Abstract:
A wind turbine that includes a hub carrying one or more blades, a generator, and a shaft operatively coupled to the hub through a first coupling is described. In operation, the rotor of the generator is directly driven by the shaft. The hub is rotatably mounted on a frame, and the shaft is mounted at least partially internally in the frame. The coupling between the shaft and the hub is adapted to transmit the torque about the hub&#39;s rotational axis from the hub to the shaft while substantially limiting the transmission of other loads. The generator is arranged in such a way that the torque about the shaft&#39;s rotational axis is transmitted from the shaft to the rotor of the generator while substantially limiting the transmission of other loads from the shaft to the generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/EP2011/050704 entitled “Wind Turbine”, filed Jan. 19, 2011 which claims priority to European Patent Application No. 10158262.5 entitled “Wind Turbine” filed Mar. 29, 2010 the disclosures of each of which are hereby incorporated herein in their entirety by reference. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention relate to wind turbines. Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox. 
         [0003]    Gearboxes form one of the most maintenance-intensive components of the wind turbine. They need to be inspected regularly and do not always fulfill their expected service life; the gearbox or some of its parts sometimes need to be replaced prematurely. This is due to the high loads and fluctuating loads to which a gearbox is subjected. Particularly, the bending loads on the blades, which may be transmitted through the rotor shaft to the gearbox are damaging. 
         [0004]    Direct drive wind turbines do not suffer from the problems related to the gearbox. However, since there is no speed increase, the generator shaft rotates very slowly. As a consequence, a large and expensive generator is generally needed to be able to generate electricity in an effective way. Additionally, when bending loads and movements (and corresponding deformations) are transmitted through the rotor shaft to the generator, it may not be possible to maintain a constant air gap between generator rotor and generator stator. Moreover, high bending loads could even cause structural damage to parts of the generator, e.g. its bearings. Replacement or repair of such generator parts may be very expensive due to the size and related cost of the generator. 
         [0005]    Also in the case of more integrated direct drive wind turbine designs, which lack a rotor shaft and which have a direct coupling between the hub or its blades and the generator&#39;s rotor (as described in, for example, DE 10255745), the bending moments and deformations are directly transmitted from the hub to the rotor and/or the stator, making it more difficult to minimize air gap variations. 
         [0006]    In offshore applications (both near-shore and far offshore), maintenance costs form an important part of the operating cost of a wind turbine. Therefore, in these kinds of applications, a direct drive configuration is often chosen so as to avoid the maintenance cost related to a gearbox. However, this does not resolve the aforementioned problems relating to the transmission of bending loads, associated deformations to the generator, and variations in the generator air gap. 
         [0007]    The cause of the transmission of the bending loads and deformations from the blades and hub to the generator lies in the wind turbine configuration. In most conventional wind turbines, the rotor hub is mounted on one end of the rotor shaft. The rotor shaft is rotatably mounted in a support structure within the nacelle on top of the wind turbine tower. The rotor thus forms an overhanging structure which transmits torque, but additionally transmits cyclical bending loads due to the loads on the blades and the weight of the hub and blades. These bending loads are transmitted to the generator (in the case of direct drive turbines) causing air gap variations. 
         [0008]    In order to solve this problem, it is known from e.g. ES 2 163 362 to provide a wind turbine tower with a forward extending frame. The rotor hub with its plurality of blades is mounted upon the frame and can rotate; the rotor hub is coupled to a rotor shaft located within the frame. Such a wind turbine has been schematically indicated in  FIG. 1 . In  FIG. 1 , a wind turbine  100  comprises a hub  110 , which is rotatably mounted upon frame  170 , at a distal end of the frame. Frame  170  is mounted upon tower  180 . A coupling element  120  couples rotor shaft  130  to hub  110 . The rotation of rotor shaft  130  is transformed with a gearbox  140  to a fast rotation of output shaft  150  which drives generator  160 . 
         [0009]    With this kind of configuration comprising a hub mounted on a frame, the loads due to the weight of hub and blades are transmitted more directly via the frame to the tower, whereas the rotor shaft transmits mainly torque to the gearbox (and/or generator), thus substantially avoiding undesired deformations in the drive train. This represents a major improvement with respect to other prior art wind turbines, but the transmission of bending loads from the blades to the rotor shaft, (and through the rotor shaft to the gearbox) cannot be avoided entirely. 
         [0010]    There thus still exists a need for a direct drive wind turbine, wherein the transfer of bending loads and movements from the rotor hub to the generator can substantially be reduced. 
