Patent Publication Number: US-2016237989-A1

Title: Vertical axis wind turbines and related methods of braking

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 62/117,242, filed Feb. 17, 2015, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to devices and methods for harnessing natural energies, and more particularly, to vertical axis wind turbines. 
     BACKGROUND 
     Ongoing depletion of nonrenewable energy resources, such as petroleum, has fostered a growing need for devices and methods that harness the power of renewable energy resources, including wind. Wind farms generally consist of one or more groupings of wind turbines that each include a rotor having blades that are driven by the wind to rotate about a turbine axis. This mechanical rotation is then converted into electrical energy. Most wind farms consist of horizontal axis wind turbines, which include blades that rotate about a horizontal turbine axis. However, when in close proximity to neighboring turbines, horizontal axis wind turbines suffer from a reduced power coefficient. 
     This deficiency of horizontal axis wind turbines has driven the development of vertical axis wind turbines. The blades of vertical axis wind turbines span in a direction generally parallel to the wind turbine axis, whereas the blades of horizontal wind turbines span in a direction generally perpendicular to the wind turbine axis. While vertical axis wind turbines are generally mounted such that the wind turbine axis and blade span directions are oriented vertically, vertical axis wind turbines may also be mounted in various other orientations relative to a ground surface, such as horizontal. Accordingly, the term “vertical” as used herein in connection with vertical axis wind turbines and related components is not limiting to a traditional vertical orientation of such components. 
     Vertical axis wind turbines can be positioned much closer together than horizontal axis wind turbines without negatively impacting performance characteristics to the same degree, or even at all. Consequently, vertical axis wind turbines have the ability to generate as much as ten times more energy per square meter than horizontal axis wind turbines, thereby yielding much higher power outputs per unit of land than horizontal axis wind turbines. Further, vertical axis wind turbines are generally smaller, less intrusive, and cheaper to produce that horizontal axis wind turbines. 
     Conventional vertical axis wind turbines generally include a plurality of curved blades rigidly fixed to a lower plate, often in the form of a disc, or to a shaft. As the blades rotate with the turbine disc or shaft about a vertical axis of the wind turbine, each blade successively passes back and forth between first and second orientations relative to the wind. In a first orientation relative to the wind, the blade receives the wind force to generate a torque in a first direction about the turbine axis, thereby successfully contributing to ongoing, power-generating rotation of the wind turbine. 
     As the blade rotates about the vertical turbine axis, it momentarily transitions to a second orientation relative to the wind in which the blade faces “backward” to the wind. In this second orientation, the backward facing blade momentarily generates a counter torque in a second, opposite direction about the turbine axis that resists the positive turbine rotation in the first direction. For example, while a first blade of a vertical axis wind turbine faces toward the wind and generates a positive torque about the turbine axis, a second blade may simultaneously be facing backward relative to the wind and generating a negative counter torque, which may be lesser in quantity that the positive torque. Nevertheless, this counter torque undesirably reduces the efficiency of the turbine in harnessing energy from the wind. 
     Accordingly, there is a need for improvements to known vertical axis wind turbines to address at least the shortcomings described above. 
     SUMMARY 
     A vertical axis wind turbine according to an exemplary embodiment of the invention includes a support structure rotatable about a turbine axis, and at least two blades, each blade operatively coupled to the support structure and being pivotable about a respective blade axis. Each of the at least two blades is pivotable between a fixed working position at which the blade receives a first amount of wind force and generates a torque for rotating the support structure about the turbine axis, and a neutral position at which the blade receives a lesser second amount of wind force. 
     In another embodiment, an exemplary method of aerodynamically braking a vertical axis wind turbine is also provided. The method includes obtaining a vertical axis wind turbine including a support structure rotatable about a vertical turbine axis, and at least one blade operatively coupled to the support structure. The at least one blade is pivotable about a blade axis between a fixed working position at which the at least one blade is configured to receive a first amount of wind force and generate a torque for rotating the support structure about the turbine axis, and a neutral position at which the at least one blade is configured to receive a lesser second amount of wind force. The method further includes braking rotation of the support structure about the turbine axis by inhibiting the at least one blade from reaching the fixed working position, such that the at least one blade generates substantially no torque about the turbine axis. Alternatively, or in addition to the inhibiting step, braking rotation of the support structure about the turbine axis may be accomplished by securing the at least one blade in a position other than the neutral position so as to inhibit the at least one blade from generating a net torque about the turbine axis. 
     Various additional features and advantages of the invention will become more apparent to those of ordinary skill in the art upon review of the following detailed description of one or more illustrative embodiments taken in conjunction with the accompanying drawings. The drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the general description given above and the detailed description given below, serve to explain the one or more embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a vertical axis wind turbine according to an exemplary embodiment of the invention. 
         FIG. 1A  is a top view of the vertical axis wind turbine of  FIG. 1 , showing a first blade at a fixed working position and a second blade at a neutral position. 
         FIG. 1B  is a top view similar to  FIG. 1A , showing pivoting of the first and second blades between fixed working positions and neutral positions while the turbine rotates about a turbine axis. 
         FIG. 1C  is a top view of the vertical axis wind turbine of  FIG. 1 , showing the blades in a first aerodynamic braking configuration. 
         FIG. 1D  is a top view of the vertical axis wind turbine of  FIG. 1 , showing the blades in a second aerodynamic braking configuration. 
         FIG. 2  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention, in which pivoting of the blades is retrained by cable members. 
         FIG. 3  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention, in which portions of the wind turbine are fitted with solar panels. 
