This paper reports on the real sea performance of the buoyancy control system of Wave Dragon, a floating wave energy converter using the overtopping principle. The device operates with the full independent control system which has been tested during three years of operation. The impact of the buoyancy control system performance on the power production is noted. This provides motivation and a target for improved control algorithms.
There is very little literature published on control strategies used in the real sea on Wave Energy Converters (WECs) and results from long term testing of such. This paper will present the Wave Dragon (WD) device, explain the first generations control strategy used onboard the prototype for the last 3 years of real sea testing and present detailed results from this period. The WD is a WEC utilizing the overtopping principle. The structure consists of a floating platform with an integrated reservoir and a ramp, as illustrated in Fig. 1. Waves overtop the ramp and enter the reservoir. Here the water is temporarily stored before it is led back to the sea via hydro turbines generating power to the grid, using the head in the reservoir. The platform is equipped with two reflectors focusing the incoming waves towards the ramp, which enhance the power production capability. A 237 tonne prototype of the WD has been grid connected and undergoing sea trials in Nissum Bredning, Denmark, since Spring 2003. A thorough introduction to the prototype testing is given by Kofoed et al. (2006). The floating platform of the WD has open bottom chambers as a part of its structure. By controlling the air pressure inside these chambers, the floating level, the heel (the quasi static inclination of the platform along the centre line - wave induced oscillations are filtered out) and trim (the quasi static inclination of the platform perpendicular to the centre line) of the platform are actively controlled.