Background of the Invention Noise emission from technical installations is a serious problem. Legislation dictates the admissible sound pressure level that a noise source in a certain location may continuously make. Regulations governing these levels vary from country to country. In Germany, the standard values are based on VDI standard 2058, and were adopted by the Technical Directive on Noise Abatement prescribed by law. The maximum allowable values depend on the character of the surroundings and the time of day. For example, 65 dB (A) are allowed in a prevailing industrial surrounding during the day, whereas only 35 dB (A) are allowed in exclusively residential surroundings during the night. These regulations also apply to the operation of wind turbines.

Noise generated by wind turbines is partly mechanical, and partly aerodynamic. Mechanical noise is generated mainly from rotating machinery in the nacelle, particularly the gearbox and the generator, although there may also be contributions from cooling fans, auxiliary equipment (such as pumps and compressors), bearings and the yaw system. Mechanical noise is often at an

identifiable frequency or tone, e. g. , caused by the meshing frequency of a stage of the gearbox.

In addition to being airborne, mechanical noise may also be transmitted though the system. For reasons of strength and stiffness, gearbox meshing is transmitted through the gearbox casing, the nacelle bed-plate, the blades and the tower. Noise is thus transferred to these structures; in addition there may be considerable resonance amplifications of the emitted sounds within the structure. For example, a hollow steel tower is just about the ideal resonating body for radiating structural born noise which is typically in the range of 0 to 500 Hz. Thus, the gearbox is a source of significant tonal-mechanical noise.

Noise created by the gearbox should be dampened so that the wind turbine's sound pressure level does not exceed the limit set by law. Generally, the propagation of sound through the air does not cause a serious problem. It is prevented by appropriate sound insulation of the nacelle. Noise propagation through solid components, however, is much more difficult to prevent. Thus, the structural born noise of a wind turbine should be reduced.

In US 6,224, 341 B1 by J. R. Fricke patented May 1,2001, and assigned to Edge Innovations & Technology, LLC, a vibration damped system is described, wherein a hollow rotating member is filled with a low-density granular material which damps the vibrations of the rotating member. In DE 199 30 751 Al by F. Mitsch filed July 2,1999, a method for reducing vibrations of components of a wind turbine is described. According to said method, a plurality of bearings made of very soft elastomeric material are used for damping the vibrations.

Summary of the Invention It is an object of the present invention to provide an impedance element capable of reducing the structural born noise of a wind turbine.

This object is solved by an impedance element according to claim 1 and a wind turbine according to claim 9. The dependent claims relate to further embodiments of the present invention.

According to a first embodiment of the present invention, an impedance element is provided that reflects vibrational energy at its surface.

By use of such an impedance element, the noise transmission of a wind power plant can be reduced. The impedance element acts as an impedance in the noise transmission path so that the structural born noise generated in the machine nacelle, by e. g. the gearbox, is reflected at the interface between the machine nacelle and the impedance element. This effect applies to longitudial (compressive) sound waves as well as to transversal (flexural) sound waves which contribute dominantly to the total amount of sound created in the nacelle. Since not only a negligible fraction of the vibrational mechanical energy is reflected at the junction of the machine nacelle and the impedance element, the vibrational energy transmitted to the tower is clearly reduced. Typically, it is desirable that 25% or more of the vibrational energy are reflected by the impedance element. Also, reflection coefficients of 0.3, 0.5, 0.75 and even up to 1 may be realized by an impedance element according to an embodiment of the present invention. Thus, the vibrational energy flowing from the machine nacelle to the tower is strongly reduced. However, even in the event of a reflection coefficient of 1 evanescent waves are induced in the impedance element by the mechanical vibrations. Since the amplitude of evanescent waves decays exponentially, little or no vibrational energy is transmitted from the machine nacelle to the steel tower anyway.

Therefore, the sound radiation by the steel tower is significantly reduced.

Furthermore, only little shear stress is induced in the impedance element due to the reflection of the vibrational energy. The reflected sound energy is damped or dissipated in the nacelle by usual damping means. In the event that the damping means of the nacelle are not sufficient to dissipate the reflected sound energy, additional damping means can be provided.

According to another embodiment of the present invention, the impedance element comprises further a material with a specific sound velocity that is much greater or much less than the specific sound velocity of the material of the tower.

Typically, the tower of the wind turbine as well as the machine nacelle, especially the bed plate of such a nacelle which forms the interface to the impedance element, are made of steel. If the specific sound velocity of steel and the specific sound velocity of the impedance element are substantially different, the impedance element and the bed plate are"out of tune". In other words, the reflection coefficient of the sound is basically determined by the ratio of the specific sound velocities (or the acoustical inertiae) of the nacelle bed plate and the impendance element. If the specific sound velocities (or acoustical inertiae) are substantially different, a considerable fraction of the vibrational energy is reflected or diffracted at the interface between the nacelle and the impedance element. Since the specific sound velocity of steel is about 5000 m/s, the impedance element typically has a specific sound velocity much greater than 5000 m/s or much less than 5000 m/s. Typicall, the specific sound velocity of the impedance element is less or equal to 2/3 or greater or equal to 1,5 times of the specific sound velocity of the nacelle material. Especially, the ratio between the specific sound velocities is about 0.62, i. e. the specific sound velocity of the impedance element is about 3100 m/s. It should be noted that the term specific sound velocity as used above relates to sound velocities at 20°C.

