Thermal Energy Storage Rates Research Articles

Analysis of charging performance of thermal energy storage system with graded metal foam structure and active flip method

The latent heat thermal energy storage (LHTES) technology based on solid-liquid phase change material (PCM) is characterized by high energy storage density, small volume change, and constant operation temperature, which is widely employed in waste heat recovery, solar thermal utilization, and equipment thermal management. This paper introduces active flipping and graded porous to strengthen natural convection and heat conduction to alleviate the issue of low thermal conductivity and non-uniform phase change of PCM. Based on the fixed grid system, the enthalpy-porosity method combined with time-dependent gravity and non-Darcy porous effect is employed to solve the solid-liquid phase change problem under different porosity gradients, dimensionless flipping times (t*), and modified Ra (Ra*) numbers. Furthermore, the effects of the Ra* on the optimal t* and porosity gradient are also discussed. The results show that the flipping method can significantly enhance flow and heat transfer within the container, improve temperature field uniformity, and increase the proportion of latent heat storage. With the increase of porosity gradient and t*, the melting time initially decreases and then increases and the optimal thermal performance is achieved at a 6 % porosity gradient and t* = 0.4. Compared to the uniform porous structure without flipping, it can shorten melting time by 49.37 % and improve thermal energy storage rate (TESR) by 76.23 %. Additionally, the optimal porosity gradient is 6 % under different Ra*, but the optimal t* decreases with the increase of Ra*. This paper explores the charging performance of the thermal energy storage system with the graded metal foam structure and active flip method, which can contribute to the study of heat transfer enhancement in LHTES technology.

4E experimental investigation of concentrated solar cell cooling via system of heat dissipator-phase change material

Low-concentrator solar cell (SC) experimentally cooled using a new composite thermal regulation system of heat dissipator (HD) and phase change material (PCM) is investigated. The impact of area ratio (AR; HD area/SC area) of 1.5 and 2 at different HD thicknesses (h) on the system performance is reported. An energy, exergy, economic, and enviroeconomic assessments for the different cooling systems used for the SC thermal regulation are presented. The results show that the SC temperature reduction increases with increasing the HD thickness and AR. Compared with reference SC, SC-PCM/HD (AR=2/h = 3) achieves a maximum temperature reduction of 24 °C compared with 9.1 °C and 21.8 °C for SC-PCM and SC-HD, respectively. Moreover, SC-PCM/HD cooling system achieves the maximum enhancement in the average electrical efficiency and power output of 11.88 % and 12 %, respectively, compared with reference SC. The PCM thermal energy storage rate and PV system efficiency increase with increasing area ratio and decreasing the HD thickness. Using HD of (AR=2/h = 1) in conjunction with PCM yields the most favorable PCM thermal performance where it enhances the thermal energy storage, rate of energy storage, and overall thermal system efficiency by 9.5 %, 40 %, and 20.36 %, respectively, compared with using PCM only. Moreover, the PCM/HD cooling system is more economical than using PCM or HD only, and reference SC, with a maximum cost saving of 41.7 % compared with reference SC. The SC-PCM/HD cooling system is the most eco-friendly option with net CO2 mitigation and ψCO2 of 146.2 kg and 5.84$, respectively compared with 143.7 kg and 5.74$, respectively for SC-PCM.

Enhancing the phase change material based shell-tube thermal energy storage units with unique hybrid fins

The poor thermal conductivity of phase change material (PCM) has limited its application to thermal energy storage system. The present work aims to improve the performance of PCM in a vertical shell-tube energy storage unit through unique hybrid fins. The enthalpy-porosity approach is used to numerically investigate the phase change phenomenon. Based on the straight and spiral fin results, the novelty designs of double side spiral fin and hybrid fins, i.e. spiral/straight hybrid fin and straight/spiral hybrid fin, are proposed to further optimize the PCM charging process. The effects of different fin structure, hybrid fin proportion and spiral fin angle on the liquid fraction, temperature and average thermal energy storage rate are discussed. The fin structures can reduce the melting time of PCM up to 100% compared to the no-fin case. Although double side spiral fin outperforms the straight fin for the PCM melting behavior, the hybrid fin configurations shows the best enhancement, especially for the straight/spiral hybrid fin. The optimal fin design for the straight/spiral hybrid fin case is that with 0.7 fin proportion and 180° spiral angle, with up to 11.8% reduction of the melting time compared to the designs of other fin proportions and spiral angles. This work demonstrates the potential of this unique hybrid fin to be integrated with PCM for efficient thermal energy storage.

