WO2009155921A1

WO2009155921A1 - A wind turbine blade with angled girders

Info

Publication number: WO2009155921A1
Application number: PCT/DK2009/000150
Authority: WO
Inventors: Find Mølholt JENSEN
Original assignee: Danmarks Tekniske Universitet
Priority date: 2008-06-23
Filing date: 2009-06-23
Publication date: 2009-12-30
Also published as: EP2297456A1; CN102066747A; US20110176928A1

Abstract

The present invention relates to a reinforced blade for a wind turbine, particularly to a blade having a new arrangement of two or more girders in the blade, wherein each of the girders is connected to the upper part and the lower part of the shell and forms an angle with another girder thereby strengthening the shell against transverse shear distortion.

Description

A WIND TURBINE BLADE WITH ANGLED GIRDERS

The present invention relates to a reinforced blade for a wind turbine, particularly to a blade having a new arrangement of one or more girders in the blade in order to prevent transverse shear distortion of the blade due to operational loads.

Typically, a wind turbine blade has an aerodynamic shell and a girder, such as a beam or a spar. The girder can be a single beam, but often two girders are used. The two girders together with the parts of the shell extending between the two girders form a so-called box profile. The top and bottom of the box profile are often referred to as the caps. Some types of blades are designed with a spar in the form of a box profile which is manufactured separately and bonded in between prefabricated surface shells. The aerodynamic shell is typically made of a laminate of fibre reinforced plastics, fibreglass and/or other materials. Typically, the aerodynamic shell is made from two shell parts that are assembled to form the shell.

Under normal operating conditions, the wind turbine blade is subjected to loads at an angle to the flapwise direction. It is common to resolve this load on the blade into its components in the flapwise and edgewise direction. The flapwise direction is a direction substantially perpendicular to a transverse axis through a cross-section of the blade. The flapwise direction may thus be construed as the direction, or the opposite/reverse direction, in which the aerodynamic lift acts on the blade. The edgewise loads occur in a direction perpendicular to the flapwise direction. The blade is further subject to torsional loads which are mainly aerodynamic and inertia loads. These loads can subject the blade to harmonic motions or oscillations at the blade's torsional eigenfrequency. Indications of loads and directions are shown in Fig. 1.

During operation of the blade, transverse shear forces are generated in the blade by the flapwise and edgewise loads. The transverse shear forces are indicated on a typical cross-section of the blade shown in Fig. 2a. The transverse shear forces are induced by the flapwise and edgewise loads because of the typical asymmetric geometry and material distribution of the blade. Further, the fact that the flapwise and edgewise loads do not act through the shear centre of the blade contributes to the generation of transverse shear forces. In a box profile, the transverse shear forces result in high in-plane bending moments in the comers of the box profile. The bending moments may be counteracted by increasing the thickness of the box profile material in the corners, but increased thickness adversely affects the weight of the blade without a justifying contribution to the strength.

In wind turbine blades where the girders are manufactured separately and bonded to the shell parts, restraints in the manufacturing process result in small material thicknesses in the section of the girder that is connected to the shell part and therefore this part of the blade has a low bending stiffness.

The low bending stiffness of the corners of the box profile combined with the high bending moments in the same area, means that the box profile is easily distorted by transverse shear forces, despite the fact that the side, top and bottom of the box profile may be relatively thick.

An example of the result of the transverse shear distortion caused by the transverse shear forces is shown in Fig. 2b. The distortion consequently changes the shape of the blade and this has an adverse effect on the blade's ultimate strength. If the transverse shear distortion exceeds a certain limit which depends on the geometry and the material distribution of the blade, the blade's resistance to crushing pressure is reduced and a sudden collapse of the blade can occur. The crushing pressure is caused by the flapwise loads and occurs in the box profile of the blade due to its longitudinal curvature. This effect is also often referred to as ovalization, c.f. Fig. 3. For a further explanation of the effects of crushing pressure, reference is made to the article "Structural testing and numerical simulation of a 34 m composite wind turbine blade" by F. M. Jensen et. al. published by Elsevier in Composite Structures 76 (2006) 52-61.

The deformation of the girder at the connection between the girder and the shell can lead to fatigue failure of the girder and/or fatigue failure of the shell and/or fatigue failure in the connection between the girder and the shell.

Finally, the deformations of the blade's surface compromise the aerodynamic efficiency of the blade, because the designed shape of the profile is no longer maintained. Thus, there is a need for a wind turbine blade in which the structural layout of the blade profile is designed against transverse shear distortion and wherein the blade structure is generally strengthened without increasing the overall weight. It is further desirable to provide a wind turbine blade with an increased torsional stiffness.

It is therefore an object of the invention to provide a wind turbine blade that is designed against transverse shear distortion by transverse shear forces caused by flapwise and edgewise loads on the blade.

It is also an object of the present invention to provide a reinforced blade profile for a wind turbine blade.

It is another object of the present invention to provide a wind turbine blade with improved resistance against deformations of the blade profile.

It is yet another object of the present invention to provide a wind turbine blade with increased overall strength and stiffness.

It is still another object of the present invention to provide a wind turbine blade with increased resistance to fatigue failure.

It is yet still another object of the present invention to provide a wind turbine blade, which can be produced at a reduced manufacturing cost compared to the existing solutions.

Further, it is an object of the invention to provide wind turbine blade capable of working under severe aerodynamic loads and to optimize the aerodynamic stability, e.g. aeroelastic stability of the blade.

It is yet another object to provide a method for constructing a reinforced aerodynamic profile.

It is also an object of the present invention to provide a wind turbine blade with improved reliability of joints between shell and the girder.

It is still another object of the present invention to provide a wind turbine blade with an increased resistance against crushing pressure. It is yet another object to provide a wind turbine blade capable of working under severe aerodynamic loads and to optimise the aerodynamic efficiency, e.g. energy output of the blade.

It is another object to provide a wind turbine blade wherein the dynamic inertia loads the blade is applying on the other structural parts of the wind turbine construction are reduced.

It is yet another object of the present invention to provide alternatives to the prior art.

According to a first aspect of the invention, the above-mentioned and other objects are fulfilled by a wind turbine blade comprising a shell having a section with an aerodynamic profile, and at least two girders for increasing the strength of the blade and extending in a longitudinal direction of the blade, and wherein each of the at least two girders is connected to the upper part and the lower part of the shell and forms an angle with another girder of the at least two girders.

According to a second aspect of the invention, the above-mentioned and other objects are fulfilled by a method of increasing the strength of a wind turbine blade having a shell with a section having an aerodynamic profile, the method comprising the step of positioning and connecting at least two girders inside the shell for extension in a longitudinal direction of the blade and connecting each of the at least two girders to the upper part and the lower part of the shell in such a way that one of the at least two girders forms an angle with another girder of the at least two girders.

The wind turbine blade may be utilized in a vertical axis wind turbine, such as a Darrieus wind turbine, a wind star turbine, etc., or in a horizontal axis wind turbine, such as common modern wind turbines usually three-bladed, sometimes two-bladed or even one- bladed (and counterbalanced), etc.

The blade according to the present invention may also be used in the aeronautical industry, for example as a helicopter wing, an airplane wing, etc.

The wind turbine blade may be applicable not only to wind, but also to a variety of water flows, including free-flow (rivers, creeks), tidal flow, oceanic currents, wave motion, ocean wave surface currents, etc. The shell of the blade may preferably, but not exclusively, comprise a composite or laminated material. The material may comprise, alone or in any combination, fibreglass, carbon fibres, or other durable and flexible materials typically with a high strength/weight ratio, such as other fibre reinforced plastic materials that may further comprise, at least in part, light-weight metals or alloys. The shell may typically be a laminate or sandwich- construction. The thickness of the shell may vary along its length and width.

