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Title:
IMPROVEMENTS RELATING TO WIND TURBINES
Document Type and Number:
WIPO Patent Application WO/2015/070870
Kind Code:
A1
Abstract:
The invention relates to a wind turbine having at least one rotor blade comprising: a transmitter arranged to transmit an electro-magnetic (EM) radiation signal having spatially dependent characteristics;a receiver arranged to receive the EM-radiation signal, the receiver being located on the blade at a location outboard with respect to the transmitter and being arranged such that a deflection of the rotor blade results in a displacement of the receiver relative to the transmitter; and a processor arranged to calculate a deflection of the rotor blade based upon spatially dependent characteristics of the received EM-radiation signal. The invention also relates to a method of determining deflection of a blade of a wind turbine rotor during rotation of the rotor, the method comprising: transmitting from a first position an electro-magnetic (EM) radiation signal having spatially dependent characteristics; receiving the EM-radiation signal at a second position, the second position being located on the blade at a location outboard with respect to the first position, and being arranged such that a deflection of the rotor blade results in a displacement of the second position relative to the first position; and calculating the deflection of the blade based upon the spatially dependent characteristics of the received EM-radiation signal.

Inventors:
OLESEN IB SVEND (DK)
Application Number:
PCT/DK2014/050378
Publication Date:
May 21, 2015
Filing Date:
November 10, 2014
Export Citation:
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Assignee:
VESTAS WIND SYS AS (DK)
International Classes:
F03D11/00; F03D1/06; F03D7/02
Domestic Patent References:
WO2014187463A12014-11-27
WO2014027032A22014-02-20
Foreign References:
EP2511522A12012-10-17
Other References:
None
Download PDF:
Claims:
processor is configured to calculate the deflection of the rotor blade based on the wavelength of the received EM-radiation signal.

9. The wind turbine of any one of Claims 4 to 8, wherein the refracting element is a refracting prism.

10. The wind turbine of any one of Claims 4 to 8, wherein the refracting element is a diffraction grating. 1 1. The wind turbine of any one of Claims 2 to 10, wherein the processor is arranged to determine a difference between the wavelength or frequency of the received EM- radiation signal and the wavelength or frequency of a reference EM-radiation signal, and to calculate bending of the blade on the basis of the determined difference in wavelength or frequency.

12. The wind turbine of any one of Claims 2 to 1 1 , comprising:

first and second receivers located on the blade at a location outboard with respect to the transmitter, the first and second receivers being spaced apart from one another in a chordwise direction of the blade, the first receiver being arranged to receive a first EM-radiation signal, and the second receiver being arranged to receive a second

EM-radiation signal; and

the processor is arranged to calculate blade twist based upon the wavelength or frequency of the received first and second EM-radiation signals. 13. The wind turbine of Claim 12, wherein the processor is arranged to determine the difference in wavelength or frequency between the received first and second EM- radiation signals, and to calculate the blade twist on the basis of the determined difference in wavelength or frequency. 14. The wind turbine of Claim 12 or Claim 13, comprising:

a plurality of pairs of receivers, each pair comprising first and second receivers, each pair of receivers being arranged at a different position along a length of the blade; and wherein

the processor is arranged to calculate the blade twist along the length of the blade on the basis of the wavelength or frequency of the first and second EM-radiation signals received at each pair of receivers.

15. The wind turbine of any preceding claim, comprising:

a signal modulator arranged to imprint a fingerprint in the EM-radiation signal, enabling the received EM-radiation signal to be distinguished from background noise.

16. The wind turbine of any preceding claim, comprising:

an EM-radiation source located remotely from the transmitter; and

a first optical fibre extending along at least a portion of the length of the blade between the source and the transmitter, and being arranged to communicate EM- radiation between the source and the transmitter.

17. The wind turbine of any preceding claim, comprising:

a detector located remotely from the receiver; and

a second optical fibre extending along at least a portion of the length of the blade between the receiver and the detector, and being arranged to communicate EM- radiation between the receiver and the detector.

18. The wind turbine of Claim 17, wherein the detector is arranged to distinguish between the received EM-radiation signal and background noise, on the basis of a fingerprint present in the received EM-radiation signal.

19. The wind turbine of any preceding claim, wherein the transmitter and the receiver are located within the wind turbine rotor blade. 20. The wind turbine of any one of Claims 1 to 18, wherein the transmitter and the receiver are located on an external surface of the wind turbine rotor blade.

21. The wind turbine of any one of Claims 1 to 18, wherein the transmitter is located on a hub of the wind turbine rotor substantially in the vicinity of the root of the wind turbine blade, and the receiver is located on an external surface of the wind turbine blade.

22. The wind turbine of any preceding claim, wherein the receiver is located substantially in the vicinity of the tip of the wind turbine blade, and the transmitter is located substantially in the vicinity of the root of the wind turbine blade.

23. A method of determining deflection of a blade of a wind turbine rotor during rotation of the rotor, the method comprising:

(d) transmitting from a first position an electro-magnetic (EM) radiation signal having spatially dependent characteristics;

(e) receiving the EM-radiation signal at a second position, the second position being located on the blade at a location outboard with respect to the first position, and being arranged such that a deflection of the rotor blade results in a displacement of the second position relative to the first position; and

(f) calculating the deflection of the blade based upon the spatially dependent characteristics of the received EM-radiation signal.

24. The method of Claim 23, wherein step (a) comprises refracting an EM-radiation signal and transmitting the refracted EM-radiation signal towards the second position. 25. The method of Claim 23 or Claim 24, comprising:

wherein step (a) comprises transmitting a broadband EM-radiation signal through a refracting element.

26. The method of Claim 25, wherein the broadband EM-radiation signal is white light, and step (c) comprises calculating the deflection of the blade based upon the colour of the refracted light.

27. The method of Claim 25, wherein the broadband EM-radiation signal comprises wavelengths substantially in the range of 1540nm to 1560nm.

28. The method of any one of Claims 25 to 27, comprising:

refracting the broadband EM-radiation signal in a plane transverse to the chordwise direction of the blade, such that each wavelength or frequency component of the broadband EM-radiation signal is refracted by a different amount in said plane.

29. The method of any one of Claims 25 to 28, wherein step (c) comprises:

determining the difference between the wavelength or frequency of the received

EM-radiation signal and a wavelength or frequency of a reference EM-radiation signal, and

calculating bending of the blade on the basis of the determined difference in wavelength or frequency.