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 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.

Influences of the number and length of longitudinal fins on the single and cyclic charging and discharging performance of vertical double-tube latent heat storage systems

Using longitudinal fins is one of the efficient ways to enhance the heat transfer performance in phase change materials (PCM) based latent heat thermal energy storage systems. However, the previous studies do not consider the influence of the fins' geometric parameters on the single and cyclic charging and discharging performance and thermal effectiveness of latent heat storage systems. This paper, thus, aims to fill the research gap by developing a three-dimensional numerical model to study the effects of the fin number (i.e., 2, 4, 6, and 8) and fin length (i.e., 11.25, 17.00, and 22.00 mm) on the PCM melting/solidification characteristics and evaluate the performance and thermal effectiveness of the vertical double-tube latent heat storage system using the enthalpy-porosity method. Paraffin wax (RT-55) is used as the PCM, and water is the heat transfer fluid. The results show that increasing the fin number to eight reduces the melting, solidification, and total cycle time by 58.0%, 61.4%, and 59.8%, respectively, while the number of daily complete melting and solidification cycles increases by 150.0%, compared with the base case (i.e., without fins). The eight-fin storage system increases the daily charging capacity, thermal energy storage, thermal energy storage rate, and average effectiveness during melting by 146.0%, 133.8%, 123.5%, and 109.0%, respectively; while during solidification, the discharging rate, the daily discharging capacity, and average effectiveness increase by 159.9%, 149.3%, and 176.2%, respectively, compared with the base case. Increasing the fin length to 22.00 mm reduces the melting, solidification, and total cycle time by 80.3%, 81.3%, and 80.8%, respectively, while the number of daily complete melting and solidification cycles increases by 418.0%, compared with the base case. The highest fin length increases the daily charging capacity, thermal energy storage, thermal energy storage rate, and average effectiveness during melting by 370.0%, 350.0%, 330.0%, and 210.0%, respectively; during solidification, the discharging rate, the daily discharging capacity, and the average effectiveness increase by 420.0%, 410.0%, and 470.0%, respectively, compared with the base case.

Melting performance analysis of finned metal foam thermal energy storage tube under steady rotation

The latent heat thermal energy storage (LHTES) technology based on solid-liquid phase change material (PCM) is of great significance for the efficient utilization of thermal energy. To address the issues of slow thermal response and non-uniform melting of the LHTES technology, a hybrid heat transfer enhancement method combined with finned metal foam and steady rotation is proposed in this work. An enthalpy-porosity model considering non-Darcy porous effects and mechanical rotation is established based on the local thermal equilibrium assumption and the fixed grid system. Four different structures (uniform metal foam, graded metal foam, finned metal foam with uniform porosity, and finned metal foam with graded porosity) are investigated firstly to identify the optimal structural arrangement under the same volume of PCM. Numerical results demonstrate that the LHTES units strengthened by the finned metal foam with graded porosity achieve the shortest melting time and largest thermal energy storage rate (TESR). The graded porous structure reduces thermal resistance, rotation enhances flow and heat transfer inside the container, and the fins expand the heat source area. Moreover, the effect of different fin types, graded porosities, and rotational speeds are further considered. The findings suggest that the increase of these three parameters does not correspond to better thermal performance; instead, there is an optimal value. Compared with the base case, the optimal configuration (fin length of 20 mm, fin width of 1.5 mm, porosity gradient of 2%, and rotational speed of 0.5 rpm) can shorten the melting time by 46.68%, increase the TESR and Nu by 74.06% and 69.02%, respectively. This paper validates the feasibility of the hybrid heat transfer enhancement method with finned metal foam and steady rotation, which can offer new insights into the engineering practice of the LHTES technology.

