Abstract:
An aerodynamic slat ( 30 ) mounted over a forward suction side ( 40 ) of a wind turbine blade ( 22 ) and a mechanism ( 51 A-F) that closes or reduces a gap ( 31 ) between slat and blade. The slat may pivot to reduce the gap, or the gap may be reduced by a device such as an extendable gate ( 58 ), or butterfly plate ( 59 ), or damper plate ( 60 ). Control logic ( 64 ) activates an actuator ( 70 ) of the mechanism to close or reduce the gap when wind conditions meet or exceed a predetermined criterion such as a rated wind condition. This reduces wind loading on the blade by separating airflow ( 53 ) over the suction side of the blade downstream of the slat. The blades can then maintain a higher angle of attack during rated wind conditions than in the prior art, allowing them to stall in gusts sooner to limit peak aerodynamic loads.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of wind turbines, and more specifically to an apparatus for aerodynamic load reduction on wind turbines in high winds, and in particular to a dual purpose slat and spoiler for wind turbine blades. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wind turbine blades have thick airfoil sections near the blade root to enable low-mass designs due to high structural efficiency. However, structural efficiency comes at the cost of decreased aerodynamic efficiency. Use of multi-element airfoils, which may include a slat and/or flap on the thick blade sections, generally improves aerodynamic performance while maintaining structural efficiency 
         [0003]    Full-span blade pitch control effectively controls the aerodynamic rotor power by altering the angle of attack along the blade. When a wind turbine is operating at rated power output, the blades are pitched more towards feather (“into the wind”) which reduces the angle of attack and the resulting aerodynamic forces. However, this creates a large increase in lift generating potential during wind gusts ( FIG. 10 ) which can quickly increase the angle of attack, leading to sharply increased aerodynamic forces and loads on the blades and other turbine components This imposes high structural strength margin requirements on all parts of the wind turbine installation, from the blades to the base of the tower, with resultant weight and expense. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show. 
           [0005]      FIG. 1  is a suction side view of a prior art wind turbine rotor with slats. 
           [0006]      FIG. 2  is a perspective view of an inboard portion of a prior art wind turbine blade with slats. 
           [0007]      FIG. 3  is a transverse sectional view of a thick airfoil section with a slat taken along line  3 - 3  of  FIG. 1 . 
           [0008]      FIG. 4  shows a slat/spoiler pivot embodiment according to aspects of the invention. 
           [0009]      FIG. 5  shows another slat/spoiler pivot embodiment according to aspects of the invention 
           [0010]      FIG. 6  shows a gate embodiment according to aspects of the invention. 
           [0011]      FIG. 7  shows a butterfly plate embodiment according to aspects of the invention. 
           [0012]      FIG. 8  shows damper embodiment according to aspects of the invention 
           [0013]      FIG. 9  shows a control system embodiment for the invention 
           [0014]      FIG. 10  is a graph of lift coefficient as a function of angle of attack as known in the art. 
           [0015]      FIG. 11  shows a slat/spoiler pivot embodiment with actuators in the rotor hub 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]      FIG. 1  shows a downwind side of a wind turbine rotor  20  with radially-oriented blades  22 , sometimes referred to as main airfoils, which rotate generally in a plane  23  or disc of rotation The suction sides  40  of the blades are seen in this view, with the wind being directed generally through/into the plane of the page Only rotating elements are illustrated in this figure, with the typical nacelle and tower of a wind turbine power plant not being shown. Each main blade  22  has a radially inboard end or root end  24  that is thick to withstand flapwise loads that are normal to the chord of the blade airfoil. The roots  24  are attached to a common hub  26  that may have a cover called a spinner  28 . Each blade may have an aerodynamic slat  30  mounted above a leading portion of each blade  22  by support structures such as aerodynamic struts  32 . Slats provide increased aerodynamic efficiency and increased lift on the thick airfoil sections, both by acting as efficient small airfoils and by delaying and reducing flow separation on the suction side of the main airfoil 
         [0017]      FIG. 2  is a perspective view of an inboard portion  36  of a blade  22  having a pressure side  38  and a suction side  40  between a leading edge  42  and a trailing edge  44  Transverse sectional profiles of the blade may gradually transition from cylindrical PC at the root  24  to an airfoil shape PA at and past the shoulder  47  which is the position of longest chord length of the blade  22 . The slat  30  may have an efficient airfoil shape and angle of attack in normal operation throughout its span between its inboard end  30 A and outboard end  30 B. The main airfoil  22  and the slat  30  have respective chord lengths C 1 , C 2 . 
         [0018]      FIG. 3  shows a thick inboard airfoil section of a wind turbine blade  22  with a chord length C 1  between leading and trailing edges  42 ,  44 . A slat  30  is mounted with a given gap distance  31  above a leading suction side portion of the airfoil on aerodynamic struts  32 . Also shown are a rotation plane  23 , absolute wind direction  46 , relative wind direction  48 , and stream lines  50  influenced by the slat over the airfoil The slat helps prevent flow separation above the suction side  40 . 
