Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell‐and‐tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid‐dynamic (CFD) model that takes into account of the phase‐change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase‐change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase‐change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano‐enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano‐enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical and experimental investigation of heat transfer in a shell and tube thermal energy storage system

A numerical and experimental investigation of phase change process dominated by heat conduction in a thermal storage unit is presented in this paper. The thermal energy storage involves a shell and tube arrangement where paraffin wax as phase change material (PCM) is filled in the shell. Water as heat transfer fluid (HTF) is passed inside the tube for both charging and discharging cycles. According to the conservation of energy, a simple numerical method called alternative iteration between thermal resistance and temperature has been developed for the analysis of heat transfer between the PCM and HTF during charging and discharging cycles. Experimental arrangement has been designed and built to examine the physical validity of the numerical results. Comparison between the numerical predictions and the experimental data shows a good agreement. A detailed parametric study is also carried out for various flow parameters and system dimensions such as different mass flow rates, inlet temperatures of HTF, tube thicknesses and radii. Numerical study reveals that the contribution of the inlet temperature of HTF has much influence than mass flow rate in terms of storage operating time and HTF outlet temperature. Tube radius is a more important parameter than thickness for better heat transfer between HTF and PCM.     

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.     

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 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.     

Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

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.     

Experimental validation of a CFD model for tubes in a phase change thermal energy storage system

A computational fluid dynamic (CFD) model for tubes in a phase change thermal energy storage system has been developed and validated with experimental results. The heat transfer fluid (HTF) flows in tubes which are configured in a unique arrangement during the charging and discharging processes. Water was used as the phase change material (PCM) which was contained in a cylindrical tank with four tubes coiled inside it. Experiments were conducted for both freezing and melting processes. A three-dimensional CFD model using Ansys code was developed and validated with experimental results. This model endeavoured to describe both the freezing and melting processes of the PCM. The inlet and outlet HTF temperatures as well as nine temperature locations in the PCM were compared with the CFD results. The average effectiveness as well as the duration of the phase change process of each experimental point was also compared with results from the CFD. From this study, it was concluded that the CFD model developed can accurately predict the behaviour of the thermal storage system during charging and discharging. The paper gives details of the CFD model and compares results from the model and experiments.     

A review on effect of phase change material encapsulation on the thermal performance of a system

This paper presents a detailed review of effect of phase change material (PCM) encapsulation on the performance of a thermal energy storage system (TESS). The key encapsulation parameters, namely, encapsulation size, shell thickness, shell material and encapsulation geometry have been investigated thoroughly. It was observed that the core-to-coating ratio plays an important role in deciding the thermal and structural stability of the encapsulated PCM. An increased core-to-coating ratio results in a weak encapsulation, whereas, the amount of PCM and hence the heat storage capacity decreases with a decreased core-to-coating ratio. Thermal conductivity of shell material found to have a significant influence on the heat exchange between the PCM and heat transfer fluid (HTF). This paper also reviews the solidification and melting characteristics of the PCM and the effect of various encapsulation parameters on the phase change behavior. It was observed that a higher thermal conductivity of shell material, a lower shell size and high temperature of HTF results in rapid melting of the encapsulated PCM. Conduction and natural convection found to be dominant during solidification and melt processes, respectively. A significant enhancement in heat transfer was observed with microencapsulated phase change slurry (MPCS) due to direct surface contact between the encapsulated PCM and the HTF. It was reported that the pressure drop and viscosity increases substantially with increase in volumetric concentration of the microcapsules.     

Energy analysis of space solar dynamic heat receivers

Energy analysis of space solar dynamic heat receivers employing solid–liquid phase change storage is developed. The heat receiver is a critical component of a solar dynamic system. Phase change thermal energy storage is used in the heat receiver. The energy analysis presented here can be used to understand the energy transfer in the heat receiver and thermal energy storage in phase change materials (PCM). The heat receiver cavity radiation mathematical model and the working fluid tube heat model are established. Energy loss, energy absorbed by gas, the latent and sensible thermal energy storage in PCM, maximum tube temperature, gas outlet temperature and liquid PCM fraction were calculated. The results are analyzed and could be used in heat receiver design.     

