Numerical study of nanoencapsulated phase change material inside double pipe heat exchanger

Nanoencapsulated phase change material (NPCM) slurry is a dispersion where the phase change material (PCM) is dispersed in fluid. Compared with fluid, these nanofluids have a higher heat capacity during the phase change and a possible enhancement, as a result of this phase change, in the heat transfer phenomenon. To appreciate the merits, in terms of energy, a numerical study has been carried out with fluid based on NPCM inside double pipe heat exchanger. The numerical simulation results have been validated using experimental heat transfer data. The Reynolds and Nusselt numbers have been determined using thermal conductivity and viscosity evaluated in the same conditions as those in numerical model. The results obtained show an improvement of this energetic criterion at low mass flow rate compared with the base fluid. Analysis of the numerical and analytical results reveal that higher inlet flow rate and NPCM concentration results in higher heat transfer rate. In addition, increasing NPCM slurry temperature decreases its performance due to fast melting of PCM inside the tube.     

PCM thermal insulation in buildings

This paper presents the results of a numerical and experimental study of phase change material (PCM) filled walls and roofs under real operational conditions to achieve passive thermal comfort. The numerical part of the study was based on a one-dimensional model for the phase change problem controlled by pure conduction. Real radiation data was used to determine the external face temperature. The numerical treatment was based upon using finite difference approximations and the ADI scheme. The results obtained were compared with field measurements. The experimental set-up consisted of a small room with movable roof and side wall. The roof was constructed in the traditional way but with the phase change material enclosed. Thermocouples were distributed across the cross section of the roof. Another roof, identical but without the PCM, was also used during comparative tests. The movable wall was also constructed as is done traditionally but with the PCM enclosed. Again, thermocouples were distributed across the wall thickness to enable measurement of the local temperatures. Another wall, identical but without the PCM, was also used during comparative tests. The PCM used in the numerical and experimental tests was composed of a mixture of two commercial grades of glycol in order to obtain the required fusion temperature range. Comparison between the simulation results and the experiments indicated good agreement. Field tests also indicated that the PCM used was adequate and that the concept was effective in maintaining the indoor temperature very close to the established comfort limits. Further economical analysis indicated that the concept could effectively help in reducing the electric energy consumption and improving the energy demand pattern. © 1997 by John Wiley & Sons, Ltd.     

Impact of the Relationship between Phase Change Temperature and Boundary Temperature on the Thermal Performance of a PCM Wall and the Presentation of PCM Thermal Performance Indexes

In the composite phase change material (PCM) building envelope, the matching relationship between the phase change temperature of the PCM and the wall's boundary temperature significantly affects the energy storage performance of the PCM building envelope. In this paper, a type of concrete hollow block with a typical structure and a common PCM were adopted to produce multiform composite PCM hollow blocks, and the temperature changing hot chamber method was performed to test the thermal performance of the hollow block walls under different temperature conditions. New indexes were proposed for the thermal performance evaluation of the PCM wall. Meanwhile, combined with experimental data, the effective heat capacity model and the enthalpy model were used to analyze the effect of correlations concerning how the relationship between phase change temperature and wall's boundary temperature influenced the thermal performance of PCM wall. Three main impact factors related to temperature were obtained through the analysis. In addition, approaches for improving the thermal performance of a composite PCM wall were put forward. This paper provides the theoretical basis, data reference and practical instruction for the proper use of a PCM wall and ways for improving the thermal performance of a composite PCM wall.     

太阳能吸热器换热管蓄热数值模拟与试验研究

对以高温共晶盐LiF—CaF2为相变材料(PCM)和以干空气为工质的相变蓄热系统，采用焓方法建立了以控制体单元为对象的单管相变蓄热模型，并对系统进行了数值分析，得到了循环工质气体出口温度、相变材料容器最高温度和平均壁温等参数的瞬态变化曲线，实验研究了吸热器换热管的蓄傲热性能，分析了工质进口温度、输入热流级工质流量对工质出口温度、PCM容器平均壁温及最高壁温的影响。计算结果和试验表明单元换热管的蓄傲热性能达到了设计要求，试验结果与数值计算吻合良好。     

