The present invention relates to a wind turbine comprising at least one wind turbine blade comprising two shell parts defining the pressure and suction sides, a trailing edge and a leading edge, wherein at least one load carrying structure is arranged between the two shell parts and extends in a longitudinal direction, the wind turbine blade further comprises means to initiate buckling at extreme load conditions.

The present invention also relates to a method of providing a wind turbine blade of a wind turbine with means for initiating buckling in extreme load conditions.

The present invention further relates to a method of reducing compression loads in a wind turbine blade as described above. The present invention finally also relates to a method of monitoring buckling in a wind turbine as described above.

Background of the Invention

Wind turbine blades of modern wind turbines have an aerodynamically shaped body formed by a shell, wherein the shell defines a suction side surface and a pressure side surface connected by a leading edge and a trailing edge. The shell is designed to provide the high aerodynamic efficiency at the lowest load condition. A load carrying structure is arranged inside the body to provide structural stiffness to the wind turbine blade and to transfer loads to the adjoining pitch bearing or hub. The load carrying structure may comprise main laminates, spar caps, beams, shear webs, boxes, leading edge reinforcements or trailing edge reinforcements. This load carrying structure typically accounts for about half of the total weight and about half of the total manufacturing costs.

Until now, the main design goal has been to provide the wind turbine blade with suffi- cient stiffness so that the shell never buckles, not even during extreme load conditions. This can be achieved by increasing the local thickness of the main laminate at the load carrying structure or by adding core materials to the other areas of the laminate of the shell so that the core materials are sandwiched between an inner laminate and an outer laminate. The core materials may be balsa wood, polyvinyl chloride (PVC) foam or polyethylene teraphalate (PET) foam. This increases the thickness of the shell while providing a lightweight structure having an increased stiffness. The use of core mate- rials increases the buckling stability of the wind turbine blade, thus preventing the shell from buckling even at extreme loadings.

However, such core materials are relative expensive and need to be cut into very complex shapes. The core materials are also difficult to position correctly within the mould during the lay-up process, thereby adding to the total manufacturing time. Some core material, like balsa wood, sucks up a lot of resin, thus adding to the total weight and costs, and furthermore acts as a heat insulator which causes problems during the curing process. Additional quality control during the wind turbine blade production is therefore required.

It is proposed to solve some of these problems by using a frame design comprising a plurality of airfoil shaped frames or other inner supports, such as a truss structure, extending in the chordwise direction, wherein each frame is interconnected in the longitudinal direction by elongated frame elements. This frame structure is made of a relative stiff material, such as aluminium or steel, and a flexible fabric is wrapped about the frame structure. These stiff frame elements prevent the wind turbine blade from buckling in these areas and provide a very stiff frame close to the blade root, thereby reducing the risk of bolt connection failing due to bending in the bolts during operation. However, this solution requires additional testing in order to verify the ro- bustness of this solution.

Buckling can be detected by sensors during full-scale tests, wherein the wind turbine test blade is subjected to static and fatigue loadings in the flapwise and edgewise directions. During the test, the trailing edge area is monitored and buckling may be de- tected as failures in the glue lines or as delamination in the laminate of the suction or pressure side. Due to the costs of such full-scale tests, this test is normally only performed on the final wind turbine blade design. Finite Element simulations can be used to estimate the buckling stability of the blade design and identify areas in which buckling may occur. However, such simulations do not always provide a true image and are thus only used as guidelines when determining the required structural strength of the wind turbine blade.

It is known that failures may occur in the glue lines or at the material transition areas between the sandwich shell and the load carrying structure. One way to solve this problem is to add extra laminate layers at this transition area, however, this increases the local thickness and thus adds to the total weight. Alternatively, a different fibre with a lower stiffness or a change in the fibre orientation can be used in those critical areas.

US 8807953 B2 discloses a wind turbine blade with trailing edge reinforcement elements added between the load carrying structure and the trailing edge. Various reinforcement elements are disclosed, wherein these reinforcement elements are adhered or mechanical mounted to the inner or outer surfaces of the suction side and the pres- sure side. The reinforcement elements are configured to withstand tension forces generated during deformation of the wind turbine blade. Fibre reinforced plastic materials are preferably used for the reinforcement elements. It is stated that this solution increases the buckling stability of the wind turbine blade and thereby allows the local thickness of the shell to be reduced. However, these additional reinforcement elements add to the complexity and the total costs of the wind turbine blade. Secondly, this solution also adds steps to the manufacturing process and increases the total manufacturing time.

It has been proposed to add spar cap reinforcements in the longitudinal direction, such as disclosed in US 9,416,768 B2, and diagonal reinforcements in the chordwise direction to the load carrying box structure, such as disclosed in US 8,632,312 B2. It has also been proposed to add plate shaped leading or trailing edge reinforcements, wherein these reinforcement elements are connected to the load carrying box structure and to either the leading edge or the trailing edge in the longitudinal direction as disclosed in US 8,485,786 B2. These solutions allow the local thickness of the main laminate or the core materials to be reduced. However, this adds to the complexity and total costs of the wind turbine blades. It also adds extra steps to the manufacturing process, thereby increasing the manufacturing time and costs. Patent application EP2423104 discloses aspects of a wind turbine blade having internal structural elements constructed with buckling regions for buckling in the chord- wise direction. There exists a need for an improved blade design which provides a lighter and cheaper wind turbine blade and allows for a faster and cheaper manufacturing process compared to traditional wind turbine blades.

Object of the Invention

An object of this invention is to provide a wind turbine and a method that solves the above-mentioned problems.

Another object of this invention is to provide a wind turbine and a method that provides a lightweight and simple wind turbine blade configuration. A further object of this invention is to provide a wind turbine and a method that allows for a faster and cheaper manufacturing process of a wind turbine blade.

Yet another object of this invention is to provide a wind turbine and a method that allow a wind turbine blade to be outfitted with a pre-buckling pattern.

Yet further object of this invention is to provide a wind turbine and a method that provide a wind turbine blade with an integrated buckling function.

