Pre-bent wind turbine blade with multiple inflection points and method of manufacturing same

Abstract

A wind turbine blade (40) for a wind turbine (10) includes an elongate body (42) defining a root end (44), a tip end (46), a leading edge (56), and a trailing edge (58). The elongated body (42) also defines a pitch axis (60) and a blade centerline axis (64) extending along the length of the blade (40). The blade centerline axis (64) defines a pre-bend in the blade (40), where the curve defining the blade centerline axis (64) includes at least a first inflection point (I1) and a second inflection point (I2) between a root end (44) and a tip end (46) of the blade (40). A method of manufacturing a wind turbine blade (40) having a blade centerline axis (64) with a plurality of inflection points is disclosed, and a molded assembly (80) for manufacturing a pre-bent wind turbine blade (40) is also disclosed.

Description

Technical Field

[0001] The present invention relates generally to wind turbines, and more particularly to a wind turbine blade having a pre-bend that reduces moment loads on a blade pitch system to extend the service life of the blade pitch system. The present invention also relates to a wind turbine having such a pre-bent wind turbine blade and a mold assembly and method for manufacturing a pre-bent wind turbine blade. Background Art

[0002] Wind turbines are used to generate electrical energy using renewable resources and without burning fossil fuels. Typically, a wind turbine converts kinetic energy from the wind into electricity. A horizontal axis wind turbine includes a tower, a nacelle at the top of the tower, and a rotor having a plurality of blades and supported in the nacelle by a shaft. The shaft couples the rotor directly or indirectly to a generator housed in the nacelle. Thus, when the wind forces the blades to rotate, the generator generates electrical energy. Wind turbines can be located on land (onshore) or in a body of water (offshore).

[0003] In recent years, wind power generation has become a more attractive alternative energy source, and the number of wind turbines, wind farms, etc. has increased significantly both onshore and offshore. In addition, the size of wind turbines has also increased significantly: the length of modern wind turbine blades extends to 80 meters, and this size is expected to increase further in the future. The increased length of wind turbine blades has introduced many interesting design considerations for wind turbine designers and manufacturers. For example, as the length of the blades increases, the backward deflection of the blades under aerodynamic loads also increases: the maximum deflection usually occurs at the tip end of the blade. If the deflection of the blades becomes significant, such as during unexpected high wind conditions, high turbulence conditions, etc., the blades may contact the tower. Tower strikes may damage the tower or blades, making it necessary to implement repair or replacement procedures to get the wind turbine running again. These procedures are expensive and may cause the wind turbine to be inoperable and offline for an extended period of time, causing the wind turbine owner to lose valuable production time. Therefore, the clearance between the blades and the tower is a constraint that is considered during the design phase of a wind turbine in order to prevent or reduce the likelihood of a potential tower strike during operation of the wind turbine.

[0004] One way to maintain an acceptable clearance between the blade and the tower during operation under potentially unexpected and harsh conditions is to limit or reduce the amount of deflection of the blade under aerodynamic loads. For example, this can be achieved by generally making the blade stiffer. In conventional methods, wind turbine blades are typically made stiffer by simply adding more material during the manufacture of the wind turbine blades (e.g., adding more fiber material and resin during the manufacture of the blades). However, this approach has many disadvantages. For example, as one can imagine, the amount of material required to make an 80 meter cantilever blade stiff enough under aerodynamic loads is not insignificant. Therefore, the material cost for making sufficiently stiff wind turbine blades is relatively high. In addition, the added material increases the overall weight of the wind turbine blades, which not only potentially has a negative impact on their aerodynamic performance, but also makes the storage, transportation and assembly of the wind turbine blades more expensive and complicated.

[0005] Another method for achieving increased clearance between the tower and the blade during operation is to use pre-bend in the wind turbine blade. Pre-bent wind turbine blades are generally known, such as from EP1019631. The broad concept of a pre-bent blade is to move away from an overly stiff blade design and allow the blade to have a certain amount of flexibility or deflection under load. By specifically designing a certain degree of flexibility into the blade, the blade can be less stiff and therefore the amount of material used to manufacture the blade and its associated weight can be significantly reduced. In addition, to address the clearance problem between the blade and the tower, the blade can be designed with a pre-bend so that in its natural non-operating state (undeflected shape), the blade bends or curves away from the tower in the upwind direction (known as flapping pre-bend). However, during aerodynamic loading of the pre-bent blade during normal operation of the wind turbine, the blade is deficient in the downwind direction compared to an aerodynamically loaded straight wind turbine blade of similar design and similar loading, but maintains increased clearance from the tower. Thus, by using flap pre-bend, longer and lighter wind turbine blades can be used without increasing the risk of tower strike.

[0006] Although pre-bent wind turbine blades have generally been successfully used for their intended purpose, there are some disadvantages to using pre-bent blades currently available in the wind turbine industry. For example, it is known that pre-bent wind turbine blades are subject to relatively large oscillating moment loads about their axis of rotation during operation of the wind turbine. Therefore, the pitch system associated with the pre-bent wind turbine blade (which controls the rotational movement of the blade about its axis of rotation or pitch axis) must resist and counteract the large moment loads in order to optimize the operation of the wind turbine and maintain control of the wind turbine. This in turn increases the wear and fatigue in the pitch system (including the pitch bearings and the pitch motors used to rotate the blades), potentially leading to premature wear and failure of the blade pitch system. When the pitch system of a blade fails, the wind turbine must be taken out of service until it can be repaired. Similar to the above, these repair procedures are expensive and may cause the wind turbine to be inoperable and offline for a period of time, thereby losing production time to the wind turbine owner.

