Patent Publication Number: US-11028822-B2

Title: Wind turbine airfoil structure for increasing wind farm efficiency

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/687,026, filed on Jun. 19, 2018, the benefit of priority of which is claimed hereby, and which is incorporated by reference herein in its entirety. 
    
    
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. The following notice applies to the software and data as described below and in the drawings that form a part of this document or portions incorporated herein by reference: Copyright Shujaut Bader, Boston, Mass., U.S.A. and Blair Perot, Boston, Mass., U.S.A. All Rights Reserved. 
     TECHNICAL FIELD 
     This document pertains generally, but not by way of limitation, to wind turbines and wind turbine farms. 
     BACKGROUND 
     Efficiency of individual wind turbines often focuses on the shape, orientation, or stiffness of turbine blades used in the wind turbine. However, individually increasing wind turbine efficiency does not always account for wind turbines installed in wind farms where wake effects from other wind turbines can affect the efficiency of an individual wind turbine. Wind turbines are often used in wind farms where multiple wind turbines are arranged in various arrays that result in upstream wind turbines obstructing or partially obstructing the free entry of wind into downstream wind turbines. Previous attempts to mitigate such problems have involved the use of tethered kites that attempt to mix fast and slow moving air within a wind farm array in order to reduce or eliminate the presence of slow moving air with the wind farm array. Tethered kite systems are described in Improving the Wind Farm Efficiency by Simple Means, LES study of a wind turbine array with tethered kites, by Evangelos Ploumakis, Delft University of Technology Masters of Science Thesis, Sep. 8, 2015. 
     Overview 
     The present inventors have recognized, among other things, that a problem to be solved can include inefficiencies that can potentially be introduced in wind turbine applications where one or more wind turbines are located downstream or partially downstream of other wind turbines. Wind passing through the blades of the upstream wind turbine produces a wake that can influence the wind passing through the blades of the downstream turbines. The inventors have recognized that airflow from the upstream turbine can be slowed as energy is extracted by the blades of the upstream wind turbine. In particular, the present inventors have recognized that a downstream wind turbine can lose 15%-20% of its operational power due to wake effects of one or more upstream wind turbines. 
     The present inventors have recognized that previous tethered kite systems can be inefficient from both a wind speed and economics standpoint. Tethered kite systems are designed to improve overall wind farm wind speeds by mixing high speed wind and low speed wind within the wind farm to produce medium speed air, thereby eliminating the slow moving wind from the entire wind farm array. These kite systems require tethering a large number of kites to the ground throughout the wind farm array. The kites are anchored separate from any particular wind turbine. Thus, the kites randomly mix air within the wind farm depending on the direction that the wind is blowing. The kites, therefore, cannot be equally efficient for all wind directions. 
     The present subject matter can help provide a solution to this and other problems, such as by using an airfoil structure to increase the speed of the airflow in the downstream wake of a wind turbine to increase the efficiency of a downstream wind turbine. The airfoil structure can be statically attached to an individual wind turbine, such as at the nacelle, to thereby always most efficiently interact with the wake field of such wind turbine regardless of wind direction. The airfoil structure can be configured to displace slow moving wind that has passed through the turbine blades with faster moving wind that is flowing above, below or laterally beyond the length of the turbine blades. In other words, the static airfoil structure can be used to pull surrounding high speed wind into the wake of the wind turbine to push the slowed down wind out of the wake field. 
     This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a conventional wind turbine system illustrating various components of a wind turbine. 
         FIG. 2  is a schematic perspective view of a conventional wind farm including a plurality of wind turbines arranged in line with each other along a wind path. 
         FIG. 3A  is a schematic perspective view of a wind turbine system incorporating an example static airfoil structure of the present application. 
         FIG. 3B  is a front view of the wind turbine system and static airfoil structure of  FIG. 3A . 
         FIG. 3C  is a side view of the wind turbine system and static airfoil structure of  FIG. 3A . 
         FIG. 4  is a schematic perspective view of a wind turbine airfoil rotor having an outboard mounted static airfoil structure with a straight span. 
         FIG. 5A  is a graph illustrating wind velocity in a wind turbine farm illustrating an upwind wind turbine and a downstream wind turbine. 
