Patent Publication Number: US-2011052404-A1

Title: Swept blades with enhanced twist response

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention is directed generally to wind turbine blade design. 
     2. Description of the Related Art 
     Wind turbine blades typically comprise a blade shell formed from one or more skins, which may themselves be formed from several layers of fabric. Swept blades, particularly swept blades that utilize sweep-twist coupling to shed loads, may benefit from fabrics and uses of fabrics which differ from those traditionally used in the construction of straight blades. In particular, the fabric of a straight blade generally does not need to be significantly curved within the plane of the fabric to accommodate the shape of the blade, while such curvature may be necessary to accommodate certain fabric layouts used in a swept blade. This curvature places additional constraints on the type of fabrics which can be used, but the geometry of swept blades can also be leveraged to provide or amplify a desired response under load though the use of specific fabrics and orientations. 
     SUMMARY OF CERTAIN EMBODIMENTS 
     In one embodiment, a swept wind turbine blade is provided, including a swept blade shell, the swept blade shell including a unidirectional fabric layer, where the unidirectional fabric layer is disposed within the blade shell such that fibers of the unidirectional fabric layer extend in a substantially constant direction over the length of the fabric layer. 
     In another embodiment, a swept wind turbine blade is provided, including a swept blade shell, the swept blade shell including a first unidirectional fabric section, where fibers of the first fabric section extend in a substantially constant direction over the length of the first fabric section, a second unidirectional fabric section, where fibers of the second fabric section extend in a substantially constant direction over the length of the second fabric section, where at least a portion of the second fabric section is located outboard of the first fabric section, and a transition region between the first and second fabric sections, where the transition region is substantially parallel to the fibers of at least one of the first and second fabric sections. 
     In another embodiment, a swept wind turbine blade is provided, including a blade shell, the blade shell including a layout axis which sweeps in an aft direction as the layout axis moves outward, a unidirectional fabric section, where a forward angle between the fibers of the unidirectional fabric section and the layout axis increases in an outboard direction of the layout axis. 
     In another embodiment, a method of fabricating a swept turbine blade is provided, the method including providing at least one swept shell mold, the mold defining at least a root section, a location of maximum chord, a first edge which is at least partially convex, a second edge which is at least partially concave in a region outboard of the location of maximum chord, and positioning at least one unidirectional fabric within the blade mold such that fibers of the unidirectional fabric extend in a substantially constant direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a wind turbine comprising three swept wind turbine blades. 
         FIG. 2  is a top plan view of a swept wind turbine blade. 
         FIG. 3  is a cross-sectional view of the swept wind turbine blade of  FIG. 2  taken along the line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a top plan view of an alternative swept wind turbine blade comprising a unidirectional fabric oriented substantially parallel to the inboard blade axis. 
         FIG. 5  is a top plan view of an alternative swept wind turbine blade comprising two types of unidirectional material oriented substantially parallel to the inboard blade axis. 
         FIG. 6  is a top plan view of an alternative swept wind turbine blade comprising two types of unidirectional material oriented at a forward angle to the inboard blade axis. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
       FIG. 1  depicts an exemplary wind turbine  10  comprising three wind turbine blades  100  extending radially from a wind turbine hub  30  mounted on a tower  40 . The wind turbine rotates in a direction  20 , such that a leading edge  110  of a blade  100  and a trailing edge  120  are oriented as shown in  FIG. 1 . 
       FIG. 2  is a top plan view of an exemplary swept wind turbine blade  100  of  FIG. 1 . The chord length of the blade is measured from the leading edge  110  to the trailing edge  120  within a twisting plane whose outer part lies near the plane of rotation of the blade  100  in its full power setting. This chord length initially increases as the distance from a blade root  130  increases, until reaching a maximum chord length  150 , and then decreases towards a tip  140  of the blade. 
     It can be seen in  FIG. 2  that the tip  140  of the blade is swept backwards, in a direction away from the leading edge  110 . The particular shape of the blade may be defined with respect to a layout axis  155 , which can be alternatively referred to as the stacking axis. In certain embodiments, the layout axis is defined as a line connecting the centers of area or other chosen reference points (such as percentage of chord from the leading edge) within transverse sections of the airfoil. 
