一种新型复合相变蓄热材料的制备与表征

相变蓄热材料是太阳能高效利用的基础与关键。文章选用54%KNO_3-46%NaNO_3作为太阳能高温热电站的蓄热材料,并选用膨胀石墨作为添加剂,分别制备了膨胀石墨(EG)质量分数为1%和2%的新型太阳能复合相变蓄热材料KNO_3-NaNO_3/EG。然后利用同步热分析仪(SDT-Q600)测量上述蓄热材料的相变温度、潜热,利用扫描电子显微镜(SEM)观测上述蓄热材料的微观结构。分析结果表明:太阳能复合相变蓄热材料KNO_3-NaNO_3/EG的相变温度为224.28℃,相变潜热为105.8 J/g;添加膨胀石墨能够明显地增强蓄热材料的导热性能,石墨对蓄热材料的熔点影响较小。     

硝酸盐/膨胀石墨储热材料的制备及热性能研究

为了得到膨胀石墨(EG)含量对NaNO₃-KNO₃/EG复合储热材料导热系数的影响,通过水溶液法,采用NaNO₃-KNO₃共晶盐作为储热材料,不同质量分数的EG作为基体材料,制备出NaNO₃-KNO₃/EG复合储热材料。分别用同步热分析仪(STA)和激光闪射仪(LFA)测量了材料的潜热和导热系数。结果表明,NaNO₃-KNO₃/EG复合材料的潜热随EG含量的增加而减小,导热系数相较于纯硝酸盐均有明显提升。添加20%的EG时,复合材料的平均导热系数可以达到3.95 W/(m·K),约为纯硝酸盐的7.4倍。将EG加入到二元硝酸盐中制备成复合储热材料是提高其传热性能的一种有前景的候选材料。     

Preparation and thermal stability of ternary sulfate molten salt

以硫酸钠、硫酸钾和硫酸镁为原料,采用在硫酸钠-硫酸钾二元共晶盐中加入硫酸镁的方法制备三元硫酸熔盐。应用TG-DSC联用分析仪、热常数分析仪、X射线衍射仪以及热循环法对复合熔盐的熔点、相变潜热、热导率、比热容、分解点以及热稳定性进行表征。结果表明：所制备的三元硫酸熔盐熔点分布在667.5～669.7 ℃之间,较二元熔盐熔点降低了160 ℃左右,硫酸镁含量为30%（质量分数）的三元硫酸熔盐相变潜热值最大为94.3 J/g,比热容最大为1.13 J/(g·K)（720℃≤T≤800℃）,导热系数为0.41 W/(m·K),分解温度为1070 ℃,经50次热循环后,相变潜热值降低约4.34%,熔点和物相保持基本恒定,具有良好的热稳定性。该研究为硫酸盐作为高温传热蓄热介质提供了依据。     

剥离膨胀石墨/熔融盐材料的制备及其性能研究

采用水溶液法配以超声波剥离和分散手段,使用二元硝酸熔融盐Na NO3-KNO3和膨胀石墨(EG)制备出剥离膨胀石墨/熔融盐复合相变储能材料。通过扫描电镜(SEM)、能量色散谱仪(EDX)、透射电镜(TEM)、X-射线衍射(XRD)、导热性能、差示扫描量热(DSC)等表征和测试手段,研究添加不同质量分率的EG对复合相变储能材料热物理性能的影响。结果表明:利用超声波处理使EG得以剥离成石墨片并均匀分散在二元硝酸熔盐中,制得的复合相变储能材料微观结构均一,导热性能大大改善,其导热系数高达4.884 W/(m·K),相变潜热随EG质量分数的增大而逐渐降低,但变化不大,相变峰值温度约224℃。该制备工艺简单,制备出的复合材料在中温储热领域具有良好的应用前景。     

谷电利用复合石蜡蓄热材料的制备及供暖墙体构造实验

针对石蜡作为相变蓄热材料导热性能差、供热能力不足的问题,向石蜡中添加膨胀石墨(EG)制备导热性能增强的复合相变材料(PCM),探讨EG与石蜡的配比对复合PCM热性能的影响.测试结果表明,当EG质量分数达到12％时,复合PCM的导热系数提升至纯石蜡的12倍,相变潜热从纯石蜡的190.8 J/g减小至152.1 J/g,相...     

基于石墨泡沫强化的相变储能材料研究进展

大多数相变储能材料导热性能差是导致其不能推广应用的一个重要因素,因此,目前相变材料研究的重点是提高相变材料的等效导热系数。石墨泡沫由于其特殊的微蜂窝三维结构,使其具有良好的传热性能,在储能领域有很好的应用前景。国内外学者对利用石墨泡沫的强化相变传热进行了一些研究,本文主要介绍了近几年石墨泡沫/相变材料的国内外实验研究和数值模拟研究进展和存在的问题。     