30. The method of any one of Claims 23 to 29, wherein the wind turbine blade comprises first and second receivers located at the second position and spaced apart from one another in the chordwise direction of the blade, and the method comprises: receiving a first EM-radiation signal at the first receiver and a second EM- radiation signal at the second receiver in step (b); and

step (c) comprises calculating blade twist based upon the characteristics of the received first and second EM-radiation signals.

31. The method of Claim 30, comprising:

determining the difference in wavelength or frequency between the received first and second EM-radiation signals at step (c), and calculating the blade twist on the basis of the determined difference in wavelength or frequency.

32. The method of Claim 30 or 31 , wherein the wind turbine blade comprises a plurality of pairs of receivers, each pair comprising first and second receivers, and each pair being arranged at a different position along a length of the blade; and the method comprises:

calculating at step (c) the blade twist along the length of the blade on the basis of the characteristics of the first and second EM-radiation signals received at each pair of receivers.

33. The method of any one of Claims 23 to 32, comprising:

embedding a fingerprint within the EM-radiation signal, enabling the received EM- radiation signal to be distinguished from a background noise signal.

34. The method of Claim 33, comprising:

using the fingerprint to distinguish between the EM-radiation signal and the background noise signal.

35. The method of any one of Claims 23 to 34, comprising:

communicating the EM-radiation signal from a remotely-located source to the first position. 36. The method of any one of Claims 23 to 35, comprising:

communicating the received EM-radiation signal to a remotely-located detector.

37. The method of any one of Claims 23 to 36, comprising:

transmitting the EM-radiation signal from the first position to the second position inside the wind turbine blade.

38. The method of any one of Claims 23 to 36, comprising:

transmitting the EM-radiation signal from the first position to the second position outside the wind turbine blade.

39. The method of any one of Claims 23 to 36, comprising:

transmitting the EM-radiation signal from the first position located on a hub of the wind turbine rotor to the second position located on an external surface of the wind turbine blade.

40. The method of any one of Claims 37 to 39, comprising:

transmitting the EM-radiation signal from the first position, the first position being located substantially in the vicinity of the root of the wind turbine blade, to the second position, the second position being located substantially in the vicinity of the tip of the wind turbine blade

Description:
Improvements relating to wind turbines

Background Modern utility-scale wind turbines have rotors comprising very long, slender blades. Figure 1 shows a typical wind turbine blade 10, which tapers longitudinally from a relatively wide root end 12 towards a relatively narrow tip end 14. A longitudinal axis L of the blade is also shown in Figure 1. The root end 12 of the blade is circular in cross section. Outboard from the root, the blade has an airfoil profile 16 in cross section. The root of the blade is typically connected to a hub of the rotor via a pitch mechanism, which turns the blade about the longitudinal pitch axis L in order to vary the pitch of the blade.

Varying the pitch of a blade varies its angle of attack with respect to the wind. This is used to control the energy capture of the blade, and hence to control the rotor speed so that it remains within operating limits as the wind speed changes. In low to moderate winds it is particularly important to control the pitch of the blades in order to maximise the energy capture of the blades and to maximise the productivity of the wind turbine.

The energy capture of a wind turbine blade generally increases moving from the root towards the tip. Hence, the inboard or root part 12 of the blade 10 tends to capture the least energy, whilst the outboard or tip part 14 of the blade tends to capture the most energy. Precise control over the pitch angle of the outboard part of the blade is therefore desirable in order to maximise the output of the wind turbine. Modern wind turbine blades are typically 50-80 metres in length, or longer in some cases, and are generally made from composite materials such as glass-fibre reinforced plastic (GFRP). The blades are therefore relatively flexible and inevitably bend and twist to an extent during operation. The relatively narrow outboard part of the blade is in particular susceptible to twisting and bending.

Whilst the pitch mechanism allows precise control over the angle of the root of the blade, this does not necessarily reflect the angle of the tip of the blade, which is more susceptible to bending and twisting as described above. The present invention provides a method and apparatus for measuring the bending and/or twist angle of the blade caused by wind loads. These parameters can then be employed in control strategies for the wind turbine. For example, accurate measurements of blade bending and twist angles can be employed in pitch control strategies allowing precise control over the angle of attack of the outboard part of the blade so that the energy capture of the blade can be maximised. These measurements may also be employed in blade load calculations and control strategies for protecting the blades from extreme loads.

Summary of the Invention

Against this background, a first aspect of the present invention provides a wind turbine having at least one rotor blade. The wind turbine comprises a transmitter arranged to transmit an electro-magnetic (EM) radiation signal having spatially dependent characteristics; a receiver arranged to receive the EM-radiation signal, the receiver being located on the blade at a location outboard with respect to the transmitter and being arranged such that a deflection of the rotor blade results in a displacement of the receiver relative to the transmitter; and a processor arranged to calculate a deflection of the rotor blade based upon spatially dependent characteristics of the received EM- radiation signal. In this way an EM-radiation signal having spatially dependent characteristics can advantageously be used to calculate blade deflection. Within the present context the deflection of the blade of a wind turbine relates to any deviation of the blade from an arbitrarily defined reference state due to bending and/or twisting. This term is defined more precisely in the ensuing detailed description of the invention section. The term spatially dependent characteristics of the received EM-radiation signal, relates to any characteristic of the EM-radiation signal, which varies with spatial location. For example, the EM-radiation signal may be selected such that the wavelength and/or frequency vary with spatial location.

In some embodiments, the processor is arranged to calculate the deflection of the blade on the basis of the wavelength or frequency of the received EM-radiation signal.

The transmitter may be arranged to transmit a refracted EM-radiation signal, and the processor may be arranged to calculate the deflection of the blade based upon the wavelength or frequency of the refracted EM-radiation signal. A refracted EM-radiation signal is one non-limiting example of an EM-radiation signal having spatially dependent characteristics, and provides a convenient way of determining blade deflection. Since each wavelength/frequency component of the EM-radiation signal is refracted by a different amount, this enables the amount of blade deflection to be determined on the basis of the wavelength/frequency of the measured refracted EM-radiation signal. Some embodiments of the wind turbine may comprise a refracting element arranged to refract incident EM-radiation. The refracting element enables a refracted EM-radiation signal to be generated by transmitting an incident EM-radiation signal through the refracting element. Preferably, the refracting element is comprised in the transmitter.

