Thermal conductivity measurement of erythritol,xylitol, and their blends for phase change material design: A methodological study

This work presents and discusses a detailed thermal conductivity assessment of erythritol, xylitol, and their blends: 25 mol% erythritol and 80 mol% erythritol using the transient plane source (TPS) method with a Hot Disk Thermal Constants Analyzer TPS‐2500S. Thereby, the thermal conductivities of xylitol, 25 mol% erythritol, 80 mol% erythritol, and erythritol were here found for respectively in the solid state to be 0.373, 0.394, 0.535, and 0.589 W m^?1 K^?1 and in the liquid state to be 0.433, 0.402, 0.363, and 0.321 W m^?1 K^?1. These obtained results are comprehensively and critically analyzed as compared to available literature data on the same materials, in the phase change materials (PCMs) design context. This study clearly indicates that these thermal conductivity data in literature have considerable discrepancies between the literature sources and as compared to the data obtained in the present investigation. Primary reasons for these disparities are identified here as the lack of sufficiently transparent and repeatable data and procedure reporting, and relevant standards in this context. To exemplify the significance of such transparent and repeatable data reporting in thermal conductivity evaluations in the PCM design context, here focused on the TPS method, a comprehensive measurement validation is discussed along various residual plots obtained for varying input parameters (ie, the heating power and time). Clearly, the variations in the input parameters give rise to various thermal conductivity results, where choosing the most coherent result requires a sequence of efforts per material, because there are no universally valid conditions. Transparent and repeatable data and procedure reporting are the key to achieve comparable thermal conductivity results, which are essential for the correct design of thermal energy storage systems using PCMs.     

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.     

Preparation and application of composite EG/Ba(OH)2·8H2O form‐stable phase change material for solar thermal storage

Composite expended graphite (EG)/Ba(OH)₂·8H₂O form‐stable phase change material (PCM) is prepared with porous adsorption method in this study to solve the problem of the leakage risk in the application process based on barium hydroxide octahydrate. In addition, the thermal properties and stability have been measured and verified. Thermal conductivity of each group composite material was enhanced by about two to four times, while the addition has little negative effect on the other properties. With microscopic features being characterized by scanning electron microscope (SEM), the experimental result demonstrates that the composite material with 7 wt% of EG is optimal to be a saturated state between two phases. After the thermal cyclings of 100, 300, and 500 times, thermal properties did not change dramatically, which can be used as an ideal material for solar thermal storage system within finite thermal cyclings. In order to simulate the operating condition of solar energy or waste heat storage, the composite material was encapsulated in the heat storage unit with pipe bundle, and heat storage/release experiences were performed to test the performance of the material and hot water supply. The results indicate that the composite material is qualified to storage and release sufficient heat. The experimental data can provide necessary technical reference for engineering design and effect prediction in practical application.     

无机水合盐相变储热材料的过冷性研究

无机水合盐相变储能材料通常存在过冷现象,影响了该类材料所蓄热量的排放性能,如果过冷严重,储存的热量不能释放出来。就无机水合盐的过冷机理及非均匀形核的动力学机理进行了探讨,加入成核剂是降低过冷度的有效措施。     

Preparation of stearic acid/halloysite nanotube composite as form‐stable PCM for thermal energy storage

A novel form‐stable composite as phase change material (PCM) for thermal energy storage was prepared by absorbing stearic acid (SA) into halloysite nanotube (HNT). The composite PCM was characterized by TEM, FT‐IR and DSC analysis techniques. The composite can contain SA as high as 60 wt% and maintain its original shape perfectly without any SA leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (SA/HNT: 60/40 wt%) were determined as 53.46°C and 93.97 J g^?1 by DSC. Graphite was added into the SA/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 65.3 and 63.9%, respectively. Because of its high adsorption capacity of SA, high heat storage capacity, good thermal stability, low cost and simple preparation method, the composite can be considered as cost‐effective latent heat storage material for practical application. Copyright © 2011 John Wiley & Sons, Ltd.     