       SUMMARY 
       [0011]    Embodiments of the invention are defined by the claims below, not this summary. A high-level overview of various aspects of the invention are provided here for that reason, to provide an overview of the disclosure, and to introduce a selection of concepts that are further described in the Detailed-Description section below. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in isolation to determine the scope of the claimed subject matter. 
         [0012]    In a first aspect, the invention provides a wind turbine comprising a hub carrying one or more blades, a generator, and a shaft operatively coupled with the hub through a first coupling, wherein in operation, the rotor of the generator is directly driven by the shaft, and wherein the hub is rotatably mounted on a frame, the shaft is provided at least partially internally of the frame, and wherein the coupling between the shaft and the hub is adapted to transmit the torque about the hub&#39;s rotational axis from the hub to the shaft while substantially limiting the transmission of other loads, and wherein the generator is arranged in such a way that the torque about the shaft&#39;s rotational axis is transmitted from the shaft to the rotor of the generator while substantially limiting the transmission of other loads from the shaft to the generator. 
         [0013]    In this aspect of the invention, the coupling between the shaft and the hub is adapted to transmit the torque about the hub&#39;s rotational axis from the hub to the shaft while limiting the transmission of other loads (e.g. bending moments, transversal and axial loads). It should be understood that the coupling cannot avoid the transmission of these other loads completely. However, the coupling may be relatively flexible with respect to these other loads, so that they are transmitted through different load paths (particularly through the frame). Also the arrangement of the generator should be understood in the same way: although the transmission of other loads (bending moments, transversal and axial loads) cannot be completely avoided, their transmission will be substantially limited. 
         [0014]    With this configuration, potentially damaging bending loads and deformations to which the hub is inevitably subjected may be avoided in the generator. The connection between the hub and the generator through the shaft is relatively stiff with respect to torsion but flexible with respect to bending loads and movements. These loads are thereby transmitted directly from the hub to the frame to the tower. 
         [0015]    In some embodiments, the shaft is connected to the generator rotor through a non-rigid second coupling, and the second coupling is adapted to transmit torque about the shaft&#39;s rotational axis from the shaft to the generator while substantially limiting the transmission of other loads. Optionally, the second coupling comprises circular splines. Another option is that the second coupling comprises a center piece from which a plurality of spokes extend substantially radially, the center piece being mounted on the shaft, and flexible elements are arranged to connect the spokes to the generator rotor. Yet a further option is that the second coupling comprises a center piece mounted on the shaft, the center piece comprising a substantially circular disc, the circular disc being connected to the generator rotor through a plurality of circumferentially arranged axial bolts, wherein the bolts are arranged within the circular disc with a plurality of flexible bushings. 
         [0016]    In other embodiments, the shaft is rigidly connected to the generator rotor, and the generator stator is supported by and flexibly connected to a fixed structure through a third coupling. The third coupling may e.g. be connected to a part of the frame, a flange connected to the frame or another suitable component. In this sense, a “fixed” structure is to be understood as a non-rotating structure that is fixed with respect to the nacelle, such as the nacelle itself, or the frame upon which the hub is mounted. It will be clear that strictly speaking, these components are not completely “fixed”, since they may rotate with respect to the tower with the help of a yaw mechanism. 
         [0017]    Preferably, this third coupling will be relatively stiff with respect to torsion, but flexible with respect to other loads (so that these loads are not transferred from the stator to the frame). 
         [0018]    In some embodiments, one or more bearings are provided within the frame to support the shaft. 
         [0019]    In some embodiments, the first coupling comprises a center piece from which a plurality of spokes extends radially, the center piece being mounted on the shaft, and the hub is provided with a plurality of circumferentially arranged axial protrusions, and flexible elements are arranged to connect the spokes to the protrusions. In other embodiments, the first coupling comprises a center piece mounted on the shaft, the center piece comprising a substantially circular disc, the circular disc being connected to the hub through a plurality of circumferentially arranged axial bolts, wherein the bolts are arranged within the circular disc with a plurality of flexible bushings. Within the scope of the invention, even further embodiments of the first coupling may be used, comprising e.g. suitably arranged elastic or visco-elastic elements, or yet other types of elements that yield to bending loads etc. 
         [0020]    In some embodiments, the before-mentioned center piece may be mounted on the shaft with a shrink disc. In other embodiments however, the center piece may be welded, bolted or connected through other suitable means. 
         [0021]    In some embodiments, the generator rotor is arranged radially outside of the generator stator. In other embodiments, the generator stator is arranged radially outside of the generator rotor. Within the scope of the invention, even other embodiments are possible, e.g. configurations wherein the generator rotor and stator are axially arranged with respect to each other. 
