Abstract:
A method for operating a wind turbine having a rotor with a number of wind turbine blades, wherein the rotor&#39;s axis of rotation is tilted in relation to the direction of the incoming wind, includes the steps of determining the azimuth angle (A) of the blades and adjusting the pitch angle of the blades in accordance with the azimuth angle (A) to ensure a substantially constant angle of attack (AoA) during at least a full rotation of the rotor. A wind turbine and use of the method are also contemplated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of pending International patent application PCT/DK2008/000188 filed on May 21, 2008 which designates the United States and claims priority from Danish patent application PA 2007 00787 filed on May 31, 2007, the content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for operating a wind turbine, a wind turbine comprising control means and use of the method. 
       BACKGROUND OF THE INVENTION 
       [0003]    A wind turbine known in the art comprises a wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, as illustrated on  FIG. 1 . 
         [0004]    One of the consequences of producing power by means of a wind turbine is the generation of noise from the mechanical system (gear, generator, cooling fans etc.) and the aerodynamic noise from the blades. With off-shore wind turbines the noise emission is not necessarily an issue but with land based wind turbines noise emission can be a problem if not carefully controlled. 
         [0005]    Two of the most important parameters that influence the magnitude of noise generated by the wind turbine are the angular speed of the blade and the angle of attack of the incoming wind that passes the rotating blade. 
         [0006]    One way of ensuring an acceptable noise level at locations neighboring the wind turbine is to measure the noise level under given circumstances once and for all and then control the operation of the wind turbine (e.g. the rotation speed of the rotor or the blades angle of attack) to ensure that the critical noise level is never reached. But the noise level at a given position near the wind turbine depends on a large number of factors which often interacts and it is therefore necessary to operate with relatively large margins of safety to the critical noise level to ensure that the critical level is not exceeded under any circumstances. 
         [0007]    From U.S. Pat. No. 6,688,841 it is also known to more or less constantly measure the sound level at a location close to a wind turbine park and then reduce the rotational speed of the rotor if a predefined level is exceeded. However, such sound level measurements are subject to uncertainty in that a large number of factors can affect the measurements such as background noise, rain, wind or other and it can be very complex and expensive to establish and operate this system if the noise level at many different locations has to be taken into consideration. 
         [0008]    An object of the invention is therefore to provide for an advantageous and cost-efficient technique for operating a wind turbine. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention provides for a method for operating a wind turbine. The wind turbine comprises a rotor with a number of wind turbine blades, wherein the rotors axis of rotation is tilted in relation to the direction of the incoming wind. The method comprises the steps of
       determining the azimuth angle of the blades and   adjusting the pitch angle of the blades in accordance with the azimuth angle to ensure a substantially constant angle of attack during at least a full rotation of the rotor.       
 
         [0012]    The rotor-plane of almost all modern wind turbines are tilted slightly for among other reasons to ensure that the blades do not hit the tower. But this tilt has the effect that when the blades are moving downwards they also move slightly forward into the incoming wind and when they move upwards they move slightly out of the incoming wind. This will result in that the blades angle of attack will vary depending on the blades azimuth position. The blades angle of attack has a great influence on the blades noise generation and by ensuring a substantially constant angle of attack during at least a full rotation of the tilted rotor a more constant level of noise emission obtained. 
         [0013]    A more constant level of noise emission is for among other reasons advantageous in that it hereby is possible to reduce the size of the margin of safety and thereby reduce the noise emission from the wind turbine or increase the overall output of the wind turbine. 
         [0014]    It should be emphasized that this does not limit the blades to operate at a constant angle of attack at all times. E.g. through a more long-term collective pitch control strategy the pitch angles of the blades can be adjusted in relation to the wind speed, the power output or other and the present method therefore only attempts to ensure a substantially constant angle of attack during at least a full rotation i.e. to compensate for the effects of the tilted rotor. 
