Latent Heat Thermal Storage Unit Research Articles

Partitions influence on rectangular latent heat thermal energy storage unit performance during melting and solidification

The growing use of renewable energy sources demands efficient storage solutions due to their variability. Thermal energy storage systems utilising phase change materials are emerging as viable options to address this challenge. This study evaluates the impact of various partition types on phase change duration in thermal energy storage systems. The well-known and validated enthalpy-porosity algorithm implemented in the Fluent 2021R2 software was used. The shortest melting time was observed in the case with four vertical tubes and three horizontal partitions, which was 82 % shorter than in the reference case. The highest change in specific enthalpy of phase change material (252.5 kJ/kg) during melting was also noted for this system. The shortest solidification time was observed for the case with four tubes and two diagonal partitions, which was 72 % shorter than in the reference case. The greatest increase in the specific enthalpy of phase change material (240.4 kJ/kg) during the solidification process was also observed in this system. In conclusion, due to the thermal conductivity of the partitions, the temperature distribution throughout the domain becomes more homogeonous, thus contributing to more uniform heat flux extraction and supply throughout the process.

Numerical investigation of performance enhancement in a PCM-based thermal energy storage system using stair-shaped fins and nanoparticles

This study numerically investigates the melting performance enhancement of phase change material (PCM) in a latent heat thermal energy storage (LHTES) unit using a novel stair-shaped fin and nano-enhanced PCM. Different fin configurations are designed and their thermal performance is compared to traditional straight fins, while the total mass of fins is kept constant. Nano-enhanced PCM is prepared by the inclusion of CuO nanoparticles into RT82 as PCM. The simulation results reveal that the stair-shaped fins can notably enhance the efficiency of phase change compared to the traditional straight fins. The configuration with more extended fin close to the lower zone of the latent heat thermal energy storage unit had the best thermal performance. The full PCM melting time for the most efficient stair-shaped fin configuration is reduced by 52% compared to the straight fins. The combined impacts of the most effective stair-shaped fin design and nanoparticles on the phase change melting performance are evaluated. The inclusion of nanoparticles with volume concentrations of 2% and 5% into the pure PCM leads to 61% and 65% reduction in charge rate, respectively, as compared to the straight fin with pure PCM.

Experimental investigation of thermal performance in a shell-and-tube phase change thermal energy storage unit with an inner square tube

Experimental investigations of phase change processes in a shell-and-tube latent heat thermal energy storage unit with an inner square tube were carried out. Paraffin OP44E was selected as a phase change material, and the water heated or cooled by constant temperature water tanks flowed into the inner square tube as the heat transfer fluid. Visualization experiments allow for the observation and analysis of the solid-liquid interface during the phase change process. Due to natural convection, the melting interface progresses from top to bottom, while the solidification interface advances from the center outward, as heat transfer is primarily governed by conduction. The experimental results demonstrated that the initial temperature of the PCM has a negligible impact on the thermal performance of the energy storage unit, particularly during the solidification process. The dimensionless analysis reveals the relationships of Nusselt numbers, Stefan numbers, and Reynolds numbers. Furthermore, four stages of conduction, mixed conduction-convection, convective enhancement, and convective attenuation were divided according to the effects of natural convection. Finally, empirical formulas for effectiveness were established to provide guidance for the heat exchanger design.

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.

Thermal energy storage, heat transfer, and thermodynamic behaviors of nano phase change material in a concentric double tube unit with triple tree fins

The contribution of this study is the proposal of a synergistic composite enhancement strategy involving tree fins and nanomaterials to improve the low thermal performance of phase change material (lauric acid) in a typical double tube latent heat thermal energy storage unit. The effects of nanoparticle concentrations and tree fin branching angles on the fluid dynamics, melting time, heat transfer, energy storage, and entropy generation characteristics were investigated. By employing tree fins, the melting time was respectively reduced by up to 60.20% and 36.05% compared to the finless case and the rectangular fins, while the energy storage power was respectively boosted by up to 143.36% and 37.45%. The increase of nanoparticle concentration was beneficial for melting heat transfer, and the dendritic fins with a bottom angle of 90° and a top angle of 30° yielded the highest energy storage performance. The total entropy generation of the melting process was mainly governed by the thermal entropy generation, and it showed a decreasing trend with the increase of time. In contrast to the finless configuration, the incorporation of fins diminished the integrated entropy generation value. Similarly, an elevation in nanoparticle concentration also led to a reduction of the integrated entropy generation value.

