WO2010000263A2

WO2010000263A2 - A reinforced blade for a wind turbine

Info

Publication number: WO2010000263A2
Application number: PCT/DK2009/000151
Authority: WO
Inventors: Find Mølholt JENSEN; Per Hørlyk NIELSEN
Original assignee: Danmarks Tekniske Universitet
Priority date: 2008-07-01
Filing date: 2009-06-23
Publication date: 2010-01-07
Also published as: WO2010000263A3

Abstract

The present invention relates to a reinforced blade for a wind turbine having elongated reinforcing members in the blade extending substantially in the plane of the profile chord in order to strengthen the blade against edgewise and flapwise forces.

Description

A REINFORCED BLADE FOR A WIND TURBINE

The present invention relates to a reinforced blade for a wind turbine, particularly to a blade having elongated reinforcing members in the blade extending substantially in the plane of the profile chord in order to strengthen the blade against edgewise and flapwise forces.

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.

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. 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 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; see Fig. 1 for an indication of the loads and the directions.

When a blade is subjected to edgewise loading the section of the shell between a trailing edge of the blade and the internal girder is deforming out of the plane of the "neutral" (or initial) plane of the surface, see Fig. 1. This deformation induces peeling stresses in the trailing edge of the blade and consequently this can lead to a fatigue failure in the adhesive joint of the trailing edge where the two shell parts are connected to each other. Furthermore, the deformation of the of the shell can lead to deformations in both the shell and the girder at the connection between the girder and the shell and this 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.

The fatigue failure in the trailing edge, the shell, girder or the connections may then ultimately cause the blade to break apart.

The deformation can also lead to buckling of the shell and this reduces the ultimate strength of the blade because the shell is load bearing. Furthermore, the deformations also compromise the aerodynamic efficiency of the blade since the designed shape of the blade profile is no longer maintained.

The edgewise loads can further cause the trailing edge of the blade to deform in a stable post buckling pattern, see Fig. 2. This is caused by bending of the blade from the leading edge towards the trailing edge. The blade material in the leading edge is then subject to tension and the trailing edge to compression. Since the trailing edge is relative thin, it cannot withstand substantial compression forces before it bends out of its neutral plane. When this happens, some of the load on the trailing edge is transferred to and distributed through part of the shell further away from the trailing edge, until equilibrium of the forces is established. Although this deformation does not immediately lead to failure, it decreases the safety margin for the general failure load of the blade and also increases the peeling and shear stresses in the trailing edge.

Subjected to flapwise loads, the section of the aerodynamic shell between the trailing edge and the internal girder is deforming out of the plane of the surface's "neutral" position in a similar way as described above for the edgewise loads. This deformation also induces shear and peeling stresses in the trailing edge of the blade. The section will deform into a state of "lowest energy level", i.e. a situation wherein as much as possible of the stress in the blade is distributed to other sections of the blade. When part of the shell deforms in this manner, it is usually referred to as an "ineffective panel". The distribution of the stresses to other parts of the blade means that these parts are subjected to a higher load. This will result in a larger tip deflection of the blade. Furthermore, 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. Under flapwise loading, crushing pressure, see Hg. 3; occur on the box profile of the blade due to its longitudinal curvature. This effect is often referred to as ovalization (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 crushing pressure loads the internal girder in compression, see Fig. 4. The flapwise loads also induce in-plane shear forces in the internal girder. During the operation of the blade, transverse shear forces occur in the blade as shown on a cross-section of the blade (see Fig 5). The shear forces are generated by the flapwise and edgewise loads because the blade has asymmetric geometry and material distribution. The transverse shear forces distort the profile as shown on Fig. 5. The distortion of the profile reduces the blade's resistance to the crushing pressure and can cause a sudden collapse of the blade.

Presently, there is thus a need for a wind turbine blade in which deformations of the shell are prevented or minimised and wherein the blade structure is strengthened without increasing the overall weight. It is also desirable to provide improvements of a blade with at least one internal girder leading to increased resistance against buckling from crushing pressure and in-plane shear in order to carry the loads in the blade.

It is therefore an object of the present invention to provide a wind turbine blade with improved resistance against deformations of the shell.

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

It is another object of the present invention to provide a wind turbine blade with reduced weight.

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

It is another object of the present invention to provide a wind turbine blade with an improved transferral of forces in the transition between the blade and the circular root.

It is yet another object of the present invention to provide a wind turbine blade that can be produced at a reduced manufacturing cost compared to the existing solutions. It is still another object of the present invention to provide a wind turbine blade with an increased resistance against crushing pressure.

It is a further 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 further an object of the present invention to provide alternatives to the prior art.

According to a first aspect of the present invention, the above-mentioned and other objects are fulfilled by a wind turbine blade A wind turbine blade comprising a shell having a section with an aerodynamic profile, and at least one elongated reinforcing member connected inside the shell for increasing the strength of the blade, each of the at least one reinforcing member having a first end and a second end and extending substantially in the plane of the profile chord in order to strengthen the blade against edgewise and flapwise forces.

