WO2010054661A2

WO2010054661A2 - Load monitoring of wind turbine blades

Info

Publication number: WO2010054661A2
Application number: PCT/DK2009/050297
Authority: WO
Inventors: Xiao Qian Li
Original assignee: Vestas Wind Systems A/S
Priority date: 2008-11-12
Filing date: 2009-11-11
Publication date: 2010-05-20
Also published as: WO2010054661A3; DK200801562A

Abstract

In order e.g. to provide a wind turbine with a blade deflection monitoring which is relatively easy to maintain there is disclosed a wind turbine including a nacelle (104) and a hub (106) rotatably mounted to the nacelle via a main shaft. The hub includes at least one wind turbine blade (110). The blade includes a blade reflector (202) which reflector displaces with the blade when the blade is subjected to a change in load. The nacelle includes an electromagnetic radiation emitter output (204) and an electromagnetic radiation receiver input (206) arranged to provide an electromagnetic radiation path from the electromagnetic radiation emitter output to the blade reflector and back towards the electromagnetic radiation receiver input, and a deflection of the blade can be determined by a monitoring device from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load.

Description

LOAD MONITORING OF WIND TURBINE BLADES

FIELD OF THE INVENTION

The invention relates to load monitoring of wind turbine blades, and in particular to load monitoring of wind turbine blades using electromagnetic radiation reflection.

BACKGROUND OF THE INVENTION

Modern wind turbines get bigger both in output and in size and the length and the size of the wind turbine blades also increases. Due to the increased size of the blades, the blades are subject to an increased load e.g. when subjected to wind. When subjected to wind a wind turbine blade will be subject to a certain load which will cause a certain deflection of the blade. This deflection will primarily have a direction out of a rotational plane of the wind turbine blade. The invention will be explained for wind turbines of the kind with a main shaft substantially parallel with a wind direction.

On-line monitoring of blade load can be used, such as for avoiding unexpected failures of the wind turbine, planning preventive maintenance, optimizing load control or maximizing energy output of a wind turbine generator of the wind turbine. Furthermore, a reason for monitoring blade deflection or blade load may be to prevent blades of the wind turbine getting to close to a tower of the wind turbine when the blade passes the tower at a rotational position approximately down-right. For this purpose there have been proposed solutions where a distance between the blade and the tower is measured by mounting sensor parts on the tower and on or in the blade which can sense a distance between the blade and the tower each time the blade passes the tower.

Although such solutions may be relatively easy to maintain, and may provide a measure in order to prevent the blade from getting too close to the tower, it has been found that they may fail in providing measures which can also, or additionally, be used in order to optimize the load control or to maximize the energy output of the wind turbine. SUMMARY OF THE INVENTION

It may be seen as an object of the present invention to provide an improved wind turbine, a method of monitoring a deflection of a wind turbine blade as well as a method of operating the wind turbine. Preferably, the invention alleviates, mitigates or eliminates one or more of the above or other inflexibilities or disadvantages singly or in any combination.

Accordingly there is provided, in a first aspect, a wind turbine comprising

- a nacelle,

- a hub rotatably mounted to the nacelle via a main shaft, the hub comprising at least one wind turbine blade, and

- a blade reflector comprised in the blade, which blade reflector displaces with the blade when the blade is subjected to a change in load, and wherein

- the nacelle comprises an electromagnetic radiation emitter output, and an electromagnetic radiation receiver input arranged to provide an electromagnetic radiation path from the electromagnetic radiation emitter output to the blade reflector and towards the electromagnetic radiation receiver input, and - a deflection of the blade can be determined by a monitoring device from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load.

A possible advantage hereby is that an improved wind turbine is provided. The improvement or advantage may lie therein that the blade deflection may be determined at one or more positions of the blade which are not necessarily substantially down-right, i.e. when the blade passes the wind turbine tower. Alternatively or additionally an improvement may lie therein that the described wind turbine is relatively easy to maintain, e.g. in that no maintenance or power requiring parts related to determining the deflection are needed in the hub or in the blades. The electromagnetic radiation input and output are provided at fixed positions in or on the nacelle. The electromagnetic radiation input and output may be the actual element for measuring the received radiation and an actual emitter for generating and emitting the electromagnetic radiation which may be positioned in the nacelle, but possibly the output and input are only one or more ends of a device for transporting the electromagnetic radiation such as an end of an optical fibre for receiving the radiation and an end of an optical fibre for emitting the radiation. Still further, the electromagnetic radiation emitter output and the electromagnetic radiation receiver input may be comprised in one single optical fibre fixed in or on the nacelle.

