Hinged wind turbine blade defining an angle in a flap-wise direction

Abstract

A horizontal axis wind turbine (1) with a wind turbine blade (5) is disclosed, the wind turbine blade (5) comprising a hinge (6) arranged to connect the wind turbine blade (5) to a blade carrying structure (4) of the wind turbine (1), at a non-zero distance from an inner tip (5a) and at a non-zero distance from an outer tip (5b) of the wind turbine blade (5). An outer blade part (7) is arranged between the hinge region and the outer tip (5b), and an inner blade part (8) is arranged between the hinge region and the inner tip (5a). The outer blade part (7) extends from the hinge region along a first direction and the inner blade part (8) extends from the hinge region along a second direction, and the first direction and the second direction form an angle, α, there between, where 0°<α<90°.

Claims

1. A horizontal axis wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure, and at least one wind turbine blade defining an aerodynamic profile between an inner tip and an outer tip, the wind turbine blade comprising: a hinge connecting the wind turbine blade to the blade carrying structure, in a hinge region of the wind turbine blade, the hinge region being arranged at a non-zero distance from the inner tip and at a non-zero distance from the outer tip, the wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, an outer blade part arranged between the hinge region and the outer tip, and an inner blade part arranged between the hinge region and the inner tip, wherein the outer blade part extends from the hinge region along a first direction and the inner blade part extends from the hinge region along a second direction, and wherein the first direction and the second direction form an angle, α, there between, where 0°<α<90°, wherein the inner tip of the wind turbine blade is free of mechanical connections that cause the pivot movement towards the maximum pivot angle of the wind turbine blade relative to the blade carrying structure.

2. The horizontal axis wind turbine according to claim 1, wherein the angle, α, is in a flap-wise direction.

3. The horizontal axis wind turbine according to claim 1, wherein the outer blade part and the inner blade part are two separate parts being joined to each other.

4. The horizontal axis wind turbine according to claim 3, wherein the wind turbine blade further comprises a hinge part interconnecting the inner blade part and the outer blade part.

5. The horizontal axis wind turbine according to claim 1, wherein the outer blade part and the inner blade part form one piece.

6. The horizontal axis wind turbine according to claim 1, wherein the angle, α, is within a range of 5° to 45°.

7. The horizontal axis wind turbine according to claim 1, wherein the inner blade part and/or the outer blade part are curved in a flap-wise direction.

8. The horizontal axis wind turbine according to claim 1, wherein the inner blade part and/or the outer blade part are curved in an edge-wise direction.

9. The horizontal axis wind turbine according to claim 1, wherein the wind turbine blade comprises a plurality of fibres arranged in parallel along the wind turbine blade, and wherein the wind turbine blade comprises a region in which an orientation of the fibres deviates from a main orientation of the fibres being substantially parallel to a leading edge or a trailing edge of the wind turbine blade.

10. The horizontal axis wind turbine according to claim 1, wherein the inner blade part is provided with a balancing mass.

11. The horizontal axis wind turbine according to claim 1, wherein the inner blade part and/or the outer blade part is provided with a winglet.

12. The horizontal axis wind turbine according to claim 1, further comprising a biasing mechanism arranged to apply a biasing force to the wind turbine blade which biases the wind turbine blade towards a position defining a minimum pivot angle relative to the blade carrying structure.

13. The horizontal axis wind turbine according to claim 1, further comprising a biasing mechanism arranged to apply a biasing force to the wind turbine blade which biases the wind turbine blade towards a position defining a maximum pivot angle relative to the blade carrying structure.

14. The horizontal axis wind turbine according to claim 1, wherein the blade carrying structure comprises one or more arms, each arm having a wind turbine blade connected thereto.

15. The horizontal axis wind turbine according to claim 14, wherein each arm extends from the hub along a direction which forms an angle, β, relative to a vertical direction, where 0°<β<30°.

16. The horizontal axis wind turbine according to claim 1, wherein a rotational axis of the hinge of each wind turbine blade is arranged relative to a line which extends between a rotational axis of the hub and a centre of the hinge, in such a manner that the rotational axis of the hinge and the line form an angle, Φ, there between which differs from 90°.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in further detail with reference to the accompanying drawings in which

(2) FIG. 1 is a front view of a wind turbine according to an embodiment of the invention,

(3) FIGS. 2-4 are side views of the wind turbine of FIG. 1 with the wind turbine blades at three different pivot angles,

(4) FIG. 5 shows part of the wind turbine shown in FIGS. 1-4,

(5) FIGS. 6 and 7 show an outer blade part of a wind turbine blade according to two different embodiments of the invention,

