Patent Publication Number: US-2015078913-A1

Title: Arrangement to reduce noise emission

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of European Application No. EP13184993 filed Sep. 18, 2013, incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The present invention relates to an arrangement to reduce noise emission, the noise emission being generated by a wind turbine rotor blade. In particular, the arrangement comprises a noise reduction device, which is connected to the wind turbine rotor blade at a rotor blade trailing edge section of the wind turbine rotor blade. Furthermore, the invention relates to a method of manufacturing such an arrangement. 
     BACKGROUND OF THE INVENTION 
     Realization of a low noise emission level is a critical and highly important design parameter in the development of wind turbine rotor blades and wind turbines. This is due to the fact that the noise emission level of a wind turbine sets a limitation on if a wind turbine is allowed to be built and how close in the proximity of residential areas the wind turbine is allowed to be built. Thus, the noise emission level may set a limitation to a rotational frequency of a rotor of the wind turbine because of the above mentioned restrictions. Furthermore, also the rotor diameter may be limited by noise constraints. 
     Noise emission, which is generated by rotating wind turbine rotor blades, represents a significant share of the total noise emission, which is generated by the wind turbine under operation. Consequently, a significant effort has been made to develop solutions which mitigate and reduce noise generated by wind turbine rotor blades. It has been disclosed in the European patent application EP 2 309 119 A1 that serrated panels attached to or integrated with a wind turbine rotor blade may have a beneficial effect on the noise emission level to a certain extent. 
     However, there exists an urgent need to provide an improved device to reduce noise emission, generated by a wind turbine rotor blade. In particular, the device should be capable to both substitute and complement existing noise emission means for wind turbine rotor blades. 
     SUMMARY OF THE INVENTION 
     This objective is achieved by the independent claims. The dependent claims describe advantageous developments and modifications of the invention. 
     In accordance with aspects of the invention there is provided an arrangement to reduce noise emission, which is generated by a wind turbine rotor blade. The arrangement comprises the wind turbine rotor blade and a noise reduction device, the noise reduction device being attached to the wind turbine rotor blade. The wind turbine rotor blade comprises a rotor blade suction side, a rotor blade pressure side, a rotor blade leading edge section and a rotor blade trailing edge section with a rotor blade trailing edge. The noise reduction means comprises an inner surface, which is facing the rotor blade trailing edge section, and an outer surface, which is opposite to the inner surface. The noise reduction device comprises a leading edge and a trailing edge, both connecting the inner surface and the outer surface. The noise reduction device is connected at the rotor blade trailing edge section by a connector. Thus, the inner surface shows a pre-defined distance to at least one side of the wind turbine rotor blade. Finally, the pre-defined distance is chosen in a way that acoustic waves, which are generated at the rotor blade trailing edge section, are reflected by the inner surface such that noise emission, which is generated by the wind turbine rotor blade, is reduced. 
     The wind turbine rotor blade is arranged and prepared for being a part of a wind turbine. A wind turbine is a device that can convert wind energy, i. e. kinetic energy from wind, into mechanical energy. Advantageously, the mechanical energy is subsequently used to generate electricity. A wind turbine is also referred to as a wind power plant or a wind charger. Furthermore, a wind turbine may also be referred to as a wind turbine engine. 
     The rotor blade trailing edge has to be understood as an edge or a rim, which is located at the periphery of the rotor blade trailing edge section. The rotor blade trailing edge section is a region or an area of the wind turbine rotor blade, which comprises the rotor blade trailing edge and the adjacent region. Exemplarily, the rotor blade trailing edge section may cover an area of 20 percent, in particular 10 percent, with regard to an area of the whole wind turbine rotor blade. Likewise, the rotor blade leading edge section also has to be understood as an area or a region around the rotor blade leading edge. 
     The noise reduction device may be substantially parallel to at least one side of the wind turbine rotor blade. However, it may also be inclined to at least one side of the wind turbine rotor blade. Thus, it is important that the distance between the noise reduction device and the wind turbine rotor blade stays constant or mechanically stable, which is meant by the notion “pre-defined”. 
