Modeling and Control of Hydrogen Systems

A metal hydride (MH) hydrogen storage can be charged directly using a proton exchange membrane (PEM) water electrolyzer. The electrolysis process regulates the hydrogen pressure during the charging process. A hierarchical control approach is taken where a higher-level controller determines the desired gas rate for charging, while a lower-level controller tracks this gas generation rate. The low-level proportional-integral (PI) controller is tuned using the relay-feedback auto-tuning approach to adapt to the nonlinear and time-varying dynamics of the electrolyzer. A self-assessment algorithm determines when to activate the autotuner and gain scheduling reduces tuning frequency. This controller is validated on a PEM electrolyzer setup, showing desirable transient behavior at varying operating conditions. The high-level controller adopts the active disturbance rejection control (ADRC) paradigm. ADRC consists of a pressure profile generator, an observer that estimates unmeasurable states, and a controller that produce a suitable gas rate to track the pressure profile. The observer also predicts the state-of-charge (SOC) of the tank. Simulation results show that the ADRC provides good disturbance and noise rejection.

Further, this work develops two types of buoyancy control devices. The first system varies its buoyancy using the collection of gases from water electrolysis and the release of stored gases using solenoid valves. The second system varies buoyancy via the gas generation and consumption of reversible fuel cells (RFC), allowing for improved energy efficiency. A dynamic model is constructed for both buoyancy varying systems. The first device is controlled by a proportional-integral-derivative-acceleration (PIDA) controller with its gain identified via the pole placement method. The second device employs a proportional-derivative-acceleration (PDA) controller with gains identified via a relay-feedback auto-tuning method. Finally, the effectiveness of both controllers is confirmed with experiments.