非均勻入口流場效應下熔融碳酸鹽燃料電池堆之性能分析

Abstract

本論文主要在探討，陽極及陰極氣體入口處使用非均勻莫耳流率( mole flow rate )時，對單體熔融碳酸鹽電池(Molten Carbonate Fuel Cell)及電池堆 (Molten Carbonate Fuel Cell Stack)的性能影響。以有限差分法(finite difference method)對質量守恆、能量守恆及化學計量守恆式等偏微分方程式進行解析。並對部份的結果用套裝軟體FlexPDE以有限元素法( finite element method)進行驗證。在分析的方法中，本文利用莫耳流率的不均勻性設計成八種不同型式(patterns)的入口流場，然後分析入口流場對單體熔融碳酸鹽電池及電池堆性能的影響。 首先，分析非均勻入口流場對單體熔融碳酸鹽電池性能的影響。在溫度場及電流密度場方面，以有限差分法進行求解，再利用套裝軟體FlexPDE加以驗證，兩者之間的數據相當吻合。對於單體的融碳酸鹽電池而言，當氣體入口處的非均勻莫耳流率偏差量為0.25時(d=0.25)，在G式樣的電池溫度比均勻流(d=0)的高出12%，而在D式樣的電流密度場要比均勻型式樣的高出37%，根據結果，在入口處之非均勻流會對電池的溫度及電池密度分佈範圍產生明顯的影響。 此外，本研究第二部份探討具交叉流氣體供應方式的熔融碳酸鹽燃料電池在高氣體使用率與非均勻陽極氣流下的性能表現。數學模式方面採用二維之質量，能量等守恆方程式，而不考慮堆疊方向的性質變化。由數值計算結果顯示陽極氣體使用率隨入口莫耳流率之減少而增加。此外，陽極端入口的非均勻流將導致在陽極出口端及陰極入口端產生不反應區域，進而影響到電池的局部電流密度與性能。 最後，本文第三部份則進一步探討非均勻入口流場對電池堆性能的影響。電池堆使用10個單體熔融碳酸鹽電池所組成，在陽極及陰極入口處之流場皆為不均勻流場。本文主要是利用近似三維之數值模擬，來分析一個具有10層單體的熔融碳酸塩燃料電池堆之溫度場及電壓分佈。在陽極及陰極入口處，假設莫耳流率分佈曲線沿電池堆方向為漸增式及均勻式設計，並將其組成四組不同的入口流場形式(patterns)。結果顯示在陰極入口處使用非均勻的莫耳流率會明顯的改變熔融碳酸塩燃料電池堆的溫度場，而且電池堆的入口有最少的莫耳流率時，那麼在此時的陰極出口處會產生最高的溫度。此外，在沿著電池堆的陽極入口處，如果具有非均勻的莫耳流率時，會強烈影響電壓的分佈。沿著電池堆的方向來看，各單體電池的平均溫度變化率約為百分之二，而平均的電池電壓變化率約為百分之四十。此結果與作者先前所探討的非均勻流對單體電池的變化率要有比較明顯的不同。

This study investigates the temperature and current density distributions in a molten carbonate fuel cell unit and stack when the inlet flows of the anode gas and the cathode gas are mal-distributed. Furthermore, this study extends the research to the temperature and current density distributions in a molten carbonate fuel cell when there is higher utilization of anode gas. In the analysis of a unit cell, the two-dimensional simultaneous partial differential equations of mass, energy and electrochemistry are solved numerically. The numerical method is reliable through the accuracy comparison between this FORTRAN program and a software package. The results indicate that the maldistribution of anode and cathode gases dominates the current density field and the cell temperature field, respectively. Moreover, the non-uniform inlet flow slightly affects the mean temperature and mean current density, but worsens the distribution of temperature and current density for most maldistribution patterns. According to the results, the variations of the cell temperature in Pattern G and the current density in Pattern D are 12% and 37% greater than those in the uniform pattern when the deviation of the non-uniform profile is 0.25. Consequently, the effect of non-uniform inlet flow in the transverse direction on the temperature and current density distribution on the cell plane is evident, and cannot be neglected. In the analysis of a MCFC stack, this study considers that the MCFC is composed by ten stacks, and the molar flow rate in each stack is different because of the inlet distributor. This study employs the procedure of calculation in a MCFC unit to calculate the results of each stack, and then averages the temperatures of up separator and down separator, which connect together between stacks. The FORTRAN program iterates the whole procedure to get the quasi three- dimensional temperature and current density distributions until the relative errors of average temperature of separators satisfy the converge criterion. The primary results show that the effect of non-uniform in the stacking direction is more apparent than that of non-uniform in the transverse direction on the thermal and electrical performance of a MCFC. Then, the second part of this dissertation, the electric performance of a planar MCFC unit with cross-flow configuration when there is higher gas utilization in anode and cathode is investigated in the final part of this dissertation. A two-dimensional model, considering the conservation equations of mass, energy and electro-chemistry is applied. The results show that the anode gas utilization increases with a decrease in the molar flow rate, and the average current density decreases when the molar flow rate drops. In addition, non-uniform inlet profile of the anode gas will induce a happening of non-reaction area in the corner of the anode gas exit and the cathode gas inlet. This non-reaction area deteriorates the average current density and then reduces the electrical performance up to 4% when the anode gas molar flow rate is 0.01242 mol/s and anode gas utilization is 73%. Finally, in the third part of this dissertation, the effects of the non-uniform inlet flow on the MCFC stack are investigated. We develop a quasi-three dimensional numerical method for analyzing three-dimensional temperature and cell voltage distribution in a ten-layer molten carbonate fuel cell. The authors consider the non-uniform profile as progressively increasing along the stacking direction, and assign it to the anode gas inlet or cathode gas inlet to form four kinds of patterns. Results indicate that the non-uniform molar flow rate of cathode gas obviously changes the temperature field of a molten carbonate fuel cell stack, and highest cell temperature occurs at the cathode gas exit in the layer with the lowest molar flow rate. Moreover, non-uniform anode gas in the stacking direction strongly affects cell voltage distribution in the molten carbonate fuel cell stack. The variation of average cell temperature and cell voltage among different layers along the stacking direction are 2% and 40%, apparently larger than the variation rate due to non-uniformity in the transverse direction in previous chapter.