Optimization of a Single-Cell Solid-Oxide Fuel Cell Using Computational Fluid Dynamics

To determine the effects of various parameters on the performance of a solid-oxide fuel cell (SOFC), a series of simulations were performed using computational fluid dynamics (CFD). The first step in this process was to create a 3-dimensional CFD model of a specific single-cell SOFC for which experimental performance data had been published. The simulation results using this baseline model were validated by comparing them to the experimental data. Numerous CFD simulations were then performed with various thermal conditions at the cell’s boundaries and with different air- and fuel-inlet temperatures. Simulations were also conducted with fuel-utilization factors of from 30% to 90% and air ratios of from 2 to 6. As predicted by theory, conditions that resulted in higher cell temperatures or in lower air and fuel concentrations resulted in lower thermodynamically reversible voltages. However, the higher temperatures also reduced Ohmic and, when operating with low to moderate current densities, activation losses, which often caused the voltages actually being produced by the cell to increase. Additional simulations were performed during which air and fuel supply pressures were varied from 1 to 15 atmospheres. Although the increased pressure did result in higher cell voltages, this benefit was significantly reduced or eliminated when the air- and fuel-compressor electrical loads were included. CFD simulations were also performed with counterflow, crossflow, and parallel-flow fuel-channel to air-channel configurations and with various flow-channel dimensions. The counterflow arrangement produced cell voltages that were equal to or slightly higher than the other configurations, and it resulted in a differential temperature across the electrolyte that was significantly less than that of the parallel-flow cell and was close to the maximum value in the crossflow cell, which limits stress caused by uneven thermal expansion. The use of wider ribs separating adjacent channels reduced the resistance to the electrical current conducted through the ribs; however, it also reduced the area over which incoming fuel and oxygen were in contact with the electrode surfaces and, consequently, impeded diffusion through the electrodes. Reducing channel height reduced electrical resistance but increased the pressure drop within the channels. Plots of voltage versus current density, together with temperature and species distributions, were developed for the various simulations. Using this data, the effect of each change was determined and an optimum cell configuration was established. This process could be used as a guide by fuel-cell designers to better predict the effect of various changes on fuel-cell performance, thereby facilitating the design of more efficient cells.