WO2014207015A1

WO2014207015A1 - Rotor blade with noise reduction means

Info

Publication number: WO2014207015A1
Application number: PCT/EP2014/063339
Authority: WO
Inventors: Stefan Oerlemans; Michael J. Asheim; Valerio LORENZONI; Anders Smaerup OLSEN; Justin JAWORSKI; Manjinder J. Singh
Original assignee: Siemens Aktiengesellschaft
Priority date: 2013-06-27
Filing date: 2014-06-25
Publication date: 2014-12-31

Abstract

The invention relates to a rotor blade of a wind turbine with noise reduction means. The rotor blade comprises a leading edge section, a trailing edge section, a pressure side and a suction side. The noise reduction means is attached to the trailing edge section. The noise reduction means comprises a passage for influencing the airflow flowing from the leading edge section to the trailing edge section, such that noise, which is generated by the interaction of the airflow and the rotor blade, is reduced.

Description

Rotor blade with noise reduction means

The present invention relates to a rotor blade of a wind turbine wherein the rotor blade comprises noise reduction means.

A drawback to the use of wind turbines is the noise nuisance that is caused during operation of the wind turbine. There are generally different source of noise nuisances at wind turbines. This invention relates to noise which is generated by the interaction of airflow and the rotor blade.

The US patent 5,533,865 proposes a serrated trailing edge of a rotor blade in order to reduce this type of noise. The serrations of the trailing edge, which may for example have a saw-tooth shape, may be integrated in the rotor blade or may for example be part of a panel that is attached to the trailing edge of the rotor blade. Different designs of such a serrated panel, e.g. with regard to the angle between adjacent serrations or with regard to the angle of the serrated panel itself relative to the chord line of the rotor blade have been disclosed.

However, it may be the case that noise is reduced by such a serrated trailing edge only up to a certain extent.

It is thus an objective of the current invention to provide ways to further reduce, compared to the state of the art, noise generated by the interaction of the airflow and the rotor blade .

This objective is solved by independent claim 1 of the present patent application. Advantageous developments and embodiments are described in the dependent claims.

According to the invention, it is provided a rotor blade of a wind turbine with noise reduction means, wherein the rotor blade comprises a leading edge section, a trailing edge sec- tion, a pressure side and a suction side. The noise reduction means is attached to the trailing edge section. The noise reduction means furthermore comprises a passage for influencing the airflow flowing from the leading edge section to the trailing edge section, such that noise, which is generated by the interaction of the airflow and the rotor blade, is reduced .

On rotor blades of a wind turbine, noise may be generated due to the interaction of turbulent eddies with the trailing edge of the rotor blade. The extent of noise radiation is highest when the axis of rotation of the turbulent eddies is substantially parallel to the trailing edge. Existing technologies to reduce this noise relate on breaking up the orthogonality between the axis of rotation of the eddies and the trailing edge, which is responsible for noise scattering. In practice, this breaking up of the orthogonality may be achieved by introducing serrations to the otherwise straight trailing edge.

This invention pursues another approach. It applies the concept of introducing passages, which may for example be holes or slits or the like, to the noise reduction means. This has the consequence that the airflow is scattered at the passages and that air is at least partially guided through the passages. Note that the airflow through the passages may be relatively small compared to the total airflow flowing across the passages from the leading edge section to the trailing edge section of the rotor blade. Nevertheless, due to the presence of the passages, noise, which is generated by the interaction of the airflow and the rotor blade, is reduced. The passage may in principle have any shape.

It has been supposed that the silent flight of owls may relate to the interaction of a turbulent eddy with a semi- infinite poroelastic edge at the wing of the owl, as it has been described for example in the scientific article of J.W. Jaworski and N. Peake: "Aerodynamic noise from a poroelastic edge with implications for the silent flight of owls", Journal of Fluid Mechanics, Vol. 723, pages 456 - 479, 2013. This invention relates to a transfer of this concept to noise mitigation of rotor blades of wind turbines.