Description of the Invention

An object of the invention is achieved by a wind turbine comprising:

- a wind turbine tower,

a nacelle arranged on the wind turbine tower,

a hub rotatably connected to the nacelle and at least two wind turbine blades connected to the hub, wherein

each wind turbine blade comprises a first shell part and a second shell part, the first and second shell parts define a suction side, a pressure side, a trailing edge and a leading edge of said each wind turbine blade, at least one load carrying structure is arranged between the first and second shell parts and extends in a longitudinal direction, the leading edge and the at least one load carrying structure de- fine a leading edge area of at least one of said first and second shell parts, and the trailing edge and the at least one load carrying structure define a trailing edge area of said at least one of said first and second shell parts, characterised in that, at least one of said at least two wind turbine blades further comprises predeter- mined means configured to initiate buckling in extreme load conditions and to regain its initial shape without significant structural damages to said one wind turbine blade in a predetermined load condition, wherein said predetermined means are arranged in at least one of said first and second shell parts and extend at least in a chordwise direction.

This provides a wind turbine blade which is capable of buckling during extreme load conditions and regaining its initial shape without any significant structural damages when the high loads no longer exist. The term "buckling" here defines the ability of the wind turbine blade to deform relative to its original shape due to compression loads and to return to its original shape while substantially maintaining its structural integrity. This also provides a significant lighter and cheaper wind turbine blade compared to conventional wind turbine blades made of glass or carbon fibre material as the core material normally accounts for about 20 % of the total weight and material costs. This further saves costs for quality control during the manufacturing process while increasing the robustness of the wind turbine blade. The present configuration additionally allows for an optimal usage of the material properties compared to conventional wind turbine blades.

This configuration allows for a faster and cheaper manufacturing method as the amount of core material used can be reduced to zero, or at least to a minimum, thereby significantly reducing the layup and curing problems normally associated with the use of core materials. The present invention also allows for additional full-scale tests since the manufacturing time and costs are reduced. This configuration is suitable for any types of a variable speed wind turbine comprising two, three or more wind turbine blades. The present wind turbine blade may be a full-scale wind turbine blade or a wind turbine blade comprising an inner blade section and an outer blade section interconnected via a pitch bearing. According to one embodiment, said predetermined means are buckling patterns integrated into said at least one of said first and second shell parts.

In conventional wind turbine blades additional reinforcements, such as trailing edge reinforcements, are mounted inside the wind turbine blade for increasing the edgewise stiffness of the wind turbine blade and partly also to provide buckling stability. This increases resistance of the shell parts or trailing edge towards buckling due to edgewise or flapwise compression loads. The suction and pressure sides have a substantially smooth surface profile which may delaminate or get damaged due to sudden loads.

In the present configuration, no additional separate reinforcement elements have to be placed in the trailing edge area or the leading edge area. Instead, one or more predetermined buckling patterns are integrated into one or both shell parts. Two or more buckling patterns may be arranged in the suction and/or pressure side. The buckling patterns may extend in a longitudinal direction and/or in a chordwise direction of the wind turbine blade. The buckling patterns define areas prone to buckle during compression loads and other areas not prone to buckle, thus the buckling can be guided into well-defined areas in a controlled manner. According to a special embodiment, said buckling patterns are shaped as waves extending substantially in the longitudinal direction, wherein said waves have a predetermined wave length and a predetermined wave height.

The buckling pattern may in example, but not limited to, have a sinusoidal or triangu- lar shaped profile extending in the longitudinal direction. Any profiles capable of initiating the buckling may be used. The buckling pattern may comprise any number of waves depending on the particular dimensions of the wind turbine blade and desired location on the suction or pressure side. The tops and valleys of the waves may substantially extend in the chordwise direction, alternatively, they may be angled relative to the chordwise direction.

The waves may have a predetermined wave length and/or wave height selected based on the desired configuration of the wind turbine blade. In example, but not limited to, the wave length may be between 0.2 metres and 5 metres, e.g. between 0.5 metres and 2 metres. In example, but not limited to, the wave height may be between 2 centimetres and 15 centimetres, e.g. between 5 centimetres and 10 centimetres. This allows the shell surface to initiate and guide the buckling along the wave pattern without causing an irreversible structural failure. This also reduces the stresses in the laminate in the shell part.

Finite element (FE) analysis is used to identify areas in conventional wind turbine blades that are most likely to buckle, however, the buckling may still occur randomly within that area. During buckling in conventional wind turbine blade waves with a very short wave length and a high amplitude are formed in these areas. This significantly increases the stresses in the laminate of the shell part and the risk of an irreversible structural failure.

According to another special embodiment, said buckling patterns comprise at least one reinforced shell area and at least one non-reinforced shell area, the at least one reinforced shell area has a first shell thickness or a first stiffness and the at least one non- reinforced shell area has a second shell thickness or a second stiffness, wherein said first shell thickness or first stiffness is greater than said second shell thickness or second stiffness.

The predetermined means may also comprise a plurality of integrated shell areas with different shell thicknesses and thus different structural stiffness. Any number of first shell areas with a first shell thickness or a first stiffness and any number of second shell areas with a second shell thickness or a second stiffness may be arranged in lon- gitudinal and/or chordwise direction. The respective first and second shell thickness or stiffness may be selected dependent of the particular location on the wind turbine blade in the longitudinal direction. In example, but not limited to, the first and second shell thickness may be between 0.1 centimetres and 10 centimetres. This also allows the shell surface to initiate and guide the buckling along the wave pattern without causing an irreversible structural failure.

In example, but not limited to, the reinforced shell areas may have a greater shell thickness, e.g. a greater number of laminate layers, than the shell thickness of the non- reinforced shell areas. Alternatively, the reinforced shell areas may have a greater stiffness than the stiffness of the non-reinforced shell areas, but have the same shell thickness.

The trailing edge area and/or leading edge area may be reinforced by use of a local overlapping laminate and/or by adding extra layers to the main laminate towards that edge as described later.

According to another embodiment, said predetermined means comprises cut-outs extending from the trailing edge towards the load carrying structure, wherein each cut- out has a predetermined width measured in the longitudinal direction and a predetermined length measured in the chordwise direction.

The predetermined means may alternatively also comprise a plurality of cut-outs arranged in one or both shell parts. The cut-outs may extend from the trailing edge to- wards the load carrying structure and have a predetermined width and length. In example, but not limited to, the cut-outs have a width between 0.5 centimetres and 5 centimetres, e.g. between 1 centimetre and 3 centimetres. The distance between the individual cut-outs may be selected depending on the particular configuration of the wind turbine blade and the width of these cut-outs. In example, but not limited to, the distance may be between 0.2 metres and 2 metres, e.g. between 0.5 metres and 1 metre. These cut-outs may be combined with the above-mentioned buckling pattern, e.g. the waves or the reinforced and non-reinforced shell areas. This enables the adjacent blade sections or blocks to deform into these cut-outs during flapwise or edgewise compression loads. This also provides sufficient stiffness to support the trailing edge while preventing the trailing edge from buckling. This further restricts the buckling of the pressure or suction side to the well-defined areas and reduces the local compression loads.

