Latent Thermal Energy Storage Research Articles

Potential of the Use of Sodium Chloride (NaCl) in Thermal Energy Storage Applications

ABSTRACTEnergy storage is a group of technologies that decrease the gap between energy supply and energy demand. Thermal energy storage (TES) reduces this gap at not only different temperatures but also at different places or power. Design criteria include the integration of the storage system into the whole application system, maximum load, and high‐energy density. Since energy density is given by the material used, the selection of the storage material is key to the success of any energy storage system. In this paper, the potential of sodium chloride (NaCl) to be used in TES, both using sensible TES and latent TES, is evaluated for low‐temperature applications and for high‐temperature ones. This material is found to have high potential for successful applications, both in buildings and in industry.

A novel photocurable polymeric solid–solid phase change material for intelligent temperature regulators

The advancement of solid–solid phase change materials (SSPCMs) with crystalline segments embedded onto crosslinked polymer backbones offers a promising solution to the leakage problems associated with traditional phase change materials. Despite its potential, this technology faces challenges, including energy-intensive synthesis processes of SSPCMs and difficulties in custom designing materials for confined spaces. Herein, we introduce a novel strategy for the rapid preparation of polyurethane acrylate-based SSPCMs via UV-initiated polymerization. By leveraging crystalline segments from polyethylene glycol, the resulting SSPCM exhibits an impressive enthalpy of 110.4 J⋅g−1, enabling seamless transitions from rigid to soft states during solid–solid phase changes. When applied in portable electronic devices, this SSPCM can be synthesized in-situ to meet various size requirements, demonstrating exceptional temperature regulation performance based on latent thermal energy storage mechanisms. Furthermore, by integrating thin layers of carbon nanotubes (CNTs) onto the rough surfaces of SSPCM films and employing a face-to-face configuration to convert temperature-sensitive mechanical changes into electric resistance variations, we have engineered high-temperature alarms for SSPCM-based temperature regulators. The advantages of this preparation strategy, combined with the thermal regulation capabilities and innovative temperature alarm design, underscore the significant potential for intelligent and adaptive applications of SSPCMs in advanced technological landscapes.

Thermal analysis of packed bed thermal energy storage system with dimpled spherical capsules

The thermal storage potential of a packed bed filled with paraffin wax capsules was examined. Heat transfer fluid (HTF) at 70 °C inlet temperature for dimpled and plain stainless-steel capsules was compared for three different flow rates, 1 L/min, 3 L/min and 5 L/min. In general, dimpled spherical capsules sustain more heat than plain spherical capsules at the same flow rate, according to instantaneous and cumulative heat stored. Spherical capsules with dimples exhibited a 30 % improvement in charging rate compared to smooth spheres, particularly at elevated flow rates. Specifically, at a flow rate of 5 L/min, dimpled capsules achieved faster charging and sustained higher temperature compared to plain capsules. Higher flow rates (3 and 5 L/min) result in steeper temperature increases and an earlier peak temperature phase compared to the lower flow rate (1 L/min). Key dimensionless heat transfer numbers were examined. The Stefan number was found to be 0.1966 for an HTF inlet temperature of 70 °C and a latent heat of fusion (Lm) of 213 kJ/kg. Heat transfer increases when Reynolds numbers increased from laminar (123.8 at 1 L/min) to turbulent (619.1 at 5 L/min). Rayleigh number decreased over time but increased for dimpled capsules, indicating improved convection. Dimpled capsules had higher Nusselt numbers and increases with flow rate, indicating better convective heat transfer. Dimpled capsules absorbed heat faster and stored more energy, giving them a viable and efficient Packed Bed Latent Thermal Energy Storage system design.

