WO2013156479A1

WO2013156479A1 - A wind turbine blade having an angled stall fence

Info

Publication number: WO2013156479A1
Application number: PCT/EP2013/057905
Authority: WO
Inventors: Dhinagaran Ramachandran; Jesper Madsen; Mohammed Mehraj Anwar
Original assignee: Lm Wp Patent Holding A/S
Priority date: 2012-04-19
Filing date: 2013-04-16
Publication date: 2013-10-24

Abstract

A wind turbine blade is described having a curved stall fence. The stall fence has a curved cross-section along at least a section of the length of the stall fence. The curved section acts to re-route airflow, particularly cross-flow along the longitudinal direction of the blade, to redirect it back in the direction of flow. This construction provides for an improvement over traditionally straight stall fences. Additionally, the stall fences may be shaped to provide an acceleration of airflow past the stall fence in the mean flow direction, by restricting airflow and inducing a Venturi effect at the stall fence.

Description

A Wind Turbine Blade having an Angled Stall Fence

Field of the Invention

The present invention relates to a wind turbine blade having an angled stall fence, preferably a stall fence having a curved or concavely-shaped cross-section.

Background of the Invention

In wind turbine blades, cross-flows of airflow sometimes propagate along the longitudinal length of the wind turbine blade, from the root end of the blade. Such cross-flows act to negatively impact on blade performance, in particular by contributing to airflow detachment from the surface of the blade, thereby affecting blade lift.

It is known to provide a stall fence which extends across the surface of a wind turbine blade, transverse to the longitudinal axis, to prevent the formation of such cross-flows. An example of such a wind turbine blade can be seen in US Patent No. 7,585,157. Such a stall fence is formed by a planar member projecting orthogonally from the surface of the wind turbine blade, presenting a barrier or wall to any cross-flows travelling along the length of the blade in the longitudinal direction. The extent to which the stall fence is able to prevent cross-flows is partly dependent on the height of the stall fence, with the taller the stall fence the greater the barrier presented to halt cross-flows. However, an increased height of stall fence adds weight to the wind turbine blade, and may affect other blade aerodynamic characteristics, e.g. lift. Accordingly, current stall fence designs are selected to have a height to reduce any negative impact on blade performance, but subsequently do not completely eliminate cross-flows during wind turbine operation, thereby resulting in a continued level of flow detachment.

It is an object of the invention to provide a wind turbine blade having a stall fence design which provides improved performance over existing stall fence systems. Summary of the Invention

Accordingly, there is provided a wind turbine blade comprising an airfoil profile body having a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord length extending therebetween, the blade having a tip end and a root end, the wind turbine blade further having a stall fence arranged on the suction side of the blade, the stall fence comprising a body protruding from the suction side of the blade and having:

a base end for attachment to a surface of the wind turbine blade body;

a tip end distal from said surface;

a first end for location towards said leading edge;

a second end for location towards said trailing edge,

a first side facing towards the root end; and

a second side facing towards the tip end,

wherein said body comprises a portion, where the first side between said base end and said tip end of the body is angled towards the root end of the blade.

Providing a stall fence having a portion angled towards the root end of the blade acts to re-route or re-direct airflow impacting on the stall fence, particularly cross-flow along the longitudinal direction of the blade, to flow back in the direction of original flow. The provision of an angled stall fence provides for greater redirection of airflow than for a traditional straight stall fence of similar height, and is more effective at preventing flow crossing the top of stall fence.

It will be understood that the wind turbine blade is preferably in excess of 30 metres in length, further preferably 40 metres.

In a preferred embodiment, said body comprises a portion, where said first side is curved or concavely-shaped along at least a portion of the length of the body between said first end and said second end. It will be understood that said concavity will be as seen from the root end of the blade. In such a configuration, preferably the base end of said body on said first side is substantially orthogonal to the surface of the wind turbine blade, and the distal end of said body on said first side is arranged at a substantially acute angle to said surface, alternatively substantially parallel to said surface. In one aspect, said stall fence comprises a substantially planar body wherein said planar body comprises a curved cross-sectional portion between said base end and said tip end of said planar body along at least a portion of the length of the planar body between said first end and said second end. This provides a section of the stall fence having a curved cross-section.

In an alternative aspect, said second side comprises a substantially level surface extending between said base end and said tip end. Accordingly, the stall fence is configured as a body having a curved or concavely-shaped portion on the side of the body facing the root end of the blade, i.e. a body having a thinner proximal portion and a thicker distal portion.

