Heat Thermal Energy Storage System Research Articles

Experimental investigation of high-temperature latent heat storage packed bed using alloy-based phase change materials

This study explores the effectiveness of a high-temperature latent heat thermal energy storage (LTES) system incorporating Al-Si-based microencapsulated phase change material (MEPCM) composite pellets within a cylindrical packed bed. A parametric analysis was conducted to examine the impact of varying pellet sizes (1, 3, and 5 mm) and airflow rates (20–50 L min−1) on the efficiency of heat storage and discharge. The experimental approach included controlled charging and discharging cycles at temperatures ranging from 500 to 800 °C, with Reynolds numbers between 17.6 and 261. The findings indicate that the system achieved a maximum round-trip efficiency of 0.93, with no substantial gains observed beyond a Reynolds number of 150. Additionally, the results reveal the importance of minimizing heat loss to improve system efficiency, particularly during the discharge phase. These insights are crucial for optimizing the design and operational parameters of high-temperature LTES systems to enhance energy storage efficiency.

Thermal performance analysis of new bionic “fishbone” fins in a triplex-tube latent heat storage system with single cycle process

Targeting the low thermal conductivity characteristics of energy storage materials in latent heat thermal energy storage (LHTES) systems, a novel bionic fish bone fin design is proposed in this paper, and the numerical simulation is employed to analyze the thermal performance of a system during single charging and discharging cycle. In assessing the system, consideration is given to the time required to complete a cycle, mean temperature change rate, and charging and discharging efficiency over the course of a cycle. The temperature response of each monitoring point within the system was first analyzed for fishbone fins of different curvatures and found that the 30° and 60° fishbone fins have the fastest heat transfer rates and shortest cycle times, respectively. Subsequently, the effect of fishbone fractal position on the thermal performance of the system was investigated. The results showed that the system achieves the best circulating thermal performance when the fishbone fins are positioned in the middle of the straight fins. An increase in the dimensionless position of the fins from 0.2 to 0.5 produced a reduction in the cycle time of 28.03 % and an increase in the energy transfer efficiency of 38.15 %. Finally, the shortest cyclic cycle and the fastest heat transfer rate were taken as the optimization objectives to optimize and predict the thermal performance of the system through the response surface method. The findings demonstrated that the system exhibited optimal thermal performance when the radian angle of the fishbone fin is 44.43° and the dimensionless position is 0.45. Compared to traditional rectangular straight fins, the optimized structure reduces the cycle time by 52.64 % and increases the average heat transfer rate by 101.58 %.

Performance enhancement of latent heat thermal energy storage systems via dynamic melting process of PCM under different control strategies

Latent heat thermal energy storage (LHTES) systems merging high energy densities with near isotherm operations have made a reliable solution to ease the intermittence difficulties of renewable energy and manage periodic energy demands to ensure supply–demand balances on electricity grids. This study experimentally and numerically explores the liquefaction characteristics of phase change material (PCM) in the LHTES tank with the dynamic melting technique, involving the liquid PCM recirculation in the liquefying process to enhance the melting outcomes. Experimental measurements are conducted to investigate the dynamic melting progression for revealing the system effectiveness in terms of the complete melt time and mean power with different control strategies modifying the formations and inlet velocities of recirculating PCM flows. The computational fluid dynamics (CFD) simulations are also conducted to resolve the liquid fraction, vorticity and temperature distributions for offering the insights of the transfiguration of heat transfer and liquefaction behaviors. The photos of ice-water interfaces and measured temperature data in the LHTES tank are thus acquired to verify the computational model generating the CFD predictions. The performance assessments signpost the optimization of dynamic melting arrangements adopting the top to bottom layout with a PCM inlet velocity of 0.22 m/s, achieving the reduction of full melting time by 48.1 % and the enhancement of estimated mean power by 132.6 %, respectively.

