We study the power output contributed from the heave and pitch modes of a wave energy converter in regular and irregular waves. The device is a vertical truncated circular cylinder allowed to oscillate in the vertical plane in finite depth water. Our results focus on the share of the pitch power from the total power. The results also highlight the effects of some parameters such as the buoy radius, height and floatation level on the power output. The results in regular waves indicate that the contribution from the pitch mode is highest at high frequencies and radii. The power per unit mass in irregular waves is found to admit a maximum at a fixed buoy height-to-radius ratio.
It is a well known fact (Falnes, 2002) that the theoretical maximum power that can be absorbed from an axisymmetric body oscillating in heave or pitch only is limited by the energy transport per unit frontage of the incident wave divided by the wave number. This maximum becomes three times higher when the modes: surge, heave and pitch are allowed. It might seem advantageous to employ all three modes simultaneously in a wave energy converter. This does not have to be the case as the power extracted is mostly less than the theoretical maximum unless some control strategy is considered (Bjarte-Larsson & Falnes, 2006). In general, it is difficult to implement a controller (Falcão, 2008) and in its absence, it is difficult to tell for a generic body whether allowing more degrees of freedom will help. It is the purpose of this work to show how much more power is actually extracted by employing the added modes to a vertical truncated circular cylindrical body in shallow water. The body considered is connected to the ocean bottom using a loose mooring cable and power is extracted from the heave and pitch modes using two power takeoff mechanisms (PTOs).
