DK201570468A1 - Wind Turbine Leading Edge Slat - Google Patents

Abstract

A wind turbine blade extending from a root end to a tip end and comprising a pressure surface and a suction surface; the blade comprising: a blade body a leading edge slat mounted at a leading edge of the blade body; wherein a trailing edge of the leading edge slat is serrated.

Description

Wind Turbine Leading Edge Slat

The present invention relates to a wind turbine with a leading edge slat, and in particular to a leading edge slat for reducing noise emission from the wind turbine blade.

Wind turbine blades for a horizontal axis wind turbine extend from a root end to a tip end. The root end of the blade is connected to a hub rotatably mounted to a nacelle. Wind turbine blades are a source of noise generation. Near the root of the blade, separated airflow may lead to noise emission from the wind turbine blade.

The present invention aims to reduce the noise emitted by wind turbine blades, particularly at near to the root of the blade.

According to a first aspect of the present invention there is provided a wind turbine blade extending from a root end to a tip end and comprising a pressure surface and a suction surface; the blade comprising: a blade body a leading edge slat mounted at a leading edge of the blade body; wherein a trailing edge of the leading edge slat is serrated.

By providing a leading edge slat on a wind turbine blade, noise from the blade can be reduced. This is because the leading edge slat will reduce separation from the suction surface of the blade which can contribute to noise. The trailing edge of the leading edge slat is serrated in order to prevent the leading edge slat from being a further source of noise.

The blade has a span length extending from the root end to the tip end, and the leading edge slat may be positioned between the root end and 30% of the span length. Preferably, the leading edge slat is positioned between the 5% and 20% of the span length. This includes a transition portion of the blade inboard of a position of maximum chord. The transition portion of the blade may be susceptible to flow separation from the suction surface of the blade so the leading edge slat is preferably located in this location.

Preferably, the leading edge slat has a chord length extending between a leading edge and the trailing edge of the leading edge slat, and the chord length of the leading edge slat is between 10% and 20% of a local chord length of the blade body.

The trailing edge of the leading edge slat may have a sawtooth, sinusoidal or rectangular serration shape.

A wind turbine having a wind turbine blade as described above may also be provided.

According to a second aspect of the present invention there is provided a method of reducing noise emissions from a wind turbine blade, comprising: providing a blade body extending from a root end to a tip end and comprising a pressure surface and a suction surface; providing a leading edge slat mounted at a leading edge of the blade body; and applying serrations to a trailing edge of the leading edge slat.

Examples of the present invention will now be described with reference to the accompanying Figures in which:

Figure 1 is a view of a horizontal axis wind turbine.

Figure 2 is a perspective view of a wind turbine blade.

Figure 3 is a plan view of a wind turbine blade according to the present invention.

Figure 4 is a cross section through the line IV-IV in Figure 3.

Figure 5 is partial perspective view of the wind turbine blade according to the invention.

Figure 1 shows a typical horizontal axis wind turbine 1. The turbine comprises a tower which supports a nacelle 12. The wind turbine 1 comprises a rotor made up of three blades 10 each having a root end 16 mounted on a hub 13. Each blade 10 comprises a leading edge 14, a trailing edge 15, and a tip 17. As is well known in the art, each blade 10 can pitch about its own pitch axis which extends longitudinally along the span of the blade, and the nacelle 12 (together with the rotor) can yaw about a vertical axis aligned with the tower.

Referring to Figure 2, this shows one of the wind turbine blades 10 in more detail. The blade 10 has a blade body 11 and it can be seen that the root end 16 of the blade 10 is generally circular. Moving in a spanwise direction S from the root end 16 towards the tip end 17 of the blade 10, it can be seen that the width (i.e. chord) of the blade 10 rapidly increases up to a maximum width (i.e. maximum chord, as indicated by the line CMax in Figure 2). The width of the blade 10 then steadily decreases moving towards the tip 17 the blade 10.

The part of the blade 10 between the root end 16 of the blade 10 and the maximum chord Cmax is referred to herein as the ‘transition portion’ 30 of the blade 20. The transition portion 30 of the blade 10 has a cross-sectional profile that transitions from a circular profile at the root end 16 of the blade 10 into an aerodynamically-optimised airfoil profile at maximum chord Cmax, as will be readily apparent to persons skilled in the art. The region of the blade 10 between the maximum chord CMax and the tip of the blade 17 is referred to herein as the ‘outer portion’ 32 of the blade 10. This portion 32 of the blade 10 has an airfoil profile of varying geometry along its length. The blade 10 extends in a chordwise direction C between the leading edge 14 and the trailing edge 15.

Figure 3 shows a plan view of the wind turbine blade 10 according to the invention. Figure 4 shows a cross section of the blade 10 through the line IV-IV in Figure 3. Figure 5 shows a part of the blade 10 in perspective view. Referring to Figures 3, 4 and 5, the blade 10 comprises a leading edge slat 40. The leading edge slat comprises an airfoil body 41 having a leading edge 42 and a trailing edge 43. At least part of the leading edge slat 40 is positioned forward (that is upstream) of the leading edge 14 of the blade 11.

Extending from the trailing edge 43 of the leading edge slat 40 is a plurality of serrations 44 having a sawtooth pattern. Alternatively, the serrations may have a sinusoidal or rectangular pattern for example. The serrations 44 may be integrally formed with the airfoil body 41 of the leading edge slat 40, or the serrations may be an add-on to the airfoil body 41 of the leading edge slat 40. For example, the serrations may be bonded to the airfoil body 41 with pressure sensitive adhesive tape.

