Modelling of Tubular SOFC in Hybrid Power Plant Systems - Cell and Stack Modelling

Hybrid power plants combining SOFC generators and micro gas turbines are promising because of their high electrical efficiencies. The DLR Institute of Technical Thermodynamics is part of a research partnership developing a concept and control system for a hybrid power plant. Since the control strategy of a hybrid power plant is complex and demanding the development process is optimised by modelling component and plant. The control system is developed on the basis of the computer model of the power plant. Therefore the models have to be highly reliable and fast concerning computing time. The poster presents the modelling of the fuel cell system in the hybrid power plant project. A model library for MATLAB Simulink containing several fuel cell models as well as balance of plant components has been developed. Balance of plant components feature ejectors for anode recirculation, reformer and desulphurisation. The reformer is modelled as a 1D fixed bed steam reformer utilising steam reforming and water gas shift reactions. The reformer is discretised along the flow direction delivering additional information on the reactions inside the vessel. A focus will be set on the 1+1D model of a tubular fuel cell. The model is discretised along the flow path, with the whole model representing the length of the tube. Each element of the model contains five calculatory areas which influence each other in terms of heat transfer, heat radiation and electrochemical reactions. The fuel cell model is based on the Nernst potential and incorporates ohmic losses as well as activation overpotential and mass transportation limitation. The calculated voltage and corresponding current is mainly dependent on temperature, pressure and concentration of the reacting species. Those governing parameters are interrelated to the neighbouring elements, therefore heat conduction and convective mass flow are integrated into the model. As cell development continues the model can be easily adjusted to new cell designs or material parameters. Due to the complexity of the model and the associated time delay an acceleration of the simulation was necessary. Certain parameters which are most likely to change during operation or have major influence on the cell have been identified. Those parameters have been varied to obtain a cell map for the calculation of current voltage behaviour. The models are being validated on a SFC-5 system installed at DLR site in Stuttgart