Nonlinear stiffness mechanism for high-efficiency and broadband raft-type wave energy converters

In this study, a simple and effective nonlinear stiffness mechanism (NSM) is proposed to enhance the capture efficiency and broaden the bandwidth of a raft-type wave energy converter (WEC). A prototype of a simple and compact NSM is first proposed in which a simple elastic element is arranged between hinged floating rafts. The governing equations of motion for the nonlinear WEC are then established based on linear wave theory and the Cummins equation. The harmonic balance method is employed to solve the set of nonlinear governing equations. A numerical simulation for a two-raft WEC is performed to demonstrate the effectiveness of the robust per-formance improvement. The numerical results show that the NSM can enhance the energy capture efficiency and broaden the low-frequency bandwidth effectively when the structural parameters are adjusted appropriately. The passive phase control mechanism of the NSM is revealed using the Lissajous figure and the potential energy diagram