NL2007438C2

NL2007438C2 - Blade for a wind turbine and wind turbine including such blades.

Info

Publication number: NL2007438C2
Application number: NL2007438A
Authority: NL
Inventors: Cornelis Joseph Besteman
Original assignee: Suzlon Blade Technology B V
Priority date: 2011-09-16
Filing date: 2011-09-16
Publication date: 2013-03-19

Description

Blade for a wind turbine and wind turbine including such blades

The present invention relates to a blade for a 5 wind turbine, comprising a root section and a tip section, an upper skin and a lower skin, a leading edge and a trailing edge, and at least one spar extending from the root section to the tip section, in particular substantially parallel to the leading edge and trailing edge and having an 10 upper edge connected to the upper skin and a lower edge connected to the lower skin. Such a wind turbine blade forms part of the prior art.

Blades for wind turbines must meet stringent and often conflicting requirements. On the one hand they must be 15 strong enough to withstand high wind loads and stiff enough to do so without deforming to such an extent that they might interfere with the tower supporting the wind turbine. On the other hand they must be relatively thin and slender, i.e. have a high aspect ratio, in order to achieve optimum 20 aerodynamic efficiency. Moreover, wind turbine blades must be light so as to reduce inertia and minimize loads on the supporting structure, i.e. the tower and nacelle. And of course they must be designed for ease of manufacture and maintenance .

25 As each new generation of wind turbines is designed to generate more power than the previous generation, wind turbine designs tend to grow. For instance, the applicant markets a series of wind turbines with power ratings ranging from about 0,5 megawatts to around 2,5 30 megawatts. Smaller wind turbines have a rotor diameter of about 50 metres, while the rotor diameter of larger wind turbines is more than 80 metres. The growth in rotor diameter makes it increasingly difficult to design wind 2 turbine blades which will meet the above-mentioned requirements .

Conventional wind turbine blades are usually made from composite materials, in particular glass fibre 5 reinforced plastic (FRP). The spar, which is relatively highly loaded, in particular due to shear loads, is often constructed as a sandwich comprising two parallel FRP skins and a plastic foam core in between. Although such a foam filled spar has sufficient strength, stiffness and 10 resistance to buckling, it has the drawback that it is relatively heavy.

One of the objects of the invention is to provide a wind turbine blade in which the above drawbacks are at least partially overcome. In accordance with the invention 15 this is achieved in that the web is non-planar over at least part of its length. Due to its shape, such a non-planar web has greater resistance to buckling due to a crushing load or shear load than a plane web having the same main dimensions. This allows a solid laminate non-planar web to be used, 20 rather than a sandwich structure having two skins and a foam core. Since such a solid laminate web is transparent the teaching of the claim surprisingly provides the double effect that the spar can be visually inspected for damage or non-conformance during manufacture. Since no foam core is 25 needed, there is no need for hot wire foam cutting, which means that less labour is required. Also capital investment may be reduced because there is no need for a foam cutting machine. Moreover, when the web is made of fibre reinforced plastic, it may be cured at higher temperatures than would 30 be possible with a foam core, thus allowing reduced curing times and increased productivity. The resin infusion process is also simplified when the web only includes fibre reinforced plastics. Logistics are simplified and storage 3 requirements reduced when no foam is involved in the production of wind turbine blades.

A substantially uniform structure may be achieved when the non-planar part of the web is corrugated.

5 In one embodiment of the invention, the non-planar part of the web is preferably wave shaped, in particular sine wave shaped. By this sharp angles which may lead to stress concentrations are avoided. The wave shape can be defined by a wave length in lengthwise direction of the web 10 and an amplitude perpendicular to a main plane of the web.

The wavelength is defined as the distance between successive corresponding points and the amplitude is defined as the distance between opposite extremes.

In a preferred embodiment of the wind turbine 15 blade the wavelength and/or the amplitude are varied along the length of the web. This allows the strength and stiffness to be adapted to local load conditions, thus leading to substantially uniform behaviour under load of the entire spar .

20 Such adaptation to local load conditions may be accomplished relatively easily when the wavelength is reduced and/or the amplitude is increased in relatively more heavily loaded parts of the spar and the wavelength is increased and/or the amplitude is reduced in relatively more 25 lightly loaded parts.

