WO1999043955A1

WO1999043955A1 - Wind turbine blade

Info

Publication number: WO1999043955A1
Application number: PCT/DK1999/000073
Authority: WO
Inventors: Stig ØYE; Casper Kildegaard; Carsten Westergaard
Original assignee: Lm Glasfiber A/S
Priority date: 1998-02-24
Filing date: 1999-02-23
Publication date: 1999-09-02
Also published as: AU2411999A

Abstract

A wind turbine blade with a load-bearing structure and a damper means for damping the natural vibrations of the blade. The damper means (5) comprises a layer of energy-absorbing material adhesively secured to the load-bearing structure (1, 2) of the blade and at least one oblong element adhesively secured to said layer and extending substantially in the longitudinal direction of the blade. The length and rigidity of the at least one oblong element are such in the longitudinal direction of the blade that a strain of the blade in the longitudinal direction due to a deflection of the blade causes a displacement in the energy-absorbing material.

Description

Title: Wind turbine blade

Technical Field

The invention relates to a wind turbine blade with a load-bearing structure and a damper means for damping the natural vibrations of the blade.

During operation of a wind turbine the blades thereof are exposed to very strong forces and are continuously excited to perform natural vibrations resulting in increased loads and risk of fatigue fractures. Naturally, this problem is taken into consideration when designing the structure of the blade. However increased loads of the blade result in an increased material consumption in order to strengthen the blade, whereby the manufacturing costs of the blades also are increased.

In operation the blades are subjected to vibrations in two main vibration directions, viz. edge- wise vibrations (ie. vibrations in a plane through the leading and trailing edge of the blade) and flap- wise vibrations (ie. vibrations in a plane perpendicular to the leading and trailing edge of the blade). Both types of fundamental vibrations are not only very important, because they per se may result in breakdown of the blade, but also because they are transferred to and influence the mode of vibration of the wind turbine itself.

During recent years the main focus has been on edge-wise vibrations which occur when the blade enters the operational area called stall. This phenomenon occurs in all types of wind turbines, both pitch and stall-regulated turbines, but is of course most common on stall-regulated turbines, as the power adjustment of these turbines is effected by moving the blade into the stall area. In the stall area the aerodynamic forces are self-amplifying, ie. if the blade is excited to perform natural vibrations caused by accidental occurrences (such as turbulence), the aerodynamic forces am- plify the movement. This is called negative aerodynamic damping. If no other forces influence the blade, instability arises, ie. the vibration become increasingly severe, until the blade breaks. However the inherent damping (also called structural damping) of the blade as well another aerodynamic phenomenon called dynamic stall counteract said self-increasing vibration situation. Dynamic stall is, however, only slightly effec- tive in edge- wise direction, but constitutes a very important factor in preventing large flap- wise vibrations.

Thus in edge- wise direction substantially only the aerodynamic damping and structural damping of the blade function. When the blade is close to stall, the aerodynamic damping becomes negative, and if the structural damping of the blade is not suffi- cientiy high, a potentially detrimental situation may arise which may result in blade failure. In recent years several blade damages and failures have occurred due to uncontrolled edge- wise vibrations during winter gales in Northern Europe. The above uncontrolled vibration situation can be avoided by increasing the damping of the blade such that the inherent damping of the blade always exceeds the negative aerodynamic damping . This can be effected in two ways - either by dynamic damping or by passive damping.

Background Art

WO 95/21327 discloses a blade having a dynamic damper means. In practice this solution has proved difficult to implement.

Furthermore DK-B 1-171333 discloses a wind turbine blade with a shell made of fϊbreglass-reinforced plastic, one or more layers of an elastic material of rubber sheet forming part of the laminate. As a result the natural vibrations of the shell itself and the noise caused by the shell vibrations are dampened. The rubber layers of the laminate have, however, no substantial damping effect on the vibrations of the blade per se, the fibreglass laminate layers on the two sides of the rubber sheet having substantially the same deflection pattern, whereby the rubber blanket is not subjected to displacement.

Brief Description of the Invention

The object of the present invention is to provide a wind turbine blade of the above type having a damper means of simple design which is easy to implement and which increases the inherent damping of the blade such that detrimental natural vibrations of the blade are avoided.

