Reliable Control and Disturbance Rejection for the Thermal Behavior of Solid Oxide Fuel Cell Systems

Chemical energy can be converted directly into electricity and process heat with the help of solid oxide fuel cells. This type of fuel cells, moreover, allows for operating points with high efficiency and reasonable exploitation of the supplied gases at high temperature levels. However, the maximum admissible cell temperature is restricted by the properties of the materials contained in the fuel cell stack. For an application in a decentralized power supply grid, control strategies are essential which can cope with varying electrical load demands and prevent local over-temperatures leading to a decreased system reliability. To derive such control laws, mathematical models are required which represent the instationary thermal behavior of a fuel cell stack for the above-mentioned load variations. For that purpose, the spatial temperature distribution in the interior of the stack module is described by means of a finite volume discretization. This approach leads to a set of coupled nonlinear ordinary differential equations, where several parameters cannot be characterized exactly due to disturbances and an imperfect system knowledge. These uncertainties are taken into consideration in the thermal subsystem in terms of parameters which can be bounded by means of intervals. In this contribution, different nonlinear control strategies are presented to account for such parameter uncertainties by designing guaranteed stabilizing control laws on the basis of suitable control Lyapunov functions. The corresponding stability analysis guarantees a safe operation of the instationary thermal subsystem for all possible values of the system parameters in spite of their uncertainties.