Abstract
Ocean wave energy conversion has attained significant research interest due to its vast untapped potential. Wave energy converters (WECs) offer promising solutions for powering blue economy (PBE) applications in remote maritime locations where grid connections are impractical or costly .For instance, supplying energy to offshore aquaculture farms presents a significant opportunity for WECs. This independence helps reduce costs, solve transmission challenges, and mitigate security risks to power infrastructure. The efficiency of WECs remains challenging due to both the variability of ocean conditions and the complexities of energy conversion. Direct drive linear generators (DDLG) as power take offs (PTOs) face challenges due to the infrequent utilization of their full stroke, heavily influenced by changing sea states. To enable a smaller, more cost-effective PTO, stroke travel must be limited in high sea states. Addressing these challenges requires a robust, efficient control framework, achievable through advanced yet adaptable control techniques.
In this study, our goal is to present a comprehensive control framework for the WEC and PTO systems to power the PBE applications. To design an effective control system for WECs, it is crucial to understand their multi-stage energy, coupled conversion processes. In the first stage, a buoy captures wave energy and converts it into mechanical motion. The second stage utilizes a DDLG PTO system, which directly converts the mechanical oscillations into electrical energy, eliminating the need for intermediate mechanical components. For these two stages, a control system (i.e., first controller) is required to synchronize the WEC's movements with the ocean waves, with the goal of maximizing power capture. The electrical power harvested from WECs exhibits significant variability and irregularity. Therefore, this raw power requires proper conditioning before it can be utilized for practical PBE applications. The final stage converts the generated electrical power using electronic converters, including AC-DC and DC-AC stages, to ensure compatibility with PBE applications. This stage will require controllers (i.e., second and third controllers) to regulate the voltage and frequency of the output power to match the specific requirements of the applications.
An example of a nonlinear point absorber WEC with DDLG PTO system and generator side converters is modeled and simulated using WEC-Sim. This study will focus on the implementation of the first controller. Given the inherent nonlinear characteristics of ocean waves and the dynamic behavior of the system, sliding mode control (SMC) for the first controller is proposed and benchmarked against the proportional Integral (PI) control. The primary objective of this controller is to maximize energy extraction by determining the optimal PTO force and monitoring the DDLG's relative travel distance to evaluate potential stroke issue. Additionally, the SMC ensures robustness against system uncertainties, which are common in real-world ocean environments. This study presents a general and flexible control framework for WECs and PTO systems, demonstrating improvements in efficiency and reliability of the whole WEC and PTO system (wave to PBE) through advanced control strategies.
Implementing these controllers enhances WEC performance, ensuring more efficient energy use for PBE applications.
The associated OREC/UMERC 2025 presentation can be here.