Abstract:
A method for controlling aerodynamic loads in wind turbine ( 20 ), includes stopping rotation of blades ( 22 ) of the turbine about a rotor shaft axis ( 38 ); stopping rotation of a nacelle ( 30 ) of the turbine about a vertical yaw axis ( 36 ); pitching each blade of the turbine about its respective pitch axis ( 43 ) into a stable pitch angle range ( 52 B- 52 C or  52 E- 52 F) in which a resulting root twisting moment ( 52 ) created by a current wind loading ( 48, 50 ) on the respective blade is in a direction urging pitch rotation of the blade toward a position of lower root twisting moment; and releasing the blades to rotate passively about their respective pitch axes during subsequent changing wind directions (V R1 ). A blade may be designed to better align a root zero twisting moment ( 52 A,  52 D) in the stable pitch angle range with a minimum ( 48 B,  48 D,  50 B,  50 D) wind loading.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to wind turbines and more particularly to minimizing aerodynamic structural loading in a parked wind turbine. 
       BACKGROUND OF THE INVENTION 
       [0002]    The cost efficiency of a wind turbine increases with rotor diameter, but blade length and design are often limited by maximum wind loads during storms. When a wind turbine is operating, a yaw control keeps the nacelle and rotor shaft aligned with the wind. However, when high winds are detected or forecast, the yaw position may be locked. In this condition, the wind can come from any direction relative to the nacelle. Winds that are broadside to the blade can cause excessive stress on the blade and all supporting parts. For this reason, blades may be actively feathered to align their chord lines with the wind. Active feathering requires constant blade pitch adjustment as wind direction changes. If the rotor is parked for safety, no power is being generated. If the power grid fails, there is no power for the active pitch controls, so an auxiliary power unit is required, which itself is subject to failure. When power for pitch control is unavailable, the blades may be subjected to excessive bending and twisting forces in high winds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The invention is explained in the following description in view of the drawings that show: 
           [0004]      FIG. 1  is a front view of a wind turbine with a first blade in a vertical azimuth position according to an aspect of the invention. 
           [0005]      FIG. 2  is a sectional top view of the vertically oriented blade taken along line  2 - 2  of  FIG. 1 . 
           [0006]      FIG. 3  is an enlargement of the blade airfoil section of  FIG. 2 . 
           [0007]      FIG. 4  shows interrelated function curves for root bending moment, root shear, and root twisting moment computed over a full range of relative wind directions. 
           [0008]      FIG. 5  shows root twisting moment curves for different amounts of sweep relative to a given “full” sweep of 1.00. 
           [0009]      FIG. 6  schematically shows a blade with a profile curve passing through its ¼ chord points from root to tip. 
           [0010]      FIG. 7  shows a profile curve for a swept back blade. 
           [0011]      FIG. 8  shows a profile curve for a blade that is swept back at an intermediate radial position. 
           [0012]      FIG. 9  shows a blade as designed per  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  is a front view of a wind turbine (WT)  20 , with three blades  22 ,  24 ,  26  mounted radially to a hub  28 , which is mounted on a rotor shaft (not visible) extending from a nacelle  30 , which is mounted on a tower  32  via a yaw bearing  34  that provides rotation of the nacelle about a yaw axis  36 . The blades, hub, and shaft constitute a rotor that rotates about a horizontal rotor shaft axis  38 . A radial position r is indicated. Each blade pivots controllably about a respective pitch axis  43 . The vertical blade is shown in the zero azimuth position. It may be locked in this position when high winds are detected or forecast in order to minimize total bending moments on the tower and to distance the blades from eddies around the tower. 
         [0014]      FIG. 2  is a sectional top view of a vertically oriented blade  22  radially mounted to a hub  28  on a horizontal rotor shaft  40 , which drives a generator (not shown) in the nacelle  30 . The blade rotates in a vertical plane  42 . The blade pivots about a pitch axis  43 , which may be located at or near the ¼ chord position as shown, or at some other position. In  FIG. 2  the yaw, azimuth, and pitch are parked (stationary), with the airfoil&#39;s chord line  44  aligned with the plane of rotation  42 . The incoming wind vector V R1  relative to the nacelle is defined as zero degrees when it is from the front and parallel to the shaft axis  46 . The relative wind angle increases clockwise in this view, so it is 90° when coming from the right, and −90° when coming from the left. An angle of attack AoA is defined between the chord line  44  of the blade and the relative wind vector V R1 , V R2 . An angle of attack is defined for any wind direction φ relative to the blade as follows: 
         [0000]        AoA ( r,θ )=φ+90°−β( r )−θ
 
       Where  
       [0000]    
       
         
           
             r=radius from the rotor axis 
             θ=pitch angle 
             φ=relative wind direction 
             β(r)=twist angle (pitch built-in to blade at a given radius) 
           
         
       
