Reducing noise emitted from wind turbines

Abstract

A wind turbine (1) having a noise reduction apparatus (20, 40, 50) comprising at least one noise reduction channel (22). The at least one noise reduction channel (22) is provided by an acoustically reflective base surface (24), an acoustically reflective first sidewall (26), an acoustically reflective second sidewall (28), and at least one acoustically absorptive transverse wall (30). The first and second acoustically reflective sidewalls (26, 28) extend from the base surface (24). The at least one acoustically absorptive transverse wall (30) extends from at least the first acoustically reflective side wall (26). A depth of the noise reduction channel (22) taken in the direction from the acoustically reflective base surface (24) to the transverse wall (30) is selected to serve as a quarter- wave resonator for at least one predetermined acoustic tonality.

Description

[0001] Reducing Noise Emitted from Wind Turbines

[0002] Technical Field

[0003] This disclosure generally relates to reducing noise in wind turbines, and particularly to reducing acoustic noise generated by wind turbines and emitted to the external environment.

[0004] Background

[0005] Wind turbines typically comprise a rotor having one or more blades extending radially from a rotor hub. The rotor hub is disposed at the front of a nacelle which is mounted at the top of a support tower secured to a foundation embedded in the ground. The nacelle contains a generator system configured to produce electrical energy from a flow of wind passing over the surface of the blades, causing the rotor hub to rotate. The generator system contains an electrical generator which is driven by the rotor hub via a main shaft and a gearbox that provides an increase in speed between the rotor hub and a generator rotor of the electrical generator. Electrical power that is generated by the electrical generator is controlled and regulated by a suitable power converter system that is coupled to an external power export connection.

[0006] As power is generated, acoustic noise is generated across a broad spectrum of frequencies by a number of different components or systems within the wind turbine, such as the movement of rotor hub and main shaft, the interaction between the gears inside the gearbox, and other operational systems within the nacelle and the wind turbine. The generated acoustic noise may have a large associated noise power due to the large size of the components, the magnitude of the power generated, the speed at which the wind turbine is being driven, and so on. In some circumstances, it may be desirable to reduce the level of noise that is emitted from the wind turbine to the external environment, particularly at certain acoustic frequencies or tonalities.

[0007] Some known prior art dampening systems require acoustic coupling between adjacent structures. Acoustic coupling allows for ‘acoustics’ or sound waves to ‘couple’ or interact between 2 or more areas / volumes. If something cuts off the transmission such that there is a non-transmitting wall or the interaction is blocked, then there is no acoustic coupling.

[0008] In order for such prior art dampening systems to function, the placement and coupling characteristics need to be finely detailed to get the desired effect of acoustic attenuation. One object of the present design is to make a solution robust and indifferent to the need for any need of acoustic coupling.

[0009] It is against this background that the invention has been devised.

[0010] Summary of the Invention

[0011] According to an aspect of the invention, there is provided a wind turbine having a noise reduction apparatus comprising at least one noise reduction channel. The at least one noise reduction channel is provided by an acoustically reflective base surface, an acoustically reflective first sidewall, an acoustically reflective second sidewall, and at least one acoustically absorptive transverse wall. The first and second acoustically reflective sidewalls extend from the base surface. The at least one acoustically absorptive transverse wall extends from at least the first acoustically reflective side wall. The depth of the noise reduction channel taken in the direction from the acoustically reflective base surface to the transverse wall is selected such that the noise reduction channel serves as a quarter-wave resonator for one or more predetermined acoustic frequencies or tonalities.

[0012] The at least one noise reduction channel is capable of causing like-tonalities within the acoustic noise to destructively interfere, thereby reducing the noise power of specific tonalities within the acoustic noise. The noise reduction apparatus therefore advantageously targets particular tonalities generated by and emitted from the wind turbine. Further, the acoustically absorptive transverse wall allows the noise reduction apparatus to absorbing incident acoustic noise, thereby reducing the overall noise power of the acoustic noise, including any targeted tonalities.

[0013] In some embodiments, the first and second acoustically reflective sidewalls either side of the noise reduction channel may be substantially parallel to each other or may be non-parallel or may diverge from one another when considered in a direction from the base surface towards an open end of the noise reduction channel. Incident acoustic noise can thus be advantageously reflected in a specific manner to reduce the noise level of particular tonalities by destructive interference.

[0014] In some embodiments, the first and second acoustically reflective sidewalls may be associated with a substrate. The substrate may provide or be integrated with a formation or underlying structure for at least one of the base surface, the first and second sidewalls, and the transverse wall to be mounted to.

[0015] In some embodiments, the substrate may be acoustically absorptive. The level of acoustic absorption can therefore be chosen separately to the manufacturing of the noise reduction channel.

[0016] In some embodiments, the substrate may provide the acoustically absorptive transverse wall. This enables the noise reduction apparatus to be manufactured in a simple manner.

[0017] In some embodiments, the substrate may be acoustically reflective. The level of acoustic reflectivity can therefore be chosen separately to the manufacturing of the noise reduction channel.

[0018] In some embodiments, the at least one acoustically absorptive transverse wall may extend away from the noise reduction channel. Acoustic noise may therefore enter the noise reduction channel in an unobstructed manner.