The length of the individual cut-outs may be determined as function of the chord length. In example, but not limited to, the cut-outs have a length between 10% and 50% of the chord length, e.g. between 20% and 30% of the chord length, measured from the trailing edge. This allows the length to be adapted to the individual positions of the cut-outs and the desired configuration of the wind turbine blade. According to a further special embodiment, each cut-out defines a gap in said at least one of said first and second shell parts, wherein at least one flexible element is either arranged inside said gap or extends over said gap. The cut-outs may be partly or fully covered by at least one flexible element, e.g. a lip or a tape, extending along the cut-out on the suction and/or pressure side. The cut-outs on the suction side and on the pressure side may be interconnected to form a gap. The sides of this gap may be closed off to prevent moisture, dust or ice from entering the interior of the wind turbine blade. Alternatively, the at least one flexible element, e.g. a filler or glue, may be arranged in the gap and partly or fully fill up this gap. If placed in the gap, the flexible element may be a hollow or solid element. This flexible element buckles or deforms during compression loads and, thus, reduces the local compression loads in the adjacent shell parts. This also prevents an air leakage to be formed between the suction and pressure sides and, in turn, maintains the aerodynamic efficiency of the wind turbine blade.

Some conventional wind turbine blades may comprise cut-outs used to prevent the trailing edge from buckling due to edgewise compression loads, however, these wind turbine blades still require the use of core materials to provide sufficient buckling sta- bility.

According to one embodiment, the one wind turbine blade has a chord length measured between the trailing edge and the leading edge and a local shell thickness as function of the chord length, wherein at least 5% of said at least one of said first and second shell parts along the chord length has a local shell thickness to chord length ratio equal to or less than 0.03.

The present configuration enables the shell thickness to be significantly reduced, thereby also significantly lowering the shell thickness to the chord length ratio, com- pared to conventional sandwich shells using core materials. In example, but not limited to, the wind turbine blade may, along at least 5 %, e.g. between 5 % to 70 %, e.g. 25 % and 50 %, of the chord length, have a local shell thickness to chord length ratio of no more than 0.03, e.g. between 0.001 and 0.03, e.g. between 0.01 and 0.02. This provides an optimal configuration allowing the wind turbine blade to buckle under high load conditions while maintaining a stable aerodynamic shape under low load conditions.

In example, a conventional wind turbine blade may have local shell thickness (incl. core material) of about 30 millimetres to 60 millimetres at a chord length of about 3 metres to 4 metres. A wind turbine blade according to the present configuration may at this inner position, but not limited to, have a local shell thickness of about 2 millimetres to 7 millimetres. In example, a conventional wind turbine blade may have a local shell thickness (incl. core material) of about 10 millimetres to 20 millimetres at a chord length of about 1 metre. A wind turbine blade according to the present configuration may at this outer position, but not limited to, have a local shell thickness of about 2 millimetres to 7 millimetres.

According to one embodiment, the one wind turbine blade has a local heat transfer coefficient, a U-value, as function of the chord length, wherein at least 5% of said at least one of said first and second shell parts along the chord length has a U-value equal to or greater than 0.1 W/(m ² · K).

The present configuration also enables the heat transfer coefficient (also called U- value) to be significantly increased compared to conventional sandwich shells using core materials. The core material used in conventional sandwich shells has a high thermal insulating effect and, thus, a negative effect on the heating process during curing. Thus, a desire to eliminate the use of core material exists.

In example, but not limited to, the wind turbine blade may, along at least 5 %, e.g. between 5 % to 70 %, e.g. between 25 % to 50 %, of the chord length, have a U-value of no less than 0.1 W/(m ² · K), e.g. between 0.1 W/(m ² · K) and 10 W/(m ² · K), be- tween 1 W/(m ² · K) and 5 W/(m ² · K). This allows for a fast heating process during curing and, thus, a significantly reduced curing time. This also allows for an accurate heat control due to the relative thin shell compared to conventional sandwich shells. The local heat transfer coefficient can be determined, but not limited to, using in-situ measurements, e.g. thermography, multiple temperature measurements or heat flux measurements. Other known techniques may be used to determine the local heat transfer coefficient.

According to one embodiment, said predetermined load condition comprises wind speeds equal to or above 6 ^m / _s .

The predetermined means, i.e. the buckling pattern, the reinforced and non-reinforced areas or the cut-outs, may be configured so that they are activated, e.g. initiate buckling, at predetermined load conditions and/or at predetermined wind speeds. In one example, the means may be activated when the wind turbine blade experiences maximum edgewise compression loads, e.g. in the 9 o'clock position or 3 o'clock position dependent on the rotational direction of the rotor.

In another example, the predetermined means may be activated at wind speeds of 6 ^m / _s to 10 ^m / _s , e.g. 7 ^m / _s , 8 ^m / _s or 9 ^m / _s , or greater. This is particularly suited for low wind speed applications, i.e. wind turbine blades designed for low wind speed areas. Alternatively, the predetermined means may be configured to function at all wind speeds. This is particularly suited if the means are configured so that the buckling has very little or no effect of the aerodynamic performance of the wind turbine blade.

According to one embodiment, a local laminate overlaps the leading edge and/or the trailing edge and extends into the leading edge area or trailing edge area.

The load carrying structure and the leading edge defines a leading edge area on the first and second shell parts and thus the suction and pressure sides. Furthermore, the load carrying structure and the trailing edge define a trailing edge area on the first and second shell parts and, thus, the suction and pressure sides. One or both shell parts may in the above-mentioned areas have a single laminate as described later.

In some conventional wind turbine blades, the trailing edge is reinforced by placing a solid trailing edge element, e.g. of wood or metal, between the trailing edges of the two shell parts and joining them together using adhesive or bolts. Such solid trailing elements are used in combination with sandwich shells. However, the transition area between the solid trailing edge element and the adjoining shell parts are likely to fail structurally due to extreme compression loads or environmental impacts.

The trailing edge and/or the leading edge may be reinforced by overlapping the respective edge with two or more layers during the layup process, thus forming an overlapping laminate. This overlapping laminate may form part of the laminate of the trailing edge area or the leading edge area, thus forming a continuous laminate. Alterna- tively, the overlapping laminate may be a local laminate which is jointed with the laminates of both the suction and pressure sides. The transition of one layer in the two laminates at this joint may be offset relative to an adjacent layer, thereby providing a stronger joint. This increases resistance of the trailing edge or leasing edge towards buckling during compression loads, thus reducing the flapwise movement during buckling. This also allows for a more even distribution of the leading edge reinforcement, e.g. the local laminate, compared to conventional leading edge reinforcements, e.g. shear webs, core materials or add-ons.

