Heat transfer of phase change materials (PCMs) in porous materials

In this paper, the feasibility of using metal foams to enhance the heat transfer capability of phase change materials (PCMs) in low- and high-temperature thermal energy storage systems was assessed. Heat transfer in solid/liquid phase change of porous materials (metal foams and expanded graphite) at low and high temperatures was investigated. Organic commercial paraffin wax and inorganic calcium chloride hydrate were employed as the low-temperature materials, whereas sodium nitrate was used as the high-temperature material in the experiment. Heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied. Composites of paraffin and expanded graphite with a graphite mass ratio of 3%, 6%, and 9% were developed. The heat transfer performances of these composites were tested and compared with metal foams. The results indicate that metal foams have better heat transfer performance due to their continuous inter-connected structures than expanded graphite. However, porous materials can suppress the effects of natural convection in liquid zone, particularly for PCMs with low viscosities, thereby leading to different heat transfer performances at different regimes (solid, solid/liquid, and liquid regions). This implies that porous materials do not always enhance heat transfer in every regime.     

A numerical investigation of heat transfer in phase change materials (PCMs) embedded in porous metals

The effects of metal foams on heat transfer enhancement in Phase Change Materials (PCMs) are investigated. The numerical investigation is based on the two-equation non-equilibrium heat transfer model, in which the coupled heat conduction and natural convection are considered at phase transition and liquid zones. The numerical results are validated by experimental data. The main findings of the investigation are that heat conduction rate is increased significantly by using metal foams, due to their high thermal conductivities, and that natural convection is suppressed owing to the large flow resistance in metal foams. In spite of this suppression caused by metal foams, the overall heat transfer performance is improved when metal foams are embedded into PCM; this implies that the enhancement of heat conduction offsets or exceeds the natural convection loss. The results indicate that for different metal foam samples, heat transfer rate can be further increased by using metal foams with smaller porosities and bigger pore densities.     

Laminar film condensation driven latent thermal energy storage in rectangular containers

This paper focuses on numerically analyzing the thermal transport phenomena in the transient conjugate problem of melting and laminar film condensation. The key focus is to identify an optimum container aspect ratio/shape and conditions for which the heat storage time and the storage capacity are minimum and maximum respectively. Since most solid–liquid phase change materials (PCMs) suffer from poor thermal conductivities, the major resistance to heat transfer comes from PCM. Hence, high thermal conductivity, low-cost metal foam is suggested for use along with PCM to minimize this resistance. The conjugate transient problem of film condensation driven solid–liquid phase change of PCM impregnated inside porous metal foam is numerically analyzed. An effective heat capacity formulation is employed for modeling the transient PCM phase change in porous foam and is solved using finite element method. It is coupled with laminar film condensation on the outside of the storage container. The model is then used for selecting the best aspect ratio for thermal energy storage (TES) containers that enables to store comparatively the maximum heat. The results of the developed model showed that the major resistance to heat transfer and hence efficient thermal energy storage depends strongly on the aspect ratio of the PCM storage containers.     

Review on thermal transport in high porosity cellular metal foams with open cells

Thermal transport in metal foams has received growing attention in both academic research and industrial applications. In this paper the recent research progress of thermal transport in metal foams has been reviewed. This paper aims to provide the comprehensive state-of-the-art knowledge and research results of thermal transport in open celled cellular metal foams, which covers the effective thermal conductivity, forced convection, natural convection, thermal radiation, pool boiling and flow boiling heat transfer, solid/liquid phase change heat transfer and catalytic reactor. The forced convection and thermal conductivity have been extensively investigated, while less research were performed on two-phase (boiling and solid/liquid phase change heat transfer) and thermal radiation in metal foams. Also most research still treats the metal foam as one type of effective continuous porous media, very few researchers investigated the detailed thermal behaviours at the pore level either by numerical or experimental approaches.     

