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Phase change materials (PCMs) are important constituents for the storage of thermal energy available from the sun. It acts as a bridge between energy demand and supply while reducing the mismatch. Organic and inorganic constituents have been used for a long time for thermal energy storage applications. In recent years, the focus has been shifted to eutectics. A eutectic is a minimum melting substance of two or more constituents. It has the advantage of having a sharp melting temperature and possessing high volumetric heat storage density. The eutectics possess a wide range of temperatures and have the properties of all of their constituents. At present organic-organic, organic-inorganic, and organic-inorganic eutectics are widely studied. In the present paper, various eutectic PCMs for low and medium temperature ranges have been analyzed. Their thermophysical properties and thermal stability and reliability concerning thermal cycling have been thoroughly discussed. The melting temperature lies in the range of −23.50 °C to 80 °C and the latent heat of fusion can be as high as 280 kJ/kg. Thermal cycle tests of up to 30000 have been conducted by various research groups, but at least 300 melt/freeze cycles are recommended so that they can be stable for a year of application. The organic eutectic PCMs are found stable in terms of deviation in melting temperature with the maximum deviation in latent heat of fusion observed was ± 20 %. The inorganic eutectic PCMs are somewhat unstable having a large deviation in melting temperature and latent heat of fusion. The low thermal conductivity of organic eutectics can be eliminated with suitable nanoparticle additives. The leakage issue can be eliminated by providing shape stabilization to the PCMs. These PCMs are suitable for various photovoltaic/thermal, buildings, textiles, solar water heating, solar air heaters, and heat recovery systems applications.

When discharging latent heat thermal energy storage (LHTES) systems, performance is influenced by the formation and adherence of a solid layer of phase change material (PCM) on heat eXchange (HX) surfaces. Super‐liquid‐repellent thin films (STFs) may be able to reduce solidifying PCM adhesion on HX surfaces during discharging, delay PCM solidification to lower temperatures, and by modifying nucleation sites potentially enable long‐term seasonal thermal storage. Techniques employed previously to fabricate sintered polymeric STF coatings include chemical vapour deposition, dip‐coating, spray‐coating, spin‐coating, layer‐by‐layer (LbL) assembly, sol‐gel, anodizing, electrodeposition, electrospinning, so on. Dip‐coating is considered attractive for fabricating thin films on simple and complex surface geometries due to process maturity, scalability, flexibility and cost‐effectiveness. To identify suitable materials for preparing STFs on metal HX surfaces using the dip‐coating process, more than 200 journal articles published in English during the period 2010 to 2022 were reviewed and the potential role of STFs in LHTES applications was assessed. The review identified key areas and applications stimulating STF material developments and formulations. The dip‐coating of potential STF materials was classified under three major themes driving current research and development (R&D) activities, that is, high performance thin films, eco‐friendly thin films and fundamental research formulations. This review provides a platform from which to develop coatings and HX systems to enable the cost‐effective implementation of STFs for improved heat transfer in future mobile/stationery LHTES systems. [ABSTRACT FROM AUTHOR]

Phase-change materials have various applications across industries from thermal energy storage through automotive battery temperature management systems to thermal stabilisation. Many of these applications are shell and tube structures with different shell shapes. However, it is not yet known how the shape of the shell affects the melting, solidification times, and heat transport processes in such structures. To fill this research gap, seventeen shell shapes/orientations were compared using a simulation study. The well-known and validated enthalpy porosity algorithm implemented in the Fluent 2021R2 software was used. The numerical calculations were preceded by the measurement of thermal conductivity, phase change enthalpy, and specific heat during melting and solidification of the phase-change material. The shortest melting time was achieved for a semi-circular shell shape in the downward position, which was 44% shorter than the reference circular case. The shortest solidification times were recorded for an isosceles trapezium in an upward orientation relative to the reference circular case. Therefore, it is possible to significantly reduce the melting time in shell-and-tube systems as a result of the appropriate selection of the shell shape. [ABSTRACT FROM AUTHOR]

Energy storage technology provides a new direction for the utilization of renewable and sustainability energy. The objective of this study is to introduce a novel, wavy, longitudinal fin design, which aims to improve heat transfer in the melting process of a Latent Heat Thermal Energy Storage (LHTES) unit. The main goal is to mitigate the negative effects caused by the refractory zone at the end of the melting phase. A two-dimensional numerical model of LHTES unit is established by using the enthalpy porosity method and verified by experimental data. Through the quantitative comparison between the traditional rectangular fin and the innovative wave fin, the influence of wave fin on the heat transfer mechanism in the heat storage process is revealed. The results show that the average heat storage rate of five and six wave fins is 3.70% and 12.98% higher than that of conventional rectangular fins, respectively, and the average temperature response of six wave fins is 17.78% higher than that of conventional rectangular fins. The addition of the wave fin weakens the negative effect of the refractory zone, but prolongs the heating time of the initial melting point. [ABSTRACT FROM AUTHOR]