(6) FIGS. 8 and 9 show possible bend directions of a wind turbine blade according to various embodiments of the invention,

(7) FIG. 10 shows a wind turbine exposed to vertical wind shear,

(8) FIG. 11 is a cross-sectional view of a wind turbine blade according to an embodiment of the invention,

(9) FIG. 12 is a graph showing angle of attack on the wind turbine blade of FIG. 11 as a function of time,

(10) FIG. 13 shows a wind turbine blade according to an embodiment of the invention at two different pivot angles,

(11) FIG. 14 shows rotor inertia as a function of rotor speed for a wind turbine according to an embodiment of the invention,

(12) FIG. 15 shows a wind turbine blade according to an embodiment of the invention at maximum pivot angle, and

(13) FIGS. 16a-16c illustrate a wind turbine blade according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(14) FIGS. 1-4 illustrate a wind turbine 1 according to an embodiment of the invention.

(15) FIG. 1 is a front view of the wind turbine 1. The wind turbine 1 comprises a tower 2 and a nacelle (not visible) mounted on the tower 2 via a yaw system. A hub 3 is mounted rotatably on the nacelle, the hub 3 comprising a blade carrying structure 4 with three arms. Three wind turbine blades 5 are each connected to the blade carrying structure 4 via a hinge 6 in a hinge region of the wind turbine blade 5. The wind turbine blade 5 is thereby arranged to perform pivot movements relative to the blade carrying structure 4 between a minimum pivot angle and a maximum pivot angle.

(16) Each wind turbine blade 5 defines an aerodynamic profile between an inner tip 5a and an outer tip 5b. The hinge 6 is arranged at a non-zero distance from the inner tip 5a and at a non-zero distance from the outer tip 5b. Thereby an outer blade part 7, extending between the hinge 6 and the outer tip 5b, and an inner blade part 8, extending between the hinge 6 and the inner tip 5a, are defined.

(17) The hinge 6 allows the wind turbine blade 5 to perform pivot movements relative to the blade carrying structure 4. A pivot angle is thereby defined between the wind turbine blade 5 and the blade carrying structure 4, depending on the position of the hinge 6 and thereby of the wind turbine blade 5 relative to the blade carrying structure 4. This determines a diameter of the rotor, and thereby the ability of the wind turbine 1 to extract energy from the wind.

(18) The outer blade part 7 extends from the hinge 6 along a first direction and the inner blade part 8 extends from the hinge 6 along a second direction. The first direction and the second direction form an angle, α, there between. The wind turbine blade 5 thereby forms a bend at or near the hinge 6. In FIGS. 2-4 it can be seen that a is approximately 25°, and that it is in a flap-wise direction. It can further be seen that the bend is formed approximately at the position of the hinge 6.

(19) FIGS. 2-4 are side views of the wind turbine 1 of FIG. 1 with the wind turbine blades 5 at three different pivot angles. The pivot angle can vary between a minimum pivot angle, defining a maximum rotor diameter, as shown in FIG. 2, and a maximum pivot angle, defining a minimum rotor diameter, as shown in FIG. 4. FIG. 3 shows the wind turbine blades 5 at an intermediate pivot angle between the maximum and minimum rotor diameter, i.e., minimum and maximum pivot angle.

(20) In FIG. 2 the wind turbine blade 5 is arranged in a position defining minimum pivot angle, and thereby maximum rotor diameter. Accordingly, the inner blade part 8 is arranged immediately adjacent to the blade carrying structure 4. The bend defined by the angle, α, between the inner blade part 8 and the outer blade part 7 ensures that the outer tip 5b, at this pivot angle, is arranged further away from the hub 3 along a radial direction than would be the case for a wind turbine blade without such a bend. This increases the maximum rotor area, and will be described in further detail below with reference to FIG. 5.

(21) FIG. 3 shows the wind turbine 1 with the wind turbine blades 5 at a pivot angle between minimum and maximum pivot angle, and a rotor diameter which is decreased compared to the maximum rotor diameter illustrated in FIG. 2. It can be seen that the inner blade part 8 has moved away from the blade carrying structure 4, and that the inner tip 5a has been moved closer to the tower 2.

(22) FIG. 4 shows the wind turbine 1 with the wind turbine blades 5 pivoted such that they define a maximum pivot angle and therefore minimum rotor diameter. Accordingly, the inner blade part 8 has been moved further away from the blade carrying structure 4 and the inner tip 5a has been moved closer to the tower 2.

