Patent Application
20090074585
1. Technical Field
The subject matter described here generally relates to fluid reaction surfaces with specific blade structures, and, more particularly, to wind turbine blades with trailing edge serrations.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If that mechanical energy is used directly by machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is further transformed into electrical energy, then the turbine may be referred to as a wind generator or wind power plant.
Wind turbines use one or more airfoils in the form of a “blade” to generate lift and capture momentum from moving air that is them imparted to a rotor. Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of the chord line is simply referred to as the “chord.”
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in
Although wind energy is one of the fastest growing sources of renewable energy, wind turbine noise is still a major obstacle to implementation. For large, modern wind turbines, aerodynamic noise is considered to be the dominant source of this noise problem, and, in particular, so-called “trailing edge noise” caused by the interaction of turbulence in the boundary layer with the trailing edge of the blade.
Around 1996, under the non-nuclear energy research “Joule III” program, the European Commission began the Serrated Trailing Edge Noise (“STENO”) project aimed at the verification of a prediction algorithm for trailing-edge noise where various serrations were designed and tested in free-field measurements on the with the Universal Wind turbine for Experiments (“UNIWEX”) in Schnittlingen, Germany. Serrations with different total lengths, aspect ratios, and geometries in cross-section profile (straight, bent, curved) were tested. According to the “Publishable Final Report,” bent, 2:1 aspect ratio serrations had almost the same aero-acoustic noise properties as the curved 2:1 aspect ratio serrations, and the larger maximum reduction in the aero-acoustic noise emitted within the moderate frequency range made the longer bent 3:1 serrations preferable to the 2:1 serrations.
As illustrated in FIG. 2, U.S. Pat. No. 7,059,833 to Stiesdal et al. discloses a conventional wind turbine blade having serrations 16 that are triangular in shape, of hexagonal cross-section and having a fairly sharp vertex angle, typically less than 30 degrees. The serrated part of the of the trailing edge is limited to the outboard part of the blade near the tip, having a length of typically 10-20 percent of the span.
FIGS. 3 and 4 from U.S. Pat. No. 7,059,833 to Stiesdal et al., illustrate a serration panel 18 that is disclosed with some preferred dimensions of the serrations suitable for use on wind turbine blades of 20-50 m length. The serration panel 18 can be manufactured from a 1000×110 mm polycarbonate sheet. A serration tooth can be an equilateral triangle with a height of 50 mm. The cross-section can be rectangular, with a thickness of 2 mm, and the panel can be bent along the long axis, as shown in
FIG. 5 is also copied from U.S. Pat. No. 7,059,833 to Stiesdal et al. and shows a schematic, cross-sectional view of the mounting of the serrated panel 18 on a wind turbine blade. A linear version of the panel may be mounted on the pressure side of the blade, projecting behind the trailing edge. The bent version of the panel 18 shown in
European Patent Application No. 1,338,793 also discloses a wind turbine blade with a serrated trailing edge where the tooth height is defined by the thickness of the boundary layer on the chord surface of the blade. In one embodiment, the tooth height is varied along the length of the blade so that the ratio of the tooth height to the thickness of the boundary layer on the upper and lower chord surface is constant along the length of the blade. This patent also discloses that the thickness of the boundary layer increases in proportion to the to the chord length of the blade according to the equation delta=c L (1/Re)/5, where delta is the thickness of the boundary layer, c is a coefficient having a value of about 0.37, L is the chord length, and Re is the Reynolds number.
In January 2003, the European 5th Framework Project SCIROCO: Silent Rotors by Acoustic Optimization was launched under the coordination of the Energy Research Center of the Netherlands with a goal of addressing this trailing edge noise problem by designing airfoils for which the boundary layer is modified so that trailing edge noise is reduced, while the aerodynamic capabilities are maintained, for varying conditions on a full-scale wind turbine. A second challenge for the project lay in the design and manufacturing of full-scale rotor blades. Since trailing edge noise is mainly generated at the outer part of the blades (where the speeds are highest), any new, low-noise airfoil designs could only be applied at the outer portion of the blade span. In addition to these aerodynamic and acoustic aspects of the problem, aero-elastic, structural, and load issues also had to be carefully considered.
These and other problems associated with such conventional approaches are addressed here by providing, in various configurations, a wind turbine blade, including a trailing edge having a plurality of serrations; a length of the serrations in each of a plurality of sections of the trailing edge being between approximately 10% and 40% of a mean chord for the corresponding section; and a length to width ratio of each of the serrations being between approximately 1:1 to 4:1. Also provided is a wind generator, including a tower supporting a rotor that is connected to a gearbox and a generator; at least one blade, extending radially from the rotor, with a trailing edge having a plurality of triangular serrations arranged substantially coplanar with a trailing edge streamline; a length of the serrations in each of a plurality of sections of the trailing edge being between approximately 18% and 22% of a mean chord for the corresponding section; and a length to width ratio of each of the serrations being between approximately 1.5:1 to 2.5:1.
