Electrospun ultrafine composite fibers of binary fatty acid eutectics and polyethylene terephthalate as innovative form‐stable phase change materials for storage and retrieval of thermal energy

In this study, four fatty acids of lauric acid (LA), myristic acid (MA), palmitic acid (PA), and stearic acid (SA) were selected to prepare six binary fatty acid eutectics of LA‐MA, LA‐PA, LA‐SA, MA‐PA, MA‐SA, and PA‐SA; thereafter, electrospun ultrafine composite fibers with the binary fatty acid eutectics encapsulated in the supporting matrices of polyethylene terephthalate (PET) were prepared as innovative form‐stable phase change materials for storage and retrieval of thermal energy. The morphological structures and thermal energy storage properties of the ultrafine composite fibers were characterized by scanning electron microscope (SEM) and differential scanning calorimeter (DSC), respectively. The SEM results indicated that the fibers had the cylindrical morphology with diameters of 1–2 µm; some had smooth surfaces, while others had wrinkled surfaces with grooves. The DSC results indicated that the phase transition temperatures of binary fatty acid eutectics were lower than those of individual fatty acids; the enthalpy values associated with melting and crystallization for the eutectics encapsulated in the composite fibers were considerably reduced, whereas there were no appreciable changes on the phase transition temperatures. Copyright © 2012 John Wiley & Sons, Ltd.     

固-液相变贮能材料的研究进展

固-液相变贮能材料具有贮能密度大、相变温度恒定、体积变化小等优点，已成为能源开发利用和材料科学研究的新热点。综述了固-液相变贮能材料的研究现状，介绍了其分类及各类材料贮能性能，并总结了其应用上的缺陷及解决方法。     

相变储能建筑材料的研究进展

相变储能建筑材料是相变材料与建材基体复合制备的一种新型储能建筑材料。本文分析了相变材料的筛选和改进方法及其封装技术的研究现状,介绍了相变材料与建材基体复合工艺,系统阐述了相变储能建筑材料的作用机理和应用现状,并指出了相变储能建筑材料在实际应用中存在的一些问题,最后展望了相变储能建筑材料的发展前景。     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Form-stable phase change materials for thermal energy storage

The present paper considers the state of investigations and developments in form-stable phase change materials for thermal energy storage. Paraffins, fatty acids and their blends, polyethylene glycol are widely used as latent heat storage component in developing form-stable materials while high-density polyethylene (HDPE), styrene-butadiene-styrene (SBS) triblock copolymer, Eudragit S, Eudragit E, poly (vynil chloride) (PVC), poly (vynil alcohol) (PVA) and polyurethane block copolymer serve as structure supporting component. A set of organic and metallo-organic materials with high transition heat in solid-solid state is considered as perspective for-stable materials to store thermal energy. Another perspective class of form-stable materials are the materials on the basis of such porous materials as expanded perlite and vermiculite impregnated with phase change heat storage materials. The technology of producing new form-stable ultrafine heat storage fibers is developed. It opens availability to produce the clothers with improved heat storage ability for extremely cold regions. The perspective fields of application of form-stable materials are discussed. The further directions of investigations and developments are considered.     

Photo-to-thermal conversion and energy storage of lauric acid/expanded graphite composite phase change materials

To make better use of solar energy, lauric acid/expanded graphite (LA/EG) composite phase change materials (PCMs) were synthesized to collect and store solar energy as latent heat thermal energy. The results of thermal characteristics show that when the mass fraction of EG is 5%, 10%, and 15%, the latent heat of LA/EG is 164.5, 156.9, and 148.0 J/g, and the thermal conductivity is 2.73, 7.98, and 10.54 W/(m·K). Leakage test shows that LA/EG PCMs with EG mass fraction of 10% and 15% are form stable after phase change. One thousand thermal cycles prove good thermal reliability of LA/EG. TG analysis indicates LA/EG PCMs have good thermal stability within operating temperature range. The Ultraviolet-visible spectra reveal that the absorbance of LA/EG composite PCMs would increase as the mass fraction of EG increases. Photothermal conversion experiment results indicate that the photothermal conversion efficiency of LA/EG composite PCMs increases as the mass fraction of EG increases, and the efficiency can reach 95% when the mass fraction of EG is 15%. Moreover, it was also found that the process of photothermal conversion can be accelerated with stronger illumination intensity or smaller heat transfer size. All the results show that the prepared LA/EG PCMs can convert solar energy into thermal energy and store it in the form of latent heat at the same time, which indicates it has promising prospect in the application of solar energy conversion and storage.     