相变蓄热供热装置热性能试验研究与系统经济性分析

为分析相变蓄热装置在充热和放热过程中的热性能,设计并搭建一套相变蓄热供热装置中试实验系统,研究主要运行参数对相变蓄热装置热性能的影响;在此基础上,结合项目案例,对相变蓄热供热系统经济性进行分析。结果表明：相变材料(Phase Change Material, PCM)凝固过程中的传热主要受相变介质内部导热控制;而在其熔化过程中自然对流对传热起重要控制作用;蓄热装置充热速率快于放热速率。提高传热流体流量有助于增强PCM中的热传递,缩短充/放热时间,但蓄热装置内PCM温度分布均匀性有所降低;为降低系统能耗,提高储放热效率,优先选用小流量进行充/放热。该相变蓄热供热项目的动态投资回收期为3.55年,具有良好的经济性。研究结果可对相变蓄热供热系统的设计及应用推广提供参考依据。     

A comparative study on the performances of different shell-and-tube type latent heat thermal energy storage units including the effects of natural convection

Numerical modeling was performed to simulate the melting process of a fixed volume/mass phase-change material (PCM) in different shell-and-tube type latent thermal energy storage units with identical heat transfer area. The effect of liquid PCM natural convection (NC) on the latent heat storage performance of the pipe and cylinder models was investigated using a 3D numerical model with FLUENT software. Result shows that NC can cause a non-uniform distribution of the solid–liquid interface, which accelerates PCM melting rate. The PCM melting rate and heat storage rate in the horizontal cylinder model are higher than those in the horizontal pipe model because of the combined effects of heat conduction and NC. A comparative study was conducted to determine the effects of horizontal and vertical shell-and-tube models with different heat transfer fluid (HTF) inlets including the effects of NC. The results indicate that the vertical model with an HTF inlet at the bottom exhibits the highest PCM melting rate and heat storage rate for the pipe models. For the cylinder models, the horizontal model and the vertical model with an HTF inlet at the bottom can achieve nearly the same completed melting time. In addition, NC has minimal effect on any model with an HTF inlet at the top.     

Flow of Microencapsulated Phase Change Material Slurry through Planar Spiral Coil

Forced convection cooling is an effective method in thermal management that relies mainly on dissipating heat by pumping heat transfer fluid (HTF) through the heat source. In this paper, we investigate the thermal properties enhancement of dielectric water as the HTF. To enhance the properties of the HTF, microencapsulated phase change materials (MPCM) will be added to the base fluid. The MPCMs are composed of phase change material (PCM) encapsulated with shell materials. The PCM inside the capsules may undergo a phase change. This leads to a significant heat gain and release. The numerical model is developed to solve for continuity, momentum, and heat transfer equations using the finite volume method. The behavior of the MPCM slurry in curved channels, generates unique patterns due to different viscosity values and the centrifugal forces. Our preliminary numerical data on MPCM slurry through planar spiral coil heat exchangers show the new patterns of velocity and heat transfer curves. The current paper studies the steady condition of laminar flow at different boundary conditions. The velocity and temperature profiles, heat transfer data with different mass fractions of MPCM additives to the base fluid, and their heat removal capabilities are quantified and discussed in detail.     

Modeling of the melting characteristics of commercial paraffin wax in an inverted equilateral triangular enclosure: Effect of convective heat transfer boundary conditions

A horizontal double-pipe heat exchanger with an inverted outer equilateral triangular tube is modeled to numerically investigate the low-temperature thermal energy storage capability of an impure phase change material (PCM). The energy source fluid (hot water) flows through the inner tube and transfers heat to the PCM (heat sink) residing in the annular gap. The results show that the inlet temperature of the heat transfer fluid (HTF) has a significant effect on the melting process compared with the mass flow rate (MFR). The configuration, as well the concentricity/eccentricity of the inner tube has a great influence on the energy storage.     