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 steps of positioning at least one elongated reinforcing member inside the shell for increasing the strength of the blade, each of the at least one reinforcing member having a first end and a second end and extending substantially in the plane of the profile chord in order to strengthen the blade against edgewise and flapwise forces.

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 wind turbine blade may preferably, but not exclusively, comprise a composite or laminated material. The material may preferably, but not exclusively, comprise fibreglass and/or carbon fibres and/or other durable and flexible materials typically with a high strength/weight ratio, such as other fibre reinforced plastic materials. This 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/or width.

The blade according to the invention may also comprise one or more girders. Wind turbine blades with one or more girders are well-known. A conventional girder has a longitudinal extension in the longitudinal direction of the blade and a transverse extension perpendicular to the profile chord of the blade. The one or more conventional girders primarily strengthen the blade along the longitudinal extension of the blade. A girder may also be referred to as a web. The conventional 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. Typically, conventional girders extend along substantially the entire length of the blade.

The wind turbine blade may comprise at least one girder to primarily strengthen and/or reinforce the blade in its longitudinal direction. However, it may also be preferred to provide the blade with two or more separated webs in the longitudinal direction of the blade, especially for facilitating handling or transporting purposes. In principle, any number of webs may be applied, however for the sake of simplicity and for keeping the overall weight of the blade as low as possible a number of one or two webs is preferred. Preferably, in a direction perpendicular to its longitudinal extension, each girder or web of the at least one internal girder extends from the lower part of the shell to the upper part of the shell in a substantially flapwise direction and is connected to the upper part and lower part, respectively, of the shell. Thus, in a wind turbine blade with a plurality of girders or webs, the shell preferably interconnects the girders or webs.

The at least one internal girder may comprise a box girder or a box beam. The sides of the box girder may vary in thickness in its longitudinal and/or transverse direction(s) and the shape and/or the perimeter length of the cross-section of the girder may also vary along its longitudinal extension.

Preferably, the box girder or box beam is of a substantially polygonal cross-section. The cross-section of the box girder or box beam may have any polygonal shape such as substantially rectangular, triangular, circular, oval, elliptical etc. but is preferably rectangular or substantially square.

Preferably, the wind turbine blade comprises a plurality of elongated reinforcing members extending substantially along the profile chord of the blade and positioned in spaced relationship along the longitudinal extension of the blade. The plurality of elongated reinforcing members may form an overlapping grid in which adjacent elongated members cross each other. Adjacent elongated reinforcing members may have interconnected ends, or connections of first ends and second ends, respectively, of adjacent elongated reinforcing members may be spaced apart. Further, adjacent elongated reinforcing members may be positioned in a non-overlapping spaced relationship.

The elongated reinforcing members may be positioned in certain sections of the blade only. Particularly, but not exclusively, the elongated reinforcing members may be located at positions wherein a substantial deformation of the section between the trailing edge of the blade and the girder is expected or established.

At least one of, and more preferred each of, the at least one elongated reinforcing member may form an oblique angle with the longitudinal extension of the blade, preferably ranging from 15° to 90°, more preferred from 25° to 90°, more preferred from 35° to 90°, more preferred from 45° to 90°.

At least one of, preferably each of, the at least one elongated reinforcing member may extend in a direction substantially along the profile chord and substantially perpendicular to the longitudinal extension of the blade.

In case of a curved blade wherein the longitudinal extension of the blade forms a nonlinear curve in space, the specified angles of each of the reinforcing members are determined with relation to the longitudinal extension of the blade in the vicinity of the reinforcing member in question. The first end and the second end of each of the at least one elongated reinforcing member may be connected to an inner surface of the shell at the trailing edge and the leading edge, respectively, of the shell; or, one of the first and second ends may be connected to a girder or another supporting structure within the blade.

In a wind turbine blade with at least one internal girder, a reinforcing member may be provided between the trailing edge and one of the at least one internal girder. If more than one internal girder is provided, the at least one reinforcing member may be provided between the trailing edge and the internal girder or web closest to the trailing edge, or the at least one reinforcing member may extend through one or more of the girders for connection to yet another girder or the leading edge of the blade or a supporting structure at the leading edge of the blade.

In a wind turbine blade with at least one internal girder, a reinforcing member may be provided between the leading edge and one of the at least one internal girder. If more than one internal girder is provided, the at least one reinforcing member may be provided between the leading edge and the internal girder or web closest to the leading edge, or the at least one reinforcing member may extend through one or more of the girders for connection to yet another girder or the trailing edge of the blade or a supporting structure at the trailing edge of the blade.

When a reinforcing member extends through a girder, the reinforcing member may optionally be connected to that girder.

A part of the blade at the trailing edge may be made solid, due to manufacturing considerations, the solid part extending in the direction towards the leading edge of the blade. The solid part may be formed by a plate fastened to the upper part and lower part of the shell and extending there at a distance from the trailing edge, and the cavity between the plate and the shell may be filled with lightweight material, such as foam. In such blades, it may not be possible to fasten the at least one reinforcing member directly to the trailing edge, but instead to an internal part of the blade, preferably as near the trailing edge as possible. The above-mentioned advantages are also obtained in such blades.