Various aspects of this technology will now be described with reference to the following figures which are not necessarily drawn to scale, but use the same reference numerals to designate corresponding parts throughout each of the several views.
Each section of the serrated trailing edge 32 may be formed separately or contiguously from any material, including aluminum, plastic, reinforced plastic, fiber reinforced plastic, glass fiber reinforced plastic, and/or other materials. For example, the serrated trailing edge 32 may be formed as one or more relatively stiff plates that do not significantly deform under the expected aerodynamic loads. In this regard, one to two millimeter thick aluminum plate is expected to provide suitable rigidity in many applications. However, less-rigid materials may also be used, and the serrated trailing edge 32 may also be integrally formed with the blade 30.
Each of the illustrated sections I through IV includes a plurality of triangular serrations 38, as best illustrated in enlarged, partial detail in
Each of the triangular serrations 38 illustrated in
The illustrated troughs 42 between serrations 38 also form an angle α which is the same as the apex angle α at the tip of the serrations. However, the angle α does not necessarily have to be the same for the apex of the triangular serrations 38 and the troughs 42, as, for example, where adjacent serrations do not have the same height to width ratios. Similarly, the troughs 42 are not necessarily V-shaped to correspond with the V-shaped apex of the triangular serrations 38. For example, some or all of the troughs 42 may be U-shaped, semicircular, elliptical, rectangular, and/or square.
The base of troughs 42 may be aligned with the unserrated trailing edge of the of the blade 30 so that only the serrations 38 extend from the unserrated trailing edge of the blade 30. Alternatively, some of the strip 40 may extend past the edge of the unserrated trailing edge of the blade 30. The serrations 38 may also be spaced apart from each other along the strip 40 and/or blade 30.
However, the position of the streamlines for a particular blade 30 may change for various wind conditions and blade configurations. Consequently, the linear and angular position of the serrated trailing edge 32 will typically be optimized for each blade 30 and its expected operating environment. Although further optimization can then be obtained by defining a length and position each of the serrations 38 along the lade 30, this would be very difficult for a large blade such as the one illustrated in
In light of these and other difficulties, the blade 30 can be divided into a suitable number of span-wise sections where each of the serrations 38 may have a similar length and angular configuration in that section. Although any number of sections may be used, a suitable tradeoff has been found using a wide range of between 1 and 10 sections, or a smaller range of between 2 and 6 sections, such as four sections. The four sections labeled with Roman numerals I through IV starling from the tip of the blade 30 that are shown in
Each of the serrations 38 may have the same configuration in each section, or the numbers listed below may be averages or medians over the entire section. Furthermore, in the examples below, it is expected that suitable results may be obtained by varying the lengths by a wide range of +/−30% and/or varying the angles by +/−20°, or by varying the lengths by a narrower range of +/−5% and/or varying the angles by +/−5° For example, the values listed below are expected to have engineering tolerances of +/−10% or +/−20°, where applicable.
In one embodiment for use with a Model No. GE46 wind turbine blade available from General Electric Corporation of Fairfield, Conn., USA, four sections may be used with the serrations 38 having a ratio of height H to width W of approximately 2:1 and further configured as follows where lengths are listed in millimeters and angles are listed in degrees:
As indicated by the angles for β listed above, each of the serrations is angled between approximately 7.5 and 5.5 degrees from the reference line 54 shown in
Field measurements that were conducted for a hybrid-rotor 2.3 MW wind generator (with a rotor diameter of approximately 94 meters) including one such blade from GE Energy at the Energy Center of the Netherlands test site in Wieringmeer. The results are illustrated in
In another embodiment for use with a Model No. GE48.7 wind turbine blade available from General Electric Corporation of Fairfield, Conn., USA, four sections may be used with the serrations 38 having a ratio of height H to width W of approximately 2:1 and further configured as follows where lengths are listed in millimeters and angles are listed in degrees.
In yet another embodiment for use with a Model No. GE40 wind turbine blade available from General Electric Corporation of Fairfield, Conn., USA, four sections may be used with the serrations 38 having a ratio of height H to width W of approximately 2:1 and further configured as follows where lengths are listed in millimeters and angles are listed in degrees listed in millimeters and angles are listed in degrees:
In the latter two examples, the angles δ may be determined from the blade geometry, and the angles γ and β may be determined from the expected position of the trailing edge streamline for the expected flow conditions.
The technology described above offers a variety of advantages over conventional approaches. For example, the turbine blades can be easily field fitted with the serrated training edge 32 which significantly decreases aerodynamic noise without substantial increases in weight or changes to existing blade molds.
It should be emphasized that the embodiments described above, and particularly any “preferred” embodiments, are merely examples of various implementations that have been set forth here to provide a clear understanding of various aspects of this technology. These embodiments may be modified without substantially departing from scope of protection defined solely by the proper construction of the following claims.