Shell and tube latent heat thermal energy storage systems are compact and effective among other types of energy storage systems. The present numerical study deals with geometry optimization of a modified design of shell and tube latent heat thermal energy storage system over melting and solidification cycles. The modified design was produced by splitting the single inner tube into three smaller inner tubes arranged in a line array with different orientations. The numerical model covered four different line array orientations: 0°, 30°, 60°, and 90°, along with a storage model consisting of a single inner tube. The surface temperature, perimeter of inner tubes, and mass of PCM in all numerical cases were assumed to be identical. The numerical model has been validated by comparing the melting fraction contours and the PCM temperature with the experimental published data. It has been found that the line array of 0° orientation has the minimum full cyclic duration with an improvement of 44.4% compared to the base case. Conversely, the line array of 90° orientation has the maximum full cyclic time; 15.83% longer compared to the base case of a single inner tube. Lastly, the orientation of the line array is more significant for melting compared to the solidification.

Phase change materials (PCMs) have emerged as promising solutions for latent heat thermal energy storage (LHTES) systems, offering considerable potential for storing energy derived from renewable sources across various engineering applications. The present study focused on optimization of solar cooling system by integrating LHTES with different PCM tank configurations. TRNSYS simulation software was selected for the study, and the collected experimental data from laboratory system prototype were used for system validation. The results indicate that the use of PCM led to a noteworthy decrease of 6.2% in auxiliary energy consumption. Furthermore, the time during which the heat carrier temperature flow exceeded 90 °C from the storage tank to the auxiliary fluid heater was extended by 27.8% when PCM was utilized compared to that of its absence. The use of PCM in LHTES is more effective under variable weather conditions. On the day when changes in weather conditions were observed, around 98% of the cooling load was provided by produced sun energy. The results of the research can be used to optimize the solar cooling system, which will help reduce the environmental impact of cooling systems running on non-renewable fuels. [ABSTRACT FROM AUTHOR]

The objective of this paper is to examine the thermal performance of a LHTES system comprised of a single tube, by means of a Computational Fluid Dynamics (CFD) computer code. Two different types of fins, straight and annular, were considered to enhance the heat conduction between the tube and the RT42 Phase Change Material (PCM). The dimensions of the fins were elected so that the total area of the fins to be almost equal, in both configurations. The RT42 PCM is a paraffin wax with medium to high thermal energy storage and chemically inert. Whereas possible, temperature dependent thermal properties, provided by the producer, used for the definition of the material model. Between the finless tube and the one with straight fins, the melting time reduction was approximately 56.7% and between the finless and the annular tube, the PCM melt time was reduced by 61.5%. Heat conduction was the driving heat transfer mechanism during the first seconds of the PCM melting process. On the other hand, natural convection was the lead heat transfer method as the liquid PCM was able to move due to density and temperature differences. The investigation of the liquid PCM travel revealed that in the finless model and the one with the straight fins the PCM movement was parallel to the axial direction whereas in the model with the annular fins the velocity of the PCM was parallel to the radial direction. [ABSTRACT FROM AUTHOR]

One important solution for societies to overcome the energy crisis is to manage the production and storage of energy. In this regard, the latent heat energy storage systems have recently gained a remarkable attention. Hence, this work is devoted to enhance performance of a latent heat tube-shell storage system by using novel spiral fins with different geometrical characteristics. Different number of vanes have been employed on the fins with constant twisting pitch. The different cases studied include the cases with single-fin, double-fin, triple-fin and quadruple-fin. These fins have been employed to augment heat transfer from a low temperature fluid to the PCM inside the shell container during the solidification process. To model the phase change, the enthalpy-porosity technique has been adopted. Various parameters have been used to assess the functionality of the system including the Nusselt number, total solidification time, temperature, solid fraction evolution, solid fraction contours, temperature contours and etc. The results demonstrated that the triple-fin and double-fin cases took the advantage in heat transfer augmentation and discharging process while the case with single-fin showed the worst performance. The results indicated that fin number is not the only influencing factor and fin length also has a dominant role.

Xylitol is a promising phase change material for thermal energy storage at low and medium temperatures, but its supercooling and low crystallization rate can hinder its performance in actual systems. This problem can be overcome with the application of external stimuli that promotes the nucleation and subsequent crystallization of the supercooled xylitol. Seeding combined with mechanical stirring or air-bubbling, the techniques used in this study, are proven to achieve the crystallization (and the release of the stored latent heat) of xylitol, but the effect was not instantaneous. Rheological measurements were performed firstly to study the influence of shear and temperature in the crystallization of xylitol. Then, the use of different mechanisms to promote the crystallization of the material was evaluated by two different laboratory-scale prototypes; one of them included a mechanical stirring system while the other employed a gas-bubbling mechanism. Thermal power and discharged energy of xylitol were evaluated in the bubbling system prototype and the influence of supercooling in both discharged power and energy were also studied. A methodology to calculate induction time is proposed to evaluate crystallization systematically in both systems. This new systematic evaluation proposed in this work is different from that of the literature and aims to be used to compare different crystallization systems.

