Wind turbine noise reduction with acoustically absorbent serrations

A wind turbine blade includes a trailing edge including a radially inboard portion and a radially outboard portion opposite the radially inboard portion. The trailing edge further includes at least one serrated portion extending at least partially between the radially inboard portion and the radially outboard portion. The serrated portion includes at least one substantially acoustically absorbent material.

BACKGROUND

The field of the invention relates generally to wind turbine generators and, more particularly, to systems and methods for reducing noise generated by wind turbine blades.

Most known wind turbine generators include a rotor having multiple blades. The rotor is sometimes coupled to a housing, or nacelle, that is positioned on top of a base, for example, a tubular tower. At least some known utility grade wind turbines, i.e., wind turbines designed to provide electrical power to a utility grid, include rotor blades having predetermined shapes and dimensions. The rotor blades transform mechanical wind energy into induced blade lift forces that further induce a mechanical rotational torque that drives one or more generators, subsequently generating electric power. A plurality of wind turbine generators in a localized geographic array is typically referred to as a wind farm or a wind park.

During operation of such known wind parks or an individual wind turbine, rotational transiting of the rotor blades through air generates aerodynamic acoustic emissions, or noise. As a consequence, at least some of these known wind parks, or an individual wind turbine, will receive noise receptor devices in their vicinity to measure the noise level. At least some of such measured acoustic noises have a decibel (dB) level that may approach local regulatory levels. To comply with the limits, at least some of the wind turbines of the park, or the individual wind turbine, may need to be put into a noise reduced operation (NRO) mode for a period of time. A wind turbine is less efficient at generating electrical energy when in a NRO mode, but produces less noise. The reduction of noise comes at the cost of annual energy production (AEP). Therefore, it is necessary to avoid the application of NROs to increase AEP.

BRIEF DESCRIPTION

In one aspect, a wind turbine blade including a trailing edge is provided. The trailing edge includes a radially inboard portion and a radially outboard portion opposite the radially inboard portion. The trailing edge further includes at least one serrated portion extending at least partially between the radially inboard portion and the radially outboard portion. The serrated portion includes at least one substantially acoustically absorbent material.

In a further aspect, a retrofit system for a wind turbine blade is provided. The wind turbine blade includes a trailing edge. The retrofit system includes a mounting structure for coupling the system to the trailing edge. The retrofit system further includes at least one serrated portion extending at least partially along the mounting structure. The serrated portion includes at least one substantially acoustically absorbent material.

In another aspect, a method for reducing noise emission from a wind turbine blade is provided. The wind turbine blade includes a trailing edge. The method includes providing a wind turbine noise abatement device including a mounting structure having at least one serrated portion. The at least one serrated portion includes at least one substantially acoustically absorbent material. The method further includes preparing the trailing edge to receive the mounting structure and coupling the mounting structure to the trailing edge.

DETAILED DESCRIPTION

The wind turbine blades and modifications to wind turbine blades through retrofitting described herein provide systems and methods for reducing noise emanating from wind turbine blades. Specifically, the systems and methods described herein use wind turbine blades including serrated trailing edges fabricated from at least one substantially acoustically absorbent material to mitigate noise produced by operation of a wind turbine. Boundary-layer turbulence interaction with the trailing edge of the wind turbine blade, while in motion, is a primary source of aerodynamic noise emanating from wind turbine blades in operation. This noise is approximated by quadrupole sources which interact in the turbulent boundary-layer to produce dipole-like noise radiation patterns. In comparison to trailing edges with straight edges, serrated portions included in a trailing edge portion of the wind turbine blade reduce coherent scattering of the noise emanating from the wind turbine blade, approximated as the quadrupole sources. Reducing coherent scattering of the quadrupole sources mitigates the noise emanating from the wind turbine blade while in operation. The reduction in noise emanating from the wind turbine blades reduces the need for the wind turbine to be put into a noise reduced operation (NRO) mode to comply with a decibel (dB) level that may approach local regulatory levels. The reduction in NRO increases the annual energy production (AEP) of the wind turbine.

The serrated portion of the trailing edge includes at least one substantially acoustically absorbent material. The substantially acoustically absorbent materials function as sound absorbers and/or turbulence dampers. In at least some of the embodiments disclosed herein, the substantially acoustically absorbent materials are substantially porous. The acoustically absorbent materials reduce a magnitude of the sound reflected from the wind turbine blade in comparison to wind turbine blades having hard surfaces and/or materials. The acoustically absorbent materials thus mitigate noise emanating from the wind turbine blade while in operation. The reduction in noise emanating from the wind turbine blades reduces the need for NRO and increases AEP.

The combination of serrated portions of the trailing edge and at least one substantially acoustically absorbent material included in the serrated portions reduces the coherent scattering strength of noise emanating from the wind turbine blade through the combination of the geometry of the serrations and the effect of the acoustically absorbent material on the reflected sound. The combination of serrated portions of the trailing edge and at least one substantially acoustically absorbent material included in the serrated portions results in greater noise mitigation than the sum of the noise mitigated by serrated portions alone and substantially acoustically absorbent materials alone.

