Abstract:
A vertical axis wind turbine which includes a rotor having three radially extending blades spaced at even intervals about a central axis. Each radially blade having an outer edge that lies on an imaginary circle of a first diameter. Each radially extending blade including a plurality of spaced airfoil sub-blades separated by gaps for the passage of air therethrough. Each of sub-blade having a leading vertical edge, and a trailing vertical edge and being positioned with the trailing vertical edge along a common radius line of the imaginary circle. Each sub-blade is skewed such that its cord line is rotated negative 45 degrees with respect the radius of the imaginary circle. The airfoil sub-blades maximize energy production by creating a secondary wind flow of a higher velocity for impingement upon blades of the rotor, and utilize backpressure during the second half of a rotation cycle to efficiently break the rotor against overspeed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/957,778 filed Aug. 24, 2007, the entire of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to wind turbines, and more particularly, relating to a vertical axis self-breaking wind turbine. 
   BACKGROUND OF THE INVENTION 
   As a result of the steadily increasing demand and cost of fossil fuels, and environmental factors related to the use of fossil fuels, the development of alternative and renewable energy sources is on the rise. 
   One area of particular attention is the conversion of wind energy into a useful form whether it be in the form of mechanical energy to drive a mechanical system such as a pump or in the form of electrical energy. Windmills have been used throughout the ages in converting wind energy into a useful form. There are two basic types of windmills or wind turbines, the horizontal axis windmill and the vertical axis wind turbine. 
   Vertical axis wind turbines have inherent advantages of stability due to gyroscopic action of the rotor, simplicity of design due to the avoidance of yaw mechanisms and blade controls, and strength of construction. However, the fact that the blades of the rotor are exposed to the force of the wind during only one-half of each cycle and then must be shielded from the wind to prevent creation of back pressure during the remaining half of each cycle has been a major problem. A variety of structural changes have been proposed in an effort to avoid or minimize formation of back pressure on the sails during their return sweep. These efforts have not been successful in overcoming the problems associated with the prior known vertical-axis wind machines. As a consequence, vertical-axis machines have not been commercially attractive and have not achieved substantial acceptance in competition with the horizontal-axis windmills. 
   The windmill construction which has been most commonly utilized for the generation of electricity is a plural-bladed propeller positioned vertically for rotation about a horizontal axis. This type of construction has been widely used because, when positioned into the wind, the entire surface of each blade of the propeller is exposed to the full force of the moving air. The commercial windmill industry has developed around the horizontal-axis construction and the aerodynamic principles and knowledge discovered in connection with atmospheric flight. Accordingly, it has become common practice to design such machines for the atmospheric/wind conditions of specific locations by varying the number and/or dimensions of the blades employed. The fewer the propeller blades, the more efficient the machines become at high wind speeds but the less efficient they are at lower wind speeds. 
   Because the blades of horizontal-axis windmills are coupled indirectly to an electric generator which is effective only at a constant design speed, and because the blades themselves become unsafe at high speeds, the horizontal-axis windmills have been capable of utilizing only a small percentage of the theoretically-available power in the wind. The multi-blade windmills have high starting torque at low wind speeds, harvesting up to 30% of the kinetic energy from the wind but become very inefficient at high wind speeds. The Dutch 4-blade machines, for instance, utilize only about 16% of the winds&#39; kinetic energy. The most common and efficient windmills today are of the two and three blade types designed for high tip speed operation. These machines harvest roughly 42% of the theoretical 59.2% kinetic energy from the wind. Such windmills operate within a narrow window or range of wind velocities defined by a cut-in wind speed of 3-5 mps (meters/sec.) and a cut-out wind speed of about 25 mps. To maintain a near constant level of torque to drive the generator has required either: complex controls, in the case of pitch control, or intricate blade designs, in the case of stall control, both of which are expensive to build and maintain. In addition, such wind machines require yaw mechanisms with motors, gearboxes, cable twist counters, etc. to keep the machine yawed against the wind. These requirements have combined to make the horizontal-axis windmills economically unattractive except in areas where alternative forms of electricity generation are not readily available. 