     Noise, which is mitigated and reduced by the above described noise reduction device, may in principal originate or be generated at any region of the wind turbine rotor blade. In particular, however, acoustic waves, which cause the noise, are generated at the rotor blade trailing edge section. Even more particularly, the acoustic waves are generated at the rotor blade trailing edge. 
     The pre-defined distance and thus the location of the noise reduction device with regard to the wind turbine rotor blade may be chosen to be inside a boundary layer of the wind turbine rotor blade or to be outside the boundary layer. 
     The boundary layer is a region close to a surface of the wind turbine rotor blade, where a mean flow speed of wind is smaller than outside of the region. The boundary layer can be both laminar and turbulent. Advantageously, the boundary layer starts laminarly at the rotor blade leading edge section, and then transitions into a turbulent boundary layer. The transition may, for instance, after the first third of the chord between the rotor blade leading edge section and the rotor blade trailing edge. Advantageously, at the rotor blade leading edge section the thickness of the boundary layer is in a range of a few millimeters, while at the rotor blade trailing edge section it is in a range of a few centimeters. If the wind flow separates from the surface of the wind turbine rotor blade, this is denoted as stall. 
     An advantage of locating the noise reduction device inside the boundary layer is that drag is lower and that possible self-noise from the noise reduction device is reduced due to a reduced mean flow speed. 
     An advantage of locating the noise reduction shield outside of the boundary layer is that the complete boundary layer is shielded and that possible self-noise from the noise reduction device is reduced due to lower turbulence outside the boundary layer. 
     A general advantage of the noise reduction device is that the noise reduction device may function as an additional airfoil at low wind speeds when the boundary layer is thin. This is due to the fact that the noise reduction device extends outside the boundary layer where the noise reduction device is aerodynamically active. Thus, a higher power generated by the wind turbine under operation is possible at low wind speeds compared to a wind turbine rotor blade without a noise reduction device. 
     The noise reduction device may comprise polyurethane and/or three dimensional printed plastic. 
     In an advantageous embodiment, the wind turbine rotor blade comprises a tip end, a root and a shoulder. Furthermore, the rotor blade trailing edge section extends between the shoulder and the tip end, and the rotor blade trailing edge section is divided into an inner half adjacent to the shoulder and an outer half adjacent to the tip end. Advantageously, the noise reduction device is located in the outer half. 
     At the root, the wind turbine rotor blade is advantageously attached to a hub of the wind turbine. The tip end is characterized as a part of the wind turbine rotor blade which is most distant from the root. 
     The shoulder is adjacent to the rotor blade trailing edge section. In particular, the chord of the wind turbine rotor blade is greatest or longest where the shoulder is located. 
     The notion of the inner half and the outer half has to be understood such that 50 percent of the rotor blade trailing edge section is located in the inner half and that 50 percent of the rotor blade trailing edge section is located in the outer half. In other words, the rotor blade trailing edge section is virtually cut into two equal parts by the inner half and the outer half. 
     In an alternative embodiment, the rotor blade trailing edge section is divided into three equal parts, namely an inner third, an intermediate third and an outer third. The inner third is adjacent to the shoulder, the outer third is adjacent to the tip end and the intermediate third is located in between the inner third and the outer third. Advantageously, the noise reduction device is located in the outer third. 
     A reason why the noise reduction device is beneficially located in the outer half or in the outer third is that rotational speeds of the wind turbine rotor blade are highest towards the tip end. Consequently, noise emission may also be highest towards the tip end. Thus, it is advantageous to place the noise reduction device towards the tip end. 
     In another advantageous embodiment, in a plane, which is perpendicular to the rotor blade trailing edge, a first line extends from the rotor blade trailing edge to a first point of the inner surface. The first point is closest to the rotor blade leading edge section. Likewise, in this plane, a second line extends from the rotor blade trailing edge to a second point of the inner surface. The second point is furthermost to the rotor blade leading edge section. It is beneficial that the noise reduction device is connected with the rotor blade trailing edge section in a way that an angle, which is between the first line and the second line, is greater than 70 degree, in particular greater than 100 degree. 
