Air turbine optimization for a bottom-standing oscillating-water-column wave energy converter

The oscillating-water-column wave energy device equipped with an air turbine is widely regarded as the simplest and most reliable, and the one that was object of the most extensive development effort. The aerodynamic performance of the air turbine plays a major role in the success of the technology. A case study was selected to investigate these issues: the existing bottom-standing plant on the shoreline of the island of Pico, in Azores Archipelago. The overall performance of the OWC plant was modelled as an integrated hydrodynamic and aerodynamic process. The hydrodynamic modelling was based on linear water wave theory. Published results from model testing of Wells and biradial turbines, together with well-known tools from dimensional analysis, were employed to determine the aerodynamic performance, and to optimize the turbine size and rotational speed. Single- and two-stage Wells turbines were considered. Unlike the biradial turbine, the performance of the Wells turbine, especially the single-stage version, was found to be severely affected by the constraints on rotor-blade tip speed (to avoid excessive centrifugal stresses and shock waves in the more energetic sea states). A power law, relating instantaneous values of the electromagnetic torque and the rotational speed, was found to apply to both types of turbines as a rotational speed control algorithm. The large runaway speed of the Wells turbine was found to be a risk to turbine integrity by excessive centrifugal stresses that requires safety measures. The much lower runaway speed of the biradial turbine is not expected to be a major problem.