FIG. 1is a schematic view of an exemplary wind turbine generator100having rotor blades102having trailing edge portions104which include a serrated portion106that, in turn, includes at least one substantially acoustically absorbent material (not shown inFIG. 1). In the exemplary embodiment, wind turbine generator100is a horizontal axis wind turbine. Alternatively, wind turbine generator100may be a vertical axis wind turbine. Wind turbine generator100includes a tower108extending from a supporting surface110, a nacelle112coupled to tower108, and a rotor114coupled to nacelle112. Rotor blades102are coupled to nacelle112. Rotor114has a rotatable hub116to which a plurality of rotor blades102are coupled. In the exemplary embodiments, rotor114has three rotor blades102. Alternatively, rotor114has any number of rotor blades102that enables wind turbine generator100to function as described herein. In the exemplary embodiment, tower108is fabricated from tubular steel and has a cavity (not shown inFIG. 1) extending between supporting surface110and nacelle112. Alternatively, tower108is any tower that enables wind turbine generator100to function as described herein including, but not limited to, a lattice tower. The height of tower108is any value that enables wind turbine generator100to function as described herein.

Blades102are positioned about rotor hub116to facilitate rotating rotor114, thereby transferring kinetic energy from wind118into usable mechanical energy, and subsequently, electrical energy. Rotor114and nacelle112are rotated about tower108on a yaw axis120to control the perspective of blades102with respect to the direction of wind118. Blades102are mated to hub116by coupling a blade root portion122to hub116at a plurality of load transfer regions124. Load transfer regions124have a hub load transfer region and a blade load transfer region (both not shown inFIG. 1). Loads induced in blades102are transferred to hub116via load transfer regions124. Each of blades102also includes a blade tip portion126.

In the exemplary embodiment, blades102have a length between 50 meters (m) (164 feet (ft)) and 100 m (328 ft), however these parameters form no limitations to the instant disclosure. Alternatively, blades102may have any length that enables wind turbine generator100to function as described herein. As wind118strikes each of blades102, blade lift forces (not shown) are induced on each of blades102and rotation of rotor114about rotation axis128is induced as blade tip portions126are accelerated. A pitch angle (not shown) of blades102, i.e., an angle that determines each of blades'102perspective with respect to the direction of wind118, may be changed by a pitch adjustment mechanism (not shown inFIG. 1). Specifically, increasing a pitch angle of blade102decreases a percentage of area130exposed to wind118and, conversely, decreasing a pitch angle of blade102increases a percentage of area130exposed to wind118. Blade lift forces are directly proportional to blade surface area130exposed to wind118. Rotor114, driven by blade lift forces, rotates a generator (not shown) coupled to rotor114and positioned within nacelle112. The generator converts mechanical motion of rotor114into electrical energy.

As speed of blade tip portion126increases, an amplitude (not shown) of acoustic emissions (not shown inFIG. 1) from blade102increases. Conversely, as the speed of blade tip portion126decreases, an amplitude of acoustic emissions from blades102decreases. Therefore, the amplitude of acoustic emissions from blades102has a known relationship to a rotational speed of blade tip portions126, typically increasing with a power of around 5/2 of the inflow velocity of wind118, and the amplitude of acoustic emissions from blades102has a known relationship to blade pitch angle. Blade pitch angle for blades102may be adjusted to reduce noise by reducing the speed of blade tip portion126and putting wind turbine generator100into an NRO mode. The acoustic emissions from blades102are mitigated by serrated portions106of trailing edge portion104and the at least one substantially acoustically absorbent material included in serrated portions106. This reduces the need for wind turbine generator100to be put into an NRO mode in order to comply with noise regulations.

FIG. 2is a schematic view of a portion of a wind turbine blade102that may be used with wind turbine generator100. Serrated portions106extend at least partially between a radially inboard portion202of blade102and a radially outboard portion204of blade102. Radially inboard portion202is located closer to nacelle112than radially outboard portion204. Radially outboard portion204includes tip portion126of blade102and radially inboard portion204includes root portion122of blade102.

In the exemplary embodiment, at least one serrated portion106terminates prior to tip portion126and prior to root portion122. In alternative embodiments, at least one serrated portion106extends from root portion122to tip portion126for substantially the length of blade102.

In some embodiments, at least one serrated portion106is entirely located within a distance of approximately thirty percent of the length of blade102from tip portion126toward load transfer region124, i.e., at least one serrated portion106extends within the radially outboard thirty percent of blade102. The remaining approximately seventy percent of blade102does not include any serrated portions106.

In alternative embodiments, at least one serrated portion106extends for a length less than the approximately radially outboard thirty percent of blade102and is located anywhere within the approximately radially outboard thirty percent of blade102. The remaining approximately seventy percent of blade102does not include any serrated portions106. In additional alternative embodiments, at least one serrated portion106is located within the approximately radially outboard forty percent of blade102as measured from the radially outboard termination of tip portion126. In still further alternative embodiments, at least one serrated portion106is located within the approximately radially outboard fifty percent of blade102as measured from the radially outboard termination of tip portion126. Similarly, in some alternative embodiments, rather than a precise approximately thirty, forty, and fifty percent of the length of blade102, at least one serrated portion106is located within the radially outboard portion of blade102in a range from approximately twenty percent to approximately forty percent of the length of blade102as measured from the radially outboard termination of tip portion126.

