Abstract:
A rotor assembly is provided that includes a hub rotatable about an axis of rotation. The assembly also includes a plurality of rotor blades spaced circumferentially about the hub. Each of said rotor blades extends from a blade root to a blade tip such that said plurality of blades are rotatable through a plane of rotation extending about said axis of rotation, said plane of rotation is defined as substantially perpendicular to said axis of rotation, wherein each of said blade roots is coupled to said hub, wherein each of said blade tips is offset a distance upstream from said plane of rotation.

Description:
BACKGROUND OF THE INVENTION 
     The field of the present disclosure relates generally to wind turbines, and more specifically to wind turbines that include coned hub assemblies. 
     At least some known wind turbine towers include a nacelle that is coupled atop a tower, wherein the nacelle includes a rotor assembly coupled via a shaft to a generator. In known rotor assemblies, a plurality of blades extends from the rotor, and the assembly is oriented such that wind contacts the rotor and blades, and thereafter the tower. This configuration is generally known as a “front-runner” assembly. Additionally, the blades are oriented such that wind passing over the blades turns the rotor and rotates the shaft, thereby driving the generator to generate electricity. 
     In at least some known rotor assemblies, wind pressure exerted against the blades may cause an elastic rearward flexing of the blades, and as a result the blade tips may be pushed in close proximity to the tower, especially during strong wind conditions. Some know wind turbine towers are configured to brake the rotor in strong winds. However, in such towers, increased braking of the rotor may result in increased pressure being induced against the blades, which may cause rearward flexing of the blades towards the tower to increase. Because known wind turbines must function effectively during strong wind conditions, it is necessary for the blade rotor to be positioned a sufficient distance from the tower so that during operation, the potential of blade contact with the tower, and associated risk of serious accidents and/or equipment damage, may be substantially reduced. 
     To facilitate reducing rearward flexing of the rotor blades during operation some known wind turbines use blades fabricated from materials that have an increased stiffness. Such materials enable the blades to withstand a higher wind pressure, without requiring that the blade hub be positioned an exaggerated distance from the vertical axis of the tower. However, such materials also increase blade production costs, and create greater loading upon turbine components as a result of the increased weight of the blades. As a result, often such turbines and require a more robust and less efficient turbine design. 
     Other known wind turbines use a tilted rotor, wherein the axis of rotation of the rotor is shifted upwards with respect to the angle of the oncoming wind. As such, the tips of the blades are shifted a distance away from the turbine tower as the blades pass through the lower most point of their rotational path. However, such a design causes uneven contact between the oncoming wind and the blades, which may induce a yaw-error to the wind turbine and thus effectively reduce system efficiency. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a wind turbine system is provided. The system includes a stator configured to generate electricity, and a rotor rotatably coupled to the stator. The rotor includes a hub rotatable about an axis of rotation, and a plurality of rotor blades spaced circumferentially about the hub, each of said rotor blades extends from a blade root to a blade tip such that said plurality of blades are rotatable through a plane of rotation extending about said axis of rotation, said plane of rotation is defined as substantially perpendicular to said axis of rotation, wherein each of said blade roots is coupled to said hub, wherein each of said blade tips is offset a distance upstream from said plane of rotation. 
     In another aspect, a rotor assembly is provided. The assembly includes a hub rotatable about an axis of rotation. The assembly also includes a plurality of rotor blades spaced circumferentially about the hub. Each of said rotor blades extends from a blade root to a blade tip such that said plurality of blades are rotatable through a plane of rotation extending about said axis of rotation, said plane of rotation is defined as substantially perpendicular to said axis of rotation, wherein each of said blade roots is coupled to said hub, wherein each of said blade tips is offset a distance upstream from said plane of rotation. 
     In yet another aspect, a method of assembling a wind turbine system is provided. The method includes providing a hub rotatable about an axis of rotation, and coupling a plurality of rotor blades circumferentially about the hub, wherein each of said rotor blades extends from a blade root to a blade tip such that said plurality of blades are rotatable through a plane of rotation extending about said axis of rotation, said plane of rotation is defined as substantially perpendicular to said axis of rotation, wherein each of said blade roots is coupled to said hub, wherein each of said blade tips is offset a distance upstream from said plane of rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view of an exemplary wind turbine. 