According to still another embodiment of the present invention, the impedance element also has a damping factor with a logarithmic decrement larger than about 0.01 Neper (Np), typically within the range of about 0.015 Np to 0.04 Np, and more typically within the range of about 0.022 Np to 0.035 Np.

An impedance element made of a material with a damping factor having a logarithmic decrement within the above range further allows to damp the fraction of vibrational energy that is not reflected but enters the impedance element. The damping of the vibrational energy is due to the absorption of the vibrational energy in the bulk of the impedance element. Thus, the amount of vibrational energy that is transmitted from the nacelle to the tower is further reduced. Compared to the damping factor of steel, which is approx. 0.002 Np, a material of the above-described type has a damping ability which is about one to two orders of magnitude larger. Therefore, such a material is suitable for interrupting the noise transmission path between the machine nacelle (gearbox) and the tower made of steel.

According to another embodiment of the present invention, the impedance element includes concrete, preferably a reaction resin concrete.

Concrete, especially reaction resin concrete, is a material that fulfills the requirements of an impedance element according to the embodiments of the present invention. Reaction resin typically includes two components, the binder and extender.

Typically, epoxy resins are used as reaction resins together with minerals to form the reaction resin concrete. Advantageously, reaction resin concrete does not need any reinforcement as reinforcement is usually made of steel and provides an undesirable noise transmission path. Therefore, providing a reinforcement would deteriorate the damping ability of the impedance element. Since the reinforcement is not required, the damping ability of reaction resin concrete can be maintained.

According to still another aspect of the present invention, the impedance element further comprises a woven material or a carbon fiber material.

It is well-known that concrete shows superior compressive strength while only having relatively poor tensile strength. In principle, this is also true for reaction resin concrete. However, the impedance element is not only subject to compressive forces but can also be subject to tensile forces, e. g. due to teetering motions of the nacelle.

Therefore, it could be desirable to improve the tensile strength of the impedance element. This may be done by adding woven materials, e. g. Kevlar, or carbon fiber materials. Typically, the carbon fiber materials are deployed in unidirectional layers.

Woven or carbon fiber materials strongly improve the tensile strength of the impedance element. Furthermore, they typically have specific sound velocities below that of reaction resin concrete and, thus, enhance the reflection coefficient of the impedance element. However, since woven and carbon fiber materials are renown for their low density, it needs to be observed that the total mass of an impedance element comprising these materials is not too low.

Brief Description of Drawings A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein: Fig. 1A shows a plan view of an impedance element according to an embodiment of the present invention.

Fig. 1B shows a side perspective view of the impedance element of Fig. 1A.

Fig. 2A shows a plan view of an impedance element according to another embodiment of the present invention.

Fig. 2B shows a side perspective view of the impedance element of Fig. 2A.

Fig. 3A shows a cross-sectional side view of an impedance element according to a further embodiment of the present invention.

Fig. 3B shows a plan view of the impedance element of Fig. 3A.

Fig. 4A shows a cross-sectional side view of an impedance element according to just a further embodiment of the present invention.

Fig. 4B shows a perspective view of a first design of a flange used in the embodiment of Fig. 4A.

Fig. 4C shows a perspective view of a second design of a flange used in the embodiment of Fig. 4A.

Fig. 5 shows a side view of a wind turbine comprising an impedance element according to an embodiment of the present invention.

Detailed Description Reference will now be made in detail to the various embodiments of the invention, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation of the invention, and is not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the present invention includes such modifications and variations.

Fig. 1A shows a plan view of a disc-shaped impedance element 1 according to an embodiment of the present invention. A side view of this impedance element is shown in Fig. 1B, wherein also the height H of the impedance element is identified.

The impedance element 1 will typically have a height (H) in the range of about 0.5 m to 2 m, typically of about 1 m to 1. 5 m. Typically, the impedance element is made of a reaction resin concrete, e. g. EPUMENT 145/B, EPUMENT 140/8B or AMPLEX 140/5 available from EPUCRET Polymertechnik GmbH & Co KG, Daimlerstral3e 18- 26, D-73117 Wangen.

The Young's modulus of these materials is within about 30 kN/mm2 to about 45 kN/mm2, their flexural or bending strength is within about 30 N/mm2 to about 40 N/rnm2, and their pressure resistance in within about 130 N/mm2 to 150 N/mm2. The damping factor of these materials has a logarithmic decrement within the range of about 0.022 Np to about 0.035 Np. The height H of the impedance element is approx.