Improving the charging performance of latent heat thermal energy storage systems using triply periodic minimal surface (TPMS) structures

The utilization of latent heat thermal energy storage (LHTES) using phase change materials (PCM) has attracted considerable attention due to their high energy density and thermal regulation. However, the inferior thermal conductivity of PCM has hindered their widespread implementation, triply periodic minimal surface (TPMS) structures could considerably ameliorate the energy storage utilization. TPMS structures offer escalated surface area and pore interconnectivity, which can enhance the apparent thermal conductivity and improve the performance of LHTES systems. This study focuses on investigating the performance and storage capabilities of three unique TPMS structures, namely Gyroid, IWP, and Primitive, at different relative densities and charging temperatures. The computational model is developed, verified, and validated with the relevant experiment results. A comparative analysis of the temperature distribution and liquid fraction evolution in TPMS LHTES units is carried out and compared to pure PCM LHTES. At a charging temperature of 80 °C, compared to the complete melting time of 7206 s for the pure PCM LHTES, the results demonstrate that the G10, IWP10, and P10-PCM LHTES achieve complete melting at 344 s, 276 s, and 368 s, respectively. This affirms the superiority of IWP structures in improving the melting rates due to their escalated surface areas and tortuous paths. The use of TPMS structures improves the total heat storage, heat storage rate, and volumetric heat storage density, while reducing the gravimetric heat storage density, compared to the pure PCM LHTES. The G10-PCM LHTES attains total heat storage, heat storage rate, volumetric, and gravimetric heat storage density of 2120 J, 6.16 W, 63.6 kWh/m3, and 227 kJ/kg, compared to 1910 J, 0.27 W, 57.3 kWh/m3, and 250 kJ/kg for pure PCM LHTES, respectively. Moreover, increasing the relative density of TPMS structures adversely reduces the total heat storage, heat storage rate, and volumetric heat storage density, and significantly increases the heat storage rates. As the charging temperature increases, the complete melting time is reduced and the charging performance indicators are improved, for the pure PCM and TPMS-PCM LHTES units.

Optimisation of Integrated Heat Pump and Thermal Energy Storage Systems in Active Buildings for Community Heat Decarbonisation

The electrification of residential heating systems, crucial for achieving net-zero emissions, poses significant challenges for low-voltage distribution networks. This study develops a simulation model to explore the integration of heat pumps within active building systems for community heating decarbonisation. The model optimises heat pump operations in conjunction with thermal energy storage units to reduce peak demand on low-voltage networks by using real-time measured electricity demand data and modelled heat demand data for 76 houses. The study employs an algorithm that adjusts thermal storage charging and discharging cycles to align with off-peak periods. Three scenarios were simulated: a baseline with unoptimised heat pumps, a fixed threshold model, and an active building model with daily optimised thresholds. The results demonstrate that the active building model achieves a 21% reduction in peak demand on the low-voltage substation compared to the baseline scenario; it also reduces the total electrical energy consumption by 12% and carbon emissions by 17%. The fixed threshold scenario shows a 16% improvement in peak demand reduction, but it also shows an increase in energy consumption and emissions. These findings highlight the potential of active buildings to enhance the efficiency and sustainability of residential energy systems, marking a significant step toward decarbonising residential heating while maintaining grid stability.

Numerical analysis of the combined influence of fin shape and location on constrained melting of phase change materials in a spherical capsule with double fins

AbstractThis study presents a novel approach to investigating the combined influence of fin position and shape on the constrained melting behavior of phase change material (PCM) within a spherical capsule (S.C.) through numerical analysis. Unlike previous research, which predominantly focused on single fin shapes or positions, this work uniquely explores the impact of double, simple, and easily manufacturable fin shapes. A two‐dimensional computational model employing the enthalpy–porosity method assesses melting behavior, temperature distribution, and PCM flow. Numerous fin shapes, namely rectangular, trapezoidal converging, trapezoidal diverging stepped, inverse stepped, and triangular, are considered in the analysis. The study reports the influence of the location of two identically shaped fins on the thermal performance. The fins' cross‐sectional area and base thickness are kept equal in all cases. The thermal performance of an S.C.‐integrated fin system is evaluated by analyzing various attributes such as total saving in the duration of melting, enhancement ratio, and Nusselt number. The results indicate that the position of the fins has a more significant impact on melting performance than the fin shape. The best performance is achieved when fins are placed in the lower half of the capsule, followed by the center and upper halves, regardless of fin shape. For rectangular fins, shifting the position of the fin from the bottom half to the center increases the melting time by 24.7% and the top half by 68.3%. The shortest melting time of 93 min is observed for lower‐half rectangular fins, followed by center‐placed triangular fins (94 min). This study offers a theoretical foundation for optimizing the performance of different technologies using latent heat thermal energy storage systems such as packed‐bed, cascaded thermal energy storage systems.

