Recent progress about expanded graphite matrix composite phase change material for energy storage

膨胀石墨基复合相变材料具有导热系数高,储能密度大以及相变过程无液体泄漏等优点,是近年来储能科学领域的研究热点.本文探讨了应用于储热系统的相变材料的性能及分类,并对膨胀石墨及其复合相变材料的制备方法进行了简要分析,最后综述了石蜡类,脂肪酸类,共晶混合物类,聚乙二醇以及乙酰胺等膨胀石墨基复合相变材料的国内外研究进展.     

Preparation and characterization of building mortar with lauric acid-stearic acid/expanded vermiculite composite phase change material

以改性膨胀蛭石为吸附材料,以月桂酸和硬脂酸为相变材料,通过熔融共混法与真空吸附法制备定型复合相变材料,然后将其掺入砂浆中制备得到蓄热砂浆。结果表明：复合相变材料经过1000次循环后相变焓为167.6 kJ/kg,变化率仅为3.6%,热稳定性良好,无渗漏现象,掺入30%体积含量复合相变材料的砂浆28 d强度为9.2 MPa。掺有该定型相变材料的蓄热砂浆具有优异的热力学性能,完全可以应用于建筑物围护结构来调节室内温度。     

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.     

Structure and thermal properties of expanded graphite/paraffin composite phase change material

Different contents of expanded graphite (EG) composite phase change material (PCM) were prepared by the melt mixing method, taking paraffin as the PCM and EG as the supporting material. Phase compositions of EG, paraffin, and EG/paraffin composite were investigated using X-ray diffraction (XRD). Microstructures of EG and EG/paraffin composite PCMs with different EG contents were observed by a scanning electron microscope (SEM). Thermal properties, such as phase-transition temperature and latent heat of the materials, were determined by differential scanning calorimetry (DSC). Mass loss and thermal properties after 100 heating cycles were measured. The results show that physical absorption exists between paraffin and EG. EG is beneficial for the PCM composite to reduce leakage of paraffin, decrease the phase change temperature and latent heat, and strengthen the thermal stability. The solid–liquid phase change latent heat of materials is larger than that of the solid–solid one. The heating cycle has little effect on the phase-transition temperature and latent heat.     

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。     

Recent progress in diatomite based composite phase change materials for thermal energy storage

硅藻土是一种含量丰富的非金属矿,具有较高的孔隙率,良好的表面结构和热物理性能,因而可作为复合储热材料的载体.本文综述了复合储热材料的种类和制备工艺,并介绍了硅藻土的结构,性能和以硅藻土为载体的复合相变储热材料的研究及应用现状.     

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%。     

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.     

Thermal properties and crystallization kinetics of pentaglycerine/graphene nanoplatelets composite phase change material for thermal energy storage

Solid-solid phase change materials (SSPCMs) used in thermal energy storage (TES) system attract much attention in recent days. Here, graphene nanoplatelets (GnPs) were introduced into pentaglycerine (PG) with mass ratios of 1 wt%, 2 wt%, and 4 wt% to obtain PG/GnPs PCMs. The structure and thermal property of PG/GnPs PCMs were characterized by SEM, XPS, FT-IR, POM, DSC, thermal conductivity tester, and heat transfer performance test system. The effect of GnPs on the crystallization kinetic of PG was investigated by isoconversional method. The results indicated that PG and GnPs were uniformly mixed together by physical reaction. GnPs reduced the subcooling and enhanced the thermal conductivity of the PG/GnPs. The heat transfer rate of PG/GnPs was improved during to the high thermal conductivity. Crystallization kinetic results presented that the activation energy increases with the GnP content. In summary, GnPs improved the thermal behaviors of PG.     

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.     

