Patent Description:
Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly ("directly driven" or "gearless") or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.

The wind turbine hub may be rotatably coupled to a front of the nacelle. The wind turbine hub may be connected to a rotor shaft, and the rotor shaft may then be rotatably mounted in the nacelle using one or more rotor shaft bearings arranged in a frame inside the nacelle. The nacelle is a housing arranged on top of a wind turbine tower that may contain and protect the gearbox (if present) and the generator (if not placed outside the nacelle) and, depending on the wind turbine, further components such as a power converter, and auxiliary systems.

There is a trend to make wind turbine blades increasingly longer to capture more wind and convert the energy of the wind into electricity. That makes blades more flexible and more prone to vibrations of the blades. Wind turbine blades vibrating excessively may get damaged. Vibrations of the rotor blades may also result in the whole wind turbine structure oscillating e.g. fore-aft oscillations, or sideways oscillations. Vibrations in the wind turbine blade may also damage other components of the wind turbine due to excessive stress.

When the wind turbine is in operation (i.e. producing energy and connected to an electrical grid), a wind turbine controller may operate auxiliary drive systems such as a pitch system or a yaw system to reduce or change loads on the blades. This way, vibrations of the blades may be counteracted. However, the problem of vibrations can be serious as well in circumstances when the wind turbine is parked and disconnected from the grid.

When a wind turbine is parked, the wind may blow against the wind turbine from unusual directions, i.e. different from when in normal operation. The airflow around the wind turbine may cause the wind turbine to vibrate. Vibrations may stress and even damage one or more wind turbine components, which may compromise the performance of the wind turbine, increase the need of repairs and reduce the lifespan of the wind turbine. As an orientation of a wind turbine blade cannot be adapted to the direction of the incoming wind, e.g. through yawing and/or pitching as opposed to when the wind turbine is operating, the effects of vibrations may be greater or different when the wind turbine is parked than when the wind turbine is operating normally and producing energy.

In particular, this may apply when the wind turbine is being installed or commissioned. For example, it may happen that an incomplete rotor is installed (e.g. a rotor having a single blade or two blades out of the total of three blades). The remaining blades may not be installed until a few days or a week later. In the meantime, the partially installed (or "incomplete") rotor may be in standstill. The rotor may or may not be locked, and the wind turbine can be exposed to varying wind conditions. This may likewise apply if the wind turbine is stopped during several hours, days or weeks, e.g. for maintenance reasons. A wind turbine blade can start to vibrate in any of these conditions depending particularly on the direction of the wind.

Document <CIT> discloses an apparatus for mitigating vortex-shedding vibrations or stall-induced vibrations on one or more rotor blades of a wind turbine during standstill. The apparatus includes at least one positioning element configured to be located between a blade tip section and a blade root section which is adapted for wrapping around at least a portion of the rotor blade. The positioning element may be a coil spring in some examples. The apparatus also includes at least one airflow modifying element coupled to the positioning element and configured to define a height relative to a surface of the rotor blade. Additionally, the apparatus includes at least one securing element operably coupled to the positioning element for temporarily securing the airflow modifying element to the rotor blade.

In an aspect of the present disclosure, a device for mitigating vibration of a parked wind turbine is provided according to claim <NUM>.

According to this aspect, a device can be provided in an inactive state. If arranged with, e.g. placed around, a portion of a wind turbine blade in the inactive state, the device may not be able be able to grip the wind turbine blade at all or the device may be able to grip the blade with a certain strength, i.e. it may be able to exert some pressure against the blade. When acting on or activating the device and causing the device to transition to an active state, the device grips the wind turbine blade more firmly than in the inactive state. In use, the device may modify the air flowing around the wind turbine blade and avoid, or at least reduce, vortex and/or stall induced vibrations.

Throughout this disclosure, a device in an active state may be understood as a device which, in use, i.e. when mounted to a wind turbine blade, presses against an external surface of the wind turbine blade more than in an inactive state.

Throughout this disclosure, a device in an inactive state may be understood as a device which does not grip a wind turbine blade when arranged over or around it, or at least that it does not do so as strongly as it does when it is in the active state.

Throughout the present disclosure, the terms "standstill" and "parked" are used interchangeably, and may be understood as a situation in which the wind turbine is not producing electricity, and the rotor is substantially standing still. The rotor may or may not be locked in standstill. For instance, a wind turbine may be parked or in standstill during installation and/or commissioning. A wind turbine may also be parked for e.g. maintenance reasons after operating normally, i.e. producing energy, or in case of a prolonged grid loss.

Herein it may be understood that a wind turbine is in operation when its rotor is rotating at a speed high enough to produce energy and the generator of the wind turbine is producing electrical power.

Throughout this disclosure, an airflow modifying element may be understood as an element configured to significantly disturb the air flow, e.g. its magnitude and/or its direction, around a wind turbine blade. In particular, the airflow modifying element may be configured to make the airflow more turbulent and/or disturb at least an airflow advancing in a spanwise direction of the wind turbine blade.

In a further aspect of the disclosure, a method for mitigating vibrations in a parked wind turbine is provided according to claim <NUM>.

Still in a further aspect of the disclosure, a method for mitigating vibrations of a parked wind turbine is provided. The method comprises arranging an inflatable device around a wind turbine blade. The method further comprises inflating the device. Inflating enables the device to disturb air flow around the wind turbine blade.

Each example is provided by way of explanation of the invention, not as a limitation of the invention.

The rotor blades <NUM> are mated to the hub <NUM> by coupling a blade root region <NUM> to the hub <NUM> at a plurality of load transfer regions <NUM>.

In examples, the rotor blades <NUM> may have a length ranging from about <NUM> meters (m) to about <NUM> or more. Rotor blades <NUM> may have any suitable length that enables the wind turbine <NUM> to function as described herein. For example, non-limiting examples of blade lengths include <NUM> or less, <NUM>, <NUM>, <NUM>, <NUM> or a length that is greater than <NUM>. As wind strikes the rotor blades <NUM> from a wind direction <NUM>, the rotor <NUM> is rotated about a rotor axis <NUM>. As the rotor blades <NUM> are rotated and subjected to centrifugal forces, the rotor blades <NUM> are also subjected to various forces and moments. As such, the rotor blades <NUM> may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position.