In a further aspect, said stall fence comprises a portion, wherein said first side between said base end and said tip end of the body is concavely-shaped towards the root end of the blade and said second side between said base end and said tip end of the body is concave towards the tip end of the blade. This provides a section of the stall fence having a bi-concave body, with inwardly-curving surfaces provided on both the first and second sides of the body.

Alternatively, said first side comprises a portion between said base end and said tip end of said body along at least a portion of the length of the planar body between said first end and said second end, wherein said portion linearly extends at an angle β to the surface of the wind turbine blade. Preferably, said angle β is less than 90 degrees, taken in the direction of the root end of the blade, preferably between 15-75 degrees, further preferably approximately 45 degrees.

In a further embodiment, along at least a further portion of the stall fence between the first end and the second end, the stall fence is arranged such that the first side between said base end and said tip end of the stall fence body is angled towards the tip end of the blade. In such a case, said at least a further portion of the first side linearly extends at an angle β to the surface of the wind turbine blade. Preferably, said angle β is greater than 90 degrees, taken in the direction of the root end of the blade, preferably between 105-165 degrees, further preferably approximately 135 degrees. By arranging for the stall fence to be angled in the direction of the tip end along a portion of the stall fence, this allows for a balancing of benefits arising by mitigating the flow separation and losses created by the use of the stall fence itself.

The wind turbine blade is provided for a rotor of a wind turbine having a substantially horizontal rotor shaft, said rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade having a longitudinal direction with a tip end and a root end and a transverse direction.

Preferably, said stall fence extends along at least 70% of the chordal length between said leading edge and said trailing edge.

Preferably, said stall fence extends in a direction substantially parallel to the mean flow direction over the airfoil, preferably substantially parallel to the chordal plane, preferably +/- 15 degrees.

Preferably, the curved or angled portion of said stall fence extends along at least 70% of the length of the stall fence between said first end and said second end.

In one embodiment, the curved or angled portion is spaced from the first end of the stall fence.

By spacing the curved or angled section from the first end, i.e. placing the stall fence such that the angled section of the stall fence is spaced from the leading edge of the blade, the negative impact on the lift of the blade at the blade leading edge due to the curvature of the stall fence is reduced.

In a further embodiment, said stall fence is curved or angled along approximately 100% of the length of the stall fence.

In one aspect, said curved cross-sectional portion of said stall fence is curved along substantially the height of said portion between said base end and said tip end. In this case, said at least a section of said at least one planar member is curved along the entire height of said planar member between said base end and said tip end. A constantly curving member acts to provide a duct to route airflow across the blade, and wherein that the curve will also direct a flow towards the blade surface, thus reenergizing the boundary layer and reducing the risk of separation of airstream.

In an alternative aspect, said curved cross-sectional portion of said stall fence extends along a sub-portion of the height of said portion between said base end and said tip end.

In this case, only a portion of the planar member is curved along the height of the planar member. This may increase the difficulty for airflow to route over the tip or tip end of the planar member. In one embodiment, the stall fence comprises a substantially straight sub-portion projecting from the base end of said stall fence at a substantially orthogonal angle to the surface of the wind turbine blade body, and a curved sub-portion extending from a distal end of said substantially straight sub-portion to the tip end of said stall fence. In this embodiment, said at least a section of said at least one planar member comprises a substantially straight portion provided at the base end of said planar member and a curved portion provided at the distal tip end of said planar member, said substantially straight portion having a constant angle βΐ to the surface of said wind turbine blade, said curved portion having a varying tangential angle β2. Preferably, said constant angle βΐ is between approximately 0-90 degrees to the surface of the wind turbine blade, preferably between approximately 45-90 degrees, further preferably approximately 90 degrees.

Preferably, a tangential angle β of the curved portion of any embodiment may vary between a first angle located towards said base end and a second angle located towards said distal tip end, wherein said first angle is arranged such that said based end is substantially orthogonal to the surface of the wind turbine blade and wherein said second angle is arranged such that said distal tip end is substantially parallel to the surface of the wind turbine blade. In a further aspect, said curved portion is arranged such that said second angle is provided in a convergent direction towards the surface of the wind turbine blade body.

Preferably, β is measured in the direction of the root end of the blade, with respect to the surface of the wind turbine blade.

