WO2025067614A1

WO2025067614A1 - A wind turbine with an anti-swaying system

Info

Publication number: WO2025067614A1
Application number: PCT/DK2024/050210
Authority: WO
Inventors: Anton Bech
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2023-09-28
Filing date: 2024-09-10
Publication date: 2025-04-03
Anticipated expiration: 2026-03-28

Abstract

A wind turbine (1) comprising a tower (2), a nacelle and a hub (4) carrying two or more wind turbine blades (5) is disclosed The wind turbine (1) further comprises an anti-swaying system arranged at an up-tower position of the wind turbine (1), the anti-swaying system comprising at least one air thruster (6) configured to produce an accelerated airflow (10) being adjustable in direction and magnitude. Furthermore, a method for reducing swaying of a wind turbine (1) by operating at least one air thruster (6) is disclosed.

Description

A WIND TURBINE WITH AN ANTI-SWAYING SYSTEM

FIELD OF THE INVENTION

The present invention relates to a wind turbine comprising an anti-swaying system for handling swaying of the wind turbine. The present invention further relates to a method for reducing swaying of a wind turbine.

BACKGROUND OF THE INVENTION

During operation of a wind turbine, the wind turbine is subjected to various impacts that may cause the wind turbine, notably the upper part of the wind turbine, to sway. This could, e.g., be due to wind acting on the wind turbine, and/or waves acting on the wind turbine, in the case that the wind turbine is an offshore wind turbine.

Such swaying movements may be detrimental for various reasons. For instance, repeated and frequent swaying movements may lead to fatigue in various parts of the wind turbine, in particular in the tower. Furthermore, in the case that personnel need to enter the wind turbine, e.g. in order to perform maintenance, repair, etc., swaying movements may be inconvenient for the personnel, e.g. causing nausea. In addition, if the tasks being performed by the personnel requires handling and/or movement of heavy components, significant swaying of the wind turbine may introduce a risk of hazardous situations.

In addition to the above, swaying of the wind turbine may also be an issue when the wind turbine is not operating. For instance, when wind turbine blades or other large components are being mounted on the wind turbine, swaying of the wind turbine causes significant relative movements between the mounting position at the wind turbine and the component, e.g. wind turbine blade, being mounted. This increases the risk of collisions between the component and the wind turbine, and in general makes it difficult to perform the task. Mounting of wind turbine blades or other large components could, e.g., take place during erection of the wind turbine, or as part of a replacement process.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a wind turbine in which swaying of the wind turbine can be reduced in an easy and efficient manner.

It is a further object of embodiments of the invention to provide a method for reducing swaying of a wind turbine in an easy and efficient manner.

According to a first aspect the invention provides a wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, and a hub configured to carry two or more wind turbine blades mounted rotatably on the nacelle, wherein the wind turbine further comprises an anti-swaying system arranged at an up-tower position of the wind turbine, the anti-swaying system comprising at least one air thruster configured to produce an accelerated airflow being adjustable in direction and magnitude.

Thus, according to the first aspect, the invention provides a wind turbine comprising a tower with a nacelle mounted thereon, via a yaw system. Accordingly, the nacelle is allowed to perform yawing movements relative to the tower in order to position a rotor of the wind turbine in accordance with the direction of the incoming wind.

The wind turbine further comprises a hub configured to carry two or more wind turbine blades. The hub is mounted rotatably on the nacelle, and the hub with the wind turbine blades may be referred to as a rotor. Accordingly, when wind turbine blades are mounted on the hub, and during operation of the wind turbine, wind acting on the wind turbine blades causes the hub to rotate, and this rotating movement is transferred to a generator, possibly via a gear system, where the mechanical energy is converted into electrical energy, which may subsequently be supplied to a power grid. The wind turbine further comprises an anti-swaying system arranged at an up- tower position of the wind turbine. In the present context the term 'up-tower position' should be interpreted to mean a position which is at or near a top of the tower where the nacelle is mounted. The up-tower position may, thus, be on top of the nacelle, on an outer sidewall of the nacelle, inside the nacelle, in a compartment mounted on the nacelle, inside the tower or on an outer surface of the tower immediately below the nacelle, e.g. within the upper 1/3 of the tower, etc. Thus, the anti-swaying system is arranged at a part of the wind turbine where swaying movements may be expected to be most significant.