         [0015]    It should also be emphasized that by the term “the rotors axis of rotation is tilted in relation to the direction of the incoming wind” is to be understood that the rotors axis of rotation is not parallel with the direction of the incoming wind i.e. the rotor-plane—which is the plane in which the blades of the rotor rotates—is arranged so that the angle of the incoming wind is not perpendicular to said rotor-plane. 
         [0016]    In an aspect of the invention, said angle of attack is ensured constant to reduce the noise emission of said rotor. 
         [0017]    Modern wind turbines are often operated close to the limits of their capacity to maximise the overall output of the wind turbine. But to do so it has to be ensured that the wind turbines noise propagation is maintained at an acceptable level. It is therefore advantageous to reduce the noise emission by ensuring a substantially constant angle of attack. 
         [0018]    In an aspect of the invention, said angle of attack is ensured constant to maintain the noise emission below a predefined level. 
         [0019]    Hereby is achieved an advantageous embodiment of the invention. 
         [0020]    In an aspect of the invention, said angle of attack is ensured constant to improve the wind turbines noise emission/output ratio. 
         [0021]    Under some circumstances it is the noise emission of the wind turbine that are the primary factor in limiting the overall power output of the wind turbine and it is therefore advantageous to improve the wind turbines noise emission/output ratio. 
         [0022]    In an aspect of the invention, the pitch angle of said blades are adjusted so that the trailing edge is moved in direction of the incoming wind and the leading edge is moved in the opposite direction at least at some point when said blades azimuth angle is between 0° and 180° i.e. when the blades are moving downwards. 
         [0023]    By moving the trailing edge into the incoming wind when the blades are moving downwards the blades position in relation to the incoming wind becomes less aggressive in that the blades angle of attack is reduced for a blade on a modern wind turbine. This is advantageous in that the noise emission hereby can be reduced substantially without changing the output. 
         [0024]    In an aspect of the invention, the pitch angle of said blades are adjusted so that the leading edge is moved in direction of the incoming wind and the trailing edge is moved in the opposite direction at least at some point when said blades azimuth angle is between 180° and 360° i.e. when the blades are moving upwards. 
         [0025]    By moving the leading edge into the incoming wind when the blades are moving upwards the blades position in relation to the incoming wind becomes more aggressive in that the blades angle of attack is increased for a blade on a modern wind turbine. This is advantageous in that the output of the blade and hereby the power output of the wind turbine can be increased substantially without changing the noise emission from the blade. 
         [0026]    In an aspect of the invention, said pitch angle of said blades are further adjusted in accordance with the current speed of the incoming wind. 
         [0027]    A lot of factors affects the blades angle of attack and it is therefore not necessarily possible to predict how much the pitch angle shall be adjusted at a given azimuth angle to give a more optimal result. It is therefore advantageous to take the current speed of the incoming wind into account it that it hereby is possible to reduce the noise emission even further or increase the output even further. 
         [0028]    In an aspect of the invention, said pitch angle of said blades are further adjusted in accordance with the current rotational speed of said rotor. 
         [0029]    It is advantageous to take the current rotational speed of the rotor into account it that it hereby is possible to reduce the noise emission even further or increase the output even further. 
         [0030]    In an aspect of the invention, said pitch angle is adjusted when said azimuth angle is between 10° and 170° and between 190° and 350°, preferably between 30° and 150° and between 210° and 330°, and most preferred between 60° and 120° and between 240° and 300°. 
         [0031]    The closer the blade is to an azimuth angle of 90° and 270° the more the blade is moving in the direction of the wind and the present angle ranges therefore presents advantageous intervals for adjusting the azimuth angle. 
         [0032]    In an aspect of the invention, said pitch angle of said blades can be adjusted individually. 
         [0033]    The pitch angle of the blades could be adjusted collectively to simplify the operation of the wind turbine but to increase the positive effects of the present method regarding noise emission, output or both it is advantageous if the blades can be adjusted individually. 
         [0034]    In an aspect of the invention, said rotors axis of rotation is tilted so that the bottom of the rotor-plane is moved into the direction of the incoming wind and the top of the rotor-plane is moved in the direction away from the incoming wind. 