[0019] In some embodiments, the at least one acoustically absorptive transverse wall may extend at least partially over the noise reduction channel. The amount of noise absorbed may therefore be increased while also increasing the number of reflections within the noise reduction channel.

[0020] In some embodiments, the first and second acoustically reflective sidewalls may be continuous with at least one of the acoustically reflective base surface and the at least one acoustically absorptive transverse wall. The noise reduction apparatus may therefore be manufactured in a simple and efficient manner.

[0021] In some embodiments, the noise reduction channel may be provided on at least one surface of a base panel. For example, the base panel may be provided by a surface of the wind turbine. The noise reduction apparatus may therefore be manufactured and installed in the wind turbine in a simple and efficient manner.

[0022] Alternatively, the noise reduction channel may be provided on one or more surfaces of the base panel. For example, the noise reduction channel may be provided on opposing surfaces of the base panel. The level of noise reduced by the noise reduction apparatus may therefore be increased when installed within the wind turbine. In some embodiments, there may be a plurality of noise reduction channels. In this sense, therefore, there may be a plurality of noise reduction channels arranged in a substantially parallel array, although a parallel arrangement is not essential. In other examples, however, there may be a single noise reduction channel that extends in a meandering path, for example in a spiral configuration or a ‘back and forth’ configuration.

[0023] In some embodiments, the plurality of noise reduction channels may have the same or different depths. This enables a plurality of tonalities to be targeted, increasing the effectiveness of the noise reduction apparatus.

[0024] In some embodiments, there may be a plurality of acoustically absorptive transverse walls. The simplicity and efficiency of manufacturing the noise reduction apparatus may therefore be increased.

[0025] In some embodiments, the acoustically reflective base surface may be provided by at least one of a surface of the wind turbine, at least one acoustically reflective element on a surface of the wind turbine, and a surface of the base panel.

[0026] According to another aspect of the invention, there is provided a noise reduction apparatus for reducing noise in a wind turbine emitting acoustic noise. The noise reduction apparatus comprises at least one formation protruding from a plane in a first direction and at least one noise reduction channel. The noise reduction channel is provided by a first acoustically reflective sidewall of the at least one formation, an acoustically reflective surface on the plane, and a second acoustically reflective sidewall of the at least one formation.

[0027] In some embodiments, the at least one formation protrudes by a first distance in the first direction from the plane corresponding to an odd multiple of a quarter of a wavelength of one or more acoustic frequencies comprised in the acoustic noise. The noise reduction channel may therefore advantageously act as a quarter-wave resonator alone or in combination with characteristics of a Helmholtz resonator.

[0028] In some embodiments, the at least one formation may be associated with at least one acoustically absorptive transverse wall. For example, the at least one formation may be provided by an acoustically absorptive structure or material. In some embodiments, the apparatus may be mounted on or to a surface of the wind turbine. For example, the acoustically reflective surface on the plane may be a surface of the wind turbine.

[0029] In some embodiments, the apparatus may comprise at least one of a plurality of formations and a plurality of noise reduction channels. For example, the plurality of formations may be separated by the plurality of noise reduction channels in an alternating manner such that each formation is adjacent to at least one noise reduction channel.

[0030] According to another aspect of the invention, there is provided a noise reduction structure for reducing noise in a wind turbine emitting acoustic noise. The noise reduction structure comprises one or more noise reduction apparatuses or baffles, wherein each baffle comprises at least one noise reduction channel. The at least one noise reduction channel is provided by an acoustically reflective base surface, an acoustically reflective first sidewall, an acoustically reflective second sidewall, and at least one acoustically absorptive transverse wall. The first and second acoustically reflective sidewalls extend from the base surface. The at least one acoustically absorptive transverse wall extends from at least the first acoustically reflective side wall.

[0031] In some embodiments, the noise reduction structure comprises an inlet and an outlet for providing a flow path through the noise reduction structure. The noise reduction structure is capable of being installed in problematic areas of the wind turbine where a flow path from an internal environment of the wind turbine to an external environment is required.

[0032] In some embodiments, one or more baffles may be disposed substantially perpendicular to the flow path. Acoustic noise travelling along the flow path will therefore be obstructed by the one or more baffles and be absorbed.

[0033] In some embodiments, the baffles may be modular components of the noise reduction structure or be integrated with the noise reduction structure. The manufacturing and assembly of the noise reduction structure is therefore simplified.

[0034] It will be appreciated that preferred and / or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also. Brief Description of the Drawings

[0035] So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the following drawings, in which like features are assigned like reference numerals, and in which:

[0036] Figure 1 is a schematic diagram of a wind turbine in which embodiments of the invention may be implemented;

[0037] Figure 2 is a simplified schematic diagram of a nacelle of the wind turbine of Figure 1;

[0038] Figures 3a and 3b show an example implementation of a noise reduction apparatus in accordance with the invention from different viewing perspectives;

[0039] Figures 4a to 4e are cross-sectional views of different examples of the noise reduction apparatus;

[0040] Figures 5a to 5d are plan views of different configurations of the noise reduction apparatus;

[0041] Figures 6a and 6b are views of another example implementation of the noise reduction apparatus from different viewing perspectives;

[0042] Figure 7 shows another example implementation of noise reduction apparatus;

[0043] Figure 8 shows a detailed view of area ‘B’ of the noise reduction apparatus of Figure 7;

[0044] Figure 9 shows a detailed view of area ‘C’ shown in area B of Figure 8; and

[0045] Figure 10 shows a graphical representation of different acoustic frequency spectrums in the wind turbine.

