Pre-bent wind turbine blade optimized for aeroelastic stability

Abstract

A pre-bent wind turbine blade extending along a longitudinal path from a root to a tip, the wind turbine blade comprising a root region and an airfoil region having a tip, the wind turbine blade comprising a chord line extending between a leading edge and a trailing edge, the wind turbine blade comprising an aerodynamic outer blade surface, the aerodynamic outer blade surface comprises a pressure side and a suction side, the airfoil region comprises a plurality of longitudinal pre-bending regions in which the wind turbine blade is pre-bent in a shimmy direction, a first pre-bending region of the plurality of longitudinal pre-bending regions is arranged closer to the root than a second longitudinal pre-bending region, and a second pre-bending region of the plurality of longitudinal pre-bending regions is arranged closer to the root than the second pre-bending region. The longitudinal path in the second pre-bending region includes a kink.

Description

Technical Field

[0001] The present disclosure relates to pre-bent wind turbine blades, wherein the longitudinal course of the blade includes a kink. Background Art

[0002] Wind power is considered to be one of the cleanest and most environmentally friendly energy sources currently available, and, in this regard, wind turbines have received increasing attention. Modern wind turbines typically include a tower, a generator, a gearbox, a nacelle, and a rotor with a rotatable hub having one or more wind turbine blades. The wind turbine blades use the known airfoil principle to capture the kinetic energy of the wind. The wind turbine blades transmit kinetic energy in the form of rotational energy so as to rotate a shaft that couples the wind turbine blades to a gearbox or directly to a generator without using a gearbox. The generator then converts the mechanical energy into electrical energy, which can be deployed to a utility grid.

[0003] A wind turbine blade generally includes a suction side shell and a pressure side shell, typically formed using a molding process, that are joined together at a join line along the leading and trailing edges of the blade. Moreover, the pressure and suction shells are relatively lightweight and have structural properties (e.g., stiffness, buckling resistance, and strength) that are not configured to withstand bending moments and other loads imposed on the wind turbine blade during operation. Thus, to increase the stiffness, buckling resistance, and strength of the wind turbine blade, the main body shell is typically reinforced using one or more structural members (e.g., opposing spar caps with a shear web configured therebetween) that join the inner pressure side and inner suction side surfaces of the shell halves. The spar caps and / or shear webs may be constructed of a variety of materials including, but not limited to, fiberglass laminate composites and / or carbon fiber laminate composites.

[0004] Wind turbine blades typically have a pre-bend in the flapwise direction to ensure that the wind turbine blades do not contact the tower under load. In a pitch-regulated wind turbine, the wind turbine blades are pitched when the rated wind speed is reached. In these situations, the tip of the wind turbine blade can sometimes be positioned farther away from the tower than the pitch axis, which is the opposite of the situation just before the rated wind speed is reached, where the tip is typically positioned closer to the tower. This large variation in flapwise bending leads to many different edge wise mode shapes of the blades in the rotor, some of which may be unstable, meaning that the aeroelastic damping is too low to resist vibration accumulation in the blades. This means that in these critical mode shapes, changes in the aerodynamic loads of the vibrating blades will enhance the vibrations or at least not be sufficient to resist the vibrations to avoid vibration accumulation on the blades in the rotor.

[0005] A further development is to construct larger wind turbine blades in segments that can be assembled on site via one or more pin joints. However, the aforementioned problems during pitch adjustment of pre-bent wind turbine blades are generally more critical in jointed blades, since the joints are heavy and located near the blade tip. Summary of the invention

[0006] Against this background, it can be seen that an object of the present disclosure is to provide a pre-bent wind turbine blade which alleviates at least some of the above mentioned disadvantages. This object may be met by aspects of the present disclosure as described hereinafter.

[0007] A first aspect of the present disclosure relates to a pre-bent wind turbine blade, preferably for an upwind wind turbine, the pre-bent wind turbine blade extending from a root to a tip along a longitudinal course, the wind turbine blade comprising a root region and an airfoil region having a tip, the wind turbine blade comprising a chord line extending between a leading edge and a trailing edge, the wind turbine blade comprising an aerodynamic outer blade surface, the aerodynamic outer blade surface comprising a pressure side and a suction side, wherein the airfoil region comprises a plurality of longitudinal pre-bent regions in which the wind turbine blade is pre-bent in a flap direction, wherein a first pre-bent region of the plurality of longitudinal pre-bent regions is arranged closer to the root than a second longitudinal pre-bent region, wherein the longitudinal course in the second pre-bent region comprises a kink.