   Today&#39;s windmill designs also have other drawbacks. They have problems with gyroscopic vibration when the machine veers with changing wind direction. They are vulnerable to high bending moments at the base or root of the blades as each blade passes by or into the wind-shade of the supporting mast as well as when being braked during tempest conditions. These bending moments lead to frequent blade replacements and high maintenance costs. Because of their massive structures, these machines, of necessity, are remotely located miles from the area of power usage, thus necessitating construction of expensive power grids to transport the energy produced to the point of consumption, (generally large cities). Consequently, an approximate eight to ten percent of the power generated never reaches its destination due to line and transformer losses. Lastly, because of opposition from environmentalists with regard to the esthetics in natural settings as well as prohibition from municipal regulating authorities due to safety hazards associated with these large-prop machines in populated areas, many areas which would be ideal for generating wind energy, such as atop large buildings, are simply off-limits due to opposing design constraints. 
   Accordingly, there is a need for a vertical axis wind turbine of an improved and simplified construction that can be utilized both in urban and rural settings, that does not depend upon wind direction or wind velocity for optimal energy production, and that utilizes the back pressure during the remaining half of a rotation cycle to apply a breaking force to the rotor of the turbine to prevent overspeed, while overcoming the drawbacks of prior vertical axis wind turbines. 
   SUMMARY OF THE INVENTION 
   The preferred embodiments of the present invention addresses this need by providing a vertical axis wind turbine having a rotor of an improved and simplified construction that more efficiently utilizes wind flow across the blades of the rotor to provide a secondary wind flow of an increased velocity against blades of the rotor, and which efficiently utilizes backpressure created by the rotor blades during the second half of a rotation cycle to break the rotor against overspeed. The specific rotor design of the wind turbine of present invention is compact and quiet making it suitable and desirable for use in urban settings, where the wind turbine may be installed on roof tops. 
   To achieve these and other advantages, in general, in one aspect, a wind turbine is provided comprising a rotor rotatable about a central axis, and including three radially extending blades spaced at even intervals about the central axis, each blade having an outer edge that lies on an imaginary circle of a first diameter, each radially extending blade including a plurality of spaced sub-blades separated by gaps for the passage of air therethrough, each of the sub-blades having a leading vertical edge, and a trailing vertical edge, each sub-blade of each radially extending blades being positioned with the trailing vertical edge along a common radius line of the imaginary circle. 
   In general, in another aspect, each sub-blade has a chord line extending between the leading vertical edge and the trailing vertical edge, and wherein each sub-blade is skewed such that the chord line is rotated negative 45 degrees with respect the radius of the imaginary circle. 
   In general, in another aspect, each sub-blade has a quarter-circular profile. 
   In general, in another aspect, the rotor further includes a central vertical support centered at the central vertical axis; and further wherein each sub-blade has a chord line extending between the leading vertical edge and the trailing vertical edge, and wherein each sub-blade is skewed such that the chord line is rotated negative 45 degrees with respect the radius of the imaginary circle. 
   In general, in another aspect, the central vertical support has a circular cross-section with a radius of 0.1 times the diameter of the imaginary circle; and further wherein each sub-blade has a quarter-circular profile with a radius equal to the radius of the central vertical support. 
   In general, in another aspect, a wind turbine is provided comprising a wind turbine, comprising a rotor rotatable about a central axis, and including three radially extending blades spaced at even intervals about the central axis, each blade having an outer edge that lies on an imaginary circle of a first diameter, each radially extending blade including a plurality of spaced sub-blades separated by gaps for the passage of air therethrough, each of the sub-blades having a leading vertical edge, and a trailing vertical edge, each sub-blade of each radially extending blades being positioned with the trailing vertical edge along a common radius line of the imaginary circle; and wherein each of the sub-blades are of a shape and are arranged such that air flowing through the gaps is accelerated and directed towards the sub-blades of the preceding radially extending blade in respect to the direction of rotation of the rotor. 
   There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood and in order that the present contribution to the art may be better appreciated. 