Numerical study on melt fraction during melting of phase change material inside a sphere

This paper presents a numerical study on the constrained melting of phase change material (PCM) inside a sphere to investigate the effect of various factors on the melt fraction. A mathematical model of melting processes of the PCM inside a sphere is developed. And experiments are conducted to verify the numerical method. On the basis of the model, the effects of the sphere radius, the bath temperature, the PCM thermal conduction coefficient and the spherical shell material on the melt fraction of PCM inside a sphere are discussed. The results show that the PCM inside a sphere melts fast as the sphere radius is small, the bath temperature increases, and the PCM thermal conductivity is high. And the metal shell with high thermal conductivity should be adopted preferentially. The present study provides theoretical guidance for the design and operation of the phase change heat storage unit with sphere containers.     

Thermal performance analysis of a flat slab phase change thermal storage unit with liquid-based heat transfer fluid for cooling applications

This paper presents the results of a thermal performance analysis of a phase change thermal storage unit. The unit consists of several parallel flat slabs of phase change material (PCM) with a liquid heat transfer fluid (HTF) flowing along the passages between the slabs. A validated numerical model developed previously to solve the phase change problem in flat slabs was used. An insight is gained into the melting process by examining the temperatures of the HTF nodes, wall nodes and PCM nodes and the heat transfer rates at four phases during melting. The duration of the melting process is defined based on the level of melting completion. The effects of several parameters on the HTF outlet temperature, heat transfer rate and melting time are evaluated through a parametric study to evaluate the effects of the HTF mass flow rate, HTF inlet temperature, gap between slabs, slab dimensions, PCM initial temperature and thermal conductivity of the container on the thermal performance. The results are used to design a phase change thermal storage unit for a refrigerated truck.     

An analysis of isothermal phase change of phase change material within rectangular and cylindrical containers

In this paper, a simple computational model for isothermal phase change of phase change material (PCM) encapsulated in a single container is presented. The mathematical model was based on an enthalpy formulation with equations cast in such a form that the only unknown variable is the PCM’s temperature. The theoretical model was verified with a test problem and an experiment performed in order to assess the validity of the assumptions of the mathematical model. With very good agreement between experimental and computational data, it can be concluded that conduction within the PCM in the direction of heat transfer fluid flow, thermal resistance of the container’s wall, and the effects of natural convection within the melt can be ignored for the conditions investigated in this study. The numerical analysis of the melting time for rectangular and cylindrical containers was then performed using the computational model presented in this paper. Results show that the rectangular container requires nearly half of the melting time as for the cylindrical container of the same volume and heat transfer area.     

Thermal performance of myristic acid as a phase change material for energy storage application

Thermal performance and phase change stability of myristic acid as a latent heat energy storage material has been studied experimentally. In the experimental study, the thermal performance and heat transfer characteristics of the myristic acid were tested and compared with other studies given in the literature. In the present study is included some parameters such as transition times, temperature range, and propagation of the solid–liquid interface as well as heat flow rate effect on the phase change stability of myristic acid as a phase change material (PCM). The experimental results showed that the melting stability of the PCM is better in the radial direction than the axial direction. The variety of the melting and solidification parameters of the PCM with the change of inlet water temperature is also studied. The results show that the better stability of the myristic acid was accomplished at low inlet water temperature compared with the obtained results at high inlet water temperature. We also observed that while the heat exchanger tube is in the horizontal position, the PCM has more effective and steady phase change characteristics than in the vertical position. The heat storage capacity of the container (PCM tube) is not as good as we expected in this study and the average heat storage efficiency (or heat exchanger effectiveness) is 54%. It means that 46% of the heat acrually lost somewhere.     