Also, similarly, in other alternative embodiments, at least one serrated portion106is located within the radially outboard portion of blade102in a range from approximately ten percent to approximately twenty percent of the length of blade102as measured from the radially outboard termination of tip portion126. In yet other alternative embodiments, serrated portions106are located elsewhere along blade102, e.g., and without limitation, serrated portions106extend substantially to load transfer region124of blade102.

In some embodiments, serrated portion106is continuous. In alternative embodiments serrated portion106is discontinuous and includes at least two discrete serrated portions106. Blade102is not serrated between discrete serrated portions106. For example, blade102includes a plurality of serrated portions106. Each serrated portion is located within a distance of thirty percent of the length of blade102from tip portion126. Alternatively, the plurality of serrated portions106are located at any position on blade102.

Serrated portion106includes a plurality of structures206extending from a base portion208to a tip portion210in a direction away from a leading edge portion212of blade102. In some embodiments, structures206have a substantially triangular shape. In alternative embodiments, structures206have other shapes that enable serrated portion106to function as described herein including, but not limited to, a triangle with a rounded or tip portion210, a trapezoid, a semicircle, a rectangle, and/or other shapes. In some embodiments, serrated portion106includes only a single repeating structure206. For example, serrated portion106includes only repeating triangular structures206. In alternative embodiments, serrated portion106includes a plurality of structures206having different shapes.

FIG. 3is a schematic perspective cutaway view, along section A-A (shown inFIG. 2) of wind turbine blade102. Blade102includes, e.g., is shaped to include, an airfoil300. Airfoil300has any shape that enables blade102to function as described herein. Trailing edge portion104includes serrated portion106and structures206thereof. Each structure206extends from base portion208to tip portion210in a direction away from leading edge portion212. In some embodiments, structures206are substantially three dimensional and include a non-nominal thickness302. In some embodiments, thickness302of structures206decreases between base portion208and tip portion210of structures206. For example, thickness302of structures206at base portion208is within the range of two millimeters (mm) to four mm. In some embodiments, structures206are shaped with thickness302such that structures206continue the shape of airfoil300. Structures206may function with the shape of airfoil300to provide some aerodynamic lift. In some embodiments, structures206do not contribute to the aerodynamic lift generated by airfoil300, but include a variable thickness302. Structures206may taper from thickness302at base portion208to tip portion210. In alternative embodiments, structures206may have a substantially unchanging thickness302. In some embodiments, structures206may have a substantially nominal thickness302.

The geometry of structures206and serrated portion106facilitates decreasing the acoustic emissions emanating from blade102by reducing coherent scattering of noise from trailing edge portion104. Compared to a straight-edged trailing edge portion (not shown inFIG. 3), blade102including serrated portion106reduces coherent scattering of acoustic emissions caused by boundary-layer turbulence interacting with trailing edge portion104. Serrated portion106reflects acoustic emissions such that the emissions are less coherent in comparison to a straight-edged trailing edge portion. This effect is caused by the geometry of serrated portion106including the geometry of structures206.

Serrated portion106includes at least one substantially acoustically absorbent material304. In some embodiments, serrated portion106includes only a single substantially acoustically absorbent material304. In alternative embodiments, serrated portion106includes a plurality of substantially acoustically absorbent materials304. Substantially acoustically absorbent material304facilitates increasing an acoustic absorption coefficient of blade102in comparison to a blade (not shown inFIG. 3) which does not include serrated portion106including at least one substantially acoustically absorbent material304. In some embodiments, substantially acoustically absorbent material304is a substantially porous material. In these embodiments, the pores of substantially acoustically absorbent material304function as sound absorbers which absorb at least a portion of the acoustic emissions caused by the boundary-layer turbulence interacting with trailing edge portion104. The substantially acoustically absorbent material304and the pores therein may also function as a turbulence damper which reduces turbulence in the boundary layer and reduces acoustic emissions caused by the boundary-layer turbulence interacting with trailing edge portion104in comparison to hard or non-acoustically absorbent materials. Serrated portion106reduces the coherent scattering strength of acoustic emissions from blade102through the geometry of serrated portion106and structures206and through the effect of substantially acoustically absorbent martial304, e.g., an acoustically absorbent material, on the reflected sound and on the turbulence activity in the boundary-layer.

Substantially acoustically absorbent material304is, or includes, any material that enables substantially acoustically absorbent material304to function as described herein. As described above, in at least some embodiments, for example, and without limitation, substantially acoustically absorbent material304may be, or include, a structural foam, a series of three dimensional structures including a plurality of pores defined therein formed by additive manufacturing, a self-supporting shell with an epoxy matrix, a micro-perforated metal, or other substantially acoustically absorbent material. In alternative embodiments, substantially acoustically absorbent material304is a fibrous material.

In some embodiments, blade102is fabricated with serrated portion106and/or structures206made of a single substantially acoustically absorbent material304. In such a case, substantially acoustically absorbent material304is self-supporting. No casing or supportive material is used. For example, substantially acoustically absorbent material304may be added to trailing edge portion104using adhesive, fasteners, or additive manufacturing. In some embodiments, blade102is manufactured with trailing edge portion104initially terminating at edge306. Serrated portion106including self-supporting substantially acoustically absorbent material304is coupled to blade102at edge306and forms part of trailing edge portion104. In alternative embodiments, any other suitable manufacturing techniques for producing blade102having trailing edge portion104including self-supporting substantially acoustically absorbent material304are used.