         FIG. 5B  is a graph illustrating wind velocity leaving the upwind wind turbine of  FIG. 5  as influenced by the static airfoil structure of  FIG. 4 . 
         FIG. 6  is a diagram illustrating various example locations for static airfoil structures relative to a span of airfoils for the wind turbine airfoil rotor. 
         FIG. 7  is a schematic perspective view of a wind turbine airfoil rotor having an inboard mounted static airfoil structure with a straight span. 
         FIG. 8  is a schematic perspective view of a wind turbine airfoil rotor having an outboard mounted static airfoil structure with a curved span. 
         FIG. 9  is a schematic perspective view of a wind turbine airfoil rotor having an inboard mounted static airfoil structure with a curved span. 
         FIG. 10  is a schematic perspective view of a wind turbine airfoil rotor having a pair of laterally mounted static airfoil structures with straight spans. 
         FIG. 11  is a schematic perspective view of a wind turbine airfoil rotor having a pair of longitudinally mounted static airfoil structures with curved spans. 
         FIG. 12A  is a schematic perspective view of a wind turbine system incorporating an example static airfoil structure having a plurality of stacked airfoils. 
         FIG. 12B  is a front view of the wind turbine system and static airfoil structure of  FIG. 12A . 
         FIG. 12C  is a side view of the wind turbine system and static airfoil structure of  FIG. 12A . 
         FIG. 13  is a perspective view of an exemplary airfoil structure comprising a plurality of frame members connected by a plurality of crossbeams and a plurality of skin support rods. 
         FIG. 14  is a top view of the airfoil structure of  FIG. 13  showing a plurality of skins laid out over the skin support rods. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic perspective view of wind turbine system  10  illustrating various components of wind turbine  12 . The various airfoil structures described herein can be incorporated into wind turbine  12 , as well as other wind turbines. Wind turbine  12  can include tower  14 , nacelle  16 , rotor  18  and blades  20 A,  20 B and  20 C. Tower  14  can include yaw motor  22 , and yaw drive  24 . Mounting shaft  26  can extend from yaw drive  24  to support nacelle  16  and other components of wind turbine  12 . Nacelle  16  can have located therein drive shaft  28 , gear box  30  and generator  32 . Rotor  18  can be mounted to drive shaft  28  and blades  20 A- 20 C can be mounted to rotor  18 . Wind turbine  12  can include other components such as pitch system  34 , controller  36 , anemometer  38 , wind vane  40  and brake  42 . Wind turbine  12  can operate in a conventional manner. Nacelle  16  can freely pivot relative to tower  14 , such as can be influenced via directionality of wind encountering blades  20 A,  20 B and  20 C. Additionally, nacelle  16  can be pivoted relative to tower  14  via yaw motor  22  and yaw drive  24 . 
     Wind W flows through blades  20 A- 20 C to cause rotation of rotor  18  on drive shaft  28  along a drive shaft axis. Input from drive shaft  28  into gear box  30  causes generator  32  to operate and produce electrical power, which can be provided to a power sub-station, an electrical grid, a battery or end-users, such as residential housing. As wind W flows through blades  20 A- 20 C, energy is extracted and the velocity of wind W is reduced accordingly. Thus, as wind W flows from wind turbine  12  to a downstream wind turbine, wind W becomes less effective at turning turbine blades and generating electrical power. 
       FIG. 2  is a schematic perspective view of wind farm  100  including a plurality of wind turbines, including wind turbine  102 A,  102 B and  102 C, arranged in line with each other along wind path P. Unobstructed wind W 1  enters blades  104 A,  104 B and  104 C of turbine  102 A and energy is extracted therefrom by blades  104 A- 104 C. Wind W 1  leaves blades  104 A- 104 C and forms wake field WF downstream of turbine  102 A. Wind W 2  within wake field WF has a speed that is reduced from the speed of wind W 1 . Thus, Wind W 2  is less effective in turning turbine blades  106 A,  106 B and  106 C. Wind W 2  forms an additional wake field downstream of turbine  102 . Speed of wind W 3  is subsequently reduced by blades  108 A,  108 B and  108 C as the process repeats as wind travels through each subsequent wind turbine. 