         [0022]    In some embodiments of the invention, the shaft comprises a front part and a rear part connected with each other. The front part and the rear part of the shaft are preferably rigidly connected with each other. The division of the shaft in a front part and rear part can make the installation process easier. It may furthermore facilitate the manufacturing of the shaft. On the other hand, the use of one integral shaft may lead to a lower total weight of the shaft. 
         [0023]    In some embodiments of the invention, the frame comprises a front part and a rear part, wherein the hub is rotatably mounted on the front part, and the rear part of the frame is rotatably mounted on a tower. The hub is thus able to rotate around its rotational axis and the rear part of the frame is able to rotate about the tower&#39;s axis. Within the scope of the invention, the frame may be formed of one integral part or may comprise two or more separate parts. In one embodiment, the frame comprises three parts: a front part carrying the hub, a middle part rotatably mounted on the wind turbine tower and a rear part carrying the generator. The frame comprising a plurality of separate parts may have advantages for the installation of the wind turbine. 
         [0024]    The frame may furthermore be of any suitable shape and configuration: the frame may e.g. have a circular, elliptical, rectangular or other cross-section. The frame may be a forged component but may also be formed by e.g. a plurality of beams or a suitable truss structure. 
         [0025]    In some embodiments of the invention, the shaft is a “traditional” solid shaft. In preferred embodiments of the invention however, the shaft may be a tubular hollow shaft. Due to the reduced loads in the shaft, the shaft may be made more lightweight. Instead of a conventional solid shaft, a tubular hollow shaft may be employed in some embodiments of the invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0026]    Particular embodiments of the invention will be described in the following, only by way of non-limiting examples, with reference to the appended drawings, in which: 
           [0027]      FIG. 1  illustrates a prior art wind turbine; 
           [0028]      FIG. 2  schematically illustrates a first embodiment of a wind turbine in accordance with an embodiment of the invention; 
           [0029]      FIGS. 3   a - 3   c  schematically illustrate some embodiments of couplings between a hub and a rotor shaft which may be used in accordance with an embodiment of the invention; 
           [0030]      FIG. 4  schematically illustrates a second embodiment of a wind turbine in accordance with an embodiment of the invention; 
           [0031]      FIG. 5  schematically illustrates a third embodiment of a wind turbine in accordance with an embodiment of the invention; 
           [0032]      FIG. 6  schematically illustrates a fourth embodiment of a wind turbine in accordance with an embodiment of the invention; 
           [0033]      FIG. 7  schematically illustrates a fifth embodiment of a wind turbine in accordance with an embodiment of the invention; 
           [0034]      FIG. 8  schematically illustrates a sixth embodiment of a wind turbine in accordance with an embodiment of the invention; 
           [0035]      FIG. 9  schematically illustrates a coupling between a generator and a frame which may be used in accordance with an embodiment of the invention; 
           [0036]      FIG. 10  schematically illustrates a seventh embodiment of a wind turbine in accordance with an embodiment of the invention; and 
           [0037]      FIGS. 11   a  and  11   b  schematically illustrate a spherical spline connection which may be used in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The subject matter of select embodiments of the invention is described with specificity herein to meet statutory requirements. But the description itself is not intended to necessarily limit the scope of claims. Rather, the claimed subject matter might be embodied in other ways to include different components, steps, or combinations thereof similar to the ones described in this document, in conjunction with other present or future technologies. Terms should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described. 
         [0039]      FIG. 2  schematically illustrates a first embodiment of a wind turbine in accordance with an embodiment of the invention. Wind turbine  1  comprises a tower  50 , upon which frame  20  is mounted. In this embodiment, frame  20  comprises a front part  20   a , a middle part  20   b , and a rear part  20   c . Hub  10  carries a plurality of blades (not shown) and is rotatably mounted with two bearings  15  upon the frame&#39;s front part  20   a.    
         [0040]    Hub  10  is connected to shaft  30  through coupling element  40 . Coupling element  40  is designed such that it transmits torque from the rotor hub  10  to shaft  30 , while substantially limiting the transfer of other loads. It will be clear that coupling element  40  may take various suitable forms.  FIGS. 3   a - 3   c  illustrates various suitable coupling elements. 
