GB2485595A

GB2485595A - Wind turbine

Info

Publication number: GB2485595A
Application number: GB1019693.9A
Authority: GB
Inventors: Garrett Moran; Kim Mittendorf; Shanshan Mcneill; Ryan Link; Rasmus Svendsen
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2010-11-19
Filing date: 2010-11-19
Publication date: 2012-05-23
Also published as: GB201019693D0

Abstract

The wind turbine comprises a tower, a rotor supported by a tower and including at least one blade 5 which is configured to rotate about a substantially horizontal axis and includes an adjustable stiffening element 21. A sensor is configured to sense a parameter from which deflection of the blade may be determined and a controller acts to adjust the stiffening element in response to signals received from the sensor. The stiffening element preferably comprises a tension element that is anchored to and off-set from a spar extending along the length of the blade and attached to a tensioning device 24 which is preferably located in the hub of the turbine. The blade may include a number of stiffening elements, one or more of which may instead act as compression elements. Also claimed is a wind turbine power plant and a method of operating a wind turbine.

Description

I

SYSTEM AND METHOD FOR REDUCING EDGEWISE VIBRATION IN A WIND

TURBINE ROTOR BLADE AND THE LIKELIHOOD OF A TOWER STRIKE IN A WIND

TURBiNE

FIELD OF THE INVENTION

The invention relates to wind turbines. More particularly, the invention relates to a system and method for reducing edgewise vibration in a wind turbine rotor blade and reducing the likelihood of a tower strike in a wind turbine.

BACKGROUND OF THE INVENTION

Edgewise vibration can cause significant damage to wind turbine rotor blades. The repeated stress caused by the vibrations can cause the fibres of the blades to break. This can result in blades requiring frequent repair and, ultimately, lead to reduced blade service life. Wind turbines are often erected in remote and hostile environments where repairing or replacing a blade can be very difficult and extremely costly.

Similarly, a wind turbine rotor blade striking the tower, often referred to as a tower strike, can cause catastrophic damage to a wind turbine rotor blade and indeed the entire wind turbine. As blades have increased in length and become more flexible so the risk of a tower strike has increased.

Many systems are known which attempt to identify the likelihood of a tower strike such that, if a tower strike is considered likely, evasive action may be taken to avert the threat. For example, the pitch of the blades or the speed of the rotor may be adjusted to reduce the loading on the blades and so reduce the bending or deflection of the blades. In an extreme case the wind turbine may be switched off until the threat has passed. These actions reduce the likelihood of a tower strike but often at the expense of reduced power output from the wind turbine.

The present invention aims to address these problems.

SUMMARY OF THE INVENTION

According to the invention there is provided a wind turbine, comprising: a tower extending along a substantially vertical axis; a rotor supported by the tower, the rotor including at least one blade configured to rotate about a substantially horizontal axis thereby defining a rotational plane, the at least one blade including an adjustable stiffening element; a sensor configured to sense a parameter from which deflection of the blade may be determined; and a controller communicating with the sensor and the stiffening element, the controller being configured to adjust the stiffening element in response to signals received from the sensor.

The inventors have appreciated that by increasing the stiffness of a wind turbine rotor blade, an undesirable movement or deflection of the blade may be countered. The deflection may be caused by edgewise vibration and thus be substantially in the rotational plane. Alternatively or in addition, the deflection may be from the rotational plane, for example in the direction of the tower. Thus, embodiments of the present invention, advantageously, discourage edgewise vibration and reduce the likelihood of a tower strike.

The stiffening element may be coupled to a spar or load bearing member. The spar may be a beam or comprise reinforced areas of the shell interconnected by a web. In a preferred embodiment, the stiffening element comprises a tension element offset from the spar, the tension element having at least one end anchored to the spar, and a tensioner for varying the tension in the tension element. When the tensioner increases the tension in the tension element, the stiffness of the blade can be increased.