Selection of phase change materials,metal foams and geometries for improving metal hydride performance

This paper reports the numerical investigation of the effect of different phase change materials (PCMs) on the metal hydride (MH) behaviour in a reactor bed during the absorption process. The feasibility of integrating metal foams (MFs) into the phase change materials to improve the hydrogen storage performance of the system was also evaluated. A two-dimensional model for a LaNi₅ hydride reactor equipped with different phase change materials has been developed. The selection of five different PCMs having a high latent heat of fusion and a range of melting temperatures were investigated. In addition, the effect of the mass and volume of the different PCMs on the hydrogen performance of the MH reactor was studied. It was found that LiNO₃·3H₂O PCM shown better performance than the other PCMs, its loading time is faster, and its mass within the reactor is enough to absorb the total heat generated from the MH during hydrogenation. Three different metals foam with three different porosities were integrated into the most suitable PCM with the appropriate dimension of a cylindrical reactor that shows the optimum performance. The obtained results indicated that the integration of the metal foams into the PCM show better heat transfer performance than the case of MH-PCM without metal foams. Two different configurations cylindrical and spherical MH reactors were investigated. The obtained results indicated that the two configurations have very similar behaviours. So, both configurations are good for the hydriding process within an MH reactor.     

Investigation of the effect of metal foam characteristics on the PCM melting performance in a latent heat thermal energy storage unit by pore-scale lattice Boltzmann modeling

Latent heat thermal energy storage (LHTES) has many advantages such as high energy density and phase change at a nearly constant temperature compared with sensible thermal energy storage or chemical energy storage techniques. However, one of its major drawbacks is the low thermal conductivity of phase change materials (PCMs) which impedes the heat transfer efficiency. High thermal conductivity metal foams could be added into the LHTES to enhance the heat transfer speed. Under this case, the investigation of the effects of metal foam porosity and pore size on the melting process is essential for improving the heat storage capability of LHTES. In this article, a pore-scale modeling of melting process in a LHTES unit filled with metal foams is carried out by enthalpy-based multiple-relaxation-time lattice Boltzmann method. The quartet structure generation set is used to generate the morphology of metal foams. In addition, a Compute Unified Device Architecture (CUDA) Fortran code is developed in this work for executing highly parallel computation through graphics processing units. The melting process in the PCMs is investigated in terms of porosity, pore size, nonuniform metal foam, hot wall temperature, and initial subcooled temperature to optimize the design of LHTES filled with metal foams.     

Analytical and experimental investigations of nanoparticles embedded phase change materials for cooling application in modern buildings

This paper presents the analytical and experimental investigations of the phase change heat transfer characteristics and thermodynamic behavior of spherically enclosed phase change material (PCM) with dispersion of nanoparticles for latent thermal energy storage (LTES) system in buildings. In this study, the heat transfer characteristics in terms of the transient temperature variations, moving interface positions, complete rate of solidification and melting were analyzed for the six different PCMs considered in pure form and with dispersed nanoparticles as well. The heat transfer characteristics of the PCMs considered were analytically modeled and experimentally evaluated for the steady state and transient conditions for various heat generation parameters during freezing and melting cycles of the LTES system. The experimental results infer that for the same thermal load conditions the rate of solidification for the PCMs decreased with the increased mass fractions of nanoparticles while compared to the pure PCMs. For the same operating conditions of the LTES system, similar heat transfer characteristics were observed for the six PCMs considered. In this paper, the analytical model solutions and experimental results for the 60% n-tetradecane: 40% n-hexadecane PCM are presented. The solidification time for the 60% n-tetradecane: 40% n-hexadecane PCM embedded with the aluminium and alumina nanoparticles were expected to reduce by 12.97% and 4.97% than at its pure form respectively. Besides, the test results indicate that by increasing the mass fraction of the nanoparticles beyond the limiting value of 0.07 the rate of solidification was not significant further. Furthermore, the rate of melting was improved significantly for the PCMs embedded with the dispersed nanoparticles than the pure PCMs. The analytical solutions obtained for the pure and dispersed nanoparticles based PCMs were validated using the experimental results. The deviations observed between the analytical solutions and the experimental results were in the range of 10%-13%. Based on the analytical and experimental results the present nanoencapsulated LTES system can be regarded as a potential substitute for the conventional LTES system in buildings for achieving enhanced heat transfer characteristics and energy efficiency.     

