Cross-Reference

[0001] This application claims the benefit of U.S. Application No. 61/373,580, filed on August 13, 201 1, and Danish Patent Application No. PA 2010 70462, filed on October 29, 2010.

Technical Field

[0002] The present invention relates to damping the vibrations of a wind turbine blade. More specifically, the present invention relates to a wind turbine blade including a damping element for dissipating edgewise vibrations.

Background

[0003] There are two primary types of natural vibrations (i.e., resonant oscillations) associated with the blade of a wind turbine. Flap wise vibrations occur in a plane perpendicular to leading and trailing edges of the blade. Edgewise vibrations generally occur in a plane through the leading and trailing edges, i.e. in the general direction of the chord of the blade's airfoil shape. Both types of vibrations place significant loads on the blade that can intensify fatigue damage and lead to failure. Therefore, it is important to avoid exciting these vibrations.

[0004] This is particularly true when a blade enters an operational condition called stall. During stall, the airflow over the upper surface of the blade becomes increasingly turbulent. If turbulence or other factors excite the blade's natural vibrations, aerodynamic forces tend amplify these movements. This occurs because of a principle called negative aerodynamic damping.

[0005] There is a high risk of damage in the situation described above, especially in stalled-controlled turbines where stall is intentionally used to control power output. Specifically, the aerodynamic forces that excite natural vibrations during operation are a function of the blade's tip speed squared. These forces are significant during stall because that condition occurs at relatively high wind speeds.

[0006] Pitch-controlled turbines do not experience the situation described above as much as stalled- controlled turbines. This is because the blades of a pitch-controlled turbine pitch away from stall to control power output. Nevertheless, the situation may still occur for a brief period of time. The blades of a pitch-controlled turbine may also experience the amplification of natural vibrations when "parked" during a storm with extremely high winds and/or high yaw error. This is especially true during grid outage or yaw system failure. In the parked condition the additional threat of vortex shedding and resulting vortex-induced vibration is present as well.

[0007] The problem of negative aerodynamic damping and vortex-induced vibrations can

theoretically be mitigated by a few approaches, the primary being increasing drag and increasing structural damping. Increasing drag through changes in airfoil geometry will negatively impact power production. Increased structural damping will not affect turbine performance, and is thus a much more attractive approach.

[0008] Several ways to increase the damping of a structural blade have been developed. For example, WO 95/21327 discloses a blade having an oscillation-reduction element oriented in the direction of unwanted oscillations. Although the patent application first describes the oscillation-reduction element using generic terms and depicts it using conventional symbols, most of the embodiments disclosed are tuned liquid dampers. These dampers are specifically designed (i.e., "tuned") to have a natural frequency substantially corresponding to the dominating natural frequency of the blade. As such, their effectiveness at damping vibrations is frequency-dependent. They also typically require maintenance and can be difficult to access and install.

[0009] Passive dampers are also known. One example of a passive damper is disclosed in WO 99/43955. However, because passive dampers are typically difficult to design and implement, the number of adequate solutions developed has been limited. There remains plenty of room for improvement in this area.

Summary

[0010] A blade for a wind turbine is provided by the disclosure below. The blade generally comprises a shell and a spar supporting at least a portion of the shell. More specifically, the shell has a leading edge and a trailing edge extending from a root to a tip of the blade and defines first and second sides extending between the leading edge and trailing edge. The spar supports the shell between the first and second sides and has a length extending in a direction from the root to the tip. The blade further includes first and second supports coupled to different locations along the length of the spar and a damping element coupled to the first and second supports. Thus, the damping element is offset from the spar.

[001 1] The spar bends back-and-forth along its length when the blade experiences edgewise vibrations. The damping element is configured to counteract movement between the first and second supports caused by this bending, thereby dissipating the edgewise vibrations. By offsetting the damping element from the spar, greater damping forces may be applied because the first and second supports magnify displacements caused by the bending. Thus, the effectiveness of the damper will be improved by increasing the offset from the spar.

[0012] Different embodiments of the damping element are disclosed as examples. The term

"damping means" refers to some or all of these embodiments, together with equivalents to such embodiments. The damping element may comprise, for example, a first portion coupled to the first support and a second portion coupled to the second support. The first and second portions are movable relative to each other, but are also coupled together by a damping force that resists such relative velocity. The damping force may be hydraulic or mechanical.

