Wind turbine blades are a means of converting power from wind into mechanical power to propel a generator of a wind turbine. Wind turbine blade design is mainly conducted under consideration of aerodynamic and mechanical considerations, i.e. the airfoil shape or profile is designed as a compromise between aerodynamic optimization and mechanical characteristics, for example strength, for various wind conditions. An optimal blade of a wind turbine exhibits a low cut in wind speed, good and efficient aerodynamic performance for various wind speeds, which means for low to high wind speed, minimal loads on blade and wind turbine structure, i.e. under turbulent and high wind speed conditions, and low acoustic noise.
It is for instance important that the blade can resist high forces and stresses close to the hub. Blades are therefore thick and wide in the region of a root section close to the hub. At the root, the blade is usually narrow and tubular to fit onto the hub and to provide sufficient strength. The blade profile becomes thinner and thinner as to obtain acceptable aerodynamic properties. The lift force will increase as the speed increases towards a tip of the blade, caused by a larger diameter at the same rotational frequency toward the tip. Decreasing the chord width towards the tip will contribute to counteract this effect. The blade tapers from a point somewhere near the root towards the tip. Furthermore the blade is twisted along its axis to account for a change in direction of the airflow to the wind resulting from rotation. The speed of a blade section is increasing the further it is located towards a tip of the blade.
Nevertheless, conventional blades of wind turbines have a static airfoil and thus exhibit limited possibilities to adjust for wind conditions. The only possible means of adjustment and optimization of the aerodynamic properties for a wind speed and turbulence is the adjustment of a pitch angle for the whole blade. From an aerodynamic point of view an optimal blade of a wind turbine would not only comprise a variable pitch angle for each section of the blade but also comprise of an adjustable airfoil to account for different wind and turbulence conditions.
It is commonly known to enhance and optimize the performance of wind turbine blades with devices added onto the wind turbine blade. Such devices are among others active or passive components such as flaps, vortex generators or stall strips. The actuation of flaps can for instance be conducted with electricity, hydraulic or piezoelectric means. The bending of a blade is considered to stay in a relation to blade loading and wind conditions. A passive solution is known, where a shape and/or angle of a flap relative to a blades chord line changes depending on the bending of a wind turbine blade.
Another difficulty with wind turbine blades is the occurrence of acoustic noise when in operation. The tip speed of a blade in operation is for instance 80 m/s. One means to reduce the noise is the attachment of a serrated plate in the region of the trailing edge projecting over the trailing edge. The EP 1314885 discloses valuable information on wind turbine blades technology and performance.
Generally, it is desirable to improve the aerodynamic performance and efficiency of wind turbine blades, i.e. in low wind speed conditions. Moreover, the mechanical loading of the blade may be to be minimized. A secondary factor is the reduction of acoustic noise of the wind turbines blades when in operation.
Further relevant state of the art is disclosed in EP 1623111 B1, US 2011/0116927 A1, WO 2004/088130 A1 and EP 2034178 A2. The EP 1314885 discloses an apparatus improving the efficiency of a wind turbine with a panel connected to the trailing edge of the wind turbine blade. EP 2034178 A2 discloses a fairing plate to avoid an air gap when the flap is deflected, but which has no mechanical function.