Recent progress in shape-stabilized phase change materials

定形相变材料是以聚合物为基体,相变物质分布在聚合物三维网状结构中的一种新型相变材料.定形相变材料在相变过程中表现为宏观固相,微观液相,支撑和力学性能优秀,不易泄漏,因其优良的加工性能和安全性能而受到广泛关注,并表现出了广阔的应用前景.本文综述了定形相变材料的制备,导热和阻燃性能等方面的研究进展,并从实验和模拟两方面综合评价了定形相变材料在建筑节能方面的使用性能,展望了定形相变材料的发展前景.     

Preparation and characterization of polyethylene glycol/active carbon composites as shape-stabilized phase change materials

Shape-stabilized phase change materials (PCMs) composed of polyethylene glycol (PEG) and mesoporous active carbon (AC) were prepared by a blending and impregnating method. Various techniques were carried out to characterize the structural and thermal properties of the composites. Lower phase change temperatures and enthalpies were observed as the weight percentage and molecular weight of PEG was decreased. The crystallinity of PEG in the PCMs decreased with the increase in the AC content. The activation energy of the PEG phase change decreased with higher PEG weight percentages. We conclude that the phase change properties of the PEG/AC PCMs are influenced by the adsorption confinement of the PEG segments from the porous structure of AC and also the interference of AC by acting as an impurity with perfect PEG crystallization.     

Modification on hydrated salt‐based phase change composites with carbon fillers for electronic thermal management

For electronic thermal management with hydrated salt phase change materials (PCM), supercooling and thermal stability usually inhibit its development. In view of this, novel of disodium hydrogen phosphate dodecahydrate (DSP)‐based composites PCM with miniaturized size is developed to solve these problems. Three kinds of carbon fillers employed as both nucleating agent and heat transfer promoter were added in DSP separately. The influences of carbon fillers' specific surface area, particle size, and adsorption mode on thermal properties of DSP‐based composites PCM were investigated experimentally. The thermal conductive chains of composites PCM were detected by energy dispersive spectroscopy. The supercooling degree was analyzed by melting‐freezing test. Temperature‐regulated property was captured by infrared imager. In the present work, the supercooling degree of GNS/DSP composites PCM is efficiently reduced to 97.24% compared with the pure DSP. Negligible change in phase change temperature of the CNTS/DSP composites PCM was confirmed after 200 times cycles. There was no obvious liquid leakage of DSP when 2 wt% of carbon fillers were added in composites PCM. The enhancement on thermal properties of DSP is a promising strategy for numerous thermal applications.     

Development and evaluation of graphite nanoplate (GNP)‐based phase change material for energy storage applications

In this study, arachidic acid (AA)/graphite nanoplate (GNP) composite material is proposed as a novel phase change material (PCM). In order to examine the influence of GNP loading into the base material (AA), composite PCMs were prepared with three different fractions (0.5, 1.0 and 2.0 in wt%). The thermal, chemical and morphological characteristics of the samples were introduced in terms of differential scanning calorimetry, thermogravimetric analysis, Fourier transform infrared, and X‐ray diffraction analyses. Moreover, the thermal conductivity values of samples in the liquid state were measured by means of 3ω method. It was found that, for GNP fractions of 0.5, 1, and 2 wt. %, the thermal conductivity of composite PCMs have been increased by 15, 30 and 43% in comparison to the pure AA. Results also revealed that the GNP loading into the pure AA led to a reduction in the degree of subcooling and a small diminution in the latent heat values. Considering the thermal cycling tests, it was also obtained that the proposed composite PCMs remained stable even at the highest GNP loading. Conduction‐dominated inward solidification process was simulated for one‐dimensional spherical computational domain. For this particular case, it was found that the increments of 15, 30 and 45% in thermal conductivity tend to increase the effectiveness by 3.5, 6.4 and 8.7%, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Adipic acid-silica composite phase change materials for thermal energy storage:Preparation and characterization

