Abstract:
A variable blade pitch wind turbine and method of passively varying the blade pitch is presented. Each blade of the turbine rotor can individually and passively rotate about a respective blade axis to continuously vary the pitch of the blades to adjust to the ever changing wind speed and direction without active controls or other mechanical or electrical induced inputs to force rotational movement of the blades. Sections of each blade can respectively rotate to change the blade twist. In one example, a second rotor is used to improve the efficiency of the wind turbine and increase output of the electric generating equipment.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part and claims benefit of U.S. patent application Ser. No. 12/541,590, filed on Aug. 14, 2009, which is herein incorporated by reference in its entirety. This application is related to U.S. patent application Ser. No. 12/541,603, filed on Aug. 14, 2009, which issued as U.S. Pat. No. 8,454,313 on Jun. 4, 2013, both of which are herein incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The general field of invention is alternative energy. 
       BACKGROUND 
       [0003]    There has long been motivation for alternative energy sources to reduce the dependency on non-renewable energy sources such as oil and coal. There has long been interest in harnessing wind to generate electricity, particularly in areas with a high or relatively continuous stream of wind energy. 
         [0004]    There have been challenges with previously proposed solutions for harnessing wind energy through use of electric wind turbines. These large windmill-type structures suffer from many disadvantages, including the inability to adjust to variable wind conditions in either or both direction and velocity. Due to the size and mass of large wind turbines, it may be costly to adapt the wind turbine design to every possible operating environment. Prior wind turbines may also suffer from inefficient and complex designs. 
         [0005]    Prior wind turbine configurations may suffer from relatively low efficiency rates due to standard configurations that are employed in operating environments not suitable for the particular wind turbine design or initial set-up. In an attempt to overcome these deficiencies, prior wind turbine designs may use various mechanisms, such as segmented blades, weights, flaps, actuators, sensors, motors and springs, in an effort to improve efficiency and control a rotational speed of the rotors to try and keep them from experiencing an overspeed condition. These prior configurations may also use additional forces, such as centrifugal force, centripetal force, and mechanical and electromechanical forces, to alter an aerodynamic orientation or shape of the blade in an attempt to control the rotational speed of the rotors. This in turn may cause a less efficient blade orientation and create a “governing” position that reduces the risk of encountering an overspeed condition at the expense of turbine efficiency. 
         [0006]    Previously disclosed methods for estimating alignment of the blades with the wind source in both direction and blade angle of attack may also negatively affect efficiency. Blade alignment is typically determined using averaged data collected over a given time duration, not in real time. This may result in a less than optimal blade orientation for the current wind conditions and a corresponding drop in turbine efficiency. 
         [0007]    Thus it is desirable to improve on prior wind turbine designs to increase the turbine&#39;s ability to harness the wind energy, improve the efficiency in converting the wind energy to electricity by responding quicker to wind speed and wind direction changes, enable the wind turbine to operate through a larger range of wind speeds, and to simplify and improve their design and operation. 
       BRIEF SUMMARY 
       [0008]    The present invention relates to electric wind turbines and methods for varying the orientation and/or shape of the rotor blades to better adapt to changing wind conditions and increase the capture of wind energy. One method of varying the shape involves varying the twist of the rotor blade as discussed in more detail herein. 
         [0009]    In one example of the wind turbine invention, a first or front rotor and a second or rear rotor are used on opposing ends of the power transmission housing or nacelle. In alternate examples of this invention, the rotor blades include separate blade sections that are able to freely rotate relative to each section through a predetermined range to balance aerodynamic forces acting on the airfoils by passively adjusting an angle of attack for each blade section in response to changing wind speeds and conditions occurring at different positions along the path of rotation and thereby changing the orientation and/or twist of the entire blade. 
         [0010]    In an alternate example, the individual rotor blade supports extend from a central hub and are able to rotate about the blade support axis independent of all other blade supports, and passively change the rotational position of the entire blade to balance the aerodynamic forces acting on the airfoils by passively adjusting an angle of attack of the blade in response to different wind speeds or changing wind conditions. 
         [0011]    In an another example, both the separate sections of the individual rotor blade and the rotor blade support are able to rotate through predetermined ranges to further increase the adaptability of the wind turbine to the wind conditions. 
         [0012]    In an alternate example of a two rotor design, the wind turbine acts similar to a turbine engine. The first rotor blades act like the stator blades in the turbine engine by setting up the wind for the rear rotor. The first rotor/stator blades also redirect the wind to provide a more efficient angle of attack on the second rotor blades. This stator and rotor effect increases the ability to harness the wind energy for increased electrical output and efficiency. 
         [0013]    In an alternate example of a two rotor design, the first rotor blades are not as long as the blades on the downwind second rotor blades to provide a passive yaw condition that improves the ability to harness the wind energy for increased electrical output and efficiency. 
         [0014]    In alternate examples where only a single rotor is used, the above mentioned examples of utilizing separate blade sections passively rotatable with respect to each section and/or passive rotation of the blade support relative to a central hub may be used depending on the geographic location and anticipated climate conditions. 