In accordance with the present invention, an internal reinforcing member extends along, or substantially along, the profile chord of the blade. Thus, a connection between one of the at least one internal reinforcing member and a 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.

Preferably, the elongated reinforcing member has a straight shape. If the shape of the elongated reinforcing member is not straight, the shape of the elongated reinforcing member could be straightened when subjected to stretching forces leading to movement of its end points and obviously, this is not desired.

The elongated reinforcing member may be constituted by any type of elongated constructional member capable of taking up loads.

The elongated reinforcing member 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.

The elongated reinforcing member may be 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 connections of the at least one reinforcing member 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 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.

The at least one elongated reinforcing member connected to an inner surface of the shell at the trailing edge of the blade and/or to an inner surface of the shell at the leading edge of the blade prevents or reduces deformations of the surface of the blade caused by forces in the edgewise and flapwise direction of the blade, in particular forces in the edgewise direction, since each of the at least one elongated reinforcing member prevents such forces from urging the two connections of the elongated reinforcing member away from each other thereby strengthening the shell against such forces. Thus, the elongated reinforcing member desirably has a high tensional strength while the elongated reinforcing member may be, but need not be, capable of resisting compression forces.

Thus, the elongated reinforcing member secures and keeps the shape of the shell substantially unchanged when the aerodynamic profile is loaded by forces in the edgewise and flapwise direction. This in turn causes the overall strength of the aerodynamic profile to increase significantly since the resistance against buckling is increased. With the elongated reinforcing member according to the invention, the dimensions of the material(s) used for the profile's shell may further be drastically reduced compared to currently available solutions and thus facilitates lower dynamic loads on the other parts of the system, improved handling and transportation characteristics of the profile and a reduction of material costs.

An elongated reinforcing member according to the present invention improves the aerodynamic efficiency of the blade since the designed shape of the blade profile is maintained to a higher degree than for a conventional blade. Due to the at least one reinforcing member, 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 member. This will decrease the potentially damaging forces in the joint between the shell parts, since the forces are distributed towards the elongated reinforcing members and other parts of the blade.

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, strengthening against deformation increases the resistance of the blade 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.

Due to reduction of deformations, 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 through the reinforcing members to other parts of the blade.

The reinforcing member have a substantial desirable effect on the edgewise stiffness of the blade. As mentioned 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. Furthermore, the at least one elongated reinforcing member 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.

By connecting or coupling the leading edge with the closest web with reinforcing members, the loads on the leading edge are distributed towards the elongated reinforcing members and the web, thereby reducing the potentially damaging forces in the joint between the shell parts. The elongated reinforcing members stabilise 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 wind turbine blades where a sandwich construction is provided, the thickness of the core can be reduced. The use of a sandwich construction in the leading edge section of the shell can be completely may be 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 member 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.

The blade has a transition from a wide aerodynamic profile to a cylindrical root section in the lower part of the blade. The root is the part of the blade that is mounted on the wind turbine axle. In this part of blade, a reinforcing member 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.

The elongated reinforcing member(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.

The reinforcing member(s) may be equipped with or may consist of active installations, such as piezoelectric installations, that may be activated by means of voltage, current, electric or magnetic field, whereby the length of the reinforcing member changes and/or stresses are imposed on the element. By this it is possible to change the curvature of the profile's surface and thereby change the aerodynamic properties of the profile. With these installations it is possible to optimize the performance of the aerodynamic profile. These installations may also be used to change the dampening of the blade and thereby the eigenfrequency of the blade. This could decrease the dynamic inertia loads the blade is applying on the other structure of the wind turbine, because a change of the eigenfrequency could reduce the amplitude of the harmonic oscillations of the blade.

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

Fig. 1 is a schematic view of a cross-section of a wind turbine blade indicating a deformation of the blade shell (or panel) between a trailing edge and an internal girder/web due to flapwise loads,

Fig. 2 is a schematic perspective view of a wind turbine blade indicating a deformation in a trailing edge of the blade in the form of a buckling pattern caused by an edgewise load that is also indicated,

Fig. 3 is a schematic view of a cross-section of a wind turbine blade indicating the crushing pressure on the blade from the bending moment acting on the blade in operation, Fig. 4 is a schematic view of part of the cross-section of a wind turbine blade, in particular showing a web in the form of a box profile and indicating the potential deformation (ovalization) caused by the crushing pressure (deformed state shown as punctured lines),

Fig. 5 is a schematic perspective view of a wind turbine blade indicating deformations caused by the influence of transverse shear forces on the blade profile,

Fig. 6 schematically illustrates in perspective a part of a wind turbine blade with elongated reinforcing members interconnecting the trailing edge and the leading edge of the blade in accordance with the invention,

Fig. 7 schematically illustrates in perspective a part of a wind turbine blade with elongated reinforcing members interconnecting the trailing edge and the web in accordance with the invention,