     The outer surfaces of typical modern wind turbine blades, also referred to herein as shells, are comprised of an inner skin, an outer skin, and a stabilizing core, as will be described in greater detail with respect to  FIG. 3  below. Typically, these skins run from the leading edge  110 , or nose of the blade, to the trailing edge  120 , or tail of the blade, so that the need to cut or join fabrics at an intermediate point is minimized or avoided, simplifying the construction of the blade. For blades with very large maximum chord lengths, fabric of sufficient width to run from the nose to the tail may be prohibitively difficult and/or expensive to obtain. Thus, production of the fabric covering for the blade length generally requires the joining of fabric sections to form a skin which extends from the nose to the tail of the blade. Of course, it is desirable to minimize the number of such joints. These skins thus typically provide constant mechanical properties, such as the shear and axial modulus of the skin, along their lengths. 
     In certain embodiments, these skins may comprise multiple types of fabric, so as to provide a resultant structure equipped to handle the loads to which a wind turbine blade will be exposed while in use. Two commonly used types of fabrics are unidirectional fabrics, in which the fibers are oriented in a single direction, and double-biased fabric, in which the fibers are oriented at an angle to one another. By utilizing a combination of unidirectional and double-biased fabrics, a structure can be provided in which the unidirectional fibers bear certain loads, primarily resisting bending of the blade, and the double-biased fabric bears other loads, providing resistance against both bending and twisting. 
       FIG. 3  is an illustration of an exemplary cross-section of the blade  100  of  FIG. 2 , taken along the line  3 - 3  of  FIG. 2 . The blade  100  comprises an upper shell  160   a  located on a first or upper surface  180   a  of blade  100  and a lower shell  160   b  located on a second or lower surface  180   b  of blade  100 , and an interior stiffening structure comprising spar caps  170   a  and  170   b  and shear web  172 , each of which are located in or between the upper and lower shells. As noted above, the shells  160   a  and  160   b  are composite structures. In particular, shell  160   a  comprises an outer skin  162   a , an inner skin  164   a , and a core  166   a  located therebetween. The outer and inner skins  162   a  and  164   a  may comprise fiberglass or another suitable material in an appropriate thickness. The particular thickness and properties of the outer and inner skins  162   a  and  164   a  may vary significantly in various embodiments. 
     The interior stiffening structure, referred to herein as a spar or main spar, comprises the pair of spar caps  170   a  and  170   b  extending adjacent the inner skins  164   a  and  164   b  of the upper and lower shells, and extending along part of the chord length of the shells, and the shear web  172  extending between the spar caps  170   a  and  170   b . In the illustrated embodiment, the spar caps  170   a  and  170   b  are disposed between the inner skin  164   a  and the outer skin  164   b  of the adjacent shell sections and of the stiffening cores  166   a  and  166   b . In such an embodiment, the skins may be formed over the spar caps and the core sections to form shells  160   a  and  160   b , and the shells may then be assembled to form a blade. In an alternate embodiment, however, the shells may be formed without the spar caps, such that the inner skin is brought into contact with the outer skin, leaving a gap between the core sections where a spar cap can later be placed. 
     In the illustrated embodiment, a single shear web  172  extends between the spar caps  170   a  and  170   b  to form essentially an I-beam structure. In certain embodiments, some or all of the spar caps  170   a  and  170   b  and shear web  172  comprise a high performance material such as carbon fiber, although these structural members may comprise multiple materials at different locations within the structural members. 
     As noted above, the skins are formed from multiple layers of fabric, which can be placed one upon another in a mold to form a stack of fabric of the desired thickness in the desired blade shell shape. Stiffness of the structure is provided by resin which can be applied to the fabric prior to or during the molding process. Fabric pieces which run from the root of the blade to the tip of the blade, or a substantial section thereof, may provide optimal performance as transition regions between fabric pieces can be avoided over the length of the blade. When forming a swept blade, if fabric is curved in the direction of the blade sweep, portions of the fabric near the leading edge  110  of the blade are stretched to accommodate the blade shape, and the portions of the fabric near the trailing edge  120  of the blade are compressed. 
