grow at an annual rate of about 10% with the present volume of about half a million 20-foot containers [5]. The refrigeration units of refrigerated containers run on a vapor compressor system powered by electric current produced by the alternators of the carrying vehicles or by direct connection to the electric grid. Due to their lightweight envelopes and the external climatic conditions the power supply of the refrigerated container is generally subject to high oscillation. For this reason the application of a PCM (Phase Change Material) layer to the external side of a container envelope is investigated here. In fact, thanks to their high value of thermal inertia, PCMs are able to lower the energy consumption of these facilities. Oró et al. [6] developed a model to assess the potential effect on the reduction in energy consumption and related CO2 emissions by using thermal energy storage (TES) as a cold production system (Spanish and European Context). They estimated that in Europe the energy and environmental benefits due to a (PCM based) thermal energy storage system in the “cold road transport” sector could be between 3% and 40%.
PCMs are organic, inorganic or eutectic substances that change their state from solid to liquid and vice versa. Owing to their high latent heat of fusion and, to a lesser extent, to their specific heat, these materials act as heat accumulators [7]. There are different PCM applications in the building sector, which have several similarities with the case under study. Specifically a large number of studies have demonstrated that the addition of PCMs to the building envelope allows the thermal load to be reduced in warm climatic contexts especially during the summer season [8], [9], [10], [11]. Moreover PCMs have been widely used in small containers [12], [13] in order to stabilize the internal temperature during short journeys of perishable goods such as food, blood or medicines. In literature many researchers have investigated PCM addition for different cold storage and transportation systems.
For example, Tassou et al. [14], [15] analyzed different ways to reduce energy consumption in refrigerated chambers and indicated eutectic systems as a possible solution. Specifically, hollow tubes, beams or plates can be filled with a eutectic solution (PCM) aimed at storing energy and producing the cooling effect necessary to maintain the refrigerated compartment at the desired thermal conditions. In the same study PCMs are charged during the night or before starting the journey, and discharged during transportation, thereby limiting the activation of the refrigeration system.
Simard and Lacroix [16] analyzed the thermal performance of a parallel plate latent heat cold storage unit (which would take up 3% of the internal compartment volume) in a typical refrigerated truck. The PCM was an aqueous-glycol (50%) mixture with a melting temperature of −48.15 °C. The results highlighted that 8 plates (1 m × 2 m × 0.05 m) filled with PCM help to maintain the internal temperature of a refrigerated compartment (2.5 m wide × 2.5 m high × 6 m long) at below 265 K for 8 h. Moreover, there is also growing interest in the integration of PCMs into various cold storage and transportation system envelopes. As an example, Ahmed et al. [17] proposed a novel method to enhance the energy performance of the walls of a refrigerated trailer using PCMs. The technology was based on the inclusion of copper pipes containing paraffin (melting temperature 7 °C) in standard vehicle walls. During the experimental tests, as a result of adding PCMs, an average peak heat transfer reduction of 29.1% was obtained as well as average energy savings of 16.3%. Tinti et al. [18] analyzed the possibility to incorporate a microencapsulated phase change material (melting point 6 °C) into standard polyurethane foam designed for thermal insulation in refrigerated transport. This technology would be very useful in contrasting all events in which a temperature transient occurs, such as a temporary blackout of the refrigeration system, the frequent opening/closing of the compartment doors, the varying solar irradiation during the day and the vehicle journey. Glouannec et al. [19] numerically and experimentally studied the heat transfer across an insulation wall of a refrigerated van used for transporting refrigerated products. Specifically, the thermal performance of the reference wall was compared with two different multilayer insulation walls containing, in the first case, reflective multi-foil insulation (RMS) and aerogel and, in the second, phase change material (60% paraffin microencapsulated within a copolymer). The test results showed that the RMS combined with aerogel layers decreased the heat flux density peak by 27% and the energy consumption by 36% with respect to the reference wall. Good results were also obtained by increasing the thermal inertia using a PCM layer. Globally, during the daytime, the energy consumption decreased by 25% compared with the reference wall.
The present study proposes an application that could reduce and shift the daily heat load phases in a refrigerated container. Specifically, the research investigates the inclusion of PCMs in a traditional refrigerated container envelope acting as a thermal shield against the heat transfer rate from the outside environment to the inside of the refrigerated compartment. For this purpose, a PCM layer was added to the external side of the sandwich container walls, acting as a solar radiation and incoming thermal flux reduction technology. In this way both a theoretical and an experimental evaluation were carried out. The aims of the theoretical analysis are the preliminary investigation of the benefits deriving from this technology under several climatic conditions and the selection of the most suitable PCM, while a methodology to predict the PCM thermal behavior is also suggested. The aims of the experimental analysis are to evaluate the behavior of this technology under real summer environmental conditions and also to provide information in order to verify the numerical method. The present paper makes three original contributions: (1) it presents one of the first applications of PCM to a refrigerated container envelope; (2) it investigates this application in the Italian climatic context and selects the most suitable PCM; (3) it sets up and validates a well-known numerical method to investigate this application.