Enhancing Heat Storage Capacity: Nanoparticle and Shape Optimization for PCM Systems

ABSTRACTPhase change material (PCM)‐based heat storage systems utilize the absorption or release of latent heat during a phase change of the storage material to store thermal energy. Nevertheless, the effectiveness of these systems is restricted by the shape and structure of their confinement, as well as the heat conductivity of the storage material. This work investigates a novel method to enhance the effectiveness of PCM systems by concurrently utilizing two techniques: the inclusion of nanoparticles and the alteration of the system's geometry. Introducing nanoparticles enhances the thermal conductivity of the storage medium while altering the shape, which improves heat transfer efficiency by adjusting the surface area available for heat exchange. RT‐35 was tested for use in latent heat thermal energy storage systems for space heating and cooling. With a melting point of 35°C, RT‐35 was chosen to moderate building temperatures by storing and releasing thermal energy for space heating and cooling. The results indicate that using nanoparticles and adjusting shape can greatly enhance the effectiveness of PCM systems. By incorporating Al2O3 nanoparticles, the melting time of the PCM was reduced by 20% compared to the pure PCM, and it is more efficient than the best case of shape modification. These findings indicate that including nanoparticles and modifying the shape are effective methods to improve the performance of heat storage devices. This technology's potential surpasses this study's limits and can be utilized in diverse applications, including solar thermal energy storage, district heating and cooling, and industrial process heat.

Electrochemistry of Phase-Change Materials in Thermal Energy Storage Systems: A Critical Review of Green Transitions in Built Environments

This article reviews recent research on phase-change materials (PCMs) used in thermal energy storage systems with the aim of enhancing their performance. The study explores various methods to improve heat transfer in PCMs, such as microencapsulation, infill materials, fins and nanofluids. Additionally, it evaluates techniques to boost heat transfer in latent heat thermal energy storage (LHTES) systems and investigates ways to increase thermal conductivity using porous and low-density materials. PCMs store thermal energy, making them suitable for use in solar energy systems when solar energy is not available. The need for eco-friendly alternatives to conventional heating and cooling in global construction and the significant energy consumption of buildings has driven research on this topic. As such, this study additionally examines current advancements in free cooling systems with latent heat storage to identify the key factors affecting their effectiveness. The findings show that using PCMs for overnight cooling maintains the room temperature within the comfort zone and reduces the cooling loads in various climates. Using machine learning methods, this study also compares recent advancements in the use of PCMs in various solar energy systems, including solar thermal power plants, solar air purifiers, solar water heaters and solar appliances. Results derived feature key factors crucial for the optimal selection of PCMs and the challenges associated with sustainable green transitions in built environments. HIGHLIGHTS This review provides an in-depth examination of PCM thermal energy storage systems. This study investigates the use of fillers, nanofluids, and nanoparticles for enhanced heat transfer. Analytical methods for boosting the PCM thermal conductivity are briefly outlined. Geometric design and thermal conductivity enhancement affect the Latent Heat Thermal Energy Storage (LHTES). The assessment also evaluates advanced free-cooling systems using the LHTES. PCMs help to store heat energy in solar systems and bridge supply gaps. GRAPHICAL ABSTRACT

Investigation of Factors Affecting Corrosion Mechanisms in Latent Heat Thermal Energy Storage Systems

