Abstract:
A wind turbine device of increased efficiency is comprised of a set of fixed airfoils that direct wind into a rotor having a plurality of blades. The fixed airfoils may extend to the ground to increase the amount of wind directed into the rotor and may be manufactured from concrete. The rotor blades have a vented portion near the axis of rotation that has been found to increase efficiency for certain blade geometries. For other blade geometries, increased efficiency is observed with no gap at the axis of rotation. The rotor may also be manufactured from composite materials to increase strength while decreasing the moment of inertia for the rotor.

Description:
CROSS REFERENCE TO RELATED CASES 
   The present patent application is based upon and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/467,773, filed on Apr. 30, 2003, entitled “Wind Turbine” by Ronald J. Taylor and Scott J. Taylor, which is hereby specifically incorporated herein by reference for all that it discloses and teaches. 

   BACKGROUND OF THE INVENTION 
   a. Field of the Invention 
   The present invention pertains generally to wind turbines and more specifically to crossflow wind turbines having a plurality of stationary airfoils positioned about a rotor having a plurality of blades. 
   b. Description of the Background 
   Radial flow windmills that harness wind energy using a plurality of exposed blades have been used both privately and commercially for some time. These machines often have a high initial cost and have limited efficiency. Further, the exposed blades are hazardous to certain wildlife such as birds. In addition, radial windmills cannot normally be operated in very high wind conditions, as they often lack sufficient structural integrity and are not mechanically designed to prevent over-speeding. 
   Crossflow turbine wind machines, such as described in U.S. Pat. Nos. 6,015,258 to Taylor and U.S. Pat. No. 5,391,926 to Staley, et al., have been developed to address some of the limitations of the radial flow wind turbines. The crossflow turbine wind machine comprises a set of fixed stators that direct wind into a rotating turbine. One of the advantages to the crossflow turbine machine is the higher efficiencies that can be achieved, and they are less dangerous. Further, the structural integrity of the machine and the serviceability of the moving components are superior to that of a radial flow windmill. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the disadvantages and limitations of the prior art by providing a crossflow wind turbine that uses various airfoil and rotor configurations and orientations, including a rotor that has gaps near the leading edges and ground airfoils to increase efficiency. 
   The present invention may therefore comprise a crossflow wind turbine that generates mechanical energy from wind comprising: a rotor having a plurality of rotor blades that are symmetrically disposed around an axis, the rotor blades disposed in the rotor so that a gap is formed between leading edges of the rotor blades; a rotor space formed in a volume that is swept out by the rotor blades, the rotor space having a drive portion in which the rotors are driven by the wind and a return portion in which the rotors return to the drive portion; a plurality of airfoils that direct wind into the drive portion and direct wind away from the return portion to cause the rotor to turn and generate the mechanical energy. 
   The present invention may further comprise a method of generating mechanical energy from wind comprising: providing a crossflow wind turbine having airfoils and a rotor that sweeps out a rotor space, the rotor space having a drive portion and a return portion; symmetrically placing a plurality of rotor blades in the rotor that form a gap between leading edges of the rotor blades; placing the airfoils around the rotor to direct the wind into the drive portion of the rotor space so that the wind drives the rotor blades in the drive portion, and to block the wind from entering the return portion of the rotor space so that the rotor blades return to the drive portion to generate the mechanical energy. 
   The present invention may further comprise a crossflow wind turbine that generates mechanical energy from wind comprising: a rotor having a plurality of rotor blades that are symmetrically disposed around an axis, the rotor blades disposed in the rotor so that a gap is not formed between leading edges of the rotor blades; a rotor space formed in a volume that is swept out by the rotor blades, the rotor space having a drive portion in which the rotors are driven by the wind and a return portion in which the rotors return to the drive portion and a plurality of airfoils that direct wind into the drive portion and direct wind away from the return portion to cause the rotor to turn and generate the mechanical energy. 
   Advantages of various embodiments of the present invention include the ability to harness wind energy with an economical wind turbine that is safe and visually appealing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
       FIG. 1  is an illustration of a perspective view of one embodiment of the present invention of a wind turbine. 
       FIG. 2  is an illustration of a perspective view of another embodiment of the present invention of a wind turbine. 