[0008] Conventional pre-bent wind turbine blades are designed to have a smoothly curved longitudinal path that steadily increases the pre-bend displacement along the longitudinal path. Typically, the pre-bend region of such conventional wind turbine blades can be approximated using a quadratic polynomial. In addition, some of these conventional pre-bent wind turbine blades include a straight section closer to the root and a pre-bent section closer to the tip. This can be referred to as a "hockey stick pre-bend". In contrast, the inventors have surprisingly found that by designing the longitudinal path of a wind turbine blade with a kink so that the longitudinal path turns away from a smooth curve (and it cannot be satisfactorily approximated using a quadratic polynomial), the torsion of the wind turbine blade is modified during operation of the wind turbine blade. Therefore, the edgewise damping of the wind turbine blade can be improved. The inventors have found that a kink that turns away from the tower and a kink that turns toward the tower (which may also be referred to as a flip kink in the present disclosure) can improve the edgewise damping of some corresponding wind turbine blades.

[0009] In the context of the present disclosure, the prebend of a wind turbine blade is defined as the displacement between the longitudinal course of the wind turbine blade and the central axis of the root of the wind turbine blade in the flap direction. The sign of the prebend is defined such that a negative prebend corresponds to a wind turbine blade that is bent towards the tower of the wind turbine (or in other words, in the downwind direction) when mounted on the wind turbine, and a positive prebend corresponds to a wind turbine blade that is bent away from the tower of the wind turbine (or in other words, in the upwind direction) when mounted on the wind turbine. Thus, the prebend of a prebent wind turbine blade typically increases from the root to the tip, with the tip being positioned farthest from the central axis of the root. In the present disclosure, unless otherwise specified, prebend refers to the prebend of a wind turbine blade in an unloaded state.

[0010] The wind turbine blade may include a plurality of regions along a longitudinal course of the wind turbine blade. One or more of the plurality of regions may be straight, defined as having a substantially constant displacement change in a flap direction between the longitudinal course of the wind turbine blade and a central axis of a root toward a tip of the wind turbine blade (in other words, a first derivative of prebend is horizontal). In contrast, one or more of the plurality of regions may be prebent, defined as having a substantially non-constant displacement change in a flap direction between the longitudinal course of the wind turbine blade and a central axis of a root toward a tip of the wind turbine blade (in other words, a second derivative of prebend is non-zero).

[0011] Additionally or alternatively, the longitudinal path with the kink defines an S-shape. An S-shape is understood to mean that the longitudinal path substantially defines an S or an inverted S (e.g. ).

[0012] It has been found that such an S-shape allows the wind turbine blade to resist the twisting mentioned above while maintaining sufficient tip-to-tower clearance to avoid tower collision.

[0013] Additionally or alternatively, the kink may be provided by a change of sign of the second order derivative of the prebending displacement in the second prebending region. In other words, the kink may be provided by a first change of sign of the second order derivative of the prebending displacement in the second prebending region. Such a change of sign of the second order derivative may also be referred to as an inflection point, wherein the second order derivative changes from a positive value to a negative value or vice versa. Additionally, the kink may be further provided by a second change of sign of the second order derivative of the prebending displacement in the first prebending region. For example, the second change of sign may be located between 0% and 30% of the blade length of the wind turbine blade along the longitudinal route, but preferably between 5% and 15% of the blade length. The third change of sign of the second order derivative of the prebending displacement may further be located between 50% and 65% of the blade length.

[0014] Additionally or alternatively, the kink in the longitudinal route in the second pre-bend zone can be formed by a first average change in the pre-bend displacement per longitudinal unit length averaged over the first pre-bend zone (in other words, a first-order derivative of the pre-bend displacement averaged over the first pre-bend zone) being greater than zero and a second average change in the pre-bend displacement per longitudinal unit length averaged over the second pre-bend zone (in other words, a first-order derivative of the pre-bend averaged over the second pre-bend zone) being less than the first average change.

