【作者】 张鹏飞；

【导师】 邱羽； 王宗亚；

【作者基本信息】 中南大学 ， 工程（专业学位）， 2024， 硕士

【摘要】 复合相变储热介质具有相变温度恒定、储能密度较高和热稳定性良好的特点,在光/电热转换与储热领域具有良好应用前景。然而,在光热利用方面,现有介质存在传热方向上热导率不足,不能有效地传输和存储热能的难题;在电热利用方面,现有介质存在电流方向上电导率普遍过低,导致驱动电压过高、难以高效加热的难题。因而,有必要提升复合相变储热介质在传热和导电方向上的传导能力,从而提高相变储热过程的光/电热转换性能。针对上述难题,本文采用实验研究和数值模拟相结合的方法,分析了复合介质热物性变化与其传储热性能的内在联系,探究了影响介质光/电热转换性能的关键因素及其调控机制,发展了3种具有高各向异性传导能力的复合相变储热介质,实现了高效的光/电热转换与储热。具体研究内容和结论如下:（1）采用压力诱导法制备了一种三羟甲基乙烷/膨胀石墨复合固-固相变介质,实验分析了介质的结构特性、传导特性及储热特性,研制了一种光热储热器和一种电热储热器,并实测了其综合性能。结果表明,压力诱导法促使石墨微粒实现定向排列,形成了相互连通的热/电传导通路,因而复合介质的热导率和电导率都随膨胀石墨质量分数的增加而显著增加。当膨胀石墨质量分数为20 wt.%时,复合介质在传导方向上的热导率和电导率分别可达12.82 W·m^-1·K^-1、4.11S·cm^-1,明显高于其在垂直于传导方向上的值。在3倍阳光下,光热储热器的光热转换存储效率可达77.30%。在3.6 V的低电压下,电热储热器的电热转换存储效率高达91.62%。最后,稳定性实验表明,复合介质在600次循环前后的性能没有明显变化。（2）提出了新型定向碳纤维/烯烃嵌段共聚物/石蜡复合固-液相变介质及其制备方法,通过熔融纺丝、预氧化、碳化、石墨化等4步制备了中间相沥青基碳纤维,通过熔融浸渍、均匀涂抹、分块切割、逐层堆叠、模压成型等5步制备了复合相变介质,实验研究了复合介质的传热储热特性。结果表明,定向排列的碳纤维在介质内形成了连续的导热路径,使其纵向热导率高达5.63 W·m^-1·K^-1,横向热导率低至0.77 W·m^-1·K^-1,各向异性度高达7.31。在光热利用中,优异的各向异性导热能力有效增强了介质表面向内部的纵向传热,减少了介质向周围的横向传热损失。同时,为了增强光吸收性能,将碳黑涂覆于复合介质表面构建了一种新型光热储热器,从而使其阳光吸收率达到0.988。在优异的各向异性热导率和高阳光吸收率的协同作用下,该光热储热器在1～3倍阳光下的光热转换存储效率可达79.17%～98.31%。（3）研制了定向碳纤维/烯烃嵌段共聚物/软脂酸复合固-液相变介质,分析了碳纤维对复合介质热导率的强化机理,揭示了复合介质实际热导率远低于理论值的原因,制备了一种光热储热器并研究了其光热性能。实验结果表明,复合介质的纵向和横向热导率分别可达5.84 W·m^-1·K^-1和1.34 W·m^-1·K^-1,但上述结果明显低于理论值。进一步的模拟研究发现,当部分碳纤维发生断裂时,会在连续的碳纤维导热路径中引入相变介质薄层,导致热传输路径变得不连续,从而极大地降低热量传输能力,进而显著降低了复合介质热导率。所制得的光热储热器实现了0.963的阳光吸收率,并在1～3倍阳光下达到了87.54%～95.08%的光热转换存储效率。最后,稳定性测试还证实,该器件具有优异的防泄漏性能、热稳定性和长期使用的循环稳定性。图93幅,表19个,参考文献87篇更多还原

