Application of a negative stiffness mechanism on pitching wave energy devices

Harnessing energy from ocean waves in an economic manner remains a challenge. Recent efforts are targeted at improving the performance of a wave energy device without resorting to reactive control. One such strategy is to use a negative stiffness mechanism. Theoretically, negative stiffness is able not only to lengthen the resonance period of the device, but also to broaden its resonance bandwidth, thus making it potentially capable of capturing energy from a broader spectrum of incoming waves. This study aims to extend the application of such mechanism to pitching wave energy devices by studying Salter-duck type devices as a specific case. We consider first a single duck, and then two lines of multiple ducks meeting at an angle. The analysis is carried out using linear frequency-domain models. For this purpose, equivalent linearised stiffness of the negative stiffness mechanism is derived. Our study confirms that negative stiffness improves the power performance of pitching devices. However, it is most effective when the pitch rotation axis is fixed. It becomes less effective when the axis is allowed to move.