Thermal and heat transfer characteristics in a latent heat storage system using lauric acid

The thermal and heat transfer characteristics of lauric acid during the melting and solidification processes were determined experimentally in a vertical double pipe energy storage system. In this study, three important subjects were addressed. The first one is temperature distributions and temporal temperature variations in the radial and axial distances in the phase change material (PCM) during phase change processes. The second one is the thermal characteristics of the lauric acid, which include total melting and total solidification times, the nature of heat transfer in melted and solidified PCM and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition parameters. The final one is to calculate the heat transfer coefficient and the heat flow rate and also discuss the role of Reynolds and Stefan numbers on the heat transfer parameters. The experimental results proved that the PCM melts and solidifies congruently, and the melting and solidification front moved from the outer wall of the HTF pipe (HTFP) to the inner wall of the PCM container in radial distances as the melting front moved from the top to the bottom of the PCM container in axial distances. However, it was difficult to establish the solidification proceeding at the axial distances in the PCM. Though natural convection in the liquid phase played a dominant role during the melting process due to buoyancy effects, the solidification process was controlled by conduction heat transfer, and it was slowed by the conduction thermal resistance through the solidified layer. The results also indicated that the average heat transfer coefficient and the heat flow rate were affected by varying the Reynolds and Stefan numbers more during the melting process than during the solidification process due to the natural convection effect during the melting process.     

Thermal Energy Storage Performance of Fatty Acids as a Phase Change Material

Thermal energy storage performance of fatty acids and a eutectic mixture as phase change materials (PCMs) has been investigated experimentally. The selected PCMs for this study were palmitic acid, myristic acid, stearic acid, and a mixture of stearic and myristic acids in eutectic combination ratio of 65.7 wt% myristic acid and 34.3 wt% stearic acid. The PCMs have a melting temperature range of 50.0°C to 61.20°C and a latent heat range of 162.0 J/g to 204.5 J/g. The inlet temperature and the mass flow rate of heat transfer fluid (HTF) were selected as experimental parameters to test the thermal energy storage performance of the PCMs. The transition times, temperature range, propagation of the solid-liquid interface, as well as heat flow rate characteristics of the employed cylindrical tube storage system were studied at varied experimental parameters. The experimental results show that the melting front moves to inward in the radial directions as well as in the axial directions from the top toward to the bottom of the PCM tube. It was observed that the convection heat transfer in the liquid phase plays an important role in the melting process. The changes in the studied HTF parameters have more effect on the melting processes than the solidification processes of the PCMs. The average heat storage efficiency calculated from data for all the PCMs is 51.5%, meaning that 48.5% of the heat actually was lost somewhere.     

Solidification process analysis of a heat storage device containing graphite foam and paraffin

石蜡相变材料的导热系数较小,严重影响了其传热速率和凝固速率。通过对填充石墨泡沫/石蜡的储能系统进行凝固过程的模拟,确定了石墨泡沫对相变储能系统性能的影响。研究结果表明石墨泡沫不仅大大缩短了相变凝固时间,也使储能系统的温度分布更加均匀;通过分析冷却水进口速度和温度对复合相变材料的凝固过程的影响,说明随着冷却水进口速度的增大和温度的降低,传热速率加快,凝固时间缩短。分析了复合材料相变区的自然对流对相变过程的影响,模拟结果证明自然对流能在一定程度上加快相变材料的凝固过程。     

Numerical investigation of the thermal mechanism of the solid-liquid phase changing process