[0007] Therefore, there is a need in the wind turbine industry for an improved pre-bent wind turbine blade that can not only achieve its purpose of reducing the likelihood of tower strike, but also reduce the moment loads acting on the blade pitch system during normal operation to reduce wear and possible failure, and avoid expensive operations associated with repairing, replacing and / or maintaining the blade pitch system. Summary of the invention

[0008] In a first aspect of the present invention, a wind turbine blade for a wind turbine is disclosed. The wind turbine blade includes an elongated body defining a root end, a tip end, a leading edge, and a trailing edge. The elongated body also defines a pitch axis and a blade centerline axis extending along the length of the blade. The pitch axis is the axis about which the blade pitch system rotates the blade, and the blade centerline axis generally defines a pre-bend (i.e., a flapping pre-bend) in the blade. The curve defining the blade centerline axis of the elongated body includes at least a first inflection point and a second inflection point between the root end and the tip end of the blade. Taking into account the bending / torsion coupling, the configuration of the blade centerline axis reduces the moment load on the blade pitch system while maintaining aerodynamic stability in the tip region of the blade. In an exemplary embodiment, the blade centerline axis may include exactly two inflection points between the root end and the tip end of the blade.

[0009] Wind turbine blades are used in horizontal axis upwind turbines, i.e. the rotor is upwind of the tower.

[0010] The first inflection point in the blade centerline axis may be located in the inner half of the blade length. For example, the first inflection point in the blade centerline axis may be located between about 20% and about 40% of the blade length. Preferably, the first inflection point in the blade centerline axis may be located between about 25% and about 35% of the blade length.

[0011] The second inflection point in the blade centerline axis may be located in the outer half of the blade length. For example, the second inflection point in the blade centerline axis may be located between about 55% and about 75% of the blade length. Preferably, the second inflection point in the blade centerline axis may be located between about 60% and about 70% of the blade length.

[0012] Preferably, the blade centreline axis does not intersect an imaginary straight line drawn between the root end and the tip end.

[0013] Preferably, the prebend defined by the blade centerline axis increases monotonically from the root end to the tip end, preferably strictly monotonically from the root end to the tip end. This means that the magnitude of the prebend increases from the root end to the tip end.

[0014] In a second aspect of the invention, a wind turbine comprises a tower, a nacelle connected to the tower, and a rotor connected to the nacelle. The rotor comprises a hub and at least one wind turbine blade, the at least one wind turbine blade being a wind turbine blade according to the first aspect described above, the at least one wind turbine blade being connected to the hub and extending from the hub. When the rotor faces the oncoming wind (such as by yaw the nacelle relative to the tower) and the blade is oriented in its operating position (i.e., the pitch orientation of the blade when the wind turbine is operating at, for example, rated wind speed), the blade defines an upwind side and a downwind side.

[0015] The wind turbine is a horizontal axis upwind turbine, ie the rotor is upwind of the tower.

[0016] The rated wind speed is the wind speed at which a wind turbine reaches its rated power. The rated power of a wind turbine is defined in IEC61400 as the maximum continuous electrical power output that the wind turbine is designed to achieve under normal operating and external conditions. Large commercial wind turbines are typically designed for a lifetime of 20 to 30 years, and their rated power output takes that lifetime into account.

[0017] The blade centerline axis may include: a first portion between the root end and the first inflection point, the first portion having a center of curvature on the upwind side of the blade; a second portion between the first inflection point and the second inflection point, the second portion having a center of curvature on the downwind side of the blade; and a third portion between the second inflection point and the tip end, the third portion having a center of curvature again on the upwind side of the blade. In addition, the wind turbine blade includes a center of gravity. When the wind turbine blade is in its operating but aerodynamically unloaded state, the center of gravity of the blade may be spaced apart from the pitch axis and located on the upwind side of the blade. However, when the blade is in its operating and aerodynamically loaded state, the center of gravity of the blade may be spaced apart from the pitch axis and located on the downwind side of the blade. Preferably, the tip end of the blade points in an upwind direction away from the tower.

[0018] When a wind turbine blade is in its operational but aerodynamically unloaded condition, this means that the blade is mounted on the turbine but it is not subjected to oncoming wind.

[0019] Preferably, when the wind turbine blade is in its operational but aerodynamically unloaded condition, the blade centreline axis does not intersect an imaginary straight line drawn between the root end and the tip end.

[0020] In a third aspect of the invention, a method of manufacturing a wind turbine blade is disclosed, the wind turbine blade having a root end, a tip end, and a shell. The method includes: providing a first mold half having a first mold cavity; providing a second mold half having a second mold cavity; and molding the shell of the blade using the first mold half and the second mold half. The contours of the first mold cavity and the second mold cavity are designed so that the shell of the blade defines a pitch axis and a blade centerline axis extending along the length of the blade, the blade centerline axis defining a pre-bend in the blade. In addition, the curve defining the blade centerline axis of the blade includes at least a first inflection point and a second inflection point between the root end and the tip end of the blade.

[0021] The molding of the outer shell of the blade can be performed in a one-step molding process (e.g., the outer shell is formed in one piece) or in a two-step molding process. For example, molding the outer shell of the blade can be a two-step process and includes: molding a first half shell in a first mold cavity; molding a second half shell in a second mold cavity; arranging the first half mold and the second half mold one above the other; and coupling the first half shell to the second half shell to form the outer shell of the wind turbine blade.

[0022] According to the method, the profiles of the first mold cavity and the second mold cavity can be designed so that the first inflection point in the blade centerline axis is located in the inner half of the blade length. For example, the first inflection point in the blade centerline axis can be located between about 20% and about 40% of the blade length. The second inflection point in the blade centerline axis can be located in the outer half of the blade length. For example, the second inflection point in the blade centerline axis can be located between about 55% and about 75% of the blade length.

[0023] According to the method, the contours of the first mold cavity and the second mold cavity can be designed so that when the blade is oriented in its operating and aerodynamic unloaded position, the centerline axis of the blade has: a first portion between the root end and the first inflection point, the first portion having a center of curvature on the upwind side of the blade; a second portion between the first inflection point and the second inflection point, the second portion having a center of curvature on the downwind side of the blade; and a third portion between the second inflection point and the tip end, the third portion having a center of curvature again on the upwind side of the blade.