         FIG. 2  is a schematic illustration of an exemplary coned-hub assembly used with the wind turbine shown in  FIG. 1 . 
         FIG. 3  is a side perspective view of the coned hub shown in  FIG. 2 . 
         FIG. 4  is a schematic illustration of an exemplary rotor blade used with the wind turbine shown in  FIG. 1 . 
         FIG. 5  is a schematic illustration of an alternative rotor blade that may be used with the wind turbine shown in  FIG. 1 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a side elevation view of an exemplary wind turbine  100 . In the exemplary embodiment, wind turbine  100  is a nearly horizontal-axis wind turbine. In another embodiment, wind turbine  100  may have an up-tilt angle (not shown) ranging from about 1° to about 15°. Alternatively, wind turbine  100  is 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  includes 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 ) defined between supporting surface  104  and nacelle  106 . In an alternate embodiment, tower  102  is a lattice tower. A height of tower  102  is selected based upon factors and conditions known in the art. 
     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 to blades  112  are transferred to hub  110  via load transfer regions  122 . 
     In the exemplary embodiment, blades  112  have a length ranging from about 50 feet (ft) (about 15 meters (m)) to about 300 ft (about 91 m). Alternatively, blades  112  may have any length that enables wind turbine  100  to function as described herein. For example, other non-limiting examples of blade lengths include 10 meters or less, 20 meters, and 37 meters. As wind strikes blades  112  from a direction  124 , rotor  108  is rotated about an axis of rotation  114 . As blades  112  are rotated and subjected to centrifugal forces, blades  112  are also 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 associated stresses, or loads, may be induced in blades  112 . Moreover, a pitch angle of blades  112 , i.e., the angle that determines a perspective of blades  112  with respect to the direction of the wind, may be changed by a pitch adjustment mechanism (not shown in  FIG. 1 ) that facilitates increasing or decreasing blade  112  speed by adjusting the surface area of blades  112  exposed to the wind force vectors. Pitch axes  118  for blades  112  are illustrated. In the exemplary embodiment, each blade&#39;s pitch is controlled individually. Alternatively, blade pitch for all blades may be controlled simultaneously. 
     Referring to  FIGS. 2 and 3 ,  FIG. 2  is a schematic illustration of an exemplary coned hub system  200  used with wind turbine  100 , and  FIG. 3  is a perspective view of coned hub system  200 . In the exemplary embodiment, coned hub system  200  includes a hub assembly  210 , hub external surface  212 , and a plurality of rotor blades  214 . Rotor blades  214  are coupled to hub assembly  210  via a pitch bearing assembly  216  that enables a pitch of each rotor blade  214  to be changed depending upon external conditions. More specifically, in the exemplary embodiment, the pitch of each blade  214  can be independently controlled via each blade&#39;s respective pitch bearing assembly  216 . Alternatively, a pitch of all rotor blades  214  may be controlled simultaneously. 
     Coned hub system  200  is coupled to nacelle structure  218  via a central shaft  220  that defines an axis of rotation  222 . In the exemplary embodiment, hub system  200  includes a hub assembly  210 , hub external surface  212 , and plurality of blades (not shown). For clarity, only a single blade  214  is illustrated in  FIG. 3 . Hub assembly  210  is fabricated such that hub external surface  212  is substantially cone-shaped. More specifically and in the exemplary embodiment, hub assembly  210  includes a first end  230  and an opposite second end  232  that are separated by a length L 1  extending along axis of rotation  222 . Hub first end  230  includes a blunt, approximately spherically-shaped portion  234  that defines a radius of curvature of R 1 . In the exemplary embodiment, hub second end  232  has a radius R 2  that is longer than R 1 . Hub radius R 1  increases linearly to hub radius R 2  along a length L 2  of hub external surface  212 . Alternatively, R 2  may define a hub external surface with a non-circular cross-sectional area. In the exemplary embodiment, an angle α 1  is defined between hub external surface  212  and axis of rotation  222 . In the exemplary embodiment, angle α 1  ranges from about 0.2 degrees to about 20 degrees. Alternatively, angle α 1  may be any angle that enables wind turbine  100  to function as described herein. 
