A rectified unidirectional rotary PTO for two-body wave energy converters

This work presents a new mechanism for a mechanical power take-off (PTO) system that can be used in a variety of wave energy converters (WECs). This mechanism uses two ball screws to convert the relative linear motion of the WEC bodies into unidirectional rotary motion. The resulting rotary motion finally causes a generator to rotate and generate electricity. Due to the oscillatory nature of the wave, the relative velocity of the WEC bodies will have a sinusoidal motion; therefore, the angular velocity of the ball screw becomes zero at two moments during one period of movement. The designed mechanism prevents the generator shaft from changing its direction of rotation at these moments. This mechanism also decouples the generator shaft from the ball screw when the angular velocity of the generator shaft exceeds the angular velocity of the ball screw due to its rotational moment of inertia. As the relative velocity of the bodies increases, the generator shaft is re-coupled to the ball screw and, in consequence, rotates at the angular velocity of the ball screw. This article examines the effect of using the present mechanism on increasing the electrical output power of the WEC at different wave frequencies.