An arrangement for stiffening a blade using a tension element offset from the spar is disclosed in W02010/091773. However, in that invention, the tension element is anchored to the spar at both of its ends and is simply used to set the stiffness of a wind turbine rotor blade before the wind turbine is erected. As wind turbine blades have become longer, loading on the blades, particularly edgewise loading, has increased. That invention provides a blade structure which can support the additional edgewise loading generated by the increased blade length. In contrast, embodiments of the present invention provide active control of a tension element to increase the stiffness of the blade to reduce for example edgewise vibrations or the likelihood of a tower strike.

Preferably a support is coupled to the spar and extends away from the spar, the tension element being coupled to the support so as to be offset from the spar. In a preferred embodiment of the invention, the tension element passes through an aperture in the support and can move through the aperture.

Preferably, there are a plurality of supports coupled to the spar and extending away from the spar at different locations along the spar, the tension element being coupled to each of the supports so as to be offset from the spar. This enables the load experienced by the spar when the tensioner increases the tension in the tension element to be spread over the spar.

In a preferred embodiment, the tensioner is coupled to a second end of the tension element. Preferably, the tensioner is provided in the hub of the wind turbine. This is advantageous, as it will be appreciated that the force at which the tensioner must pull the tension element to increase the tension in the tension element will be very large. Thus, the tensioner is likely to be large and bulky. In addition, location in the hub is desirable as power can then be supplied to the tensioner without having to run cables down the blades.

Repair of the tensioner, if needed, is also easier as the tensioner is more assessable if it is mounted in the hub. However, the tensioner may be installed in the blade instead.

The tension element may be anchored to the spar at the tip end of the spar or at a point away from the tip end of the spar. Generally spars are larger at the root of the blade than the tip, therefore the spar may be able to belier withstand the loads exerted by the tension element on the spar if the tension element is anchored at a point away from the tip end of the spar.

In a preferred embodiment, the end of the tension element anchored to the spar is embedded in the spar and may extend around the spar. Thus, the tension element may be securely attached to the spar. Moreover, the tension element may thereby be attached to the spar over a region of the spar. Thus, the load exerted by the tension element on the spar, may be spread over a region of the spar.

The tension element may be a cable, which may be made of steel or nylon. In a preferred embodiment the tension element comprises a rope having a plurality of nylon chords. This tension element is particularly strong.

In an alternative embodiment the tension element may be a rod, which may be made of carbon fibre. The rod may in addition or alternatively be used as a compression element.

In a further embodiment, the stiffening element comprises a compression element offset from the spar, the compression element having at least one end anchored to the spar; and a compressioner for varying the compression in the compression element.

The tension or compression element may be arranged on the trailing edge side of the wind turbine blade or the leading edge side of the wind turbine blade. The tension or compression element may further be arranged on the upper shell side of the wind turbine blade or the lower shell side of the wind turbine blade. Arrangement on the trailing or leading edge side of the wind turbine blade enables deflections of the blade in the edgewise direction, to be countered, and thus edgewise vibration to be reduced.

Arrangement on the upper shell side or the lower shell side of the wind turbine blade, enables deflections of the blade from the rotational plane, that is for example in the direction of the tower to be countered, and thus the likelihood of a tower strike to be reduced.

The at least one blade may include a plurality of stiffening elements.

In one embodiment, the at least one blade includes at least one tension or compression element arranged on the trailing edge side of the wind turbine and at least one tension or compression element arranged on the leading edge side of the wind turbine. This embodiment is particularly desirable for reducing edgewise vibrations as deflections in both the direction of the leading edge and the direction of the trailing edge can be countered.

In a preferred embodiment, the at least one blade includes at least one tension or compression element arranged on the trailing edge side of the lower shell side of the wind turbine blade and at least one tension or compression element arranged on the leading edge side of the lower shell side of the wind turbine blade. This embodiment is particularly desirable for reducing the likelihood of a tower strike as the arrangement may be used to bend the blade in the flapwise direction, away from the tower, and thus deflections from the rotational plane may be countered.

