固態氧化物燃料電池複合系統分析與元件熱傳分析;Analysis of Solid Oxide Fuel Cell Hybrid System and Heat Transfer Characteristics

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题名:	固態氧化物燃料電池複合系統分析與元件熱傳分析;Analysis of Solid Oxide Fuel Cell Hybrid System and Heat Transfer Characteristics
作者:	鄭天鈞;Cheng, Tien-Chun
贡献者:	機械工程學系
关键词:	質子傳導型固態氧化物燃料電池;系統模擬;熱輻射;隨溫度變化之性質;pSOFC;system simulation;radiative heat transfer;temperature-dependent properties
日期:	2017-06-22
上传时间:	2017-10-27 16:15:17 (UTC+8)
出版者:	國立中央大學
摘要:	本研究發展了一個質子傳導型固態氧化物燃料電池(pSOFC)複合系統，此複合系統包含了一個20kW的pSOFC與渦輪機，並討論不同燃料當量比與空氣當量比條件下，系統的效率與運作特徵。研究採用Matlab / Simulink / Thermolib 軟體進行分析計算，操作壓力討論範圍從1到10，燃料當量比討論範圍從1.25到1.45，空氣當量比討論範圍從2到3.5。結果顯示在pSOFC複合系統中，燃料電池的運作溫度會直接影響輸出功率大小，當燃料當量比增加時，雖然電池發電效率會因為剩餘燃料增加而降低，但較高的運作溫度可使系統輸出更多電功；而當空氣當量比增加時，對燃料電池的效率影響不大，但系統輸出功率則因操作溫度降低而減少。提高操作壓力可以獲得較佳的電池性能與較大的渦輪機輸出功率，但同時也會大幅增加燃料與空氣入口壓縮機之消耗功率。在系統效率的部份，空氣當量比增加可以提昇系統熱電聯產效率，但卻會降低系統電效率，此是因為壓縮機加壓耗能所導致；系統加壓可以提昇系統效率，但在高壓力與高空氣當量比時，因為壓縮機耗能太大，進而使得系統電效率反而下降。添加燃料分流設計可以在維持系統效率的情況下，有效的調整電池運作溫度；空氣分流設計除了能增加系統的操作性之外，調整過的冷卻流道安排亦明顯提昇系統熱能利用，大幅提昇系統電效率的表現。此外，本研究更針對系統中高溫元件可能面臨的散熱需求，討論元件熱傳特徵與機制分析。討論中考慮了熱輻射機制與自然對流的影響，並且設定了兩種輻射性質隨溫度變化的介質，使用無因次化的方法，分析在各種條件下，同特性之介質對系統熱傳特徵的影響。其中使用有限體積法求解連續方程式、動量方程式、與能量方程式；輻射傳遞方程式是使用 scheme的離散座標法求解；操作參數包含瑞利數(Ra)、由到，傳導-輻射參數(Nr)由10到0.005，光學厚度(τ)由0.1到10。結果顯示輻射性質隨溫度變化之介質，對系統熱傳現象有很明顯的影響；輻射性質不同所造成的差異，會影響系統溫度梯度分佈，進而影響其他的熱傳特徵；而且在熱輻射機制較弱的條件下，此一現象仍能被觀察到。介質的不同輻射性質所造成的影響，會在Nr降低或Ra增加時，產生較大的差異，因為Nr降低代表系統熱輻射機制所佔權重增加；而Ra增加是指自然對流增強，溫度場受旋轉流動扭曲，在局部產生較大的溫度梯度變化，進而使不同輻射特性之介質產生較大的差異。光學厚度增減對此介質差異造成的影響較為不同，太薄或太厚的光學厚度，均會使介質差異造成的流動影響變得不明顯，太薄的的光學厚度表示介質受熱輻射影響較小，太厚的光學厚度則象徵輻射能量會被侷限僅跟鄰近的介質作能量交換；然而，在熱對流能力的表現上，兩種介質隨著光學厚度增厚，而產生了明顯的趨勢差異，case A介質隨著光學厚度增加而抑止了熱對流現象，case B介質則隨著光學厚度增加到一定程度、熱輻射影響變顯著後，會間接的增強熱壁的熱對流現象。;The performance of an intermediate-temperature proton-conducting solid oxide fuel cell (pSOFC) hybrid system is investigated in this work. The hybrid system consists of a 20-kW pSOFC, a micro gas turbine (MGT), and heat exchangers. Heat exchangers are used to recover waste heat from pSOFC and MGT. The performance of the system is analyzed by using Matlab / Simulink / Thermolib. Flow rates of air and hydrogen are controlled by assigning different stoichiometric ratio (St). St considered in this study is 2 – 3.5 for air, and 1.25 - 1.45 for hydrogen. System operation pressure is controlled from 1 to 10 atm. Results show that the operation temperature of pSOFC affects the output power greatly. As fuel St increases, although the cell efficiency drops due to unreacted fuel, the higher cell operation temperature causes more electric power output from system. As air St increases, the cell efficiency does not change too much, but the system output power is reduced because of decreased operation temperature. Increasing operation pressure leads to better pSOFC performance and larger MGT output power, but it also increases the consumption power of compressors of air and fuel. The CHP efficiency can be increased by increasing air St or operation pressure, but it makes the system efficiency lower because of larger consumption power of air compressor. System operability can be enhanced effectively by attaching fuel and air bypass designs. Stack temperature can be adjusted by changing bypass ratio and keep the system performance. Air bypass design also enhances system efficiency via better heat utilization. In addition, this paper also investigates the heat transfer characteristics of high-temperature components. The effects of temperature dependence of radiative properties of a medium on radiation and natural convection interaction in a rectangular enclosure are studied. The radiative transfer equation is solved using the discrete ordinates method, and the momentum, continuity, and energy equations are solved by the finite volume method. Effects of the conduction-to-radiation parameter (Nr), Rayleigh number (Ra), and optical thickness are discussed. Results show that temperature dependence of radiative properties affects the temperature gradient, and hence the energy transport even in relatively weak radiation condition. As Nr is decreased or Ra is increased, the effects of temperature dependence of radiative properties become more significant. Lower Nr means larger weighting of thermal radiation contribution than convection. Larger Ra means stronger natural convection, and the local temperature difference is enhanced due to convection vortex. This, in turns, enhances the effects of different radiative properties. Effects of radiative properties on convection flow diminish when the medium is either optically thin or optically thick. However, optical thickness affects thermal convection greatly, and the two types of media show completely different trends. In case A, thermal convection is suppressed as the optical thickness is increased. In case B, thermal convection is enhanced when radiation effects become obvious.
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