         FIGS. 4A-4F  are cross-sectional views of various exemplary shapes defining a cross-sectional shape of a wind turbine blade. 
         FIG. 5  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention, in which the pivoting edges of the blades are supported by a turbine shaft. 
         FIG. 6  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention similar to the embodiment of  FIG. 5 . 
         FIG. 7A  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention, including a plurality of inner blades and plurality of outer blades. 
         FIG. 7B  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention similar to the embodiment of  FIG. 7A . 
         FIG. 7C  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention similar to the embodiment of  FIG. 7A . 
         FIG. 7D  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention similar to the embodiment of  FIG. 7A . 
         FIG. 8  is a perspective view of a vertical axis wind turbine according to another exemplary embodiment of the invention, including a single turbine blade. 
         FIG. 9  is a perspective view of an exemplary turbine support frame for supporting a vertical axis wind turbine, shown schematically. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides vertical axis wind turbines according to various exemplary embodiments for providing improved efficiency in harnessing wind energy and generating power. 
       FIGS. 1-1D  show a vertical axis wind turbine  10  according to a first exemplary embodiment. The wind turbine  10  generally includes a turbine shaft  12 , a blade support structure  14  operatively coupled to the turbine shaft  12 , and at least one blade operatively coupled to the support structure  14 . The wind turbine  10  is shown having first and second blades  16 ,  18 , though it will be appreciated that additional blades may be provided in alternative embodiments. 
     The support structure  14  and the turbine shaft  12  are rotatable about a turbine axis A defined by the turbine shaft  12 . As described below, each of the blades  16 ,  18  is configured to receive a wind force exerted by a wind W when in a fixed working position, and thereby generate a torque that rotates the support structure  14  about the turbine axis A. Advantageously, each of the blades  16 ,  18  pivots about a respective blade axis during rotation of the wind turbine  10 , so as to mitigate production of undesirable counter-torques, as described below. The wind turbine  10  may further include a generator  20  for converting rotational mechanical energy of the wind turbine  10  into electrical energy, and a friction brake  22  for selectively braking rotation of the wind turbine  10 . 
     The support structure  14  may include a lower support member  24  and optionally an upper support member  26  (shown in phantom), for supporting respective lower and upper ends of the blades  16 ,  18 . The support members  24 ,  26  are shown in the Figures in the form of disc-like structures extending generally orthogonally to the turbine axis A. However, it will be appreciated that the support members  24 ,  26  may be formed with various alternative configurations suitable for supporting the upper and lower ends of the blades  16 ,  18 . For example, the support members  24 ,  26  may be formed as perforated or otherwise non-solid disc-like structures, or the support members  24 ,  26  may be formed with various arm-like structures extending radially outward from a central hub, for example. 
     Optionally, the turbine shaft  12  may extend fully between the lower and upper support members  24 ,  26 , as indicated in phantom in  FIG. 1 . For example, the wind turbine  10  may be mounted vertically relative to a support surface (e.g., a ground surface or building surface) so as to be fully supported near the lower support member  24 . In that case, the turbine shaft  12  may include a first shaft portion that extends fully between the upper and lower support members  24 ,  26 , and a second shaft portion that extends outwardly from the lower support member  24  for supporting the wind turbine  10 . In another exemplary embodiment (not shown), the wind turbine  10  may be mounted relative to a ground surface (e.g., vertically or horizontally) so as to be supported near both the upper and the lower support members  24 ,  26 . In that case, the turbine shaft  12  may include a first shaft portion that extends outwardly from the lower support member  24 , and a second shaft portion that extends outwardly from the upper support member  26 , without a shaft portion that extends between the upper and lower support members  24 ,  26 . In such case, the upper and lower support members  24 ,  26  may be connected by the blades  16 ,  18 . In both embodiments, the wind turbine  10  may be supported at the one or more shaft portions extending outwardly from the support members  24 ,  26 , for example as described in greater detail below in connection with  FIG. 9 . 
     Still referring to  FIGS. 1-1D , and in particular to  FIG. 1 , each of the turbine blades  16 ,  18  may include an elongate blade strut  28  that defines a respective blade axis about which the blade  16 ,  18  pivots relative to the support structure  14 . For example, each blade  16 ,  18 , via its blade strut  28 , may be pivotably coupled to the upper and lower support members  24 ,  26  with bearing units  30 . As shown in the Figures, the blades  16 ,  18  may be mounted such that their blade struts  28  and corresponding blade axes extend generally parallel to the turbine axis A. In alternative embodiments, the blades  16 ,  18  may be mounted such that their blade axes are angled relative to the turbine axis A. 
     As shown, each blade strut  28  may extend for the full span of its respective blade  16 ,  18 , though the struts  28  may be formed with various other configurations in alternative embodiments. For example, each blade strut  28  may be formed with upper and lower strut portions that extend outwardly from respective upper and lower ends of the blade  16 ,  18 . Furthermore, it will be appreciated that in place of traditional struts, a blade  16 ,  18  may be provided with various other suitable mechanical features that extend fully or partially along the blade span and enable pivoting of the blade  16 ,  18  about the blade axis relative to the support structure  14 . In that regard, it will be further appreciated that the blades  16 ,  18  may be operatively coupled to the support structure  14  in a variety of manners using known mechanical coupling components suitable to enable the blade pivoting motions described herein. 
