Vibration damping system for a rotor blade of a wind turbine

Abstract

A rotor blade (2) for a wind turbine (1) comprising a vibration damping system (10) is provided. The vibration damping system comprises at least one liquid damping device (11) mounted in the rotor blade, wherein the liquid damping device (11) comprises a vessel (20) that is partially filled with a liquid (21) and with particles (22). The at least one liquid damping device (11) is arranged and tuned to damp vibrations of the rotor blade (2) that occur during a standstill of the rotor blade (2) when the rotor blade (2) is mounted to the wind turbine (1).

Description

[0001] 2025PF00023 1

[0002] Description

[0003] Vibration damping system for a rotor blade of a wind turbine

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to a rotor blade for a wind turbine comprising a vibration damping system. It further relates to a method of assembling such rotor blade .

[0006] BACKGROUND

[0007] In recent years, wind turbines and correspondingly also wind turbine rotor blades have become larger in size and lighter in construction. As a result, the ratio of aerodynamic to inertial loading may increase . In addition, the structural damping of the rotor blade may be compromised by the introduction of new composites . Large rotor blades may thus suffer from reduced intrinsic damping and may be more susceptible to vibrations, which may be induced by aerodynamic interactions, including stall and vortex induced vibrations . It has been found that especially larger rotor blades and component to which they are mounted experience increased fatigue loading caused by blade vibrations .

[0008] It is thus desirable to reduce extreme loads and fatigue loads experienced by a wind turbine rotor blade that forms part of a wind turbine rotor . In particular, it is desirable to provide protection from such loads also for existing rotor blades . The document US2012 / 251318A1 describes dampers for a rotor blade that are specifically configured for counteracting vibration modes occurring during operation. The document WO99 / 32789A1 provides a solution that damps vibrations of the rotor blade that occur during the frequent load changes experienced by the blade, wherein a damper is positioned in the tip of the rotor blade . The document WO99 / 43955A1 describes an increasing of the structural damping of the rotor blade by providing damping means in form of fiberglass-reinforced polyester bars that extend in the2025PF00023 2

[0009] longitudinal direction of the rotor blade . While these solutions may improve the structural damping of the rotor blade during operation, they are mostly not suited for existing rotor blades . Also, it is desirable to further improve the reduction of fatigue loads without significant changes to the structure of the rotor blades .

[0010] SUMMARY

[0011] Accordingly, there is a need to mitigate at least some of the drawbacks mentioned above, in particular to reduce the fatigue loads experienced by a rotor blade, e . g. due to vibrations .

[0012] This need is met by the features of the independent claims . The dependent claims describe embodiments of the invention.

[0013] According to an aspect of the present invention, a rotor blade for a wind turbine comprising a vibration damping system is provided. The vibration damping system comprises at least one liquid damping device mounted in the rotor blade, wherein the liquid damping device comprises a vessel that is partially filled with a liquid and with particles . The at least one liquid damping device is arranged and tuned to damp vibrations of the rotor blade that occur during a standstill of the rotor blade when the rotor blade is mounted to a wind turbine (in particular forms part of a wind turbine rotor of the wind turbine) .

[0014] Using such a vibration damping system for damping vibrations of the rotor blade of a wind turbine may provide several advantages . By using such a vibration damping system, the probability of stall and vortex induced vibrations or vibrations due to seismic events of the rotor blades during standstill of the wind turbine may be reduced significantly. In particular, the vibration damping system may dampen the vibrations so that the energy of these vibration events may be absorbed by the liquid damping device rather than by the rotor blades of the wind turbine or by further components of2025PF00023 3

[0015] the wind turbine . Vibrations of the rotor blade may cause transmission of vibrational energy to the hub and drivetrain of the wind turbine, which may in turn lead to increased fatigue loads on these components . By reducing these vibrations at the rotor blade level, the transmission of loads to other critical parts of the turbine (e . g. , the bearing systems, gearboxes, and generator) may be minimized, reducing maintenance costs and increasing the overall reliability of the wind turbine . By filling the vessel only partially with the liquid, the liquid may oscillate inside the vessel when vibrations are occurring. These oscillations, induced by the vibrations, may be damped more effectively by providing the particles in the liquid, as the motion of the particles may induce local turbulence or chaotic flow, which may aid in energy dissipation through increased mixing within the liquid.

[0016] Severe stall induced vibrations (SIV) and vortex induced vibrations (VIV) may occur when vortices shed form the blade form a coherent wake structure, which in wind turbines may occur if the rotor either locked or in idling mode . Stall and vortex induced vibrations may be suppressed when the coherent structure of the shed vortices is destroyed, preventing vortex lock-on. Therefore, stall and vortex induced vibrations may rarely occur during normal operation but may appear under standstill conditions which may occur due to grid loss, maintenance of the wind turbine, storm conditions, or during erection of the wind turbine . Consequently, conventional systems that damp vibrations during operation may not consider SIV and VIV, as they do not pose a problem during operation.