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.

Revolutionizing the latent heat storage: Boosting discharge performance with innovative undulated phase change material containers in a vertical shell-and-tube system

Abstract This paper examines the impact of various parameters, including frames, zigzag number, and enclosure shape, on the solidification process and thermal energy storage rate of a vertical phase change material (PCM) container. The study also assesses the effects of the flow rate of the heat transfer fluid as well as changing the materials of the PCM between RT35 and RT35HC. In addition, the study compares the framed versus unframed systems and, subsequently, the best case was tested with various zigzag pitch numbers before changing the zigzag-shaped structure to arc and reversed-arc. The findings are examined by contrasting the different scenarios’ liquid fractions, temperature distributions, solidification rates, and heat storage rates. The results show that the framed geometry is 66% faster to reach the target temperature compared with the unframed geometry and employing a zigzag enclosure in a PCM can significantly improve the solidification time and heat recovery rate. As the number of pitches in the zigzag enclosure increases, the improvement rate decreases but still improves the solidification time and heat recovery rate. The reversed-arc-shaped structure has the best performance compared with the other undulated surfaces. For the system with RT35HC, the discharge time is 55% higher compared with that of the system with RT35, while the discharge rate is 8.2% higher for the former during the first 3000 s of the discharging process.

Numerical study of the melting rate enhancement of nano phase change material in a modified shell and tube heat exchanger

The current investigation employs numerical analysis using ANSYS Fluent software to explore the melting behaviour of phase change material (PCM) and nano-PCM in a semi-circular shell and tube heat exchanger, utilizing dual side heating for enhanced conduction and convection rates of heat transfer. The study incorporates aluminium oxide nanoparticles at volume concentrations of 2 %, 4 % and 6 % within paraffin wax and assessing their impact on melting and thermal energy storage rates. Notably, the 2 % nanoparticle volume concentration exhibits an enhanced storage rate as compared to higher concentrations. The increase in nanoparticle concentration from 0 % to 2 %, 2 % to 4 % and 4 % to 6 % results in percentage reduction in melting time of 21 %, 3.51 % and 2.8 % respectively. Semi-circular configuration shows significant improvement in the energy storage rate as compared to the circular triplex tube configuration. The maximum energy storage rate of PCM for the semi-circular and circular configurations is 1.15 kW and 1.04 kW respectively, marking a 9.56 % increase over the circular configuration. Moreover, the semi-circular and circular configurations using nano-PCM achieve a maximum energy storage rate of 1.19 kW and 1.14 kW, representing a 4.20 % increase over the circular configuration.

Geometry modification of a vertical shell-and-tube latent heat thermal energy storage system using a framed structure with different undulated shapes for the phase change material container during the melting process

This study aims to modify the melting performance of a vertical latent heat double-pipe heat exchanger by geometry modification of the phase change material (PCM) container. To modify the configuration of the PCM container placed in the outer tube, a framed structure is embedded in the inner surface of the outer tube. Different framed structures i.e. smooth, arc-shaped, reverse arc-shaped, and zigzag-shaped structures are examined and compared with that of the unframed enclosure. Moreover, different numbers of pitches are also examined. The results show that the melting duration drops by almost 55 % and the thermal energy storage rate improves by 115 % using a smooth framed structure for the PCM container compared with that of using an unframed structure. The thermal energy storage rate and melting time are ∼9.5 W and 244.4 min, respectively, for the system using an unframed structure. The analysis of the pitch number for the zigzag-shaped structure reveals that increasing the number of pitches from 0 to 11 enhances the thermal energy storage rate by 36.5 %. The evaluation of different shapes of the framed structure shows that the case with reverse arc-shaped structure is the most enhanced unit which has the thermal energy storage rate of ∼34 W and melting time of 63.15 min heated by water at the Reynolds number 1000 and inlet water temperature of 50 °C. The sensitivity analysis of the HTF characteristics reveals that by increasing the Re from 1000 to 2000, the thermal energy storage rate increases by around 6 W. Furthermore, by increasing the inlet temperature of HTF from 50 °C to 55 °C, the thermal energy storage rate enhances by around 10 W. This paper shows the capability of geometry modification in enhancing the thermal energy storage rate of the systems.