In a further embodiment, the at least one blade further includes at least one tension or compression element arranged on the trailing edge side of the upper shell side of the wind turbine blade and at least one tension or compression element arranged on the leading edge side of the upper shell side of the wind turbine. Advantageously, this embodiment may be used to introduce bend twist coupling into the blade. Bend twist coupling refers to the dynamic relationship between bending and torsional motion of a blade. In this embodiment the tensioners coupled to each of the tension or compression elements may be operated to increase the tension or compression in the tension or compression elements by different amounts. Thus twist may advantageously be introduced into the blade. Thus the blades may be pitched so as to reduce the loading in the blades and therefore, for example, the likelihood of a tower strike.

The invention also resides in a wind turbine power plant comprising: a plurality of wind turbines, at least one of the wind turbines comprising a tower extending along a substantially vertical axis, a rotor supported by the tower, the rotor including at least one blade configured to rotate about a substantially horizontal axis thereby defining a rotational plane, the at least one blade including an adjustable stiffening element, and a sensor configured to sense a parameter from which deflection of the blade may be determined; the wind turbine power plant further comprising a controller communicating with the sensor and the stiffening element, the controller being configured to adjust the stiffening element in response to signals received from the sensor.

The invention further resides in a method of operating a wind turbine having a tower extending along a substantially vertical axis and a rotor supported by the tower, the rotor including at least one blade, the method comprising allowing the rotor to be driven by the wind so that the blade rotates about a substantially horizontal axis, the at least one blade including a stiffening element, sensing a parameter with a sensor from which blade deflection may be determined, the sensor communicating with a controller, and adjusting the stiffness of the stiffening element in response to signals received from the sensor.

In embodiments where the stiffening element comprises a tension element offset from the spar, the tension element having at least one end anchored to the spar; and a tensioner for varying the tension in the tension element, adjusting the stiffness of the stiffening element in response to signals received from the sensor comprises operating the tensioner to vary the tension in the tension element.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a view of a wind turbine; Figure 2 shows a schematic diagram of a system embodying the invention; Figure 3 shows a schematic illustration of a wind turbine rotor blade Figure 4a and 4b show cross sections through a conventional wind turbine rotor blade; Figure 5 shows a schematic view of a wind turbine rotor blade having an adjustable stiffening element embodying the invention; Figures 6a and 6b show the blade shown in Figure 5 before and after operation of the adjustable stiffening element; Figure 7 illustrates an embodiment of the adjustable stiffening element arranged along the blade; Figure 8 illustrates a second embodiment of the adjustable stiffening element arranged along the blade; Figure 9 shows a cross section of a conventional wind turbine rotor blade and an adjustable stiffening element embodying the invention arranged for use in countering flapwise bending; and Figure 10 shows a further adjustable stiffening element embodying the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As shown in Figure 1, a wind turbine 1 generally comprises a nacelle 3 mounted for rotation on a tower 2. A rotor 4 comprising a plurality of rotor blades 5 and a hub 6 is mounted to the nacelle 3. The hub 6 drives a generator (not shown) housed within the nacelle 3, typically via a rotor shaft extending from the nacelle front and one or more gear stages, to generate power.

The wind turbine I of Figure 1 is a horizontal axis wind turbine. The tower 2 extends along a substantially vertical axis and the blades 5 are configured to rotate about a substantially horizontal axis thereby defining a rotational plane.

Figure 2 shows a schematic drawing of a system embodying the invention. A sensor 7 is arranged or configured to sense a parameter from which deflection of a blade 5 may be determined. The sensor 7 may be configured to measure deflections from the rotational plane, for example towards the tower 2. Alternatively or in addition, the sensor 7 may be configured to measure deflections in the rotational plane. Thus, deflections in the plane of the leading and trailing edge, the edgewise direction, may be measured or determined. It will be appreciated that, although the edgewise direction will have a large component in the rotational plane, generally the edgewise direction will not coincide exactly with the rotational plane since the edgewise direction depends on the pitch angle and twist of the blade 5. Thus, deflections of the blade 5 in the edgewise direction will generally have a component from as well as in the rotational plane. Similarly, deflections in the flapwise direction will generally have a component in as well as from the rotational. plane.

Preferably, where there are a plurality of blades 5, as in this example, one or more sensors 7 are arranged to determine deflection of each of the blades 5.