夏季间歇运行工况下相变温度对相变回填地埋管换热器传热性能的影响

为探究相变温度对相变材料回填地埋管换热器传热性能的影响,建立管内流体换热、回填区域相变换热及土壤换热的三维耦合传热数值模型,利用焓-多孔介质模型对相变区域相变问题进行处理,研究夏季间歇运行工况下不同相变温度回填材料对埋管换热器传热性能的影响。结果表明:添加PCM,可有效提高换热量,短期内缓解埋管周围热积聚,利用相变温度18℃的PCM回填,单位井深换热量至少比普通材料回填提高49.54%;在间歇运行初期,换热量随相变温度的升高逐渐减小,低相变温度的PCM可明显改善埋管换热量,但随着时间的进行,较高相变温度PCM回填对换热器换热量的改善效果优于前期低相变温度。此外,在运行期间,不同相变温度的PCM表现出不同的熔化、凝固特性,当PCM的熔化、凝固过程交替进行时,可减缓土壤温度在运行期间内波动幅度。     

Heat transfer behaviour of thermal energy storage components using composite phase change materials

对基于复合相变材料储热单元的储热性能进行了研究。建立了复合材料和储热单元体内部的二维传热模型,考察了复合材料物性和结构尺寸及传热流体操作条件（流体流速）对单元体储热性能的影响,对比了两种不同结构单元体的储热性能,并搭建实验平台进行了实验对比研究。对比结果表明,模型结果与实验结果趋于一致,验证了模型的准确性。复合材料物性和结构尺寸及传热流体操作条件对单元体储热性能有较大的影响。相比较单管储热单元体,同心管储热单元体有着更优的储热特性,在相同的操作条件下,同心管储热单元体的储热、放热时间较单管储热单元体分别减少10%和15%。     

Performance evaluation of a solar thermal energy storage system using nanoparticle-enhanced phase change material

This paper presents a numerical investigation on the thermal performance of a solar latent heat storage unit composed of rectangular slabs combined with a flat-plate solar collector. The rectangular slabs of the storage unit are vertically arranged and filled with phase change material (PCM: RT50) dispersed with high conductive nanoparticles (Al₂O₃). A heat transfer fluid (HTF: water) goes flow in the solar collector and receives solar thermal energy form the absorber area, then circulates between the slabs to transfer heat by forced convection to nanoparticle-enhanced phase change material (NEPCM). A numerical model based on the finite volume method and the conservation equations was developed to model the heat transfer and flow processes in the storage unit. The developed model was validated by comparing the obtained results with the experimental, numerical and theoretical results published in the literature. The thermal performance of the investigated latent heat storage unit combined with the solar collector was evaluated under the meteorological data of a representative day of the month of July in Marrakesh city, Morocco. The effect of the dispersion of high conductive nanoparticles on the thermal behavior and storage performance was also evaluated and compared with the case of base PCM without additives.     

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.     

相变蓄热装置内部温度场的理论与实验研究

对螺旋盘管相变蓄热装置性能和相变材料 (PCM)的传热特性开展理论和试验研究,建立相变蓄热装置物理和数学模型,对蓄热温度场进行了数值模拟和实验测试。结果表明 ：自然对流换热对PCM的熔化过程影响很大,当考虑自然对流时,相变蓄热速率加快,相变分层现象明显;实验实测温度与模拟温度相近,说明所建立的模型适用于相变装置内部温度场的模拟。     

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.     

Numerical heat transfer analysis of the packed bed latent heat storage system based on an effective packed bed model

Due to the complexity of the fluid flow and heat transfer in packed bed latent thermal energy storage (LTES) systems, many hypotheses were introduced into the previous packed bed models, which consequently influenced the accuracy and authenticity of the numerical calculation. An effective packed bed model was therefore developed, which could investigate the flow field as the fluid flows through the voids of the phase change material (PCM), and at the same time could account for the thermal gradients inside the PCM spheres. The proposed packed bed model was validated experimentally and found to accurately describe the thermo-fluidic phenomena during heat storage and retrieval. The proposed model was then used to do a parametric study on the influence of the arrangement of the PCM spheres and encapsulation of PCM on the heat transfer performance of LTES bed, which was difficult to perform with the previous packed bed models. The results indicated that random packing is more favorable for heat storage and retrieval as compared to special packing; both the material and the thickness of the encapsulation have the apparent effects on the heat transfer performance of the LTES bed.     