The refracting element is preferably arranged to refract the EM-radiation signal in a plane transverse to a chordwise direction of the blade. For example, the plane is preferably substantially perpendicular to the chordwise direction, and the different wavelength/frequency components of the EM-radiation signal are refracted by different amounts in the plane. As the blade is deflected, a different one of the components of the refracted EM-radiation signal is measured by the receiver. The amount of blade deflection is correlated to the wavelength/frequency of the measured refracted EM- radiation signal, which correlation can be used to calculate blade deflection. Specifically, the amount of blade deflection may be calculated from the wavelength/frequency of the measured refracted EM-radiation signal. It is to be appreciated that the term transverse to a chordwise direction of the blade is used herein to mean that the EM-radiation signal is refracted in a plane at an angle with respect to the chordwise direction of the blade. The transmitter is preferably arranged to generate the refracted EM-radiation signal by transmitting a broadband EM-radiation signal through the refracting element. Within the present context, a broadband EM-radiation signal relates to any EM-radiation signal that comprises a range of wavelength/frequency components, such that when transmitted through the refracting element, the different components are refracted by different amounts.

In certain embodiments the broadband EM-radiation signal is white light, and the processor is configured to calculate the deflection of the rotor blade based upon the colour of the received EM-radiation signal.

In alternative embodiments, the broadband EM-radiation signal comprises wavelengths substantially in the range of 1540nm to 1560nm, and the processor is configured to calculate the deflection of the rotor blade based on the wavelength of the received EM- radiation signal. In certain embodiments, the refracting element may relate to a refracting prism, or alternatively to a diffraction grating, or any other device with such functionality.

The processor may be arranged to determine a difference between the wavelength or frequency of the received EM-radiation signal and the wavelength or frequency of a reference EM-radiation signal, and to calculate bending of the blade on the basis of the determined difference in wavelength or frequency.

In certain embodiments, the wind turbine further comprises first and second receivers located on the blade at a location outboard with respect to the transmitter, the first and second receivers being spaced apart from one another in a chordwise direction of the blade, the first receiver being arranged to receive a first EM-radiation signal, and the second receiver being arranged to receive a second EM-radiation signal. The processor is arranged to calculate blade twist based upon the wavelength or frequency of the received first and second EM-radiation signals. The use of first and second receivers spaced apart in the chordwise direction enables the twist of the blade to be determined.

Preferably, the processor is arranged to determine the difference in wavelength or frequency between the received first and second EM-radiation signals, and to calculate the blade twist on the basis of the determined difference in wavelength or frequency. Twisting of the blade results in a relative displacement of the first and second receivers with respect to the EM-radiation signal, such that each receiver measures a different wavelength/frequency as the blade twists. The amount of blade twist may therefore be calculated on the basis of the difference in measured wavelength/frequency.

In certain embodiments, the wind turbine comprises a plurality of pairs of receivers, each pair comprising first and second receivers, and each pair of receivers being arranged at a different position along a length of the blade. The processor is arranged to calculate the blade twist along the length of the blade on the basis of the wavelength or frequency of the first and second EM-radiation signals received at each pair of receivers. By configuring the wind turbine with a plurality of pairs of receivers as described, the blade twist may be determined accurately along the length of the blade, even in conditions where the blade is subject to a non-uniform twist along its length.

The wind turbine may comprise a signal modulator arranged to imprint a fingerprint in the EM-radiation signal, enabling the received EM-radiation signal to be distinguished from background noise. Preferably, the detector is arranged to distinguish between the received EM-radiation signal and background noise, on the basis of a fingerprint present in the received EM-radiation signal. Use of a fingerprint is a convenient way of enabling the received EM-radiation signal to be distinguished from any background noise, improving the accuracy of the calculated blade deflection. Any type of signal modulation may be used to embed the fingerprint in the EM-radiation signal.

In certain embodiments, the wind turbine comprises an EM-radiation source located remotely from the transmitter, and a first optical fibre extending along at least a portion of the length of the blade between the source and the transmitter. The first optical fibre is arranged to communicate EM-radiation between the source and the transmitter. This decreases the amount of hardware apparatus present in the blade and also helps to minimise the weight of the blade. Similarly, the wind turbine may comprise a detector located remotely from the receiver, and a second optical fibre extending along at least a portion of the length of the blade between the receiver and the detector. The second optical fibre is arranged to communicate EM-radiation between the receiver and the detector. Use of a remotely located detector helps minimise the weight of the blade.

The use of optical fibres also provides other advantages, that are now explained in further detail. Modern wind turbines are very tall structures, and the blades are particularly susceptible to lightning strikes. Therefore, most wind turbine blades incorporate lightning protection systems for conducting the electrical energy from lightning strikes safely to ground. The present invention aims to avoid the use of metal parts or electrical components on wind turbine blades as these can attract lightning strikes in preference to the lightning receptors on the blade, which may cause damage to the blade, by using optical fibres where possible as waveguides for the EM-radiation. In some embodiments, the wind turbine is arranged such that the transmitter and the receiver are located within the wind turbine rotor blade. This arrangement of transmitter and receiver protects the transmitter and the receiver from the surrounding environment, and ensures that EM-radiation signal measurements are unaffected by external weather conditions. Alternatively, the wind turbine may be arranged such that the transmitter and the receiver are located on an external surface of the wind turbine rotor blade. This arrangement ensures that the transmitter and receiver are accessible for maintenance purposes. In yet a further alternative embodiment, the transmitter may be located on a hub of the wind turbine rotor substantially in the vicinity of the root of the wind turbine blade, and the receiver is located on an external surface of the wind turbine blade.

A second aspect of the present invention relates to a method of determining deflection of a blade of a wind turbine rotor during rotation of the rotor. The method comprises the steps of:

(a) transmitting from a first position an electro-magnetic (EM) radiation signal having spatially dependent characteristics;

(b) receiving the EM-radiation signal at a second position, the second position being located on the blade at a location outboard with respect to the first position, and being arranged such that a deflection of the rotor blade results in a displacement of the second position relative to the first position; and

(c) calculating the deflection of the blade based upon the spatially dependent characteristics of the received EM-radiation signal.

In certain embodiments step (a) comprises refracting an EM-radiation signal and transmitting the refracted EM-radiation signal towards the second position. Use of a refracted EM-radiation signal provides a convenient way of determining blade deflection. Since each wavelength/frequency component of the EM-radiation signal is refracted by a different amount, this enables the amount of blade deflection to be calculated on the basis of the wavelength/frequency of the measured refracted EM-radiation signal.