The anti-swaying system comprises at least one air thruster configured to produce an accelerated airflow being adjustable in direction and magnitude. When activated, the accelerated airflow creates a force in a direction opposite to the direction of the accelerated airflow, and the magnitude of the created force is proportional to the magnitude of the accelerated airflow. Thus, by appropriately varying the direction and magnitude of the accelerated airflow, a force can be created which counteracts the swaying movements of the wind turbine, thereby at least reducing the amplitude of the swaying movement to an acceptable level. This is an easy and efficient manner of reducing swaying movements of a wind turbine.

Accordingly, swaying movements of the wind turbine can be reduced whenever this is required, e.g. when personnel needs to enter the wind turbine, when large components, such as wind turbine blades, need to be installed or dismantled, or when swaying simply exceeds a level which may introduce an increased risk of fatigue.

The at least one air thruster may be arranged in or on the nacelle. According to this embodiment, the at least one air thruster is arranged at or near an upper extremity of the wind turbine, and thus in a position where swaying movements may be expected to be most significant. Furthermore, by arranging the at least one air thruster in or on the nacelle, it is ensured that the at least one air thruster moves along when the nacelle performs yawing movements relative to the tower. Thereby the relative position of the rotor and the at least one air thruster is maintained regardless of the yaw position of the nacelle, and interference between the rotor and the at least one air thruster is thereby avoided.

As an alternative, the at least one air thruster may be arranged in or on an upper part of the tower, or in or on a compartment or similar mounted on the nacelle or the upper part of the tower.

The at least one air thruster may be movable between an operational position and a storage position. In the operational position, the at least one air thruster is capable of producing the accelerated airflow, whereas in the storage position it is not. Accordingly, when it is required to reduce swaying movements, the at least one air thruster may be moved to the operational position, and when it is not required to reduce swaying movements, the at least one air thruster may be moved to the storage position. For instance, the at least one air thruster may be moved to the operational position when personnel is about to enter the wind turbine, immediately prior to instalment or replacement of wind turbine components, such as wind turbine blades, when swaying of the wind turbine exceeds a predefined level, or when any other suitable circumstances are occurring.

The storage position may, e.g., be a retracted position, e.g. inside the nacelle or inside a compartment mounted on the nacelle or an upper part of the tower. In this case the at least one air thruster is protected from wind and weather conditions when it is not in use, and it will not affect the aerodynamic properties of the wind turbine.

On the other hand, the operational position may, e.g., be a position where the at least one air thruster is exposed to the ambient air, and the accelerated airflow can therefore be produced essentially uninhibited.

Thus, according to this embodiment, the at least one air deflector may by default be arranged in the storage position, where it does not interfere with the operation of the wind turbine, and it/they may only be moved to the operational position when it is required to reduce swaying of the wind turbine, such as under the circumstances mentioned above. The at least one air thruster may be mounted at the up-tower position of the wind turbine via an azimuth rotating system allowing the at least one air thruster to perform azimuth rotating movements relative to the wind turbine.

According to this embodiment, the entire air thruster is able to rotate relative to the wind turbine, e.g., relative to the nacelle, about an azimuth axis, i.e. about a substantially vertical axis. Performing such azimuth rotating movements of the air thruster causes the direction of the accelerated airflow produced by the air thruster to change. Accordingly, the direction of the accelerated airflow, and thereby of the force created by the accelerated airflow, can be selected to match the direction of a swaying movement of the wind turbine, simply by appropriately rotating the at least one air thruster by means of the azimuth rotating system.

Alternatively or additionally, the at least one air thruster may be provided with an adjustable air deflector allowing the produced accelerated airflow to be directed in a desired direction, so as to match the direction of a swaying movement of the wind turbine.

The at least one air thruster may comprise at least one adjustable fan, e.g. a ducted fan. Such fans are suitable for producing an accelerated airflow. The fan may be adjustable in terms of direction and/or magnitude of the produced accelerated airflow. The fan may, e.g., be a variable speed fan, so as to enable the magnitude of the accelerated airflow to be adjustable by adjusting the speed of the fan.

As an alternative, the at least one air thruster may comprise a Voith Schneider rotor, a centrifugal ventilator, or any other suitable kind of air thruster being capable of producing an appropriate accelerated airflow.