When the hub includes a first hub reflector and a radiation path can be provided to and from the blade reflector via the first hub reflector a possible advantage is that the radiation path may in a relative easy way be provided via the hub and possibly enclosed within the nacelle, hub and blades.

When the first hub reflector comprises a reflection function and a beam splitting function a possible advantage is that possibly a plurality of radiation paths can be provided.

When the first hub reflector partially reflects an incoming electromagnetic radiation and partially passes the incoming radiation, a possible advantage is that a radiation beam passing through the first hub reflector may optionally be provided.

When the hub further comprises a second hub reflector, a possible advantage is that a first path and a second radiation path may possibly be provided via the second hub reflector.

When the first path is a blade deflection path which can be provided from the electromagnetic emitter output to the first hub reflector, and to the blade reflector and back via the first hub reflector to the electromagnetic receiver input, and the second path is a reference path which can be provided from the electromagnetic emitter output to the first hub reflector and via the second hub reflector back to the first hub reflector and to the electromagnetic receiver input, a possible advantage that reference paths which may alternatively or additionally be used as a calibration path for calibrating the equipment can possibly be provided.

When the first path is compared with the second path for determination of blade deflection by an interferometry analysis, a possible advantage is that a consistent analysis method is obtainable with the system.

When the first hub reflector is fixed in the hub and rotates with the hub a possible advantage is that a relative simple construction is provided in the hub.

When the first hub reflector comprises a reflection part dedicated and following each individual wind turbine blade an embodiment of the invention is provided. Alternatively one single and monolithic first hub reflector which can be used for all blades may be provided.

When the first hub reflector is arranged in a 45 degree angle relatively to a main shaft of the wind turbine in the positions when the blade for which it is dedicated is upright or downright, a possible advantage is that this angle may provide a 90 degree reflection of the radiation towards the dedicated wind turbine blade. It is to be understood that it is the reflection means inside the first hub reflector that are arranged and of a kind so that that the 90 degree reflection of the radiation is provided.

When the second hub reflector is rotation symmetrical around a centre of a main shaft of the wind turbine and positioned with a centre of the hub reflector on a centreline of the main shaft, a possible advantage is that only one second hub reflector is possibly needed in the hub.

When the electromagnetic radiation emitter output and the radiation electromagnetic radiation receiver input and any additional optical means are arranged so as to emit and receive airborne electromagnetic radiation in a direction of a main axis of the wind turbine and symmetrically relatively to a centreline of a main shaft of the wind turbine, an embodiment of the present invention is provided. When the electromagnetic radiation is emitted to the hub and received from the hub with the electromagnetic emitter output and receiver input via a hollow main shaft of the wind turbine, a possible advantage is that one way of providing a free radiation path between the nacelle and the hub is provided.

When the electromagnetic emitter output and receiver input is positioned such as to emit and receive electromagnetic radiation from and to the hub via an outside of the main shaft of the wind turbine another way of providing a free radiation path between the nacelle and the hub is provided. This way of providing the path may be simpler than e.g. to provide the path through a hollow main shaft. Though, for a solution with the beams being sent and received to and from the hub outside the main shaft it may be helpful to provide further reflectors. These reflectors may be positioned in the hub such as to provide one or more a radiation beam paths which are suited for being reflected into an interior of the wind turbine blades.

In accordance with embodiments of the present invention the electromagnetic radiation output is a laser light output generated and emitted by a laser, and the electromagnetic radiation output is provided from one or more optical fibres operably coupled to an electromagnetic radiation emitter.