     It will be understood that the junctions between the shell sections of the blade may not be located directly at the leading and trailing edges of the blade. Thus, the leading and trailing edges of a mold for a blade shell may not correspond directly to the leading and trailing edges of the eventual blade. Nevertheless, a blade mold for a swept turbine blade will generally have a first edge which is at least partially convex which will form the edge of the blade shell located near the leading edge. Similarly, the blade mold will generally have a second edge which is at least partially concave which will form the edge of the blade shell located near the trailing edge, although other portions of the trailing side of the blade mold may be convex, particularly around the region of maximum chord. The location of maximum chord for a shell section may have a length which is less than the maximum chord length of the finished blade, because at least one of the blade shells may not extend all the way to the leading or trailing edge of the finished blade. In some embodiments, the leading joint between an upper and lower blade shell may be located at the stagnation point, rather than directly at the leading edge. The blade mold will also have other sections, such as a root region to form the base of the blade shell, a location of maximum chord as noted above, a transition section between the root and the location of maximum chord, and an outboard section at greater radius than maximum chord. 
     Blade skins may comprise a combination of multiple types of fabric, including biased or double-biased fabric, in which the fibers are oriented at angles to the fabric direction, and unidirectional fabric, in which the fibers are oriented in the same direction, usually aligned with the fabric direction. When applied within a swept blade, unidirectional fabric is curved as discussed above to generally follow the curve of the blade, or the curve of the layout axis, such that the fibers are oriented generally parallel to the layout axis of the blade. This arrangement provides the maximum bending resistance, but will place constraints on the type of unidirectional fabrics which can be used in swept blades. 
     When the unidirectional fabric is curved to be used in a swept blade, the fabric must allow the fibers to shear somewhat relative to one another to accept this curvature. Many unidirectional fabrics are unable to accept the curvature required by some swept blade designs due to the inability to shear in this manner. 
       FIG. 4  illustrates an exemplary swept blade  200  in which unidirectional fiber is used, but is not curved to follow the curvature of the blade. The blade  200  comprises a layout axis  255  running from the root  230  of the blade to the tip  240  of the blade, which curves aft as the blade sweeps in the aft direction. The shell  260  comprises unidirectional fabric in which the fibers (the orientation of which are illustrated by the shading) are oriented substantially parallel to the inboard blade axis  205  of the blade  200 . 
     Because the fibers of the unidirectional fabric are oriented in a constant direction across the length of the blade, the fibers in the illustrated embodiment begin substantially parallel to the layout axis  255  near the root  230  of the blade but the layout axis  255  makes a gradually increasing angle with the fibers of the unidirectional fabric as the layout axis approaches the tip  240  of the blade. This increasing angle between the fibers and the layout axis will result in an enhanced twist response near the tip of the blade. This increase in the ability of the tip  240  of the blade to twist in response to thrust loading is particularly helpful when the blade is exposed to very turbulent conditions, as twist of the blade tip can shed loads and decrease the spikes in loading experienced as a turbine blade is exposed to these turbulent conditions. 
     In particular embodiments, it may be desirable to utilize premium material near the tip of the blade, as reduced thickness and weight of the outboard sections of the blade can have increased effects on its efficiency. Premium material may be a material which has a higher strength to weight or stiffness to weight ratio than that used in inboard sections. For example, it may be desirable to use carbon fiber in outboard sections when fiberglass is used in inboard sections. Any reduction in airfoil thickness or weight at these outboard sections can be highly desirable. 
     However, difficulties may arise in constructing a transition zone to transfer loads between an inboard material such as fiberglass and a lighter, stronger outboard material such as carbon fiber. In a blade in which the unidirectional fiber orientation tracks the curvature of the blade, a transition zone must be formed to transfer loads from fibers of the inboard material to fibers of the outboard material, and such a transition zone will generally be thick, potentially increasing both the weight and thickness of an outboard section of the blade. In addition to the mechanical tradeoffs of increased weight and thickness, the formation of the transition zone may be more labor-intensive and introduces a possible point of failure. A transition zone may be avoided by forming the entire shell section from the premium material, but use of premium unidirectional material over the entire length of the blade may not be cost effective, as the reduced weight and increased stiffness of premium material may not be necessary in inboard sections of the blade. 
       FIG. 5  illustrates a swept blade  300  which utilizes an uncurved unidirectional material to gradually introduce a transition between an inboard material and an outboard material without the need for a bulky transition region. The device is similar to the blade  200  of  FIG. 4 , and comprises a shell comprising a unidirectional fabric oriented such that the fibers extend substantially parallel to the inboard blade axis  305 . The blade  300  is swept aft along a layout axis  355 . The blade  300  differs from blade  200  of  FIG. 4  in that the shell comprises a first shell section  390  extending from the root  330  of the blade and a second shell section  395  near the tip of the blade. The boundary  398  between the first shell section  390  and the second shell section  395  extends generally parallel to the fibers of the unidirectional fabrics, such that few or none of the fibers are terminated at the boundary. 