         [0041]    In a first embodiment of  FIG. 3   a , coupling element  40   a  connects shaft  30  to hub  10  (not shown in  FIG. 3   a ). Coupling element  40   a  comprises a circular disc  46 , mounted on shaft  30  with a shrink disc  45 . A plurality of holes  48  has been provided in disc  46  to provide access to the hub. The annular rim of disc  46  comprises a plurality of holes, in which bolts are provided to connect the disc to the hub. Bolts  41  are provided in flexible bushings  42   a . These bushings  42   a  may be made of a suitable elastic or flexible material. With this arrangement, the connection between the hub and shaft  30  substantially limits the transfer of any loads other than the torque from the hub. Reference sign  39  indicates a closing element, connected to shrink disc  45 , which substantially closes off the shaft and may serve to protect the inside of the shaft from the environment. 
         [0042]    Another solution is shown in  FIG. 3   b : coupling  40   b . A center piece is provided on shaft  30 . Three spokes  44  extend radially from the center piece. The spokes  44  create openings  47  at their ends. Protrusions from the hub (not shown) can be fitted in these openings  47 . Flexible elements  42   b  connect the spokes  44  to the protrusions on the hub. Annular segments  49  with access holes  48  are provided between spokes  44 . Also with this embodiment, the torque is transmitted from the hub, while the transfer of other loads is substantially limited. It shall be clear that the number of spokes may vary freely in this particular embodiment. 
         [0043]    A further option is shown in  FIG. 3   c : coupling  40   c . Similarly as in coupling  40   b , a center piece mounted on shaft  30  has a plurality of radially extending spokes  44 . Flexible elements  42   b  are provided at their distal ends. The spokes may be fitted between suitable protrusions from the hub. 
         [0044]    The flexible elements shown in the couplings  40   a ,  40   b  and  40   c  may take many suitable forms. They may be e.g. elastic or visco-elastic. They may be made from e.g. elastomers or from both elastomers and metals. In some embodiments, the stiffness (or flexibility, or elasticity) of the flexible elements may be adjustable. In preferred embodiments, they may be pre-loaded. The most important aspect of the flexible elements is that due to their arrangement and their properties, they yield in a certain extent to all loads, but securely transmit the torque from the hub. 
         [0045]    With further reference to  FIG. 2 , coupling element  40  is mounted on shaft  30  through a shrink disc  45 . Within the scope of the invention however, coupling element  40  may be mounted on shaft  30  in any other suitable way (e.g. welded, bolted, screwed, interference fit etc.) It may be seen in  FIG. 2  that shaft  30  extends internally of frame  20 . A second coupling element  70  is provided which transmits the torque from shaft  30  to the generator rotor  62 , while simultaneously limiting the transfer of other loads. The second coupling is similar to the first coupling in the sense that substantially only torque is transmitted. This second coupling may therefore also take a similar shape as the first couplings shown in  FIGS. 3   a - 3   c : in some embodiments, the second coupling comprises a center piece from which a plurality of spokes extend substantially radially, the center piece being mounted on the shaft, and flexible elements are arranged to connect the spokes to the generator rotor. In other embodiments, the second coupling comprises a center piece mounted on the shaft, the center piece comprising a substantially circular disc, the circular disc being connected to the generator rotor through a plurality of circumferentially arranged axial bolts, wherein the bolts are arranged within the circular disc with a plurality of flexible bushings. The invention however is not limited to such examples. 
         [0046]    Generator rotor  62  is mounted on frame  20   c  through suitable bearings  65 . The generator stator  64  is radially arranged outside the generator rotor  62 . Generator housing  61  is provided for protection from weather influences. Due to the first and second flexible couplings, the transfer of any load other than torque from the hub to the generator is substantially avoided. Since bending loads and accompanying deformations are not transferred, the air gap between generator rotor and stator can be maintained relatively stably. 
         [0047]    A further embodiment of the invention is schematically illustrated in  FIG. 4 . The same reference signs have been used to denote the same elements. The main difference between the embodiments of  FIGS. 4 and 2  is in the generator  60 , and more particularly the arrangement of the housing  61 . In  FIG. 4 , the generator is completely closed, which makes the generator structurally stronger. On the other hand, it comprises more material which may make this embodiment more expensive than the embodiment of  FIG. 2 . 
         [0048]    Yet a further embodiment is shown in  FIG. 5 . In this embodiment, a further bearing  85  is provided between generator housing  61  and generator rotor  62 . This bearing further reduces the air gap variations, by minimizing relative movement between the generator stator and rotor. 
         [0049]    In the embodiment of  FIG. 6 , a single bearing  85  is provided between the generator housing  61  and generator rotor  62 . Also a single bearing  65  is provided between generator rotor  62  and frame  20   c . In this embodiment, coupling element  70  and shrink disc  75  are arranged completely inside generator housing  61 . 
