The present invention relates to a wind turbine.

Modern wind turbines are commonly used to supply electricity into the 5 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.

Gearboxes form one of the most maintenance-intensive components of

10 the wind turbine. They need to be inspected regularly and do not always fulfil 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.

15 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

20 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 either to the generator (in the case of direct drive turbines) causing air gap variations or to the gearbox causing fluctuating loads in the gearbox.

25 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 and can rotate upon said frame; the rotor hub is coupled to a rotor shaft located within said frame. Such a wind turbine has been schematically indicated in figure 1. In figure 1 , a wind turbine 100 comprises a hub

30 110, which is rotatably mounted upon frame 170, at a distal end of said 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.

In a prior art coupling, a centre piece is mounted on a rotor shaft with a shrink disc. Along the circumference of the annular rim of the centre piece a plurality of holes is provided. Bolts provided in elastic bushings are used to connect the centre piece to the hub. The elastic bushings make the coupling more flexible in the longitudinal direction of the rotor shaft.

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 avoiding to a certain extent the undesired loads and deformations in the drive train. This represents an 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.

Furthermore, mounting the coupling with the plurality of bolts is a cumbersome, and time consuming and therefore expensive task. Disassembling the coupling for maintenance, inspection or repair is of course just as cumbersome. It is furthermore important in this configuration that the rotor shaft be perfectly aligned because a misalignment leads to stresses in the rotor shaft, the coupling and the hub. These stresses may lead to e.g. fatigue problems. The installation process is thus further complicated because of the need to perfectly align the rotor shaft with respect to the hub.

Also, having such a quantity of bolts and flexible elements (e.g. approximately 30 each per wind turbine) significantly raises the part count, which may complicate logistics.

There thus still exists a need for a wind turbine in which the transmission of bending loads from the blades to the rotor shaft is further reduced. There also exists a need for a wind turbine with a reduced part count and which is easier to install and maintain.

It is an objective of the present invention to provide a wind turbine which at least partially fulfils the aforementioned needs. In a first aspect, the present invention provides a wind turbine comprising a hub with one or more blades, said hub being rotatably mounted on a frame and operatively coupled to a shaft, wherein said shaft is provided at least partially internally of said frame, and a centre piece from which a plurality of spokes extend substantially radially is mounted on said shaft, and the hub is provided with a plurality of circumferentially arranged axial protrusions, and flexible elements are arranged to connect the spokes to said protrusions.

In this aspect of the invention, the stiffness of the coupling between the hub and the shaft with respect to loads other than torque may be reduced significantly as compared to prior art systems. This means that torque from the hub is effectively transmitted to the shaft, but that the transmission of all other loads is substantially reduced. Also, the part count may be significantly reduced and the mounting of the connection between the hub and rotor shaft is made much easier. A further aspect of this configuration is that there is no further need to perfectly align the rotor shaft with respect to the hub. The possible misalignment due to the manufacturing tolerances can be absorbed by the flexible elements.

In this sense, "flexible" elements are to be understood to be elements that deform or yield ("give in") relatively easily to loads in at least one direction. They may be made from any suitable material, e.g. elastomer materials, or combinations of metals with elastomers or yet other suitable materials. The elements may obtain their flexible properties due to their shape, material, positioning, mounting or combinations of these.

In some embodiments, said flexible elements are pre-loaded: the elements are compressed between the protrusions and spokes, so that they cannot go loose during operation of the wind turbine.

In some embodiments of the invention, each of said protrusions is connected (through said flexible elements) to a pair of said spokes. Each of the hub's protrusions is thus located between a pair of spokes. In one embodiment, the hub comprises three protrusions and the centre piece comprises three pairs of spokes. In other embodiments, a different number of protrusions and pairs of spokes may be used, e.g. two, four, five or six. Configurations having three or more protrusions and pairs of spokes have advantages of more balanced dynamic loading.

In some embodiments of the invention, said centre piece further comprises annular segments between neighbouring pairs of spokes. In preferred embodiments said annular segments comprise at least one hole. These holes may provide access to the hub or the hub's bearings. They thereby facilitate maintenance and inspection.

In further embodiments, each of said spokes is connected to a pair of said protrusions. In yet other embodiments, the wind turbine comprises the same number of spokes and protrusions. It will be clear that the number of spokes and protrusions may be freely varied also in these embodiments.

Optionally, said centre piece is mounted on said shaft with a shrink disc. In other embodiments, the centre piece may be connected to the shaft in a different way: e.g. a bolted connection, welding, brazing, using adhesive, through a process of thermal interference, or a form fit using e.g. serrations provided on the shaft, combinations of the above, or yet other methods. The choice for a suitable connection method may depend e.g. on the possible need for the coupling to be disassembled.