In the example, the wind turbine controller <NUM> is shown as being centralized within the nacelle <NUM>, however, the wind turbine controller <NUM> may be a distributed system throughout the wind turbine <NUM>, on the support system <NUM>, within a wind farm, and/or at a remote-control center. The wind turbine controller <NUM> includes a processor <NUM> configured to perform the methods and/or steps described herein. Further, many of the other components described herein include a processor.

As used herein, the term "processor" is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific, integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that a processor and/or a control system can also include memory, input channels, and/or output channels.

<FIG> is an enlarged sectional view of a portion of the wind turbine <NUM>. In the example, the wind turbine <NUM> includes the nacelle <NUM> and the rotor <NUM> that is rotatably coupled to the nacelle <NUM>. More specifically, the hub <NUM> of the rotor <NUM> is rotatably coupled to an electric generator <NUM> positioned within the nacelle <NUM> by the main shaft <NUM>, a gearbox <NUM>, a high-speed shaft <NUM>, and a coupling <NUM>. In the example, the main shaft <NUM> is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle <NUM>. A rotation of the main shaft <NUM> drives the gearbox <NUM> that subsequently drives the high-speed shaft <NUM> by translating the relatively slow rotational movement of the rotor <NUM> and of the main shaft <NUM> into a relatively fast rotational movement of the high-speed shaft <NUM>. The latter is connected to the generator <NUM> for generating electrical energy with the help of a coupling <NUM>. Furthermore, a transformer <NUM> and/or suitable electronics, switches, and/or inverters may be arranged in the nacelle <NUM> in order to transform electrical energy generated by the generator <NUM> having a voltage between 400V to <NUM> V into electrical energy having medium voltage (<NUM> - <NUM> KV). Said electrical energy is conducted via power cables from the nacelle <NUM> into the tower <NUM>.

The gearbox <NUM>, generator <NUM> and transformer <NUM> may be supported by a main support structure frame of the nacelle <NUM>, optionally embodied as a main frame <NUM>. The gearbox <NUM> may include a gearbox housing that is connected to the main frame <NUM> by one or more torque arms <NUM>. In the example, the nacelle <NUM> also includes a main forward support bearing <NUM> and a main aft support bearing <NUM>. Furthermore, the generator <NUM> can be mounted to the main frame <NUM> by decoupling support means <NUM>, in particular in order to prevent vibrations of the generator <NUM> to be introduced into the main frame <NUM> and thereby causing a noise emission source.

In some examples, the wind turbine may be a direct drive wind turbine without gearbox <NUM>. Generator <NUM> operate at the same rotational speed as the rotor <NUM> in direct drive wind turbines. They therefore generally have a much larger diameter than generators used in wind turbines having a gearbox <NUM> for providing a similar amount of power than a wind turbine with a gearbox.

For positioning the nacelle <NUM> appropriately with respect to the wind direction <NUM>, the nacelle <NUM> may also include at least one meteorological measurement system which may include a wind vane and anemometer. The meteorological measurement system <NUM> can provide information to the wind turbine controller <NUM> that may include wind direction <NUM> and/or wind speed. In the example, the pitch system <NUM> is at least partially arranged as a pitch assembly <NUM> in the hub <NUM>. The pitch assembly <NUM> includes one or more pitch drive systems <NUM> and at least one sensor <NUM>. Each pitch drive system <NUM> is coupled to a respective rotor blade <NUM> (shown in <FIG>) for modulating the pitch angel of a rotor blade <NUM> along the pitch axis <NUM>. Only one of three pitch drive systems <NUM> is shown in <FIG>.

In the example, the pitch assembly <NUM> includes at least one pitch bearing <NUM> coupled to hub <NUM> and to a respective rotor blade <NUM> (shown in <FIG>) for rotating the respective rotor blade <NUM> about the pitch axis <NUM>. The pitch drive system <NUM> includes a pitch drive motor <NUM>, a pitch drive gearbox <NUM>, and a pitch drive pinion <NUM>. The pitch drive motor <NUM> is coupled to the pitch drive gearbox <NUM> such that the pitch drive motor <NUM> imparts mechanical force to the pitch drive gearbox <NUM>. The pitch drive gearbox <NUM> is coupled to the pitch drive pinion <NUM> such that the pitch drive pinion <NUM> is rotated by the pitch drive gearbox <NUM>. The pitch bearing <NUM> is coupled to pitch drive pinion <NUM> such that the rotation of the pitch drive pinion <NUM> causes a rotation of the pitch bearing <NUM>.

Pitch drive system <NUM> is coupled to the wind turbine controller <NUM> for adjusting the pitch angle of a rotor blade <NUM> upon receipt of one or more signals from the wind turbine controller <NUM>. In the example, the pitch drive motor <NUM> is any suitable motor driven by electrical power and/or a hydraulic system that enables pitch assembly <NUM> to function as described herein. Alternatively, the pitch assembly <NUM> may include any suitable structure, configuration, arrangement, and/or components such as, but not limited to, hydraulic cylinders, springs, and/or servomechanisms. In certain embodiments, the pitch drive motor <NUM> is driven by energy extracted from a rotational inertia of hub <NUM> and/or a stored energy source (not shown) that supplies energy to components of the wind turbine <NUM>.

The pitch assembly <NUM> may also include one or more pitch control systems <NUM> for controlling the pitch drive system <NUM> according to control signals from the wind turbine controller <NUM>, in case of specific prioritized situations and/or during rotor <NUM> overspeed. In the example, the pitch assembly <NUM> includes at least one pitch control system <NUM> communicatively coupled to a respective pitch drive system <NUM> for controlling pitch drive system <NUM> independently from the wind turbine controller <NUM>. In the example, the pitch control system <NUM> is coupled to the pitch drive system <NUM> and to a sensor <NUM>. During normal operation of the wind turbine <NUM>, the wind turbine controller <NUM> may control the pitch drive system <NUM> to adjust a pitch angle of rotor blades <NUM>.

