A dynamic model for studying the cold start dynamics of a high temperature solid oxide fuel cell (SOFC) auxiliary power unit (APU) for automotive applications was set up. The SOFC APU system is implemented with a combustion chamber, which burns the residual fuel and air from the fuel cell to provide high temperature gas to preheat the SOFC stack through an energy recovery unit. A nonlinear real-time sliding observer for stack temperature evaluation was coded into the system dynamics to monitor the temperature variation. The simulation results show that a 100W APU system in this research takes about 2 minutes for start-up without considering the thermal limitation of the cell material. This small scale APU system is able to provide additional power to automotive electronics when the engine is off.
A second example is to design a stand-alone power plant for either automotive power application or a distributed power generation system. A conceptual turbo fuel cell system, including SOFC stack, afterburner, turbo generator and heat exchanger subsystems, is modeled using a modified filling-and-emptying approach. The simulation results show that the start-up time for the example turbo fuel cell system (200 kW SOFC plus 50 kW MGT) may take up to more than one hour.
In summary, a modified filling-and-emptying model based on mass and energy conservation laws has been developed to configure the general system dynamics of high temperature SOFC systems. Transient simulations of two example SOFC systems were carried out via a self-written dynamic code based on the Matlab/Simulink platform. A nonlinear sliding observer for on-line tracking temperature dynamics has been set up for startup control, whose targets are either on rapid or safety start-up. A model-based controller, including PI and sliding controls, has been designed for these purposes.

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