       FIG. 3  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 4  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 5  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 6  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 7  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 8  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 9  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 10  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 11  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 12  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
       FIG. 13  is an illustration of a cross-sectional view of another embodiment of the present invention of a wind turbine. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a perspective view of a first embodiment of a crossflow wind turbine  100 . Crossflow wind turbines differ from propeller type (radial flow) turbines in that the wind generally flows across the axis of rotation in crossflow turbines, rather than generally along the axis of rotation in propeller type turbines. As shown in  FIG. 1 , a vertical rotor  102  is propelled by the wind and is mechanically coupled to an electrical generator located in the base  108 . The rotor  102  is generally disposed with a vertical axis, but can have other orientations as desired. Three airfoils  104 ,  106 , and one not visible behind the rotor  102 , serve to support the upper portion of the rotor  102  and also to direct the wind into a drive portion of a rotor space  118  of the rotor  102  for increased efficiency and to block wind from a return portion of the rotor space  118 . The rotor space  118  is the volume that is swept out by the rotor blades of rotor  102 , such as rotor blades  120 ,  122 , during the rotation of rotor  102 . A ground airfoil portion  110  of airfoil  104  extends across the base  108 , and functions to direct additional wind into the rotor  102 . Airfoil  106  similarly has a ground airfoil  112 . An optional vent hole  14  may be present in the top of the wind turbine. 
   The wind turbine  100  may be over 210 feet tall in some embodiments. The distance from the tip of the ground airfoil  110  to the tip of ground airfoil  112  may be over 200 feet in such embodiments. Such an embodiment may be suitable for a large wind farm application of a permanent power generation facility. In other embodiments, such as a portable wind turbine generator, the entire height of the turbine  100  may be only three or six feet. The general concepts embodied in the present invention are scalable to wind turbines of many different sizes, as is appreciated by those skilled in the arts. 
   The rotor  102  may be composed of several sections that are connected at joints such as the flange joint  116 . Such embodiments may allow the rotor sections to be fabricated in sections that may be shipped to a wind turbine site for assembly. The rotor  102 , in the embodiment of  FIG. 1 , is comprised of three blades. In some embodiments, the rotor blades may be individually manufactured and assembled in sections. 
   The rotor blades may be manufactured from a variety of materials, using a variety of methods. For example, the rotor blades may be fabricated from sheet metal, such as steel or aluminum, using fasteners or welded connections. In other embodiments, the rotor blades may be constructed of reinforced composite material using a variety of manufacturing techniques, including hand laid-up and autoclave cured fiberglass, or graphite composite, or any automated or semi-automated composite manufacturing technique desired. In still other embodiments, the rotor blades may be manufactured of molded or formed plastic. An advantage of lighter weight rotors is that less wind speed may be required to start the rotational motion of the rotor. In yet other embodiments, the rotor blades may be constructed with a sail cloth or another engineered fabric exterior over a structural frame. Carbon fiber may also be used. In fact, any suitable material or manufacturing technique may be used by those skilled in the arts while keeping within the spirit and intent of the present invention. 
   The airfoils, such as airfoil  104 , may be manufactured by a variety of techniques. For example, the airfoil  104  may be a poured concrete slab that is lifted into place in a fashion similar to conventional ‘tip up’ building construction techniques. In such an example, the airfoils may be fabricated on-site and lifted into place. In another example, the airfoils may be continuously poured in a vertical manner using techniques common to the construction trade. In still other examples, the airfoils may be constructed of metal or other suitable material such as carbon fiber, fiberglass, etc. Some embodiments for airfoils may incorporate a rigid framework over which sail cloth is placed, or another engineered fabric or plastic type material, that forms an air-directing airfoil. Such a framework may be constructed of metal, concrete, or any other suitable material. 
   In some embodiments, the airfoils may be constructed of a combination of manufacturing techniques. For example, a steel column may support panels of concrete, sail cloth, metal, or other materials. In another example, a prestressed concrete post may have panels of various sorts attached thereto. Those skilled in the art may construct an embodiment of the present invention using any suitable materials while keeping within the spirit and intent of the present invention. 
