探討遲滯溫度機制及微奈米顆粒對流體比熱容與熱傳導率之影響

Abstract

全球約有81%的電力是由石化燃料提供，石化燃料會造成嚴重的環境汙染並加重溫室效應，因此發展再生能源便是一個非常重要的議題。太陽能為最有潛力之再生能源，傳統太陽能發電易受到日照時數與天氣狀況的限制，聚熱式太陽能發電廠則因能將太陽能轉為熱能儲存而不受此限。但目前聚熱式太陽能發電廠佔地廣且除能容量有限，因此加入相變材料，便是一種提升工作系統熱儲存的方法。在相變材料的應用過程中，遲滯溫度通常伴隨著出現，遲滯溫度的定義一般來說是熔點與凝固點的溫度差。遲滯溫度會使系統運作的效率降低，造成額外的熱損失，並使系統無法利用相變材料的潛熱以提升熱儲存。本研究合成並量測兩種殼層材料的鋅金屬微米粒子，結果顯示，合成的微米粒子具有非常好的熱穩定性，且鋅金屬/氧化鋁球殼粒子對降低遲滯溫度有較好的表現，目前推論造成遲滯溫度的主要因素為異質成核效應與加熱速率效應。此外，本研究亦建立熱傳導率量測系統，結果顯示熱傳導率隨粒子濃度增加而增加，且在1.2 vol.% Hitec添加氧化鋁奈米粒子之奈米流體中得到最高的熱傳導率，其值為0.908 ± 0.005 W/m-K，與未添加奈米粒子的熔鹽Hitec相比，其熱傳導率提升了21%，熱傳導率提升的原因是經棒式超音波震盪處理後的奈米流體，具有良好粒子分散性。另外，本研究量測Hitec添加氧化鋁奈米粒子奈米流體之比熱，量測結果顯示比熱隨著奈米粒子濃度升高而降低，其原因為氧化鋁奈米粒子之比熱小於Hitec，加入較多的奈米粒子會使比熱下降比例變大。本研究的實驗結果未來可應用在提升太陽能電廠工作流體的熱儲存與熱傳遞。

Around 81% of electricity is provided by fossil fuel. Fossil fuel could cause severe environmental pollution and deteriorate the greenhouse effect. Therefore, developing renewable energy becomes an urgent issue. Most of the renewable energy comes from solar energy. Photovoltaics would be limited by the diurnal limit of solar energy. On the contrary, concentric solar power plants would not be restricted by the diurnal limit because it can store solar energy as heat. However, concentric solar power plants occupy a large size of land and can only store a limited amount of heat. Therefore, adding phase change materials would be a solution to enhance the heat storage. During the applications of the phase change materials, a phenomenon called hysteresis temperature is widely observed. Hysteresis temperature is defined as the temperature difference between the melting point and crystallization point of a phase change material. Hysteresis temperature would lower the efficiency of the system, cause extra heat loss, and diminish the advantage of the phase change material. In this work, core-shell particles with two different shell materials (Zn microparticles coated with TiO2 and Al2O3) have been synthesized and investigated. The synthesized particles showed good thermal stability. Moreover, Zn microparticles coated with Al2O3 perform could lower the hysteresis temperature. Besides, the mechanism causing the hysteresis was investigated and it was found that the hysteresis resulted from heating rate effect and heterogeneous nucleation. Moreover, a thermal conductivity measurement system has been built. After the ultrasonication procedure, 1.2 vol.% Hitec doped with Al2O3 nanoparticle showed the highest thermal conductivity in this work. The thermal conductivity was 0.908 ± 0.005 W/m-K and was 21% enhancement compared with that of pure Hitec. The reason for the enhancement of thermal conductivity is that the nanoparticles in the fluid disperse very well after ultrasonication procedure. In addition, the measurements of the specific heat capacity of Hitec nanofluid have also been done. Based on the results, the specific heat capacity of Hitec nanofluid decreased with the increasing concentration. Compared to Hitec, Al2O3 nanoparticles have lower specific heat capacity, the addition of the Al2O3 nanoparticles will lower the specific heat capacity. The result in this work would be able to apply on the concentric power plants to enhance the heat storage and the heat transfer.