     In other words, it is advantageous to place or to arrange the noise reduction device with regard to the rotor blade trailing edge such that a sufficiently large angular region is covered by the noise reduction device. Consequently, acoustic waves, which are generated at the rotor blade trailing section, in particular at the rotor blade trailing edge, are reflected by the inner surface. The angular region has to be understood as being centered at the trailing edge. 
     In another advantageous embodiment, in the plane, which is perpendicular to the rotor blade trailing edge, a third line can be drawn from the rotor blade trailing edge to the inner surface, wherein the third line is substantially perpendicular to the surface of the rotor blade trailing edge section which is facing the inner surface. 
     Then, in a first alternative, the first point and the second point lie on different sides with regard to an intersection of the third line and the inner surface. Descriptively spoken, the first point, thus the leading edge lies “before” the rotor blade trailing edge and the second point, thus the trailing edge, lies “behind” the rotor blade trailing edge. An advantage of this alternative is a possibly large angular region which is covered by the noise reduction device, which means that the acoustic waves may be reflected to a large extent. 
     In a second alternative, both the first point, thus the leading edge, and the second point, thus the trailing edge, are located “before” the rotor blade trailing edge. This arrangement of the noise reduction device with regard to the wind turbine rotor blade may be sufficient, since most of the acoustic waves may be radiated in an upstream direction, i. e. towards the rotor blade leading edge section. 
     In general, it is advantageous that the position of the noise reduction device with regard to the wind turbine rotor blade and in particular the angle between the first line and the second line are chosen such that they are aligned with a local flow direction of the wind. 
     In another advantageous embodiment, the noise reduction device comprises a tip side, which is facing the tip end of the wind turbine rotor blade, and a root side, which is facing the root of the wind turbine rotor blade. Additionally, the noise reduction device is continuously connected via the tip side and/or via the root side with the rotor blade trailing edge section. 
     In other words, the noise reduction device may be designed in a way that it is gradually transient into the existing rotor blade trailing edge section. Thus, end sections of the noise reduction device, where noise can develop or escape, are avoided. Additionally or alternatively, noise can be reduced by replacing the spanwise edge of the noise reduction device by porous metal foam, for instance retimet or recemat. Hereby, a tip vortex is diffused, which has been shown to effectively mitigate or even eliminate edge noise. 
     In another advantageous embodiment, the inner surface and/or the surface of the rotor blade trailing edge section which is facing the inner surface comprises acoustic absorption material such that the acoustic waves are at least partly absorbed. 
     In other words, the inner surface and/or the surface of the rotor blade trailing edge section which is facing the inner surface are treated by an absorption material in order to enhance absorption of the acoustic waves at the treated surfaces. On the one hand, the acoustic waves, which are generated at the rotor blade trailing edge section, may directly be absorbed, at least partly, by the treated surfaces. On the other hand, the acoustic waves may first be reflected and subsequently be, at least partly, absorbed by the treated surfaces. 
     The acoustic absorption material may comprise perforate and/or mesh facing sheets. For example, the performated sheets may comprise punched aluminium, mechanically drilled carbon fibre composites, laser drilling or microperforates. As bulk material, also felt fibres, ceramic foam, ceramic fibre or metal casing ceramic hollow spheres are advantageous. 
     In particular, the acoustic absorption material may come as a single layer or as a double layer. They are also referred to as acoustic liners. 
     In another advantageous embodiment, the inner surface is substantially parallel to the surface of the rotor blade trailing edge section which is facing the inner surface. 
     An advantage of parallel surfaces is ease of manufacturing. 
     In another advantageous embodiment, the trailing edge comprises serrations, and/or the leading edge comprises serrations, and/or the rotor blade trailing edge section comprises serrations. In particular, the serrations are periodic serrations. 
