Abstract:
A method of controlling noise emission from a wind turbine with a rotor blade includes providing wind shear data comprising wind shear values as a function of height over ground, determining an expected noise emission based on the wind shear data and controlling the wind turbine to reduce noise emission from the wind turbine in accordance with the expected noise emission.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Application No. 12154930.7 EP filed Feb. 10, 2012. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    A method of controlling noise emission from a wind turbine with at least one rotor blade is provided. Further, a wind turbine including a control unit configured to carry out the method of controlling noise emission is provided. 
       BACKGROUND OF INVENTION 
       [0003]    As more and more wind power plants are installed, wind parks move closer to densely populated areas. Accordingly, the impact of the acoustic emissions on nearby residences caused by such wind power plants will become an increasingly important problem. Implementation of closed-loop noise control functions is difficult because the sound level will not only be impacted by noise generated by the wind turbine but also by any other sound sources in the proximity of the installation. Hence, restriction of the sound emission based on measurements of the noise level or the noise spectrum will be impractical. Thus, known solutions typically involve defining a plurality of modes of operation, where compromises between optimal power generation performance and acceptable noise level have been reached. In other words, the power output of the wind turbine may be lowered for the sake of reducing the emission of noise. The reduced cost-effectiveness of the wind turbines makes such solutions undesirable. 
       SUMMARY OF INVENTION 
       [0004]    A first aspect provides a method of controlling noise emission from a wind turbine comprising at least one rotor blade. The method comprises steps of:
       providing wind shear data comprising wind shear values as a function of height over ground;   determining an expected noise emission based on the wind shear data; and   controlling the wind turbine to reduce noise emission from the wind turbine in accordance with the expected noise emission.       
 
         [0008]    The claimed method is based on the understanding that wind speed varies largely with height over ground. Typically, wind speed measurements are only carried out at a single place of the wind turbine, usually by an anemometer arranged at the top of the nacelle of the wind turbine. This means that the controller of the wind turbine was not able to take the phenomenon of wind shear into account when selecting a combination of rotor blade pitch and rotational speed of the rotor that is expected to lower noise emission for a given wind speed. Therefore, wind shear data comprising information about wind speeds and wind directions as a function of height over ground are used to predict the noise emission more precisely. In the simplest case, the wind shear data may be predefined and fixed. 
         [0009]    A control setting suitable for reducing noise emission for e.g. normal wind conditions may prove to yield less noise reduction during high wind shear conditions. Taking wind shear data that quantifies the degree of wind shear into account can also lead to modified control settings (e.g. rotor speed and/or blade pitch) that vary with the wind shear conditions. 
         [0010]    A preferred embodiment of the method comprises determining an azimuth of the at least one rotor blade. Furthermore, the expected noise emission is determined in accordance with the determined azimuth and the wind turbine is controlled in accordance with the expected noise emission and the determined azimuth. 
         [0011]    The phenomenon of wind shear poses another problem in that for increasing rotor diameters the differences in wind speed at the top and the bottom of the rotor become large and result in periodic variations of the emitted noise level. Such variations can be perceived as being even more disturbing than a continuously high noise level. In addition the inventors have recognised that the different wind speeds and directions found depending on the current rotor azimuth mean that the optimal settings for the wind turbine as a whole and the rotor blade in particular vary during a revolution of the rotor or rotor blade. Accordingly, better results can be achieved by controlling the wind turbine and the at least one rotor blade depending on the current azimuth of the rotor or rotor blade. This will often result in periodic settings having a period which depends on the rotational speed of the rotor. 
         [0012]    Preferably, the method further comprises measuring at least one environmental parameter. The wind shear data will then be determined in accordance with the measured environmental parameter. The environmental parameter can be used to select suitable wind shear data from a plurality of predefined wind shear datasets or it can be used in a mathematical model of the wind shear to derive information on the current wind shear and to compute current wind shear data. Measuring the environmental parameter makes the control more flexible and yields a better performance for varying environmental conditions. 
         [0013]    For example, measuring the environmental parameter may comprise measuring at least one of a temperature, a wind speed, a wind direction, an atmospheric pressure, an intensity of sunshine and air humidity. The more measurands and measurements are included, the more precisely the wind shear data can be determined and the more precisely can the noise emission be predicted. 
