Gyroscopic power take-off wave energy point absorber in irregular sea states

Abstract

The Gyroscopic power take-off (GyroPTO) wave energy point absorber has the operational principle somewhat similar to the so-called gyroscopic hand wrist exerciser. Inside the float of the GyroPTO, there is a mechanical system made up of a spinning flywheel with its spin axis in rolling contact to a ring. At certain conditions, the ring starts to rotate at a frequency equal to the peak angular frequency of the wave excitation. In this synchronized state, the flywheel is running at almost constant speed, so the generated power from the generator becomes constant as well. In this paper, the performance of GyroPTO in irregular sea waves is investigated. To improve the stability (synchronization) margin of the device, a magnetic coupling mechanism has been added between the spin axis and the flywheel, which also makes the semi-active control of the device possible. Theoretical modeling of the GyroPTO is carried out using analytical rigid body dynamics, and a 4-DOF nonlinear model is established. Further, linear wave theory has been applied to calculate the hydrodynamic moments acting on the float. Rational approximation is performed on the frequency response function of the radiation damping moments, leading to an extended state vector formulation of the coupled structure- wave system. Simulation results show that magnetic coupling successfully improves the stability of the flywheel in irregular sea states. With given significant wave height and peak frequency, it is shown that the synchronization of the device is more easily obtained in narrow-banded wave than in broad-banded wave. As a result, larger values of generator gain can be chosen in narrow-banded waves, leading to larger power output. Making use of both the generator and the magnetic coupling, semi-active control algorithm might further improve the performance of the GyroPTO in real sea.