     Each turbine blade  16 ,  18  includes a free edge  32  that extends away from the respective blade axis and defines a blade chord  34  extending transverse to the blade axis. Each blade  16 ,  18  is shown in the Figures in the form of a rectangular plate having a free edge  32  that extends generally parallel to the blade axis, so as to define a chord  34  of constant length. However, it will be appreciated that the blades  16 ,  18  may be formed with various other shapes, and with chords  34  of varying length, suitable to achieve desired wind turbine performance characteristics. As described below in connection with  FIGS. 4A-4F , each blade  16 ,  18  may be formed with a transverse cross-section of various shapes. 
     Each blade  16 ,  18  may be restrained at its fixed working position by a first blade stop member shown in the form of working position pin  36 , and optionally at its neutral position by a second blade stop member shown in the form of neutral position pin  38 . The pins  36 ,  38  may be anchored to and project upwardly from the lower support member  24  of the support structure  14  to engage corresponding lower portions of the blades  16 ,  18 . As such, placement of the pins  36 ,  38  relative to the support structure  14  defines the working and neutral positions of the blades  16 ,  18 . Although not shown, corresponding sets of blade stop members may also be provided on the upper support member  26  to engage upper portions of the blades  16 ,  18 . Moreover, it will be appreciated that the blade stop members may take various alternative forms suitable to inhibit pivoting of the blades  16 ,  18 , for example as described below in connection with  FIG. 2 . 
     In the embodiment of  FIGS. 1-1D , the blade axes are spaced from the turbine axis A such that when each blade  16 ,  18  is at its fixed working position, its free edge  32  is positioned radially inward of its blade axis. In other words, each blade  16 ,  18  is mounted to the lower support member  24  such that its free edge  32  is oriented generally toward the turbine shaft  12  when at the fixed working position. To achieve this configuration, the blade struts  28  defining the blade axes may be positioned near an outer periphery of the lower support member  24 . In exemplary embodiments, each blade axis may be spaced radially outward from the turbine axis A by a distance greater than a length of the blade chord  34 . Moreover, the blade axes may be spaced radially equidistant from the turbine axis A, and with uniform circumferential spacing about the turbine axis A. While only first and second blades  16 ,  18  are shown, it will be appreciated that various alternative quantities of blades may be provided, wherein the blades are mounted with uniform spacings in radial and circumferential directions. 
     As the wind turbine  10  rotates about the turbine axis A, each blade  16 ,  18  freely pivots about its respective blade axis between a fixed working position and a neutral position. This pivoting motion may be induced fully by a force exerted by the wind W, and unassisted by an actuating device such as a motor. Referring to  FIGS. 1 and 1A , the wind turbine  10  is shown in an exemplary first rotational position about the turbine axis A, in which the first blade  16  is oriented at an exemplary fixed working position and the second blade  18  is oriented at an exemplary neutral position. When at the fixed working position, a blade  16 ,  18  receives a first amount of wind force and thereby generates a torque for rotating the support structure  14  about the turbine axis A, as indicated by directional arrows in  FIG. 1A . When at the neutral position, a blade  16 ,  18  is oriented relative to the wind force direction such that the blade  16 ,  18  receives a lesser second amount of wind force, such as no wind force, for example. Thus, to the extent that the blade  16 ,  18  at the neutral position generates any counter-torque about the turbine axis A, the counter-torque is significantly less than the torque generated in the opposite direction by the blade  16 ,  18  at the fixed working position. In this manner, a power generation process using wind turbine  10  is kept highly efficient. 
     It will be appreciated that the radial spacing of the blade axis from the turbine axis A, in combination with the length of the chord  34 , determines a torque arm distance of the blade  16 ,  18  relative to the turbine axis A. In that regard, it will be further appreciated that the span (e.g., height) of the blade  16 ,  18 , in combination with the torque arm distance, determines the amount of torque generated by the blade  16 ,  18  about the turbine axis A when the blade  16 ,  18  is acted upon by a wind W. As such, the blade chord length, blade span, and the radial spacing of the blade axis from the turbine axis A may be selectively adjusted to tune performance characteristics of the wind turbine  10 . 
     As shown in  FIG. 1A , each of blades  16  and  18  moves between a fixed working position and a neutral position. In the fixed working position, as shown by first blade  16  in  FIGS. 1 and 1A , the blade chord  34  forms a working position angle relative to a radial line extending from the turbine axis A to the blade axis (the working position angle is not indicated by reference numeral in the Figures due to an exemplary illustrated angle of zero degrees). Similarly, in the neutral position, as shown by second blade  18  in  FIGS. 1 and 1A , the blade chord  34  forms a neutral position angle  0  relative to a radial line extending from the turbine axis A to the blade axis. In various embodiments, the fixed working position and the neutral position are defined such that the neutral position angle  0  is greater than the working position angle. This relationship ensures that a blade  16 ,  18  at the neutral position is exposed to less wind force than a blade  16 ,  18  restrained at the fixed working position. Consequently, and advantageously, any counter-torques otherwise produced by the blades  16 ,  18  about the turbine axis A during rotation of the wind turbine  10  are substantially reduced, if not entirely eliminated. 
     As shown in  FIG. 1A , each working position pin  36  is located on the lower support member  24  so as to define a fixed working position of the blade  16 ,  18 . In the exemplary embodiment shown, each working position pin  36  is located so as to define a fixed working position at which the blade chord  34  extends toward and aligns with the turbine axis A, thereby forming an exemplary working position angle of zero degrees. By comparison, each neutral position pin  38  is located on the lower support member  24  so as to form an exemplary neutral position angle  0  that is acute (i.e., larger than the working position angle). 