Melting and solidification in periodically modulated thermal convection

Melting and solidification in periodically time-modulated thermal convection are relevant for numerous natural and engineering systems, for example, glacial melting under periodic sun radiation and latent thermal energy storage under periodically pulsating heating. It is highly relevant for the estimation of melt rate and melt efficiency management. However, even the dynamics of a solid–liquid interface shape subjected to a simple sinusoidal heating has not yet been investigated in detail. In this paper, we offer a better understanding of the modulation frequency dependence of the melting and solidification front. We numerically investigate periodic melting and solidification in turbulent convective flow with the solid above and the melted liquid below, and sinusoidal heating at the bottom plate with the mean temperature equal to the melting temperature. We investigate how the periodic heating can prevent the full solidification, and the resulting flow structures and the quasi-equilibrium interface height. We further study the dependence on the heating modulation frequency. As the frequency decreases, we found two distinct regimes, which are ‘partially solid’ and ‘fully solid’. In the fully solid regime, the liquid freezes completely, and the effect of the modulation is limited. In the partially solid regime, the solid partially melts, and a steady or unsteady solid–liquid interface forms depending on the frequency. The interface height can be derived based on the energy balance through the interface. In the partially solid regime, the interface height oscillates periodically, following the frequency of modulation. Here, we propose a perturbation approach that can predict the dependency of the oscillation amplitude on the modulation frequency.

Enhancing the melting rate of RT42 paraffin wax in a square cell with varied copper fin lengths and orientations: A numerical simulation

Latent thermal energy storage units are especially preferred in renewable thermal energy systems for their significant capacity to store heat. Nevertheless, the phase change materials in these units have low thermal conductivity, prompting researchers to tackle this problem using several methods, such as incorporating fins. This numerical study contributes to improving the charging rate of a square cell (50 × 50 mm) used paraffin wax RT42 as phase change material by adding 4 copper fins inside, 1 mm thick and of different lengths. The fins were positioned on the left wall, where a constant-temperature heat flux was applied, while the other walls were thermally insulated. Four configurations of the cell were examined: without fins, with 10 mm length fins, with 20 mm length fins, and with 30 mm length fins, respectively. The results demonstrated that the presence of fins significantly improved the charging rate of the square cell. Compared to the baseline configuration without fins, the time required for the paraffin wax RT42 to completely melt was reduced by 22.22 %, 33.33 %, and 55.56 % with fins of 10 mm, 20 mm, and 30 mm lengths, respectively. Configuration four was also compared with a setup where similar fins were positioned on the bottom wall while maintaining the heat flux on the left wall. It was observed that the total melting time doubled compared to configuration four, suggesting that the optimal placement for the heat flux is on the wall where the fins are located. The study contributes to enhancing and developing the charge rates of square and rectangular PCM cells such as these used for cooling PV panels.

Flexible loofah sponge-based phase change composite for thermal protection and wearable thermal management

To address the escalating demand for sophisticated thermal management in electronics, energy batteries, and wearable devices, extensive research has been dedicated to developing shape-stabilized and highly thermally conductive phase change composites (PCCs). However, their inherent rigidity and limited flexibility pose significant challenges for practical applications. In response, we have engineered a novel and flexible PCC by integrating loofah sponge (LS) and styrene-isoprene-styrene block copolymer (SIS) as co-supporting framework with paraffin wax (PW) as heat storage medium, further enhanced with carbon nanotubes (CNTs). The process began with the vacuum adsorption of PW into an ammonia-softened LS, which forming a fibrous network. This was followed by the introduction of a mixed melt of SIS and PW, physically cross-linked and dispersed with CNTs, into the LS fibrous network. The composite was then cooled and shaped to produce the (final) PCC. The resultant PCC exhibited remarkable ductility with an elongation of approximately 780 % and a maximum tensile strength of ∼2 MPa. Furthermore, it maintained thermal reliability and dimensional stability over long-term usage, with a melting heat enthalpy as high as 146.4 J/g. This flexible PCC, which combines high latent thermal energy storage, temperature regulation, and thermally induced assembly properties, shows considerable promise for thermal protection and portable thermal management in electronic devices.

Thermal conductivity enhancement of polyethylene glycol/NF composite as stabilized phase change materials for thermal energy storage

Polyethylene glycol (PEG) has the advantages of large latent heat of phase transition, a wide range of phase transition, and cheap price, but the application of PEG in the latent thermal energy storage (TES) systems is hindered due to its lower heat transfer rate and leakage. Porous metal foam has excellent thermal conductivity and high strength, which can effectively improve the problems of PEG. In this study, PEG/nickel foam (NF) composite phase change materials (PCM) were prepared by vacuum molten impregnation method using NF as substrate. It was found that PEG had good compatibility with NF, and the highest impregnation rate of PEG6000/NF reached 85.73 %. The thermal conductivity of PEG2000/NF can be increased to 1.58 1.58 W·m−1 K−1, nearly 4 times larger than that of pure PEG. The phase change temperature range of the composite PCM is 51–63 °C, and the phase change enthalpy is 67.8–126.3 J·g−1. Furthermore, the PEG/NF composite demonstrated outstanding photothermal conversion properties and high thermal stability, which can be widely used in the field of building temperature control and device heat dissipation.