   Numerous objects, features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon a reading of the following detailed description of presently preferred, but nonetheless illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of descriptions and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
   For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated preferred embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and together with the description serve to explain the principles of the invention, in which: 
       FIG. 1  is a diagrammatic perspective view of the vertical axis self-breaking wind turbine constructed in accordance with the principles of the present invention; 
       FIG. 2  is a diagrammatic front elevation of the vertical axis-self breaking wind turbine; 
       FIG. 3  is a horizontal cross-sectional showing the rotor construction; 
       FIG. 4  shows the geometrical detail of a sub-blade of a first configuration; 
       FIG. 5  show the geometrical detail of a sub-blade of a second configuration; 
       FIGS. 6   a - 6   i  are graphic representations of the wind flow about the rotor construction through a complete 360 degree rotation cycle; 
       FIG. 7  is a graphic representation of a torsional damper in connection with the rotor at a first angular rotation; and 
       FIG. 8  is the graphic representation of  FIG. 7  at a second angular rotation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
   In  FIGS. 1 and 2 , there is shown a wind turbine  10  constructed in accordance with the principals of the present invention. The wind turbine  10  includes a rotor  12  mounted for rotation about a central axis  14  for converting wind energy into a useable form, such as for example, electrical energy or mechanical energy. While not shown, the wind turbine  10  may be operatively coupled to an electric generator to produce electrical power for any number of applications, such as providing power to a residential home. Likewise, the wind turbine  10  may be operatively coupled to any number of mechanical systems for providing motive power to the system. For example, the wind turbine  10  may be operatively coupled to a pump used for pumping water from a well. For purpose of discussion and illustration, the central axis  14  is described as being a vertical axis. However, this is not meant to preclude the wind turbine  10  from having a different axis of rotation. 
   Rotor  12  may be arranged for rotation with respect to a support member  28  in any suitable fashion. Support structures of varying designs may be desired depending upon the size of the rotor  12 . Rotor  12  includes a triangular upper end plate  16  and triangular lower end plate  18  interconnected by a central support  20 , and a plurality of radially extending blades  22 ,  24 , and  26 . 
   In  FIGS. 3 and 4 , blades  22 ,  24  and  26  are positioned symmetrically about the central axis  14 , and subdivide the rotor  12  into three equisized sectors. Each blade  22 ,  24  and  26  has an outer edge  30  that lies on the circumference of an imaginary circle  62  of a diameter D 1  whose magnitude is variable since the rotor  12  may be constructed in various sizes. Circle  62  is centered on the central axis  14 . For the purpose of this specification, measurements of the wind turbine  10  will be expressed in terms of diameter D 1  of circle  62 . Blades  22 ,  24  and  26  are positioned on radii  32 ,  34 , and  36  of circle  62  respectively at 120 degree intervals around central axis  14 . 
   Each blade  22 ,  24 , and  26  is vertically subdivided into multiple airfoil sub-blades S 1 , S 2 , and S 3 . Each blade  22 ,  24  and  26  may be divided into more or less sub-blades. Sub-blades S 1 , S 2  and S 3  are spaced forming vertical slots or gaps  31  between adjacent sub-blades S 1  and S 2 , S 2  and S 3 , and between sub-blade S 3  and central support  20  through which air can pass. Each sub-blade S 1 , S 2 , and S 3  includes a leading vertical edge  38 , a trailing vertical edge  40  and a chord line  42  extending therebetween. The trailing vertical edge  40  of each sub-blade S 1 , S 2 , and S 3  of each blade  22 ,  24  and  26  is positioned along radii  32 ,  34  and  36  respectively, such that each vertical edge  40  lies in a common vertical plane at an equal spaced distance. The spacing between sub-blades S 1  and S 2 , S 2  and S 3 , and between sub-blade S 3  and central support  20  are equal, which can be expressed in terms of 0.4D1 divided by the total number of sub-blades. The sub-blade S 1 , S 2 , and S 3  are skewed with the chord line  42  rotated through an angle α a of about −45 degrees with respect to the radii  32 ,  34  and  36  respectively. 
   With continued reference to  FIG. 3 , each sub-blade S 1 , S 2 , and S 3  is preferably quarter-circular shape in cross-section. Sub-blades S 1 , S 2 , and S 3  have a radius of curvature r 2 , which may be expressed as approximately equal to 0.1D 1 , which also represents the radius r 1  of central support  20 . Sub-blades S 1 , S 2 , and S 3  have an arc length A L , which may be expressed as approximately equal to 0.16D 1 , which also represents one-quarter of the circumference of central support  20 . 