The leading edge slat 40 is mounted at the leading edge 14 of the blade body 11. The leading edge slat 40 extends longitudinally along the leading edge 14, from a point close to the root end 16 of the blade 10 to a point at approximately 30% span, including along the transition portion 30 of the blade 10 inboard of maximum chord CMax- In a preferred embodiment, the leading edge slat 40 extends longitudinally along the leading edge 14, from approximately 5% span to a point at approximately 20% span, including along the transition portion 30 of the blade 10 inboard of maximum chord Cmax- In other embodiments of the invention, the leading edge slat 40 may have a different longitudinal extent.

Referring to Figure 4, which is a cross-sectional view through the blade 20 at maximum chord Cmax, the blade body 11 comprises a pressure side 50 and a suction side 52, which are made primarily from glass-fibre reinforced plastic (GFRP). The pressure side 50 comprises a pressure surface 54 of the blade 10, and the suction side 52 comprises a suction surface 56 of the blade 10. The pressure surface 54 and the suction surface 56 meet at the leading edge 14 and the trailing edge 15 of the blade 10.

The leading edge slat 40 will further comprise one or more support members (not shown) arranged to attach the slat 40 to the blade body 11. The support member (not shown) of the leading edge slat 40 comprises a flat plate and is arranged to maintain the position of the leading edge slat 40 on the blade body 11. In other embodiments, aerodynamic struts or cylindrical rods are used to attach the leading edge slat 40 to the blade body 11.

During operation of the wind turbine, when the wind turbine blades are rotating, the inboard part of the rotor which includes the transition portion of the blade typically sees the largest inflow angles (that is the angle of attack of the oncoming wind). This large inflow angle may lead to separated flow, where the airflow can separate from the suction side of the wind turbine blade. The separated flow from the suction side of the wind turbine blade may lead to a leading edge stall. The separated flow will be turbulent airflow which will lead to noise emissions from the rotor blade.

The leading edge slat 40 comprises an airfoil 41 which is mounted ahead of the leading edge 14 of the blade with a configuration that assists in turning the oncoming air around the leading edge 14 at high angles of attack. This results in the leading edge slat 40 delaying leading edge stall of the blade 10 and increasing the lift coefficient of the blade 10. This will reduce the noise emitted from the inner part of the blade 10 as the airflow does not separate from the suction side of the wind turbine blade.

However, a disadvantage of the leading edge slat 40 is that the air flow through the slot 60 (where the slot 60 is formed between the leading edge slat 40 and the blade body 11) may generate additional noise. But, by design of the trailing edge 43 of the leading edge slat 40 with serrations 44, a major part of this additional noise can be eliminated. The resulting effect is that the noise emission is dramatically reduced from the inboard part of the rotor blade, without introducing significant additional blade noise sources.

The serrations 44 on the leading edge slat 40 reduce noise that would be otherwise generated at the trailing edge 43 of the leading slat 40 through a turbulent boundary layer. Turbulent boundary layer trailing edge noise may be due to the scattering of turbulent fluctuations within the boundary layer at the trailing edge, resulting in noise generation. The serrated trailing edge, as compared to a straight trailing edge, will reduce the noise generated by the trailing edge 43 of the leading edge slat 40. The addition of the leading edge 40 with the serrated trailing edge 44 may reduce the low frequency noise emissions by up to 3 dBs.

In an example, the chord of the leading edge slat 40 is between 10% and 20% of the local chord of the blade body 11 - where the chord of the leading edge slat is taken between its leading edge 42 and its trailing edge 43. The serrations have a length (in the chordwise direction) of between 10% and 20% of the chord of the leading edge slat 40. The width of an individual serration (that is, the length of its base, in the spanwise direction of the blade) is between 10% and 20% of the length of the serration.

In a particular embodiment, the leading edge slat 40 may be movable, such that it can translate or rotate with respect to the blade 10. Alternatively, the leading edge slat may be able to retract into the blade body 11 depending on current wind conditions.

Claims (8)

1. A wind turbine blade extending from a root end to a tip end and comprising a pressure surface and a suction surface; the blade comprising: a blade body a leading edge slat mounted at a leading edge of the blade body; wherein a trailing edge of the leading edge slat is serrated.

2. A wind turbine blade according to claim 1, wherein the blade has a span length extending from the root end to the tip end, and the leading edge slat is positioned between the root end and 30% of the span length.

3. A wind turbine blade according to claim 2, wherein the blade has a span length extending from the root end to the tip end, and the leading edge slat is positioned between the 5% and 20% of the span length.

4. A wind turbine blade according to any one of the preceding claims, wherein the leading edge slat is positioned in a transition portion of the blade.

5. A wind turbine blade according to any one of the preceding claims, wherein the leading edge slat has a chord length extending between a leading edge and the trailing edge of the leading edge slat, and the chord length of the leading edge slat is between 10% and 20% of a local chord length of the blade body.

6. A wind turbine blade according to any one of the preceding claims, wherein the trailing edge of the leading edge slat has a sawtooth, sinusoidal or rectangular serration shape.

7. A wind turbine having a wind turbine blade according to any one of the preceding claims.

8. A method of reducing noise emissions from a wind turbine blade, comprising: providing a blade body extending from a root end to a tip end and comprising a pressure surface and a suction surface; providing a leading edge slat mounted at a leading edge of the blade body; and applying serrations to a trailing edge of the leading edge slat.