Biomimetic optimized vertically aligned annular fins for fast latent heat thermal energy storage

Latent heat thermal energy storage (LHTES) technology has been widely used in balancing energy demand and intermittent energy supply, as well as thermal management in buildings, but suffers from slow heat storage rates. Here, biomimetic topology-optimized vertically aligned annular fins are proposed to accelerate heat storage performance. Taking the maximum average temperature as the optimization objective function, optimized biomimetic fins can shorten the melting time by 45.9% compared with traditional fins as verified by experimental measurements. Such a fast heat storage rate is also superior to that of pitch-varying fins and diameter-varying fins by 18.4% and 23.5%, respectively. Meanwhile, the temperature uniformity is greatly improved by biomimetic fins with a reduction of average temperature difference in phase change materials (PCMs) by 75.3%. The underlying mechanism can be attributed to accelerated melting of PCMs close to the outer bottom boundary due to unique bifurcation structures of topology optimized fins. The fractal dimension of optimized fins is similar to that of leaf vein structures, demonstrating that proposed topology structures are close to Natural’s optimal solutions. This study provides novel biomimetic structures for accelerating thermal energy storage rate and promotes the application of biomimetics in LHTES techniques.

Sea urchin skeleton-inspired triply periodic foams for fast latent heat storage

The latent heat storage technology has been widely applied in various thermal management fields, but its extensive deployment is limited due to the poor thermal conductivity of phase change material. Here, inspired by the microstructure and functions of sea urchin skeleton, four different metal foam skeletons based on triply periodic minimal surface (TPMS) are introduced to enhance latent heat thermal energy storage performances, which are evaluated by both experiment and numerical simulation. The metal foam-PCM (MFPCM) based on the Primitive structure has the fastest thermal energy storage rate with melting time prominently reduced by 20% compared to the traditional structure (Lattice). The underlying mechanism can be attributed to a more continuous and compact internal structure of TPMS compared with traditional MFPCM by thermal resistance analysis. In addition, the effect of gradient porosity is investigated as well, and the positive gradient in porosity has the fastest melting rate. The present study provides a new idea to design high-performance MFPCM and promotes the application of bionics in accelerating latent heat thermal energy storage.

Thermal buffering performance of passive phase change material micro-pillar array systems on temperature regulation of microfluidic chips

The microfluidic chip is supposed as an attractive strategy obtaining widely application potentials, while it generally suffers from acute temperature fluctuation owing to external force field, flowing resistance or chemical reaction. A passive phase change material (PCM) micro-pillar array module is novelty designed to deal with the temperature fluctuation of working fluid utilized in microfluidic chips. PCM enables to store or release thermal energy trough solid-liquid phase transition to guarantee suitable temperature of working fluid passing through the PCM micro-array module. Numerical model is constructed based on enthalpy-porosity and local thermal non-equilibrium theory between PCM and fluid. Results indicate that peak temperature of fluid decreases, whereas valley temperature of fluid increases with the elapse of time. Temperature difference between inlet and outlet fluids tends to be steady under leveling effect of the PCM micro-array module. Semi-sinusoidal fluctuation of fluid inlet temperature also contributes to periodic variation of thermal energy storage rate and liquid fraction of PCM. PCM, configurational, dimensional and operational parameters of the PCM micro-array module are evaluated and it is revealed that system performance can be substantially affected by these parameters. Thermal buffering performance indicates that temperature of outlet fluid can be moderated with more buffering effect on shifting valley temperature over shaving peak temperature. In conclusion, the built PCM micro-array module is beneficial to passively regulate and control thermal performance of microfluidic chips.