Note that the leading edge section and the trailing edge section do not necessarily comprise a strict edge but it may have a round shape. The notion trailing edge section and leading edge section refers to a portion of at maximum 10 percent of the rotor blade each comprising the leading edge of the rotor blade and the trailing edge of the rotor blade, respectively .

Also note that although it is mentioned that the noise reduction means is attached to the trailing edge section, this does not necessarily imply that the trailing edge section and the noise reduction means are two separate parts. It could be advantageous to manufacture the whole rotor blade including the noise reduction means in one single piece. This alternative shall expressively be comprised by this patent application, too.

The noise, which is generated by the interaction of the airflow and the rotor blade, and which is reduced by the noise reduction means relate to noise that is generated during operation, i.e. during rotation of the rotor blade. It may, however, also relate to noise that is generated by a rotor blade that is in a stand-still position i.e. that is not rotating about the rotor axis of rotation.

According to a first embodiment of the invention, the noise reduction means is flexible such that it passively bends in a loaded state of the rotor blade, compared to the unloaded state of the rotor blade.

If the noise reduction means is flexible, it has the advantage that it passively bends according to the wind speed and/or the angle of incidence of the airflow relative to the chord line of the rotor blade. Thus, the load penalty of the noise reduction means is minimized. If a certain load penalty can be tolerated, the stiffness of the noise reduction means may be increased up to a fully rigid and stiff noise reduction means.

An example of a flexible noise reduction means is a flexible panel that is made of rubber. An example of a rigid noise reduction means is a panel which is made of a metal and which comprises a thickness which is sufficiently large to withstand loads applied on it during operation of the rotor blade. An advantage of a rigid noise reduction means is its contribution to the lift of the rotor blade.

In another embodiment of the invention, the noise reduction means comprises a panel. Furthermore, the passage for influencing the airflow is provided by openings in the panel, which are arranged and prepared to deflect the airflow flowing from the leading edge section to the trailing edge section .

The panel is to be understood as having a predetermined length, width and thickness. The configuration of the panel may, however, change, depending on the flexibility of the panel. Thus, the notion panel includes a rigid as well as a flexible panel. The passage of a part of the airflow is concretely realized by a plurality of openings which are inserted in the panel. These openings are arranged and prepared in a way that the airflow is deflected or scattered at the edges of the openings. Due to the scattering at the edges of the openings and/or the passage of a part of the airflow through the openings the noise, which is generated by the interaction of the airflow and the rotor blade is reduced. So, the efficiency of noise radiation is minimized since the edge of the opening is at least partly not orthogonal to the trailing edge. In the case of for example a circular opening most of the circumference of the edge of the opening is not orthogonal with regard to the trailing edge. Thus, noise radiation can efficiently be minimized. In another advantageous embodiment, the cross sections of the openings, in a view from the suction side to the pressure side of the rotor blade, substantially have the shape of an ellipse or a quadrilateral.

Particularly advantageous shapes of the cross section of the openings are a circle, in the case of an ellipse or a rhombus or a rectangle in the case of a quadrilateral. Note that a quadrilateral is referred to a polygon with four sides or edges and four vertices or corners. A quadrilateral is also referred to as a quadrangle.

The mentioned shapes of the openings, in other words the mentioned cross sections of the openings, have the advantage that they are both efficient with regard to the noise reduction capability and easy to manufacture. Particularly circles, rhombi or rectangles have been proven to be a good compromise between the challenges of efficiency and ease of manufacturing .

In another advantageous embodiment, the edges of the openings comprises a corrugated structure such that indentations, a wavy shape or the like.

This has the advantage that whistle tones that might be generated by smooth edges of the openings might be reduced due to scattering of the airflow at the edges with the corrugated structure .

In another advantageous embodiment, the maximum cross sectional dimension of the openings is between 0.1 millimeters and 5 millimeters, in particular between 0.2 millimeters and 2 millimeters .