以己二酸为相变材料,二氧化硅溶胶为基体,采用水热法制备己二酸/二氧化硅复合相变储热材料.应用X射线衍射仪(XRD),红外光谱分析(FT-IR),场发射扫描电子显微镜(FE-SEM),同步热分析仪(TG)和差示扫描量热仪(DSC)测试所制备复合材料的显微结构,相变温度和相变焓.结果表明:当反应条件为pH=5,反应温度为150 ℃,反应时间为4 h时,己二酸与二氧化硅之间是简单的物理嵌合关系,复合材料粒径较均匀,平均粒径在1~2 μm;当己二酸的质量分数为30%时,复合材料相变温度峰值为139.0 ℃,相变焓为48.82 J/g;当己二酸的质量分数为50%时,复合材料相变温度峰值为140.5 ℃,相变焓为71.89 J/g.     

Effect of graphene and its derivatives on thermo-mechanical properties of phase change materials and its applications: a comprehensive review

Phase change materials (PCMs) play a leading role in overcoming the growing need of advanced thermal management for the storage and release of thermal energy which is to be used for different solar applications. However, the effectiveness of PCMs is greatly affected by their poor thermal conductivity. Therefore, in the present review the progress made in deploying the graphene (Gr) in PCMs in the last decade for providing the solution to the aforementioned inadequacy is presented and discussed in detail. Gr and its derivatives ((Gr oxide (GO), Gr aerogel (GA) and Gr nanoplatelets (GNPs)) based PCMs can improve the thermal conductivity and shape stability, which may be attributed to the extra ordinary thermo-physical properties of Gr. Moreover, it is expected from this review that the advantages and disadvantages of using Gr nanoparticles provide a deep insight and help the researchers in finding out the exact basic properties and finally the applications of Gr can be enhanced.In this work, Gr and its derivatives based PCMs was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction spectroscopy (XRD), and scanning electron microscopy (SEM) by which crystal structure was known, phase was identified along with the knowledge of surface structure respectively. The increase in the mass fraction (%) of the filler (Gr and its derivatives) led to even better thermo-physical properties and thermal stability. The thermal characterization was also done by differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA) and thermal conductivity tests. The enthalpy of freezing and melting showed that Gr and its derivatives based PCMs had a very high energy storage capability as reflected in its various applications.     

Preparation and characterization of urea/ammonia bromide composite phase change material

In order to find phase change materials that can be better applied in the field of solar collectors, the phase change heat storage properties of urea (U) and ammonium bromide (AB) were studied. The eutectic ratio of UAB was found by the Edison method, and different additives were added to UAB by dipping and mixing. The structure and property of the composites were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), differential scanning calorimetry (DSC), and TG/TGA, respectively. The results showed that the eutectic ratio of UAB was 6:3; it was weakly acidic after melting not layered. When 6-wt% EG and 1-wt% nanometer titanium dioxide (TiO₂) were added, the thermal conductivity of the material increased by 3.62 times; the thermal diffusivity was increased by 6.77 times. When the mass fraction of EG is between 1% and 6%, the larger the mass fraction of EG, the greater the thermal conductivity of the material. After 1000 cycles, the composite material performance was stable; at the same time, the used materials can be used as fertilizers to solve the problem of material pollution, which indicated that the prepared UAB/TiO2/EG composite PCMs had great application prospect in the field of solar collectors.     