In certain embodiments step (a) comprises transmitting a broadband EM-radiation signal through a refracting element. This provides a convenient way to generate a refracted EM-radiation signal from an incident broadband EM-radiation signal.

In certain embodiments the broadband EM-radiation signal may be white light, and step (c) of the method comprises calculating the deflection of the blade based upon the colour of the refracted light. Alternatively, the broadband EM-radiation signal may comprise wavelengths substantially in the non-visible range of 1540nm to 1560nm. In certain embodiments the method comprises refracting the broadband EM-radiation signal in a plane transverse to the chordwise direction of the blade, such that each wavelength or frequency component of the broadband EM-radiation signal is refracted by a different amount in the plane. This ensures that as the blade is deflected, a different wavelength or frequency component is measured. Accordingly, a correlation exists between the blade deflection and the wavelength/frequency of the measured refracted EM-radiation signal, which correlation can be used to calculate the amount of blade deflection. Step (c) of the method may comprise determining the difference between the wavelength or frequency of the received EM-radiation signal and a wavelength or frequency of a reference EM-radiation signal; and calculating bending of the blade on the basis of the determined difference in wavelength or frequency. In certain embodiments, the wind turbine blade comprises first and second receivers located at the second position and spaced apart from one another in the chordwise direction of the blade. The method further comprises: receiving a first EM-radiation signal at the first receiver and a second EM-radiation signal at the second receiver in step (b); and step (c) comprises calculating blade twist based upon the characteristics of the received first and second EM-radiation signals. As mentioned previously in relation to the first aspect of the invention, the use of first and second receivers spaced apart in a chordwise direction of the blade enables the twist of the blade to be determined.

The method may further comprise determining the difference in wavelength or frequency between the received first and second EM-radiation signals at step (c), and calculating the blade twist on the basis of the determined difference in wavelength or frequency.

In certain embodiments, the wind turbine blade comprises a plurality of pairs of receivers, each pair comprising first and second receivers, and each pair being arranged at a different position along a length of the blade; and the method comprises: calculating at step (c) the blade twist along the length of the blade on the basis of the characteristics of the first and second EM-radiation signals received at each pair of receivers. As mentioned previously in relation to the first aspect of the invention, the use of a plurality of pairs of receivers enables the blade twist to be determined accurately when the blade is subject to a non-uniform twist along the length of the blade. Preferably, the method comprises embedding a fingerprint within the EM-radiation signal, enabling the received EM-radiation signal to be distinguished from a background noise signal. Furthermore, the method may comprise using the fingerprint to distinguish between the EM-radiation signal and the background noise signal. The ability to distinguish between the received EM-radiation signal and background noise improves the accuracy of the calculated blade deflection.

Preferably, the method comprises communicating the EM-radiation signal from a remotely-located source to the first position. For example, via a first optical fibre extending along at least a part of the length of the wind turbine blade. Similarly, the method may comprise communicating the received EM-radiation signal to a remotely- located detector. For example, via a second optical fibre extending longitudinally along at least a part of the length of the blade between the second position and the detector. This arrangement provides the same advantages as described previously in relation to the corresponding feature of the first aspect of the invention.

The method may comprise transmitting the EM-radiation signal from the first position to the second position inside the wind turbine blade. This ensures that the measured EM- radiation signals are unaffected by external weather conditions. Alternatively, the method may comprise transmitting the EM-radiation signal from the first position to the second position outside the wind turbine blade. In yet a further alternative, the method may comprise transmitting the EM-radiation signal from the first position located on a hub of the wind turbine rotor to the second position located on an external surface of the wind turbine blade.

Preferably, the method comprises transmitting the EM-radiation signal from the first position, the first position being located substantially in the vicinity of the root of the wind turbine blade, to the second position, the second position being located substantially in the vicinity of the tip of the wind turbine blade. In this way, the calculated deflection provides an accurate description of the blade tip deflection, which may be used in pitch control strategies.

Figures Figure 1 , which is a perspective illustration of an exemplary wind turbine blade having a circular cross-section at the root, and an airfoil cross-section profile outboard from the root, has already been described above by way of background to the present invention. In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of non-limiting example, with reference to the following figures, in which:

Figure 2a is a schematic illustration of the cross-section of the tip of the blade of Figure 1 having a blade tip angle of 0 degrees; whilst Figure 2b illustrates a blade tip angle of θ>0 degrees; and

Figure 3a is a schematic illustration of the cross-section of the blade taken along its longitudinal axis when the blade is straight (i.e. the blade tip bending angle or=0 degrees); whilst Figure 3b illustrates a bent blade having a bending angle a>0 degrees.

Figure 4 is a front view of a rotor-hub assembly of a horizontal axis wind turbine, configured in accordance with an embodiment of the invention;

Figure 5a is a perspective view of a wind turbine blade configured to determine blade tip bending in accordance with the embodiment of Figure 4; whilst Figure 5b is a cross-sectional side view of the straight wind turbine blade of Figure 5a taken along its longitudinal axis L; and Figure 5c is the cross-sectional view of the wind turbine blade of Figures 5a and 5b when bent as a result of wind loads;

Figure 6a is a perspective view of a wind turbine blade configured to determine both blade tip bending and blade tip twist angle in accordance with an embodiment of the present invention; whilst Figure 6b is a cross-sectional view of the blade tip of Figure 6a, taken in a plane perpendicular to the blade's longitudinal axis; and

Figure 7 is a side perspective view of a wind turbine rotor configured to determine blade tip bending and/or twist in accordance with an alternative embodiment of the present invention, wherein the detector and the receiver are located outside the blade. Detailed Description The tip angle of the blade is defined herein as the angle between the chord line of the blade at the tip and a reference axis in a plane perpendicular to the longitudinal axis L of the blade, as will now be described by way of example with reference to Figures 2a and 2b. The chord line is the straight line D connecting the leading edge 18 of the blade to the trailing edge 20 of the blade 10. The chordline D is also readily viewable in Figure 1.

Figures 2a and 2b illustrate a cross-section of the tip of the wind turbine blade 10 in a plane perpendicular to the longitudinal axis L and taken along the line A-A in Figure 1. In Figure 2a the blade 10 has a first tip angle, whilst in Figure 2b the blade 10 has a second tip angle. The tip angle is marked Θ in Figures 2a and 2b.