Concentrated Solar Power (CSP) plants integrated with Latent Heat Thermal Energy Storage (LHTES) systems offer a promising solution for dispatchability, reliability, and economic concerns generally associated with renewable energy technologies. These systems, however, require an operational life of up to 30 years to compete with power plant systems operating on fossil fuels. This is a significant challenge due to the high temperatures and corrosive eutectic salts utilised in LHTES systems. Additionally, these systems and its subcomponents are expected to be under varying degrees of stress due to the diurnal cyclic temperature variations inherent in the plant’s operational cycle. Hence, it is crucial to understand the various factors that can affect the operational life of materials used in such applications and conduct thorough material compatibility studies to assess the combined impact of these operating conditions on corrosion mechanisms. This study presents a novel static immersion test approach using modified Compact Tension (CT) specimens manufactured from 316L to investigate the effects of Na2CO3:NaCl (59.45:40.55 wt.%) salt, elevated temperatures (700 ℃ for up to 1000 hours), and stress on corrosion induced in the alloy. The post-exposure results are characterised with Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) shows that corrosion mechanisms are significantly affected by factors such as high operating temperatures leading to changes in both corrosion morphology and rate, high stresses causing localised preferential corrosion, as well as corrosive salt and oxygen availability affecting the type of corrosion induced.

Conditions for economic efficiency of latent heat thermal energy storage systems at nuclear power plants

In conditions of uneven schedules of electricity consumption and construction of power plants using renewable energy sources, nuclear power plants will participate in regulating the unevenness of daily energy consumption. At the same time, nuclear power plants have high economic feasibility of operating with a maximum installed capacity utilization factor due to significant capital investments at a relatively low fuel price. The use of nuclear power plants to cover uneven energy consumption will reduce the possibility of a return on investment, which will only be realized with greatly inflated capacity charges. At the same time, the annual operating costs will increase significantly. A possible decrease in the economic efficiency of a nuclear power plant is due to the fact that, at the request of the system operator of the energy system, the NPP will be to reduce electricity generation during off-peak hours or sell electricity on the market at significantly lower prices. In this regard, the problem of increasing economic efficiency of nuclear power plants under conditions of uneven daily energy consumption becomes particularly relevant. The solution to the problem could be the accumulation of excess energy and its subsequent use. Previously, the authors developed a method of additional redundancy of auxiliary needs of nuclear power plants based on the use of low-power steam turbines. In the present paper, schemes for increasing efficiency of using low-power steam turbines at nuclear power plants when regulating the load unevenness in the power system using thermal energy storage systems based on the latent heat thermal energy storage are developed. The design characteristics of the accumulator as part of a nuclear power plant are determined to ensure operation of the heat storage system throughout the entire period of energy consumption. In this research, the lithium nitrate was considered as a phase change material. The technical and system conditions under which the maximum economic effect is achieved have been determined. The boundary conditions under which a return on investment is achieved are shown.

A shell-tube latent heat thermal energy storage: Influence of metal foam inserts in both shell and tube sides

Enhancing heat transfer in latent heat thermal energy storage systems is of utmost importance to facilitate the efficient absorption and release of thermal energy. The primary objective of the current study is to investigate the influence of a metal foam layer on heat transfer enhancement in both the heat transfer fluid side and the shell side. The research explores the incorporation of a fixed 30 % foam layer, which can either extend along the tube wall or penetrate deeper into the shell, moving away from the heat transfer fluid tube. A local thermal non-equilibrium two-temperature heat equation model is utilized to mathematically model the interactions within the metal foam embedded in the heat transfer fluid tube and the phase change material domain. The governing equations are solved using the finite element method. The results indicate that adjustments to the inlet pressure and the metal foam shape parameter (FL) can significantly reduce the melting time, with a variation of approximately 40 %. Specifically, at a constant inlet pressure of 750 Pa, the energy storage power at a 90 % charging increases from 32.2 W (FL = 0.75) to 48.7 W (FL = 1.37). A modification of FL shape parameter increased the power output by 34 %.