         [0019]      FIG. 4  shows a slat/spoiler embodiment  51 A according to aspects of the invention. The trailing edge  52  of the slat  30  pivots toward the main airfoil  22  via a pivot axis or bearing  54  actuated by means such as a servo motor, electromechanical solenoid, or hydraulic piston located for example in the blade, in a support strut  32 , or in the rotor hub. In the shown pivoted position, the slat  30  stalls and partly or completely closes the gap between the slat and the main airfoil, causing the slat to act as a spoiler. This separates airflow  53  from the suction side  40  of the main airfoil, causing a loss of lift. Employing this effect during high operational wind speeds (after rated power has been achieved) reduces the amount of lift and power generated by the inboard blade sections equipped with spoiler slats To make-up for this reduced inboard power production the entire blade must then be pitched such that the outboard blade runs at a higher angle of attack, closer to stall, and therefore the potential for large aerodynamic load changes in the event of a gust for the entire blade (inboard and outboard) is reduced This effect can also be deployed during parked conditions or other non-operational states such that the spoiler limits the maximum possible lift that can be generated by the equipped sections in the event of extreme wind speeds. One benefit is that longer wind turbine blades are possible, allowing higher-efficiency wind turbines Another benefit is reduction in installation cost by reducing overall strength requirements and weight. Another benefit is reduced pitch activity and thus reduced wear on the pitch control system. Another benefit is reduction in pitch system cost, since it does not have to be as fast to react as quickly to gusts The axis of the pivot bearing  54  may be located at any position along the slat, such as at the aerodynamic center of the slat  30  in one embodiment to minimize actuation force, or at 25-50% of the slat chord length from the leading edge of the slat in other non-limiting embodiments 
         [0020]      FIG. 5  shows a slat/spoiler embodiment  51 B in which the leading edge  56  of the slat  30  pivots toward the main airfoil  22  in high winds. The minimum length of the gap between the slat  30  and the main airfoil  22  may be partly or completely closed by the pivot action The slat  30  pivots about a pivot bearing  54  on the support struts  32  under control of an actuator, such as a servo motor, electromechanical solenoid, hydraulic piston or other suitable means located in or on the blade  22 , in a support strut  32 , or in the rotor hub. Embodiments  51 A and  51 B may use the same or similar hardware, the difference being the direction of pivot, which may be determined based on wind conditions and the amount of aerodynamic braking wanted. 
         [0021]      FIG. 6  shows a slat/spoiler embodiment  51 C in which an extendable gate  58  forms a gate valve in the gap  31  that partially or completely or closes the gap between the slat  30  and the main airfoil  22 . The gate  58  may be extended and retracted by an actuator in the main airfoil, such as a motor driven helical or pinion drive, electromechanical solenoid, or a hydraulic piston, as non-limiting examples. 
         [0022]      FIG. 7  shows a slat/spoiler embodiment  51 D in which a rotatable butterfly plate  59  partially or completely closes the gap between the slat  30  and the main airfoil  22 . The butterfly plate may be rotated by an actuator in the strut  32 , in the main airfoil  22 , or in the rotor hub. 
         [0023]      FIG. 8  shows a slat/spoiler embodiment  51 E in which a damper plate  62  forms a valve that partially or completely closes the gap  31  between the slat  30  and the main airfoil  22  The damper plate  60  may be rotated by an actuator in the strut  32 , or in the main airfoil, or in the rotor hub In embodiments  51 C,  51 D, and  51 E, the slat  30  may be fixed and stationary with respect to the blade  22 . 
         [0024]      FIG. 9  shows a control logic unit  64  that uses available sensor inputs such as wind speed  66 , pitch  67 , and rotor speed  68  and/or derived parameters to activate the spoiler function of the embodiments herein via actuators  70  when one or more predetermined thresholds are reached For example, the spoiler function (i e reduction of the gap) may be activated when the wind reaches or exceeds a rated condition. This may be determined, for example, by wind speed and possibly other factors such as wind variability or aerodynamic loading on the rotor. Wind variability may be derived for example from instantaneous changes in wind speed or by derived metrics means such as statistical variance, or a combination of higher order wind speed derivatives. 
         [0025]      FIG. 10  shows the lift coefficient on a wind turbine blade as a function of angle of attack Gusts can quickly increase both the wind speed and angle of attack During normal operation a gust causes a stall after a small increase in lift  72  During post-rated (high wind) operation the angle of attack is conventionally reduced to reduce lift. But this enables a greater increase in lift  74  caused by gusts before stall occurs, allowing high peaks in aerodynamic loading and subsequent structural stresses and fatigue. The invention allows the angle of attack to remain higher during post-rated operation, thus protecting the blade from overstress by enabling more rapid stall on the main airfoil during gusts than in the prior art. 
         [0026]      FIG. 11  shows an embodiment  51 F in which each slat  30  extends from a respective pivot bearing  78  the rotor hub  26  Each slat pivots about a spanwise axis  80  positioned for example at 25-50% of the slat chord length C 2  from the leading edge of the slat or positioned along an aerodynamic center of the slat. This embodiment may be implemented with cantilever slats without support struts Thus, it provides a relatively simple retrofit, for example, by installing a replacement spinner with attached slats  30 , actuators  70 , and power and logic connections  76 . 
         [0027]    The invention builds upon the use of multi-element airfoils by incorporating aerodynamic load control capabilities. These additional abilities reduce operational and non-operational aerodynamic blade loads and provide a mechanism for controlling rotor torque and power in addition to full-span pitch control The spoiler mechanisms of the embodiments herein have aerodynamic and structural synergy with the slat. 
         [0028]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims