Investigating controller performance in hybrid SOFC systems with unknown nonlinearities

Solid oxide fuel cells (SOFCs) offer many benefits over various other fuel cells as a result of their high operating temperatures. Our prior work has addressed the conflicting goals of preventing fuel starvation and improving load-following in SOFCs through a combination of current regulation and hybridization with an ultra-capacitor. The associated controllers are largely model independent, excepting certain generalized assumptions that were made about the unknown and nonlinear dynamics of the SOFC´s fuel supply system (FSS). In this paper we formulate a generalized structure of the control problem and controller from our earlier work, and relax the conditions imposed on the FSS. Thereafter, we analytically determine conditions of instability. We obtain inequality conditions that impose restrictions on the controller gains and the unknown nonlinearity. The conditions are helpful in predicting the limits of performance of the controller. The results are verified through simulations and through hardware-in-the-loop tests.