The upper part of the shell has a flat surface and during normal operation of the blade, the upper part of the shell is the suction side of the blade. The lower part of the shell has a more curved surface and during normal operation of the blade, the lower part of the shell is the pressure side of the blade. Thus, the upper part of the shell is also denoted the suction side of the shell, and the lower part of the shell is also denoted the pressure side of the shell.

In a conventional blade with two or more girders, the girders extend substantially in parallel with each other and substantially perpendicular to the profile chord of the blade. The profile chord of the blade is an imaginary surface that contains the leading edge and the trailing edge of the blade and extends therebetween. Thus, the edgewise direction is a direction in parallel with the profile chord and the flapwise direction is a direction perpendicular to the profile chord.

The blade according to the invention may comprise two or more girders. Each of the girders according to the invention has a longitudinal extension in parallel with the longitudinal extension of the blade and a transverse extension, i.e. the extension of the girder between the upper part and the lower part of the shell, which forms an oblique angle with another girder of the blade. Thus, compared to the girders of a conventional blade, at least one of the girders in a blade according to the invention is tilted around a longitudinal axis of the girder to form an angle that is different from 90° with the profile chord of the blade. For example, in one preferred embodiment, none of the girders have a transversal extension that is perpendicular to the profile chord of the blade. In another embodiment, some, but not all, of the girders have a transversal extension that is perpendicular to the profile chord of the blade.

The two or more girders strengthen the blade along the longitudinal extension of the blade. A girder may also be referred to as a web. The girder or web may be constituted by any type of elongate constructional member capable of taking up loads, such as a beam or a spar, e.g. shaped as an l-profile, preferably made from fibre reinforced plastics or other suitable material.

The girder may be subjected to tensile and compressive forces when the blade is loaded. To prevent the girder from buckling when subjected to compression forces, the girder can be stiffened with flanges on top of the girder or stringers on the side or may have corrugations or stiffeners to prevent buckling failure. Further, the girder or part of the girder may made as a sandwich construction with a foam material with laminates on each side.

Preferably, but not exclusively, the at least two girders may be positioned at an angle of 5° - 125° in relation to each other, the angle may for example range from 5° - 125°, such as from 10° - 110°, such as from 20° - 100°, such as from 30° to 90°; or the angle may range from 10° - 90°, such as from 20° to 90°, such as from 30° to 90°. .

Feet or flanges may be provided at the edges of the individual girders facilitating connection, particularly by bonding means, to the respective surfaces of the shell of the blade. However, the connections may be obtained in other ways, such as by secondary lamination, welded, adhered, melted, glued, bonded, fused, mechanical connections, etc, or any combination of such connection measures.

Each of the girders has a substantially straight shape, such as the shape of a rod or a planar member. If the shape of the girder is not straight, the shape of the girder could be straightened when subjected to tension leading to movement of its end connections thereby changing the aerodynamic profile.

The connections to the inner surface of the shell may in principle be located anywhere on the inner surface but it should be observed that the chosen location is suitable for the girder to be able to provide a reasonable and useful reinforcement of the profile.

Girders that are tilted or angled with respect to each other prevent or inhibit deformation of a cross-section of a wind turbine blade caused by transverse shear forces as for example illustrated in Fig. 2b and thus, strengthen the shell against transverse shear forces. This, in turn, increases the blade's resistance to the crushing pressure and thereby increases the ultimate strength of the wind turbine blade. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade. Thus, the overall strength of the blade is increased, and design of a blade with lower total weight is facilitated.

The invention also increases the torsional stiffness of the blade. An increase of the torsional stiffness of the blade will also increase the torsional eigenfrequency of the blade and in return decrease the dynamic inertia loads of the blade on other parts of the wind turbine. Furthermore, the increase in the torsional stiffness improves the aero-elastic stability of the blade significantly.

The girders may extend along substantially the entire length of the blade.

In an embodiment of the present invention, two or more girders are positioned end to end or in spaced relationship along at least a part of the longitudinal extension of the blade in such a way that neighbouring girders are mounted with different angles in relation to the profile chord of the blade. The distance between adjacent ends of neighbouring girders may not exceed 2xD, wherein D is the spanning distance of one of the girders, i.e. the distance between two opposing connections to the shell of the girder. The value of parameter D may be identical for two or more neighbouring girders.

However, since the width of the cross-section of the wind turbine blade typically decreases towards the tip of the blade, the distance D₂ of a girder located closer to the tip will be smaller than the distance D₁ of a girder located closer to the hub of the wind turbine. The resulting maximum distance between two neighbouring girders may preferably be calculated based on the minimum of the two distances, i.e. distance D₂, or based on the mean value of D₁ and D₂. It has been found that values of the resulting distance D fulfilling this relationship, there is a good balance between the girders' ability to take up the shear forces, the total weight of the wind turbine blade and the blade's stiffness. However, the maximum distance between two girders may in stead be based on other requirements, such as, but not limited to, a need for a particularly strong wind turbine blade design, e.g. when the wind turbine is intended to be subjected to repeatedly severe weather conditions, such as when erected at open sea.

In an embodiment with two or more girders mounted with a mutual distance along the edgewise direction of the blade, some or all of the girders may be divided into sections and positioned end to end along the longitudinal extension of the blade thereby facilitating handling and transportation. In principle, any number of girders may be used, however for the sake of simplicity and for keeping the overall weight of the blade as low as possible, a few, such as two girders are preferred.

The girder may comprise one or more elements selected from the group consisting of rods, plates, tubes, e.g. rods assembled in a lattice to form a lattice girder constituting the girder. The girders may be made of any suitable material. Fibre reinforced plastic is presently preferred.

The girder may further be provided with one or more cut-outs.

In an embodiment of the invention, the girders may be positioned in certain sections of the blade only.

In an embodiment of the invention, tilted girders, i.e. girders with a transversal extension that is not perpendicular to the profile chord of the blade, are located in positions wherein a substantial transverse distortion of the blade is expected or established.

In order to facilitate assembly of the girder with the shell parts, a cavity between the girder and the shell may be filled with a lightweight foamed material to facilitate positioning of the girder.

In an embodiment of the invention, the one or more girders may be individually designed so that the bending and torsion of the blade is coupled to withstand the high loads of strong wind gusts. This leads to lower fatigue loads on the blade and also facilitate a higher energy output of the wind turbine.

Below the invention will be described in more detail with reference to the exemplary embodiments illustrated in the drawings, wherein

Fig. 1 schematically illustrates in perspective a part of a wind turbine blade and arrows indicating the directions of flapwise, edgewise, and torsional loads, respectively,

Fig. 2a is a schematic cross-section of a wind turbine blade with arrows indicating directions of transverse shear forces in the blade,

Fig. 2b schematically illustrates deformation of a cross-section of a wind turbine blade caused by transverse shear forces, Fig. 3 is a schematic cross-section of a wind turbine blade with arrows indicating crushing pressure on the blade,

Fig. 4 schematically illustrates in perspective a part of a wind turbine blade with four girders interconnecting the upper and lower part of the shell,

Fig. 5 schematically illustrates in perspective a part of a wind turbine blade similar to the blade of Fig. 4, but with other mutual angles between the four girders,

Fig. 6 schematically illustrates in perspective a part of a wind turbine blade with two girders interconnecting the upper and lower part of the shell,

Fig. 7 schematically illustrates in perspective a part of a wind turbine blade similar to the blade of Fig. 6, wherein the girders have cut-outs,

Fig. 8 schematically illustrates in perspective a part of a wind turbine blade with four girders interconnecting the upper and lower part of the shell similar to the blade of Fig. 4, however wherein two of the girders are of a sandwich construction,