The longitudinal axis L is perpendicular to the plane of the page in Figures 2a and 2b. The x-axis in Figures 2a and 2b is generally parallel to a rotor axis about which the blades turn substantially in the L-y plane. It will be appreciated that, in reality, the wind turbine blades of the rotor are arranged to have a certain cone angle to avoid tower strike and the blades bend and flex in use, hence the rotor plane would not strictly coincide with the L-y plane, but this approximation is used to simplify the discussion and understanding of the present invention.

The direction of rotation of the rotor about the rotor axis is indicated by R in Figures 2a and 2b. As the rotor turns through an angle of 2π radians (i.e. 360° degrees), the blade traces a circle in the L-y plane. The wind direction is indicated as W in Figures 2a and 2b. In Figures 2a and 2b the wind direction is illustrated as being perpendicular to the L-y plane, although in practice the direction of the wind relative to the L-y plane varies, and may be incident at different angles. In Figure 2a the blade tip angle Θ is arbitrarily defined as 0° degrees when the chordline D is parallel to the x-axis, and therefore perpendicular to the L-y plane.

Figure 2b illustrates the blade tip turned through an angle © with respect to the x-axis. In the subsequent discussion of the invention, the above definition of the blade tip angle will be applied. In other words, the blade tip angle Θ is defined with respect to an arbitrary axis (for example, the x-axis of Figures 2a and 2b) formed generally perpendicular to the plane of rotation (the L-y plane of Figures 2a and 2b) of the blade. However, it will be appreciated that the tip angle could be defined relative to another arbitrary reference and so this definition should not be interpreted as limiting the scope of the present invention.

Blade tip twisting occurs when the blade tip angle e b i ade ti p differs from the pitch angle of the blade defined in the vicinity of the root of the blade 12. In the following description of the invention the twist angle will be denoted Θ, and it is to be appreciated that the twisting angle is defined relative to the pitch angle of the root of the blade 12. The total blade tip angle 6 b i ade tip is the sum of the blade's pitch angle θ ΓΟΟί defined at its root and the twist angle Θ.

Blade bending typically occurs when the wind turbine blade is subjected to a large external load perpendicular to the blade's longitudinal axis L. This can cause the blade to bend, which may result in significant displacement of the blade tip from the straight longitudinal axis L. Blade tip bending is best defined herein with respect to Figures 3a and 3b.

Figure 3a is a side perspective view of the wind turbine blade 10 taken along the longitudinal axis L, when the blade is straight. Figure 3b shows the blade 10' when bent, e.g. when the outboard part of the blade is subject to wind loads. As a result of these wind loads, the blade tip 14 in Figure 3b is deflected by an amount δ perpendicular to the longitudinal axis L. The straight line L' in Figure 3b is a line that extends between the deflected tip of the blade and the longitudinal axis L at a point C at the root of the blade, where the blade remains substantially straight. A blade tip bending angle or may be defined as the angle between L' and L In the subsequent discussion of the invention, the above definition of the blade tip bending angle will be applied. However, it will be appreciated that the tip bending angle could be defined relative to another arbitrary reference and so this definition should not be interpreted as unduly limiting the scope of the present invention. Figure 4 schematically illustrates a rotor-hub 22 assembly as featured in a horizontal axis wind turbine. The illustrated rotor-hub assembly 22 comprises three turbine blades 24a, 24b, 24c affixed to a central hub 26 via respective pitch mechanisms (not illustrated). The blades 24a, 24b, 24c have a cross-sectional profile 16 as illustrated in Figure 1 , and are arranged to cause an anti-clockwise rotation of the rotor-hub, as indicated by the directional arrows 28, when wind is incident on the blades 24a, 24b, 24c in a substantially planar direction perpendicular to and into the plane of the page.

Each blade 24a, 24b, 24c of the rotor-hub 22 assembly is configured with at least one optical transmitter 30, and at least one optical receiver 32. The at least one optical transmitter 30 is separated from the at least one optical receiver 32 in the longitudinal direction of the blade 24a, 24b, 24c. The optical receiver 32 is located substantially in the vicinity of the tip of the blade 24a, 24b, 24c, to enable accurate determination of the bending angle a at the blade tip. The bending angle a may not be constant throughout the blade's length, and therefore by positioning the optical receiver 32 substantially in the vicinity of the blade tip ensures that the determined bending angle a is an accurate reflection of the state of the blade tip. The at least one optical transmitter 30 is located substantially in the vicinity of the root of the blade 24a, 24b, 24c. This also improves the accuracy with which the bending angle a is determined. In use the root is unlikely to bend, and therefore provides a convenient reference point with respect to which the bending of the blade tip may be determined.

The at least one optical transmitter 30 located on each blade 24a, 24b, 24c comprises an optical refracting element 34. The optical transmitter 30 is configured to illuminate the optical refracting element 34 with a broadband light signal. The broadband light signal is refracted into its component wavelengths/frequencies when transmitted through the refracting element 34. A refracted light signal is received by the optical receiver 32 located in its respective blade 24a, 24b, 24c. The blade tip bending angle a is calculated on the basis of the characteristics of the detected refracted light signal, specifically on the basis of the wavelength and/or frequency of the refracted light signal detected at the optical receiver 32.

In a preferred embodiment, the optical transmitter 30, comprising the optical refracting element 34, and the optical receiver 32 are located within the turbine blade 24a, 24b, 24c as illustrated in Figures 5a and 5b. In order to avoid electrically conducting material present within the blades 24a, 24b, 24c, optical fibres are used to transmit the broadband optical light signal from a remote light source located inside the hub 26 to the optical transmitter 30 located substantially in the vicinity of the root of the blade 24a, 24b, 24c. Similarly, optical fibres are used to transmit the detected refracted light signal from the optical receiver 32 located substantially in the vicinity of the blade tip, to an optical sensor 42, located remotely from the optical receiver 30 within the hub 26.

For example, each optical transmitter 30 is provided with a first optical fibre 36 extending longitudinally along the blade, as illustrated in Figure 4. Each optical transmitter 30 is coupled to the broadband light source 38 located in the hub 26 at one end, by the first optical fibre 36. A second optical fibre 40 is operatively coupled to the optical sensor 42 at one end, and to the optical receiver 32 at its other end. Within the present context the term broadband light source is used to describe any light source, which emits light having a plurality of different wavelength/frequency components. For example, a light source emitting white light is one example of a broadband light source.