         [0015]    The invention further includes a variable twist wind turbine rotor blade that passively adjusts the blade twist to balance the aerodynamic forces on the airfoils to adjust the angle of attack in response to the immediate wind conditions confronting the blade. The change or variation in twist, in one example, uses separate blade sections that are independently rotatable with respect to the blade support such that the change in rotation of the separate blade sections alters an overall shape of the airfoil or twist of the blade. In another example, the blade sections are rotatable with respect to other blade section(s). In a third example, the blade support is freely rotatable in a two dimensional plane parallel to the mounting surface with respect to a hub to change an orientation of a portion of the blade or the entire blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
           [0017]      FIG. 1  is a partial front perspective view of an exemplary wind turbine employing two rotors; 
           [0018]      FIG. 2  is a partial front perspective view of the wind turbine of  FIG. 1 , with a nacelle removed to illustrate power generation equipment housed within the nacelle; 
           [0019]      FIG. 3  is a front perspective view of a front rotor employed with the wind turbine of  FIGS. 1 and 2 ; 
           [0020]      FIG. 4  is a perspective view of a first or front blade assembly with individual blade sections shown arranged in an aligned design position; 
           [0021]      FIG. 5  is a perspective view of the blade assembly of  FIG. 4  with the individual blade sections shown arranged in an unaligned position; 
           [0022]      FIG. 6  is an end view of a blade section that includes twist; 
           [0023]      FIG. 7  is an enlarged partial view of a portion of a blade identified as A in  FIG. 4 ; 
           [0024]      FIG. 8  is an enlarged partial perspective view of a portion of a blade identified as B in  FIG. 6  illustrating a pivot limiter; 
           [0025]      FIG. 9  is a partial section taken along section lines  8 - 8  in  FIG. 7  illustrating a bearing assembly; 
           [0026]      FIG. 10  is a partial section taken along section lines  8 - 8  in  FIG. 7  illustrating an alternatively configured bearing assembly; 
           [0027]      FIG. 11  is an enlarged partial perspective view of a portion of a blade identified as A in  FIG. 4  illustrating an alternatively configured pivot limiter; 
           [0028]      FIG. 12  is a partial section taken along section line  11 - 11  in  FIG. 10 ; 
           [0029]      FIG. 13  is an end view of adjacent blade sections identified as  12  in  FIG. 6 , showing the blades sections in the aligned position; 
           [0030]      FIG. 14  is an end view of adjacent blade sections identified as  13  in  FIG. 6 , showing the blade section in an unaligned position; 
           [0031]      FIG. 15  is an end view of adjacent blade section identified as  14  in  FIG. 6 , showing the blade sections in a second unaligned position; 
           [0032]      FIG. 16  is an end view of a blade section with a blade support positioned along a chord line of the blade; and 
           [0033]      FIG. 17  is an end view of a blade section with the blade support located in an alternative position. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Various examples of an inventive wind turbine  20  are shown in  FIGS. 1-17 . With particular reference to  FIGS. 1 and 2 , a wind turbine  20  may include a tower  22  (partially shown) anchored to a foundation (not shown) secured to the earth by means known to those skilled in the field. Tower  22  is typically 200-300 feet in height depending on the application. Secured to a top of the tower  22  is a nacelle  24  that houses power generation equipment  26  (see  FIG. 2 ). Although the nacelle  24  is illustrated atop tower  22 , it is within the scope of the invention that the nacelle  24  could be mounted to other structures or towers of alternate heights and tower constructions. 
         [0035]    In the example shown in  FIGS. 1 and 2 , wind turbine  20  includes a front rotor  28  positioned at one end of the nacelle  24  and a rear rotor  30  positioned at the opposite end of the nacelle  24 , as generally illustrated. Hereinafter, front rotor  28  is referred to as first rotor  28 , and rear rotor  30  is referred to as second rotor  30 . Each rotor  28 ,  30  includes a respective front hub  32  and rear hub  34 . 
         [0036]    Referring to  FIG. 3 , an example of first rotor  22  is illustrated in detail. In one example, wind turbine  20  may include only first rotor  22  connected to the power generation equipment  26  located inside the nacelle  24 . Alternatively, wind turbine  20  may include multiple first rotors  22 , for example, arranged on opposite sides of the nacelle  24 . The first rotor  22  may include at least two spindles  36  (three shown) connected to the front hub  32  and extending radially outward from a rotational axis  38 , as generally shown. In the example illustrated, first rotor  28  includes at least two elongate front blade supports  40  (three shown). Blade supports  40  connect to first hub spindles  36  through a mounting collar and mechanical fasteners. Blade supports  40  extend radially outward from rotational axis  38 , each defining a blade axis  42 . 