In one embodiment, the tangential angle β varies between approximately 90 degrees at said first angle and approximately 0 degrees at said second angle. In this embodiment, the tip end of the curved section points substantially in the direction of the root end of the wind turbine blade. The curving of the stall fence in the direction of the root end will serve to redirect any cross flow back in the direction of the root end of the blade, and to prevent flow from tipping over the distal end of the stall fence.

In a further enhancement, the tangential angle β may vary up to approximately 0 to - 45 degrees at said second angle.

In this embodiment, the tip end of the curved section of the stall fence is directed towards the surface of the blade. This section of the stall fence acts to form a partly- closed duct, or at least a portion of restricted airflow, which may act to increase the air velocity and reduce the air pressure at the curved section.

In a further feature of the invention, the rate of curvature of said at least a section of said at least one planar member may vary between said first end and said second end.

Accordingly, the rate of change of the tangential angle β may vary from a first rate towards said first end and a second rate towards said second end. Preferably, said first rate corresponds to a substantially straight section of said stall fence and said second rate corresponds to an area of maximum curvature of said stall fence.

The angle of the curvature may be varied such that towards the blade leading edge the curvature is minimised, thereby reducing any negative effects on blade lift, etc., at the leading edge. At the blade trailing edge, where the lift characteristics of the blade are less important than at the leading edge, the curvature is maximised, thereby providing increased redirection effect for any cross-flow.

In one embodiment, the at least one stall fence may be formed from several individual sections. Preferably, said sections may be assembled to form a single continuous stall fence. Alternatively, said sections may be provided on said wind turbine blade spaced from each other, said spaced sections acting to form an array of individual stall fences, said total array forming a virtual stall fence. There is also provided a wind turbine comprising at least one wind turbine blade as described above.

Description of the Invention

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a wind turbine;

Fig. 2 shows a schematic view of a wind turbine blade according to the invention;

Fig. 3 shows a schematic view of an airfoil profile of the blade of Fig. 2; Fig. 4 shows a suction-side view of the blade of Fig. 2 having a stall fence according to the invention;

Fig. 5 shows a cross-sectional view of an example of a prior art stall fence; Fig. 6 shows a cross-sectional view of a first embodiment of stall fence according to the invention;

Figs. 7-11 show further embodiments of stall fences according to the invention; and

Fig. 12 indicates the measurement plane for the drawings of Figs. 6-11.

Fig. 1 illustrates a conventional modern upwind wind turbine according to the so- called "Danish concept" with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 nearest the hub and a blade tip 14 furthest from the hub 8. The rotor has a radius denoted R. Fig. 2 shows a schematic view of a first embodiment of a wind turbine blade 10 according to an embodiment of the invention. The wind turbine blade 10 has the shape of a conventional wind turbine blade and comprises a root region 30 closest to the hub, a profiled or an airfoil region 34 furthest away from the hub and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 comprises a leading edge 18 facing the direction of rotation of the blade 10, when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region 30 due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade 10 to the hub. The diameter (or the chord) of the root region 30 is typically constant along the entire root area 30. The transition region 32 has a transitional profile 42 gradually changing from the circular or elliptical shape of the root region 30 to the airfoil profile 50 of the airfoil region 34. The chord length of the transition region 32 typically increases substantially linearly with increasing distance r from the hub.

The airfoil region 34 has an airfoil profile 50 with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10. The width of the chord decreases with increasing distance r from the hub. It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub.

Fig. 3 shows a schematic view of an airfoil profile 50 of a typical blade of a wind turbine depicted with the various parameters, which are typically used to define the geometrical shape of an airfoil. The airfoil profile 50 has a pressure side 52 and a suction side 54, which during use - i.e. during rotation of the rotor - normally face towards the windward (or upwind) side and the leeward (or downwind) side, respectively. The airfoil 50 has a chord 60 with a chord length c extending between a leading edge 56 and a trailing edge 58 of the blade. The airfoil 50 has a thickness t, which is defined as the distance between the pressure side 52 and the suction side 54. The thickness t of the airfoil varies along the chord 60. The deviation from a symmetrical profile is given by a camber line 62, which is a median line through the airfoil profile 50. The median line can be found by drawing inscribed circles from the leading edge 56 to the trailing edge 58. The median line follows the centres of these inscribed circles and the deviation or distance from the chord 60 is called the camber f. The asymmetry can also be defined by use of parameters called the upper camber and lower camber, which are defined as the distances from the chord 60 and the suction side 54 and pressure side 52, respectively.