       FIG. 3A  is a schematic perspective view of wind turbine system  200  incorporating an example static airfoil structure  202  of the present application.  FIG. 3B  is a front view of wind turbine system  200  and static airfoil structure  202  of  FIG. 3A .  FIG. 3C  is a side view of wind turbine system  200  and static airfoil structure  202  of  FIG. 3A .  FIGS. 3A-3C  are discussed concurrently. 
     Wind turbine system  200  can include wind turbine  204 , which can be mounted on tower  206 . In particular, nacelle  208  can be mounted to tower  206  via a mounting shaft and rotor  210  can be mounted to nacelle  208  via a drive shaft. Blades  212 A,  212 B and  212 C can be mounted to rotor  210 . Airfoil structure  202  can be mounted to wind turbine  204 , such as at nacelle  208 . Airfoil structure  202  can comprise airfoil  214  and struts  216 A,  216 B and  216 C. 
     Nacelle  208  can rotate relative to tower  206 . Airfoil structure  202 , rotor  210  and blades  212 A- 212 C are coupled to nacelle  208  so as to be static relative to rotation between nacelle  208  and tower  206 . Nacelle  208  can be rotated such that the major axis A of nacelle  208  aligns in the direction of incoming wind, with rotor  210  and blades  212 A- 212 C being positioned upstream of airfoil structure  202 . Blades  212 A,  212 B and  212 C can be mounted to rotor  210  to extend radially relative to axis A. 
     Airfoil structure  202  can be positioned to affect the wake field of wind that has passed through blades  212 A- 212 C. Tower  206  is positioned inboard of nacelle  208 . In the example shown, airfoils structure  202  is positioned outboard of nacelle  208 . In particular, struts  216 A- 216 C can be positioned outboard of nacelle  208 . Struts  216 A- 216 B can have a length sufficient to place airfoil  214  radially outward of blades  212 A- 212 C. As can be seen in  FIG. 3C , struts  216 A- 216 C are connected to nacelle  208  downstream of airfoils  212 A- 212 C and are configured to position airfoil  214  radially outward of airfoils  212 A- 212 C and partially aft of airfoils  212 A- 212 C. 
     As shown in  FIG. 3C , airfoil structure  202  can have an angle of attack α relative to axis AA. Angle of attack α can be defined as the angle between horizontal axis AA and an axis extending along the chord length of airfoil structure  202  between the leading edge and the trailing edge, as defined below. Motor  218 , for example, can be added to airfoil structure  202  to adjust angle of attack α. Motor  218  can be located within airfoil structure  202  and can be powered by any suitable source such as the wind turbine itself, the grid or a battery. Angle of attack α can be adjusted to suit wind conditions. For example, angle of attack α can be set to zero in severe wind conditions to reduce stress on airfoil structure  202 . However, the present inventors have found that an angle of attack α of fifteen degrees works well in most or all wind conditions. 
       FIG. 4  is a schematic perspective view of wind turbine airfoil rotor  300  having outboard mounted static airfoil structure  302  with a straight span. Wind turbine airfoil rotor  300  schematically represents airfoils  212 A- 212 C and rotor  210  of  FIGS. 3A-3C  as a rotating disk. Airfoil structure  302  is schematically shown placed relative to airfoil rotor  300 . Airfoil structure  302  can be supported relative to airfoil rotor  300  via any suitable means, such as by struts  216 A- 216 B and nacelle  208  of  FIGS. 3A-3C . 
     Airfoil structure  302  can include leading edge  304 , trailing edge  306 , pressure side  308  and suction side  310 . Airfoil structure  302  can extend between first end  312  and second end  314 . Leading edge  304  can face toward incoming wind and pressure side  308  can face down toward the ground and airfoil rotor  300 . The span of airfoil structure  302  between first end  312  and second end  314  can be straight. Airfoil structure  302  is shown radially outward of airfoils  212 A- 212 C in an outboard direction. Airfoil structure  302  is also located radially in-line with airfoils  212 A- 212 C. Airfoil structure  302  can be configured to push faster moving air down into the wake of airfoil rotor  300 . 