[0017] Such stall and vortex induced vibrations may not only cause the rotor blades but also the entire wind turbine to vibrate Vibrations that resonate with the natural frequencies of the rotor blades of the wind turbine may not only occur during stall and vortex induced vibrations, but also during seismic events to which the wind turbine is exposed to . Such stall and vortex induced vibrations and seismic induced vibrations2025PF00023 4

[0018] may influence the fatigue wear and the lifetime of the components of the wind turbine . This influence may become more critical as the size of the rotor blades increases . By the disclosed solution, such vibrations that occur during the standstill of the wind turbine rotor can be damped efficiently, and increased fatigue loads and lifetime reduction may be avoided.

[0019] The vibration damping system may comprise one, two, or more liquid damping devices . The vibration damping system including its at least one liquid damping device may thus be tuned to damp vibrations of the rotor blade that occur during a standstill of the rotor blade . The one, two or more liquid damping devices may for example be distributed at positions along the length of the rotor blade to achieve such damping.

[0020] The vessel may comprise a material that ensures a long lifetime and which is in particular not degraded by humidity and by strongly fluctuating temperature conditions inside a rotor blade . It may for example comprise stainless steel, a fiber reinforced plastic material, e . g. a filament wound composite, or another metallic alloy. Thus, the liquid damping device lends itself to being retrofitted and may further provide a long lifetime and low maintenance .

[0021] In an embodiment, the at least one liquid damping device is arranged at a position along the longitudinal direction of the rotor blade of less than 70%L, preferably between 10%L to 60%L, more preferably between 15%L and 55%L, wherein L is the length of the rotor blade in longitudinal direction and the blade extends from 0%L at a root end of the blade to 100%L at a tip end of the blade . Additional liquid damping devices may be provided within this positional range or at other positions .

[0022] By providing the at least one damping device at a position of the rotor blade of less than 70%L, vibrations that are occurring during standstill in the middle portion of the rotor blade may be mitigated. The middle portion of the rotor2025PF00023 5

[0023] blade typically experiences higher levels of aerodynamic loading compared to the tip end and root end, in particular in standstill . A standstill vibration mode may cause large vibration amplitudes in this range, and thus increased fatigue loading. By mitigating vibrations at standstill of the wind turbine in this region, fatigue accumulation of the rotor blade may be reduced, which may lead to a longer operational life of the rotor blade . Repair costs for the rotor blades and downtime of the wind turbine may thus be mitigated .

[0024] In an embodiment, a dynamic viscosity of the liquid at a temperature of 20°C is less than 600 mPa*s, preferably less than 100 mPa*s, more preferably less than 50 mPa*s . The viscosity may for example be measured by a viscometer, such as a glass capillary viscometer, e . g. an Ostwald viscometer .

[0025] When a vessel containing a liquid is subj ected to vibration, the motion of the vessel may disturb the liquid inside . This disturbance may create a wave motion (also called sloshing or sloshing waves) within the liquid. These waves typically propagate through the liquid as the vessel moves in a direction, causing the surface of the liquid to oscillate . By using a liquid that comprises a viscosity less than 600 mPa*s it may be possible for the liquid to flow inside the vessel more easily. Waves that propagate through the liquid may thus be damped similar to a damped oscillator and the vibrations may thus be absorbed by the wave motion of the liquid. Further, it may allow movement of the particles within and / or on the surface of the liquid, thus improving energy dissipation. Especially if stall and vortex induced vibrations and seismic induced vibrations and thus vibrations with a low frequency may occur, these vibrations may be absorbed efficiently by using a liquid comprising a viscosity of less than 600 mPa*s .

[0026] In an embodiment, the vibration damping system is tuned to damp frequencies of vibrations below 5 Hz, preferably below 2 Hz, more preferably below 1 Hz, for example between 0.1 Hz2025PF00023 6

[0027] and 1 Hz, preferably between 0.2 Hz and 0.9 Hz . Thus, it may be possible to damp frequencies of vibrations that are associated with stall and vortex induced vibrations and seismic induced vibrations of a rotor blade in standstill more effectively.

[0028] In an embodiment, the vessel extends in a chord direction over less than 70%c, 60%c or 50%c, e . g. over a length of between 20%c and 70%c, or between 30%c and 60%c, wherein c is a chord length of the rotor blade at the (longitudinal) position of the vessel in the rotor blade . The position of the vessel in the rotor blade may be determined by the position of its center of mass .