The effect of fin geometry on the thermal performance of a conical shell double tube latent storage unit

Thermal energy storage systems are widely used in a variety of applications. Latent heat thermal energy storage units are generally recognized as one of the most efficient ways to utilize and store renewable energy because of their high energy density and constant charging/discharging temperature. Conversely, the low thermal conductivity of phase change materials in these units limits their applications. The present paper investigates the performance of a novel double-tube phase-change unit of a conical shape. To aid the melting rate, fins are added to enhance heat transfer. A two-dimensional ANSYS computational fluid dynamics model has been developed. The results have been validated against published data from the literature with very good agreement. The study considered different geometrical modifications in terms of fin angle and fin length. Their effect on melting rate and energy stored was carefully discussed. Results have shown that longer fins accelerate the PCM melting rate. The melting rate was enhanced by up to 42.6% for the 20 mm fin. However, the best orientation angle varies with the considered fin length. Energy and power have revealed the best performance for a short fin of length 5 mm and an angle of 60° from the vertical.

A comparative analysis on charging performance of triplex-tube heat exchanger under various configurations of composite phase change material

In present work, the melting performance of triplex-tube latent heat thermal energy storage (LHTES) unit was numerically studied using equal volumes of PCM and metal foam composite PCM (CPCM) in various arrangements. For the n-eicosane (as PCM), the study was conducted at the fixed Rayleigh number (Ra) = 4.08x107, Prandtl number (Pr) = 62.9, and Stefan number (Ste) = 0.14. The results showed that positioning the metal foam on the bottom side and distributing segmented CPCM with alternating PCM zones effectively improved the system performance. Moreover, this also prevents the overheating of thermal layers in the LHTES unit. While the model labelled M2 exhibited the highest economic efficiency among all isotropic models, its low dimensionless thermal energy storage (TES) density (i.e., q’ ∼ 0.6) led this study to focus on models falling under the category having a TES density of ∼ 0.8. Compared to a pure PCM model, the configurations under equal volume ratio category demonstrated up to ∼ four times higher TES rate (p’) and the significant reduction of ∼ 75 % in melting time. The optimized isotropic model achieved the highest TES rate per unit cost with peak value of ∼ 3 at a price ratio (N) of 1. Lastly, the testing of metal foam anisotropy on the chosen design showed a substantial increase in melting/heat storage rates. The largest drop of ∼ 33 % in the total melting time was noticed for model M2 as compared to isotropic case.

Improving phase change heat transfer in an enclosure partially filled by uniform and anisotropic metal foam layers

Anisotropic metal foams exhibit exceptional potential for improving the efficiency of latent heat thermal energy storage (LHTES) units by enhancing heat transfer and thermal energy storage rates. This study explores the largely uncharted area of thermal behavior of anisotropic metal foams in varied configurations and enclosure designs. The research focuses on an LHTES unit with a specific channel configuration, constructed using copper and containing three layers of copper metal foam, one of which is anisotropic and infused with paraffin wax. The finite element method was utilized to manage the complexities emerging from phase change, and the anisotropic angle varied from 0 to 90° in three different placements of the AMFL: top, middle, or bottom of the enclosure. The ideal design was achieved with an AMFL in the middle with a 0°angle, resulting in a 3.7 % reduction in melting time and approximately a 2.3 % reduction in solidification time. However, AMFL designs at the middle placement with a 75° anisotropic angle were less effective, hampering the melting and solidification processes, thus potentially extending charging and discharging times. The study concludes that the placement and angle of the AMFL layer are vital for the heat transfer capabilities of phase change materials. AMFL at the middle with a 0° angle optimally leverages temperature gradients, enhancing heat transfer compared to other investigated cases. An AMFL in the middle with a 0° angle exhibits approximately 7.1 % and 6.3 % improvements for melting fractions of 0.9 and 0.95, respectively, underscoring its potential for efficient thermal energy storage.

