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標題: | 質子交換膜燃料電池系統之輸送現象探討及操作參數分析 Studies of Transport Phenomenon and Analysis of Operational Parameters for Proton-Exchange-Membrane Fuel Cells |
作者: | Chung-Hsien Chen 陳崇憲 |
指導教授: | 顏溪成 |
關鍵字: | 雙極板,流道型態,均勻性,交叉型,蛇紋型, Stack Bipolar plate,Flow Distribution,Flow Uniformity,Inter-digitated Flow Channel,Serpentine Flow Channel, |
出版年 : | 2008 |
學位: | 博士 |
摘要: | 本研究主要利用數值模擬、實驗研究等方式來探討質子交換膜燃料電池組之流動現象及操作參數等對於系統之性能影響。首先利用數值模擬方式來探討質子交換膜標準單電池(5公分*5公分)之性能表現,並藉由實驗方式來驗證其結果;在質子交換膜燃料標準應用上,主要在於大面積單電池之應用,藉由大面積(16公分*16公分)單電池之實驗方式,來進一步探討各種雙極板流道設計對於性能之影響,並提出雙極板設計之建議;最後則利用計算流體力學來模擬72個單電池所組成之電池組的壓力變化及氣體分配,並提出動量平衡理論與壓損模型來說明氣體分配之作用機制,以作為解決電池組氣體分配之解決方案。
首先標準單電池驗證上,主要提出一質子交換膜燃料電池之數值計算,模型中包含了兩極的流道、擴散層、觸媒層與薄膜層,並在一特定的操作條件與設計參數下模擬燃料電池的傳輸現象。此研究分別以加濕氫氣與空氣作為燃料及氧化劑。流場可視為層流、穩態、單相流,並滿足理想氣體定律。電化學熱質傳輸現象的統御方程式被同時求解,其中Stefan-Maxwell方程式描述各成份氣體的質傳,而Bulter-Volmer方程式則定義了觸媒層中的電化學反應。計算區域忽略了流道固體部分且假設其電位損失極小,故計算時電位是定義在擴散層與集電板的交界面上來模擬輸出電流。結果指出藉由一劑量比的方式可適當地捕捉I-V極化曲線,然而此模型忽略了液態水氾濫的效應,所以在低電位時與實驗結果有較大的差異。 在大面積單電池雙極板流道型態實驗上,對於雙極板流道上則以交叉型、蛇紋型等2種流道型態,並分以多重蛇紋型、多重Z型蛇紋型及多重Z型交叉型等3種流道實際加以驗證。在交叉型流道設計上,具備有效移除水份、高反應效率等優勢,惟會導致高壓降等高能源損耗因素,故在實際應用上較為少見。 最後,則藉由計算流體力學的方法模擬燃料電池組的壓力變化與氣體分配。在研究中建立了由72個單電池所形成的電池組二維模型,並將流道視為由多孔性介質所填滿,藉此模擬流道所造成的壓損。為了簡化分析不考慮電化學反應與熱質傳輸現象,僅以空氣作為工作流體評估電池組的氣體分配情形。在本文中藉著改變多孔隙介質的滲透率與歧道寬度,來進行各案例的比較,評估在不同設計參數下氣體分配均勻性的優劣程度。此外本文也提出了一動量平衡理論與壓損模型來解釋支配氣體分配的作用機制。由模擬結果發現,當流道壓損增加與歧道寬度變大時,氣體分配之均勻性可獲得提昇。然而過大的流道壓損對於燃料電池組的實際應用上是不利的,因此增加歧道寬度是解決電池組氣體分配問題的較佳方案。 In this study, we want to discover the transport phenomena of PEMFC by simulation and experiment with different condition. In first section, we simulation the standard cell (5 cm * 5 cm) of PEMFC by three-dimensional model and verify the result (I-V curve) by experimental. In the next, a large-scale area (16 cm *16 cm) PEMFC with different flow channel has been investigated by experiment. The relationship between the different operational parameter and flow pattern has been studied. Finally, the 72 cells stack model of PEMFC has been simulated by two-dimensional, computational fluid dynamic. The pressure variation and the flow distribution in the manifold of a PEMFC stack have been studied. At first, a three-dimensional computational model of proton exchange membrane fuel cell (PEMFC) is presented in this study. This model includes flow channels, gas diffusion layers (GDL), catalyst layers, membrane on both anode and cathode side and simulates transport phenomena within fuel cells under particular operating conditions and design parameters. In this study, humid hydrogen and air are provided as fuel and oxidant respectively. The flow is laminar, steady and ignores the effects of two phase flow. Therefore, all fluids are treated gases and obeyed ideal gas law. The Governing equations for flow, heat, and mass transfer coupled with the electrochemical reactions are solved simultaneously, where Stefan-Maxwell equations describe mass transfer of each species and Bulter-Volmer equations are used to define the electrochemical reactions on the catalyst layer. The computational domain neglects ribs and the voltage loss through those is assumed negligible. The voltage is defined on the interface of current collectors and GDL to simulate output current. The results refer the test method of stoichmetric ratio can catch the I-V polarization curve appropriately. This model ignores liquid water flooding therefore there occur larger differences between modeling and experimental results on low voltages. Next, a large-scale are of Proton exchange membrane fuel cells (PEMFC) with different flow channel has been investigated experimentally. Interdigitated channel geometry has the advantages of effective water removal and higher reaction efficiency through forcing gas transport in the diffusion layer. In this study, multiple-Z type flow pattern has been adopted with and without the interdigitated channels. The performance of single PEM fuel cell with an interdigitated flow field has been improved with specific operating conditions. The experimental results under the effects of gas humidification temperature and reactant gas flow rate, etc. have been comprehensively discussed in this work. It can be found experimentally that the multiple-Z interdigitated flow channel has better performance as compared with the conventional Z type in the high current-density region of current-voltage (I-V) polarization curves. However, the improvement in the low current-density region is insignificant. The pressure drop loss of multiple-Z interdigitated flow field is about twice as compared to the conventional one. Finally, the pressure variation and the flow distribution in the manifold of a fuel-cell stack are simulated by a computational fluid dynamics (CFD) approach. Two dimensional stack model composed of 72 cells filled with porous media is constructed to evaluate pressure drop caused by channel flow resistance. In order to simplify this model, electrochemical reactions, heat and mass transport phenomena are ignored and air is treated as working fluid to investigate flow distribution in stacks. Design parameters such as the permeability of the porous media, the manifold width and the air feeding rate were changed to estimate uniformity of the flow distribution in the manifold. A momentum-balance theory and a pressure-drop model are presented to explain the physical mechanism of flow distribution. Modeling results indicate that both the channel resistance and the manifold width can enhance the uniformity of the flow distribution. In addition, a lower air feeding rate can also enhance the uniformity of flow distribution. However excessive pressure drop is not beneficial for realistic applications of a fuel-cell stack and hence enhanced manifold width is a better solution for flow distribution. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/24834 |
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