FIG. 4is a schematic cross-sectional view, detail view A, of an exemplary alternative embodiment of serrated portion106shown inFIG. 3. In some alternative embodiments, serrated portion106includes a single substantially acoustically absorbent material304encased with a casing material402. Casing material402is configured to substantially allow acoustic emissions from blade102to pass through casing material402. Casing material402is made of a material which has a substantially high acoustic transmission coefficient. For example and without limitation, casing material402is or includes poly-paraphenylene terephthalamide or other suitable materials with a substantially high acoustic transmission coefficient. Additionally or alternatively, casing material402has a geometry or other characteristics which allow for the transmission of acoustic emissions through casing material402. For example and without limitation, casing material402is perforated plate made of metal, plastic, or other suitable materials.

Casing material402supports and/or protect substantially acoustically absorbent material304encased within casing material402. In some embodiments, substantially acoustically absorbent material304is not self-supporting. In such a case, casing material402supports substantially acoustically absorbent material304. Casing material402allows at least a portion of acoustic emissions from blade102to pass through casing material402. The acoustic emissions passing through casing material402are at least partially absorbed, dampened, or otherwise mitigated by substantially acoustically absorbent material304within casing material402.

In some embodiments, blade102is fabricated with casing material402and substantially acoustically absorbent material304integral to blade102. Casing material402and/or substantially acoustically absorbent material304are not added after fabrication of remainder of blade102. For example and without limitation, blade102is fabricated with substantially acoustically absorbent material304, e.g., foam, forming a core which is encased with a skin of casing material402, e.g., poly-paraphenylene terephthalamide. Alternatively, substantially acoustically absorbent material304, e.g., foam, is encased in casing material402, e.g., fiberglass, which forms the skin of blade102. Casing material402, e.g., the fiberglass skin, is then perforated.

In an alternative embodiment, blade102is fabricated with casing material and substantially acoustically absorbent material304being added to blade102to complete trailing edge portion104. For example and without limitation, serrated portion106is made by encapsulating substantially acoustically absorbent material304, e.g., foam, in an aerodynamically smooth envelop forming casing material402, e.g., perforated plate. Serrated portion106is then coupled, e.g., using an adhesive or fasteners, to blade102to complete trailing edge portion104. Alternatively, substantially acoustically absorbent material304and/or casing material402is added to blade102to complete trailing edge portion104using one or more additive manufacturing techniques.

In some embodiments, serrated portion106is fabricated by generating a composite of multiple plies of substantially acoustically absorbent material304. The composite is cut, e.g., water jet cut, to form structures206and serrated portion106. In some embodiments, serrated portion106is fabricated by molding a polymer. This allows for serration portions106of varying thicknesses302to be fabricated to match the thickness of trailing edge portion104. In some embodiments, serration portion106is formed from structural foam. For example and without limitation, serration portion106is formed from an open cell foam substantially acoustically absorbent material304such as foamed polymethylmethacrylate (PMMA) or foamed polyvinylchloride. Serrations portion106, including structures206, are formed by hot pressing the foam into a contour and/or by cutting with a water jet. In some embodiments, serration portion106is formed by water cutting a substantially acoustically absorbent material such as polymethacrylimide. The flow resistivity of an open cell foam typically ranges from 10-250 kiloRayleighs per meter (krayl/m). Structural polymer foams tend to be at the low end of the range as they are designed for minimal epoxy uptake. Nonstructural acoustic foams (as in anechoic chambers) are at the high end of the range as they have large pores and are made from different polymeric materials. A dense structural polymer foam would typically be in the lower part of that range. A dense structural polymer foam typically gives an acoustic absorption coefficient within a range of approximately 0.1 and approximately 0.5 for acoustic emissions starting at approximately one kiloHertz (kHz) when included in serration portion106having a thickness302within a range of approximately two millimeters (mm) and approximately four mm. Other materials and configurations as described herein may result in varying acoustic absorption coefficients.

In some embodiments, substantially acoustically absorbent material304and serration portion106are formed by partially impregnating layers of material with a resin or to hot press layers of material together with a thermoplastic weave in between layers. This results in substantially acoustically absorbent material304with characteristics that allow air to travel through the material. In some alternative embodiments, substantially acoustically absorbent material304and/or serration portion106is formed as a three dimensional weave. Composites are woven into complete three dimensional shapes which results in a substantially acoustically absorbent material304.

In some embodiments, clogging of substantially acoustically absorbent material304is mitigated by a combination of pore size and placement location on blade102. Pores of substantially acoustically absorbent material304may be susceptible to clogging by airborne particles or debris including, but not limited to, hail, snow, insect debris, dust, soot aggregates, sand, sea spray, ice crystals, pollen, hair, and large bacteria. In some embodiments, substantially acoustically absorbent material304is chosen to include substantially small pores which resist clogging. In some embodiments, serrated portion106is placed near tip portion126of blade102. This results in an angular velocity of substantially acoustically absorbent material304which resists clogging of the pores. In still further embodiments, turbulence at trailing edge portion104prevents clogging of the pores.