Thermal effect of the anisotropic metal foam on the melting performance of phase change material: A pore-scale study

Recently, metal foams have been attractive in reinforcing the heat storage process of phase change materials (PCMs) beneficial from their high thermal conductivity, interconnected structure, and large specific surface. Nevertheless, the melting of PCM is not uniform caused by convective heat transfer, so it cannot be sufficiently accelerated by a traditional isotropic metal foam. In this study, we numerically investigated the melting performance of PCM in isotropic and anisotropic metal foams. Results illustrated that the X-anisotropic structure can more efficiently enhance the melting performance as compared to the isotropic original metal foam, but the Z-anisotropic structure suppresses it. Moreover, the X-anisotropy-0.5 shows the fastest melting, its complete melting time is significantly reduced by 7.99 % compared with the original model, and its heat storage power is 8.68 % higher than the original one. Instead, the Z-anisotropic models prolong the complete melting time by 11.10 % ˗ 28.55 %. It is owing that the anisotropic structure would promote the heat transfer along one direction but suppress its vertical direction. This article provides a feasible routine to accelerate the melting of metal foam-based PCMs, thus improving the heat storage power of the latent heat thermal energy storage system.

Numerical study and experimental validation of a porous biochar supported form stable composite for thermal energy storage

This study numerically investigates the performance of the melting process for a biochar-based phase change material latent heat energy storage system. The melting processes of a composite made from softwood biochar combined with paraffin RT28, were compared numerically using the equilibrium porous medium model in ANSYS Fluent. The set up was validated through results from previous literature and experimental work. The enhancement in melting was compared to the pure PCM melting scenario and the effects of different paraffins, boundary conditions and inclination angle were investigated for real world application. Softwood biochar composites significantly surpass the performance of paraffin alone, reducing the melting time by 79 %. Different paraffins within the softwood composite produced different melting times, ranging from 2565 s to 4790 s. In terms of inclination angle adjusting to 90° produced steadier heating and reduced the melting time by 63 % in the composite. These findings offer insights into optimising latent heat thermal energy storage systems for improved performance and reduced energy wastage, highlighting the potential of biochar composites in sustainable energy solutions.

Thermal performance of cascaded latent heat thermal energy storage units constructed based on solid-liquid interface information

A novel approach to constructing cascaded latent heat thermal energy storage units is proposed to enhance the thermal performance of horizontally placed shell-and-tube phase change heat storage systems. This method is based on the solid-liquid interface variation patterns during the solidification and melting processes. It creates interfaces between different phase change materials within the cascaded latent heat thermal energy storage unit to shorten melting time and increase thermal performance by optimizing the unit's internal structure to balance heat transfer rates across all areas. The enthalpy-porosity method was used to numerically study the phase change phenomenon. The inner tube of the units was set as an isothermal boundary, and the outer shell was insulated. Findings from this cascaded latent heat thermal energy storage system demonstrate that, compared to single-latent heat thermal energy storage units, the average heat transfer rates during solidification and melting increased by 27.3% and 36.7%, respectively. The average heat transfer rate during solidification improved by 11.7% compared to traditional concentric circular cascaded latent heat thermal energy storage units. These results indicate that this method can significantly enhance the thermal performance of latent heat thermal energy storage units, providing valuable insights for future structural optimization.

Numerical modeling of discharge modes and evaluation of the major characteristics of latent heat thermal energy storage at a nuclear power plant

Under conditions of uneven power consumption schedules, construction of renewable power plants and low maneuverability of nuclear power plants (NPPs), combining NPPs with latent heat thermal energy storage systems (LHTES) can improve efficiency and maneuverability of NPPs. Combination of the LHTES with an additional low-capacity steam turbine will allow not only meeting peak electricity loads, but also increase the NPP safety by additional reservation of its own needs using this multifunctional turbine in emergency situations of complete blackouts. A scheme for combining a PCM-based thermal storage system with a two-circuit nuclear power plant is proposed, where the LHTES is charged with part of fresh steam from steam generators in the night hours of off-peak loads in the power grid and heating feed water above the nominal temperature during the peak loads. Non-stationary heat transfer between the heat storage material and feed water during the LHTES discharge was modeled using the finite volume method. The design characteristics of the LHTES as part of the NPP and the weight-size parameters of the thermal accumulator have been determined. The rational phase change material and the types of finned tubes have been determined. A solution has been proposed to ensure stable water temperature at the outlet of the PCM-based thermal accumulator during its discharge within the NPP that will significantly increase efficiency of its performance. Heat losses from the LHTES to the environment during the day have been calculated.