         [0035]    Blades of modern wind turbines are very long and slender and therefore also relatively flexible. To ensure that the blade do not strike the tower even when heavily loaded in high wind speeds the blade has to pass the tower in a safe distance at all times. This could be ensured by positioning the rotor hub in a great distance from the top of the tower but this would complicate the wind turbine design and entail large torque moments. It is therefore advantageous to tilt the rotors axis of rotation so that the blades pass the tower in a safe distance. 
         [0036]    In an aspect of the invention, said rotor is coned in that one or more of said blades are entirely or partly angled forward in the direction of the incoming wind such as between 1° and 7° forward in relation the rotor-plane. 
         [0037]    As explained above the blades of modern wind turbines are very flexible and to ensure that the blades pass the tower in a safe distance it is advantageous that the blades are mounted so that they point slightly forward in the direction of the incoming wind, making the rotor-plane cone slightly. However, if the blades are angled too much forward their efficiency is reduced and the present angle range therefore provides for an advantageous interval in relation to safety and functionality. 
         [0038]    The invention also provides for a wind turbine comprising control means for performing a method according to any of the preceding claims. 
         [0039]    Providing a wind turbine with control means for performing the previously described method is advantageous in that it hereby is possible in a simple and cost-efficient manner to reduce the noise emission from the wind turbine and/or increase the output of the wind turbine. 
         [0040]    In an aspect of the invention, said wind turbine comprises means for adjusting the pitch angle of the blades individually. 
         [0041]    In an aspect of the invention, said wind turbine is a variable speed pitch wind turbine. 
         [0042]    The noise emission from a variable speed pitch wind turbine is often more unpredictable that the noise emission from other types of wind turbines and it is therefore particularly advantageous if the wind turbine is a variable speed pitch wind turbine. 
         [0043]    Even further the invention provides for use of a method according to any of the previously mentioned methods for reducing noise emission from a wind turbine. 
         [0044]    The blades angle of attack is a principal factor in controlling the blades noise emission and by adjusting the pitch angle of the blades in accordance with the azimuth angle to ensure a substantially constant angle of attack during at least a full rotation of the tilted rotor is therefore an advantageous method for reducing the noise emission from the wind turbine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The invention will be described in the following with reference to the figures in which 
           [0046]      FIG. 1  illustrates a large modern wind turbine as seen from the front, 
           [0047]      FIG. 2  illustrates a cross section of an embodiment of simplified nacelle known in the art, as seen from the side, 
           [0048]      FIG. 3  illustrates a large modern wind turbine with a blade in a 90° azimuth position, as seen from the front, 
           [0049]      FIG. 4  illustrates a cross section of the blade illustrated in a 90° azimuth position in  FIG. 3 , as seen from the side, 
           [0050]      FIG. 5  illustrates a large modern wind turbine with a blade in a 270° azimuth position, as seen from the front, 
           [0051]      FIG. 6  illustrates a cross section of the blade illustrated in a 270° azimuth position in  FIG. 5 , as seen from the side, 
           [0052]      FIG. 7  illustrates the noise emission at different times for a prior art wind turbine and a wind turbine according to the invention, 
           [0053]      FIG. 8  illustrates a system of coordinates over the angle of attack over the wing span versus azimuth angle for wind speeds at 8 meters/second, and 
           [0054]      FIG. 9  illustrates a system of coordinates over the angle of attack over the wing span versus azimuth angle for wind speeds at 11 meters/second. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0055]      FIG. 1  illustrates a wind turbine  1 , comprising a tower  2  and a wind turbine nacelle  3  positioned on top of the tower  2 . The wind turbine rotor  4 , comprising three wind turbine blades  5  mounted on a hub  6 , is connected to the nacelle  3  through the low speed shaft which extends out of the nacelle  3  front. 
         [0056]    In another embodiment the wind turbine rotor  4  could comprise another number of blades  5  such as one, two or four. 