Fig. 9 schematically illustrates in perspective a part of a wind turbine blade with a spar in the form of a box profile which is manufactured separately and bonded in between prefabricated surface shells,

Fig. 10 schematically illustrates in perspective a part of a wind turbine blade with a plurality of girders with alternating angles in relation to the profile chord of the blade,

Fig. 11 shows examples of connections of girders to the shell,

Fig. 12 shows a schematic cross-section of a wind turbine blade with a U-profile enclosed by upper and lower shell parts,

Fig. 13 shows a schematic cross-section of a wind turbine blade with a U-profile closed with a box cap part and connected to a leading edge section and a trailing edge section,

Fig. 14 shows a schematic cross-section of a wind turbine blade with a U-profile divided into an upper and lower section by a floor and enclosed by upper and lower shell parts, Fig. 15 shows a schematic cross-section of a wind turbine blade with a U-profile closed with a box cap part and divided into an upper and lower section by a floor and connected to a leading edge section and a trailing edge section, and

Fig. 16 shows a schematic cross-section of a wind turbine blade with a U-profile closed with a box cap part and connected to a leading edge section and a trailing edge section, and a floor divided into three sections by the U-profile.

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The figures are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the invention, while other details have been left out. Throughout, the same reference numerals are used for identical or corresponding parts.

In addition to the shown embodiments, the invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

Fig. 1 schematically illustrates in perspective a part of a wind turbine blade 1 and arrows indicating the directions of flapwise F, edgewise E, and torsional T loads, respectively. The dashed line 12 indicates the longitudinal extension of the blade. The co-ordinates system 14 has an x-axis in the edgewise direction, an y-axis in flapwise direction, and a z- axis in the direction of the longitudinal extension of the blade 1. The cross-section S1 is parallel with the xy-plane of the co-ordinate system 14, and S1 is also shown in Figs. 2a, 2b, and 3.

Fig. 2a is a schematic cross-section S1 of a wind turbine blade and arrows indicating directions C of transverse shear forces in the blade,

Fig. 2b schematically illustrates deformation of a cross-section S1 of a wind turbine blade 1 caused by transverse shear forces. The illustrated blade 1 is twisted clockwise by the transverse shear forces.

Fig. 3 is a schematic cross-section of a wind turbine blade 1 having a shell 2 with leading edge 3 and trailing edge 4. The wind turbine blade 1 has a box profile with two girders 5 and caps 10 and 11 of the shell 2 located between the girders. The aerodynamic and inertia forces working on a blade in operation induce a bending moment on the blade and create a crushing pressure indicated by arrows B. The crushing pressure is also referred to as the Brazier effect (reference is made to the article "Structural testing and numerical simulation of a 34 m composite wind turbine blade" by F. M. Jensen et. al. published by Elsevier in Composite Structures 76 (2006) 52-61).

Fig. 4 schematically illustrates in perspective a part of a wind turbine blade 20 with four girders 22, 24, 26, 28 interconnecting the upper part 30 and the lower part 32 of the shell 34. Each of the girders 22, 24, 26, 28 forms similar sized oblique angles of alternating values larger than or lesser than 90° with respect to the profile chord of the blade 20. . Each of the girders 22, 24, 26, 28 comprises feet 36, each of which provides a large surface for bonding with the respective upper and lower parts 30, 32 of the shell 34. In the illustrated embodiment, each of the girders 22, 24, 26, 28 is made in one piece. Each leg of the shaped girders 22, 24, 26, 28 constitutes a straight reinforcing element.

Optionally, a foamed material may be located in one or more of the cavities formed between one or more girders and the inner surface of the shell 34. Further, the shape of the foamed material may match the shape of the respective cavity whereby the foamed material may guide the positioning of the one or more girders 22, 24, 26, 28 in question during assembly of the blade 20.

In the illustrated embodiment, the feet 36 of girder 22 and girder 24 abut each other at the upper part 30 of the shell 34. Likewise, the feet 36 of girder 26 and girder 28 abut each other at the upper part 30 of the shell 34. In another embodiment, the feet at the upper part 30 of the shell 34 are placed with a mutual distance.

Further, some of the lower part 32 of the shell 34 may be thickened to form a lower thickened cap part and some of the upper part 30 of the shell 34 may be thickened to form an upper thickened cap part. The position and width of the cap parts may be determined in the same way as for a conventional blade.

In the illustrated embodiment, the tilted girders 22, 24, 26, 28 significantly reduce transverse shear distortion of the blade 20 due to operational loads of the blade 20. The reduction of the transverse shear distortion of the profile also increases the blade 20's resistance to crushing pressure and thereby increases the ultimate strength of the wind turbine blade 20. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade. Provision of tilted girders also increases the reliability of the blade, because deformation of the girder at the connection between the girder and the shell is reduced and this increases the resistance against fatigue failure of the girder and/or fatigue failure of the shell and/or fatigue failure in the connection between the girder and the shell.

Provision of tilted girders also increases the torsional stiffness of the blade. An increase of the torsional stiffness of the blade will also increase the torsional eigenfrequency of the blade and in turn decrease the dynamic inertia loads of the blade on other parts of the wind turbine. Furthermore, the increase in the torsional stiffness improves the aero-elastic stability of the blade significantly.

Rg. 5 schematically illustrates in perspective a part of a wind turbine blade 20 with four girders 22, 24, 26, 28 interconnecting the upper part 30 and the lower part 32 of the shell 34 similar to the blade of Fig. 4 with the difference that the girder 22 of the blade of Fig. 5 is mounted substantially perpendicular to the profile chord of the blade 20 for increasing the blade's resistance against crushing pressure while the other three girders 24, 26, 28 are mounted with the same angles as the corresponding girders of the blade of Fig. 4, i.e. each of the girders 24, 26, 28 forms similar sized oblique angles of alternating values larger than or lesser than 90° with respect to the profile chord of the blade 20. Each of the girders 22, 24, 26, 28 comprises feet 36, each of which provides a large surface for bonding with the respective upper and lower parts 30, 32 of the shell 34. In the illustrated embodiment, each of the girders 22, 24, 26, 28 is made in one piece. Each leg of the shaped girders 22, 24, 26, 28 constitutes a straight reinforcing element.

In the illustrated embodiment, the feet 36 of girder 22 and girder 24 abut each other at the upper part 30 of the shell 34. Likewise, the feet 36 of girder 26 and girder 28 abut each other at the upper part 30 of the shell 34. In another embodiment, the feet at the upper part 30 of the shell 34 are placed with a mutual distance Further, some of the lower part 32 of the shell 34 may be thickened to form a lower thickened cap part and some of the upper part 30 of the shell 34 may be thickened to form an upper thickened cap part. The position and width of the cap parts may be determined in the same way as for a conventional blade.

In the illustrated embodiment, the girders 22, 24, 26, 28 significantly reduce transverse shear distortion of the blade 20 due to operational loads of the blade 20. The reduction of the transverse shear distortion of the profile also increases the blade 20's resistance to crushing pressure and thereby increases the ultimate strength of the wind turbine blade 20. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade.

Provision of tilted girders also increases the reliability of the blade, because deformation of the girder at the connection between the girder and the shell is reduced and this increases the resistance against fatigue failure of the girder and/or fatigue failure of the shell and/or fatigue failure in the connection between the girder and the shell.

Fig. 6 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 interconnecting the upper part 30 and the lower part 32 of the shell 34. Each of the girders 22, 24 forms similar sized oblique angles larger than or lesser than 90° with respect to the profile chord of the blade 20. Each of the girders 22, 24 comprises feet 36, each of which provides a large surface for bonding with the respective upper and lower parts 30, 32 of the shell 34. In the illustrated embodiment, each of the girders 22, 24 is made in one piece. Each leg of the shaped girders 22, 24 constitutes a straight reinforcing element.