[0029] In an embodiment, the vessel extends in a thickness direction over less than 70%t, 60%t or 50%t, e . g. between 20%t and 70%t, wherein t is a thickness of the rotor blade taken perpendicular to the chord direction at the (longitudinal) position of the vessel in the rotor blade . The thickness t may generally be defined as the maximum of the blade thickness in the direction perpendicular to the chord direction .

[0030] The liquid damping device may be sized and arranged according to the available space in chord and / or thickness direction inside the rotor blade of the wind turbine . Thus, it may be possible to integrate the vibration damping system into already existing rotor blade setups more easily (e . g. for retrofitting) . Extending the vessel over a longer distance of the chord direction and / or in thickness direction may allow providing more mass inside the rotor blade, as the volume of the vessel may be increased, and more liquid and thus mass may be disposed in the vessel .

[0031] The damping device may be tuned to damp an edge mode of the rotor blade . In particular, the damping device may be configured to damp a base frequency of an edge-to-edge mode of the rotor blade at its position in the blade . Edge-wise vibrations may thus be damped efficiently. For example, the2025PF00023 7

[0032] length of the vessel (in longitudinal direction, in particular in chord direction) may be set to provide the damping at the respective frequency.

[0033] A volume of the vessel may be set to adjust the mass of the damping device, which may determine an amount of damping. For example, the diameter of a cylindrical vessel may be set in accordance with the desired amount of damping and the space / mass constraints of the rotor blade .

[0034] In an embodiment, the particles are configured to float on a surface of the liquid.

[0035] Using particles that are configured to float on the surface of the liquid may increase the apparent viscosity of the surface layer of the liquid. The particles may provide additional resistance to the flow of the liquid, which may increase the damping of surface waves or oscillations . As the particles move across the surface of the liquid, they may create more friction and thus energy dissipation, which may help to dampen the waves in the liquid more effectively.

[0036] Using larger particles may create more drag on the surface of the liquid, which may lead to a greater energy dissipation through friction. Small particles may be less effective at damping larger surface waves but may still contribute to surface viscosity, which may help with damping smaller waves or higher frequencies of the waves . As the particles move over the surface of the liquid, additional friction between the liquid and the particles may be created. This friction may convert vibrational energy into heat . Furthermore, the movement of the particles may directly oppose the motion of the surface wave, which may decrease its amplitude over time .

[0037] In a further embodiment, the particles may comprise nonfloating particles . Embodiments with only floating particles (positive buoyancy) , only non-floating particles (negative buoyancy) , or floating and non-floating particles are conceivable . Non-floating particles may comprise a relatively high weight, so that the liquid damping device comprises a2025PF00023 8

[0038] high inertia, which may dampen the vibrations more efficiently.

[0039] In an embodiment, the particles are hollow and preferably comprise a spherical or cylindrical shape .

[0040] Hollow particles may comprise a much lower density than solid particles of the same size . This may lead to a reduction in the overall mass of the particles . Thus, a light weight of the liquid damping device may be ensured. The increase in structural loading of the blade by the damping device may thus be kept small . The lower density may also mean that the hollow particles are more buoyant in the liquid, which may ensure that the hollow particles stay suspended in the liquid and prevent settling. This may be advantageous in maintaining consistent surface coverage with particles and thus damping performance over time . Using hollow particles may provide more surface area on the surface of the liquid per unit mass compared to solid particles . This may increase the ability of the particles to interact with the liquid surface, leading to greater frictional forces and more effective energy dissipation .

[0041] Particles comprising a cylindrical shape may provide more resistance to movement because their elongated shape may cause more friction as they slide over the surface of the liquid or through the liquid. Furthermore, particles comprising a cylindrical shape may comprise a larger contact area with the liquid and may interact more severely, which may lead to better energy dissipation through frictional forces .

[0042] In an embodiment, the particles floating on the surface of the liquid cover at least 10% (e . g. , between 10%-100%, preferably at least 20% (e . g. , between 20%-90%) , more preferably at least 40% (e . g. , between 40%-90%) of the surface of the liquid. A preferred range may be 40%-80% surface coverage .2025PF00023 9

[0043] The concentration of particles covering the surface of the liquid may influence how effectively they can dampen vibrations . A higher concentration may lead to more collisions between the particles, resulting in more energy dissipation. If the particle concentration is too high, the particles may form a dense layer, which may reduce flowability. It may then even be possible that particles are settling below the surface of the liquid, which may potentially reduce the damping effect . At a low particle concentration, individual particles may be more likely to move independently and the overall effect on the surface behavior may be reduced. This may lead to less frictional energy dissipation and less effective damping of the surface waves and consequently the vibrations . By using a high enough number of particles that cover the surface of the liquid, the interaction of the liquid and the particles may be equated to a viscoelastic behavior, in which viscous (dissipative) properties may be observable .

