Latent Heat Storage Device Research Articles

Comprehensive enhancement of melting-solidifying process in latent heat storage based on eccentric fin-foam combination

A novel fin-foam combination, applying upward fins to accelerate natural convection and downward foams to dominate thermal conduction, was established to comprehensively enhance the melting and solidifying performance of horizontal shell-and-tube latent heat storage devices. Enhancement mechanisms of the current design were numerically investigated and compared with the typical eccentric and concentric design the whole melting-solidifying process. Among investigated enhancement designs, the fin-foam combination achieved the best results, reducing the melting time and the solidifying time by 47.9% and 55.4% respectively. Meanwhile, the relative difference between the melting and solidifying time was reduced to 12%, indicating a significant mitigation of the buckets effect of solidifying caused by eccentric designs. Further, the effect of the total enhancement material usage was investigated. Results showed that the marginal effect of the heat transfer enhancement started at 9 fins and 0.91 porosity, but the relative difference between melting and solidifying time then dropped below 0.61%. The economic assessment showed that increasing the amount of enhancement material can significantly improve the storage capacity per unit cost when the price ratio of the enhancement material to PCM is less than 10, indicating a considerable applicability and cost performance of fin-foam combination under different power density demands.

Thermo-hydraulic performance evaluation of lattice structures with triply periodic minimal surfaces for latent heat storage devices

Triply periodic minimal surface (TPMS) structures exhibit significant potential in latent heat storage (LHS) applications. In this study, numerical investigations were conducted to study the thermal storage performance of four TPMS lattice types: Primitive, IWP, FRD, and Gyroid lattices, used as metallic skeletons in LHS devices. The volumes of both the TPMS skeletons and the phase change material (PCM) were kept constant, with PCM embedded either inside or outside the lattices. The melting process of PCM, the heat storage energy density, and the power density of the LHS units for different configurations were examined. Additionally, a performance evaluation coefficient was introduced to assess the heat transfer and flow characteristics of heat transfer fluid (HTF) flowing on both sides of the lattices. The results indicated that when using Primitive and FRD lattices, embedding the PCM within the internal space of the lattices resulted in a 20 % and 9.8 % higher energy storage compared to embedding it outside the lattices. The internal space of IWP and Gyroid lattices was more suitable for HTF flow channels, resulting in performance evaluation coefficients (PECs) that improved by 27.7 % and 86.3 %, respectively, when HTF flowed through the internal channels of these lattices. Moreover, the Gyroid lattice exhibited the most balanced overall heat transfer performance due to its minimal flow resistance, whereas the FRD lattice had the lowest flow performance due to its complex structure. Therefore, the Primitive and Gyroid lattices could be considered as new heat transfer structures for LHS devices, while the FRD lattice was more suitable for replacing traditional fin structures in heat sink devices.

Application of bionic topology to latent heat storage devices

Latent heat storage is employed to address the intermittent supply of renewable energy, and enhancing heat transfer in phase change materials (PCMs) is pivotal for achieving effective heat storage. Currently, the predominant method for improving heat transfer is through the integration of high thermal conductivity fins into heat storage devices. This study introduces an innovative design of a three-tube phase change heat storage device created through bionic topology optimization. The impact of parameters such as penalty factors and filtering radius on the topology structure is analyzed, and the optimal range of fin volume fraction is determined based on energy storage density and energy storage time. Considering the influence of natural convection on heat transfer properties, four distinct heat storage models, including topology-optimized fins, are compared. The heat transfer and flow properties during the processes of melting and solidification, as well as their field synergy, are investigated. The study reveals that the optimized fins exhibit more bionic fractal structures and a larger, more uniform heat transfer contact area. Compared to traditionally optimized fin structures, this research reduces the heat storage time of the three-tube heat storage device by 21.96 % and the heat release time by 38.13 % without compromising the maximum heat storage capacity. The topology-optimized fins demonstrate a fractal dimension similar to natural fractal structures such as branches, leaf veins, antlers, and snowflakes. This study presents a novel bionic structural design approach for high-performance latent heat storage systems.