These dimensions are advantageous if a standard industrial size rotor blade comprising a length between 50 meters and 150 meters is used. Thus, relatively small openings are sufficient to efficiently scatter the airflow and reduce noise generated by it. Also note that it is not necessary to have a large portion of the airflow effectively flowing through the openings. It may be sufficient to just provide a relatively small opening in order to efficiently reduce noise. In another advantageous embodiment, the porosity, which is defined as the fraction of open space of the panel is between 1 percent and 50 percent, in particular between 3 percent and 30 percent. As an example, if the size and the number of openings of a given panel amount to 10 percent of the area of the panel, the porosity of the panel is referred to a number of 10 percent . Given the broad range of porosities between 1 percent and 50 percent, it becomes clear that both with a moderate fraction of open space of the panel of a few percent and with a large fraction of open space up to the half of the total space or area of the panel noise may be efficiently reduced.

According to another advantageous embodiment, the porosity of the panel varies in spanwise direction of the rotor blade and/or chordwise direction of the rotor blade. It may for example be beneficial to have a less dense distribution of openings close to the trailing edge of the rotor blade compared to a more dense distribution of the openings further away from the trailing edge of the rotor blade. This has been proven to reduce whistling, i.e. to reduce a whis- tling tone of the noise, which is generated by the interaction of the rotor blade and the airflow.

Another way to avoid or reduce a whistling tone is to irregularly distributing the openings over the panel. This irregu- lar distribution relates to the size of the openings as well as the shape of the openings . In another advantageous embodiment, the chordwise dimension of the panel is between 2 percent and 20 percent, in particular between 5 percent and 15 percent.

If, for example, the chord length of the airfoil has a value of 5 meters, a chordwise dimension of the panel of 50 centimeters is advantageous. A chordwise dimension of the panel in the given range between 2 percent and 20 percent is a good compromise between a significant impact regarding noise reduction and other consequences such as an enhancement of the lift of the rotor blade for instance and the additional weight and drag of such a panel.

Another advantageous embodiment features a thickness of the panel between 0.1 millimeters and 2 millimeters, in particular between 0.2 millimeters and 1 millimeter.

An advantageous material that the panel is made of is mylar, which is a flexible plastic sheet material.

In a first alternative, the panel is entirely serrated. In a second alternative, a part of the panel is serrated. For example, the panel comprises an inner part, which is also denoted as a base part, which does not feature any serrations and which is adjacent to the trailing edge of the rotor blade, and the panel comprises an outer part, which is further away from the trailing edge and which comprises serrations . openings may be distributed on all serrations entirely or may cover some parts of the serrations or some serrations .

An advantage of a serrated panel comprising openings is that two means or ways of reducing noise, which is generated by the interaction of the airflow and the rotor blade, are combined with each other. This may lead to a significant reduction of the noise. In another advantageous embodiment, the noise reduction means comprises strips.

Advantageously, the strips are arranged substantially in chordwise direction of the rotor blade.

The strips may, for instance, have the shape of an ellipsis. It may also have a rectangular shape.

In another advantageous embodiment, the strips overlap in an unloaded state of the rotor blade and open up the passage for influencing the airflow flowing from the leading edge section to the trailing edge section of the rotor blade such that the noise, which is generated by the interaction of the airflow and the rotor blade, is reduced in a loaded state of the rotor blade .

In another advantageous embodiment, the strips are spaced from each other such that the passage for influencing the airflow flowing from the leading edge section to the traili edge section of the rotor blade is permanently provided.

An advantage of a permanent spacing to each other of the strips is that by preventing the strips from touching each other it is prevented that the strips create noise. It may be beneficial to increase the gap between adjacent strips in streamwise direction, in particular gradually increase the gap width, in order to gradually equalize the pressure difference in streamwise direction. In another alternative, the gap width between adjacent strips is irregularly distributed in streamwise direction of the rotor blade.

Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, of which :

Figure 1 shows a wind turbine.

Figure 2 shows a rotor blade of a wind turbine in a top view. Figure 3 shows a panel with openings at the trailing edge of a rotor blade . Figure 4 shows different states of a flexible panel with openings .

Figure 5 shows a poroelastic trailing edge with a low density of pores.

Figure 6 shows a poroelastic trailing edge with a high density of pores.

Figure 7 shows a poroelastic trailing edge with a density of pores that varies in chordwise direction of the rotor blade.

