Nonlinear control with wave observer to maximize harvested power for point absorber wave energy converters

This paper designs control laws that maximize the harvested power for submerged point absorber wave energy converters (PAWECs) constrained to heave motion. First, a mathematical model of a submerged PAWEC driven by nonlinear hydrodynamic, friction, restoring, and harvested power forces is derived. Second, a new disturbance observer is designed that globally practically exponentially estimate the wave velocity and its time derivatives. Next, the optimal operating points are derived such that when the PAWEC operates at these points, the harvested power is maximized. Fourth, nonlinear control laws are designed to track the optimal operating points based on the proposed disturbance observer and Lyapunov's direct method. In addition to maximize the harvested power, these optimal control laws ensure that the harvested power is always nonnegative; i.e., no power is drawn from the network to the PAWEC at any time. For comparison, a control law that maximizes the hydrodynamic extraction power is also designed. Several simulations are carried out and illustrate that the maximizing harvested power control yields better results than others.