Numerical simulation of a multi-layer latent heat thermal energy storage system

A computational model for the prediction of the thermal behaviour of a compact multi-layer latent heat storage unit is presented. The model is based on the conservation equations of energy for the phase change material (PCM) and the heat transfer fluid (HTF). Electrical heat sources embedded inside the PCM are used for heat storage (melting) while the flow of an HTF is employed for heat recovery (solidification). Parametric studies are performed to assess the effect of various design parameters and operating conditions on the thermal behaviour of the unit. Results indicate that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, on the thermal diffusivity of the liquid phase, on the thickness of the PCM layer and on the HTF inlet mass flowrate and temperature. It is, on the other hand, nearly independent of the wall thermal diffusivity and thickness and of the maximum operating temperature. Correlations are proposed for the total energy stored and the output heat load as a function of the design parameters and the operating conditions. © 1998 John Wiley & Sons, Ltd.     

Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system

The phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipes energy storage system. This study deals with three important subjects. First is determination of the eutectic composition ratio of the palmitic acid (PA) and stearic acid (SA) binary system and measurement of its thermophysical properties by differential scanning calorimetry (DSC). Second is establishment of the phase transition characteristics of the mixture, such as the total melting and solidification temperatures and times, the heat transfer modes in the melted and solidified PCM and the effect of Reynolds and Stefan numbers as initial heat transfer fluid (HTF) conditions on the phase transition behaviors. Third is calculation of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, the heat recovery rates and heat fractions during the phase change processes of the mixture and also discussion of the effect of the inlet HTF parameters on these characteristics. The DSC results showed that the PA–SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 °C and has a latent heat of 181.7 J g⁻¹, and thus, these properties make it a suitable PCM for passive solar space heating and domestic water heating applications with respect to climate conditions. The experimental results also indicated that the eutectic mixture of PA–SA encapsulated in the annulus of concentric double pipes has good phase change and heat transfer characteristics during the melting and solidification processes, and it is an attractive candidate as a potential PCM for heat storage in latent heat thermal energy storage systems.     

Thermal Performance of a Phase Change Material‐Based Latent Heat Thermal Storage Unit

An experimental analysis is presented to establish the thermal performance of a latent heat thermal storage (LHTS) unit. Paraffin is used as the phase change material (PCM) on the shell side of the shell and tube‐type LHTS unit while water is used as the heat transfer fluid (HTF) flowing through the inner tube. The fluid inlet temperature and the mass flow rate of HTF are varied and the temperature distribution of paraffin in the shell side is measured along the radial and axial direction during melting and solidification process. The total melting time is established for different mass flow rates and fluid inlet temperature of HTF. The motion of the solid–liquid interface of the PCM with time along axial and radial direction of the test unit is critically evaluated. The experimental results indicate that the melting front moves from top to bottom along the axial direction while the solidification front moves only in the radial direction. The total melting time of PCM increases as the mass flow rate and inlet temperature of HTF decreases. A correlation is proposed for the dimensionless melting time in terms of Reynolds number and Stefan number of HTF. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21120     

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.     

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。     

Enrichment of heat transfer in a latent heat storage unit using longitudinal fins

The charging and discharging rates of a phase change material (PCM) in a horizontal latent heat storage unit (LHSU) is largely influenced by the lower thermal conductivity of the PCM. In the present research, four different configurations of longitudinal fins are proposed to augment the heat transfer in horizontal shell and tube type LHSUs. Numerical investigations are reported to establish the thermal performance augmentation with rectangular, triangular, and Y‐shaped (bifurcated) fins. From the results, it has been inferred that all fin configurations provide a faster charging and discharging rate. In the present set of geometric dimensions of LHSU considered, a reduction in charging time of 68.71% is evaluated for case III (three rectangular fins with one fin positioned in the area of the heat transfer fluid [HTF] surface) and case V (two bifurcated fins with one fin positioned in the area of the HTF surface). Moreover, overall cycle (charging + discharging) time is reduced by 58.3% for case III. Employment of fins results in a faster rate of absorption and extraction of energy from the PCM.     

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.     