The wind turbine blade according to the invention is characterised in that the damper means comprises at least one oblong element extending substantially in the longitudinal direction of the blade and having such a rigidity in the longitudinal direction and being secured to the load-bearing structure such that at least a portion thereof is subjected to a strain in longitudinal direction at deflection of the blade, said strain differing from the strain in the load-bearing structure, and that the oblong element in said portion is secured to the load-bearing structure by means of a flexible, energy-absorbing element.

The invention is based on the realization that, when a blade is subjected to edge-wise or flap- wise vibrations and consequently deflects, its load-bearing structure and an oblong element of a certain rigidity secured thereto are subjected to different strains in the longitudinal direction of the blade, if the oblong element is not secured to the load-bearing structure such that it is subjected to the strain of the blade. By intercon- necting the load-bearing structure and the oblong element by means of a flexible, energy-absorbing element, this difference in strain is converted into an internal movement in the energy-absorbing element absorbing energy and consequently damping the vibrations of the blade. For obtaining the optimum energy absorption the energy- absorbing element should be placed in the portion or portions in which the largest longitudinal displacement between the oblong element and the load-bearing structure occurs, ie. at the ends of the oblong element when this is not fixed directly to the structure and furthest away from the fastening point when the oblong element is fixed directly to the structure at a fixation point.

The damper means is most advantageously arranged in the portion of the load-bearing structure of the blade, where the structure is subjected to the highest strain when the blade vibrates in the mode of vibration which is to be dampened. For damping edge- -wise vibrations, the damper means is thus most advantageously arranged adjacent the leading edge or the trailing edge of the blade or both places and in any circumstance spaced apart from the neutral axis. Correspondingly, the damper means for damping flap-wise vibrations is most advantageously arranged furthest away from the neutral axis during this mode of vibration. In blades, wherein the blade shell itself forms part of the load-bearing structure, the damper means is advantageously arranged directly on the shell and preferably on the inner face thereof. At subsequent mounting of the damper means on existing blades, the damper may, however, also be arranged on the surface of the blade. In blades, wherein the blade shell does not form part of the load-bearing structure, the damper means is advantageously arranged on the main beam of the blade.

The energy-absorbing element of the damper means may advantageously be made of an energy-absorbing material. As the energy-absorbing material on one side is secured to the load-bearing structure of the blade and to the oblong element on the other side, the longitudinal displacement or the difference in strain between the load-bearing structure and the oblong element causes an internal displacement of movement in the energy-absorbing material at a deflection of the blade during its vibration, whereby the vibration energy is absorbed in the material. This energy absorption dampens the vibration in the blade, whereby an effective enhancement of the structural damping of the blade or of the damping caused by the load-bearing structure of the blade is obtained in a simple manner.

According to the invention the oblong element may be secured to the load-bearing structure by means of one or several interspaced block-shaped elements of an energy-absorbing material.

Furthermore, according to the invention at portions of the oblong element may be secured to the load-bearing structure of the blade by means of a layer of energy- absorbing material adhesively secured to the oblong element and the load-bearing structure.

According to the invention the energy-absorbing material may have a loss coefficient η of between 5 x 10^"3 and 5 and a Young modulus of between 10^" and 10 GPa. An

2 energy-absorbing material with a loss coefficient η of between 5 x 10^" and 5 and a

3 Young modulus E of between 10^" and 1 GPa is considered most advantageous. Typical materials having the said properties are butyl rubber, polymer foam and urethane rubber which have a high loss coefficient η, but also other materials such as polyethylene, polypropylene, nylon, polyester and wood, such as balsa may be used depending on especially the rigidity of the at least one oblong element and the load-bearing structure of the blade. In general , the properties of the energy-absorbing material are chosen under consideration of the properties of the load-bearing structure and of the oblong element such that the said movement in the energy-absorbing material is effected and the intended damping of the vibrations of the blade is obtained.

Preferably, the at least one oblong element is bar-shaped and may have any cross -sec- tional shape, e.g. round, triangular or rectangular.

Furthermore, according to the invention the damper means may comprise a plurality of aligned oblong elements. The individual oblong elements may be of the same or different lengths. Experience has shown that in certain situations by using several aligned successive oblong elements a more effective damping is obtained than by using a single oblong element of the same length as the total length of several smaller oblong elements.

Moreover according to the invention the damper means may comprise a plurality of juxtaposed oblong elements.

Furthermore according to the invention the row of oblong elements may be adhesively secured to an additional layer of energy-absorbing material which in turn may be adhesively secured to at least one additional oblong element, the at least one additional element being ananged so as to extend over at least one interspace between the oblong elements in the subjacent row of the elements. As a result, when the load-bearing structure of the blade is strained, a movement in the layer of energy-absorbing material between the load-bearing structure and the first row of oblong elements as well as a movement in the additional layer of energy-absorbing material between the first row of oblong elements and the additional at least one oblong element are effected. Overall, a highly effective damping of the vibration of the blade is thus obtained.

In a further development of the above embodiment of the invention the damper means may comprise additional superposed layers of energy-absorbing material alternating with rows of oblong elements. A strain of the load-bearing structure of the blade causes a shear in the layer of energy-absorbing material between each row of oblong element, whereby the intended energy absorption and thus the desired damping of the vibrations of the blade may be obtained in a simple manner.

Moreover according to the invention the damper means may comprise an uppermost oblong element being secured relative to the load-bearing structure of the blade so as to substantially follow the deflection pattern thereof. As a result, when the blade is stained, a movement occurs both in the layer of energy-absorbing material between the uppermost oblong element and a subjacent row of oblong elements and in the layer of energy -absorbing material between the latter and the load-bearing structure of the blade. In a typical wind turbine blade made from fibreglass-reinforced polyester the upper oblong element may advantageously be formed of one of the laminate layers of the blade.

Finally according to the invention the at least one oblong element may be embedded in a matrix of an energy-absorbing material. In practice this embodiment may be carried out by prefabricating a unit comprising the matrix and the oblong elements embedded therein in a mould and then glue the unit on to the desired portion of the load-bearing structure of the blade. Optionally the embedment of the oblong element may be performed directly in the desired area of the load-bearing structure of the blade.

Brief Description of the Drawings

The invention is explained in greater detail below with reference to the accompanying drawings, in which

Fig. 1 is a diagrammatic and perspective view of a wind turbine blade according to the invention,

Fig. 2 is a diagrammatic, cross-sectional view through a wind turbine blade according to the invention, and in which different possible anangements of a damper means are shown,

Figs. 3 to 6 is a longitudinal sectional view through different embodiments of a damper means pertaining to the wind turbine blade according to the invention,

Figs. 7a and 7b illustrate the damping of the vibrations in a blade according to prior art and according to a conesponding blade according to the invention, and

Fig. 8 is a diagrammatic, longitudinal sectional view through a wind turbine blade according to the invention comprising an alternative embodiment of the damper means.

Best Mode for Carrying Out the Invention

The wind turbine blade shown in Figs. 1 and 2 has a load-bearing structure comprising a shell laminate 1 and a main beam 2 extending between the upper face and lower face of the blade substantially perpendicular to a plane through the leading edge 3 and trailing edge 4 of the blade. The shell laminate 1 and the main beam 2 are made of fibreglass- reinforced polyester.

During operation of a wind turbine, the blade vibrates in edge-wise direction (ie. in the plane through the leading edge 3 and the trailing edge 4) as indicated by the anow k in Fig. 1, and in flap- wise direction (ie. perpendicular to said plane through the leading edge 3 and the trailing edge 4) as indicated by the anow f in Fig. 1.

For damping said vibrations the wind turbine blade according to the invention comprises a damper means which is described in detail below and which in short comprises a layer of an energy-absorbing material adhesively connected to the load-bearing structure of the blade, and at least one oblong element adhesively connected to said layer and extending substantially in the longitudinal direction of the blade.

Fig. 2 illustrates possible positions of said damper means extending entirely or partially in the longitudinal direction of the blade. Fig. 2 thus illustrates a damper means 5a ananged on the inner face of the shell laminate adjacent the leading edge 3 of the blade and acting to dampen edge-wise vibrations, and a damper means 5b ananged on the inner face of the shell laminate 1 adjacent the trailing edge 4 of the blade and also acting to dampen edge- wise vibrations. Fig. 2 furthermore illustrates two damper means 5c, 5d ananged on the main beam 2, one adjacent the upper face of the blade and one adjacent the lower face thereof and substantially acting to dampen flap-wise vibrations. Finally Fig. 2 also illustrates a damper means 5e arranged on the inner face of the shell laminate 1 adjacent the base of the main beam 2 and which also primarily acts to dampen the flap-wise vibrations.

