Analysis of contact melting of phase change materials inside a heated rectangular capsule

Close-contact melting processes of phase change material (PCM) inside a horizontal rectangular capsule are studied. The PCM is heated by the capsule at constant heat flux at the top and isothermally at the bottom, and the sides are adiabatic. The theoretical formulas of the dimensionless melting rate and the thickness of the liquid layer during the heat transfer process are obtained by analysing, which is convenient for engineering predictions. Finally, the influences on the melting process are discussed, and conclusions are drawn.     

Effect of internal void placement on the heat transfer performance – Encapsulated phase change material for energy storage

The effect of an internal air void on the heat transfer phenomenon within encapsulated phase change material (EPCM) is examined. Heat transfer simulations are conducted on a two dimensional cylindrical capsule using sodium nitrate as the high temperature phase change material (PCM). The effects of thermal expansion of the PCM and the buoyancy driven convection within the fluid media are considered in the present thermal analysis. The melting time of three different initial locations of an internal 20% air void within the EPCM capsule are compared. Latent heat is stored within an EPCM capsule, in addition to sensible heat storage. In general, the solid/liquid interface propagates radially inward during the melting process. The shape of the solid liquid interface as well as the rate at which it moves is affected by the location of the internal air void. The case of an initial void located at the center of the EPCM capsule has the highest heat transfer rate and thus fastest melting time. An EPCM capsule with a void located at the top has the longest melting time. Since the inclusion of a void space is necessary to accommodate the thermal expansion of a PCM upon melting, understanding its effect on the heat transfer within an EPCM capsule is necessary.     

矩形腔内相变材料接触熔化的分析

对矩形腔内相变材料紧密接触熔化过程进行了理论分析。应用努谢尔特液体边界层理论，求得了便于工程计算用的接触熔化传热过程的理论解。分析结果与实验数据进行了比较，吻合程度良好。     

Latent heat thermal energy storage using cylindrical capsule: Numerical and experimental investigations

This paper is aimed at analyzing the melting behavior of paraffin wax as a phase change material (PCM) encapsulated in a cylindrical capsule, used in a latent heat thermal energy storage system with a solar water heating collector. The heat for melting of PCM in the capsule is provided by hot water surrounding it. Since it is observed experimentally that the phase change occurs in a range of temperature, the present analysis considers this range instead of constant phase change temperature and the deviation between the results of these two is presented. The numerical analysis has been carried out by using enthalpy method and the results are verified with the experimental data. The experiments have been done by visualization technique without disturbing the actual process of melting. Three distinct stages of melting process have been identified as revealed by visualization studies. Results indicate that the melting process is chiefly governed by the magnitude of the Stefan number, Ste, phase change temperature range and the capsule radius. The analysis shows that the agreement between analytical and experimental results is significantly improved when the results are obtained considering phase change temperature range and the natural convection in the liquid phase instead of considering the process to be conduction dominated only.     

Experimental and computational study of constrained melting of phase change materials (PCM) inside a spherical capsule

An experimental and computational investigation directed at understanding the role of buoyancy-driven convection during constrained melting of phase change materials (PCM) inside a spherical capsule is reported. The computations are based on an iterative, finite-volume numerical procedure that incorporates a single-domain enthalpy formulation for simulation of the phase change phenomenon. A Darcy’s Law-type porous media treatment links the effect of phase change on convection. Paraffin wax n-octadecane was constrained during melting inside a transparent glass sphere through the use of thermocouples installed inside the sphere. The melting phase front and melting fraction of the PCM are analyzed and compared with numerical solution obtained from the CFD code Fluent. Following a short period of symmetric melting due to prominence of diffusion, expedited phase change in the top region of the sphere and a wavy surface at the bottom of the PCM are observed. The computational predictions point to the strong thermal stratification in the upper half of the sphere that results from rising of the molten liquid along the inner surface of the sphere thus displacing the colder fluid. The waviness and excessive melting of the bottom of the PCM is shown to be underestimated by the experimental observation. This discrepancy is linked to the use of a support structure to hold the sphere. Measured temperature data and computational results near the bottom indicate the establishment of an unstable fluid layer that promotes chaotic fluctuations and is responsible for waviness of the bottom of the PCM. On the other hand, the comparison between the measured and computed temperatures in the top half of the sphere show the stable nature of the molten liquid layer. The computational results start to deviate from the thermocouple readings as one moves lower from the top of the sphere. This delay in predicting the melting instant is linked to the thermal stratification within the “constant temperature bath” that encloses the capsule.     

