Nonlinear stiffness concept in wave energy conversion

Abstract

The thesis investigates the nonlinear stiffness system (NSS) concept in improving wave energy conversion for a point absorber (PA) type wave energy converter (WEC). One semi-submersible sphere is adopted together with the meshing and analytical approaches in calculating the nonlinear Froude-Krylov force. The analytical approach is improved to solve the nonlinear Froude-Krylov force at the instantaneous wetted surface in both regular and irregular wave conditions more efficiently through fitting Bessel function and exponential function with suitable polynomial series according to various wave frequencies. The static analysis of one classical NSS with mechanical compression springs (NSMc) describes three types of configurations: bi-stable, QZS (quasi-zero stiffness), and mono-stable configuration. The response in regular wave conditions indicates that with the nonlinear approach the resonance response is pushed even to a longer period range, besides broadening the response bandwidth. One procedure is established to determine the feasible region of NSMc parameters. Due to the practical consideration, the potential improvements in wave energy conversion associated with the NSMc’s advantages are dramatically weakened. Then the alternative NSS with pneumatic cylinders (NSPn) is investigated in the sequence, showing that in its feasible region, the annual energy production (AEP) does not reduce too much. For the wave climate featuring a long period in the nearshore region of Rio de Janeiro, Brazil, PA could harvest more wave energy with the NSPn, than with the latching control. Moreover, the lower peak-to-average power ratio shows that using the NSPn, the process of power absorption is smoother.