Economic plantwide control of a hybrid solid oxide fuel cell - gas turbine system

This study presents a novel approach for hybrid solid oxide fuel cell (SOFC) - gas turbine (GT) system control. The approach is based on a plantwide top-down strategy which leads to a proper selection of the control structure. As a result, while maintaining the system stability and dynamic performance during transients, the system operation is also kept at optimal conditions using simple PI controllers. The optimization cost function is defined to be the unit price of the net produced electricity. The system inlet fuel flow and the power drawn from the SOFC are considered as the main system disturbances. The two-dimensional disturbance space is discretized. Rigorous optimization problems are performed in each disturbance node and variation of system efficiency and constraint values are evaluated. Three different operating regions, with different sets of active constraints, are characterized. Equivalent to the identified operating regions, an efficiency map is also provided which illustrates the achievable system efficiencies in presence of disturbances. Based on the optimization outcomes, the identification of appropriate controlled variables is accomplished in each region. It revealed that to keep near-optimal operation under various operating conditions; the combustion chamber outlet temperature should be treated as a key controlled variable. Finally, dynamic simulations are performed and the proficiency of the proposed control structure is justified. According to the results, maximum SOFC temperature transients in all regions would be less than 2 K.min(-1) while variations on the hybrid system output power are kept below 3 % and efficiency loss is restricted to 2 %.