[0013] The blade may include a single damping element or a plurality of damping elements. For the latter embodiments, the blade also includes a plurality of supports coupled to the spar at different locations along its length. Each damping element is then coupled to two of the supports. There may even be more than one damping element coupled to any given support. For example, in a variation of the embodiment mentioned above, the blade further includes a third support coupled to the spar at a location spaced from the second support and a damping element coupled to the second and third supports so as to be offset from the spar (like the damping element coupled to the first and second supports).

[0014] These and other aspects will be made more apparent by the detailed description and claims below, as well as by accompanying drawings. Note that when describing the same type of elements, numerical adjectives such as "first" and "second" are merely used for clarity. They are assigned arbitrarily and may be interchanged. As such, the use of these adjectives in the claims may or may not correspond to the use of the same adjectives in the detailed description (e.g., a "first element" in the claims might refer to any such "element" and not necessarily the ones labeled "first" in the detailed description below). Brief Description of the Drawings

[0015] Fig. 1 is a perspective view of one embodiment of a wind turbine.

[0016] Fig. 2 is a perspective view of a blade on the wind turbine of Fig. 1.

[0017] Fig. 3 is a sectional view taken along line 3—3 in Fig. 2.

[0018] Fig. 4 is a sectional view similar to Fig. 3 showing an alternative embodiment of the blade

[0019] Fig. 5 is a sectional view similar to Fig. 3 showing yet another embodiment of the blade.

Detailed Description

[0020] Fig. 1 shows one embodiment of a wind turbine 10. The wind turbine generally comprises a tower 12, a nacelle 14 supported by the tower 12, and a rotor 16 attached to the nacelle 14. The rotor 16 includes a hub 18 rotatably mounted to the nacelle 14 and a set of blades 20 coupled to the hub 18. More specifically, each blade 20 includes a root 22 coupled to the hub 18 and a tip 24 spaced from the hub 18. The blades 20 convert the kinetic energy of the wind into mechanical energy used to rotate the shaft of a generator (not shown), as is conventional. However, as will be described in greater detail below, one or more of the blades 20 are specially designed to reduce certain vibrations that create loads and increase the potential of damage or failure.

[0021] Figs. 2 and 3 schematically illustrate one of the blades 20 in further detail. The blade 20 includes a shell 26 defined by first and second sides 28, 30 extending between a leading edge 32 and a trailing edge 34 and forming an airfoil cross-section. A spar 36 extends from the root 22 toward the tip 24 between the first and second sides 28, 30 to support at least a portion of the shell 26. The blade 20 may be constructed using any materials and techniques suitable for wind turbines. For example, the first and second sides 28, 30 may be separate shell components constructed by laying materials in a mold and curing resin. The resin may be pre-impregnated in the materials (e.g., pre-preg glass fibers) and/or introduced separately (e.g., using an infusion process), depending on the technique used. The mold may then be closed so that the components can be glued together to form the shell 26.

[0022] Certain conditions may cause the blade 20 to experience vibrations in the plane of its rotation. The tip 24 moves back and forth in an edgewise direction 38 between the leading and trailing edges 32, 34 during these vibrations. The blade 20 may also experience vibrations in a flapwise direction 40, where the tip 24 moves perpendicular to the plane of rotation. [0023] As mentioned in the background section above, previous attempts to dampen edgewise vibrations have focused on applying forces in the opposite direction of movement of the tip 24 (i.e., the edgewise direction 38). However, applicant has discovered that the geometry of the blade 20 allows edgewise vibrations to be dampened by applying forces in a different manner. More specifically, when the tip 24 moves in the edgewise direction 38 toward the leading edge 32, the root 22 tends to resist such movement because of its connection to the hub 18. This results in the shell 26 and spar 36 bending during edgewise vibrations. Because linear distance between points along the length of the spar 36 changes as the blade oscillates, one or more damping elements 42 may be arranged to apply forces in this direction (i.e., primarily in a radial direction) rather than the edgewise direction 38 to dissipate edgewise vibrations. Only one damping element 42 is shown in Fig. 3 for illustrative purposes.