Fig. 3 is longitudinal sectional view through a first embodiment of a damper means 5 which for instance may be ananged adjacent the trailing edge 4 of the blade and thus acts to dampen edge- wise vibrations. The damper means 5 comprises a layer 6 of an energy-absorbing material adhesively secured to the shell laminate 1 of the blade. The layer 6 of an energy-absorbing material may for instance be a self-adhesive strip of butyl 9 rubber having a thickness of about 3 mm.

The damper means 5 further comprises six oblong elements 7 to 7 adhesively secured to the layer 6 and aligned substantially in the longitudinal direction of the blade and evenly spaced apart. Each of the oblong elements is formed of a thin bar of fibre-reinforced plastics, such as fibreglass-reinforced polyester, and with a width of for instance 40 mm and a thickness of for instance 3 mm.

When the blade is subjected to edgewise vibrations, the shell laminate 1 is subjected to strain in the longitudinal direction. The strain is transfened to the energy-absorbing layer 6 and causes an inner movement or displacement thereof relative to the oblong elements 7. During this movement or displacement the energy-absorbing material absorbs energy and thus dampens the vibrations of the blade.

Fig. 4 illustrates a damper means 5 which a modification of the damper means in Fig. 3 , said damper means in addition to the energy-absorbing layer 6 and the oblong ele- ments 7 secured thereto further comprising an additional layer 16 of an energy-absorbing material and adhesively secured to the oblong elements 7 and an additional oblong element 17 adhesively secured to the additional adhesive layer 16. When seen in the longitudinal direction, the length of the additional oblong element 17 conesponds to the total length of the subjacent oblong elements 7. At a strain of the shell laminate 1 of the blade, a movement or inner displacement of the layer 6 in relation to the row of oblong element 7 takes place. In addition a displacement or movement of the additional energy- absorbing layer 16 takes place in relation to the row 7 of oblong elements and the additional oblong element 17. As a result, an increased energy absorption and thus an increased damping is obtained.

2 Fig. 5 illustrates a damper means 5 , which also is a modification of the damper means shown in Fig. 3, an additional layer 16 of energy-absorbing material being ananged above the row of damper means 7 and a second row of oblong elements 27 being adhesively secured to said additional layer. The second row of oblong elements 27 are ar- 10 ranged such in relation to the row of oblong elements 7 that each oblong element 27 extend over the space between two subjacent elements so as to overlap these. Furthermore the second row of oblong elements 17 are adhesively secured to a third layer 26 of energy-absorbing material, a third row of oblong elements 37 being adhesively con- nected thereto. The third row of oblong elements 37 are arranged such that each element extends over the interspace between the subjacent oblong element 27 and thus overlaps two adjacent elements in this row. Specifically, the elements in the third row are arranged and shaped as the elements 7 in the first row. At a strain of the shell laminate of the blade relative movements occur in the three energy-absorbing layers 6, 16,26 and thus an energy-absorption which dampens the vibrations of the blade.

3

Fig. 6 illustrates an embodiment of a damper means 5 , substantially conesponding to the embodiment shown in Fig. 3, however, modified such that the additional or uppermost oblong element 47 is secured in relation to the shell laminate 1. This feature is illustrated by means of two screws 8,9. In practice the fixation of the element 47 may be made by gluing the upper oblong element 47 to the shell laminate 6. Optionally the upper oblong element 47 may be formed of one or more laminate layers which substantially follow the deflection or strain of the shell laminate 1.

Moreover the screw 10 illustrates that one or more of the oblong elements at one end may be secured in relation to the shell laminate 1. In the example shown the oblong

3 element 7 .

An example illustrating the advantages obtained by means of the present invention in relation to prior art is rendered below.

EXAMPLE

A blade of 21.5 meters according to prior art was excited to perform a free natural vibration in edge-wise direction. Fig. 7a illustrates the deflection in mm of the tip 11 of the blade versus the time in seconds. Furthermore the envelope of the amplitude of the 11 blade is drawn in Fig. 7a.

The blade according to prior art was modified into a blade according to the invention by ananging a damper means adjacent the trailing edge of the blade. The damper means was substantially formed as shown in Fig. 3 in that it comprised five rows of oblong elements, each consisting of four oblong elements. Each oblong element had a length of 3 metres, a width of 40 mm and a height of 3 mm and was made from fibreglass -reinforced polyester. Each row of oblong elements extended towards the tip from a point substantially at the widest portion 12 of the blade, confer Fig 1. The energy -absorbing material 6 was a self-adhesive butyl rubber strip having a thickness of 3 mm.