       FIG. 5A  is a graph illustrating wind velocity in a wind turbine farm illustrating upwind wind turbine  400 A and downstream wind turbine  400 B. Wind turbine  400 A can include static airfoil structure  402 . Wind turbine  400 A can comprise a side view of airfoil rotor  300  of  FIG. 4 . Static airfoil structure  402  can comprise airfoil structure  302  of  FIG. 4 .  FIG. 5B  is a graph illustrating wind velocity leaving the upwind wind turbine  400 A of  FIG. 5  as influenced by static airfoil structure  402 . 
     The horizontal Y-axis ( FIG. 5A ) represents distance from the center of wind turbine  400 A, the horizontal X-axis ( FIG. 5B ) represents distance from the center of wind turbine  400 A, the left-hand vertical Z-axis represents distance from the center of wind turbine  400 A, and the right-hand vertical Z-axis ( FIG. 5A ) represents wind speed, Uy, which may be meters per second. As can be seen in  FIGS. 5A and 5B , wind speed increases the further away from the Y and X axes the wind is located, that is further away from the ground. Additionally,  FIG. 5A  shows that wind turbine  400 A produces wake field WF 1  that includes slower moving air than is above and below wind turbine  400 A. Wind from wake field WF 1  feeds into wind turbine  400 B and, as can be seen in  FIG. 5A , wake field WF 2  includes even slower air than wake field WF 1 . However, as can be seen in  FIG. 5B , airfoil structure  402  can force faster moving air from above (outboard of) wind turbine  400 A into zone Z 1  of wake field WF 1 . Thus, wake field WF 2  can include zone Z 2  of faster moving air relative to what would have occurred without the presence of airfoil structure  402 . 
       FIG. 6  is a diagram illustrating various example locations for static airfoil structures  500 A,  500 B,  500 C,  500 D and  500 E relative to a span of airfoils for wind turbine airfoil rotor  502  of wind turbine  504 . Wind turbine  504  can include tower  506  and nacelle  508 .  FIG. 6  also shows wind turbine  510 , which can include rotor  512 , tower  514  and nacelle  516 . Wind turbine  510  can include static airfoil structures  518 A and  518 B.  FIG. 6  illustrates the various locations of airfoils structures  500 A- 500 E and airfoil structures  518 A and  518 B. Various embodiments of the present disclosure can include any one, all, or any combination of airfoils structures  500 A- 500 E and airfoil structures  518 A and  518 B. 
     Airfoil structure  500 A is radially outward of the tips of rotor  502 , radially in-line with rotor  502  and outboard of nacelle  508 . 
     Airfoil structure  500 B is radially inward of the tips of rotor  502 , axially aft of rotor  502  and outboard of nacelle  508 . 
     Airfoil structure  500 C is radially inward of the tips of rotor  502 , axially aft of rotor  502  and inboard of nacelle  508 . 
     Airfoil structure  500 D is radially inward of the tips of rotor  502 , axially aft of rotor  502  and inboard of nacelle  508 . 
     Airfoil structure  500 E is radially outward of the tips of rotor  502 , radially in-line with rotor  502  and inboard of nacelle  508 . 
     Airfoil structure s  518 A and  518 B are positioned similarly to airfoil structures  500 A and  500 E, respectively. 
       FIG. 7  is a schematic perspective view of wind turbine airfoil rotor  600  having inboard mounted static airfoil structure  602  with a straight span. 
     Airfoil structure  602  can include leading edge  604 , trailing edge  606 , pressure side  608  and suction side  610 . Airfoil structure  602  can extend between first end  612  and second end  614 . Leading edge  604  can face toward incoming wind and pressure side  608  can face down toward the ground away from airfoil rotor  600 . The span of airfoil structure  602  between first end  612  and second end  614  can be straight. Airfoil structure  602  is shown radially outward of airfoil rotor  600  in an inboard direction. Airfoil structure  602  is also located radially in-line with airfoil rotor  600 . Airfoil structure  602  can be configured to push slower moving air down out of the wake of airfoil rotor  600 . 
       FIG. 8  is a schematic perspective view of wind turbine airfoil rotor  700  having outboard mounted static airfoil structure  702  with a curved span. 
