TECHNICAL FIELD

The invention relates to a wind turbine comprising a rotor with at least one blade.

BACKGROUND

There is a general desire for wind turbine rotors to produce as much energy as possible for a given wind speed. However, the inner part of a wind turbine rotor blade has to meet requirements related to structure, manufacturing and transport, which requirements counteract optimal lift requirements. The structural requirements lead to the airfoil section being increasingly thicker towards a root section of the blade.

SUMMARY

The object of the invention is to increase the power output from a wind turbine.

This object is reached with a wind turbine of the type mentioned initially, wherein a flap is provided so as to at least partly extend along at least a part of a threshold thickness section of the blade in which the airfoil thickness is at least 36%, the flap being arranged so that a slot is provided between the threshold thickness section and the flap, the chord of the flap being at least 15% of the chord of the threshold thickness section.

Such a threshold thickness section would normally be provided in the inner part of the blade, in which the airfoil thickness is larger that at sections closer to the blade tip. The flap could be provided so as to extend at least partly along at least a part of a trailing edge of the threshold thickness section, the flap being arranged so that the slot is provided between the trailing edge and the flap. As seen in a lateral cross-section of the blade, the flap could be arranged entirely downstream of the threshold thickness section or partly or entirely overlapping the threshold thickness section. A flap at a trailing edge of a main wing portion, arranged so that a slot is provided between the main wing portion and the flap, is often referred to as a slotted flap. Slotted flaps are known from aircraft applications, and have also been suggested for wind turbine blades, as in the patent documents WO92/01865, DE2908761 or DE4132453. Such traditional use of slotted flaps take advantage of the fact that the slot provides an airflow from the pressure to the suction side of the wing or blade, thereby preventing flow separation on the suction side, thereby increasing the maximum lift coefficient.

Here the airfoil thickness is defined as a ratio between the absolute thickness and the airfoil chord length. The inventors have found that at airfoils with a thickness of 36% and at thicker airfoils, at low angle of attacks, especially around zero degrees, a flow separation takes place on the pressure side of the airfoil. In addition, the inventors have found that a flap arranged downstream or partly downstream of the airfoil with a slot between the airfoil and the flap, wherein the chord of the flap being at least 15% of the chord of the threshold thickness section, will prevent such flow separation on the pressure side of the airfoil. Thereby, there will be a significant increase in lift of the blade. (The flap and threshold thickness section chords relate to their respective chords in a common cross-section perpendicular a longitudinal direction of the blade.)

The flap can either be incorporated in the initial design of the blade or added to an existing blade to increase the maximum lift. In addition to the flap preventing flow separation on the pressure side, the slot allows air from the pressure side to flow up through the slot and delay separation on the suction side of the section. The delayed separation allows higher maximum lift to be achieved. In the case of adding a flap to an existing blade there is also an increase in area/chord which contributes to the increase of lift. By providing the flap according to the invention, the blade root can be designed to better comply with the requirements of structure, manufacturing and transport. For example, a lower twist, a smaller chord and a higher thickness to chord ratio compared to a traditional root can be accomplished while not compromising lift performance.

Preferably, where the airfoil thickness of the threshold thickness section is between 36% and 45%, the chord of the flap is not more than 60% of the chord of the threshold thickness section. Preferably, where the airfoil thickness of the threshold thickness section is between 45% and 65%, the chord of the flap is 20%-100% of the chord of the threshold thickness section. Preferably, where the airfoil thickness of the threshold thickness section is between 65% and 85%, the chord of the flap is 25%- 150% of the chord of the threshold thickness section.

Preferably, the thickness of the flap is between 10% and 25% in relation to its chord. Preferably, the camber of the flap is between 0% and 25%.

Preferably, where the airfoil thickness of the threshold thickness section is between 36% and 45%, the flap leading edge is located on, or on an extension of, the threshold thickness section chord line, or at a distance from the threshold thickness section chord line such that an imaginary line which is parallel to the threshold thickness section chord line and intersects the flap leading edge is at a distance from the threshold thickness section chord line, towards the suction side of the threshold thickness section, which is no larger than 10%, preferably no larger than 5%, of the threshold thickness section chord, or at a distance from the threshold thickness section chord line, towards the pressure side of the threshold thickness section, which is no larger than 30%, preferably no larger than 10%, of the threshold thickness section chord.

Preferably, where the airfoil thickness of the threshold thickness section is between 45% and 85%, the flap leading edge is located on, or on an extension of, the threshold thickness section chord line, or at a distance from the threshold thickness section chord line such that an imaginary line which is parallel to the threshold thickness section chord line and intersects the flap leading edge is at a distance from the threshold thickness section chord line, towards the suction side of the threshold thickness section, which is no larger than 10%, preferably no larger than 5%, of the threshold thickness section chord, or at a distance from the threshold thickness section chord line, towards the pressure side of the threshold thickness section, which is no larger than 50%, preferably no larger than 10%, of the threshold thickness section chord. Preferably, where the airfoil thickness of the threshold thickness section is between 36% and 45%, the flap leading edge is located on an imaginary line which is perpendicular to the threshold thickness section chord line and intersects an intersection between the threshold thickness section chord line and the threshold thickness section trailing edge, or located at a distance from said imaginary line, which distance is not larger than 20%, preferably no larger than 10%, of the threshold thickness section chord. It should be noted that in some embodiments the distance between the flap leading edge and the threshold thickness section leading edge could be shorter that the distance between the leading and trailing edges of the threshold thickness section, e.g. if the flap leading edge is locate on the same side of the threshold thickness section chord line as the pressure side of the threshold thickness section.

