Computational Fluid Dynamics Analysis on Transport Phenomena in Solid Oxide Electrochemical Cells

Abstract: The thesis comprises an analysis of the heat transfer phenomenon in solid oxide fuel cells (SOFCs) and a description of various transport phenomena in a solid oxide electrolysis cell. Both devices are encompassed under the solid oxide electrochemical cell concept. Moreover, a CFD model of an SOEC was developed to study the behavior of the cell under different conditions of operating and structural parameters. In the first part, the different heat sources that take place within an operating SOFC have been described together with the regions where they are located with special attention on their relationship with the chemical and electrochemical reactions that occur in an SOFC. The kinetics of the internal reforming reactions and of the electrochemical reactions are believed to be one of the main issues for improving the SOFC performance. A literature study on the different kinetic expressions based on the overall reaction schemes for methane steam reforming reaction in Ni-YSZ catalysts on SOFC anodes has been performed revealing the difficult comparison between the reported reaction rate expressions due to different experimental conditions. Furthermore, from a CFD analysis on a 2D planar SOFC, it is found that the main heat source contributor of the cell is the heat generation due to the electrochemical reactions followed by the heat consumption by the steam reforming reaction. In the second part, various transport processes in SOECs are described. Besides, a FVM based CFD model was developed and applied for a cathode-supported planar SOEC operating in cross-ow configuration arrangement. The behavior, in terms of current density, temperature distribution and the hydrogen production in an SOEC, has been investigated for different cases, such as varied operating voltages, the reduced porosity of the porous materials, the presence of hydrogen at the inlet of the cathode channel and in parallel-ow configuration. The predicted results show that higher current densities are obtained for higher operating voltages being the anodic current density higher than the cathodic one. However, no significant difference was observed when decreasing the porosity of the cell nor when hydrogen was present at the inlet of the cathode channel. Yet, the parallel-ow configuration yields lower current density values although they remain in the same order of magnitude as those from the cross-flow arrangement. The temperature simulation reveals various profiles depending on the operating voltage emphasizing the three thermal operating modes of an SOEC: endothermic, thermo-neutral and exothermic. A decrease in the porosity leads to higher temperature values in the cell due to an increase in the joule heating. The presence of hydrogen in the water channel inlet also gives higher temperature values. Per contra, the parallel-flow arrangement reveals a temperature decrease along the ow direction although operating in exothermic mode. Higher hydrogen molar fractions at the outlet of the cathode channel were obtained with higher operating voltages due to the higher current densities generated and the exothermic operating mode. However, no difference was observed when reducing the porosity value. The hydrogen fueled cell yields lower hydrogen production and so does the parallel-ow arrangement due to the lower current densities revealed.