The anti-swaying system may comprise at least two independently adjustable air thrusters. According to this embodiment, the air thrusters may not be capable of producing an accelerated airflow with a sufficient magnitude to counteract swaying movements of the wind turbine on their own. Airflows produced by two or more air thrusters may, thus, be required in order to obtain the desired reduction of the swaying movements.

Furthermore, since the at least two adjustable air thrusters are independently adjustable, they may be adjusted so as to direct their accelerated airflows along non-parallel directions. This will allow complex non-linear swaying movements of the wind turbine to be counteracted and reduced by appropriately selecting direction and magnitude of the produced accelerated airflows of the respective air thrusters.

The anti-swaying system may further comprise a controller configured to receive sensor input representing a swaying movement of the wind turbine, and to generate control output for the at least one air thruster, based on the received sensor input, so as to cause the at least one air thruster to produce an accelerated airflow creating a force which counteracts the swaying movement of the wind turbine.

According to this embodiment, the at least one air thruster is dynamically operable, based on sensor input being indicative of the current swaying movements of the wind turbine. Accordingly, the swaying movements can be continuously and dynamically counteracted and reduced, essentially in real time.

The wind turbine may be an offshore wind turbine, i.e. a wind turbine arranged offshore, such as at sea or on a lake. Swaying movements may in particular be an issue for offshore wind turbines, since offshore wind turbines are impacted by waves, in addition to being impacted by wind.

According to a second aspect the invention provides a method for reducing swaying of a wind turbine, the wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub configured to carry two or more wind turbine blades mounted rotatably on the nacelle, and an anti-swaying system arranged at an up-tower position of the wind turbine, the anti-swaying system comprising at least one air thruster, the method comprising the steps of: - obtaining sensor input being representative for a swaying movement of the wind turbine,

- generating control signals for the at least one air thruster, based on the sensor input, and

- operating the at least one air thruster in accordance with the generated control signals, wherein the step of operating the at least one air thruster causes the at least one air thruster to produce an accelerated airflow creating a force which counteracts the swaying movement of the wind turbine.

Thus, according to the second aspect, the invention provides a method for reducing swaying of a wind turbine. The wind turbine may advantageously be a wind turbine according to the first aspect of the invention, and the remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here.

More particularly, the wind turbine comprises a tower, a nacelle, a hub configured to carry two or more wind turbine blades, and an anti-swaying system arranged at an up-tower position of the wind turbine, the anti-swaying system comprising at least one air thruster. This has already been described in detail above with reference to the first aspect of the invention.

In the method according to the second aspect of the invention, sensor input being representative for a swaying movement of the wind turbine is initially obtained. The sensor input may, e.g., be obtained from one or more sensors mounted directly on the wind turbine, e.g. in or on the nacelle, in or on the tower, in or on one or more wind turbine blades, etc. The one or more sensors may advantageously be arranged at an upper part of the wind turbine where swaying movements may be expected to be most significant.

The one or more sensors may, e.g., be or include accelerometers, gyroscopes, vibration sensors, etc. Next, control signals for the at least one air thruster are generated, based on the sensor input, and the at least one air thruster is then operated in accordance with the generated control signals, and thereby based on the sensor input being representative for the swaying movement of the wind turbine.

More particularly, the at least one air thruster is operated in such a manner that the at least one air thruster is caused to produce an accelerated airflow creating a force which counteracts the swaying movement of the wind turbine. As described above, the sensor input is representative for the swaying movement of the wind turbine, e.g. in terms of direction and magnitude of the swaying movement. Such a swaying movement may be counteracted by applying a force in an opposite direction with the same magnitude. Accordingly, the control signals should cause the at least one air thruster to produce an accelerated airflow, which results in a force which follows and counteracts a force vector of the swaying movement.

For instance, if the swaying movement is a linearly oscillating movement, then the accelerated airflow of the at least one air thruster should be directed along the direction of the linearly oscillating movement, and the magnitude of the accelerated airflow should be varied in the same manner as the linearly oscillating movement, and in counterphase therewith. In the case that the swaying movement is more complex, variations in the direction of the accelerated airflow may also be required in order to create a force which counteracts the swaying movement.

Thus, simply by appropriately operating the at least one air thruster in accordance with the sensor input, the swaying movement can be counteracted accurately and efficiently, thereby significantly reducing the amplitude of the swaying movement. Moreover, this is obtained regardless of the origin or nature of the swaying movement.