According to an embodiment, a power generator <NUM>, for example comprising a battery and electric capacitors, is arranged at or within the hub <NUM> and is coupled to the sensor <NUM>, the pitch control system <NUM>, and to the pitch drive system <NUM> to provide a source of power to these components. In the example, the power generator <NUM> provides a continuing source of power to the pitch assembly <NUM> during operation of the wind turbine <NUM>. In an alternative embodiment, power generator <NUM> provides power to the pitch assembly <NUM> only during an electrical power loss event of the wind turbine <NUM>. The electrical power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine <NUM>, and/or failure of the wind turbine controller <NUM>. During the electrical power loss event, the power generator <NUM> operates to provide electrical power to the pitch assembly <NUM> such that pitch assembly <NUM> can operate during the electrical power loss event.

In the example, the pitch drive system <NUM>, the sensor <NUM>, the pitch control system <NUM>, cables, and the power generator <NUM> are each positioned in a cavity <NUM> defined by an inner surface <NUM> of hub <NUM>. In an alternative embodiment, said components are positioned with respect to an outer surface of hub <NUM> and may be coupled, directly or indirectly, to the outer surface.

A schematic perspective view of a wind turbine blade <NUM>, e.g. one of the rotor blades <NUM> shown in <FIG>, is illustrated as an example in <FIG>. The rotor blade <NUM> includes a blade root <NUM>, a blade tip <NUM>, a leading edge <NUM> and a trailing edge <NUM>. The blade root <NUM> is configured for mounting the rotor blade <NUM> to the hub <NUM> of a wind turbine <NUM>. The wind turbine blade <NUM> extends lengthwise between the blade root <NUM> and the blade tip <NUM>. A span <NUM> defines a length of the rotor blade <NUM> between said blade root <NUM> and blade tip <NUM>. A chord <NUM> at a given position of the blade is an imaginary straight line joining the leading edge <NUM> and the trailing edge <NUM>, the cross-section generally having airfoil shaped cross-section. As is generally understood, a chordwise direction is substantially perpendicular to a spanwise direction. Also, the chord <NUM> may vary in length <NUM> as the rotor blade <NUM> extends from the blade root <NUM> to the blade tip <NUM>. The wind turbine blade <NUM> also includes a pressure side <NUM> and a suction side <NUM> extending between the leading edge <NUM> and the trailing edge <NUM>. A tip region <NUM> may be understood as a portion of a wind turbine blade <NUM> that includes the tip <NUM>. A tip region may have a length of <NUM>%, <NUM>%, or <NUM>% of the span or less. A root region <NUM> may be understood as a portion of the blade that includes root <NUM>. A root region may have a length of e.g. <NUM>%, <NUM>% of the span or less.

The rotor blade <NUM>, at different spanwise positions, has different aerodynamic profiles and thus can have airfoil shaped cross-sections <NUM>, such as a symmetrical or cambered airfoil-shaped cross-section. Close to a root of the blade, the cross-section of the blade may be rounded, even circular or almost circular. Closer to a tip of the blade, the cross-section of the blade may be thinner and may have an airfoil shape.

When a wind turbine is parked or stopped, vibrations caused by the air flowing around the wind turbine, in particular around the wind turbine blades, may stress and damage the wind turbine blades and the wind turbine. The wind turbine rotor may or may not be locked in these situations.

At least two types of oscillations or vibrations may happen particularly when the turbine is parked. The first ones are so-called vortex induced vibrations (VIVs), and these can arise when an angle of attack for a blade or airfoil portion is around <NUM> degrees. Vortex shedding may contribute to enhance the wind turbine blade oscillation. The second type of oscillations are stall induced vibrations (SIVs) which can arise when the angle of attack is close to stall angles (e.g. <NUM> degrees - <NUM> degrees). The angle of attack may be understood as a geometrical angle between a flow direction of the wind and the chord of a rotor blade or a local chord of a rotor blade section.

Devices <NUM> as described herein may reduce vibrations when the wind turbine is parked. The performance of the wind turbine may not be negatively affected as the device(s) may be removed before the wind turbine starts normal operation. One or more devices <NUM> may be particularly useful during installation and/or commissioning of a wind turbine. It may be also useful if the wind turbine is stopped, e.g. for maintenance.

A device <NUM> for mitigating vibrations of a parked wind turbine is provided. The device comprises one or more airflow modifying elements. The device is configured to transition from an inactive state to an active state. The device is configured to grip a wind turbine blade more firmly in the active state than in the inactive state. The device comprises a shape memory material, and the device is configured to transition from the inactive state to the active state in response to the activation of the shape memory material; or the device comprises one or more inflatable structures, and the device (<NUM>) is configured to transition from the inactive state to the active state by inflating the inflatable structures; or the device comprises one or more vacuum chambers, and the device (<NUM>) is configured to transition from the inactive state to the active state by applying vacuum to the vacuum chambers.

<FIG> schematically show some examples of such devices <NUM>.

In some examples, e.g. in <FIG>, the device may comprise a shape memory material, and the device may be configured to transition from the inactive state to the active in response to the activation of the shape memory material. A shape memory material may be understood as a material that has one or more properties that are affected in a controlled manner by some external stimuli such that the material is able to change from a first state (e.g. a deformed temporary shape) to a second state (e.g. an original default shape) in response to the stimuli.

, activating on the device, e.g. by applying a stimulus such as heat or light to it, may change its shape. In this way, in an inactive state, the device may have a first shape, and when for example heat is applied to the device, its shape changes, causing it to grip a wind turbine blade. In the active state, the device has therefore a second different shape.

An example of a device <NUM> comprising a shape memory material is illustrated in <FIG>. In this figure, the device is shown in an active state attached to a wind turbine blade <NUM>. The device may be entirely made of one or more shape memory materials in some examples. In other examples, one or more shape memory materials may be included only in some portions of the device, e.g. along one or more longitudinal portions of the device <NUM>. By activating such portions, the device <NUM> may be secured to the blade. Shrinking of the device <NUM> may, no matter whether the device includes shape memory material(s) along its entire length or only along some portions of it, may occur along a length of the device and/or along a height <NUM> of the device. In some examples, the device <NUM> may shrink along a length of the device and it may expand along a height of the device. This may enable a suitable attachment to the blade <NUM>.