         [0037]    In one example of wind turbine  20 , blade supports  40  are rigidly fixed to the spindles  36  and do not translate or rotate relative to spindles  36  or front hub  32 . In an alternate example, blade supports  40  are configured to rotate relative to spindles  36  about blade axis  42 . In this example, spindles  36  may include a bearing within spindle  36  to secure blade support  40  while permitting rotation about blade axis  42 . Blade support  40  may be freely rotatable about blade axis  42  through a predetermined and limited angle of rotation through mechanical stops or barriers similar to that described later with respect to the blade sections. Rotation of blade support  40  is entirely passive in the illustrated embodiments. Wind turbine  20  does not use any electrical, electro-mechanical or mechanical mechanisms, including sensor, flaps, actuators, motors, springs or flyweights, to impart a force or energy to forcibly rotate blade support  40  relative to the respective spindle  36 . Rather, the force that produces rotation of blade support  40  relative to spindle  36  is the result of aerodynamic forces acting on a blade  44  attached to blade support  40  that act to passively adjust an angle of attack of blade  44  to help optimize performance and efficiency of the wind turbine  20 . 
         [0038]    Blade supports  40  radially extend from the rotational axis  38  and define a geometric plane  48  through which part, or substantially all, of blade supports  40  lie in and rotate through in operation. In one example, blade supports  40  are hollow, rigid tubular rods made from steel. Other lengths and numbers of blade supports  40 , as well as different materials and mechanical connections may be used without deviating from the invention. In one example, blade supports  40  are made from composite materials, such as carbon fiber reinforced resin. 
         [0039]    With continued reference to  FIGS. 1-3 , an example of first rotor  28  includes three blades  44  connected to a respective blade support  40  and extending radially outward along blade axis  42 . Blades  44  include an inner end  50  and a radially distant outer or distal end  52 . Each blade  44  includes a leading or cutting edge  54 , which is the first portion to contact air in the direction of rotation (clockwise about rotational axis  38  as viewed from the perspective of  FIG. 3 ), and a trailing edge  56  on an opposite side of the blade  44 . The ends  50 ,  52  and edges  54 ,  56  define a perimeter of a wind bearing surface  58  and an opposing leeward surface  60  on the opposite side of the blade  44 . Wind bearing surface  58  substantially faces toward wind source  46 . 
         [0040]    Blade  44  may be made from a variety of materials, including, but not limited to, reinforced resin or polycarbonate. An outer skin of the blade  44  defines a substantially hollow interior cavity that may include internal reinforcing ribs as needed to meet a particular application and performance requirements. Other materials, such as lightweight ferrous and non-ferrous metals, composites, as well as other materials and constructions, may be also be employed. Although three blades  44  for first rotor  28  are illustrated, alternatively a greater or less number of blade supports  40  and associated blades  44  may be used to suit particular application and performance requirements. 
         [0041]    In one example of wind turbine  20 , first rotor blades  44 , when assembled and connected to the respective blade supports  40 , form a single piece blade unit that is rigidly fixed to blade support  40 . Blade  44  does not translate or rotate relative to blade support  40 . Instead, blade support  40  supporting the single unit fixed blade  44  is capable of rotating about blade axis  42  with respect to first hub spindle  36 , as previously described. Rotation of blade support  40  adjusts, modifies or varies an angle of attack of the respective blade  44  relative to the wind source  46 . Blade support  40  and blade  44  passively rotate in substantial unison in response to changes in the aerodynamic forces acting along the pressure and leeward surfaces  58 ,  60  of the blade  44 , respectively, to increase an ultimate power output of generator  26  and/or increase an efficiency with which power is generated. Passive rotation of blade support  40  and blade  44  occurs without active controls or other artificially generated (e.g., electrical or mechanical) forces, and is in response to the aerodynamic forces acting on the blade  44 . Changes in the wind source  46 , such as speed and direction, affect the aerodynamic forces acting on blade  44  and cause support  40  and blade  44  to rotate relative to spindle  36  to adjust the angle of attack of blade  44 . As described in greater detail subsequently, the response characteristics of blade  44  to changes in wind source  46  may be tailored for a particular application by adjusting the location of blade support  40  relative to a maximum thickness of blade  44 . The position of blade support  40  (which defines the blade axis of rotation  42 ) relative to blade  44  determines, at least in part, how blade  44  responds to changes in the wind source  46  to change the angle of attack. The passively adjustable control system for regulating the angle of attack of blade  44  operates continuously in real time to accommodate changes in wind source  46  to optimize the performance and efficiency of wind turbine  20 . 
         [0042]    Blade  44  may alternatively be rotatably attached to support  40 , rather than configuring blade  44  and blade support  40  as a unitized structure that rotate together, as previously described. With this configuration, blade support  40  may be prevented from rotating relative to spindle  36 . Blade  44  may include one or more bearings, which may be configured substantially the same or similar to that shown in  FIGS. 9 and 10 , and as further described below, for mounting blade  44  to blade support  40 . 
