Modelling and Dynamics of a Compressible Flap-like Variable-shape Wave Energy Converter

The purpose of this thesis is the development of a system of compressible, flap-like variable Wave Energy Converters (WECs) aimed at maximizing energy capture. Traditional WECs have fixed geometries and, consequently, limited efficiency. An alternative approach is to adapt the geometry of the converters to the incoming wave conditions, based on the acting wave forces, thereby improving power production without the need for costly reactive power systems. Numerous studies have demonstrated that variable-geometry devices can significantly enhance energy absorption. This thesis project, carried out in collaboration with Mondragon Unibertsitatea (Spain) under the supervision of Dr. Markel Peñalba Retes, explores the concept of shape-changing, multibody flap-type WECs coupled with a Power Take-Off (PTO) system, in order to transform the mechanical energy of ocean waves into electrical power. The study focuses on developing both analytical and numerical models of compressible, variable-shape flap systems inspired by soft robotics, particularly by soft pneumatic network actuators (sPNAs) used in robotic grippers. Each flap, made of soft elastomeric material, deforms under wave impact, transferring air from one chamber to another through an external pipe. This airflow can then drive a hydraulic PTO system for power extraction. The computational tools used in this work are Python and MATLAB. Python, through the open-source hydrodynamic solver Capytaine, is employed to compute the hydrodynamic coefficients — namely, added mass, radiation damping, and hydrostatic stiffness matrices in the frequency domain. To apply Cummins’ equation and represent the system in the time domain, a state-space identification method is implemented. MATLAB is then used to obtain the displacement and velocity of the flaps, which serve as input for the hydraulic PTO model, allowing power generation to be evaluated. This work was initiated at the Marine Offshore Renewable Energy (MOREnergy) Lab and continued during a two-month research period at Mondragon Unibertsitatea, where the study of the complete model was carried out.