     
         [0019]    An exemplary relative wind direction V R2  for this calculation is shown at −75°. If the pitch and twist angles are zero at the given radius r, then the angle of attack for V R2  is: AoA=−75°+90°−0°−0°=15°. The AoA is used for computing aerodynamic loads. However, the graphs shown herein use the wind direction convention of V R1  (0 to 180° clockwise, and −0 to −180° counterclockwise from the shaft axis  46 ), with the airfoil chord  44  parallel to the rotation plane  42  as shown in  FIG. 2 . This provides graphs that show where to direct the pitch relative to the rotor shaft axis  46  to reach a stable valley of the wind loading curves as later described. 
         [0020]      FIG. 3  shows a enlargement of the blade airfoil section of  FIG. 2 , with a leading edge LE, trailing edge TE, pressure side PS, and suction side SS. A pitch axis  43  may be located at or near the aerodynamic center of the airfoil along at least part of the blade span. However, the aerodynamic center can vary with radial position relative to the pitch axis due to the changing shape, taper, and sweep of the blade. The pitch axis may be outside the blade on swept portions of the blade and when the blade is bent forward by pre-bend or backward by the wind. The drag vector D has the same direction as the wind. The lift vector L is perpendicular to the wind. The positive direction for the lift vector is shown, even though lift for some wind directions urges the airfoil backwards. Vector lengths are not to scale. The aerodynamic moment M A  is positive clockwise in this view. 
         [0021]      FIG. 4  shows function curves computed for root bending moment  48 , root shear  50 , and root twisting moment  52 . These curves are interrelated by vertical lines a, b, c, d, e, f, which cross positions on the bottom curve  52  where the root twisting moment is zero. In three cases a, c, e, the slope of the twisting moment  52  is positive at the zero crossing point. In the other cases b, d, f, the slope of the twisting moment curve  52  is negative at the zero crossing point. Where the slope is positive, direction changes in the relative wind V R1  urge the blade toward zero twist, because a higher wind angle causes positive blade twist, which rotates the blade clockwise in  FIG. 2 , thus reducing the relative wind angle; and a lower wind angle causes negative twist, which rotates the blade counterclockwise to a greater relative wind angle. This condition may be called a stabilizing slope about the zero crossing point of the twist moment curve. Stable pitch angle ranges under consideration herein are  52 B- 52 C and  52 E- 52 F. 
         [0022]    Two of the stable zero twist positions  52 A and  52 D occur within respective pairs of stress valleys  48 A/ 50 A and  48 C/ 50 C. Thus, if the blade pitch control is released to allow the pitch to change freely anywhere in the range of  52 B- 52 C or  52 E- 52 F, the blade will passively seek the respective zero twisting moment position  52 A,  52 D, and will stay within the wind load valleys  48 A/ 50 A or  48 C/ 50 C. For exemplary purposes herein, the shapes of the two wind load curves  48  and  50  are nearly the same in that they have peaks and valleys at essentially the same positions. So the term “wind load valley” or “load valley”, means a valley in either one of the curves  48  or  50  or a combination of them. The minima  48 B,  48 D,  50 B,  50 D of the bending and shear curves  48 ,  50  occur with the chord line  44  generally aligned or anti-aligned with the wind V R1 . The zero twist line “a” occurs with the leading edge LE into the wind, while zero twist line “e” occurs with the trailing edge TE into the wind. In the particular model used for these graphs, the minima  48 D and  50 D are closer to the zero line “e” than the minima  48 B,  50 B are to the zero line “a”. To utilize this fact, an option is to pitch the trailing edge into the wind before releasing the pitch control. However, another option is to design the blade so that the minima  48 B and/or  50 B are closer to, or aligned with, the zero twist line “a”. For example, the blade may be designed such that the average distance of the two minima  48 B and  50 B from the zero twist line “a” is minimized. Alternatively, the blade may be designed such that each of the valley minima are at or proximate (within 5-10 degrees) the zero twist line. 
         [0023]      FIG. 5  shows root twisting moment curves for five different amounts of sweep relative to a given “full” sweep of 1.00. While not meant to be limiting but simply as an example, if full sweep is 2 meters backward at the blade tip, then 0.50 sweep is 1 meter backward at the tip. The inventor realized that stress could be minimized under passive pitch control if the positions of the minima of the wind load curves  48  and  50  were modified to coincide with at least one of the zero twist lines “a” and/or “e”. Sweep amount and/or shape can be used to adjust the zero twist crossing points a 1 -a 5  as shown. 
         [0024]      FIG. 6  schematically shows a front profile or planform of a blade  22 , with a blade profile curve  54 A passing through its ¼ chord points from root  56  to tip  58 .  FIG. 7  shows a profile curve  54 B for a swept back blade. This curve may fit a function such as Sweep (r)=S*(r/R)̂2, where S is the amount of sweep at the tip  58 , such as 2.0 meters, and r/R is the proportion of the blade span (radial position/rotor radius).  FIG. 8  shows a profile curve  54 C for a blade that is swept back at an intermediate radial position. The backward sweep may be continued to a chord line  60 , and then swept forward to the tip  58 , forming a mildly backward-pointing V-shaped or U-shaped profile. The chord  60  may be chosen because increasing the influence of its built-in pitch or twist moves the zero twist point “a” of  FIG. 4  closer to one or more of the minima  48 B,  48 D,  50 A,  50 D. Thus the blade profile curve  54 C may be shaped to specify a blade planform that is swept back at any particular radial position where the sweep is most effective to align a zero twist line “a” of  FIG. 4  with a minimum of a load curve  48 ,  50 .  FIG. 9  shows a planform of a blade  22 C designed per  FIG. 8 . Other blade modifications can be made to achieve this result, such as increasing the chord length of the blade at the chosen radial position or changing the camber shape. 
         [0025]    The neutral pitch condition allowing the blade pitch to rotate passively may be provided by motor-driven gears that are engaged and disengaged by a solenoid as with an engine starter. For example, a solenoid may move a drive pinion on the motor shaft to mesh with a ring gear on the pitch shaft of the blade. Alternate means, such as a clutch or a hydraulic drive system with pressure relief valving to allow free rotation may be used. The pitch drive system may be designed to disengage from the blade in a default condition without power, thereby allowing the blade to passively pitch without angular limit. A damping mechanism such as a partial brake or clutch may be provided to prevent flutter. 
         [0026]    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.