         [0043]    With reference to  FIGS. 4 and 5 , blades  44  may include multiple sections that may be independently pivoted relative to one another to vary a twist of blades  44 . Each blade  44  may be divided or separated into at least two sections that can rotate relative to blade support  40  about blade axis  42 . In the illustrated example, blade  44  includes a first blade section  62 , a second blade section  64  positioned adjacent and radially outward of first blade section  62  along blade support  40 , and a third blade section  66  positioned adjacent and radially outward of second section  62 . Second blade section  64  is disposed between first blade section  62  and third blade section  66 . Alternatively, blade  44  may be divided into fewer or more blade sections to accommodate the design and performance requirements of a particular application. Unlike the previously discussed example, where blade  44  is rigidly fixed to blade support  40 , each of first  62 , second  64  and third  66  blade sections may be rotatably connected to blade support  40  to permit rotation of the respective section about support  40  and blade axis  42  independent of the adjacent blade sections. 
         [0044]    In  FIG. 4 , each of the three blade sections  62 ,  64  and  66  is shown aligned with the adjacent blade section. This arrangement may be referred to as the “aligned” or “design” position. In this position a longitudinal end of a particular blade section is aligned generally with a longitudinal end of an adjacent blade section to produce a design twist of the blade. 
         [0045]    Each blade section  62 ,  64  and  66  is free to rotate about blade axis  42  independent of the other. The twist in blade  44  may be modified by rotating one or more of the blade sections  62 ,  64  and  66  relative to one another about support  40  to independently optimize an angle of attack of the individual blade section for the wind source  46 . For example, in  FIG. 5  blade sections  62 ,  64  and  66  are shown in an “unaligned” position. In this configuration one or more of the blade sections  62 ,  64  and  66  have passively rotated relative to the other blade sections in response to the aerodynamic forces acting on the individual blade sections  62 ,  64  and  66  caused by wind source  46  passing over the blade sections. In the exemplary arrangement shown in  FIG. 5 , blade section  64  is rotated clockwise (as viewed from a perspective looking inward from the tip  52  of blade  44  along blade axis  42 ) relative to blade section  62 , and blade section  66  is rotated clockwise from blade section  64 . Each blade section  62 ,  64  and  66  may also be rotated in an opposite direction, for example, counter-clockwise. In practice, the angular rotation of blade sections  62 ,  64  and  66  about blade axis  42  may be more or less than shown in  FIG. 5 , and each blade section may rotate more or less than the other blade sections. The rotational positioning of the blade section  62 ,  64  and  66  relative to one another may remain fairly constant, or may periodically or continuously change depending on the consistency of wind source  46 . 
         [0046]    The blade  44  and blade sections  62 ,  64  and  66  may be configured as a substantially straight blade or may include a twist. The exemplary blade  44  and blade sections  62 ,  64  and  66  are illustrated in the figures as having little or no twist. Alternatively, the blade  44  and one or more of the blade sections  62 ,  64  and  66  may include a twist. An example of a twisted blade section  62  is illustrated in  FIG. 6 . The twist results in the leading edge  54  of the blade being angularly rotated at a radial outer section of the blade (for example, at second end cap  70 ) relative to a radial inner section of the blade (for example at first end cap  68 ). All or a portion of the blade  44  and blade sections  62 ,  64  and  66  may include a twist. It is not necessary that the twist be uniform along the entire length of the blade or blade section. One or more of the blade sections  62 ,  64  and  66  may include a twist, and if one blade section includes twist it is not necessary that the other blade sections also include a twist. For example, blade  44  may include a combination of twisted and straight blade sections. 
         [0047]    With reference to  FIG. 6 , each of the blade sections  62 ,  64  and  66  includes a first end cap  68  radially closer to the rotational axis  38  (see  FIG. 1 ) and a second end cap  70  radially distant from the first end cap  68 . The first and second end caps  68  and  70 , in combination with the previously described portions of the blade wind bearing surface  58  and leeward surface  60 , generally provide a closed section defining a hollow cavity interior  72  within blade  44 . 
         [0048]    With reference to  FIGS. 9 and 10 , the individual blade sections may be rotatably connected to support  40  through one or more bearings  74 . Bearing  74  may include various configurations. For example, with particular reference to  FIG. 9 , bearing  74  may include a roller bearing  76  having an outer race  78  connected to first end cap  68 , an inner race  80  fixedly connected to blade support  40  and multiple ball bearings  80  slidably disposed between outer race  76  and inner race  78 . Roller bearing  76  may be positioned within interior cavity  72  of a respective blade section (for example, second section  64  as illustrated in  FIGS. 4 and 5 ). Outer race  78  may be connected to either first end cap  68  or second end cap  70  using fastener  84 , depending on which end of a blade section the bearing is located. Inner race  80  may be connected to blade support  40  using fasteners  86 . 
         [0049]    With reference to  FIG. 10 , bearing  74  may alternatively be configured to include concentric interlocking guides. For example, bearing  74  may include an outer guide block  88  rotatably engaging an inner guide block  90 . The guide blocks  88  and  90  may be located within interior cavity  72  of a respective blade section, for example second section  64  (see  FIGS. 4 and 5 ). Outer guide block  88  may be connected to either first end cap  68  or second end cap  70  using fastener  84 , depending on which end of a blade section the bearing is located. Inner guide block  90  may be connected to blade support  40  using fasteners  86 . Alternatively, inner guide block  90  may be configured as an integral part of blade support  40 . Inner guide block  90  may include one or more flanges  92  extending radially outward from an outer perimeter of the inner guide block. Flanges  92  slidably engage corresponding slots  94  extending around an inner circumference of the outer guide block  88 . Similarly, outer guide block  88  may include one or more flanges  96  extending radially inward from the inner circumference of the outer guide block  88 . Flanges  96  slidably engage corresponding slots  98  extending circumferentially around the outer perimeter of inner guide block  90 . 