[0024] In a fourth aspect of the invention, a mold assembly for manufacturing a wind turbine blade having a root end, a tip end, and a shell is disclosed. The mold assembly includes a first mold half having a first mold cavity and a second mold half having a second mold cavity. The first mold half and the second mold half can be configured to form a shell of the blade. The contours of the first mold cavity and the second mold cavity are designed so that the shell of the wind turbine blade defines a pitch axis and a blade centerline axis extending along the length of the blade, the blade centerline axis defining a pre-bend in the blade. The curve defining the blade centerline axis of the blade includes at least a first inflection point and a second inflection point between the root end and the tip end of the blade.

[0025] The first mold half may be configured for forming a first blade shell in a first mold cavity, and the second mold half may be configured for forming a second blade shell in a second mold cavity separately from the molding of the first blade shell. Furthermore, the first mold half and the second mold half can be arranged one above the other to connect the first blade shell and the second blade shell to form an outer shell of the blade.

[0026] It will be appreciated that preferred and / or optional features of each aspect of the invention may be combined with other aspects of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above and the detailed description given below, serve to explain the invention.

[0028] Figure 1 is a perspective view of a wind turbine having pre-bent wind turbine blades according to an embodiment of the present invention;

[0029] Figure 2 and Figure 3 is a schematic diagram of the leading and distal ends of a conventional pre-bent wind turbine blade, showing the first moment acting on the blade pitch system;

[0030] Figure 4 and Figure 5 is a schematic diagram of the top and tip ends of a conventional pre-bent wind turbine blade, illustrating the second moment acting on the blade pitch system;

[0031] Figure 6 and Figure 7 are two views of a wind turbine blade with pre-bend according to an embodiment of the present invention;

[0032] Figure 8 is a graph showing the blade centerline axis of a conventional pre-bent blade and a pre-bent blade of the present invention when they are in an aerodynamic no-load state;

[0033] Fig. 9 is a graph showing the blade centerline axis of a conventional pre-bent blade and a pre-bent blade of the present invention when in an aerodynamically loaded state;

[0034] Fig.10 is a graph showing the curvature of the blade centerline axis of a conventional pre-bent blade and a pre-bent blade of the present invention when they are in an aerodynamic no-load state;

[0035] Fig.11 is a graph showing the blade centerline axis of the pre-bent blade of the present invention in an aerodynamically loaded state and showing two inflection points along the curve; and

[0036] Fig.12 is a schematic diagram of a mold assembly for manufacturing a wind turbine blade having an inventive pre-bent configuration according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] refer to Figure 1 , wind turbine 10 includes a tower 12, a nacelle 14 disposed at an apex of tower 12, a rotor 16 operatively coupled to a generator (not shown) housed within the nacelle, and a gearbox (not shown) also housed within nacelle 14. In addition to the generator and gearbox, nacelle 14 may house various components required to convert wind energy into electrical energy and to operate wind turbine 10 and optimize the performance of wind turbine 10. Tower 12 supports loads presented by nacelle 14, rotor 16, and other wind turbine components housed within nacelle 14, and operates to elevate nacelle 14 and rotor 16 to an altitude above ground level or sea level, as the case may be, where airflows with lower turbulence and higher velocities are generally found.

[0038] The rotor 16 may include a central hub 18 and a plurality of blades 20 attached to the central hub 18 at locations distributed around the circumference of the central hub 18. In a representative embodiment, the rotor 16 includes three blades 20, however, the number may vary. The blades 20 projecting radially outward from the central hub 18 are configured to interact with a passing airflow to generate a rotational force that rotates the central hub 18 about its longitudinal axis.

[0039] The rotor 16 may be coupled to the gearbox directly or indirectly via a drive shaft (not shown). In either case, the gearbox transmits the rotation of the rotor 16 to the generator through a coupling (not shown). Wind exceeding a minimum speed may activate the rotor 16, causing the rotor 16 to rotate in a direction substantially perpendicular to the wind, thereby applying torque to the input shaft of the generator. As will be appreciated by one of ordinary skill in the art, the electricity generated by the generator may be supplied to a power grid (not shown) or an energy storage system (not shown) for later release to the power grid. In this way, the kinetic energy of the wind may be utilized by the wind turbine 10 for power generation.

[0040] The blades 20 of the wind turbine 10 are of a type that include a pre-bend, and more specifically include a flapping pre-bend, so as to increase the distance between the blade 20 (e.g., its tip) and the tower 12 during operation of the wind turbine 10. In addition, each of the blades 20 includes a pitch system 22 ( Figure 1 ). As is generally known in the wind turbine industry, the pitch system 22 allows the blade 20 to rotate about a pitch axis of the blade 20 to change the orientation of the blade 20 relative to the oncoming wind. Although not shown, the pitch system 22 generally includes a pitch bearing having an inner race coupled to the hub 18 or the blade 20, an outer race coupled to the other of the hub 18 and the blade 20, and a plurality of bearing elements (e.g., rollers, etc.) disposed between the two races to provide relative motion therebetween. The pitch system 22 also includes one or more pitch motors and a suitable transmission (e.g., a ring gear) such that when the pitch motor is actuated, the blade 20 rotates about its pitch axis relative to the central hub 18. As described above, the pitch system of a conventional pre-bent wind turbine blade is subject to large oscillating moment loads during normal operation of the wind turbine. Various aspects of the present invention are directed to reducing these large oscillating moment loads on the blade pitch system to extend the working life of its components and reduce the repair, replacement and / or maintenance activities and costs associated with the blade pitch system.