Sorption kinetics of salt-in-porous-matrix composites: The effect of expanded natural graphite on cooling power

Sorption heat transformers and thermal energy storage systems are emerging technologies that utilize and store low-grade waste heat for heating and cooling applications. The performance of sorption systems is not only affected by systems’ operating conditions, and overall systems’ design but also by sorption material or composite parameters such as thermal diffusivity, composition, and pore structure, among others. In this study, CaCl2-based salt-in-porous-matrix composites of different compositions and coating thicknesses were synthesized. During synthesis, salt to silica gel and polyvinyl alcohol to silica gel ratios were fixed and the thermal additive (expanded natural graphite) to silica gel ratio was varied with care from 0 to 0.26 (or 0 to 20.5 wt.%, additive to silica gel ratio). The thickness of samples varied from 2.3 to 8.3 ± 0.1 mm. The composites were characterized by a transient plane source (thermal conductivity and thermal diffusivity), nitrogen adsorption porosimetry (specific surface area and total pore volume), and thermogravimetric sorption analysis (water sorption equilibrium) methods. A custom-built gravimetric large pressure jump (G-LPJ) testbed was used to study water sorption kinetics (water uptake vs. time) for selected samples. The thermal conductivity and diffusivity of the studied composite samples have shown significant enhancements, e.g., 240% (0.11 W/(m·K) vs. 0.37 W/(m·K)) and 310% (0.21 mm2/s vs. 0.87 mm2/s), respectively, by adding 12.5 wt.% expanded natural graphite (additive to silica gel ratio is 0.14) as a thermally conductive additive (additive to silica gel ratio) because of thermal percolation effect. This ratio of expanded natural graphite to silica gel was found to be optimal for studied composition. The results indicate that sorption composites with higher thermal diffusivity offer notably higher specific cooling power and improved sorption kinetics, compared to the composites without expanded natural graphite of the same thickness (850 W/kg vs. 480 W/kg at 70% water conversion for samples with thickness of 5.3 mm).

Surrogate-based multi-objective design optimization of tree-shaped fins with uniform branch end distribution for latent heat thermal energy storage

The enhancement of latent heat thermal energy storage (LHTES) systems through fin geometry optimization remains a critical challenge for leveraging the full potential of renewable energy sources. This study focuses on optimizing the geometries of tree-shaped fins to enhance power and energy densities in LHTES systems. The goal is to find branch designs with high energy and power density through a novel surrogate model-based optimization strategy that explores a broad design space. The surrogate models applied, including linear regression, principal component analysis-based linear regression, artificial neural networks, and random forest, are evaluated for their predictive performance. The random forest model demonstrates superior accuracy in predicting targets. The optimization process results in a Pareto-optimal design with a volume fraction of 33.9%. This optimal design substantially enhances the system's power density by 61.6% compared to conventional plate fins at an equivalent energy density. This optimized design improves energy and power density, achieving a uniform end-to-branch distribution, which is a pivotal factor for consistent temperature distribution and improved thermal efficiency. By integrating surrogate-based optimization with broad ranges of the tree-shaped fin design, this research has significantly improved the operational efficiency of LHTES systems. This research promises more effective thermal management and provides a methodological framework for design innovation in thermal energy storage.