     In alternative embodiments, the working position pins  36  and neutral position pins  38  may be repositioned to various other locations on the lower support member  24  to define various other working position angles and neutral position angles  0 , wherein the neutral position angle  0  is larger than the working position angle. In one embodiment, the neutral position pins  38  may be relocated, or even removed, so as to define a neutral position angle  0  of approximately 90 degrees (i.e., the blade chord  34  being parallel to the wind force direction), for example. In such case, the wind force received by the blade  16 ,  18  at the neutral position, and the resulting torque generated by the blade  16 ,  18  about the turbine axis A, is approximately zero. 
     In operation, the wind turbine  10  is subjected to a wind W having a direction. Depending on the starting rotational orientation of the wind turbine  10  relative to the wind direction, the wind W forces one of the blades  16 ,  18  to pivot to its fixed working position, and the other of the blades  16 ,  18  to pivot to its neutral position. For example, as shown in  FIG. 1A , the wind W may force the first blade  16  to pivot into the fixed working position against the respective working position pin  36 , and force the second blade  18  to pivot into the neutral position against the respective neutral position pin  38 . In this configuration, the first blade  16  receives the wind force and thereby generates a torque about the turbine axis A in a first direction, indicated by the directional arrows. Advantageously, because the second blade  18  is less exposed to the wind W than the first blade  16 , the second blade  18  produces very little, if any, counter-torque that resists the torque generated by the first blade  16 . As a result, the wind turbine  10  may rotate about the turbine axis A with minimal resistance and optimal efficiency. Further, the ability of the blades  16 ,  18  to freely and independently pivot when exposed to a wind W allows the wind turbine  10  to self-start without assistance provided by external devices, such as a starter motor. 
     As shown in  FIG. 1B , the wind turbine  10  has rotated more than 90 degrees from its position in  FIG. 1A  in which the first blade  16 , restrained at its fixed working position, was oriented perpendicular to the direction of the wind W. As a result, as shown in  FIG. 1B , the sides of the blades  16 ,  18  now exposed to the wind W are opposite from the sides exposed to the wind in the position of  FIG. 1A . As a result, the wind W now forces the first blade  16  to pivot toward its respective neutral position pin  38 , and forces the second blade  18  to pivot toward its respective working position pin  36 . As the wind turbine  10  continues to rotate, the second blade  18  pivots fully to its fixed working position and the first blade  16  pivots fully to its neutral position. As the wind turbine  10  further rotates past an additional 90 degrees from the rotational position at which the second blade  18  is perpendicular to the wind direction, the first and second blades  16 ,  18  again begin to pivot back toward their original working or neutral positions. In this manner, the first and second blades  16 ,  18  freely oscillate between fixed working positions and neutral positions as the wind turbine  10  continues to rotate about the turbine axis A. 
     As each blade  16 ,  18  pivots to its neutral position, the blade  16 ,  18  contacts and thereby exerts an impact force on its respective neutral position pin  38 . In embodiments similar to that of  FIGS. 1-1D  in which the blade axes are spaced from the turbine axis A such that the free edges  32  of the blades  16 ,  18  are positioned radially inward of their respective blade axes, the impact force exerted on the neutral position pin  38  advantageously generates a secondary torque about the wind turbine axis A that enhances the primary torque generated by the blade  16 ,  18  in the fixed working position. In exemplary embodiments, this secondary torque may enhance the primary torque by up to 40%, for example. The amount of secondary torque generated may be adjusted by repositioning the neutral position pins  38  on the lower support member  24 . Alternatively, as described above, the neutral position pins  38  may be removed or otherwise omitted from the wind turbine  10 , such that the blades  16 ,  18  freely pivot to neutral positions that are parallel to the wind direction. In such case, the blades  16 ,  18  do not generate secondary torques when pivoting to their neutral positions. 
     Referring to  FIGS. 1C and 1D , two exemplary methods of aerodynamically braking the wind turbine  10  are shown. These methods may be used in combination with, or in substitute for, frictional braking provided by the friction brake  22 . In that regard, the friction brake  22  may be any suitable friction braking device known in the art that engages the turbine shaft  12  or either of the upper and lower support members  24 ,  26 , for example. 
     Referring to  FIG. 1C , a first exemplary method of aerodynamically braking the wind turbine  10  includes securing each of the blades  16 ,  18  in their fixed working positions, each blade  16 ,  18  forming a similar working position angle. The wind turbine  10  is oriented rotationally such that each secured blade  16 ,  18  contacts the wind at the same angle, and thus receives the same amount of wind force. For example, as shown in  FIG. 1C , the wind turbine  10  is oriented such that both blades  16 ,  18  are perpendicular to the wind direction. Consequently, each blade  16 ,  18  generates an equal and opposite torque about the turbine axis A, such that the blades  16 ,  18  collectively generate a net torque of zero. As a result, the wind turbine  10  does not rotate in either direction. However, it will be appreciated that if the wind W momentarily shifts direction so as to exert a greater force on one of the blades  16 ,  18  than the other, a torque will be generated about the turbine axis A, which may cause the wind turbine  10  to rotate such that the blades  16 ,  18  become parallel to the new wind direction. 
     Still referring to  FIG. 1C , the blades  16 ,  18  may be secured using an additional set of blade stop members shown in the form of pins  40 , which may cooperate with the working position pins  36  to clamp the blades  16 ,  18  in their fixed working positions. In alternative embodiments, the additional blade stop members may include various other mechanical devices suitable to engage and inhibit the blades  16 ,  18  from pivoting. 
     While the blades  16 ,  18  are shown secured in exemplary fixed working positions forming working position angles of zero degrees, the blades  16 ,  18  may be secured in any desired pivot position other than the neutral position, provided that both blades  16 ,  18  are similarly oriented. For example, both blades  16 ,  18  may be secured in a pivot position that is between the exemplary fixed working position and neutral position shown in  FIGS. 1-1B . 