     Serrated rotor blade trailing edge sections are known for noise reduction purposes as disclosed for instance in the European patent application EP 0 652 367 A1. Furthermore, it is known that serrated rotor blade trailing edge sections may improve wind turbine rotor efficiency as disclosed for example in the European patent EP 1 314 885 B1. It is therefore beneficial to also use serrations for the trailing edge and the leading edge of the noise reduction device, and alternatively combine these serrations with a serrated rotor blade trailing edge section. In other words, it is possible that the noise reduction device is added to an existing serrated rotor blade trailing edge section which has been, amongst other reasons, being serrated already for noise reduction purposes. Also is it possible to substitute a serrated rotor blade trailing edge section by the noise reduction device. 
     Thus, the rotor blade trailing edge section where the noise reduction device is attached to can be plane or serrated. 
     In another advantageous embodiment, the connector is attached to a support structure. The support structure, which in particular comprises a shape of a plate, is attached to the rotor blade trailing edge section. 
     An advantage of the support structure is that the noise reduction device can easily and efficiently be manufactured separately from the wind turbine rotor blade and can subsequently be attached to the wind turbine rotor blade. 
     In another advantageous embodiment, the arrangement comprises a first noise reduction device, which is connected with the rotor blade suction side, and/or a second noise reduction shield, which is connected with the rotor blade pressure side. 
     It may be the case that for a wind turbine rotor blade under operation noise, noise emission at the rotor blade suction side is more significant compared to noise radiation at the rotor blade pressure side. Then it is beneficial that at least the first noise reduction device (at the suction side) is installed; the second noise reduction device is optional. 
     Furthermore, embodiments of the invention relate to a method of manufacturing such an arrangement to reduce noise emission. 
     The method advantageously comprises the following: 
     a) casting the wind turbine rotor blade, 
     b) attaching a pre-fabricated noise reduction device to the wind turbine rotor blade as a retrofit and/or during manufacturing of the arrangement. 
     If the noise reduction device is attached to the wind turbine rotor blade during manufacturing, it is advantageous that the noise reduction device is an integral part of the wind turbine rotor blade with which it is cased together. 
     The aspects defined above and further aspects of the present invention are apparent from the examples of embodiments to be described hereinafter and are explained with reference to the examples of embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  shows an arrangement to reduce noise emission, comprising a wind turbine rotor blade and a noise reduction device, 
         FIG. 2  shows a part of such an arrangement in a perspective view, 
         FIG. 3  shows acoustic waves, originating from a rotor blade trailing edge of a wind turbine rotor blade, 
         FIG. 4  shows an angular region covered by a noise reduction device and a pre-defined distance of an inner surface of the noise reduction device with regard to one side of a wind turbine rotor blade, 
         FIG. 5  shows a first noise reduction device and a second noise reduction device inside a boundary layer, 
         FIG. 6  shows a noise reduction device with a closed tip side and a closed root side, 
         FIG. 7  shows a noise reduction device with a closed root side in a first view, 
         FIG. 8  shows the noise reduction device with a closed root side in a second view, 
         FIG. 9  shows a first embodiment of a serrated noise reduction device, 
         FIG. 10  shows a second embodiment of a serrated noise reduction device and 
         FIG. 11  shows a third embodiment of a serrated noise reduction device. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows an arrangement  10  to reduce noise emission, which is generated by a wind turbine rotor blade  11 . The arrangement  10  comprises a wind turbine rotor blade  11  and a noise reduction device  20 . The wind turbine rotor blade  11  comprises a root  17  and a tip end  16 . The root  17  and the tip end  16  are virtually connected by a rotor blade longitudinal axis  19 . Furthermore, the wind turbine rotor blade  11  comprises a rotor blade leading edge section  12  and rotor blade trailing edge section  13 . The rotor blade trailing edge section  13  comprises a rotor blade trailing edge  35 . The rotor blade trailing edge  35  has to be understood as an edge or a rim or a line, while the rotor blade trailing edge section  13  and the rotor blade leading edge section  12  each has to be understood as a section or a region or an area of the wind turbine rotor blade  11 . The rotor blade trailing edge  35  extends from a shoulder  18  of the wind turbine rotor blade  11  to the tip end  16 . The rotor blade trailing edge section  13  is additionally divided into an inner half  32 , which is adjacent to the shoulder  18 , and an outer half  33 , which is adjacent to the tip end  16 . Thus, 50 percent of the rotor blade trailing edge section  13  comprises the inner half  32  and 50 percent of the rotor blade trailing edge section  13  comprises the outer half  33 . In the concrete example of the invention shown in  FIG. 1 , the noise reduction device  20  is attached to the rotor blade trailing edge section  13  in the outer half  33  of the rotor blade trailing edge section  13 . 