Investigation on Phase Change Behavior of Paraffin Phase Change Material in a Spherical Capsule for Solar Thermal Storage Units

In this work, the melting and solidification behaviour of paraffin phase change material encapsulated in a stainless steel spherical container has been studied experimentally. A computational fluid dynamics analysis has also been performed for the encapsulated phase change material (PCM) during phase change process. In the melting process, the hot air, used as the heat transfer fluid enters the test section and flows over the spherical capsule resulting in the melting of phase change material. In the solidification process, the ambient air flows over the capsule and received heat from phase change material resulting in the solidification of phase change material. In the computational fluid dynamics, the constant wall boundary condition is employed for both melting (75°C) and solidification (36°C) processes since the internal conductive resistance offered by the PCM is much higher compared to the outer surface convective resistance. The time required for complete solidification and melting of the phase change material obtained from the computational fluid dynamics analysis are validated with the experimental results and a reasonable agreement is achieved. The reason for the deviation between the results are analyzed and reported.     

Analysis of close-contact melting for octadecane and ice inside isothermally heated horizontal rectangular capsule

Close-contact melting characteristics of phase change materials (PCM) inside horizontal rectangular capsules are examined experimentally. The capsules are heated isothermally and three kinds of aspect ratios (H/W = 3, 1 and 1/3) are provided. Octadecane and ice are used, respectively, as PCM. A method of analysis applying Nusselt's liquid film theory to the close-contact melting heat transfer in the rectangular capsule is presented. The analytical results show good agreement with the experimental data. For the melting of ice, it is found that the effect of natural convection resulting from density inversion of water at 4°C becomes significant for large Stefan numbers.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.     

Exergy analysis of two phase change materials storage system for solar thermal power with finite-time thermodynamics

A mathematical model for the overall exergetic efficiency of two phase change materials named PCM1 and PCM2 storage system with a concentrating collector for solar thermal power based on finite-time thermodynamics is developed. The model takes into consideration the effects of melting temperatures and number of heat transfer unit of PCM1 and PCM2 on the overall exergetic efficiency. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the air which assumes that the temperature of the air varies in its flow path. The results show that the overall exergetic efficiency can be improved by 19.0-53.8% using two PCMs compared with a single PCM. It is found that melting temperatures of PCM1 and PCM2 have different influences on the overall exergetic efficiency, and the overall exergetic efficiency decreases with increasing the melting temperature of PCM1, increases with increasing the melting temperature of PCM2. It is also found that for PCM1, increasing its number of heat transfer unit can increase the overall exergetic efficiency, however, for PCM2, only when the melting temperature of PCM1 is less than 1150 K and the melting temperature of PCM2 is more than 750 K, increasing the number of heat transfer unit of PCM2 can increase the overall exergetic efficiency. Considering actual application of solar thermal power, we suggest that the optimum melting temperature range of PCM1 is 1000-1150 K and that of PCM2 is 750-900 K. The present analysis provides theoretical guidance for applications of two PCMs storage system for solar thermal power.     

Melting of NEPCM within a Cylindrical Tube: Numerical Study Using the Lattice Boltzmann Method

This article presents a numerical investigation on melting of phase change material (PCM) enhanced by nanoparticles inside a cylindrical tube using the lattice Boltzmann method. Water (ice) and copper particles are chosen as the base fluid (PCM) and nanoparticles, respectively. Results show that the melting rate is the same for all regions of the cylinder for a low Rayleigh number, while it intensifies at the top half of the cylinder for a moderate Rayleigh number. Also, existence of strong unstable flow in the bottom portion of the cylinder at a Rayleigh number of 10⁶ causes the melting rate to keen after a definite time. Nanoparticles have no significant effect on the melting rate at the beginning of melting, where the conduction mode of heat transfer dominates between the hot wall and solid PCM, while full melting of PCM occurs earlier by the increase of solid concentration.     