[0044] At a concentration between 40%-80% of particles covering the surface of the liquid, the particles may act as a reinforced network within the liquid. This network may improve the viscous damping as the particles move in response to external vibrations, providing both resistance to flow and energy dissipation. This may especially be beneficial for damping vibrations across different frequencies .

[0045] The volume of the particles may be less than 30%, less than 20%, preferably less than 10% of the inner volume of the vessel .

[0046] In an embodiment, the at least one liquid damping device is arranged such that a longitudinal axis of the liquid damping device is in a direction pointing towards a leading edge and a trailing edge of the wind turbine blade . The longitudinal axis of the liquid damping device may in particular be perpendicular to a longitudinal axis of the rotor blade, for example (approximately) parallel to a chord direction of the rotor blade .2025PF00023 10

[0047] Such arrangement may ensure an efficient damping of the edge mode of the rotor blade, as such mode may cause the liquid in the damping device to flow in the longitudinal direction thereof, e . g. in its length direction. Further, this may allow an efficient tuning of the damping device by adjusting the extension of the vessel in length direction.

[0048] Further, such orientation of the damping device may allow an efficient damping of edge modes in any orientation of the wind turbine rotor blade . In standstill, the rotor blade may be positioned at any rotational angle of the wind turbine rotor (e . g. at all azimuths) with the rotor blades being pitched out of the wind. The arrangement may allow efficient damping in most of the respective blade positions .

[0049] Preferably, the vessel comprises an axisymmetric shape . The symmetry axis may correspond to the longitudinal axis of the liquid damping device .

[0050] In an embodiment, the vessel comprises a cylindrical or a box-like shape, wherein the longitudinal axis corresponds to a cylinder axis of the cylindrical shape or to a central axis of the box-like shape . Such central axis of the box like shape may be a central symmetry axis, e . g. an axis through the center of gravity of the end faces .

[0051] A vessel comprising a box-like shape may comprise corners and flat walls . In such a vessel, surface waves may be more easily confined or restricted to certain areas, which may concentrate wave energy in specific regions, depending on the shape and size of the vessel . This may allow the particles to interact more effectively in those areas, which may improve damping of the vibration induced waves in the liquid.

[0052] A vessel comprising a cylindrical shape may comprise a curvature along its sides, which may result in reduced space for the wave crest and thus in higher wave amplitudes . The curvature of the walls may influence the speed and energy of2025PF00023 11

[0053] surface waves . An efficient energy dissipation may thus be achieved in a cylindrical vessel .

[0054] In an embodiment, the vessel is partially filled with 30%-70%, preferably with 40%-60% of the liquid. The combined volume of liquid and particles may be between 30%-80%, preferably between 40% and 70%, e . g. between 40% and 60%, of the inner volume of the vessel . The remainder of the vessel may be filled with a gas, e . g. air, nitrogen or another gas .

[0055] Using a vessel that is partially filled with liquid may provide a more efficient energy absorption and dissipation, as the liquid may oscillate inside the vessel and thus efficiently dissipate energy of slow vibrations . Furthermore, as the walls of the vessel are in contact with the liquid, the frictional resistance between the liquid and the walls may be increased by such filling fraction, which may further improve energy dissipation. For a cylindrical vessel, the surface area may be become maximal between 40% and 60% filling level .

[0056] In an example embodiment, the liquid is or comprises a glycol component . For example, the glycol component may be one or a combination of ethylene glycol, diethylene glycol, or the like .

[0057] By using a liquid that is or comprises a glycol component, the liquid may be naturally frost-proof, non-volatile and corrosion resistant . Thus, the liquid inside the liquid damping device may be provided as a long-life component which may not require maintenance over time . This may consequently reduce the operational costs of employing the liquid damping device / vibration damping system.

[0058] In an embodiment, the vibration damping system is arranged in a shear web or in a spar of the rotor blade . Preferably, the liquid damping device is arranged in an opening (or a hole) in the shear web or spar of the rotor blade, in particular mounted in or to such opening.2025PF00023 12

[0059] Accordingly, the vibration damping system may be integrated in the construction of the rotor blade of the wind turbine . It may thus be possible to easily incorporate the vibration damping system into a rotor blade design. Furthermore, the vibration damping system may be fitted into already existing rotor blade setups . By providing the liquid damping device in an opening in the shear web or spar, the tank of the liquid damping device may be held in place by the hole in the shear web / spar . Consequently, it may be possible to arrange the vibration damping system with only few or without additional fixation means . Furthermore, the liquid damping devices may be arranged such that it is not in contact with a pressure side or a suction side of the rotor blade . Also, such arrangement may result in a favorable wight distribution.