Fig. 8 schematically illustrates in perspective a part of a wind turbine blade with elongated reinforcing members interconnecting the trailing edge and the web in accordance with the invention,

Fig. 9 schematically illustrates in perspective a part of a wind turbine blade with elongated reinforcing members interconnecting the trailing edge and the leading edge of the blade in accordance with the invention,

Fig. 10 schematically illustrates in perspective a part of a wind turbine blade with elongated reinforcing members interconnecting the leading edge and the web in accordance with the invention,

Fig. 11 shows a schematic cross-section of a root section of a wind turbine blade elongated reinforcing members interconnecting the trailing edge and the web in accordance with the invention,

Fig. 12 shows a schematic cross-section of a root section of a wind turbine blade elongated reinforcing members interconnecting the trailing edge and the web in accordance with the invention,

Fig. 13 shows an example of connections of an elongated reinforcing inside the blade in accordance with the invention, and Fig. 14 shows another example of connections of an elongated reinforcing inside the blade in accordance with the invention.

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 shows a cross-section of a wind turbine blade 1 indicating (by punctured line) a deformation of the blade shell (or panel) 2 between a trailing edge 4 and an internal girder/web 5 due to flapwise loads originating from the aerodynamic and inertia forces on the blade in operation. The flapwise direction is illustrated by arrow A in Fig. 3. The shell 2 shown in this embodiment comprises two shell parts, designated in this example as upper part 6 and lower part 7. The upper and lower shell parts are connected by bonding in joints 8 and 9 (not indicated), preferably in or close to the leading and trailing edges 3 and 4 respectively, of the blade.

Fig. 2 shows a perspective view of a wind turbine blade 1 indicating a deformation (drawn exaggerated for the purpose of clarity) in the trailing edge 4 of the blade in the form of a buckling pattern caused by an edgewise load indicated by arrow F.

Fig. 3 shows a principle cross-section of a wind turbine blade 1 having a shell 2 with leading edge 3 and trailing edge 4. Also indicated is a box profile with two webs 5 and sections 10 and 11 of the shell 2 located between the webs. 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). The flapwise direction is illustrated by arrows A. Fig. 4 shows a schematic partial view of a cross-section of the blade 1. The blade is shown in a loaded or ovalized state, indicated by the punctured line. The figure also indicates a cross-section of the blade in a neutral or un-loaded position (fully drawn line). The figure is intended to support the understanding of how the forces on the blade cause its cross-sectional profile/shape to vary. The repeated exposure to ovalization adds to fatiguing the blade structure over time.

Fig. 5 schematically shows two cross-sections of a wind turbine blade 1. Fig. 5a indicates the transverse shear forces (arrows C) on the blade profile and Fig. 5b indicates in principle the resulting deformed blade profile from the influence of the transverse shear forces. The blade 1 is illustrated as being "twisted clockwise" by the transverse forces.

Fig. 6 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 and an upper thickened cap part 26 and a lower thickened cap part 28 of the shell 30. The two girders 22, 24 and thickened cap parts 26, 28 of the blade 20 constitute a box profile. The illustrated embodiment further has a plurality of elongated reinforcing members 38 interconnecting the trailing edge 40 and the leading edge 42 of the blade and extending through the two girders 22, 24 substantially along the profile chord of the blade. The elongated reinforcing members 38 may optionally be connected to one of, or both of, the girders 22, 24. The elongated reinforcing members 38 form an angle with the longitudinal extension of the blade 20 that is approximately 45°. In another embodiment this angle may be different, preferably ranging from 25° to 90°. Further, the angle may vary along the longitudinal extension of the blade to provide useful reinforcement of the blade, for example in order to compensate for a twist of the blade along the longitudinal extension of the blade, or, in order to compensate for varying thickness of the shell of the blade, etc.

The elongated reinforcing members 38 form an overlapping grid of crossing members 38 in which adjacent elongated reinforcing members 38 have mutually interconnected first ends at connections 44 to the trailing edge 40 of the shell 30 and mutually interconnected second ends at connections 46 to the leading edge 42 of the shell 30.

In another embodiment, the connections of first ends and second ends, respectively, of adjacent elongated reinforcing members 38, are spaced apart. Further, adjacent elongated reinforcing members may be positioned in a non-overlapping spaced relationship. In the illustrated embodiment, each of the elongated reinforcing members 38 is bonded to the internal surface of the shell 30.

Some or all of the elongated reinforcing members 38 may be flexible wires with high tensional strength without a capability of resisting compression forces.

Some or all of the elongated reinforcing members 38 may be rods capable of resisting both compression forces and tension forces.

In both cases, the elongated reinforcing members prevent forces in the edgewise and flapwise direction of the blade from urging the two connections away from each other thereby strengthening the shell against deformation by forces in the edgewise and flapwise direction.

Thus, the at least one elongated reinforcing member secures and keeps the shape of the shell substantially unchanged. Strengthening against deformation increases the resistance of the blade against buckling of the shell thereby increasing the ultimate strength of the blade because the shell is load bearing.