   The base  108  may be used to house various components such as gearboxes, generators, control equipment and the like. The base  108  may be constructed above ground as shown or may be constructed below grade in other embodiments. In still other embodiments, the mechanical and electrical equipment for the wind turbine  100  may be located partially below grade. In embodiments with the base  108  above ground, the walls of the base  108  may be slanted to direct airflow into the rotor  102 . 
     FIG. 2  illustrates a perspective view of another embodiment of a wind turbine  200 . The rotor  202  comprises eight blades while six airfoils  204  support the rotor  202  and direct airflow into the rotor  202 . A mechanical enclosure  206  may contain a generator and other electrical and mechanical equipment. The mechanical energy may be used directly in pumps or other mechanical devices including reverse osmosis desalination, or with generators/alternators that produce electrical energy that can be used for various purposes. The ground airfoils  208  may direct airflow into the drive portion of the rotor space. 
   The rotor  202  may contain a plurality of stiffening ribs  210  disposed between the various blades of the rotor  202 . The ribs  210  may help disperse the loads seen by the blades of the rotor  202  during high wind conditions. In some embodiments, the rotor blades may be stiff enough to not require the ribs  210 . In other embodiments, the ribs  210  may be used to lower the weight of the blades while giving the same overall structural integrity. Such tradeoffs may be made by those skilled in the arts in mechanical and structural design. 
     FIG. 3  illustrates a cross-sectional view of an embodiment of a wind turbine  300 . The incoming airflow is represented by arrow  301  which shows wind blowing from the left. Airfoils  302 ,  304 , and  306  are symmetrically disposed about the embodiment  300 . Three rotor blades  308 ,  310 , and  312  have a slight curve. A center axis shaft  314  is present in the embodiment  300  that may be used to support the rotor blades  308 ,  310  and  312  along the length of the rotor blades. A gap  317  is also present between the leading edge of each blade. The gap  317  has been shown through computational fluid dynamics models to reduce the drag on the rotor and thereby increase efficiency under certain conditions. Different configurations of rotor blades that are shown in the various embodiments disclosed below can be used with the airfoil configuration of  FIG. 3  and other airfoil configurations disclosed herein. Similarly, the various airfoil configurations disclosed herein can be used with the various rotor configurations to achieve desired results. 
   As also shown in  FIG. 3 , the rotor blades  308 ,  310 ,  312  sweep out a volume that is generally shown by the circle  320  which is the rotor space. The rotor space has two different portions, a drive portion  316  and a return portion  318 . The drive portion  316  is the portion of the rotor space  320  in which the rotor blades are driven by the wind flowing from direction  301 . The return portion  318  is the portion where the rotor blades return to the drive portion  316 . As shown in  FIG. 3 , airfoil  302  blocks wind flowing from direction  301  from substantially entering the return portion  318 . In addition, airfoil  302  directs wind into the drive portion  316 . Airfoil  306  also directs wind into the drive portion  316 . The drive portion  316  may vary in accordance with the magnitude of the flow  301 , i.e., the magnitude of the wind speed. For example, high wind flowing from direction  301  may be guided by airfoil  306  so that the drive portion  316  extends into part of the return portion  318  at the right side of the rotor space  320  where the rotor blade  310  is disposed. In other words, the division between the drive portion  316  and the return portion  318  may not be exactly as shown in  FIG. 3  or any of the figures and may vary in accordance with the wind speed. Further, the direction of flow  301  to the wind may greatly affect the operability and efficiency of the device of  FIG. 3 . As shown in  FIG. 3 , the wind flow  301  is from the 9 o&#39;clock position and is able to produce high efficiency because the wind flow  301  is guided by airfoil  302  into the drive portion  316  and away from the return portion  318 . Similar efficiencies occur when the wind comes from the 1 o&#39;clock position and the 5 o&#39;clock position. Of course, as the direction of the flow  301  changes, the drive portion and the return portion of the rotor space  320  also change. 