[0046] Detailed Description

[0047] Embodiments of the invention are directed to a reducing an overall noise power of acoustic noise generated by and emitted from a wind turbine as well as particular frequencies, or tonalities, within the acoustic noise. In particular, the noise reduction apparatus may have a broad sound reduction index in the frequency domain and may be capable of reducing the overall noise level as a frequency broadband reduction. In addition, the noise reduction apparatus may be capable of targeting particular tonalities within the overall noise. The examples of the invention described herein therefore may provide an advantage in reducing the acoustic noise that would otherwise be emitted from the wind turbine to the external environment, particularly at certain tonalities that may still be dominant after a frequency broadband reduction.

[0048] The proposed approach involves implementing a noise reduction apparatus within the wind turbine. At least one noise reduction channel reflects incident acoustic noise off of associated acoustically reflective surfaces. For example, a depth of the noise reduction channel may be selected such that the noise reduction channel serves as a quarter-wave resonator for one or more predetermined acoustic tonalities. Destructive interference between reflected acoustic noise and incoming acoustic noise may therefore occur, thereby reducing particular targeted tonalities within the acoustic noise. At least one acoustic absorbative surface provided by the noise reduction apparatus may also absorb incident acoustic noise to reduce the overall level of noise, including the particular targeted tonalities.

[0049] The configuration of the noise reduction apparatus may be modified to alter the absorptive and reflective acoustic characteristics of the noise reduction apparatus. The depth of the noise reduction channel may be adjusted to target different tonalities, and the surface area of the absorptive surface may be increased or decreased depending on the requirements of the noise reduction apparatus.

[0050] The noise reduction apparatus may be provided on at least one internal surface within the wind turbine, thereby enabling the reduction of the acoustic noise to occur before it is emitted to the external environment. For example, the noise reduction apparatus may be provided in problematic areas of the wind turbine that have poor acoustic absorptive properties and may typically allow acoustic noise to propagate from the wind turbine to the external environment in a relatively unobstructed manner.

[0051] Specific examples of a noise reduction apparatus in accordance with embodiments of the invention are described herein. First, to provide context for the invention, an individual wind turbine 1 in which the noise reduction apparatus may be implemented in is described with reference to Figure 1. The example shown is based on a full-scale architecture, but it should be appreciated that the wind turbine 1 is referred to here by way of example only, and it would be possible to implement embodiments of the invention into many different types of wind turbine systems and power plant architectures. It is noted that the topology illustrated in Figure 1 is entirely illustrative, and many architectures are known that would be suitable for use with embodiments of the invention.

[0052] The wind turbine 1 shown in Figure 1 is a three-bladed upwind horizontal-axis wind turbine (HAWT), which is the most common type of turbine in use. The wind turbine 1 comprises a rotor 2 having three blades 3 extending radially from, and equi-angularly spaced around, a rotor hub 4. It is noted that although three blades are common, different numbers of blades may be used in alternative implementations. The rotor 2 is supported by its rotor hub 4 at the front of a nacelle 5, which is mounted at the top of a support tower 7 that is secured to a foundation (not shown) embedded in the ground.

[0053] Figure 2 shows a simplified schematic of a generator system 10 within the nacelle 5 and configured to produce electrical energy from a flow of wind passing over the surface of the blades 3, causing the rotor hub 4 to rotate. The nacelle 5 contains an electrical generator 12 which is driven by the rotor hub 4 via a main shaft 14 and a gearbox 16 that provides an increase in speed between the rotor hub 4 and a generator rotor (not shown) of the generator 12 to produce electrical energy. It will be appreciated that the nacelle 5 and generator system 10 may contain additional systems and components in accordance with typical wind turbine systems, omitted in Figure 2 for simplicity.

[0054] As power is generated, the generator system 10 and each of the associated components will generate acoustic noise with associated noise powers across a broad spectrum of frequencies or tonalities. The resulting acoustic noise may subsequently be emitted to an external environment outside of the nacelle 5 and the wind turbine 1. For example, the gearbox 16 will generate acoustic noise from gear interaction, with typically different frequencies arising from different gears mating. For planetary gear stages, different acoustic frequencies may be generated from sun-planet gear interaction and planet-ring gear interaction. For other following planetary gear stages, the rotational speed will be different and hence the frequencies generated will also be different. Acoustic noise may, of course, also be generated from other elements, such as the generator 12 or main shaft 14.