Performance optimization of latent heat storage device based on surrogate-assisted multi-objective evolutionary algorithm and CFD method

The design of fin structures and the formulation of heat conductivity enhancers have always been critical factors influencing the performance of latent heat storage devices. However, achieving the optimal parameters of fin structures and the fraction of heat conductivity enhancers when adjusting multiple parameters simultaneously remains a time-consuming and labor-intensive task. In this paper, a combination of machine learning methods and computational fluid dynamics (CFD) was employed to optimize the parameters of fin structures and the mass fraction of expanded graphite with minor computational resources and time costs, thereby obtaining a set of Pareto optimal latent heat storage (LHS) devices that simultaneously considers energy storage power and energy storage capacity. The combination essentially is a surrogate assistant multi-objective evolutionary optimization algorithm, which utilizes batch recommendation strategies to recommend multiple high-quality candidate solutions that achieve a balance between two conflicting optimization objectives in CFD simulations. This method significantly reduces the number of model iterations and saves the time and computational resources required for multi-objective optimization. The results showed that the proposed algorithm only required 80 cases of CFD calculations to obtain two well-fitted Gaussian process models, leading to a 91 % improvement in computational efficiency. According to the recommendations from the algorithm, the LHS device with 78 fins of 2.0 mm thickness and 20 % mass fraction of expanded graphite could achieve the optimal trade-off of energy storage power and energy storage capacity. Compared to the prototype, the energy storage power and total energy storage capacity increased by 46.7 % and 22.1 %, respectively. The method proposed in this study can provide guidance for the optimal configuration design of LHS devices.

Analysis of the influence of asymmetric V-shaped fins on the melting of phase change materials in latent heat storage units

Improving the melting performance of latent heat storage devices is highly important for achieving efficient utilization of thermal energy. To achieve this goal, this paper proposes an innovative thermal storage unit with asymmetric V-shaped fins. The influence of structural parameters including eccentricity, fin angle, height and shape on the melting process of phase change materials is analysed. The results show that compared with concentric tubes without eccentricity, when the eccentricity is increased to the point where the bottom fins are in contact with the outer tube, the total melting time is shortened by 33.56%, and the average heat storage rate is increased by 32.95%. At the same time, increasing the angle and height of the top branch fins can strengthen the natural convection in the heat storage unit and shorten the total melting time by 13.54%. It is worth noting that changing the upper-side branch fin parameters has little effect on the melting process. In addition, when the shape of the branch fins constituting the V-shaped fin changes from rectangular to trapezoidal, the total melting time decreases by 3.11%. In summary, the use of asymmetric V-shaped fins can improve the melting performance and heat storage performance of eccentric heat storage units, and changing the fin shape can further shorten the total melting time and increase the average heat storage rate.

Thermal performance investigation and optimization of a novel v-shaped finned latent heat storage unit

The low thermal conductivity of phase change materials limits the thermal performance of heat storage devices, hindering their widespread application in engineering. To address this challenge, this paper focused on the fin design of a horizontal triplex-tube latent heat storage device. A novel V-shaped fin with a secondary arrangement is proposed. Numerical simulation is employed to investigate the effects of fin angle, fin length, and heat transfer fluid temperature on the thermal performance of the storage system. Moreover, the fin parameter is optimized by the response surface method. The results demonstrate that, compared to traditional rectangular fins, both the primary and secondary V-shaped fins can significantly enhance the thermal performance. The secondary V-shaped fin has the most significant effects, reducing the melting time of the phase change material by 48.5%, and increasing the average heat storage rate by 83.46%. Additionally, the thermal performance of the heat storage unit exhibits a trend of initially increasing and then decreasing with the increase of fin angle and length, suggesting the existence of optimal structural parameters. Consequently, the optimized secondary V-shaped fin reduces the melting time by 52.9% and increases the average heat storage rate of phase change materials by 104.48%.

