Improved power take-off (PTO) controller design for wave energy converters is considered a critical component for reducing the cost of energy production. However, the device and control design process often remains sequential, with the space of possible final designs largely reduced before the controller has been considered. Control co-design, whereby the device and control design are considered concurrently, has resulted in improved designs in many industries, but remains rare in the wave energy community. In this paper we demonstrate the use of a new open-source code, WecOptTool , for control co-design of wave energy converters, with the aim to make the co-design approach more accessible and accelerate its adoption. Additionally, we highlight the importance of designing a wave energy converter to maximize electrical power, rather than mechanical power, and demonstrate the co-design process while modeling the PTO's components (i.e., drive-train and generator, and their dynamics). We also consider the design and optimization of causal fixed-structure controllers. The demonstration presented here considers the PTO design problem and finds the optimal PTO drive-train that maximizes annual electrical power production. The results show a 22.0% improvement in the optimal controller and drive-train co-design over the optimal controller for the nominal, as built, device design.

Ocean wave energy is a renewable energy which remains too costly for large-scale electricity generation. The oscillating water column (OWC) wave energy converter (WEC) is a promising device-type with a rectifying air turbine and generator which convert alternating airflow induced by the water motion into kinetic energy then into electric energy. Applying control at each stage of energy conversion could increase the electric energy output of the device. As researchers overcome the modeling challenges of OWC, such as the nonlinearities due to air compressibility and power take-off (PTO) dynamics, we can integrate specific control algorithms to test their ability to improve the efficiency of the OWC. Herein, we present a state-space model of an array of OWC WECs restricted to heave motion with nonlinear PTO dynamics. We apply second-order sliding mode control (SMC) which commands a smooth torque signal to a direct-drive generator to maintain a reference turbine angular velocity. Because the algorithm can yield high turbine torques, we investigate a simple feed-forward relation for the control of a valve to limit the turbine airflow and discard mechanical power. We find that implementing the SMC algorithm and valve control can improve electric energy conversion most effectively in less energetic sea states.

The oscillating water column (OWC) wave energy converter (WEC) together with a self-rectifying air turbine and generator which convert alternating airflow induced by the water motion into kinetic energy and then into electric energy is a promising device for the advancement of marine renewable energy. As researchers overcome the modeling challenges of OWCs, such as the nonlinearities due to air compressibility and power take-off (PTO) dynamics, we can integrate control algorithms to improve the efficiency of the OWC. Herein, we present a nonlinear model predictive controller (NMPC) that maximizes the generated electro-mechanical power while maintaining the efficiency of a self-rectifying turbine attached to an OWC WEC. To achieve this goal, a good trade-off between maximizing generated power and turbine efficiency is found by adjusting the importance of both objectives in the computation of the control signal. Furthermore, we apply the proposed NMPC to an array of three WECs that takes into account the hydrodynamic interactions between the devices in the computation of optimal generator torque control signal.