[0015] This can be expressed as follows: where the function p'(x) is the first derivative of the prebending displacement, x is the longitudinal distance from the root along the longitudinal path, and a R1 is the starting point of the first pre-bend area along the longitudinal route, b R1 is the end point of the first prebend area along the longitudinal route, a R2 is the starting point of the second pre-bend region along the longitudinal path, and b R2 It is the end point of the second pre-bend area along the longitudinal route.

[0016] Additionally or alternatively, the second pre-bend region may be arranged between the first pre-bend region and the third pre-bend region.The third pre-bend region may comprise a tip of the wind turbine blade.

[0017] Additionally, the third average variation of the pre-bending displacement per longitudinal unit length over the third pre-bending region may be greater than the second average variation and preferably greater than the first average variation.

[0018] This can be expressed as follows: Among them, a R3 is the starting point of the third pre-bend area along the longitudinal route, b R3 is the end point of the third pre-bending zone along the longitudinal route. The remaining expression terms are the same as those previously disclosed. The expression is applicable to a wind turbine blade that is kinked toward the tower. However, another wind turbine blade may be kinked away from the tower. In this case, the kink portion of the longitudinal route in the second pre-bending zone may be formed by a first average change in the pre-bending displacement per longitudinal unit length averaged over the first pre-bending zone being greater than zero, and by a second average change in the pre-bending displacement per longitudinal unit length averaged over the second pre-bending zone being greater than the first average change and a third average change in the pre-bending displacement per longitudinal unit length over the third pre-bending zone being less than the second average change and preferably greater than the first average change. Therefore, the expression will be as follows: Additionally or alternatively, the wind turbine blade may comprise a straight region, preferably arranged between the root and the first pre-bent region of the wind turbine blade.

[0019] The straight region may be defined as having a substantially constant pre-bending displacement per longitudinal unit length within the straight region (in other words, the first derivative of the pre-bending displacement is substantially constant within the straight region).

[0020] Additionally or alternatively, the second pre-bent region may be located in an outboard portion of the blade, preferably along a longitudinal course from 40% to 95%, more preferably from 50% to 95%, even more preferably from 60% to 95%, even more preferably from 70%, 71%, 72%, 73% or 74% to 95%, and most preferably from 75% to 95% of the blade length of the wind turbine blade. By arranging the second pre-bent region further outboard, it has been found that the edgewise damping can be further improved.

[0021] Additionally or alternatively, the longitudinal extent of the first pre-bend region and / or the second pre-bend region may be at least 5%, preferably at least 10%, of the blade length of the wind turbine blade along the longitudinal course.

[0022] Additionally or alternatively, the longitudinal extent of the first pre-bent region may be at least 15%, preferably at least 20%, more preferably at least 30%, or even more preferably at least 40%, or more preferably at least 50%.

[0023] Additionally or alternatively, the longitudinal extent of the second pre-bent region may be at least 15%, preferably at least 20%, more preferably at least 30%, or even more preferably at least 40%.

[0024] Additionally or alternatively, the longitudinal extent of the third pre-bent region may be at least 15%, preferably at least 20%, more preferably at least 30%, or even more preferably at least 40%.

[0025] Additionally or alternatively, a boundary between the first pre-bend region and the second pre-bend region may be located in the range of 40% to 65% of the blade length of the wind turbine blade. A boundary between the second pre-bend region and the third pre-bend region may be located in the range of 70% to 95% of the blade length of the wind turbine blade.

[0026] Additionally or alternatively, the wind turbine blade may include a first blade segment and a second blade segment extending from the chord-wise joint in opposite directions.

[0027] Additionally, the chord-wise joint may be located at from about 70% to about 90% of the blade length of the wind turbine blade from the blade root of the wind turbine blade, such as about 85% of the blade length of the wind turbine blade from the blade root. Additionally, in such an embodiment, the first blade segment may correspond to a blade tip segment of the wind turbine blade, and the second blade segment may correspond to a blade root segment of the wind turbine blade.

[0028] Additionally or alternatively, the first blade segment may include a beam structure having a receiving end having at least one span-wise extending pin extending therefrom. The second blade segment may include a receiving section that receives the beam structure of the first blade segment. The receiving section includes a chord-wise member having a pin joint slot defined therethrough. In such an embodiment, fastening the first blade segment and the second blade segment together in opposite directions from the chord-wise joint may include inserting the beam structure of the first blade segment into the receiving section of the second blade segment and fastening the span-wise extending pin of the receiving end of the beam structure within the pin joint slot of the receiving section.