       FIG. 2  shows a part of the arrangement  10  shown in  FIG. 1  in a perspective view. Moreover, the arrangement  10  of  FIG. 1  is shown in  FIG. 2  along the cut A to A′. In this view, a rotor blade suction side  14  and a rotor blade pressure side  15  of the wind turbine rotor blade  11  become apparent. In  FIG. 2  the noise reduction device  20  substantially comprises a shape of a plate and comprises two connectors  22 . 
     In  FIG. 3 , again a wind turbine rotor blade  11  with a rotor blade leading edge section  12  and a rotor blade trailing edge  35  is shown. Furthermore, it can be seen that acoustic waves  31  are generated or originate at the rotor blade trailing edge  35 . 
     In  FIG. 4 , a rotor blade trailing edge section  13  and a noise reduction device  20 , which is attached or connected to the rotor blade trailing edge section  13  by a connector  22  is shown. The noise reduction device  20  comprises an inner surface  23  which is facing a surface of the rotor blade trailing edge section  13 . Furthermore, the noise reduction device  20  comprises an outer surface  24 , which is opposite to the inner surface  23 . Additionally, the noise reduction device  20  comprises a leading edge  27 , which is facing a rotor blade leading edge section  12  of the wind turbine rotor blade  11 . Furthermore, the noise reduction device  20  comprises a trailing edge  28 , which is opposite to the leading edge  27 . 
     As the noise reduction device  20  is connected to the rotor blade trailing edge section  13  in a mechanically stable way, the inner surface  23  features a pre-defined distance  21  to the surface of the rotor blade trailing edge section  13 . In the example shown in  FIG. 4 , the inner surface  23  is substantially parallel to the surface of the rotor blade trailing edge section  13 , which the inner surface  23  is facing. However, in an alternative embodiment, the noise reduction device  20 , i.e. its inner surface  23 , can also be inclined with regard to the facing surface of the rotor blade trailing edge section  13 . 
     An important aspect of the invention is that the pre-defined distance  21  between the inner surface  23  and the rotor blade trailing edge section  13  is chosen in a way that acoustic waves  31 , which are generated at the rotor blade trailing edge section  13 , in particular at a rotor blade trailing edge  35  of the rotor blade trailing edge section  13 , are reflected by the inner surface  23 . A benefit of this is that noise emission, which is generated by the wind turbine rotor blade  11 , is mitigated and reduced. 
     An important design parameter of the inventive arrangement  10  is the angular region, which is covered by the noise reduction device  20  and which is centered at the rotor blade trailing edge  35 . This angular region is defined as follows: 
     A first point  371  and a second point  381  are assigned to the inner surface  23 . The first point  371  is defined as a point which, in a plane perpendicular to the rotor blade trailing edge  35 , is closest to the rotor blade leading edge section  12 . The second point  381  is most distant to the rotor blade leading edge section  12 . Then, a first line  372  extends from the rotor blade trailing edge  35  to the first point  371  Likewise, a second line  382  extends from the rotor blade trailing edge  35  to the second point  381 . Given this definitions, an angle  36  between the first line  372  and the second line  382  can be determined, wherein the angle  36  is centered at the intersection of the first line  372  and the second line  382 , i. e. the rotor blade trailing edge  35 . In the advantageous embodiment shown in  FIG. 4  the angle  36  is 95 degree. 