     Referring to  FIG. 1D , a second exemplary method of aerodynamically braking the wind turbine  10  includes inhibiting each of the blades  16 ,  18  from reaching a fixed working position at which either blade receives the wind force and generates a torque about the turbine axis A. More specifically, the blade stop pins  36 ,  38 , or other blade stop members, may be manipulated or removed to allow each of the blades  16 ,  18  to pivot so that the blade chords  34  become parallel to the wind direction. As a result, neither blade  16 ,  18  generates a torque about the turbine axis A, and the wind turbine  10  does not rotate. Advantageously, because the blades  16 ,  18  in this braking embodiment are not being restrained in a position in which they generate a torque, the components of the wind turbine  10  are not stressed. 
     Referring to  FIG. 2 , a vertical axis wind turbine  50  according to another exemplary embodiment of the invention is shown. The wind turbine  50  is largely similar in structure and function to wind turbine  10  of  FIG. 1 , as indicated by use of similar reference numerals in  FIG. 2 . However, wind turbine  50  includes blade stop members in the form of tethers  52 ,  54 . Each tether  52 ,  54  has a first end  56  anchored to the lower support member  24 , and a second end  58  coupled to the respective blade  16 ,  18 , for example at a lower corner of the blade  16 ,  18  near the free edge  32 . In embodiments in which the wind turbine  50  includes an upper support member  26 , an additional set of tethers (not shown) may be provided to couple the upper portions of the blades  16 ,  18  to the upper support member  26 . These upper tethers may be mounted to the blades  16 ,  18  and the upper support member  26  in a manner similar to that described in connection with the lower tethers  52 ,  54 . 
     Each tether  52 ,  54  restrains the respective blade  16 ,  18  at both its fixed working position and at its neutral position. Accordingly, a length of the tether  52 ,  54  and a location at which the first end  56  is anchored to the lower support member  24  may be selected as desired to define the fixed working position and the neutral position. It will be appreciated that the location at which the first end  56  is mounted to the lower support member  24  may define a middle point in the pivoting range of the blade  16 ,  18  between the fixed working position and the neutral position. 
     In an exemplary embodiment, either or both of the tethers  52 ,  54  may be selectively adjustable in length, for example at the first end  56  or at the second end  58 , for adjusting the working position angle and the neutral position angle (see  FIG. 1A ) of the respective blade  16 ,  18 . Adjustment of the tether length may be performed manually or automatically, for example with assistance of a powered drive. Further, one or both of the tethers  52 ,  54  may be selectively lengthened to allow the first and second blades  16 ,  18  to pivot to become parallel with the wind direction, thereby achieving an aerodynamic braking effect similar to that described above in connection with  FIG. 1D . In another exemplary embodiment, one or both of the tethers  52 ,  54  may be selectively releasable at the first end  56 , the second end  58 , or a location therebetween, to achieve a similar aerodynamic braking effect. 
     Advantageously, the tethers  52 ,  54  may enable quieter operating conditions than the blade stop pins  36 ,  38  provided on wind turbine  10 . When blades  16 ,  18  of wind turbine  50  pivot to their fixed working position or neutral position, they exert a tension force on the tethers  52 ,  54 . This application of tension on the tethers  52 ,  54  may produce a quieter noise, if any, compared to the noise of the blades  16 ,  18  contacting the pins  36 ,  38  of wind turbine  10 . 
     Referring to  FIG. 3 , a vertical axis wind turbine  60  according to another exemplary embodiment of the invention is shown. The wind turbine  60  is largely similar in structure and function to wind turbine  10  of  FIG. 1 , as indicated by use of similar reference numerals in  FIG. 3 . As shown, the wind turbine  60  includes a plurality of solar panels  62  mounted to various portions of the wind turbine  60 . In particular, the solar panels  62  may be mounted to the side surfaces of the first and second blades  16 ,  18 , and to a top surface of the upper support member  26 . It will be appreciated that the solar panels  62  may be mounted to various other surfaces of the wind turbine  60 , depending on the orientation in which the wind turbine  60  is to be supported relative to the sun. The solar panels  62  may be electrically connected to an electrical storage bank (not shown) for storing electrical energy produced by the solar panels  62 . 
     Each turbine blade of the various exemplary wind turbines disclosed herein may be formed with a transverse cross-section selected from a variety of shapes, such any one or combination of those shown in  FIGS. 4A-4F , for example.  FIG. 4A  shows a first exemplary blade cross-section  64  having a generally rectangular shape, which defines a turbine blade in the form of a generally flat plate.  FIG. 4B  shows a second exemplary blade cross-section  66  having a generally rectangular shape similar to that of blade cross-section  64  of  FIG. 4A , but including rounded ends.  FIG. 4C  shows a third exemplary blade cross-section  68  having a generally oblong or oval-like shape.  FIG. 4D  shows a fourth exemplary blade cross-section  70  having a symmetrical airfoil shape.  FIG. 4E  shows a fifth exemplary blade cross-section  72  having a generally V-like shape.  FIG. 4F  shows a sixth exemplary blade cross-section  74  having a generally rectangular shape with upwardly turned ends. It will be appreciated that the particular cross-section of the blades may be selected to optimize wind turbine performance in view of specific operating conditions. 
     Referring to  FIG. 5 , a vertical axis wind turbine  80  according to another exemplary embodiment of the invention is shown. The wind turbine  80  is generally similar in structure to wind turbine  10  of  FIG. 1 , as indicated by use of similar reference numerals in  FIG. 5 , except as otherwise described below. 