A multi-valve flexible heat pump system with latent thermal energy storage for defrosting operation

There has been growing interest in using air-source heat pumps as an alternative to potentially replace fossil-fuel-based heating technologies for heat decarbonisation. One of the technical issues that hinders the wide uptake of air source heat pumps is the decreasing energy efficiency caused by frequent defrosting operations under low ambient temperatures. Currently, the most widely used defrosting method is the reverse-cycle defrosting but this operation would have to interrupt the heating supply during the defrosting process. The authors recently proposed, developed, and demonstrated a flexible heat pump concept that recovers partial of the subcooled heat from the hot refrigerant exiting the condenser and stores it in an integrated thermal storage. The stored heat could later be used to save compressor power and defrosting applications. In this paper, we proposed an innovative method by using a multi-valve design of the flexible heat pump to achieve the defrosting function. In this new concept, defrosting is achieved by condensing the refrigerant inside the evaporator and simultaneously ensuring uninterrupted heating during the heat storage discharge process. A model has been established to study the heat pump performance during a full charge/discharge cycle of the storage. Results indicated that with R410a, R134a and R1234yf, the proposed flexible heat pump has the potential to respectively save up to 11.2%, 10.8% and 13.2% of compressor power, in comparison with a heat pump using the conventional reverse cycle defrosting method. Concurrently with the defrosting operation, the same heating capacity is ensured without interruption by the flexible heat pump.

Melting performance improvement of phase change materials with thermal energy storage unit using nanoparticles

The world is facing two major problems today, as imbalance of supply and demand of the energy and the continues deterioration of the environment. Latent thermal energy storage with phase change material plays a vital rule in resolving this problem. The current study investigates the numerical simulation of phase change material with novel fins configuration in the triplex-tube storage unit. But their low thermal conductivity is the main problem by affecting the energy storage. To enhance the heat transfer rate within the system, novel longitudinal triangular and traditional rectangular fins with a combination of hybrid nanoparticles (Cu-TiO2) are employed. In summary, utilizing novel fins to improve heat transfer rate in conjunction with nanoparticles is highly effective for improving the phase change material melting process in thermal energy storage systems. The usage of nanoparticles speeds up the melting rate 33.5%- Case A, 20%-Case B, 19.4%-Case C, and 18% for Case D. Moreover, when compared to rectangular-shaped fins, the use of longitudinal triangular -shaped fins accelerates the PCM melting rate. For instance, the melting rate in Case C (fins with longitudinal triangular form) is higher than in Case F (fins with a rectangle shape).

Improvement of Latent Heat Thermal Energy Storage Rate for Domestic Solar Water Heater Systems Using Anisotropic Layers of Metal Foam

Latent Heat Transfer Thermal Energy Storage (LHTES) units are crucial in managing the variability of solar energy in solar thermal storage systems. This study explores the effectiveness of strategically placing layers of anisotropic and uniform metal foam (MF) within an LHTES to optimize the melting times of phase-change materials (PCMs) in three different setups. Using the enthalpy–porosity approach and finite element method simulations for fluid dynamics in MF, this research evaluates the impact of the metal foam’s anisotropy parameter (Kn) and orientation angle (ω) on thermal performance. The results indicate that the configuration placing the anisotropic MF layer to channel heat towards the lower right corner shortens the phase transition time by 2.72% compared to other setups. Conversely, the middle setup experiences extended melting periods, particularly when ω is at 90°—an increase in Kn from 0.1 to 0.2 cuts the melting time by 4.14%, although it remains the least efficient option. The findings highlight the critical influence of MF anisotropy and the pivotal role of ω = 45°. Angles greater than this significantly increase the liquefaction time, especially at higher Kn values, due to altered thermal conductivity directions. Furthermore, the tactical placement of the anisotropic MF layer significantly boosts thermal efficiency, as evidenced by a 13.12% reduction in the PCM liquefaction time, most notably in configurations with a lower angle orientation.