         [0050]    It is understood that other forms of bearings and ways to enable rotation of the blade sections about support  40  and blade axis  42  may be used. It is further contemplated that bearing  74  may be positioned in other orientations, for example other than in the interior cavity  72  of the blade section. 
         [0051]    With reference to  FIG. 7 , blade configurations employing separate blade sections rotatable with respect to one another may include a clearance or gap  100  between adjacent blade sections. Gap  100  may serve several purposes, for example, reducing frictional contact between adjacent sections as they are capable of rotation relative to one another and providing clearance for additional components positioned between the sections. For example, bearings  74  (see  FIGS. 9 and 10 ) may be positioned outside blade interior cavity  72  between adjacent blade sections. It is desirable for aerodynamics, however, that clearance or gap  100  be minimized. 
         [0052]    With continued reference to  FIG. 7 , the blade sections may employ a blade section pivot limiter  103  for controlling the maximum amount or degree of angular movement about blade axis  42  that can take place between adjacent blade sections. Although illustrated as being used in connection with blade sections  62  and  64 , pivot limiter  103  may also be used to limit the maximum amount of angular movement about blade axis  42  between blade sections  64  and  66 . Continuing to refer  FIG. 7 , blade support  40  passes through apertures  102  extending through the blade end caps, for example, second end cap  70  of the first blade section  64  and the first end cap  68  of the second blade section  64 . In the illustrated example, to limit or control the maximum rotation about blade axis  42  of second blade section  64  relative to first blade section  62 , two pins  104  may be secured to first end cap  68 . Pins  104  may be oriented to extend axially down from first end cap  68  and into angular slots  106 , as generally shown. On rotation of second section  64  relative to first section  62 , a predetermined maximum amount of angular differentiation is achieved through a hard mechanical stop of the pins  104  at ends  108  of the slots  106 . Pins  104  may be made from ferrous or non-ferrous metals, or other materials suitable for a particular application. 
         [0053]    With reference also to  FIGS. 13-15 , in one example of angular slots  106  and pins  104 , a nominal blade section orientation is established, for example, at the time of installation, wherein pin  104  is positioned in an approximate center of the respective slots  106  in a middle portion between the two extreme ends  108  of the slots, as shown for example in  FIGS. 7 and 13 . The nominal blade section orientation corresponds to the aligned position, as illustrated for example in  FIG. 4 . In this example, the predetermined maximum angular movement of pins  104  in the respective slot  106  is plus and minus thirty degrees about blade axis  42 . In other words, the blade section predetermined maximum angular movement or range of movement is up to clockwise thirty degrees or up to counterclockwise thirty degrees about blade axis  42  from the nominal, aligned position. For example, in  FIG. 14  blade section  64  is illustrated rotated clockwise (as viewed from the perspective of  FIG. 14 ) relative to blade section  62 . In the illustrated example, blade section  64  is positioned at nearly the maximum clockwise angular position allowed by pivot limiter  103 , as indicated by the location of the pins  104  adjacent the end  108  of the slot  106 . In  FIG. 15 , blade section  64  is illustrated rotated counter-clockwise (as viewed from the perspective of  FIG. 15 ) relative to blade section  62 . In the illustrated example, blade section  64  is positioned at nearly the maximum counter-clockwise angular position allowed by pivot limiter  103 , as indicated by the location of the pins  104  adjacent the end  108  of the slot  106 . Although blade section  64  is described in this example as rotating relative to blade section  62  about blade support  40 , it is also possible for blade section  62  to rotate relative to blade section  64 , or for both blade sections to rotate about blade support  40 . The limitation being that the maximum relative angular rotation between the blade sections not exceed the maximum allowed by pivot limiter  103 . 
         [0054]    Although  FIG. 7  illustrates where the pins  104  and slots  106  are positioned or integral within the end caps  68  and  70  of adjacent blade sections, these structures may take other forms, for example a separate plate or collar positioned in the gap  100 . 
         [0055]    With reference to  FIGS. 11 and 12 , an alternately configured pivot limiter  110  for limiting the maximum amount or degree of angular rotation of the blade sections about blade axis  42  is illustrated. Although shown employed in connection with blade section  64 , pivot limiter  110  may also be used to limit the maximum relative angular rotation between other blade sections, such as blade sections  64  and  66 . With continued reference to  FIGS. 11 and 12 , blade support  40  extends through second end cap  70  of first blade section  62  and first end cap  68  of the second blade section  64 , substantially as previously described in connection with pivot limiter  103 . A stop block  112  may be positioned within the interior cavity  72  of the blade section between the wind bearing surface  58  and the leeward surface  60 . The stop block  112  may be attached to one or both of the wind bearing and leeward surfaces  58  and  60 , respectively. The stop block  112  includes an aperture  114  through which the blade support  40  extends. The stop block and the blade section  64  may freely rotate around the blade support  40  within predetermined limits determined by the configuration of the pivot limiter  110 . The stop block  112  may extend an entire axial length of the blade section or only a portion. More than one pivot block  112  may be disposed within a blade section. 
