| 摘要: | The high operating temperature enables solid oxide fuel cell hybrid system to obtain high efficiencies from integration with gas turbines for large scale stationary applications. High temperature solid oxide fuel cell means that the components of the stack need to be made of expensive materials. For smaller scale applications, there is a trend to move to lower temperatures of operation into the so-called intermediate temperature solid oxide fuel cell. Biofuel such as methanol, ethanol and isooctane can be considered to be a favorable biofuel because they are the most environmentally friendly and renewable energy sources. These fuels are easy to be used for solid oxide fuel cell hybrid system and safe in storage and handling.
In this thesis, the hybrid system consists of a proton solid oxide fuel cell (pSOFC) stack, a micro gas turbine, a combustor, compressors, heat exchangers and external steam reformer. The effect of operating parameters such as compressor pressure, fuel utilization, steam-to-carbon ratio (S/C) and bypass ratio are investigated for each fuels. Then, several case studies conducted in this system model are presented. Finally, the performances of the system using different fuels are compared in terms of pSOFC power output, turbine power output, H2 production, pSOFC efficiency, power system efficiency and combine heat and power (CHP) efficiency.
The simulation is based on the thermodynamic analysis and developed using Matlab Version 7.6 / Simulink Version 7.1 / Thermolib 5.1.1.5353 and validated using the published data in the literature. The results from the thermodynamic model showed that the increasing of the compressor pressure can result in the decrease in the pSOFC efficiency but power system and CHP efficiency are enhanced. The increasing of the fuel utilization can result in the increasing of the pSOFC efficiency. However, power system efficiency decreases for methanol fuel but increases for ethanol and isooctane fuels. CHP efficiency increases when fueled by ethanol and isooctane but reduces when fueled by methanol. The variation of the S/C has an impact on the hybrid system. A minimum S/C for each fuel is required to prevent carbon formation but increase S/C will decrease the power system and CHP efficiency. Practically in hybrid system, the value of S/C is usually below the optimum value. From the effect of bypass ratio, it was found that the minimum bypass ratio is 0.05, 0.15 and 0.2 for methanol, ethanol and isooctane. Below those values, steam reformer has insufficient heat to reform the fuel to hydrogen. From the comparison between fuels, it was found that methanol is more attractive than ethanol and isooctane. Finally, due to the new design system configuration of solid oxide fuel cell hybrid system, it is possible to use higher molecular weights fuel such as ethanol and isooctane in external steam reforming.
由於固態氧化物燃料電池(SOFC)有著較高的操作溫度，因此在大型固定式裝置上，可搭配渦輪機組成熱電共生(hybrid)系統以獲得較高的效率。但是高溫型SOFC也同時意味著需要使用能耐高溫的昂貴材料，所以在小型裝置的應用上，目前的研究趨勢是朝向較低的操作溫度發展，也就是所謂中溫SOFC。
在生質燃料中，甲醇，乙醇和異辛烷，由於其環保性和可再生性，因此被認為是較佳的生質燃料。同時，這些生質燃料適合利於SOFC hybrid系統上，而且也能安全地儲存和處理。
在這篇論文中，SOFC hybrid系統是由質子傳導型固態氧化物燃料(pSOFC)電池堆、微型燃氣渦輪機、燃燒器、空氣壓縮機、熱交換器，以及外部蒸汽重組器等所組成，並針對各個燃料的壓縮機壓力，燃料利用率，水碳比(S/C)和分流比率等操作參數的影響進行分析研究。最後，在不同燃料系統的性能上，比較了pSOFC電池堆的功率輸出、渦輪機的功率輸出、氫氣產量、pSOFC電池堆效率、系統電效率和熱電效率(CHP)。
本研究是從熱力學分析的角度，使用Matlab 7.6版 / Simulink7.1版 / EUtech Thermolib 5.1.1.5353版進行模擬，並以文獻中已知的數據進行驗證。從熱力學模型的結果可知，增加壓縮機的壓力會導致pSOFC電池堆的效率減少，但系統電效率和熱電效率皆增加。燃料利用率的增加，會導致pSOFC電池堆效率增加。然而，使用甲醇燃料系統電效率和熱電效率皆降低，但使用乙醇和異辛烷為燃料時，系統電效率和熱電效率皆增加。在SOFC hybrid系統中水碳比的變化有顯著的影響。為了預防積碳的產生，每種燃料皆有其理想之最低水碳比，但在系統模擬上，增加水碳比將會降低系統電效率和熱電效率。事實上，在SOFC hybrid系統中，水碳比的最佳值通常低於理論最佳值。從分流比率的影響可發現，甲醇，乙醇和異辛烷最低分流比率分別是0.05、0.15和0.2，低於此值，蒸汽重組器將沒有足夠的熱量來進行重組。從燃料之間的比較可發現，甲醇比乙醇和異辛烷更具吸引力。最後，藉由本研究所設計之SOFC hybrid系統，對於較高分子量的燃料，例如乙醇和異辛烷，亦可適用於外部蒸汽重組器。 |
