Crystallization of supercooled spherical nodules in a flow

The general context of this work concerns the latent heat storage (cold storage) by encapsulated spherical nodules of 7.7 cm in diameter containing a Phase Change Material (PCM) and filling a cylindrical tank (about 2500 nodules per m³). During the storage process, a cold heat transfer liquid flows through the tank to crystallize the PCM inside the nodules. The stored energy is released when a hotter heat transfer liquid flows through the tank to induce the melting of the PCM. As the velocity in the tank is relatively small (a few mm·s⁻¹), natural convection is expected on the coolant when the latent heat is released at the crystallization of the PCM. The present study concerns a single nodule surrounded by a flowing heat transfer liquid at a temperature T_C lower than the melting temperature T_F. This text presents an enthalpic modelling of the phase change inside the nodule coupled with a CFD simulation of the external flow to describe the mutual influence of the natural convection and the kinetics of crystallization. This study is afterwards extended to the case of two superposed nodules to investigate the influence of the crystallization of the lower nodule on the upper one.     

Numerical and experimental study of spherical capsules packed bed latent heat storage system

A numerical model to simulate a storage system composed of spherical capsules filled with PCM placed inside a cylindrical tank fitted with a working fluid circulation system to charge and discharge the storage tank. The simplified transient one-dimensional model is based on dividing the tank into a number of axial layers whose thickness is always equal or larger than a capsule diameter. It is also assumed that the temperature of the working fluid is uniform and equal to the average temperature of the layer. The solidification process inside the spherical capsule is treated by using a conductive one-dimensional phase change model with convective boundary condition on the external surface. The convection present in the liquid phase of the PCM is treated by using an effective heat conduction coefficient in the liquid region of the PCM. The solution of the differential equations is realized by the finite difference approximation and a moving grid inside the spherical capsules. The geometrical and operational parameters of the system are investigated both numerically and experimentally and their influence on the charging and discharging times was investigated.     

Investigation on Phase Change Behavior of Paraffin Phase Change Material in a Spherical Capsule for Solar Thermal Storage Units

In this work, the melting and solidification behaviour of paraffin phase change material encapsulated in a stainless steel spherical container has been studied experimentally. A computational fluid dynamics analysis has also been performed for the encapsulated phase change material (PCM) during phase change process. In the melting process, the hot air, used as the heat transfer fluid enters the test section and flows over the spherical capsule resulting in the melting of phase change material. In the solidification process, the ambient air flows over the capsule and received heat from phase change material resulting in the solidification of phase change material. In the computational fluid dynamics, the constant wall boundary condition is employed for both melting (75°C) and solidification (36°C) processes since the internal conductive resistance offered by the PCM is much higher compared to the outer surface convective resistance. The time required for complete solidification and melting of the phase change material obtained from the computational fluid dynamics analysis are validated with the experimental results and a reasonable agreement is achieved. The reason for the deviation between the results are analyzed and reported.     

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.     

High temperature latent heat thermal energy storage using heat pipes

A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.     

Thermal performance enhancement of melting and solidification process of phase‐change material in triplex tube heat exchanger using longitudinal fins

Efficient application of intermittent renewable energy sources, like solar, waste heat recovery, and so forth, depends on a large extent on the thermal energy storage methods. Latent heat energy storage with the use of phase‐change material (PCM) is the most promising one because it stores large energy in the form of latent heat at a constant temperature. The current study investigates melting and solidification of PCM in the triplex tube heat exchanger (TTHX) numerically. The two‐dimensional numerical model has been developed using Ansys Fluent 16.2, which considers the effects of conduction as well as natural convection. To overcome the limitation imposed by the poor thermal conductivity of PCM, use of fins is the better solution. In the current study, longitudinal fins are used for better performance of TTHX, which increases heat‐transfer area between PCM and heat‐transfer fluid. The effects of location of fins, that is, internal, external, and combined internal‐external fins, are observed. All three configurations improve melting as well as solidification process. During the melting process, internal and combined internal‐external fins are equally efficient, in which maximum 59% to 60% reduction in melting time is achieved. For solidification, internal‐external fins combination gives maximum 58% reduction in solidification time.     

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.     