As mentioned previously, the use of optical fibres 36, 40 avoids electrically conducting material located in the blades 24a, 24b, 24c, which would be susceptible to lightning strikes in adverse weather conditions. The remotely located optical sensor 42 is located within the hub 26 in this example, and is configured to measure the wavelength and/or frequency of the detected refracted light signal. The optical sensor 42 is connected to a processor (not shown) arranged to determine the blade tip bending angle or, on the basis of the wavelength and/or frequency of the detected refracted light signal.

The details of how the blade tip bending angle a is determined from the wavelength and/or frequency of the detected refracted light signal will now be explained in detail with reference to the remaining figures.

Figure 5a schematically illustrates a perspective view of one of the turbine blades 24a of Figure 4. White light is emitted from the second optical fibre 40 and illuminates the optical refracting element 34. The white light is refracted into its component wavelengths/frequencies as it is transmitted through the optical refracting element 34, forming a rainbow fan of light 44. The amount by which each component wavelength/frequency is refracted depends upon the properties of the optical refracting element 34, such as its refractive index, as well as the wavelength/frequency of the component light. For present purposes a detailed analysis of optical refraction theory is not required. It suffices to appreciate that the different wavelength/frequency components of the white light will be refracted by different amounts - no two wavelength/frequency components will be refracted by the same amount.

The optical refracting element is arranged such that each different constituent wavelength/frequency component of the white light is refracted by a different amount in a plane transverse to the chordwise direction of the blade. Preferably, the white light is refracted in a plane substantially perpendicular to the chordwise direction of the blade. The chordwise direction of the wind turbine blade is highlighted in Figures 1 and 5a as the line D. In other words, the optical refracting element is arranged such that the angle of refraction is comprised in the L-y plane formed preferably perpendicular to the chordwise direction D.

Figure 5b is a schematic cross-sectional side view of the blade 24a taken in the L-y plane when the blade is straight. The figure shows how the diffracted light forms a rainbow fan of light 44 when the optical refracting element 34 is arranged to refract the light in the L-y plane. In this example, the white light is split into its component wavelengths (i.e. the various colours of the visible spectrum), with each wavelength/frequency being refracted at a different angle in the L-y plane. When the blade 24a is straight, the optical receiver 32 measures a first wavelength/frequency component λ .

Figure 5c is a schematic cross-sectional side view of the blade 24a in the L-y plane when the blade 24a is bent, i.e. when the outboard part of the blade is subject to wind loads. In this case, the optical receiver 32 measures a second wavelength/frequency component ^, which is different to the first wavelength/frequency component λ that is measured when the blade is straight (Figure 5b). This arises because the bending of the blade results in the position of the optical receiver 32 being displaced relative to the rainbow fan of light 44 and to the transmitter 30. Figure 5b and 5c clearly show how a different wavelength/frequency component of the refracted light is measured by the optical receiver 32 as the blade 24a bends.

The blade tip bending angle or is determined from the wavelength/frequency of the detected refracted light signal, knowing the refractive properties of the optical refracting element 34. Equation 1 below illustrates the relationship between the longitudinal distance d (see Figure 5a) between the optical transmitter 30 (comprising the optical refracting element 34) and the optical receiver 32; the displacement δ (see Figure 3b) of the receiver in the L-y plane; and the bending angle or a = arc tan (^j eq.1

Consider the following example, provided herein for illustrative purposes only, where the longitudinal distance d between the optical transmitter 30 (comprising the optical refracting element 34) and the optical receiver 32 is 1 ,000mm, and the displacement δ of the optical receiver 32 is 1 mm. Using equation 1 , the bending angle a is then

, lmm . _ _ _ 0

a = arc tan ( ) = 0. 057° The relationship between a change in the wavelength Αλ of the detected refracted light signal and the bending angle a is given by equation 2 below:

Αλ = γ * a eq.2 where γ is the dispersion coefficient of the optical refracting element 34, and has the units of nm per angular degree (i.e. nm/°). Equation 2 may be expressed in terms of a change in measured frequency using equation 3 below, which highlights the relationship between the frequency and the wavelength λ of a wave = ν/λ eq.3 where v is the speed of light within the propagating medium, and is associated to the speed of light in vacuum c, by equation 4 below: v = c/n eq.4 where n is the refractive index of the propagating medium.

Continuing with the above provided worked example, consider the dispersion coefficient of the optical refracting element 34 being 100nm/° Using equation 2 above, the change in wavelength Αλ of the refracted light signal associated with the bending angle or=0.057° is

100nm/° * 0.057° = 5.7nm

This shows that a 0.057° bending angle would manifest itself as a wavelength change of 5.7nm in the detected refracted light signal.

In practice, the wavelength/frequency detected at the sensor is monitored and may be used to determine the blade bending angle a using equation 5 below, which is a rearrangement of equation 2 above:

In practice, the optical receiver 32 and the optical transmitter 30 are calibrated prior to use, such that a reference wavelength/frequency of the refracted light signal associated with a straight blade is determined. The reference wavelength/frequency can be determined when the blade is originally manufactured, for example, or in conditions where it is known that the blade is not subject to any external loads causing bending of the blade. The reference wavelength/frequency is then used as a benchmark with respect to which subsequent refracted light signal measurements are compared to determine the change in wavelength/frequency resulting from the bending of the blade. Using equation 5, the bending angle a is then determined.

For example, if the reference wavelength is 500nm, and the wavelength of the measured refracted light signal is 505.7nm, then the bending angle is or=0.057° for y = 100nm/°.

It is to be appreciated that the above examples are for illustrative purposes only. The provided lengths and angles are not indicative of real world lengths, but are provided to facilitate the reader's understanding of the invention. Real-world turbine blades are known to be of the order of 50m to 80m in length. Accordingly, the longitudinal distance of separation between the optical transmitter 30 and the optical receiver 32 is expected to be significantly larger than the 1000mm set out in the above provided illustrative example. Figure 6a schematically illustrates an alternative arrangement comprising two optical receivers 32, 46. The second optical receiver 46 is displaced from the first optical receiver 32 in the chordwise direction of the turbine blade 48. This arrangement also enables the twist angle of the blade tip to be determined. As mentioned previously and as illustrated, the chordwise direction is taken to be perpendicular to the blade's longitudinal axis L, in the direction of the line D.

As mentioned by way of introduction, when the blade tip 14 is subjected to large loads, this can result in the blade tip twisting about the longitudinal axis relative to the blade root 12, and forming a twist angle Θ with respect to the untwisted blade root 12. Figure 6b, which is a cross-sectional schematic of the blade tip of Figure 6a, taken in the x-y plane perpendicular to the blade's longitudinal axis L, shows how twisting of the blade tip introduces a relative vertical displacement χ parallel to the y-axis, between the two optical receivers 32, 46. The line A-A comprises the position of the first optical receiver 32' when the blade is in an untwisted state. In other words, if the blade 48 was not twisted about the longitudinal axis L, then the first optical receiver 32 would be positioned at the point marked 32'.