A combined experimental and computational study on the melting behavior of a medium temperature phase change storage material inside shell and tube heat exchanger

A combined experimental and numerical study is performed aiming to understand the role of buoyancy-driven convection during constrained melting of phase change materials (PCMs) inside a shell and tube heat exchanger. A series of experiments is conducted to investigate the effect of increasing the inlet temperature of the heat transfer fluid (HTF) on the charging process (melting) 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. It was observed from experimental results that the melting front appeared at different times at positions close to the HTF tube and progressing at different rates outwards towards the shell. The computational results show that by increasing the inlet water temperature to 80 °C, the total melting time is decreased to 37%.     

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 performance of molten salt phase change thermal energy storage system with enhanced tubes

Based on enthalpy method, numerical studies were performed for high temperature molten salt phase change thermal energy storage (PCTES) unit used in a dish solar thermal power generation system. Firstly, the effects of the heat transfer fluid (HTF) inlet temperature and velocity on the PCTES performance were examined. The results show that although increasing the HTF inlet velocity or temperature can enhance the melting rate of the phase change material (PCM) and improve the performance of the PCTES unit, the two parameters will restrict each other for the fixed solar collector heat output. Then three enhanced tubes were adopted to improve the PCTES performance, which are dimpled tube, cone-finned tube and helically-finned tube respectively. The effects of the enhanced tubes on the PCM melting rate, solid–liquid interface, TES capacity, TES efficiency and HTF outlet temperature were discussed. The results show that compared with the smooth tube, all of the three enhanced tubes could improve the PCM melting rate. At the same working conditions, the melting time is 437.92 min for the smooth tube, 350.75 min for dimpled tube which is reduced about 19.9% and 320.25 min for cone-finned tube which is reduced about 26.9% and 302.75 min for helically-finned tube reduced about 30.7%. As a conclusion, the thermal performance of PCTES unit can be effectively enhanced by using enhanced tube instead of smooth tube. Although, the HTF pressure drops for the enhanced tubes are also larger than that of the smooth tube, the largest pressure drop (1476.2 Pa) is still very lower compared with the working pressure (MPa magnitude) of the dish solar generation system. So, the pressure drops caused by the enhanced tubes could almost be neglected.     

Eutectic compound (KNO3/NaNO3 : PCM) quasi‐encapsulated into SiC‐honeycomb for suppressing natural convection of melted PCM

The phase change eutectic compound, KNO₃/NaNO₃ (50/50 mol%) (phase change material (PCM)), which is used as the thermal energy storage material in the solar thermal power plant, was quasi‐encapsulated into the SiC‐honeycomb (SCH) for suppressing the natural convection occurring at the liquid state of PCM. The performance of the SCH as the material suppressing natural convection of PCM was investigated experimentally. PCM with three kinds of mixing ratios of SCH of 10%, 20%, and 30%, was prepared and packed in their respective stainless can with oil‐flowing pipe in the center, which is called thermal energy storage unit (TESU). Three units were linked together and stacked vertically by the connector at the inlet/outlet oil pipe. The time variation of temperature at the fixed positions inside the TESU in charging/discharging process and temperature gradient in the radial direction inside TESU when PCM was liquid state were investigated. It is concluded that the natural convection is suppressed by mixing the SCH with PCM up to around 30% in weight, because the PCM is quasi‐encapsulated into cell holes and porous structures of SCHs. And thus, the heat transfer of the PCM + 30%SCH composite is controlled mainly by its thermal conduction, which is also supported through comparison of simulation result with experimental one. And so, we conclude that SCH has a function as the quasi‐encapsulating material for suppressing the natural convection of PCM. Copyright © 2015 John Wiley & Sons, Ltd.