In accordance with a second aspect of the invention there is provided a method of monitoring a deflection of a wind turbine blade comprising providing the wind turbine blade with a blade reflector which displaces with the blade when the blade is subjected to a change in load, and providing a nacelle of the wind turbine with an electromagnetic radiation emitter output and an electromagnetic radiation receiver input, and arranging an electromagnetic radiation path from the electromagnetic radiation emitter output to the at least one blade reflector and towards the electromagnetic radiation receiver input, and determining a deflection of the blade from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load. In accordance with the method aspect the method may furthermore include arranging a first hub reflector in the hub, and providing a radiation path to and from the at least one blade reflector via the first hub reflector. The deflection of the blade can be determined in one or more positions of the blade. Such position may include one or more of; when the blade is substantially at its uppermost position, when the blade is substantially at its lowermost position and preferably both at the uppermost and lowermost position. In accordance with a third aspect of the invention the wind turbine is monitored according to the method of monitoring described and operated in response to the determined deflection.

It must be understood that any advantage mentioned may be seen as a possible advantage provided by the invention, but it may also be understood that the invention is particularly, but not exclusively, advantageous for obtaining the described advantage.

In general the various aspects and advantages of the invention may be combined and coupled in any way possible within the scope of the invention.

These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 shows a wind turbine, and

FIG. 2 is an illustration which shows part of the wind turbine with a blade in an uppermost position, and

FIG. 3 is an illustration which shows part of the wind turbine with a blade in an lowermost position, and FIG. 4 is an illustration of radiation paths from and to the nacelle via the hub when the wind turbine is in an uppermost position, and

FIG. 5 is an illustration of radiation paths from and to the nacelle via the hub when the wind turbine is in a lowermost position, and

FIG. 6 is an illustrational side-view of first and second hub reflectors and a cross- sectional view of the first hub reflector, and

FIG. 7 is an illustration which shows part of the wind turbine with blades in positions different from the uppermost or lowermost position according to an embodiment of the invention, and

FIG. 8 illustrates a radiation path from and to the nacelle through a hollow main shaft, and

FIG. 9 illustrates another radiation path from and to the nacelle which is provided outside a main shaft, and

FIG. 10 is an illustration of the method in accordance with embodiments of the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a wind turbine 102 with a nacelle 104, and a hub 106 rotatably mounted to the nacelle 104 via a main shaft. The nacelle 104 is rotatably mounted on a wind turbine tower 108. The hub 106 of the wind turbine includes three wind turbine blades 110 which rotates around a main shaft centre axis of the wind turbine in a rotational plane substantially perpendicular to the main shaft centre axis. When a wind turbine blade 110 is subjected to a change in load, such as due to wind, the wind turbine blade displaces out of the rotational plane.

FIG. 2 is a cross sectional side view of a wind turbine blade 110 in its uppermost right-up position, the hub 106 and the nacelle 104. In an interior of the wind turbine blade there is fixed a blade reflector 202 which is optically aligned with a pitching axis of the wind turbine blade 102. The blade reflector is positioned in a distance from a blade root so as to displace, such as to tilt, when the blade is deflected out of the rotational plane. Preferably the blade reflector is a retro- reflector which is rotational symmetric around the pitching axis so that it, e.g. in a zero deflection situation, will reflect incoming electromagnetic radiation independently of a pitching angle of the wind turbine blade.

Furthermore the figure shows the nacelle provided with an electromagnetic radiation emitter output 204 and an electromagnetic radiation receiver input 206 for providing an electromagnetic radiation path from the electromagnetic radiation emitter output to the blade reflector 202 and towards the electromagnetic radiation receiver input. Airborne, preferably non optical fibre encapsulated, beams of electromagnetic radiation are shown with dashed lines 208 in the figure. In the shown embodiment the blade reflector 202 is positioned inside the blade 110 and the electromagnetic radiation emitter output 204 and an electromagnetic radiation receiver input 206 are positioned inside the nacelle 104, and the radiation path runs via one or more optical devices in the hub 106. Alternatively, though less preferred, the blade reflector may be positioned on an outside of the blade and the radiation output and input positioned and pointed towards a blade reflector positioned on the outside of the blade.