In the present paper a new multi-objective optimisation procedure for the design of a shell-and-tube Latent Heat Thermal Energy Storage (LHTES) is proposed. A simple arrangement of a cylindrical shell with multiple vertical tubes has been examined. The optimisation considers, as design variables, the number of tubes, the tube internal radius and the device height-to-diameter ratio, H / D , while the storage volume is kept constant. This analysis aims to detect the set of solutions which optimises the LHTES performances evaluated in terms of charging and discharging times and overall thermal energy capacity. To accomplish the multi-objectives optimal thermal storage design, a simplified mathematical model of the LHTES has been employed. This model can evaluate the prescribed performances for a given set of design variables. The proposed optimisation procedure evaluates new solutions along the most promising directions in the design variables domain, leading to a significant improvement in storage performances. The Design of the Experiment, together with the Pareto dominance relationship, gives a starting optimal solutions subset. The proposed optimisation procedure permits to enhance the starting optimal solutions subset letting approach the Pareto barrier. The paper shows that, at the end of the optimisation procedure, the designer can select the solutions on the Pareto barrier with the best performance and the corresponding design variables for each chosen solution. The proposed optimisation procedure will also allow for maintaining low computational costs due to the low number of the new design variables evaluated only in the promising directions. [ABSTRACT FROM AUTHOR]

Changes in the energy sector, associated with the move away from fossil fuels, pose a challenge for appropriate thermal energy management in residential buildings. The important element to deal with the variability of renewable energy in thermal systems is latent heat thermal energy storage. Due to the low thermal conductivity of phase change materials, a number of techniques are proposed to enhance the heat transfer process. In this research, the global sensitivity of fin geometrical parameters on the melting and solidification times and energy efficiency of these processes was investigated. The computational model of the phase change was developed using the finite volume method with the enthalpy-porosity model and Boussinesq approximation. Numerical simulations were carried out according to the design of experiments technique. The multi-dimensional response surface was developed, and the multi-objective optimisation was done. The research shows that the melting process is most influenced by the position of the top fin (α angle) and the solidification process by the position of the bottom fin (γ angle). The angle of the tree fin (β) has a different effect on both processes, with the energy efficiency decreasing during melting and increasing during solidification. Maximum values for the energy efficiencies of melting ( η m = 0.973 ) and solidification ( η s = 0.988 ) were obtained for α = 18. 2 ° , β = 89. 0 ° , L = 10.7 mm and γ = 21. 0 ° . [ABSTRACT FROM AUTHOR]

Phase-change materials have various applications across industries from thermal energy storage through automotive battery temperature management systems to thermal stabilisation. Many of these applications are shell and tube structures with different shell shapes. However, it is not yet known how the shape of the shell affects the melting, solidification times, and heat transport processes in such structures. To fill this research gap, seventeen shell shapes/orientations were compared using a simulation study. The well-known and validated enthalpy porosity algorithm implemented in the Fluent 2021R2 software was used. The numerical calculations were preceded by the measurement of thermal conductivity, phase change enthalpy, and specific heat during melting and solidification of the phase-change material. The shortest melting time was achieved for a semi-circular shell shape in the downward position, which was 44% shorter than the reference circular case. The shortest solidification times were recorded for an isosceles trapezium in an upward orientation relative to the reference circular case. Therefore, it is possible to significantly reduce the melting time in shell-and-tube systems as a result of the appropriate selection of the shell shape.

This review paper critically analyzes the most recent literature (64% published after 2015) on the experimentation and mathematical modeling of latent heat thermal energy storage (LHTES) systems in buildings. Commercial software and in-built codes used for mathematical modeling of LHTES systems are consolidated and reviewed to provide details on the selection of appropriate tools. Insights on software's computing speed, model simplicity, accuracy (by considering the convective term in the melting process), and application of artificial neural networks are reviewed in detail. Moreover, the overall research status of the experiments conducted on the phase change material-based LHTES systems with different experiment configurations is reviewed. The analysis shows that ANSYS Fluent is the most widely used software for specific heat transfer phenomenon in storage tanks, while self-developed models with simplified terms are evaluated as more flexible and easier to apply. For hybrid systems, self-developed MATLAB, mature parts in ESP-r, TRNSYS, and EnergyPlus are compatible. Further, most of the experimental investigations are conducted on the laboratory scale, providing data for model validation. To provide a clear guidance for the future market application, the scope for future works is presented. With this review, it would be easier to develop a unified, simplified, visual, and accurate simulation platform for the PCM-based thermal energy storage in buildings. [ABSTRACT FROM AUTHOR]