[0041] In order to understand certain aspects of the present invention, a better understanding of the dynamic loading of pre-bent wind turbine blades is believed to be instructive. Figure 2For example, a conventional pre-bent wind turbine blade 24 is shown from a front view (i.e., from the wind direction) when the blade 24 is in a three o'clock position on the wind turbine 10. The blade 24 has a pitch axis 26 about which the blade 24 rotates when the pitch system 28 is actuated. Gravity acts on the flexible cantilever blade, and at different positions throughout the rotation, the blade 24 deflects downward under the influence of gravity, such as Figure 2 These deflections are called the gravitational induced shimmy deflections. As shown in the figure, the resultant aerodynamic thrust load T is at the lift center L. c The lift center L on the blade 24 acts on the blade 24. When the blade is at the three o'clock position as shown in the figure, the lift center L on the blade c Deflected downward, the lift center L c Located below the pitch axis 26 .

[0042] like Figure 3 The best shown in ( Figure 3 2 shows a schematic cross-sectional view of a blade 24 through the lift center and viewed from the tip toward the root). The lift center L c The position of is generally below the pitch axis 26 so that a portion of the thrust load T on the blade 24 generates a moment M1 about the pitch axis 26. As shown in the figure, the moment M1 is in the counterclockwise direction and is caused by the thrust load combined with the flap deflection of the blade 24.

[0043] Figure 4 For example, a conventional pre-bent wind turbine blade 24 is shown from a top view (looking down at the trailing edge) when the blade 24 is in a three o'clock position on the wind turbine 10. During operation of the wind turbine, the blade 24 deflects in the downwind direction under aerodynamic loads. The blade 24 has a center of gravity CoG, and the weight W of the blade 24 is t Acting on the center of gravity CoG. Figure 5 As shown ( Figure 5 A schematic cross-sectional view of a blade 24 through the center of gravity, viewed from the tip toward the root, is shown, similar to Figure 3 ), the position of the center of gravity CoG is generally located behind the pitch axis 26 (e.g., downwind), so that the weight Wt of the blade 24 acting on the center of gravity CoG generates a moment M2 about the pitch axis 26. As shown in the figure, the moment M2 is relatively quiet in the clockwise direction and is due to the weight Wt of the blade. t This is caused in conjunction with the flapping deflection of blades 24 (due to aerodynamic loads) during operation of the wind turbine.

[0044] Therefore, during operation of the wind turbine, the two moments M1 and M2 oppose each other. Conceptually, these two moments M1, M2 can then be used to counteract the moment loads on the blade pitch system. However, large modern wind turbine blades are much more flexible in the flap direction than in the swing direction. Therefore, the moment M2 is usually always much greater than M1 (i.e., M2>>M1). There are many options to make the two moments M1, M2 more balanced and provide a more significant counteracting effect. For example, one option is to limit the amount of swing deflection or flapping deflection that the blade experiences under load. As described above, this approach can be a very expensive solution for reducing the moment loads on the pitch system and may also cause other challenges. However, the present invention provides a more feasible and economical solution by manipulating the position of the center of gravity CoG of the blade during normal operation of the wind turbine (i.e., when the blade is under aerodynamic loading and undergoing flapping deflection) to thereby reduce the M2 moment. Reducing the M2 moment relative to the M1 moment provides a greater counteracting effect and reduced moment loads on the blade pitch system.

[0045] Changing the location of the center of gravity CoG can be achieved by changing the pre-bend configuration in the wind turbine blade. This is an interesting concept in the context of pre-bent wind turbine blades. In this regard, designers of pre-bent blades have primarily (if not exclusively) focused on the configuration of the outboard or tip region of the wind turbine blade. This makes perfect sense, as the goal of a pre-bent blade is to move the tip end of the blade further away from the tower during operation of the wind turbine (when the blade is deflected in the direction of the wind) in order to reduce the likelihood of a tower strike. Little attention is typically paid to the configuration of the blade along the inboard or root region of a wind turbine blade. This is of significance in this context, as blade designs are known to require more substantial structure closer to the root end of the blade than closer to the tip end of the blade. Thus, for example, the root region of a wind turbine blade is typically very thick and heavy compared to the tip region of the blade. Due to the distribution of the materials (e.g., fibers, resins, inserts, etc.) that make up the blade, the center of gravity CoG of the blade is typically located more towards the root end of the blade, as shown in FIG. Figure 4 As roughly shown in .

[0046] By manipulating the configuration of the pre-bend in the root region of the blade, the position of the center of gravity CoG can be changed in a manner that reduces the M2 moment, thereby resulting in a reduction in the moment load acting on the blade pitch system during operation of the wind turbine. More specifically, the pre-bend is configured to move the center of gravity CoG forward in an operating but unloaded configuration, for example, from a position similar to Figure 4From the top view perspective shown, moving in the upwind direction, the center of gravity CoG does not move as far in the downstream direction relative to the pitch axis of the blade when the blade is loaded. In other words, when the blade is deflected in the flapping direction during operation, the moment arm of the weight load acting at the center of gravity CoG (measured from the pitch axis) is reduced, thereby reducing the M2 moment.

[0047] While manipulating the pre-bend configuration in the root region of the blade to change the position of the center of gravity CoG may provide certain advantages with respect to the moments acting on the blade pitch system, care must be taken with the pre-bend configuration in the tip region of the blade. More specifically, the absolute pre-bend in the blade (which is defined as the distance between the pitch axis and the blade at its tip when the blade is in an aerodynamically unloaded state) is typically selected such that during normal operation of the wind turbine (and the blade is deflected in the flapping direction), a minimum clearance distance is met between the blade tip and the wind turbine tower. By way of example, a current pre-bend blade (e.g., having a length of 80 meters) may have an absolute pre-bend at the blade tip of between about 2 m and about 4 m such that there is sufficient clearance between the blade tip and the tower during operation. Because there may be manufacturing and transportation challenges in providing pre-bend in the blade, providing excessive absolute pre-bend in the blade that provides excessive clearance between the blade tip and the tower during normal operation is typically avoided.