   As set forth above, a gap  317  is formed between the leading edges of each of the rotor blades  308 ,  310 ,  312 . As shown in  FIG. 3 , the leading edges of the rotor blades  308 ,  310  and  312  are not overlapping. The curvature of the rotor blade  308 ,  310 ,  312  functions to assist in capturing wind flowing from direction  301  in the drive portion  316  and reducing resistance in the return portion  318 . The curvature of the rotor blades  308 ,  310 ,  312  also causes wind to flow across the surface of the rotor blades, such as rotor blade  308 , and direct wind into the return portion  318  to drive another rotor blade, such as rotor blade  310 . This process further increases the efficiency of the embodiment of  FIG. 3 . Hence, the gaps cause the wind to flow across the rotor blades through the gap and into the return portion  318  for the purpose of driving additional rotor blades in the return portion  318 , which increases efficiencies under certain conditions. 
     FIG. 4  illustrates a cross-sectional view of another embodiment of a wind turbine  400  that is similar to the embodiment of  FIG. 3 , but has smaller gaps between the leading edges of the rotor blades  408 ,  410  and  412 . The incoming airflow is represented by arrow  401  from the left side of  FIG. 3 . Airfoils  402 ,  404 , and  406  are also symmetrically disposed about the embodiment  400 . Three rotor blades  408 ,  410 , and  412  also have a slight curve. A center axis shaft  414  is present in the embodiment  400 . A gap  417  is present between the leading edge of each of the rotor blades  408 ,  410 ,  412 . The gap  417  is smaller than the gap  317  of embodiment  300 , which provides higher efficiencies in some conditions. The gap  417  increases the performance of the wind turbine  400  under some conditions for the same reasons as set forth above in the explanation of the embodiment of  FIG. 3 . The size of the gap in the position of the rotor blades  408 ,  410 ,  412  with respect to the other blades and the axis  414  controls the manner in which the wind flows across the rotor blade and is directed from the drive portion  416  into the return portion  418  of the rotor space  420 . As such, the efficiency in operation of the device is affected by these matters. As shown in  FIG. 4 , the rotor blades are capable of directing wind from the drive portion  416  into the return portion  418  to drive other rotor blades in the return portion  418  through the gap  417 . Again,  FIG. 4  shows the leading edges of the rotor blades  408 ,  410 ,  412  as non-overlapping rotor blades. 
     FIG. 5  illustrates a cross-sectional view of another embodiment of a wind turbine  500 . The incoming airflow is represented by arrow  501  from the left. Airfoils  502 ,  504 , and  506  are symmetrically disposed about the embodiment  500 . Three rotor blades  508 ,  510 , and  512  have a slight curve. The rotor blades are supported at the ends so that no center axis shaft is necessary in embodiment  500 . The leading edges of the rotor blades  508 ,  510 ,  512  form a gap around the axis of rotation. As shown in  FIG. 5 , the rotor blades are overlapping. The overlapping blades function to further channel the flow of wind across the rotor blade and onto the surface of another rotor blade. This process functions to further increase the efficiency of the device under certain conditions by creating multiple driving surfaces formed from multiple rotor blades. For example, wind captured by rotor blade  508  may flow across the surface of rotor blade  508  and be directed onto the surface of rotor blade  510  so as to drive multiple rotor blades. Airfoil  502  directs wind away from the return portion  520  and into the drive portion  518  of the rotor space  522 . The trailing edge  516  of the rotor blade  508  has an angled fin to further assist in capturing wind. 
     FIG. 6  is a cross-sectional view of another embodiment of a wind turbine  600 . The incoming airflow is represented by arrow  601  from the left side of  FIG. 6 . Airfoils  602  and  604  are asymmetrically disposed about the embodiment  600 . Airfoil  602  blocks the wind from the return portion  620  of rotor space  622 . Airfoil  604  guides the wind into the drive portion  618  of the rotor space  622 . Three rotor blades  608 ,  610 , and  612  have a slight curve and are overlapping in a fashion similar to rotor blades  508 ,  510 ,  512  of  FIG. 5 . As such, rotor blades  608 ,  610 ,  612  operate in a fashion similar to rotor blades  508 ,  510 ,  512 . No center axis shaft is present in embodiment  600 , and the rotor blades  608 ,  610 ,  612  can be supported in various ways such as top and bottom plates. The gap formed between the leading edges provides increased efficiencies for the same reasons as set forth above. The trailing edge  616  of the rotor blade  608  has an angled fin which further aids in capturing wind and holding the wind as the wind falls off of the rotor blades during rotation of the rotor blades through the drive portion  618 . 