Effect of unsteady heat source condition on thermal performance for cascaded latent heat storage packed bed

The intermittency and fluctuations of heat sources such as solar energy and industrial waste heat have a significant impact on the thermal properties of thermal energy storage systems. The dynamic heat transfer properties of latent heat storage devices under unsteady heat sources need to be further studied to better understand the impacts of thermal fluctuations. In this study, the concentric dispersion model is used to investigate the melting process of a cascaded packed bed phase change heat storage system under sinusoidal fluctuating heat sources. The effects of period and amplitude variations on the thermal performance of the system are investigated, using several metrics for thermodynamic evaluation. The study shows that the effect of low-frequency fluctuation period on the melting of packed bed thermal energy storage (PBTES) is differentiated by the limitation of the total melting time. The thermal performance of the system is enhanced when the fluctuation period is much larger than the melting time, especially the melting time is significantly reduced by 24 % when the period is 120 min, and the opposite is true when the fluctuation period is approximately equal to the melting time. The effect of high-frequency features is smaller. The influence of the fluctuation amplitude on the melting process lasts throughout the process. When the amplitude is 15 °C, the complete melting time is shortened by 23 % compared with the steady condition. At the same time, the increase in the amplitude of the fluctuations enhances the sensitivity of the thermal properties of the system to the variability of the fluctuation period, further amplifying the effect of the period on the enhancement or reduction of the thermal properties of the system. This work provides a certain reference for the design of PBTES under unsteady heat sources.

Experimental study of thermal performance in a rectangular finned-tube latent heat storage device with composite polyethylene wax/expanded graphite

Phase change materials and geometric structure of latent heat storage (LHS) devices constitute crucial factors affecting the performance of phase change thermal energy storage systems. In this study, focusing on the LHS system with a heat source temperature of 150 °C, a rectangular finned-tube latent heat storage device was designed and fabricated. Polyethylene wax (PEW) with a 10% mass fraction of expanded graphite was adopted to prepare the composite phase change material. A series of experiments was conducted to evaluate the thermodynamic performance of the device with varying inlet temperatures and mass flow rates. The experimental results showed that the process of heat storage and release was predominantly governed by thermal conduction due to the high viscosity of PEW. Although the temperature variations in various directions within the LHS device remain nearly consistent, the heat transfer in the non-fin regions on the rectangular LHS device walls and corners was comparatively inferior due to the absence of natural convective heat transfer. In comparison to the mass flow rate, the inlet temperature exerts a more significant impact on accelerating the heat storage/release processes. While an increase in the mass flow rate enhanced the heat transfer capacity, its slightly accelerates the heat transfer duration. Moreover, the heat release ratio of the device exhibits an initial increase followed by a decrease with an increasing mass flow rate. In practical applications, prioritizing an increase in inlet temperature difference proves to be more effective than adjusting mass flow rate to enhance the heat storage performance.

Effect of enhancement in metal foam pore density on heat transfer of phase-change materials

To explore the effect of metal foam(MF)pore density on the melting and heat transfer of phase-change materials(PCMs), a semi-cylindrical visualization experimental setup was established to experimentally study copper MFs with different pore densities coupled with paraffin wax. The results showed that as pore density increases, the melting time of the composite PCMs decreases compared to that of pure paraffin wax by 11.5%, 16.2% and 18.1% for pore densities of 5, 20, and 30 PPI, respectively. Composite PCMs with a pore density of 30 PPI exhibited fastest melting and lowest maximum temperature difference during melting. Considering the relative thermal energy storage(TES) capacity and relative TES rate, composite PCMs with a pore density of 30 PPI exhibited optimal overall thermal storage performance. Furthermore, heat conduction was the primary means of thermal transmission during the melting of the bottom paraffin. However, the melting of the upper paraffin was nevertheless influenced by natural convection, Hence, as pore size decreased, the influence on the weakening of natural convection became more significant. The results of the study provide guidance for the optimal design of latent heat storage devices for semi-cylinders.

