質子交換膜燃料電池中流道幾何形狀對其性能影響之探討

Abstract

本論文係以數值模擬方法來探討不同幾何形狀的蛇形流場板對質子交換膜燃料電池的性能影響，並分析此項研究參數所得出的不同變數之分佈變化。本模擬使用商用套裝軟體CFD-ACE+來建構一個穩態、三維、雙相流、多物種並包含電化學反應的數值模型。論文首先進行模式驗證，是以CNC製作尺寸為10×10×1.2公分，彎道角度90°和寬度及深度均為1mm的石墨流場板，並和其他元件，如端板、集電板、防漏墊片、氣體擴散層及質子交換膜 (含反應面積為25平方公分之觸媒層)組合成一燃料電池。組裝完成後，進行燃料電池測試，其測試條件為：測試溫度為353K，陽極氫氣入口流量為796 ml/min，陰極氧氣入口端流量為659 ml/min。實驗結果顯示，其量測所得的I-V性能圖與模擬結果最大的誤差在5%以內，吻合度相當好，因此再進行參數分析；其內容是先分析流場、溫度場及其他電化學變數在質子交換膜燃料電池內的基本現象。從模擬結果得知，電流密度、溫度以及水含量等三項變數彼此的分佈情形呈現緊密的正相關性。而其中存在於邊緣肋條的些許差異來自於等溫的邊界條件。另外，這三項變數的量從陽極流道入口逐漸向陽極流道出口遞減，顯示其分佈情形主要受制於氫氣濃度的影響。除此之外，當電池操作溫度超過348K時，液態水的生成變得相當微弱且未在電池內產生水氾濫的情形。接著的參數研究是在兩個不同操作溫度(323K和353K)下，不同彎道角度(45°、60°、90°、120°與135°)的流場板對質子交換膜燃料電池的性能影響。數值計算結果顯示，因為質傳擴散速率的提升，結合60°與120°的流場板可以得到最好的性能，尤其在低操作電壓下(0.4–0.6 V)更加明顯。然而，不同角度的流場板的性能差異隨著操作溫度的降低而增加，說明彎道角度對性能的影響跟操作溫度呈負相關。另一方面，從薄膜溫度分佈圖中得知不同角度流場板的溫度分佈是相似的，說明改變流道角度並未能降低薄膜的溫度變化。接著探討不同彎道寬度的流場板在相同的操作條件下對質子交換膜燃料電池的性能影響。從模擬結果得知，因為隨物質擴散能力的提升，具有較寬彎道的流場板可以得到較好的性能。然而，從電流密度及溫度的分佈圖中得知，該兩項變數在較寬彎道流場的薄膜中分佈地相當的不均勻並且伴隨著熱點存在於彎道裡，此現象對薄膜的使用壽命有相當程度的傷害。

This study numerically investigated how the geometry of serpentine flow pattern influences performance of proton exchange membrane fuel cell (PEMFC), and analyzed how these parameters lead to different distributions of model variables. Three-dimensional simulations were carried out with a steady, two-phase, multi-component and electrochemical model, using CFD-ACE+, the commercial CFD code. This thesis begins with a model validation. In this part, a graphite flow-field plate, whose size is10×10×1.2 cm with a flow channel of 90° bend angle and 1 mm width and depth, is manufactured by CNC. It combines with other components, such as end plates, current collection plates, anti-leakage gaskets, gas diffusion layers, and proton exchange membrane (including a 25 cm2 reaction area of catalyst layer), to form a PEMFC. After that, a series of performance tests is carried out. The test conditions are: the cell operating temperature is 353K, the hydrogen inlet flow rate at anode 796 ml/min, and the oxygen inlet flow rate at anode 659 ml/min. The measured I-V curve agrees very well with the predicted one that the maximum discrepancy is within 5%. Based on the above justification, it starts to carry out the parametric study. Initially, the fundamental behaviors of the flow field, temperature and electrochemical variables inside a PEMFC are analyzed to serve as comparison base lines for varying the bend width, bend angle and temperature, respectively. From the numerical results, it shows a close and positive correlation between the distributions of current density, temperature and water content with only a slight discrepancy existing at the marginal rib due to isothermal boundary conditions. Also, these three variables decrease gradually from anodic inlet toward anodic outlet, indicating that their distributions are principally dictated by the hydrogen concentration. Additionally, with the cell temperature increased beyond 348K, liquid water formation doesn’t appear to be considerable nor result in flooding effect inside the cell. Next, the effects of bend angle on the PEMFC performance is studied with various angles (45°, 60°, 90°, 120° and 135°) with cell temperature of 323K and 353K. The numerical results indicate that the combination of 60° and 120° enables flow pattern to achieve the highest performance, especially at low operating voltages, due to the increase mass diffusion rate. Also, the differences in performance for different angles become more noticeable with decreasing cell temperature, implying that the influence of bend angle on the performance is inversely proportional to the operating temperature. On the other hand, the temperature distributions of flow patterns in the membrane with different angles are more or less similar, indicating that the variation of temperature in the membrane is not reduced from the change of bend angles. The effects of bend width on the PEMFC performance are subsequently studied under the same operating conditions applied in the previous parametric study. Simulation results show that flow pattern with wider bend width achieves the highest performance compared to patterns with narrower width because of the enhanced mass diffusion. However, the distributions of current density and temperature in the membrane with wider bend show a high non-uniformity with the existence of hot spot at bending areas that is fatal to membrane lifetime.