     For illustrative purposes, a plane  240  that is substantially perpendicular to axis of rotation  222  is illustrated in  FIG. 3 . In the exemplary embodiment, rotor blade  214  is coupled to hub assembly  210  as described herein. Rotor blade  214  is coupled substantially perpendicularly to hub external surface  212  such that an angle α 2  is defined between a rotor blade mid-chord  242  and plane  240 . As such, α 2  is substantially equivalent to α 1 . In the exemplary embodiment, rotor blade  214  is coupled to hub assembly  210  such that angle α 2  is approximately equal to angle α 1 . Alternatively, rotor blade  214  may be coupled to hub assembly  210  such that angle α 2  is greater than angle α 1 , and such that each angle α 1  and angle α 2  is a magnitude that enables the wind turbine  100  to function as described herein, and which prevents rotor blade  214  from striking tower  102  (shown in  FIG. 1 ). 
       FIG. 4  illustrates an exemplary rotor blade  400  coupled to hub system  200 . Configurations of exemplary rotor blade are applicable to rotor blades of any length L 4 . For example, and not by way of limitation, in some embodiments, blades  400  have a length L 4  of approximately 0.5 meters. In other configurations, blades  400  have a length L 4  of approximately 50 meters. Other non-limiting examples of blade lengths L 4  include 10 meters or less, 20 meters, 37 meters, and 50 meters. In the exemplary embodiment, rotor blade  400  includes a root  402  and tip  404 , a spanwise axis E and a pitch axis P. More specifically, in the exemplary embodiment, rotor blade  400  has a curved span  406 , wherein the magnitude of the curve is defined by a radius of curvature R 3 . In the exemplary embodiment, radius of curvature R 3  is substantially constant along spanwise axis E from root  402  to tip  404 . In the exemplary embodiment, tip  404  is offset a distance L 3  defined by the radius of curvature R 3  into a direction of oncoming wind  408  and away from the support tower (not shown). In the exemplary embodiment, distance L 3  ranges from about 0.2 meters to about 5 meters. Alternatively, tip  404  may be offset a distance that enables wind turbine  100  to function as described herein. Such a system provides a wind turbine that operates to prevent rotor blade  214  from striking tower  102  (shown in  FIG. 1 ) and reduce loads upon turbine components. 
       FIG. 5  illustrates an alternative rotor blade configuration that may be coupled coned hub system  200 . In the exemplary embodiment, rotor blade  500  includes a root  502  and tip  504 , a spanwise axis E, a pitch axis P and a span length L 4 . More specifically, in the exemplary embodiment, rotor blade  500  includes a partial span L 5  this is substantially planar and a partial span L 6  that is arcuate. Measured from root  502 , partial span L 5  ranges from about ⅓ of span length L 4  to about ½ of span length L 4  measured from root  502 . Partial span L 6  is then defined as the difference between span length L 4  and partial span L 5 , and in the exemplary embodiment, has a radius of curvature R 4  that is substantially constant along spanwise axis E. Tip  504  is offset a distance L 7 , defined by the radius of curvature R 4 , into a direction of oncoming wind  508  and away from the support tower (not shown). Similar to the embodiment illustrated in  FIG. 4 , tip offset L 7  ranges from about 0.2 meters to about 5 meters. Alternatively, tip may be offset in an upstream direction any distance that allows wind turbine to function as described herein. Such a system provides a wind turbine that operates to prevent rotor blade  214  from striking tower  102  (shown in  FIG. 1 ) and reduce loads upon turbine components. 
     Exemplary embodiments of a wind turbine using a combination of a pitched hub and contoured, forward-pitched rotor blades are described in detail above. The exemplary blades described herein may be used to facilitate substantially reducing the occurrence of tower strikes by the blades, even under strong wind conditions. In general, the above-described system used arcuate blades or partially arcuate blades that are coupled to a coned hub such that the blade is at least partially angled into the oncoming wind. Because each of the blades coupled to the turbine are substantially identical, the need for heavier, stiffer blades typically used to prevent tower strikes is facilitated to be eliminated. Additionally, the system and methods described herein provide a wind turbine that operates with a higher efficiency than known wind turbines that use rotor blades fabricated from heavier and/or stiffer materials. Such a system also provides a wind turbine that operates to prevent the rotor blades from striking the wind turbine tower as well as to reduce loads upon turbine components. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     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.