In an exemplary configuration, the leading edge and, optionally, the leading edge area may be reinforced by adding more layers to the laminate and/or the overlapping laminate while the trailing edge and, optionally, the trailing edge area may be configured to buckle during flapwise or edgewise compression loads. Thus, only the trailing edge area may be capable of buckling during high load conditions. A flexible structural adhesive, e.g. a MMA glue by the tradename Plexus ^® , may be used to bond the first and second shell parts together at the trailing edge and/or the leading edge. This allows the glue line to deform when compressed, thereby reducing stresses in the glue line due to buckling. After curing, the trailing edge and/or the leading edge may then be grinded, polished or otherwise worked into the finished shape after which a protective coating may be applied. Alternatively, the protective coating or layer may be applied during layup process. This allows the trailing and leading edges to obtain their desired shapes. The trailing edge may have a local thickness of about 10 millimetres to 30 millimetres. One or more trailing edge extenders may be mounted to or integrated into the original trailing edge at predetermined positions along the longitudinal direction. The trailing edge extenders may have a noise reducing profile, thus allowing noises to be reduced.

According to one embodiment, the at least one load carrying structure comprises a first main laminate located between the trailing edge area and the leading edge area of the first shell part and a second main laminate located between the trailing edge area and the leading edge area of the second shell part, wherein one of the first and second main laminates have a width measured in the chordwise direction which is greater than the width of the other main laminate.

The load carrying structure may comprise a first laminate arranged in the first shell part and a second main laminate arranged in the second shell part. These two main laminates may be interconnected by one or more shear webs, I-beams or boxes. The shear webs, I-beams or boxes may be jointed to the respective main laminate by an adhesive, e.g. a flexible structural adhesive as mentioned above. Trailing and/or leading edge reinforcements may be integrated into the first and/or second main laminate by extending the width of the respective main laminate. Both main laminates may be extended equally into the trailing edge area and/or the leading edge area. Alternatively, one main laminate may extend further into the trailing edge area and/or the leading edge area than the other main laminate. This increases the buckling stability of the load carrying structure and increases the resistance of the trailing edge or the leading edge towards buckling.

According to one embodiment, at least one of the leading edge area and the trailing edge area comprises a single laminate having at least two layers of a fibre reinforced material.

Conventional sandwich shells consist of two separate laminates between which a core material is located. Each of these laminates typically comprises three layers of a glass or carbon fibre reinforced material where the fibres are oriented at +45 degrees and - 45 degrees angles.

In the present configuration, the first and/or second shell parts in the trailing edge area and/or the leading edge area may comprise a single laminate comprising a plurality of layers of the same material. The material may be a reinforced plastic material with unidirectional fibres, chopped stand mat (e.g. CSM) or continuous filament mat (e.g. Unifilo ^® ). This provides a thick shell capable of buckling and retaining its original shape without significant structural damages. This also reduces the total costs and weight of the wind turbine blade.

Optionally, the material and/or resin used may be selected so that, when cured, the shrinkage of the resin may pre-tension the material and, thus, postpone the buckling into a higher load condition and/or wind speed.

This single laminate may have a relatively constant shell thickness along the chord length. The shell thickness may be between 2 millimetres and 7 millimetres as described above. This is particularly suited for wind turbine blades with a longitudinal length of at least 50 metres.

According to one embodiment, the at least one load carrying structure has a third shell thickness measured towards the leading edge area and a fourth shell thickness measured towards the trailing edge area, the one wind turbine blade further has a fifth shell thickness measured at or near the trailing edge or leading edge, wherein said fourth shell thickness is greater than said third shell thickness or said fifth shell thickness.

Trailing and/or leading edge reinforcements may also be integrated into the first and/or second main laminate by adding extra layers at one end towards the trailing edge area or the leading edge area. Alternatively, additional layers may be added at both ends to increase both the third and fourth thickness of the respective main laminate. This allows the main laminate to have different local shell thickness and/or the main laminate to have a greater shell thickness than the shell thickness at or near the trailing or leading edge. According to one embodiment, the one wind turbine blade further comprises a predetermined number of stiffening elements having a stiffness greater than the stiffness of said first or second shell part, wherein a first of said stiffening elements is located between 30 centimetres and 90 centimetres from a blade root of the one wind turbine blade.

The wind turbine blade has a longitudinal length of at least 35 metres, e.g. at least 50 metres, corresponding to a relative length of 1. The predetermined means may be located at a relative length between 0.25 and 1 measured from the blade root, i.e. in an outer blade section. This allows for an optimal placement of the buckling pattern relative to the edgewise and/or flapwise compression loads in the wind turbine blade.

The wind turbine blade may further comprise one or more stiffening elements configured to further increase the buckling stability of the wind turbine blade. Any number of stiffening elements may be used. The stiffening elements may be located at a relative length between 0 and 0.25 measured from the blade root, i.e. in an inner blade section. The stiffening elements may be configured as bulkheads, stringers or frames and/or have a stiffness significantly greater than the stiffness of the single laminate of the shell part. The stiffening elements may be made of a stiff material, such as steel, aluminium or fibre reinforced materials (e.g. GFRP or CFRP), or a composite material. This stiff material may have a Youngs modulus of 50 GPa or higher. This eliminates the need for core materials in that blade section and reduces the risk of a bolt connection failure. A first stiffening element may be positioned between 30 centimetres and 90 centimetres, e.g. between 50 centimetres and 70 centimetres, from the blade root. The remaining stiffening elements may be positioned towards to the outer blade section. The individual stiffening elements may be spaced apart by 0.5 metres to 2.5 metres, e.g. 1 meter to 2 metres. This reduces the risk of the bolt connection bending due to ovalisa- tion of the blade root area and reduces the risk of the inner blade buckling during operation.

