Pressurized SOFC Hybrid Systems: Control System Study and Experimental Verification

Abstract

In this paper two different advanced control approaches for a pressurized SOFC hybrid system are investigated and compared against traditional proportional–integral–derivative (PID). Both advanced control methods use model predictive control (MPC): in the first case, the MPC has direct access to the plant manipulated variables, in the second case the MPC operates on the setpoints of PIDs which control the plant. In the second approach the idea is to use MPC at the highest level of the plant control system to optimize the performance of bottoming PIDs, retaining system stability and operator confidence. Two MIMO (multi-input multi-output) controllers were obtained: fuel cell power and cathode inlet temperature are the controlled variables; fuel cell by-pass flow, current and fuel mass flow rate (the utilization factor kept constant) are the manipulated variables. The two advanced control methods were tested and compared against the conventional PID approach using a SOFC hybrid system model. Then, the MPC controller was implemented in the hybrid system emulator test rig developed by the Thermochemical Power Group (TPG) at the University of Genoa. Experimental tests were carried out to compare MPC against classic PID method: load following tests were carried out. Ramping the fuel cell load from 100% to 80% and back, keeping constant the target of the cathode inlet temperature, the MPC controller was able to reduce the mismatch between the actual and the target values of the cathode inlet temperature from 7 K maximum of the PID controller to 3 K maximum, showing more stable behavior in general.

Solid oxide fuel cell hybrid system: A detailed review of an environmentally clean and efficient source of energy

“…The system was analysed using computational models. Therefore, it is very different from the SOFC/mGT emulator including a real gas turbine as reported in previous studies [32,[43][44][45][46][47]. The differences and the limitations in emulation activities in the model are that only the mGT can operate at partial load [49].…”

“…At the University of Genoa, Italy a cathode-anode emulator test rig based on microturbine technology (a T100 machine) was developed. The cathode and anode sides of this emulator is based on vessels (without ceramic materials) designed considering a similitude approach with the stack by Rolls-Royce Fuel Cell Systems [43][44][45]. In this layout, the recuperator and the combustor have been kept inside the turbine cabinet, and the pipes related to the simulation of the hybrid volume have been connected to the original turbine.…”

“…Thus, to gain an acceptable emulation of a real plant, part of the anodic loop was included into the cathodic volume to partially heat the anodic flow [43,28]. Also, the TPG's facility can be operated in cyber-physical mode for tests in conjunction with a real-time model [44,45].…”

“…However, validated solutions for some research issues (cost decrease, SOFC/mGT matching problem and control system) have since been developed [28,47,52]. Hence, an increase of studies and funding resources on SOFC hybrid systems would be essential to provide realistic solutions for the problems [28,44,45,47,60,76,80]. A promising positive trend concerning these systems is shown by…”

Verification of the control system performance using viability theory

“…Safety of the system is a major issue that is investigated using reachability analysis [2]. There is also a rich literature in experimental verification of control systems [3], [4], [5].…”