Numerical and Experimental Investigation on Heat Transfer Performance of a Solar Single Storage Tank

The single-tank latent heat thermal energy storage(LHTES) of solar energy mainly consists of two modules: the first one is the phase change material(PCM) module heated by solar energy; the second is a module of heat transfer between melted PCM and the user's low-temperature water. This paper mainly focuses on the former one. To investigate the heat transfer performance of the paraffin-based solar single storage tank and find a more suitable experimental configuration, as basic research work, we established a single-tank thermal storage platform and then conducted a numerical simulation on the heat transfer process with Fluent. The result of numerical simulation shows that the test situation was basically reflected and the data agreed well with the experiment results. The numerical simulation analysis is accurate and the method is reliable. To obtain the heat transfer performance of paraffin in a single tank and strengthen heat transfer, the aspect ratio, the melting temperature of paraffin, and the heating power of the electric heater were analyzed based on simulation. The results show that the heat transfer gets more uniform when the aspect ratio is lower. This results in an increase in the liquid fraction of 61.83% to 76.47% one hour after heating when the aspect ratio of the tank reduced from 2.8 to 1.1. The higher the melting temperature of paraffin, the longer it takes for PCM to reach a stable state. And the curvature of liquid heating is greater than that of solid heating at the bottom layer. Under the constant total work, the heating power has little effect on the heat transfer performance of the paraffin. This study will provide some reference value for the optimization design of single-tank LHTES systems in the future.     

Analysis of hydrogen storage performance of metal hydride reactor with phase change materials

Using phase change materials (PCM) as thermal energy storage material in metal hydride reactor bed is an effective method to store the heat emitted during hydrogen charging and retrieving it later during discharging. The present work examines the effect of a PCM on the behaviour of the metal hydride in the reactor bed. A two-dimensional model was developed to describe the mass and heat transfer inside the metal hydride and the PCM as well as the interaction between them. The results were compared with other numerical simulation and experimental data. In the simulations, thermal conductivity and the latent heat were varied in order to evaluate the effect of these parameters on the kinetics of absorption, desorption and melting of the phase change material.     

Exergetic optimization of solar collector and thermal energy storage system

This paper deals with the exergetic optimization of a solar thermal energy system. This consists of a solar collector (SC) and a rectangular water storage tank (ST) that contains a phase change material (PCM) distributed in an assembly of slabs. The study takes into account both conduction and convection heat transfer mode for water in the SC, and also the phase change process for the PCM in the ST. An analytical solution for the melting process in the PCM is also presented. The results of the study are compared with previous experimental data, confirming the accuracy of the model. Results of a numerical case study are presented and discussed.     

An experimental and numerical investigation of heat transfer during technical grade paraffin melting and solidification in a shell-and-tube latent thermal energy storage unit

The latent thermal energy storage system of the shell-and-tube type during charging and discharging has been analysed in this paper. An experimental and numerical investigation of transient forced convective heat transfer between the heat transfer fluid (HTF) with moderate Prandtl numbers and the tube wall, heat conduction through the wall and solid–liquid phase change of the phase change material (PCM), based on the enthalpy formulation, has been presented. A fully implicit two-dimensional control volume Fortran computer code, with algorithm for non-isothermal phase transition, has been developed for the solution of the corresponding mathematical model. The comparison between numerical predictions and experimental data shows good agreement for both paraffin non-isothermal melting and isothermal solidification. In order to provide guidelines for system performance and design optimisation, unsteady temperature distributions of the HTF, tube wall and the PCM have been obtained by a series of numerical calculations for various HTF working conditions and various geometric parameters, and the thermal behaviour of the latent thermal energy storage unit during charging and discharging has been simulated.     

Experimental and computational evolution of a shell and tube heat exchanger as a PCM thermal storage system

A combined experimental and numerical study has been designed to study thermal behavior and heat transfer characteristics of Paraffin RT50 as a phase change material (PCM) during constrained melting and solidification processes inside a shell and tube heat exchanger. A series of experiments are conducted to investigate the effects of increasing the inlet temperature of the heat transfer fluid (HTF) on the charging and discharging processes of the PCM. 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. The molten front at various times of process has been studied through a numerical simulation. The experimental results show that by increasing the inlet HTF temperature from T_H = 70 °C to 75 and 80 °C, theoretical efficiency in charging and discharging processes rises from 81.1% to 88.4% and from 79.7% to 81.4% respectively.