Design and model testing of an optimized ducted marine current turbine

Abstract

As a part of the MegaWattForce Project, the Guinard Energies Company studies a new current turbine design with a ducted rotor. The project has tended toward couples of water turbines fixed on a rotating arm free to rotate about a vertical axis fixed on the sea-floor. The couple of generators can orientate themselves in the current direction, to capture the maximum of current power (the water turbine can be asymmetric and, thus, fully optimized). Another specificity of this project consists in putting the rotor of the water turbine inside an asymmetric duct to increase the velocity of the flow in the turbine.The validation of the project requires analysis, calculation and experimentation. A first stage was to study the different concepts of ducts and to set the main dimensions (diameter, length, convergent, etc.) while characterizing their influence on the water turbine output. Concerning this part, Reynolds Average Navier–Stokes calculations enable to know the flow rates in the duct. This study has showed that the flow acceleration factor, inside the duct, could reach a value of 1.4 the current velocity. Finally, two duct geometries have been chosen to be tested at model scale.On a second stage, DGA Hydrodynamics designed several rotor geometries using again the RANS calculations. A specific test set-up has been developed to simulate the turbine at model scale in the large towing tank of DGA Hydrodynamics. From a complete set of experimental data recorded, numerical simulation calculations have been validated and some issues on boundary layer separation of the internal flow downstream the rotor plane have been raised. A second RANS calculation along with new experimental tests led to an optimum design. In this second campaign of experiments, a new rotor with fixed pitch but different sets of pitches (a rotor with swiveling blades) was tested in order to assess the optimal pitch.The final geometry of the rotor have shown great results during towing tests with a power coefficient up to 0.75 on a large operating range.