Figure 8 shows a poroelastic trailing edge which ends in chordwise direction of the rotor blades. Figure 9 shows a panel with openings that have the shape of a rhombus .

Figure 10 shows elliptically-shaped openings of a panel. Figure 11 shows a serrated poroelastic trailing edge with pores on one of the serrations.

Figure 12 shows a poroelastic trailing edge with serrations which comprise slits.

Figure 13 shows feather-shaped strips of a poroelastic trailing edge.

And figure 14 shows a poroelastic trailing edge with strips which is permanently spaced from each other.

The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements may be provided with the same reference signs. In Figure 1, a wind turbine 10 is shown. The wind turbine 10 comprises a nacelle 12 and a tower 11. The nacelle 12 is mounted at the top of the tower 11. The nacelle 12 is mounted rotatable with regard to the tower 11 by means of a yaw bearing. The axis of rotation of the nacelle 12 with regard to the tower 11 is referred to as the yaw axis.

The wind turbine 10 also comprises a hub 13 with one or more rotor blades 20. Preferably, the wind turbine 10 comprises three rotor blades 20. The hub 13 is mounted rotatable with regard to the nacelle 12 by means of a main bearing. The hub 13 is mounted rotatable about a rotor axis of rotation 14.

The wind turbine 10 furthermore comprises a main shaft, which connects the hub 13 with a rotor of a generator 15. If the hub 13 is connected directly to the rotor, the wind turbine is referred to as a gearless, direct drive wind turbine. Alternatively, the hub 13 may also be connected to the rotor via a gearbox. This type of wind turbine is commonly referred to as a geared wind turbine.

The generator 15 is accommodated within the nacelle 12. It comprises the rotor and a stator. The generator 15 is arranged and prepared for converting the rotational energy from the rotor into electrical energy.

In the concrete example of Figure 1, the wind turbine 10 comprises three rotor blades 20 (of which two rotor blades 20 are depicted in Figure 1) . The rotor blades 20 are mounted rotatable with regard to the hub 13 by means of a pitch bearing. The rotor blades 20 may thus be pitched about a pitch axis 16 in order to optimize the orientation with regard to the wind flow impinging on the wind turbine 10. Each of the rotor blades 20 comprises a root section 23 and a tip section 21. The root section 23 refers to the section of the rotor blade 20 which is closest to the hub 13. The tip section 21 refers to the section of the rotor blade 20 which is furthest away of the hub 13, thus being opposite to the root section 23.

Figure 2 shows a rotor blade 20 of a wind turbine. The rotor blade 20 comprises a root section 21 with a root 211 and a tip section 22 with a tip 221. The root 211 and the tip 221 are virtually connected by the span 26 which follows the shape of the rotor blade 20. If the rotor blade were a rectangular shaped object, the span 26 would be a straight line. However, as the rotor blade 20 features a varying thickness, the span 26 is slightly curved or bent as well. Note that if the rotor blade 20 was bent itself, then the span 26 would be bent, too. The rotor blade 20 furthermore comprises a leading edge section 24 with a leading edge 241 and a trailing edge section 23 with a trailing edge 231.

The trailing edge section 23 surrounds the trailing edge 231. Likewise, the leading edge section 24 surrounds the leading edge 241.

At each spanwise position, a chord line 27 which connects the leading edge 241 with the trailing edge 231 can be defined. Note that the chord line 27 is perpendicular to the span 26. The shoulder 28 is defined in the region where the chord line comprises a maximum chord length.

Furthermore, the rotor blade 20 can be divided into an in- board section which comprises the half of the rotor blade 20 adjacent to the root section 21 and an outboard section which comprises the half of the rotor blade 20 which is adjacent to the tip section 22.