Numerical study of thermal performance of a PCM in a modifying tube-bundle latent heat thermal storage

The main aim of this study is to numerically investigate the thermal performance of a newly designed modifying tube-bundle Latent Heat Thermal Storage (LHTS). The new LHTS design proposed a half-cylindrical tube attached to the Heat Transfer Fluid tube, which implies making the PCM indirect contact with the HTF tube. This reduces the heat transfer resistance by eliminating the metal construction of the tubes. For this purpose, a 3D numerical model was developed based on mass, momentum and energy conservations using ANSYS Fluent software. After validating the model developed by comparing numerical results with those obtained experimentally by a published study, the PCM liquid fraction and amount of power stored during the melting and solidification were studied. Water and lauric acid as a working fluid and PCM were implemented in the model. In addition, three different Heat Transfer Fluid (HTF) inlet temperatures 15°C,20°Cand25°C, and HTF’s flow rates in terms of Reynolds number 1000,1500and2000 were tested. The thermal behaviour of the present LHSU design was compared with that of the same volume double pipe LHTS unit. Results showed that the melting and solidification processes of the proposed storage were very uniform, with convection and conduction heat dominating PCM melting and solidification, respectively. The increase in the heat transfer contact area between the HTF and the tube bundle contributed significantly to the PCM melting and solidification acceleration, making each tube bundle behave as a single storage unit. Therefore, the time required for complete melting and solidifying of the PCM was half (50%) of the time needed for the same amount in the double pipe LHTS. Similarly, the melting and solidification time in the preset storage was shortened by about 22% and 41.6%, respectively, compared with the regular tube LHTS unit.

Natural convection and field synergy principle analysis of the influence of fins redistribution on the performance of a latent heat storage unit in a successive charge and discharge

The present study focuses on analyzing the impact of enhancing the heat exchanges on the shell side by surface extension on the kinetics of melting/solidification in a latent heat thermal energy storage (LHTES) unit. Particular attention is paid to the influence of fins redistribution on the performance of the LHTES unit, natural convection, and Phase Change Material (PCM) melting/solidification kinetics. The particularity of this study lies in the improvement of the LHTES unit performances in a successive charging-discharging operation by only redistributing the fins while keeping the same total heat transfer surface, PCM volume, and compactness. The first part of the study consists of evaluating the impact of fin redistribution for the same total heat transfer surface on the LHTES unit performance. Attention was paid to the fully charging and discharging times and the heat duty absorbed or released during the melting or solidification process. For this purpose, 4 configurations of LHTES units having 4 and 8 radial fins with the same total transfer surface were analyzed, with the aim of ensuring a maximum performance on charging process. In a second part, an improved configuration was proposed in order to minimize the successive charging and discharging time for a cyclic operation of the LHTES unit. The coordination factor derived from the field synergy principle, PCM liquid fraction, the heat duty exchanged, phase change kinetics, and liquid PCM velocity were used as performance key factors. They were used to analyze and compare the different configurations in order to find the best configuration for a successive charging-discharging operation. The selected improved configuration has been found to enhance both the charging and discharging processes. It leads to the reduction of the successive charging-discharging overall duration by 10 % to 47 % compared to the other configurations investigated with the same heat transfer surface. The enhanced LHTES unit developed in the present study is intended to improve the thermal comfort of buildings by valorizing and storing renewable and waste energies.