     Unlike wind turbine  10 , wind turbine  80  includes first and second blades  82 ,  84  having respective blade axes that are positioned relative to the turbine axis A such that when the blade  82 ,  84  is at its fixed working position, its free edge  86  is positioned radially outward of its blade axis. In other words, each blade  82 ,  84  is mounted such that its free edge  86  is oriented generally away the turbine shaft  12  when at the fixed working position. In the exemplary embodiment shown in  FIG. 5 , each blade  82 ,  84  is hingedly connected to and supported by the turbine shaft  12 . In particular, the first blade  82  is supported by a first hinge  88  that defines the blade axis about which the first blade  82  pivots, and the second blade  84  is supported by a second hinge  90  that defines the blade axis about which the second blade  84  pivots. In alternative embodiments, the blades  82 ,  84  may include struts (e.g., similar to struts  28 ) instead of hinges  88 ,  90 , the struts being pivotably coupled to the support structure  14  at positions slightly radially outward from the turbine shaft  12 . 
     As shown in  FIG. 5 , the hinges  88 ,  90  and blades  82 ,  84  may be positioned at generally diametrically opposite positions on the turbine shaft  12 . While only two blades  82 ,  84  are shown, additional blades may be provided in alternative embodiments. Further, the wind turbine  80  may additionally include working position pins  36  for defining the fixed working positions of the blades  82 ,  84 . The wind turbine  80  may be formed without neutral position pins  38 , so as to permit the blades  82 ,  84  to pivot to a neutral position parallel with the wind direction without contacting a blade stop member. Because each blade  82 ,  84  is mounted such that its free edge  86  is positioned radially outward of its blade axis, the blades  82 ,  84  pivot from the fixed working position to the neutral position in a direction opposite the direction of rotation of the wind turbine  80  about the turbine axis A. Accordingly, if neutral position pins were included on wind turbine  80 , the blades  82 ,  84  could contact them at the neutral position to generate a secondary torque about the turbine axis A that undesirably opposes the primary torque generated by the blades  82 ,  84  at the fixed working position. 
     Referring to  FIG. 6 , a vertical axis wind turbine  100  according to another exemplary embodiment of the invention is shown. The wind turbine  100  is generally similar in structure to wind turbine  80  of  FIG. 5 , as indicated by use of similar reference numerals in  FIG. 6 , as except as otherwise described. In particular, the wind turbine  100  includes a third blade  102  hingedly connected to and supported by the turbine shaft  12  with a third hinge  104 . As shown, the blades  82 ,  84 ,  102  are equally spaced circumferentially about the turbine shaft  12 . Furthermore, the upper and lower support members  24 ,  26  may be omitted from the wind turbine  100 , so that each blade  82 ,  84 ,  102  may pivot a full range permitted by its respective hinge  88 ,  90 ,  104 , without contacting a blade stop member. As such, it will be appreciated that the fixed working position of each blade  82 ,  84 ,  102  is defined by the degree to which the hinge  88 ,  90 ,  104  permits the blade  82 ,  84 ,  102  to pivot. The hinges  88 ,  90 ,  104  may be formed with various features suitable to limit the pivot ranges of the blades  82 ,  84 ,  102  as desired. 
     Referring to  FIGS. 7A-7D , vertical axis wind turbines according to additional exemplary embodiments of the invention are shown. Each wind turbine includes a plurality of inner blades  112  and a plurality of outer blades  114  positioned radially outward of the inner blades  112 . While each wind turbine is shown having blades supported by only a lower support member  24 , it will be appreciated that an upper support member  26  may also be provided. 
     Referring to  FIG. 7A , an exemplary vertical axis wind turbine  110  includes three inner blades  112  circumferentially spaced about the turbine shaft  12 , and three outer blades  114  positioned radially outward of the inner blades  112  and also circumferentially spaced about the turbine shaft  12 . As shown, the outer blades  114  may be positioned so as to not extend radially inward of a circular border  116  defined by the radially outermost portions of the circumferentially arranged inner blades  112 . 
     Each of the inner blades  112  includes an inner blade strut  118  defining a respective inner blade axis about which the inner blade  112  pivots. Similarly, each of the outer blades  114  includes an outer blade strut  120  defining a respective outer blade axis about which the outer blade  114  pivots. In the exemplary embodiment of  FIG. 7A , the inner blade struts  118  are spaced radially outward from the turbine shaft  12 , generally along the circular border  116 , such that free edges  122  of the inner blades  112  are positioned radially inward of their respective inner blade axes when at the respective fixed working positions. Additionally, the outer blade struts  120  are spaced radially outward from the circular border  116 , near a periphery of the lower support member  24 , such that free edges  124  of the outer blades  114  are positioned radially inward of their respective outer blade axes, and generally along the circular border  116 , when at the respective fixed working positions. As such, both the inner blades  112  and the outer blades  114  are mounted to the lower support member  24  such that their free edges  122 ,  124  are oriented generally toward the turbine shaft  12  when the blades  112 ,  114  are at their fixed working positions. 
     With the configuration of  FIG. 7A , it will be appreciated that the plurality of inner blades  112  and the plurality of outer blades  114  each pivot between fixed working positions and neutral positions in a manner generally similar to that described above in connection with wind turbine  10  of  FIG. 1 . In that regard, because each inner blade  112  and outer blade  114  is mounted such that its free edge  122 ,  124  is oriented generally toward the centrally positioned turbine shaft  12 , each blade  112 ,  114  pivots from its fixed working position to its neutral position in the same direction as the rotation of the wind turbine  110  about the turbine axis A. 