         [0014]    Measuring the environmental parameter preferably comprises measuring at a plurality of heights between ground level and a maximum height of the wind turbine (i.e. the maximum height of the rotor of the wind turbine). In some such embodiments the wind shear data may be derived directly form the measurements at the plurality of heights. The measurements can be carried out using a LIDAR system (Light Detection and Ranging), a SODAR system (Sonic Detecting and Ranging), another suitable atmospheric measurement device or simply by placing a plurality of measurement instruments such as anemometers, barometers, thermometers and the like on a mast or on the tower of the wind turbine. Especially in the latter case it may suffice for some cases to only carry out measurements at the height of the nacelle and below and to compute wind shear data for the heights above the nacelle based on these measurements. This has an advantage in that additional support structures for the measurement devices are unnecessary. 
         [0015]    The method may further comprise steps of determining a wind speed and an actual power output of the wind turbine and comparing the actual power output with an expected power output for the determined wind speed. In such embodiments, the provided wind shear data will be determined based on a result of the comparison of the actual power output and the expected power output. If the actual power output is lower than expected for the given wind speed, it may be concluded that the wind exposes less of a laminar flow and that there is increased wind shear. Accordingly a noise level will be higher and the power generation should be throttled. 
         [0016]    Preferably, one of the actual power output or the expected power output is compensated in accordance with the measured environmental parameter prior to comparing the actual power output to the expected power output. This avoids the risk of misinterpreting the result of the comparison because of a contamination of the rotor blades by wake, dust, snow, insects or other detrimental factors that may temporarily cause a loss of turbine performance. At least some such influences can be identified by means of the measurement of the environmental parameter. 
         [0017]    The method may further comprise measuring a blade load of the at least one rotor blade as a function of the azimuth of the at least one rotor blade. In this case the provided wind shear data will be determined based on the measured blade load. The forces applied to the rotor blade vary with the wind speed. Accordingly measuring the blade load can give information about the wind speed at the present azimuth or height of the rotor blade. This is especially useful for determining wind speeds at heights greater than that of the nacelle below which the wind speed can be measured easily by placing anemometers on the tower of the wind turbine (see above). Even though the wind applies a specific force to every section of the rotor blade which sum up to yield a total blade load, the wind speed can be determined for different heights with sufficient precision by observing the variation of blade load as a function of azimuth of the rotor blade. 
         [0018]    The method may further comprise determining at least one of a time of day and a day of the week. Then controlling the wind turbine to reduce noise emission from the wind turbine may be carried out in accordance with the determined time of day and/or day of the week. The advantage of this is twofold: firstly, the acceptable noise level may be set depending on the current time, e.g., the noise level may be lower during week-ends and outside working hours. Secondly, it may be observed that wind shear depends on the time of day. For example it may be found that wind shear is often higher in the time towards sunrise and the wind turbine can be controlled taking this phenomenon into consideration. 
         [0019]    The method may also further comprise determining a direction of wind. In this case the step of controlling the wind turbine to reduce noise emission from the wind turbine is carried out in accordance with the determined direction of wind. For example, the wind turbine may be controlled to produce less noise when the wind direction is such that the noise will be carried by the wind to nearby settlements and to produce more noise if the wind direction is such that the noise will be carried away from nearby settlements. 
         [0020]    Generally controlling the wind turbine may comprise setting at least one of rotor speed and blade pitch of the at least one rotor blade in accordance with the expected noise emission. Rotor speed and blade pitch both have a direct impact on power generation as well as on the emitted noise level. The blade pitch may be set for all rotor blades or for each rotor blade individually. A controller may output pitch positions, rotations-per-minute targets, dB noise limits or a combination of these which result in changes of rotor speed and blade pitch. In the case of a wind park a central controller could output such control values individually for each wind turbine and—where appropriate—for each rotor blade. 
         [0021]    Controlling the wind turbine to reduce noise emission may be conditional on the expected noise emission being greater than a threshold noise emission. Wear and tear of the wind turbine will be lower if the number of control actions like blade pitching is kept to a minimum. Accordingly it is advantageous to restrict the controlling to reduce noise emission to a necessary minimum. If the expected noise emission remains below the threshold noise emission, no specific controlling action needs to be taken. 
         [0022]    The threshold noise emission may be a function of at least one of time of day and day of the week. As mentioned above, the controlling of the noise emission may take week-ends and common leisure times into account and reduce the noise emission even more during times when the noise emission will be perceived as even more disturbing. 
