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.     

Thermal Performance Enhancement of Triplex Tube Latent Thermal Storage Using Fins-Nano-Phase Change Material Technique

Latent heat thermal energy storage (LHTES) systems using a phase change material (PCM) can reduce the heat-transfer rates during charging/discharging processes because of their inherently low thermal conductivity. In this study, heat-transfer enhancement using various configurations of longitudinal fins employing both a PCM and a nano-PCM in a large triplex-tube heat exchanger (TTHX) was numerically investigated via the Fluent 15 software. The results showed that the thermal conductivity of the pure PCM (0.2 W/m K) can be observably enhanced by dispersing 10% alumina (Al₂O₃) to 25%. Therefore, the melting time is reduced to 12%, 11%, and 17% for the internal, internal-external, and external fins, respectively, compared with the case of the PCM without nanoparticle. It is concluded that the model of external fins-nano-PCM embedded in a large TTHX is the most efficient model for achieving complete PCM melting in a short time (188 min), where improving the thermal performance to 14% and 11% compared with the TTHX with internal and internal-external fins-nano-PCM, respectively. The simulation results are validated and agree well with experimental results for the PCM and nano-PCM.     

水平管壳式相变蓄热装置的强化传热研究

基于焓法模型对水平管壳式相变蓄热装置热性能的增强进行研究,首先分析蓄热过程中传统管壳式装置内材料的传热及流动机理;然后引入椭圆元素并对比椭圆内管及外壳的强化传热效果;最后对热源温度、相变材料导热系数及初始温度对装置热性能的作用规律进行探讨。结果显示,椭圆外壳的强化传热效果优于内管,同等条件下,长短轴之比为2的椭圆外壳可使蓄热时间缩短53.5%。热源温度升高,椭圆外壳的强化传热效果进一步增强,相变材料的导热系数及初始温度对装置热性能的影响较小。     

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。     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.     

A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins

An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature-time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.     

增强石蜡相变材料传热性能的实验研究

添加高导热颗粒和增大换热面积是当前增强石蜡相变材料传热性能的主要研究方向。以此为基础搭建试验台结合数据采集系统对石蜡在圆管外的熔化凝固过程进行了实验测试,并对各测点的温度变化趋势进行分析,研究了添加不同纳米颗粒和加入金属肋片对换热过程的影响。结果表明:在石蜡溶液中添加纳米颗粒能够起到减小过冷度的效果同时有效增强相变材料的传热性能,添加纳米氧化铜颗粒的传热性能增强效果要优于添加氧化锌颗粒和二氧化硅颗粒;在储热系统中加入肋片能够显著提高相变储能系统的热性能,强化换热过程。     

三套管式相变蓄热器强化传热研究

基于三套管式相变蓄热器的特点,提出应用T字形翅片来强化相变蓄热器的传热性能。研究结果表明:添加翅片可有效地降低蓄热器中相变材料的凝固和融化时间,直翅片和T字形翅片的混合强化结构能使凝固过程比未强化结构节省74%的时间,使融化过程节省60%的时间。因此直翅片和T字形翅片的混合使用可以达到进一步强化传热的目的。     

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.     

Performance enhancement of a solar dynamic LHTS module having both fins and multiple PCMs

Latent heat thermal storage units span a wide and varied range of applications in the domestic, industrial and space based activities. Numerical investigations on the performance enhancement of a solar dynamic latent heat thermal storage (LHTS) unit employing multiple phase change materials (PCM) and fins are made. The LHTS unit has been studied for the charging mode alone. Enthalpy based formulation of the energy equations governing the behaviour of the LHTS system has been made and compared with the response of a single PCM unit. The governing conjugate equations have been solved employing finite difference techniques. The results show an appreciable enhancement in the rate of melting of PCM and nearly uniform exit temperature of heat transfer fluid (HTF) in the multiple PCM LHTS unit.     

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.     

Heat transfer intensification in horizontal shell and tube latent heat storage unit

Employment of latent heat storage unit (LHSU) utilizing phase change material (PCM) in a substantial scale is constrained by the poor thermal conductivity of PCMs. Future utilization of LHSU will therefore to a great extent rely on the heat transfer intensification techniques. Present research is on enhancement techniques in which heat transfer mechanism is altered without altering the mass of PCM and heat transfer surface area. The intensification mechanisms considered in the present research include imparting eccentricity to heat transfer fluid (HTF) pipe, imparting rotation to the LHSU and providing multi HTF tube. Numerical investigations are reported here towards comparative evaluation of the thermal characteristics associated with such intensification mechanisms for horizontal LHSU. In the present study stearic acid (melting point 55.7–56.6?°C) is used as PCM and water is used as HTF. Results infer that all the three mechanisms offer quicker melting rate. For the geometric configuration of LHSU considered in the present research, a reduction in melting time of 47.75% is evaluated for rotating LHSU. The rate of energy storage is higher for both eccentric and rotating LHSU. Solidification process is however not accelerated by such techniques. On the contrary, eccentric and multi HTF tube LHSU takes more time for solidification.     

