Impact of tsunamis on wave energy converters

Abstract

With an increasing emphasis on renewable energy resources, wave power technology is fast becoming a realistic solution. However, the recent tsunami in Japan was a harsh reminder of the ferocity of the ocean. It is known that tsunamis are nearly undetectable in the open ocean but as the wave approaches the shore its energy is compressed creating large destructive waves. The question posed here is whether a nearshore wave energy converter (WEC) could withstand the force of an incoming tsunami. WECs of this type are usually located close to the boundary of dominance between linear and non linear e ects. An analytical 3D model is developed within the framework of a linear theory and applied to an array of xed plates [1]. The time derivative of the velocity potential allows the hydrodynamic force to be calculated, and the hydrostatic force can be calculated from the di erence in free surface heights on either side of the device. Results show that the loading for a typical tsunami is invariant with depth and maximum loading is felt at the center of the plate. By comparison with the loading from a typical swell, it is shown that the maximum force of a tsunami on a nearshore WEC will be approximately one hundreth of the magnitude of a regular sea state. We therefore conclude that an array of WECs will withstand a tsunami. A preliminary study on the non linear e ects on nearshore WECs, in particular the e ects of a sloping sea bed and multiple waves, is attempted through a comparison between the velocities of resonant and non resonant states. If after the rst wave recedes the device is left on dry land, a second wave may act as a shock on the plate and do more damage than it would to a partially submerged device. This e ect is demonstrated using a two dimensional non linear shallow water solver,VOLNA [2]. It is believed that dangerous con gurations could be found with more detailed investigations.