         [0057]      FIG. 2  illustrates a simplified cross section of a nacelle  3  of a prior art wind turbine  1 , as seen from the side. Nacelles  3  exists in a multitude of variations and configurations but in most cases the drive train in the nacelle  3  almost always comprise one or more of the following components: a gearbox  15  (typically a epicyclical gearbox), a coupling (not shown), some sort of breaking system  16  and a generator  17 . A nacelle  3  of a modern wind turbine  1  can also include a converter  18  (also called an inverter) and additional peripheral equipment such as further power handling equipment, control cabinets, hydraulic systems, cooling systems and more. 
         [0058]    The weight of the entire nacelle  3  including the nacelle components  15 ,  16 ,  17 ,  18  is carried by a nacelle structure  19 . The components  15 ,  16 ,  17 ,  18  are usually placed on and/or connected to this common load carrying nacelle structure  19 . In this simplified embodiment the load carrying nacelle structure  19  only extends along the bottom of the nacelle  3  e.g. in form of a bed frame to which some or all the components  15 ,  16 ,  17 ,  18  are connected. In another embodiment the load carrying structure  19  could comprise a gear bell which through the main bearings could transfer the load of the rotor  4  to the tower  2 , or the load carrying structure  19  could comprise several interconnected parts such as latticework. 
         [0059]    The nacelle  3  further comprise a main bearing (not shown) for ensuring that the rotor  4  can rotate substantially freely in relation to the nacelle structure  19  and the fixed drive train parts  15 ,  16 ,  17 ,  18  of the nacelle  3 . In this embodiment the of a drive train the main bearing is integrated in the gearbox  15  in that the rotor  4  is connected directly to the gearbox  15  via the hub  6 . Because the main bearing  14  is incorporated in the gearbox  15 , the gearbox structure has to be able to transfer the entire load of the rotor  4  to the tower  2  by means of the nacelle strengthening structure  19 . 
         [0060]    In this embodiment the drive train is established in a normal operation angle NA of 6° in relation to a plane perpendicular to a centre axis through the tower  2  i.e. a horizontal plane. In another embodiment the rotor  4  and/or drive train could be tilted between 1° and 10° and preferably between 1.5° and 8°. 
         [0061]    By tilting the drive train the rotor-plane  24  is tilted accordingly making the incoming wind  13  reach the rotor-plane  24  in an angle different from perpendicular. It is evident to a person skilled in the art the even though the rotor-plane  24  is tilted NA it might under some conditions occur that the incoming wind  13  reaches the rotor-plane  24  substantially perpendicularly in that the direction of the wind can change e.g. due to high turbulence, changes in the wind direction, obstacles, changes in the terrain or other. 
         [0062]    The rotor  4  is tilted e.g. to ensure that the blades  5  do not hit the tower  2 , to compensate for the differences in wind speed at the top and bottom of the rotor  4  and/or other. 
         [0063]    To further ensure tower clearance of the blades  5 , the blades  5  of most modern wind turbines  1  also cone. 
         [0064]    In this embodiment of a wind turbine  1  the longitudinal axis  8  of the blades  5  is substantially perpendicular to the rotation axis  7  of the rotor  4 . But in another embodiment the blades  5  could be mounted in an angle that would make them point slightly forwards in relation to the rotors axis of rotation  7  (i.e. the blades cone slightly) or the rotor  4  could be provided with means for adjusting the angle of the longitudinal axis  8  of the blades  5  in relation to the rotors axis of rotation  7 . This coning can be established by providing the blades with a angle bend somewhere along the length of the blade or by establishing the hub  6  or the mounting surface of the hub  6  in the desired angle. Traditionally the blades  5  cone approximately 2° and about 4° is the maximum coning angle in most cases. 
         [0065]    In another embodiment of the invention the coning angle of the blades  5  could be established and/or controlled individually. 
