| In this thesis, several models of solid oxide fuel cells (SOFC) were developed in cell and system levels. These models were used in several case studies to simulate the performance of the cells and systems studied. In addition, the effectiveness of SOFC in reducing greenhouse gases was assessed through a case study.;In system level, integrated SOFC systems were modeled using energy and exergy analyses. The analyses were done using the models developed for SOFC in cell level and through development of thermodynamic models for other components of integrated systems (e.g. gasifier, afterburner, and heat exchanger). These integrated systems included a gas turbine and SOFC-based cogeneration system and two SOFC and biomass gasification-based cogeneration systems. Performance assessment parameters, e.g. electrical efficiency, fuel utilization efficiency, power-to-heat ratio, and exergetic efficiency, as well as the exergy destructions and losses were calculated in these systems.;The models developed in cell level were validated using the published data in the literature and used to simulate the performance of several cases. The results from the thermodynamic model showed that lower recirculation ratio, which quantifies the amount of depleted fuel that is recirculated to the fuel channel inlet, and higher fuel utilization increased the performance of the system. From the carbon deposition model, it was found that in order to operate the SOFC with the minimum recirculation ratio as required for higher electrical efficiency, the maximum possible operating temperature level and fuel utilization ratio should be chosen to prevent carbon deposition. It was also shown that gases produced from advanced gasification systems, such as twin-fluid bed and multi-solid fluid bed, yield higher electrical efficiency for SOFC compared to those produced from downdraft and updraft gasifiers. The heat transfer model yielded that the counterflow configuration takes slightly more time to reach the steady state condition, and it has a better electrical efficiency for low Reynolds numbers. The study on the effect of excess air coefficient on the performance of the SOFC showed that taking this coefficient higher provides better electrical efficiency.;The system level models were used to simulate the performance of several cases. The case study, in which a SOFC and gas turbine based cogeneration system was simulated, pointed out that this system has a better thermodynamic performance compared to its competing technologies. The simulation of SOFC and biomass gasification system showed that selecting steam as the gasification agent yields higher electrical efficiency, power-to-heat ratio, and exergetic efficiency.;In cell level, a thermodynamic model, a carbon deposition model, and a quasi 2-D transient heat transfer model were developed. The thermodynamic model is capable of determining the performance of a SOFC including polarization curve, power output, and electrical efficiency. This model takes into account the recirculation of depleted fuel and internal reforming processes. The original model was improved by addressing problems associated with carbon deposition. The occurrence of carbon deposition was investigated using C-H-O triangular phase diagrams and calculation of carbon activities. More detailed modeling of SOFC was accomplished by including the heat transfer mechanisms inside the fuel cell such as conduction, convection and radiation. In this heat transfer model, the transient behaviour of the cell was simulated during the heat-up and start-up stages. Several parametric studies, such as effect of Reynolds number and excess air coefficient on the performance of the cell, were conducted to better examine co- and counter-flow configurations of SOFC.;Greenhouse gas emissions from uncontrolled and controlled landfill sites were compared through a case study. In the controlled landfill sites, the following systems were used for utilizing the landfill gas: flaring, internal combustion engine, gas turbine, and SOFC. The results showed that the SOFC has a better potential to reduce GHG emissions among the different technologies studied. |