In the illustrated embodiment, the feet 36 of girder 22 and girder 24 is attached to the upper part 30 of the shell 34 with a mutual distance.

5 Further, some of the lower part 32 of the shell 34 may be thickened to form a lower thickened cap part and some of the upper part 30 of the shell 34 may be thickened to form an upper thickened cap part. The position and width of the cap parts may be determined in the same way as for a conventional blade.

In the illustrated embodiment, the girders 22, 24 significantly reduce transverse shear 10 distortion of the blade 20 due to operational loads of the blade 20. The reduction of the transverse shear distortion of the profile also increases the blade 20's resistance to crushing pressure and thereby increases the ultimate strength of the wind turbine blade 20. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a 15 conventional blade.

20 Provision of tilted girders also increases the torsional stiffness of the blade. An increase of the torsional stiffness of the blade will also increase the torsional eigenfrequency of the blade and in turn decrease the dynamic inertia loads of the blade on other parts of the wind turbine. Furthermore, the increase in the torsional stiffness improves the aero-elastic stability of the blade significantly.

25 Fig. 7 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 interconnecting the upper part 30 and the lower part 32 of the shell 34 similar to the wind turbine blade of Fig. 6 with the difference that the girders of Fig. 7 have cut-outs 38 to decrease their weights. Each of the girders 22, 24 forms similar sized oblique angles larger than or lesser than 90° with respect to the profile chord of the blade

30 20. Each of the girders 22, 24 comprises feet 36, each of which provides a large surface for bonding with the respective upper and lower parts 30, 32 of the shell 34. In the illustrated embodiment, each of the girders 22, 24 is made in one piece.

In the illustrated embodiment, the feet 36 of girder 22 and girder 24 abut each other at the upper part 30 of the shell 34. In another embodiment, the feet at the upper part 30 of the shell 34 are placed with a mutual distance

In the illustrated embodiment, the girders 22, 24 significantly reduce transverse shear distortion of the blade 20 due to operational loads of the blade 20. The reduction of the transverse shear distortion of the profile also increases the blade 20's resistance to crushing pressure and thereby increases the ultimate strength of the wind turbine blade 20. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade.

Fig. 8 schematically illustrates in perspective a part of a wind turbine blade 20 with four girders 22, 24, 26, 28 interconnecting the upper part 30 and the lower part 32 of the shell 34 similar to the blade of Fig. 4 with the difference that the girders 22, 24 of the blade of Fig. 8 comprise a plate 40 of a laminated or sandwiched construction that is capable of withstanding tension and compression forces, comprising a layer of a lightweight foamed material provided between two layers of a fibre reinforced plastic material. Each of the girders 22, 24, 26, 28 forms similar sized oblique angles of alternating values larger than or lesser than 90° with respect to the profile chord of the blade 20. Each of the girders 22, 24, 26, 28 comprises feet 36, each of which provides a large surface for bonding with the respective upper and lower parts 30, 32 of the shell 34. In the illustrated embodiment, each of the girders 22, 24, 26, 28 is made in one piece. Each leg of the shaped girders 22, 24, 26, 28 constitutes a straight reinforcing element.

In the illustrated embodiment, the feet 36 of girder 22 and girder 24 abut each other at the upper part 30 of the shell 34. Likewise, the feet 36 of girder 26 and girder 28 abut each other at the upper part 30 of the shell 34. In another embodiment, the feet at the upper part 30 of the shell 34 are placed with a mutual distance

Provision of tilted girders also increases the reliability of the blade, because deformation of the girder at the connection between the girder and the shell is reduced and this increases the resistance against fatigue failure of the girder and/or fatigue failure of the shell and/or fatigue failure in the connection between the girder and the shell. Provision of tilted girders also increases the torsional stiffness of the blade. An increase of the torsional stiffness of the blade will also increase the torsional eigenfrequency of the blade and in turn decrease the dynamic inertia loads of the blade on other parts of the wind turbine. Furthermore, the increase in the torsional stiffness improves the aero-elastic stability of the blade significantly.

Fig. 9 schematically illustrates in perspective a part of a wind turbine blade 20 with a spar 42 which is manufactured separately and bonded in between prefabricated surface shells 32, 34. The spar 42 has two girders 22, 24 interconnecting the upper part 30 and the lower part 32 of the shell 34, and caps 44, 46 located between the girders 22, 24 and bonded to the upper part 30 and lower part 32, respectively, of the shell 34. Each of the girders 22, 24 may form similar sized oblique angles of values larger than or lesser than 90° with respect to the profile chord of the blade 20. Each of the girders 22, 24 constitutes a straight reinforcing element.

In the illustrated embodiment, the girders 22, 24 significantly reduce transverse shear distortion of the blade 20 due to operational loads of the blade 20. The reduction of the transverse shear distortion of the profile also increases the blade 20's resistance to crushing pressure and thereby increases the ultimate strength of the wind turbine blade 20. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade. Provision of tilted girders also increases the reliability of the blade, because deformation of the girder at the connection between the girder and the shell is reduced and this increases the resistance against fatigue failure of the girder and/or fatigue failure of the shell and/or fatigue failure in the connection between the girder and the shell.

Fig. 10 schematically illustrates in perspective a part of a wind turbine blade 20 with a plurality of girders 22, 24 that interconnect the upper part 30 and the lower part 32 of the shell 34 and are positioned end to end along at least a part of the longitudinal extension of the blade 20 in such a way that neighbouring girders 22, 24 are mounted with different angles in relation to the profile chord of the blade 20. The distance between adjacent ends of neighbouring girders may not exceed 2xD, wherein D is the spanning distance of one of the girders, i.e. the distance between two opposing connections to the shell 34 of the girder. The value of parameter D may be identical for two or more neighbouring girders.

Each of the girders 22, 24 comprises feet 36, each of which provides a large surface for bonding with the respective upper and lower parts 30, 32 of the shell 34. In the illustrated embodiment, each of the girders 22, 24 is made in one piece, and each leg of the shaped girders 22, 24 constitutes a straight reinforcing element. The upper feet 36 of the girders 22, 24 may be attached to the upper shell 30 along a common longitudinal axis of the blade 20 or they may be laterally displaced with a mutual distance in the edgewise direction of the blade 20.

Fig. 11 schematically shows a cross-section of a wind turbine blade 20 similar to the embodiment of Fig. 6, but with different types of connections 50, 52 between the tilted girders 22, 24 and the shell 30. For illustration purposes, the girders 22, 24 shown in Fig. 11 have different types of connections; however, preferably the girders of an embodiment have the same type of connections to the shell.

The tilted girder 24 is connected to the shell 30 with anchors 36. The anchors 36 are bonded to the inner surface of the upper part 32 and lower part 34, respectively, of the shell 30. The girder 24 is received between two receiving surfaces of the anchors 36 and girder 24 is bonded or adhered to the anchors 36.

In another embodiment, the connection between the girder and the shell is made by lamination applied directly on the girder and the shell. This could be done using fibre reinforced plastic and is often referred to as secondary lamination.

The tilted girder 22 is also connected to the shell 30 with anchors 36. The anchors 36 are bonded to the inner surface of the upper part 32 and lower part 34, respectively, of the shell 30. The girder 24 is received between two receiving surfaces of the anchors 36 and girder 24 is fastened to them mechanically, namely nuts engaging with a threaded rod that has been inserted through holes in the anchors and the girder 22. In a further embodiment the connection between the girder and the shell or the anchor are releasably interconnected. The releasable interconnection may comprise any suitable kind of joint such as a snap-fit, press-fit, groove-and-tongue connection or other simple mechanical connection. A releasable interconnection may be used to provide an aerodynamic profile with an increased degree of flexibility.