复合相变储能材料的传热性能研究进展

总结近年来国内外相变储能材料的研究状况,包括相变储能材料的制备、传热性能、相变过程数值模拟和应用等,并对复合相变储能材料的传热性能研究方法的前景作了展望。     

Thermal performance of stearic acid/carbon nanotube composite phase change materials for energy storage prepared by ball milling

In this work, stearic acid/carbon nanotubes composite phase change materials (SA/CNTs composite PCMs) were fabricated by ball milling for the first time to enhance the heat conduction of SA and prevent the delamination of SA and CNTs components. The results of suspension stability study conducted using a gravity sedimentation method showed that polyvinylpyrrolidone (PVP) used as dispersant has the best effect on the stability of composite PCMs. Then, the thermal cycling test further proved the stability of prepared composite. The SEM and FT‐IR results revealed that ball milling led to the formation of highly homogeneous composites. The thermal properties of the fabricated SA/CNTs composites with CNTs contents of 2, 6, and 10 wt.% characterized by differential scanning calorimetry (DSC) demonstrated that their phase change temperatures varied slightly while the latent heat decreased with the increased CNTs content. Furthermore, the thermal conductivity of the SA/CNTs composites were greater than that of pure SA by 61.5%, 92.3%, and 119.2%, respectively. The addition of CNTs also increased the thermal release rates of the prepared PCMs and decreased their storage rates. Therefore, the produced materials can be potentially used in thermal management.     

纳米复合相变材料固液相变储能过程研究进展

对纳米复合相变材料固液相变储能过程的若干最新研究进行了回顾,从相变储能系统的动态性能和典型的凝固、熔化传热过程两方面总结了相关研究的进展,并重点评述了数值模拟研究中纳米复合相变材料有效热物性预测方法的适用性及其与实验结果之间的偏差,最后对纳米复合相变材料固液相变储能过程的未来发展和重点研究方向进行了展望。     

Heat transfer behaviour of thermal energy storage components using composite phase change materials

对基于复合相变材料储热单元的储热性能进行了研究。建立了复合材料和储热单元体内部的二维传热模型,考察了复合材料物性和结构尺寸及传热流体操作条件（流体流速）对单元体储热性能的影响,对比了两种不同结构单元体的储热性能,并搭建实验平台进行了实验对比研究。对比结果表明,模型结果与实验结果趋于一致,验证了模型的准确性。复合材料物性和结构尺寸及传热流体操作条件对单元体储热性能有较大的影响。相比较单管储热单元体,同心管储热单元体有着更优的储热特性,在相同的操作条件下,同心管储热单元体的储热、放热时间较单管储热单元体分别减少10%和15%。     

Dynamics of energy storage in phase change drywall systems

Experimental evaluations of manufactured samples of laminated and randomly mixed phase change material (PCM) drywalls have been carried out and compared with numerical results. The analysis showed that the laminated PCM drywall performed thermally better. Even though there was a maximum 3% deviation of the average experimental result from the numerical values, the laminated PCM board achieved about 55% of the phase change process as against 48% for the randomly distributed drywall sample. The laminated board sample also released about 27% more latent heat than the randomly distributed type at the optimum time of 90 min thus validating previous simulation study. Given the experimental conditions and assumptions the experiment has proved that it is feasible to develop the laminated PCM technique for enhancing and minimising multi‐dimensional heat transfers in drywall systems. Further practical developments are however encouraged to improve the overall level of heat transfer. Copyright © 2005 John Wiley & Sons, Ltd.     

Discharge performance of a thermal energy storage unit with paraffin-expanded graphite composite phase change materials

本工作对石蜡（PA）及石蜡/膨胀石墨（97% PA/3% EG和95% PA/5% EG）复合相变储热材料的热性能进行了探究,考察了不同直径储热单元在干燥介质温度为25℃,风速为0.8 m/s条件下的放热性能。结果表明,在石蜡中添加膨胀石墨后,复合材料导热系数较纯石蜡分别提高了178.10%和214.30%,可以有效改善石蜡的导热性能,缩短放热时间;储热单元直径对放热性能有显著影响,随着石蜡相变储热单元直径的增大,放热时间线性增加;膨胀石墨的添加可以明显缩短放热时间,随膨胀石墨含量的增加,相同直径储热单元的放热时间逐渐缩短;膨胀石墨对储热单元放热性能的改善效果随直径变化而不同,在一定范围内随储热单元直径的增大而效果逐渐显著,达到极值后随直径的增大效果逐渐减弱,本实验条件下,最优储热单元直径在35~50 mm之间。结合实际生产需求,最优直径为35 mm。     

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。     

Analysis of an encapsulated phase change material‐based energy storage system for high‐temperature applications

The capability of an encapsulated phase change material (EPCM)‐based thermal energy storage (TES) system to store a large fraction of latent energy at high temperatures was examined. A 3‐dimensional simulation of a prototype heat exchanger was conducted employing sodium nitrate as the phase change material (PCM). The k‐ω SST model was used to capture the turbulent flow of the HTF, while the melting front was tracked using the enthalpy‐porosity method. The results show that the use of metal deflectors yields a nearly constant heat transfer coefficient over the capsule's surface. Despite this, the presence of the void in the capsule and natural convection within the molten PCM influenced the storage characteristics of the system affecting the shape of the isotherms and melting front. Furthermore, the EPCM capsules consecutively undergo the same heat transfer starting from the capsule closest to the inlet. The EPCM capsules store 80% of the energy lost by the HTF. The 17.7 kg of sodium nitrate stores 14.5 MJ of energy where 20% of the energy stored is via latent heat. Of the energy released by the heat transfer fluid, 80% was absorbed by the EPCM capsules with the remaining energy going into the test section walls. A total of 14.5 MJ of energy was stored by the 17.7 kg of NaNO₃, of which 20% is attributed to the latent heat. The fraction of energy stored as latent heat would be larger if a smaller operating temperature range was used. Thus, an EPCM‐based latent heat TES system is capable of storing a large fraction of the supplied energy and presents efficient means of storing thermal energy for high‐temperature applications. Additionally, the strong agreement between the numerical and experimental works demonstrates that the numerical methods employed can predict the behavior of an EPCM capsule not only within a single capsule but on the system scale as well. Therefore, the applied numerical methods can be used for further design and optimization of EPCM‐based latent heat TES systems.     

