The present paper concerns an OWC spar-buoy, possibly the simplest concept for a floating oscillating-water-column (OWC) wave energy converter. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends, fixed to a floater that moves essentially in heave. The length of the tube determines the resonance frequency of the inner water column. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a turbine that drives an electrical generator. It is well known that the frequency response of point absorbers like the spar buoy is relatively narrow, which implies that their performance in irregular waves is relatively poor. Phase control has been proposed to improve this situation. The present paper presents a theoretical investigation of phase control by latching of an OWC spar-buoy in which the compressibility of air in the chamber plays an important role (the latching is performed by fast closing and opening an air valve in series with the turbine). In particular such compressibility may remove the constraint of latching threshold having to coincide with an instant of zero relative velocity between the two bodies (in the case under consideration, between the floater and the OWC). The modelling is performed in the time domain for a given device geometry, and includes the numerical optimization of the air turbine rotational speed, chamber volume and latching parameters. Results are obtained for regular waves.
Experimental and numerical comparisons of a dual-flap floating oscillating surge wave energy converter in regular waves