The relative vertical displacement along the y-axis between the two optical receivers 32, 46 is introduced by the twisting of the blade tip 14 relative to the blade's root end 12. Since the root end comprises the optical transmitter 30, the twisting of the blade tip 14 relative to the root end 12, is analogous to twisting of the two optical receivers 32, 46 relative to the optical transmitter 30. The twist angle Θ is given by

Θ = arcsin(-) eq.6 where ε is the distance of separation between the two optical receivers 32, 46.

As a result of this twisting, each one of the optical receivers 32, 46 detects a different wavelength/frequency component of the refracted light signal as the various wavelengths/frequencies of light fan out in the L-y plane. The vertical displacement along the y-axis between the two optical receivers 32, 46 is proportional to the difference in wavelength Αλ between the components of the refracted light signal detected by each one of the optical receivers 32, 46, as expressed by equation 7 below:

X = kA eq. 7 where k is the constant of proportionality, k is a characteristic of the optical refracting element and has the units of mm/nm, and describes the refracting characteristics of the refracting element. With reference to Figure 6b, k quantifies the amount of vertical displacement associated with a unit change in the difference in wavelength measured by each respective optical receiver 32, 46 for a specific distance of separation d between the optical receivers 32, 46 and the optical transmitter 30. Defined in this way, k is a function of d. k may be determined empirically at the source of manufacture for a given configuration of receivers, transmitters, and refracting element. Alternatively, and by way of non-limiting illustrative example, the refracting characteristics of the refracting element may be defined in terms of a refracting angle , which has the units of °/nm. In other words, Φ describes the angular refraction of incident light per unit wavelength as the light is transmitted through the refracting element. When the refracting element is configured to refract incident light in the L-y plane, the cross-section of the refracted light forms a right-hand triangle in the L-y plane, and the longitudinal distance of separation between the optical transmitter 30 and receivers 32, 46 is d, then equation 7 may be rewritten as: χ = d ΐαη(ΦΔλ) eq.8 where ΦΔλ is the total angle through which the measured light signal is refracted. As with k, Φ may be determined empirically. In contrast with k, Φ is not a function of the distance of separation d between the transmitter and the receivers. Equation 8 only holds where the cross-section of the refracted light in the L-y plane forms a right angle triangle. Equation 8 is introduced for illustrative purposes only to show that the refracting element's refraction characteristics may also be expressed in terms of an angle of refraction. For completeness it is to be appreciated that the precise form of equation 8 will be dependent on the cross-sectional geometry in the L-y plane of the refracted light. Equation 6 may be re-expressed in terms of the difference in wavelength detected by the two optical receivers 32, 46

Θ = arcsin— ) eq.9

Likewise, equation 9 may be re-expressed in terms of Φ e = arc sm[ ^— -] eq.10

It is important to note that the difference in wavelength Αλ of equations 7, 8, 9 and 10 is different to the difference in wavelength of equation 2. In equation 2, the difference in wavelength is the difference in wavelength between the refracted reference signal measured when the blade is straight and the wavelength of the refracted signal measured when the blade is bent. The difference in wavelength Αλ of equations 7, 8, 9 and 10 is the difference between the two wavelength signals measured respectively and simultaneously by the two optical receivers 32, 46 when the turbine blade is twisted about the longitudinal axis L relative to the blade's root 12.

The following illustrative example shows how equation 9 can be used to determine the blade tip twist angle Θ.

Consider a configuration of optical receivers wherein the distance of separation ε in the chordwise direction between the first and second optical receivers 32, 46 is 200mm, and the difference in wavelength measured by the two receivers is 0.1 mm. Using equation 9, the twist angle Θ is found to be 0.0286°.

The above illustrative examples assume that the first and second optical receivers 32, 46 are both located substantially in the vicinity of the blade tip 14. This is not a strict requirement of the present invention. Reliable blade tip bending and twist angles may be determined regardless of precisely where the optical transmitter 30 and the optical receivers 32, 46 are located along the blade's length, provided that certain assumptions are made and that the transmitter 30 and receivers 32, 46 are displaced longitudinally along the blade's length relative to each other. For example, if it is assumed that the bending and/or twist of the blade is uniform, then it is possible to define a bending angle and/or twist angle per unit length of the blade. The total bending angle and/or twist angle is then determined by multiplying the calculated bending angle and/or twist angle per unit length, by the total length of the blade.

For example, if the blade length is 50m, and the twist angle Θ is found to be 0.0286° when the optical transmitter 30 is longitudinally separated from the optical receivers 32, 46 by a distance of 1 m (e.g. 1000mm), then the twist angle per meter of longitudinal blade length is 0.0286 m. Assuming a uniform twist throughout the blade's longitudinal length with respect to the blade root 12, the total twist angle of the blade is then 50m * 0.0286 m = 1.4°. In other words, the total twist angle at the tip of the blade is 1.4° with respect to the blade root 12. An advantage of the embodiment illustrated in Figures 6a and 6b is that this arrangement of receivers enables both bending angle and twist angle to be determined.

The bending angle a is determined as described previously. If both optical receivers 32, 46 detect substantially the same wavelength/frequency of diffracted light, and that wavelength/frequency is different from the reference wavelength/frequency, then the blade is only subject to bending. If however, both receivers 32, 46 simultaneously detect different wavelengths/frequencies, then the blade is subject to twisting. If neither one of the receivers 32, 46 detects the reference wavelength/frequency, and both receivers 32, 46 detect a different wavelength/frequency, then the blade is subject to both twisting and bending.

In short, to determine bending, the difference in wavelength/frequency between the measured diffracted light signal and the wavelength/frequency of the reference diffracted signal is required. To determine twist, the difference in wavelength/frequency between the diffracted light signals measured simultaneously by the two optical receivers 32 ,46 is required.

To more accurately determine the turbine blade's bending angle and twist angle along the blade's length, alternative arrangements comprising a plurality of different pairs of optical receivers arranged along the turbine blade's length are envisaged. Such arrangements are particularly useful where the blade bending angle and the blade twist angle are non-uniform throughout the blade's length. In such embodiments, each pair of optical receivers is located at different longitudinal positions along the blade's length. In this way, each pair of optical receivers enables an accurate bending and/or twist angle of the blade to be determined at the associated longitudinal blade position.