In a further embodiment the wavelength and/or the amplitude are gradually varied from the root section of the blade towards the tip section. By this the web is adapted at its best to the local load conditions at the very point of 30 the blade which results in an optimal use of resources and subsequently in better performance of the blade.

An optimum design in terms of structural efficiency and buckling resistance may be obtained when the 4 amplitude of the wave shape is at most substantially equal to the wavelength, in particular less than half of the wavelength and more in particular about a quarter of the wavelength .

5 In one embodiment of the wind turbine blade of the invention, the wavelength is of the same order of magnitude as the distance between the upper and lower edges of the at least one spar. This results in a structure that combines good strength, stiffness and buckling resistance and is 10 relatively easy to manufacture at reasonable costs.

For instance, in a wind turbine blade having a length of 43 metres, the spar may have a an average height (distance between the skins of the blade) of approximately 40 centimetres, and the average wavelength of the web may 15 also be around 40 centimetres. The average amplitude of the wave shape of the web may then be in the order of 10 centimetres. For most turbine blades having wave shaped spar webs the amplitude of the wave shape may be between 2 and 8 centimetres over a large part of the span.

20 When the upper edge of the at least one spar is formed by an upper flange and the lower edge is formed by a lower flange, and when the web has an upper edge bonded to the upper flange and a lower edge bonded to the lower flange, a strong bond between the web and the flanges is 25 ensured.

A very efficient, easy to manufacture and lightweight wind turbine blade is obtained when the upper and lower flanges are embedded in the upper and lower skins, respectively .

30 In order to optimize the connection and load transfer between the flanges and the web, the upper and lower edges of the web preferably extend substantially 5 perpendicularly to its main plane. These bent edges provide a relatively large connecting surface.

Although a variety of strong lightweight materials may be considered, it is preferred that the web comprises a 5 fibre reinforced matrix material, in particular a glass fibre reinforced epoxy resin. Fibre reinforced plastics combine high strength and stiffness with relatively low weight and very low maintenance. Moreover, such fibre reinforced plastics allow relatively complex shapes like the 10 aerodynamic profiles of wind turbine blades to be formed relatively easily.

The invention also relates to a wind turbine. Conventionally a wind turbine comprises a hub supporting at least two blades. The wind turbine of the present invention 15 is characterized in that it is provided with blades of the type described above.

The invention is now illustrated by way of an example, with reference being made to the annexed drawing, in which: 20 Fig. 1 is a perspective view of a wind turbine having three blades,

Fig. 2 is a perspective sectional view of a blade of the wind turbine of fig. 1, in which one of the skins has been removed, 25 Fig. 3 is a cross-sectional view of a blade of the wind turbine of fig. 1,

Figs. 4, 5 and 6 are perspective detail views of part of the spar of the blade of figs. 2 and 3,

Fig. 7 is a diagram of the spanwise distribution 30 of loads in the spar of a typical wind turbine blade,

Fig. 8 is a perspective detail view of part of an alternative embodiment of the spar, and 6

Fig. 9 is a chordwise longitudinal section of part of yet another embodiment of the spar, showing variations of wavelength and amplitude along the length of the spar.

A wind turbine 1 includes a tower 2, a nacelle 3, 5 a hub 4 and a plurality of blades 5. The nacelle 3 houses an electrical generator (not shown here) and a bearing (not shown either) for supporting the hub 4.

Each blade 5 has a length or spanwise dimension from its root 6 to its tip 7 which may amount to several 10 tens of metres. For instance, for a wind turbine 1 that is capable of generating 2 MW of electrical power each blade 5 may be almost 45 metres long from root to tip. Each blade 5 may have a chord length c, i.e. a distance between a leading edge 10 and a trailing edge 11, which varies from 2-2.5 15 metres at the root 6 to a maximum of 3-4 metres at about 25 percent of the span S. The chord length may then gradually decrease to about 0.5-1 metres near the blade tip 7. Blade thickness t may vary between 10 and 25 percent of the chord length c.