金属基相变材料的研究进展及应用

金属基相变材料由于具有储能密度高、热稳定性好、热导率高等优点,在潜热热能储存系统中具有极大的优势。本文回顾了金属基相变材料的发展历程,归纳了金属基相变材料的性能参数,总结了各种热物性的测量方法,探讨了金属基相变材料与容器材料的相容性问题,分析了金属基相变材料在太阳能热发电、工业余热回收和电力削峰填谷中的应用前景。金属基相变材料的高温腐蚀性是目前限制其在热控制中应用的主要因素。为了实现金属基相变材料的广泛应用,需要重点解决金属基相变材料的封装问题。     

Application of energy storage enclosure with phase change materials in building energy saving

相变材料具有高效的能量储存功能。相变材料与建材基体结合,制成一种具有储热功能的围护结构。该围护结构可发挥相变储热功能,降低建筑室内温度波动,增强建筑热舒适度,能够更加有效地减少建筑物运行能耗,从而实现建筑节能。本文对当前现有相变材料的优缺点、相变材料在不同围护结构中的应用、相变储能围护结构对室内热环境的影响及对建筑运行能耗的影响、相变储能围护结构应用的经济性等方面进行了分析,提出相变储能围护结构在工程应用中所存在的不足及其发展前景。     

管壳式相变储能换热器的优化设计

叙述了利用优化设计理论对管壳式相变储能换热器进行优化设计的方法。以该装置的成本为优化目标,储热量,放热时间,传热量,加工和防腐要求作为约束条件,得出了最佳的管子半径,厚度及管子根数,并给出了一个计算实例。     

Numerical analysis and parameters optimization of shell-and-tube heat storage unit using three phase change materials

In this paper, a mathematical model of shell-and-tube latent heat thermal energy storage (LHTES) unit of two-dimension of three phase change materials (PCMs) named PCM1, PCM2 and PCM3 with different high melting temperatures (983 K, 823 K and 670 K, respectively) and heat transfer fluid (HTF: air) with flowing resistance and viscous dissipation based on the enthalpy method has been developed. Instantaneous solid–liquid interface positions and liquid fractions of PCMs as well as the effects of inlet temperatures of the air and lengths of the shell-and-tube LHTES unit on melting times of PCMs were numerically analyzed. The results show that melting rates of PCM3 are the fastest and that of PCM1 are the slowest both x, r directions. It is also found that the melting times of PCM1, PCM2 and PCM3 decrease with increase in inlet temperatures of the air. Moreover, with increase in inlet temperatures of the air, decreasing degree of their melting times are different, decreasing degree of the melting time of PCM1 is the biggest and that of PCM3 is the smallest. Considering actual application of solar thermal power, we suggest that the optimum lengths are L₁ = 250 mm, L₂ = 400 mm, L₃ = 550 mm (L = 1200 mm) which corresponds to the same melting times of PCM1, PCM2 and PCM3 are about 3230 s and inlet temperature of the air is about 1200 K. The present analysis provides theoretical guidance for designing optimization of the shell-and-tube LHTES unit with three PCMs for solar thermal power.     

Modification of fly ash as a carrier of paraffin wax based phase change energy storage material for waste heat recovery

以石蜡为相变材料,改性粉煤灰为载体,无水乙醇为溶剂,采用溶液插层法合成了石蜡/改性粉煤灰相变储能材料.经过柠檬酸改性后的粉煤灰,骨架结构保持完整,不仅去除了其中杂质,而且对相变材料吸附能力有所改善.差示扫描量热(DSC)结果表明,PCM-3储能材料的熔化潜热值和熔化温度分别为327.85 kJ/kg和53.63 ℃;红外(FT-IR)测试表明,储能材料化学稳定性良好,由于吸附后的石蜡与粉煤灰载体间具有很强的表面张力,可以有效防止熔化后石蜡从载体中流失.借助相变蓄热式换热器测试了储能材料在工业余热回收中的应用效果,发现换热器出口水流温度在一定时间内可以近似保持恒定温度.     

内通流体套管式潜热蓄热器充放热过程的实验分析

通过建立与工业实际相似的加肋同心套管式潜热蓄热器模拟实验台，对潜热蓄热器内通流体时的充热、放热过程进行了实验研究。实验得出了流体的出口温度、充热量和放热量随时间的变化规律。