In another advantageous embodiment, the panel comprises ser- rations, and the openings are at least partly arranged at the serrations . Figure 3 shows a trailing edge section 23 of a rotor blade 20. The trailing edge section 23 comprises a trailing edge 231 at which a panel 31 is attached. The panel 31 comprises a plurality of openings 32. The panel 31 in figure 3 is elas- tic, which means that it deflects according to the velocity and the direction of airflow 41 flowing from a leading edge section of the rotor blade 20 to the trailing edge section 23. However, the panel 31 may alternatively also be ridged, i.e. stiff. The airflow 41 which interacts with the rotor blade 20 is at least partially scattered at the edges of the openings 32. As a consequence, noise, which is generated by the interaction of the airflow 41 and the rotor blade 20, is reduced . Figure 4 shows a cross-sectional view of the trailing edge section 23 that is shown in figure 3. It can be seen that the panel 31 may be in different states according to the forces or loads that is acting on it. The reference sign 311 shows the panel as it is in an un-deflected state. In other words, this is the state of the panel 31 if no wind pressure is acting on it .

The reference signs 312 and 313 shows the panel 31 in a first deflected state and a second deflected state, respectively. The first deflected state 312 is present at moderate wind speeds, while the panel in the second deflected state 313 occurs at heavy or strong wind speeds .

By deflection of the panel, the noise is efficiently reduced, while at the same time no or only a small additional load is added to the rotor blade.

Figure 5 shows a trailing edge section 23 with a panel 31. The panel 31 comprises openings 32 at a relatively low densi- ty. The density of the openings 32 is similar in both spanwise direction 261 and chordwise direction 271.

Figure 6 shows a similar rotor blade with the sole difference of a higher density of openings 32, compared to figure 5. Figure 7 shows yet another embodiment with a variable density of the openings 32 in the chordwise direction 271. Note that in spanwise direction 261 the density of the openings 32 is relatively constant. Figure 8 shows a panel 31 with openings in a cross-sectional view. As the panel 31 is attached to the trailing edge section 23 of the rotor blade 20, the panel 31 is also denoted as a poroelastic trailing edge. The panel 31 is designed such that it deflects more in the outer part, wherein the outer part refers to the part which is further away from the trailing edge 231, compared to the inner part close to the trailing edge 231. As the pressure difference between the top and the bottom of the panel 31 at the inner part is more than at the outer part, less dense openings in the inner part and more dense openings in the outer part are preferred.

Figure 9 shows a top view on a panel 31 comprising openings 32. The openings have the shape of a rhombus. The openings 31 have irregular shapes and an irregular distribution across the panel 31. The irregular distribution is both in spanwise direction 261 and in chordwise direction 271. Figure 10 shows another design of the cross section of the openings 31. In this embodiment, the cross section is elliptical and is elongated along the chordwise direction 271 of the rotor blade. Figure 11 shows a serrated panel 31 with a plurality of openings 32. The panel 31 comprises a base which is fully covered with openings 32 and three serrations 42. One of the three serrations is entirely covered with openings 32. Figure 12 shows another realization of openings which are arranged at serrations 42. In this embodiment, the openings 32 have the shape of rectangles . These rectangles have a length that is significantly larger than their width and may thus also be described as slits. These slits also have the tech- nical effect of scattering the airflow that is flowing along the panel, thus noise which is generated of the interaction of the airflow and the panel 31 is reduced. Figure 13 shows a panel 31 with feather-shaped strips 43. The strips 43 overlap partially. They overlap and touch each other in an unloaded state of the rotor blade but open up such that a passage between the strips 43 is created in a loaded state of the rotor blade. Due to the passage, noise is re- duced. The feather-shaped strips 43 are also referred to as owl wings as they resemble the feathers of birds, in particular owls. Note that the size of the strips 43 may vary along the spanwise direction 261. Advantageously, the size of the strips 43 in chordwise direction decreases towards the tip of the rotor blade.

Yet another embodiment is a panel with rectangular strips 43 which are permanently spaced from each other by passages 44. In the concrete example of figure 14, these passages have a varying and irregular size, in particular a varying width. It is common to all embodiments of poroelastic trailing edges that flexible plastic sheet material, for example Mylar, with a thickness between 0.1 mm and 1 mm may be chosen. This flexible plastic sheet material may be applied to the trailing edge of the rotor blade and may comprise a chordwise extent of about 5 to 15 % of the local chord length. The porosity which is defined as the fraction of open space of the panel may advantageously by in the range between 3 % and 30 %. The whole diameter may advantageously be between 0.2 mm and 2 mm. An irregular opening pattern is advantageous for the avoidance of resonances .