20 Since a wind turbine blade 5 is relatively highly loaded, it is important that it combines high strength and stiffness. The blade 5 includes an upper skin 8 and a lower skin 9. These upper and lower skins 8, 9 meet at the leading edge 10 and the trailing edge 11 of the blade 5.

25 Approximately halfway between the leading and trailing edges 10, 11 the upper and lower skins 8, 9 are mutually connected by a spar 12. The spar 12 extends from a location near the root 6 to the tip 7 of the blade 5. The root end of the spar 12, which is located radially outwardly from the actual 30 blade root 6, is provided with a flange 18 for connecting the spar 12 to the hub 4.

The spar 12 comprises a web 13 and upper and lower flanges 14, 15. The web 13 has upper and lower edges 16, 17 7 which are connected to the upper and lower flanges 14, 15.

In the shown embodiment the edges 16, 17 extend perpendicular to the web 13 so as to form a surface for bonding to the upper and lower flanges 14, 15. These flanges 5 14, 15 are in turn connected to the upper and lower skins 8, 9 of the blade 5. In the illustrated embodiment both the upper and lower skins 8, 9 and the spar 12 are manufactured from glass fibre reinforced plastic and the upper and lower flanges 14, 15 of the spar 12 are embedded in the upper and 10 lower skins 8, 9, respectively. To this end the upper and lower skins 8, 9 include additional layers of glass fibre reinforcement at the location of the upper and lower flanges 14, 15 of the spar.

The web 13 is the most highly loaded part of the 15 spar 12. The spar 12 is subjected to three main loads; bending, stretching and crushing loads. Bending loads are due to the fact that the blade 5 is only attached to the wind turbine structure at its root 6. Therefore the aerodynamic loads acting on the blade 5 over its entire 20 length will lead to bending of the blade 5. These bending loads will mainly act perpendicular to the plane swept by the blades 5, although some minor bending loads may act in the chordwise direction of each blade 5. Stretching loads are due to the centrifugal forces acting on the blade 5 25 during rotation and act in radial or spanwise direction. And finally, crushing loads are mainly the result of deformation (bending and torsion) of the blade 5 and spar 12 and act perpendicular to the chordwise plane of the blade 5.

In accordance with the invention the web 13 is 30 non-planar over at least part of the length of the spar 12 in order to withstand the high loading. In the illustrated embodiment the web 13 is wave shaped. Although various wave shapes may be considered, the web 13 as shown here is shaped 8 as a sine wave. A sine wave has a smooth contour without any discontinuities which might lead to stress concentrations . The sine wave is characterized by its wave length L, i.e. the spanwise distance between two adjacent peaks P or 5 troughs T, and its amplitude A, i.e. the chordwise distance between a peak P and a trough T.

It has been found to be advantageous for the wavelength L to be of the same order of magnitude as the height h of the spar 12, i.e. the distance between its 10 flanges 14, 15. It has also been found that the amplitude A of the wave shape should be substantially smaller than the wavelength L. Preferably the amplitude A should be less than half of the wavelength L and more preferably less than about a quarter of the wavelength L. Over large parts of the 15 length of the spar 12 the amplitude A may be between 20 and 80 millimetres.

In order to obtain a substantially uniform behaviour of the blade 5 under load it is preferred to have the wave length L and/or amplitude A vary with in accordance 20 with the spanwise load distribution. Whenever the load increases the wavelength L may be reduced and/or the amplitude A may be increased. Conversely, whenever the load decreases the wavelength L may be increased and/or the amplitude A may be reduced. From the diagram in fig. 7 it 25 follows that the wavelength L should be relatively small and/or the amplitude A should be relatively large near the root 6 and from about 65 percent of the span outward to the tip 7 of the blade 5.

In this way a spar 12 is created which is both 30 strong and stiff, and which has a high resistance to buckling. Moreover, the spar 12 including the non-planar web 13 is lighter and easier to manufacture than a foam-filled sandwich spar having the same structural characteristics. It 9 has been found that for a blade 5 having a length of 45 metres the weight saving that can be achieved by the wave shaped web 13 may amount to more than 300 kilograms, while manufacturing cost savings in the order of € 1,500 may be 5 achieved. Moreover, the non-planar web has major advantages in terms of ease of maintenance.