The method may further comprise the step of deriving at least a direction, an amplitude and a phase of the swaying movement of the wind turbine from the sensor input, and the generated control signals may reflect the derived direction, amplitude and phase. The direction, the amplitude and the phase of a swaying movement may be sufficient to characterise the swaying movement, at least in terms of what is required in order to counteract the swaying movement. For instance, applying a force along the same direction, which varies with the same amplitude and with an opposite phase as that of the swaying movement will result in counteracting of the swaying movement, and thereby in a reduction of the swaying of the wind turbine. Accordingly, the control signals being generated based on the sensor input should ensure that the accelerated airflow produced by the at least one air thruster results in such a force.

The step of operating the at least one air thruster may comprise manipulating the at least one air thruster to direct the produced accelerated airflow along a direction of the swaying movement of the wind turbine. As described above, such an airflow will result in a force which counteracts the swaying movement. In the case that the swaying movement is a linearly oscillating movement, the accelerated airflow may be directed along the direction of the linearly oscillating movement. In the case of a more complex swaying movement, a dominating or primary direction of the swaying movement may be identified, and the accelerated airflow may be directed along this dominating or primary direction, so as to provide a reduction of the swaying movement that is as efficient as possible.

The step of manipulating the at least one air thruster may comprise causing the at least one air thruster to perform azimuth rotating movements relative to the wind turbine. According to this embodiment, the entire air thruster, or a relevant part thereof, is rotated relative to the wind turbine, e.g. relative to the nacelle, with the purpose of changing the direction of the accelerated airflow. This has already been described in detail above with reference to the first aspect of the invention.

As an alternative, the direction of the accelerated airflow may be changed by manipulating an air deflector arranged at or near an outlet of the at least one air thruster. The method may further comprise the step of tracing a main direction of the swaying movement of the wind turbine, and the step of operating the at least one air thruster may comprise adjusting a direction of the accelerated airflow in accordance with the traced main direction of the swaying movement of the wind turbine.

According to this embodiment, the main direction of the swaying movement is continuously traced, and the at least one air thruster is operated in such a manner that the direction of the accelerated airflow at least substantially coincides with the main direction of the swaying movement. This allows for complex swaying movements to be counteracted in an accurate and efficient manner.

The step of operating the at least one air thruster may cause an amplitude of the swaying movement of the wind turbine to be reduced, and the method may further comprise the step of mounting one or more wind turbine blades on the hub and/or dismantling one or more wind turbine blades from the hub when the amplitude of the swaying movement has been reduced below a threshold level.

According to this embodiment, the swaying of the wind turbine is counteracted and reduced in order to allow one or more wind turbine blades to be mounted on and/or dismantled from the hub. Accordingly, in this case the relative movements between the hub and the wind turbine blade being mounted or dismantled are reduced, thereby significantly reducing the risk of collisions between the wind turbine blade and the rest of the wind turbine. Furthermore, in the case of mounting of the wind turbine blade, this allows the wind turbine blade to be accurately moved to its installing position in an easy manner.

Mounting of wind turbine blades could, e.g., be performed as part of erection of the wind turbine, or as part of replacement of one or more wind turbine blades. Similarly, dismantling of wind turbine blades could, e.g., be performed as part of dismantling of the wind turbine, or as part of replacement of one or more wind turbine blades. BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a side view of a wind turbine according to a first embodiment of the invention,

Figs. 2 and 3 are top views of the wind turbine of Fig. 1,

Fig. 4 is a top view of a wind turbine according to a second embodiment of the invention,

Fig. 5 illustrates a nacelle of a wind turbine according to a third embodiment of the invention,

Fig. 6 illustrates a nacelle of a wind turbine according to a fourth embodiment of the invention,

Figs. 7 illustrates an air thruster for a wind turbine according to a fifth embodiment of the invention, and

Fig. 8 illustrates an air thruster for a wind turbine according to a sixth embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of a wind turbine 1 according to a first embodiment of the invention. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted rotatably on the tower 2 via a yaw system, so as to allow the nacelle 3 to perform yawing movements relative to the tower 2 in accordance with the direction of the incoming wind.