In the active state, the device <NUM> has a spiral or helicoidal shape, thereby surrounding a wind turbine blade <NUM>. In an inactive state, the device has a different shape or at least a different size. For example, the device may still have a spiral or helicoidal shape in an inactive state, but with loosen or less tight loops. When subjected to an appropriate stimulus, it may shrink and press the outer surface, e.g. pressure side <NUM> and suction side <NUM> surfaces, of the wind turbine blade <NUM>. Appropriate stimulus may e.g. include heat, light, electric fields and magnetic fields.

In some other examples, the device <NUM> in the inactive state may be substantially straight, for example a straight tube, and only when acted on it starts to curve around the wind turbine blade <NUM> for gripping the blade. Storing and transporting substantially straight, non-curved, devices may be easier and more efficient that storing and transporting curved devices. Still in other examples, the shape of the device in the inactive state may be different from the options mentioned herein.

The device <NUM> may comprise one or more shape memory materials, including shape memory alloys, shape memory polymers and/or shape memory hybrids.

When in use, the device <NUM> may help to modify the air flowing around the wind turbine blade <NUM>. For example, the air flowing around the blade may encounter the device, which hinders the advancing of the air flow. The air may become more turbulent after leaving behind the device. Hindering the advancing of the air flow may decorrelate the vortex sheddings at the spanwise location of the device and other vortex shedding at other spanwise positions of the blade. Making the vortex sheddings uncoherent along a length <NUM> of the blade <NUM> may reduce VIVs.

Also, the helicoidal shape of the device <NUM> may alter the separation patterns of the local flow. The negative slopes of the lift curve may therefore be reduced and SIVs may accordingly be mitigated.

The device may comprise several loops in the active state. For example, a first loop <NUM> and a second loop <NUM> can be provided. Having more than one loop, e.g. two, three, four, or more loops, may help to enhance the effect of decorrelation of the vortex sheddings.

<FIG> shows several examples of partial cross-sections of the body <NUM> of the device <NUM> of <FIG>. The body <NUM> of the device <NUM>, or the device as such, may be seen as an air flow modifying element. Cross-sections in this figure have been taken along line A-A of <FIG>. Only a portion of the device <NUM> and the blade <NUM> is illustrated. The device <NUM> has a height <NUM> such that when the device is mounted to a wind turbine blade, the air flow can be disturbed. A height <NUM> may be about <NUM>% to <NUM>% of a chord length <NUM>. A chord length may be understood as an average length of the chords along the length <NUM> of the blade <NUM>. A height <NUM> may be measured in a direction substantially perpendicular to a local surface of the wind turbine blade <NUM> to which the device is attached in some examples. A blade side <NUM> of the device <NUM>, i.e. a side of the device configured to face a blade external surface, does not need to touch the surface of the wind turbine blade along all the length of the device, and thus a direction in which the height <NUM> is measured does not need to be always substantially perpendicular to the outer surface of the wind turbine blade. Thus, <FIG> is merely illustrative. A length of the device <NUM> is perpendicular to the plane of <FIG> in the cross-sections shown.

It should be noted that the device of <FIG> may have a varying cross-section along its length. That is to say, a height <NUM> and a width (dimension substantially perpendicular to the height <NUM>) of the device <NUM> may vary along the length of the device.

Device <NUM> may comprise a heating element <NUM> for heating the shape memory material. A heating element may be understood as an element in direct contact with the device which can be heated. When the element is heated, the temperature of the device increases and triggers its change of shape. For example, a film, a wire or a tube which can be heated may be provided extending along the length the device, e.g. inside the device. In some other examples, the device <NUM> may comprise one or more wires of shape memory material(s) along a total or partial length of the device, and one or more coils may heat the wires. One or more coils may e.g. wrap the wires. Still in some other examples, an illuminating element may be provided. In general, an activating element configured to stimulate the shape memory material may be provided.

In some other examples, a heating element <NUM> does not need to be included with the device <NUM>. The ohmic resistance of the shape memory material(s) may in some examples be sufficient for self-heating when circulating a current through the shape memory material(s). Alternatively, a stimulus such as heat or light may be directly applied by an operator to the device <NUM>. For example, the device <NUM> may be illuminated with specific light from a light source, and the light triggers the contraction of the device. The light may for example be infrared light.

An energy source <NUM> may be provided for increasing the temperature of the heating element. For example, a battery may be configured to apply a voltage to the heating element such that heat is dissipated through the element, increasing its temperature. Then, the device <NUM> is heated too, thereby changing its shape and/or causing it to shrink. An energy source may be provided in general for supplying energy to the activating element.

In some examples, the energy source may be provided with the device <NUM>, e.g. inside the device, such that each device that is to be attached to a wind turbine blade <NUM> has its own energy source for activating the device. In some other examples, the energy source may not be provided with the device, but it may be an external energy source which can be connected to the heating element <NUM> of the device <NUM>. An operator may carry the energy source and use it to heat each device <NUM> to fit them around a wind turbine blade <NUM>.

The number and arrangement of energy sources <NUM> to be used to heat a device <NUM>, or at least the position at which the transfer of energy takes place from the energy source(s) <NUM> to the heating element <NUM>, may be selected depending on how it is desired that the deformation of the device <NUM> proceeds. For example, one energy source <NUM> or connection <NUM> to a heating element <NUM> may be provided at a longitudinal end <NUM> of a device <NUM>, or a plurality of energy sources or connections to a heating element <NUM> may be provided along a length of the device <NUM>. The way in which the device shrinks and changes its shape in each case will be different.

A device <NUM> may be arranged in a tip region <NUM>. The tip region may be the portion of the wind turbine blade that vibrates the most when the wind turbine is parked. Therefore, it may be particularly advantageous to fit the device in this region of the blade.

More than one device <NUM> may be attached to the wind turbine blade <NUM>. The distance between two consecutive devices attached to a blade in a spanwise direction may be such that the air flow encounters two obstacles on its way (if the devices <NUM> were too close, they may act as a one bigger single device instead of providing two local disruptions of the air flow around the blade <NUM>). This may help to further decorrelate vortex sheddings along a length <NUM> of the wind turbine blade and to attach the air flow to the blade. In some examples, two devices <NUM> may be separated between one and five chord lengths <NUM>. In this aspect, a chord length may be understood as an average length of the chords along the length of the blade <NUM>.