In a wind turbine blade of the invention, two girders form part of a U-profile having a bottom part that extends between the two girders forming an oblique angle with relation to each other, and the shell has a first shell part and a second shell part. The wind turbine blade further comprises a box cap part for closing the open end of the U-profile so that the girder and the box cap part, when assembled, constitutes a load carrying box profile of the blade. The bottom part of the U-profile and the box cap part conform to the outer surface of the blade, and the box cap part, the first shell part and the second shell part are connected to the U-profile.

Examples of such wind turbine blades are illustrated in Figs. 12 - 16. A wind turbine blade with a U-profile with parallel girders is disclosed in WO 2008/092451.

Fig. 12 schematically illustrates in perspective a part of a wind turbine blade 20 wherein the first shell part constitutes an upper shell part 30 of the blade 20, and the second shell part constitutes a lower shell part 32 of the blade 20. The blade 20 has two girders 22, 24 interconnecting the upper part 30 and the lower part 32 of the shell 34, and wherein the two girders 22, 24 form part of a U-profile 54 also having a bottom part 56 extending between the two girders 22, 24 and shaped to conform with the aerodynamic profile of the blade 20. Each of the girders 22, 24 forms similar sized oblique angles different from 90° with respect to the profile chord of the blade 20 and an oblique angle with respect to each other. Each of the girders 22, 24 comprises feet 36, each of which provides a large surface for bonding with the upper part 30 of the shell 34 and the box cap part 58. In the illustrated embodiment, the U-profile 54 is made in one piece. Each leg of the girders 22, 24 constitutes a straight reinforcing element.

Furthermore, a box cap part 58 is provided that is connected to the girders 22, 24. The box cap part 58 is formed so as to conform to the outer surface of the blade 20. The U- profile 54 and the box cap part 58 constitutes, when assembled, a load carrying box profile of the blade 20. The box cap part 58 and the bottom part 56 of the U-profile 54 are covered by two shell parts, an upper shell part 30 and a lower shell part 32 that defines the aerodynamic shape of the blade including the leading and trailing edges of the blade. The U-profile 54, box cap part 58 and upper and lower shell parts 30, 32 are preferably manufactured as individual components which are assembled into a blade.

Fig. 13 schematically illustrates in perspective a part of a wind turbine blade 20 wherein the first shell part constitutes a leading edge section 60 of the blade 20, and the second shell part constitutes a trailing edge section 62 of the blade 20, and wherein the bottom part 56 of the U-profile 54 constitutes a part of the outer surface of the blade 20 substantially opposite the box cap part 58 also constituting a part of the outer surface of the blade 20. Similar to the wind turbine blade of Fig. 12, the blade 20 has two girders 22, 24 interconnecting the upper part and the lower part of the shell 34, and wherein the two girders 22, 24 form part of a U-profile 54 also having a bottom part 56 extending between the two girders 22, 24. Each of the girders 22, 24 forms similar sized oblique angles different from 90° with respect to the profile chord of the blade 20 and an oblique angle with respect to each other. The upper ends of the girders 22, 24 are provided with feet or flanges 36 to which the box cap part 58 is connected.

The blade further comprises a leading edge section 60. The leading edge section 60 is shaped as an open profile defining the leading edge section of the blade and is attached to the flange 36 of the girder 22 at the upper side of the U-profile 54 and attached to the U-profile 54 by a flange 50 of the leading edge section 60 in a region in the vicinity of the lower side of the blade 20.

The leading edge section 60 is preferably attached to the U-profile 54 by a adhering technique in which e.g. resin is applied to the surfaces to be adhered which in turn are pressed against each other.

Similarly, the trailing edge section 62 is shaped as an open profile defining the trailing edge of the blade 20 and attached to the U-profile 54 in a manner similar to the manner according to which the leading edge section 60 is attached to the U-profile 54.

In Figs. 12 and 13, the shell regions of attachment of the box cap part 58 and the U-profile 54, respectively, are in general thinner than the remaining parts of the shell.

Also in both embodiments, the box cap part 58 and the U-profile 54 may be geometrically reversed so the box cap part 58 is located at the lower side and the bottom part 56 located at the upper side of the blade 20. Further, some of the lower part of the shell 34 may be thickened to form a lower thickened cap part and some of the upper part of the shell 34 may be thickened to form an upper thickened cap part. The position and width of the cap parts may be determined in the same way as for a conventional blade.

The U-profile 54 is provided to primarily strengthen and/or reinforce the blade in its longitudinal direction and is preferably of a fibre reinforced plastic material or other suitable material.

Similar to the other embodiments, in embodiments with the U-profile 54 with angled girders 22, 24; the angled girders significantly reduce transverse shear distortion of the blade 20 due to operational loads of the blade 20. The reduction of the transverse shear distortion of the profile also increases the blade 20's resistance to crushing pressure and thereby increases the ultimate strength of the wind turbine blade 20. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade.

In some embodiments of the invention, the U-profile 54 may itself comprise individual parts. Preferably, in such embodiments, the U-profile 54 is assembled with two angled girders connected by a bottom part in the shape of another, geometrically reversed but otherwise similar, box cap part.

The U-profile 54 may substantially extend along the length of the blade 20. However, it may also be preferred to provide the blade wherein the U-profile 54 is divided into two or more sections positioned end to end along the longitudinal direction of the blade, especially for facilitating handling or transporting purposes. The bottom part 56 of the U-profile 54 may preferably form a part of a cap part of the blade.

The connections, or joints, between the U-profile 54, the box cap part 58, and the first part 30, 60 of the shell 34, and the second part 32, 62 of the shell 34, respectively, may be established by bonding, laminating or mechanical means.

When the U-profile 54 and the box cap part 58 are joined, they constitute the load carrying box profile of the blade. Preferably, the load bearing fibres in the cap parts are situated at the outermost part of the blade where they provide optimum strength for counteracting the bending moment of the blade.

Each of the first part 30, 60 and the second part 32, 62 of the shell 34 may preferably be made in one piece, respectively, by traditional laminating techniques.

Each of the individual components of the wind turbine blade 20 may be manufactured using an autoclave for improving the quality. Preferably, at least the box cap part 58 and the U-profile 54 are subjected to autoclaving. The laminate of the component in the relevant mould is covered by a bag that is put under vacuum. The laminate and the mould are placed in the autoclave, which is pressurized and heated during the cure of the laminate. This result in a laminate with fully wetted fibres, an improved distribution of resin, low resin content and a very low void content.

The individual components of the wind turbine blade 20 may preferably be manufactured separately. The individual components may then subsequently be bonded or laminated together to form a complete blade 20. The provision of a blade 20 made from smaller individual components means that the components may be manufactured in moulds of a size that may fit in autoclaves that are economically feasible. Consequently, the provision of a blade constructed of individual components facilitates the use of an autoclave in the manufacturing of components for the blade which in turn produces blades of a higher quality than with the known techniques.

In embodiments, one or more of the individual components is/are at least partly made of a carbon fibre reinforced plastic material. Since the use of an autoclave is facilitated by the invention it is particularly, but not exclusively, preferable to use carbon fibres for reinforcing the plastic materials of the components according to the invention because it is possible to fulfil the demands in terms of level of stretched fibres, controlled resin distribution and content and also low void content. Particularly, it may be advantageous to manufacture the box cap part 58 in an extra good quality since the box cap part 58 may constitute the cap part of the blade 20 and an improved capability of taking up compression forces in the blade 20 is therefore clearly beneficial.