A hub 4 carrying a number of wind turbine blades 5, two of which are shown, is mounted rotatably on the nacelle 3. Thus, wind acting on the wind turbine blades 5 causes the hub 4 to rotate. The rotational movement is fed to a generator, possibly via a gear system, where it is transformed into electrical energy which may be supplied to a power grid.

The wind turbine 1 is further provided with an anti-swaying system comprising an air thruster 6 mounted on the nacelle 3. The air thruster 6 is capable of producing an accelerated airflow which is adjustable in direction and magnitude.

In the case that a swaying movement, illustrated by arrow 7, of the wind turbine 1 is detected, control signals for the air thruster 6 are generated which causes the air thruster 6 to produce an accelerated airflow which creates a force which counteracts the swaying movement 7 of the wind turbine 1. More particularly, the air thruster 6 is manipulated to direct the accelerated airflow along a direction defined by the swaying movement 7, and to cause the magnitude of the accelerated airflow to vary with an amplitude and a frequency corresponding to the detected swaying movement 7. More particularly, the accelerated airflow is in the same direction as the swaying movement 7, resulting in a reacting force in the opposite direction, and therefore in antiphase therewith. This counteracts the swaying movement 7 of the wind turbine 1, and therefore reduces the swaying.

Figs. 2 and 3 are top views of the wind turbine 1 of Fig. 1. In Figs. 2 and 3 it can be seen that the air thruster 6 is in the form of an adjustable fan 8, e.g. a ducted fan, driven by a motor 9. The motor 9 may be configured to vary the speed of the adjustable fan 8, thereby varying the magnitude of the accelerated airflow, illustrated by arrow 10, produced by the air thruster 6.

The air thruster 6 is mounted on the nacelle 3 via an azimuth rotating system (not shown) allowing the air thruster 6 to rotate relative to the nacelle 3 about an azimuth axis being substantially parallel to the length direction of the tower 2. This allows the direction of the accelerated airflow 10 to be varied by appropriately rotating the air thruster 6 to a suitable azimuth position.

In Fig. 2, the swaying movement 7 of the wind turbine 1 is directed along a substantially fore-aft direction of the wind turbine 1, and thereby substantially parallel to the incoming wind acting on the wind turbine blades 5. Accordingly, the air thruster 6 has been rotated to an azimuth position which ensures that the accelerated airflow 10 produced by the air thruster 6 is arranged substantially parallel to the direction of the swaying movement 7.

In Fig. 3, the swaying movement 7 of the wind turbine 1 is directed along a direction which differs from the fore-aft direction described above with reference to Fig. 2. Accordingly, the air thruster 6 has been rotated to an azimuth position which ensures that the accelerated airflow 10 produced by the air thruster 6 is arranged substantially parallel to this direction, so as to cause the swaying movement 7 to be counteracted by the force created by the accelerated airflow 10.

It should be noted that even though the wind turbine 1 of Figs. 1-3 has been shown with wind turbine blades 5 mounted on the hub 4, swaying movements of a similar wind turbine 1 without wind turbine blades 5 mounted thereon could be reduced in a similar manner, e.g. in order to allow wind turbine blades 5 to be mounted on the hub 4 without risking collisions between the wind turbine blades 5 and the rest of the wind turbine 1.

Fig. 4 is a top view of a wind turbine 1 according to a second embodiment of the invention. The wind turbine 1 of Fig. 4 is very similar to the wind turbine 1 of Figs. 1-3, and it will therefore not be described in further detail here. However, the wind turbine 1 of Fig. 4 is provided with two air thrusters 6 which may be operated individually. The air thrusters 6 illustrated in Fig. 4 are essentially identical to the air thruster 6 illustrated in Figs. 1-3 and described above.

Applying two air thrusters 6 enables a counteracting force to be produced which is larger than a maximum counteracting force produced by a single air thruster 6. This allows for handling of larger and more significant swaying movements of the wind turbine 1, without having to apply an excessively large air thruster.

When operating the air thrusters 6 illustrated in Fig. 4 in order to counteract a swaying movement of the wind turbine 1, the air thrusters 6 may be rotated to azimuth positions which cause the accelerated airflows produced by the respective air thrusters 6 to be substantially parallel to each other. This will result in a large counteracting force along this direction.