The influence of the metal foam layer shape on the thermal charging response time of a latent heat thermal energy storage system

In the current study, the impact of various foam shapes and configurations was addressed for the first time on the thermal energy storage rate. A partial layer of metal foam was used to improve the heat transfer rate in a channel-shaped latent heat thermal energy storage unit for solar thermal energy storage. The channel was heated at the right vertical wall while half of the channel was filled with a layer of metal foam. The enthalpy porosity model was utilized for simulating the melting heat transfer in the channel. The finite element method was employed to solve the governing equations. Several design configurations for the placement of the metal foam layer were examined. The findings showed that placing the metal foam layer in an L shape form along the left and bottom walls could yield maximum thermal energy storage power (0.57 kW). Diagonal placement of the porous layer along the right and bottom walls gives the lowest thermal energy storage power (0.23 kW). Therefore, changing the shape of the metal foam layer could alter the storage power by about 60 % for a fixed amount of metal foam. Therefore, the shape of the metal foam layer is an important design parameter that should be selected carefully.

Numerical evaluation of thermal energy storage rate in planar and cylindrical phase change material composites

Efficient design of high-performance phase change material (PCM) composites remains challenging, due to a lack of understanding regarding the relationships between the performance characteristics, and relevant design parameters. In this work, we adopt the effective composite approximation, as justified through experimental validation, and numerically investigate the effects of individual design variables on the thermal energy storage rate into phase change material composites under a constant temperature boundary condition. We isolate and describe the impact of (1) geometry, (2) volume fraction of metal, (3) time, (4) material thermophysical properties, and (5) the magnitude of the thermal boundary condition on the heat absorption rate normalized by heated area, by volume, and by mass of the active composite PCM. Finally, we assess the accuracy of the quasi-steady state approximation, as well as a modified version of the quasi-steady state approximation, which incorporates a term for sensible heating. These results are used to illustrate key relationships and trends which affect the volume fraction of metal in a composite PCM which maximizes the thermal energy storage rate across different performance metrics.

Evaluation of different melting performance enhancement structures in a shell-and-tube latent heat thermal energy storage system

Latent heat thermal energy storage employing phase change materials is widely used in energy storage systems. To further improve the low thermal conductivity of phase change materials in these systems, it is essential to investigate different thermal enhancement techniques. In this work, two principal thermal enhancement techniques (i.e., finned tubes and conductive metal foams) are numerically investigated for melting processes in a shell-and-tube latent heat thermal energy storage system. Topology optimised structures are used as fins, and the simulation predictions are validated by experimental results using additive manufactured topology optimised fins. For metal foams, different filling ratios (i.e., whole-foam structure and half-foam structure) are considered. Compared to the configuration without enhancement, the thermal energy storage rates are 3.3–5.8 times higher. In addition, the results show that the topology optimised fins can achieve the best performance, but can only be an economical solution when the unit price ratio between the enhancement technique and the phase change materials is less than 6. For the first time, the thermal enhancement performance and economic efficiency of these two principal techniques are quantitatively analysed. The results would be useful for appropriate energy storage design solutions in practice.

Combined effects of nanoparticles and ultrasonic field on thermal energy storage performance of phase change materials with metal foam

To further improve the performance of thermal energy storage (TES) system with phase change materials (PCMs), this paper proposed a novel method, i.e. combining the additions of TiO2 nanoparticles, metal foam and the provision of ultrasonic field, investigated its synergetic effects in enhancing conduction and convection heat transfer. The thermal characteristics, including the TES time distributions and the energy consumption of the TES system, were discussed to evaluate the combined effects of TiO2 nanoparticles and ultrasonic field on the TES rate and TES efficiency. The results showed that the latent TES time reduction index reached 46.50%, when the TiO2 nanoparticles concentration was 5.0 wt% and the ultrasonic power was 100 W, while the TES efficiency dropped to 10.66%. Increasing TiO2 nanoparticles concentration and ultrasonic power positively improved the TES rate due to conduction heat transfer enhanced by nanoparticles and convection heat transfer enhanced by the acoustic streaming effect and the cavitation effect of the ultrasonic field, but which negatively reduced the TES efficiency mainly due to the energy consumption of the ultrasonic field. Therefore, the effects of the ultrasonic field introduced at four action stages on the TES rate and TES efficiency were compared, and it confirmed that introducing ultrasonic field at the latent TES stage was better than that in the sensible TES stage. Additionally, the proposed novel combined method needed to consider the priority relationship between TES rate and TES efficiency for designing the TES system, favoring the potentials for further advances in TES applications.

