A tower for a wind turbine and a wind turbine The present invention relates to a tower for a wind turbine and to a wind turbine.

Modern wind turbines typically comprise a tower supporting a nacelle located at an upper end of the tower. Further, the wind turbine comprises a rotor connected to a generator which is arranged inside the nacelle. The rotor typically comprises three rotor blades which further increase the dimensions of the wind turbine. Because of the dimensions of wind turbines flying objects, e.g. planes or helicopters, can be disturbed. Thus, wind turbines usually require means to warn approaching objects. Such requirements may be prescribed by local regula ^¬ tions. Such warning means include aviation and/or obstruction lights. When equipping the tower with a light it should be clearly visible from the outside while easily accessible from the inside for maintenance and service.

EP 1 842 004 Bl shows a tower of a wind turbine provided with a flash light inserted in a through-hole of an outer wall of the tower. One part of the flash light which comprises a light source protrudes out of the through-hole. The light source is arranged outside the outer wall of the tower and faces an en ^¬ vironment surrounding the tower.

DE 103 58 962 Al shows a tower of a wind turbine. The tower comprises an outer wall encompassing an inner space. A light source is supported at a separate platform of the tower. Fur ^¬ ther, a through-hole is provided in the outer wall above the light source. The light source is supplying several fiber op ^¬ tic cables with light, wherein the fiber optic cables are con- nected to obstruction lights located at an outer face of the outer wall of the tower. It is one object of the present invention to provide an im ^¬ proved tower for a wind turbine and an improved wind turbine.

Accordingly, a tower for a wind turbine is provided. The tower comprises an outer wall encompassing an inner space and including a through-hole having a central axis, and a light source configured to emit light through the through-hole into an environment surrounding the tower. The light source lies on the central axis of the through-hole and is arranged inside the inner space of the outer wall.

In contrast to known towers, the serviceability and mainte ^¬ nance of the light source is improved since the light source can be easily accessed by maintenance personnel operating in- side the tower. Further, by means of arranging the light source on the central axis of the through-hole a compact lighting arrangement with reduced failure susceptibility is provided . In particular, the outer wall of the wind turbine constitutes a main structure supporting a nacelle and a rotor of the wind turbine. Preferably, a cross-section of the outer wall has a circular ring shape. Preferably, the inner space of the tower is defined by an inner shell surface of the outer wall. The through-hole preferably has a circular shape. Alternatively, the through-hole may have a rectangular or elliptical contour. In particular, the central axis passes a center relative to the contour of the through-hole. The "light source lies on the central axis" means that the central axis intersects the light source. The light source is thus arranged in line with and in front of the through-hole. Preferably, the light emitted by the light source is guided at least partially through a volume of air or vacuum on its path from the light source towards the environment surrounding the tower. Thus, there is no need to use e.g. fiber optic cables. Preferably, the light source comprises light-emitting diodes, a light bulb, a halogen lamp and/or a fluorescent lamp.

According to an embodiment the tower further comprises a hous- ing arranged at least partially inside the through-hole, wherein the light emitted by the light source passes through the housing.

The housing may be insertable into the through-hole. The hous- ing comprises preferably a tube, in particular having a re ^¬ flective inner surface configured to guide the light.

According to a further embodiment, the housing supports the light source at its inner end.

Further, the light source may be detachably mounted to the housing such that serviceability is simplified. Furthermore, the light source may be supported inside the inner space ex ^¬ clusively by means of the housing or the housing and struts, i.e. there are no other components holding or supporting the light source.

According to a further embodiment, the tower further comprises an optical element. The housing supports the optical element at its outer end. The optical element is configured to modify the light on its path into the environment.

Thus, a suitable obstruction light can be provided. Prefera ^¬ bly, the light source is associated with a single through-hole only, i.e. does not supply light to the outside through other through-holes. Preferably, the optical element is a lens or a prism. Preferably, the optical element protrudes into the en ^¬ vironment past the outer wall. Thus, the optical element should be robust, highly transparent, heat dispersive, re- sistant against weathering and ageing effects. Preferably, the optical element comprises glass, crystal and/or high-density polymer, e.g. Polymethyl Methacrylate (PMMA) . The optical ele- ment may also be arranged completely inside the through-hole (without protruding into the environment) .

According to an embodiment, the optical element is configured to disperse the light.

