  Citation: Zdun, K.; Uhl, T. Improvement of Properties of an Insulated Wall for Refrigerated Trailer-Numerical and Experimental Study. Energies 2022, 15, 51. https:// doi.org/10.3390/en15010051 Academic Editors: Wei-Hsin Chen, Aristotle T. Ubando, Chih-Che Chueh and Liwen Jin Received: 14 October 2021 Accepted: 16 December 2021 Published: 22 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Improvement of Properties of an Insulated Wall for Refrigerated Trailer-Numerical and Experimental Study Konrad Zdun * and Tadeusz Uhl Faculty of Mechanical Engineering and Robotics, University of Science and Technology AGH, 30-059 Krakow, Poland; tuhl@agh.edu.pl * Correspondence: zdun@agh.edu.pl Abstract: In the paper, we report our research on the improvement of thermal efficiency of refrigerated trailers by modification of their wall structure by placing a layer of phase change material inside them. The research was carried out in the field of transport, meeting the requirements of all classes provided for in the ATP agreement for refrigerated trailers. As part of the research, we formulated a numerical model of the proposed design of the refrigerator walls, which was subsequently validated by comparing the modeling results with the results of experimental tests carried out on a test bench designed specifically for this purpose. Based on the validated simulation conditions, we formulated the numerical model of a full-scale refrigerated semi-trailer, which was numerically tested under the conditions specified in the ATP Agreement. The results proved that adding a 6 mm layer of the SP-24 phase change material in each of the walls of the cold store allows the temperature inside the trailer to be kept below −20 ◦ C for a period of 24 h without the need to supply cold from the outside during operation. The passive refrigerated semi-trailer system implemented in this manner with 6 mm PCM layer allows for a reduction in primary energy consumption by up to 86% in a period of 22 h. The mentioned percentage did not take into account the efficiency of the cooling system of the phase change material. Keywords: phase change materials (PCMs); refrigerated trailer; cold storage; latent heat storage 1. Introduction Low-temperature cargo transports is an important link of the cold supply chain. The rapid development of mankind requires the transport of more and more products, especially food and medicines, which are subject to strictly defined temperature conditions. This, on a growing scale, is carried out with the use of refrigerated trailers, each of which have an individual refrigeration unit driven by low-efficiency diesel engines. It is estimated that worldwide transport of goods in low temperatures is responsible for some 14% of the total amount of CO 2 emitted [1], and diesel-driven refrigeration systems are responsible for up to 40% of these emissions [2]. The scale of the issue of emissions connected to the cold chain is so substantial that it has attracted the attention of numerous groups of researchers, who have taken steps to reduce the environmental footprint of this industry [3–8]. The replacing of diesel units by the use of central, highly efficient refrigeration units in cooperation with cold thermal energy storage in semi-trailers is particularly promising and is currently enjoying great interest among technologies that seem promising in this respect [3,5,6]. This technology is in line with the general trend of searching for solutions reducing greenhouse gas emissions from industry and energy plants with the use of thermal energy storage [9]. The aforementioned cold thermal energy storage can be implemented in cold storage in two ways [10]: (a) they can be integrated with the walls to improve their insulation [4] or (b) in the form of the cooling source for the cooling system [3]. It is also possible to combine both solutions, where the cold storage is implemented as a layer in the cold store Energies 2022, 15, 51. https://doi.org/10.3390/en15010051 https://www.mdpi.com/journal/energies