[0024] Advantageously, the damping element 42 is offset from the spar by first and second supports 46, 48. Accordingly, the first and second supports 46, 48 are coupled to the spar 36 at different locations along its length, and the damping element 42 is coupled to the first and second supports 46, 48. The first and second supports 46, 48 are substantially rigid so that most or all of the bending forces causing displacement between the attachment locations on the spar 36 are transferred to the damping element 42. The displacements between the attachment locations on the spar 36 are magnified due to the offset of the supports from the spar 36. The damping element 42 applies a force to counteract this movement such that the damping force is magnified as well (compared to a non-offset damping element).

[0025] Fig. 3 illustrates the first and second supports 46, 48 extending from the spar 36 toward the trailing edge 34 so that the damping element 42 is positioned between the spar 36 and the trailing edge 34. Such an embodiment typically allows for greater magnification of the damping force because there is more room for the first and second supports 46, 48 to extend (and thereby offset the damping element 42) from the spar 36 within the shell 26. It is even possible for the first and second supports 46, 48 to offset the damping element 42 different distances from the spar 36 to better take advantage of the available space. For example, Fig. 4 shows an embodiment where the first support 46 is coupled to the spar 36 at a location spaced slightly further from the root 22. The chord (distance between the leading edge 32 and trailing edge 34) of the blade 20 is greatest in this section because the blade 20 then transitions from an airfoil shape to a circular cross-section to facilitate mounting to the hub 18 (Fig. 1). The increased offset results in greater displacements between the ends of the first and second supports 46, 48, thereby providing the damping element 42 with an increased stroke length to improve damping effectiveness.

[0026] Although Figs. 3 and 4 show the damping element 42 on the side of the spar 36 facing the trailing edge 34, in an alternative embodiment not shown the first and second supports 46, 48 may be positioned on the opposite side of the spar 36. In other words, the first and second supports 46, 68 may extend from the spar 36 toward the leading edge 32 so that the damping element 42 is positioned between the spar 36 and the leading edge 32. Combinations of the above-described arrangements are also possible. Indeed, as will be described in greater detail below, there may be a plurality of supports and damping elements in alternative embodiments.

[0027] The first and second supports 46, 48 are shown schematically in Fig. 3 as truss-like structures. Again, their purpose is to transfer bending forces and magnify displacements. With this in mind, various shapes and types of structures will be apparent to those skilled in wind turbine blade design. The structures may be formed together with the spar 36, for example, by incorporating them on the same structure on which glass and/or carbon fibers are placed to form the spar 36. Such an integral construction may be particularly effective at transferring bending forces to the damping element 42. Alternatively, the structures may be formed separately and attached to the spar 36, by bolting, gluing, or other fastening techniques. Furthermore, the structures need not be similar (i.e., the first support 46 having a different construction than the second support 48).

[0028] The coupling between the first and second supports 46, 48 and the spar 36 need not even be rigid in all directions given that their function is to transfer edgewise bending motion into radial motion. Indeed, it may be advantageous to design the first and second supports 46, 48 so that they are coupled to the spar 36 with one or more degrees-of- freedom that permit relative movement in the fiapwise direction 40. One example of such an embodiment is a pivotal or hinge connection (not shown) between the first and second supports 46, 48 and the spar 36 that only permits rotation about an axis aligned along the length of the spar 36 (radial direction). This may provide relative movement in a cross-sectional plane along the length of the blade, but not in a cross-sectional plane parallel to the plane of rotation. Thus, in such an embodiment there is still no movement of the attachment locations along the length of the spar 36, allowing bending forces to be transferred to the damping element 42. [0029] Similar considerations apply to the coupling between the damping element 42 and the first and second supports 46, 48. That is, the damping element 42 may be coupled to either or both of the first and second supports 46, 48 in a manner that permits relative movement in directions other than a radial direction. There may be pivotal connection(s) like the one mentioned above, for example. The damping element 42 may even be coupled to either or both of the first and second supports 46, 48 with rotational degrees-of- freedom in all directions by using a universal joint, ball-and-socket joint, or the like.