Numerical investigation and optimization of the melting performance of latent heat thermal energy storage unit strengthened by graded metal foam and mechanical rotation

In this paper, a combined passive graded metal foam and active mechanical rotation strategy is proposed to simultaneously solve the problem of slow melting rate and non-uniform phase change problem of the latent heat thermal energy storage (LHTES) technology. The enthalpy-porosity method and fixed grid structure are employed to numerically investigate the effects of porosity range, the number of graded layers, and the rotational angular velocity. Moreover, the response surface method (RSM) is further selected to optimize the structural parameters and obtain the function of melting performance and various factors. The result shows that there is an optimal porosity range and the total melting time decreases first and then increases with the increase of porosity range. When the porosity range is 12 % and the graded layer is three, it can shorten the melting time by 35.54 % and increase the thermal energy storage rate (TESR) by 51.57 %. In addition, the melting performance is gradually improved as the number of graded layers increases. The strengthening effect of graded metal foam with just four layers is close to that of the maximum layers considered in this paper. The trend of the influence of rotation speed on the melting performance of the LHTES unit is consistent with the porosity range, indicating the existence of the optimal rotational speed. The RSM optimization structure, with a 14.969 % porosity range, 5-layer graded number, and 2.267 rpm rotational speed, shortens the melting time by 42.42 % and increases the TESR by 62.75 % compared with the stationary case with uniform metal foam. This work verifies the effectiveness of active rotation coupling with passive graded porous structures, which could provide a new strengthening approach to enhance LHTES.

Performance of a rotating latent heat thermal energy storage unit with heat transfer from different surfaces

Due to the rapid growth in the demand for fast and efficient latent heat thermal energy storage system, multiple heat transfer enhancement techniques have been proposed and widely investigated. Actively or passively, rotation of the energy storage unit affects the internal natural convection and the heat transfer performance. Hence, this study is conducted to evaluate the potential of the heat transfer enhancement by rotation. A two-dimensional model with the phase change material filled in the annular space of a double tube energy storage unit is developed. The numerical simulations for the heat transfer through the internal or external surfaces of the annular space are respectively conducted. Both solidification and melting of the phase change material are taken into account by the enthalpy-porosity method. According to different heat transfer surfaces and the occurrence of solidification or melting, four arrangements are formed. It is found that the rotation of the thermal energy storage unit can reduce the solidification time as high as 46 % compared to its still counterpart, but variation of the rotation speed only produces a minor influence. The effect of rotation on the melting depends on the heat transfer direction (or surface). When the heat is transferred into the solid phase change material through the internal wall, the melting time can be gradually reduced as the rotation speed increases with a maximum of 69 %. However, when the heat is transferred through the external wall, influence of the rotation results in adverse effect. This study can guide the design of innovative high performance latent heat thermal energy storage system containing rotation unit.

Study on melting process of latent heat energy storage system by nano-enhanced phase change material under rotation condition

The utilization of phase change material in latent heat thermal energy storage technology is hindered by its limited thermal conductivity. This research aims to enhance the melting properties of a triplex-tube latent heat thermal energy storage unit through active strengthening (rotation mechanism) and passive strengthening (nanoparticle, longitudinal fin) heat transfer methods. A numerical investigation is conducted to examine the influence of nanoparticle percentage and rotation speed on various melting properties, including melting time, liquid fraction, heat absorption rate, and temperature response. The results demonstrate that the inclusion of 2.5 % and 5 % Al2O3 nanoparticles can significantly reduce the melting time by 58.49 % and 57.59 % respectively, while also increasing the average heat absorption rate by 126.53 % and 123.37 % at a rotation speed of 0.1 rpm. The findings from the Taguchi method suggest that rotation speed has a greater impact on melting duration and heat absorption rate compared to nanoparticle concentration. Moreover, maintaining a constant rotation speed, and a higher nanoparticle proportion can lead to an increase in dynamic viscosity and the settling of nanoparticles, which adversely affects the melting process efficiency.

Design and optimization of a bionic-lotus root inspired shell-and-tube latent heat thermal energy storage unit

