Abstract:
A main bearing of a wind turbine, wherein the main bearing is reinforced by a reinforcement element which includes a cavity to reduce a torsional deformation of the main bearing, is provided. A wind turbine for generating electricity including such a main bearing is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to European Application No. 15180483.8 having a filing date of Aug. 11, 2015 the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a main bearing of a wind turbine. In particular, the following relates to the main bearing of a wind turbine which comprises a reinforcement element in order to reduce torsional deformation of the main bearing. Furthermore, the following relates to a wind turbine comprising such a main bearing. 
       BACKGROUND 
       [0003]    The main bearing of a wind turbine is typically subject to considerable deformation during operation of the wind turbine. This is particularly valid for conventional wind turbines with one single main bearing connecting rotatably the hub of the wind turbine with the generator rotor of the wind turbine. These deformations of the main bearing are unfavorable for the life time of the bearing due to wear, for instance. 
         [0004]    The deformations that the main bearing experiences can be of radial, axial and torsional nature. In this context, the terms “radial”, “axial” and “torsional” relate to the rotational axis of the rotor, in other words to the rotational axis of the main bearing. 
         [0005]    Conventionally, stiffener rings are applied to the main bearing. These stiffener rings are applied in particular to one or both sides of the main bearing in order to reinforce the main bearing and to better withstand the loads which are acting on it. Such stiffener rings are particularly efficient with regard to radial deformations of the main bearing. However, this type of classical reinforcement is neither cost nor mass effective with respect to torsional deformation of the main bearing. 
       SUMMARY 
       [0006]    An aspect relates to a concept of efficiently addressing the torsional deformation of the main bearing and propose a cost and mass effective solution to reduce torsional deformation of the main bearing. 
         [0007]    According to embodiments of the invention there is provided a main bearing of a wind turbine. The main bearing is reinforced by a reinforcement element which comprises a cavity to reduce torsional deformation of the main bearing. 
         [0008]    The torsional deformation is also known as warping or rotation of the main bearing, in particular the rings of the main bearing. 
         [0009]    By introducing a reinforcement element with a cavity, torsional stiffness is introduced which in turn effectively reduces the torsional deformations of the main bearing. In structural mechanics, it is known that torsional stiffness is ideally achieved with an element comprising a thin walled circular section. One of the least effective sections for torsional stiffness is a flat plate section. This is however the classical reinforcement means for bearings as the commonly used stiffener rings exactly represent such flat plates. 
         [0010]    It has been recognized by the inventor that this concept of an element comprising a cavity is advantageously used and applied to the main bearing of a wind turbine; and that application of this concept and transfer of this concept to main bearings of wind turbines efficiently reduces torsional deformations of the main bearing. 
         [0011]    Advantageously the reinforcement element is centered around the main bearing and thus the cavity represents a toroidal shape. 
         [0012]    Thus, in particular, for a main bearing comprising an outer ring and an inner ring, wherein both rings are arranged coaxially with regard to each other, the reinforcement element reduces the torsional deformation of the outer ring and/or the inner ring. 
         [0013]    In other words, the reinforcement element is particularly beneficial for reducing the deformation of the bearing rings, at least reducing the deformation of one of the bearing rings. 
         [0014]    The coaxial arrangement of the inner ring and the outer ring can also be described by a coaxial arrangement about a common axis of symmetry. 
         [0015]    Regarding the cross section of the cavity as viewed in a cross-sectional view encompassing the common axis of symmetry of the outer ring and the inner ring, a first advantageous alternative is a circular cross section. 
         [0016]    This has the advantage that according to structural mechanics a circular cross section has the potential of a maximum torsional stiffness. 
         [0017]    In a second alternative, the cross section of the cavity is at least substantially rectangular. This has the advantage that on the one hand it still allows for a good torsional stiffness and on the other hand the manufacturing of a reinforcement element with a cavity comprising a rectangular cross section is facilitated. In other words, a rectangular cross section represents a good compromise between torsional efficiency and inexpensive manufacturing. 
         [0018]    Embodiments of the invention are also directed towards a wind turbine for generating electricity comprising a reinforced main bearing as described above. 
