在眾多替代能源中，氫氣因其含量豐富、環保無汙染且具有高能量密度的特性而成為未來能源中最具潛力之能量載體之一。考量儲氫能力及安全性，金屬氫化物被視為一理想且適合儲存與運輸氫氣的系統; 然而，金屬氫化物儲釋氫的過程中會排放與吸收大量的熱，並伴隨著複雜的熱質傳變化。因此，如何在系統內部規劃有效的熱管理將深切的影響其儲存與釋放氫氣的性能表現。本論文藉由高導熱性發泡金屬的添加來增強系統內部熱傳，並利用數值模擬工具(COMSOL Multiphysics)建立一金屬氫化物(LaNi5)儲氫行為之計算模型，以探討發泡金屬對儲氫系統吸氫表現之影響。
本研究團隊先前的研究結果指出，在固定的反應器體積且儲氫量需求下，存在一最佳金屬發泡物體積比(ϕ_mf)使儲氫時間得以最短。此外，本團隊先前也發現在固定發泡金屬總含量下，在空間中置入不同體積比之發泡金屬能進一步提升系統性能。結合此兩發現，本論文將利用之前得到之最佳ϕ_mf進行系統空間中不同比例發泡金屬之配置，期望藉此可獲得比先前研究更佳的充氫效率。計算結果顯示在空間中添加不同比例發泡金屬之方法對於熱傳效能已相當良好且幾乎沒有改善空間之系統並無顯著的效果;然而，對於為了增加系統儲氫空間而減少ϕ_mf或增加反應器尺寸之系統(即熱傳表現較差)，此方法確實能縮短儲氫時間，改善系統儲氫之效能。計算結果也顯示，不恰當的發泡金屬比例分佈不僅無法減短充氫時間，更可能使反應變慢，所以必須通過適當設計才能得到一最佳的比例。因此，我們檢視此改善機制，發現不同比例分佈將使得系統整體熱阻及流阻改變，並且最佳比例分佈產生於最低熱阻與最低流阻之間。換言之，此比例是在熱傳率和流體流率抗衡下所決定，並且隨操作條件而有所偏移。為此，我們定義一參數(ω)量化此偏移，進而發現不僅僅是ϕ_mf，儲氫量及反應器尺寸都將進一步影響ω值。故在指定的反應器操作條件下，ω值將有助於了解最佳ϕ_mf分佈之決定及反應器之設計，進而減少蒐尋最佳參數之時間成本。

Hydrogen is regarded as an ideal choice of alternative energy because it is highly abundant, environmentally friendly and possesses high calorific value. Among existing methods of storage, metal hydride systems are thought to be effective due to their high volumetric capacity and safety. However, significant heat energy exchanges occur during the hydrogen storing and releasing processes, and hence thermal management becomes a serious issue in metal hydride reactors (MHRs). In this study, metal foam is added in the reactors to augment the overall thermal conductivity and to improve the heat transfer behavior in the reactor. To examine the effect of metal foam addition, a 2-D mathematical model is constructed and used to simulate the hydriding process of MHRs by means of COMSOL Multiphysics software.
Previously, it was found in works of our group that there exists an “optimal” volume fraction of metal foam (ϕ_mf) to minimize the charging time if a prescribed amount of hydrogen is to be stored in the reactor. Besides, it is also found that a spatially varying ϕ_mf distribution will reduce the charging time of the reactor with a given overall ϕ_mf. Combining those two findings, we expect a further improvement in the adsorption rate of MHRs with spatially distributed ϕ_mf based on the optimal choice of ϕ_mf obtained before. It is shown that the effectiveness of spatially ϕ_mf distributions is no longer powerful when ϕ_mf is close to the optimal value and the heat transfer rate has not much room for further enhancement.
In addition, it is found that the optimal distribution resulting in the shortest charging time is actually determined under the compromise of the heat transfer and fluid flow rate. To measure quantitatively the relative importance of thermal and fluid resistances, a parameter (ω) is defined. It is then shown that ω is not only affected by the value of ϕ_mf, but also by the hydrogen storage capacity and the size of the MHR. The concept of ω therefore can help expedite the determination of the optimal ϕ_mf distribution, and thus is useful to the design of MHRs.

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