Many different arrangements and types of sensor 7 may be used and will occur to those skilled in the art.

For example, in one embodiment, one or more sensors 7 may be mounted on each of the blades 5 so as to sense deflection in the flapwise and/or edgewise direction. In this case, the sensors 7 may be attached to the surface of the blade 5, for example by an adhesive, or embedded, wholly or partially, in the blade 5. The sensors 7 may be strain gauges or optical fibre sensors for sensing strain or deformation. Alternatively, the sensors 7 may be accelerometers. Thus, in this embodiment, deflection of the blades 5 may be measured directly.

In an alternative embodiment, deflection of the blades 5 from the rotational plane may be measured indirectly by measuring the distance between the blades 5 and the tower 2. For example, a proximity sensor such as a laser may be mounted to the tower 2, blade tip, nacelle 3 or hub 6, and configured to detect the distance between the blades 5 and the tower 2 each time the blades 5 pass the tower 2. Assuming the tower 2 remains stationary, by detecting a change in the distance between the blade 5 and the tower 2, deflection of the blades 5 from the rotational plane may be measured.

Particularly when determining deflection of the blade 5 towards the tower 2, and preferably when determining deflection in the edgewise direction, preferably the sensor 7 is arranged to determine deflection at least in the region of the tip of the blade 5. Due to the length of wind turbine rotor blades 5, the tip of a blade 5 is most likely to experience the greatest bending force and is therefore the part of the blade 5 which, in the event of a tower strike, is most likely to strike the tower 2 first. In addition, the amplitude of blade vibrations is generally largest, and thus most easily detectable, at the tip of the blades 5.

In a further embodiment, the sensor 7 may be configured to sense a parameter of the wind, for example wind speed, to determine deflection of the blades 5 from the rotational plane.

If, for example, the wind speed upstream of the wind turbine 1 is determined to be very high, it may be expected that the force at which the wind will hit the blades 5 will also be high and therefore it may be predicted or determined that deflection of the blades 5 toward the tower will be large. Many systems for sensing upwind conditions exist. For example a lidar or sonar system may be mounted to the hub 6 of the wind turbine 1 so as to sense wind conditions in front of the wind turbine 1.

Referring back to Figure 2, the sensor 7 is in communication with a controller 8. The communication may be direct or indirect. The sensor 7 and the controller 8 may be connected by wires or cables. Alternatively, the sensor 7 and the controller 8 may be connected wirelessly.

The controller 8 may be an onboard controller located, for example, in the hub 6 of the wind turbine 1 or a central wind power plant controller (not shown) which may control a plurality of wind turbines. In this example, each of the blades 5 includes an adjustable stiffening element 9 for adjusting the stiffness of the blade 5. The controller 8 is configured to adjust each of the stiffening elements 9 in response to signals received from the sensor 7.

If, in view of the signals received from the sensor 7, it is determined for example that a tower strike is likely or edgewise vibration is large, the controller 8 may send a signal to the stiffening elements 9 to increase the stiffness of the blade 5. In one embodiment, the controller 8 comprises a processor for determining whether the deflection of the blades 5 exceeds a predetermined minimum value. Different predetermined minimum values may be set for deflection from the rotational plane and for deflection in the edgewise direction. If the deflection is determined to exceed at least one of the predetermined values, the controller 8 may send a signal to adjust at least one of the stiffening elements 9 to temporarily increase the stiffness of the blade 5 such that the deflection of the blade 5 may be countered.

Where the deflection is in the direction of the tower 2, increasing the stiffness of the blade 5 in the flapwise direction may thereby prevent or at least reduce the likelihood of a tower strike. Where the deflection is in the edgewise direction increasing the stiffness of the blade 5 in the edgewise direction may thereby reduce the vibrations, that is dampen the vibrations.

After the deflection has been countered, the stiffness of the blade 5 may then be reduced to its previous level. This is desirable as increasing the stiffness of the blades 5 increases the natural frequency of vibrations in the blade 5, and thus maintaining the stiffness at an increased level could lead to damage of the wind turbine rotor blade 5.