[0048] In addition to the above, pre-bent wind turbine blades may exhibit instabilities at the blade tip under dynamic loads. The instability typically manifests itself as an uncontrolled, dynamically complex rotational motion near the blade tip, which results in reduced aerodynamic performance and shortened fatigue life of the blade. It is believed that the primary cause of the instability lies in the bend-torsion coupling associated with pre-bent blades. Current pre-bent blades address this instability by controlling the rate of change of the pre-bend (e.g., the slope of the pre-bend) in the near region of the blade tip. Therefore, while it is desirable to manipulate the pre-bend configuration in the root region of the blade for the purpose of improving the moment loads on the pitch system, it is not desirable to manipulate the pre-bend configuration in the tip region of the blade, which fully addresses the bend-torsion instability to which the pre-bent blades are susceptible.

[0049] Figure 6 and Figure 7 A pre-bent wind turbine blade 40 according to an embodiment of the present invention is shown. The pre-bent wind turbine blade 40 is configured to Figure 1The blade 40 is used in the wind turbine 10 shown. The blade 40 is configured as an elongated structure or body having an airfoil shell 42 and an inner spar (not shown) including a spar cap and one or more shear webs that provide structural support for the blade 40. The elongated blade 40 includes a root end 44 and a tip end 46, the root end 44 being configured to be coupled to the central hub 18 when mounted to the rotor 16, and the tip end 46 being longitudinally opposite the root end 44. Figure 6 and Figure 7 In the orientation shown, the outer shell 42 may include a first lower shell half 48 defining a suction surface 50 of the blade 40 and a second upper shell half 52 defining a pressure surface 54 of the blade 40. The lower shell half 38 and the upper shell half 42 are coupled together along a leading edge 56 and a trailing edge 58 that are positioned opposite each other across the chord of the blade 40.

[0050] The blade 40 is configured to rotate about a pitch axis 60 under the influence of a blade pitch system (not shown). A section 62 of the blade 40 proximate the root end 44 may be generally cylindrical in shape so as to be connected to a (circular) pitch bearing of the pitch system by which the blade 40 is attached to the hub 18. The central axis of the cylindrical section 62 is generally coaxial with the pitch axis 60 of the blade 40. The cylindrical section 62 extends longitudinally along a relatively short distance and is configured to accommodate an insert (not shown) at the root end 44 of the blade 40 for receiving a stud that physically connects the blade 40 to the pitch bearing of the pitch system, as is generally known.

[0051] The blade 40 also defines a blade centerline axis 64. The concept of the blade centerline axis 64 is well known in the wind turbine industry and is generally defined by a set of points located midway between the suction surface 50 and the pressure surface 54 and at chord-wise locations defined by the maximum blade thickness. The blade centerline axis may also be referred to as the blade twist axis. The blade centerline axis 64 is defined by the body of the blade 40 and generally does not include various additional elements, such as blade extenders, winglets, or other items attached to the blade 40. The blade centerline axis 64 characterizes several features of the blade 40, such as the pre-bend configuration, the overall shape of the blade 40, the position of the blade 40 relative to the pitch axis 60 in unloaded and loaded conditions, etc. This may be due to the fact that the blade 40 is not twisted at all. Figure 7 Therefore, details of the blade 40 (eg, shape, pre-bend, moments, etc.) will be described below with reference to certain features in the blade centerline axis 64 alone or in combination with the pitch axis 60 of the blade 40 .

[0052] Figure 8 is a graph illustrating the blade centerline axis 64 of a blade 40 according to an embodiment of the present invention as compared to the blade centerline axis 68 of a conventional pre-bent wind turbine blade 24 . Figure 8The perspective shown in corresponds to a top view of the blade 24, 40 at the three o'clock position and aerodynamically unloaded, which provides a clear indication of the (flapping) pre-bent configuration of the blade 24, 40. In this graph, the abscissa is the radial dimension of the blade 24, 40 (e.g., from a perspective located on the rotor 16), where zero is adjacent to the root end of the blade, and the abscissa is scaled by the length R of the blade. The ordinate in the graph is the pre-bend of the blade 24, 40 in meters. The abscissa substantially represents the pitch axis of the blade 24, 40 and is generally collinear.

[0053] Figure 8 Several features of the pre-bent blade 40 are illustrated relative to the conventional pre-bent blade 24. Turning first to the root region of the blades 24, 40 in circle A, the pre-bending in the blade 40 begins earlier (e.g., closer to the root end 44) ​​than the conventional blade 24 and begins at a higher rate (e.g., slope) than the conventional blade 24. The effect of this is that it causes the heavier portion of the blade 40 in the root section to bend in the upwind direction (from the Figure 8 ) and moves away from the pitch axis 60. The center of gravity CoG1 of blade 24 and the center of gravity CoG2 of blade 40 are both shown in the figure. Because more mass has been transferred in the upwind direction, the center of gravity of blade 40 has been correspondingly shifted in the upwind direction. The relevance of this will be explained in more detail below.

[0054] Steering Figure 8 By substantially maintaining the pre-bend configuration near the tip of the blades 24, 40, it is believed that the blade 40 will have similar performance to that of the blade 24 as it relates to the bending-torsion instability at the tip of the blade 40. Thus, the pre-bend in the blade 40 provided by the blade centerline axis 64 demonstrates a method in which the pre-bend is manipulated in the root region of the blade 40 while maintaining the pre-bend configuration near the tip region of the blade 40.