     In  FIG. 5 , a wind turbine rotor blade  11  with a rotor blade suction side  14  and a rotor blade pressure side  15  is shown. Additionally, a boundary layer  34  of the wind turbine rotor blade  11  is shown. The boundary layer  34  can be understood as a region close to the surfaces of the wind turbine rotor blade  11 , where a mean flow speed of wind is considerably smaller than in an outer region, which is further away from the surfaces of the wind turbine rotor blade  11 . Note that the boundary layer  34  of the suction side  14  and the boundary layer  34  of the pressure side  15  merge at the rotor blade trailing edge and continue downstream. The boundary layer thickness at the suction side  14  and at the pressure side  15  slightly increases downstream. 
     It is advantageous to arrange the noise reduction device  20  inside the boundary layer  34 . Additionally, it is advantageous to arrange a first noise reduction device  41  in connection with the rotor blade suction side  14  and a second noise reduction device  42  in connection with the pressure side  15 . 
       FIG. 6  shows a rotor blade trailing edge section  13  with a rotor blade trailing edge  35 . Furthermore, it shows a noise reduction device  20  with a leading edge  27 , a trailing edge  28 , a tip side  43  and a root side  44 . The tip side  43  is defined as facing the tip end  16  of the wind turbine rotor blade  11  and the root side  44  is defined as facing a root  17  of the wind turbine rotor blade  11 . 
     In the advantageous embodiment shown in  FIG. 6  it can be seen that the noise reduction device  20  is closed at the tip side  43  and the root side  44 . An advantage of this arrangement is that edge noise can be mitigated efficiently and considerably. Note that the noise reduction device  20  is however open at the leading edge  27  and the trailing edge  28 . This is important as the wind shall be able to flow between the noise reduction device  20  and the rotor blade trailing edge section  13  of the wind turbine rotor blade  11 . 
       FIG. 7  and  FIG. 8  show a rotor blade trailing edge section  13  of a wind turbine rotor blade from two different perspectives. In particular, a rotor blade suction side  14  of the wind turbine rotor blade is shown. A noise reduction device  20  is attached to the rotor blade trailing edge section  13  via a plurality of connectors  22 . A leading edge  27 , which is facing to the rotor blade leading edge, a trailing edge  28 , which is opposite to the leading edge  27 , a tip side  43 , which is facing to the tip of the wind turbine rotor blade, and a root side  44 , which is facing to the root of the wind turbine rotor blade, can be attributed to the noise reduction device  20 . 
     The specific feature that shall be illustrated in  FIGS. 7 and 8  is the transient root side  44 . In other words, the noise reduction device  20  is continuously connected with the trailing edge section  13  at the root side  44 . 
       FIG. 9 ,  FIG. 10  and  FIG. 11  show advantageous embodiments of serrated noise reduction device  20 . In each of the Figures the noise reduction device  20  is mounted on a support structure  25 , which comprises a shape of a plate. An advantage of the support structure  25  is a better mechanical stability and ease of manufacturing. Concretely, instead of mounting or attaching the pre-fabricated noise reduction device  20  directly to the rotor blade trailing section  13 , which might be tricky and difficult in practice, the noise reduction device  20  can be attached by connector  22  to the support structure  25  beforehand, i.e. separately, and subsequently the support structure  25  with the noise reduction device  20  can directly be connected or attached to the wind turbine rotor blade trailing edge section  13 . If the support structure  25  has for instance the shape of a plate as shown in the  FIGS. 9 to 11 , attaching the support structure  25  to the rotor blade trailing edge section  13  is easy to perform. 
     In  FIG. 9 , the noise reduction device  20  comprises serrations  29  at a leading edge  27  of the noise reduction device  20 . In  FIG. 10 , the noise reduction device  20  comprises serrations at a trailing edge  28  of the noise reduction device  20 . In  FIG. 11 , the noise reduction device  20  comprises serrations at both the leading edge  27  and the trailing edge  28 . The serrations  29  may have a random shape or profile. However, it is advantageous that the serrations  29  comprise a substantially periodic structure.