In embodiments, the box cap part 58 has a radius of curvature of at least 2 Meters. However, in certain embodiments the box cap part 58 is completely linear or flat. The box cap part 58 may preferably be so designed that it is possible to produce the part on a nearly flat table or mould, further facilitating the use of an autoclave, and also makes it much easier to place plastic material in the mould of the box cap part 58. The easy access to the mould facilitates manufacture of the box cap part 58 with accurate placing of the material, stretching of the fibres and also allows for a preferably mechanical pressurisation of the fibre material during the lay-up. Such a pressurisation will further lower the void content if prepreg materials are used. A nearly flat mould is particularly suited in connection with use of machines for lying up of fibres due to the unhindered access from the sides. The use of machines may further improve the level of stretched fibres and reduce the void content. The box cap part 58 according to the invention may also be well suited for adjustments or adaptations subsequent to curing using CAD/CAM milling machines. With these machines, narrow tolerances in regard to width, length and thickness of the box cap part 58 may be achieved. It may also be possible to shape the edges of the closing part so that they are prepared for connections with the other components. One particularly suitable type of joint for load transfer is a scarf joint used for bonding of the closing part to the other parts. However, any suitable type of joint may be applied.

Another advanced method of manufacturing the box cap part 58 is by pultruding, wherein yarns of fibres are drawn thought a matrix which has the shape of a cross-section of the box cap part 58. In the matrix resin is added and cured as the complete part is drawn continuously from the matrix. This method has the potential of achieving the highest quality possible for fibre reinforced plastics.

In embodiments, one or both of the sides of the U-profile 54 comprise a sandwich construction. However, one or both sides may be made as a single skin laminate but as the girder is subjected to compression forces, sandwich construction is a particularly useful solution enabling the sides to take up such forces. As the connection with the box cap part 58 are typically at the end of the U-profile 54, the girders' resistance to deflection/buckling is not compromised, as it would be by joints positioned e.g. at the middle of their span.

In embodiments, the U-profile 54 further comprises connection means at an end region of the sides thereof. In embodiments, the connection means comprises a flange integrated with the end region of the side(s). The flange may serve as a connection surface for bonding, laminating or mechanically fastening the girder to the respective other components. However, the flange and other suitable connection means may also be attached to the girder in stead of being integrated therewith.

In other embodiments, the box cap part 58 may further comprise one or more stiffening means provided in the longitudinal direction of the blade. Particularly, when the box cap part 58 constitutes a cap part of the blade it may be designed to have increased resistance against global deflection/buckling. A simple way to do this is to connect one or more stiffeners to the inner surface of the cap part in the along at least a part of the length of the blade. The stiffener may be of any suitable shape. Alternatively, the one or more stiffeners may be made integral with the box cap part 58. The caps' resistance to deflection/buckling may also be increased with the use of a sandwich construction or a hollow profile with internal stiffeners. These features may advantageously be included in the manufacture of the box cap part 58 and the U-profile 54. The stiffeners may comprise any suitable material but are preferably made of same material as the main part itself.

With this configuration the load carrying box profile may be assembled separately and subsequently bonded in between two shell parts. The shell parts will then cover the complete surface of the box profile and surround the U-shaped girder and the box cap part 58 completely. This facilitates that shell parts from a conventionally manufactured blade maybe used with the novel box cap part 58 and the U-profile 54.

The illustrated wind turbine blade may be produce by a method comprising the steps of

producing a girder substantially shaped as a U-profile 54 with a bottom part 56 conforming to a predetermined aerodynamic profile,

producing a first shell part 30, 60 having the shape of a part of the predetermined aerodynamic profile,

producing a second shell part 32, 62 having the shape of a part of the predetermined aerodynamic profile, and producing a box cap part 58 having the shape of a part of the predetermined aerodynamic profile,

connecting the box cap part 58 to the girder thereby forming a load carrying box profile of the blade, and

connecting the first shell part 30, 60 and the second shell part 32, 62 to the U-profile 54.

The U-profile 54 with box cap part 58 extends along the length of the blade. Typically, the U-profile 54 with box cap part 58 extends through-out the entire span of the wing. However, the load on the tip of the blade is typically low and the size of the tip is typically small allowing the tip to be formed without the U-profile 54 and box cap part 58. Thus, the U-profile 54 with box cap part 58 may in some preferred embodiments extend only to an extent where the load carrying effect of the girder with box cap part 58 plays a role. It has been found that an extension of at least 70%, such as at least 80%, preferably at least 90% or even at least 95% of the lengthwise extension of the blade is sufficient to provide a suitable stiffness of the blade.

In embodiments where the U-profile 54 with box cap part 58 does not extend through-out the whole lengthwise extension of the blade, the tip may preferably be made as a single piece which is attached to the U-profile 54.

The box cap part 58 may be of a sandwich construction.

As shown in Figs. 14 - 16, the wind turbine blade 20 may further have a at least one internal reinforcing floor 64 connected to an inner surface of the shell 34 at the trailing edge of the blade and to an inner surface of the shell at the leading edge of the blade in order to prevent or reduce deformations of the surface of the blade 20, in particular deformations caused by edgewise loading of the blade structure. Although the illustrated wind turbine blades also include the U-profile, wind turbine blades without the U-profile having angled girders and a floor may also be provided.

A wind turbine blade with a floor is disclosed in WO 2008/086805.

Fig. 14 shows schematically a cross-section of a wind turbine blade 20 similar to the blade 20 shown in Fig. 12, and further comprising a reinforcing floor 64 extending from a position in the vicinity of the connection joint 66 at the leading edge to a position in the vicinity of the connection joint 68 at the trailing edge of the blade 20. The U-profile is divided into a lower U-profile section 70 and two upper girders 72, 74 abutting the reinforcing floor 64 on both sides thereof thereby facilitating positioning of the components of the blade 20 in a convenient sequence during manufacture of the blade 20. During manufacture of the blade 20, the reinforcing floor 64 is positioned subsequent to the positioning of the lower U-profile section 70 in the lower part 32 of the shell, followed by positioning of the upper girders 72, 74, closure of the U-profile 54 by the box cap part 58, and finally positioning of the upper shell part 30.

Fig. 15 shows schematically a cross-section of a wind turbine blade 20 similar to the blade 20 shown in Fig. 13, and further comprising a reinforcing floor 64 extending from a position in the vicinity of the connection joint 66 in the leading edge section 66 to a position in the vicinity of the connection joint 68 in the trailing edge section 62 of the blade 20. The U-profile is divided into a lower U-profile section 70 and two upper girders 72, 74 abutting the reinforcing floor 64 on both sides thereof thereby facilitating positioning of the components of the blade in a convenient sequence during manufacture of the blade 20.

Fig. 16 shows schematically a cross-section of a wind turbine blade 20 similar to the blade 20 shown in Fig. 15; however, wherein the U-profile 54 is not divided by the reinforcing floor 64; but instead, the reinforcing floor 64 is divided into sections by the U-profile 54. Each section of the reinforcing floor 64 is individually connected to the U-profile 54 and the connection joints 66, 68, respectively. In some embodiments, only the section of the reinforcing floor 68 between the trailing edge and the girder closest to the trailing edge is present. In some embodiments, only the section of the reinforcing floor between the leading edge and the girder closest to the leading edge is present. In some embodiments, only the section of the reinforcing floor between the girders is present. In some embodiments, any two of the above-mentioned three sections of the reinforcing floor are present.

In general, the profile chord of the blade is an imaginary surface that contains the leading edge and the trailing edge of the blade and extends therebetween. Thus, in accordance with the present invention, an internal reinforcing floor extends along, or substantially along, the profile chord of the blade.