[0060] In some embodiments, the damping device further comprises a mechanical mount by which the vessel is mounted to one or more surfaces inside the rotor blade . For example, the vessel may be mounted in a hollow space inside the rotor blade, preferably in a trailing edge cavity, to a shear web or spar of the rotor blade and / or to a suction side shell or a pressure side shell of the rotor blade . Such mount may for example comprise an attachment portion, e . g. a plate, that is bolted or adhered to the respective structure of the blade, and / or part of the mount may be embedded in the respective blade structure . In such embodiments, the vessel may or may not reach through an opening in the shear web or spar of the blade . The mount may for example comprise an attachment portion for attachment to a shear web or spar of the blade, and may further comprise an attachment portion for attachment to a pressure side and / or suction side of a blade shell .

[0061] In another embodiment, the liquid damping device comprises a pressure release valve, which is configured to reduce pressure inside the liquid damping device . This may be particularly useful in situations where lightning strikes the rotor blade of the wind turbine and liquid in the vessel may evaporate .2025PF00023 13

[0062] In an embodiment, the at least one liquid damping device comprises plural liquid damping devices, wherein the plural liquid damping devices are arranged at different longitudinal positions along the length of the rotor blade . For example, at least 2, 3, 4, or more liquid damping devices may be provided .

[0063] By employing more than one liquid damping device, vibrations of the rotor blade may be absorbed more efficiently, as the vibrational energy may be dissipated by the different liquid damping devices at different positions . Furthermore, by employing plural liquid damping devices, the damping mass may be increased so that the vibration damping system can absorb a higher vibrational energy.

[0064] In an embodiment, the plural liquid damping devices comprise at least a first liquid damping device arranged at a position between 40%L and 60%L, at least a second liquid damping device arranged at a position between 20%L and 30%L and / or at least a third liquid damping device arranged at a position between 70%L and 80%L . In other embodiments, the plural liquid damping devices may be arranged at a position of less than 70%L, preferably less than 60%L .

[0065] In an embodiment, the plural liquid damping devices are arranged at different longitudinal positions along the rotor blade that correspond to vibrational antinodes of one or more vibrational modes of the rotor blade (during standstill) . For example, a liquid damping device may be arranged at each of at least two antinodes, preferably at each of at least three antinodes .

[0066] At the position of vibrational antinodes of the vibrational modes of the rotor blade, the vibrations may comprise the maximum vibration intensity, e . g. maximum vibration amplitude, of the respective mode . Consequently, by arranging the liquid damping devices at positions that coincide with vibrational antinodes of maximum amplitude of one or more2025PF00023 14

[0067] vibrational modes, the vibrations may be absorbed more efficiently. As a result, fewer liquid damping devices may be required to damp the vibrations . A more cost-efficient vibration damping system may thus be provided. Preferably, the mode comprises at least a base mode of an edge mode .

[0068] It should be clear that the frequency or frequencies to which the liquid damping device is to be tuned depends on the size and structure of the rotor blade . The position and dimensions of the vessel can be adjusted in accordance with the respective frequency to be dampened and the type of liquid and particles used in the vessel . In particular, the length in edge direction may be adapted to adjust the tuning frequency and the vessel volume may be adapted to achieve the desired damping power .

[0069] According to a further aspect of the invention, a wind turbine comprising a rotor blade having any of the configurations described herein is provided.

[0070] According to a further aspect of the invention, a method of assembling a rotor blade is provided. The method comprises providing a rotor blade and mounting a vibration damping system to the rotor blade . Mounting the vibration damping system comprises mounting at least one liquid damping device in the rotor blade, wherein the liquid damping device comprises a vessel that is partially filled with a liquid and with particles, wherein the at least one liquid damping device is arranged and tuned to damp vibrations of the rotor blade that occur during a standstill of the rotor blade when the rotor blade is mounted to a wind turbine . The method may be performed so as to obtain a rotor blade having any of the configurations described herein.

[0071] By such wind turbine or method, advantages may be obtained that correspond to the advantages outlined above with respect to the rotor blade .2025PF00023 15

[0072] It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. In particular, the features of the different aspects and embodiments of the invention can be combined with each other unless noted to the contrary.

[0073] BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The forgoing and other features and advantages of the invention will become further apparent from the following detailed description read in conjunction with the accompanying drawings . In the drawings, like reference numerals refer to like elements .

[0075] Fig. 1 is a schematic drawing showing a wind turbine in a parked position.

[0076] Fig. 2 is a schematic drawing showing a perspective view of a liquid damping device of a vibration damping system of a rotor blade according to an embodiment .

[0077] Fig. 3 is a schematic drawing showing a rotor blade comprising a plurality of liquid damping devices according to an embodiment .