Another example of a device <NUM> for mitigating vibrations of a parked wind turbine is provided in <FIG>. In <FIG>, the device is fitted around a wind turbine blade <NUM> and comprises a base <NUM> and a plurality of protrusions <NUM>. The base <NUM> is configured to face a surface of a wind turbine blade <NUM>, e.g. pressure side <NUM> and suction side <NUM> surfaces. The protrusions <NUM> are configured to extend away from the surface of the wind turbine blade in the active state. A top view of the device <NUM> of <FIG> in an extended configuration is provided in <FIG>. A cross section of the device of <FIG> along line B-B is shown in <FIG>.

The base <NUM> may comprise a shape memory material. The above description in relation to shape memory materials and the device <NUM> of <FIG> also apply to base <NUM>. For example, the base <NUM> may include a heating element <NUM> (in general an activating element), such that when the heating element <NUM> increases its temperature, the temperature of the base <NUM> increases as well, and the base <NUM> shrinks around a wind turbine blade <NUM>.

It is also possible that the base <NUM> comprises more than one activating element <NUM>. For example, two or more heating elements <NUM>, e.g. three, four, five or more heating elements may be arranged inside the base <NUM>, e.g. substantially parallel along a length <NUM> or a width <NUM> of the base <NUM>. A plurality of heating elements <NUM> may also be included in the device <NUM> of <FIG>. In use, a length <NUM> of the base <NUM> may be measured along a spanwise direction and a width <NUM> of the base <NUM> may be measured along a chordwise direction. In <FIG> the length <NUM> is bigger than the width <NUM> of the base <NUM>, but the opposite, i.e. dimension <NUM> greater than dimension <NUM>, may also be possible.

In some other examples, the base <NUM> does not need to include a heating element <NUM>, but the base <NUM> may be caused to transition from an inactive state to an active state by an operator stimulating the base <NUM>, e.g. by directly heating it or by illuminating it with light of a suitable wavelength.

Still in some other examples, the base <NUM> may not comprise one or more shape memory materials.

Independently of the base <NUM> including or not one or more shape memory materials, the base <NUM> may be provided in a closed configuration. For example, the base <NUM> may have an annular cross-section and/or the base <NUM> may have a tubular shape. A base <NUM> in a closed configuration may be referred to as a sleeve. In other examples, the base <NUM> may be provided in an open configuration. In an open configuration, two opposite edges <NUM>, <NUM> of the device <NUM>, e.g. the edges configured to extend in a spanwise direction when placing the device around the blade <NUM>, may not be joined.

One or more tensioning elements <NUM> may be provided along a length <NUM> of the base <NUM>. A tensioning element, which extends between edges <NUM> and <NUM> of the base <NUM> in <FIG>, may be configured to extend, when the device is in use <NUM>, along a spanwise direction. A tensioning element <NUM> is configured to provide a reliable tension to the base <NUM> and to accommodate variations in shape of a blade <NUM> and/or variations in shape between different blades <NUM>. If a perimeter in cross-section changes along a length <NUM> of the blade <NUM>, the tensioning element(s) <NUM> may deform, and in particular may stretch and compensate for that change by bridging a gap between two edges of the base <NUM>, thus adapting the base <NUM> to the blade <NUM>. The stretching capability of the tensioning element(s) <NUM> may also facilitate using a same base <NUM> for blades <NUM> of different sizes. A tensioning element <NUM> may e.g. be a deformable, resilient or elastic element such as a rubber strap winding or an elastic garment.

A base <NUM> may comprise one or more shape memory materials and one or more tensioning elements <NUM> in some examples. In other examples, tensioning elements <NUM> may not be provided in a base <NUM> which already comprises one or more shape memory materials.

In some examples, the base <NUM> of the device <NUM> may comprise a release element <NUM>. The release element <NUM> may be configured to removably attach the base <NUM> to the wind turbine blade <NUM>. The release element may therefore help to detach the base <NUM> from the blade <NUM> when the device <NUM> is no longer needed, i.e. before starting to operate the wind turbine <NUM>. A release element may be a zipper or a hook-and-loop fastener such as Velcro™. The release element <NUM>, which extends between edges <NUM> and <NUM> of the base <NUM> in <FIG>, may be configured to extend, when the device is in use <NUM>, along a spanwise direction.

One or more tensioning elements <NUM> may be used in combination with a release element <NUM>. This may be seen in <FIG>. In this example, a zipper <NUM> may for example be used to removably attach the base <NUM> to the blade <NUM>, and two tensioning elements <NUM> may help to adapt the base <NUM> to the blade <NUM> contour when closing the zipper. The tensioning elements <NUM> may be provided substantially adjacent the release element <NUM>.

In some examples, the base <NUM> may comprise perforations for facilitating the removal of the device <NUM>. Perforations may be arranged in a specific pattern. Such pattern including a plurality of perforations may be referred to as frangible or tearable pattern. For example, perforations may form a substantially straight line along a length <NUM> or a width <NUM> of the base <NUM>. The line may be referred to as tearable or frangible line. By acting on the perforated pattern, the base <NUM> may be separated from the blade <NUM>.

Release element(s) <NUM> and/or frangible or tearable pattern(s) may be provided in other examples of a device <NUM>. For example, the device <NUM> of <FIG> may include a removable attachment or a tearable pattern for removing the device <NUM> from a blade <NUM>. Such ways of removing a device <NUM> may be useful if the device <NUM> includes a one-way shape memory material, i.e. if transitioning from an active state back to the inactive state is not possible.

The protrusions <NUM> of the device <NUM> extend from its base <NUM>. The protrusions <NUM> may be configured to be arranged along at least the leading edge <NUM> in the activated state of the device. The protrusions can be grouped in rows <NUM>. For example, two rows <NUM> of protrusions may be provided. Each of the rows <NUM> may be configured such that, when the device <NUM> is arranged on the wind turbine blade <NUM> and the base <NUM> is caused to contract, one row extends along the leading edge of the wind turbine blade and the other row extends along the trailing edge <NUM> of the wind turbine blade.

