Wind turbine blade with optimized tip shape

Abstract

The invention relates to a wind turbine blade having a profile that tapers along a longitudinal axis xx' from a base to an end. At a given distance from the end, the profile widens on either side of the axis xx' to form a spatula (12) that extends to the end. Figure for abstract: Fig. 1A

Description

Title of the invention: Wind turbine blade having an optimized tip shape Technical field

[0001] The invention relates to wind turbine blades, in particular wind turbines in a range between low power and medium power. Background

[0002] So-called "medium power" wind turbines are intended to produce a power of around 500 kW and reach a height of around 50 m with a rotor diameter of the same order.

[0003] While high-power wind turbines have a rotor designed to rotate at a fixed speed, of the order of 21 revolutions per minute, rotors of wind turbines in the range up to medium power can rotate at variable speed, for example from 0 to 150 revolutions per minute. To optimize efficiency, the blades are generally helical along their longitudinal axis and, for larger models, the blades have an adjustable pitch by a mechanism housed in the hub.

[0004] The variable speed of the rotors poses challenges for optimizing the blades over the entire speed range. Summary

[0005] A wind turbine blade is generally provided having a profile that tapers along a longitudinal axis xx' from a base to an end. At a given distance from the end, the profile widens on either side of the axis xx' to form a spatula that extends to the end.

[0006] The end of the blade may comprise a fin oriented in the direction of the wind.

[0007] The spatula may have, at the level of the trailing edge of the blade, a straight edge substantially parallel to the longitudinal axis xx' of the blade.

[0008] The spatula may have, at the level of the leading edge of the blade, a convex curvilinear edge.

[0009] The blade may be helical along the axis xx' up to the spatula and the spatula may generally be flat.

[0010] The given distance from the tip may be between 10% and 25% of the useful length of the blade.

[0011] The fin may be of generally triangular shape in projection on a plane yz perpendicular to the axis xx', and have a vertex further from the axis xx' than the trailing edge of the spatula. Summary description of the drawings

[0012] Embodiments will be set out in the following description, given without limitation in relation to the attached figures among which:

[0013] [Fig.1A] represents a front view of an embodiment of a wind turbine blade;

[0014] [Fig.lB] shows the blade profile at different positions along a long axis vertical xx' of the blade; and

[0015] [Fig. IC] represents a side view of the blade. Detailed description

[0016] A wind turbine blade according to the embodiments described below has been designed in the context of wind turbines in an intermediate range between low power and medium power, for example a wind turbine having a rotor diameter ranging from 8 to 32 m and a corresponding height of 20 to 50 m. In this range, the rotational speeds are generally variable from 0 to 150 revolutions per minute, and the rotors may have five blades.

[0017] Under these conditions of variable rotational speed, the blade profiles are difficult to optimize and the blades generally provide lower efficiency than a blade rotating at a fixed speed for which it has been optimized. This state of affairs is hardly improved by the ability to adjust the pitch of the blades.

[0018] A conventional wind turbine blade, regardless of the power range and rotational speed, generally has a profile that tapers along the longitudinal axis of the blade from its base (near the center of the rotor) to its tip. The "profile" of the blade refers to the shape of its cross-section at different points along the longitudinal axis. This profile is generally elongated along an axis that is assumed to be aligned with the direction of the relative wind. The helical shape of the blade serves to compensate for the variation in angular velocity of the relative wind along the longitudinal axis of the blade.

[0019] The inventors found that by widening the profile again at the tip of the blades, it was possible to significantly improve efficiency over the entire range of rotation speeds.

[0020] Figures 1A to 1C show an embodiment of a blade according to different views.

[0021] [Fig.lA] shows a front view of the blade, namely in an xy plane corresponding to the rotor plane, which is in principle perpendicular to the wind direction zz'. The xx' axis is the longitudinal axis of the blade. In the drawings, for ease of understanding, the blade pitch is set for the maximum rotational speed, which positions the blade tip profile parallel to the xy plane.

[0022] [Fig.lB], to which reference will be made at the same time, illustrates cross-sections, or profiles of the blade, at different points on the xx' axis. Indicative radii are reported in mm on the xx' axis, and the base of the blade starts at radius 500.

