(305g) Detailed Steady State Modeling of An Anode-Supported Tubular Solid Oxide Fuel Cell (SOFC)

High reactivity, comparatively inexpensive anode and cathode material, high power density, high level of usable heat and the possibility of using flexible fuels have made SOFC an attractive choice for utility and industrial applications. A detailed steady state model of a SOFC can be a useful tool to evaluate the effects of the operating and design parameters and thus can be used to design new cells. The model can also be used for optimization studies of the cell dimensional and operating parameters. In this talk, the development of an optimization-oriented detailed steady state model will be presented.

To attain this goal, a detailed two dimensional steady state model for an anode-supported tubular SOFC is developed. The model includes: (i) mass transport phenomena in the anode and cathode gas flow channels for the reactants and the products, (ii) diffusion from the gas flow channels through the porous electrodes to the reaction sites, and (iv) ohmic resistances. The ohmic resistances of the electrodes are modeled by an equivalent circuit model considering the current path length from the current collectors to the TPB (Triple Phase Boundary). A detailed kinetic study of the electrochemical reactions at the TPB is done where the rate parameters are found out by comparing the simulation results with experimental data.

For this industrial cell, the cell temperature is maintained by a furnace. However spatial variation in temperature cannot be ruled out. To consider the effects of these variations, energy conservation equations are written. Energy conservation equations are written in the gas flow channels, and in the solid. Energy conservation equations in the flow channels account for heat transfer between electrodes and gases, heat transport due to mass transport between electrodes and flow channels, and heat transfer due to the heating element in the furnace. In electrodes, heat transfer takes place due to conduction and exchange of mass. In addition, radiative energy transfer between the heating element and the cathode is considered in the cathode. Further, heat generation due to ohmic resistance is considered in the electrolytes and in the electrodes. In the anode and cathode TPBs, heat generation due to electrochemical reactions is modeled.

It is observed in the study that the model mismatch is very significant when H2 flowrate decreases to a very low value. The experimental data clearly show two distinct slopes in the I-V curve for the low flow condition indicating to a different dominating effect in the low-flow regime. Observing the characteristic of the experimental data and based on experimental characterizations1, it appears that adsorption at the TPB may play a key role when the H2 concentration reaches to a very low value. To capture this effect, a detailed kinetic model of the electrochemical reactions is considered at the TPB. The kinetic parameters are estimated by comparing the simulation results with the experimental data.

A MAPLE-MATLAB environment is used to solve the steady state model. The computational issues in solving the model will be discussed. The model is validated using data from a commercial SOFC over a wide range of cell temperatures, reactant flow rates, and DC polarizations. Critical observations made in the validation process will also be presented.

1Metzer P., Friedrich, K. A., Steinhagen H., Schiller, G. SOFC characteristics along the flow path, Solid State Ionics, 177, 2045-2051, 2006