Figure 3 shows a schematic illustration of a wind turbine rotor blade 5. The blade 5 has a leading edge 10 and a trailing edge 11. The root 12 of the blade 5 is the end mounted to the hub 6. The opposite end is the tip 13 of the blade 5.

Figures 4a and 4b show representative cross sections through a wind turbine rotor blade 5.

These figures illustrate how the blade 5 is typically comprised of a sheD 14 which is moulded from upper and lower shell halves 15, 16. The shell 14 is typically made from composite material formed of reinforcement fibres, such as carbon or glass, with a resin matrix. The shell 14 is designed to be as lightweight as possible and is arranged to transfer aerodynamic forces onto a load carrying or load bearing member 17 which extends along the length of the blade 5 from the root 11 towards the tip 12. Such load bearing members 17 are referred to as spars.

Two main types of spars are commonly used. The arrangement shown in Figure 4a comprises load bearing fibres 18, 19 embedded into the upper and lower shell halves 15, 16 respectively and spaced apart by a web 20. Alternatively, it is common to use a beam having a square or rectangular cross section as shown in Figure 4b. Where the load bearing member 17 comprises strengthening fibres, those fibres are embedded in the shell 14 during manufacture of the shell 14. Where a separate beam is used, the beam is manufactured first, and the shell 14 is added afterwards, for example, the beam is placed between the shell halves 15, 16 before they are joined to form the shell 14.

Figure 5 illustrates one embodiment of an adjustable stiffening element 21 arranged for adjusting the stiffness of the blade 5 in the edgewise direction. Figure 5 shows a schematic plan of a wind turbine rotor blade 5 and it will be appreciated that the stiffening element 21 to be described is an internal feature of the blade 5. Although not shown in this figure, the stiffening element 21 is coupled to the spar 17 of the blade 5.

The stiffening element 21 comprises a tension element 22 which extends longitudinally along the blade 5 and which is offset or spaced apart from the spar by a plurality of supports 23. The tension element 22 comprises two ends. One end is anchored to the spar (not shown) at the tip end of the spar. The other end is anchored to a tensioner 24 for varying the tension of the tension element 22. In this example, the tensioner 24 is arranged in the hub 6 of the wind turbine I (not shown). The plurality of supports 23 are spaced along the spar. Although not shown in this figure, the supports 23 comprise an aperture through which the tension element 22 can move.

In this example, the tension element 22 is a cable. The tension element 22 is preferably made from a high strength material such as steel or nylon and in a preferred embodiment is made from multiple nylon chords weaved into a rope. The cross section of the tension element 22 may take a variety of shapes.

Figures 6a and 6b illustrate the blade 5 and stiffening element 21 before and after adjustment. In Figure 6a the blade 5 is bending or deflecting in the edgewise plane. When the controller 8 sends a signal to increase the stiffness of the blade 5, the tensioner 24 is activated to increase the tension in the tension element 22. In other words, the tensioner 24 exerts a pulling force on the tension element 22. Since the tension element 22 can move freely through the apertures of the supports 23, the tension is transmitted down the tension element 22 to the end of the tension element 22 anchored to the blade 5. The supports 23, which offset the tension element 22 from the spar, act to redirect the force applied by the tensioner 24 so as to introduce a force component to counter the deflection or bend in the edgewise plane. That is, the force component is in the opposite direction to the deflection of the blade 5. The supports 23 can be considered to be moment arms.

Thus, in this example the stiffening element 21 acts to straighten the blade 5. In Figure 6b the blade 5 is straight again. After the blade 5 has been returned to a desired position, the tensioner 24 reduces the tension in the tension element 22.

It will be appreciated that a plurality of supports 23, although preferable, is not essential.

Where there are a plurality of supports 23, each support 23 which is arranged at a point where the blade 5 is bending may act to redirect the force applied by the tensioner 24.

Thus, the load experienced by the spar may be spread over a length of the spar.

The tension element 22 is anchored or attached to the spar 5 in such a way that it does not break free from the spar when the tensioner 24 increases the tension in the tension element 22. In a preferred embodiment the end of the tension element 22 may be embedded in the spar and may extend around the spar. This may provide a stronger attachment and in addition may spread the load at the point of attachment over a larger area of the spar.

