Abstract:
A blade for a wind turbine, includes a shell; a spar member for supporting the shell; and a stiffener, secured to an inside surface of the shell, for enhancing a buckling resistance of the blade.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The subject matter described here generally relates to fluid reaction surfaces with specific blade structures that are formed with a main spar, and, more particularly, to wind turbine blade spars with stringers. 
         [0003]    2. Related Art 
         [0004]    A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If that mechanical energy is used directly by machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is further transformed into electrical energy, then the turbine may be referred to as a wind generator or wind power plant. 
         [0005]    Wind turbines use one or more airfoils in the form of a “blade” to generate lift and capture momentum from moving air that is then imparted to a rotor. Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of the chord line is simply the “chord.” 
         [0006]    Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called “horizontal-axis wind generator” is schematically illustrated in  FIG. 1  and available from GE Energy of Atlanta, Ga. USA. This particular configuration for a wind turbine  2  includes a tower  4  supporting a drive train  6  with a rotor  8  that is covered by a protective enclosure referred to as a “nacelle.” The blades  10  are arranged at one end of the rotor  8 , outside the nacelle, for driving a gearbox  12  that is connected to an electrical generator  14  at the other end of the drive train  6  along with a control system  16 . 
         [0007]    As illustrated in the cross-section for the blade  10  shown in  FIG. 2 , wind turbine blades are typically configured with one or more “spar” members  20  extending spanwise inside of the shell  30  for carrying most of the weight and aerodynamic forces on the blade. The spars  20  are typically configured as I-shaped beams having a web  22 , referred to as a “shear web,” extending between two flanges  24 , referred to as “caps” or “spar caps.” However, other spar configurations may also be used including, but not limited to “C-,” “L-,” “T-,” “X-,” “K-,” and/or box-shaped beams. The spar caps  24  are typically secured to the inside surface of the shell  30  that forms the suction and pressure surfaces of the blade. In configurations, the spar caps  24  form part of the inside surface of the shell  30 . The spar  20  may also be utilized without caps  24  and/or the web  22  may be formed integrally with other portions of the blade  10 , including the shell  30 . 
         [0008]    Modern wind turbine blades  10  have become so large that, even with the structural features described above, they can still suffer from buckling failure at stresses that are smaller than the ultimate strength of materials from which they are constructed. For example, so-called “self buckling” can occur where the vertical length of the blade  10  exceeds a certain critical height, while “dynamic buckling” can occur for even smaller loads that are suddenly applied to the blade, and then released. It is well known that the buckling resistance of a columnar structure can generally be increased, without increasing its weight, by distributing the material in the structure as far as possible from the principle axes of its cross section so as to increase its moment of inertia. However, the profile of the blade  10  is controlled by aerodynamic, rather than structural, considerations. Furthermore, current manufacturing techniques for wind turbine blades  10  also generally require a core over which a skin material can be draped in order to form the contour of the airfoil. And, due to the large surface area of the blade  10 , even small increases in the overall skin thickness can lead to undesirable increases in the weight of the blade  10 . 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    These and other aspects of such conventional approaches are addressed here by providing, in various embodiments, a blade for a wind turbine including a shell; a spar member for supporting the shell; and a stiffener, secured to an inside surface of the shell, for enhancing a buckling resistance of the blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various aspects of this technology invention will now be described with reference to the following figures (“FIGs.”) which are not necessarily drawn to scale, but use the same reference numerals to designate corresponding parts throughout each of the several views. 
           [0011]      FIG. 1  is a schematic side view of a conventional wind turbine. 
           [0012]      FIG. 2  is a schematic, cross-sectional view of the blade taken along chord section line II-II in  FIG. 1 . 
           [0013]      FIG. 3  is a schematic, cross-sectional view of another wind turbine blade. 
           [0014]      FIG. 4  is a schematic partial cross-section of a blade taken along chord section line IV-IV shown in  FIG. 3 . 
           [0015]      FIG. 5  is an enlarged partial cross-section of the blade shown in  FIG. 3 . 
           [0016]      FIG. 6  is a schematic, partial orthographic view of a wind turbine blade. 
           [0017]      FIG. 7  is another schematic, partial orthographic view of a wind turbine blade. 
           [0018]      FIG. 8  is yet another schematic, partial orthographic view of a wind turbine blade. 