With provision of the at least one elongated reinforcing member according to the invention, the dimensions of the material(s) used for the profile's shell may further be drastically reduced compared to currently available solutions and thus facilitates lower dynamic loads on the other parts of the system, improved handling and transportation characteristics of the profile and a reduction of material costs. Due to the at least one reinforcing member, the deformations in the shell between the trailing edge and the web are reduced. This will decrease the potentially damaging forces in the joint between the shell parts.

As a result, the joint between the shell parts in the trailing edge is less exposed to damaging peeling and shear forces and therefore improves the reliability of the adhesive joint of the trailing edge. This means that 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, strengthening against deformation increases the resistance of the blade 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. The elongated reinforcing members also increase the blades resistance to buckling of the trailing edge caused by edgewise loads and thereby increase the safety margin for the general failure load of the blade and also decrease the peeling and shear stresses in the trailing edge.

The elongated reinforcing members also increase the blades resistance of deforming out of the plane of the surface's "neutral" position of the section of the aerodynamic shell between the trailing edge and the internal girder. This decreases the shear and peeling stresses in the trailing edge of the blade and result in a smaller tip deflection of the blade.

Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the shell is maintained to a higher degree than for a conventional blade.

The elongated reinforcing members have a substantial desirable effect on the edgewise stiffness of the blade and provide a higher edgewise eigenfrequency. A higher edgewise eigenfrequency decreases the dynamic inertia loads that the blade is applying on the other structure of the wind turbine.

Furthermore, the elongated reinforcing members increase the torsional stiffness of the blade. This will increase the torsional eigenfrequency of the blade and in return decrease the dynamic inertia loads that the blade is applying on the other structure of the wind turbine.

At the leading edge, the elongated reinforcing members reduce the potentially damaging forces in the joint between the shell parts and stabilise the shell at the leading edge and in the vicinity of the leading edge and increase the resistance of the shell against buckling in the leading edge section. As a result, the weight of the blade can be further reduced without compromising strength and stiffness.

The resistance of the web against buckling increases when a reinforcing member is connected to a web. This will allow for a reduction of the weight of the blade, and a reduction of material costs.

In the cylindrical root section in the lower part of the blade, an elongated reinforcing member significantly reduces the stresses in the trailing edge section in the part of the blade proximal to the root and reduces the risk of failure in the connection between the shell parts in the trailing edge of the blade Fig. 7 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 and an upper thickened cap part 26 and a lower thickened cap part 28 of the shell 30. The two girders 22, 24 and thickened cap parts 26, 28 of the blade 20 constitute a box profile. The illustrated embodiment further has a plurality of elongated reinforcing members 38 interconnecting the trailing edge 40 and the rearmost girder 24 of the blade substantially along the profile chord of the blade.

The elongated reinforcing members 38 form an angle with the longitudinal extension of the blade 20 that is approximately 45°. In another embodiment this angle may be different, preferably ranging from 25° to 90°. Further, the angle may vary along the longitudinal extension of the blade to provide useful reinforcement of the blade, for example in order to compensate for a twist of the blade along the longitudinal extension of the blade, or, in order to compensate for varying thickness of the shell of the blade, etc.

The elongated reinforcing members 38 form an overlapping grid of crossing members 38 in which adjacent elongated reinforcing members 38 have mutually interconnected first ends at connections 44 to the trailing edge 40 of the shell 30.

In another embodiment, the connections of first ends and second ends, respectively, of adjacent elongated reinforcing members 38, are spaced apart. Further, adjacent elongated reinforcing members may be positioned in a non-overlapping spaced relationship.

In the illustrated embodiment, each of the elongated reinforcing members 38 is bonded to the internal surface of the shell 30 and to the girder 24.

In both cases, the elongated reinforcing members prevent forces in the edgewise and flapwise direction of the blade from urging the two connections away from each other thereby strengthening the shell against deformation by forces in the edgewise and flapwise direction. Thus, the at least one elongated reinforcing member secures and keeps the shape of the shell substantially unchanged. Strengthening against deformation increases the resistance of the blade against buckling of the shell thereby increasing the ultimate strength of the blade because the shell is load bearing.

As a result, the joint between the shell parts in the trailing edge is less exposed to damaging peeling and shear forces and therefore improves the reliability of the adhesive joint of the trailing edge. This means that 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, strengthening against deformation increases the resistance of the blade 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.

The elongated reinforcing members also increase the blades resistance to buckling of the trailing edge caused by edgewise loads and thereby increase the safety margin for the general failure load of the blade and also decrease the peeling and shear stresses in the trailing edge.

Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the shell is maintained to a higher degree than for a conventional blade. The elongated reinforcing members have a substantial desirable effect on the edgewise stiffness of the blade and provide a higher edgewise eigenfrequency. A higher edgewise eigenfrequency decreases the dynamic inertia loads that the blade is applying on the other structure of the wind turbine.