According to a further embodiment, the tower further comprises a lens arranged inside the housing in the light path from the light source towards the optical element.

Thus, adapting the characteristics of the light emitted by the light source is facilitated. Preferably, the lens has a con ^¬ cave shape viewing form the environment surrounding the tower. According to a further embodiment, the tower further comprises a reflective arrangement arranged between the lens and the light source, the reflective arrangement converging towards the lens . Preferably, the reflective arrangement is a reflective tube. It is understood that an inner surface of the tube is reflec ^¬ tive. Thus, the light emitted by the light source can be fo ^¬ cused and efficiency is increased since absorption of light is reduced .

According to a further embodiment, the housing comprises a cy ^¬ lindrical portion arranged at least partially inside the through-hole and a flange portion connected to the cylindrical portion and extending radially along an inner surface of the outer wall.

Thus, the through-hole is reliably covered by means of the housing and a reliable connection between the housing and the outer wall of the tower can be achieved. Preferably, the flange portion is fixed to the inner surface of the outer wall by means of studs or screws. According to a further embodiment, the tower comprises at least one strut supporting the light source, the strut being attached to the flange portion and/or the inner surface of the outer wall and to the light source.

Thus, the housing and the light source can be mounted from the inside of the tower, thus increasing the safety of personnel mounting or servicing such lighting arrangements. Preferably, several struts, e.g. 2, 3 or 4, are provided. Preferably, the struts are fixed to the flange portion and/or to the inner surface of the outer wall by means of studs or screws.

According to a further embodiment, the tower further comprises a seal configured to seal the through-hole of the outer wall against the housing.

Preferably, the seal comprises a first portion arranged be ^¬ tween an inner surface of the through-hole and an outer sur ^¬ face of the housing, and a second portion arranged between the inner surface of the outer wall and an outer surface of the flange portion of the housing. Thus, a reliable seal can be provided .

Further, a tower for a wind turbine is provided. The tower comprises an outer wall encompassing an inner space and having a through-hole, a mirror, and a light source configured to emit light towards the mirror and arranged inside the inner space. The mirror is configured to reflect the light through the through-hole into the environment.

This has the advantage that the light source and/or the mirror can be arranged at a place having a good accessibility. Fur ^¬ ther, the through-hole can be arranged at any place of the outer wall. The use of a mirror ensures a simple light path. Further, optical fibers do not allow sharp angles. Reflecting the light in a sharp angle by means of the mirror leads to a more compact design. The "mirror" means a reflective element configured to reflect sufficient light into the environment.

Furthermore, a wind turbine comprising such a tower is provid- ed .

"Wind turbine" presently refers to an apparatus converting the wind' s kinetic energy into rotational energy, which may again be converted to electrical energy by the apparatus.

The embodiments and features described with reference to the tower - comprising the light source lying on the central axis of the through-hole - of the present invention apply mutatis mutandis to the tower - comprising the mirror - of the present invention.

Further possible implementations or alternative solutions of the invention also encompass combinations - that are not ex ^¬ plicitly mentioned herein - of features described above or be- low with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of the invention.

Further embodiments, features and advantages of the present invention will become apparent from the subsequent description and dependent claims, taken in conjunction with the accompanying drawings, in which:

Fig. 1 shows a perspective view of a wind turbine according to an embodiment;

Fig. 2 shows, in a cross-sectional view II-II from Fig. 1, a first embodiment; and Fig. 3 shows, in a cross-sectional view II-II from Fig. 1, a second embodiment. In the Figures, like reference numerals designate like or functionally equivalent elements, unless otherwise indicated.

Fig. 1 shows a wind turbine 1 comprising a rotor 2 connected to a generator (not shown) arranged inside a nacelle 3. The nacelle 3 is arranged at the upper end of a tower 4 of the wind turbine 1.

The rotor 2 comprises three rotor blades 5. Alternatively, on- ly two or more than three rotor blades 5 can be provided. The rotor blades 5 are connected to a hub 6 of the wind turbine 1. Rotors 2 of this kind may have diameters ranging from, for ex ^¬ ample, 30 to 200 meters or even more. Further, the tower comprises an obstruction light 7 for warning approaching objects. The tower 4 may comprise several ob ^¬ struction lights 7 arranged around a circumference of the tow ^¬ er and/or arranged at different heights of the tower 4. Fig. 2 shows a cross-sectional view II-II from Fig. 1.

