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In order to achieve global carbon neutrality in the middle of the 21st century, efficient utilization of fossil fuels is highly desired in diverse energy utilization sectors such as industry, transportation, building as well as life science. In the energy utilization infrastructure, about 75% of the fossil fuel consumption is used to provide and maintain heat, leading to more than 60% waste heat of the input energy discharging to the environment. Types of low-grade waste heat recovery technologies are developed to increase the energy efficiency. However, due to the spatial and temporal mismatch between the need and supply of the thermal energy, much of the waste thermal energy is difficult to be recovered. Thermal energy storage (TES) technologies in the forms of sensible, latent and thermochemical heat storage are developed for relieving the mismatched energy supply and demand. Diverse TES systems are developed in recent years with the superior features of large density, long-term, durable and low-cost. These technologies are vital in efficient utilization of low-grade waste heat and expected for building a low or zero carbon emission society. This paper reviews the thermal storage technologies for low carbon power generation, low carbon transportation, low carbon building as well as low carbon life science, in addition, carbon capture, utilization, and storage are also considered for carbon emission reduction. The conclusion and perspective are raised after discussing the specific technologies. This study is expected to provide a reference for the TES technologies in achieving zero-carbon future.

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.

Recent contributions to thermochemical heat storage (TCHS) technology have been reviewed and have revealed that there are four main branches whose mastery could significantly contribute to the field. These are the control of the processes to store or release heat, a perfect understanding and designing of the materials used for each storage process, the good sizing of the reactor, and the mastery of the whole system connected to design an efficient system. The above-mentioned fields constitute a very complex area of investigation, and most of the works focus on one of the branches to deepen their research. For this purpose, significant contributions have been and continue to be made. However, the technology is still not mature, and, up to now, no definitive, efficient, autonomous, practical, and commercial TCHS device is available. This paper highlights several issues that impede the maturity of the technology. These are the limited number of research works dedicated to the topic, the simulation results that are too illusory and impossible to implement in real prototypes, the incomplete analysis of the proposed works (simulation works without experimentation or experimentations without prior simulation study), and the endless problem of heat and mass transfer limitation. This paper provides insights and recommendations to better analyze and solve the problems that still challenge the technology.

Hyperloop transport operates in a low-pressure environment in which convective heat transfer is strongly limited, making conventional air-based cooling ineffective. One promising thermal management approach is therefore to absorb the waste heat generated during travel in a thermal energy storage (TES) system and dissipate it during stops. In this context, latent heat storage based on water–ice systems is particularly attractive because of its high energy density and nearly constant-temperature heat absorption. However, experimental validation of such systems beyond laboratory scale is still lacking. This study therefore investigated a large-scale direct contact latent heat storage (DCLHS) system for Hyperloop thermal management, using water as heat transfer fluid and ice as phase change material. The system was evaluated for two ice morphologies, crushed ice and ice block, under both constant and time-variant cooling power profiles representative of Hyperloop operation. The objective was to assess thermal performance, exergy efficiency, and hydraulic stability at application-relevant scale, and to identify morphology-dependent trade-offs relevant for system integration. The results show that the large-scale system can operate reliably under dynamic loads and that upscaling leads to smoother thermal behavior and reduced boundary effects. Crushed ice demonstrated superior thermal responsiveness, maintaining outlet temperatures close to the phase change temperature and achieving exergy efficiencies up to 0.72 at cooling powers up to 3.8 kW while enabling stable operation at 15 °C. In contrast, the ice block configuration provided higher volumetric energy density but exhibited delayed thermal response and required substantially higher mass flow rates, which limited operation to approximately 25 °C and reduced exergy efficiency to 0.03–0.35. Overall, the results show that large-scale DCLHS is a feasible option for Hyperloop thermal management, while also revealing that system behavior at larger scale is strongly influenced by storage morphology.

Magnesium hydroxide, Mg(OH)2, is recognized as a promising material for medium-temperature heat storage, but its low thermal conductivity limits its full potential application. In this study, thermal enhancement of a developed magnesium hydroxide-potassium nitrate (Mg(OH)2-KNO3) material was carried out with aluminum oxide (Al2O3) nanomaterials. The theoretical results obtained through a molecular dynamics (MD) simulation approach showed an enhancement of about 12.9% in thermal conductivity with an optimal 15 wt% of Al2O3. There was also close agreement with the experimental results within an error of ≤10%, thus confirming the reliability of the theoretical approach and the potential of the developed Mg(OH)2-KNO3 as a medium heat storage material. Further investigation is, however, encouraged to establish the long-term recyclability of the material towards achieving a more efficient energy storage process.