Enhancement of heat transfer through the incorporation of copper metal wool in latent heat thermal energy storage systems

The design of thermal energy storage (TES) tank is the key part that can limit charging and discharging process. Most research findings highlight that the use of fins augments the heat transfer rate. This work experimentally investigates the use of aligned copper wools as fillers to enhance the thermal performance of a lab-scale shell-and-tube TES tank filled with phase change material (PCM). Two copper wools with different fibre thicknesses were chosen and discretely laid around the TES tank tubes in two design patterns. Accordingly, five shell-and-tube TES tank configurations were obtained, including the reference, for performance evaluation. The TES tank was loaded with n-octadecane as PCM for all the cases studied. The results showed up to a 16 % reduction in melting time with the inclusion of copper wool. The TES tank significantly increased the mean power during charging (53 %) and discharging (205 %). The addition of metal wool into the TES tank enables the PCM to release the heat at a constant temperature during the entire phase transition process. And the overall efficiency of the TES tank was found to get improved. Therefore, a copper wool integrated TES tank would be a beneficial addition to thermal energy storage systems.

Improving the performance of a shell and tube latent heat thermal energy storage through modifications of heat transfer pipes: A comprehensive investigation on various configurations

The modification of the geometric configurations of heat transfer pipes in shell and tube Latent Heat Thermal Energy Storage (LHTES) systems not only enhances the melting process of the phase change material (PCM) but also improves the overall performance of these systems. This study aims to investigate ways to enhance the performance of LHTES systems by employing heat transfer pipes with various fin and twisted tape arrangements in a horizontal orientation. The Finite Volume Method and Enthalpy-Porosity method are employed to simulate the melting process. Stearic acid is used as the PCM material, while water serves as the heat transfer fluid. Eight different geometric configurations are modelled in the LHTES system: base case, horizontal fins, vertical fins, helical fins, horizontal tape, vertical tape, twisted tape and helical fins with twisted tape. The results show that within the time range of 0 and 29 min, the combined configuration of helical fins with twisted tape consistently demonstrates the fastest melting process. After 29 min, the configuration with vertical fins exhibits a marginally faster melting process than the combined configuration of helical fins with twisted tape. The configurations involving tapes also contribute to accelerated melting, although to a lesser extent than those with fins. Particularly, twisted tape proves highly effective in facilitating faster melting. The complete melting process times for configurations with vertical fins, helical fins, and combined helical fins with twisted tape are 38.7 %, 23.5 % and 32.7 % faster compared to the base case which is ∼69 min. Among the configurations, using tapes results in higher flow resistance and surface area compared to the base case. The attractive features of these configurations make them ideal for creating efficient and space-saving energy storage systems. This study provides crucial insights into essential heat and mass transfer processes, which can be leveraged to develop advanced LHTES systems for enhanced performance and sustainable energy solutions.

Integration of solar thermal collectors and heat pumps with thermal energy storage systems for building energy demand reduction: A comprehensive review

Solar energy, coupled with innovative technologies, holds the promise of propelling buildings towards net-zero and carbon neutrality. In this regard, this review explores the integration of solar technologies, heat pumps, and thermal energy storage systems to reduce building energy demand. It thoroughly examines various types of solar thermal collectors (STCs), including both concentrating devices like compound parabolic concentrators and parabolic troughs, as well as non-concentrating designs such as flat plate and evacuated tube collectors. It examines innovative strategies for enhancing STC performance, such as alternative profiles for compound parabolic concentrators and the insertion of thermal fins in absorber tubes. Furthermore, the review categorizes solar-assisted heat pumps (SAHPs) into indirect expansion (IDX) and direct expansion (DX) systems, describing their advantages and limitations. It performs a comparison between IDX-SAHPs and DX-SAHPs, explaining their effectiveness and applicability. Eventually, the review explores thermal energy storage materials, categorizing them into sensible heat storage, latent heat storage, and thermochemical heat storage (TCHS). It discusses the advantages and disadvantages of each category, as well as notable materials within them. A comparative examination between open and closed systems for TCHS further enriches the discussion. Throughout the review, recent advancements and key findings from relevant studies are summarized in separate tables, offering valuable information into practical strategies for sustainable building energy systems. Regarding the provided sections, this review plays a crucial role in introducing scholars to practical methods employed in recent years to integrate the STCs with heat pumps and thermal energy storage systems.