         [0066]    In a further embodiment the wind turbine  1  could comprise two blades  5  rigidly connected at or near the rotors axis of rotation  7  where the blades  5  would be connected to the hub  6  through some sort of hinge allowing that the two blades  5  simultaneously could tilt slightly in relation to the rotors axis of rotation  7 . This tilt could then be actively controlled or it could occur as a reaction to specific wind turbine or wind conditions. 
         [0067]      FIG. 3  illustrates a large modern wind turbine  1  with a blade  5  in a 90° azimuth position A, as seen from the front. 
         [0068]    In this embodiment of a wind turbine  1  according to the invention one of the wind turbines blades  5  is illustrated in a 90° azimuth angle A. 
         [0069]    Given the direction of rotation  14  of the rotor  4  of the present wind turbine  1  and because the rotor-plane  24  is tilted the blade  5  in the 90° azimuth angle A is moving downwards and thereby slightly into the incoming wind  13 . 
         [0070]      FIG. 4  illustrates a cross section of the blade  5  illustrated in a 90° azimuth position A in  FIG. 3 , as seen from the side. 
         [0071]    When the blade  5  is rotating, the tilt NA and conning of the rotor  4  results in a velocity component in the axial direction  22 , that constantly changes magnitude for the blade  5  as function to the azimuth position A. 
         [0072]    When the blade  5  is in azimuth position A 90° the axial velocity  20  of the blade  5  is maximum due to the tilt and because of the tilt NA and that the blade  5  is moving downwards the direction of this axial velocity  20  is against the incoming wind  23 . This entails that the blade  5  “see” a higher incoming wind speed  23  and therefore higher angle of attack AoA an also higher relative wind speed  21  passing the blade  5 . 
         [0073]    As previously explained one of the most important factors regarding noise emission is the blades angle of attack AoA and the blade  5  on the present wind turbine  1  will therefore produce more noise when it is moving downwards and particularly when the blade  5  is in and near the 90° azimuth angle A. 
         [0074]    By the method according to the invention it is attempted to compensate for this extra axial speed component  20  by attempting to keep the angle of attack AoA at a substantially constant desired angle by adjusting the pitch angle of the blade  5  so that the trailing edge  26  is moved in direction of the incoming wind  13  and the leading edge  23  is moved slightly out of the incoming wind  13  when the azimuth angle A is between 0° and 180° i.e. when the blades are moving downwards. 
         [0075]    It is important to point out that the due to other control schemes it can be advantageous to make the angle of attack AoA vary even over a single rotation of the rotor but according to the present invention it is advantageous to maintain the angle of attack AoA substantially constant to level out the influence of the tilted NA rotor-plane  24  and thereby at least seen over one rotation of the rotor  4  level out the noise and/or power generation of the blade  5 . 
         [0076]    Exactly how much and when the pitch angle A is adjusted depends on a number of factors. First of all it depends on the azimuth angle A and as previously explained it is at and near 90° and 270° azimuth angle that the problem is most profound when the rotor-plane  24  is tilted so that the incoming wind  13  is not perpendicular with the rotor-plane  24 . How much and when the pitch angle A is adjusted can also depend on the actual pitch angle of the blade  5  in the given situation and the actual pitch angle can e.g. depend on the speed of the incoming wind  23 , the rotational speed of the rotor  4 , the blade design and other factors. 
         [0077]      FIG. 5  illustrates a large modern wind turbine  1  with a blade  5  in a 270° azimuth position A, as seen from the front. 
         [0078]    In this embodiment of a wind turbine  1  according to the invention one of the wind turbines blades  5  is illustrated in a 270° azimuth angle A. 
         [0079]    Given the direction of rotation  14  of the rotor  4  of the present wind turbine  1  and because the rotor-plane  24  is tilted the blade  5  in the 270° azimuth angle A is moving upwards and thereby slightly out of the incoming wind  13 . 
         [0080]      FIG. 6  illustrates a cross section of the blade  5  illustrated in a 270° azimuth position A in  FIG. 5 , as seen from the side. 