         [0019]    The wind turbine comprises a generator rotor which is at least partly surrounded by a rotor housing and a hub. The generator rotor and the hub are rotatably connected via a main bearing, and the main bearing is reinforced by a reinforcement element. The reinforcement element comprises a cavity to reduce torsional deformation of the main bearing. 
         [0020]    Advantageously, the reinforcement element is connected to the rotor housing of the wind turbine. 
         [0021]    The rotor housing is a typical element of the wind turbine which surrounds at least partly the generator rotor of the wind turbine. The generator rotor is the rotating part of the generator. Thus, the rotor housing is rotating as well with regard to the stationary components of the wind turbine such as the nacelle or the tower. 
         [0022]    The rotor housing is located in close proximity to the stator shaft which is stationary with regard to the nacelle of the wind turbine. In direct-drive wind turbines the generator rotor is directly connected or even built as one single piece to the rotor and the hub at which the rotor blades are mounted. In geared wind turbines the gear drive is arranged between the generator rotor and the rotor at the hub of the wind turbine where the rotor blades are mounted. In any case, the generator rotor is surrounded at least partly by the rotor housing. 
         [0023]    Assuming that the outer ring is the rotating bearing component with regard to the nacelle, one beneficial way to arrange the reinforcement element is to place the reinforcement element radially more outwards from the rotor axis of rotation and connect it with the rotor housing. This has the beneficial effect that not only the torsional stiffness of the main bearing is enhanced but also the air gap between the generator rotor and the stationary stator shaft is stabilized. 
         [0024]    In other words, it is ensured that the rotor housing and the generator rotor is stabilized against torsional deformation as well. As a consequence of the stabilized air gap of the generator, the air gap as such can be decreased which potentially gives more power to the generator. This advantage particularly applies to direct drive wind turbines. 
         [0025]    In an advantageous embodiment of the invention, the reinforcement element comprises a first side which is partly limiting the cavity, and the first side is formed by a part of the rotor housing. 
         [0026]    In other words, the reinforcement element is directly connected to the rotor housing and is even using as one of the walls of the reinforcement element a part of the rotor housing as such. 
         [0027]    This has the advantage of avoiding an extra wall of the reinforcement element and to directly increase the stiffness of the rotor housing. 
         [0028]    Advantageously, a second side of the reinforcement element, which is partly limiting the cavity, is formed by a part of the outer ring of the main bearing. 
         [0029]    This has the advantage that the reinforcement element uses as a second wall of the cavity the outer surface of the outer ring. Thus, again, material is saved and the torsional stiffness of the main bearing and the rotor housing is directly improved. 
         [0030]    In another advantageous embodiment, a third side of the reinforcement element, which is partly limiting the cavity, is formed by a so-called “connection element”. 
         [0031]    This connection element is also referred to as a “connector ring”. Assuming that the outer ring has an annular shape, which is typical for the outer ring of a main bearing of a wind turbine, the reinforcement element is advantageously placed in direct contact to the outer ring. A simple and efficient means to obtain the reinforcement element with the cavity is to add the connection element to the outer ring and the rotor housing. 
         [0032]    Thus, with simple and lightweight means, the reinforcement element with the cavity is achieved. 
         [0033]    With regard to a fourth side of the reinforcement element, this may also be realized by the connection element or it may be realized by the rotor housing. In particular, a suitable recess might be added or might already be present at the rotor housing, thus only the connection element has to be added in order to obtain a cavity which is substantially closed and which considerably increases the torsional stiffness of the main bearing. 
         [0034]    The connection element may be embedded or screwed to the rotor housing. The connection element may be a panel, i.e. a plate with an annular shape. 
         [0035]    In the following an advantageous embodiment of the present invention is given. This represents one practical realization. It is not limiting; similar concepts may also lead to satisfying results for reducing the torsional deformations of the main bearing. 
       BRIEF DESCRIPTION 
       [0036]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         [0037]      FIG. 1  is a wind turbine; and 
         [0038]      FIG. 2  is an embodiment of a reinforced main bearing of a wind turbine. 
     
    
     DETAILED DESCRIPTION 
       [0039]    In  FIG. 1 , a wind turbine  10  is shown. The wind turbine  10  comprises a nacelle  12  and a tower  11 . The nacelle  12  is mounted at the top of the tower  11 . The nacelle  12  is mounted rotatable with regard to the tower  11  by means of a yaw bearing. The axis of rotation of the nacelle  12  with regard to the tower  11  is referred to as the yaw axis. 