Preferably, where the airfoil thickness of the threshold thickness section is between 45% and 85%, the flap leading edge is located on an imaginary line which is perpendicular to the threshold thickness section chord line and intersects an intersection between the threshold thickness section chord line and the threshold thickness section trailing edge, or located at a distance from said imaginary line, which distance is not larger than 30%, preferably no larger than 10%, of the threshold thickness section chord.

Preferably, where the airfoil thickness of the threshold thickness section is between 36% and 45%, the flap chord line is parallel to the threshold thickness section chord line, the flap is angled towards the pressure side of the threshold thickness section so that the angle between the flap chord line and the threshold thickness section chord line no larger than 45 degrees, preferably no larger than 25 degrees, or the flap is angled towards the suction side of the threshold thickness section so that the angle between the flap chord line and the threshold thickness section chord line no larger than 30 degrees, preferably no larger than 10 degrees.

Preferably, where the airfoil thickness of the threshold thickness section is between 45% and 85%, the flap chord line is parallel to the threshold thickness section chord line, the flap is angled towards the pressure side of the threshold thickness section so that the angle between the flap chord line and the threshold thickness section chord line no larger than 45 degrees, preferably no larger than 25 degrees, or the flap is angled towards the suction side of the threshold thickness section so that the angle between the flap chord line and the threshold thickness section chord line no larger than 20 degrees, preferably no larger than 10 degrees.

Preferably, vortex generating means, which could be in the form of a plurality of vortex generators, are provided on the suction side of the flap. Preferably, the extension of the vortex generating means normal to the surface of the suction side of the flap is between 0.25% and 1% of the flap chord. Preferably, the vortex generating means are located at distances from the flap leading edge corresponding to 20%-40% of the flap chord. This will improve the flow attachment at the flap, which will further improve the lift of the blade(s).

Preferably, the flap extends along at least 50% of the threshold thickness section.

Preferably, the threshold thickness section presents a camber between 0% and 20% of the threshold thickness section chord.

Preferably, where the airfoil thickness of the threshold thickness section is between 36% and 45%, the trailing edge thickness of the threshold thickness section is not more than 15% of the threshold thickness section chord, where the airfoil thickness of the threshold thickness section is between 45% and 65%, the trailing edge thickness of the threshold thickness section is not more than 30% of the threshold thickness section chord, and where the airfoil thickness of the threshold thickness section is between 65% and 85%, the trailing edge thickness of the threshold thickness section is not more than 45% of the threshold thickness section chord.

The threshold thickness section can optionally present vortex generating means, which could be in the form of a plurality of vortex generators, provided on the suction side of the threshold thickness section. The extension of the vortex generating means normal to the surface of the suction side of the threshold thickness section can, where the airfoil thickness of the threshold thickness section is between 36% and 65%, be between 0.5% and 2% of the threshold thickness section chord, and where the airfoil thickness of the threshold thickness section is between 65% and 85%, be between 1% and 3% of the threshold thickness section chord. The vortex generating means can, where the airfoil thickness of the threshold thickness section is between 36% and 65%, be located at distances from the threshold thickness section leading edge corresponding to 10%-20% of the threshold thickness section chord. The vortex generating means can, where the airfoil thickness of the threshold thickness section is between 65% and 85%, be located at distances from the threshold thickness section leading edge corresponding to 5%-15% of the threshold thickness section chord.

Further embodiments of the invention include the flap being arranged to be detached from the blade for transport.

The objects above are also reached with a wind turbine comprising a rotor with at least one blade, characterized in that vortex generating means are provided on the suction side of a flap arranged to at least partly extend downstream of at least a part of the blade, the flap being arranged so that a slot is provided between the threshold thickness section and the flap.

DESCRPTION OF THE FIGURES

Below, embodiments of the invention will be described with reference to the drawings, in which

- fig. 1 is a schematic plan view of a wind turbine blade with a flap according to one embodiment of the invention,

- fig. 2 is a schematic cross-section oriented as indicated by the line H-II in fig. 1,

- fig. 3 shows a detail of fig. 2,

- fig. 4 shows a view similar to the one shown in fig. 4 in an alternative embodiment of the invention, fig. 5 — fig. 7 show respective views of a vortex generator provided on the blade or on the flap, in fig. 1 - fig. 4, and fig. 8 - fig. 15 show results of CFD simulations of a main airfoil, with and without a slotted flap.