[0055] The nacelle 5 is enclosed by an external surface 6 which may absorb and / or reflect a small amount of the generated acoustic noise. The external surface 6 may comprise areas with relatively thin thicknesses or poor absorptive characteristics such that acoustic noise is able to propagate through the nacelle 5 and the external surface 6 to the external environment in a relatively unobstructed and unchanged manner. In addition, some wind turbines may be required to operate in challenging environments with suboptimal conductions, for example such as an offshore environment. These wind turbines may require additional systems to function. For example, the internal environment of the nacelle 5 may need to be closely controlled to ensure that adverse external environmental conditions, such as a large presence of salt in the air and a high level of humidity, do not effect the operation of the wind turbine 1 and the generation of electrical power. The nacelle 5 may therefore comprise one or more openings 18 to provide air inlets and outlets to, for example, deliver cool air into the nacelle 5 and vent or purge warmer air from within the nacelle 5. Acoustic noise generated by the systems and components within the nacelle 5 may therefore propagate through the openings 18 in a relatively unobstructed manner and be emitted to the external environment.

[0056] In particular, each component may generate and emit acoustic tonalities which have higher noise powers compared to other neighbouring frequencies within the generated acoustic noise. These tonalities may therefore stand out within the acoustic noise emitted to the external environment. Additionally, a frequency broadband reduction to the acoustic noise may be ineffective at reducing dominant tonalities which may continue to stand out within the acoustic noise after the reduction.

[0057] Components of the wind turbine may also exhibit resonance if they are subjected to tonalities which substantially correspond to associated natural frequencies of the components. Corresponding tonalities may therefore be amplified, further increasing their respective noise powers above the general acoustic noise.

[0058] To overcome the above issues, a noise reduction apparatus 20 may be advantageously employed within the wind turbine 1 to reduce particular tonalities within the acoustic noise as well as the overall acoustic noise. Dominant tonalities within the acoustic noise may therefore be advantageous targeted and reduced concurrently with applying a frequency broadband reduction to the acoustic noise, as shall be discussed herein.

[0059] Figures 3a and 3b show perspective views of the noise reduction apparatus 20 capable of reducing acoustic noise generated by the systems and components of the wind turbine 1 in an embodiment of the invention. It should be noted that these Figures are representative only and the skilled reader will appreciate that embodiments of the invention may be applicable to many different configurations, as will be discussed.

[0060] In an embodiment, the noise reduction apparatus 20 comprises one or more acoustic elements. The noise reduction apparatus 20 may comprise a first acoustic reflective element and a second acoustic absorptive element. The first reflective element may comprise at least one noise reduction channel 22 (only 3 of which are labelled in each of Figures 3a and 3b). The noise reduction channel 22 may be a single continuous channel 22 or may comprise a plurality of channels 22, for example as illustrated in Figures 3a and 3b. The noise reduction channel 22 may be configured to reflect acoustic noise incident on the channel. The reflected acoustic noise may be directed out of the channel 22 such that it may encounter incoming acoustic noise at the open end of the channel 22. The channel 22 may be configured such that the interaction between the reflected noise and the incoming noise results in one or more tonalities within the acoustic noise destructively interfering. The second absorptive element may comprise at least one absorptive surface 30 (only 3 of which are labelled in each of Figures 3a and 3b). The absorptive surface 30 may be a single absorptive surface 30 across the noise reduction apparatus 20 or may comprise a plurality of absorptive surfaces 30, for example as illustrated in Figures 3a and 3b. The absorptive surface 30 may be configured to absorb incident acoustic noise.

[0061] Figure 4a shows a cross-sectional view of area ‘A’ of the noise reduction apparatus 20 shown in Figure 3a. In an embodiment, the noise reduction channel 22 is provided by an acoustically reflective base surface 24, an acoustically reflective first sidewall 26, and an acoustically reflective second sidewall 28. The first and second acoustically reflective sidewalls 26, 28 extend from the base surface 24 of the channel 22. At least one acoustically absorptive transverse wall 30 extends from at least the first acoustically reflective sidewall 26. The base surface 24, the first and second sidewalls 26, 28, and the transverse wall 30 may be separate components. In other examples, the first and second sidewalls 26, 28 may be continuous with at least one of the base surface 24 and the transverse wall 30.

[0062] In some embodiments, a depth of the channel 22 taken in a direction from the base surface 24 to an open end of the channel 22 may be selected such that the channel 22 acts as a resonator for one or more predetermined tonalities. For example, specific tonalities within the acoustic noise may be targeted by selecting the depth of the channel 22 to correspond to an odd multiple of a quarter of a wavelength of one or more tonalities. The depth of the channel 22 may thus be represented by the equations d = A / 4 or d = c / (4f), where A = c / f, d is the depth of the channel 22, A is the wavelength of the targeted tonalities, c is the speed of sound, and f is the frequency of the targeted tonalities. Tonalities of corresponding wavelengths may thus enter the channel 22 and reflect off of the acoustically reflective base surface 24, first sidewall 26, and / or second sidewall 28 and exit through the open end of the channel 22. In one example, the reflected tonalities (along with tonalities of non-corresponding wavelengths within the acoustic noise) may exit the channel 22 in an unobstructed manner. Upon exit, the reflected tonalities may have a phase difference approximately half of the wavelength with like-tonalities in incoming acoustic noise. Due to this phase difference, like-tonalities may interact at the open end of the channel 22 and destructively interfere, thus reducing specific tonalities within the acoustic noise emitted by the wind turbine 1. In this manner, the channel 22 acts as a quarter-wave resonator.