Lipid‐derived monoamide as phase change energy storage materials

One of the major shortcomings of current organic phase change materials (PCMs) is their relatively low melting points, typically below 80°C, which limits their integration into thermal energy storage (TES) systems. The present work was aimed at developing lipid‐derived PCMs with increased melting points which would be suitable for TES applications requiring higher melting points without compromising other key properties such as enthalpy. The introduction of an amide group into the structure of linear saturated fatty acids was used as a means to increase intermolecular interactions and therefore crystallization and melting points. A series of six linear monoamides with differing chain length and symmetry about the amide group were investigated for thermal stability, thermal transition, flow behavior, and crystal structure to establish the structure‐property relationships relevant to TES. The presence of the highly polar amide group in the aliphatic fatty acid–derived molecules resulted in notable improvement in performance compared with the analogous monofunctional molecules: Increases in melting points (79°C‐96°C) and high enthalpies of fusion (155‐201 J/g) were recorded. Fundamental relationships between structure, processing, and macroscopic physicochemical properties, never before elucidated, were revealed in the study. The study revealed a step‐like variation of macroscopic properties: a surprising outcome of the competition between intermolecular attractions, symmetry effects, and mass transfer limitations. The predictive structure‐function relationships established in this work will allow the straightforward engineering of monoamide architectures that can extend the range of organic PCMs and deliver thermal properties desirable for TES applications.     

Experimental and numerical investigation of the melting process and heat transfer characteristics of multiple phase change materials

The melting and heat transfer characteristics of multiple phase change materials (PCMs) are investigated both experimentally and numerically. Multiple PCMs, which consist of three PCMs with different melting points, are filled into a rectangle-shaped cavity to serve as heat storage unit. One side of the cavity is set as heating wall. The melting rate of multiple PCMs was recorded experimentally and compared with that of single PCM for different heating temperatures. A two-dimensional mathematical model to describe the phase change heat transfer was developed and verified experimentally. The properties of multiple PCMs, including the effect of the melting point difference (combined type), thermal conductivity, and latent heat, on the heat transfer performance of the PCM were analyzed numerically. The results show that, the melting time decreases before it increases, with an increasing melting point difference for the multiple PCMs. In addition, the melting point decreases with increasing distance from the heating wall. Most of these types of multiple PCMs melt faster than the single PCM, and the multiple PCMs, with the melting point arranged as 322 K/313 K/304 K, has the shortest melting time in this study. The melting rate of the multiple PCMs, 322 K/313 K/304 K, accelerates faster than for the single PCM as the thermal conductivity, latent heat, and heating wall temperature increase. Finally, generalized results are obtained using a dimensionless analysis for both single and multiple PCMs.     

End group modification of polyethylene glycol (PEG): A novel method to mitigate the supercooling of PEG as phase change material

Polyethylene glycol (PEG) is a kind of phase change material with high phase change enthalpy and good compatibility with the environment. However, there is relatively large supercooling for PEG, limiting their practical applications. To reduce their supercooling degree, herein we use three different small molecules (acryloyl chloride, acetyl chloride, and thionyl chloride) to modify PEG and study the effects of different end group modification on their phase change properties. Fourier‐transform infrared (FTIR) and proton nuclear magnetic resonance (¹H‐NMR) spectroscopy are used to confirm the molecular structure of the PEG with different molecular weights and functionalities after chemical modification. Crystal structures of the PEG before and after the modification are verified by X‐ray diffractometry (XRD) method and show no change. Differential scanning calorimetry (DSC) results show that the end group modification is quite effective for mitigating the supercooling of PEG with double ‐OH end groups but not effective for PEG with mono ‐OH end group. The mechanism for the change of supercooling behavior is proposed. The costs of different modification methods are estimated and compared.     

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。     

Preparation and properties of solid-solid phase change materials based on polyethylene glycol and EPDM cross-linked resin containing cyclodextrin cavity

β-cyclodextrin with vinyl group (β-CD-MA) was synthesized from β-cyclodextrin (β-CD) and glycidyl methacrylate and identified by FT-IR and ¹H-NMR spectroscopy analysis. A novel adsorption resin containing cyclodextrin cavity moieties was prepared by suspension polymerization technique with ethylene-propylene-diene (EPDM), styrene (St) and β-CD-MA as comonomers in this study. The adsorption resin was used as a solid skeleton for adsorption of phase change material such as polyethylene glycol (PEG) to prepare solid-solid phase change materials (SSPCMs). Structure and phase change behaviors of SSPCMs were studied by FT-IR and DSC, and the phase change process of the SSPCM was observed by using reflection mode of POM. The experimental results showed that the introduction of cyclodextrin cavity structure increased the adsorption capacity of PEG and improved the phase change latent heat of SSPCMs. Otherwise, during the whole process of phase change, the surface morphology of SSPCMs could be observed and appeared bright or dark change. During the transition, the whole PCMs remained in solid state.     