In a blade with a floor, whether the floor extends through the U-profile as shown in Figs. 14 and 15; or the U-profile extends through the floor as shown in Fig. 16; a connection between one of the at least one internal reinforcing floor and a respective one of the at least one internal girder is preferably located with a shortest distance to the shell that is larger than 0.16 times, more preferred larger than 0.33 times, the total distance between the upper part of the shell and the lower part of the shell along a transversal extension of the respective girder at the connection. For example, the connection may be located halfway or approximately halfway between the upper part of the shell and the lower part of the shell along a transversal extension of the respective girder at the connection.

Further, the at least one internal reinforcing floor may be connected to the inner surface of the shell and to one girder of the U-profile. The connection on the inner surface of the shell and on the girder may in principle be positioned anywhere thereon, but it should be observed that the chosen positioning causes the reinforcing floor to be able to provide a reasonable and useful reinforcing effect in the blade. The connection of a reinforcing floor between connecting points on the inner surface of the shell and the girder prevents or minimises the problematic deformations below. The connections may comprise any suitable kind of joint such as welded, glued, melted, fused or other simple mechanical connections such as bolt-and-nut connections. The reinforcing floor itself may comprise the connections or it may comprise additional connections or connection parts adapted to engage or cooperate with the other connections.

An extent of the trailing edge in the direction towards the leading edge may be made solid or, due to manufacturing considerations, embodiments may comprise a cavity between the lower and upper shell parts and a plate fastened between the two parts at a distance from the trailing edge. The cavity may be filled with lightweight material such as foam. Thereby, it may not be possible to fasten the reinforcing floor directly to the trailing edge, but instead to a part of the shell as near the trailing edge as possible. By connecting the reinforcing floor to a part of the shell near the trailing edge, instead of directly to the trailing edge, one can still obtain the advantages discussed above.

The reinforcing floor may comprise a plate shaped element. The plate element may be solid or hollow or any suitable combination thereof. The thickness of the plate may vary along different sections of the plate or it may be substantially equally thick over its entire area. However, it is required that the plate element is able to take up in-plane compression forces in the floor and the material and the dimensions of the floor must have this capability. The material may preferably, but not exclusively, be a fibre reinforced plastic material or another material such as metal, metal alloy, wood, plywood, veneer, glass fibre, carbon fibre and other suitable materials such as e.g. one or more composite materials. The reinforced plastic material may be manufactured from materials such as, but not limited to glass fibres, carbon fibres or aramid fibres thus providing a high strength and a low weight.

The mentioned materials may also be combined to any construction. Thus, in another embodiment the at least one reinforcing element is a laminate or a sandwich construction having relatively hard/durable outer surfaces, such as a fibre reinforced plastic, and an inner core of another material, such as, but not limited to, a softer and/or lighter material such as a foamed material.

Additionally, the plate element may comprise one or more stiffeners for e.g. maintaining strength and stiffness while minimising the weight of the construction. The stiffeners may comprise any suitable shape and material such as rods or bars or lattices of a fibre reinforced plastic material or another light-weight material such as aluminium.

Furthermore, in embodiments the plate element may comprise one or more cut-outs in order to reduce weight and/or increase the stiffness of the plate element. The cut-outs may be provided in any suitable pattern.

The reinforcing floor may made of or formed by elongated reinforcing members constituted by any type of elongated constructional members capable of taking up loads and assembled into the reinforcing floor or sections thereof.

The elongated reinforcing members may comprise one or more elements selected from the group consisting of a rod, a plate, and a tube, capable of resisting both compression forces and tensional forces.

Since, the reinforcing member need not be capable of resisting compression forces, the elongated reinforcing member may further comprise one or more elements selected from the group consisting of a wire, a rope, a thread, a fibre, and a web of fabric.

The elements may have any suitable cross-section, for example a substantially round or polygonal cross-section, such as substantially rectangular, triangular, circular, oval, elliptical, etc, but is preferably circular or oval.

The elements may be applied individually or may be applied as a number of individual elements together forming a "thicker" element. Particularly, the element may comprise fibres of very high stiffness and strength such as, but not limited to, aramid fibres. The elongated reinforcing members may be made of any suitable material. Fibre reinforced plastic is presently preferred for rods, plates and tubes.

In one embodiment, the elongated reinforcing member is required to have a high tensional strength only; i.e. preferably, the elongated reinforcing member need not carry other loads so that the elongated reinforcing member may be thin whereby its weight and cost are kept at a minimum.

The interconnections of the reinforcing members may comprise any suitable kind of joint such as welded, glued, melted, fused or other simple mechanical connections. The elongated reinforcing member itself may comprise the connections or it may comprise additional connections or connection parts adapted to engage or cooperate with other reinforcing members and/or the connections on the inner surface of the shell. The additional connections or connection parts must be sufficiently rigid to maintain their shape when subjected to tension in order to properly cooperate with the elongated reinforcing member to prevent the connections on the shells from being displaced away from each other.

The connections may be releasable connections that may comprise any suitable kind of joint, such as a snap-fit, press-fit, groove-and-tongue connection or other simple mechanical connection. A releasable interconnection may be used to provide an aerodynamic profile with an increased degree of flexibility.

By connecting or coupling the trailing edge with the closest girder using a reinforcing floor that can withstand compression forces, the deformations in the shell between the trailing edge and the web are reduced since the greater part of the forces causing the deformations are taken up by and distributed through the reinforcing floor and the web. This will decrease the potentially damaging forces in the joint between the shell parts, as the forces are distributed towards the floor and the web.

As deformations are reduced, the shell is kept in its original shape or position to a much higher degree. The result is that the "ineffective" panels of the shell carry an increased part of the load on the blade, and thereby decrease the load taken up by other parts of the blade. This results in an increased stiffness of the blade in the flapwise direction and thereby decreases the tip deflection. Along with this, the aerodynamic efficiency of the blade is increased since the blade profile will remain closer to its originally designed shape. The coupling will also increase the resistance of the trailing edge against buckling due to the edgewise loads because the damaging forces are distributed to the web through the floor.

As a result, the joint between the shell parts in the trailing edge is less exposed to damaging peeling and shear forces and the weight of the blade can be reduced since a less strong construction of the blade is needed. The lower weight reduces the dynamic inertia loads originating from the operation of the blade on the other parts of the wind turbine structure. Furthermore the aerodynamic efficiency of the blade is increased.

The reinforcing floor have a substantial desirable effect on the edgewise stiffness of the blade. As presented above, it prevents the deformation of the shell, which in itself has a positive effect on the edgewise stiffness, but it will also carry some of the edgewise loads.

This will take load off of other parts of the blade which means the edgewise stiffness is increased substantially. Such increased edgewise stiffness provides a higher edgewise eigenfrequency. It is an advantage to have a higher edgewise eigenfrequency because it decreases the dynamic inertia loads the blade is applying on the other structure of the wind turbine, because an increase of the eigenfrequency reduces the amplitude of the harmonic oscillations of the blade.

The floor also reduces the transverse shear force distortion of the profile of the blade, and this increases the blade's capability of taking up crushing pressure. This again helps maintaining the blade profile closer to its original shape and thus potentially increases the power output from the turbine.

By connecting or coupling the leading edge with the closest web using a reinforcing floor that can withstand compression forces, the loads on the leading edge are distributed towards the floor and the web, thereby reducing the potentially damaging forces in the joint between the shell parts. The reinforcing floor stabilises the shell in and in the vicinity of the leading edge section and increases the resistance of the shell against buckling in the leading edge section. When the buckling resistance is increased, the thickness of the laminated material used for shell can be reduced or, in embodiments where a sandwich construction is provided, the thickness of the core can be reduced. In embodiments the use of a sandwich construction in the leading edge section of the shell can be completely omitted and instead a single kind of material may be used for the leading edge. As a result, the weight of the blade can be further reduced without compromising strength and stiffness, a more simple construction of the blade is provided and consequently the blade can be produced at a lower total price.