[0063] In other examples, the channel 22 may be configured to comprise the characteristics of both a quarter-wave resonator and a Helmholtz resonator. Such a configuration may take a form as shown in Figure 4b, whereby the open end of the channel 22 is restricted by converging non-parallel first and second sidewalls 26, 28.

[0064] It will be appreciated that the configuration of the noise reduction apparatus 20 may be adjusted to alter the acoustic characteristics of the noise reduction apparatus 20. For example, the noise reduction apparatus 20 may comprise one or more channels 22 which act as quarter-wave resonators, Helmholtz resonators, or a mixture of the two. The one or more channels 22 may be configured to target one or more tonalities within predetermined frequency ranges, for example such as a range of approximately 500 to 2000 Hz. The targeted tonalities may be chosen or selected to correspond to one or more dominant tonalities in the acoustic noise. In this manner, the one or more channels 22 may be tuned to reduce the one or more dominant tonalities within the acoustic noise. It will, however, be appreciated that this range is not limiting and other frequencies and frequency ranges may be equally appropriate in accordance with the embodiments of the invention.

[0065] In some embodiments, the first and second sidewalls 26, 28 may be substantially parallel to each other and the transverse wall 30 may extend away from the channel 22, for example as illustrated in Figure 4a. Incident noise on each channel 22 may therefore be reflected off of the base surface 24, the first sidewall 26, and / or the second sidewall 28 such that the reflected noise is directed out of the channel 22 in an unobstructed manner. The reflected noise will have the same acoustic characteristics as incoming noise emitted from the wind turbine 1, therefore tonalities within the reflected noise will therefore destructively interfere with like-tonalities in the incoming noise. In this manner, the channel 22 may act as a quarter-wave resonator. Incident noise that does not fall onto the channels 22 may be incident on the transverse wall 30. Due to an acoustically absorptive nature of the transverse walls 30, the incident noise may be absorbed, thus enabling the noise reduction apparatus 20 to apply a frequency broadband reduction across a range of acoustic frequencies while concurrently targeting specific tonalities.

[0066] In another example, the first and second sidewalls 26, 28 may be non-parallel or may diverge from one another when considered in a direction from the base surface 24 towards the open end of the channel 22, as illustrated in Figure 4b. An area of the open end of the channel 22 may therefore be reduced compared to, for example, the configuration of the channel 22 illustrated in Figure 4a. Noise travelling into the channel 22 therefore experience an increased number of reflections before exiting the channel 22 due to the configuration of the first and second sidewalls 26, 28, thus increasing the effectiveness of the destructive interference of like-tonalities within the channel 22. Further, the area of the transverse wall 30 is increased, therefore increasing the amount of noise that is absorbed by the first and / or second transverse walls 30.

[0067] The first and second sidewalls 26, 28 may be associated with a substrate 29. The substrate 29 may be provided by a material associated with acoustic characteristics. The substrate 29 may have acoustically absorptive properties and may be applied to or be part of the transverse wall 30, for example as illustrated in Figure 4b. In other examples, the substrate 29 may have acoustically reflective properties and may be applied to or be part of the first and / or second sidewalls 26, 28. This advantageously allows the noise reduction apparatus 20 to be manufactured and assembled in a simple manner.

[0068] The noise reduction apparatus 20 may comprise a formation or underlying structure 31 on which the channel 22 may be mounted or fixed to or be integrated with such that the first and second sidewalls 26, 28, the base surface 24, and the transverse wall 30 may also be mounted or fixed to or be integrated with the formation 31. The formation 31 may take the form of a ridge, column, or the like, and may protrude from a plane in a first direction, the plane being substantially parallel to the base surface 24. The substrate 29 and the formation 31 each may be provided by respective acoustically absorptive materials, such as absorptive foam, or respective acoustically reflective materials, such as metal or plastic columns.

[0069] The substrate 29 may be applied to the formation 31 or be provided by the formation 31. For example, Figure 4c shows an example implementation where the substrate 29 is provided by the formation 31. The formation 31 is made from an acoustically absorptive material and so provides the transverse wall 30 as well as a structure for the first and second sidewalls 26, 28 to be mounted or fixed to. Figure 4d shows another example implementation where the substrate 29 is again provided by the formation 31. The formation 31 in Figure 4d, however, is made from an acoustically reflective material and so provides the first and second sidewalls 26, 28 as well as a structure for the transverse wall 30 to be mounted or fixed to. The simplicity in which the noise reduction apparatus 20 may be manufactured and assembled is therefore further increased.

[0070] The noise reduction channel 22 may be provided on a base panel 32. The base panel 32 may provide the base surface 24 and may be a surface of the wind turbine 1 , a surface of the nacelle 5, such as the external surface 6, or at least one acoustically reflective element on a surface of the wind turbine 1. The wind turbine surface may be any applicable or appropriate surface within the wind turbine 1 on which acoustic noise generated by and emitted from the wind turbine 1 may be incident on. The noise reduction channel 22, and thus the base surface 24, the first sidewall 26, the second sidewall 28, and the transverse wall 30, may be provided as separate components or as one or more integrated components on at least one surface of the base panel 32, thus advantageously allowing the noise reduction apparatus 20 to be directly applied and installed onto surfaces within the wind turbine 1.