An experimental and numerical study on phase change material melting rate enhancement for a horizontal semi-circular shell and tube thermal energy storage system

In the present study, the melting characteristics of a lauric acid in a semi-circular and circular latent heat thermal energy storage unit have been studied. The low melting rate of phase change materials in the lower half of a circular heat storage unit can be enhanced by confining the phase change material in the upper region by configuring the outer shell of a conventional circular latent heat thermal energy storage unit into a semi-circular shape. The recent literature shows that no work has been carried out on the effect of the melting performance of phase change material in a semi-circular latent heat thermal energy storage unit hence an effort has been made to investigate the melting performance of phase change material experimentally and numerically for semi-circular latent heat thermal energy storage unit. The enhancement in the melting rate at three different inner tube position values from the bottom surface of outer semi-circular shell has been investigated and it is found that the enhancement is more when the inner tube is nearest to the bottom surface of the outer shell.The semi-circular heat storage unit is also compared with circular heat storage unit by considering the equal volume of phase change material in both the cases. The semi-circular latent heat thermal energy storage system shows enhancement in melting performance of phase change material as compared to circular latent heat thermal energy storage system. The semi-circular latent heat thermal energy storage system melts the phase change material completely in almost half time as compared to circular latent heat thermal energy storage system. It is found that the thermal energy stored in phase change material for semi-circular latent heat thermal energy storage system is 416.4 kJ which is 12.04 % more as compared to thermal energy stored of phase change material for circular latent heat thermal energy storage for the same time duration of 4800 sec. It is also found that the maximum thermal energy storage rate attained in semi-circular latent heat thermal energy storage system is 0.15 kW whereas in circular latent heat thermal energy storage system it is 0.13 kW which is 25 % higher as compared to that of circular latent heat thermal energy storage system.

Melting behavior of the latent heat thermal energy storage unit with fins and graded metal foam

Metal foam can effectively improve the melting rate of latent heat thermal energy storage units (LHTESU). However, the existing metal foam structure can’t simultaneously solve the problem of non-uniform melting caused by natural convection and slow melting rate in horizontal shell-and-tube LHTESU. To this end, the structure with fins and graded metal foam is proposed to further enhance thermal performance. The porosity of the metal foam increases gradually from the inner tube to the outer shell to reduce thermal resistance and improve thermal conduction globally. The fins (the porosity is 0) are employed to further enhance local heat transfer and balance the difference of melting rate between upper and lower parts. The correctness of the current model is verified by the one-phase Stefan problem with explicit solutions and research results in previous literature. With consideration of natural convection, the enthalpy-porosity method is adopted to compare the melting behavior of LHTESU strengthened by uniform metal foam, graded metal foam, fins and uniform metal foam, and fins and graded metal foam. The results show that the existence of the fins improves the uniformity of the temperature field and significantly shortens the melting time. Based on the same amount of metal materials, the proposed structure obtains the best strengthening effect in the four different structures. Furthermore, the effects of different structural parameters (the fins angle, fins thickness, and porosity gradient) on the melting process are discussed. It can be found that there are optimal values for the three structural parameters. Compared with the graded metal foam structure, the new structure (the fins angle is 25°, the fins thickness is 3 mm and the porosity gradient is 2%) further reduces the total melting time by 27.23% and increases the thermal energy storage rate by 36.52%.