System frequency tuning for heaving buoy wave energy converters

In this paper, we propose a control technique that maximizes power absorption in point absorber wave energy converter systems. This technique involves the tuning of the Eigen-frequency of the converter system to meet the incident waves energy frequency, providing near-resonance operation. The system was modeled, where the effects of radiation forces due to oscillator-generated waves were considered. The radiation forces depend on the hydrodynamic coefficients, the added mass, and the hydrodynamic damping, which are calculated through solving the hydrodynamic boundary value problem for heaving bodies. The power absorbed through the power take-off system corresponds to the external damping of the system. The frequency tuning is performed by changing the effective stiffness of the system through the introduction of an external stiffness that is connected in parallel to the buoy stiffness. We propose changing the effect of this external stiffness by means of a stepless V-belt variable speed drive, which changes the speed ratio continuously. The effect of changing the drive speed ratio on the system Eigen-frequency was investigated. The effects of the power take-off system equivalent damping on both the absorbed power and the absorption efficiency for a frequency-tuned system are presented. Numerical results showed promising power absorption and efficiency values, compared with other control techniques.