   The asymmetrical design of the embodiment  600  may be beneficial in locations where the wind is predominately from one direction, which is often the case in locations of high wind. Such embodiments may have the benefit of lower costs, since fewer structural components may be needed to construct the wind turbine. Further, the asymmetric nature of the wind turbine may be optimized for increased performance in the direction of the prevailing wind. 
     FIG. 7  illustrates a cross-sectional view of another embodiment of a wind turbine  700 . The incoming airflow is represented by arrow  701  from the left side of  FIG. 7 . Airfoils  702 ,  704 ,  706 , and  708  are disposed symmetrically on two sides of the rotor space  720 . Three rotor blades  710 ,  712 , and  714  are similar in design to those of embodiment  600 . 
   Airfoils  702 ,  704 ,  706 ,  708  are disposed in a symmetric design that is capable of capturing wind from two opposing directions. Airfoils  702 ,  704 ,  706 ,  708  are arranged in a manner that provides optimized performance from wind flowing from direction  701  as well as wind flowing from the opposite direction  703 . In many locations with high prevailing wind, the wind direction may often be from a primary direction  701 . In such locations, the secondary direction is often opposite from the primary direction  701 . The embodiment  700  may take advantage of such a phenomena by being oriented to perform at maximum efficiency in the two main wind directions. As shown in  FIG. 7 , airfoil  702  blocks wind flowing from direction  701  from entering the return portion  718  of the rotor space  720 . Airfoil  706  assists in guiding wind flowing from direction  701  into the drive portion  716 . Similarly, airfoil  708  blocks wind flowing from direction  703  from the drive portion  716 , which becomes the return portion. Airfoil  704  guides wind flowing from direction  703  into the return portion  718 , which becomes the drive portion of the rotor space  720 . 
     FIG. 8  illustrates a cross-sectional view of another embodiment of a wind turbine  800 . The incoming airflow is represented by arrow  801  from the left side of  FIG. 8 . Airfoils  802 ,  804 , and  806  are symmetrically disposed about the rotor space  820 . Two rotor blades  808  and  810  are barrel-shaped and are separated at the center by a gap between leading edges  812  and  814 . The leading edges are not overlapping. The shape of the rotor blades  808 ,  810  together with the gap provided between the leading edges  812 ,  814 , respectively, allows wind to be channeled from one rotor blade to another. In other words, when a rotor blade is in a position to catch the wind flowing from direction  801  in the drive portion  816 , the wind will move along the surface of the rotor blade and be transferred to the other rotor blade through the gap between the leading edges  812 ,  814 . In this fashion, driving forces can be generated in both the drive portion  816  and return portion  818  of the rotor space  820 . 
   The embodiment  800  illustrates the use of two rotor blades and three airfoils. Many different combinations of rotor blades and airfoils may be used while keeping within the spirit and intent of the present invention. Further, the embodiment  800  illustrates the use of different shapes of rotor blades. The rotor blades  808  and  810  are shown as lines. However, in practice the blades  808  and  810  will have some thickness and shape such as an airfoil design. Those skilled in the arts will appreciate that a line may represent the general shape of an airfoil design. However, the rotor blade may require some thickness for structural integrity and the thickness may be constructed in an aerodynamic airfoil shape to further enhance efficiency of the wind turbine. 
     FIG. 9  illustrates a cross-sectional view of another embodiment of a wind turbine  900 . The incoming airflow is represented by arrow  901  from the left side of  FIG. 9 . Airfoils  902 ,  904 , and  906  are symmetrically disposed about the embodiment  900 . Two rotor blades  908  and  910  are barrel-shaped and are separated at the center by a gap between leading edges  912  and  914 . The rotor blades  908 ,  910  are shaped to provide a large overlapping area between the rotor blades. This allows wind flowing from direction  901  to be easily transferred from one rotor blade in the drive portion  916  to another rotor blade in return portion  918  of the rotor space  920 . Again, this design provides driving forces in both the drive portion  916  and the return portion  918 . 
