Abstract:
A blade for a wind turbine and a method for making same are provided. The blade includes a skin having a braided fiber sock. One or more stiffeners are attached to the braided fiber sock.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to wind turbine blades and more particularly to a braided blade structure and method of making same. Such blades are particularly suitable for (but are not limited to use in) wind turbine configurations. 
         [0002]    Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing, or nacelle, that is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors, e.g., 70 meters (m) (˜230 feet (ft)) or more in diameter. Blades, attached to rotatable hubs on these rotors, transform mechanical wind energy into a mechanical rotational torque that drives one or more generators. The generators are generally, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid. Gearless direct drive turbines also exist. 
         [0003]    Contemporary blades are typically at least partially fabricated of a laminated (i.e., layered) fiber/resin composite material. In general, reinforcing fibers are deposited into a resin within a range of predetermined orientations. The fiber orientations are often determined by a range of expected stress and deflection factors that a blade may experience during an expected blade lifetime. The planar interface regions between the laminations are often referred to as interlaminar regions and are normally the weakest element of a composite material. Loads are normally carried in the planes of the laminations and such loads are transferred from the planes of the laminae to an attachment or interface with another component, i.e., the hub. This transfer typically occurs via interlaminar shear, tension, compression, or a combination thereof As a consequence, when load within the laminar planes is increased, stress on the interlaminar regions increases as well. In the event that an interlaminar shear stress limit (i.e., the shearing stress resulting from the force tending to produce displacement between two lamina along the plane of their interface) of an interlamination region is exceeded, the potential for delamination (the separation of a laminated material along the plane of the interlaminar regions) is increased. Delamination results in a reduction in laminate stiffness and may lead to material strain, i.e., elastic deformation of a material as a result of stress. 
         [0004]    Some examples of stress factors are vertical wind shear, localized turbulence (including interaction of the rotor with the tower), gravity, wind flow variations and start-stop cycles. Vertical wind shear is typically defined as the relationship between wind speeds and height above the surface of the earth, i.e., altitude. In general, as the altitude increases, wind speed increases. Given a blade can be 35 meters (˜115 ft.) or more in length, and the subsequent large diameter of rotation (at least twice the blade length plus the diameter of the associated hub), wind speed can increase 5% to 10% above that at the hub centerline from the hub centerline up to the end of the blade at the blade tip with the blade pointing straight upward. The wind speed may also decrease 5% to 10% below that at the hub centerline from the hub centerline down to the end of the blade at the blade tip with the blade pointing straight downward. As the blades rotate, the cyclic increasing and decreasing of the wind shear induces a cyclic bending moment resulting in both in-plane and interlaminar stresses within the blades. 
         [0005]    Localized turbulence includes stationary wakes and bow waves induced by the blades and by the near proximity of the rotating blades to the tower. As the blades rotate through these localized regions, additional in-plane and interlaminar stresses are induced within the blades. Also, as the blades rotate, gravity induces fluctuating bending moments within the blades that also induce in-plane and interlaminar stresses. Cyclic acceleration and deceleration of the blades due to the aforementioned wind flow variations and start-stop cycles induce cyclic stresses on the blades as well. 
         [0006]    The blades are typically designed and manufactured to withstand such stresses including the cumulative impact of such stresses in a variety of combinations. The blades are also designed and manufactured to withstand the cumulative impact of a predetermined number of stress cycles, commonly referred to as fatigue cycles. Upon exceeding the predetermined number of fatigue cycles, the potential for material delamination may increase. 
         [0007]    As described above, blades are typically attached to a rotating hub at attachment regions designed and fabricated to receive the blades. The blades also have integral attachment regions. The hub and the blade attachment regions act as load transfer regions. For example, the weight of the blades and the aforementioned cyclic stresses are transferred to the hub attachment regions via the blade attachment regions. Also, as described above, the majority of the load is carried through the planes of the laminations except in the immediate vicinity of attachments and interfaces. As blade size and weight increase, the laminations and interfaces of the blade attachment regions may have an increased potential for exceeding interlaminar shear stress limits. 