[0071] Equally, the base panel 32 may be a separate component and may be integrated with one or more of the base surface 24, the first sidewall 26, the second sidewall 28, and the transverse wall 30. For example, the noise reduction apparatus 20 may be formed as a single component or unit and provide at least one of the base surface 24, the first sidewall 26, the second sidewall 28, the transverse wall 30, and the base panel 32. The noise reduction apparatus 20 may therefore be quickly and efficiently installed within the wind turbine 1.

[0072] In one example, the formation 31 may be acoustically absorptive and may be integrated with or provide the transverse wall 30 and may provide an underlying structure for the base surface 24, the first sidewall 26, and the second sidewall 28 to be mounted on or fixed to, for example as illustrated in Figure 4e. Alternatively, the formation 31 may be acoustically reflective and may be integrated with or provide at least one of the base surface 24 and the first and second sidewalls 26, 28. The formation 31 may therefore provide an underlying structure for the transverse wall 30 to be mounted on. The noise reduction apparatus 20 may be placed as a unitary structure on a separate base panel 32 which may be a wind turbine surface, thereby allowing the noise reduction apparatus 20 to be installed easily and quickly and a single unit onto wind turbine surfaces.

[0073] Figures 3a and 3b show one example implementation of the noise reduction apparatus 20 in which a plurality of channels 22 extend in a parallel array and are separated by a plurality of transverse walls 30 in an alternating manner. However, it will be appreciated that the noise reduction apparatus 20 may be configured in other ways.

[0074] For example, Figure 5a shows a plan view of the noise reduction apparatus 20 in an embodiment comprising a single channel 22 and a single transverse wall 30. The channel 22 and the transverse wall 30 may be arranged in a structure substantially resembling a rectangularly-shaped labyrinth.

[0075] Figure 5b shows a plan view of the noise reduction apparatus 20 in another embodiment comprising a single channel 22 and a continuous transverse wall 30. In this example, the channel 22 and the transverse wall 30 may be arranged in a structure substantially resembling a spiral-shaped labyrinth.

[0076] Figure 5c shows a plan view of a noise reduction apparatus 20 in another embodiment comprising a plurality of channels 22 and a transverse wall 30. The channels 22 and the transverse wall 30 may be arranged in a structure substantially resembling a rectangularly-shaped labyrinth, and the channels 22 may be separated by the transverse wall 30. Similarly, a noise reduction apparatus 20 with a plurality of transverse walls 30 and a single channel 22 may be equally applicable.

[0077] Figure 5d shows a plan view of a noise reduction apparatus 20 comprising a plurality of channels 22 separated by a plurality of transverse walls 30. The channels 22 and the transverse walls 30 may be arranged in a structure substantially resembling a matrix or grid whereby each channel 22 is separated by a transverse wall 30 in an alternating manner as illustrated in Figure 5d.

[0078] It will be appreciated that Figures 5a to 5d may represent the whole or part of the noise reduction apparatus 20 and that additional configurations not shown may be equally applicable to the embodiments of the invention. For example, the noise reduction apparatus 20 may comprise an array of channels 22 and / or transverse walls 30, whereby the array comprises a large number, such as hundreds or thousands, of channels 22 and / or transverse walls 30 depending on the configuration and construction of the noise reduction apparatus 20.

[0079] As well as on surfaces of the wind turbine 1 and the nacelle 5, the noise reduction apparatus 20 may be installed in specific problematic surfaces and areas in the wind turbine 1 which may typically allow acoustic noise to propagate to the external environment with relative ease and without substantial obstruction and / or absorption. Additionally, in certain areas of the wind turbine 1 it may be necessary to provide a relatively unobstructed flow path to the external environment, for example for air flow through the openings 18 of the nacelle 5.

[0080] Figure 6a shows a particular view of an example implementation of the noise reduction apparatus 40 according to another embodiment of the invention, the noise apparatus comprising a plurality of channels 22 and a plurality of transverse walls 30 on opposing sides of the base panel 32. The channels 22 on one side of the base panel 32 may have a same first depth or similar depth to the channels 22 on the opposing side of the base panel 32, or may be different. This advantageously enables the noise reduction apparatus 40 to target a plurality of tonalities at once. Other surfaces of the base panel 32 may be equally applicable depending on the configuration of the noise reduction apparatus 40. Figure 6b shows another view of the noise reduction apparatus 40 of Figure 6a. The embodiments described in relation to the noise reduction apparatus 20 shown in Figures 3a and 3b are equally applicable to the noise reduction apparatus 40 shown in Figures 6a and 6b.

[0081] The noise reduction apparatus 40 may be capable of absorbing acoustic noise on more than one surface of the base panel 32, thereby increasing the reduction of incident acoustic noise. The noise reduction apparatus 40 may be installed within the wind turbine 1 as a singular module or as part of a larger acoustically absorptive structure. For example, a plurality of noise reduction apparatuses 20, 40 may be integrated in a singular structure.

