Air Turbine and Primary Converter Matching in Spar-Buoy Oscillating Water Column Wave Energy Device

The oscillating water column (OWC) equipped with an air turbine is possibly the most reliable type of wave energy converter. The OWC spar-buoy is a simple concept for a floating OWC. 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 and fixed to a floater that moves essentially in heave. The air flow displaced by the motion of the OWC inner free-surface, relative to the buoy, drives an air turbine. The choice of air turbine type and size, the regulation of the turbine rotational speed and the rated power of the electrical equipment strongly affect the power performance of the device and also the equipment’s capital cost. Here, numerical procedures and results are presented for the power output from turbines of different sizes equipping a given OWC spar-buoy in a given offshore wave climate, the rotational speed being optimized for each of the sea states that, together with their frequency of occurrence, characterize the wave climate. The new biradial self-rectifying air turbine was chosen as appropriate to the relatively large amplitude of the pressure oscillations in the OWC air chamber. Since the turbine is strongly non-linear and a fully-nonlinear model of air compressibility was adopted, a time domain analysis was required. The boundary-element numerical code WAMIT was used to obtain the hydrodynamic coefficients of the buoy and OWC, whereas the non-dimensional performance curves of the turbine were obtained from model testing.