ZrC nanoparticle-modified microencapsulated phase change materials with enhanced thermal conductivity and photo-thermal conversion performance

A photo-thermal energy storage microcapsule modified by KH550-treated ZrC nanoparticles was prepared via in situ polymerization. ZrC nanoparticles were treated by KH550 to stabilize the paraffin-ZrC emulsion, which could ensure ZrC nanoparticles were incorporated into the microcapsule shell. In this article, microcapsules modified by different mass fractions of modified ZrC nanoparticles (paraffin@MUF-ZrC) were prepared to investigate the influence of ZrC nanoparticles on microstructure, thermal storage, thermal stability, thermal conductivity, and photo-thermal conversion behavior. According to the SEM images, the obtained paraffin@MUF-ZrC microcapsules exhibited good morphology with regular spherical shape and uniform particle size distribution. DSC, TGA, and thermal conductivity analysis were used to characterize the thermal storage properties, thermal stability, and thermal conductivity of paraffin@MUF-ZrC microcapsules, respectively. It showed that these three thermal performance indices improved as the mass fraction of modified ZrC nanoparticles increased. Specifically, the thermal properties of paraffin@MUF-4%ZrC microcapsules were better than paraffin@MUF microcapsules. As a result, specific enthalpy was slightly increased by 14.92% (121.74 J/g) and thermal conductivity increased by 225.16% (0.4962 W/m·K). Moreover, the optical absorption capability also increased. The paraffin@MUF-4%ZrC microcapsules showed remarkable optical absorption capability of 70.23%, which was a 247.67% improvement compared to the 20.20% of the paraffin@MUF microcapsules. Moreover, it was found that paraffin@MUF-4%ZrC microcapsules could heat up from 30°C to 78°C under simulated solar radiation, showing excellent photo-thermal conversion behavior. Based on good thermal storage properties, thermal conductivity, and light absorption capability over the full spectrum, the new photo-thermal energy storage microcapsules have good application prospects in solar thermal conversion and energy storage systems.     

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.     

Preparation of macro-capsules containing shape-stabilized phase change materials and description of permeation kinetics of its wall

Through in situ polymerization, a kind of macro-capsule was prepared by using silica gel as the shell material and shape-stabilized phase change materials (SSPCM) containing 50 wt% of n-octadecane (OD) of high density polyethylene (HDPE) as the core. The surface and its construction of capsules, the permeability of the capsule wall and the release kinetics parameters of the OD in the system of petroleum ether were experimental investigated by a scanning electron microscope (SEM), thermal cycles and the extraction release kinetics. The results showed that the wall thickness of the macro-capsules was about 20–50 μm under the experimental conditions. The SSPCM surface, modified with chromic acid, is rough, and there are many tiny holes about 3 μm in diameter in it. For these reasons, either the hydrophilicity of the SSPCM surface or the cohesion between the core and the wall have been greatly improved, and then, the weight loss percentage (WLP) of the macro-capsules is decreased by about 1.5 and 2.5 times relative to that of un-modified and modified SSPCM, respectively. Additionally, after the macro-capsules were re-sprayed by using a calcium chloride solution, its wall was more compact. From the fitting parameters of the power exponent, there were two different release mechanisms, quasi-Fickian diffusion and anomalous transport for the un-encapsulated SSPCM and the macro-capsules, respectively.