Encapsulated phase change materials for thermal energy storage: Experiments and simulation

In the present study, encapsulated phase change materials (PCMs) were used for the storage of thermal energy. Both experiments and simulation were performed to evaluate the characteristics of encapsulated PCMs. Tests were conducted in a packed bed to determine the performance of the encapsulated PCM. In the preparation of encapsulated PCMs, the coacervation technique was used. The performance of the encapsulated PCM was evaluated in terms of encapsulation ratio, hydrophilicity, and energy storage capacity. The experiments were designed, based on surface response method, to optimize the processing conditions. It was found that a higher coating to paraffin ratio led to a higher paraffin encapsulation ratio. The hydrophilicity value of encapsulated paraffin depended mainly on the ratio of paraffin to coating. The higher the ratio, the lower was its product hydrophilicity. When the paraffin to coating ratio was constant, the higher concentration of HCHO led to a lower hydrophilicity of the product. The encapsulated paraffin has shown large energy storage and release capacity (20–90 J g^?1) during its phase changes depending on different ratios of paraffin to coating. Thermal cyclic test showed that encapsulated paraffin kept its geometrical profile and energy storage capacity even after 1000 cycles of operation. In the experiments and simulation of fluid heating process in encapsulated PCM charged packed bed, results showed that Eulerian granular multiphase model in FLUENT 4.47 is suitable for simulation of such a system. Copyright © 2002 John Wiley & Sons, Ltd.     

Micro-encapsulated paraffin/high-density polyethylene/wood flour composite as form-stable phase change material for thermal energy storage

Six novel polymer-based form-stable composite phase change materials (PCMs), which comprise micro-encapsulated paraffin (MEP) as latent heat storage medium and high-density polyethylene (HDPE)/wood flour compound as supporting material, were prepared by blending and compression molding method for potential latent heat thermal energy storage (LHTES) applications. Micro-mist graphite (MMG) was added to improve thermal conductivities. The scanning electron microscope (SEM) images revealed that the form-stable PCMs have homogeneous constitution and most of MEP particles in them were undamaged. Both the shell of MEP and the matrix prevent molten paraffin from leakage. Therefore, the composite PCMs are described as form-stable PCMs. The differential scanning calorimeter (DSC) results showed that the melting and freezing temperatures as well as latent heats of the prepared form-stable PCMs are suitable for potential LHTES applications. Thermal cycling test indicated the form-stable PCMs have good thermal stability although it was subjected to 100 melt–freeze cycles. The thermal conductivity of the form-stable PCM was increased by 17.7% by adding 8.8 wt% MMG. The results of mechanical property test indicated that the addition of MMG has no negative influence on the mechanical properties of form-stable composite PCMs. Taking one with another, these novel form-stable PCMs have the potential for LHTES applications in terms of their proper phase change temperatures, improved thermal conductivities, outstanding leak tightness of molten paraffin and good mechanical properties.     

PolyHIPE composite based-form stable phase change material for thermal energy storage

A novel shape-stabilized n-hexadecane/polyHIPE composite phase change material (PCM) was designed and thermal energy storage properties were determined. Porous carbon-based frameworks were produced by polymerization of styrene-based high internal phase emulsions (HIPEs) in existence of the surface modified montmorillonite nanoclay. The morphological and mechanical properties of the obtained polyHIPEs were investigated by scanning electron microscopy analysis and the compression test, respectively. The polyHIPE composite with the best pore morphology and the highest compression modulus was determined as a framework to prepare the form stable n-hexadecane/polyHIPE composite phase change material using the one-step impregnation method. The chemical structure and morphologic property of composite PCM was investigated by FT-IR and polarized optical microscopy analysis. Thermal stability of the form-stable PCM (FSPCM) was examined by TG analysis. The n-hexadecane fraction engaged into the carbon foam skeleton was found of as 55 wt% from TG curve. differential scanning calorimetry analysis was used for determining melting temperature and latent heat storage capacity of FSPCM and these values were determined as (26.36°C) and (143.41 J/g), respectively. The results indicated that the obtained composite material (FSPCM) has a considerable potential for low temperature (18°C-30°C) thermal energy storage applications with its thermal energy storage capacity, appropriate phase change temperatures and high thermal stability.     