In conventional wind turbine blades, this ovalisation problem is solved by adding extra core materials as well as extra laminate layers in the shell parts and in the load car- rying structure. This increases the shell thickness as well as the thickness of the shear web or box beam, however, this also adds to the total cost and weight of the wind turbine blade. Another object of the invention is achieved by a method of providing a wind turbine blade of a wind turbine with means for initiating buckling in extreme load conditions, the method comprises the steps of:

providing a wind turbine blade with a first shell part and a second shell part, the first and second shell parts define a suction side, a pressure side, a trailing edge and a leading edge of said each wind turbine blade, at least one load carrying structure is arranged between the first and second shell parts and extends in a longitudinal direction, the leading edge and the at least one load carrying structure define a leading edge area of at least one of said first and second shell parts, and the trailing edge and the at least one load carrying structure define a trailing edge area of said at least one of said first and second shell parts,

placing said wind turbine blade in a stand, the stand comprises at least one actuator unit for applying a load to the wind turbine blade,

applying a static load to the wind turbine blade via said at least one actuator unit, shaping at least one pre-buckling pattern in at least one of the first and second shell parts by further applying static load to the wind turbine blade until buckling is initiated in said at least one of the first and second shell parts,

removing said static load via the at least one actuator unit,

removing the wind turbine blade from the stand. This provides a simple method of outfitting the wind turbine blade with pre-buckling patterns after the manufacturing process. Alternatively, the means described above can be integrated into the wind turbine during the manufacturing process, e.g. the lay-up process. The term "pre-buckling pattern" is defined as a pattern which alters the initial stiffness and, thus, changes the structural integrity of the wind turbine blade without causing any irreversible or visible structural damages. This method provides a wind turbine blade capable of buckling in extreme load conditions and regaining its initial shape without any significant structural damages when the high loads no longer exist. The wind turbine blade can be manufactured using known manufacturing techniques and, optionally, cut-outs can be formed in the wind turbine blade after completing the manufacturing process. The wind turbine blade are then loaded into a stand and suitably secured. The wind turbine blade may be placed in one or more receiving elements, e.g. support pads or plates. The wind turbine blade, e.g. the blade root, may be secured to the stand, e.g. via a mounting or support frame or mounting brackets, so that the wind turbine blade is held in place during loading.

One or more actuator units, e.g. linear actuators, may be connected to the wind turbine blade via the receiving elements and/or the frame or bracket mentioned above. A first set of actuator units may be used to apply edgewise loads to the wind turbine blade and at least a second set of actuator units may be used to apply at least flapwise loads. The actuator units may be driven using any suitable energy source, e.g. a hydraulic energy source, and operated via any suitable control unit, e.g. a control panel. Other types of means, e.g. a winch, may be used to apply loading to the wind turbine blade.

One or more static loads are applied to the wind turbine blade via the actuator units in the edgewise and/or flapwise direction until buckling is initiated. The loading may be applied in one or both directions so buckling is initiated in one or both shell parts and/or in one or both edges of the wind turbine blade. The wind turbine blade is loaded up to a predetermined level so that no irreversible structural damages occur in the wind turbine blades.

The loading is then stopped and the wind turbine blade is returned to its initial shape by removing the loading. This buckling creates a pre-buckling pattern in one or both shell parts which will initiate and guide any future buckling along this pattern in extreme load conditions as described above. The buckling alters the stiffness of the shell part, e.g. the single laminate, along the created pattern relative to the stiffness of the adjacent shell areas. In example, but not limited to, said pattern may be created by breaking or weakening a portion of the fibres in the material of the laminate. This allows the wind turbine blade, e.g. the shell parts, the trailing edge and the leading edge, to substantially maintain its structural integrity without causing any irreversible structural damages.

The wind turbine blade is finally removed from the stand. The wind turbine blade can subsequently be transported and installed at the erection site, or be partly or fully assembled with the hub and then transported to the erection site.

A further object of the invention is achieved by a method of reducing loads in a wind turbine blade of a wind turbine, the wind turbine comprises a wind turbine tower, a nacelle arranged on the wind turbine tower, a hub rotatably connected to the nacelle and at least two wind turbine blades connected to the hub, wherein each wind turbine blade comprises a first shell part and a second shell part, the first and second shell parts define a suction side, a pressure side, a trailing edge and a leading edge of said each wind turbine blade, at least one load carrying structure is arranged between the first and second shell parts and extends in a longitudinal direction, the leading edge and the at least one load carrying structure define a leading edge area of at least one of said first and second shell parts, and the trailing edge and the at least one load carrying structure define a trailing edge area of said at least one of said first and second shell parts, the method comprises the steps of:

- rotating one of said at least two wind turbine blades in one rotational direction into a first rotational position, wherein the trailing edge is subjected to a minimum compression load,

further rotating the one wind turbine in the one rotational direction into a second rotational position, wherein the trailing edge is subjected to a maximum compres- sion load,

characterised in that, the method further comprises the step of:

decreasing the aerodynamic efficiency of the one wind turbine blade when rotating into the second rotational position by activating, e.g. passively, the predetermined means in said one wind turbine blade, wherein the predetermined means initiates buckling in the at least one of said first and second shell parts.

This provides a wind turbine blade which is capable of buckling during extreme load conditions and regaining its initial shape without any significant structural damages when the high loads no longer exist as described above. This allows the buckling to be used to alter the aerodynamic performance of the wind turbine blade in a controlled manner. This also allows a significant lighter and cheaper wind turbine blade and a more optimal usage of the material properties compared to conventional wind turbine blades made of glass or carbon fibre material as the amount of core material normally used can be omitted or at least reduced to a minimum.

The means, i.e. the buckling pattern, the reinforced and non-reinforced areas or the cut-outs, described above may be passively activated at predetermined load conditions and/or at predetermined wind speeds. The means may be activated when the wind speed exceeds the rated wind speed, e.g. of 6 metres per second [ ^m / _s ] to 10 ^m / _s . This allows the wind turbine blade to start buckling when the maximum or nominal power output is reached.

Alternatively, these means may be activated when the wind turbine blade, e.g. the trailing edge, experiences maximum edgewise compression loads which occurs due to gravity when the trailing edge is facing the ground or sea level. The wind turbine blade experiences minimum edgewise compression loads when the trailing edge is facing away from the ground or sea level. The means may thus be activated due to gravity when the wind turbine blade is rotated into the 9 o'clock position or the 3 o'clock position dependent on the rotational direction of the rotor. This allows the wind turbine blade to start buckling when the compression load of the trailing edge exceeds a predetermined threshold.