[0078] Fig. 4 is a schematic drawing showing a liquid damping device mounted in a rotor blade according to an embodiment .

[0079] DETAILED DESCRIPTION

[0080] In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings . It is to be understood that the following description of the embodiments is given only for the purpose of illustration and is not to be taken in a limiting sense . It should be noted that the drawings are to be regarded as being schematic representations only, and elements in the2025PF00023 16

[0081] drawings are not necessarily to scale with each other .

[0082] Rather, the representation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art . As used herein, the singular forms "a, " "an, " and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise . The terms "comprising, " "having, " "including, " and "containing" are to be construed as open-ended terms (i . e . , meaning "including, but not limited to, ") unless otherwise noted .

[0083] Fig. 1 shows a wind turbine 1 that in a standstill condition, which comprises three rotor blades 2, each being configured in accordance with an embodiment . The rotor blades form part of a wind turbine rotor mounted to a nacelle 3, which is supported by a tower 5. In these standstill conditions, the rotor blades 2 of the wind turbine 1 may experience wind from various directions . The blades may have been pitched out of the wind (not shown in Fig. 1 ) . The nacelle 3 may for example be locked at 45° relative to the wind. As a result, the airfoils may be in a post-stall condition of the lift curve, which may result in stall induced vibrations (SIV) in the rotor blades . In other situations, the wind may impinge on the rotor blades 2 with larger angles that are close to perpendicular ( 90° ) to the chord direction of the pitched-out rotor blades 2 . Vortex shedding may lead to a vibrational edge mode from the leading edge to the trailing edge (shock edge mode) . However, the blade only provides structural damping for such edge-to-edge vibration modes ( flap modes may still benefit from aerodynamic damping) . Accordingly, in such standstill conditions, the rotor blades may suffer from increased fatigue wear due to stall induced vibrations (SIV) and vortex induced vibrations (VIV) . Further, seismic activity may transfer vibrations via tower 5 and may likewise result in vibrations experienced by rotor blades 2 .

[0084] Fig. 2 schematically illustrates a perspective view of a liquid damping device 11 of a vibration damping system 10 accord to an embodiment . Each of the blades 2 may be equipped2025PF00023 17

[0085] with a respective damping system 10, either during manufacturing or system 10 may be retrofitted. The liquid damping device 11 comprises a vessel 20. The vessel 20 is partially filled with a liquid 21 and particles 22. When the vessel 20, which contains the liquid 21, is subj ected to vibrations, the motion of the vessel 20 will disturb the liquid 21 inside the vessel . This disturbance may create a wave motion (also called sloshing or sloshing waves) within the liquid 21. These waves typically propagate through the liquid 21 as the vessel 20 moves in a direction, causing the surface 23 of the liquid 21 to oscillate . The damping device may thus be a sloshing damper .

[0086] In the present example, the vessel 20 comprises a cylindrical shape and thus comprises a uniform curvature along the side walls . The curvature of the walls may influence the size, speed and direction of surface waves of the liquid 21 and may improve energy dissipation.

[0087] The vessel 20 in figure 2 is filled to about 50% with liquid 21. As can be seen in figure 2, for a cylindrical vessel, the area of the surface of the liquid that is in contact with the walls of the vessel is maximized at 50% filling level . The frictional interaction between the walls of the vessel 20 and the liquid surface 23 may then be increased which may lead to a better energy dissipation of the liquid.

[0088] The particles 22 of the present example are configured to float on the surface 23 of the liquid 21. Using particles 22 that are configured to float on the surface 23 of the liquid 21 may increase the apparent viscosity of the surface layer of the liquid 21. The particles 22 may provide additional resistance to the flow of the liquid 21, which may increase the damping of surface waves or oscillations . As the particles 22 move across the surface 23 of the liquid 21, they may create more friction and thus energy dissipation, which helps to dampen the waves in the liquid 21 more

[0089] efficiently.2025PF00023 18

[0090] In addition, the particles 22 of the present example comprise a cylindrical shape . This may provide more resistance to the movement of the liquid 21, because the elongated shape of the particles 22 may cause more friction as the particles 22 move over the surface 23 of the liquid 21. Furthermore, the particles 22 may comprise a larger contact area with the liquid 21, which may lead to better energy dissipation through the frictional forces between the particles 22 and the liquid 21. Particle-particle interaction may further provide an improved energy dissipation for such particle shape .

[0091] In figure 2, the particles 22 cover at least 50% of the surface 23 of the liquid 21. By using particles that cover at least 50% of the surface of the liquid, the interaction of the liquid and the particles may be equated to a viscoelastic behavior, in which viscous (dissipative) properties may be observable . Thus, the vibrations may be damped more efficiently.