三元硝酸盐/陶瓷基相变材料的制备及热物性研究

为提高硝酸盐相变材料导热性能和研究三元硝酸盐的共晶特性,采用饱和溶液法配制了不同质量配比的KNO_3、LiNO_3和Ca(NO_3)_2·4H_2O样品,测试了其熔点和相变潜热,并采用直接浸泡法制备了KNO_3-LiNO_3-Ca(NO_3)_2/陶瓷基相变材料,测试其相变温度、相变潜热及导热系数等热物性参数。实验筛选出相变温度较高,潜热较大的三元硝酸盐KNO_3-LiNO_3-Ca(NO_3)_2·4H_2O为60∶30∶10配比,其相变温度为116.8℃,相变潜热为46.7 J/g。实验结果表明:所测三元硝酸盐/陶瓷基相变材料和三元硝酸盐的相变温度基本一致,单位相变潜热是三元硝酸盐试样的35.43%;三元硝酸盐/陶瓷基相变材料的导热系数在常温下相较三元硝酸盐提高了9.27倍,且随温度的升高呈下降趋势;三元硝酸盐/陶瓷基相变材料的分解温度为620℃,较纯三元硝酸盐提高了20℃,增强了其高温稳定性。     

Progress in hydrated salt based composite phase change materials

水合盐相变储能材料具有相变温度适中、导热系数大、潜热值高、价格低廉等优点,因而具有广阔的使用前景。然而,过冷、相分离、循环稳定性差等诸多问题限制了水合盐的实际应用。许多学者将水合盐与其它材料结合,构成复合相变材料,成功地解决了以上问题。前人对水合盐复合相变材料的研究以解决水合盐在使用过程中的上述问题居多。然而近年来,有研究者制备复合相变材料以改善水合盐的热物性,如相变温度、导热系数、潜热值等,取得了一定成果,但这一研究思路仍需进行进一步探索。文章将制备水合盐复合相变材料的目的作为线索,总结了水合盐复合相变材料的研究思路,详细介绍了国内外相关的研究工作和研究成果,并指出了今后水合盐复合相变材料的研究重点。     

热适应复合相变材料的制备与热性能

根据电子器件散热技术领域对热适应复合材料的性能要求,选取导热系数高且密度低的膨胀石墨作为无机支撑材料,石蜡作为有机相变材料,制备出高导热系数和储热密度的热适应复合相变材料.采用扫描电镜(SEM)、差示扫描量热仪(DSC)、偏光显微镜(POM)和Hot Disk热常数分析仪等多种测试技术,对复合相变材料进行分析研究;通过储/放热实验和1000次热循环实验研究了复合相变材料的传热性能和热稳定性.实验结果说明该复合相变材料具有形状稳定、导热率高、储热密度大等特点,并具有良好的热稳定性和使用寿命.     

石蜡/膨胀石墨复合相变储热材料的研究

以膨胀石墨为基体,石蜡为相变储热介质,利用膨胀石墨对石蜡良好的吸附性能,制备出了石蜡／膨胀 石墨复合相变储热材料。由于毛细作用力和表面张力的作用,石蜡在固-液相变时,很难从膨胀石墨的微孔中渗 透出来。实验结果表明,石蜡/膨胀石墨复合相变储热材料没有改变膨胀石墨的结构和石蜡的固-液相变温度, 且其结合了石墨高的导热系数和石蜡大的相变潜热,因而储热密度较高,导热性能好。其相变潜热与对应质量 分率下的石蜡相当,储/放热时间比石蜡明显减少。     

Capric-myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage

Phase change materials (PCMs) can be incorporated with building materials to obtain novel form-stable composite PCM which has effective energy storage performance in latent heat thermal energy storage (LHTES) systems. In this study, capric acid (CA)-myristic acid (MA) eutectic mixture/vermiculite (VMT) composite was prepared as a novel form-stable PCM using vacuum impregnation method. The composite PCM was characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. Thermal properties and thermal reliability of the composite PCM were determined by differential scanning calorimetry (DSC) analysis. The CA-MA eutectic mixture could be retained by 20 wt% into pores of the VMT without melted PCM seepage from the composite and therefore, this mixture was described as form-stable composite PCM. Thermal cycling test showed that the form-stable composite PCM has good thermal reliability and chemical stability although it was subjected to 3000 melting/freezing cycling. Thermal conductivity of the form-stable CA-MA/VMT composite PCM was increased by about 85% by introducing 2 wt% expanded graphite (EG) into the composite. The increase in thermal conductivity was confirmed by comparison of the melting and freezing times of the CA-MA/VMT composite with that of CA-MA/VMT/EG composite. The form-stable PCM including EG can be used as energy absorbing building material such as lightweight aggregate for plaster, concrete compounds, fire stop mortar, and component of interior fill for wallboards or hollow bricks because of its good thermal properties, thermal and chemical reliability and thermal conductivity.     

Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form-stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.     