     Further, as shown in  FIGS. 7A-7D , each inner blade  112  and outer blade  114  may cooperate with one or more respective blade stop members that define the fixed working position and, optionally, the neutral position for the blade  112 ,  114 . While the blade stop members are shown in the form of working position pins  36  and neutral position pins  38 , alternatively various other blade stop structures may be used, such as tethers  52 ,  54  shown in  FIG. 2 , for example. 
     Referring to  FIG. 7B , a vertical axis wind turbine  130  according to an exemplary alternative embodiment is shown. The wind turbine  130  is generally similar to wind turbine  110  of  FIG. 7A , as indicated by use of similar reference numerals in  FIG. 7B , except as otherwise described. In particular, unlike wind turbine  110 , the inner blades  112  and the outer blades  114  of wind turbine  130  are mounted such that their free edges  122 ,  124  are oriented generally away from the turbine shaft  12  when at the respective fixed working positions. More specifically, the inner blades  112  are mounted such that their free edges  122  are positioned radially outward of their respective inner blade axes. Similarly, the outer blades  114  are mounted such that their free edges  124  are positioned radially outward of their respective outer blade axes. The outer blade struts  120  may be positioned generally along the circular border  116 , while the inner blade struts  118  may be positioned adjacent to the turbine shaft  12 . In alternative embodiments, the inner blades  112  may be supported directly on the turbine shaft  12  with hinge-like structures similar to those described above in connection with  FIG. 5 , for example. 
     With the configuration of  FIG. 7B , it will be appreciated that the plurality of inner blades  112  and the plurality of outer blades  114  may each pivot between fixed working positions and neutral positions in a manner generally similar to that described above in connection wind turbine  80  of  FIG. 5 . In that regard, because each inner blade  112  and outer blade  114  is mounted such that its free edge  122 ,  124  is oriented generally away from the centrally positioned turbine shaft  12 , each blade  112 ,  114  pivots from its fixed working position to its neutral position in a direction opposite the direction of rotation of the wind turbine  130  about the turbine axis A. 
     Referring to  FIG. 7C , a vertical axis wind turbine  140  according to yet another exemplary alternative embodiment is shown. The wind turbine  140  generally combines the configuration of inner blades  112  of wind turbine  130  of  FIG. 7B , with the configuration of outer blades  114  of wind turbine  110  of  FIG. 7A , as indicated by use of similar reference numerals in  FIG. 7C . Thus, the wind turbine  140  includes inner blades  112  that are mounted such that their free edges  122  are oriented generally away from the turbine shaft  12  when at their fixed working positions, and outer blades  114  that are mounted such that their free edges  124  are oriented generally toward the turbine shaft  12  when at their fixed working positions. As such, the outer blades  114  pivot from their fixed working positions to their neutral positions in a first direction that is the same as the direction of rotation of the wind turbine  140  about the turbine axis A. In contrast, the inner blades  112  pivot from their fixed working positions to their neutral positions in a second direction that is opposite the direction of rotation of the wind turbine  140  about the turbine axis A. 
     Referring to  FIG. 7D , a vertical axis wind turbine  150  according to yet another exemplary alternative embodiment is shown. The wind turbine  150  includes blades  112 ,  114  arranged in an opposite configuration of wind turbine  140  of  FIG. 7C . In that regard, wind turbine  150  of  FIG. 7D  combines the configuration of inner blades  112  of wind turbine  110  of  FIG. 7A , with the configuration of outer blades  114  of wind turbine  150  of  FIG. 7B , as indicated by use of similar reference numerals in  FIG. 7D . Thus, the wind turbine  150  includes inner blades  112  that are mounted such that their free edges  122  are oriented generally toward the turbine shaft  12  when at their fixed working positions, and outer blades  114  that are mounted such that their free edges  124  are oriented generally away the turbine shaft  12  when at their fixed working positions. As such, the inner blades  112  pivot from their fixed working positions to their neutral positions in a first direction that is the same as the direction of rotation of the wind turbine  150  about the turbine axis A. In contrast, the outer blades  114  pivot from their fixed working positions to their neutral positions in a second direction that is opposite the direction of rotation of the wind turbine  150  about the turbine axis A. 
     While the wind turbines  110 ,  130 ,  140 ,  150  of  FIGS. 7A-7D  are shown having three inner blades  112  and three outer blades  114 , it will be appreciated that various alternative quantities and corresponding circumferential arrangements of inner and outer blades  112 ,  114  may be provided in alternative embodiments. Additional concentric rings of outer blades may be provided in alternative embodiments as well, each separated by a circular border similar to border  116 , for example. 
     Referring to  FIG. 8 , a vertical axis wind turbine  160  according to another exemplary embodiment of the invention is shown. The wind turbine  160  generally includes a turbine shaft  12 , a blade support structure  162  operatively coupled to the turbine shaft  12 , and a single blade  164  operatively coupled to the support structure  162 . The support structure  162  includes a lower support member shown in the form of a lower support disc  166 , and an upper support member shown in the form of an upper support disc  168 . The turbine shaft  12  defines a turbine axis A and includes a central portion that extends fully between the lower and upper support discs  166 ,  168 , and a lower portion that extends outwardly from the lower support disc  166  and at which the wind turbine  160  may be mounted to an external structure. 