[0029] Additionally or alternatively, the chord-wise joint may preferably be located in the second pre-bend region.

[0030] Additionally or alternatively, the aerodynamic outer surface of the pre-bent wind turbine blade may be integrally and / or continuously formed throughout the third pre-bent region. For example, the wind turbine blade itself may form the pre-bent longitudinal course without any blade extension or blade attachment. Preferably, the aerodynamic outer surface may be integrally and / or continuously formed, potentially from the blade tip to the blade root, except for one or more chord-wise joints connecting adjacent blade segments.

[0031] Those skilled in the art will appreciate that any one or more of the above aspects of the present disclosure and its embodiments may be combined with any one or more of the other aspects of the present disclosure and its embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the present disclosure will be described in more detail below with respect to the accompanying drawings. The drawings illustrate one way of implementing the invention and are not to be interpreted as limiting to other possible embodiments falling within the scope of the appended claim set.

[0033] Figure 1 is a schematic perspective view of a wind turbine.

[0034] Figure 2 It is used for Figure 1 Schematic perspective view of a wind turbine blade of a wind turbine shown in FIG.

[0035] Figure 3is a schematic side view of a wind turbine having conventional pre-bent wind turbine blades.

[0036] Figure 4 is a wind turbine blade viewed upward during edgewise vibration. Figure 3 Schematic illustration of the perspective of a ground observer shown in .

[0037] Figure 5a is a schematic side view of a kinked pre-bent wind turbine blade superimposed on a conventional pre-bent wind turbine blade.

[0038] Figure 5b Is it Figure 5a A detailed view of the tip region of a pre-bent wind turbine blade indicated by circle C.

[0039] Figure 6a is a graph showing a conventional wind turbine blade P along the length L of the corresponding wind turbine blade using a dashed line. C The pre-bend of the kinked wind turbine blade P is shown with a solid line. K The pre-bend of the wind turbine blade P is shown by using dotted lines to illustrate the flip kink portion. K2 The x-axis represents the position along the blade x normalized to the blade length L, and the y-axis represents the prebend p(x) normalized to the blade length L.

[0040] Figure 6b It is a graphic Figure 6a Wind turbine blades P C , P K , P K2 Graph of the first derivative of the pre-bend.

[0041] Figure 7a It is a graphic Figure 6a Wind turbine blades P C , P K , P K2 Graph of the second derivative of the pre-bend.

[0042] Figure 7b is a graph illustrating the pre-bending of a kinked jointed wind turbine blade along the length L of the wind turbine blade. DETAILED DESCRIPTION

[0043] In the following description of the drawings, the same reference numerals refer to the same elements and thus may not be described with respect to all drawings.

[0044] Figure 1A conventional modern upwind wind turbine 2 according to the so-called "Danish concept" is shown, having a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor axis, which may include a pitch angle of several degrees. The rotor comprises a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 closest to the hub and a blade tip 14 farthest from the hub 8.

[0045] Figure 2 A schematic view of an exemplary wind turbine blade 10 is shown. The wind turbine blade 10 has the shape of a conventional wind turbine blade having a root end 17 and a tip end 15, and includes a root region 30 closest to the hub, a profiled or airfoil region 34 farthest from the hub, and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 includes a leading edge 18 that faces in the direction of rotation of the blade 10 when the blade is mounted on the hub 8, and a trailing edge 20 that faces in the opposite direction of the leading edge 18.

[0046] The airfoil region 34 (also referred to as the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, while the root region 30 has a substantially circular or elliptical cross section due to structural considerations, which, for example, makes it easier and safer to mount the blade 10 to the hub. The diameter (or chord) of the root region 30 can be constant along the entire root region 30. The transition region 32 has a transition profile that gradually changes from the circular or elliptical shape of the root region 30 to the airfoil profile of the airfoil region 34. The chord length of the transition region 32 typically increases as the distance r from the hub increases. The airfoil region 34 has an airfoil profile with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10. The width of the chord decreases as the distance r from the hub increases.