Latent heat thermal energy storage (LHTES) with Phase Change Materials (PCM) represents an interesting option for Thermal Energy Storage (TES) applications in a wide temperature range. A tubular encapsulation model of an LHTES with PCM was developed, and the calculated data were analyzed. In addition, a parametric analysis for the preferable system geometry is presented. Organic paraffin RT18 with a melting point of 18 °C was utilized as PCM for different geometries of LHTES, and the addition of internal and external fins and their influence on LHTES thermal conductivity was investigated. One-step heat exchange from outdoor air to PCM and from PCM to water characterizes the LHTES system in solidification and melting processes, respectively. A 2D axisymmetric model was developed using Comsol Multiphysics 6.0. The LHTES unit performance with PCM organic paraffin RT18 encapsulated in electrospun fiber matrices was analyzed. The study results show that longer internal fins shorten the melting and solidification time. Direct contact of PCM electrospun fiber matrix with 23 °C water showed instant melting, and the phase change process was accelerated by 99.97% in the discharging cycle. [ABSTRACT FROM AUTHOR]

Phase change material (PCM) features an attractive option due to its solar thermal storage capability to assist the cooling/heating process especially during night operation, thus contributing to the reduction of energy cost and carbon footprint. This study aims to analyse the emergence of PCM in the application of solar thermal energy. Subsequently, to envisage Technology Readiness Level (TRL) and commercialisation opportunity based on historical and contemporary research trends. This review encompasses of peer-reviewed literatures from Scopus database for one decade between 2010 and 2019. Based on the review, there is a moderate growth on the research related to PCM-solar thermal at 22% of emergence rate from the past one decade. China has dominated in this research development by concurring approximately 22% from the number of research articles published globally. It can be concluded that the application of PCM in solar thermal energy system is at TRL 5 which reflects research and development (R&D) progress is at intermediate prototypical development based on the trend of academic publication. Furthermore, based on the review, PCM features great potential in commercialisation opportunity due to its vital contribution as a frontier material/substance in overcoming the challenges of energy and environmental insecurity.

Phase change materials (PCMs) have emerged as promising solutions for latent heat thermal energy storage (LHTES) systems, offering considerable potential for storing energy derived from renewable sources across various engineering applications. The present study focused on optimization of solar cooling system by integrating LHTES with different PCM tank configurations. TRNSYS simulation software was selected for the study, and the collected experimental data from laboratory system prototype were used for system validation. The results indicate that the use of PCM led to a noteworthy decrease of 6.2% in auxiliary energy consumption. Furthermore, the time during which the heat carrier temperature flow exceeded 90 °C from the storage tank to the auxiliary fluid heater was extended by 27.8% when PCM was utilized compared to that of its absence. The use of PCM in LHTES is more effective under variable weather conditions. On the day when changes in weather conditions were observed, around 98% of the cooling load was provided by produced sun energy. The results of the research can be used to optimize the solar cooling system, which will help reduce the environmental impact of cooling systems running on non-renewable fuels.

In this paper, an experimental investigation is revealed on the solidification process of the latent heat thermal energy storage (LHTES) system, in which the heat energy emitted into the atmosphere with the exhaust gases of ICE vehicles is stored by phase change material (PCM) enhanced with nanoparticles (Al2O3). In the study using RT55 paraffin wax as the PCM, the interior heating process of a typical sedan automobile in cold climate conditions is used as the heat release medium for the LHTES system. Experimental studies carried out in real climatic conditions were repeated for pure paraffin (RT55) and five dif-ferent Al2O3 fractions (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, and 25 wt.%). Ex-periments are performed in an optical PCM container and are completed after 1200 seconds (20 minutes). The heat energy stored in the PCM container is discharged to the automobile cabin via the closed-circuit liquid circulation system by the heating radiator system in the automobile cabin. The findings showed that the solidification process is improved considerably up to 10 wt.% Al2O3 fractions compared with pure paraffin RT55, and the temperature in the cabin could be increased by approximately 29%. In addition, the Al2O3 frac-tion increased by more than 10 wt.%, which has a negative effect on the im-provement in the solidification process, but higher solidification ability and in-cabinet temperature were obtained with all Al2O3 fractions compared to pure RT55. [ABSTRACT FROM AUTHOR]

In this article, latent heat thermal energy storage (LHTES) is analyzed for nano phase change material enclosed in a hexagonal triplex-tube with solidification acceleration. Optimized curve shape fins are adjusted inside the tube. The shape of the structure is a hexagonal annular. Therefore, both inner hexagonal walls are cold. Corresponding to six vertices of the hexagonal shaped tube, parallel but symmetrically curved fins are adjusted to every corner. Nanoparticles Enhanced Phase Change Material (NEPCM) is filled inside the annulus, and the mathematical model is constructed based upon the system of nonlinear partial differential equations through the heat equation. Single wall carbon nanotubes (SWCNT) and Copper driven through liquid phase and solid phase within the entire duct. Galerkin Finite Element Method (GFEM) is employed to model the solidification procedure. The smaller symmetric fins achieving an angle of 10o≤β1≤45o and the larger symmetric fins making an angle of 10o≤β2≤45o with the vertical rod. However, the length of rod is 10mm≤L≤20mm. It is found that the optimized curve shape fin penetrate the intensity of thermal storage into the Phase change materials (PCM) and the shape of the fin prevents the phase trade manner from final unsolidified PCM near the outer shell, in which the phase alternate front passes the top of the fins. It is further analyzed that at the mean position of the triplex tube, the solidification process is minimum due to the high thermal performance of nanoparticle volume fraction and time perfection.