In the cylindrical root section in the lower part of the blade, an elongated reinforcing member significantly reduces the stresses in the trailing edge section in the part of the blade proximal to the root and reduces the risk of failure in the connection between the shell parts in the trailing edge of the blade

Fig. 8 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 and an upper thickened cap part 26 and a lower thickened cap part 28 of the shell 30. The two girders 22, 24 and thickened cap parts 26, 28 of the blade 20 constitute a box profile. The illustrated embodiment further has a plurality of elongated reinforcing members 38 interconnecting the trailing edge 40 and the rearmost girder 24 of the blade 20 substantially along the profile chord of the blade 20.

The elongated reinforcing members 38 form an angle with the longitudinal extension of the blade 20 that is approximately 90°.

The elongated reinforcing members 38 are positioned in spaced relationship along at least a part of the longitudinal extension of the blade 20. In one embodiment, the distance between adjacent neighbouring elongated reinforcing members 38 does not exceed 2xD, wherein D is the maximum distance between the trailing edge and nearby girder in a cross-section of the blade containing the elongated reinforcing member in question. The value of parameter D may be identical for two or more neighbouring elongated reinforcing members. 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 an elongated reinforcing member located closer to the tip will be smaller than the distance D1 of an elongated reinforcing member located closer to the hub of the wind turbine. The resulting maximum distance between two neighbouring elongated reinforcing members 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 elongated reinforcing members' 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 elongated reinforcing members 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 of the invention, the elongated reinforcing members may be positioned in certain sections of the blade only possibly without any predetermined or calculated maximum distance. Particularly, but not exclusively, the elongated reinforcing members may be located at positions wherein a substantial deformation of the section between the trailing edge of the blade and the girder is expected or established.

Fig. 9 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 and an upper thickened cap part 26 and a lower thickened cap part 28 of the shell 30. The two girders 22, 24 and thickened cap parts 26, 28 of the blade 20 constitute a box profile. The illustrated embodiment further has a plurality of elongated reinforcing members 38 interconnecting the trailing edge 40 and the leading edge 42 of the blade 20 and extending through the two girders 22, 24 substantially along the profile chord of the blade 20.

The elongated reinforcing members 38 are positioned in spaced relationship along at least a part of the longitudinal extension of the blade 20. In one embodiment, the distance between adjacent neighbouring elongated reinforcing members 38 does not exceed 2xD, wherein D is the maximum distance between the trailing edge 40 and nearby girder in a cross-section of the blade containing the elongated reinforcing member in question. The value of parameter D may be identical for two or more neighbouring elongated reinforcing members.

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 an elongated reinforcing member located closer to the tip will be smaller than the distance D1 of an elongated reinforcing member located closer to the hub of the wind turbine. The resulting maximum distance between two neighbouring elongated reinforcing members 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 elongated reinforcing members' 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 elongated reinforcing members 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 of the invention, the elongated reinforcing members may be positioned in certain sections of the blade only possibly without any predetermined or calculated maximum distance. Particularly, but not exclusively, the elongated reinforcing members may be located at positions wherein a substantial deformation of the section between the trailing edge of the blade and the girder is expected or established.

The resistance of the web against buckling increases when a reinforcing member is connected to a web. This will allow for a reduction of the weight of the blade, and a reduction of material costs. In the cylindrical root section in the lower part of the blade, an elongated reinforcing member significantly reduces the stresses in the trailing edge section in the part of the blade proximal to the root and reduces the risk of failure in the connection between the shell parts in the trailing edge of the blade

Fig. 10 schematically illustrates in perspective a part of a wind turbine blade 20 with two girders 22, 24 and an upper thickened cap part 26 and a lower thickened cap part 28 of the shell 30. The two girders 22, 24 and thickened cap parts 26, 28 of the blade 20 constitute a box profile. The illustrated embodiment further has a plurality of elongated reinforcing members 38 interconnecting the leading edge 42 and the foremost girder 22 of the blade 20 substantially along the profile chord of the blade 20.

The elongated reinforcing members 38 are positioned in spaced relationship along at least a part of the longitudinal extension of the blade 20. In one embodiment, the distance between adjacent neighbouring elongated reinforcing members 38 does not exceed 2xD, wherein D is the maximum distance between the leading edge and the nearby girder in a cross-section of the blade containing the elongated reinforcing member in question. The value of parameter D may be identical for two or more neighbouring elongated reinforcing members.

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 an elongated reinforcing member located closer to the tip will be smaller than the distance D 1 of an elongated reinforcing member located closer to the hub of the wind turbine. The resulting maximum distance between two neighbouring elongated reinforcing members 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 elongated reinforcing members' 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 elongated reinforcing members 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 of the invention, the elongated reinforcing members may be positioned in certain sections of the blade only possibly without any predetermined or calculated maximum distance. Particularly, but not exclusively, the elongated reinforcing members may be located at positions wherein a substantial deformation of the section between the trailing edge of the blade and the girder is expected or established.

With provision of the at least one elongated reinforcing member according to the invention, the dimensions of the material(s) used for the profile's shell may further be drastically reduced compared to currently available solutions and thus facilitates lower dynamic loads on the other parts of the system, improved handling and transportation characteristics of the profile and a reduction of material costs. Furthermore, strengthening against deformation increases the resistance of the blade 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.

Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the shell is maintained to a higher degree than for a conventional blade. At the leading edge, the elongated reinforcing members reduce the potentially damaging forces in the joint between the shell parts and stabilise the shell at the leading edge and in the vicinity of the leading edge and increase the resistance of the shell against buckling in the leading edge section. As a result, the weight of the blade can be further reduced without compromising strength and stiffness.

Fig. 11 shows a schematic cross-section of a root section of the wind turbine blade 20 of Fig. 8 having elongated reinforcing members 38 interconnecting the trailing edge 40 and the rearmost girder 24.

The blade 20 has a transition from a wide aerodynamic profile to a cylindrical root section in the lower part of the blade 20 proximate the rotor of the wind turbine. The root is the part of the blade that is mounted on the wind turbine shaft. In this part of the blade 20, an elongated reinforcing member 38 connected to the trailing edge 40 is a very efficient structure for transfer of stresses from the blade shell 30 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.

Preferably, the connection 44 of each of the elongated reinforcing members 38 to the trailing edge 40 of the shell 30 in the transition region between the aerodynamic part of the shell 30 and the root is located at the transition between the aerodynamic part of the lower part 34 of the shell 30 and the flattened trailing edge 40.

Fig. 12 shows a schematic cross-section of a root section of the wind turbine blade 20 of Fig. 8 having elongated reinforcing members 38 interconnecting the trailing edge 40 and the rearmost girder 24.

In the illustrated embodiment, the connection 44 of each of the elongated reinforcing members 38 to the trailing edge 40 of the shell 30 in the transition region between the aerodynamic part of the shell 30 and the root is located at the transition between the aerodynamic part of the upper part 32 of the shell 30 and the flattened trailing edge 40. In both cases, the elongated reinforcing members prevent forces in the edgewise and flapwise direction of the blade from urging the two connections away from each other thereby strengthening the shell against deformation by forces in the edgewise and flapwise direction.

The elongated reinforcing members also increase the blades resistance of deforming out of the plane of the surface's "neutral" position of the section of the aerodynamic shell between the trailing edge and the internal girder. This decreases the shear and peeling stresses in the trailing edge of the blade and result in a smaller tip deflection of the blade. Furthermore, the aerodynamic efficiency of the blade is also improved since the designed shape of the shell is maintained to a higher degree than for a conventional blade.

Fig. 13 shows a schematic cross-section of the wind turbine blade 20 of Fig. 6 having elongated reinforcing members 38 interconnecting the trailing edge 40 and the leading edge 42 of the blade 20.

In the illustrated embodiment, each of the elongated reinforcing members 38 is connected to an internal supporting structure proximate the trailing edge 40 and leading edge 42, respectively, of the blade 20. Each of the elongated reinforcing members 38 are mechanically connected to the supporting structure by leading each of the reinforcing members 38 through suitable openings in the supporting structure proximate the trailing edge 40 and leading edge 42, respectively, and fastening them by means of a mechanical connection 44, such as a nut engaging with a threaded section of the ends of the reinforcing members 38.

Fig. 14 shows a schematic cross-section of the wind turbine blade 20 of Fig. 6 having elongated reinforcing members 38 interconnecting the trailing edge 40 and the leading edge 42 of the blade 20. In the illustrated embodiment, each of the elongated reinforcing members 38 is connected to the trailing edge 40 and leading edge 42, respectively, of the shell 30 with anchors 44. The anchors 44 are bonded to the inner surface of the shell 30. Each of the elongated reinforcing members 38 are connected to the anchor 44 by interconnecting pins inserted through the anchor and the member.

The anchor 44 may alternatively be laminated to the shell 30 and the member 38. This can be made using fibre reinforced plastic and is also known as secondary lamination.

Any suitable connection means or methods between the reinforcing member 38 and the web 22, 24, between the reinforcing member 38 and the inner surface of the shell 30 or between the web 22, 24 and the inner surface of the shell 30 may of course be applied in any one of the embodiments described in this application, especially, but not exclusively, bonding, laminating and mechanical means.

For all embodiments, the elongated reinforcing members may be positioned in spaced relationship along substantially the entire longitudinal extension of the blade 20 or along substantially the entire longitudinal extension of the girder or web 22, 24 or, the elongated reinforcing members may be positioned in spaced relationship along a part of the longitudinal extension of the blade. Further, the wind turbine blade may be divided into a number of sections in the longitudinal direction of the blade facilitating for example handling and transportation, and the elongated reinforcing members positioned at sections ends may divided correspondingly.

For all embodiment s of the invention, the elongated reinforcing members may be positioned in certain sections of the blade only. Particularly, but not exclusively, the elongated reinforcing members may be located at positions wherein a reinforcement of the blade is useful. A force in the flapwise direction applied to the caps between the two webs 22, 24 urges the caps towards the inner volume of the shell 30 and also urges the two connections away from each other. However, the reinforcing member keeps the two connections in substantially mutually fixed positions and thus prevents the distance between the connections from increasing thereby strengthening the blade 20 against forces in the flapwise direction. Thus, the reinforcing member 38 desirably has a high stiffness.