To summarize the following parameters may be varied for the noise reduction means: The elasticity of the noise reduction means, e.g. the panel, the overall opening density, the chordwise opening density and also the spanwise opening density. All or some of these parameters may be combined and optimized in order to optimize the noise reduction means which have to be tailored and customized for the requirements of a particular design of a rotor blade.

Claims

Patent claims

1. Rotor blade (20) of a wind turbine (10) with noise reduction means (30) ,

wherein

- the rotor blade (20) comprises a leading edge section (24), a trailing edge section (23), a pressure side (251) and a suction side (252) , and

- the noise reduction means (30) is attached to the trailing edge section (23) ,

characterized in that

the noise reduction means (30) comprises a passage (44) for influencing the airflow (41) flowing from the leading edge section (24) to the trailing edge section (23), such that noise, which is generated by the interaction of the airflow (41) and the rotor blade (20), is reduced.

2. Rotor blade (20) according to claim 1,

wherein the noise reduction means (30) is flexible such that it passively bends in a loaded state of the rotor blade (20), compared to the unloaded state of the rotor blade (20) .

3. Rotor blade (20) according to one of the preceding claims, wherein

- the noise reduction means (30) comprises a panel (31), and

- the passage (44) for influencing the airflow (41) is provided by openings (32) in the panel (31), which are arranged and prepared to deflect the airflow (41) flowing from the leading edge section (24) to the trailing edge section (23) .

4. Rotor blade (20) according to claim 3,

wherein the cross sections of the openings (32) in a view from the suction side (252) to the pressure side (251) of the rotor blade (20) have substantially the shape of

- an ellipse, in particular a circle, or

- a quadrilateral, in particular a rhombus or a rectangle.

5. Rotor blade (20) according to claim 3 or 4, wherein the maximum cross-sectional dimension of the openings (32) is between 0.1 millimeters and 5 millimeters, in particular between 0.2 millimeters and 2 millimeters.

6. Rotor blade (20) according to one of claims 3 to 5, wherein the porosity, being defined as the fraction of open space of the panel (31), is between 1% and 50%, in particular between 3% and 30%.

7. Rotor blade (20) according to claim 6,

wherein the porosity of the panel (31) varies in spanwise (261) and/or chordwise direction (271) of the rotor blade (20) .

8. Rotor blade (20) according to one of claims 3 to 7, wherein the size and/or the shape of the openings (32) are irregularly distributed over the panel (31) .

9. Rotor blade (20) according to one of claims 3 to 8, wherein the chordwise dimension of the panel (31) is between 2% and 20%, in particular between 5% and 15%.

10. Rotor blade (20) according to one of claims 3 to 9, wherein the thickness of the panel (31) is between 0.1 milli- meters and 2 millimeters, in particular between 0.2 millimeters and 1 millimeter.

11. Rotor blade (20) according to one of claims 3 to 10, wherein

- the panel (31) comprises serrations (42), and

- the openings (32) are at least partly arranged at the serrations ( 42 ) .

12. Rotor blade (20) according to claim 1 or 2,

wherein the noise reduction means (30) comprises strips (43) .

13. Rotor blade (20) according to claim 12,

wherein the strips (43) comprise the shape of a feather.

14. Rotor blade (20) according to claim 12 or 13,

wherein the strips (43)

- overlap in an unloaded state of the rotor blade (20), and

- open up the passage (44) for influencing the airflow (41) flowing from the leading edge section (24) to the trailing edge section (23) of the rotor blade (20) such that the noise, which is generated by the interaction of the airflow (41) and the rotor blade (20), is reduced in a loaded state of the rotor blade (20) .

15. Rotor blade (20) according to claim 12,

wherein the strips (43) are spaced from each other such that the passage (44) for influencing the airflow (41) flowing from the leading edge section (24) to the trailing edge sec- tion (23) of the rotor blade (20) is permanently provided.