         [0023]    A second aspect provides a non-transitory computer readable storage medium comprising program code which, when executed on a controller of a wind turbine or of a wind park, carries out the method. 
         [0024]    A third aspect is directed at a wind turbine comprising at least one rotor blade and a control unit adapted to carry out the method. 
         [0025]    These and other features, aspects and advantages will become better understood with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows a first diagram illustrating wind speed V w , as a function of height h; 
           [0027]      FIGS. 2A ,  2 B and  2 C show second diagrams illustrating wind speed V w , as a function of height h wherein the wind speed is split into vector components V wx , V wy , and V wz ; 
           [0028]      FIG. 3  shows a wind turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0029]      FIG. 1  shows a first diagram illustrating wind speed V w , as a function of height h. The curve of the wind speed is of merely exemplary nature and only related to real examples in that the wind speed is typically lower at lower heights over ground than at greater heights. 
         [0030]    In the diagram h N  denominates the height of the wind turbine&#39;s nacelle while h min  and h max  represent the minimum and maximum heights of the tips of the rotor blades. As can be seen from the diagram, the forces applied to the rotor blades will vary largely with rotor azimuth because of the differences in wind speed at different heights. The different wind speeds also mean that noise emission will vary depending on rotor azimuth. If a rotor blade is pitched for low noise emission at one rotor azimuth, the selected pitch may be unsuitable for a different rotor azimuth and cause production of an unnecessary amount of noise for the second rotor azimuth. This is because the wind speed varies largely as does the difference between the respective wind speeds at h min  and h max . 
         [0031]    Accordingly, it is not feasible to provide a predetermined optimum setting for the wind turbine with regard to noise emission only considering a single wind speed value and ignoring wind shear. For this reason, embodiments take wind shear into account. In some embodiments, the same settings are applied to all rotor blades of the wind turbine in order to provide a compromise which generates minimum noise for a fixed setting for all rotor blades. In preferred embodiments, the rotor azimuth is considered along with the wind shear data. In this case the blade pitch of each rotor blade may be varied cyclically as a function of the azimuth of the rotor blade. 
         [0032]      FIGS. 2A ,  2 B and  2 C show second diagrams illustrating wind speed V w , as a function of height h wherein the wind speed is split into vector components V wx , V wy , and V wz . In  FIG. 1  only an amount of wind speed was shown and it was silently assumed that wind generally moves in parallel to the ground. However, this is not the case and wind shear can be more accurately described using vectors.  FIG. 2A  shows the wind speed measured in a direction parallel to the normal to the rotor plane. As can be seen the curve largely corresponds to that of  FIG. 1  because the absolute value of a vector is dominated by its largest component which in the case of wind is usually parallel to the normal of the rotor plane. However, for the purpose of noise reduction the other components of the wind speed and the differences therein should be considered in order to yield better results. The second component of the wind speed shown in  FIG. 2B  is different from zero whenever the nacelle is not rotated along the tower to face the wind. Commonly control of the wind turbine will always try to minimise this component by rotating the nacelle of the wind turbine accordingly. However, this can only be done at a limited speed. Accordingly expected noise emission may be found to be higher if the direction of wind changes faster than the nacelle can follow. The third component shown in  FIG. 2C  describes the wind speed parallel to the vertical axis, e.g. the tower axis. This component will generally be rather low but can also contribute to noise generation. It will be found that rising temperatures and falling air pressures will often give rise to a wind speed component away from the ground. 
         [0033]      FIG. 3  shows a wind turbine. The wind turbine comprises rotor blades  1 ,  2 , and  3  revolving around a nacelle  4  located at the top of a tower  5 . Heights h N , h min , and h max  are indicated in the figure. Considering the wind speeds illustrated in the preceding figures that vary as a function of height, it is clear that the forces applied to the individual rotor blades as well as to the rotor as a whole will be different when the rotor is in a second position  6  (dashed line). The direction of wind will vary in addition to the amount of wind speed which leads to the conclusion that the wind shear should be taken into account in order to reduce noise emission. 
         [0034]    While the invention has been described by referring to preferred embodiments and illustrations thereof, it is to be understood that the invention is not limited to the specific form of the embodiments shown and described herein, and that many changes and modifications may be made thereto within the scope of the appended claims by one of ordinary skill in the art.