Enhanced thermal conductivity of copper-doped polyethylene glycol/urchin-like porous titanium dioxide phase change materials for thermal energy storage

A composite phase change material (PCM) of copper-doped polyethylene glycol (PEG) 2000 impregnated urchin-like porous titanium dioxide (TiO₂) microspheres (PEG/TiO₂) was successfully synthesised. The urchin-like porous TiO₂ structures contain hollow cavities that can provide a high PEG loading capacity of up to 80 wt%. Copper nanoparticles were uniformly dispersed on the outer and inner surfaces of the 0.8PEG/TiO₂ as additives to enhance the thermal conductivity of the composite PCM. The latent heat of the Cu/PEG/TiO₂ porous composite PCM reached 133.8 J/g, and the thermal conductivity was 0.58 W/(mK), which was 152.2% higher than that of TiO₂ and 38.1% higher than 0.8PEG/TiO₂. Moreover, the Cu/PEG/TiO₂ porous composite PCM has excellent thermal stability and reliability.     

Microencapsulation of coco fatty acid mixture for thermal energy storage with phase change material

Thermal energy storage systems provide several alternatives for efficient energy use and energy conservation. Microcapsules of natural coco fatty acid mixture were prepared to be used as phase change materials for thermal energy storage. The coacervation technique was used for the microencapsulation process. Several alternatives for the capsule wall material were tried. The microcapsules were characterized according to their geometric profiles, phase transition temperatures, mean particle sizes, chemical stabilities, and their thermal cycling. The diameters of microcapsules prepared in this study were about 1 mm. Coco fatty acid mixtures have kept their geometrical profiles even after 50 thermal cycles for melting and freezing operations in temperature range from 22 to 34°C. It was found that gelatin+gum Arabic mixture was the best wall material for microencapsulating coco fatty acid mixtures. Copyright © 2005 John Wiley & Sons, Ltd.     

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。     

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。     

Preparation and properties of caprylic‐nonanoic acid mixture/expanded graphite composite as phase change material for thermal energy storage

To satisfy the application demands for latent heat storage in the temperature range from 5°C to 15°C, an original composite phase change material (PCM), CA‐NA/EG (caprylic‐nonanoic acid/expanded graphite), was prepared and characterized. For CA‐NA/EG, the mass ratio of CA and NA was 8:2, and the mass percentage of the CA‐NA in CA‐NA/EG composite PCM was determined as 90% by leakage test. The melting and freezing points of the CA‐NA/EG were 6.84°C and 9.34°C, and corresponding latent heats were 108.75 kJ/kg and 107.67 kJ/kg. In addition, its thermal conductivity, thermal stability and reliability were investigated by thermal conductivity apparatus (TCA), thermal gravimetric analyzer (TGA), and accelerated thermal cycle test for 100 melt/freeze cycles, respectively. The results showed that the CA‐NA/EG had a good thermal stability and an excellent thermal reliability. Moreover, the thermal conductivity of CA‐NA/EG had an improvement of 25% than that of the CA‐NA. On the other hand, the accelerated thermal cycle test also indicated that the CA‐NA/EG had no supercooling during all melt/freeze cycles. Therefore, the prepared composite PCM, CA‐NA/EG, can be applied for low‐temperature thermal energy storage owing to its proper melting temperature, acceptable latent heat and thermal conductivity, excellent thermal stability and reliability.     

Synthetic and physical characterization of phase change materials microencapsulated by complex coacervation for thermal energy storage applications

This paper presents a comprehensive study of encapsulated phase change materials (PCMs). In order to investigate some synthesis parameters, microencapsulated paraffin with gelatin/gum Arabic wall system was prepared by the complex coacervation method and the performance of these microcapsules was evaluated by optical microscopy and differential scanning calorimetry. Further investigations were carried out on the impact of physical parameters on the melting time by studying the constrained melting transformation of an encapsulated PCM in a spherical shell subjected to a constant temperature media. Results indicate successful production of PCM microcapsules with high melting enthalpy (116 kJ/kg), and the effects of diameter and thermal conductivity on melting time of PCMs were demonstrated. Copyright © 2014 John Wiley & Sons, Ltd.