Description of the Power Take-off System on board the Wave Dragon Prototype

The paper describes the power take-off system of the overtopping based wave energy converter Wave Dragon (WD). Focus is put on the hydro turbine arrangement used for the extraction of the potential energy in the water obtained by wave overtopping of the ramp into the reservoir. INTRODUCTION WD is an offshore wave energy converter of the overtopping type, invented in the late 1980’s by M.Sc. Erik Friis-Madsen. Each unit will have a rated power of 4-10 MW depending on how energetic the wave climate is at the deployment site. As part of the development activities towards a full size production plant, a grid connected prototype of the WD is presently being tested in a Danish fjord (a scale 1:4.5 of a North Sea production plant). WD consists of three main elements (see figure 1): • Two patented wave reflectors focusing the waves towards the ramp, linked to the main structure. The wave reflectors have the verified effect of increasing the significant wave height substantially and thereby increasing energy capture by 70 % in typical wave conditions. • The main structure consisting of a patented doubly curved ramp and a water storage reservoir. • A set of low head propeller turbines for converting the hydraulic head in the reservoir into electricity. When waves have been focused by the reflectors they overtop the ramp and fill the reservoir, which is situated at a higher level than the surrounding sea. This hydraulic head is utilized for power production through the hydro turbines. WD uses the energy in the water directly via water turbines, i.e. a one-step conversion system, which yields a very simple construction and has only one kind of moving parts: the turbines. This is essential for any device operating offshore, where maintenance is difficult to perform and where the extreme forces, fouling etc. seriously affect any moving parts. Reservoir Ramp Reflectors