Abstract
Cross-flow tidal turbines (CFTTs) have proven advantages over horizontal axis turbines in terms of high power density per unit area, simplicity of design, and operation independence from inflow direction. However, they suffer from an unsteady flow regime which can comprise dynamic blade stall and thus problems of material fatigue or even failure. Active pitch control mechanisms on blade level have been shown to provide a potential solution, when continuously adjusting the pitching angle of each individual blade during the whole rotational cycle of the turbine.
As part of the research of the OPTIDE project, in this study, electric drive systems embedded in the blades of a CFTT flume model are proposed aiming to realize an active pitch control with high efficiency and fast response. The blade embedded actuation allows for reasonable flow conditions. For full scaled on-site applications this is required to reduce hydrodynamic losses and to protect the actuators and electronics from the harsh environmental and operation conditions. Based on the expected hydrodynamic loads from numerical flow simulations (CFD), several types of actuators are considered. The first type has brushless DC motors installed at both sides of each blade. Along with a gear box with proper reduction ratio, the actuators are able to provide required torque within expected cycle period. The second type of actuator drives the blade directly, which always results in faster pitching action, higher drive efficiency, more accurate positioning of blades, as well as simpler structure. Specifically, the shaft of each blade is designed as the primary of a limited angle torque motor, while the blades are used as the secondary with magnets inside. To mitigate the potential saturation effect on the iron of the primary, the blade can also be used as the primary, where there always exists much larger space for windings. In this case, the magnets are now located at the shaft. By doing this, it is expected to output larger torque in a wider range of pitching angle as compared with the original one, while almost the same power is required. An experimental test bench for a single blade with both types of actuators is built to verify their ability of a fast and accurate pitching control. This also lays the foundation of identifying the optimal pitching angle for the control and inhibition of dynamic blade stall at various flow conditions and blade positions within the rotational cycle of the turbines. After successful optimization and testing of the model scaled mechatronical design, the actuators will be up scaled for realistic applications.