As a result of the flapwise load, crushing pressure and shear forces is generated in the webs. These forces can cause the web to collapse, because the web buckles out of the plane of the web. When the web buckles due to the crushing pressure, the whole side of the web bends outwards in one direction. The buckling due to shear forces in the web shows a distinct wave pattern bending outwards to one side in one part of the web and to the other side in a neighbouring part of the web. When a reinforcing floor is connected to a web (either the web towards the trailing edge or the web towards the leading edge, in case two webs are used), it supports the part of the web that tries to buckle, and this increases the resistance of the web to buckling, and therefore a thinner core is needed in the sandwich construction in the web. This will allow for a reduction of the weight of the blade, and a reduction of material costs.

In the lower part of the blade, it comprises a transition from a wide aerodynamic profile to a cylindrical root section. The root is the part of the blade that is mounted on the wind turbine axle. In this part of blade, a reinforcing floor in the trailing edge is a very efficient structure for transfer of stresses from the blade shell to the circular cylindrical root. Thereby the stresses in the trailing edge section in the part of the blade proximal to the root are significantly reduced and the risk of failure in the connection between the shell parts in the trailing edge of the blade are minimised.

Furthermore, a connection or coupling of both the trailing and the leading edges with the web will increase the torsional stiffness of the blade. This will increase the torsional eigenfrequency of the blade and in return decrease the dynamic inertia loads the blade is applying on the other structure of the wind turbine, because an increase of the torsional eigenfrequency reduces the amplitude of the harmonic oscillations of the blade.

In embodiments, the floor(s) used in the connection or coupling between the trailing and/or leading edge(s) and the web may be specially tailored so that the bending and torsion of the blade is coupled. This is used to take the load of the blade when strong wind gusts occur. This leads to lower fatigue loads on the blade and also facilitate a higher energy output of the wind turbine. One or more individual components of the wind turbine blade, such as shell parts, girders, the U-profile, the box cap part, the spar, floors, etc., may at least partly be made of fibre reinforced plastics, such as glass fibre reinforced plastics, carbon fibre reinforced plastics or plastics reinforced with aramid fibres, etc; or of wood, such as bamboo, birch or plywood, etc; or of a material based on plant fibres with high cellulose content, such as bast fibres, such as flax, jute, etc, etc.

The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible members or steps. Also, the mentioning of references, such as "a", "an", etc., should not be construed as excluding a plurality. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is possible and advantageous.

Claims

1. A wind turbine blade comprising

a shell having a section with an aerodynamic profile, and

two girders for increasing the strength of the blade and extending in a longitudinal direction of the blade, and wherein each of the two girders is connected to the upper part and the lower part of the shell and forms an angle with the other girder of the two girders thereby strengthening the shell against transverse shear distortion.

2. A wind turbine blade according to claim 1 , wherein

the two girders form part of a U-profile having a bottom part that extends between the two girders and the shell has a first shell part and a second shell part, and wherein

the wind turbine blade further comprises a box cap part for closing the open end of the U- profile, the girder and the box cap part constituting, when assembled, a load carrying box profile of the blade, and wherein

the bottom part of the U-profile and the box cap part conform to the outer surface of the blade, and wherein

the box cap part, the first shell part and the second shell part are connected to the U- profile.

3. A wind turbine blade according to claim 2, wherein the first shell part constitutes a leading edge section of the blade, and the second shell part constitutes a trailing edge section of the blade, and wherein the bottom part of the U-profile constitutes a part of the outer surface of the blade substantially opposite the box cap part.

4. A wind turbine blade according to claim 2, wherein the first shell part constitutes an upper shell part of the blade, and the second shell part constitutes a lower part of the blade.

5. A wind turbine blade according to any of the preceding claims, further comprising a reinforcing floor connected inside the shell for increasing the strength of the blade and having a cross section transversely to the longitudinal extension of the blade that extends substantially in a direction from the trailing edge to the leading edge of the blade.

6. A wind turbine blade according to any of the preceding claims, wherein the angle ranges from 5° - 125°, such as from 10° - 110°, such as from 20° - 100°, such as from 30° to 90°.

7. A wind turbine blade according to any of claims 1 -5, wherein the angle ranges from 10° 5 - 90°, such as from 20° to 90°, such as from 30° to 90°.

8. A wind turbine blade according to any of the preceding claims, further comprising more than two girders.

9. A wind turbine blade according to any of the preceding claims, wherein the two girders comprise at least one element selected from the group consisting of a rod, a plate, and a

10 tube.

10. A wind turbine blade according to any of the preceding claims, wherein at least one of the girders comprises cut-outs.

11. A wind turbine blade according to any of the preceding claims, wherein at least one of the at least two girders comprise a plate of a sandwich construction.

15 12. A wind turbine blade according to any of the preceding claims, wherein two girders of the at least two girders form part of a spar that is manufactured in one piece with an upper cap and a lower cap interconnecting respective adjacent longitudinal edges of the two girders.

13. A wind turbine blade according to any of the preceding claims, comprising a plurality 20 of girders positioned in spaced relationship along the longitudinal extension of the blade with a mutual distance that is less than 2xD, wherein D is the distance between opposing connections of one of the plurality of straight girders to the upper part and lower part of the shell, respectively.

14. A wind turbine blade according to any of the preceding claims, wherein at least one of 25 the girders is made of fibre reinforced plastics, such as glass fibre reinforced plastics, carbon fibre reinforced plastics or plastics reinforced with aramid fibres.

15. A wind turbine blade according to any of the preceding claims, wherein at least one of the girders is made of wood, such as bamboo, birch or plywood.

16. A wind turbine blade according to any of the preceding claims, wherein at least one of the girders is made of a material based on plant fibres with high cellulose content, such as bast fibres, such as flax, jute, etc.

17. A method of producing a wind turbine blade with increased strength against

5 transverse shear distortion and having a shell with a section having an aerodynamic profile, the method comprising the step of positioning and connecting two girders inside the shell for extension in a longitudinal direction of the blade and connecting each of the girders to the upper part and the lower part of the shell in such a way that one of the girders forms an angle with the other girder.

10 18. A method of producing a wind turbine blade according to claim 17, comprising the steps of

producing a U-profile with the two girders and a bottom conforming to a predetermined aerodynamic profile,

producing a first shell part having the shape of a part of the predetermined aerodynamic 15 profile,

producing a second shell part having the shape of a part of the predetermined aerodynamic profile,

producing a box cap part having the shape of a part of the predetermined aerodynamic profile,

20 connecting the box cap part to the U-profile at the open end of the U-profile thereby forming a load carrying box profile of the blade, and

connecting the first shell part and the second shell part to the U-profile.

19. A method according to claim 18, wherein the first shell part constitutes a leading edge section of the blade and the second shell part constitutes a trailing edge section of the

25 blade and wherein the bottom part of the U-profile constitutes a part of the outer surface of the blade substantially opposite the box cap part.

20. A method according to claim 18, the first shell part constitutes an upper shell part of the blade and the second shell part constitutes a lower part of the blade.

21. A method according to any of claims 17 - 20, further comprising the step of positioning and connecting a reinforcing floor inside the shell for extension substantially in a direction from the trailing edge to the leading edge of the blade.

22. A method according to any of claims 17 - 21 , wherein the individual components are manufactured separately.

23. A method according to any of claims 17 - 22, wherein one or more of the individual components are at least partly made of fibre reinforced plastics, such as glass fibre reinforced plastics, carbon fibre reinforced plastics or plastics reinforced with aramid fibres; or of wood, such as bamboo, birch or plywood; or of a material based on plant fibres with high cellulose content, such as bast fibres, such as flax, jute, etc.