SOFC Modeling Considering Mass and Heat Transfer, Fluid Flow with Internal Reforming Reactions

Sammanfattning: Fuel cells are promising for future energy systems, since they are energy efficient and, when hydrogen is used as fuel, there are no emissions of greenhouse gases. Fuel cells have during recent years various improvements, however the technology is still in the early phases of development. This can be noted by the lack of a dominant design both for singe fuel cells, stacks and for entire fuel cell systems. A literature study is preformed to compile the current position in fuel cell modeling. A deeper investigation is made to find out if it is possible to use a multiscale approach to model solid oxide fuel cells (SOFCs) and combine the accuracy at microscale with for example the calculation speed at macroscale to design SOFCs, based on a clear understanding of transport phenomena and functional requirements. It is studied what methods can be used to model SOFCs and also to sort these models after length scale. Couplings between different methods and length scales, i.e., multiscale modeling, are outlined. Multiscale modeling increases the understanding for detailed transport phenomena, and can be used to make a correct decision on the specific design and control of operating conditions. It is expected that the development and production costs will decrease and the energy efficiency increase (reducing running cost) as the understanding of complex physical phenomena increases. In this thesis a CFD approach (COMSOL Multiphysics) is employed to investigate the effects on the temperature distribution from inlet temperature, oxygen surplus, ionic conductivity and current density for an anode-supported intermediate temperature solid oxide fuel cell (IT-SOFC). The developed model is based on the governing equations of heat-, mass- and momentum transport. A local temperature non-equilibrium (LTNE) approach is introduced to calculate the temperature distribution in the gas- and solid phase separately. This basic model is extended to include internal reforming reactions and effects on mass- and heat transfer, and on fluid dynamics. The results show that the temperature increase along the flow direction is controlled by the degree of surplus air. It is also found that the ohmic polarization in the electrolyte and the activation polarization in the anode and the cathode have major influence on the heat generation and cell efficiency. If a counter flow approach is employed the inlet temperature for the fuel stream should be close to the outlet temperature for the air flow to avoid a too high temperature gradient close to the fuel inlet. The temperature is lowered, when hydrocarbon fuels (e.g., methane) is used, due to the reforming reactions.