As an alternative, different azimuth positions may be selected for the two air thrusters 6. This will cause the accelerated airflows produced by the respective air thrusters 6 to be directed non-parallel to each other. Accordingly, the air thrusters 6 will be able to counteract swaying movements of the wind turbine 1 which are not simple linear oscillating movements, i.e. swaying movements of a more complex movement pattern can be counteracted by appropriately positioning and operating the respective air thrusters 6.

In the embodiment illustrated in Fig. 4, the air thrusters 6 are mounted on two side compartments 11 mounted on the sides of the nacelle 3. It is noted that, as an alternative, the air thrusters 6 may be mounted directly on the nacelle 3, similarly to the situation illustrated in Figs. 1-3, or at another up-tower position of the wind turbine 1.

Fig. 5 is a schematic drawing of a nacelle 3 for a wind turbine according to a third embodiment of the invention. The nacelle 3 has an air thruster 6, in the form of a Voith Schneider rotor, mounted thereon. A Voith Schneider rotor is a vertical axis rotor with a number of pitchable blades 12. The Voith Schneider rotor illustrated in Fig. 5 has four pitchable blades 12. When the Voith Schneider rotor rotates about the vertical axis, it creates an accelerated airflow. The magnitude of the accelerated airflow is determined by the rotational speed of the Voith Schneider rotor. The direction of the accelerated airflow is adjusted by cyclically adjusting the pitch angles of the pitchable blades 12 according to their positions along the rotational movement.

The air thruster 6 of Fig. 5 is movable between an operating position, where it extends above the nacelle 3, and a storage position in which the air thruster 6 is retracted into the nacelle 3. In Fig. 5, the air thruster 6 is shown in the operating position. When it is desired to move the air thruster 6 to the storage position, the air thruster 6 is lowered until it is completed accommodated inside the nacelle 3 and upper part 13 of the air thruster 6 forms a lid closing off the nacelle 3. Accordingly, the air thruster 6 may by default be arranged in the storage position inside the nacelle 3, but whenever reduction of swaying movements of the wind turbine is required, the air thruster 6 may be moved to the operating position shown in Fig. 5, and the air thruster 6 may then be operated so as to produce an accelerated airflow which creates a force which counteracts the swaying movements.

Fig. 6 is a schematic drawing of a nacelle 3 of a wind turbine according to a fourth embodiment of the invention. The nacelle 3 has an air thruster 6 comprising an adjustable fan 8 mounted thereon. The air thruster 6 is, thus, capable of producing an accelerated airflow which is adjustable in direction and magnitude, so as to create a force which counteracts swaying movements of the wind turbine, similarly to what is described above with reference to Figs. 1-3. The remarks set forth above in this regard are therefore equally applicable here.

Similarly to the air thruster 6 shown in Fig. 5, the air thruster 6 of Fig. 6 is movable between an operational position in which the air thruster 6 extends out of the nacelle 3 and a storage position in which the air thruster 6 is completely accommodated inside the nacelle 3. The operational position as well as the storage position is illustrated in Fig. 6. The air thruster 6 is moved between the operational position and the storage position by pivoting the air thruster 6 about pivot axis 14.

When the air thruster 6 is in the storage position, the opening 15 through which the air thruster 6 passes is closed by means of lid 16.

It should be noted that in Figs. 5 and 6, the nacelle 3 is not drawn to scale, and it would normally be significantly larger.

Fig. 7 is a perspective view of an air thruster 6 for a wind turbine according to a fifth embodiment of the invention, in the form of a centrifugal ventilator. In a centrifugal ventilator an impeller is rotated, as indicated by arrow 17, in order to suck air in an axial direction and expel an accelerated airflow 10 in a tangential direction. The accelerated airflow 10 creates a force 18 in an opposite direction. The magnitude of the accelerated airflow 10, and thereby of the created force 18, can be adjusted by adjusting the rotational speed of the impeller.

The air thruster 6 may be mounted on a wind turbine, e.g. on the nacelle of the wind turbine, via an azimuth rotating system, similar to the embodiments described above with reference to Figs. 1-4. Thereby the direction of the accelerated airflow 10 and the created force 18 may be varied by appropriately rotating the air thruster 6 to a suitable azimuth position. Accordingly, the air thruster 6 of Fig. 7 can be applied for reducing swaying movements of a wind turbine by creating a counteracting force. Fig. 8 is a top view of an air thruster 6 for a wind turbine according to a sixth embodiment of the invention, in the form of a centrifugal ventilator, similarly to the air thruster 6 illustrated in Fig. 7. However, the air thruster 6 of Fig. 8 is further provided with an air deflector 19 arranged at the outlet for the accelerated airflow 10. Accordingly, the direction of the accelerated airflow 10, and thereby of the created force 18, can be varied by appropriately manipulating the air deflector 19, notably by pivoting the air deflector 19 about pivot axis 20 between the two extreme positions illustrated in Fig. 8.