Preferably, the reinforcing member 38 has a straight shape, such as the shape of a rod or a stretched wire or a planar member. In the event that the shape of the reinforcing member is not straight, the shape of the reinforcing member could be straightened when subjected to stretching forces leading to movement of its end points and obviously, this is not desired.

The at least one reinforcing member may comprise a bar or a rod-like element. The element may be solid or hollow or any suitable combination thereof.

The at least one reinforcing member may comprise wire, rope, cord, thread or fibres. They 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. If suitable, glass fibres may also be used.

Further, the reinforcing member may be solid or hollow or any suitable combination thereof. The member material may comprise any of metal, metal alloy, wood, plywood, veneer, glass fibre, carbon fibre and other suitable materials such as e.g. one or more composite materials.

In embodiments, the elongated reinforcing member(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.

Although the present invention has been described in connection with the specified embodiments it should not be construed as being in any way limited to the presented examples. 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 elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. 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 not possible and advantageous.

Claims

1. A wind turbine blade comprising

a shell having a section with an aerodynamic profile, and

at least one elongated reinforcing member connected inside the shell for increasing the strength of the blade, each of the at least one reinforcing member having a first end and a second end and extending substantially in the plane of the profile chord forming an angle with the longitudinal extension of the blade in order to strengthen the blade against edgewise and flapwise forces.

2. A wind turbine blade according to claim 1 , wherein the at least one reinforcing member comprises a plurality of elongated reinforcing members extending substantially along the profile chord of the blade and positioned in spaced relationship along the longitudinal extension of the blade.

3. A wind turbine blade according to claim 1 or 2, wherein the longitudinal extension of at least one of the at least one reinforcing members is substantially perpendicular to the longitudinal extension of the blade.

4. A wind turbine blade according to claim 1 or 2, wherein the longitudinal extension of at least one of the at least one reinforcing members forms an angle with the longitudinal extension of the blade ranging from 15° to 90°.

5. A wind turbine blade according to claim 1 or 2, wherein the longitudinal extension of at least one of the at least one reinforcing members forms an angle with the longitudinal extension of the blade ranging from 25° to 90°.

6. A wind turbine blade according to claim 1 or 2, wherein the longitudinal extension of at least one of the at least one reinforcing members forms an angle with the longitudinal extension of the blade ranging from 35° to 90°.

7. A wind turbine blade according to claim 1 or 2, wherein the longitudinal extension of at least one of the at least one reinforcing members forms an angle with the longitudinal extension of the blade ranging from 45° to 90°.

8. A wind turbine blade according to any of the preceding claims, wherein one of the first end and second end of the at least one elongated reinforcing member is connected to an inner surface of the shell at one of the trailing edge and the leading edge of the blade.

9. A wind turbine blade according to claim 8, wherein the first end of the at least one elongated reinforcing member is connected to an inner surface of the shell at the trailing edge of the blade and the second end of the at least one elongated reinforcing member is connected to an inner surface of the shell at the leading edge of the blade.

10. A wind turbine blade according to any of the preceding claims, wherein the shell has a cylindrical root section for mounting of the blade on a wind turbine shaft, and wherein at least one of the at least one elongated reinforcing member is connected to the inner surface of the shell at the trailing edge in a transition region between the shell with the aerodynamic profile and the root section.

11. A wind turbine blade according to claim 10, wherein at least one of the at least one elongated reinforcing member is connected to the lower part of the shell at the trailing edge proximate the aerodynamic part of the lower part of the shell.

12. A wind turbine blade according to claim 10, wherein at least one of the at least one elongated reinforcing member is connected to the upper part of the shell at the trailing edge proximate the aerodynamic part of the upper part of the shell.

13. A wind turbine blade according to any of the preceding claims, further comprising at least one girder, and wherein the at least one elongated reinforcing member is connected to at least one of the at least one internal girder.

14. A wind turbine blade according to any of claims 1 - 12, further comprising at least one girder, and wherein the at least one elongated reinforcing member extends through at least one of the at least one internal girder.

15. A wind turbine blade according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member is a flexible wire with high tensional strength without a capability of resisting compression forces.

16. 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 steps of

positioning at least one elongated reinforcing member at an angle with the longitudinal extension of the blade inside the shell for increasing the strength of the blade, each of the at least one reinforcing member having a first end and a second end and extending substantially in the plane of the profile chord in order to strengthen the blade against edgewise and flapwise forces.

17. A method according to claim 16, wherein the step of positioning and connecting includes connecting the first end of the at least one elongated reinforcing member to the inner surface of the shell at the trailing edge of the blade.

18. A method according to claim 17, wherein the shell further has a cylindrical root section for mounting of the blade on a wind turbine shaft, and wherein the method further comprises the step of connecting the first end of at least one of the at least one elongated reinforcing member to the inner surface of the shell at the trailing edge in a transition region between the section of the shell with the aerodynamic profile and the root section.