The deflection of the blade 110 can be determined by a monitoring device (not shown) by various different analysis from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input 206 when the at least one blade 110 is subjected to change in load. This may include methods such as to determine a change of position of returning radiation and/or may include comparison of the received radiation with the emitted radiation and/or comparison with a reference path of electromagnetic radiation.

In the shown embodiments the electromagnetic radiation output is a laser light beam generated by a laser 210 and transferred via an optical fibre 214 towards an electromagnetic radiation output 204 at the end of the optical fibre 214. The electromagnetic radiation input is in the shown example an opening directly in the electromagnetic radiation receiver 212. The electromagnetic input receiver is the embodiment here suited for receiving and generating a signal in response to the received electromagnetic radiation, e.g. the laser light. The lines 216 resemble power and signal cables to and from a powering and monitoring device (not shown). It follows from the figure that the electromagnetic radiation input and output are fixed in the nacelle.

In that the electromagnetic radiation input 206 and output 204 are positioned in the nacelle a possible advantage is that there are no maintenance or power requiring parts related to determining the deflection needed in the hub or in the blades. This has the possible advantage of not requiring any wiring such as by sliding contacts between nacelle and hub and/or between hub and blades.

Furthermore a laser interferometers reference or zero output may be established for the individual blades when each blade stands at upright or down-right or at other designated positions, preferably under no deflection. Still further, a possible advantage is that the invention allows one single set of radiation emitter/receiver or a laser interferometer to monitor from one to multiple blades.

Although it may be preferred to position and fixate the electromagnetic radiation generator, i.e. the laser 210, and the electromagnetic radiation receiver in the nacelle as shown, these devices may be positioned elsewhere outside the hub (not shown) and only the electromagnetic input and output are positioned and fixed in the nacelle.

In the shown embodiment the electromagnetic radiation are provided in an airborne, non optical fibre encapsulated form to and from given positions inside the nacelle 104. These positions are the position of the electromagnetic radiation input, i.e. the end of the optical fibre 214 and the opening in the electromagnetic radiation input receiver 212. Though, in other embodiments (not shown) there may be provided some guidance of this airborne, preferably non optical fibre encapsulated radiation in the hub and/or in part of the blades. This guidance may be provided as individual optical fibres provided for guiding the electromagnetic radiation to and from the blade reflector in each of the blades.

In the shown embodiment the electromagnetic radiation emitter output and the electromagnetic radiation receiver input as well as the respective receiver and generator are provided in separate devices. Though, these input and outputs may be comprised in one single optical fibre (not shown) fixed in or on the nacelle and/or the laser 210 and the laser light receiver may be comprised in one single combined transceiver device (not shown).

For a detailed illustration and description of possible radiation paths in the centre of the hub - please refer to FIG. 4 and 5.

FIG. 3 illustrates the same part of the wind turbine as shown in figure 1, but in this situation the wind turbine blade 110 is in its lowermost down-right position. Any deflection of the blade in this position can be determined as already described for figure 1, although now the radiation path is directed towards the blade reflector in the blade in this lowermost position, i.e. the hub and the blade is now rotated 180 degrees around the main shaft centre axis.

FIG. 4 illustrates in more detail that the hub comprises a first hub reflector 402 and a radiation path can be provided to and from the blade reflector 202 via hub, and as in the shown embodiment, via the first hub reflector 402. Furthermore it follows from the figure that the first hub reflector 402 comprises a reflection function and a beam splitting function in that the first hub reflector partially reflects an incoming electromagnetic radiation towards the blade reflector 202 and partially passes the incoming radiation towards a second hub reflector 408.

The figure illustrates that two electromagnetic radiation paths can be provided via the first hub reflector 402; a first path and a second path. The first radiation path may be a blade deflection path which can be provided from the electromagnetic emitter output 204 to the first hub reflector 402 and to the blade reflector 202 and back via the first hub reflector 402 to the electromagnetic receiver input 206.

The second radiation path may be a reference path which can be provided from the electromagnetic emitter output 204 to the first hub reflector 402 and via the second hub reflector 408 back to the first hub reflector 402 and to the electromagnetic receiver input 206. Hereby, e.g. a determination of the deflection of the wind turbine blade may be determined by an interferometry analysis using the reference path. Such a reference path alternatively or additionally be used for compensating for any influence of temperature and the like variables on the equipment.