In some embodiments of the invention, the rotor shaft may be a substantially tubular hollow shaft. Due to the reduced loads on the shaft, the shaft may be tubular, instead of a conventional solid shaft. In other embodiments of the invention however, a conventional solid shaft may be used.

In some embodiments of the invention, the stiffness of said flexible elements can be adapted. In this aspect, the stiffness of the elements can be adjusted (e.g. during maintenance) in accordance with circumstances.

In some embodiments of the invention, the stiffness of at least one of said flexible elements is different from the stiffness of at least another of said flexible elements.

In some embodiments of the invention, said flexible elements are elastic. In other embodiments of the invention, said flexible elements are visco- elastic. The flexible elements may be elastic in the sense that their deformation is proportional to the applied load. They may also be visco-elastic in the sense that they exhibit time-dependent strain. Depending on the vibrations that generally occur in the wind turbine, application of elastic, visco-elastic or yet other elements may be beneficial.

In some embodiments, the flexible elements may each comprise a hydraulic chamber. Preferably in these embodiments, the elements comprising a hydraulic chamber are connected by one or more hydraulic circuits. In preferred embodiments, the flexible elements that are compressed by the wind turbine rotor torque may be connected to a common hydraulic circuit, such that radial loads are equally divided between the various struts and protrusions. Similarly, the flexible elements that are being decompressed by the wind turbine rotor torque may also be connected to another common hydraulic circuit. This kind of configuration allows the reduction of radial loads transmitted from the hub to the generator rotor which may be caused e.g. by radial misalignment of the axes of the generator rotor and wind turbine rotor.

In some implementations, the hydraulic circuits of the flexible elements may be actively controlled to influence e.g. the torsional eigenfrequency of the drive train or to counteract vibrations using active damping or reduce load peaks by releasing hydraulic fluid from such circuits.

In another aspect, the invention provides a method of assembling a wind turbine substantially as hereinbefore described, comprising: mounting the hub on the frame, providing the rotor shaft in the frame, mounting the centre piece on the rotor shaft, positioning a first flexible element between a first protrusion and a first spoke, applying pressure to compress said first flexible element, and positioning a second flexible element between said first protrusion and a second spoke or between a second protrusion and said first spoke.

Particular embodiments of the present invention will be described in the following, only by way of non-limiting examples, with reference to the appended drawings, in which:

Figure 1 illustrates a prior art wind turbine;

Figure 2 schematically illustrates a first embodiment of wind turbine according to the present invention; Figure 3 schematically illustrates another embodiment of a coupling between the hub and rotor shaft according to the present invention;

Figure 4 schematically illustrates a further embodiment of a coupling between the hub and rotor shaft according to the present invention;

Figure 5 schematically illustrates yet another embodiment of a wind turbine according to the present invention;

Figures 6a - 6d illustrate a method of mounting flexible elements in a coupling according to the present invention. Figure 2 schematically illustrates a first embodiment of wind turbine according to the present invention. Hub 10 is rotatably mounted on frame 20. The hub carries a plurality of blades (not shown) which may be mounted in blade root fitting 15. The hub comprises a number of protrusions 14. In the particular embodiment shown in figure 2, six protrusions were provided, but within the scope of the invention, this number may be freely varied.

A rotor shaft (not shown) may be provided in the central opening 31 of centre piece 40. Centre piece 40 may thus be mounted on the rotor shaft through e.g. welding, a bolted connection, an interference fit or in yet other ways. In this embodiment, six radially extending spokes 44 are provided on centre piece 40, and twelve flexible elements 42 connect the spokes 44 to the protrusions 14.

Figure 3 illustrates an alternative coupling of the rotor shaft to the hub in accordance with the present invention. A centre piece 40a is mounted on rotor shaft 30 through a shrink disc 45. Said shrink disc 45 is provided around a tubular extension of said centre piece (not visible in figure 3) and is mounted so as to compress said tubular extension and thereby establish a secure fit. Six substantially radially extending spokes 44 are provided on said centre piece. These spokes are provided in pairs. Openings 47 are created between the distal ends of the pairs of spokes 44. Suitable protrusions on the hub (not shown in this figure) can be fitted in these openings. Flexible elements 42 are provided to connect the spokes 44 to these protrusions from the hub. In this embodiment, annular segments 49 connect pairs of spokes 44 to each other. These annular segments 49 may serve to evenly distribute the loads. Access holes 48 have been provided which facilitate inspection and maintenance of the hub and components provided within the hub. Reference sign 51 indicates a closing element which substantially closes off the shaft and may protect its inside from the environment.

