Hydromechanical simulations of wave energy conversion : Linear aspects

Sammanfattning: Hydrodynamic simulations of buoy movement is crucial when designing a point absorbing wave power plant. The wave energy converter (WEC) that is developed in the Lysekil project by Uppsala University is studied in this thesis. The buoy motion in response to an incoming wave can be simulated using potential linear wave theory by deriving a transfer function from the Fourier transformed equation of motion and solve it. The buoy response is found from the convolution between the incoming wave and the transfer function in time domain. It have been found that this approach shows good agreement with experimental results for normal operating conditions.The hydromechanical parameters exciting force, added mass and radiation damping have been simulated in WAMIT, using BIEM, and in COMSOL, using FEM. This was done to compare the simulation model built in COMSOL with the commercial software WAMIT. The results were comparable. The hydromechanical parameters were used to calculate buoy movement and line force in response to a 30 minutes sea state measured at the Lysekil research site. The simulated buoy movement and line force was compared to experimental results, and showed good agreement.A WEC is a complicated dynamical system. The buoy motion, and thereby the translator velocity, depends on the buoy geometry and dimensions, the mass of the moving parts of the system and on the damping force from the generator. The damping force from the generator is dependent on the translator velocity. The above mentioned approach for motion simulation can be used if the damping force from the generator is linear and described by the velocity multiplied with a constant damping coefficient. In this thesis have it been studied how the buoy draft and radius of cylinder buoys are influencing the performance of a WEC, assuming a constant generator damping coefficient. The results were compared to the experimental power from the WECs L2 and L3, two identical generators with cylinder buoys of different dimensions. It was concluded that the experimental difference in power absorption between L2 and L3 could be derived from the difference in draft and buoy mass, rather than buoy radius. The largest part of the difference have however been concluded to derive from the incorrect line lengths.