Calculations of Transport Phenomena in Solid-Oxide Fuel Cells

This paper presents the results of a collaborative research project of computer modeling of transport phenomena within the passages of solid-oxide fuel cells. From a mechanical design viewpoint, fuel cells may be considered to be similar to heat exchangers with internal heat generation due to ohmic heating. This is a function of load-driven factors. The thermomechanical design of the units is of paramount importance, as the reaction rates are a function of temperature, pressure, and species concentrations, i.e., the process is fully coupled. The design goal of the project is to ensure uniform flow and temperature distribution throughout the stack, to optimize performance and minimize the risk of failure. We developed computer models to predict the performance of cells and stacks of cells, so as to minimize the development of expensive experimental protypes and test rigs. The standard techniques of heat transfer and computational fluid dynamics were substantially modified to be applicable in this context. Three distinct approaches were considered. In all cases two fluids; air and fuel, each containing different chemical species were considered. The equations for fluid flow, heat and mass transfer with electro-chemical reactions occurring were discretized and solved using a finite-volume method. Detailed numerical simulations of a single cell and stacks of up to 54 cells were performed using fine three-dimensional meshes of up to 4.6 million cells. Simplified models based on a distributed resistance (porous media) analogy, and also traditional presumed flow methods used in heat exchanger and furnace design, were also employed. These latter approaches have the advantage of being readily executable on small personal computers. The three methodologies are described and compared in detail.