         [0081]    When the blade  5  is in azimuth position A 270° the flow situation is opposite of what described under  FIG. 4 . The axial velocity  20  of the blade  5  due to the tilt NA is maximum and the direction is away from the incoming wind  13 . That means the blade  5  “see” a lower incoming wind speed  23  and therefore lower angle of attack AoA an also lower relative wind speed  21  passing the blade  5 . 
         [0082]    Since the blades angle of attack AoA is lower than average in this situation the blade most likely will generate less noise or the noise level will at least not be raised. However, the lower angle of attack AoA can result in a lower output of the blade  5  because the blade  5  is no longer operating at the optimal angle of attack AoA. 
         [0083]    In this embodiment of the invention the pitch angle of the blade  5  is therefore adjusted so that the leading edge  25  is moved in direction of the incoming wind  13  and the trailing edge  26  is moved in the opposite direction at least at some point when the blades azimuth angle A is between 180° and 360° i.e. when the blade  5  are moving upwards. 
         [0084]      FIG. 7  illustrates the noise emission at different times for a prior art wind turbine  1  and a wind turbine according to the invention. 
         [0085]    In the present system of coordinates the axis of abscissas represents the amount of noise that the blade  5  emits NE and the axis of the ordinate represents time T. 
         [0086]    The upper dotted line  10  illustrates a specific critical noise level that is not to be exceeded if the wind turbine  1  is to comply with the maximum noise emission requirements set out for the specific site. 
         [0087]    The distance  9  illustrates the margin of safety to the maximum allowable noise emission  10  which is usually maintained to allow for sudden peaks in the noise emission e.g. due to wind gusts without the maximum allowable noise emission  10  being exceeded. 
         [0088]    The dotted curve  11  illustrates the noise emission from a specific blade  5  of a prior art wind turbine  1  with a tilted rotor-plane  24  during five full rotations. As illustrated the noise emission fluctuates greatly with the azimuth angle A of the blade  5  making the average noise level relatively low and thereby at least to some degree also making the output of the blade  5  relatively low. 
         [0089]    The solid curve  12  illustrates the noise emission from a specific blade  5  of a wind turbine  1  according to the invention with a tilted rotor-plane  24  also during five full rotations. When the pitch angle of the blade  5  occasionally or constantly is adjusted to achieve a more constant angle of attack AoA of the blade  5  the noise emission of the blade  5  also becomes more constant and predictable and it is therefore possible to operate the blade  5  at a higher average noise emission and thereby higher average output without increasing the risk of exceeding the maximum allowable noise emission  10 . 
         [0090]      FIGS. 8 and 9  illustrates two systems of coordinates over the angle of attack AoA over the wing span versus azimuth angle A for wind speeds at 8 meters/second and 11 meters/second respectively. 
         [0091]    The axis of abscissas represents the angle of attack AoA and the axis of the ordinate represents the wing span where 0 is equivalent to the root of the blade  5  and  1  equivalent to the tip of the blade  5 . 
         [0092]    The first curves (solid)  27  illustrates a azimuth angle A of 90°, the second curves (broken line)  28  illustrates a azimuth angle A of 0°, the third curves (dash-dot)  29  illustrates a azimuth angle A of 180° and the fourth curves (dotted line)  30  illustrates a azimuth angle A of 270°. 
         [0093]    The two systems of coordinates illustrates that the variation of the azimuth angle A yields large variation of the angle of attack AoA when the blade  5  is in or near azimuth position A 90° and 270°. The difference in angle of attack AoA is especially large at the inner part of the blade  5  due to the influence from the axial velocity  20  from the tilt NA has a relative lager influence on the inner part of the blade  5  because of the much slower speed. 
         [0094]    The difference between the azimuth position 0° and 180° (blade in top and bottom position) is due to tilt, coning or the combination of tilt and coning of the rotor  4 . 
         [0095]    The invention has been exemplified above with reference to specific examples of wind turbines  1 , blades  5 , control methods and other. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.