Thermal energy storage (TES) is crucial in the efficient utilization and stable supply of renewable energy. This study aims to enhance the performance of shell-and-tube latent heat thermal energy storage (LHTES) units, particularly addressing the issue of the significant melting dead zones at the bottom, which are responsible for the long charging time. This paper proposed a new heat transfer enhancement technique inspired by the air channel distribution inside the root of the lotus. Numerical simulations are used to explore its melting behavior and heat storage performance, and a comparison is made with conventional shell-and-tube TES units. The results indicate that compared to the single-tube type, the bionic-lotus root type reduced the total melting time by 89.1 %, increased the average temperature by 13.2 °C, and enhanced the average effective power density by 7.6 times. Subsequently, multi-objective optimization was conducted based on response surface methodology, and an industrial standard type was constructed according to existing pipeline products to meet the manufacturing standards for mass production. The melting time was further decreased by 5.8 %, and the average effective power density increased by 6.6 %. The results of this study provide a promising and prospective solution for enhancing shell-and-tube LHTES units, with the potential to increase the efficiency, reduce the footprint and manufacturing costs of energy systems equipped with TES.

On the performance of a latent heat thermal storage unit integrated with a helically coiled copper tube and organic phase change materials: A lab-scale experimental study

In this study, the performance of a latent heat thermal storage (LHTS) unit integrated with a helically coiled copper tube and organic phase change materials is investigated using a lab-scale experimental setup. The effects of various operating conditions such as the coil thickness, testing fluid temperature/flow rate, PCM type, turbulator, and stirrer rotational speed on phase change behavior are investigated. The evaluation is based on PCM temperature variations, total heat transfer coefficient, and thermal effectiveness. The effects of testing fluid temperature, PCM type, turbulator, and stirring speed on phase change behavior are found to be more pronounced compared to those of the coil thickness and the fluid volume flow rate. Considering the melting process, it is found that the average temperature of the PCM in the LHTS unit with a turbulator is higher than that of the case without turbulator by nearly 10 °C which indicates an improvement in the heat transfer between water-PCM. The average PCM temperature is found to increase by 2 °C and 3.5 °C when a stirrer with a rotational speed of 200 and 400 rpm is employed, respectively. The total heat transfer coefficient for the LHTS unit at a stirring speed of 400 rpm (200 rpm) shows a significant increase of 39.6% (27%) compared to the scenario without a stirrer.

Numerical study of solidification and melting behavior of the thermal energy storage system with non-uniform metal foam and active rotation

This paper focuses on the strengthening study of the latent heat thermal energy storage (LHTES) unit and proposes a coupling strengthening method with non-uniform graded metal foam and active rotation. The non-uniform graded metal foams are employed to enhance heat conduction, and the rotation method is selected to make full use of natural convection. The solidification and melting behavior under the coupling effect of non-uniform metal foam and active rotation are numerically analyzed. The aluminum foam is partially concentrated and divided into two parts (concentrated part and diluted part) and is divided into the even-layered or uneven-layered porous structures according to the different regions area. Under the same volume of phase change material (PCM), the effects of layered forms (even-layered and uneven-layered approach), concentrated porosity, concentrated ratio, and rotational speed are performed. It can be concluded that the metal foam should concentrated near the inner tube and the uneven-layered structure can further improve the phase change heat transfer performance compared to the even-layered structure. Moreover, the thermal performance of the LHTES unit improves firstly and then decreases as the concentrated porosity, concentrated ratio, and rotational speed increase. The optimal values are obtained at 0.86 concentrated porosity, 0.3 concentrated ratio, and 0.5 rpm rotational speed. Compared with the static case with uniform metal foam, the melting time, solidification time, and total melting and solidification time can be significantly reduced by 40.30 %, 28.66 %, and 33.18 %, respectively. Similarly, the thermal energy storage rate can be correspondingly increased by 62.57 %, 40.82 %, and 54.11 %, respectively.

Modelling a packed-bed latent heat thermal energy storage unit and studying its performance using different paraffins

ABSTRACT Thermal systems, including those utilising solar energy and waste heat recovery, often have a mismatch between the energy supply and demand. It is crucial to implement a form of Thermal Energy Storage (TES) to effectively utilise the energy source. This study evaluates the thermal performance of a packed bed Latent Heat Thermal Energy Storage (LHTES) unit that is incorporated with a solar flat plate collector. The results show that the time required to charge the tank is reduced by 7% when the porosity is increased from 0.49 to 0.61, and also when the flow rate is raised from 2 to 4 kg/min, the charging time decreases by 2.5%. Additionally, studies were done to investigate the performances of different kinds of paraffin (RT30, RT28HC, WAX, RT58, and P56-58), and compare the heat capacities of each TES tank which resulted in the RT58 TES tank having the highest heat capacity. Abbreviations: BC: boundary conditions; HTF: heat transfer fluid; LHS: latent heat storage; LHTES: latent heat thermal energy storage; PCM: phase change material; SHS: sensible heat storage; TES: thermal energy storage

