Heat Transfer Enhancement for PCM Thermal Energy Storage in Triplex Tube Heat Exchanger

Thermal energy storage is critical for reducing the discrepancy between energy supply and energy demand, as well as for improving the efficiency of solar thermal energy systems. Among the different types of thermal energy storage, phase-change materials (PCM) thermal energy storage has gained significant attention recently because of its high energy density per unit mass/volume at nearly constant temperature. This study experimentally investigates the using of a triplex tube heat exchanger (TTHX) with PCM in the middle tube as the thermal energy storage to power a liquid desiccant air-conditioning system. Four longitudinal fins were welded to each of the inner and middle tubes as a heat transfer enhancement in the TTHX to improve the thermal performance of the thermal energy storage. The average temperature of the PCM during the melting process in the TTHX with and without fins was compared. The PCM temperature gradients in the angular direction were analyzed to study the effect of the natural convection in the melting process of the thermal storage. The energy storage efficiency of the TTHX was determined. Results indicated that there was a considerable enhancement in the melting rate by using fins in the TTHX thermal storage. The PCM melting time is reduced to 86% by increasing of the inlet heat transfer fluid. The average heat storage efficiency calculated from experimental data for all the PCMs is 71.8%, meaning that 28.2% of the heat actually was lost.     

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 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.     

采用肋片强化相变蓄热器传热的模拟分析

为了增加同心套管式相变蓄热器的蓄能效果,采用环形肋片强化相变储能设备的传热,利用Fluent软件模拟了这种结构中石蜡相变的融化过程,得到了石蜡熔化过程温度场分布及熔化时间的规律,根据这些规律分析了肋片间距及厚度等参数对贮热管放热效果的影响。分析结果表明：石蜡的总融化时间随肋片间距增加而延长即传热效果变差,但是随着肋片厚度的增加而缩短,即传热效果变优,但是当或肋间距超过40mm和厚度超过2mm后,进一步增加肋片间距或者厚度对传热效果的影响变得不明显。     

偏心管翅式相变储热单元性能强化的模拟

对管翅式相变储热单元进行了二维非稳态模拟研究。在考虑自然对流与外管传热的情况下对比研究了同心管翅、偏心管翅以及翅片接触外管三种储热单元的传热特性。考虑了内管壁温度、外管材料、翅片厚度对储热性能的影响。结果表明,与采用同心管翅时相比,由于自然对流的影响,偏心管翅储热单元有效削弱了固-液界面分布不均匀现象,完全融化时间减少了29.3%,而当翅片接触金属外管时,通过翅片的传热外管温度迅速升高,增大了换热面积,完全融化时间减少了近49.3%。可见,翅片接触外管储热单元不仅削弱了自然对流引起固-液界面分布不均匀现象,而且利用了外管的传热,强化了储热换热性能。     

组合相变材料储热的实验研究

分析了在同一储热系统中采用两种或两种以上相变材料的特点,介绍了用于组合相变材料储热研究的实验装置,给出了四种相变材料水平柱状储热单元储热系统的初步测试结果,并对其进行了讨论。     

Thermal Insulation for a Pipe Using Phase Change Material

This research studies the effectiveness of phase change material (PCM) as a thermal insulation for a pipe. The proposed PCM insulation can be used for a pipe when the operating time is limited. The objective of using PCM is to utilize its latent heat from fusion to minimize heat loss from the pipe by absorbing and storing it to be discharged later to the pipe. The finite element method is employed to solve the problem, and both conduction and natural convection of liquid PCM are considered modes of heat transfer. The effectiveness of the PCM insulation is evaluated by comparing its thermal performance with insulation without phase change. Both time-dependent and time-independent boundary conditions are examined. For the time-independent case, the PCM insulation reduces the heat loss from the pipe for a significant amount of time if the Rayleigh number is low. For the time-dependent case, heat loss is effectively reduced with the PCM insulation for a significant amount of time. The high resolution capturing of the solid/liquid moving boundary and the details of flow structure are also presented.     

Performance Design of High-Temperature Chloride Salts as Thermal Energy Storage Material

The chloride salts have great potential used as high-temperature thermal energy storage(TES) medium for the concentrated solar power system.In this study,LiCl,KCl and CaCl₂ were selected as energy storage materials in order to further broaden the working temperature of ternary chloride salt and improve its energy storage density.The new high-temperature energy storage ternary chloride composed of LiCl,KCl,and CaCl₂ was developed based on the phase diagram generated by FactS...     