     FIG. 10  illustrates a cross-sectional view of another embodiment of a wind turbine  1000 . The incoming airflow is represented by arrow  1001  from the left side of  FIG. 10 . Airfoils  1002 ,  1004 , and  1006  are symmetrically disposed about the embodiment  1000 . Two rotor blades  1008  and  1010  are straight and are separated at the center by a gap between leading edges  1012  and  1014 . Wind flowing from direction  1001  drives the rotor blade in the drive portion  1016  and transfers wind to the other rotor blade in the return portion  1018  of the rotor space  1020 . The trailing edge  1016  has a large fin that catches exiting wind. 
     FIG. 11  illustrates a cross-sectional view of another embodiment of a wind turbine  1100 . The incoming airflow is represented by arrow  1101  from the left side of  FIG. 11 . Airfoils  1102 ,  1104 , and  1106  are symmetrically disposed about the embodiment  1100 . Two rotor blades  1108  and  1110  are substantially straight with a curved portion near the leading edges  1012  and  1014 . The trailing edge  1116  has a large fin. Again, wind flowing from the direction  1101  drives the rotor blade in the drive portion  1118  and transfers wind to the other rotor blade in the return portion  1120  of the rotor space  1022 . The large fins at the end help to catch wind and drive the rotor blades. 
     FIG. 12  illustrates a cross-sectional view of another embodiment of a wind turbine  1200  that does not have gaps and that has increased efficiency. The incoming airflow is represented by arrow  1201  from the left side of  FIG. 12 . Airfoils  1202 ,  1204 , and  1206  are symmetrically disposed about the embodiment  1200 . Two rotor blades  1208  and  1210  are curved and join at the center axis shaft  1212  so that no gap is formed. In this manner, air is trapped by the rotor blades and continues to force the rotor blade around its axis, rather than being exhausted through a gap. This increases efficiencies under certain conditions. Air flowing from direction  1201  is substantially blocked from entering the return portion  1216  by airfoil  1202 . This allows the rotor blade  1210  to return in the return portion  1216  with minimal force from the airflow  1201 . Rotor blade  1208  is driven by the wind in the drive portion  1214  of the rotor space  1218  as described above. 
     FIG. 13  illustrates a cross-sectional view of another embodiment of a wind turbine  1300 . The incoming airflow is represented by arrow  1301  from the left side of  FIG. 13  that also does not have a central gap. Airfoils  1302 ,  1304 ,  1306 , and  1308  are symmetrically disposed about the embodiment  1300 . Two rotor blades  1310  and  1312  are curved and are joined at the center in the same manner as the embodiment of  FIG. 12 , which results in increased efficiencies for the same reasons as set forth above. As shown in  FIG. 13 , airfoil  1302  blocks the wind from entering the return portion  1316  while airfoil  1308  directs the airflow into the drive portion  1314  of the rotor space  1318 . When the wind flows from the opposite direction, airfoil  1306  performs the same function as airfoil  1302 , while airfoil  1304  performs the same function as airfoil  1308 . Similarly, wind can flow from the top of  FIG. 13 , or the bottom of  FIG. 13 , and the airfoils operate in substantially the same fashion. In this manner, the device of  FIG. 13  can operate with wind coming primarily from four different directions. 
   The present invention therefore provides a unique system of both blocking wind in a return portion of a rotor space and directing wind to rotor blades in a drive portion of a rotor space. Various configurations of airfoils and rotors can be used to achieve these results. A gap formed between leading edges of rotor blades can function to allow wind to flow across the rotor blades in a drive portion and be channeled through a gap between the leading edges and into a return portion of the rotor space to drive another rotor blade. This channeling of the wind through the central gaps allows multiple rotor blades to be driven by wind coming from a single direction. Other embodiments do not provide a central gap, which increases efficiencies in certain conditions. While certain embodiments are specifically adapted to operate with wind coming primarily from a predetermined direction, other embodiments are arranged to operate efficiently with wind coming from two or more directions. In this manner, the particular airfoil and rotor design for any particular environment can be achieved based upon prevailing winds in the area. In addition, airfoils can be used near the bottom portion of a vertically oriented wind turbine to direct ground winds or low winds up into the wind turbine in an efficient manner. 
   The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.