FIG. 5is a schematic cross-sectional view of one embodiment of serrated portion106including a plurality of substantially acoustically absorbent materials304. A first substantially acoustically absorbent material502, a second substantially acoustically absorbent material504, and a third substantially acoustically absorbent material506are encased within casing material402. First substantially acoustically absorbent material502, second substantially acoustically absorbent material504, and third substantially acoustically absorbent material506are oriented vertically, relative to a low pressure side of airfoil300(not shown inFIG. 5), within casing material402. In an alternative embodiment, first substantially acoustically absorbent material502, second substantially acoustically absorbent material504, and third substantially acoustically absorbent material506are oriented horizontally, relative to a low pressure side of airfoil300(not shown inFIG. 5), within casing material402. First substantially acoustically absorbent material502at least partially overlaps second substantially acoustically absorbent material504. Second substantially acoustically absorbent material504at least partially overlaps third substantially acoustically absorbent material506.

FIG. 6is a schematic cross-sectional view of another embodiment of serrated portion106including a plurality of substantially acoustically absorbent materials304. First substantially acoustically absorbent material502, second substantially acoustically absorbent material504, and third substantially acoustically absorbent material506are layered longitudinally and extend radially along blade102within serrated portion106. First substantially acoustically absorbent material502, second substantially acoustically absorbent material504, and third substantially acoustically absorbent material506are encased within casing material402. First substantially acoustically absorbent material502, second substantially acoustically absorbent material504, and third substantially acoustically absorbent material506substantially follow the contours of casing material402.

Referring now toFIGS. 5 and 6, in some embodiments, serrated portion106includes a plurality of substantially acoustically absorbent materials304. In some embodiments, the plurality of substantially acoustically absorbent materials304includes materials of different types. In alternative embodiments, the plurality of substantially acoustically absorbent materials304include sections of a single substantially acoustically absorbent material304with each section having a different orientation. In some embodiments, the plurality of substantially acoustically absorbent materials includes three substantially acoustically absorbent materials304. In alternative embodiments, the plurality of substantially acoustically absorbent materials304includes, for example and without limitation, two, four, five, or more substantially acoustically absorbent materials.

Referring now toFIGS. 3-6, in some embodiments, serrated portion106forms a portion of a lightning protection system (not shown). Serrated portion106is at least partially metallic such that serrated portion106is electrically conductive. In some embodiments, substantially acoustically absorbent material304is at least partially metallic. For example and without limitation, substantially acoustically absorbent material304is micro-perforated metal. In some embodiments, casing material402is at least partially metallic. For example and without limitation, casing material402is perforated plate made of metal. In some embodiments, casing material402is at least partially metallic and substantially acoustically absorbent material304is not metallic. In some embodiments, substantially acoustically absorbent material304is at least partially metallic and casing material402is not metallic. In further embodiments, both substantially acoustically absorbent material304and casing material402are at least partially metallic. Serrated portion106is coupled to a lighting protection system such that the at least partially metallic portion of serrated portion106is coupled to the lighting protection system. The combination of at least partially metallic serrated portion106and the lighting protection system may shield components of wind turbine generator100, including but not limited to a generator, electricity transmission equipment, and control equipment, from lighting which strikes wind turbine generator100.

Referring now toFIGS. 7-10, each discussed individually below, an un-serrated wind turbine blade702is retrofitted to include serrated portions106and at least one substantially acoustically absorbent material304using a retrofit system902(e.g., a wind turbine noise abatement device). This is in contrast to fabricating a blade102which includes serrated portions106and at least one substantially acoustically absorbent material304. After retrofitting, un-serrated wind turbine blade702includes serrated portion106and at least one substantially acoustically absorbent material304as described herein with respect to blade102. In some embodiments, retrofit system902includes a mounting structure1002for coupling retrofit system902to trailing edge portion104of un-serrated wind turbine blade702. Retrofit system902also includes at least one serrated portion106extending at least partially along mounting structure1002. Serrated portion106includes at least one substantially acoustically absorbent material304. Serrated portion106includes structures206as described herein. In some embodiments, serrated portion106and/or retrofit system902includes additional components and/or features described herein including, without limitation, one or more of a plurality of layered substantially acoustically absorbent materials304(not shown), casing material402, a lighting protection system, adhesive, and fasters.

FIG. 7is a schematic perspective cutaway view of an exemplary un-serrated wind turbine blade702. Un-serrated wind turbine blade702includes leading edge portion212and trailing edge portion104. During retrofitting, a portion704of trailing edge portion104is removed from un-serrated wind turbine blade702. For example, portion704of trailing edge portion104is cut from trailing edge portion104. This prepares trailing edge portion104to receive retrofit system902.

FIG. 8is a schematic perspective cutaway view of un-serrated wind turbine blade702with portion704(shown inFIG. 7) of trailing edge portion104removed. The removal of portion704of trailing edge portion104creates mounting surface802. In some embodiments, mounting surface802is substantially vertical relative to a high pressure side of un-serrated wind turbine blade702. The portion of trailing edge portion104removed is substantially equal in length, extending radially along un-serrated wind turbine blade702, as serrated portion106to be added to un-serrated wind turbine blade702. Portion704of trailing edge portion104removed is cut from trailing edge portion104, or otherwise removed, at the location along the radial length of un-serrated wind turbine blade702at which serrated portion106is desired to be added. For example, portion704of trailing edge portion104may be removed from un-serrated wind turbine blade702near tip portion126(not shown inFIG. 8) such that serrated portion106will be added to un-serrated wind turbine blade702within a distance of thirty percent of the length of un-serrated wind turbine blade702from tip portion126, e.g., serrated portions106will be within the radially outboard thirty percent of un-serrated wind turbine blade702. As described herein, serrated portion106is located elsewhere relative to un-serrated wind turbine blade702in alternative embodiments. In some embodiments, mounting surface802is prepared to facilitate attachment of retrofit system902. For example and without limitation, mounting surface802is planed, sanded, cleaned, or otherwise prepared.