     Airfoil structure  702  can include leading edge  704 , trailing edge  706 , pressure side  708  and suction side  710 . Airfoil structure  702  can extend between first end  712  and second end  714 . Leading edge  704  can face toward incoming wind and pressure side  708  can face down toward the ground and airfoil rotor  700 . The span of airfoil structure  702  between first end  712  and second end  714  can be curved. Airfoil structure  702  is shown radially outward of airfoil rotor  700  in an outboard direction. Airfoil structure  702  is also located radially in-line with airfoil rotor  700 . Airfoil structure  702  can be configured to push faster moving air down into the wake of airfoil rotor  700 . 
       FIG. 9  is a schematic perspective view of wind turbine airfoil rotor  800  having inboard mounted static airfoil structure  802  with a curved span. 
     Airfoil structure  802  can include leading edge  804 , trailing edge  806 , pressure side  808  and suction side  810 . Airfoil structure  802  can extend between first end  812  and second end  814 . Leading edge  804  can face toward incoming wind and pressure side  808  can face down toward the ground away from airfoil rotor  800 . The span of airfoil structure  802  between first end  812  and second end  814  can be curved. Airfoil structure  802  is shown radially outward of airfoil rotor  800  in an inboard direction. Airfoil structure  802  is also located radially in-line with airfoil rotor  800 . Airfoil structure  802  can be configured to push slower moving air down out of the wake of airfoil rotor  800 . 
       FIG. 10  is a schematic perspective view of wind turbine airfoil rotor  900  having a pair of laterally mounted static airfoil structures  902 A and  902 B with straight spans. 
     Airfoil structures  902 A and  902 B can each include leading edge  904 , trailing edge  906 , pressure side  908  and suction side  910 . Airfoil structures  902 A and  902 B can each extend between first end  912  and second end  914 . Leading edges  904  can face toward incoming wind, while pressure sides  908  can face toward airfoil rotor  900  and suction sides  910  can face away from airfoil rotor  900 . The span of airfoil structures  902 A and  902 B between first end  912  and second end  914  can be straight. Airfoil structures  902 A and  902 B are shown radially outward of airfoil rotor  900  in lateral directions. Airfoil structures  902 A and  902 B are also located radially in-line with airfoil rotor  900 . Airfoils structure  902 A and  902 B can be configured to pull faster moving into the wake of airfoil rotor  900 . 
       FIG. 11  is a schematic perspective view of wind turbine airfoil rotor  1000  having a pair of longitudinally mounted static airfoil structures  1002 A and  1002 B with curved spans. 
     Airfoil structures  1002 A and  1002 B can each include leading edge  1004 , trailing edge  1006 , pressure side  1008  and suction side  1010 . Airfoil structures  1002 A and  1002 B can each extend between first end  1012  and second end  1014 . For airfoil structure  1002 A, leading edge  1004  can face toward incoming wind and pressure side  1008  can face down toward the ground and airfoil rotor  1000 . For airfoil structure  1002 B, leading edge  1004  can face toward incoming wind and pressure side  1008  can face down toward the ground away from airfoil rotor  1000 . The span of airfoil structures  1002 A and  1002 B between first end  1012  and second end  1014  can be curved. Airfoil structures  1002 A and  1002 B are shown radially outward of airfoil rotor  1000  in outboard and inboard directions, respectively. Airfoil structures  1002 A and  1002 B are also located radially in-line with airfoil rotor  1000 . Airfoil structure  1002 A can be configured to push faster moving air down into the wake of airfoil rotor  1000  and airfoil structure  1002 B can be configured to push slower moving air out of the wake of airfoil rotor  1000 . 
     In the illustrated embodiment, airfoil structure  1002 A encircles approximately 30 percent of the circumference of airfoil rotor  1000  while airfoil structure  1002 B encircles approximately 50 percent of the circumference of airfoil rotor  1000 . In various embodiments, airfoil structures  1002 A and  1002 B can be split in a 25/50, 25/45, 30/50, 30/45, 35/50 or 35/45 relationship, respectively, relative to a 100 percent span of the circumference of airfoil rotor  1000 . 
     In other embodiments, airfoil structures  1002 A and  1002 B can completely encircle airfoil rotor  1000 . In various embodiments, airfoil structures  1002 A and  1002 B can be split in a 40/60, 45/55, 50/50, 55/45 or 60/40 relationship, respectively, relative to a 100 percent span of the circumference of airfoil rotor  1000 . 