Design optimization of a novel annular fin on a latent heat storage device for building heating

The poor heat transfer performance of the heat exchanger leads to low heat storage efficiency of the latent heat storage device. To improve the heat transfer performance of the heat exchanger, a novel annular fin with inclined angle is proposed to enhance heat transfer of the heat exchanger. 12 fins with different structural parameters are designed and welded to the inner tube wall of the heat exchanger. The simulation model is in good agreement with the literature and the model is considered to be reliable. Based on the enthalpy-porosity model, the liquid fraction, temperature distribution, melting front, energy storage density and heat storage rate were used to compare and optimize the structural parameters of the annular fins. Results demonstrated that the difference in complete melting time due to the number of fins, fin arrangement, fin length and fin angle were 43 min, 85 min, 17 min and 10 min, respectively. The fin arrangement was the most significant in reducing the heat storage time. Compared to N = 4 and uniformly distributed fins, non-uniformly distributed fins reduced the total melting time of stearic acid by 53.13 % and 20.54 %. To be conclusive, the addition of non-uniformly distributed annular fins with 70° inclination enhanced the heat transfer in the heat exchanger and saved a significant amount of heat storage time.

Numerical studies on a fin-foam composite structure towards improving melting phase change

The low thermal conductivity of phase change materials limits the large-scale application of latent heat storage technology. It is meaningful to improve the thermal conductivity of phase change materials by corresponding means. In order to improve the performance of latent heat storage device, this paper takes square latent heat storage device as the research object, and uses fin and foam metal to enhance its melting performance. A 3-D numerical model is established and verified by visualization experiment. The quantitative comparison of melting properties of four different structures (pure paraffin, fin, metal foam, fin-metal foam) is presented. The results show that compared with pure paraffin structure, the complete melting time of phase change materials corresponding to fin, metal foam, and fin-metal foam structure is reduced by 47.48%, 79.53%, and 83.68%, respectively. The temperature uniformity increased by 28.97%, 79.37%, and 91.12%, and the total heat storage decreased by 6.0%, 4.6%, and 11.64%, respectively. It shows that the addition of fin and foam metal is beneficial to improve the melting performance and the overall temperature uniformity of the device, but it has a negative effect on total heat storage. Dynamic temperature studies were conducted to further explore the effect of fins and metal foam on the internal melting process compared with pure paraffin structure.

Solidification time and solid fraction of vertical concentric shell and tube latent heat storage device : A dimensionless parametric study and correlations development

ABSTRACT Thermo-hydraulic behavior of a Phase Change Material (PCM) plays a crucial role in deciding the performance of Latent Heat Storage (LHS) devices. Discharge or Solidification is the prime portion that determines the performance of the LHS device. This work numerically models solidification in a vertical tube-in-tube LHS device. The phase change is accounted for using the fixed grid enthalpy porosity method. An independent numerical model built is validated with the experimental model. Dimensionless parameters, namely Rayleigh Number, Stefan number, Reynolds Number, and L/D ratio, vary in the range of 9.19x105 to 39.45x105, 0.2 to 0.5, 600 to 3000, and 1 to 15, respectively. Among all parameters, the Stefan Number has the greatest impact on the solid fraction and solidification time. For estimating the Solidification Time and Solid Fraction, correlations are proposed.

A novel bionic packed bed latent heat storage system filled with encapsulated PCM for thermal energy collection

• A bionic structure brings a more homogeneous temperature difference and a smaller pressure drop compared with conventional systems. • The parameters related to heat transfer and phase transition acquired from numerical study were validated by experiments. • A non-local thermal equilibrium model was employed to characterize the temperature distribution of PCM and HTF in porous media. • The enhancement of flow resistance and heat exchange performance was evaluated. The main content of this work is to propose a novel bionic solution to overcome the nonuniformity of flow and temperature distribution, which is an inherent problem and restriction for conventional latent heat storage devices. By learning from animal circulatory systems, the inner space and flow channel network are distributed hierarchically as arteries, veins, capillaries and ventricles. A conceptual configuration of the bionic system is presented, and its numerical model is established to demonstrate the flow and heat transfer phenomena. The encapsulated PCM that is used in this study is fabricated and parameters related have been measured by experiments. A numerical model of a 3D continuous PCM bed is established to help research the flow of HTF through the surfaces of PCM particles and the heat transfer between them. Then, a model of a simplified bionic device is developed where the PCM region is set as a porous domain. The results show the flow and temperature fields are distributed uniformly, along with a much smaller global pressure drop. By allocating the thermal load on cascaded layers with stepwise PCMs, a more homogeneous global temperature difference can be achieved.

Improvement in Thermal Storage Effectiveness of Paraffin with Addition of Aluminum Oxide Nanoparticles.