Similarly, in those applications where it is only required to determine the blade bending angle accurately along the blade's length, a plurality of different optical receivers may be arranged at different positions along the blade's length. The bending angle associated with each optical receiver position may then be determined as described previously. Fingerprints may be used to enable background signal noise to be effectively removed from the one or more detected refracted signals. For example, the transmitted broadband signal emitted from the optical transmitter 30 may be modulated with a periodic waveform, which is readily distinguishable from noise, which tends to have a random waveform. This enables the optical detector to filter the received diffracted light signal from background noise. For example, the transmitted broadband signal may be modulated with a pulsed periodic waveform, which is easily distinguishable from background noise. In order to amplify the detected refracted signal, the fibre optics may be provided with an optical amplifier, such as a doped fibre amplifier, which amplifies the intensity of the optical signal propagating through the fibre optics. The optical amplifier may be comprised in the second optical fibre 40, and improves signal detection by the detector. An erbium doped optical fibre amplifier is one non-limiting example of a doped fibre amplifier that may be used with the present invention.

Figure 7 is a schematic illustration of an alternative embodiment of the present invention, where the optical receiver 32 is located on the external surface of the turbine blade. The optical receiver 32 is located preferably on the external surface, substantially in the vicinity of the blade tip. The optical transmitter 30 is located in the vicinity of the root of the blade 50 on the hub 26, although alternatives are also envisaged wherein the optical transmitter 30 is located on the external surface of the blade in the vicinity of the root. The second optical fibre 40 is coupled at one end to the optical receiver 32 and at its other end to the optical detector 42. The second optical fibre 40 is preferably arranged within the blade 50, in order to protect it from external events, which could damage the fibre.

In the illustrated embodiment the optical transmitter 30 is also the optical source. Alternatively, the optical transmitter 30 could be coupled to a light source located within the rotor hub 26 as described above in relation to previous embodiments.

The bending angle of the blade is determined in the same manner as described above in relation to the previous embodiments.

To determine blade twist, two optical receivers (not shown) arranged in the chordwise direction, as described previously in relation to Figure 6a, are affixed to the external surface of the blade 50, and blade twist is determined in the same manner as described in relation to Figure 6a. Although this example is not illustrated in Figure 7, it is to be appreciated that this example also falls within the scope of the present invention. The external surface of the blade 50 may be configured with a plurality of different optical receivers located at different positions along the blade's length, to enable the bending angle to be determined precisely at different blade lengths. Similarly, the external surface of the blade 50 may be configured with a plurality of pairs of different optical receivers located at different positions along the length of the blade, each pair being arranged in the chordwise blade direction as described previously. Such an arrangement enables the twist and bending angles to be determined accurately along the length of the blade.

The optical detector may relate to an analog-to-digital (ADC) converter. The optical fibres used in the afore-described embodiments may be either single or multi- mode. Multi-mode fibres have greater light gathering characteristics and so may be preferable in certain applications.

In a further alternative embodiment, a plurality of different sources may be used to emit EM-radiation. For example, a plurality of light sources may be used, with each source arranged to emit light having spatially localised propagation characteristics, and each source emitting light having a different wavelength/frequency, and at a different angle relative to the one or more receivers. A deflection of the blade may then be determined as described in relation to previous embodiments, on the basis of the detected wavelength/frequency of light. Sources emitting light having a Gaussian beam profile, such as lasers, are one non-limiting example of light sources emitting light having spatially localised propagation characteristics. Use of a refracting element is not required in such alternative embodiments. The herein described embodiments may be used in pitch control strategies and/or in other strategies to control stress loads on the blade, in order to avoid damage to the blades caused by excessive loads.

Whilst the preceding described embodiments comprise a white light broadband light source, other broadband light sources may also be used in conjunction with the present method. Specifically, the present method may be used with any light source emitting light comprising a spectrum of different wavelengths/frequencies. For example, a light source emitting light in the range 1540-1560 nm, which is invisible to the naked eye, may be used in place of the white light source. The herein described method may be used to calculate the blade tip angle for a wind turbine comprising any number of turbine blades. Whilst the herein illustrated

embodiments relate to wind turbines comprising three blades, this is non-limiting. The present method may be used with wind turbines comprising any number of blades, and may be used to accurately determine blade bending and/or twist angle whilst the rotor is in use.

Furthermore, knowledge of blade bending and twist can be used to calculate the remaining lifetime of the blade. All herein provided embodiments are provided for illustrative purposes only and are not to be construed as limiting to the invention. It is to be appreciated that alternative embodiments are envisaged comprising suitable combinations of features of the previously described embodiments, and such alternatives fall within the scope of the present invention.

Claims

1. A wind turbine having at least one rotor blade and comprising:

a transmitter arranged to transmit an electro-magnetic (EM) radiation signal having spatially dependent characteristics;

a receiver arranged to receive the EM-radiation signal, the receiver being located on the blade at a location outboard with respect to the transmitter and being arranged such that a deflection of the rotor blade results in a displacement of the receiver relative to the transmitter; and

a processor arranged to calculate a deflection of the rotor blade based upon spatially dependent characteristics of the received EM-radiation signal.

2. The wind turbine of Claim 1 , wherein the processor is arranged to calculate the deflection of the blade on the basis of the wavelength or frequency of the received EM- radiation signal.

3. The wind turbine of Claim 2, wherein the transmitter is arranged to transmit a refracted EM-radiation signal, and the processor is arranged to calculate the deflection of the blade based upon the wavelength or frequency of the refracted EM-radiation signal.

4. The wind turbine of Claim 3, further comprising a refracting element arranged to refract incident EM-radiation.

5. The wind turbine of Claim 4, wherein the refracting element is arranged to refract the EM-radiation signal in a plane transverse to a chordwise direction of the blade.

6. The wind turbine of Claim 4 or Claim 5, wherein the transmitter is arranged to generate the refracted EM-radiation signal by transmitting a broadband EM-radiation signal through the refracting element.

7. The wind turbine of Claim 6, wherein the broadband EM-radiation signal is white light, and the processor is configured to calculate the deflection of the rotor blade based upon the colour of the received EM-radiation signal. 8. The wind turbine of Claim 6, wherein the broadband EM-radiation signal comprises wavelengths substantially in the range of 1540nm to 1560nm, and the