Elsevier

Applied Thermal Engineering

Volume 102, 5 June 2016, Pages 873-881
Applied Thermal Engineering

Thermal performance analysis of PCM in refrigerated container envelopes in the Italian context – Numerical modeling and validation

https://doi.org/10.1016/j.applthermaleng.2016.04.050Get rights and content

Highlights

  • A refrigerated container with PCMs was evaluated in the Italian climatic context.

  • The numerical results were validated by an experimental campaign.

  • A 4.23% of mean bias was achieved comparing the numerical and experimental results.

  • PCMs application leads to a reduction in peak heat load of 20%.

  • An energy rate reduction of 4.65% was obtained in the PCMs added container.

Abstract

Due to external climatic conditions, radiation and temperature, refrigerated containers are subjected to high thermal stresses during storage in yards, warehouses, ships or during transport by rail or road. Moreover the consequent high thermal load has a great influence on both the electric and fuel energy consumption and on combined greenhouse gas emissions into the atmosphere. The aim of this research is the theoretical evaluation, using a previously validated Finite Element Method (FEM), of the related energy benefits deriving from the application of PCMs (Phase Change Materials) to a traditional refrigerated container envelope. Specifically the numerical analysis was performed for several kinds of PCMs, climatic conditions and exposures. The study also provides a numerical tool to be used in the prediction of the thermal performance of refrigerated container envelopes with PCM in the Italian context. An experimental analysis was carried out in order to test the accuracy of the numerical model and to validate it. Results show that PCM application to a 20’ ISO container envelope can reduce and shift the daily heat load phases with respect to a traditional envelope fitted only with insulating materials.

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)

Cited by (51)

  • Advances in thermal energy storage: Fundamentals and applications

    2024, Progress in Energy and Combustion Science
  • Review of the modeling approaches of phase change processes

    2023, Renewable and Sustainable Energy Reviews
View all citing articles on Scopus
View full text