GT2003-38094 ANALYSIS OF ELECTROCHEMICAL PERFORMANCE AND EXERGY LOSS IN SOLID OXIDE FUEL CELL

ABSTRACT A solid oxide fuel cell (SOFC) is expected to be applied to the distributed energy systems because of its high thermal efficiency and exhaust gas utilization. The exhaust heat from the SOFC can be transferred to the electric power by a gas turbine, and the high efficiency power generation can be achieved by constructing the SOFC and gas turbine hybrid system. In this study, the local processes in the electrodes and electrolyte of unit SOFC are analyzed taking into account the heat conduction, mass diffusion, electrode reactions and the transport of electron and oxygen ion. The temperature and concentration distributions perpendicular to the electrolyte membrane are shown. The effects of the operating conditions on the cell performance are also shown. Furthermore, the entropy generation and exergy loss of each process in the electrodes and electrolyte are analyzed and the reason for generating the exergy loss in the SOFC is clarified. It is noted that two electrode reactions are responsible for the major exergy loss. In the previous simulation models of the unit cells and stacks, the electrode reaction wasn't separated by the electrolyte into the single electrode reactions, and the heat conduction in the electrodes and electrolyte wasn't analyzed in the direction perpendicular to the electrolyte membrane. Therefore, the temperature distribution inside the electrodes and electrolyte wasn't shown. The exothermic electrode reactions and the heat generation of the irreversible processes affect the temperature distribution in the electrodes and electrolyte. In the internal reforming SOFC, the temperature difference inside the electrodes and electrolyte becomes large because the endothermic steam reforming reaction of the fuel occurs near the anode. The estimation of the temperature distribution in the electrodes and electrolyte is effective in verifying the validity and accuracy of the conventional numerical simulations of unit cells and stacks. Furthermore, the exergy loss caused by the processes in the electrodes and electrolyte wasn't analyzed in the previous studies. The evaluation of exergy losses can clarify how to improve the performance of the fuel cell. In this paper, the local processes in the electrodes and electrolyte of the unit SOFC are analyzed taking into account the mass diffusion, heat transfer, electrode reactions and transport of electron and oxygen ion, and the temperature and gas concentration distributions in the direction perpendicular to the electrolyte membrane are shown. The effects of a variation in operating conditions on the thermal efficiency of the cell are also shown. Furthermore, the entropy generation and exergy loss caused by the irreversible processes in the electrodes and electrolyte are analyzed, and the major contribution for the exergy loss in the cell is identified. NOMENCLATURE INTRODUCTION In recent year, a solid oxide fuel cell (SOFC) is one of the promising candidates for the distributed energy system. The advantages of the SOFC in the distributed system are the high efficiency power generation and the exhaust heat utilization. It is effective that the SOFC is integrated with a micro gas turbine because the exhaust heat from the SOFC can be transformed to the electric power by the gas turbine. The electric generation efficiency of the SOFC and gas turbine hybrid systems becomes much higher than that of conventional gas turbine systems. The numerical analysis of the performance characteristics and transport phenomena in the SOFC has been carried out. Campanar