Claims

1. A wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2) via a yaw system, and a hub (4) configured to carry two or more wind turbine blades (5) mounted rotatably on the nacelle (3), wherein the wind turbine (1) further comprises an anti-swaying system arranged at an up-tower position of the wind turbine (1), the anti-swaying system comprising at least one air thruster (6) configured to produce an accelerated airflow (10) being adjustable in direction and magnitude.

2. A wind turbine (1) according to claim 1, wherein the at least one air thruster (6) is arranged in or on the nacelle (3).

3. A wind turbine (1) according to claim 1 or 2, wherein the at least one air thruster (6) is movable between an operational position and a storage position.

4. A wind turbine (1) according to any of the preceding claims, wherein the at least one air thruster (6) is mounted at the up-tower position of the wind turbine (1) via an azimuth rotating system allowing the at least one air thruster (6) to perform azimuth rotating movements relative to the wind turbine (1).

5. A wind turbine (1) according to any of the preceding claims, wherein the at least one air thruster (6) comprises at least one adjustable fan (8).

6. A wind turbine (1) according to any of the preceding claims, wherein the antiswaying system comprises at least two independently adjustable air thrusters (6).

7. A wind turbine (1) according to any of the preceding claims, wherein the antiswaying system further comprises a controller configured to receive sensor input representing a swaying movement (7) of the wind turbine (1), and to generate control output for the at least one air thruster (6), based on the received sensor input, so as to cause the at least one air thruster (6) to produce an accelerated airflow (10) creating a force (18) which counteracts the swaying movement (7) of the wind turbine (1).

8. A wind turbine (1) according to any of the preceding claims, wherein the wind turbine (1) is an offshore wind turbine.

9. A method for reducing swaying of a wind turbine (1), the wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2) via a yaw system, a hub (4) configured to carry two or more wind turbine blades (5) mounted rotatably on the nacelle (3), and an anti-swaying system arranged at an up-tower position of the wind turbine (1), the anti-swaying system comprising at least one air thruster (6), the method comprising the steps of:

- obtaining sensor input being representative for a swaying movement (7) of the wind turbine (1),

- generating control signals for the at least one air thruster (6), based on the sensor input, and

- operating the at least one air thruster (6) in accordance with the generated control signals, wherein the step of operating the at least one air thruster (6) causes the at least one air thruster (6) to produce an accelerated airflow (10) creating a force (18) which counteracts the swaying movement (7) of the wind turbine (1).

10. A method according to claim 9, further comprising the step of deriving at least a direction, an amplitude and a phase of the swaying movement (7) of the wind turbine (1) from the sensor input, and wherein the generated control signals reflect the derived direction, amplitude and phase.

11. A method according to claim 9 or 10, wherein the step of operating the at least one air thruster (6) comprises manipulating the at least one air thruster (6) to direct the produced accelerated airflow (10) along a direction of the swaying movement (7) of the wind turbine (1).

12. A method according to claim 11, wherein the step of manipulating the at least one air thruster (6) comprises causing the at least one air thruster (6) to perform azimuth rotating movements relative to the wind turbine (1).

13. A method according to any of claims 9-11, further comprising the step of tracing a main direction of the swaying movement (7) of the wind turbine (1), and wherein the step of operating the at least one air thruster (6) comprises adjusting a direction of the accelerated airflow (10) in accordance with the traced main direction of the swaying movement (7) of the wind turbine (1).

14. A method according to any of claims 9-13, wherein the step of operating the at least one air thruster (6) causes an amplitude of the swaying movement (7) of the wind turbine (1) to be reduced, and wherein the method further comprises the step of mounting one or more wind turbine blades (5) on the hub

(4) and/or dismantling one or more wind turbine blades (5) from the hub (4) when the amplitude of the swaying movement (7) has been reduced below a threshold level.