         [0008]    Many fabrication issues can occur with known composite blade strictures, such as, entrained air bubbles, wrinkles, mis-aligned or off-prescribed orientation fibers and non-uniform compaction. All of these issues, alone or in combination, may lead to unwanted delaminations in the blade. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    One aspect of the present invention provides, a blade for a wind turbine including a skin having a braided fiber sock. One or more stiffeners are attached to the braided fiber sock. 
         [0010]    In another aspect, the present invention provides a blade for a wind turbine. The blade has a skin, which includes an inner braided fiber sock and an outer braided fiber sock. A mandrel connects the inner braided fiber sock to the outer braided fiber sock. 
         [0011]    In yet another aspect, the present invention provides a method of making a wind turbine blade. The method includes the steps of, providing a plurality of fibers, braiding the plurality of fibers into a fabric preform and infusing a resin into the fabric preform to form a hardened shell. 
         [0012]    It will be seen that many configurations of the present invention can reduce blade weight at tops of towers while giving the designer several ways to ad just the strength and stiffness of blades to achieve improved structural performance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic illustration of an exemplary wind turbine system; 
           [0014]      FIG. 2  is an illustration of a blade preform during a braiding process, according to an aspect of the present invention; 
           [0015]      FIG. 3  is a cross-sectional illustration of a wind turbine blade, according to an aspect of the present invention; 
           [0016]      FIG. 4  is a cross-sectional illustration of a wind turbine blade, according to another aspect of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  is a schematic illustration of an exemplary wind turbine  100 . In the exemplary embodiment, wind turbine  100  is a horizontal axis wind turbine. Alternatively, wind turbine  100  may be a vertical axis wind turbine. Wind turbine  100  has a tower  102  extending from a supporting surface  104 , a nacelle  106  mounted on tower  102 , and a rotor  108  coupled to nacelle  106 . Rotor  108  has a rotatable hub  110  and a plurality of rotor blades  112  coupled to hub  110 . In the exemplary embodiment, rotor  108  has three rotor blades  112 . In an alternative embodiment, rotor  108  may have more or less than three rotor blades  112 . In the exemplary embodiment, tower  102  is fabricated from tubular steel and has a cavity (not shown in  FIG. 1 ) extending between supporting surface  104  and nacelle  106 . In an alternate embodiment, tower  102  is a lattice tower or a combination of lattice and tubular tower construction. 
         [0018]    Various components of wind turbine  100 , in the exemplary embodiment, are housed in nacelle  106  atop tower  102  of wind turbine  100 . For example, rotor  108  is coupled to an electric generator (not shown in  FIG. 1 ) that is positioned within nacelle  106 . Rotation of rotor  108  causes the generator to produce electrical power. Also positioned in nacelle  106  is a yaw adjustment mechanism (not shown in  FIG. 1 ) that may be used to rotate nacelle  106  and rotor  108  on axis  116  to control the perspective of blades  112  with respect to the direction of the wind. The height of tower  102  is selected based upon factors and conditions known in the art. 
         [0019]    Blades  112  are positioned about rotor hub  110  to facilitate rotating rotor  108  to transfer kinetic energy from the wind into usable mechanical energy, and subsequently, electrical energy. Blades  112  are mated to hub  110  by coupling a blade root portion  120  to hub  110  at a plurality of load transfer regions  122 . Load transfer regions  122  have a hub load transfer region and a blade load transfer region (both not shown in  FIG. 1 ). Loads induced in blades  112  are transferred to hub  110  via load transfer regions  122 . 
         [0020]    In the exemplary embodiment, blades  112  may have a length between about 35 meters (m) (˜115 feet (ft)) to about 52 m (˜171 ft) or more. Alternatively, blades  112  may have any length. As the wind strikes blades  112 , rotor  108  is rotated about rotation axis  114 . As blades  112  are rotated and subjected to centrifugal forces, blades  112  are subjected to various bending moments and other operational stresses. As such, blades  112  may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position and an associated stress may be induced in blades  112 . Moreover, a pitch angle of blades  112 , i.e., the angle that determines blades  112  perspective with respect to the direction of the wind, may be changed by a pitch adjustment mechanism (not shown in  FIG. 1 ) to facilitate increasing or decreasing blade  112  speed by adjusting the surface area of blades  112  exposed to the wind force vectors. Pitch axis  118  for blades  112  are illustrated. In the exemplary embodiment, the pitches of blades  112  are controlled individually. Alternatively, blades  112  pitch may be controlled as a group. 