It can be seen from the figure that the first hub reflector 402 is fixed in the hub and rotates with the hub and that the second hub reflector 408 is rotation symmetrical around a centre of the main shaft centre axis 404 of the wind turbine and positioned with a centre of the second hub reflector on a centreline of the main shaft. It also follows from the figure that the electromagnetic radiation emitter output 204 and the radiation electromagnetic radiation receiver input 206 and any additional optical means are arranged so as to emit and receive a non- enclosed airborne electromagnetic radiation in a direction of a main axis of the wind turbine and symmetrically relatively to the main shaft centre axis 404 of the wind turbine.

FIG. 5 illustrates in larger detail the electromagnetic radiation emitted to and from the nacelle via the hub and via the blade reflector as illustrated and described in figure 4. The difference to figure 4 is that now the wind turbine blade (not shown) with its blade reflector 202 is now at its lowermost position. Due to the configuration of the wind turbine, e.g. the first hub reflector 402 and the second hub reflector the radiation, and the rotation of the first hub reflector 180 degrees relatively to the main shaft centre axis the radiation path now runs towards the blade reflector 202 in the blade when the blade is in its lowermost down-right position.

The left-side of FIG. 6 shows the cross-section A-A in the right-side of FIG. 6.

FIG. 6 (left) shows that the first hub reflector 402 comprises a reflector part 604 dedicated, following and aligned with each individual wind turbine blade. The reflector part 604 is dedicated for reflection towards and from a certain blade in its uppermost and lowermost position. The reflection part is embodied as a stripe like reflector 604.

In order to determine the reflection of three blades, in their uppermost and lowermost position, three reflector parts as the above described are needed. However, only one of the three reflector parts is shown in the cross-sectional view A-A. Each of the reflector parts 604 of the first hub reflector 402 is arranged in a 45 degree angle relatively to the main shaft centre axis 404 when the blade for which it is dedicated is upright or downright. The reflector parts 604 lie in the same rotational plane, and are each separated 60 degrees from each other in the rotational plane, in that a given wind turbine blade passes the uppermost position or the lowermost position every 60 degrees when the wind turbine has three blades.

Although the outgoing electromagnetic radiation hits the first hub reflector 402, as shown with the dot 603, only the radiation path 602 is shown in figure 6 and not the path towards an uppermost blade.

The shown path 602 in FIG. 6 (right) resembles the radiation path described in figure 4 as the second path or the reference path. The shown path returns through the first hub reflector 402 as illustrated with the dot 601 in the reflection stripe. In this embodiment the first hub reflector both comprises a function of letting some of the electromagnetic radiation pass through and a function of reflecting some of the electromagnetic radiation - although only the radiation path obtained by passing some of the radiation is shown as described.

FIG. 7 illustrates an embodiment of the invention where the electromagnetic radiation is continuously generated or generated by pulses and emitted. Though in the shown example there is none of the blades in the uppermost or lowermost position and i.e. with the described embodiment there will not be received any signal in response to a blade deflection. The only electromagnetic radiation received will be the radiation which has been through the reference radiation path via the second hub reflector. Alternatively the electromagnetic radiation is only emitted continuously or in pulses for a period when one of the blades is around its lowermost or uppermost position.

In the described embodiments the deflection is determined when each blade is in its uppermost or lowermost position by positioning and arranging the electromagnetic radiation output and the electromagnetic radiation input so as to emit and receive a non-enclosed airborne electromagnetic radiation in a direction of a main axis of the wind turbine and symmetrically relatively to the main shaft centre axis 404 of the wind turbine and direct above and direct below the main shaft centre axis 404 in the up-right and down-right direction.

Less deflection determinations may be provided by only generating electromagnetic radiation at certain moments, e.g. for every tenth round or the like. Alternatively or additionally, the deflection may only be determined for the uppermost or for the lowermost position of each blade by having the stripe like reflectors dedicated only the blade in its upper-most position or its lower-most position. Though, at least for detecting malfunctioning blades, it may be preferred to be able to determine the deflection of any of the blades, whereas e.g. for optimising pitch angles of the blades it may e.g. only be needed to determine the deflection of one of the blades in its uppermost position every tenth round. Normally the load on the blade due to the wind is at its maximum in the uppermost position of the blade.