由于相变换热储能技术可以协调能量在时间和空间尺度的分配,成为了目前研究的热点问题。本工作用焓值法分别对充填低温无机盐相变材料的二维和三维管壳式相变储能换热器模型的储/放热特性进行了模拟研究,采用Boussinesq近似研究了液相区密度变化引起的自然对流的影响。研究表明换热器的入口温度对相变换热效率影响显著;在储热过程中自然对流发挥了重要作用,换热效率与液相区的运动状态直接相关,而放热过程中的热交换主要依靠热传导完成;三维模拟的结果表明换热管出口温度与管壁的平均努赛尔数高度相关,且换热管水平放置的换热效率略低于竖直放置。     

Development of graphite foam infiltrated with MgCl2 for a latent heat based thermal energy storage (LHTES) system

Thermal energy storage (TES) systems that are compatible with high temperature power cycles for concentrating solar power (CSP) require high temperature media for transporting and storing thermal energy. To that end, TES systems have been proposed based on the latent heat of fusion of the phase change materials (PCMs). However, PCMs have relatively low thermal conductivities. In this paper, use of high-thermal-conductivity graphite foam infiltrated with a PCM (MgCl₂) has been investigated as a potential TES system. Graphite foams with two porosities were infiltrated with MgCl₂. The infiltrated composites were evaluated for density, heat of fusion, melting/freezing temperatures, and thermal diffusivities. Estimated thermal conductivities of MgCl₂/graphite foam composites were significantly higher than those of MgCl₂ alone over the measured temperature range. Furthermore, heat of fusion, melting/freezing temperatures, and densities showed comparable values to those of pure MgCl₂. Results of this study indicate that MgCl₂/graphite foam composites show promise as storage media for a latent heat thermal energy storage system for CSP applications.     

Melting in a side heated tall enclosure by a uniformly dissipating heat source

Melting of an organic phase change material (PCM) n-triacontane (C₃₀H₆₂) in a side heated tall enclosure of aspect ratio 10, by a uniformly dissipating heat source has been studied computationally and experimentally. While heat transfer data for melting in enclosures under isothermal wall boundary condition are available in the literature, other boundary conditions, such as constant heat flux often arise in applications of PCM for transient thermal management of electronics. An implicit enthalpy–porosity approach was utilized for computational modeling of the melting process. Experimental visualization of melt front locations was performed. Comparisons between experimental and computational heat transfer data and melt interface locations were good. Fluid flow and heat transfer characteristics during melting suggested that natural convection plays a dominant role during initial stages of melting. At later times, the strength of natural convection diminishes as melting is completed. Correlations of heat transfer rate and melt fraction with time were obtained.     

均匀及梯度泡沫金属复合PCM熔化特性模拟研究

基于六面通圆孔的均匀泡沫金属结构,构建了泡沫金属复合相变材料(PCM)三维模型,采用高性能计算显卡(GPU)加速的多松弛时间格子玻尔兹曼方法模拟了均匀及梯度泡沫金属复合PCM的瞬态熔化过程。结果表明：随着均匀泡沫金属孔隙率的降低,复合PCM的传热速率提高,潜热储能的能力减弱;对于固定平均孔隙率的不均匀泡沫金属,孔隙率沿导热方向上递增的模型具有最佳的强化传热效果,其完全熔化时间比填充均匀骨架模型和孔隙率在导热方向上递减的骨架模型分别缩短了4.2%和25%,当孔隙率梯度变化方向与导热方向一致时,在高温壁面附近填充低孔隙率泡沫金属能显著强化传热;当两者方向垂直时,熔化速率取决于平均孔隙率,与梯度分布几乎无关。     

Experimental and numerical investigation of the melting process and heat transfer characteristics of multiple phase change materials

