Thermodynamic Modeling and Energy Analysis of a SOFC-PEMFC Combination in a Gas Turbine Cycle

This paper studies the performance of a hybrid system that comprises a SOFC (Solid-Oxide-Fuel-Cell) combined with a PEMFC (polymer electrolyte membrane fuel Cell) which is integrated into a Gas Turbine power plant. Detailed modeling, thermodynamic, kinetic, geometric models are developed, implemented and validated for the synthesis/design and operational analysis of the combined hybrid system. In this system, the PEMFC makes use of the internal reforming ability of the SOFC to produce hydrogen which is necessary for the PEMFC operation. The heat released in the SOFC is utilized in the internal reforming process. Different levels of modeling for the SOFC, the PEMFC and the integrated system are presented. The overall system performance is analyzed by employing individual models and further applying thermodynamic laws for the entire cycle. The paper also introduces different methods of using shift reactors where CO reacts with H₂O to produce CO₂ and H₂ to further increase the efficiency of the system by introducing a new factor to control parasitic energy consumption. In addition to this, the paper also suggests cooling the H₂ stream before entering the PEMFC stack using the exhaust air of the Gas turbine. The main components of the SOFC+PEMFC system are a SOFC stack, shift reactors, selective oxidizer and a PEMFC stack. The fuel cells are connected in series for fuel feeding. Furthermore, although the efficiency of the SOFC increases with increasing operating pressures, the paper describes that the efficiency of the SOFC-PEMFC combination also varies with changing the temperatures. Energy and entropy balances are performed not only for the whole system but also for each component to evaluate the distribution of irreversibility and thermodynamic inefficiencies. According to the study, around 5% efficiency improvement was obtained with a parallel SOFC-PEMFC system as compared with a stand-alone SOFC. Alternative methods of improving the efficiencies are also introduced.