[0092] The vessel has a length extending along a longitudinal axis 25 of the vessel . The damping device 11 may be oriented such that the direction of the vibration to be damped causes movement of the vessel approximately in the direction of the longitudinal axis 25. The length along the longitudinal axis 25 may be adjusted to tune the damping frequency of the damping device 1, for example to tune it to at least one of a base frequency of a VIV and / or a SIV, in particular to the base frequency of a respective edge mode .

[0093] It is noted that other shapes of the vessel and / or of the particles may also be employed, as outlined above .

[0094] Fig. 3 shows a perspective view of a rotor blade 2 of a wind turbine 1. The rotor blade 2 comprises the vibration damping system 10, which may have the configuration described further above and with respect to Fig. 2. The vibration damping system 10 comprises plural liquid damping devices 11 that are arranged at different positions along the longitudinal2025PF00023 19

[0095] direction 30 of the rotor blade . The longitudinal direction of the rotor blade is defined by the blade root end 31 and the tip end 32 of the rotor blade . The rotor blade length L is defined in this direction 30. The plural liquid damping devices 11 are arranged such that the longitudinal axis 25 of at least one, preferably each liquid damping device 11 points in a direction towards a leading edge 33 and a trailing edge 34 of the wind turbine rotor blade 2. The device 11 may be oriented such that the longitudinal axis 25 is perpendicular to the longitudinal direction 30, e . g. substantially parallel to a chord direction of blade 2 at the position of the device 11 .

[0096] As can be seen in Fig. 3, the liquid damping devices 11 are arranged at different longitudinal positions along the length of the rotor blade 2. The longitudinal positions of the different liquid damping devices 11 are chosen such that at least two, preferably at least three devices 11 are positioned between 10%L and 70%L . This may essentially correspond to a middle portion of the rotor blade 2. The middle portion of the rotor blade 2 typically experiences high levels of vibrational loading in standstill . By mitigating vibrations at standstill of the wind turbine 1 in this region, fatigue loading of the rotor blade 2 may be reduced, which may lead to a longer operational life of the rotor blade 2. It is also possible to provide further devices 11 at positions larger than 70%L .

[0097] Preferably, the longitudinal positions correspond to vibrational antinodes of vibrational modes of the rotor blade that are excited due to, e . g. stall and vortex induced vibration or seismic induced vibrations, in particular to antinodes at which the vibration amplitude is maximal .

[0098] Preferably, the antinodes include at least one or more antinodes of the lowest order edge mode . By arranging the liquid damping devices 11 at antinodes of maximal vibration amplitude, these vibrations may be absorbed more efficiently by the liquid damping devices 11, so that the vibrations are2025PF00023 20

[0099] not transferred to the wind turbine blade 2 or components of the wind turbine 1 .

[0100] A damping device 11 may for example be arranged at each of the positions 20%L-30%L, 40%-60%L, and 70%L to 80%L . It is also possible to arrange two or more damping devices at the same longitudinal position, e . g. above one another . However, it is preferred to provide a single damping device 11 at each longitudinal position.

[0101] Fig. 4 shows a damping device 11 mounted in a trailing edge cavity 39 of a rotor blade 2. The mounting may be employed with any of the embodiments described herein. The cavity 39 is bounded by the pressure side shell 37 and the suction side shell 38 (which may be respective half-shells or part of an integral shell) and the shear web 36. The vessel 20 may extend through the shear web 36, e . g. through an opening provided therein, or may only extend within the trailing edge cavity 39. The liquid damping device 11 comprises a mount 26 by which the vessel 20 is mounted. The mount 26 may comprise a shear web mount 26 by which the vessel is secured to the shear web 36. The shear web mount 36 may comprise a holder that is attached to the shear web (e . g. bolted, adhered, etc . ) and to which the vessel 20 is mounted (e . g. bolted, clamped, etc . ) . The mount 26 may further comprise a shell mount 28, which may be configured to be attached to the pressure side shell 37 and / or to the suction side shell 38. Mounting to the pressure side shell 37 is preferred. The shell mount may comprise an attachment part configured to be attached to the rotor blade shell (e . g. , by bolting, adhering, embedding, etc . ) and a holding part that mounts the vessel 20 to the attachment part, e . g. by bolting, clamping or the like . In the present example, the vessel is clamped with a bracket and a struct is used to support the bracket on an attachment part bolted or adhered to the blade shell .

[0102] Other configurations are conceivable . The mount may in particular be adapted to the respective structure of the blade . For example, if the blade has a spar, the vessel may2025PF00023 21

[0103] be mounted to the spar and / or extend through the spar . In other examples, the vessel 20 may be mounted in a leading edge cavity, or in both cavities .