In general, the deflection of the blades may be determined in any other position or more positions than the described and also blade positions different from the uppermost and lowermost position. In the described embodiment, this may be provided by positioning the electromagnetic radiation emitter output and input at positions such as at 90 degrees clockwise from the upper-most position and 180 degrees counter clockwise from this position, or by positioning additional sets of electromagnetic radiation emitter output and inputs.

FIG. 8 shows an embodiment of the invention where the electromagnetic radiation is emitted to the hub and received from the hub with the electromagnetic emitter output 204 and receiver input 206 via a hollow main shaft 802 of the wind turbine.

FIG. 9 shows an embodiment of the invention where the electromagnetic emitter output 204 and the electromagnetic receiver input 206 are positioned such as to emit and receive electromagnetic radiation from and to the hub via an outside of the main shaft of the wind turbine. For such solution it may be helpful to provide further retro-reflectors 904. These reflectors may be positioned in the hub such as to provide one or more a radiation beam paths which are suited for being reflected into an interior of the hub and/or an interior of the wind turbine blades. In general the inner surfaces of the hub or the blades or any of the optical elements, i.e. hub reflectors, beam splitter etc., described may be provided with at least some electromagnetic radiation absorbable surfaces in order to prevent unwanted and possibly disturbing radiation paths to occur.

FIG. 10 illustrates a method of monitoring 1001 a deflection 1012 of a wind turbine blade 110 including providing 1002 the wind turbine blade with a blade reflector which displaces with the blade when the blade is subjected to a change in load, and providing 1004 a nacelle of the wind turbine with an electromagnetic radiation emitter output and an electromagnetic radiation receiver input, and arranging 1006 an electromagnetic radiation path from the electromagnetic radiation emitter output 204 to the at least one blade reflector 202 and towards the electromagnetic radiation receiver input 206, and determining 1011 the deflection 1012 of the blade from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load.

As shown with dashed lines the deflection may be determined by arranging 1008 a first hub reflector 402 in the hub, and providing a radiation path 1010 to and from the at least one blade reflector via the first hub reflector 402. The deflection of the blade may at least be determined when the blade 102 is substantially at its uppermost position 1014. As illustrated with the arrow 1016, the wind turbine may be operated, e.g. so as to pitch one or more of wind turbine blades in response to the determined deflection, or as illustrated by a service technician 1018, to request maintenance of the wind turbine.

Although the present invention has been described in connection with preferred embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims.

In this section, certain specific details of the disclosed embodiment are set forth for purposes of explanation rather than limitation, so as to provide a clear and thorough understanding of the present invention. However, it should be understood readily by those skilled in this art, that the present invention may be practised in other embodiments which do not conform exactly to the details set forth herein, without departing significantly from the spirit and scope of this disclosure. Further, in this context, and for the purposes of brevity and clarity, detailed descriptions of well-known apparatus, circuits and methodology have been omitted so as to avoid unnecessary detail and possible confusion.

In short, it is disclosed that in order e.g. to provide a wind turbine with a blade deflection monitoring which is relatively easy to maintain there is disclosed a wind turbine including a nacelle and a hub rotatably mounted to the nacelle via a main shaft. The hub includes at least one wind turbine blade. The at least one blade includes a blade reflector which reflector displaces with the blade when the blade is subjected to a change in load. The nacelle includes an electromagnetic radiation emitter output and an electromagnetic radiation receiver input arranged to provide an electromagnetic radiation path from the electromagnetic radiation emitter output to the blade reflector and back towards the electromagnetic radiation receiver input, and a deflection of the blade can be determined by a monitoring device from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load.

In the claims, the term "comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus, references to "a", "an", "first", "second" etc. do not preclude a plurality.

Claims

1. A wind turbine comprising

- a nacelle, and

- the nacelle comprises an electromagnetic radiation emitter output, and an electromagnetic radiation receiver input arranged to provide an electromagnetic radiation path from the electromagnetic radiation emitter output to the blade reflector and back towards the electromagnetic radiation receiver input, and

- a deflection of the blade can be determined by a monitoring device from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load.