[0047] The shoulder 38 of the blade 10 is defined as the location where the blade 10 has its maximum chord length. The shoulder 38 is typically provided at the boundary between the transition region 32 and the airfoil region 34.

[0048] It should be noted that the chords of different sections of the blade do not typically lie in a common plane, as the blade may be twisted and / or bent (i.e. pre-bent), thereby providing the chord plane with a correspondingly twisted and / or bent path, which most often occurs in order to compensate for the local velocity of the blade which depends on the radius from the hub.

[0049] exist Figure 3 The rotor plane P is shown with a R 1. An example of a wind turbine 2 with segmented blades 10 on a hub 8 of FIG. 1. The segmented blades 10 each include a chord-wise joint 26 and a root segment 22 and a tip segment 24 extending in opposite directions from the chord-wise joint 26. The rotor plane PR Offset from tower 4 by a distance D PR , if the blade 10 is straight, the distance D PR The clearance C from the tip to the tower will be limited TT As shown here, the rotor plane P R The rotor plane may be vertically aligned, but may also be tilted relative to vertical, which is referred to as a wind turbine with tilted blades. In other embodiments, the rotor plane may be a cone, which may also be referred to as a wind turbine with tapered blades. Both tilting and taper further increase the tip to tower clearance C. TT In this Figure 3 Each blade 10 is generally pre-bent and has a longitudinal course L extending away from the rotor plane P. R and the swing direction F of the tower 4 are smoothly bent to pre-bend the displacement P C , wherein the tip end 15 of each blade 10 is arranged farthest from the tower 4, thereby increasing the tip-to-tower clearance C compared to a straight blade. TT .

[0050] Figure 4 The diagram is shown when viewed from the ground and around Figure 3 The wind turbine blade 10 at the 6 o'clock position of the hub 8 shown in FIG. Figure 3 10 is the view from the perspective of the observer O shown in . In other words, the lowest substantially vertical position of the blade 10. As discussed in the background section, during the pitching of the wind turbine blade, the wind turbine blade may experience edgewise vibration in some cases. Due to the pre-bend P of the blade 10, the resulting wind direction V res The angle of attack changes during the period of the edgewise vibration. These edgewise vibrations are caused by Figure 4 The three airfoils shown in FIG. These airfoils correspond to the cross-section at point B at the tip 14 of the blade 10, which is also Figure 3 . The center airfoil corresponds to the midpoint of the vibration, while the two adjacent airfoils correspond to the extreme points of the edgewise vibration. The angle of attack of the leftmost airfoil is above its critical angle of attack and is therefore stalled. In contrast, the angles of attack of the two other airfoils are not stalled. This introduces instabilities that may worsen the edgewise vibration.

[0051] Go to Figure 5a , showing two wind turbine blades superimposed: a conventional blade with conventional pre-bend and a blade 10 with kink pre-bend. Figure 5b A detailed view along the dotted line ellipse C is shown in FIG. In this figure, the difference between conventional pre-bend and kink pre-bend can be seen more clearly. For a conventional blade with conventional pre-bend, along the longitudinal line L C When moving, the pre-bending displacement P CThe increase of is generally smooth. In other words, the first derivative of the prebend is substantially constant. In contrast, a blade with a kinked prebend can be divided into three regions 40, 42, 44 along the longitudinal extent of the blade. The first prebend region extends from 0% to 80% of the length of the blade, the second prebend region extends from 80% to 90% of the length of the blade, and the third prebend region extends from 90% to 100% of the length of the blade. In the first prebend region 40, the prebend displacement P of the kinked blade is K The pre-bending displacement of the blade 10 is substantially the same as that of a conventional blade with conventional pre-bending. At the starting point of the second pre-bending region 42, the pre-bending displacement P of the blade 10 is K From the pre-bending displacement P of the conventional blade 10 C The longitudinal path L with the kink 43 is turned away and includes a kink 43. As shown, the longitudinal path L with the kink 43 defines an S-shape. In particular, in the second pre-bending region 42, the first derivative of the pre-bending displacement is reduced compared to the first pre-bending region 40. This is further explained below. In the third pre-bending region 44, the pre-bending displacement P is reduced as it approaches the end of the third pre-bending region 44. K Pre-bending displacement P toward conventional blade 10 C At the end of the third pre-bending region, that is, at the distal end 15, the pre-bending displacement P K , P C The pre-bending displacement P of the blade 10 K The pre-bending displacement PK of the blade 10 is quite similar, and is slightly larger than the pre-bending displacement P of the conventional blade 10. C .