This study presents a novel analytical solution of the phase change problem in a shell and tube arrangement of latent heat thermal energy storage (LHTES) system. The transient melting behavior of phase change material (PCM) is captured using time‐dependent boundary conditions and moving interface energy equation. An approximate mathematical model is developed with the help of dimensionless number (η), exponential integral function, Lambert W function, and Swamee and Oijha approximation. The proposed analytical solution is used for investigating the thermal performance of low, medium, and high melting temperature PCM for building, solar absorption cooling, and concentrated solar power (CSP) applications, respectively. The results obtained from the analytical approach are compared with a numerical model based on the enthalpy method. It is found that the root mean square difference between the average PCM temperature obtained from analytical solution and detailed numerical analysis is 3.3°C, 1.2°C, and 0.1°C for the low, medium, and high‐temperature systems, respectively. The temporal variation of melt front position and the effect of inner tube temperature are further investigated using the analytical solution. The mathematical model could be proposed as an effective tool for the assessment of shell and tube LHTES systems. [ABSTRACT FROM AUTHOR]

In the present paper a new multi-objective optimisation procedure for the design of a shell-and-tube Latent Heat Thermal Energy Storage (LHTES) is proposed. A simple arrangement of a cylindrical shell with multiple vertical tubes has been examined. The optimisation considers, as design variables, the number of tubes, the tube internal radius and the device height-to-diameter ratio, H/D, while the storage volume is kept constant. This analysis aims to detect the set of solutions which optimises the LHTES performances evaluated in terms of charging and discharging times and overall thermal energy capacity. To accomplish the multi-objectives optimal thermal storage design, a simplified mathematical model of the LHTES has been employed. This model can evaluate the prescribed performances for a given set of design variables. The proposed optimisation procedure evaluates new solutions along the most promising directions in the design variables domain, leading to a significant improvement in storage performances. The Design of the Experiment, together with the Pareto dominance relationship, gives a starting optimal solutions subset. The proposed optimisation procedure permits to enhance the starting optimal solutions subset letting approach the Pareto barrier. The paper shows that, at the end of the optimisation procedure, the designer can select the solutions on the Pareto barrier with the best performance and the corresponding design variables for each chosen solution. The proposed optimisation procedure will also allow for maintaining low computational costs due to the low number of the new design variables evaluated only in the promising directions.

Changes in the energy sector, associated with the move away from fossil fuels, pose a challenge for appropriate thermal energy management in residential buildings. The important element to deal with the variability of renewable energy in thermal systems is latent heat thermal energy storage. Due to the low thermal conductivity of phase change materials, a number of techniques are proposed to enhance the heat transfer process. In this research, the global sensitivity of fin geometrical parameters on the melting and solidification times and energy efficiency of these processes was investigated. The computational model of the phase change was developed using the finite volume method with the enthalpy-porosity model and Boussinesq approximation. Numerical simulations were carried out according to the design of experiments technique. The multi-dimensional response surface was developed, and the multi-objective optimisation was done. The research shows that the melting process is most influenced by the position of the top fin (α angle) and the solidification process by the position of the bottom fin (γ angle). The angle of the tree fin (β) has a different effect on both processes, with the energy efficiency decreasing during melting and increasing during solidification. Maximum values for the energy efficiencies of melting (ηm=0.973) and solidification (ηs=0.988) were obtained for α=18.2°, β=89.0°, L=10.7 mm and γ=21.0°.

The single-tank latent heat thermal energy storage (LHTES) of solar energy mainly consists of two modules: the first one is the phase change material (PCM) module heated by solar energy; the second is a module of heat transfer between melted PCM and the user's low-temperature water. This paper mainly focuses on the former one. To investigate the heat transfer performance of the paraffin-based solar single storage tank and find a more suitable experimental configuration, as basic research work, we established a single-tank thermal storage platform and then conducted a numerical simulation on the heat transfer process with Fluent. The result of numerical simulation shows that the test situation was basically reflected and the data agreed well with the experiment results. The numerical simulation analysis is accurate and the method is reliable. To obtain the heat transfer performance of paraffin in a single tank and strengthen heat transfer, the aspect ratio, the melting temperature of paraffin, and the heating power of the electric heater were analyzed based on simulation. The results show that the heat transfer gets more uniform when the aspect ratio is lower. This results in an increase in the liquid fraction of 61.83% to 76.47% one hour after heating when the aspect ratio of the tank reduced from 2.8 to 1.1. The higher the melting temperature of paraffin, the longer it takes for PCM to reach a stable state. And the curvature of liquid heating is greater than that of solid heating at the bottom layer. Under the constant total work, the heating power has little effect on the heat transfer performance of the paraffin. This study will provide some reference value for the optimization design of single-tank LHTES systems in the future. [ABSTRACT FROM AUTHOR]

