Model-based selection of full-scale Wells turbines for ocean wave energy conversion and prediction of their aerodynamic and acoustic performances

Abstract

One of the most intensively studied principles of harnessing the energy from ocean waves is the oscillating water column device. The oscillating water column converts the motion of the water waves into a bi-directional airflow, which in turn drives an air turbine. The bi-directional axial Wells turbine as a candidate for oscillating water column power take-off systems was the object of considerable research conducted in the last decades. However, there is a lack of consistent data to support practical design considerations when pre-selecting a turbine for an oscillating water column power plant. Furthermore, to minimize the overall environmental impact of this technology requires the assessment of the aero-acoustic noise associated with a Wells turbine’s operation. The effect of cascade solidity and hub-to-tip ratio on the aero-acoustic performance of Wells turbine rotors is assessed systematically by numerical simulations and model scale testing. Based on the data from the study on these generic design parameters, new Wells turbine design charts were developed. For a given plant site, main machine dimensions are identified and a time domain model is used to predict the annual energy output and the annual equivalent sound power level. Maximum values of total-static peak efficiency and low sound emission can be achieved by selecting moderate values of cascade solidity and hub-to-tip ratio. On the other hand, high hub-to-tip ratio rotors in combination with high solidity are recommended for a maximum range of operation without stall, as the pressure head is varied. The designer of an oscillating water column system usually specifies selected operating conditions on dimensional turbine characteristics. Combinations of suitable rotor diameter and rotational speeds are selected using the design chart approach. When a fully operating, full-scale Wells turbine at fixed speed is assessed, higher cascade solidity maximizes the annual energy output at minimum acoustic emission. The drawback is a slightly larger rotor diameter when, e.g. compared with a lower cascade solidity.