Dynamic thermal performance analysis and experimental study of cascaded packed-bed latent thermal energy storage integrated solar trough collectors

The instability of the renewable energy significantly impacts the thermal performance of solar thermoelectric systems. In this paper, a coupling system consisting of solar trough collector and double-layer cascaded packed-bed latent heat storage system (PLTES) is constructed to investigate thermal performance and operating parameters under dynamic conditions. Additionally, the experimental platform was upgraded to evaluate the dynamic charging/discharging process of the cascaded PLTES. The results show that under dynamic inlet heat flow, the PLTES exhibits only one peak temperature difference, and the average temperature is lower than that in the steady-state. The cascade-packed structure promotes a more uniform temperature and liquid phase distribution, resulting in a longer heat flow output. Higher mass flow rates of the heat flow increase convective heat transfer and reduce the temperature difference, though they have little effect on the energy efficiency. Conversely, an increase in capsule diameter significantly reduces the energy efficiency. Experimental results indicate that under dynamic conditions, the total charge-discharge capacity of the cascade system decreases by 0.5 MJ and 0.9 MJ, and the energy efficiency decreases by 0.15. Nevertheless, the energy efficiency of the cascaded PLTES (0.57) is higher than that of the single layer (0.5).

Review of organic and inorganic waste-based phase change composites in latent thermal energy storage: Thermal properties and applications

Systematization and analysis of standalone waste materials that can serve as phase change materials (PCMs), and composites consisting of commercially available PCMs combined with organic and inorganic waste materials is presented. Within the conducted review research, obtained thermal properties of so far investigated waste-derived PCMs were presented, assessing, among other, their latent heat storage capacity, thermal conductivity, and thermal and cyclic stability. Refined coconut oil and Allanblackia oil exhibited exceptional thermal and cyclic stability along with high values of latent heat, amounting 105 kJ kg−1 and 81 kJ kg−1, respectively. In some cases incorporation of waste materials significantly improved thermal properties, e.g. the addition of carbonized waste tire rubber to dodecyl alcohol increased its thermal conductivity to 0.431 W m−1K−1, representing an increase by a factor of 2.3. This enhancement led to a 17.2 % reduction in heating times and a 20 % reduction in cooling times compared to using pure PCM. Besides providing extensive data on thermophysical properties of phase change composites, the study provides a comprehensive overview of their applications, economic feasibility, and environmental implications. This extensive review shows the potential of waste-derived PCMs to contribute to a circular economy by repurposing waste materials for sustainable energy solutions and reducing environmental impacts. The findings indicate that while valorising wastes or by-products as latent thermal energy storage materials is feasible, more research efforts are required towards potential commercialization.

On the numerical simulation of a phase change material melting inside periodic structures: from validation to optimization

Nowadays, numerical models are considered powerful design tools, which are largely used in several engineering applications. When applied to heat transfer applications, they have already been shown to be reliable, especially in single phase systems. When considering two-phase heat transfer, and in particular solid-liquid phase change processes, the numerical tools must be carefully handled. Thus, this paper follows the critical literature on the application of numerical methods to simulate and optimize latent thermal energy storages to show how, in most cases, the common geometrical and numerical approximations should not be applied because they might lead to misleading solutions. Starting from reliable experimental results collected during melting process of a paraffin wax inside 3D periodic structures, different simulation approaches are proposed, and their advantages and disadvantages are discussed. The results show that, when dealing with certain complex 3D structures, 2D approximations cannot reliably capture the phase-change phenomenon. The validated tool also allowed to identify the optimum pore size to maximize the solid-liquid phase change. This work can be considered as a reference for future numerical studies on the solid-liquid process inside 3D structures.