         [0056]    The pivot limiter  110  may include an elongated stop pin  116 . An end  118  of the stop pin  116  may be attached to the blade support  40 . The stop pin  116  extends generally radially outward from the blade support  40 . At least a portion of the stop pin  116  is movably positioned within an elongated slot  120  formed in the pivot block  112 . The slot  120  may extend entirely through the pivot block  112  and be open to an exterior of the blade section, as illustrated for example in  FIGS. 11 and 12 , or may alternatively extend only partially through the pivot block  112 . The slot  120  may include a longitudinal length L that may be greater than its width W. The longitudinal axis of the slot  120 , which extends axially along the longitudinal length of the slot, may be oriented generally perpendicular to the blade axis  42 . 
         [0057]    The length L of the slot  120  determines the maximum allowable axial rotation of blade section  64  relative to blade section  62 . On rotation of second section  64  relative to blade support  40 , a predetermined maximum amount of angular rotation may be achieved through a hard mechanical stop of the stop pin  116  at an end  122  of the slot  120 . Stop pin  116  may be made from ferrous or non-ferrous metals or other materials suitable for the particular application. Structures other than the pin  116  and slot  120  may be employed as a stop for controlling the maximum angular rotation of the blade section. 
         [0058]    In one example, the axial lengths of the individual first  62 , second  64  and third  66  blade sections between first end cap  68  and second end cap  70  along blade axis  42  are not equal to reduce potential harmonic motion conditions or effects. It is understood that equal length blade sections may also be used with that consideration in mind or otherwise addressed through other measures. 
         [0059]    One way of determining the axial lengths of the individual sections of blade  44  is to consider the blade twist. Conventionally, wind turbine blades are not “flat” in their geometry. They are designed with a twist such that the ratio of the rotational speed of the tip versus the wind speed is approximately 8:1, so that a blade has a change in angle along the blade length. For example, the blade at one-eighth of the axial length from the tip experiences only one degree difference in the angle relative to the tip, while the innermost eighth of the blade experiences a 45 degree difference in the angle relative to the tip. In theory, a large number of blade sections are desirable such that the change in the angle over the length of each section would be minimized and roughly equal to the other sections. Such a design would provide the most efficient use of the wind. It would also give an infinite number of cross section speed ratios and closely match any wind speed. In practice, however, manufacture of multiple sections is difficult, and multiple sections increase part count and maintenance expenses. Balancing these consideration results in one possible design described herein, where first  62 , second  64  and third  66  blade sections are used. Although illustrated with three separate sections, first blade section  62 , second blade section  64  and third blade section  66 , it is understood that a fewer or greater number of blade sections may be used to suit the particular application or performance requirements. 
         [0060]    The wind source  46  flowing over the blade  44  or blade sections  62 ,  64  and/or  66  generate aerodynamic forces acting along the wind bearing surface  58  and leeward surface  60  of the blade or blade section. The aerodynamic forces may result in a pitching moment tending to rotate the blade or blade section around the blade pivot axis  42 . Changes in the wind source  46 , such as wind speed or direction, may alter the aerodynamic forces and resulting pitching moment acting on the blade or blade section. Adjusting the angle of attack or twist of the blade or blade section may also cause changes in the aerodynamic forces and pitching moment acting on the blade or blade section. The blade or blade section react to these changes by passively rotating clockwise about blade pivot axis  42 , as shown for example in  FIG. 14 , or counter-clockwise, as shown for example in  FIG. 15 . Rotation of blade  44  or blade sections  62 ,  64  and/or  66  changes the angle of attack and/or twist of the blade or blade section relative to the wind source  46 , which in turn alters the aerodynamic forces and resulting pitching moment acting in the blade or blade section. The blade or blade section will passively rotate in the appropriate direction until the aerodynamic forces are such that the resulting pitching moment is substantially zero. This process will continue whenever there are changes in the wind source  46 . 
         [0061]    Due to the passive nature of the permitted rotation, the blade  44  or blade sections  62 ,  64  or  66  freely rotate with respect to blade support  40  within the limits imposed by pivot limiter  110 . In the example of pivot limiter  110  using pins  116  and slots  106 , as shown for example in  FIG. 7 , the pins  116  will “float” or freely move within the slot  106  to allow continuous adjustment of the blade or blade section orientation or blade twist to compensate for changes in the aerodynamic forces and pitching moment acting on the blade  44  or individual blade sections  62 ,  64  and  66  without further controls or other generated stimulus from the wind turbine  20 . In the alternate example described, the blade sections  62 ,  64  and/or  66  will further rotate relative to one another for added adjustment to the blade twist as they rotate about blade pivot axis  42 . 