组合相变材料储热系统的储热速率研究

建立了组合式柱内封装相变材料熔化－固化循环相变储热系统的物理模型,用有限差分法进行了数值模拟求解。结果表明,与采用单一相变材料的传统储热系统相比,在给定相变材料组合方式和传热流体进口温度条件下,传热流体流量存在最佳值;选用三种石蜡作用相变材料和水作传热流体的模拟计算结果表明,相变速率可提高１５％￣２５％左右。     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

潜热储热系统强化传热研究进展

近年来,潜热储热系统在太阳能和工业废能的利用中发挥着极其重要的作用,因此用于潜热储热的相变材料受到普遍关注.文章对国内外潜热储热系统众多强化传热技术进行了综述与讨论.     

螺旋盘管式相变储热单元储热性能

以石蜡作为相变材料,制作了内通流体螺旋盘管结构的相变储热单元。在对储热单元储热过程进行传热分析的基础上,利用实验手段对储热单元在不同工况下的储热性能进行了研究。通过对其储热过程中相变材料相变过程的分析,提出储热器设计的优化方案。利用实验数据得到其准则关联式,为其在工程中的应用提供了依据。     

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.     

Experimental and numerical investigation of thermal energy storage with a finned tube

A latent heat thermal energy storage system using a phase change material (PCM) is an efficient way of storing or releasing a large amount of heat during melting or solidification. It has been determined that the shell‐and‐tube type heat exchanger is the most promising device as a latent heat system that requires high efficiency for a minimum volume. In this type of heat exchanger, the PCM fills the annular shell space around the finned tube while the heat transfer fluid flows within the tube. One of the methods used for increasing the rate of energy storage is to increase the heat transfer surface area by employing finned surfaces. In this study, energy storage by phase change around a radially finned tube is investigated numerically and experimentally. The solution of the system consists of the solving governing equations for the heat transfer fluid (HTF), pipe wall and phase change material. Numerical simulations are performed to investigate the effect of several fin parameters (fin spacing and fin diameter) and flow parameter (Re number and inlet temperature of HTF) and compare with experimental results. The effect of each variable on energy storage and amount of solidification are presented graphically. Copyright © 2005 John Wiley & Sons, Ltd.     

Approximate analytical model for PCM solidification in a rectangular finned container with convective cooling boundaries

Thermal energy storage units that utilize latent heat storage materials have received increased attention in the recent years because of their relatively large heat storage capacities and isothermal behavior during charging and discharging. In this study, an analytical approach is presented for the prediction of temperature during the solidification in a two-dimensional rectangular latent heat storage using a phase change material (PCM) with internal plate fins. The basic energy equation is formulated accounting for the presence of a heat thermal fluid (HTF) on the walls. A two-dimensional numerical model is developed based on the enthalpy method to predict the distribution temperature of the fin and solid–liquid interface in storage. Results from the analytical solution and numerical model show a good agreement. The developed analytical model estimates satisfactorily the solidification time of PCM in storage, which is useful in the design of PCM-based thermal energy storages and cooling systems.     

Thermal Energy Storage Behavior of a Paraffin during Melting and Solidification

Abstract

In this study, experiments are conducted to investigate charging and discharging characteristics of a paraffin as a phase change material (PCM). A vertical tube-in-shell geometry is designed to store the PCM. The thermophysical properties of the paraffin examined are determined through the differential scanning calorimeter (DSC) analysis. A series of experiments are carried out to investigate the effect of increasing the inlet temperature and the mass flow rate of the heat transfer fluid (HTF) both on the charging and discharging processes (i.e., melting and solidification) of the PCM.     

管内流体流动管外PCM发生相变的贮能系统热性能研究

建立了分析空调贮能系统中管内流体流动管外PCM发生相变的相变的贮能器热性能的数学模型，并进行了数值计算。其中，把传热流体看作是沿轴向的—维无粘流动，对PCM相变过程的求解用显热容法。计算结果与文献中的计算结果吻合较好。所得结论对该类贮能系统的设计和性能优化有一定指导作用。     

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.     

Numerical simulation of a shell-and-tube latent heat thermal energy storage unit

A theoretical model was developed to predict the transient behavior of a shell-and-tube storage unit with the phase change material (PCM) on the shell side and the heat transfer fluid (HTF) circulating inside the tubes. The multidimensional phase change problem is tackled with an enthalpy-based method coupled to the convective heat transfer from the HTF. The numerical predictions are validated with experimental data. A series of numerical experiments are then undertaken to assess the effects of various thermal and geometric parameters on the heat transfer process and on the behavior of the system. Results show that the shell radius, the mass flow rate, and the inlet temperature of the HTF must be chosen carefully in order to optimize the performance of the unit.     

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.