[0030] Like the first and second supports 46, 48, the damping element 42 is also shown schematically in Fig. 3. This is because those skilled in the art will appreciate that many different types of damping elements are available for resisting the linear displacement between two locations (the first and second supports 46, 48). The damping element 42 may be, for example, a piston-like damper. Such a damping element includes a first portion 52 coupled to the first support 46 and a second portion 54 coupled to the second support 48. The first and second portions 52, 54 are movable relative to each other, but are also coupled together by a damping force that resists such relative velocity. The damping force may be hydraulic, such as when the first portion 52 is a piston and the second portion 54 is a cylinder containing fluid or gas acted on by the piston. Alternatively, the damping force may be mechanical, such as when the first portion 52 is coupled to the second portion 54 by friction.

[0031] PCT application no. PCT/US 10/26198 ("the Ί98 application"), the disclosure of which is fully incorporated herein by reference, discloses some examples of such embodiments. The damping elements in that application are also arranged to apply forces in directions other than the edgewise direction. Although the application discloses the damping elements being configured to move primarily in the flapwise direction to dissipate edgewise vibrations, many of the damping elements themselves could equally be used in the manner described above.

[0032] For instance, the Ί98 application describes an embodiment of a damping element where the first and second portions 52, 54 are cylindrical elements. One of the cylindrical elements is configured to at least partially receive the other in a telescopic manner, with friction between the two resisting the telescopic movement. The friction is created by a joint material located circumferentially between the first and second portions. The first and second portions are constructed from composite fibers, hard plastics, metals, or other relatively stiff materials, while the joint material is constructed from rubber, epoxy resins, thermoplastics, or the like. This provides the joint material with a lower stiffness but a damping capacity (energy dissipated per cycle of stress) than the first and second portions.

[0033] Another embodiment is disclosed in the PCT application where the first and second portions 52, 54 are plates. A joint material is located between the plates to serve the same purpose as in the other embodiment, i.e., to help dissipate forces. In either embodiment, the joint material may simply be applied to (e.g., coated on) the first and/or second portions, or be formed as a separate component glued or otherwise attached to the first and/or second portions.

[0034] Again, these embodiments are merely examples, as there are many types of damping elements for applying forces to counteract the movement between two points and dissipating vibrations.

Additionally, the number of damping elements 42 and their placement on and along the spar 36 may vary. Fig. 3 illustrates a single damping element 42 with a relatively long length. The first and second supports 46, 48 are coupled to the spar 36 near the root 22 and tip 24, respectively, such that the damping element 42 has a length at least half the length of the spar 36. Fig. 4 illustrates the first support 46 being spaced further from the root 22, but the damping element 42 still has a length at least one -third of the spar 36. In alternative embodiments, however, the first and second supports 46, 48 may be coupled to the spar 36 closer together so that the damping element 42 has a much smaller length. Fig. 5 illustrates one such embodiment.

[0035] The shorter length of the damping element 42 makes it possible to include many of them, if desired. As shown in Fig. 5, there may be a plurality of supports coupled to the spar 36 at different locations along the length of the spar 36. Each damping element 42 is coupled to two of the supports. One or more of the supports may even serve as the connection point for two of the damping elements 42. In Fig. 5, a third support 58 is coupled to the spar 36 at a location spaced from the second support 48. Thus, the second support 48 serves as a connection point for the damping element 42 coupled to the first support 46 and the damping element 42 coupled to the third support 58. Similar statements apply to the third support 58, but with reference to a fourth support 60, a damping element 42 coupled to the fourth support 60, and the damping element 42 coupled to the second and third supports 48, 58. A fifth and sixth support 62, 64, together with the damping element 42 coupled to them, are shown as being independent from (i.e., not associated with) the other supports 46, 48, 58, 60 and damping elements 42 to also illustrate such an arrangement as a possibility when including a plurality of damping elements 42. [0036] By including a plurality of supports and damping elements with short lengths, the damping forces can be distributed more along the length of the spar 36. Less force is transferred by the supports to any particular damping element when compared to the embodiment of Fig. 3. As a result, the supporting structures, their connections/junctions with the spar 36, and the damping elements 42 may be more robust and/or have simplified designs.

[0037] Additionally, including a plurality of supports and damping elements allows for strategic positioning. The locations for these components may be selected where they are easy to install or access in addition to being effective at damping edgewise vibrations. There may even be damping elements 42 coupled to supports on opposite sides of the spar 36, as mentioned above.

[0038] Additional advantages and modifications will be readily apparent based upon the above description, given that the embodiments described are merely examples of the invention defined by the claims below. Moreover, individual features of the various embodiments may be combined in different ways.