[0055] Fig. 9 is a graph showing a comparison of the blade centerline axis 64 of the blade 40 and the blade centerline axis 68 of a conventional pre-bent wind turbine blade 24 under aerodynamic loading during normal operation of the wind turbine 10 (e.g., at rated wind speed). Figure 8As shown, the center of gravity CoG1 of blade 24 and the center of gravity CoG2 of blade 40 are both located on the upwind side of the blade and are spaced apart from the pitch axis of blades 24, 40. However, as shown in the figure, under aerodynamic loads, blades 24, 40 deflect in the flapping direction as expected. This causes a significant portion of the blade mass to be in the downwind direction (from Figure 8 ) and moves across the pitch axis of blades 24, 40. The center of gravity CoG1 of blade 24 and the center of gravity CoG2 of blade 40 are both shown in the figure as being on the downwind side of blades 24, 40 in an aerodynamically loaded state. Although both the centers of gravity CoG1 and CoG2 have moved to the other side of the pitch axis 60, the relevant feature is that the center of gravity CoG2 of blade 40 is not as severely deflected in the flapping direction as the center of gravity CoG1 of blade 24. Therefore, the distance between the pitch axis 60 and the center of gravity CoG2 of blade 40 (i.e., the moment arm of the moment M2 on the pitch system generated by the weight of blade 40) is reduced compared to the moment M2 on the pitch system generated by the weight of blade 24. Therefore, when the wind turbine blade is in its operating but aerodynamically unloaded state, the center of gravity CoG2 of blade 40 is spaced apart from the pitch axis 60 and is located on the upwind side of blade 40. However, when the blade 40 is in its operational and aerodynamically loaded state, the center of gravity CoG2 of the blade 40 is spaced apart from the pitch axis 60 and is located on the downwind side of the blade 40 .

[0056] Advantageously, blade centerline axis 64 does not intersect an imaginary straight line 100 drawn between root end 44 and tip end 46 of the blade. This prevents the center of gravity CoG2 of blade 40 from being too far upwind of the pitch axis when the blade is in its operational but aerodynamically unloaded state, so that if the blade is pitched in the aerodynamically unloaded state or even in low wind conditions, the moment arm acting on the pitch system is not too large.

[0057] Thus, the pre-bent wind turbine blade 40 as described above provides: i) a reduction in moment loads acting on the blade pitch system compared to conventional pre-bent blades; and ii) good aerodynamic stability is maintained in the tip region of the blade. The former feature results in a wind turbine blade pitch system with fewer failures and maintenance requirements, as well as an extended operating life, in particular due to lower fatigue loads. Thus, the costs associated with repair, replacement and maintenance operations on the blade pitch system are reduced. The latter feature results in the blade 40 operating stably and efficiently over a wide range of wind speeds, and further avoids any blade damage due to aerodynamic instability caused by bend-torsion coupling.

[0058] Achieving these two results by manipulating the pre-bend configuration provides a unique blade geometry. In the exemplary embodiment, blade 40 includes a pre-bend such that when blade 40 is mounted to wind turbine 10 and oriented in a normal operating position (but aerodynamically unloaded) and viewed from the top, blade 40 is configured to bend or curve in an upwind direction, similar to a standard pre-bent blade. Thus, tip end 46 of blade 40 is positioned further upwind than root end 44 of blade 40. However, the configuration of the pre-bend in blade 40 is different from the configuration of a standard pre-bent blade (such as blade 24).

[0059] More specifically, and in accordance with one aspect of the present invention, the blade 40 is pre-curved to have a "double S" configuration that is generally characterized by a pair of inflection points I1 and I2 located along the length of the blade 40 between the root end 44 and the tip end 46. As used herein, the inflection point I refers to a location along the blade 40 at which the curvature defined by the second derivative of the curve defining the blade centerline axis 64 of the blade 40 is zero. When the inflection point I occurs at an interior portion of the blade 40 (i.e., not at the root end 44 or the tip end 46), then the curvature changes sign (i.e., changes from negative to positive or vice versa, depending on how the curvature is defined) on either side of the inflection point.

[0060] Fig.10 is a graph showing the curvature of the blade centerline axis 64 of the blade 40 compared to the blade centerline axis 68 of a conventional pre-bent wind turbine blade 24 from a perspective when the blades 24, 40 are mounted to the wind turbine 10 and oriented in a normal operating position (but aerodynamically unloaded) and viewed from the top. As shown in the figure, the curvature of the standard pre-bent blade 24 varies but does not include an inflection point along the length of the blade. However, in contrast, the blade 40 includes a pair of inflection points I1, I2 at an inner portion of the blade 40 where the curvature changes sign. In an exemplary embodiment, the first inflection point I1 may be located in the inner half of the blade 40, i.e., between the root end 44 and the midpoint (50% of the blade length R) of the blade 40, and the second inflection point I2 may be located in the outer half of the blade 40, i.e., between the midpoint and the tip end 46 of the blade 40. In a preferred embodiment, the first inflection point I1 may be located between about 20% and about 40% of the blade length R. In an even more preferred embodiment, the first inflection point I1 may be located between about 25% and about 35% of the blade length R. In a similar manner, in a preferred embodiment, the second inflection point I2 may be located between about 55% and about 75% of the blade length R. In an even more preferred embodiment, the first inflection point I2 may be located between about 60% and about 70% of the blade length R.

[0061] exist Fig.11In a reference frame of , i.e., from a perspective when the blade 40 is mounted on the wind turbine 10 and oriented in a normal operating position facing the oncoming wind (but aerodynamically unloaded) and viewed from the top, the curvature of the blade may be designated as negative along a first portion 74 of the blade 40 from the root end 44 to the first inflection point I1. With this designation, the first portion 74 of the blade 40 may have a center of curvature C1 on the upwind side of the blade 40 such that the portion 74 of the blade 40 has a concave configuration. The curvature of the blade may be designated as positive along a second portion 76 of the blade 40 from the first inflection point I1 to the second inflection point I2. With this designation, the second portion 76 of the blade 40 may have a center of curvature C2 on the downwind side of the blade 40 such that the portion 76 of the blade 40 has a convex configuration. The curvature of the blade may be designed to be negative along a third portion 78 of the blade 40 from the second inflection point I2 to the tip end 46. With this designation, the third portion 78 of the blade 40 may again have a center of curvature C3 on the upwind side of the blade 40 such that this portion 78 of the blade has a concave configuration.

