多元烷烃相变微胶囊设计、制备及其深冷储能特性

The high-efficient cryogenic energy storage technology is one of the key factors to improve the efficiency of liquid air energy storage system. This project proposes to design and fabricate the multi-alkane based microencapsulated phase change materials (MPCMs) for cryogenic energy storage. Meanwhile, a fundamental research on phase transition properties of these MPCMs will also be performed. At first, the effects of mass fraction, carbon atoms odd-even combination and crystalline structure on the thermodynamic properties of multi-alkane mixtures will be analyzed. And the multi-alkane mixtures with the phase transition temperature between -70˚C—-60˚C will be designed. Secondly, using the corresponding multi-alkane mixtures as core materials, investigations will be conducted to elucidate the reaction and microstructure evolution mechanism during the MPCMs fabricating process. The effects of key factors such as homogeneous rate, core/shell ratios, types and cooling rate of emulsifier on appearance of microencapsulation, cold storage performances and life cycles will be revealed. Through characterization tests, the fabricating parameters will be optimized, thus to produce the high performance cryogenic-temperature multi-alkane MPCMs. Finally, the internal evolutionary phase change process of MPCMs will be clarified and the effects of size, shell apparent morphology and interface thermal resistance on interior phase on the phase change behaviors of MPCMs will be obtained. This project will provide a theoretical framework for the practical application of the multi-alkane based MPCMs in the field of cryogenic liquid air storage system.

高效的储冷技术是提高深冷液态空气储能系统效率的关键因素之一。本项目针对低温储冷多元烷烃体系相变微胶囊的设计与相变特性开展基础研究。探索组合质量分数，碳原子奇偶搭配以及晶体结构对复合烷烃相变材料热力学性能的影响规律，设计相变温度在-70˚C—-60˚C之间的多元烷烃复合体系；以此体系为芯材阐明微胶囊制备过程中反应机理与微结构演化机制，揭示均质速率、芯壁比、乳化剂类型及冷却速率等关键因素对微胶囊形貌、储冷性能与寿命的影响规律，通过性能测试与表征优化关键工艺参数，制备出高性能的多元烷烃低温相变材料微胶囊；明晰微胶囊内部相变动态演变过程，获得胶囊粒径，囊壁界面形貌及界面热阻对其内部相变行为影响规律。该研究成果将为低温复合烷烃类相变材料微胶囊在深冷液态空气储能中的应用提供理论基础。

本项目研究了C8到C14的二元和三元共晶体系。二元体系中，C8-C9体系表现出包晶现象，C9-C10和C9-C11二元体系表现出简单的共晶行为，C10-C11体系表现为完全互溶体系，C11-C12、C11-C14和C12-C14二元体系表现出可能的共晶行为。三元体系中，C8-C9-C10体系表现出简单的共晶行为，C11-C12-C14三元体系存在很可能的共晶点。最终优选出90wt%C9-10wt%C11低温二元烷烃共晶体系，其最低共熔温度为-58℃，熔化相变焓为101.8J/g。在烷烃混合体系固液相平衡热力学预测方面，对于二元混合烷烃体系熔化温度，UNIQUAC模型表现了最佳的准确性。最小误差为0.38%。对于三元体系，理想模型预测的共熔点的精确度最高，最小误差为0.75%。.针对以上优选的复合烷烃（C12-C14）为芯材、密胺树脂为壁材成功制备了相变微胶囊，明确了“三步法”制备工艺 。性能表征显示微胶囊形貌完整，具有优异的热力学特性和循环耐久性，融化温度和融化焓分别为-8.2˚C和111.3J/g，封装效率达74.05%。对于纯烷烃微胶囊，通过碳纳米管和纳米金属氧化物对壁材和芯材进行改性，将微胶囊的导热性提升了382%。此外，本项目对纳米胶囊开展了初步研究，制备了以C14为芯材、MUF‒TiO2复合材料为壳材的纳米胶囊。原位聚合制备后分别采用溶胶-凝胶法和共混法改性。对NEPCMs样品的物理、化学和热力学性能进行了表征。结果表明，溶胶-凝胶改性显著增加了纳米胶囊的平均粒径。所有NEPCMs均表现出较高的热稳定性。.最后，建立了单一烷烃C9微胶囊相变的二维数值模型，利用格子玻尔兹曼方法中的焓法，结合浸润边界等边界处理方法，从微观尺度对胶囊相变过程中的相界面演化过程进行了数值研究，获得了胶囊相变速率及热力学特性。结果表明，微胶囊的相变过程受其尺寸影响明显，熔化过程几乎完全均匀而对称。熔化过程Nu的变化与凝固过程相近。Nu的大小也与相界面的位置存在关联，相界面附近的区域Nu更大。.本项目以一作或通讯身份发表SCI论文20篇，项目成果为设计低温复合烷烃类储冷相变材料微胶囊奠定理论基础，也为其在深冷液态空气储能中的应用提供了支撑，还可拓展应用至LNG液化、惰性气体分离、干冰制取等低温领域，具有广阔的应用前景。