Phase change materials: classification, use, phase transitions, and heat transfer enhancement techniques: a comprehensive review

Currently, there is great interest in producing thermal energy (heat) from renewable sources and storing this energy in a suitable system. The use of a latent heat storage (LHS) system using a phase change material (PCM) is a very efficient storage means (medium) and offers the advantages of high volumetric energy storage capacity and the quasi-isothermal nature of the storage process. In recent years, phase change materials (PCMs) have become an interesting research area due to their advantages especially in thermal energy storage (TES). Indeed, there are a large number of PCMs that melt and solidify over a wide temperature range, making them interesting thermal energy storage media in several applications. In the literature, research on PCMs and their associated applications has attracted and still attracts great interest from various researchers and scientists. Most of the research studies on phase change materials (PCMs) have been generally devoted to the development of PCM-based energy storage technologies, the promotion of PCM-based renewable energy sources, and the encouragement of sustainable/profitable (economic) use of PCM-based energy. In order to get an overview of current progress and trends, to highlight research and to identify gaps, from the literature reviews undertaken on this research topic, it is useful to review the major research studies conducted in this field. Our analysis showed that the literature lacks many comprehensive analyses and studies on the applications of PCMs, the phase transition processes (melting and solidification) of PCMs and the factors that influence these transitions, and in particular the calculation models of the thermal performance parameters of a PCM performing a phase transition and the thermal performance parameters of a PCM-based TES system (referred to as LHS unit). To address these questions, we have presented in this review article a detailed overview of the literature on (a) relevant practical applications of PCMs, (b) characteristics and performances of phase transition processes, (c) major factors influencing PCM transition processes such as geometric design of the PCM tank and its orientation, imposed boundary and operating conditions, thermophysical properties of the material (PCM), and (d) models for calculating thermal performance parameters for a PCM performing a phase transition and for an LHS unit. In addition, several techniques aimed at improving heat transfer in PCMs have been introduced and discussed. The findings indicate that there are three types of PCMs: eutectic, inorganic, and organic. Numerous other industries also use PCMs, such as solar energy (including thermal energy storage through the use of photovoltaic and latent heat systems); buildings; HVAC systems; textiles; the biomedical, food, and agricultural industries; the automotive sector; and desalination. Besides PCMs classification and use, it was found that during phase transitions of PCMs heat transfer is dominated by conduction and natural convection. During melting, conduction heat transfer is dominant in the early stages, and as the PCM melts, natural convection dominates. Unlike melting, solidification is dominated by conductive heat transfer. On the other hand, boundary conditions, material properties, and enclosure configuration and orientation all found having an impact on melting and solidification. In this context, by increasing, for example, thermal conductivity, viscosity, wall-imposed temperature, and PCM initial temperature, as well as by decreasing PCM latent heat of melting, PCM melting point, and PCM system orientation, the melting process rate increases. However, by increasing thermal conductivity, viscosity, melting point, and PCM system orientation, as well as by lowering the latent heat of melting, the initial PCM temperature, and the imposed wall temperature, the solidification process rate increases. Lastly, introducing external fields and adding high thermal conductivity additives like fins, metal foam, and nanoparticles can greatly increase the rate at which PCM melts and solidifies.