         [0050]    In the embodiment of  FIG. 7 , the generator rotor  62  is arranged radially outside of the generator stator  64 . Generator housing  61  is thus formed by the rotor. Bearings  95  are provided between generator housing  61  and frame  20   c . A flexible coupling  70 , similarly to the ones shown before, is provided between the generator rotor and the rotor shaft  30  to transfer the shaft&#39;s torque and substantially limit the transfer of other loads. 
         [0051]    Such a flexible coupling is not provided in the embodiment of  FIG. 8 . The connection between shaft  30  and generator rotor  62  is rigid. The unwanted deformations in the generator and accompanying air gap instability are avoided in a different way: firstly (as in other embodiments), a flexible coupling is provided between rotor hub  10  and shaft  30 . Secondly, the generator stator  64  (and housing  61 ) is supported by and flexibly connected to frame  20   c  through a third coupling  90 . The third coupling  90  is a non-rotatable coupling which is relatively stiff with respect to torsion but relatively flexible with respect to other loads. 
         [0052]    A preferred example of such a coupling  90  which may be used in the invention is shown in  FIG. 9 . A center piece  91  may be mounted on frame  20   c . A plurality of spokes  92  extends radially from the center piece. The generator housing  61  comprises a plurality of radial protrusions  94 . These radial protrusions are connected to spokes  92  by suitable flexible elements  93 . It will be clear that many different suitable flexible elements of many different suitable materials could be employed. The most important characteristic of the flexible elements is that they yield easily to loads out of the plane of coupling  90 . 
         [0053]    The combination of the first coupling between hub and shaft and the third coupling between generator stator and frame ensures that air gap variations can be minimized. 
         [0054]      FIG. 10  shows yet a further embodiment of the invention. A rigid coupling  80  is provided between rotor shaft  30  and generator housing  61  (and generator rotor  62 , which is once again arranged radially external to the generator stator  64 ). Bearings  65  are provided between the generator rotor and frame  20   c . Furthermore, a bearing  35  is provided between rotor shaft and frame  20   b . In this embodiment, the connection  99  between frame middle part  20   b  and frame rear part  20   c  is such that the transfer of loads other than loads in the plane of the connection are substantially limited. This may be achieved in various possible ways, e.g. with an arrangement shown in  FIG. 9 , another suitable arrangement of elastic or flexible elements between the frame parts, or the provision of flexible bushings for bolts or screws used to connect the frame parts together. 
         [0055]    In this embodiment, bearing  35  was provided at the junction between frame middle part  20   b  and frame rear part  20   c . In other embodiments, bearing  35  may be placed at a different position. 
         [0056]      FIGS. 11   a  and  11   b  very schematically illustrate another way of a non-rigid coupling between the rotor shaft  30  and the generator rotor  62 , which transmits the torque from the shaft to the generator rotor but substantially limits the transfer of other loads. The connection shown uses splines  33  provided on rotor shaft  30  and mating splines  63  provided on the generator rotor. Radially extending splines  33  are shaped like circular segment. Mating splines  63  have a shape that is complementary to splines  33 , such that splines  33  fit in them. 
         [0057]    When subjected to bending loads, the splines  33  would slide relative to splines  63 . When subjected to torque, the loads are transferred directly through splines  33  and  63 . Thus, also using this kind of connection one can ensure that torque from the rotor shaft is transferred while substantially limiting the transfer of other loads. 
         [0058]    Although in the embodiments shown in the figures, rotor shaft  30  was depicted as a hollow tubular shaft, in other embodiments of the invention, the shaft may be a solid shaft. 
         [0059]    And although in the embodiments shown in the figures, the frame  20  was depicted as comprising three separate parts, in other embodiments of the invention, the frame may be unitary or may comprise two or four or more different parts. Within the scope of the invention, the frame may furthermore take a different shape and structure. 
         [0060]    The invention is moreover not limited in any way to the kind of bearings used to mount the hub on the frame or to mount the generator on the frame. Suitable fluid bearings, particularly hydrodynamic or hydrostatic bearings, may be employed. Alternatively, suitable rolling element bearings, such as roller bearings, double-tapered roller bearings, or ball bearings may also be used. The bearings may further be purely radial bearings or radial and axial bearings. 
         [0061]    The invention is furthermore not limited in any way to the kind of generator employed in the wind turbine. Any suitable kind of synchronous or asynchronous generator may be used. In one preferred embodiment of the invention, the generator rotor is provided with permanent magnets. 
         [0062]    Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention herein disclosed should not be limited by the particular disclosed embodiments described before, but should be determined only by a fair reading of the claims that follow.