Section snippets
Materials and method
The numerical analysis was carried out using the COMSOL Multiphysics [20] software finite element method. To verify the thermal improvement deriving from PCM application to the external envelope, unidirectional and time-dependent simulations were performed (Fig. 1).
A 20’ ISO [21] refrigerated container with dimensions 6.058 m × 2.438 m × 2.591 m was considered during the numerical analysis. The considered global heat load inside the container takes into account the contribution of the incoming heat
Results and discussion
In order to assess the increase in envelope thermal inertia obtained with the addition of a PCM layer, two case studies were analyzed. The first investigates the envelope system without PCM, considering the layer of polyurethane foam sandwiched between the two layers of metal sheet (reference case). The second considers the integration, in turn, of different PCMs into the traditional insulated envelope.
Fig. 3 reports the daily heat load on the inside of the refrigerated container compared with
Conclusion
This paper presents the first numerical investigation of the energy behavior (in the Italian climatic context) of a refrigerated container envelope fitted with PCMs. Due to the light weight of the envelope, refrigerated containers are subject to overheating problems when they are irradiated, especially during the summer. A low thermal inertia envelope, even if characterized by high thermal resistance, leads to overheating when irradiated by the sun with a consequent high energy consumption. In
Acknowledgements
The computing resources and the related technical support used in this study were provided by CRESCO/ENEAGRID High Performance Computing infrastructures and their staff [36]. CRESCO/ENEAGRID High Performance Computing infrastructures is funded by ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development and by Italian and European research programs.
References (36)
- et al.
Transport and distribution of foods: today’s situation and future trends
Int. J. Refrig.
(1999) - et al.
Simulation and measurement on the full-load performance of a refrigeration system in a shipping container
Int. J. Refrig.
(2000) - et al.
Energy management and CO2 mitigation using phase change materials (PCM) for thermal energy storage (TES) in cold storage and transport
Int. J. Refrig.
(2014) - et al.
Materials used as PCM in thermal energy storage in buildings: a review
Renew. Sustain. Energy Rev.
(2011) - et al.
Numerical study on the thermal performance of a ventilated facade with PCM
Appl. Therm. Eng.
(2013) - et al.
Numerical and experimental analyses of PCM containing sandwich panels for prefabricated walls
Energy Build.
(2006) - et al.
A statistical approach for the evaluation of the thermal behavior of dry assembled PCM containing walls
Build. Environ.
(2006) - et al.
A methodology for investigating the effectiveness of PCM wallboards for summer thermal comfort in buildings
Build. Environ.
(2013) - et al.
Experimental and numerical analysis of a chilly bin incorporating phase change material
Appl. Therm. Eng.
(2013) - et al.
Methodology of temperature prediction in an insulated container equipped with PCM
Int. J. Refrig.
(2008)
Food transport refrigeration – approaches to reduce energy consumption and environmental impacts of road transport
Appl. Therm. Eng.
A review of emerging technologies for food refrigeration applications
Appl. Therm. Eng.
Study of the thermal behavior of a latent heat cold storage unit operating under frosting conditions
Energy Convers. Manage.
Reducing heat transfer across the insulated walls of refrigerated truck trailers by the application of phase change materials
Energy Convers. Manage.
Thermographic analysis of polyurethane foams integrated with phase change materials designed for dynamic thermal insulation inrefrigerated transport
Appl. Therm. Eng.
Experimental and numerical study of heat transfer across insulation wall of a refrigerated integral panel van
Appl. Therm. Eng.
Thermal analysis of the application of pcm and low emissivity coating in hollow bricks
Energy Build.
Simulation of phase change drywalls in a passive solar building
Appl. Therm. Eng.
Cited by (51)
Three-dimensional numerical simulation of solid-liquid phase change in the cavity of composite structures based on TPMS
2024, Applied Thermal EngineeringA review on thermal energy storage using phase change materials for refrigerated trucks: Active and passive approaches
2024, Journal of Energy StorageAdvances in thermal energy storage: Fundamentals and applications
2024, Progress in Energy and Combustion ScienceReview of the modeling approaches of phase change processes
2023, Renewable and Sustainable Energy ReviewsPerformance study and heating simulation on novel latent heat thermal energy storage device suit for air source heat pump
2023, Journal of Energy StorageTechno-economic analysis of a refrigerated warehouse equipped with on-shelf phase change material for cold storage under different operating strategies
2023, Energy Conversion and Management