2. A wind turbine according to claim 1 wherein the hub comprises a first hub reflector and a radiation path can be provided to and from the blade reflector via the first hub reflector.

3. A wind turbine according to claim 2, wherein the first hub reflector comprises a reflection function and a beam splitting function.

4. A wind turbine according to claim 3, wherein the first hub reflector partially reflects an incoming electromagnetic radiation and partially passes the incoming radiation.

5. A wind turbine according to any of the preceding claims, wherein the hub further comprises a second hub reflector.

6. A wind turbine according to claim 5, wherein two electromagnetic radiation paths can be provided via the first hub reflector; a first path and a second path, and

- the first path is a blade deflection path which can be provided from the electromagnetic emitter output to the first hub reflector, and to the blade reflector and back via the first hub reflector to the electromagnetic receiver input, and

- the second path is a reference path which can be provided from the electromagnetic emitter output to the first hub reflector and via the second hub reflector back to the first hub reflector and to the electromagnetic receiver input.

7. A wind turbine according to claim 6, wherein the first path can be compared with the second path for determination of blade deflection by an interferometry analysis.

8. A wind turbine according to any of the preceding claims 2-7, wherein the first hub reflector is fixed in the hub and rotates with the hub.

9. A wind turbine according to any of the preceding claims 2-8, wherein the first hub reflector comprises a reflection part dedicated and following each individual wind turbine blade.

10. A wind turbine according to any of the preceding claims 2-9, wherein the first hub reflector is arranged in a 45 degree angle relatively to a main shaft of the wind turbine when the blade for which it is dedicated is upright or downright.

11. A wind turbine according to any of the preceding claims 5-10, wherein the second hub reflector is rotation symmetrical around a centre of a main shaft of the wind turbine and positioned with a centre of the second hub reflector on a centreline of the main shaft.

12. A wind turbine according to any of the preceding claims 2-11, wherein the electromagnetic radiation emitter output and the radiation electromagnetic radiation receiver input and any additional optical means are arranged so as to emit and receive airborne electromagnetic radiation in a direction of a main axis of the wind turbine and symmetrically relatively to a centreline of a main shaft of the wind turbine.

13. A wind turbine according to any of the preceding claims 2-12, wherein the electromagnetic radiation is emitted to the hub and received from the hub with the electromagnetic emitter output and receiver input via a hollow main shaft of the wind turbine.

14. A wind turbine according to any of the preceding claims 2-12, wherein the electromagnetic emitter output and the electromagnetic receiver input are positioned such as to emit and receive electromagnetic radiation from and to the hub via an outside of the main shaft of the wind turbine.

15. A wind turbine according to any of the preceding claims, wherein the electromagnetic radiation output is a laser light output generated by a laser.

16. A wind turbine according to any of the preceding claims, wherein the electromagnetic radiation output is provided from one or more optical fibres operably coupled to an electromagnetic radiation emitter.

17. A method of monitoring a deflection of a wind turbine blade comprising

- providing the wind turbine blade with a blade reflector which displaces with the blade when the blade is subjected to a change in load, and

- providing a nacelle of the wind turbine with an electromagnetic radiation emitter output and an electromagnetic radiation receiver input, and

- arranging an electromagnetic radiation path from the electromagnetic radiation emitter output to the at least one blade reflector and towards the electromagnetic radiation receiver input, and

- determining the deflection of the blade from a change in the electromagnetic radiation received in the electromagnetic radiation receiver input when the at least one blade is subjected to the change in load.

18. A method of monitoring a deflection of a wind turbine blade according to claim 17, further comprising - arranging a first hub reflector in the hub, and - providing a radiation path to and from the at least one blade reflector via the first hub reflector.

19. A method of monitoring a deflection of a wind turbine blade according to any of the claims 17 or 18, wherein the deflection of the blade is at least determined when the blade is substantially at its uppermost position.

20. A method of operating a wind turbine, comprising

- monitoring the wind turbine according to any of the claims 17-19, and - operating the wind turbine in response to the determined deflection.