Research on melting and solidification processes for enhanced double tubes with constant wall temperature/wall heat flux

Latent heat thermal energy storage (LHTES) improves the energy utilization efficiency between energy supply and energy demand of heating storage in buildings and liquid desiccant air conditioning systems. The present work is focused on validated numerical investigation of the thermal performances of LHTES inside enhanced double tubes. The effects of the number of fins ranging from 2 to 10 and boundary conditions of the inner tube wall on the melting and solidification processes are examined. The results indicate that number of fins and wall boundary conditions play an important role in the thermal performances of LHTES. It is noted that recirculation flow in the liquid phase change material region is formed remarkably. The enhancement ratio for constant wall temperature is more significant than that of constant wall heat flux during the melting process. However, the discrepancy of the enhancement ratio for different inner wall temperatures is limited during the solidification process.     

Enhanced thermal performance analysis of buried heat transfer tubes thermal storage system filled with microcapsule particles

The heat transfer enhancement performance of a phase change buried tubes thermal storage system is influenced by major parameters such as arrangement of heat transfer tubes, fin structure and fin geometry size. We developed a three-dimensional numerical model with two different arrangements and five different enhanced heat transfer structures respectively. For the sake of analysis the effects of arrangement of heat transfer tubes, fin structure and fin geometry size. In addition, we applied the enthalpy-transforming model to obtain the liquid fraction and location of the solid-liquid interface at different time in the phase change process. The numerical results show that the melting time of the thermal storage system model with a triangle arrangement is about 6.1% longer than that of the model with a square arrangement. Besides, the melting time of the model with 55 mm tube pitch is about 16.7% shorter than that of tube pitch with 60 mm. Moreover, the buried tube thermal storage system models with circle fins have the shortest melting time, which is 18 seconds. Melting time of the model with circle fins is about 40% shorter than that of the model with smooth tube. In addition, the melting time of the model with 3 mm fin thickness is 10 seconds, which is the shortest. The model with thicker fins means the shorter time of melting process. Moreover, the melting time of the model with 10.5 mm fin spacing is about 23.5% shorter than that of the model with 12.5 mm fin spacing, which is 13 seconds. In conclusion, the main factor of the melting time is the heat transfer area. It provides a guidance for the design and reconstruction of the type of heat storage structure.     

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.     

Thermal Energy Storage Performance of Fatty Acids as a Phase Change Material

Thermal energy storage performance of fatty acids and a eutectic mixture as phase change materials (PCMs) has been investigated experimentally. The selected PCMs for this study were palmitic acid, myristic acid, stearic acid, and a mixture of stearic and myristic acids in eutectic combination ratio of 65.7 wt% myristic acid and 34.3 wt% stearic acid. The PCMs have a melting temperature range of 50.0°C to 61.20°C and a latent heat range of 162.0 J/g to 204.5 J/g. The inlet temperature and the mass flow rate of heat transfer fluid (HTF) were selected as experimental parameters to test the thermal energy storage performance of the PCMs. The transition times, temperature range, propagation of the solid-liquid interface, as well as heat flow rate characteristics of the employed cylindrical tube storage system were studied at varied experimental parameters. The experimental results show that the melting front moves to inward in the radial directions as well as in the axial directions from the top toward to the bottom of the PCM tube. It was observed that the convection heat transfer in the liquid phase plays an important role in the melting process. The changes in the studied HTF parameters have more effect on the melting processes than the solidification processes of the PCMs. The average heat storage efficiency calculated from data for all the PCMs is 51.5%, meaning that 48.5% of the heat actually was lost somewhere.     

Numerical simulations on performance enhancement of a cross-flow latent thermal energy storage heat exchanger

基于列管式换热器具有传热面积大、结构紧凑、操作弹性大等优点,使其在相变储能领域具有广阔的应用前景。本文建立一种新型列管式相变蓄热器模型,在不考虑自然对流的情况下,利用Fluent软件对相变蓄热器进行二维储热过程的数值模拟。本文主要研究斯蒂芬数、雷诺数、列管排列方式、肋片数以及相变材料的导热系数对熔化过程的影响,并对熔化过程中固液分界面的移动规律进行了分析。模拟结果表明,内肋片强化换热效果明显,特别是对应用低导热系数相变材料[导热系数小于1 W/(m·K)]的列管式蓄热器,相对于无肋片结构,加入肋片（N_fn=2）可缩短熔化时间52.6%。     

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.     

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.     

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.