[0082] Figure 7 shows a particular view of an example implementation of the noise reduction apparatus 50 according to another embodiment of the invention, whereby the noise reduction apparatus 50 is provided by a noise reduction structure 50 comprising at least one baffle 45. The baffle 45 may be a noise reduction apparatus 20, 40 as described in relation to any of Figures 3 to 6.

[0083] The noise reduction structure 50 may comprise an inlet 52 and an outlet 54 for providing a flow path FP through the noise reduction structure 50. The baffle 45 may be disposed or arranged within the noise reduction structure 50 to be substantially perpendicular to the flow path FP, and thus substantially perpendicular to the direction of travel of acoustic noise travelling along the flow path FP. The noise reduction structure 50 may be enclosed on at least one side by a baffle wall 56, upon which the baffle 45 may be mounted. The baffle wall 56 may additionally ensure that acoustic noise can only enter the noise reduction structure 50 through the inlet 52 and cannot be emitted from the wind turbine 1 without travelling along the flow path FP. The flow path FP may be provided by a single flow path or by a plurality of individual flow paths. Figure 8 shows a close-up view of area ‘B’ of the noise reduction structure 50 shown in Figure 7. The noise reduction structure 50 may comprise an array of baffles 45, whereby there may be a large number of baffles within the noise reduction structure 50. The baffles 45 may be separate components or may be integrated with one another. The baffles 45 may be arranged such that at least one of the baffles 45a is arranged to be perpendicular to the flow path FP. Acoustic noise travelling along the flow path FP will therefore be incident on the perpendicular baffle 45a, thus allowing the baffle 45a to reduce the overall noise level and particular tonalities within the acoustic noise. One or more of the baffles 45b may additionally be arranged to be non-perpendicular to the flow path FP such that reflections of the acoustic noise within the noise reduction structure 50 is increased, thereby increasing the acoustic absorption potential of the noise reduction structure 50.

[0084] In some embodiments, different baffles within the noise reduction structure 50 may comprise channels 22 with varied depths. Figure 9 shows a detailed view of area ‘C’ of the area B of the noise reduction structure 50 shown in Figure 8. In some embodiments, the baffle 45a may comprise at least one channel 22 with a depth of D1 on a first surface of the base panel 32 of baffle 45a and at least one channel 22 with a depth of D2 on a second surface of the base panel 32 of baffle 45a. The baffle 45b may comprise at least one channel 22 with a depth of D3 on a first surface of the base panel 32 of baffle 45b, at least one channel 22 with a depth of D4 on the first surface of the base panel 32 of baffle 45b, and at least one channel 22 with a depth of D5 on a second surface of the base panel 32 of baffle 45b. Depths D1 to D5 may each be selected such that the channels 22 corresponding to heights D1 to D5 may each respectively target specific tonalities, thereby enabling the noise reduction structure 50 to target a plurality of tonalities within the acoustic noise. In addition, by configuring the noise reduction structure 50 with baffles 45 with varying channel depths, reflections of acoustic noise within the noise reduction structure 50 are increased.

[0085] The dimensions of the noise reduction apparatus 20, 40, 50 may be selected based on the dimensions of wind turbine surfaces and / or problematic areas within the wind turbine 1 in or on which the noise reduction apparatus 20, 40, 50 may be installed. For example, the depth of the channel 22 may be between 1 and 20 cm, preferably between 5 and 10 cm. The width of each channel taken between the first and second sidewalls 26, 28 may be between 1 and 20 cm, preferably between 5 and 10 cm. It will be appreciated that the described dimensions are not limiting and other appropriate dimensions in accordance with the embodiments of the invention described. Figure 10 shows a graphical representation of three example acoustic frequency spectrums, with the horizontal or x-axis representing frequency in Hz and the vertical or y-axis representing amplitude in dB. It will be appreciated that the example acoustic frequency spectrums shown are not limiting and may comprise additional or different tonalities not illustrated in Figure 10.

[0086] Acoustic frequency spectrum X represents a baseline acoustic noise generated by the wind turbine 1. The acoustic noise comprises a tonality T with a relatively large noise power and amplitude compared to other neighbouring tonalities within the baseline spectrum X. Tonality T therefore stands out amongst the other tonalities of the acoustic noise.

[0087] Acoustic frequency spectrum Y represents the acoustic noise generated by the wind turbine 1 after a broadband frequency reduction has been applied to it. As previously discussed, a frequency broadband reduction may be ineffective at reducing the noise power and amplitude of specific problematic tonalities within the acoustic noise. As can be seen in Figure 10, the amplitude of all tonalities within spectrum Y has been reduced compared to the baseline spectrum X. However, the amplitude of tonality T remains larger than other neighbouring tonalities in the same manner as in baseline spectrum X, and thus still stands out in the acoustic noise.