The melting and heat transfer characteristics of multiple phase change materials (PCMs) are investigated both experimentally and numerically. Multiple PCMs, which consist of three PCMs with different melting points, are filled into a rectangle-shaped cavity to serve as heat storage unit. One side of the cavity is set as heating wall. The melting rate of multiple PCMs was recorded experimentally and compared with that of single PCM for different heating temperatures. A two-dimensional mathematical model to describe the phase change heat transfer was developed and verified experimentally. The properties of multiple PCMs, including the effect of the melting point difference (combined type), thermal conductivity, and latent heat, on the heat transfer performance of the PCM were analyzed numerically. The results show that, the melting time decreases before it increases, with an increasing melting point difference for the multiple PCMs. In addition, the melting point decreases with increasing distance from the heating wall. Most of these types of multiple PCMs melt faster than the single PCM, and the multiple PCMs, with the melting point arranged as 322 K/313 K/304 K, has the shortest melting time in this study. The melting rate of the multiple PCMs, 322 K/313 K/304 K, accelerates faster than for the single PCM as the thermal conductivity, latent heat, and heating wall temperature increase. Finally, generalized results are obtained using a dimensionless analysis for both single and multiple PCMs.     

Numerical Analysis on a Latent Thermal Energy Storage System with Phase Change Materials and Aluminum Foam

Abstract

A latent heat thermal energy storage system with phase change material (PCM) is numerically studied. To enhance the heat transfer inside the system, a highly conductive metal foam is employed with ceramic nanoparticles. The latter method of enhancement leads to a new class of material called Nano-PCM. The system under investigation is a 70-L tank filled up with pure PCM or Nano-PCM and several pipes are situated where the heat transfer fluid (HTF) flows. The pipe surfaces are assumed at constant temperature above the PCM melting temperature to simulate the heat transfer from the HTF. The enthalpy-porosity theory is applied to simulate the PCM phase change, while the porous media formulation is assumed to describe the metal foam behavior. The nano-PCM is modeled with single-phase model where the properties are the weighted-average between the fluid base and the nanoparticles. The simulations are accomplished for charging-discharging process at different porosities and nanoparticle concentration. The results are given in term of average melting fraction evolution, average temperature as function of time, average stored energy. The metal foam significantly improves the heat transfer between PCM and HTF respect to the addition of nanoparticles, reducing the charging and discharging time more than one order of magnitude.     

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.     

An experimental optimization study on a tube-in-shell latent heat storage

Thermal energy storage (TES) using phase change materials (PCMs) has recently received considerable attention in the literature, due to its high storage capacity and isothermal behaviour during the storage (melting or charging) and removal (discharging or solidification). In this study, a novel modification on a tube-in-shell-type storage geometry is suggested. In the proposed geometry, the outer surface of the shell is inclined and it is the objective of this study to determine the optimum range for the inclination angle of the shell surface. Paraffin with a melting temperature of 58.06°C, which is supplied by the Merck Company, is used as the PCM. The PCM is stored in the vertical annular space between an inner tube through which the heat transfer fluid (HTF), hot water, is flowing and a concentrically placed outer shell. At first, the thermophysical properties of this paraffin are determined through the differential scanning calorimeter (DSC) analysis. Temporal behaviour of the PCM undergoing a non-isothermal solid–liquid phase change during its melting or charging by the HTF are determined for different values of the inlet temperature and the mass flow rate of the HTF. The new geometry is shown to respond well with the melting characteristics of the PCM and to enhance heat transfer inside the PCM for a specific range of the shell inclination angle. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

Lattice Boltzmann simulation of tree-shaped fins enhanced melting heat transfer

Abstract

A thermal lattice Boltzmann model is developed to simulate the melting process with natural convection in a cavity filled with tree-shaped solid fins, in which the velocity field and temperature field distribution functions are considered. The present model incorporates the total enthalpy and a free parameter in the equilibrium distribution function to handle conjugate heat transfer. The results indicate that natural convection of liquid phase change material (PCM) plays a significant role in the melting heat transfer of PCM. Increasing the number of branching levels leads to a more rapid melting process, and selecting appropriate bifurcation angle has more efficient heat transfer performance.