A review on thermal energy storage with phase change materials enhanced by metal foams

Due to the mismatch between energy demand and energy production, in the last years the study of latent thermal energy storage systems has been significantly growing. This work aims to provide an overview of the studies and the experiments carried out on thermal energy storage systems, that use phase change materials combined with metal foams. A histogram that shows how the number of works on these subjects has been increasing over the years is reported; consequently, a general survey on phase change materials is conducted, identifying the temperature ranges and the latent heat which are most used in thermal energy storage systems and defining the strengths and weaknesses of the various types of phase change materials. Furthermore, the paper illustrates the models and methods currently used to describe the phase transition of phase change materials in metal foams. Subsequently, how metal foams improve the desirable characteristics of phase change materials is described, and then it is represented in what percentage the different pore sizes (PPI) are used and what the most functional porosity is for individual works. Lastly, through the summary of the analyzed papers some specific articles are considered, in order to explain the state-of-the-art and to clarify some fundamental concepts.

Charge/discharge Profiling of Organic Latent Heat Material Embedded in Polymer Matrix for Heat Energy System

The operational aspect of a latent thermal energy storage (LTES) system is highly dependent on the performance of its latent heat storage material (LHSM). In this work, performance evaluation of the LHSM is conducted using low-density polyethylene (LDPE) and linear LDPE. As an initial assessment, three different organic LHSMs are used for the evaluation. The thermal assessment indicates that the heat of fusion for the LHSM varies between 142.45 J/g and 198.8 J/g. Thermal decomposition demonstrates that the LHSM is suitable for operating a thermal power system under 100 °C. The addition of polymer clearly influences the charge/discharge rate for each LHSM. For example, it has a positive influence on paraffin wax, which increases the charge rate up to 54.4 %. A positive outcome is also observed for palmitic acid, which has the highest charge rate at around 4.4 °C/min. Interestingly, the charge/discharge rate for stearic acid is decreased by the presence of a polymer. It implies that the melting/solidification rate can be decreased, which may be advantageous for passive thermal systems. In general, the addition of LDPE/LLDPE alters the performance of LHSM. It makes the polymer suitable to be taken as a stabilizer for the LHSM.

Progress and challenges of latent thermal energy storage through external field-dependent heat transfer enhancement methods

The development of Energy Storage technologies is critical to achieving a cleaner energy future. As one of the most widely used energy storage technologies, Latent Thermal Energy Storage (LTES) still suffers from poor charging and discharging performance subjected to the low thermal conductivity of Phase Change Materials (PCMs) and inefficient heat transfer process. Heat transfer enhancement techniques, such as using fins and adding highly conductive materials, have been widely developed and optimized to overcome these challenges. Recently, additional novel methods integrating adjustable external fields such as gravity, magnetic field, and electric field have been proposed to enhance the heat transfer performance of LTES, due to their advantages including easy adjustment of the field parameters according to the evolution of the heat storage process. Firstly, this work briefly summarizes the progress of conventional heat transfer enhancement methods. Secondly, the advancement of heat transfer enhancement by integrating adjustable external field effects has been analyzed and discussed. Finally, the potential of external fields to improve the heat storage performance of LTES under fluctuating thermal sources is discussed considering the wide existence of fluctuating energy supply and load in real applications. The main contributions from this review are: (a) The emerging heat transfer enhancement methods for LTES are comprehensively summarized and discussed for the first time; (b) The potential of different heat transfer enhancement techniques is demonstrated to overcome the poor heat storage performance under fluctuating thermal sources when considering real applications of LTES.

Parametric analysis of system performance and cost of heating systems with heat pump and latent thermal energy storage

This paper presents a parametric analysis examining the impact of phase change material (PCM) melting temperature, latent thermal energy storage (LTES) volume, and solar collectors’ surface area on the performance and costs of heating systems integrating a heat pump and LTES by using dynamic system simulations. The computational models, implemented in TRNSYS software, and the numerical procedure were validated by comparing the numerical results with experimental measurements. The computational models represent the entire heating systems and thus consider the mutual interaction of heat production, distribution and automatic regulation system as well as a building which represent the heat load, in transient boundary conditions. Two main system designs with different LTES roles were considered – a system in which the role of the LTES is to provide more favorable operating conditions and a system in which the LTES role is to store the heat on a sufficiently high temperature to be used directly for heating. System performance was analyzed in terms of the share of renewable energy in total delivered energy, power consumption, stored energy in LTES, seasonal performance factor, average solar collector efficiency and total annual costs. The results showed that, regarding the system in which LTES was used as a heat source for the heat pump, a better overall performance and lower costs were observed in systems with PCMs with melting temperatures in range between 19 and 29 °C, compared to systems with PCMs with melting temperatures outside this range. For system in which the heat from the LTES was used directly for heating, the highest values of system performance criteria and the lowest cost were achieved with the PCM of 54 °C melting temperature. Almost all of the considered system performance criteria and total costs were higher when the LTES volume and solar collector surface area were larger as well.