The means are specifically configured so that, when buckling, the aerodynamic shape of the wind turbine blade is altered and, thus, the initial chamber line is changed. This decreases the aerodynamic efficiency and reduces the compression loads of the wind turbine blade. The wind turbine blade is then operated at a different angle-of-attack. This also reduces the maximum noise generated by the wind turbine blade. When the wind turbine blade rotates away from this position, the means may be deactivated. The shell parts return to their initial aerodynamic shape and, thus, the chamber line returns to its initial position. This increases the aerodynamic efficiency and increases the compression loads of the wind turbine blade. The wind turbine blade is then operated at its initial angle-of-attack. These means can thus be used as a passive load reduction method so that the wind turbine blade experiences substantially the same compression loads and noise level in the 9 o'clock position and the 3 o'clock position.

According to one embodiment, the method further comprises the step of:

- providing a symmetrical aerodynamic profile, wherein the first and second shell parts are arranged symmetrically relative to a chord of the one wind turbine blade.

The first and second shell parts may form a symmetrical aerodynamic profile in the chordwise direction. Optionally, the original trailing edge may be extended by mount- ing or integrating a trailing edge extender. The trailing edge extender may extend along up to 70 % of the longitudinal length. Alternatively or additionally, the trailing edge extender may define up to 60% of the total chord length while the remaining chord length may be defined by the wind turbine blade. This reduces the flapwise compression loads to a minimum and reduces the noise. This also significantly reduc- es the flapwise bending so that the buckling is substantially driven by the trailing edge compression.

This configuration may allow the wind turbine blade and, thus, the rotor to be placed in a tilted position relative to the longitudinal direction of the wind turbine tower. Conventional wind turbine blades are normally placed in a tilt angle of 4 to 5 degrees in order to increases the tip-to-tower clearance so that the tip end does not hint the wind turbine tower during rotation. Conventional wind turbine blades are normally also oversized in regards to actual requirements. This is done in order to keep the load unbalance and, thus, the fatigue loads within acceptable tolerances.

The present configuration enables the wind turbine blade to be placed in a tilt angle of greater than 4 to 5 degrees since the buckling can be used to balance the compression loads and reduce the fatigue loads. The wind turbine blade of the present invention may be placed in a tilt angle of up to and equal to 15 degrees. The wind turbine blade may thus be shaped as straight blades, or even back-swept blades, i.e. the tip end is bend backwards the wind turbine tower. This reduces the additional manufacturing costs and transport costs normally associated with forward-swept wind turbine blades. The buckling may optionally be used to negatively or positively twist the wind turbine blade away from a stall situation. In example, the wind turbine blade may be twisted positively 5 degrees to 10 degrees or negatively 2 degrees to 3 degrees due to the buckling. This allows the wind turbine blade to achieve maximum load reduction.

Yet another object of the invention is achieved by a method of monitoring buckling in a wind turbine blade of a wind turbine, the wind turbine comprises a wind turbine tower, a nacelle arranged on the wind turbine tower, a hub rotatably connected to the nacelle and at least two wind turbine blades connected to the hub, wherein each wind turbine blade comprises a first shell part and a second shell part, the first and second shell parts define a suction side, a pressure side, a trailing edge, and a leading edge of said each wind turbine blade, at least one load carrying structure is arranged between the first and second shell parts and extends in a longitudinal direction, the leading edge and the at least one load carrying structure define a leading edge area of at least one of said first and second shell parts, and the trailing edge and the at least one load carrying structure define a trailing edge area of said at least one of said first and second shell parts, characterised in that, the method further comprises the steps of:

arranging at least one measuring unit inside one of said at least two wind turbine blades,

- measuring at least one signal using the at least one measuring unit, the at least one signal is indicative of buckling of the leading edge area or the trailing edge area, determining a buckling signal based on said at least one signal,

comparing the buckling signal to a predetermined threshold,

generating an event signal if the buckling signal exceeds the predetermined threshold,

optionally, adjusting the operation of the wind turbine based on said event signal by applying at least one corrective action, wherein the at least one corrective action reduces compression loads in the one wind turbine blade. This provides a simple and easy method of monitoring the effect of buckling in the wind turbine blade as described above. The wind turbine blade may be outfitted with any suitable sensors or measuring units for detecting buckling in the trailing edge area and/or leading edge area. The signal from these sensors or measuring units may be used as an input for a local control unit or a remote control unit which then uses this signal to determine a buckling signal indicative of the current buckling. The control unit may then compare this buckling signal to a predetermined threshold or tolerance range. If the buckling signal exceeds the threshold or is outside the tolerance range, then an event signal is generated indicating that the wind turbine is about to buckle.

This event signal may be used to simply alert or inform the operator that corrective actions are required. The operator may then apply one or more corrective actions via the remote control unit to the wind turbine so that the load conditions are reduced to acceptable tolerances. Alternatively, the event signal may be used by the local control unit to automatically apply one or more corrective actions. The corrective action may in example, but not limited to, be adjustment of the pitch angle, reduction of the rotor speed, reduction of the power set-point, reduction of the rotor thrust or any other suitable correction action. Buckling may be detected as movement or deformation in at least one direction relative to a reference position, wherein this relative movement or deformation can be registered or picked up by the measuring unit.

According to one embodiment, said at least one measuring unit is integrated into the first or second shell part or arranged on the at least one load carrying structure inside the one wind turbine blade, wherein the at least one measuring unit captures an image of the first or second shell part or measures a refractive signal from the first or second shell part. The sensors or measuring units may be mounted to or integrated into the wind turbine blade during or after manufacturing. The sensors or measuring units may be positioned relative to the trailing edge area and/or the leading edge area in order to directly or indirectly detect the buckling. Deflection sensors, e.g. strain gauges or optical fibres, may be integrated into the laminate of the respective shell parts or mounted to an inner surface of the laminate. Vibration sensors, e.g. accelerometers, may also be used to detect buckling. These vibration sensors may be positioned inside the wind turbine blade or the nacelle. The vibra- tion signal thereof may thus be suitable analysed and evaluated in order to determine the buckling of the wind turbine blade.

Distance measuring units, e.g. laser or ultrasonic units, may be positioned at the load carrying structure to provide a substantially stable platform. The distance measuring unit may comprise a transmitter configured to transmit an electromagnetic signal, e.g. a laser beam or an ultrasonic signal, towards the respective shell parts. The distance measuring unit may further comprise a receiver configured to receive a refractive signal, e.g. from one or more reflector on the shell part.

Image capturing units, e.g. cameras, may alternatively be positioned on the load carrying structure. Alternatively, the image capturing unit may be positioned on the nacelle or wind turbine tower and may face the passing wind turbine blade. The image capturing unit may then capture images of the trailing edge area and/or the leading edge area which are then suitable processed by the control unit in order to determine the buckling.