Thermal Energy Storage Module Design Using Energy and Exergy Analysis

In this paper, irreversibility of a thermal energy storage system is numerically investigated. The system consists of two concentric cylinders. The outer cylinder is filled with phase change material (PCM), while working fluid flows inside the inner pipe. The system works periodically. The related governing equations are solved by a control volume-based finite difference method. The effects of different parameters such as PCM size and melting point temperature are examined on the irreversibility of the system. The results show that the irreversibility of thermal storage module is strongly affected by the size of PCM (diameter and length of the external cylinder) and melting temperature. Based on the obtained results, the irreversibility of the system can be reduced by proper selection of PCM size and melting temperature.     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

Numerical study on performance enhancement of latent heat storage unit

建立了考虑液态相变材料自然对流的壳管式相变蓄热单元的三维模型,数值分析了自然对流对相变蓄热过程的影响.对比研究了外侧强化传热管和双侧强化传热管对相变蓄热单元蓄热性能的强化效果.结果表明,液态相变材料的自然对流,会引起固-液界面分布不均匀现象,采用外翅片管可以有效削弱这一现象;采用外侧强化传热管和双侧强化传热管,都可以缩短相变材料完全熔化以及整个蓄热过程所需时间.与采用光管时相比,采用外侧强化传热管时,完全熔化时间减少了18.0%;采用双侧强化传热管时,完全熔化时间减少52.5%.可见,采用带有外翅片的强化传热管,不仅可以削弱自然对流引起的固-液界面不均匀性问题,而且可以强化相变蓄热单元的蓄热性能.     

Numerical analysis and parameters optimization of shell-and-tube heat storage unit using three phase change materials

In this paper, a mathematical model of shell-and-tube latent heat thermal energy storage (LHTES) unit of two-dimension of three phase change materials (PCMs) named PCM1, PCM2 and PCM3 with different high melting temperatures (983 K, 823 K and 670 K, respectively) and heat transfer fluid (HTF: air) with flowing resistance and viscous dissipation based on the enthalpy method has been developed. Instantaneous solid–liquid interface positions and liquid fractions of PCMs as well as the effects of inlet temperatures of the air and lengths of the shell-and-tube LHTES unit on melting times of PCMs were numerically analyzed. The results show that melting rates of PCM3 are the fastest and that of PCM1 are the slowest both x, r directions. It is also found that the melting times of PCM1, PCM2 and PCM3 decrease with increase in inlet temperatures of the air. Moreover, with increase in inlet temperatures of the air, decreasing degree of their melting times are different, decreasing degree of the melting time of PCM1 is the biggest and that of PCM3 is the smallest. Considering actual application of solar thermal power, we suggest that the optimum lengths are L₁ = 250 mm, L₂ = 400 mm, L₃ = 550 mm (L = 1200 mm) which corresponds to the same melting times of PCM1, PCM2 and PCM3 are about 3230 s and inlet temperature of the air is about 1200 K. The present analysis provides theoretical guidance for designing optimization of the shell-and-tube LHTES unit with three PCMs for solar thermal power.     

A new model of phase change process for thermal energy storage

Thermal energy can be converted into mechanical energy through the melting process of a phase change material (PCM). A PCM mixed with an insoluble liquid has higher energy converting efficiency during the whole melting process, where the massive microvacuum formed during the freezing process is filled by the insoluble liquid, which increases utilization of the volume change. The traditional theoretical model of the phase change process is unable to sufficiently describe the mixed PCM; therefore, a new model aimed at analyzing the characteristics of the volumetric change rate, as well as the freezing and melting times of the mixed PCM, is theoretically constructed. In this paper, the effective heat capacity method is used, and the effects of porosity are considered when the PCM is in the solid state. Comparisons of this model with the traditional model are carried out using both simulations and experiments for different pressures and geometric structures. Our results indicate that the introduced model has better accuracy when describing the phase change process of the pure PCM mixed with an insoluble liquid.     