Many types of support 23 are possible. For example the support 23 may be a strut comprising a pair of legs or three legs, each of which is coupled to the spar. The struts may be coupled to the spar by anchor points moulded into the spar. These anchor points may comprise metal plates having eyelets to which corresponding eyelets at the ends of the struts may be attached.

Figure 7 illustrates an arrangement of the stiffening element 21 described above in more detail. A spar 17 is illustrated extending along the length of the blade 5. In this embodiment, a first and second tension element 22 are arranged along the length of the blade 5. The first along the trailing edge 11 side of the spar 17 and the second along the leading edge 10 side of the spar 17. Referring to figure 4b, where the spar 17 is a beam, the supports 23 may be anchored to the spar 17 at the sides of the spar 17 connecting the upper and lower shell. The supports 23 extend towards the leading and trailing edge 10, 11 respectively. Two tensioners 24 are provided in the hub 6 of the wind turbine (not shown) each connected to a tension element 22 for varying the tension in the first and second tension element 22 respectively.

Advantageously, in this embodiment, the tensioners 24 may be activated to increase the stiffness of both the leading and the trailing edge 10, 11. Thus, edgewise vibration may be reduced or dampened in both the direction of the leading edge 10 and the direction of the trailing edge 11. Alternatively the tension element 22 may be arranged along only the leading edge side or trailing edge side 10, 11 of the spar 17.

Figure 8 shows an alternative embodiment of the stiffening element 21 described above.

This embodiment is exactly the same as the embodiment described above except the tension elements 22 extend only an intermediate distance along the spar 17, such as half or a third of the way from the root 12 of the blade towards the tip 13 of the blade 5. Many spars 17 have an enlarged portion at the root end 12 of the blade 5 with a width equal to the root diameter. This enlarged portion tapers gradually towards the point of maximum chord width at which point the spar becomes narrower. Thus, at a point away from the tip end of the spar 17, the spar 17 is more likely to be able to withstand the loads exerted on the spar 17.

Figure 9 shows a cross section of a wind turbine blade 5 comprising the stiffening element 21 described above arranged along the lower shell 15 side of the wind turbine blade 5.

This embodiment can be used to minimize deflections from the rotational plane, in the direction of the tower 2, and therefore to reduce the likelihood of a tower strike. In this example, the spar 17 is a beam with a substantially rectangular cross section. The wind turbine blade 5 comprises at least two of the stiffening elements 21 described above. The tension element 22 of the first stiffening element 21 is arranged at the corner of the spar abutting the lower shell 15 at the leading edge 10 side of the wind turbine rotor blade 5.

The tension element 22 of the second stiffening element 21 is arranged at the corner of the spar 17 abutting the lower shell 15 at the trailing edge 11 side of the wind turbine rotor blade 5. As described above, both of the tension elements 22 are offset from the spar 17.

Thus, in this embodiment, when the tensioners 24 are activated, the stiffness in the flapwise direction of the blade 5, as well as the edgewise direction of the blade 5, may be increased and thus the arrangement may be used to bend the blade 5 in the flapwise direction, away from the tower 2. Thus, deflections of the blade 5 in the direction of the tower 2 may be countered. Having two stiffening elements 21 enables the stiffness of the blade 5 in the flapwise direction to be increased by a large amount. In an alternative embodiment only one tension element 22 may be arranged at one corner of the spar 17 abutting the lower shell 22.

In the embodiments described above, the tension element 22 is a cable. In an alternative embodiment, the tension element 22 described may be a rod. The rod may be made of carbon fibre, Advantageously, by using a rod, compression as well as tension may be used to stiffen the blade 5.