Description of the Drawing

The invention is described by example only and with reference to the drawings, wherein:

Fig. 1 shows an exemplary embodiment of a wind turbine according to the invention,

Fig. 2 shows a conventional wind turbine blade,

Fig. 3 shows an exemplary embodiment of a wind turbine blade according to the invention,

Fig. 4 shows a first embodiment of the wind turbine blade with measuring units for measuring buckling,

Fig. 5 shows a second embodiment of the wind turbine blade with varying shell thickness,

Fig. 6a-b show a third embodiment of the wind turbine blade with a local laminate overlapping the trailing edge and the leading edge,

Fig. 7 shows a fourth embodiment of the wind turbine blade with varying widths of the main laminates, Fig. 8 shows a fifth embodiment of the wind turbine blade with a buckling pattern, Fig. 9 shows a first exemplary embodiment of the buckling pattern,

Fig. 10 shows a second exemplary embodiment of the buckling pattern,

Fig. 11 shows a sixth embodiment of the wind turbine blade with cut-outs and a flexible element,

Fig. 12 shows a seventh embodiment of the wind turbine blade with stiffening elements,

Fig. 13 shows the wind turbine blade placed in a stand,

Fig. 14 shows the wind turbine blade with an initial chamber line and an altered chamber line, and

Fig. 15 shows an exemplary graph of the compression loads of the trailing edge as function of the rotational position.

In the following text, the figures will be described one by one, and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

Position number list

I . Wind turbine

2. Wind turbine tower

3. Nacelle

4. Wind direction

5. Hub

6. Wind turbine blade

7. Remote control unit

8. Conventional wind turbine blade

9. First shell part

10. Second shell part

I I . Leading edge

12. Trailing edge 13. Load carrying structure

14. Shear web

15. Main laminate

16. Leading edge area

17. Trailing edge area

18. Measuring unit, distance measuring unit or camera

19. Measuring unit, deflection sensor

20. Local laminate, leading edge

21. Local laminate, trailing edge

22. Single laminate

23. Glue

24. Buckling pattern

25. Blade root

26. Tip end

27. Cut-out

28. Flexible element

29. Stiffening elements

30. Stand

31. Mounting bracket

32. Actuator unit

33. Maximum compression load

34. Minimum compression load

35. AoA (Angle of Attack) Detailed Description of the Invention

Fig. 1 shows a wind turbine 1 comprising a wind turbine tower 2 and a nacelle 3 arranged on top of the wind turbine tower 2. The nacelle 3 is connected to a yaw system for yawing the nacelle 3 relative to a wind direction 4. A rotatable hub 5 is arranged relative to the nacelle 3 and at least two wind turbine blades 6 are connected to the hub 5. Here, three wind turbine blades are shown. A rotation shaft is connected the hub 6 and to a drive train for producing an electrical power output.

The wind turbine 1 further comprises a local control unit capable of communicating with a remote control unit 7 via a communications link, e.g. a SCADA system. The remote control unit 7 is configured to receive and monitor the operation of the wind turbine 1 and, optionally, regulate the operation by transmitting one or more control signals to the local control unit.

Fig. 2 shows a cross section of a conventional wind turbine blade 8 where a first shell part 9 defines the suction side and a second shell part 10 defines the pressure side. The two shell parts 9, 10 together define a leading edge 11 and a trailing edge 12.

The wind turbine blade 8 comprises a load carrying structure 13 in the form of a shear web and two spar caps with increased thickness for providing structural strength and buckling stability. The two shell parts 9, 10 have a sandwich structure with an enclosed core material for preventing buckling of the pressure and suction sides and re- si sting buckling of the trailing edge 12.

Fig. 3 shows an exemplary embodiment of the wind turbine blade 6 wherein the shell parts 9', 10' is formed by a single laminate having at least two layers of a fibre reinforced material. The single laminate has a shell thickness T of 2 millimetres to 7 mil- limetres while the sandwich structure of fig. 2 has a significant greater shell thickness.

The wind turbine blade 6 has a load carrying structure 13' comprises at least one shear web 14 or a box beam (shown in fig. 5) extending between the two shell parts 9', 10' and a main laminate 15 integrated in both shell parts 9', 10'. Here, two shear webs 14 are shown. The load carrying structure 13' and the leading edge 11 define a leading edge area 16. The load carrying structure 13' and the trailing edge 12 define a trailing edge area 17. The two shell parts 9', 10' have at a predetermined position along the longitudinal length a local shell thickness to chord length ratio measured along the chord length. The shell part 9', 10' has, along 5% to 70% of the chord C, a local shell thickness to chord length ratio of 0.001 to 0.03. The two shell parts 9', 10' further have at a prede- termined position along the longitudinal length a local heat transfer coefficient, a U- value, determined as function of the chord length. The shell part 9', 10' has, along 5% to 70% of the chord, a U-value of 0.1 W/(m ² · K) to 10 W/(m ² · K). This enables the wind turbine blade 6 and, thus, the shell parts 9', 10' to buckle during extreme load conditions.

Fig. 4 shows a first embodiment of the wind turbine blade 6 with measuring units 18, 19 for measuring the buckling of the shell part 9', 10'.

The measuring unit 18 in form of a distance measuring unit or camera can be mounted on the load carrying structure 13' for measuring the relative movement (indicated by dotted lines) of the trailing edge area 17 and, optionally, the trailing edge 12. The measuring unit 19 in form of a deflection sensor can be integrated into the single laminate of the shell part 9', 10' for measuring the relative movement of the trailing edge area 17 and, optionally, the trailing edge 12.

The measuring units 18, 19 are connected to the local control unit which determines a buckling signal based on the signals from the measuring units 18, 19. The buckling signal is optionally compared to a predetermined threshold indicative of a buckling range. The local control unit then adapts the operation of the wind turbine 1 and, thus, the wind turbine blade 6 as function of this buckling signal, or if the buckling signal exceeds the predetermined threshold.

Fig. 5 shows a second embodiment of the wind turbine blade 6 with varying shell thickness T. The main laminate 15' of the load carrying structure 13' has a shell thickness Ti measured towards the leading edge area 16 and a shell thickness T ₂ measured towards the trailing edge area 17. The shell part 9', 10' further has a shell thickness T ₃ measured at or near the trailing edge 12. The main laminate 15' is reinforced by adding extra layers to the laminate towards at least one end, e.g. the trailing edge 12. The shell thickness T ₂ is thus increased compared to at least the shell thickness T ₃ . This increases the resistance of the main laminate 15' towards buckling while allowing the corresponding edge, e.g. the trailing edge 12, to buckle.