Airfoil profiles are often characterised by the following parameters: the chord length c, the maximum camber f, the position df of the maximum camber f, the maximum airfoil thickness t, which is the largest diameter of the inscribed circles along the median camber line 62, the position dt of the maximum thickness t, and a nose radius (not shown). These parameters are typically defined as ratios to the chord length c. Wind turbine blades are generally formed from fibre-reinforced plastics material, e.g. glass fibres and/or carbon fibres which are arranged in a mould and cured with a resin to form a solid structure. Modern wind turbine blades can often be in excess of 30-40 metres in length, having blade root diameters of several metres. With reference to Fig. 4, a plan view of the suction side 54 of the wind turbine blade 10 is shown. The wind turbine blade 10 further comprises a stall fence 100 provided on the suction side 54, the stall fence extending from a first end 101 adjacent said leading edge 56 and a second end 103 adjacent said trailing edge 58. Preferably, the stall fence 100 extends across the chordal length of the wind turbine blade, but it will be understood that the stall fence 100 may alternatively be arranged to extend across only a portion of the chordal length of the blade. In particular, the stall fence 100 may be arranged such that the first end 101 and/or the second end 103 of the stall fence 100 is spaced from the leading edge 56 or trailing edge 58 respectively of the blade 10. In one arrangement, the stall fence 100 may extend along approximately 70% of the chordal extent of the blade 10.

Preferably, the stall fence 100 is arranged to be substantially parallel with the mean flow direction at the blade 10, indicated by arrow Y, such that the stall fence 100 does not affect the lift force generated by the airflow over the blade profile. It will be understood however that the stall fence 100 may be offset from the mean flow direction, preferably within approximately +/- 15 degrees of the mean flow direction over the blade profile.

While a single stall fence 100 is shown in Fig. 4, it will be understood that a plurality of stall fences may be provided on a single blade, to provide greater prevention of cross-flow. Furthermore, while the stall fence 100 is preferably provided towards the root end 16 of the blade 10, it will be understood that the stall fence 100 may be provided at any location along the length of the blade 10 between the root end 16 and the tip end 14.

With reference to Fig. 5, a cross-sectional view of a prior art stall fence is indicated at 70. Such a stall fence 70 comprises a body projecting from a base end 72 to a distal tip end 74, the body attached to the surface of a wind turbine blade 10 at the base end 72. The stall fence body may be tapered towards said tip end 74, the stall fence 70 providing a barrier to impede cross-flow of air along the length of the blade from the root end to the tip end. However, laminar cross-flow (indicated by arrow A) may still cross over the stall fence 70 by tipping over the distal tip end 74 of the stall fence 70 and continuing in a longitudinal direction along the blade length.

With reference to the Figures shown in Figs. 6-11, it will be understood that the drawings represent a cross-section of a portion of a stall fence according to the invention, for example taken along line X-X as indicated in Fig. 4. However, it will be understood that the indicated cross-sectional profiles may be located at any point along the length of the stall fence, as required by the appropriate design and constructional needs. In this regard, Fig. 12 indicates the measurement plane for the appropriate angles of the stall fence cross-sections, wherein an angle of 0 degrees extends substantially in the direction of the root end 16 of the blade 10, an angle of 90 degrees extends substantially perpendicular to the adjacent surface 54 of the blade 10, and an angle of 180 degrees extends substantially in the direction of the tip end 14 of the blade 10. With reference to Fig. 6, a cross-sectional view of an embodiment of stall fence according to the invention is illustrated at 100. The stall fence 100 comprises a substantially planar body projecting from a base end 102 to a distal tip end 104, the body arranged to be attached to the surface of a wind turbine blade 10 at the base end 102. The stall fence 100 has a first face 106 arranged to substantially face in the direction of the root end 16 of a blade 10, and a second face 108 arranged to substantially face in the direction of the tip end 14 of a blade 10.

At least a portion of the stall fence 100 between the first end 101 and the second end 103 is shaped to curve between the base end 102 and the tip end 104, such that the first face 106 is substantially concave and the second face 108 is substantially convex, i.e. the stall fence is shaped to curve in the direction of the root end 16 of the blade 10. As a result of this curvature of the stall fence 100, airflow across the length of the blade 10 from the root end 16 to the tip end 14 will be redirected back towards the blade root end 16 (indicated by arrow B in Fig. 6). Accordingly, the cross-flow is prevented from easily tipping over the distal tip end 104 of the stall fence 100, providing an improved performance over prior art stall fences 70.