(23) The bend of the wind turbine blade 5, i.e., the angle, α, ensures that the inner tip 5a is arranged further away from the tower 2 than would be the case if the wind turbine blade 5 had not been provided with the bend. Accordingly, the attachment point between the wind turbine blades 5 and the blade carrying structure 4 can be moved closer to the tower 2, without risking collisions between the wind turbine blades 5 and the tower 2 at large pivot angles. Thereby coning can be avoided, or a reduced coning angle can be applied. Thereby the centre of mass of the wind turbine blades 5 can be moved closer to the tower 2, thereby reducing the loads on the wind turbine 1, in particular on the hub 3, the drive train and the tower 2. Furthermore, this position of the centre of mass of the wind turbine blades 5 reduces inertia of the rotor when the wind turbine blades 5 are in a position defining minimum pivot angle, which will normally be the case at low wind speeds. This will be described in further detail below with reference to FIGS. 13 and 14.

(24) FIG. 5 shows part of the wind turbine shown in FIGS. 1-4. The dashed outline 9 represents an outer blade part with the same length as the outer blade part 7, but without the bend defining the angle, α, which is described above with reference to FIGS. 1-4.

(25) In FIG. 5 the wind turbine blade 5 is arranged at a position defining minimum pivot angle. It can be seen that the outer tip 5b, at this pivot angle, is arranged further away from the hub 3 by the distance, 4, than the outer tip of the dashed outline 9 representing the wind turbine blade without the bend. Accordingly, the bend provides an increased rotor diameter, and thereby increased power production, of the wind turbine 1 at the minimum pivot angle. This is desirable, since the minimum pivot angle often occurs at low wind speeds.

(26) FIGS. 6 and 7 show outer blade parts 7 of a wind turbine blade according to two different embodiments of the invention. FIG. 6 shows an outer blade part 7 extending from a hinge part 10 defining a hinge region, along a straight line 12. The straight line 12 is non-perpendicular to a mounting surface of the hinge part 10, and it forms an angle, α, with a direction 11, along which an inner blade part (not shown) attached to an opposite mounting surface of the hinge part 10 extends.

(27) The hinge part 10 is provided with pins, one of which 10a is shown, for connecting the hinge part 10 to mating parts on a blade carrying structure, thereby forming a hinge.

(28) FIG. 7 shows an outer blade part 7 which extends along a curved line 13 in the flap-wise direction. Accordingly, the angle, α, between the outer blade part 7 and the inner blade part (not shown) increases along the outer blade part 7 in the direction towards the outer tip 5b.

(29) FIGS. 8 and 9 show possible bend directions of a wind turbine blade 5 according to various embodiments of the invention. FIG. 8 shows a side view of the wind turbine blade 5 hinged onto the blade carrying structure 4 via a hinge 6. FIG. 8 shows possible bending of the wind turbine blade 5 in the flap-wise direction. The inner blade part 8 may be curved in the flap-wise direction, either in an outwards direction relative to the blade carrying structure, having an inner tip 5a-3, or in an inwards direction relative to the blade carrying structure, having an inner tip 5a-1, or it may follow a straight line as indicated by inner tip 5a-2.

(30) Similarly, the outer blade part 7 may be curved in the flap-wise direction, either in the outwards direction relative to the blade carrying structure, having an outer tip 5b-3, or in the inwards direction relative to the blade carrying structure, having an outer tip 5b-1, or it may follow a straight line as indicated by outer tip 5b-2. Any combinations of the inner tips 5a-1, 5a-2, 5a-3 and the outer tips 5b-1, 5b-2, 5b-3 could be applied.

(31) FIG. 9 shows a front view of the wind turbine blade 5 and possible bending of the wind turbine blade 5 in an edge-wise direction. Bending in the edge-wise direction can be formed either as an alternative to the flap-wise bending illustrated in FIG. 8, or in addition to flap-wise bending.

(32) According to this embodiment, the inner blade part 8 may be curved in the edge-wise direction, either towards the leading edge having an inner tip 5a-4, or in a direction towards the trailing edge having an inner tip 5a-6, or it may follow a straight line as indicated by inner tip 5a-5.

(33) Similarly, the outer blade part 7 may be curved in the edge-wise direction, either towards the leading edge having an outer tip 5b-4, or in a direction towards the trailing edge having an outer tip 5b-6, or it could follow a straight line as indicated by outer tip 5b-5. Any combinations of the inner tips 5a-4, 5a-5, 5a-6 and the outer tips 5b-4, 5b-5, 5b-6 could be applied.