X-ray computed tomography analysis of calcium chloride hexahydrate solidification

Latent Thermal Energy Storages (LTESs) are receiving increasing interest, since they offer many advantages to the field of Thermal Energy Storages (TESs), such as a nearly isothermal solid–liquid phase transition and a high energy storage density. To boost the transition towards a climate-neutral future, a lot of work is surely needed on Phase Change Materials (PCMs) which are used inside LTESs. Salt hydrates, a particular class of PCMs, are very attractive in terms of cost and volumetric latent enthalpy but present drawbacks that still limit their effective deployment in real applications, such as supercooling and phase separation (or segregation). The current work assesses the suitability of X-ray Computed Tomography (XCT) as a novel technique in the field of LTESs to study the solidification of phase-change materials. In this study, calcium chloride hexahydrate (CaCl2·6H2O) is chosen as a representative PCM and the results derived are shown to be repeatable. For the first time, the direct measurement of the volumetric liquid fraction is correlated to temperature measurements allowing for the visualisation of supercooling and the validation of an enthalpy-porosity numerical model, which could be useful in the design of LTES. XCT methodology offers unmatchable features as compared to the other method to measure the volumetric liquid fraction. Given its accuracy and the rigorous protocol, it can be considered as a benchmark for new liquid fraction measurement technologies but it also permits to study how the different PCMs melt and solidify, giving new insights on the mechanisms at the basis of the most crucial challenges, among those: subcooling, segregation, and metastable phase change.

Performance improvement of novel latent thermal energy storage system with two-layer metal foam porosities

A 2D numerical analysis was conducted to examine the melting performance of a shell-and-tube-type phase change material with a two-layer copper foam having higher and lower porosity levels. The design parameters for these two layers of metal foam consisted of three different volume fill ratios (VFR) (30 %/70 %, 50 %/50 %, and 70 %/30 %) and three different shapes (circular, semi-circular, and elliptical) for the lower porosity layer. The results showed that a semicircular-shaped low porosity layer with a 30 % VFR had the least melting time of 475 s. When the VFR increased to 50 %, the elliptical and circular shapes had the least melting time of 394 s. However, at a VFR of 70 %, only the elliptical shape had the least melting time of 323 s. Furthermore, the results were compared with the uniform porosity metal foam system, and deduced that the two-layer system did not always have the lower melting time compared with that of the uniform porosity system because it depended on the VFR and shape of the different porosities filled in the different layers. Additionally, for an average porosity of the metal foam, an optimal combination of two different porosities was observed for the melting performance.

Carbon-decorated diatomite stabilized lauric acid-stearic acid as composite phase change materials for photo-to-thermal conversion and storage

Diatomite is a promising supporting matrix in the preparation of composite phase change material for latent thermal energy storage. Raw diatomite has the advantages of porous space but with a shortage of thermal conductivity than carbon materials. In this work, the carbon-decorated diatomite (DC) matrix was synthesized by a novel template-carbonization method at different calcination temperatures of 600 °C, 800 °C, and 1000 °C. Then, the matrix was used to stabilize binary lauric-stearic acid of LA-SA for the preparation of diatomite-based composite phase change material which is used to explore the application of composite in photo-to-thermal conversion/storage. Results show that the carbon-decorated process on diatomite causes an increment of specific surface area and pore space. Thus, more phase change material being loaded in the matrix leads to a larger heat storage capacity of the composite. The designed composite with a matrix carbonization temperature of 1000 °C, LA-SA/DC-1000, has a loadage of 49.92 %, melting temperature of 31.48 °C, and latent heat of 70.07 J g-1 which is improved by 23 %. Compared with the raw diatomite-based composite, the enhancements of thermal conductivity were 90 % for that of LA-SA/DC-1000. Moreover, the corresponding composite exhibits better photo-to-thermal conversion performance and transient temperature response.