Enhancing Heat Transfer and Energy Storage Performance of Shell-and-Tube Latent Heat Thermal Energy Storage Unit with Unequal-Length Fins

Previous studies in literatures adequately emphasized that inserting fins into phase change material is among the most promising techniques to augment thermal performance of shell-and-tube latent heat thermal energy storage unit.In this study,the novel unequal-length fins are designed from the perspective of synergistic benefits of heat transfer and energy storage performance,and the effects of arrangement,number and total length of unequal-length fins are numerically investigated.Results show t...     

Impact of Phase Change Material's Thermal Properties on the Thermal Performance of Phase Change Material Hollow Block Wall

ABSTRACT

Composite phase change material (PCM) hollow block wall (CPCMHBW) can be established by introducing PCM into the holes of generally used hollow block wall, and good thermal insulation performance will probably produce together with the energy storage function from PCM simultaneously. In this article, the impact of PCM's thermal properties on the thermal performance of CPCMHBW has been analyzed, using two-dimensional enthalpy model. The conclusions include: complete melting and freezing processes and a bit amount of remaining PCM which has not melted or solidified, are fundamental and necessary for high performance; furthermore, that the average surrounding temperature equals to PCM's central phase change temperature determines whether the PCM's function can be used; besides, the PCM's total latent heat controls wall's thermal storage level; in addition, relatively low block material's thermal conductivity and Fourier number (better smaller than 1.0 W·m^?1·K^?1 and 59.83) and medium PCM's corresponding values (lies in the ranges 0.2–0.7 W·m^?1·K^?1 and 0.80–2.80) generate optimum thermal performance. Finally, the thermal factors are ranked with the functions in descending order.     

Comparison of Thermal Performance between Annular Fins and Longitudinal Fins in Latent Heat Storage Unit

As a simple and effective method of heat transfer enhancement,fins are widely used in latent heat storage systems.However,the choice of annular fins and longitudinal fins has always been controversial.In this paper,the melting process of phase change material(PCM) in annular fins and longitudinal fins latent heat storage units with the same volume is numerically simulated.To ensure the same thermal penetration,three-dimensional spaces occupied with fins are specially controlled to be the same.Co...     

泡沫铜强化石蜡相变蓄热特性的数值分析

通过构建泡沫金属内固液相变传热模型,对方腔蓄热单元中泡沫铜强化石蜡相变蓄热特性进行数值分析。数值模型采用考虑泡沫金属真实结构的等效导热系数通用模型,并兼顾石蜡在融化前后与金属骨架之间的热非平衡效应。通过求解模型得到方腔内石蜡固液界面演化规律与温度分布,进而对蓄热过程进行火用分析。结果表明:当前数值模型能较好地预测泡沫铜内固液相变传热;泡沫铜显著改善了石蜡相变的空间均匀性,减小了蓄热区温度梯度,使蓄热速率和火用效率得到有效提高。     

Numerical Analysis on a Latent Thermal Energy Storage System with Phase Change Materials and Aluminum Foam

Abstract

A latent heat thermal energy storage system with phase change material (PCM) is numerically studied. To enhance the heat transfer inside the system, a highly conductive metal foam is employed with ceramic nanoparticles. The latter method of enhancement leads to a new class of material called Nano-PCM. The system under investigation is a 70-L tank filled up with pure PCM or Nano-PCM and several pipes are situated where the heat transfer fluid (HTF) flows. The pipe surfaces are assumed at constant temperature above the PCM melting temperature to simulate the heat transfer from the HTF. The enthalpy-porosity theory is applied to simulate the PCM phase change, while the porous media formulation is assumed to describe the metal foam behavior. The nano-PCM is modeled with single-phase model where the properties are the weighted-average between the fluid base and the nanoparticles. The simulations are accomplished for charging-discharging process at different porosities and nanoparticle concentration. The results are given in term of average melting fraction evolution, average temperature as function of time, average stored energy. The metal foam significantly improves the heat transfer between PCM and HTF respect to the addition of nanoparticles, reducing the charging and discharging time more than one order of magnitude.     

Numerical Analysis of Phase Change Material Characteristics Used in a Thermal Energy Storage Device

In this study, a numerical analysis is performed to investigate the freezing process of phase change materials (PCM) in a predesigned thermal energy storage (TES) device. This TES device is integrated with a milk storage cooling cycle operating under predefined practical conditions. Using this cooling unit, 100 litres of milk is kept cool at 4°C for 48 hours before it is collected. A 2-D model of the TES device is developed in COMSOL Multiphysics to analyze the phase change performance of water-based PCMs. The variations of thermal properties with temperature during the phase change are considered in the analysis. The model is used for exploring the solidification process of PCMs inside the TES device. Temperature variations with time, ice formation, and the impacts of boundary conditions are investigated in detail. Water PCM shows better characteristics in the solidification process in comparison to eutectic PCMs, which is mainly due to the differences between phase change temperatures of the PCMs.     

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.     

