Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

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.     

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.     

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.     

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。     

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.     

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     

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.     

Thermal performance simulations of a packed bed cool thermal energy storage system using n-tetradecane as phase change material

The objective of this paper is to study the thermal performance of latent cool thermal energy storage system using packed bed containing spherical capsules filled with phase change material during charging and discharging process. According to the energy balance of the phase change material (PCM) and heat transfer fluid (HTF), a mathematical model of packed bed is conducted. n-tetradecane is taken as PCM and aqueous ethylene glycol solution of 40% volumetric concentration is considered as HTF. The temperatures of the PCM and HTF, solid and melt fraction and cool stored and released rate with time are simulated. The effects of the inlet temperature and flow rate of HTF, porosity of packed bed and diameter of capsules on the melting time, solidification time, cool stored and released rate during charging and discharging process are also discussed.     

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.     

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.     

Experimental Investigation on Heat Storage/Retrieval Characteristics of a Latent Heat Storage System

The charging and discharging characteristics of a latent thermal energy storage (LTES) system were experimentally studied. Pure paraffin and paraffin/expanded graphite (EG) composite containing 7% and 10% mass fraction of EG were used as the phase-change materials (PCMs). Various experiments were conducted with different heat transfer fluid (HTF) temperatures and flow rates for heat storage and retrieval, respectively. The time durations of the charging and discharging processes, the mean power, and the energy efficiency of the system, which are the important factors of the LTES system, were discussed. The results showed that natural convection played a crucial role in the heat transfer during the charging process of paraffin, but heat conduction was the main heat transfer mechanism during the discharging process of paraffin. The higher the flow rate was, the higher the charging and discharging rate would be. Large temperature difference between the HTF and the initial state of PCM would accelerate the charging and discharging processes. During the charging process, the large temperature difference would result in the accelerated phase-change process due to the enhanced natural convection that could be seen clearly when the PCM was paraffin. While no significant difference was found for different initial temperatures during the discharging process. The performance of the LTES was affected prominently by the PCMs, HTF temperatures, and flow rates. The energy efficiency was higher for the 10 wt% EG PCMs, and the mean power during the discharging process was larger accordingly.     

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。     

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.     

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.     

Analysis and optimization of a latent thermal energy storage system with embedded heat pipes

Latent thermal energy storage system (LTES) is an integral part of concentrating solar power (CSP) plants for storing sun’s energy during its intermittent diurnal availability in the form of latent heat of a phase change material (PCM). The advantages of an LTES include its isothermal operation and high energy storage density, while the low thermal conductivity of the PCM used in LTES poses a significant disadvantage due to the reduction in the rate at which the PCM can be melted (charging) or solidified (discharging). The present study considers an approach to reducing the thermal resistance of LTES through embedding heat pipes to augment the energy transfer from the heat transfer fluid (HTF) to the PCM. Using a thermal resistance network model of a shell and tube LTES with embedded heat pipes, detailed parametric studies are carried out to assess the influence of the heat pipe and the LTES geometric and operational parameters on the performance of the system during charging and discharging. The physical model is coupled with a numerical optimization method to identify the design and operating parameters of the heat pipe embedded LTES system that maximizes energy transferred, energy transfer rate and effectiveness.     

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.     

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.     

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.     

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.