Different Model Predictive Control Approaches for Controlling Point Absorber Wave Energy Converters

Ocean wave energy is a promising renewable energy source that can be converted into useful electrical energy using wave energy converters (WECs). In order for WECs to become a commercially viable alternative to established methods of energy generation, operating the WEC in an optimal fashion is a key task. Thus, model predictive control (MPC) is an encouraging control method that exploits the entire power potential of a WEC on the one hand, while respecting the constraints on motions and forces on the other hand. This work focuses on one subclass of WECs, namely, buoy type point absorbers with a linear generator as the power take-off (PTO) system and presents different MPCs whose goals are to optimize the generator force. For a linear one-body model of the point absorber it is shown that optimizing the power generation is possible without using an optimal velocity reference trajectory. Therefore, the control of a linear two-body model with a similar approach can be demonstrated, even though no adequate reference trajectory can be calculated in this case. Moreover, possibilities to deal with the occurance of infeasibility problems are introduced and implemented. Due to possible nonlinear effects, such as the mooring forces, a nonlinear model predictive controller is proposed, whose performance is compared to a linear MPC, also controlling the nonlinear system. The proposed controllers are validated and compared through simulation for regular and irregular sea states.