Energy performance assessment of a novel enhanced solar thermal system with topology optimized latent heat thermal energy storage unit for domestic water heating

In this study, we introduce an innovative approach by incorporating a Topology-Optimized Latent Heat Thermal Energy Storage (TO-LHTES) unit with fins into a solar water heating system. Employing EnergyPlus software, we initially assess the energy and power requirements essential for meeting domestic hot water needs within the Moroccan context. Afterward, Computational Fluid Dynamics (CFD) analyses explore diverse Phase Change Materials (RT35, RT50, and RT60) under varying operational conditions, including injected temperature, velocity, and stored temperature. Our investigation extends to different climates, evaluating the energy savings potential. The study's outcomes reveal the remarkable efficacy of the enhanced solar system featuring the TO-LHTES unit, particularly with specific configurations—such as an injected temperature of 20 °C, injected velocity of 0.02 m/s, stored temperature of 80 °C, and RT50 as the chosen Phase change material. Given the poor thermal conductivity of many PCMs, effective heat distribution and storage necessitate innovative methods. While various finned heat exchanger structures have been explored in existing literature, a notable gap exists in non-intuitive heat exchanger concepts specifically tailored for LHTES units. This work contributes by presenting an innovative TO-LHTES unit design, advancing the understanding and applicability of latent heat storage in solar water heating systems. Importantly, a maximum energy savings of 63.2% was achieved.

Thermodynamic performance improvement of the horizontal shell-and-tube latent heat thermal storage unit by splitter plates and upper-and-lower cascade PCMs

A new horizontal shell-and-tube latent heat thermal storage (LHTES) unit is proposed by using symmetric splitter plates to structure upper-and-lower (UAL) cascade PCMs in the paper. PCM 1 and PCM 2 are filled in the upper and lower sector regions, respectively. The thermodynamic performance of UAL cascade PCMs design (Case 3) is calculated. The obtained results are compared with bare tube (Case 1) and non-cascaded splitter plate design (Case 2) by various evaluation indexes, including liquid fraction, average temperature, complete melting time, thermal energy storage rate, exergy generation number, and exergy efficiency. Results imply that the UAL cascade PCMs design can melt completely at nearly the same time and attain the best melting uniformity for two PCMs in the sector region. Compared with Case 1 and Case 2, Case 3 can improve the full melting time, thermal energy storage rate, and exergy efficiency. Besides, Case 3 has advantages in minimizing the entropy generation number compared with other cases. The results can give some reference value to the structural design of the horizontal shell-and-tube LHTES units.

Investigation of phase change heat transfer in a rectangular case as function of fin placement for solar applications

This paper attempts to maximize solar energy utilization by employing a latent heat thermal energy storage (LHTES) unit integrated with a solar receiver of concentrating collectors or thermal management unit for photovoltaic panels. The study investigated an LHTES unit with a rectangular shape and equipped with a T-shaped fin subjected to heat flux. The paper examined the effect of eight T-fin positions under the applied heat flux on the thermal performance of phase change material (PCM). The PCM melting process was modeled using the enthalpy-porosity method using the finite element approach. The obtained numerical results revealed that PCM melting rate strongly depends on fin position, with superiority for fins located at the upper part of the chamber when heat flux is used as the primary heating source. Under applied heat flux, locating the fin at the upper section of the vertical LHTES unit enhanced the melting rate of the PCM, reaching 10 % better than the fin's middle and bottom positions, which is against the cases reported for the fixed temperature heating process that prefer bottom fins. Thus, these results recommend using T-shaped fins in the upper parts of PCM-based units used for solar collector receivers and thermal management of photovoltaic panels.