Anode-Oxidizing Critical Operating Conditions of Large Planar Solid Oxide Fuel Cell

Abstract

In a solid oxide fuel cell power system, large planar cells are common central parts for electric generation. In practice, the operating conditions of a planar cell may have significant impact on the electrical efficiency and the lifespan of the cell, thus the operating and maintenance cost of the power system. Given varying electric load, the power system would dynamically adjust the operating conditions of each cell or cell module, so that the energy and heat balance in the system is maintained, the fuel consumption is minimized and the cells are protected from reactant starvation, local overheating etc. To realize this functionality, we seek a fundamental understanding of the quantitative connections among fuel cell properties, operating conditions and optimal electrical efficiency.We established a quasi-2D model for quick evaluation of fuel and oxidant concentration and current density along the flow direction. For fuel cells with very low charge transfer impedance i.e. 0.02 Ω*cm^2 operating with low fuel flow rates, the fuel concentration at the exit may reach an equilibrium where the local Nernstian potential across the cell equals the cell voltage. In such cases, the fuel utilization remains almost constant regardless of varying fuel flow rate, given a certain cell voltage. For fuel cells with local charge transfer impedance of 0.5 Ω*cm^2 operating with high fuel flow rates, such phenomenon was not observed because the fuel concentration at the exit depends strongly on the fuel flow rate. Given the fuel molar fraction at the fuel inlet, the electrical efficiency of a fuel cell has its maximum at certain cell voltage, which depends strongly on the charge transfer impedance of the fuel cell. At constant fuel flow rates, the optimal cell voltage increases with lowering charge transfer impedance. The optimal electrical efficiency appears lower with higher inlet fuel molar fraction. Given fixed and uniform fuel cell temperature, the air flow rate has little impact on the electrical efficiency for air-fuel stoichiometric ratios above 1.3.Besides the above findings, further studies are necessary on the effects of temperature distribution as well as porous electrode diffusion resistance. Figure 1