       FIG. 12A  is a schematic perspective view of wind turbine system  1100  incorporating static airfoil structure  1102  having a plurality of stacked airfoils  1104 A,  1104 B and  1104 C. Airfoils  1104 A- 1104 C can be connected to each other via struts  1106 A and  1106 B. Airfoils  1104 A- 1104 C can be connected to nacelle  1108  via struts  1110 A,  1110 B,  1110 C and  1110 D. Struts  1110 A- 1110 D can be connected to tower  1112  via struts  1114 A and  1114 B. Wind turbine  1116  can include turbine airfoils  1118 A,  1118 B and  1118 C, and rotor  120 . 
       FIG. 12B  is a front view of the wind turbine system  1100  and static airfoil structure  1102  of  FIG. 12A .  FIG. 12C  is a side view of wind turbine system  1100  and static airfoil structure  1102  of  FIG. 12A . In the example shown, airfoils  1104 A- 1104 C can be disposed is a stacked configuration downstream of turbine airfoils  1118 A- 1118 C. Airfoils  1104 A- 1104 C can be stacked so that lateral side edges of each of airfoils  1104 A- 1104 C are vertically aligned. Airfoils  1104 A- 1104 C can be centered relative to rotor  1120  such that the center of airfoil  1104 B is centered on rotor  1120  and airfoils  1104 A and  1104 C are equally spaced above and below airfoil  1104 B, respectively. 
       FIG. 13  is a perspective view of exemplary airfoil structure  1200  comprising frame members  1202 A,  1202 B,  1202 C,  1202 D,  1202 E and  1202 F connected by crossbeams  1204 A and  1204 B and a plurality of skin support rods  1206 , such as skin support rods  1206 A,  1206 B and  1206 C. Airfoil structure  1200  can also include roller  1208  and leading edge body  1210 . 
     Frame members  1202 A- 1202 F can have the profile of an airfoil, such as with a leading edge, a trailing edge, a pressure side and a suction side. In an example, frame members  1202 A- 1202 F can each have a NACA  2412  airfoil profile. Frame members  1202 A- 1202 F can be rigid bodies that can be used to support a skin structure forming outer, pressure side and suction side surfaces of airfoil structure  1200 . Frame members  1202 A- 1202 F can be made of strong and lightweight materials, such as steel, carbon fiber, fiberglass, wood or aluminum, and can include weight reducing cut-outs. 
     Crossbeams  1204 A and  1204 B can comprise rigid bodies that support frame members  1202 A- 1202 B as a rigid unified structure. Crossbeams  1204 A and  1204 B can be made of material capable of resisting twisting and bending from wind-induced forces. Such materials can include steel, carbon fiber, fiberglass, wood and aluminum. 
     Support rods  1206  can comprise elongate rigid bodies configured to support the skin structure in the pressure side and suction side shape. For example, support rods  1206  can comprise small diameter poles extending through or into frame members  1202 A- 1202 F near or at the pressure and suction side surfaces. Thus, a pliable or rigid skin structure can be applied over support rods  1206  to provide airfoil structure  1200  with aerodynamic surfaces for inducing lift or, in other words, for bending flow of air over airfoil structure  1200 . 
     Leading edge body  1210  can comprise an aerodynamically shaped body that can provide multiple functions, such as connecting frame members  1202 A- 1202 F and providing a cover for roller  1208 . Roller  1208  can be disposed at least partially within leading edge body  1210  and can be configured to rotate between frames  1202 A and  1202 F. One or more pliable skin structures, such as cloth, fiber, plastic or metallic sheets, can be wound around roller  1208 . Roller  1208  can be powered, such as with electric motor  1212 , to wind and unwind, or furl and unfurl, the skin structures along the pressure and suction sides of airfoil structure  1200 . Electric motor  1212  can be located within airfoil structure  1200  and can be powered by any suitable source such as the wind turbine itself, the grid or a battery. 