FIG. 9is a schematic perspective cutaway view of un-serrated wind turbine blade702shown inFIG. 7after having been retrofit with retrofit system902. Retrofit system902is attached to mounting surface802. In some embodiments, mounting structure1002(shown inFIG. 10and discussed further below) is coupled to mounting surface802. For example, mounting structure1002and/or mounting surface802may be treated with an adhesive. Mounting structure1002and mounting surface802are brought together and coupled together via the adhesive. In alternative embodiments, mounting structure1002and mounting surface802are coupled together using other techniques and/or components including, but not limited to, fasteners, welding, or other joining techniques. The resulting wind turbine blade102includes a portion of un-serrated wind turbine blade702and retrofit system902.

After retrofitting with retrofit system902, the resulting wind turbine blade102has an increased acoustic absorption coefficient relative to the unmodified un-serrated wind turbine blade702due to the inclusion of substantially acoustically absorbent material304. Additionally, the resulting wind turbine blade102has reduced coherent scattering of acoustic emissions in relative to the unmodified un-serrated wind turbine blade702due to the geometry of serrated portion106and structures206.

FIG. 10is a schematic perspective view of an alternative retrofit system902for adding serrated portion106to trailing edge104of un-serrated wind turbine blade702. This alternative retrofit system902is used to retrofit un-serrated wind turbine blade702without removing portion704of trailing edge portion104as shown inFIGS. 7-9. Retrofit system902includes mounting structure1002. Mounting structure is a substantially rectangular plate1004. Serrated portion106extends laterally from rectangular plate1004and includes structures106. Serrated portion106and structures206include at least one substantially acoustically absorbent material304(shown inFIGS. 3-6 and 9). Substantially rectangular plate1004includes an attachment surface1006. Attachment surface1006is attached to trailing edge portion104of un-serrated wind turbine blade702. A portion of trailing edge portion104is not removed prior to attaching retrofit system902. Rather, attachment surface1006is coupled to a low pressure side of un-serrated wind turbine blade702at trailing edge portion104. Alternatively, attachment surface1006is coupled to a high pressure side of un-serrated wind turbine blade702at trailing edge portion104. Attachment surface1006may be coupled to un-serrated wind turbine blade702using techniques and/or components including, but not limited to, adhesives, fasteners, welding, or other joining techniques. Retrofit system902including serration portion106is fabricated as described herein with respect to serration portion106.

FIG. 11is an exemplary graphical view, i.e., graph1100of a comparison between the normalized noise directivities of a wind turbine blade having a hard, un-serrated trailing edge and a wind turbine blade having an acoustically absorbent, un-serrated trailing edge. The product of the acoustic wave number and the chord of the wind turbine blade is 5. The trailing edge of each wind turbine blade is centered at origin1106. Graph1100includes an x-axis1108representative of normalized acoustic emissions extending horizontally from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. Graph1100includes a y-axis1110representative of normalized acoustic emissions extending vertically from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. First plot1102corresponds to noise generated by a wind turbine blade having an acoustically absorbent, un-serrated trailing edge. Second plot1104corresponds to noise generated by a wind turbine blade having a hard, un-serrated trailing edge. The direction in which the noise from either wind turbine blade is at the highest value is indicated with a value of 1.0. In this case, noise generated by a wind turbine blade having a hard, un-serrated trailing edge is greatest at point1112and point1114of plot1104. Point1114corresponds to noise emitted above and forward from the trailing edge located at origin1106. Point1112corresponds to noise emitted below and forward from the trailing edge located at origin1106.

The inclusion of at least one substantially acoustically absorbent material304in a wind turbine blade results in a reduction of the noise generated as shown by comparing first plot1102to second plot1104. The reduction in noise generated due to substantially acoustically absorbent material304is shown as gap1116between first plot1102and second plot1104, first plot1102generally having lesser normalized values of noise emission. The reduction in noise varies based on directivity. For example, the inclusion of substantially acoustically absorbent material304results in a greater reduction in noise forward of the trailing edge located at origin1106than rearward of the trailing edge.

