This paper looks at the effect that a mono-directional Power Take Off (PTO) has on the motions and power extraction of a Wave Energy Converter (WEC). A mono-directional damper is one where the resisting force only operates in one direction. The model comprises of a cylinder that is restrained to move in heave, reacting against an internal mass (also limited to move in heave) that is connected to the cylinder by a spring and mono-directional damper in parallel.
This paper presents experimental results for a variety of different damping constants for mono-directional dampers which operate in both in expansion and compression. The response amplitude operators to monochromatic waves for both the cylinder, the internal mass and the relative motion between them are presented. A Pierson Moskowitz spectrum is used to look at the effect of an irregular sea state and the power extraction is calculated from this spectrum. The results show a greater response of the cylinder, but also in certain cases the internal mass and relative motion, when a mono-directional damper is present than when there is no damper. It is postulated by the authors that this is due to a “latching” effect. This increase in motion results in a greater than expected power extraction.