While the curvature of the stall fence 100 is arranged such that the base end 102 of the stall fence 100 projects orthogonally from the adjacent surface of a wind turbine blade 10 and the distal tip end 104 projects substantially parallel to said surface, it will be understood that any other rate or length of curvature may be employed.

It will be understood that further different shapes and configurations of stall fence may be used. Accordingly, different embodiments of stall fence according to the invention are shown in Figs. 7-11.

In Fig. 7, a cross-section is shown of a portion of a stall fence 200 having a base end 202 for attachment to a wind turbine blade 10 and a distal tip end 204. The stall fence 200 further comprises a first face 206 arranged to substantially face in the direction of the root end 16 of a blade 10, and a second face 208 arranged to substantially face in the direction of the tip end 14 of a blade 10. The stall fence 200 is arranged such that while said second face 208 linearly extends in substantially orthogonal direction relative to the adjacent surface of a wind turbine blade 10, said first face 206 is substantially concave, i.e. the first face 206 of the stall fence 200 is shaped to curve in the direction of the root end 16 of the blade 10, with the body of the stall fence 200 having a relatively thin proximal portion and a relatively thick distal portion.

In Fig. 8, a cross-section is shown of a portion of a stall fence 300 having a base end 302 for attachment to a wind turbine blade 10 and a distal tip end 304. The stall fence 300 further comprises a first face 306 arranged to substantially face in the direction of the root end 16 of a blade 10, and a second face 308 arranged to substantially face in the direction of the tip end 14 of a blade 10. In this embodiment, the stall fence 300 is arranged such that said first face 306 is substantially concave and said second face 308 is substantially concave, i.e. the first face 306 is shaped to curve in the direction of the root end 16 of the blade 10 and the second face 308 is shaped to curve in the direction of the tip end 14 of the blade 10. Accordingly, the body of the stall fence 300 comprises a relatively thin proximal portion and a relatively thick distal portion. Such a cross-section may be used at a location where the flow separation is relatively mild (e.g. at a radial location which is far from the root end, and possibly after a stall fence). In such a location, it is desired to turn the attached flow towards the tip end on one side of the stall fence, while preventing the mildly separated flow from moving across the stall fence (giving more aerodynamic benefit). In Fig. 9, a cross-section is shown of a portion of a stall fence 400 having a base end 402 for attachment to a wind turbine blade 10 and a distal tip end 404. In this embodiment, the stall fence 400 comprises a substantially planar body which is arranged to linearly extend from the surface of a wind turbine blade 10 at a constant angle β in the direction of the root end 16 of the blade 10. Preferably, said angle β is less than 90 degrees, preferably between 15-75 degrees, e.g. 45 degrees.

In Fig. 10, a cross-section is shown of a portion of a stall fence 500 having a base end 502 for attachment to a wind turbine blade 10 and a distal tip end 504. The stall fence 500 further comprises a first face 506 arranged to substantially face in the direction of the root end 16 of a blade 10, and a second face 508 arranged to substantially face in the direction of the tip end 14 of a blade 10. Similar to the embodiment in Fig. 7, the stall fence 500 is arranged such that while said second face 508 linearly extends in substantially orthogonal direction relative to the adjacent surface of a wind turbine blade 10, said first face 506 is substantially concave, such that the first face 506 of the stall fence 500 is shaped to curve in the direction of the root end 16 of the blade 10. Furthermore, the first face 506 is further arranged to curve towards the surface of the blade 10, providing the stall fence 500 with an overhang portion at the distal tip end 504 of said first face 506. This section of the stall fence acts to form a partly-closed duct, or at least a portion of restricted airflow, which may act to increase the air velocity and reduce the air pressure at the curved section.

It will be understood that a stall fence according to the invention may comprise any combination of the features of the above embodiments. For example, in Fig. 11, a cross-section is shown of a portion of a stall fence 600 having a base end 602 for attachment to a wind turbine blade 10 and a distal tip end 604. The stall fence 600 further comprises a first face 606 arranged to substantially face in the direction of the root end 16 of a blade 10, and a second face 608 arranged to substantially face in the direction of the tip end 14 of a blade 10. In this embodiment, the first face 606 comprises a first subsection 610 linearly extending from the blade surface at a constant angle βΐ and a second subsection 612 provided at the distal end of said first subsection 610, the second subsection 612 curved in the direction of the blade root end 16 having a varying tangential angle β2. It will be further understood that the rate of curvature of the stall fences 100,200,300,500,600 may be varied between the respective base ends and distal tip ends. In Fig. 7, tangential angle β is shown for a point on the curve provided on the first face 206 of stall fence 200. It will be understood that the rate of change of tangential angle β may vary between the base end and distal tip end of a curved stall fence.