As Fig. 7a, Fig. 7b illustrates the modes of vibration of the described blade according to the invention, the envelope of the amplitude also being shown.

A comparison of the two envelopes shows that the blade according to the invention (Fig. 7b) had a considerably higher inherent damping than the blade according to prior art (Fig. 7a).

As the blade shown in Figs. 1 and 2, the wind turbine blade shown in a diagrammatic sectional view in Fig. 8 comprises a shell laminate 1 and a main beam 2. The blade is provided with a damper means comprising a comparatively rigid, oblong, bar- or beam- shaped element 50 extending from the blade root 51 along the main beam 2, while being spaced apart therefrom, and two block-shaped elements 52,53 of an energy-absorbing material, such as rubber, which is secured to the main beam 2 and the bar 50 respectively. The inner end of the bar 50 is fixed directly to the load-bearing structure of the blade at the blade root 51. This fixation of the inner end of the bar is, however, not mandatory. At deflection of the blade, the main beam 2 is subjected to a strain different from the strain to which the bar 50 is subjected, whereby the block-shaped elements 52,53 of energy-absorbing material are subjected to shear stresses and the vibration of the blade is dampened. 12

In a modified embodiment of the shown damper means, the damper means may comprise the bar 50 secured at the blade root and only one of the block-shaped elements 52 and 53. Furthermore as indicated above the damper means may comprise the two block- shaped elements 52 and 53 and a bar 50 extending therebetween without being fixed to the blade root forming a part of the load-bearing structure of the blade. In conclusion it should be noted that the damper means may also be secured to other parts of the load-bear ing structure of the blade than the main beam 2, and that the means may comprise several bars secured to the load-bearing structure by means of one or more block-shaped elements.

Claims

13 Claims

1. Wind turbine blade having a load-bearing structure and a damper means (5) for damping the natural vibrations of the blade, characterised in that the damper means comprises at least one oblong element (7,50) extending substantially in the longitudinal direction of the blade and having such a rigidity in the longitudinal direction and being secured to the load-bearing structure (1,2) such that at least a portion thereof is subjected to a strain in longitudinal direction at deflection of the blade, said strain differing from the strain in the load-bearing structure (1,2), and that the oblong element (7,50) in said portion is secured to the load-bearing structure (1,2) by means of a flexible, energy-absorbing element (6,52,53).

2. Wind turbine blade according to claim 1, characterised in that the oblong element (50) is secured to the load-bearing structure (2) by means of one or several interspaced block-shaped elements (52,53) of an energy-absorbing material.

3. Wind turbine blade according to claim 1, characterised in that at least portions of the oblong element (7) are secured to the load-bearing structure of the blade by means of a layer (6) of energy-absorbing material adhesively secured to the oblong element (7) and the load-bearing structure (1,2).

4. Wind turbine blade according to claim 2 or 3, characterised in that the

3 energy-absorbing material has a loss coefficient ╬╖ of between 5 x 10^" and 5 and a Young modulus of between 10^" and 10 GPa.

5. Wind turbine blade according to claim 2 or 3, characterised in that the

2 energy-absorbing matenal has a loss coefficient ╬╖ of between 5 x 10^" and 5 and a

3

Young modulus of between 10^" and 1 GPa.

6. Wind turbine blade according to one or more of the preceding claims, characterised in that damper means (5) comprises a plurality aligned oblong 14 elements (7).

7. Wind turbine blade according to one or more of the preceding claims, characterised in that the damper means (5) comprises a plurality of successive oblong elements (7).

8. Wind turbine blade according to claim 3 and one or more of the preceding claims, characterised in that the row of oblong elements (7) is adhesively secured to an additional layer (16) of energy-absorbing material which in turn is adhesively secured to at least one additional oblong element (17, 27,47), the at least one additional element (17,27.47) being ananged so as to extend over at least one interspace between the oblong elements (7) in the subjacent row of elements.

9. Wind turbine blade according to one or more of the preceding claims,

3 characterised in that the damper means (5 ) comprises an uppermost oblong element (47) being fixed relative to the load-bearing structure (1) of the blade so as to substantially follow the deflection pattern thereof.

10. Wind turbine blade according to one or more of the preceding claims, characterised in that the at least one oblong element is embedded in a matrix of an energy- absorbing material.