     Unlike the wind turbines of  FIGS. 1-7D , the support structure  162  of wind turbine  160  is mounted to the turbine shaft  12  such that a central axis of the support structure  162  is offset from the turbine axis A. As shown in  FIG. 8 , the turbine shaft  12  extends through the support structure  162  near the radially outer edges of the lower and upper support discs  166 ,  168 . The single blade  164  is mounted to the support structure  162  at a position generally diametrically opposite from the turbine shaft  12  and turbine axis A. The single blade  164  includes a blade strut  170  that extends between the lower and upper support discs  166 ,  168  in a direction generally parallel to the turbine axis A, and is pivotably mounted to the support discs  166 ,  168 , for example with bearing units  172 . The single blade  164  is mounted such that its free edge  173  is positioned radially inward of the blade strut  170 , and is oriented toward the turbine axis A. 
     The single blade  164  is shown in an exemplary fixed working position at which the blade abuts and is restrained by a first blade stop member, shown in the form of working position pin  36 . The wind turbine  160  may further include a second blade stop member, shown in the form of neutral position pin  38 , for restraining the single blade  164  at a neutral position. The working position pin  36  and neutral position pin  38  function in the manner as described above in connection with  FIGS. 1-1B . In that regard, the working position pin  36  restrains the single blade  164  at the exemplary working position shown, at which the single blade  164  receives a wind force and generates a torque to rotate the support structure  162  about the turbine axis A. Once the support structure  162  rotates more than 90 degrees past an orientation at which the single blade  164  is perpendicular to the wind direction, the wind W engages an opposite side of the single blade  164  and thereby forces the single blade  164  to pivot toward its neutral position. The single blade  164  pivots in the same rotational direction as the rotation of the support structure  162  about the turbine axis A. Advantageously, as described above, pivoting of the single blade  164  to its neutral position decreases generation of undesirable counter-torque that would otherwise oppose the torque generated by the single blade  164  at its fixed working position. 
     Because the blade strut  170  and the turbine shaft  12  are positioned at generally opposite sides of the support structure  162 , the torque arm of the single blade  164  relative to the turbine axis A may be approximately twice that of either one of the blades  16 ,  18  of the wind turbine  10  of  FIG. 1  formed with the same diameter. Accordingly, and advantageously, the single blade  164  may generate approximately twice the amount of torque generated by either one of the blades  16 ,  18 , while incorporating fewer moving parts. In exemplary embodiments, the single blade  164  may be formed with a blade chord  165  that is less than or equal to a radius of the support structure  162 . Further, the chord  165  and a span of the single blade  164  may be adjusted as desired to achieve various performance characteristics of the wind turbine  160 . 
     As a result of the single blade  164  and support structure  162  being offset from the turbine axis A, they generate an unbalanced centrifugal force during rotation. To balance this centrifugal force and mitigate undesired vibrations of the wind turbine  160  during operation, the support structure  162  may be provided with one or more counterweight elements, shown in the form of thickened portions  174  having increased mass and formed integrally with the lower and upper support discs  166 ,  168 . The thickened portions  174 , or other counterweight elements, are provided at a circumferential location on support structure  162  that generally opposes the circumferential location at which the single blade  164  is mounted, so as to extend radially outward from an opposite side of the turbine shaft  12 . It will be appreciated that the counterweigh elements may take various other forms suitable to offset the otherwise unbalanced centrifugal force generated by the single blade  164  and support structure  162  during rotation. Moreover, the counterweight elements may be formed integrally with or coupled to the support structure  162 . The mass and positioning of the counterweight elements may be tuned as desired depending on the mounting location and dimensions of the single blade  164 . 
     In the various embodiments shown and described herein, the geometric shape and mounting location of the wind turbine blades may be modified as desired to adjust certain performance characteristics of the wind turbine, including power generation and noise production. In various embodiments, the amounts of power and noise generated by a wind turbine may be determined by the speed at which the blades pivot about their blade axes, as well as the speed at which the wind turbine rotates about its turbine axis. In one exemplary wind turbine configuration, the blades may be shaped and mounted so as to maximize power generated by the wind turbine, without regard for noise reduction. In a second exemplary wind turbine configuration, the blades may be shaped and mounted so as to minimize noise production, without regard for power generation. In a third exemplary wind turbine configuration, the blades may be shaped and mounted so as to balance power generation with noise reduction. 
     Referring to  FIG. 9 , any one of the vertical axis wind turbines disclosed herein (indicated generically at  180 ), such as wind turbine  10  for example, may be rotatably mounted to and supported by an exemplary wind turbine support frame  182 . The support frame  182  may be formed from a plurality of interconnected arm-like members defining a base portion  184  and a body portion  186  extending upwardly from the base portion  184 . The base portion  184  may house a generator and a friction braking device (collectively indicated at  187 ) that engage a turbine shaft of the wind turbine  180 . The body portion  186  may at least partially enclose the wind turbine  180  and may include a lower hub  188 , supported by lower arms  190 , that rotatably supports a lower portion of the wind turbine shaft, and optionally an upper hub  192 , supported by upper arms  194 , that rotatably supports an upper portion of the wind turbine shaft, for example using bearing units (not shown). 
     The support frame  182  may support the wind turbine  180  vertically such that the base portion  184  rests on a support surface, such as a ground surface or a building surface, for example. Alternatively, the support frame  182  may support the wind turbine  180  horizontally. For example, both the base portion  184  and the body portion  186  may be rested on a support surface, or the entire support frame  182  may be attached (e.g., at its ends) to an external structure that suspends the support frame  182  and the wind turbine  180  above a support surface. Moreover, while the support frame  182  is shown having a particular structural configuration, it will be appreciated that the support frame  182  may be formed with various alternative configurations suitable to support a vertical axis wind turbine according to any one of the exemplary embodiments of the invention. 
     While the present invention has been illustrated by the description of specific embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features discussed herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.