         [0062]    With reference also to  FIGS. 16 and 17 , the response characteristics of the blade  46  or blade sections  62 ,  64  and  66  to changes in the wind source  46  to maintain an optimum angle of attack for the prevailing wind conditions may be controlled by selective placement of the blade support  40  within the blade or blade section. For purposes of discussion, blade section  62  is illustrated in  FIGS. 15 and 16 , but the control method is also applicable to the blade  46  and blade section  64  and  66 . A pivot point  123  of blade section  62  about blade pivot axis  42  coincides substantially with a center of blade support  40 . One option for controlling the response characteristic of the blade is to position blade support  40  along a chord line  124  of the blade section  62 . Exemplary blade section  62  is shown configured as a symmetrical non-cambered airfoil, with the chord line  124  substantially centered between the wind bearing surface  58  and leeward surface  60  along the entire length of the blade section. Because blade section  62  is substantially symmetrical, the chord line  124  also coincides with a camber line of the blade. 
         [0063]    Dimension  126  represents a maximum thickness of the blade section  62  measured perpendicular to the cord line  124 , which for this particular blade section occurs at a distance  128  from the blade leading edge  54 . Due to the symmetry of blade section  62 , the chord line  124  bisects the blade, such that a dimension  130  measured perpendicular from the chord line  124  to the blade surface at the blade maximum thickness is substantially half the maximum blade thickness  126 . Dimension  132  is a distance measured along the cord line  124  from the blade leading edge  54  to the pivot point  123 , or center of the blade support  40 . The blade support  40  is preferably positioned forward of a maximum thickness plane  133 , coinciding with the maximum blade thickness, to provide an aerodynamically stable airfoil capable of passively responding to changes in wind conditions by pivoting the blade section  62  to maintain a suitable angle of attack. Generally, the closer blade support  40  is positioned to the maximum thickness plane  133  the less aerodynamically stable the blade will be. Preferably the blade support  40  is positioned relative to the maximum thickness plane, such that dimension  132  is not more than approximately 95% of dimension  128  to create an aerodynamically stable airfoil. 
         [0064]    Positioning the blade support  40  outside of the preferred range (i.e., distance  132  between 0-95% of distance  128 ) may require the use of additional control mechanisms to accommodate the aerodynamic instability that may occur and to maintain control of the blade orientation, and in particular, blade angle of attack. For example, the blade may require use of adjustable trailing and/or leading edge flaps capable of actively modifying the aerodynamic shape and/or orientation of the blade to accommodate the unstable forces acting on the airfoil as a result of locating blade support  40  outside the preferred range. This may of course add considerable cost, weight and complexity to the wind turbine. Also, it may not be feasible to employ movable flaps if the blade section includes twist, such as shown for example in  FIG. 6 . Wind turbine  20  does not require use of any additional controls, such as flaps, to maintain control over the orientation of the blade relative to the wind source  46 . The aerodynamic shape of the blade section  62  does not change as the blade section passively rotates about the blade support  40  in response to changes in the wind source  46 . Selective placement of the blade support  40  within the blade section as a means for controlling the response characteristics of the blade may be used effectively with blades having straight and/or twisted sections. Dimension  132  may change based on the position of the particular chord in the blade section. 
         [0065]    In an alternative configuration illustrated for example in  FIG. 17 , the pivot point  123  may be positioned offset from the chord line  124  by a distance  134 . In this configuration the pivot point is still positioned forward of the maximum thickness plane  133  and is located at a distance of not more than 95% of the distance  128  from the leading edge  54 . This configuration also results in an aerodynamically stable airfoil capable of passively altering the aerodynamic forces and corresponding pivot moment acting on the blade by rotating the blade about the blade support  40  to maintain an optimum blade angle of attack for the prevailing wind conditions. 
         [0066]    In the exemplary configuration shown in  FIGS. 1 and 2  and described with reference to power transmission equipment  26  above, wind turbine  20  may include the second or rear rotor  30 . Second rotor  30  may include the same construction and configuration as described for first rotor  28 . For example and without limitation, second or rear rotor blades  136 , like first rotor blades  44 , may be of the configuration where each blade  136  is one unit fixed to a respective blade support  138  and wherein the blade supports  138  themselves rotate about blade axis  140  relative to the second hub  34 . In the alternative, as described above with respect to first rotor blades  44 , second rotor blades  136  may include separate blade sections that are capable of rotational movement with respect to the respective blade support  138  and one another through the structures described. Construction of second hub  34  can be the same as first hub  32 . 
         [0067]    One difference between the first rotor  28  and the second rotor  30  is that, in a preferred configuration, second rotor  30  rotates in an opposite direction about rotational axis  38  relative to first rotor  28 . In the example shown in  FIGS. 1 and 2 , the second rotor  30  rotates in a counter-clockwise direction (as viewed from the perspective of  FIG. 1 ), and thus the leading edge is on the opposite side of the blade when compared to blades of the first rotor  28 . The blade orientation of the second rotor  30  is also different from the first rotor  28  in this configuration due to the opposite rotation. This counter-rotation between the first and second rotors  28  and  30  provides the benefit of reducing the torsional forces and stresses on the nacelle  24  and tower  22  through at least partial cancellation of opposing forces. 