In an alternative embodiment of the spar 12 (fig. 8), the non-planar part of the web 13 comprises straight corrugations, rather than a wave shape. These corrugations 10 are formed by mutually offset flat parts 19 that are substantially parallel to the spanwise direction of the spar 12 and connecting parts 20 running at an angle to the spanwise direction.

The strength and stiffness of the spar 12 may be 15 locally varied by changing the wavelength L and/or the amplitude A of the wave shape or the corrugations of the web 13. For instance, in a relatively heavily loaded area I the wavelength 1^ may be reduced, resulting in increased "waviness" or non-planarity. The same effect may be achieved 20 by a local increase of the amplitude Ai. And of course, these two measures may be combined. In a relatively lightly loaded area II on the other hand, the wavelength L2 may be increased and/or the amplitude A2 may be reduced. It is even conceivable that the web 13 is planar at locations (area 25 III) where only very light loads act on the blade 5.

Although the invention has been illustrated above by refer ence to an exemplary embodiment thereof, the skilled person will appreciate that many adaptations and modifications may be made. For instance, the web might have 30 another wave form than a sine wave, e.g. a sequence of semicircles. Also combinations of different wave forms along the length of the spar may be envisaged. The web and flanges of the spar may also be made from other materials than the 10 glass fibre reinforced plastics described here, and may include carbon or aramid fibres. Consequently, the scope of the invention is defined solely by the following claims.

Claims

A blade (b) for a wind turbine (1), comprising: - a foot part (6), an end part (7), an upper skin (8) and a lower skin (9), a leading edge (10) and a rear edge (11), and - at least one girder (12) extending substantially from the foot part (6) to the end part (7), in particular parallel to the front edge (10) and the rear edge (11), - which beam (12) has an upper edge connected to the upper skin (8), a lower edge connected to the lower skin (9) and a body plate (13) which connects the upper and lower edges, characterized in that the body plate (13) is not flat over at least a part of its length.

Sheet (5) according to claim 1, characterized in that the non-flat part of the web plate (13) is pleated. 20

3. Sheet (5) according to claim 1 or 2, characterized in that the non-flat part of the body plate (13) is corrugated, in particular sinusoidally corrugated, the wave form being determined by a wavelength (L) in the longitudinal direction of the body plate (13) and an amplitude (A) perpendicular to a major surface of the body plate (13).

Sheet (5) according to claim 3, characterized in that the wavelength (L) and / or the amplitude (A) vary along the length of the web plate (13).

Sheet (5) according to claim 4, characterized in that the wavelength (L) is reduced and / or the amplitude (A) is increased in relatively more heavily loaded parts of the beam (12) and the wavelength (L) is increased and / or the amplitude (A) is reduced in relatively lighter areas.

Sheet (5) according to claim 4 or 5, characterized in that the wavelength (L) and / or the amplitude (A) vary gradually from the base part (6) to the end part (7).

Sheet (5) according to one of claims 3 to 6, characterized in that the amplitude (A) is at most substantially equal to the wavelength (L), in particular smaller than half the wavelength (L), and more particularly about a quarter of the wavelength (L). 15

Sheet (%) according to one of claims 3 to 7, characterized in that the wavelength (L) is of the same order of magnitude as the distance (h) between the top and bottom edges of the at least one beam (12). 20

Sheet (5) according to one of the preceding claims, characterized in that the upper edge of the at least one beam (12) is formed by an upper flange (14) and the lower edge is formed by a lower flange (15), and the The body plate (13) has an upper edge (16) connected to the upper flange (14) and a lower edge (17) connected to the lower flange (15).

10. Sheet (5) according to claim 9, characterized in that the upper and lower flanges (14, 15) are embedded in the upper and lower skin (8, 9), respectively.

A blade (5) according to claim 9 or 10, characterized in that the upper and lower edges (16, 17) of the web plate (13) extend substantially transversely to the main surface thereof. 5

Sheet (5) according to one of the preceding claims, characterized in that the web plate (13) comprises a fiber-reinforced matrix material, in particular a glass fiber-reinforced epoxy resin. 10

A wind turbine (1), comprising a hub (4) carrying at least two blades (5) according to any one of the preceding claims.