[0052] Figure 6a The figure shows a conventional pre-bend P C Conventional blades and pre-bent blades with kink K The blades have a flip-knotted pre-bend P K2 The length of the blade is pre-bent. K The blades are kinked towards the tower, and have a flip kink pre-bend P K2 The blades are kinked away from the tower. Figure 6b Graphics and Figure 6a The pre-bending displacement P' of the same blade C , P' K , P' K2 The first derivative of . Figure 6b The best seen in the figure is the pre-bending displacement P of the conventional blade. C The length of the conventional blade increases gradually. In contrast, the pre-bending displacement P of the kinked blade K , P K2In the first pre-bending region 40, the pre-bending displacement increases sharply from 0% to 10% of the blade length and remains somewhat constant from about 10% to 50%. In the first pre-bending region 40, a first average variation 40 of the pre-bending displacement per longitudinal unit length averaged over the first pre-bending region 40 is P'K , 40 P'K2 (In other words, the first derivative of the prebend displacement averaged over the first prebend region 40) is greater than zero. In the second prebend region 42, for the prebend with kink P K For the blade, the second average change 42 of the pre-bending displacement per longitudinal unit length averaged on the second pre-bending region 42 is P'K (In other words, the first derivative of the prebend averaged over the second prebend region 42) is less than the first average variation 40 P' , and for the pre-bend with flip kink P K2 The leaves are opposite, such as Figure 6b In the third pre-bend region 44, for the pre-bend P K For the blade of the present invention, the third average change 44 of the pre-bending displacement per longitudinal unit length in the third pre-bending region 44 is P'K Greater than the second average change 42 P'K and greater than the first average change of 40 P'K For pre-bend P with flip kink K2 The average change of the blades is 42 P'K2 Greater than the third average change 44 P'K2 , the third average change is 44 P'K2 It is greater than the first average change of 40 P'K2 Comparing it with the conventional blade shows the pre-bending displacement P' for the conventional blade C The changes in Figure 6b In the illustrated embodiment, the first pre-bend region 40 extends from 0% to about 57%, the second pre-bend region 42 extends from about 57% to about 75%, and the third pre-bend region 44 extends from about 75% to 100%.

[0053] This can be achieved by studying Figure 7a The second derivative of the pre-bending displacement shown in is further illustrated. As can be seen, the pre-bending displacement P" of the conventional blade C The second derivative increases linearly from the root to the tip. At the same time, there is a kink prebend P" K The blades and pre-bend P" with flip kink K2The second derivatives of the prebend displacement of the blades are substantially the same in the first prebend region 40, but differ from conventional blades. In particular, the second derivatives of the two kinked blades change sign in the second prebend region providing the kink 43. For the blade with the kink prebend, this occurs at approximately 66% of the blade length, while for the blade with the flip kink prebend, this occurs at approximately 68%. The exact location may vary depending on the characteristics of a given blade.

[0054] Go to Figure 7b , shows a graph illustrating the pre-bending displacement of a segmented blade with a joint. In this embodiment, the blade includes a chord-wise joint 26 and a root segment 22 and a tip segment extending in opposite directions from the chord-wise joint 26. In this figure, the pre-bending displacement P K The S-shape is particularly visible, but in other embodiments, it may be less obvious, for example, Figure 6a The chord-wise joint 26 is located in the second pre-bend region 42 at the kink 43. In other embodiments, the chord-wise joint 26 may be located on either side of the kink 43.