The high latent heat thermal energy storage (LHTES) potential of phase change materials (PCMs) has long promised a step-change in the energy density for thermal storage applications. However, the uptake of PCM systems has been limited due to their relatively slow charging response, limited life, and economic considerations. Fortunately, a concerted global research effort is now underway to remove these remaining technical challenges. The bibliometric analysis of this review reveals that a major focus is now on the development of nano-enhanced phase change materials (NePCM), which have the potential to mitigate many of these technical challenges for PCM-based thermal energy storage systems. As such, our bibliometric analysis has zeroed in on research in the field of thermal energy storage using NePCMs since 1977. It was found that journal articles were the most frequently used document type, representing 79% of the records and that the pace of new work in this specific area has increased exponentially over these two decades, with China accounting for the highest number of citations and the most publications (168), followed by India and Iran. China has also played a central role in the collaboration network among the most productive countries, while Saudi Arabia and Vietnam show the highest international collaboration level.

A series of two-dimensional axis models with structure mesh were created to simulate the charging process inside a multi-layer latent heat storage system (LHTES) that applied in disk solar thermal power system. 60% NaNO 3 mixed with 40%KNO 3 were taken as phase change materials (PCMs). PCMs were separated into three ring-shape volume, which made it necessary to balance the heat transfer of the three volume and maximize the proportion of latent heat in energy storage. The phase change material reserves, liquid fraction, latent heat fraction (the proportion of latent heat in total absorbed energy), volume difference of melting fraction (parameter defined to describe the melting difference between three volumes) and volume difference of latent heat fraction (parameter defined to describe the latent heat fraction difference between three volumes) of 4 models with different structure parameter were taken into comparison. The result showed that the phase change material reserve reach to max, melting difference and latent heat fraction difference are minimum when the thickness ratio of the three volume in radial direction are 11:18:8 (when volume ratio was 231:1044:736). The best configuration made the ratio of heat exchange area to heat storage volume optimum and made the heat transfer for charging process close to balance in three volumes. • This manuscript explores a model that has rarely been studied before. • The present manuscript provide an idea and method for evaluating multi-layer LHTES with different structural parameters. • The 11:18:8 thickness ratio of the three PCMs storage volume in radial direction is a suitable structure parameter. [ABSTRACT FROM AUTHOR]

Solar energy offers over 2,945,926 TWh/year of global Concentrating Solar Power (CSP) potential, that can be used to substitute fossil fuels in power generation and mitigate 2.1 GtCO 2 of greenhouse gas (GHG) emission to support Sustainable Development Goals (SDGs) set by the United Nations (UN). Thermal energy storage (TES) is required in CSP plants to improve dispatchability, reliability, efficiency, and economy. Of all TES options, the latent heat thermal energy storage (LHTES) together with phase change materials (PCMs) exhibit the highest potential in terms of efficiency and economy. PCM properties thus become the ultimate decider of CSP performance. This paper reviews over 200 of published articles and websites to describe the history, identify the technologies, economics, and current trends of PCM as LHTES material for CSP plants. A list of 163 PCM candidates suitable for LHTES systems in low-temperature (T < 220 °C), medium-temperature (220 °C < T < 420 °C), and high-temperature (T > 420 °C) applications is presented for the first time. Research gaps are identified to determine the future directions. • CSP shows remarkable potential in global electricity generation. • Intermittency of solar has made the requirement of a TES for CSP system. • We evaluate the advantages of LHTES for storing thermal energy. • LHTES shows 5–14 times the energy density of SHTES allowing for more compact storage and lower inventory requirements. • Selection of a compatible PCM is crucial for operating the LHTES at desired performance level. [ABSTRACT FROM AUTHOR]

Latent heat stored in phase change materials (PCM) can greatly improve energy efficiency in indoor heating/cooling applications. This study presents the materials and methods for the formation and characterization of a PCM layer for a latent heat thermal energy storage (LHTES) application. Four commercially available PCMs comprising the classes of organic paraffins and organic non-paraffins were selected for thermal storage application. Pure organic PCM and PCM in water emulsions were experimentally investigated. PCM electrospun microfibers were produced by a co-axial electrospinning technique, where solutions of Polycaprolactone (PCL) 9% w/v and 12% w/v in dichloromethane (DCM) were used as the fiber shell materials. PCM emulsified with sodium dodecyl sulfate (SDS), and Polyvinylalcohol 10% w/v (PVA) constituted the core of the fibers. The thermal behavior of the PCM, PCM emulsions, and PCM electrospun fibers were analyzed with differential scanning calorimetry (DSC). A commercial organic paraffin with a phase change temperature of 18 °C (RT 18) in its pure and emulsified forms was found to be a suitable PCM candidate for LHTES. The PVA-PCM electrospun fiber matrix of the organic paraffin RT18 with a PCL concentration of 12% w/v showed the most promising results leading to an encapsulation efficiency of 67%.