[0023] The blade is mounted by its base, on the left, in a hub, not shown, using a cylindrical part 10. Between radii 500 and 840, the profile is not functional. The profile starts from radius 500 of a circular section to arrive at its greatest functional width at radius 840. The leading edge of the blade (at the bottom) remains closer to the axis xx' than the trailing edge (at the top). These edges are substantially rectilinear.

[0024] As seen in [Fig. 1B], the profile with radius 840 is for example in the shape of a drop whose barycenter is centered on the axis xx'. The blade being helical, the profile here has its greatest inclination, of approximately 30 degrees relative to the xy plane. The "drop" can also have a convex part downwind, and a concave part in the wind.

[0025] Between radii 840 and 4000, the profile continuously narrows, the leading edge always remaining closer to the xx' axis than the trailing edge. As shown in [Fig.lB], the blade profile gradually straightens to become parallel to the xy plane at radius 4000. At radius 2400, at the center of the blade, the profile is still drop-shaped, while at radius 4000, the profile is flattened, forming a wing whose hollow is turned towards the wind.

[0026] Up to radius 4000, the profile is conventional. From radius 4000, according to one embodiment, the profile widens again on either side of the axis xx' to form a spatula 12 which ends at the end of the blade at radius 4760. As shown by way of example, the spatula has at the trailing edge of the blade a rectilinear edge substantially parallel to the axis xx'. At the leading edge of the blade, the spatula has a convex curvilinear edge. Thus, the spatula resembles a hand with the thumb hidden under the palm.

[0027] As shown in [Fig.lB], the entire spatula has a flattened wing profile substantially parallel to the xy plane. The hollow portion of the wing profile faces into the wind.

[0028] According to one embodiment, the end of the spatula is provided with a fin 14 oriented in the direction of the wind. As shown in [Fig.lB], the fin may be of generally triangular shape in projection on a plane yz perpendicular to the axis xx', and have a vertex further from the axis xx' than the trailing edge of the spatula. Thus, the spatula resembles a hand with the thumb hidden under the palm and the ends of the fingers raised.

[0029] [Fig. 1C], provided to complete the definition of the blade, is a side view of the blade, namely along the yy' axis. In this view, it can be seen that the profile of the blade tapers from radius 840 to radius 4000. From radius 4000, namely at the level of the spatula, it can be seen that the profile has a substantially constant thickness, reflecting the flatness of the spatula.

[0030] Dimensions are indicated as an example in the figures for a rotor blade with a radius of 4.76 m. The length of the spatula is here 0.76 m over a useful length of the blade of 4.76 - 0.84 = 3.92 m, namely that the spatula has a length equal to 19.4% of the useful blade length. In practice, the spatula length can vary between 10 and 25% of the useful blade length to benefit from a significant improvement in efficiency.

[0031] Estimates made with the dimensions shown revealed a 10% increase in efficiency compared to a conventional blade which was also 33% longer.

[0032] It is difficult to show with current simulation tools the relationships of the various elements of the spatula with the efficiency. It is assumed that these elements reduce in particular the turbulence at the end of the blade. As the turbulence decreases, the noise also decreases.

Claims

Claims

1. Wind turbine blade having a profile tapering along a longitudinal axis xx' from a base towards an end, characterized in that the profile, at a given distance from the end, widens on either side of the axis xx' to form a spatula (12) which extends to the end, characterized in that the spatula (12) has, at the trailing edge of the blade, a rectilinear edge substantially parallel to the longitudinal axis xx' of the blade and, at the leading edge of the blade, a convex curvilinear edge.

2. A blade according to claim 1, wherein the tip of the blade comprises a fin (14) facing in the direction of the wind.

3. A blade according to claim 1, wherein the blade is helical along the axis xx' to the spatula and the spatula is generally planar.

4. A blade according to claim 1, wherein the given distance from the tip is between 10% and 25% of the useful length of the blade.

5. Blade according to claim 2, in which the fin (14) is of generally triangular shape in projection on a plane yz perpendicular to the axis xx', and has a vertex further from the axis xx' than the trailing edge of the spatula (12).

Images