A control strategy to mitigate fuel cell degradation effects in a solid oxide fuel cell (SOFC) gas turbine (GT) hybrid system was implemented on the Hybrid Performance facility at the National Energy Technology Laboratory. In this experiment, a cyber-physical approach was employed to emulate the SOFC component and to couple it to a physical recuperated gas turbine. An empirical degradation model simulated fuel cell performance decay over time depending on operating parameters. A combination of virtual and physical actuators was manipulated with the goal of ensuring safe fuel cell performance over time, maintaining constant voltage and minimizing thermal stresses as fuel cell power degraded. Three single-input single-output controllers were used for this purpose. In particular, a gain scheduling approach was used for voltage control to account for different degraded conditions of operation because stability could not be maintained with the initial controller gains. In addition, a bypass valve control was designed to maintain constant temperature difference across the cell, and turbine load was manipulated to keep constant speed.
This work presents the control design and implementation on the Hybrid Performance facility and illustrates the impact of fuel cell degradation on the entire system long-term performance. Controllers design was based on empirical transfer functions and stability analysis. Issues related to coupling phenomena between controlled variables are discussed. The results show the potential for an adequate control of the system to extend fuel cell operating lifetime.