DETAILED DESCRIPTION Fig. 1 is a schematic plan view of a wind turbine blade 1. The airfoil of the blade thickens towards the root, and within a section indicated by a double arrow A, the airfoil thickness is 36% or more, and this section is herein referred to as a threshold thickness section. A flap 2 is arranged to extend downstream the blade 1 , and is fixed to the blade with a suitable arrangement 3 schematically indicated in fig. 1. A part of the flap 2 extends downstream of a part of the threshold thickness section A. Another part of the flap 2 extends downstream of a section of the blade 1 being further from the blade root compared to the threshold thickness section A. The flap is arranged so that a slot is provided between the blade 1 and the flap 2.

Referring to fig. 2, the chord line cA of the threshold thickness section A extends from its leading edge Al to the middle of its trailing edge A2, which in this example forms a straight line in the cross-section. The threshold thickness section thickness is defined as the absolute thickness tA divided by the threshold thickness section chord line length LcA, herein also referred to simply as the threshold thickness section chord. The threshold thickness section presents a pressure side A3 and a suction side A4.

The flap 2 is located in relation to the threshold thickness section A so that the flap leading edge 21 is located on an extension of the threshold thickness section chord line cA, and downstream of the threshold thickness section trailing edge, i.e. at a distance H2 from an imaginary line which is perpendicular to the threshold thickness section chord line cA and intersects an intersection between the threshold thickness section chord line cA and the threshold thickness section trailing edge A2.

Fig. 3 indicates by a double arrow t2 the absolute thickness of the flap 2, and the flap chord with a double arrow Lc2, i.e. the length of the flap chord line c2 extending from the flap leading edge 21 to the flap trailing edge 22. The flap thickness is defined as the absolute thickness t2 divided by the flap chord Lc2.

The flap is angled towards the pressure side A3 of the threshold thickness section A so that an angle aA2 between the flap chord line c2 and the threshold thickness section chord line cA is formed. As can be seen in fig. 2, the threshold thickness section A presents a concave curvature A5 of the pressure side A3 extending from the trailing edge A2. In general, preferable, such a concave curvature A5 extends from the trailing edge A2, and its extension is 5-15%, preferably about 10%, of the threshold thickness section chord LcA.

Fig. 4 shows an alternative embodiment differing from the embodiment shown in fig. 2 and fig. 3 in that the flap leading edge 21 is located at a distance from the threshold thickness section chord line cA such that an imaginary line which is parallel to the threshold thickness section chord line cA and intersects the flap leading edge 21 is at a distance V2 from the threshold thickness section chord line cA, towards the pressure side A3 of the threshold thickness section A.

As can be seen in fig. 1 - fig. 4, vortex generating means 11, 12 in the form of vortex generators are provided on the blade 1 as well as on the flap 2. The vortex generators are distributed in the spanwise direction. Each vortex generator 11, 12 could be provided as illustrated in fig. 5 - fig. 7, described in the international application PCT/DK2005/000324 incorporated herein by reference. Fig. 5 shows a vortex generator as seen from an upwind position, fig. 6 shows it in a side view, and fig. 7 shows it in a top view, the flow direction being indicated with arrows. The extension of the flap vortex generators 12 normal to the surface of the suction side of the flap is indicated with a double arrow H in fig. 6.

Fig. 8 and fig. 9 illustrate an important advantage of the invention. Fig. 8 and fig. 9 show results of CFD (Computational Fluid Dynamics) simulations of a main airfoil at an angle of attack of zero degrees, with and without a slotted flap with a chord being 25% of the main airfoil chord. The main airfoil has a thickness of 36%, and the inventors have found that a flow separation takes place on the pressure side of the airfoil, as can be seen in fig. 8. Such a separation does not take place at thinner airfoils at this angle of attack. The inventors have also found that a slotted can remove or reduce this pressure side separation, as can be seen in fig. 9. This is reflected in the dramatic difference of the lift coefficient (cl), i.e. the lift coefficient is 0.2 in the simulation in fig. 8 and 1.2 in the simulation in fig. 9. Fig. 10 and fig. 11 illustrate results of CFD simulations of a main airfoil at an angle of attack of zero degrees, with and without a slotted flap with a chord being 67% of the main airfoil chord. The flap in fig. 11 is angled downward 10 degrees in relation to the main airfoil chord line, and also located below the main airfoil chord line. The main airfoil has a thickness of 62%, and without the flap a pronounced flow separation can be observed on the pressure side. As can be seen in fig. 11, this separation is completely avoided in the presence of the flap. This is reflected in the dramatic increase of the lift coefficient for the main airfoil alone from 0.35 in fig. 10

Fig. 12 - fig. 15 illustrate results of CFD simulations of the main airfoil in fig. 10 and fig. 11 at an angle of attack of 10 degrees. In fig. 13 - fig. 15, the flap is angled downward 10 degrees in relation to the main airfoil chord line. In fig. 13 the leading edge of the flap is located on the extension of the main airfoil chord line, while in fig. 14 and fig. 15, the flap is located at respective distances below the main airfoil chord line, said distance in fig. 15 being larger than said distance in fig. 14. As can be seen, the main airfoil pressure side flow separation is avoided in the presence of the flap. In the simulation in fig. 12 the lift coefficient is -0.2, in fig. 13, 14 and 15 the lift coefficient of the main airfoil alone is 1.0, 1.1 and 0.9, respectively.