In this paper, an integrated waste heat recovery system consisting of a thermoelectric generator and a latent heat thermal energy storage system was constructed to convert the wasted heat energy in a spark-ignition engine into useful energy. The TEG used in the integrated system has a three-layer structure formed of two cold-side heat exchangers placed on the upper and lower surfaces of the rectangular mid-exhaust heat exchanger. A total of 2x12 = 24 pieces of TEM were used between the lower and upper TEG_ec_hex and mid-TEG_ex_hex. For LHTES, a 3-layer heat exchanger consisting of a vertical copper tube placed in the middle of two cylindrical coils in a cylindrical tank with a diameter of 280 mm and a height of 350 mm was used. Three different configurations (Case_1, Case_2 and Case_3) were designed for the integrated system, in which exhaust gases, engine coolant, and HTF are used in different combinations. As a result, while the TEG_power is 69.395 W in Case_1, it decreases to 64.375 W in Case_2 and 62.417 W in Case_3. The total energy generated in the TEG at 7200 s is 500.27 kJ, 463.99 W, and 449.97 kJ for Case_1, Case_2, and Case_3, respectively. The stored energy in PCM_hex at 7200 s is 1372.91 kJ in Case_1, but it increases to 2255.16 kJ in Case_2 and to 2559.39 kJ in Case_3. While the fuel energy recovered by the integrated system is 1.03% in Case_1, it increases to 1.49% in Case_2 and 1.66% in Case_3. • An integrated WHR system consisting of TEG and LHTES for waste heat recovery in a SI engine is designed. • When the engine coolant is heated in TEG and taken to LHTES, the fuel energy recovered by the integrated WHR increases. • While energy recovery is 500.27 kJ with TEG in 7200 s, it increases to 2559.39 kJ with the integrated WHR system. • While the fuel energy recovered is 0.27% with TEG, it increases to 1.66% with the integrated WHR system. [ABSTRACT FROM AUTHOR]

Currently, energy storage systems are considered a key solution when mismatch occurs between energy supply and demand, allowing a more efficient energy deployment and use. The present paper is focused on the study of a latent heat thermal energy storage (LHTES) system based on a packed bed of encapsulated phase change material (PCM) of spherical shape, conceived as an auxiliary component of a micro-grid to be built in a Research Center located in southwestern Sardinia (Italy). The main purpose of this work was to perform numerical simulations for predicting the performance of the TES system, designed to store the surplus thermal energy produced during the weekend by a heat pump fed by a photovoltaic (PV) plant. The stored energy would then be utilized during the weekdays to integrate the air-conditioning system supply. The numerical simulations were based on a one-dimensional (1-D) two-equation transient model, able to return the thermocline profile of the water and the PCM separately. The behavior of the LHTES device during charge and discharge phases was reproduced, as well as during the standby periods. Finally, two characteristic indexes of the PV system were evaluated, to investigate the effect of TES on grid interchanges, self-consumption, and self-sufficiency.

Abstract In the present contribution the authors investigate by means of numerical simulations the thermal performance of two geometries of Latent Heat Thermal Energy Storage (LHTES). The numerical model takes into account the mass, momentum and energy equations for the PCM together with an Entalphy-Porosity model for the phase change and a Bussinesq approximation for the buoyancy force. Indeed in such a system the convective motion within the melted PCM plays an important role, as confirmed by several studies. The scope of the research has been the enhancement of heat transfer performance of LHTES increasing the convection by means of modifications on the shape of the PCM enclosure. In particular, in this study two geometries have been considered to investigate the influence of the convective motion on the overall melting time. The first case consists of two concentric cylinders with the axis aligned with the vertical direction, in order to realize an internal heating of the Phase Change Material (PCM) that is confined between the internal and the external cylinder. It is a well-known "shell-and-tube" configuration, generally adopted in LHTES systems, even in CSP applications. In the second case, the authors propose a simple cylindrical geometry filled with the PCM, with the cylinder axis aligned with the vertical position. Thus, the heated surface corresponds to the external lateral surface of the cylinder. The PCM volume (V) and the net heat transfer surface (A) have been kept constant in the two cases, in order to be able to compare them under the same operative conditions. The tests reveal a sensible difference between the thermal performance of the two proposed solutions. The external heating configuration shows an important enhancement of the convective motion within the PCM that induces a more efficient heat transfer with respect to the standard "shell-and-tube" configuration. The complete melting time of the PCM with external heating is reduced by about 50%. [ABSTRACT FROM AUTHOR]

This paper provides a review of the solid–liquid phase change materials (PCMs) for latent heat thermal energy storage (LHTES). The commonly used solid–liquid PCMs and their thermal properties are summarized here firstly. Two major drawbacks that seriously limit the application of PCMs in an LHTES system, that is, low thermal conductivity and liquid leakage, are discussed. Various methods for enhancing the thermal conductivity and heat transfer of solid–liquid PCMs are explained. Previous studies regarding form‐stable composite PCMs and microencapsulated PCMs are also presented. Furthermore, applications of the solid–liquid PCMs used in LHTES and thermal management systems are introduced and analyzed. Finally, future outlooks and research topics are proposed. [ABSTRACT FROM AUTHOR]