石墨泡沫/共晶盐复合相变材料制备

选择KNO3/NaNO3二元体系按照质量比4∶6制备共晶盐,对共晶盐进行了熔点及熔化潜热的测量;将石墨泡沫这一新型材料作为强化基体,共晶盐作为相变材料（PCM）,采用熔融浸渗法制备了适用于太阳能热发电系统储能装置的石墨泡沫/共晶盐复合相变材料。采用扫描电镜对复合相变材料表面的微观结构进行了表征,并对其熔点、潜热、等效导热系数等热物性参数进行了测试。结果表明：共晶盐与石墨泡沫复合效果比较理想;复合前后共晶盐的熔点和潜热几乎没有发生变化;复合相变材料的等效导热系数得到了显著提升,石墨泡沫对相变材料起到了导热强化作用,满足高温蓄热的要求。     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.     

Comparative study of single and multi-layered packed-bed thermal energy storage systems for CSP plants

The Multi-layered Thermal Energy Storage (TES) tank consists of three regions–top and bottom part is packed with suitable Phase Change Materials (PCM) and low-cost pebbles are placed in the middle region, whereas entire tank portion is filled by solid fillers in Single-layered tank system. For a storage tank operating between 563 and 663 K with bed dimensions of 12 and 14.38 m using Solar salt as Heat Transfer Fluid (HTF), it is observed that the duration of discharge for multi-layered tank is 5.32 h whereas it is 4.19 h for single-layered tank with a Reynolds number of 10. The effect of intermediate melting temperature range of PCMs are also analyzed by taking PCMs with sharp as well as intermediate melting ranges. Further, comparison of single and multi-layered systems is carried out by analyzing the temperature profiles and width of both PCM layers. The width of top and bottom PCM layers of tank is varied from 0 to 30% to analyze its effect on the discharging duration. It is observed that multi-layered system provides extra discharge of 1 h with introduction of PCM at top and bottom with a width of 10%. Discharge duration increases with increase in PCM width whereas the percentage increase in duration of discharge with increase in PCM width is comparatively less. It is also seen that PCMs with sharp melting point performs better compared to one having intermediate range of melting temperatures. Multi-layered configuration concept offers best possibilities as integration to CSP plants with desired efficiency.     

Thermal performance of PCM thermal storage unit for a roof integrated solar heating system

The thermal performance of a phase change thermal storage unit is analysed and discussed. The storage unit is a component of a roof integrated solar heating system being developed for space heating of a home. The unit consists of several layers of phase change material (PCM) slabs with a melting temperature of 29 °C. Warm air delivered by a roof integrated collector is passed through the spaces between the PCM layers to charge the storage unit. The stored heat is utilised to heat ambient air before being admitted to a living space. The study is based on both experimental results and a theoretical two dimensional mathematical model of the PCM employed to analyse the transient thermal behaviour of the storage unit during the charge and discharge periods. The analysis takes into account the effects of sensible heat which exists when the initial temperature of the PCM is well below or above the melting point during melting or freezing. The significance of natural convection occurring inside the PCM on the heat transfer rate during melting which was previously suspected as the cause of faster melting process in one of the experiments is discussed. The results are compared with a previous analysis based on a one dimensional model which neglected the effect of sensible heat. A comparison with experimental results for a specific geometry is also made.     

Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs)

In this paper the experimental investigation on the solid/liquid phase change (melting and solidification) processes have been carried out. Paraffin wax RT58 is used as phase change material (PCM), in which metal foams are embedded to enhance the heat transfer. During the melting process, the test samples are electrically heated on the bottom surface with a constant heat flux. The PCM with metal foams has been heated from the solid state to the pure liquid phase. The temperature differences between the heated wall and PCM have been analysed to examine the effects of heat flux and metal foam structure (pore size and relative density). Compared to the results of the pure PCM sample, the effect of metal foam on solid/liquid phase change heat transfer is very significant, particularly at the solid zone of PCMs. When the PCM starts melting, natural convection can improve the heat transfer performance, thereby reducing the temperature difference between the wall and PCM. The addition of metal foam can increase the overall heat transfer rate by 3-10 times (depending on the metal foam structures and materials) during the melting process (two-phase zone) and the pure liquid zone. The tests for investigating the solidification process under different cooling conditions (e.g. natural convection and forced convection) have been carried out. The results show that the use of metal foams can make the sample solidified much faster than pure PCM samples, evidenced by the solidification time being reduced by more than half. In addition, a two-dimensional numerical analysis has been carried out for heat transfer enhancement in PCMs by using metal foams, and the prediction results agree reasonably well with the experimental data.