     In an example, the skin structure can function as an awning or umbrella structure. Under low wind conditions the skin structure can remain unrolled or unfurled and can hang or slide along frame members  1202 A- 1202 F. Under high load conditions frame members  1202 A- 1202 F can be put into tension as the wind pulls on airfoil structure  1200 , which can be supported by frame members  1202 A- 1202 F. In severe weather the skin structure sheets can be rolled up or furled up onto roller  1208  or another suitable structure, to reduce wind tension on airfoils structure  1200 . 
       FIG. 14  is a top view of airfoil structure  1200  of  FIG. 13  showing skins  1210 A,  1210 B,  1210 C,  1210 D and  1210 E laid out over skin support rods  1206  ( FIG. 13 ). Each of skins  1210 A- 1210 E can be rolled up on roller  1208  ( FIG. 13 ). In various embodiments, roller  1208  can be divided into a number of rollers to match the number of skins.  FIG. 14  illustrates skins  1210 A- 1210 E forming a suction side of airfoil structure  1200 . Additional skins can be added to form a pressure side of airfoil structure  1200 . Alternatively, skins  1210 A- 1210 E can be configured to wrap around both of the pressure and suction sides of airfoil structure  1200 . Alternatively, skins for the pressure side can be omitted. 
     Further description and illustration of the various static airfoils structures described herein is provided in the Appendices of U.S. Provisional Patent Application Ser. No. 62/687,026. The Appendices include reference to particular embodiments having specific material and dimensions that are example embodiments. The particular configurations described and illustrated in the Appendices can be modified based on particular applications and design preferences. 
     This disclosure is directed to using static fixed airfoils in proximity to a wind turbine to control the airflow coming out of the turbine for use with, for example, a downstream turbine. These control devices have at least three beneficial effects. (1) They gather air from “higher up” where the air is moving faster on average (and therefore has more potential energy in it). (2) They throw the used (and slowed down air) air downwards. This means that any turbines in the wind farm behind the lead turbines do not get “stale” (already slowed down) air. (3) These control devices provide a large stabilizing lifting force for floating off-shore turbines, which are likely to eventually dominate the wind-power sector. 
     In examples, various configurations can use a streamlined (airfoil shaped) structure, at an angle of attack close to 15°, close to the upstream wind turbine so as to create a significant downwash of the faster wind from upper layers of atmospheric boundary layer. This downwash can energize the wake and as a result feed faster wind to the downstream turbine which would otherwise receive low speed wind due to its presence in the wake of the upstream turbine. 
     This design is simple, practical to implement, and improves the performance of wind turbines especially in “farm” settings. Wind turbine power output is proportional to the third power of the velocity. This means a small increase in inflow wind speeds (such as 10%) produces a large effect on the final power output (over 30%). Simulations have shown power increase in downstream wind turbines as large as 48%. Various examples of the benefits of the structures described herein can be found in Improving the Wind Farm Efficiency by Increasing the Rate of Vertical Mixing and Kinetic Energy Entrainment Using Novel Airfoil-Shaped Designs Around the Wind Turbines by Shujaut H. Bader, which can be found in the Appendices of U.S. Provisional Patent Application Ser. No. 62/687,026. 
     The devices described herein can be retrofitted to existing wind turbines. They can be constructed nearby (and not on) a wind turbine and subsequently mounted or attached to the wind turbine. They can enhance the competitiveness of wind energy and enable the acceleration of off-shore wind energy (because they solve a separate stability problem). 
     The devices and methods described herein can be used by installers of wind turbines, which tend to be utility companies. Wind turbine manufacturers can also utilize the devices and methods described herein. The devices described herein can be sold with a wind turbine unit or as an add-on feature. For example, floating off-shore wind turbines can be sold as a whole package (turbine, floating structure, mooring system, etc.) and the static airfoil structures described herein can be a component of that package to facilitate structure stability. 
     In addition, it is possible that the static airfoil devices described herein can allow full power recovery and closer spacing of all the wind turbines in a wind farm, which can lower capital and operating costs. 
     Various Notes 
     Each of the non-limiting examples described herein can stand on its own, or can be combined in various permutations or combinations with one or more of the other examples. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. For example, various static airfoil structures are described herein as being directly radially outward of a turbine airfoil rotor. However, the various static airfoil structures can be slightly forward of the turbine airfoil rotor, slightly behind the turbine airfoil rotor or completely behind the turbine airfoil rotor. 
     The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.