[0062] The wind turbine blade 40 may be formed by a molding process, as is generally known in the art. Fig.12 A molding assembly 80 for forming the wind turbine blade 40 described above is shown. The molding assembly 80 includes a lower mold half 82 having a lower mold cavity 84 defining a contoured surface 86. The molding assembly 80 also includes an upper mold half 88 having an upper mold cavity 90 defining a contoured surface 92. As is known in the wind turbine industry, fiber materials, resins, and possibly foam cores, etc., can be introduced into the cavities 84, 90 of the lower mold half 82 and the upper mold half 88 and cured to form a lower blade shell 94 and an upper blade shell 96. The lower blade shell 94 and the upper blade shell 96 can then be coupled to each other, such as by flipping the upper mold half 88 to be positioned above the lower mold half 82, and the blade shells 94, 96 and any spar structure disposed between the blade shells 94, 96 are bonded to each other. The blade shells 94, 96 form the outer shell 42 of the wind turbine blade 40. The contoured surfaces 86 , 92 of the mold cavities 84 , 90 are configured to define a blade pre-bend configuration (defined by the blade centerline axis 68 ) having the features shown and described above.

[0063] Although in Fig.12A two-step molding process is described in , but the present invention is not limited to any particular type of molding process. For example, a one-step molding process can be used to mold the integrated shell 42 of the wind turbine blade 40. In this embodiment, two half molds can be used in conjunction with one or more removable inserts (not shown) to provide a molded assembly that together forms a lower blade shell 94 and an upper blade shell 96 in a single molding step. Other variations of the molding process may also be implemented, but the other variations are still within the scope of the present invention. In these various alternative molding processes, the contoured surfaces 86, 92 of the mold cavities 84, 90 of the half molds 82, 88 ultimately define the shape of the blade and the blade centerline axis 64.

[0064] From a manufacturing perspective, blades having a double S configuration may provide certain advantages over standard pre-bent wind turbine blades. In this regard, the lower mold half 82 and the upper mold half 88 must have a height that accommodates the camber height of the respective mold cavities 84, 90. The pre-bending increases the camber height of the mold cavities 84, 90, thereby increasing the overall height of the mold halves 82, 88. Figure 8 As best shown in FIG. 1 , the camber height H of the mold cavities 84 and 90 is b is the vertical distance from the line 100 connecting the root and tip ends of the cavities 84, 90 to the farthest point of the cavities 84, 90. Figure 8 As shown, the standard pre-bent blade has a camber height H b，1 The camber height H of the blade 40 having the double S structure according to the present invention is usually greater than b，2 Thus, the mold halves 82, 88 for the blade 40 having the double-S configuration have a reduced height compared to the mold halves of conventional pre-bent wind turbine blades. This results in an overall reduction in the footprint of the molding assembly 80 on the factory floor, which can be invaluable. Additionally, the facility's ceiling height, walkways, and other equipment used to mold the blades can all be present at a reduced height, which can provide additional benefits.

[0065] The prebend defined by the blade centerline axis increases monotonically from the root end to the tip end, preferably strictly monotonically from the root end to the tip end. Figure 8 , Fig. 9 and Fig.11 As can be seen in , the convention for pre-bending is to measure it negatively in front of the tower (i.e. upwind). When pre-bending increases monotonically, it is the absolute value or magnitude that increases, i.e., when moving along the blade from the root end to the tip end, the blade moves further away from the tower. In other words, the blade does not bend backwards at any point, and this means that the camber height H b，2 can be kept to a minimum (compared to the camber height of the blades deflected backwards towards the tower).

[0066] In addition, the reduced camber height H of the blade 40b，2 It makes it easier to transport the blade because the blade has a smaller "footprint" when mounted on a transport vehicle. This is particularly true for transporting the blade by rail. The blade 40 can be mounted on two railway vehicles in a known manner and because of the smaller footprint of the blade there will be less overhang between the blade and the railway track, which means that the railway vehicles can take sharper corners than with a blade having a relatively greater camber height.

[0067] Reduced arch height H b，2 Another advantage is that it facilitates the manufacture of the blade 40. During blade manufacture, composite material is placed into the mold cavities 84, 90 to create the outer shell 94, 96 of the blade. Fig.12 As can be seen in the figure, the mold cavity is bent to the shape of the airfoil profile between the leading and trailing edges. Fig.12 There will also be a curvature of the mold cavity in the direction of the page plane in . This results in a double curvature of the mold cavity 84, 90. Considering the arch height H of a standard pre-bent blade b，1 and the arch height H of the blade 40 b，2 , it will be appreciated that there is less double curvature in the mold cavity for the blade 40 of the present invention. This aids in manufacturing because it may be difficult to lay down composite materials on a double-curved surface. For example, fiberglass fabric may wrinkle on a double-curved surface, and a pre-curved composite pultrusion (which may be used for a spar cap) may not follow the correct path in the mold cavity. (This can be explained by thinking of laying a piece of paper into a curved cylinder - this can be done without causing any wrinkles in the paper. Now, if a piece of paper is laid into a bowl with double curvature, it would be impossible to lay the piece of paper onto the surface of the bowl without forming wrinkles in the paper.) By reducing the amount of double curvature, the layup process is simplified.

[0068] Although the present invention has been illustrated by the description of various preferred embodiments, and although these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will be apparent to those skilled in the art. Therefore, the various features of the present invention may be used alone or in any combination, depending on the needs and preferences of the user.

Claims

1. A wind turbine blade (40) for a wind turbine (10), the wind turbine blade (40) comprising: an elongated body (42) defining a root end (44), a tip end (46), a leading edge (56), and a trailing edge (58), the elongated body (42) further defining a pitch axis (60) and a blade centerline axis (64) extending along the length of the blade (40), the blade centerline axis (64) defining a prebend in the blade (40), The curve defining the blade centerline axis (64) of the elongated body (42) includes at least a first inflection point (I1) and a second inflection point (I2) between a root end (44) and a tip end (46) of the blade (40).

2. The wind turbine blade (40) of claim 1, wherein: A first inflection point (I1) in the blade centerline axis (64) is located in the inner half of the blade length (R).

3. The wind turbine blade (40) according to claim 1 or 2, wherein: A first inflection point (I1) in the blade centerline axis (64) is located between about 20% and about 40% of the blade length (R).