The output of the latent heat storage devices (LHSDs), based on some phase change materials (PCMs), depends upon the thermophysical properties of the phase change material used. In this study, a paraffin-based nanofluid, blended with aluminum oxide (Al2O3) nanoparticles, is used as PCM for performance evaluation. A three-dimensional (3D) numerical model of regenerative type shell-and-tube LHSD is prepared using COMSOL Multiphysics® 4.3a software to estimate the percentage of melt and the average temperature of the analyzed nanofluids. The results of this study are in close agreement with those reported in the literature, thereby ensuring the validation of the numerically predicted results. The effects of adding the nanoparticles on the rate of melting, as well as solidification and rate of stored/liberated energy, are studied. The results revealed that, by adding 10% nanoparticles of Al2O3, the melting rate of pure-paraffin-based LHSD improved by about 2.25 times. In addition, the rate of solidification was enhanced by 1.8 times. On the other hand, the heat of fusion and specific heat capacities were reduced, which, in turn, reduced the latent and sensible heat-storing capabilities. From the outcomes of the present research, it can be inferred that combining LHSD with a solar water heater may be used in technologies such as biogas generation.

Innovative ladder-shaped fin design on a latent heat storage device for waste heat recovery

Latent heat energy storage system provides an alternative solution to solving the imbalance problem of energy supply and demand. To improve the phase change efficiency, a novel ladder-shaped fin is proposed to accelerate melting process. Under the same mass of fin materials, two groups of fin shapes (totally eight cases) are innovatively designed. Upon being verified by experiments in literature, numerical models account for comprehensive descriptions on melting front propagation with emphasizing temperature development and free convection in the liquid phase. Results demonstrate that the ladder-shaped fins can better optimize the melting channel of phase change material than the straight fin. Compared to the original straight fin case, a maximal 52.2% of the total melting time can be saved. The angle change of fins has a significant effect on reducing the melting time of the whole PCM. In Group I where fins are arranged vertically and horizontally, the total melting time is much shorter than that of each corresponding case in Group II (45° from the vertical axis). For the original straight fin in Group II, a 36.8% reduction in total melting time is obtained if turning fins by 45°clockwise. To be conclusive, it is more beneficial to add fins to mobile heat accumulators than to have no fins, saving more energy charging time.

Experimental and numerical investigations of solar charging performances of 3D porous skeleton based latent heat storage devices

Volumetric-absorption-based solar charging via phase change processes is an emerging technology to harvest solar energy, however, how pore-scale radiation transport interacts with latent heat thermal energy storage processes is still vague. In this paper, volumetric-absorption-based solar charging processes at pore scale are investigated by experiments and numerical simulations based on Monte Carlo ray tracing coupled with the Finite Volume Method. The solar radiation transport, temperature distribution, liquid fraction, and solar thermal energy storage efficiency are systematically evaluated under different radiation intensities and skeleton thermal conductivities. Compared to the traditional surface-absorption-based mode, solar thermal energy storage efficiency of the volumetric-absorption mode is enhanced by 94% due to presence of multi-region heat sources. The solar thermal energy storage efficiency reaches a peak value of 54.09% at a radiation intensity of 10 kW·m−2. That is because low radiation intensities increase the melting time while too high intensities lead to large temperature nonuniformity, leading to high heat losses for both scenarios. Increasing the skeleton thermal conductivity can continuously improve the efficiency by reducing temperature nonuniformity, but further enhancement becomes marginal for thermal conductivity over 90 W·m−1·K−1. This work paves the way for the design and deployment of efficient integrated solar thermal conversion and latent heat storage systems.

