固態氧化物燃料電池堆熱應力與變形受潛變機制影響之分析;Creep Effect on Stress and Strain Distributions in a Solid Oxide Fuel Cell Stack

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题名:	固態氧化物燃料電池堆熱應力與變形受潛變機制影響之分析;Creep Effect on Stress and Strain Distributions in a Solid Oxide Fuel Cell Stack
作者:	鄭光志;Jheng, Guang-Jhih
贡献者:	機械工程學系
关键词:	固態氧化物燃料電池;熱應力分析;潛變;solid oxide fuel cell;thermal stress analysis;creep
日期:	2019-07-23
上传时间:	2019-09-03 16:34:18 (UTC+8)
出版者:	國立中央大學
摘要:	本研究以一款平板式SOFC系統電池堆為研究對象，利用有限元素法，進行熱應力與結構變形的數值模擬分析。為了符合SOFC電池堆的實際運作狀況，本研究將潛變機制在高溫穩態運轉階段對組件結構應力及應變分佈的影響納入考量，包含將金屬連接板、玻璃陶瓷密封膠及二者接合件之各項高溫機械性質與潛變性質，融入所建構之有限元素模型中，以求得在高溫穩態運轉階段，SOFC電池堆各組件熱應力與應變分佈隨時間變化的趨勢，藉此評估SOFC電池堆結構強度的穩定性及耐久壽命。由模擬結果得知，本研究中所使用的平板式SOFC電池堆，其電池片、鎳網、金屬連接板在各階段的最大等效應力皆小於該材料可承受的臨界應力；而玻璃陶瓷密封膠在完成組裝階段及電池停機階段的最大等效應力在其邊角處皆大於臨界應力。在長時間運作的高溫潛變機制作用下，所有元件的等效應力皆隨高溫運轉時間愈長而有下降的趨勢，而應變則變化不大，顯示有應力鬆弛的現象。此外，金屬連接板與玻璃陶瓷密封膠在邊角處介面上的最大正向應力值與最大剪應力值都會超過可承受的介面臨界應力，有可能於此處產生密封膠的剝離失效。本研究也探討平板式SOFC電池堆在模擬步驟與其他元件不變的情況下，將原先使用玻璃陶瓷密封膠的地方改由金屬硬銲密封取代，進行結構熱應力分析。結果顯示，與原先使用玻璃陶瓷密封膠時的各元件應力相比，電池片與金屬連接板的最大等效應力在使用硬銲合金密封時於完成組裝階段及電池停機階段分別產生了50%與100%的應力提升，主要的原因來自於硬銲合金與相鄰元件的熱膨脹係數不匹配的程度比玻璃陶瓷密封膠來得大。 ;The purpose of this study is to characterize thermal stress distribution in a planar solid oxide fuel cell (pSOFC) stack. A 3-D finite element model for a multiple-cell pSOFC stack is constructed to solve the thermal stress distribution at different stages including after-assembly, start-up, steady operation, and shutdown. The effect of creep mechanism on the variation of stress/strain distribution is taken into account by applying to the finite element model the previously obtained creep properties of the given component materials. Combing the numerical results with mechanical strength of relevant materials, assessment for structural integrity of the given pSOFC stack is also conducted in the present study. Simulation results indicate the maximum equivalent stress in cell assembly, nickel mesh, and interconnect/frame at each stage of the pSOFC operation conditions is smaller than the critical value. However, the maximum equivalent stress in glass-ceramic sealant at the corners of the bonding region is greater than the critical one at after-assembly and shutdown stages. During long-term operation, the equivalent stress of all components decreases with an increase in operating time, while the strain barely changes. It indicates that stress relaxation takes place in the pSOFC stack at high-temperature operation stage due to a constant-strain creep mechanism. In addition, the maximum interfacial normal and shear stresses on the edge or corner of the interface between interconnect/frame and glass-ceramic sealant exceed the critical value such that debonding may occur at the interface. Structural thermal stress analysis is also carried out for the case that the glass-ceramic sealant in the given pSOFC stack is replaced by braze alloy under a condition that the simulation procedure and other components are unchanged. The results show that the maximum equivalent stress in cell assembly and interconnect/frame is respectively increased to an extent of 50% and 100% at after-assembly and shutdown stages when using braze alloy in replacement of glass-ceramic sealant. The main reason for the significant stress increase is that the braze alloy has a larger mismatch of coefficient of thermal expansion with adjacent components.
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