This paper reports a numerical study to optimize the buoy of a wave energy converter which uses the heave motion to absorb wave energy. To proceed to the buoy evaluation the equation of motion in the frequency domain is expressed as a function of the complex amplitude of the displacement, which can be determined from the amplitude of the excitation force and the hydrodynamic coefficients of added mass and damping. From the stroke and the PTO characterisation the mean power absorption by the device is computed as well as the capture width. In the study it was assumed a PTO connected to the ground which can be described through two terms: one is proportional to the strock and the other proportional to the velocity. If the stroke is lower than a prescribed maximum, the (optimal) mechanical spring and damping coefficients are computed from maximum power absorption. If larger, the mechanical spring and damping coefficients are set to limit the stroke to the prescribed maximum value.
The methodology developed to optimize the buoy consists in the definition of a cylinder with the adequate length to tune the resonance at the desired frequency. Then, the specified cylinder is transformed into two buoy components, keeping the same hydrostatic coefficient as the original cylinder. The upper component (the “surface buoy”), which crosses the free surface, may have a high hydrodynamic damping. Its volume, as low as possible, is set up according to the stroke required. The other component (the “submerged mass”) carries the remaining mass (cylinder mass minus the surface buoy mass) and is placed deeper in the water, at a depth dependent on its volume, to avoid the interference with the hydrodynamic damping of the surface buoy. A non-dimensional analysis has been performed allowing the establishment of relations between the maximum power increment (as the cylinder is progressively transformed into a more suitable shape) and the fraction of the cylinder volume reduction. It was also possible to relate the buoy centre of gravity with the absorbed power reduction if the submerged mass position is not deep enough to avoid any interference with the surface buoy.