[0055] Reference Numbers List 2 Wind Turbines 4 Tower 6. Cabin 8 Hub 10 blades 13 Shell 14 Leaf tip 15 distal end 16 Blade root 17 Root end 18 front edge 20 Back edge 22 root segments 24 terminal segments 26 Chordal joint 27 Outer blade surface 28 Pressure side 29 Suction side 30 Root Zone 32 Transition Zone 34 Airfoil area 36 terminal area 38 Shoulders 40 First pre-bending area 40 P' First average prebend change 42 Second pre-bend area 42 P' Second average prebend change 43 Kink Department 44 Third pre-bend area 44 P' Third average prebend change L Longitudinal route P R Rotor plane D PR Rotor plane offset C TT Clearance from tip to tower P Pre-bending displacement O Observer F Swing direction V res The wind direction formed

Claims

1. A pre-bent wind turbine blade for an upwind wind turbine, the pre-bent wind turbine blade extending along a longitudinal course (L) from a root (16) to a tip (14), the wind turbine blade (10) comprising a root region (30) and an airfoil region (34) having the tip (14), the wind turbine blade (10) comprising a chord line extending between a leading edge (18) and a trailing edge (20), the wind turbine blade (10) comprising an aerodynamic outer blade surface (27), the aerodynamic outer blade surface (27) comprising a pressure side (28) and a suction side (29), wherein: The airfoil region (34) comprises a plurality of longitudinal pre-bend regions (40, 42, 44), wherein the wind turbine blade is pre-bent in a flap direction (F), wherein a first pre-bend region (40) of the plurality of longitudinal pre-bend regions is arranged closer to the root than a second longitudinal pre-bend region (42), wherein the longitudinal course in the second pre-bend region comprises a kink (43).

2. The pre-bent wind turbine blade according to claim 1, wherein: The longitudinal course with the kink defines an S-shape.

3. A pre-bent wind turbine blade according to any one of the preceding claims, wherein: The kink portion is pre-bent by displacement (P" K ,P" K2 ) changes sign in the second pre-bending region to provide.

4. The pre-bent wind turbine blade according to claim 3, wherein: The kink portion is pre-bent by displacement (P" K ,P" K2 ) also changes sign in the first pre-bending region to provide a second derivative of .

5. A pre-bent wind turbine blade according to any one of the preceding claims, wherein: The second pre-bending area is arranged between the first pre-bending area and the third pre-bending area (44).

6. A pre-bent wind turbine blade according to any one of the preceding claims, wherein: The kink in the longitudinal path in the second pre-bending zone is determined by a first average change in pre-bending displacement per longitudinal unit length averaged over the first pre-bending zone (40 P'K ) is greater than zero, and a second average change (42 P'K ) is less than the first average change.

7. A pre-bent wind turbine blade according to claim 6 when dependent on claim 5, wherein: a third average change (44) of the pre-bending displacement per longitudinal unit length in the third pre-bending area; P'K ) is greater than said second average variation and preferably greater than said first average variation.

8. The pre-bent wind turbine blade according to claim 5, wherein: The kink in the longitudinal path in the second pre-bending zone is determined by a first average change in pre-bending displacement per longitudinal unit length averaged over the first pre-bending zone (40 P'K2 ) is greater than zero, a second average change (42) of the prebending displacement per longitudinal unit length averaged over the second prebending region P'K2 ) is greater than the first average change and the third average change of the prebending displacement per longitudinal unit length on the third prebending area (44 P'K2 ) is smaller than the second average variation and preferably larger than the first average variation.

9. A pre-bent wind turbine blade according to any of the preceding claims, comprising a straight region preferably arranged between the root of the wind turbine blade and the first pre-bent region.

10. A pre-bent wind turbine blade according to any one of the preceding claims, wherein: The second pre-bent region is located in an outer portion of the blade, preferably from 40% to 95% of the blade length of the wind turbine blade along the longitudinal path.

11. The pre-bent wind turbine blade according to claim 10, wherein: The second pre-bend region is located along the longitudinal path from 75% to 95% of the blade length of the wind turbine blade.

12. A pre-bent wind turbine blade according to any one of the preceding claims, wherein: The longitudinal extent of the first pre-bend region and / or the second pre-bend region is at least 5% of the blade length of the wind turbine blade along the longitudinal course.

13. The pre-bent wind turbine blade according to any one of the preceding claims, comprising a first blade segment (22) and a second blade segment (24) extending in opposite directions from a chord-wise joint (26).

14. The pre-bent wind turbine blade according to claim 13, wherein: The chord-wise joint is located in the second pre-bend region.

15. A pre-bent wind turbine blade according to any preceding claim when dependent on claim 5, wherein: The aerodynamic outer surface of the pre-bent wind turbine blade is formed integrally and / or continuously throughout the third pre-bent region.

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