Figure 10 illustrates an embodiment where the tension element 22 is a rod. The rod extends along the trailing edge side of the blade 5 and is offset from the spar 5. The rod is anchored to the blade 5 at one end, towards the tip end of the spar 17, and connected to a tensioner 24 in the hub 6 of the wind turbine (not shown) at its other end. In this embodiment the rod is offset from the blade 5 using a plurality of supports 23 similar to those described above, but may be offset from the blade 5 by other means provided that the rod can move along the blade 5. The embodiment works using the same principle as when the tension element 22 is a cable. Due to the offset of the rod from the spar 17 when the tension in the rod is increased by the tensioner 24, the rod acts to pull the blade 5 in a direction perpendicular to the length of the spar 17 so as to stiffen the blade 5 and counter deflections of the blade 5. The rod may be arranged along various sides of the spar 17 as discussed above in relation to the cable embodiment.

In a further embodiment, the rod is a compression element and the tensioner 24 a compressioner. All other features of the stiffening element 21 are the same as described above. In This embodiment, the compressioner acts to push the compression element so as to increase compression in the element. This also acts to stiffen the blade 5. It will be appreciated that, when the compression element is compressed, the element acts to move the blade 5 in the opposite direction to that which the blade 5 is moved when the element is under tension. In a preferred embodiment, the rod may alternately be tensioned and compressed to act to counter deflection for example in the direction of the leading and trailing edge 10, 11 respectively. The tensioner 24 and compressioner in such an embodiment may be the same element.

In a further embodiment, two tension or compression elements 22 may be arranged as illustrated in Figure 9 at the lower shell 15 side of the wind turbine blade 5. Two further tension or compression elements 22 may be arranged similarly at the upper shell 16 side of the blade 5, one at the corner of the spar 17 towards the leading edge 10 side of the blade and a second at the corner of the spar 17 towards the trailing edge 11 side of the spar 17.

Each of the tension or compression elements 22 may be coupled to a tensioner 24 or compressioner which is arranged in the hub 6 of the wind turbine (not shown). For example, when a tower strike is considered likely, the tensioners 24 or compressioners may be controlled individually to vary the tension or compression in the elements by different amounts. Advantageously, in this embodiment, therefore the tension or compression of the elements may be adjusted such that flapbend-twist or edgebend-twist may be introduced into the blade 5. Thus, the pitch of the blades 5 may be adjusted, and so loading on the blades 5 may be reduced to reduce deflection of the blades from the rotational plane.

Thus, advantageously, embodiments of the present invention provide a system and method for reducing edgewise vibration and the likelihood of a tower strike. Although in all of the described embodiments the tension element 22 is coupled to a tensioner at one end, this need not always be the case. It may be possible to anchor the tension element at both of its ends to the spar for example, and arrange the tensioner intermediate the ends to vary the tension in the element. The tensioner 24 need not be provided in the hub although this arrangement is particularly advantageous. The invention is defined solely by the following claims.