Due to the low thermal conductivity of the phase change material and low thermal diffusion inside the phase change material, this study seeks to improve the melting response of a triple‐tube latent heat storage system via employing annular fins by optimizing their structural parameters, including the fin number, location, and dimensions. Natural convection effects are numerically evaluated considering different numbers and the locations of the fins, including fin numbers of 4, 10, 16, 20, and 30 in a vertical system orientation. The fins are attached to the inner and outer sides of the annulus, accommodating the phase change material between the inner and center tubes. The fins' number and location are identical on both sides of the annulus, and the volume of the fins is the same across all scenarios evaluated. The results show that the higher the number of fins used, the greater the heat communication between the fins and the phase change material layers in charge, resulting in faster melting and a higher rate of heat storage. Due to the limited natural convection effect and lower heat diffusion at the heat exchanger's bottom, an additional fin is added, and its thickness is assessed. The results show that the case with equal fin thickness, that is, both original fins and the new fin, performs the best performance compared with that for the cases with an added fin with thicknesses of 0.5, 1, and 2 mm. Eliminating an extra fin from the base of the system for the case with 30 fins increases the charging time by 53.3%, and reduces the heat storage rate by 44%. The overall melting time for the case with an added fin to the bottom is 1549 s for the case with 30 fins which is 85.8%, 34.2%, 18%, and 8.8% faster than the cases with 4, 10, 16, and 20 fins, respectively. This study reveals that further attention should be given to the position and number of annular fins to optimize the melting mechanism in phase‐changing materials‐based heat storage systems. This study seeks to improve the melting response of a triple‐tube latent heat storage system via annular fins by optimizing their structural parameters including the fin number, location, and dimensions. It reveals that further attention should be given to the position and number of annular fins to optimize the melting mechanism in phase‐changing materials‐based heat storage systems.

The efficient utilization of solar energy technology is significantly enhanced by the application of energy storage, which plays an essential role. Nowadays, a wide variety of applications deal with energy storage. Due to the intermittent nature of solar radiation, phase change materials are excellent options for use in several types of solar energy systems. This overview of the relevant literature thoroughly discusses the applications of phase change materials, including solar collectors, solar stills, solar ponds, solar air heaters, and solar chimneys. Despite the complexity of their availability and high costs, phase change materials are utilized in the majority of solar energy techniques because of the considerable technical improvements they provide. While numerous studies have investigated the progress of phase change materials used in solar energy applications such as photovoltaic systems, it is vital to understand the conceptual knowledge of employing phase change materials in various types of solar thermal energy systems. Investigations into the use of phase change materials in solar applications for the purpose of storing thermal energy are still being carried out to upgrade the overall performance. This paper briefly reviews recently published studies between 2016 and 2023 that utilized phase change materials as thermal energy storage in different solar energy systems by collecting more than 74 examples from the open literature. This study focuses on demonstrating the maturity of phase change materials and their integration into solar energy applications. Based on the findings, proposals for new research projects are made.

With countries proposing the goal of carbon neutrality, the clean transformation of energy structure has become a hot and trendy issue internationally. Renewable energy generation will account for the main proportion, but it also leads to the problem of unstable electricity supply. At present, large-scale energy storage technology is not yet mature. Improving the flexibility of coal-fired power plants to suppress the instability of renewable energy generation is a feasible path. Thermal energy storage is a feasible technology to improve the flexibility of coal-fired power plants. This article provides a review of the research on the flexibility transformation of coal-fired power plants based on heat storage technology, mainly including medium to low-temperature heat storage based on hot water tanks and high-temperature heat storage based on molten salt. The current technical difficulties are summarized, and future development prospects are presented. The combination of the thermal energy storage system and coal-fired power generation system is the foundation, and the control of the inclined temperature layer and the selection and development of molten salt are key issues. The authors hope that the research in this article can provide a reference for the flexibility transformation research of coal-fired power plants, and promote the application of heat storage foundation in specific coal-fired power plant transformation projects.

Abstract Thermochemical heat storage (THS) systems have major advantages over other thermal storage systems, notably high energy density and low heat loss when hermetically sealed. There are several review papers available that discuss THS. Unlike other published review articles, this paper presents a literature survey and a review that add insights into the current state-of-the-art THS technologies, covering: the THS materials, THS reactor design and THS as thermal batteries. Emphasis is placed on THS for solar thermal energy storage and also for industrial waste heat recovery. At the materials level, in addition to a review on THS material sorbents, emphasis is placed on innovative composite THS materials with salt mixtures and metal-organic frameworks materials. Reactor design is one of the major fields of THS system development. In this paper, we also review several types of innovative reactor designs, including hybrid THS systems, towards obtaining advanced reactor concept, numerical studies in THS studies mainly covering the heat and mass transfer in the reactor designs, and also the implementation of THS systems as thermal batteries. Among the main conclusions, it is found that, although several advancements have been achieved in these fields in the last decade, further research is needed for advancing THS technology to be commercially viable. This paper will provide a wide range of information including the research gaps and critical issues in this field. The authors aim to allow readers to identify gaps/issues in the current research towards improving the practicality of THS systems.