Ideally, a wind turbine blade of the airfoil type is shaped similarly to the profile of an aeroplane wing, where the chord plane width of the wind turbine blade as well as the first derivative thereof increase continuously with decreasing distance from the hub.
This results in the blade ideally being comparatively wide in the vicinity of the hub. This again results in problems when having to mount the wind turbine blade to the hub, and, moreover, this causes great loads during operation of the wind turbine blade, such as storm loads, due to the large surface area of the wind turbine blade.
Therefore, over the years, construction of wind turbine blades has developed towards a shape, where the wind turbine blade consists of a root region closest to the hub, an airfoil region comprising a lift-generating profile furthest away from the hub and a transition region between the root region and the airfoil region. The airfoil region has an ideal or almost ideal profiled contour shape with respect to generating lift, whereas the root region has a substantially circular cross-section, which reduces the loads and makes it easy and safe to mount the wind turbine blade to the hub. The root region diameter may advantageously be constant along the entire root region. Due to the circular cross-section, the root region does not contribute to the energy production of the wind turbine and, in fact, lowers this a little because of drag. As it is suggested by the name, the transition region has a shape gradually changing from the circular shape of the root region to the airfoil profile of the airfoil region. Typically, the width of the wind turbine blade in the transition region increases substantially linearly with increasing distance from the hub.
When the wind turbine blade is impacted by incident airflow, the profiled contour generates a lift. When the wind turbine blade is mounted on a wind turbine, the wind turbine hub begins to rotate due to the lift. Incident flow is here defined as the inflow conditions at a profiled contour section during normal use of the wind turbine blade, i.e. rotation on a wind turbine rotor. Thus, the incoming flow is the inflow formed by the resultant of the axial wind speed and the rotational component, as it is seen by the local section of the profiled contour.
As for instance wind turbine blades for wind turbines have become increasingly bigger in the course of time and may now be more than 70 meters long, the demand for optimised aerodynamic performance has increased. The wind turbine blades are designed to have an operational lifetime of at least 20 years. Therefore, even small changes to the overall performance of the wind turbine blade may accumulate over the lifetime of a wind turbine blade to a high increase in financial gains, which surpasses the additional manufacturing costs relating to such changes.
As the requirement for effectiveness of a wind turbine is increased, there is a need for increasing the effectiveness or performance of wind turbines or wind turbine blades.
However, the increase in blade length also imposes challenges to all stages involved in the manufacturing and installment of the wind turbine blades; the moulds for manufacturing blade parts such as blade shells are becoming longer, higher and heavier, which means that the manufacturing halls have to be longer and need to have a higher clearance to the ceiling, the turning mechanisms used for assembling blade shells have to be more powerful or additional turning apparatuses have to be used; it becomes increasingly difficult to transport the blades and the logistics of transporting the blades have to be planned in detail; the installment of the blades on a wind turbine becomes increasingly difficult; and the wind turbine itself needs to be dimensioned for the higher rotor mass as well as having more powerful pitch bearings and motors to pitch the heavier blades.
US 2011/0206529 describes a spar assembly for a rotor blade of a wind turbine. The spar assembly comprises a first spar cap that is disposed adjacent to a first interior surface of the shell and a second spar cap that is disposed adjacent to a second interior surface of the shell. In one embodiment, the assembly is divided into two longitudinal sections, the inner assembly being a tubular assembly and the outer assembly being a C-shaped assembly. However, the type of spar cap does not vary in the longitudinal direction of the blade. Further, the blade shell is manufactured as a single piece.
US 2008/0124216 discloses a turbine blade assembly with a plurality of blades mounted on a central rotor. Each of the blades has a proximal section and a variable pitch section.
EP 2 378 115 discloses a blade with a configurable winglet. In one embodiment, the winglet comprises a pitch axis which is angled compared to a longitudinal axis of the blade itself so that the angle of the winglet may be varied compared to the rotor plane.
WO 03/060319 discloses a wind turbine provided with a hub extender. The hub extender comprises a first flange attached to the hub of the blade, and a second flange for attaching a rotor blade. The planes of the flanges form an acute angle so that the rotor blade when mounted to the hub extender is angled away from the tower of the wind turbine.
WO 2011/0067323 discloses a sectional blade for a wind turbine and comprising a first blade section and a second blade section. The two blade sections are attached to each other via a spar bridge in an interlocking way so that the two blade sections are fixed in relation to each other.