(34) Curving a wind turbine blade 5 in the edge-wise direction is sometimes referred to as ‘sweep’. When a wind turbine blade 5 provided with sweep passes the tower of a wind turbine, the tower is passed gradually, since the sweep ensures that only a portion of the wind turbine blade 5 is arranged adjacent to the tower at any given time. This reduces the loads on the wind turbine during tower passage, in particular loads on the wind turbine blade 5 and on the tower.

(35) FIG. 10 shows a wind turbine 1 exposed to vertical wind shear 14, i.e. wind speed variations in a vertical direction z. In the situation illustrated in FIG. 10, the wind speed is low close to the base of the tower 2 and increases with increasing height and towards the hub 3. Such variations in wind speed at different heights create different loads on the wind turbine blades 5 as they rotate along with the hub 3. This causes deflections of the wind turbine blades 5 which vary periodically for each full turn of the rotor. This will normally lead to periodical variations in suction pressure on the wind turbine blades 5 and periodical variations in angle of attack.

(36) FIG. 11 is a cross-sectional view of a wind turbine blade 5 according to an embodiment of the invention, and a local angle of attack AOA on the wind turbine blade 5. The local angle of attack AOA in a cross section of the wind turbine blade 5 is defined as the angle between the chord of the wind turbine blade 5 and the relative wind speed, where the relative wind speed is the resultant vector of the local incoming wind speed vector V(z) and the local rotational speed vector ω.Math.r of the wind turbine blade 5. During rotation V(z) changes as the blade 5 rotates, due to the wind shear, and thereby the angle of attack AOA changes.

(37) The wind turbine blade 5 of FIG. 11 has a design which introduces bend/twist couplings of the wind turbine blade 5. This could, e.g., be provided by means of sweep, as described above with reference to FIG. 9, or by means of off-axis fibre placement. This has the consequence that increased deflections of the wind turbine blade 5 in the flap-wise direction causes torsional twist of the wind turbine blade 5 towards a lower angle of attack.

(38) FIG. 12 is a graph showing angle of attack AOA as a function of time for a wind turbine blade without bend/twist couplings (solid line 15) and for the wind turbine blade of FIG. 11 (dashed line 16). It can be seen from curve 15 that for the wind turbine blade without bend/twist couplings, the angle of attack varies in a substantially sinusoidal manner as a function of time. For curve 16, representing the wind turbine blade of FIG. 11, the variations in angle of attack are significantly reduced. This is due to the bend/twist coupling described above. The reduced variations in angle of attack significantly reduce the loads on the wind turbine blade.

(39) It should be noted that the remarks set forth above with reference to FIGS. 10-12 are equally applicable to a situation in which the wind turbine is subjected to wind weer, i.e. variations in wind speed along a horizontal direction, rather than along a vertical direction. Wind weer causes similar variations in deflection of the wind turbine blade during a full rotation of the rotor as wind shear, and therefore bend/twist couplings will reduce variations in angle of attack in the same manner as described above in this case.

(40) FIG. 13 shows a wind turbine blade 5 according to an embodiment of the invention at two different pivot angles. The wind turbine blade 5 is very similar to the wind turbine blades 5 illustrated in FIGS. 1-5, and it will therefore not be described in detail here. The solid line shows the wind turbine blade 5 at minimum pivot angle, and the dashed line shows the wind turbine blade 5 at a larger pivot angle.

(41) The inertia of the wind turbine blade 5 can be calculated as:
Σ.sub.ir.sub.i.sup.2.Math.m.sub.i,
where i indicates infinitesimal portions of the wind turbine blade 5, r.sub.i is the distance between the portion i and the rotational axis of the rotor, and m.sub.i is the mass of the portion i. One of these infinitesimal portions is shown in FIG. 13.

(42) The bend of the wind turbine blade 5 arranges the centre of mass of the wind turbine blade 5 closer to the rotational axis of the rotor, when the wind turbine blade 5 is arranged at or near minimum pivot angle. Thereby the inertia of the wind turbine blade 5 is reduced, and can easily be overcome by wind acting on the wind turbine blades 5, and thereby it is easy to start the wind turbine 1 at cut-in wind speed.

(43) FIG. 14 is a graph showing rotor inertia of a wind turbine as a function of rotor speed. It can be seen that at low rotor speed the drivetrain losses are relatively high. During start-up of a wind turbine it is therefore desirable to increase the rotor speed to a level where the drivetrain losses decrease as fast as possible. The reduced inertia described above with reference to FIG. 13 ensures this.

