藉由奈米結構提升熱能轉換與儲存

Abstract

在全球所產生的電力中，約只有一半被有效的運用，而其餘的一半便以廢熱的形式浪費，因此，該如何減少能量的浪費以及廢熱的應用便是本研究所探討之目的。本論文分為兩個部分:應用奈米晶體增強太陽熱能系統之能量儲存以及藉由奈米結構提升熱能轉換效率。 研究顯示只需轉換少部分的太陽能就足夠全世界的能源需求，但太陽日照易受氣候影響且不能提供無太陽光時之能源需求。相反的，太陽熱能系統因為可以將太陽光能儲存為熱能以提供無日照時之電力需求為一非常有吸引力之供電方式，但目前太陽熱能系統佔地龐大且儲能有限，使其實際應用上受到相當的限制。本研究利用奈米粒子之潛熱提升太陽熱能系統之儲能，目前本研究已合成錫/氧化矽核殼奈米粒子，並添加此奈米粒子於太陽熱能系統之工作流體Hitec熔鹽內，結果顯示，氧化矽殼可以防止錫氧化，並且使錫/氧化矽核殼奈 米粒子之融化熱穩定的維持在28.36 J/g。當添加5 wt%錫/氧化矽核殼奈米粒子於HITEC熔鹽，可以使熔鹽比熱提升30%，此方法可有效增強熔鹽的熱容，且免卻了複雜的製造過程，具有低成本的優勢。 在提升熱能轉換效率方面，本研究先對石墨烯之熱傳導率進行量測，分子動力學模擬顯示石墨烯垂直於平面之熱傳導率由於聲子受到邊界散射的影響會隨著石墨烯層數增加而降低，因而導致熱傳導率隨層數增加而增加。因此藉由其在垂直平面方向較低的熱傳導率，石墨烯極有可能成為一良好的熱電材料。目前3 omega 量測結果發現石墨烯於垂直於薄膜方向之熱傳導率在320 K時為0.008 W/m-K，但此石墨烯層並非此量測系統之主要熱阻，此可能造成量測誤差。鑑於此原因，本研究之未來方向為藉由氧電漿來使石墨烯產生缺陷，進而降低其熱傳導率使其成為量測之主要熱阻。

Approximately half of the power generated in the world is eventually turned into waste heat. Therefore, reduction of the amount of waste heat and optimization of the usage of waste heat are important. This study focuses on enhancing thermal energy storage and thermal energy conversion to fulfill these two objects of reducing and optimizing waste heat. It was shown that only small fraction of solar energy will be enough for world’s total energy consumption. However, its contribution on world’s energy supply is very tiny due to the diurnal nature of solar energy. Solar-thermal power system which stores sunlight as thermal energy and utilizes the energy when the sunlight is not available is a promising candidate for renewable energy. Nevertheless, current solar-thermal power plants require large amount of land and store only a limited amount of energy, resulting in a restricted application of the power plants. This project focuses on enhancing energy storage of the solar-thermal power system by using latent heat of nanoparticles. We have successfully synthesized the Sn/SiO2 core-shell nanoparticles. In addition, the nanoparticles have been doped into the Hitec molten-salt for evaluating the effectiveness of the nanoparticles in energy storage. The results show that the specific heat capacity of the Hitec molten-salt doped with 5 % Sn/SiO2 core-shell nanoparticles could be increased about 30 % compared to that of Hitec salt alone by using the latent heat of the nanoparticles. The insight from study could be applied in enhancing energy storage in solar-thermal power plants in the world. Molecular dynamics simulation shows that cross-plane thermal conductivity of graphene decreases with increasing the number of layers due to the increased boundary scattering. As a result, although graphene has a large in-plane thermal conductivity, it might exhibit a low cross-plane thermal conductivity. The experiment results show that the thermal conductivity of graphene in the cross-plane direction is 0.008 W/m-K at 320K. However, the thermal conductivity of the graphene layer is not the dominant thermal resistance in the system and is smaller than the system uncertainty. These two issues affect the data accuracy. In the future, we will make defects in graphene by oxygen plasma which can reduce the thermal conductivity of graphene to facilitate the thermal conductivity measurement.