The tower 4 comprises an outer wall 8 encompassing an inner space 9 by means of its inner surface 10. The outer wall 8 in ^¬ cludes a through-hole 11 having a central axis 12. The

through-hole 11 has a circular cross-section, wherein a diameter d of the through-hole 11 may vary from, for example, 20 to 60 mm or even more. The obstruction light 7 comprises a light source 13 configured to emit light 14 through the through-hole 11 into an environment 15 surrounding the tower 4. The light source 13 lies on the central axis 12 of the through-hole 11 and is arranged inside the inner space 9 of the tower 4.

The obstruction light 7 further comprises a housing 16 arranged at least partially inside the through-hole 11. The housing 16 comprises a cylindrical portion 17 (e.g. a tube) arranged at least partially inside the through-hole 11 and a flange portion 18 connected to the cylindrical portion 17 and extending radially along the inner surface 10 of the outer wall 8. The light 14 emitted by the light source 7 passes through the housing 16. The housing 16 supports the light source 13 at its inner end 19.

The obstruction light 7 further comprises an optical element 20. The housing 16 supports the optical element 20 at its out ^¬ er end 21. A portion of the housing 16 including the optical element 20 may protrude past the outer wall 8 into the envi- ronment 15. The optical element 20, e.g. a lens or prism, is configured to modify the light 14 on its path into the envi ^¬ ronment 15. Further, the optical element 20 is configured to disperse the light 14. The light 14 travels freely through the through-hole 11 and is then dispersed on the outside of the tower 4.

The obstruction light 7 further comprises a lens 22, e.g. a concave lens, arranged inside the housing 16 in the light path from the light source 13 towards the optical element 20.

The obstruction light 7 further comprises a reflective tube 23 arranged between the lens 22 and the light source 13. The re ^¬ flective tube 23 converges towards the lens 22. The optical element 20, the housing 16, the lens 22 and/or the tube 23 may be provided as one module which is insertable into the

through-hole 11 and detachable from the through-hole 11 as one unit .

The obstruction light 7 further comprises struts 24 supporting the light source 13. The struts 24 are attached to the flange portion 18 and to the light source 13. Further, the light source 13 is mounted detachably to the housing 16, e.g. by means of bolts 25. The light source 13 may be a separate mod ^¬ ule mountable to the housing 16. Alternatively, the optical element 20, the housing 16, the lens 22, the tube 23 and/or the light source 13 may be provided as one module which is insertable in the through-hole 11 and detachable from the through-hole 11 as one unit.

Alternatively, the struts 24 may be attached to the inner sur- face 10 of the outer wall 8 and the light source 13 such that the light source 13 is mounted detachably to the outer wall 8.

The obstruction light 7 further comprises a seal 26 configured to seal the through-hole 11 of the outer wall 8 against the housing 16. The seal 26 comprises a first portion 27 arranged between an inner surface 28 of the through-hole 11 and an out ^¬ er surface 29 of the cylindrical portion 17 of the housing 16, and a second portion 30 arranged between the inner surface 10 of the outer wall 8 and flange portion 18 of the housing 16.

Fig. 3 shows the cross-sectional view II-II from Fig. 1.

In contrast to Fig. 2, the obstruction light 7 comprises a mirror 31 arranged inside the inner space 9. Further, the mir- ror 31 may be arranged inside and connected to the housing 16. Alternatively, the mirror 31 may be arranged outside the hous ^¬ ing 16 and connected to the outer wall 8 of the tower 4. The light source 13 is configured to emit light 14 towards the mirror 31. Further, the mirror 31 is configured to reflect the light 14 through the through-hole 11 into the environment 15. Furthermore, the mirror 31 reflects incident light at an angle a relative to a reflective mirror surface 32. The angle a equals, e.g., 45°. The angle a also may measure between 10 and 80°, 20 and 70°, 30 and 60° or 40 and 50°. Further, the mirror surface 32 may have a concave or convex shape. The light source 13 is, for example, arranged beneath the through-hole 11. Alternatively, the strut 24 may be connected to the inner surface 10 of the outer wall 8. Thus, the light source 13 can be mounted separate from the housing 16. It is understood that the light source 13 comprises a casing for safety purposes. Although the present invention has been described in accord ^¬ ance with preferred embodiments, it is obvious for the person skilled in the art that modifications are possible in all em ^¬ bodiments .