- 學位論文
流道設計對質子交換膜燃料電池性能及局部傳遞現象之影響
Effect of flow channel designs on the cell performance and the local transport phenomena of proton exchange membrane fuel cells
摘要
摘要 本文係利用計算流體力學軟體CFDRC建立三維質子交換膜燃料電池全電池模型，主要探討流道設計(直通型、蛇型、指叉型、Z型指叉、直通指叉混合及棋盤型流道)之燃料電池內部電質傳現象。此外藉由操作參數(電池操作溫度、入口加濕溫度、氣體擴散層孔隙度、氣體擴散層滲透率、入口操作壓力、陰極燃料入口流速、流體同向流與反向流)，探討不同流道設計在實際應用上對燃料電池性能及流道壓損的影響，藉此選擇較佳的燃料使用率，相對較小壓損之流道設計；再加入壁面熱傳條件，觀察電池內、外部熱傳遞對燃料電池整體熱分佈以及性能之影響。 模擬結果顯示，隨著入口加濕溫度升高、降低電池操作溫度與流體方向成同向流時，整體的I-V性能曲線與功率密度曲線較佳。氣體擴散層方面，隨著孔隙度及滲透率加大，可降低燃料質傳阻抗，使燃料氣體傳輸至陰極觸媒層參與電化學反應量變多，促使電池性能提升。操作壓力方面，提高燃料氧化劑操作壓力可明顯提升電池性能，且陰極側加壓會使得加壓氣體提高觸媒層表面燃料氣體濃度。因此，提高氣體壓力能夠減少活性與濃度過電位損失，促使電池性能提升。陰極入口燃料流速方面，隨著陰極入口流速增加，提高了流道中氧含量，促使電池性能提升。接著改變電池內部熱傳導係數時，電池內部的熱源分佈也受到影響，較高的熱傳導係數，電池的電流密度也較高；改變電池外部熱對流係數時，影響電池整體散熱，當熱對流係數越高時，電流密度也隨之增加。 隨著操作溫度提高其氫穿透率越高。而隨著入口加濕溫度的提高其氫穿透率降低，且提高入口溼度可增加薄膜的適應性及電滲透性，有利於提高電池性能。且隨著增加擴散層孔隙度及滲透率，其氫穿透率增加。增加陽極入口操作壓力在不同流道幾何結構設計下皆會造成氫穿透率增加，主因於增加入口壓力時，陽極端之氫氣偏壓隨之增加，使之產生較大的壓力差穿過質子交換膜。 在同時考慮上述主要操作參數對燃料電池極化性能曲線、局部電流密度分佈、質傳現象、溫度分佈及壓力損失的影響後發現，雖然指叉型流道之進、出口壓力差略大於直通型流道與直通指叉混合型流道，且電池性能略低於蛇型流道與Z型指叉流道，但壓力損失較小，因此選擇指叉型流道設計，將最符合實際應用的經濟效益。
並列摘要
Abstract The purpose of the thesis is to establish a three dimensional PEM fuel cell model. The main objective is to investigate the phenomena of the elector/mass transfer inside a proton exchange membrane fuel cell with flow fields design (parallel flow field, serpentine flow field, interdigitated flow field, Z-type interdigitated flow field, parallel-interdigitated mixed flow field and grid flow field). In addition, the effects of operation condition (fuel cell temperature, inlet humidification temperature, porosity of diffusion layer, permeability of diffusion layer, inlet flow pressure, inlet velocity and gaseous fuel flow direction) on the cell performance and flow channel pressure drop of the PEM fuel cells under the real operating conditions are examined in detail by different air flow rates. Add the condition of the effect of the wall heat transfer; observed the effect of heat distribution and performance of internal and external heat transfer for fuel cell. The simulation results show that the inlet humidification increase to 80oC and decreasing the cell temperature to 50oC while the flow direction is counter-flow provides the best cell performance. While in the higher gas diffusion layer porosity and permeability that can reduce mass transfer impedance and dead zone area, allows more fuel gas transport into the catalyst layer of cathode and participate in the electrochemical reaction, and improves the cell performance. The performance of PEM fuel cell can also significantly improved by increasing the pressure of the cathode flow channel by incorporating forced convection. As increasing the inlet velocity it can promoted oxygen content on the flow channel and improve the cell performance. The heat distribution is effected on fuel cell when the heat conduction coefficient changed. If the heat conduction coefficient is getting larger, the current density will increase too. If the heat convection coefficient is getting larger, the current density will increase too, and then affect the cooling rate. H2 crossover rate increases with increasing the cell temperature, gas diffusion layer porosity and permeability. The H2 crossover rate decreases with increasing the inlet humidification, due to the increase in membrane flexibility when inlet humidification was increased. A monotonic increase in the H2 crossover rate with increasing backpressure of the anode will result in a H2 partial pressure increase, which then creates a larger pressure difference across the PEM. For the effects of the main operating condition on the cell performance curve, local current density distribution, phenomena of the mass transfer, temperature distribution and flow channel pressure drop, although the interdigitated flow field have a slightly larger pressure drop than those with parallel flow field or parallel-interdigitated mixed flow field, but the cell performance is lower the serpentine flow field and Z-type interdigitated flow field. For this reason, the interdigitated flow field behaves with the economic benefits in practical application.