【Abstract】 The phase change material is widely used in solar/electro-thermal conversion and storage because of its constant phase change temperature,high energy storage density,and high thermal stability.However,in the solar-thermal utilization,the thermal conductivity of the phase change material is insufficient in the direction of heat transfer,which cannot transfer and store heat effectively.In the electric-thermal utilization,the electrical conductivity of the phase change material is generally too low in the direction of current,resulting in high driving voltage and low heating performance.Therefore,it is necessary to improve the conductive ability of the phase change composite in the directions of heat conduction and electrical conduction,thus improving the solar/electro-thermal conversion performance of the phase change heat storage process.To solve the above problems,combining experiments and simulations,firstly,the internal relationships between thermophysical properties and heat transfer performance of the phase change composite were analyzed.Then,the key factors affecting the solar/electro-thermal conversion performance of the composite and its regulatory mechanisms were explored.Then,three kinds of phase change composites with high anisotropic conductivity were developed to realize efficient solar/electro-thermal conversion and heat storage.The specific research contents and conclusions are as follows:（1）A 1,1,1-Trimethylolethane/expanded graphite solid-solid phase change composite was prepared by pressure-induced method,and its structural,conductive,and thermal storage properties were experimentally analyzed.After that,a solar-thermal device and an electrical-thermal device were developed and tested.The results show that the graphite particles are oriented by the pressure-induced method,forming interconnected thermal/electrical conduction pathways,and thus the thermal and electrical conductivities of the composite increase significantly with increasing graphite loading.When the graphite loading is 20 wt.%,the thermal and electrical conductivity of the composite in the conduction direction can reach 12.82 W·m^-1·K^-1 and 4.11 S·cm^-1,respectively,which are higher than their values in the direction perpendicular to the conduction direction significantly.The solar-thermal conversion and storage efficiency of the solar-thermal device can reach77.30%under 3 suns.At a low voltage of 3.6 V,the electro-thermal conversion and storage efficiency of the electrical-thermal device can reach91.62%.Finally,stability experiments show that the performance of the composite has no significant change before and after 600 cycles.（2）An oriented carbon fiber/olefin block copolymer/paraffin wax solid-liquid phase change composite was prepared by a simple and scalable method.In this method,intermediate-phase asphalt-based carbon fibers were prepared in four steps,including melt spinning,pre-oxidation,carbonization,and graphitization,and the phase change composite was prepared in five steps,including melt impregnation,uniform brushing,cutting,layer-by-layer stacking,and molding.The heat transfer and storage properties of the composite were experimentally investigated.The results show that the oriented arrangements of carbon fibers in the composite form continuous thermal conduction paths,enabling the lengthwise thermal conductivity as high as 5.63 W·m^-1·K^-1,transverse thermal conductivity as low as 0.77 W·m^-1·K^-1,and the degree of anisotropy is as high as 7.31.In the solar thermal utilization,the excellent anisotropic thermal conductivity enhances the lengthwise heat transfer from the surface to the interior of the composite effectively,and reduces the transverse heat transfer loss.Meanwhile,to enhance the solar absorption,carbon black was coated on the surface of the composite to construct a novel solar thermal device,resulting in a total solar absorptance of 0.988.Due to the synergy of the excellent anisotropic thermal conductivity and the high solar absorptance,the solar thermal device can reach quite high solar-thermal conversion and storage efficiencies of 79.17%～98.31%under 1～3 suns.（3）An oriented carbon fiber/olefin block copolymer/soft stearic acid solid-liquid phase change composite was developed,the enhancing mechanisms of carbon fibers on the thermal conductivity of the composite were analyzed,and the reason why the actual thermal conductivity of the composite was much lower than its theoretical value was revealed.After that,a solar-thermal device was prepared and tested.The experimental results show that the lengthwise and transverse thermal conductivities of the composite can reach 5.84 W·m^-1·K^-1 and 1.34 W·m^-1·K^-1,respectively,but the above results are obviously lower than the theoretical values.Further simulation studies revealed that a thin layer of the phase change material would be introduced into the continuous carbon fiber thermal conduction path when some of the carbon fibers are fractured,resulting in the heat transfer path becoming discontinuous,reducing the heat transfer capability greatly,and thus reducing the thermal conductivity of the composite significantly.The solar-thermal device can achieve a solar absorptance of 0.963 and high solar-thermal conversion and storage efficiencies of 87.54%～95.08%under 1～3 suns.Finally,stability tests also confirm that the device has excellent leakage resistance,thermal stability,and cycling stability for long-term use.更多还原