         [0021]    In some configurations, one or more microcontrollers in a control system (not shown in  FIG. 1 ) are used for overall system monitoring and control including pitch and rotor speed regulation, yaw drive and yaw brake application, and fault monitoring. Alternatively, distributed or centralized control architectures are used in alternate embodiments of wind turbine  100 . 
         [0022]      FIG. 2  illustrates a wind turbine blade in the process of being fabricated, according to aspects of the present invention. A three-dimensional braiding process is used for forming fiber “socks” by the continuous intertwining of fibers. During the braiding process, a plurality of fibers  210  in a matrix array are moved simultaneously across a braiding frame  220 . A fiber extends from a carrier member (not shown) and is intertwined with fibers from other carrier members (not shown) as they are simultaneously moved. The fibers  210  are gathered by the braiding frame  220  and intertwined to form a multi-axial braid. This braiding process is characterized by an absence of planes of delamination in the preform and results in a tough, delamination resistant composite article when the blade preform  230  is impregnated with resin (such as epoxy), metal or other known matrix materials. 
         [0023]    The fibers  210  can include, but are not limited to, fibers such as fiberglass, carbon, aromatic polyamides, aramid or para-aramid (e.g., Kevlar®, a registered trademark of E. I. du Pont de Nemours and Company) either alone or in combination. The fibers  210  form a one piece textile “sock” preform  230  that once infiltrated with resin, becomes the “skin” of a wind turbine blade  112 . 
         [0024]      FIG. 3  illustrates a cross-sectional view of a wind turbine blade, according to one aspect of the present invention. The “sock” preform  330  can incorporate integral stiffeners  340 , which can be co-braided with the preform  330 . The stiffeners  340  improve the span-wise flex of the blade and can be designed to improve the overall characteristics of blade  112 . The stiffeners  340  may be comprised of unidirectional fibers or filled with continuous unidirectional fibers by a secondary processing operation to provide additional stiffness to the integral stiffeners. A pre-cured composite spar  350  (or stringer) can be added in the span-wise direction to provide increased structural rigidity and shear capability. The spar can also be produced from a braided sock preform for further improvements in quality and structural efficiency. 
         [0025]      FIG. 4  illustrates a cross-sectional view of a wind turbine blade formed using a sandwich-type construction, according to another aspect of the present invention. Two sock preforms, an inner perform  460  and an outer perform  470 , can be utilized to encapsulate a mandrel  480  that is “trapped” and co-cured to become the core of the wind turbine blade&#39;s skin. The mandrel  480  can be constricted of multi-piece design to facilitate geometry and assembly and may be formed of Balsa wood or any of a number of other core materials such as foam, non-metallic material, small cell material or honeycomb shaped material. The mandrel may also be inflatable and can include pockets to conform to the integral stiffeners (not shown in  FIG. 4 ). A pre-cured composite spar  450  (or stringer) may be added in the span-wise direction to provide increased structural rigidity and shear capability. 
         [0026]    By incorporating an engineered textile structure in wind turbine blade  112 , the geometry and braid fiber architecture can be optimized to reduce weight and material usage relative to known blade construction without compromising performance or reliability. One advantage the present invention provides is in the one-piece construction of the blade&#39;s “skin” surface or sandwich construction. 
         [0027]    It will thus be appreciated that many configurations of the present invention reduce fabrication issues with blades, such as, entrained air bubbles, wrinkles, off-axis fibers, regions of disband (delamination) and non-uniform compaction. The construction of the blade also reduces blade weight, and is particularly useful in reducing overall blade/rotor weight in wind turbines at tops of towers. Reduced weight blades will also help to reduce wear on yaw and pitch motors, gears, bearings and other components due to lower blade inertia. Reduced blade weight also reduces the cost and possibility of damage during shipment, as well as, facilitating assembly at wind turbine locations. Many configurations of the present invention also give designers a plurality of ways to adjust the strength and stiffness of blades to achieve improved structural performance. 
         [0028]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.