An experimental and analytical study of solidification around an array of horizontal cylinders subjected to free convection flow

In this paper the solidification of a liquid around an array of cooled horizontal cylinders has been investigating experimentally and analytically. The horizontal cylinders can be arranged in various configurations. In this paper one typical example for such a configuration is studied. This is a cyclic system cylinders with cylinders of a diameter of 54 mm a length of 1793 mm and a spacing between them of 100 mm. The cylinders are arranged in an equilateral triangular array. The theoretical part of the paper deals with a simplified solution of the problem of liquid solidification around the horizontal cylinders under the influence of a free convection flow. A simple quasi steady-state model for the solidification process is derived. The model describes the phase change phenomenon with an imposed boundary condition on the solidification front which assumes a distribution of the heat transfer coefficient from the surrounding free convection flow. The simplified theoretical model, which is based on the Happel-Brenner cell method, can be used for a fast preliminary design of latent heat storage devices. In addition, the model considers the effect of a contact layer between the cooled cylinders and the surrounding ice layers. It can be shown, that the solution of the problem can be reduced to the solution of a system of integro-differential equations, which has been solved numerically. In the experimental part of this work, the thickness of the solidified layer was measured by using a newly built test apparatus. Both, the thickness of the solidified layer and the average liquid temperature depend on time. The local heat transfer coefficient at the solidification front was also determined experimentally. Experiments have been done with timely varying fluid temperatures between 293 K to 274 K. The experimental results are found in good agreement with the theoretical model for the obtained results for time dependent thickness of the frozen crust.

Exergy analysis of a novel multi-stage latent heat storage device based on uniformity of temperature differences fields

For shell-and-tube type latent heat storage devices, heat transfer rate reduces with time due to the increasement of ineffective heat transfer area. To solve this problem, a latent heat storage device, which is made of ‘replaceable multi-stage’ phase change plates (PCPs), is proposed in this paper. When one PCP has no heating ability, it is replaced by a new PCP. Effects of the ‘replaceable multi-stage’ configuration are evaluated using unmatched coefficient, which indicates uniformity of temperature differences distribution, and exergy destruction, which is influenced by the unmatched coefficient. Two replacement methods are numerically studied and based on the recommended method, effects of stage number and length of the device on heat transfer performances are investigated. It is found that, with length of the device being fixed, larger stage number results in smaller unmatched coefficient and lower exergy destruction. Therefore, higher and more stable supply air temperature can be realized. However, more frequent replacement actions are required if stage number is increased. For space heating applications, a 2-stage heating unit with length of each plate being 30 cm is recommended, which can heat indoor air from 20 °C to 29.5 °C for 8 h.

A novel 3-D model of an industrial-scale tube-fin latent heat storage using salt hydrates with supercooling: A model validation

Latent heat storage using salt hydrates based on tube-fin heat exchanger is a promising heat storage approach for distributed building heating. In spite of abundant experimental investigations, there is quite a lack of numerical works focusing on the industrial-oriented design and optimization on this technology. The present work develops a reasonably accurate 3-D model for the evaluation of the thermal behavior of a pilot-scale tube-fin latent heat storage device using salt hydrates, considering the balance between computing effort and simulation accuracy. The availability and accuracy of the proposed model are validated through comparisons with experimental results in which numerical discrepancies and model applicability are comprehensively discussed. The results show that the model is able to portray a clear heat transfer process of a 100 kWh-level pilot-scale latent heat storage device during a continuous 24-h charge-discharge operation, with considering detailed thermal behaviors of PCMs including non-isothermal melting and solidification with supercooling effect. The calculation errors on the transient temperature variation of the heat transfer fluid and storage medium, as well as the overall thermal performances of the storage device are less than 7%. The CPU time consumed for the calculation is around half of the actual operation time.

Controlled short time large scale synthesis of magnetic cobalt nanoparticles on carbon nanotubes by flash annealing

Nanopatterned arrays of discrete cobalt nanostructures showing characteristic parameter-dependent sizes are formed from continuous thin films on a carbon nanotube substrate using millisecond pulsed intense UV light. The nanoparticles exhibit ferromagnetic behavior with magnetic remanence and coercivity depending on the particle size. The end-state particle size is shown to be a function of initial thin film thickness and excitation energy and is therefore tunable. The evolutionary process from continuous thin films to a discrete morphology is thermodynamically driven by the large surface energy difference between metastable thin films and the underlying carbon nanotube substrate. Evidence of the Danielson model of the dewetting process is observed. These arrays can find applications as platforms for the self-assembly of magnetically susceptible materials, such as iron or nickel nanostructures, into a conduction matrix for applications in energy extraction from a latent heat storage device.