本文針對LaNi5儲氫罐之吸放氫熱流特性進行數值模擬分析，儲氫罐內部分成上下兩種結構，下層為儲氫合金所構成的多孔性介質區，上層則為供合金膨脹所預留的膨脹區域，對此結構建立了吸氫與放氫兩種皆可適用的數學模型。由於吸氫時會生成熱，放氫時則須提供合金熱能來轉換成化學能，因此利用能量方程式來分析熱的問題，並且假設反應時合金與氣體間皆達熱平衡。氫氣和合金的質量變化以連續方程式描述。對於多孔介質的合金區域與膨脹區分別使用佛許海默-布里克曼與那維爾-史托克斯方程式來描述流動的問題，最後將上述方程式利用COMSOL Mutilphysics 3.2 (COMSOL Inc., Sweden) 進行有限元素法數值計算分析。結果發現吸放氫時，內部流場會受到溫度分布不均勻的影響，造成氣體密度的改變而有明顯的變化。此外，由於壁面有較好的熱交換效果，可以有效的排出吸氫產生的多餘熱量或適時提供放氫反應所需的熱，因此有較快速的反應變化；罐內靠中心的部分，熱傳需要穿越合金才能達反應位置，過程中多了合金所造成的熱阻，因此內部位置的合金無法有效利用。針對一些可調整的外在因素做為控制變因，模擬計算不同環境條件下的差異，結果發現改變外部流體溫度和入口設定的氣體壓力大小，對於吸氫與放氫的速率有明顯的影響，但是都有其影響的極限存在；而改變膨脹區域高度和吸放氫氣出入口直徑大小，對於整體吸放氫的速率皆無顯著影響。 A study of the hydrogen storage and usage using LaNi5 metal hydride is presented. The metal hydride tank is considered to consist of a porous medium (hydride bed) and an expansion volume (gaseous phase). For this storage structure a mathematical model including both hydriding and dehydriding processes is developed. When metal hydrides absorb hydrogen, they release heat. Conversely, when they absorb heat, the alloys release hydrogen. Therefore we use the energy equation to analyze the associated heat transfer problem based on assuming that thermal equilibrium has been reached between the metal hydride and hydrogen gas. The mass balance between the hydrogen gas and metal hydride is described in terms of the continuous equation. The Forchheimer-Brinkman and Navier-Stokes equations are used to describe the gas flow within the porous medium and expansion volumes respectively. The mathematical model developed was solved using a finite element code, COMSOL Multiphysics 3.2 (COMSOL Inc., Sweden). Results show during the process of either absorbing or releasing hydrogen, the inside flow influenced by the fact that the temperature profile is not uniform in the tank has a major impact on the changes of the gas density. Hydride absorption or desorption both take place faster neat the tank wall because there is better heat exchange near the wall. Using the simulation model, we also study how the hydrogen reaction rates change with the parametric values. Result show changing the external cooling/heating temperature or the inlet/outlet gas pressure has profound influences on the hydriding and dehydriding speeds. In contrast, the inlet diameter and expansion height have virtually nothing to do with the hydriding and dehydriding rates.