In a further alternative, it will be understood that the curvature and/or the rate of change of tangential angle β of a curved stall fence 100,200,300,500,600 may vary between first and second ends 101 ,103 of the stall fence, e.g. the stall fence may have little or no curvature towards the leading edge 56 of the wind turbine blade 10, to minimise the leading edge lift effect, and may have maximum curvature or deflection towards the trailing edge 58, to maximise the prevention of cross-flow where the effects on blade lift by the presence of the curved stall fence will be minimal.

It will be understood that the above stall fences may comprise an integral base member provided at the base end of the stall fence for attachment to the surface of the blade, as shown in Figs. 6-7. Alternatively, the stall fence may be formed integrally with the blade surface.

The stall fences are preferably provided on the suction side of a wind turbine blade, but it will be understood that the stall fences may extend to the pressure side of the wind turbine blade. The herein described embodiments present a wind turbine blade having an angled stall fence arranged to effectively prevent cross-flow along the length of the wind turbine blade.

The invention is not limited to the embodiment described herein, and may be modified or adapted without departing from the scope of the present invention.

Claims

1. A wind turbine blade comprising an airfoil profile body having a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord length extending therebetween, the blade having a tip end and a root end, the wind turbine blade further having a stall fence arranged on the suction side of the blade, the stall fence comprising a body protruding from the suction side of the blade and having: a base end for attachment to a surface of the wind turbine blade body;

a tip end distal from said surface;

a first end for location towards said leading edge;

a second end for location towards said trailing edge,

a first side facing towards the root end; and

a second side facing towards the tip end,

2. The blade of claim 1, wherein said body comprises a portion where said first side is concavely shaped along at least a portion of the length of the body between said first end and said second end.

3. The blade of claim 2, wherein said stall fence comprises a substantially planar body wherein said planar body comprises a curved cross-sectional portion between said base end and said tip end of said planar body along at least a portion of the length of the planar body between said first end and said second end.

4. The blade of claim 2, wherein said second side comprises a substantially level surface extending between said base end and said tip end.

5. The blade of claim 2, wherein said stall fence comprises a portion where said first side between said base end and said tip end of the body is concavely shaped towards the root end of the blade and said second side between said base end and said tip end of the body is concavely shaped towards the tip end of the blade.

6. The blade of any one of claims 2-5, wherein a tangential angle β of the curved or concave portion varies between a first angle located towards said base end and a second angle located towards said distal tip end, wherein said first angle is arranged such that said base end is substantially orthogonal to the surface of the wind turbine blade and wherein said second angle is arranged such that said distal tip end is substantially parallel to the surface of the wind turbine blade.

7. The blade of claim 6, wherein β varies between approximately 90 degrees at said first angle and approximately 0 degrees at said second angle, β measured with respect to the surface of the wind turbine blade in the direction of the root end of the blade.

8. The blade of any one of claims 2-7, wherein the rate of curvature of said portion of said at least one planar member varies between the first end and the second end of said stall fence.

9. The blade of claim 1 , wherein said first face comprises a portion between said base end and said tip end of said body along at least a portion of the length of the planar body between said first end and said second end, wherein said portion linearly extends at an angle β to the surface of the wind turbine blade, said angle β less than 90 degrees.

10. The blade of claim 6, wherein said angle β is between approximately 15-75 degrees.

11. The blade of any preceding claim, wherein said stall fence extends along at least 70% of the chordal length between said leading edge and said trailing edge.

12. The blade of any preceding claim, wherein said stall fence extends in a direction substantially parallel to the mean flow direction over the airfoil.

13. The blade of any preceding claim, wherein the angled portion of said stall fence extends along at least 70% of the length of the stall fence between said first end and said second end.

14. The blade of any preceding claim, wherein the stall fence comprises a substantially straight sub-portion projecting from the base end of said stall fence at a substantially orthogonal angle to the surface of the wind turbine blade body, and an angled or curved sub-portion extending from a distal end of said substantially straight sub-portion to the distal tip end of said stall fence.

15. A wind turbine comprising at least one wind turbine blade as claimed in any one of claims 1-14.