The protrusions <NUM> may help to create turbulent air flow, provoking earlier local flow separation, increasing the drag and reducing the lift. The protrusions may thus may help to reduce VIVs and/or SIVs.

The device <NUM> may comprise more than two rows <NUM>. For example, three, four, five or more rows <NUM> of protrusions <NUM> may be provided. The rows <NUM> may extend in a spanwise direction when the device <NUM> is in use.

Including protrusions to be placed over a pressure <NUM> and/or a suction <NUM> side of a blade <NUM> may enhance the effect of creating turbulence and therefore decreasing stall and coherence between vortex sheddings.

In some other examples, the protrusions <NUM> are not arranged in spanwise rows <NUM>. Different arrangements of the protrusions <NUM> may be provided.

The protrusions <NUM> may be configured to have a same shape when the device <NUM> is in the inactive state than when it is in the active state. For example, protrusions <NUM> may have an elongated protruding shape regardless the state (inactive or active) in which the device is. The protrusions <NUM> may comprise one or more materials conferring certain sturdiness to it. Therefore, as the device is robust, once the device <NUM> is tightened around the wind turbine blade <NUM>, the functionality of altering the air flow around the blade is immediately obtained.

In some other examples, the protrusions <NUM> are inflatable structures <NUM> and can therefore be inflated. A protrusion <NUM> may include an internal chamber <NUM> and a wall <NUM>. It may be understood that the wall <NUM> delimits, at least in part, the chamber <NUM>. The wall <NUM> is joined to the base <NUM>. When air or a suitable gas, e.g. carbon dioxide, helium or nitrogen, is introduced in the internal chamber <NUM>, the wall <NUM> is pushed and the protrusion <NUM> is inflated. A gas, or in general a fluid, may be suitable if it is able to push the walls <NUM> and inflate the inflatable structure.

The base <NUM> may be first fitted around the wind turbine blade by making the base <NUM> to contract and then the protrusions <NUM> may be inflated. The protrusions <NUM> may be inflated one by one, e.g. one after the other. But if their internal chambers <NUM> are connected, i.e. there is effectively a single internal chamber over the base <NUM>, the protrusions <NUM> may me be inflated substantially at a same time. Or at least, all the protrusions <NUM> may be inflated by using a single gas inlet.

Still in some other examples, it may be possible that the protrusions <NUM> include a shape memory material, and the protrusions <NUM> are caused to protrude from the base <NUM> when applying a stimulus to the base <NUM>. In these examples, the protrusions <NUM> would be retracted in the inactive state of the device, and they would extend when the device transitions to an active state, e.g. by heating the device <NUM> or the base <NUM>.

Still in some other examples, the base <NUM> may be inflatable. The base <NUM> may be configured to be inflated separately from the protrusions <NUM>, or they may be provided in a single piece and they may be inflated together.

Still in some other examples, the base <NUM> may be caused to transition from the inactive state to the active state by applying vacuum to the base <NUM>. For example, a pneumatic pump may be used for causing the base <NUM> to grip the wind turbine blade <NUM>. The base <NUM> may be configured to this end, e.g. the base may be configured for creasing and shrinking around the blade <NUM> when vacuum is applied to it. The base <NUM> may have a valve to connect a vacuum pump for applying vacuum. The base <NUM> may be seen as a vacuum chamber in some examples. The base <NUM> may comprise more than one vacuum chambers in other examples. In general, a device <NUM> may comprise one or more vacuum chambers, and the device <NUM> may be configured to transition from an inactive state to an active state by applying vacuum to the one or more vacuum chambers.

It is noted that the device <NUM> of <FIG> may be provided similarly to the device of <FIG> in that the body <NUM> of the device <NUM> may comprise a base <NUM> and a protrusion <NUM>. The description of the base <NUM> and a protrusion <NUM> in connection with <FIG> may apply to this other example of device. A cross-section of this device is illustrated in <FIG>. Therefore, protrusions <NUM> may be sturdy and have its functional shape already in the inactive state of the device <NUM>, or protrusions <NUM> may be inflatable and the air disturbing functionality may e.g. be obtained after the device is transitioned to the active state by inflating the protrusions, or protrusions <NUM> may comprise a shape memory material and its functionality may be gained when changing from the inactive to the active state of the device <NUM>. Vacuum may be applied to the body <NUM> or to the base <NUM> for causing the device <NUM> to grip the blade <NUM>.

Also, the device of <FIG> may be totally or partially inflatable. For example, the device may comprise an internal fluid chamber <NUM> and one or more walls <NUM> delimiting the fluid chamber. When a suitable fluid, e.g. a suitable gas, is introduced in the fluid chamber, the wall(s) expand and the device adopts its spiral or helical shape. The device therefore attaches more firmly to the blade <NUM>. The device <NUM> may comprise more than one fluid chamber. A cross-section of the device may be non-constant along a length of the device. For example, inflating one or more fluid chambers may cause that a height <NUM> of the device in cross-section varies along the length of the device. The device may therefore acquire a bellow shape.

The device <NUM> of <FIG> may be attached for example to a tip region <NUM>, for the reasons already mentioned before. Likewise, more than one device <NUM> may be fitted to the wind turbine blade <NUM> in a spanwise direction for acting on the air flow at two different local positions along the blade. For example, a device such as the one in <FIG> may be provided at the tip region <NUM> for increasing damping, and a device such as the one in <FIG> may be provided closer to the root <NUM> for decreasing vortex street formation. Two consecutive devices of <FIG> may in some examples be separated between <NUM> and <NUM> chord lengths <NUM>, and more in particular between <NUM> and <NUM> chord lengths <NUM>, one chord length being understood as an average chord length of the blade.

Still another example of device <NUM> configured to disturb the air flowing around a wind turbine blade is provided in <FIG>.

