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Control and management of cathode airflow in a solid oxide fuel cell gas turbine hybrid power system was analyzed using the Hybrid Performance (HyPer) hardware simulation at the National Energy Technology Laboratory (NETL), U.S. Department of Energy. This work delves into previously unexplored operating practices for HyPer, via simultaneous manipulation of bypass valves and the electric load on the generator. The work is preparatory to the development of a Multi-Input, Multi-Output (MIMO) controller for HyPer. A factorial design of experiments was conducted to acquire data for 81 different combinations of the manipulated variables, which consisted of three air flow control valves and the electric load on the turbine generator. From this data the response surfaces for the cathode airflow and other operational parameters with respect to bypass valve positions or electric load were analyzed. The control of airflow through the cathode during system startup and during large load swings is of particular interest. This work presents two algorithms for controlling air mass flow through the cathode. The first was an initial trajectory based on steepest ascent method to minimize valve movement. The second and more optimal trajectory determines the trajectories taking into account safe operation with the least penalty in efficiency particularly during load variations. This was achieved by identifying step changes during a load change where the load division between the solid oxide fuel cell and gas turbine are optimal. These load changes result in changes in cathode airflow. The hot air, cold air, and bleed air bypasses are used to achieve these cathode mass flow rates. The hot air bypass is the main method of variation in cathode airflow and is varied to its full range. The cold air bypass valve however, is held as close as possible to an identified efficient location of 20% open. To maintain stability the bleed air bypass is used only when the compressor flow reaches below a determined minimum of 2.02kg/s. These criteria work to create an effective control scheme that showcases complex relationships in which load swings can be managed in the HyPer system.

Control scheme for cathode airflow during load swings in a solid oxide fuel cell/gas turbine system