           [0019]      FIG. 9  is a schematic partial cross-section of a blade taken along chord section line IX-IX shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 3  is a schematic, cross-sectional view of a wind turbine blade  30  for use with the wind generator  2  shown in  FIG. 1  and/or any other suitable wind turbine. For example, the blade  10  shown in  FIGS. 1 and 2  may be replaced with the blade  30  and/or modified to include any of the features of the various configurations of the blades  30  illustrated in  FIGS. 3-7 , and/or combinations of those features. 
         [0021]      FIGS. 3-7  illustrate various structures corresponding to means for enhancing the buckling resistance of the blade  30 . For example, in  FIG. 3 , stiffener strips  32  through  50  are secured to an inside surface of the shell  26 . In particular, the flange strips  32  are long, thin, and narrow structures that are secured to the flange  24 . As illustrated in the enlarged, schematic partial cross section of  FIG. 5 , one or both of the flange strips  32  may include various layers such as a crown skin layer  322  and/or a core layer  324 , where the core layer and/or skin layer may be formed from materials including, but not limited to, balsa wood, foam, and reinforced composites such as glass reinforced plastic. The core layer  324  may also be hollowed in order to further reduce weight. 
         [0022]    Buckling factor analysis for various configurations suggests that continuous strips, with a 50 millimeter by 25 millimeter rectangular, cross sections may provide the greatest enhancement for the least increase in weight. However, other configurations may also be used including, but not limited to, 75×75, 75×50, and 50×50 millimeter dimensions, and/or non-rectangular, discontinuous, and transverse stiffeners that are not necessarily arranged on the flange  24 . 
         [0023]    Alternatively, or in addition to flange strips  32 , a continuous stiffener  34  may be arranged to extend spanwise across the blade  30  and secured to the shell  26  at a position which is displaced from the flange  24 . Stiffeners with non-rectangular cross-sections may also be used, such as the round stiffener  36  shown in  FIG. 3  and/or elliptical stiffeners, triangular stiffeners, pentagonal stiffeners, and so on. The stiffeners do not necessarily need to extend across the entire span of the blade  30 . For example, the stiffener  38  extends only part way across the span of the blade  30 ) and has an angled top surface resulting in one of many possible variations on a non-rectangular cross section. Various end configurations may also be provided for the stiffeners. For example, the stiffener  40  has one rounded end and one angled end. 
         [0024]    The stiffener  42  illustrates a square plan configuration which extends equal distances in both the chordwise (or “cross”) and spanwise directions of the blade  30 . However, other plan configurations may also be used including elliptical, circular, triangular, pentagonal, and etc. A transverse rectangular stiffener strip  44  extends substantially chordwise across the blade  30  in  FIGS. 4 and 9 , while the angled stiffener strip  46  extends substantially chordwise and spanwise across the blade  30 . Other configurations that extend both substantially chordwise and spanwise across the blade  30  include the cross stiffener  48  and the grid stiffeners  50  shown in  FIGS. 4 and 8 . 
         [0025]    The stiffeners are not necessarily required to have the same thickness across the span and/or chord of the blade  30 . For example,  FIG. 6  illustrates another pair of flange strips  32  that are thickened in the central regions where buckling resistance needs to be enhanced the most.  FIG. 7  illustrates other stiffeners  34  having variable cross-sections along the span of the blade  30 . The grid stiffeners  50  may also have variable width, thicknesses, and/or spacings between members. 
         [0026]    The various stiffeners may also be arranged at other locations in the blade  30  than shown and described here. In fact, the buckling resistance of the blade  30  may be significantly enhanced by arranging the stiffeners in areas of the blade with the longest chord. As illustrated in  FIG. 8 , a grid stiffener  50  may be arranged with one or more spanwise rectangular stiffener strips  34  arranged substantially parallel to the trailing edge of the blade  30 . Additional transverse strips  44  are then arranged to extend chordwise from the outermost of the strips  34  to the edge of the flange  24  (not shown in  FIG. 8 ). Various spacings may be provided between the stiffener strips  34  and transverse strips  44  that form the grid stiffener  50  illustrated in  FIG. 8 . For example, the spacing may be about the width of one to two stiffener strips. 
         [0027]    The various embodiments described above provide enhanced buckling resistance for wind turbine blades. It should be emphasized that the embodiments described above, and particularly any “preferred” embodiments, are merely examples of various implementations that have been set forth here to provide a clear understanding of various aspects of this technology. It will be possible to alter many of these embodiments without substantially departing from scope of protection defined solely by the proper construction of the following claims.