[0088] Advantageously, the noise reduction apparatus 20, 40, 50 can be configured to target tonality T by equating the depth of the noise reduction channel 22 to an odd multiple of a quarter of the wavelength of tonality T, as discussed previously. The acoustically absorptive transverse wall 30 advantageously also applies a broadband reduction such that the noise power of the acoustic noise is also reduced, including tonality T. The resulting acoustic noise, represented in Figure 10 by acoustic frequency spectrum Z, therefore exhibits a significant reduction in amplitude and thus noise power for targeted tonality T as well as an overall reduction in amplitude across all of the tonalities in the acoustic noise. The amplitude of tonality T in spectrum Z can be seen to be relatively similar to the amplitude of other neighbouring frequencies, thus preventing tonality T from standing out in the acoustic noise.

[0089] It will be appreciated that the noise reduction apparatus 20 may have additional configurations and arrangements not described or illustrated which are appropriate and / or applicable to the described embodiments of the invention. References:

[0090] 1 - wind turbine

[0091] 2 - rotor

[0092] 3 - blades

[0093] 4 - rotor hub

[0094] 5 - nacelle

[0095] 6 - nacelle external surface

[0096] 7 - support tower

[0097] 10 - generator system

[0098] 12 - generator

[0099] 14 - main shaft

[0100] 16 - gearbox

[0101] 18 - openings

[0102] 20 - noise reduction apparatus

[0103] 22 - noise reduction channels

[0104] 24 - acoustically reflective base surface

[0105] 26 - acoustically reflective first sidewall

[0106] 28 - acoustically reflective second sidewall

[0107] 29 - substrate

[0108] 30 - acoustically absorptive surface / transverse wall 31 - formation or underlying structure

[0109] 32 - wind turbine surface / base panel

[0110] 34 - underside surface of transverse walls

[0111] 40 - multi-sided noise reduction apparatus

[0112] 45 - baffles

[0113] 50 - baffle structure

[0114] 52 - inlet of baffle structure

[0115] 54 - outlet of baffle structure

[0116] 56 - walls of baffle structure

[0117] A - area of noise reduction apparatus in fig 3a

[0118] B - area of baffle structure in fig 7

[0119] C - area of baffle structure in fig 8

[0120] D1-D5 - channel depths

[0121] FP - flow path

[0122] T - example tonality X - baseline acoustic frequency spectrum generated by the wind turbine Y - acoustic frequency spectrum post broadband reduction

[0123] Z - acoustic frequency spectrum post combined broadband and tonality reduction

Claims

Claims1. A wind turbine (1) having a noise reduction apparatus (20, 40, 50) comprising:at least one noise reduction channel (22) provided by:an acoustically reflective base surface (24);an acoustically reflective first sidewall (26), extending from the base surface (24); and an acoustically reflective second sidewall (28), extending from the base surface (24); and at least one acoustically absorptive transverse wall (30) extending from at least the first acoustically reflective sidewall (26),wherein a depth of the noise reduction channel (22) taken in the direction from the acoustically reflective base surface (24) to the transverse wall (30) is selected to serve as a quarter-wave resonator for at least one predetermined acoustic tonality.

2. The wind turbine (1 ) of Claim 1 , wherein the first and second acoustically reflective sidewalls (26, 28) either side of the channel (22):are substantially parallel to each other; ordiverge from one another when considered in a direction from the base surface (24) of the channel (22) towards an open end of the channel (22).

3. The wind turbine (1) of Claims 1 or 2, wherein the first and second acoustically reflective sidewalls (26, 28) are associated with a substrate (29).

4. The wind turbine (1) of Claim 3, wherein the substrate (29) is acoustically absorptive.

5. The wind turbine (1) of Claims 3 or 4, wherein the substrate (29) provides the acoustically absorptive transverse wall (30).

6. The wind turbine (1) of Claim 3, wherein the substrate (29) is acoustically reflective.

7. The wind turbine (1) of any one of the preceding claims, wherein the at least one absorptive transverse wall (30) extends away from the respective channel (22).

8. The wind turbine (1) of any one of the preceding claims, wherein the at least one acoustically absorptive transverse wall (30) extends at least partially over the respective channel (22).

9. The wind turbine (1) of any one of the preceding claims, wherein the channel (22) is configured to comprise the characteristics of both a quarter-wave resonator and a Helmholtz resonator.

10. The wind turbine (1) of any one of the preceding claims, wherein the first and second acoustically reflective sidewalls (26, 28) are continuous with at least one of the acoustically reflective base surface (24) and the at least one acoustically absorptive transverse wall (30).

11. The wind turbine (1) of any one of the preceding claims, wherein the noise reduction channel (22) is provided on at least one surface of a base panel (32).

12. The wind turbine (1) of any preceding claim, further comprising an array of noise reduction channels (22).

13. The wind turbine (1) of Claim 12, wherein the array of noise reduction channels (22) comprise a first depth or a plurality of depths.

14. The wind turbine (1) of any one of the preceding claims, further comprising an array of acoustically absorptive transverse walls (30) extending from at least one of the first and second sidewall (26, 28).

15. The wind turbine (1) of any preceding claim, wherein the acoustically reflective base surface (24) is provided by at least one of:a surface of the wind turbine (1);at least one acoustically reflective element on a surface of the wind turbine (1); anda surface of a base panel (32).

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