(44) Furthermore, the increased rotor diameter provided by the bend of the wind turbine blades ensure that the rotor speed is increased even faster once the region labelled ‘Area booster’ is reached.

(45) FIG. 15 shows a wind turbine blade 5 according to an embodiment of the invention. The wind turbine blade 5 is arranged at a position defining maximum pivot angle. Thereby the outer blade part 7 is arranged substantially parallel to a rotational axis of the hub 3, while the inner blade part 8 is arranged with an angle thereto, due to the angle, α, between the inner blade part 8 and the outer blade part 7.

(46) It can be seen that the angle, α, between the inner blade part 8 and the outer blade part 7 causes the inner tip 5a to be arranged further away from the tower 2 than would be the case if a wind turbine blade without a bend was used. Accordingly, due to the bend, the wind turbine blade 5 can be mounted on the blade carrying structure 4 at a position closer to the tower 2, and thereby the centre of mass for the hub 3, the blade carrying structure 4 and the wind turbine blades 5 can be moved closer to the tower 2, without risking collisions between the wind turbine blades 5 and the tower 2, and without having an increased coning angle. This reduces uneven loads on the wind turbine 1.

(47) Furthermore, the centrifugal force acting on a given infinitesimal part of the wind turbine blade 5, can be calculated as:
r.sub.i.Math.m.sub.i.Math.ω.sup.2,
where r.sub.i is the distance between the infinitesimal part and the rotational axis of the hub 3, m.sub.i is the mass of the infinitesimal part and ω is the rotational speed of the hub.

(48) Thus, for a given rotational speed, ω, the centrifugal force acting on the infinitesimal part of the wind turbine blade 5 is given by the distance, r.sub.i, and the mass, m.sub.i. It can be seen from FIG. 15 that the distance between the inner blade part 8 and the rotational axis of the hub 3 is larger than would be the case if a wind turbine blade without a bend was applied. This allows a given centrifugal force, and thereby a desired behaviour of the wind turbine blade 5, to be obtained with a lower mass of the inner blade part 8. This reduces the total mass of the wind turbine blade 5, and thereby the loads on the wind turbine 1 as well as manufacturing costs.

(49) FIGS. 16a-16c illustrate a wind turbine blade 5 according to an embodiment of the invention at two different pivot angles, Ψ. Wind turbine blade 5 is shown at minimum pivot angle, and wind turbine blade 5′ is shown at a larger pivot angle. FIG. 16a is a side view of the wind turbine blade 5, FIG. 16b is a front view of the wind turbine blade 5, and FIG. 16c is a cross sectional view along the line A-A shown in FIG. 16b.

(50) In FIGS. 16a and 16b, a line 17 extending perpendicularly to the rotational axis 18 of the hub (not shown), and interconnecting the rotational axis 18 of the hub and the position of the hinge 6 is shown. It should be noted that the line 17 is shown for illustrative purposes, and is not a structural part of the wind turbine.

(51) As best seen in FIG. 16b, the rotational axis 19 of the hinge 6 forms an angle, Φ, with respect to the line 17, which is slightly smaller than 90°. Thereby, when the pivot angle, Ψ, is increased, the wind turbine blade 5 is also rotated about its torsional axis. This is illustrated in FIG. 16c, where the wind turbine blade 5′ is rotated about its torsional axis relative to the wind turbine blade 5.

(52) As illustrated in FIG. 16c, this affects the angle of attack, δ, in such a manner that, given that the relation between the rotational speed, ω, of the rotor and the wind speed, v.sub.wind, is fixed, the angle of attack, δ, of the wind turbine blade 5 can be controlled for any wind speed. In the case that Φ<90°, a close to constant angle of attack, δ, can be maintained passively throughout the wind speeds where the wind turbine produces power, by adjusting the pivot angle, Ψ, as illustrated in FIG. 16c. Alternatively, in the case that Φ>90°, it is possible to passively control at which wind speed the blade will stall, hence pushing it towards maximum pivot angle.

(53) Additionally, at hinge angles Φ≠90°, the torsional movement of the blade carrying structure 4 will not be directly coupled to the edge-wise movement of the wind turbine blade 5. This introduces a considerable amount of damping of the torsional movements in the blade carrying structure 4 from the partly edge/flap coupled blade movements.

(54) Additionally, at hinge angles Φ≠90°, the hinge 6 has a direct impact on blade direction of movement during blade vibrations. As the aeroelastic damping is sensitive to this direction of vibration relative to the incoming wind, the damping can be adjusted by the skew hinge angle and therefore be an efficient method to eliminate risk of, e.g., edgewise blade vibrations.