Figs. 6a-b show a third embodiment of the wind turbine blade 6 with a local laminate 20, 21 overlapping the trailing edge 12' and the leading edge I V . The leading edge 1 1 ' is reinforced by arranging a local laminate 20 over the initial leading edge of the shell part 9', 10' . The trailing edge 12' is also reinforced by arranging a local laminate 21 over the initial trailing edge of the shell part 9', 10' . Here, both edges 1 1 ', 12' are reinforced as indicated in fig. 6a, however, only one edge can be reinforced. This increases the resistance of the leading edge 1 1 ' and the trailing edge 12' towards buckling.

The transition between the individual layers in the single laminate 22 and in the local laminate 20, 21 are offset relative to each other as indicated in fig. 6b. A suitable glue 23, e.g. flexible structural adhesive, is used to bond to two shell parts 9', 10' together. Fig. 7 shows a fourth embodiment of the wind turbine blade 6 with varying widths of the main laminate 15. The first part 9' comprises a first main laminate 15" having a first width Wi measured in the chordwise direction. The second part 10' comprises a second main laminate 15" ' having a second width W ₂ measured in the chordwise direction.

The main laminates 15 are here reinforced by extending the width of one main laminate, e.g. the first main laminate 15", relative to the other main laminate, e.g. the second main laminate 15" ' . Here, the first width Wi is greater than the second width W ₂ , however, the second width W ₂ , can also be greater than the first width Wi. This in- creases the resistance of the pressure or suction side towards buckling.

Fig. 8 shows a fifth embodiment of the wind turbine blade 6 with an integrated buckling pattern 24 extending in the chordwise and lengthwise directions of the wind tur- bine blade 6. The buckling pattern 24 is located at a relative length of 0.25 to 1 measured from the blade root 25 towards the tip end 26.

Fig. 9 shows a first exemplary embodiment of the buckling pattern 24 seen along the A-A line marked in fig. 9. Here, the buckling pattern 24 is defined by waves extending in the longitudinal direction where the wave tops extend in the chordwise direction. The waves has a wave length Li of 0.2 metres to 5 metres and a wave height H of 2 centimetres to 15 centimetres. The buckling is thus guided along these soft waves which will regain their initial shape without significant structural damage.

Fig. 10 shows a second exemplary embodiment of the buckling pattern seen along the A-A line marked in fig. 9. Here, the buckling pattern 24 is defined by a plurality of non-reinforced shell areas and reinforced shell areas. The reinforced and non- reinforced areas are arranged in a predetermined order. This also guides the buckling along these areas which will regain their initial shape without significant structural damage.

The areas can be reinforced by adding more layers to the single laminate 22 and, thus, increasing the shell thickness T ₅ compared to the shell thickness T ₄ of the non- reinforced areas as indicated in fig. 10. Instead of increasing the shell thickness, the stiffness of these areas can be increased.

Fig. 11 shows a sixth embodiment of the wind turbine blade 6 with cut-outs 27 arranged in the trailing edge 12. Here, four cut-outs are shown, but any number of cut- outs can be arranged along the trailing edge 12.

Each cut-out 27 has a width W ₃ measuring in the chordwise direction which is determined as function of the chord C. Each cut-out 27 further has a length L ₂ measured in the longitudinal direction. The cut-outs 27 has a length L ₂ of 0.5 centimetres to 5 cen- timetres. The buckling may thus be limited to the surface areas between these cut-outs 27.

A flexible element 28 can be arranged in the gap formed by the cut-out 27 and fully fill up this gap. The flexible element 28' can also be arranged at the pressure and sue- tion side and extend over this gap so that the gap is closed off. This allows the wind turbine blade 6 to maintain its aerodynamic performance.

Fig. 12 shows a seventh embodiment of the wind turbine blade 6 with stiffening ele- ments 29 arranged towards the blade root 25. The stiffening elements 29 have a stiffness significantly greater than the stiffness of the single laminate 22. Here, the stiffening elements 29 are shaped as bulkheads for increasing the resistance towards buckling and ovalisation. Fig. 13 shows the wind turbine blade placed in a stand 30 configured to shape or create a pre-buckling pattern in the wind turbine blade 6. The stand 30 comprises a mounting bracket 31 for securing the wind turbine blade 6 to the stand. Other receiving elements are optionally used to further secure the wind turbine blade 6. At least one set of actuator units 32 is used to apply a static load to the wind turbine blade 6.

Once the wind turbine blade 6 is secured, the stand 30 is operated to shape a pre- buckling pattern in the wind turbine blade 6 by moving the actuator units 32 in edgewise and/or flapwise direction (indicated by arrow). After the pre-buckling pattern is formed, the actuator units 32 and, thus, the wind turbine blade 6 are returned to their initial positions. The wind turbine blade 6 is finally demounted and removed from the stand 30.

Fig. 14-15 show the wind turbine blade 6 wherein the buckling is used to passively reduce compression loads during rotation. When the wind turbine blade 6 is rotated in the anti-clockwise direction, the compression load of trailing edge 12 varies due to gravity between a maximum compression load 33 and a minimum compression load 34 as indicated in fig. 15. The x-axis of the graph in fig. 15 defines the rotational angle of the wind turbine blade while the y-axis defines the compression loads. The point 0.0 indicates the compression loads when the tip end 26 of the wind turbine blade 6 is placed in the uppermost position, i.e. in the 12 o'clock position.

The bucking pattern 24 and, optionally, the cut-outs 27 are configured to be activated at a predetermined wind speed or at a predetermined compression load. When these means, e.g. the buckling pattern 24 and the cut-outs 27, are not activated, the shell parts 9', 10' have their original shape. The wind turbine blade 6 thus has an initial chamber line CL and is operated according to an initial AoA 33 as indicated in fig. 14.

When the wind speed or the compression load exceeds this activation threshold, e.g. when rotated in the 9 o ^' clock position, then the means is activated and the wind turbine blade 6 starts to buckle. This alters the aerodynamic shape and efficiency of the shell parts 9", 10" as indicated in fig. 14 and, thus, alters the chamber line CL' of the wind turbine blade. This further alters the AoA 33' of the wind turbine blade 6 and thereby reduces the maximum compression load 33.

As the wind turbine blade 6 is rotated further in the same direction, then the means is deactivated and the shell parts 9', 10' regain their initial aerodynamic shape and efficiency. This further causes the AoA 33 to return its initial value.