1500 mm for a tower of 80 m to 100 m height. Accordingly, the mass of the impedance element is between about 2000 to 5000 kg. Typically, the specific sound velocity of such an impedance element is about 2900 m/s to 3200 m/s. Accordingly, the ratio of the specific sound velocities of the impedance element and the nacelle bed plate are between 0.58 and 0.64.

Furthermore, the impedance element is reinforced with carbon fiber material (not shown) that has been added in unidirectional layers during the manufacturing

process of the impendance element. Alternatively, Kevlar can be added during the manufacturing process. Thus, the tensile strength of the impedance element is enhanced compared to an impedance element without carbon fiber reinforcement.

Additionally, the reinforcement with woven or carbon fiber materials further lowers the sound velocity of the impedance element.

Figs. 2A and 2B show an alternative embodiment of the present invention, wherein the impedance element 1 is ring-shaped and has a center opening O. The materials used for the impedance element may be the same as described in the above embodiment. Typically, the height H of the ring-shaped impedance element is greater than the height of the disc-shaped impedance element to still provide an impedance element of sufficient mass that is capable of effectively damping the structural born noise. Additionally, the ring-shaped impedance element of increased height may bear higher mechanical loads than a ring-shaped impedance element of smaller height.

Although the impedance element is shown with circular cross-section, also other shapes are possible, e. g. polygonal shapes like a hexagon or octagon.

Furthermore, the impedance element is not necessarily cylindrical but can also be cone-shaped. Thus, the particular shape of the impedance element can be adapted to the respective requirements of an individual wind turbine.

Alternatively, all of the above described embodiments can also be realized using concrete without reaction resin. In this case, the height H of the impedance element may have to be increased, if necessary, to compensate for the lack of reaction resin.

The impedance elements may be either pre-produced or produced on-site.

When an on-site pour of the impedance element is considered, an accordingly adapted mould needs to be provided. After manufacturing the impedance element, it may be attached to adjacent parts of the wind turbine, e. g. the top section of the steel tower and the bottom part of the machine nacelle, by a grouted joint or a flange connection.

In the event that the impedance element is pre-produced in a shop, the manufacturing of the impedance element can be done under controlled conditions and with high quality. Typically, a pre-produced impedance element is provided with at least one

flange for forming a flange connection with an adjacent part of the wind turbine. In the following, such impedance elements with flanges are described in detail.

Fig 3A shows a cross-sectional view of a further embodiment of the present invention. Therein, at least one upper and lower flange (s) 2,3 are provided in the impedance element 1. Impedance element 1 can be attached to upper and lower parts of a wind turbine by use of these flanges. For example, upper flange 2 can be fixed to the bed plate of the machine nacelle and lower flange 3 can be fixed to an upper part of the steel tower. Upper and lower flanges 2,3 are typically made of steel.

Therefore, it is important that the flanges 2,3 do not touch each other, otherwise they would form a noise transmission path through impedance element 1. Thus, the damping ability of impedance element 1 would be deteriorated.

Still a further embodiment of the present invention is shown in Fig. 3B, which is a plan view of the impedance element 1. Therein, upper flanges 2 and lower flanges 3 are distributed along the circumference of the impedance element. Typically, an upper flange 2 is horizontally spaced along the circumference of the impedance element 1 from a lower flange 3 to provide improved insulation of the flanges from each other. In this embodiment of the present invention, the lower flanges 3 are offset from the upper flanges 2 by approx. 45°. This offset is typically 360° divided by the total number of flanges in order to evenly space the flanges along the circumference of the impedance element. Thereby, no noise transmission path through impedance element 1 is generated, so that the noise generated on the upper side of impedance element 1 by, e. g. the gearbox, is only transmitted to the lower side of impedance element 1, i. e. to the steel tower of the wind turbine, in a strongly damped manner.

Alternatively, the flanges 2,3 may be designed with cylindrical heads as it is shown in Fig. 4A. Furthermore, the shank portion of the flanges 2,3 can be threaded as it is shown in Figs. 4B and 4C. Especially, a male thread (Fig. 4B) or a female thread (Fig. 4C) can be provided.

Finally, Fig. 5 shows a wind turbine, according to an aspect of the present invention, that includes a steel tower 4, a machine nacelle 5 and a rotor 6 with rotor blades 7. The rotor 6 is driven by the lift of the rotor blades 7 and connected to a rotor

shaft (not shown). The rotor shaft is coupled to a gearbox (not shown) disposed in the machine nacelle 5. The noise of the gearbox generated during operation is transmitted via machine nacelle 5 to impedance element 1. There, a considerable amount of vibrational energy is reflected back to machine nacelle 5. Only a fraction of the mechanical energy enters the bulk of impedance element 1. Due to its damping ability, i. e. a damping factor with a logarithmic decrement between 0.022 Np and 0.035 Np, the bulk of impedance element 1 further reduces the structural born noise generated by the gearbox. Therefore, only little or no noise is transmitted to steel tower 4 via impedance element 1 ; thus, the total emission of structural born noise of the wind turbine is effectively reduced.

Having thus described the invention in detail, it should be apparent that various modifications can be made in the present invention without departing from the spirit and scope of the following claims.