[0104] While specific embodiments are disclosed herein, various changes and modifications can be made without departing from the scope of the invention. The present embodiments are to be considered in all respects as illustrative and non-restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims

2025PF00023 22Patent claims1. A rotor blade for a wind turbine ( 1 ) comprising a vibration damping system ( 10) , wherein the vibration damping system comprises at least one liquid damping device ( 11 ) mounted in the rotor blade (2 ) , wherein the liquid damping device ( 11 ) comprises a vessel (20) that is partially filled with a liquid (21 ) and with particles (22 ) , wherein the at least one liquid damping device ( 11 ) is arranged and tuned to damp vibrations of the rotor blade (2 ) that occur during a standstill of the rotor blade (2 ) when the rotor blade (2 ) is mounted to the wind turbine ( 1 ) .

2. The rotor blade according to claim 1, wherein the at least one liquid damping device ( 11 ) is arranged at a position along the longitudinal direction of the rotor blade of less than 70%L, preferably between 10%L and 60%L, wherein L is the length of the rotor blade (2 ) in longitudinal direction (30) and the rotor blade extends from 0%L at a root end (31 ) of the rotor blade (2 ) to 100%L at a tip end (32 ) of the rotor blade ( 2 ) .

3. The rotor blade according to claim 1 or 2, wherein a dynamic viscosity of the liquid (21 ) at a temperature of 20°C is less than 600 mPa*s, preferably less than 100 mPa*s, more preferably less than 50 mPa*s .

4. The rotor blade according to any of the preceding claims, wherein the vibration damping system ( 10) is tuned to damp frequencies of vibrations below 2 Hz, preferably below 1 Hz, more preferably between 0.1 Hz and 1 Hz .

5. The rotor blade according to any of the preceding claims, wherein the vessel extends in a chord direction of the rotor blade over less than 70%c, 60%c or 50%c, wherein c is a chord length of the rotor blade at the position of the vessel (20) in the rotor blade (2 ) .2025PF00023 236. The rotor blade according to any of the preceding claims, wherein the vessel extends in a thickness direction over less than 70%t, 60%t or 50%t, wherein t is a thickness of the rotor blade taken perpendicular to a chord direction of the rotor blade at the position of the vessel (20) in the rotor blade ( 2 ) .

7. The rotor blade according to any of the preceding claims, wherein the particles (22 ) are configured to float on a surface (23) of the liquid (21 ) in the vessel .

8. The rotor blade according to claim 7, wherein the particles (22 ) floating on the surface (23) of the liquid cover at least 10%, preferably at least 20%, more preferably at least 40% of the surface (23) of the liquid (21 ) .

9. The rotor blade according to any of the preceding claims, wherein the at least one liquid damping device ( 11 ) is arranged such that a longitudinal axis of the liquid damping device ( 11 ) is in a direction pointing towards a leading edge (33) and a trailing edge (34 ) of the wind turbine rotor blade (2 ) .

10. The rotor blade according to claim 9, wherein the vessel (20) comprises a cylindrical or a box-like shape, wherein the longitudinal axis corresponds to a cylinder axis of the cylindrical shape or a central axis of the box-like shape .

11. The rotor blade according to any of the preceding claims, wherein the liquid fills 30%-70%, preferably 40%-60%, of an inner volume of the vessel (20) .

12. The rotor blade according to any of the preceding claims, wherein the vibration damping system ( 10) is arranged in or mounted to a shear web or a spar of the rotor blade (2 ) , wherein preferably, the vibration damping system ( 10) is arranged in an opening in the shear web or spar of the rotor blade ( 2 ) .2025PF00023 2413. The rotor blade according to any of the preceding claims, wherein the at least one liquid damping device ( 11 ) comprises plural liquid damping devices ( 11 ) , wherein the plural liquid damping devices ( 11 ) are arranged at different longitudinal positions along the length of the rotor blade (2 ) .

14. The rotor blade according to claim 13, wherein the plural liquid damping devices ( 11 ) are arranged at different longitudinal positions along the rotor blade (2 ) , wherein the different longitudinal positions correspond to vibrational antinodes of one or more vibrational modes of the rotor blade (2 ) .

15. A method of assembling a rotor blade, comprising:- providing a rotor blade;- mounting a vibration damping system ( 10) in the rotor blade, wherein mounting the vibration damping system comprises mounting at least one liquid damping device (11 ) in the rotor blade, wherein the liquid damping device ( 11 ) comprises a vessel (20) that is partially filled with a liquid (21 ) and with particles (22 ) , wherein the at least one liquid damping device ( 11 ) is arranged and tuned to damp vibrations of the rotor blade (2 ) that occur during a standstill of the rotor blade (2 ) when the rotor blade (2 ) is mounted to a wind turbine ( 1 ) .

Images