• A six-fin tube-and-shell heat exchanger model is established and enthalpy-porosity technique is used to simulate the PCM melting process. • The mechanism of the effect of mechanical vibration on PCM melting heat transfer has been revealed. • The influence trend of amplitude and frequency on PCM heat transfer is obtained. • The difference in the effect of amplitude and frequency on the heat transfer of PCM was found and the reason for the existence of the difference was explained. The optimization of heat transfer in latent heat thermal energy storage units (LHTES) has received a lot of research during the past decade, and the use of fins to enhance heat transfer in phase change materials (PCM) is one of the efficient ways. However, the existing studies do not consider the influence of mechanical vibration which widely exists in LHTES. To fill the research gap in this important topic, a horizontal shell-and-tube six-fin latent heat thermal energy storage unit is investigated in this study, and the effects of mechanical vibration are investigated by numerically simulating the PCM charging process in LHTES using the Enthalpy-porosity technique. In this paper, the charging rate of PCM, variation of key parameters such as Nusselt number, temperature and liquid fraction are analyzed in detail under vibration conditions. And the following conclusions can be obtained. (1) Mechanical vibration can significantly increase the charging rate of PCM, under the condition of vibration amplitude of 10mm and frequency of 1Hz (the melting time of PCM is reduced by 46.6%). (2) The enhancement effect of mechanical vibration on the melting rate of PCM increases with the increase of amplitude and frequency, but the enhancement efficiency keeps decreasing. These observations result from that the mechanical vibration changes the velocity of the liquid PCM, so the forced convective heat transfer of the PCM is significantly enhanced. Furthermore, the melting process of PCM is sensitively affected by increasing the amplitude than by increasing the frequency. Increasing the amplitude or frequency alone (amplitude from 10mm to 40mm; frequency from 1Hz to 4Hz) can reduce the total melting time of PCM by 75% and 68.7%, respectively. [ABSTRACT FROM AUTHOR]

The traditional cooling methods cannot meet the requirements of safety, stability, reliability and no-power at the same time under some special circumstances. In this study, a new coupled cooling method of Latent Heat Thermal Energy Storage (LHTES) combined with Pre-cooling of Envelope (PE) is proposed and the numerical model of the coupled cooling method is developed. In the current study, a refuge chamber is selected as a case study. A semi-analytical method is used to analyze the cold storage performance of the Surrounding Rock (SR). Afterwards, a numerical model of the coupled cooling system, which takes the heat source, SR, Phase Change Material (PCM) and air heat transfer into consideration, is further established. The study identified that the simplified semi-analytical calculation formula with the diagram of the cold storage quantity of SR are very helpful for engineering calculation. The influence of the Fourier and Biot number on the cold storage capacity of SR can be easily analyzed. In addition, the whole-field model of the coupled cooling system is completely developed based on the PCM unit. [ABSTRACT FROM AUTHOR]

This study presents an innovative new alternative for utilizing phase change materials as energy storage media in solar power towers. The solar power tower prilling or granulation concept combines the existing molten salt design with a prilling tower incorporated into the central receiver. In this way heat transfer limitations normally associated with these systems can be overcome since only very small particles (diameter 1–2 mm) are utilized. Three design cases were considered and a detailed model of the process was developed. The model demonstrates the design dependence on several key variables including particle diameter and the necessity of considering the system in conjunction with the steam generation cycle. Pinch considerations within this cycle and the tower itself determine the operational feasibility. It was found that the receiver design and prilling feed systems are crucial aspects which must be investigated further to refine the concept design and complete a full techno-economic assessment and optimization. The “best case scenario” resulted in a reduction of the circulating salt flow by one third compared to the base case. Preliminary indications are that this would result in a 20% reduction of the overall plant capital cost, but additional costs such as solids handling and a complex receiver design have to be taken into account. [ABSTRACT FROM AUTHOR]

PCMs-metal foams composite has been used in LHTES for improving the charge/discharge rate of the system. Due to the high exergy efficiency of cold energy storage (CES) systems, cold charging/discharging behaviours of the PCM-metal foams composite in CES was particularly analysed. The product solution method was adopted to synthesize the analytical solutions for multi-dimensional problems. The heat transfer model for the PCM-metal foams composite was established based on the concept of average volume of the composite materials. The internal convection was ignored due to the relatively small thermal resistance caused by the buoyancy force inside the PCM capsule. The case study of paraffin showed that the effective thermal diffusivity and effective thermal conductivity were improved by more than 21 times due to the embedding of Al foams in the PCM. As a result, the temperature of the PCM-metal foams composite became more uniform, while the cold discharge rate was improved by approximately 8 times. The comparative study of different PCMs with/without Al foams indicated the value of effective thermal diffusivity in weighing the charge/discharge rate in LHTES systems. [ABSTRACT FROM AUTHOR]