FIG. 12is an exemplary graphical view, i.e., graph1200of a comparison between the normalized noise directivities of a wind turbine blade having a hard, un-serrated trailing edge and a wind turbine blade having an acoustically absorbent, un-serrated trailing edge. The product of the acoustic wave number and the chord of the wind turbine blade is 18. The trailing edge of each wind turbine blade is centered at origin1106. Graph1200includes an x-axis1108representative of normalized acoustic emissions extending horizontally from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. Graph1200includes a y-axis1110representative of normalized acoustic emissions extending vertically from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. First plot1102corresponds to noise generated by a wind turbine blade having an acoustically absorbent, un-serrated trailing edge. Second plot1104corresponds to noise generated by a wind turbine blade having a hard, un-serrated trailing edge. The direction in which the noise from either wind turbine blade is at the highest value is indicated with a value of 1.0. In this case, i.e., where the product of the acoustic wave number and the chord of the wind turbine blade is 18, noise generated by a wind turbine blade having a hard, un-serrated trailing edge is greatest at point1212and point1214of plot1104. Point1214corresponds to noise emitted above and forward from the trailing edge located at origin1106. Point1212corresponds to noise emitted below and forward from the trailing edge located at origin1106.

At higher frequencies, in this case where the product of the acoustic wave number and the chord of the wind turbine blade is 18, the inclusion of at least one substantially acoustically absorbent material304results in a reduction of the noise generated as shown by comparing first plot1102to second plot1104. The reduction in noise generated due to substantially acoustically absorbent material304is shown as gap1216between first plot1102and second plot1104, first plot1102generally having lesser normalized values of noise emission. The reduction in noise varies based on directivity. For example, the inclusion of substantially acoustically absorbent material304results in a greater reduction in noise forward of the trailing edge located at origin1106than rearward of the trailing edge. Comparing graph1100(shown inFIG. 11) of noise emissions for a wind turbine blade having a product of acoustic wave number and chord length of 5 to graph1200(shown inFIG. 12), the inclusion of at least one substantially acoustically absorbent material304results in a greater mitigation of noise produced by the wind turbine blade at higher frequencies. Gap1216is generally larger than gap1116.

FIG. 13is an exemplary graphical view, i.e., graph1300of a comparison between the normalized noise of a wind turbine blade having a hard, serrated trailing edge and wind turbine blade102(shown inFIGS. 1-4, 6 and 9) having an acoustically absorbent, serrated trailing edge. The product of the acoustic wave number and the chord of the wind turbine blade is 5. The trailing edge of each wind turbine blade is centered at origin1106. Graph1300includes an x-axis1108representative of normalized acoustic emissions extending horizontally from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. Graph1300includes a y-axis1110representative of normalized acoustic emissions extending vertically from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. Third plot1302corresponds to noise generated by wind turbine blade102having an acoustically absorbent, serrated trailing edge. Fourth plot1304corresponds to noise generated by a wind turbine blade having a hard, serrated trailing edge. The direction in which the noise from either wind turbine blade is at the highest value is indicated with a value of 1.0. In this case, noise generated by a wind turbine blade having a hard, serrated trailing edge is greatest at point1312and point1114of plot1304. Point1314corresponds to noise emitted above and forward from the trailing edge located at origin1106. Point1312corresponds to noise emitted below and forward from the trailing edge located at origin1106.

The inclusion of at least one substantially acoustically absorbent material304and serrated portion106results in a reduction in noise generated in comparison to the inclusion of serrated portion106alone as shown by comparing third plot1302to fourth plot1304. The reduction in noise generated due to substantially acoustically absorbent material304and serrated portion106is shown as gap1316between third plot1302and fourth plot1304, third plot1302generally having lesser normalized values of noise emission. The reduction in noise varies based on directivity. For example, the inclusion of substantially acoustically absorbent material304and serrated portion106results in a greater reduction in noise forward of the trailing edge located at origin1106than rearward of the trailing edge. Additionally, the inclusion of at least one substantially acoustically absorbent material304and serrated portion106results in a reduction in noise generated in comparison to the inclusion of acoustically absorbent material304alone as shown by comparing third plot1302(shown inFIG. 13) to first plot1102(shown inFIG. 11). In the case of wind turbine blades having a product of the acoustic wave number and the chord length equal to 5, wind turbine blade102with serrated portion106and acoustically absorbent material304, represented by third plot1302(shown inFIG. 13) generates less noise than a wind turbine blade with an acoustically absorbent material alone, represented by first plot1102(shown inFIG. 11).

FIG. 14is an exemplary graphical view, i.e., graph1400of a comparison between the normalized noise of a wind turbine blade having a hard, serrated trailing edge and wind turbine blade102having an acoustically absorbent, serrated trailing edge. The product of the acoustic wave number and the chord of the wind turbine blade is 18. The trailing edge of each wind turbine blade is centered at origin1106. Graph1400includes an x-axis1108representative of normalized acoustic emissions extending horizontally from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. Graph1400includes a y-axis1110representative of normalized acoustic emissions extending vertically from the trailing edge of the wind turbine blade on a linear scale from a value of 0 to a value of 1. Third plot1302corresponds to noise generated by wind turbine blade102having an acoustically absorbent, serrated trailing edge. Fourth plot1304corresponds to noise generated by a wind turbine blade having a hard, serrated trailing edge. The direction in which the noise from either wind turbine blade is at the highest value is indicated with a value of 1.0. In this case, noise generated by a wind turbine blade having a hard, serrated trailing edge is greatest at point1412and point1414of plot1304. Point1414corresponds to noise emitted above and rearward from the trailing edge located at origin1106. Point1412corresponds to noise emitted below and rearward from the trailing edge located at origin1106.