Claims

CLAIMS1. A wind turbine, comprising: a tower extending along a substantially vertical axis; a rotor supported by the tower, the rotor including at least one blade configured to rotate about a substantially horizontal axis thereby defining a rotational plane, the at least one blade including an adjustable stiffening element; a sensor configured to sense a parameter from which deflection of the blade may be determined; and a controller communicating with the sensor and the stiffening element, the controller being configured to adjust the stiffening element in response to signals received from the sensor.
2. A wind turbine according to claim 1, wherein the parameter is a parameter of the wind.
3. A wind turbine according to claim 1, wherein the sensor is configured to measure deflections of the blade.
4. A wind turbine according to claim 3, wherein the sensor is configured to measure deflections of the blade from the rotational plane.
5. A wind turbine according to claim 3, wherein the sensor is configured to indirectly measure deflections of the blade from the rotational plane by measuring a distance between the blade and the tower.
6. A wind turbine according to claim 3, wherein the sensor is configured to measure deflections of the blade in the rotational plane.
7. A wind turbine according to any of claims 3 to 6, wherein the sensor comprises a strain-gauge sensor or accelerometer mounted on the blade.
8. A wind turbine according to any of claims 3 to 7, wherein the controller is configured to adjust the stiffening element only when the sensor measures deflections that exceed a predetermined minimum value.
9. A wind turbine according to claim any preceding claim, wherein the at least one blade comprises: a shell having a leading edge and trailing edge extending from a root to a tip, the shell defining first and second sides extending between the leading edge and trailing edge; and a spar supporting at least a portion of the shell between the first and second sides, the stiffening element being coupled to the spar.
10. A wind turbine according to claim 9, wherein the stiffening element comprises a tension element offset from the spar, the tension element having at least one end anchored to the spar; and a tensioner for varying the tension in the tension element.
11. A wind turbine according to claim 10, further comprising at least one support coupled to the spar and extending away from the spar, the tension element being coupled to the at least one support so as to be offset from the spar.
12. A wind turbine according to claim 11, wherein the tension element passes through an aperture in the at least one support.
13. A wind turbine according to any of claims 10 to 12, wherein the tensioner is provided in the hub of the wind turbine.
14. A wind turbine according to any of claims 10 to 13, wherein the tension element is anchored to the spar at the tip end of the spar.
15. A wind turbine according to any of claims 10 to 13, wherein the tension element is anchored to the spar at a point away from the tip end of the spar.
16. A wind turbine according to any of claims 13 to 15, wherein the end of the tension element anchored to the spar is embedded in the spar.
17. A wind turbine according to any of claims 10 to 16, wherein the end of the tension element anchored to the spar extends around the spar.
18. A wind turbine according to any of claims 10 to 17, wherein the tension element is a cable or rope.
19. A wind turbine according to claim 9, wherein the stiffening element comprises a compression element offset from the spar, the compression element having at least one end anchored to the spar; and a compressioner for varying the compression in the tension element.
20. A wind turbine according to claim 19, comprising a further stiffening element comprising a tension element offset from the spar, the tension element having at least one end anchored to the spar; and a tensioner for varying the tension in the tension element.
21. A wind turbine according to any of claims 9 to 20, wherein the element is a rod.
22. A wind turbine according to any of claims 9 to 21, wherein the at least one blade includes a plurality of the stiffening elements, at least one tension or compression element arranged on the trailing edge side of the wind turbine blade and at least one tension or compression element arranged on the leading edge side of the wind turbine blade.
23. A wind turbine according to claim 22, wherein the at least one blade includes at least one tension or compression element arranged on the trailing edge side of the lower shell side of the wind turbine blade and at least one tension or compression element arranged on the leading edge side of the lower shell side of the blade.
24. A wind turbine according to claim 23, wherein the at least one blade further comprises at least one tension or compression elements arranged on the trailing edge side of the upper shell side of the wind turbine blade and at least one tension or compression element arranged on the leading edge side of the upper shell side of the blade.
25. A wind turbine power plant comprising a plurality of wind turbines, at least one of the wind turbines comprising a tower extending along a substantially vertical axis, a rotor supported by the tower, the rotor including at least one blade configured to rotate about a substantially horizontal axis thereby defining a rotational plane, the at least one blade including an adjustable stiffening element and a sensor configured to sense a parameter from which deflection of the blade may be determined; the wind turbine power plant further comprising a controller communicating with the sensor and the stiffening element, the controller being configured to adjust the stiffening element in response to signals received from the sensor.
26. A method of operating a wind turbine having a tower extending along a substantially vertical axis and a rotor supported by the tower, the rotor including at least one blade, the method comprising: allowing the rotor to be driven by the wind so that the blade rotates about a substantially horizontal axis, the at least one blade including a stiffening element; sensing a parameter from which blade deflection may be determined with a sensor, the sensor communicating with a controller; and adjusting the stiffness of the stiffening element in response to signals received from the sensor.
27. A method according to claim 26, wherein the blade defines a rotational plane when rotating, and wherein the sensor measures deflections from the rotational plane.
28. A method according to claim 26, wherein the blade defines a rotational plane when rotating, and wherein the sensor measures defiections in the rotational plane.
29. A method according to any of claims 26 to 28, wherein the stiffening element comprises a tension element offset from the spar, the tension element having at least one end anchored to the spar; and a tensioner for varying the tension in the tension element, wherein adjusting the stiffness of the stiffening element in response to signals received from the sensor comprises operating the tensioner to vary the tension in the tension element.