The device <NUM> of <FIG> comprises one or more inflatable structures <NUM>. The device <NUM> may be configured to transition from the inactive state to the active state by inflating the inflatable structures. <FIG> show two examples of cross sections along line C-C of <FIG>. In <FIG>, the device <NUM> comprises four inflatable structures <NUM>. In <FIG>, the device <NUM> comprises a single inflatable structure <NUM>. An inflatable structure may comprise one or more walls <NUM> delimiting an internal chamber <NUM>. As explained with respect to protrusions <NUM> of <FIG>, air or in general any suitable fluid, e.g. any suitable gas, may be introduced in the chamber(s) <NUM> for inflating the structure(s) <NUM>, and thus the device <NUM>.

The device <NUM> may have a blade side <NUM>, an outer side <NUM> which is opposite to the blade side <NUM>, and two lateral sides <NUM>, <NUM>. The blade side <NUM> is configured to face an outer surface of a wind turbine blade <NUM>. If the device <NUM> includes more than one inflatable structure <NUM>, it may have one or more internal sides <NUM>. These sides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may form the walls that delimit each chamber <NUM>. For example, as it may be seen in <FIG>, lateral side <NUM>, a portion of the blade side <NUM>, an internal side <NUM> and a portion of the outer side <NUM> form the walls that enclose an internal chamber <NUM>.

In an inactive state, the device <NUM> is deflated. Introducing air or a suitable fluid or gas in the chamber(s) <NUM> causes the device <NUM> to transition to the active state. The air introduced in the chamber(s) <NUM> presses the blade side <NUM> of the device against an outer surface of the blade <NUM>. The device <NUM> therefore grips the blade <NUM>.

An outer side <NUM> of the device <NUM> may be configured to have an accordion, wavy, serrated or zigzag shape in the activated state. Thus, when in use, the air flow is presented with irregularities along its path over the device <NUM>. Such a shape of the outer side <NUM> may promote turbulence creation, and thus may help to mitigate VIVs.

The device <NUM> may be or may include a multistable inflatable structure. A multistable, e.g. a bistable, structure may comprise a plurality of relatively rigid two-dimensional panels linked by flexible joints or hinges. When inflated, e.g. by compressed air, the device unfolds and locks into a three-dimensional structure. A gas canister may be used for inflating the structure. Due to the rigidity and geometry of the panels, the shape of the device in the active state is not lost when removing the inflating source, and keeping an inflating gas inside the device, such as with common inflatable devices, is not required. Herein, a common inflatable device may mean a device that loses its shape if the inflating gas is not kept trapped inside the device, e.g. by a gas stopper or a plug. Therefore, a common inflatable device may be a monostable structure.

If a common or monostable inflatable device is used, a fluid or gas supply device may be provided with the device to compensate for fluid or gas leaks from the device, and thus the change of its shape in the activated state.

In <FIG>, the device <NUM> is provided in a closed configuration. In other examples, the device <NUM> may be provided in an open configuration, as in <FIG>. , the device may comprise two edges <NUM>, <NUM> configured to extend in a substantially spanwise direction.

A device with a shape as what has been described with regard to <FIG> may also be obtained by incorporating a base <NUM> including a shape memory material, the inflatable structures <NUM> being arranged on/around the base <NUM>, see e.g. <FIG>. The base <NUM> may also be configured to be attached by applying vacuum. The concept of the multistable inflatable structure may be applied to other shapes of the device too. For example, a device as or similar to the one shown in <FIG> may be obtained by a multistable structure. In general, a device <NUM> may comprise or may be formed by a multistable structure and the device may be caused to transition from the inactive state to the active state by inflating the multistable structure.

A wind turbine blade <NUM> comprising one or more of the devices <NUM> as described above attached to it may be provided. The one or more devices may grip the wind turbine blade, e.g. they may be fitted around the wind turbine blade. The device may be in the active state.

In a further aspect, a method <NUM> for mitigating vibrations in a parked wind turbine <NUM> is provided. The method, which is schematically illustrated in <FIG>, may be used particularly during installation and/or during commissioning of the wind turbine. The method may also be used when the wind turbine may be stopped after it has been operating (i.e. producing energy), e.g. during maintenance or repair.

The method comprises, at block <NUM>, arranging a device <NUM> as described herein in an inactive state with a wind turbine blade <NUM>. The wind turbine blade <NUM> comprises a root, a tip and exterior surfaces defining a pressure side <NUM>, a suction side <NUM>, a leading edge <NUM> and a trailing edge <NUM>, each surface extending in a generally spanwise direction from the root the tip. Any of the devices described above with regard to <FIG> may be used.

A device <NUM> may be arranged around a wind turbine blade <NUM>. For example, arranging may comprise sliding the device <NUM> along the blade. A device such as the one in <FIG> or <FIG> may e.g. be arranged surrounding a tip region <NUM> of the blade <NUM> and then may be slid in a spanwise direction. In some other examples, the device may be simply rested against the blade or placed over the blade <NUM>. For example, instead of being slid in a spanwise direction, the device <NUM> of <FIG> may be rested against or placed over the blade <NUM>, e.g. as a substantially straight element. The device may be then curved by activating it. A device <NUM> may be particularly placed around a tip region <NUM>, as it may be the portion of the blade <NUM> more prone to vibrating.

The method further comprises, at block <NUM>, securing the device <NUM> to the wind turbine blade <NUM> by causing the device <NUM> to transition from the inactive state to an active state, wherein the device grips the blade more firmly in the active state than in the inactive state.

Causing the device to transition from the inactive state to an active state may comprise stimulating a shape memory material of the device <NUM>. For example, a shape memory material may be stimulated by heating it or illuminating it with light from a suitable wavelength. An activating element, e.g. a heating element <NUM>, may be used in some examples for activating, e.g. heating, the shape memory material. Other options for stimulating the shape memory material may be applying an electric or a magnetic field.

All the device <NUM> may be stimulated for changing its shape and/or shrinking it if it is entirely made from one or more shape memory materials, as e.g. the device of <FIG>. Only a portion of the device <NUM> may be stimulated in other examples. For example, a base <NUM> of the device of <FIG> may be stimulated.

Causing the device <NUM> to transition from the inactive state to an active state may comprise inflating at least a portion of the device. For example, the device <NUM> may be inflated by introducing a suitable fluid, e.g. a suitable gas, into one or more inflatable structures <NUM>, as explained with respect to <FIG>. If multistable, e.g. bistable, structures are used, they may similarly be inflated.