4. A wind turbine blade (40) according to any one of the preceding claims, wherein: A first inflection point (I1) in the blade centerline axis (64) is located between about 25% and about 35% of the blade length (R).

5. A wind turbine blade (40) according to any one of the preceding claims, wherein: A second inflection point (I2) in the blade centerline axis (64) is located in the outer half of the blade length (R).

6. A wind turbine blade (40) according to any one of the preceding claims, wherein: The second inflection point (I2) on the blade centerline axis (64) is located between about 55% and about 75% of the blade length (R).

7. A wind turbine blade (40) according to any one of the preceding claims, wherein: The second inflection point (I2) on the blade centerline axis (64) is located between about 60% and about 70% of the blade length (R).

8. A wind turbine blade (40) according to any one of the preceding claims, wherein: The blade centerline axis (64) does not intersect an imaginary straight line drawn between the root end (44) and the tip end (46).

9. A wind turbine blade (40) according to any one of the preceding claims, wherein: The prebend defined by the blade centerline axis (64) increases monotonically from the root end (44) to the tip end (46), preferably strictly monotonically from the root end to the tip end.

10. A wind turbine (10), comprising: Tower (12); a nacelle (14) connected to the tower (12); and A rotor (16) connected to the nacelle (14), the rotor (16) comprising a hub (18) and at least one wind turbine blade (40) connected to the hub (18), the at least one wind turbine blade (40) being a wind turbine blade according to any one of claims 1 to 9.

11. The wind turbine according to claim 10, wherein: When the rotor (16) faces an oncoming wind and the at least one blade (40) is in an operational position, the at least one blade (40) defines an upwind side and a downwind side.

12. A wind turbine (10) according to claim 10 or 11, wherein: The blade centerline axis (64) includes: a first portion (74) between the root end (44) and the first inflection point (I1), the first portion (74) having a center of curvature (C1) on the upwind side of the blade (40); a second portion (76) between the first inflection point (I1) and the second inflection point (I2), the second portion (76) having a center of curvature (C2) on the downwind side of the blade (40); and A third portion (78) between the second inflection point (I2) and the tip end (46) has a center of curvature (C3) on the upwind side of the blade (40).

13. A wind turbine (10) according to any one of claims 10 to 12, wherein: The at least one blade (40) comprises a center of gravity (CoG), and wherein: In an aerodynamically unloaded state, the center of gravity (CoG) is located on the upwind side of the pitch axis (60) of the at least one blade (40).

14. The wind turbine (10) of claim 13, wherein: In an aerodynamically loaded state when the wind turbine is operating at a rated wind speed, the center of gravity (CoG) is on the downwind side of a pitch axis (60) of the at least one blade (40).

15. A wind turbine according to any one of claims 10 to 14, wherein: The tip end of the blade is away from the tower and points in the upwind direction.

16. A method of manufacturing a wind turbine blade (40) having a root end (44), a tip end (46) and a shell (42), the method comprising: providing a first mold half (82) having a first mold cavity (84); providing a second mold half (88) having a second mold cavity (90); using the first mold half (82) and the second mold half (88) to mold the outer shell (42) of the blade (40), wherein the first mold cavity (84) and the second mold cavity (90) are contoured such that the outer shell (42) of the blade (40) defines a pitch axis (60) and a blade centerline axis (64) extending along the length of the blade (40), the blade centerline axis (64) defining a prebend in the blade (40), and The curve defining the blade centerline axis (64) of the blade (40) includes at least a first inflection point (I1) and a second inflection point (I2) between a root end (44) and a tip end (46) of the blade (40).

17. The method according to claim 16, wherein: Using the first half mold (82) and the second half mold (88) to mold the outer shell (42) of the blade (40) further includes: Molding a first blade shell (94) in the first mold cavity (84); molding a second blade shell (96) in the second mold cavity (90); arranging the first mold half (82) and the second mold half (88) one above the other; and The first blade shell (94) is coupled to the second blade shell (96) to form an outer shell (42) of the wind turbine blade (40).

18. The method according to claim 16 or 17, wherein: The first mold cavity (84) and the second mold cavity (90) are contoured so that a first inflection point (I1) in the blade centerline axis (64) is located in the inner half of the blade length (R).

19. The method according to any one of claims 16 to 18, wherein: A first inflection point (I1) in the blade centerline axis (64) is located between about 20% and about 40% of the blade length (R).

20. The method according to any one of claims 16 to 19, wherein: A second inflection point (I2) in the blade centerline axis (64) is located in the outer half of the blade length (R).

21. The method according to any one of claims 16 to 20, wherein: The second inflection point (I2) in the blade centerline axis (64) is located between about 55% and about 75% of the blade length (R).

22. A mold assembly (80) for manufacturing a wind turbine blade (40), the wind turbine blade (40) having a root end (44), a tip end (46) and a shell (42), the mold assembly (80) comprising: a first mold half (82) having a first mold cavity (84); a second mold half (88) having a second mold cavity (90), wherein the first mold half (82) and the second mold half (88) are arrangable to form a shell (42) of the blade (40); wherein the first mold cavity (84) and the second mold cavity (90) are contoured such that a shell (42) of the wind turbine blade (40) defines a pitch axis (60) and a blade centerline axis (64) extending along the length of the blade (40), the blade centerline axis (64) defining a prebend in the blade (40), and The curve defining the blade centerline axis (64) of the blade (40) includes at least a first inflection point (I1) and a second inflection point (I2) between a root end (44) and a tip end (46) of the blade (40).

23. The mold assembly (80) of claim 22, wherein: The first half mold (82) is configured to form a first blade shell (94) in the first mold cavity (84), wherein the second half mold (88) is configured to form a second blade shell (96) in the second mold cavity (84) separately from the molding of the first blade shell (94), and wherein the first half mold (82) and the second half mold (88) can be arranged one above the other to connect the first blade shell (94) and the second blade shell (96) to form an outer shell (42) of the blade (40).

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