High Density Polyethylene/Paraffin Composites as Form-stable Phase Change Material for Thermal Energy Storage

Abstract

This article focuses on the preparation and thermo-physical properties of paraffin/high density polyethylene (HDPE) composites as form-stable solid-liquid phase change material (PCM) for thermal energy storage. In the paraffin/HDPE blend, the paraffin (P) dispersed into the HDPE serves as a latent heat storage material when the HDPE, as a supporting material, prevents the melted paraffin leakage thanks to its structural strength. Therefore, this type composite is form-stable and can be used as a PCM without encapsulation for thermal energy storage. In this study, two paraffins with melting temperatures of 48°C–50°C and 63°C–65°C were used. The mass percentages of paraffins in the composites could go high as 76% without any seepage of the paraffin in melted state. The dispersion of the paraffin into the network of the solid HDPE was investigated using scanning electronic microscope (SEM). The melting temperatures and latent heats of the form-stable P1/HDPE and P2/HDPE composite PCMs were determined as 44.32°C and 61.66°C, and 179.63 and 198.14 Jg^?1, by the technique of differential scanning calorimetry (DSC), respectively. Furthermore, the thermal conductivity of the composite PCMs were improved as about 33.3% for the P1/HDPE and 52.3% for the P2/HDPE by introducing the expanded and exfoliated graphite to the samples in the ratio of 3 wt%. The results reveal that the prepared form-stable composite PCMs have great potential for thermal energy storage applications in terms of their satisfactory thermal properties, improved thermal conductivity and cost-efficiency because of no encapsulation for enhancing heat transfer in paraffin.     

太阳能复合相变蓄能炕的设计及热性能研究

为解决我国北方农村冬季采暖过程中能源浪费及环境污染的问题,设计了一种以太阳能为热源,石蜡为相变蓄能材料的蜂窝活性炭-石蜡复合相变蓄能炕采暖系统。在供水温度为40 ℃的条件下,通过对比试验,分析了纯石蜡蓄能炕、蜂窝活性炭孔密度分别为20,30以及40 PPI的复合蓄能炕的热性能。实验结果表明：新型蓄能炕可以满足人体对于夜间睡眠的热舒适性需求。蜂窝活性炭孔密度为20 PPI的复合蓄能炕升温及降温速度最快,分别为0.35和0.39 ℃;炕面层温度标准差最低,为0.39 ℃,比30 PPI的低18.75%,比40 PPI的低23.53%,比纯石蜡的低41.79%;睡眠层平均温度以及炕面热流密度最高,分别为28.10 ℃和11.88 W/m2。可见蜂窝活性炭的加入有助于提高蓄能炕的热性能,且蜂窝活性炭的孔密度越小,复合蓄能炕的热性能就越好。与传统火炕相比,纯石蜡蓄能炕在经济性方面也占优势。     

Thermal Performance Investigation of Mechanical Draft Cooling Tower Using Psychrometric Gun Technique

This paper presents an experimental work that studies the performance of a counterflow cooling tower with expanded wire mesh packing. Due to complicated configuration of the expanded wire mesh packing surface, it is not possible to measure the temperature of the air in the intermediate section of the cooling tower, but only that of the exit air and water temperature. In the experimental study a new concept of psychrometric gun technique is used to measure dry/wet bulb temperature of air at intermediate points of tower packing. First, the paper elaborates on the effect of atmospheric conditions, water mass flow rate and inlet temperature on the variation of the thermodynamics properties of moist air inside the cooling tower and thermal performance characteristics. Second, exergy analysis is applied to study the cooling tower potential of performance using the psychrometric gun technique. An analytical model was compared with experimental data.     

静电纺丝用于制备有机相变储热纤维的研究进展

相变储热材料是指由于材料相转变吸热或放热过程而本身温度不变,从而实现储能的一类功能材料。有机相变材料如石蜡类、多元醇类及硬脂酸类因无腐蚀、无毒、无过冷等优点已成为重要的低温相变储热材料。这类材料通过固-液相变调节微环境温度,可用于服装、建筑及军事等方面,市场前景广阔。但相变过程易泄漏及热导率低等限制其了实际应用。静电纺丝是有效解决该问题的方法之一,高分子在高压静电作用下形成纤维,相变材料被高分子作为支撑材料所固定,很好地解决了泄漏问题。此外,通过加入高热导率材料可提高相变材料的吸放热速率,改善有机相变材料热导率低的问题。本文总结了近年来静电纺丝用于制备相变调温纤维的研究报道,分析了目前该类材料的研究现状,并讨论未来研究方向。为储热相变材料的进一步研究提供参考。     

潜热储能系统的传热及热力学优化研究

介绍了相变潜热储能系统的组成,特点及其传热和热力学优化研究。