Abstract:
The invention concerns a rotor blade, in particular a rotor blade of a wind power installation. The object of the present invention is to provide measures for still further improving the CR-value and also the level of acoustic power of rotor blades of wind power installations. A rotor blade for a wind power installation includes a high pressure side and a low pressure side. A substantially constant, area-covering electrostatic field is provided on the low pressure side.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention concerns a rotor blade, in particular a rotor blade of a wind power installation. 
   2. Description of the Related Art 
   Rotor blades for wind turbines are known in many different forms. The goal is usually to design the rotor blades of a wind power installation in such a way that in operation they have a level of acoustic power which is as low as possible and in addition present such a low air resistance value that a very small amount of losses of wind energy is caused by the rotor blades. 
   Usually the approaches adopted for reducing the level of acoustic power and also for reducing the CR-value (coefficient of resistance) involve changing or improving an external design which is suitably adapted for the rotor blade. 
   BRIEF SUMMARY OF THE INVENTION 
   The object of the present invention is to provide measures for still further improving the CR-value and also the level of acoustic power of rotor blades of wind power installations. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       FIG. 1  shows a front view of a wind power installation including rotor blades according to the invention. 
       FIGS. 2A ,  2 B show plan views of the suction side of embodiments of a rotor blade according to the invention. 
       FIGS. 3A ,  3 B,  3 C show simplified cross-sectional views of embodiments of a rotor blade according to the invention taken along line A-A of  FIG. 2A . 
       FIG. 4  shows the characteristics of the CR-value in dependence on the voltage of the electrical field. 
       FIG. 5  shows the characteristics of the coefficient of power (Cp) in dependence on the voltage of the electrical field. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is attained by a rotor blade having the features of claim  1 . Advantageous developments are set forth in the appendant claims. 
     FIG. 1  shows a front view of a wind power installation  10  including rotor blades  14  according to the invention. The wind power installation  10  has a rotor  12  which carries the rotor blades  14 . The present invention provides that an electrical field  16  is produced at least on the suction side of the rotor blade  14 . That electrical field  16  is preferably an electrostatic field with a voltage of for example about −4 kV at the top side of the rotor blade  14 . 
     FIG. 2A  is a plan view of the suction side of one embodiment of a rotor blade  14  according to the invention. The electrical field  16  is provided in the region from the leading edge of the rotor blade  14  as far as the trailing edge at at least 30% of the rotor blade  14  in the rotor blade tip region, that is to say the region which is furthest away from the rotor blade root of the rotor blade  14 .  FIG. 2B  shows another embodiment of a rotor blade  14  in which the electrical field  16  is provided to cover lager region of the rotor blade  14 . 
   Preferably a network-like conductor structure  18  is provided on the suction side of the rotor blade  14  to produce the electrical field  16 . The network-like conductor structure  18  can comprise a conductor matrix (for example of copper), wherein a plurality of conductors are disposed in mutually juxtaposed transverse relationship with each other in a mesh configuration and the spacing parallel to each other is in the region of between about 1 and 10 mm, preferably in the region of 4 mm or less. 
   It was now surprisingly found that the application of an electrical field  16  of a value of between about −2 and −10 kV, preferably about −4 kV, causes the power value of the rotor blade  14  to rise, to a factor of between about 10 and 15%, preferably 12%. At the same time the level of acoustic power of the rotor blade  14  according to the invention is reduced by about 1 dB or less. 
   To maintain the electrical field  16  on the suction side an electrical power with rotor blades  14  of a length of about 20 m of about 5 kV per rotor blade has to be provided and with a rotor blade length of about 32 m the electrical power must be about 15 kV per rotor blade. 
   The graphs in  FIG. 4  and  FIG. 5  show the characteristics of the CR-value in dependence on the voltage of the electrical field and the characteristics of the coefficient of power (Cp) in dependence on the voltage of the electrical field. 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               CR % 
               Voltage (V) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               100 
               0 
             
             
                 
               90 
               1.6 
             
             
                 
               80 
               3.9 
             
             
                 
               70 
               5.3 
             
             
                 
               60 
               6.6 
             
             
                 
               50 
               7.5 
             
             
                 
               40 
               8.2 
             
             
                 
               30 
               8.8 
             
             
                 
               20 
               9.2 
             
             
                 
               10 
               9.3 
             
             
                 
               0 
               9.4 
             
             
                 
                 
             
           
        
       
     
   
   It can be seen in that respect that the coefficient of power Cp reaches power values at about −4 kV and −3.9 kV respectively as a maximum and falls again at voltage values of less than −3.9 and −4 kV respectively. 
     FIG. 3A  shows a simplified cross-sectional view of an embodiment of a rotor blade  14  according to the invention taken along line A-A of  FIG. 2A . In this embodiment, the network-like conductor structure  18  is only on the suction side of the rotor blade  14 . The size and shape of the electrical field will be determined by the value of the voltage placed on the conductor structure  18 , as well as the shape of the blade and whether the blade has internal or external metal components, such as a front metal surface. 
   A layer that forms the outer layer of the blade which contacts the wind can be on top of the network conductor  18  in one embodiment. This layer can be a fiberglass, protective coating, plastic, laminate sheeting, polyvinyl sheeting that covers the network-like conductor structure  18  in order to give preferable properties to the surface of the rotor blade  14  and to protect the network-like conductor structure  18 . 
   In another embodiment, an electrical field  16   b  for example with a voltage in the same direction or in opposite relationship can be applied at the pressure side of the rotor blade  14  as shown in  FIG. 3B . In the embodiment in  FIG. 3B , a network-like conductor structure  18   b  is installed on the pressure side of the rotor blade  14  as well as a network-like conductor structure  18   a  on the suction side of the rotor blade  14 . 
   In yet another embodiment shown in  FIG. 3C , a network-like conductor structure  18   c  is installed inside the rotor blade  14 . According to this embodiment, the installation of the network-like conductor structure  18   c  does not affect the surface properties of the rotor blade  14 . 
   The application of the electrical field  16  at the rotor blade  14  can comprise a galvanic connection between the rotor blade  14  and a voltage or charge supply device within the wind power installation  10 . Preferably there are provided means for separating that galvanic connection, which means can be in the form of switches which already permit galvanic separation at the rotor blade  14 , at the rotor blade root or the hub or within the wind power installation  10 . It is also possible to provide more than one switch within the galvanic connection. 
   The galvanic separation between the voltage supply means and the rotor blade  14  is preferably interrupted when a thunderstorm is approaching. In that respect it is also possible for the interruption to take place automatically when a corresponding thunderstorm situation is detected. That can be measured by a sensor that detects, for example severe power fluctuations because severe power fluctuations of the wind power installation  10  or severe fluctuations in the wind are an indication of gusts which usually precede a thunderstorm. It is however also possible to detect an approaching thunderstorm by measurement of the electrical voltage within the air. Usually that voltage rises or falls when a thunderstorm approaches and can therefore be taken as a relatively reliable indication of such a thunderstorm. 
   Shutting down the field therefore by galvanic separation between the rotor blade  14  and the voltage supply means serves for protecting the entire wind power installation  10 , in particular the rotor blades  14 . It will be appreciated that it is also possible to control a corresponding interruption in the voltage supply means with other means which are already known and with which the approach of a thunderstorm can be detected. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.