         [0040]    The wind turbine  10  also comprises a hub  13  with three rotor blades (of which two rotor blades  16  are depicted in  FIG. 1 ). The hub  13  is mounted rotatable with regard to the nacelle  12  by means of a main bearing  20 . The hub  13  is mounted rotatable about a rotor axis of rotation  14 . 
         [0041]    The wind turbine  10  furthermore comprises a generator rotor, which connects the hub  13  with the generator  15 . The hub  13  is connected directly to the generator  15 , thus the wind turbine  10  is referred to as a gearless, direct-driven wind turbine. As an alternative, the hub  13  may also be connected to the generator  15  via a gear box. This type of wind turbine is referred to as a geared wind turbine. 
         [0042]    The generator  15  is accommodated within the nacelle  12 . A generator  15  is arranged and prepared for converting the rotational energy from the hub into electrical energy. 
         [0043]      FIG. 2  shows a cross sectional view through a part of the wind turbine. In particular, it shows the stator shaft  21  which is a part of the generator and which is stationary with regard to the nacelle. The inner ring  25  of the main bearing is rigidly connected with the stator shaft. Thus, the inner ring  25  is also stationary with regard to the nacelle. The second bearing component of the main bearing is the outer ring  24 . 
         [0044]    The outer ring  24  is separated from the inner ring  25  by a gap in which rolling elements  26  are located. These rolling elements may for example be balls or cylinders or the like. For the main bearings of wind turbines, tapered roller bearings are often used. The outer ring  24  is rotatable with regard to the nacelle in the inner ring  25 . The hub  13  of the wind turbine is fixedly and rigidly connected to the outer ring  24  of the main bearing. The rotor blades are attached to the hub  13  (not shown). 
         [0045]    The rotor housing  23  is attached and connected to the outer ring  24  and protects the rotating parts of the main bearing and of the generator. The components described so far are well-known and conventional components of main bearings of wind turbines. 
         [0046]    In order to increase the stiffness of the main bearing and consequently reduce deformations of the main bearing stiffener rings, which for example are applied radially more outwards than the outer ring  25  or which are applied at the front surface or rear surface of the outer ring, are conventionally applied in order to increase the stiffness of the main bearing. But these conventional stiffener rings are on the one hand mass and cost effective and on the other hand they do not efficiently reduce the torsional stiffness of the main bearing. 
         [0047]    Embodiments of the present invention include a hollow box or chamber added to the main bearing in order to increase its torsional stiffness. In the present embodiment, this is realized by adding an annular, flat connection element  32  radially outwards of the outer ring  24 . This connection element  32 , which is also referred to as a connector ring, is screwed by a pair of screws  33  to the outer ring  24  and to a recess of the rotor housing  23 . As a result, a hollow chamber, in other words a cavity, is received and obtained. 
         [0048]    This cavity  31  has four sides: a first side  311  which is formed by the rotor housing  23 ; a second side  312  which is formed by the outer surface of the outer ring  24 ; a third side  313  which is formed by the connection element  33  and a fourth side  314  which is formed by the rotor housing  23 , in particular by the recess of the rotor housing  23 . 
         [0049]    As a result, by simple and lightweight means, a reinforcement element with a cross section which effectively reinforces the structure against not only radial deformations but also against harmful torsional deformations is obtained. 
         [0050]    Note that as an additional advantage of the reinforcement element with the cavity which is attached to the rotor housing  23  of the generator, an increased stiffness of the rotor housing  23  is also obtained. This in turn ensures a more stable air gap between the generator rotor and the stator of the direct-driven generator. Thus the air gap is stabilized and may as a consequence be decreased. This in turn has the potential of increasing the power which can be generated by the generator. 
         [0051]    The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements are provided with the same reference signs. 
         [0052]    Although the present invention has been described in detail with reference to the preferred embodiment, it is to be understood that the present invention is not limited by the disclosed examples, and that numerous additional modifications and variations could be made thereto by a person skilled in the art without departing from the scope of the invention. 
         [0053]    It should be noted that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.