Causing the device to transition from the inactive state to an active state may comprise applying vacuum to at least a portion of the device <NUM>. The device may comprise one or more vacuum chambers to which vacuum may be applied. For example, a pneumatic pump may be connected to a valve of a base <NUM>, and applying vacuum may cause the base <NUM>, and thus the device <NUM>, to crease and shrink around the blade <NUM>.

If the transitioning to the active state does not enable the device or a portion thereof to modify the air flow around a wind turbine blade <NUM>, the method may further comprise activating the air flow modifying elements. For example, protrusions <NUM> or structures <NUM> may be inflated.

Additional devices <NUM> may be arranged and secured to the blade too. Two consecutive devices <NUM> may be separated between one and five chord lengths <NUM> along a length of the blade, e.g. two devices as in <FIG> or two devices as in <FIG>. In other examples, two consecutive devices <NUM> may be separated between one and five chord lengths <NUM> in a spanwise direction, e.g. two devices as in <FIG>. Combining devices <NUM> of different types, e.g. devices of different examples, may be possible.

The steps of arranging <NUM> and securing <NUM> may be performed on the ground, e.g. after a blade <NUM> and a device <NUM> have arrived at a wind turbine installation site. In some other examples, these steps <NUM>, <NUM> may be performed onto a blade <NUM> already attached to a hub <NUM> on top of a wind turbine tower <NUM>, e.g. if the wind turbine <NUM> has been stopped for maintenance or simply if it has been decided to install first a blade <NUM> and then to attach the device <NUM> to it. Still in some other examples, these steps may be performed before transporting a wind turbine blade <NUM> to an installation site. For example, one or more devices, e.g. inflatable devices, may be arranged an secured to the blade before carrying it to the installation site. The devices <NUM> may protect the blade while it is stored and/or transported.

If the device <NUM> is arranged and secured on an uninstalled blade <NUM>, the method may further comprise installing the wind turbine blade <NUM>. The blade <NUM> may be first attached to the hub <NUM> and the hub <NUM> and the blade <NUM> may be lifted together, or the hub <NUM> may be mounted first and then the blade <NUM> with the device <NUM> may be lifted and connected to the hub <NUM>.

Once the blade <NUM> with the device <NUM> is mounted to the wind turbine, the air flow modifying elements <NUM>, <NUM>, <NUM> may reduce wind turbine <NUM> vibrations, e.g. vortex induced vibrations and/or stall induced vibrations.

The device <NUM> may stay gripped around the blade <NUM> until operation of the wind turbine is started or resumed. The method may further comprise removing the device <NUM> from the wind turbine blade <NUM> before starting operation.

Removing a device <NUM> may be performed in different ways. If the device <NUM> comprises a removable attachment <NUM>, removing may comprise detaching a removable attachment <NUM>. For example, a zipper or hook-and-loop fasteners may be detached. If the device <NUM> includes a frangible pattern, tearing the pattern may separate the device from the blade. If the device has been inflated, the air or gas trapped in one or more chambers <NUM> may be released. A valve of an inflatable structure <NUM> may be opened to do so, or a wall <NUM> may be pierced. If vacuum has been applied, a valve of a vacuum chamber may be opened to let air in. For example, a valve of a base <NUM> may be opened to release the device <NUM> from a blade <NUM>. In case of a multistable structure, suctioning the air or gas may be needed to allow the device to fold along the flexible joints joining the pieces forming the walls of the device. If the device includes memory shape material, the memory shape material may be stimulated to revert to the inactive state of the device. Other ways of removing a device from the blade may be possible.

Still in a further aspect of the invention, another method <NUM> for mitigating vibrations of a parked wind turbine is provided. The method is schematically illustrated in <FIG>. Any of the devices described with respect to <FIG> may be used in this method.

The method comprises, at block <NUM>, arranging an inflatable device <NUM> around a wind turbine blade <NUM>. An inflatable device <NUM> described with respect to <FIG>, <FIG> may for example be used.

The device <NUM> may be a monostable device. , if a fluid or gas with which the device is inflated leaves an internal chamber <NUM> of the device, the device loses its shape.

The device <NUM> may be a multistable, e.g. a bistable device. In this case, the shape of the device is not lost when the inflated configuration is achieved.

An outer surface <NUM> of the inflated device may have a zigzag or a similar shape, as described before with respect to <FIG>, <FIG>. Such a shape may help to disturb the air flow and make it more turbulent.

The method further comprises, at block <NUM>, inflating the device. Therefore, the device changes from a deflated state to an inflated state, and the device presses against the wind turbine blade <NUM> more firmly in the inflated state than in the deflated state. Inflating the device enables the device to disturb air flow around the wind turbine blade.

A suitable fluid, e.g. a suitable gas, may be introduced in one or more inflatable structures <NUM>. Compressed air may for example be used to inflate a multistable structure. A gas cartridge may be used.

The device <NUM> may be arranged around a tip region <NUM> of the wind turbine blade <NUM>. More than one device may be attached to the wind turbine blade. The device(s) <NUM> may be removed from the blade <NUM> before the wind turbine starts or resumes operation.

The explanations provided with respect to <FIG> and the explanations of method <NUM> regarding these figures may be applied to method <NUM>.

Claim 1:
A device (<NUM>) for mitigating vibrations of a parked wind turbine (<NUM>), wherein the device (<NUM>) comprises one or more airflow modifying elements (<NUM>, <NUM>, <NUM>);
the device is configured to transition from an inactive state to an active state; and
the device is configured to grip a wind turbine blade (<NUM>) more firmly in the active state than in the inactive state; characterized in that
the device (<NUM>) comprises a shape memory material, and the device is configured to transition from the inactive state to the active state in response to the activation of the shape memory material; or in that
the device (<NUM>) comprises one or more inflatable structures (<NUM>), and the device (<NUM>) is configured to transition from the inactive state to the active state by inflating the inflatable structures (<NUM>); or in that
the device (<NUM>) comprises one or more vacuum chambers, and the device (<NUM>) is configured to transition from the inactive state to the active state by applying vacuum to the vacuum chambers.