蓄能互联热泵系统相变装置熔化与凝固过程特性分析

介绍了一种新型的蓄能互联热泵系统。利用数值模拟的方法对填充石蜡C17的球型蓄热单元的熔化与凝固过程进行研究,分析了球壁温度、相变单元尺寸和相变材料初始温度三种影响因素对熔化过程和球壁温度对凝固过程的影响。通过对两个过程对比发现相变单元尺寸对相变过程影响最大,在相同温差条件下完全熔化时间少于完全凝固时间,熔化过程中始终存在的石蜡-壁面与液相石蜡-固相石蜡之间的对流换热过程增加了熔化速率。     

The effects of geometric parameters on the thermal performance of a rectangular natural circulation loop containing PCM suspensions

ABSTRACT

A numerical study that uses the finite difference method to model the fluid flow and heat transfer of a rectangular natural circulation loop that contains phase change material (PCM) suspensions is presented to investigate how geometric parameters affect the thermal performance. Parametric simulations were performed using different geometrical parameters in the following ranges: the dimensionless length of the heated section = 0.4–1; the relative elevation of the cooled section compared with the heated section = 0.5–2; and the aspect ratio of the loop = 0.25–1. The results determine the important geometric parameters that affect the heat transfer performance of the loop with the PCM suspension. In several of the geometric configurations, the heat transfer performance of the loop is significantly affected by the latent heat contribution associated with the melting/freezing of the PCM particles.     

Numerical analysis of an integrated storage solar heater

This work aims to evaluate the performance of an integrated phase change material (PCM) solar collector. The dynamic behavior of the system is investigated via a theoretical model based on the first law of thermodynamics and oriented to deliver a maximum outlet water temperature. A parametric study is used to assess the effects of the inlet water temperature, the PCM thicknesses and properties and the mass flow rates on the outlet water temperature and the melt fraction. A comparison with a conventional solar water heater without heat storage is made. Results indicate that charging and discharging processes of PCM offer six stages. It is observed that the complete solidification time is longer than the melting one. The latent heat storage system increases the heating requirements at night. The rise is most enhanced for higher inlet water temperature, melting PCM temperature and PCM thickness and for lower mass flow rate.     

Analytical solution of heat transfer in a shell-and-tube latent thermal energy storage system

An analytical solution of a latent heat storage unit (LHSU) consisting of a shell-and tube was obtained by using the Exponential Integral Function and the variables separation technique. The working fluid (water) circulating by forced convection inside the inner tube charges and discharges the storage unit. The comparison between analytical predictions and experimental data shows good agreement. Extensive parametric studies were conducted in order to examine the effect of the pertinent parameters (such as natural convection, mass flow rate of HTF, outer tube radius, pipe length etc.) on the melting and solidification processes of paraffin as a PCM. In order to provide guidelines for system performance and design optimisation, unsteady temperature distributions within PCM during melting/solidification, energy stored, position of moving interface and thermal efficiency have been obtained by a series of numerical calculations and represented graphically.     

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.     

Dynamic discharging characteristics simulation on solar heat storage system with spherical capsules using paraffin as heat storage material

The dynamic characteristics of solar heat storage system with spherical capsules packed bed during discharging process are studied. According to the energy balance of solar heat storage system, the dynamic discharging processes model of packed bed with spherical capsules is presented. Paraffin is taken as phase change material (PCM) and water is used as heat transfer fluid (HTF). The temperatures of PCM and HTF, solid fraction and heat released rate are simulated. The effects of inlet temperature of HTF, flow rate of HTF and porosity of packed bed on the time for discharging and heat released rate are also discussed. The following conclusion can be drawn: (1) the heat released rate is very high and decreases rapidly with time during the liquid cooling stage, it is stable at the solidification cooling stage, then it decreases to zero at the solid cooling stage. (2) The time for complete solidification decreases when the HTF flow rate increases, but the effect is not so obvious when the HTF flow rate is higher than 13 kg/min; (3) compared to the HTF inlet temperature and flow rate, the influence of porosity of packed bed on the time for complete solidification is not so significant.     