         [0068]    With continued reference to  FIGS. 1 and 2 , second rotor  30  has a second set of blades  136  longer than blades  44  of the first rotor  28  and that extend past the distal or radially-extreme end  52  of the blades  44  of the first rotor  28 . In these lengths and orientations, there is a blade sweep overlap between the blades of the first rotor  28  and those of the second rotor  30 , which is the distance between the radially lowest end of the second set of blades  136 , and the radially distant or top of the first set of blades  44 . This overlap is useful to increase the efficiency of the wind turbine  20  by extracting additional wind energy with the second rotor  30  from the same wind that already encountered the first rotor  28 . That is, the counter rotating and expanding vortex from the first set of (front) blades  44  is at least partly captured by the second set of (rear) blades  136 . 
         [0069]    Through having the second rotor  30  and the described orientation of the blades with respect to one another, the second rotor  30  provides additional conversion of wind energy into electricity over prior single blade assembly designs. The wind turbine design acts similar to a turbine engine. The first rotor blades act like the stator blades in the turbine engine by setting up the wind for the rear rotor. The first rotor  28  also redirects the wind to provide a more efficient angle of attack on the second rotor blades  136 . The difference in this wind turbine design, as shown for example in  FIG. 2 , is that the first rotor  28  rotate unlike the stationary turbine engine stator blades. The first rotor  28  and second rotor  30  are both connected to a common shaft for driving the power generation equipment  26 , therefore producing more power than a stationary stator system or single rotor design. 
         [0070]    With particular reference to  FIG. 2 , one example of the second rotor  30  includes an angled blade portion  142  near the distal end of blades  136 . As illustrated, this angled portion  142  may be biased toward the first rotor  28 . A suitable angle of portion  142  from the remaining portion of the blade  136  is up to about forty degrees with respect to the blade axis  140 , although other angles may be used depending on the application and performance requirements. In the example where blades  136  include multiple sections, angled portion  142  is preferably not at a joint or gap  144  between adjacent sections, but may be depending on the application and performance requirements. Alternatively, a tip section  146  of blades  136  may be substantially straight, similar to first rotor blades  44 , depending on the performance requirements of the particular application. 
         [0071]    In operation, a two rotor configuration creates an added benefit of passive yaw control. The rotor/stator effect of the first rotor  28 , as described above, sets a direction of the wind source  46  into the second rotor  30 . The second rotor  30 , due to the longer blade length and counter rotating rotor, acts as a tail to cause the first rotor  28  to steer into the wind source  46 . This steering effect is a result of the wind source  134  interacting with the wind turbine  20 , eliminating the need to use any electrical, mechanical or electro-mechanical mechanisms to control the orientation of the wind turbine relative to the wind source. 
         [0072]    As described above, the orientation of blades  44  of first rotor  28  passively adjust according to the wind conditions at the point of contact with the respective blade wind bearing surface  58 . The blade  44  or respective blade sections,  62 ,  64  and  66 , will rotate relative to blade support  40 , and in the examples illustrated, with respect to adjacent blade sections about blade axis  42 . There are no electrical, mechanical or electro-mechanical actuators, mechanisms or links interconnecting the blades. This enables independent passive variation of the blade orientation, and optionally the geometry or twist of the blades, to occur continuously throughout the rotation of the rotor about rotational axis  38  based solely on the aerodynamic forces acting on the blade to achieve an optimized angle of attack in response to the varying wind conditions provided by wind source  46 . 
         [0073]    In operation, where a variation in blade orientation is desired to accommodate changes in wind speed and direction, several optional methods have been disclosed. The blades  44  and  136  may be configured to be a single unit fixed to a rotatable blade support  40   138 , respectively, a multi-section blade wherein the blade sections are capable of rotating with respect to one another, a multi-section blade wherein the blade sections are mounted on a fixed blade support  44  and are capable of rotating independently of one another with respect to the fixed blade support  44 , or an example where the blade includes both the rotatable blade sections as well as a rotatable blade support  44 ,  138 . In each of the examples, the individual blade sections and/or the blade support  44 ,  138  are capable of independently and passively rotating through their predefined angles of movement to adjust or accommodate changing wind conditions. This independent passive adaptability, for example to accommodate often different wind speeds and directions at the lowest point in the swept area of the blade  44 ,  136  rotation versus wind speed and direction at the apex of blade  44 ,  136  rotation about rotational axis  38  at any given moment, provides a significant advantage over prior designs by increasing efficiency of operation and electrical output. Note that although all blade sections of a multi-section blade are described in the example as freely rotating, one blade section of the three, or more blade sections where there are more than three blade sections, could be fixed to the blade support  40 ,  138 . 
         [0074]    While the invention has been described in connection with certain embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.