     The shape of the boundaries will be determined based at least in part on the shape and curvature of blade itself, and the outboard material of the second shell section  595  can be introduced at any location, depending on the amount of premium material necessary for a particular blade design. Given the shape of the second blade section  395 , the percentage of premium material in the blade shell will gradually increase. This gradual introduction of the premium material allows the load to be taken up slowly by the premium material, reducing the need for a thick transition zone between the inboard material and the outboard material. 
     In alternate embodiments, the unidirectional fiber may be oriented at an angle to the inboard axis of the blade.  FIG. 6  illustrates an embodiment of such a blade  400 , which comprises an inboard blade axis  405  and is swept aft along a layout axis  455 . Like the blade  300  of  FIG. 5 , the blade  400  comprises a first shell section  490  of unidirectional material extending from the root  430  of the blade and a second shell section  495  which may comprise a different type of unidirectional material near the tip  440  of the blade. A boundary  498  extends between the first and second shell sections  490  and  495 . 
     The fibers of the unidirectional fabric of both the first and second shell sections  490  and  495  are, oriented parallel to one another, and at an angle slightly forward of the inboard blade axis  405 . In contrast to the blade  300  of  FIG. 5 , it can be seen that the boundary  498  between the first and second shell sections  490  and  495  is oriented at an angle to the blade, reducing the length of the boundary  498 . Thus, in an embodiment with an angled unidirectional fabric, the load must be more quickly taken up by the second material, but the shorter boundary may reduce the additional weight added by the boundary  498 , 
     It can also be seen that near the tip  440  of the blade, the forward angle between the fibers of the unidirectional fabric and the layout axis  455  is greater than in the blade  300  of  FIG. 5 . This increase in the fabric angle will further increase the twist response of the blade tip  440  of the blade when the blade is under thrust loading. 
     As the forward angle between the unidirectional fabric fibers and the inboard blade axis increases, additional strips of fabric may be required, particularly for larger blades, as the width of the fabric used may become a limiting factor. As additional fabric strips are used, additional transition regions between adjacent fabric strips become necessary, increasing the weight and thickness of the blade shell at these transition regions. Thus, larger angles of unidirectional fabric may not be practical, particularly for larger blades. 
     In other embodiments, the unidirectional fabric may be angled aft of the inboard blade axis, rather than forward of the inboard blade axis, as discussed with respect to  FIG. 10 . In still other embodiments, multiple sections of unidirectional fabric need not be oriented such that the fibers are parallel to one another. For example, an inboard section of unidirectional fabric may be oriented more parallel to the inboard blade axis, while an outboard section may be oriented at a different angle, either forward or aft of the inboard blade axis. A fiber line of one of the two fabric sections may still provide a natural transition zone between the two sections, while gradually introducing load not precisely on a particular fiber line of the other fabric section. 
     Fabrication of a blade such as the blade  300  of  FIG. 5  or the blade  400  of  FIG. 6  may comprise placing a section of unidirectional fabric within the blade mold such that the fabric remains parallel to or at a substantially constant angle to the inboard blade axis. It will be understood that the unidirectional fabric will generally not remain flat, as the fabric will conform to the three dimensional shape of the mold. If multiple fabric pieces are required to form a single layer of material across the length of the blade, transition regions may be formed in any appropriate manner, including alternating inboard and outboard sections of fabric. 
     Various other combinations of the above embodiments and methods discussed above are contemplated. It will be understood that the above fabrics and fabric configurations may be used either alone or in conjunction with other fabrics and configurations discussed above and known to persons of ordinary skill in the art. For example, these fabrics and techniques may be used in the fabrication of only one of the skins which forms the blade shell. It is also to be recognized that, depending on the embodiment, the acts or events of any methods described herein can be performed in other sequences, may be added, merged, or left out altogether (e.g., not all acts or events are necessary for the practice of the methods), unless the text specifically and clearly states otherwise. 
     While the above detailed description has shown, described, and pointed out novel features as applied to various embodiments, various omissions, substitutions, and changes in the form and details of the device of process illustrated may be made. Some forms that do not provide all of the features and benefits set forth herein may be made, and some features may be used or practiced separately from others