A numerical and experimental investigation of different containers and PCM options for cold storage modular units for domestic applications

This paper presents the results of an experimental and numerical study on the solidification of different phase change materials (PCM) encapsulated in spherical and cylindrical shells of different materials and diameters subject to constant surface temperature. The main objective is to determine the time for complete solidification of the PCM and how it is affected by the variations of the surface temperature, material and diameter of spherical shells. As a result of the study, it is expected to define a pair of container and PCM to operate adequately and efficiently together with refrigeration units. The experiments were realized using glass and plastic spherical shells of diameters 0.035, 0.076, 0.106 and 0.131 m, soft drink cans and small plastic water bottles with surface temperatures of ?5, ?10, ?12, ?15, ?18, ?20 and ?25 °C. The phase change materials used are water and mixtures of water with 3.75%, 7.5%, 15%, 25%, 30%, 40% and 50% Glycol content. The results are presented and discussed.     

Thermal performance of a eutectic mixture of lauric and stearic acids as PCM encapsulated in the annulus of two concentric pipes

Thermal performance characteristics of a eutectic mixture of lauric and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipe-energy storage system. This study deals with three important subjects: The first one is to determine the eutectic composition ratio of the lauric acid (LA) and stearic acid (SA) binary system, and to measure its thermophysical properties by DSC. The second one is to establish the thermal characteristics of the mixture such as total melting and solidification times, the heat transfer modes in melted and solidified PCM, and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition behaviors. The final one includes the calculations of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, and heat fractions during the melting and solidification processes of the mixture, and also the discussion of the effect of inlet HTF parameters on these characteristics. The LA–SA binary system in the mixture ratio of 75.5:24.5 wt % forms a eutectic, which melts at 37°C and has a latent heat of 182.7 J g⁻¹, and, thus, these properties make it an attractive phase change material used for passive solar space heating applications such as building and greenhouse heating with respect to the climate conditions. The experimental results indicated that the mixture encapsulated in the annulus of two concentric pipes has good thermal and heat transfer characteristics during the melting and solidification processes, and it has potential for heat storage in passive solar space heating 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 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.     

Enhancement of latent heat energy storage using embedded heat pipes

Latent heat thermal energy storage (LHTES) utilizing heat pipes or fins is investigated experimentally. Photographic observations, melting and solidification rates, and PCM energy storage quantities are reported. Heat pipe effectiveness is defined and used to quantify the relative performance of heat pipe-assisted and fin-assisted configurations to situations involving neither heat pipes nor fins. For the experimental conditions of this study, inclusion of heat pipes increases PCM melting rates by approximately 60%, while the fins are not as effective. During solidification, the heat pipe-assisted configuration transfers approximately twice the energy between a heat transfer fluid and the PCM, relative to both the fin-assisted LHTES and the non-heat pipe, non-fin configurations.     

Developing microencapsulated 12‐hydroxystearic acid (HSA) for phase change material use

Phase change materials (PCM) have an increasingly more important role as a thermal energy storage (TES) media. However, leakage problem of PCM causes limitation during their integration in TES systems. Therefore, the encapsulation of PCMs is attracting research interest to extend usage of PCMs in real TES applications in recent years. In this study, hydroxystearic acid (HSA) was encapsulated with polymethyl methacrylate (PMMA) and different PMMA comonomer shells via emulsion polymerization method for the first time in literature. HSA with high melting temperature range (74–78°C) can widen the scope of using PCMs, and the encapsulated form can make it more versatile. The chemical structures, morphologies, and thermophysical properties of capsules were determined by FT‐IR, SEM, DSC, TGA, and thermal infrared camera. Among the produced HSA capsule candidates, PMMA‐HEMA is the most promising with latent heat of 48.5 J/g with melting range of 47 to 85°C. SEM analysis indicated that the capsules have spherical shape with compact surface at nano‐micro (100–440 nm) size range; however, some capsules exhibited agglomeration.     

Thermal energy storage system using stearic acid as a phase change material

The thermal performance and phase change stability of stearic acid as a latent heat energy storage material has been studied experimentally. The thermal performance and heat transfer characteristics of the stearic acid were tested and compared with other studies given in the literature. In the present study, parameters such as transition times, temperature range and propagation of the solid–liquid interface as well as the effect of the heat flow rate on the phase change stability of stearic acid as a phase change material (PCM) were studied. The experimental results showed that the melting stability of the PCM is better in the radial direction than in the axial direction. The variation in the melting and solidification parameters of the PCM with the change of inlet water temperature is also studied. We 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 50.3%. This indicates that 49.7% of the heat is actually lost somewhere.