   As can be further seen, the vertical slots or gaps  31  between adjacent sub-blades S 1  and S 2 , S 2  and S 3 , and between sub-blade S 3  and central support  20  decrease in the direction of rotation from the trailing vertical edges  40  towards the leading vertical edges  38 . As wind flows through the gaps  31  its velocity is increased as a result of a throttling effect created by the narrowing space. As the wind continues to flow, exiting the slots or gaps  31 , it may be directed towards a corresponding sub-blade of the preceding blade in the direction of rotation ( FIG. 6   c ). This throttle effect increases the wind velocity impinging against the sub-blades of the preceding blade which are positioned in rotation to most efficiently capture the wind flow and create a higher torque moment than would be created in absence of the throttling effect. 
   Central support  20  has circular cross-section of a diameter D 2  and is centered on the central axis  14 . The central support diameter D 2  is approximately equal to 0.2D 1 . Central support  20  may be solid or of a hollow tubular construction to better resist rotational moments and to prevent buckling of the central support. 
   While less desired, it is contemplated each sub-blade may have a L-shape profile with a long leg  44  having a length L 1  and a short leg  46  having a length L 2 , as shown in  FIG. 5 . Short leg  46  length L 2  may be approximately equal to 0.4L 1  and L 1 +L 2  is approximately equal to 0.16D 1 , where L 1  is approximately equal to 0.11D 1 . This profile is less desired over the preferred semi-circular profile because the sharp edges of the L-shape profile creates more drag against the rotation of the rotor  12 . 
   The graphical representation of  FIGS. 6   a - 6   i  depict the direction of wind, as indicated by the lined arrows, as it flows across blades  22 ,  24 ,  26  and sub-blades S 1 , S 2 , and S 3  of the present invention through a 360 degree rotation cycle. Through this graphical representation, those skilled in the art can understand and how the torque or spin moments would appear through rotation of the rotor  12 . The self-braking aspect of the rotor  12  will also become apparent, which is a result of the particular construction of rotor  12  which prevents overspeed. This is significant advantage over prior wind turbines which require additional mechanical systems to act against the rotation of the rotor to prevent overspeed. Further, the self-braking design of rotor  12  of the present invention permits the wind turbine  10  to be used in higher wind velocities than previously capable in prior wind turbines. 
   In  FIGS. 7 and 8 , the rotor  12  is fitted with a torsional damper  48  to absorb torsional vibration generated in the rotor as a result of non-equal torque moments created at each blade  22 ,  24  and  26  through a 360 degree rotation cycle as each blade transitions into and out of the direction of wind flow. The torsional damper  48  operates to smooth and eliminate output torque pulsations by transitionally optimizing the torque moments of blades  22 ,  24 , and  26  in direct alignment or at their maximum angles of attack with respect to the relative wind flow throughout the rotor  12  and provide balance, strength, and stabilization to the entire rotor element. 
   Torsional dampers are well known in the art, and one of ordinary skill in the field would be readily capable of selecting a torsion damper of a particular construction and operation to suite the particular needs of the wind turbine  10  of the present invention. However, for exemplary purposes only, the torsional damper  48  herein is operatively coupled to rotor  12 , and includes three equal sized closed ended tubes  50 ,  52 , and  54  arranged symmetrically about the central axis  14 , and generally forming a triangle configuration as shown. Freely movable weights  56 ,  58 , and  60  are positioned into tubes  50 ,  52  and  54  respectively for reciprocation therein. The conservation of rotational momentum causes the weights  56 ,  58  and  60  to slide within the closed ended tubes  50 ,  52 , and  54  to absorb and release rotational moment from and into the rotor  12  as the rotor  12  experiences torque surges. Through this graphical representation of  FIGS. 7 and 8 , those skilled in the art can understand how movement of the weights  56 ,  58  and  60  will act to conserve the rotational momentum of the rotor  12 , and how the weights will absorb and release torque moments from and into the rotor. 
   The choice of materials among strong, dimensionally stable metals, composites, etc. will involve a compromise between: light materials which can minimize start-up inertia of the rotor  12  and enhance the response to light winds; and heavier materials which can make the rotor act somewhat as a flywheel and dampen the effect of wind gusts. In either case, the rotor components should be manufactured to close tolerances and be dynamically balanced to minimize structural noise and vibration. The present wind turbine  10  has been designed to accommodate different methods of assembly, either in the factory, when practicable, or at the installation site when shipping and handling costs make this advisable. 
   A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.