Development of pentadecane/diatomite and pentadecane/sepiolite nanocomposites fabricated by different compounding methods for thermal energy storage

Global warming is one of the most important consequences of excess energy consumption. Phase change materials (PCMs) have prominent advantages in thermal energy storage owing to their high latent heat capacities and small temperature variations during the phase change process. However, leakage is a major problem that limits the use of PCMs. Leakage may occur in encapsulated PCMs or in composites where the PCM is attached to the surface of a supporting material or within the pores of that material. In this study, pentadecane/diatomite and pentadecane/sepiolite nanocomposites were fabricated by using unmodified and microwave‐irradiated diatomite and sepiolite samples and by using different compounding processes, such as direct impregnation, vacuum impregnation, and ultrasonic‐assisted impregnation methods. The microstructures and the chemical and thermal properties of the composites were characterized by scanning electron microscopy, Fourier‐transform infrared spectroscopy, and differential scanning calorimetry. Subsequently, the thermal reliability and stability and the thermal conductivity of the PCM composites were also investigated. A melting temperature of 9.25°C and a latent heat capacity of 58.73 J/g were determined for the pentadecane/diatomite composite that was prepared with the direct impregnation method using a microwave‐treated diatomite sample. The pentadecane/sepiolite composite prepared in the melting temperature range 7.98°C to 8.53°C and latent heat capacity range 41.05 to 46.02 J/g. The results of the thermal analysis indicate that fabricated diatomite‐based or sepiolite‐based PCM composites have good potential as thermal energy storage materials.     

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.     

Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage

This paper deals with the preparation, characterization, and determination of thermal energy storage properties of polyethylene glycol (PEG)/diatomite composite as a novel form-stable composite phase change material (PCM). The composite PCM was prepared by incorporating PEG in the pores of diatomite. The PEG could be retained by 50 wt% into pores of the diatomite without the leakage of melted PEG from the composite. The composite PCM was characterized by using SEM and FT-IR analysis technique. Thermal properties of the composite PCM were determined by DSC analysis. DSC results showed that the melting temperature and latent heat of the composite PCM are 27.70 °C and 87.09 J/g, respectively. Thermal cycling test was conducted to determine the thermal reliability of the composite PCM and the results showed that the composite PCM had good thermal reliability and chemical stability. TG analysis showed that the impregnated PEG into the diatomite had good thermal stability. Thermal conductivity of the composite PCM was improved by adding expanded graphite in different mass fractions. Thermal energy storage performance of the composite PCM was also tested.     

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.     

Heat transfer enhancement of fatty acids when used as PCMs in thermal energy storage

Phase change materials (PCM) used in latent heat storage systems usually have very low thermal conductivities. This is a major drawback in maintaining the required heat exchange rate between PCM and heat transfer fluid. This paper investigates the enhancement of the heat transfer between PCM and heat transfer fluid, using high thermal conductivity as additives like stainless steel pieces, copper pieces and graphite–PCM composite material. In the experiments, palmitic–lauric acid (80:20) (PL) and stearic–myristic acid (80:20) (SM) were used as PCMs. Test results show that heat transfer enhancement of copper pieces was better at 0.05 Ls^?1 flow rate compared to 0.025 Ls^?1. Using copper as an additive increased the heat transfer rate 1.7 times for melting and 3.8 times for freezing when flow rate was 0.050 Ls^?1. Decreasing the flow rate from 0.050 to 0.025 Ls^?1, increased the melting times 1.3 times and freezing times 1.8 times, decreasing heat transfer rates accordingly. The best result of heat transfer enhancement was observed for the PCM–graphite composite. However, changing the flow rate did not affect the heat transfer rate when graphite was used as additive. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

Constructal enhancement of thermal transport in metal foam-PCM composite-assisted latent heat thermal energy storage system

The aim of this study is to augment thermal transport in latent heat thermal energy storage (LHTES) system by the optimum allocation of metal foam-phase change material (PCM) composite. This study emphasizes on the optimal volume and distribution of metal foam-PCM composite (MFPC) to enhance melting performance without delay in the total melting time. Therefore, a MFPC is designed according to constructal theory. The fundamental principle of the theory is to configure high thermal conductivity agents at optimal thermal energy flow path for effective heat exchange. A numerical code based on local thermal nonequilibrium approach equipped enthalpy porosity method is formulated, and evaluated. The results of the proposed configuration show that the provision of MFPC only at high local temperature gradient enhances the conductive transport with improvement in the overall thermal transport. It is derived that the elimination of metal foam volume at low temperature gradient incorporates the advantageous effect of natural convective transport, which is seen to be suppressed. Additionally, the proposed configuration may increase the volume of PCM, thus, the TES capacity. It also reduces the total weight and economy of energy storage system. The overall melting rate is improved by 11.11% in comparison with the LHTES with full volume of this high thermal conductivity agent.     

Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage

As a phase change material (PCM), acetamide (AC) can be a potential candidate for energy storage application in the active solar systems. Its utilization is however hampered by poor thermal conductivity. In this work, AC/expanded graphite (EG) composite PCM with 10 wt% (mass fraction) EG as the effective heat transfer promoter was prepared; its thermal properties were studied and compared with those of pure AC. Transient hot-wire tests showed that the addition of 10 wt% EG led to about five-fold increase in thermal conductivity. Investigations using a differential scanning calorimeter revealed that the melting/freezing points shifted from 66.95/42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ kg⁻¹. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage process could be attributed to the weakened natural convection in liquid (melted) AC because of the presence of EG.