Figure 4 illustrates yet another coupling of the rotor shaft to the hub in accordance with the present invention. A centre piece 40b is mounted on rotor shaft 30 using a shrink disc 45. Radially extending spokes 44 can be fitted between pairs of protrusions on the hub (not shown in this figure). Flexible elements 42 connect the three spokes 44 to the pairs of protrusions on the hub.

Figure 5 illustrates yet a further embodiment of a wind turbine according to the present invention. A hub 10 is connected to a rotor shaft 30. The rotor shaft 30 is connected to a first stage of a gearbox. Reference sign 60 is used to indicate this connection. Similarly to the embodiment of figure 3, centre piece 40c carries three pairs of spokes 44. Each pair of spokes defines a space in between them. Each protrusion 14 on the hub is connected to such a pair of spokes using flexible elements 42. A notable difference with the embodiment of figure 3 is that the centre piece 40c does not comprise annular segments connecting the spokes.

In the embodiment of figure 5, centre piece 40c is welded on shaft 30. For installations and repair, it is important that at least one of the connection of the shaft with a gearbox stage or the connection of the shaft with the centre piece is removable. In the shown embodiment for example, the shaft comprises a flange which is bolted to the first stage of the gearbox.

In some embodiments, the flexible elements depicted in figures 2 - 5 may each comprise a hydraulic chamber. Preferably in these embodiments, the elements comprising a hydraulic chamber are connected by one or more hydraulic circuits. This kind of configuration allows the reduction of radial loads transmitted from the hub to the generator rotor which may be caused e.g. by radial misalignment of the axes of the generator rotor and wind turbine rotor.

In preferred embodiments, the flexible elements that are compressed by the wind turbine rotor torque may be connected to a common hydraulic circuit, such that radial loads are equally divided between the various spokes. Similarly, the flexible elements that are being decompressed by the wind turbine rotor torque may also be connected to another common hydraulic circuit. Figures 6a - 6d illustrate a method of mounting flexible elements in a coupling according to the present invention. Figure 6a illustrates a first step of such a method. One flexible element 42 is positioned between a first protrusion 14 and a distal end of a first spoke 44. Furthermore illustrated in figure 6a are assembly holes 16 on protrusion 14 which serve to mount a first assembly tool. Similarly, a second spoke 44' comprises such assembly holes 51.

Figure 6b illustrates a following step in the mounting process: first assembly support 61 and second assembly support 62 are screwed on the first rotor hub protrusion 14 and second spoke 44' respectively using the assembly holes. A (hydraulic) piston 63 is provided between these assembly supports.

By subsequently actuating the piston 63, figure 6c, the assembly supports 61 and 62 may be pushed apart and the first flexible element 42 is compressed. The first rotor hub protrusion 14 and second spoke 44' are hereby also pushed apart. This process creates sufficient space to fit a second flexible element 42' on the other side of the protrusion 14 (behind the assembly supports), as indicated with an arrow in figure 6c.

The piston may subsequently be released. The piston 63 and assembly supports 61 and 62 may then be removed. As a result of this process, flexible elements 42 and 42' are pre-loaded between the protrusions on the hub 14 and the spokes 44, 44' of the centre piece (figure 6d).

The described mounting process may be carried out, one-by-one or may be carried out for several flexible elements at the same time: a first set of flexible elements is then mounted, multiple assembly supports are mounted and multiple pistons are subsequently actuated to compress said first set of flexible elements.

The method of mounting the flexible elements as illustrated in figures

6a-6d, was shown for a configuration comprising a plurality of protrusions on the hub, each protrusion being located between a pair of spokes (a similar configuration as shown in e.g. figure 3). It will be clear however that a similar method may be used when mounting the flexible elements in other embodiments of the present invention, such as e.g. the embodiments shown in figure 2 (equal number of spokes and protrusions) and figure 4 (each spoke positioned between a pair of protrusions). It will furthermore be clear that the protrusions on the hub, the spokes on the centre piece and the flexible elements may take many other suitable shapes than the ones shown in the figures. The cross-section of the flexible elements may e.g. be circular, rectangular, square or other. The protrusions on the hub may e.g. be substantially thin-walled as shown in figure 2, or more solid as shown in figure 5.

The present 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.

The present invention is furthermore not limited to the use of a gearbox in the wind turbine. The same coupling of the hub to the shaft may be used e.g. in a direct drive configuration.

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 present 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 present 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.