At higher frequencies, in this case where the product of the acoustic wave number and the chord of the wind turbine blade is 18, the inclusion of serrated portion106and acoustically absorbent material304results in a reduction of the noise generated as shown by comparing third plot1302to fourth plot1304. The reduction in noise generated due to substantially acoustically absorbent material304and serrated portion106is shown as gap1416between third plot1302and fourth plot1304, third plot1302generally having lesser normalized values of noise emission. The reduction in noise varies based on directivity. For example, the inclusion of substantially acoustically absorbent material304and serrated portion106results in a greater reduction in noise rearward of the trailing edge located at origin1106than forward of the trailing edge. This is because the directivity of the noise levels with the higher amplitudes is rearward of the trailing edge at origin1106. Comparing graph1300(shown inFIG. 13) of noise emissions for wind turbine blades having a product of acoustic wave number and chord length equal to 5 to graph1400(shown inFIG. 14), the inclusion of at least one substantially acoustically absorbent material304and serrated portion106results in a greater mitigation of noise produced by the wind turbine blade at higher frequencies. Gap1416is generally larger than gap1316.

The inclusion of both serrated portion106and at least one acoustically absorbent material304in trailing edge portion104of wind turbine blade102results in a greater reduction in noise than the inclusion of serrated portion106alone or at least one acoustically absorbent material304alone. This effect is seen at least by comparingFIG. 14toFIG. 13. Comparing the noise emissions of a un-serrated, hard wind turbine blade, represented by second plot1104(shown inFIG. 12) to noise emissions of a serrated, hard wind turbine blade, represented by fourth plot1304(shown inFIG. 14), serrated portions106mitigate noise emissions alone. Similarly, Comparing the noise emissions of a un-serrated, hard wind turbine blade, represented by second plot1104(shown inFIG. 12) to noise emissions of a un-serrated, acoustically absorbent wind turbine blade, represented by first plot1102(shown inFIG. 12), acoustically absorbent material304mitigates noise emissions alone. However, the combination of serrated portions106and acoustically absorbent material304has a greater cumulative mitigation of noise emissions than either alone. This effect is shown in comparing wind turbine blade102having serrated portions106and acoustically absorbent material304, represented by third plot1302(shown inFIG. 14) to an un-serrated, hard wind turbine blade, represented by second plot1104(shown inFIG. 12). The reduction in noise emitted by wind turbine blade102having serrated portion106and acoustically absorbent material304is greater than the mitigation in noise achieved by a wind turbine blade with serrated portion106alone or a wind turbine blade with acoustically absorbent material304alone.

Referring now toFIGS. 11-14, there is a directivity dependent reduction in noise due to the inclusion of at least one acoustically absorbent material304in trailing edge portion104. The inclusion of at least one acoustically absorbent material304in trailing edge portion results in a more significant reduction of noise when paired with serrated portion106in comparison to a straight trailing edge portion104. The reduction in noise achieved by inclusion of serrated portion106and/or substantially acoustically absorbent material304is greater at higher frequencies. The greatest reduction in noise occurs at the loudest part of the directivity.

The above described wind turbine blade and retrofit system provide for reduction in noise generated by wind turbine blades. Specifically, the turbine blade and retrofit system include serrations fabricated from an acoustically absorbent material to mitigate noise produced by the wind turbine blade. Boundary-layer turbulence interaction with the trailing edge of the wind turbine blade, while in motion, is a primary source of aerodynamic noise emanating from wind turbine blades in operation. The serrated portions included in a trailing edge portion of the wind turbine blade reduce coherent scattering of the noise emanating from the wind turbine blade. Reducing coherent scattering of the quadrupole sources mitigates the noise emanating from the wind turbine blade while in operation. The substantially acoustically absorbent materials function as sound absorbers and/or turbulence dampers. The acoustically absorbent materials reduce a magnitude of the sound reflected from the wind turbine blade in comparison to wind turbine blades having hard surfaces and/or materials. The acoustically absorbent materials thus mitigate noise emanating from the wind turbine blade while in operation.

Moreover, the reduction in noise emanating from the wind turbine blades reduces the need for the wind turbine to be put into NRO mode to comply with a dB level that may approach local regulatory levels. The reduction in NRO increases the AEP of the wind turbine. Therefore, the serrated portions and at least one acoustically absorbent material included in the trailing edge portion of the wind turbine blade enhance power generation of wind turbine generators.

An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) reduced coherent scattering of noise emanating from a wind turbine blade through the use of serrated portions included in the trailing edge portion of the blade; (b) absorption of noise emanating from a wind turbine blade through the use of at least one acoustically absorbent material included in the trailing edge portion of the blade; (c) mitigation of noise emanating from a wind turbine blade through the combination of both serrated portions and at least one substantially acoustically absorbent material; (d) increased AEP by reducing acoustic emissions and reducing the need for NRO; and (e) reduced noise emanating from existing turbine blades through modification of the existing turbine blades with a retrofit system to include serrated portions and at least one substantially acoustically absorbent material.

Exemplary embodiments of methods, systems, and apparatus for reducing acoustic emissions of wind turbine blades are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the systems and methods may also be used in addition to other systems and methods for reducing noise emissions of wind turbine blades such as blade pitch control systems, blade speed control systems, and the like, and are not limited to practice with only the systems and methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in addition to many other applications, equipment, and systems that reduce wind turbine blade acoustic emissions.