Technical Field

The present invention relates to wind turbine technology and specifically to a system and method for monitoring the deflection of wind turbine blades.

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 particularly 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 mentioned above. The present invention provides a method and apparatus for measuring the bending 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 angle 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

A first aspect of the invention relates to a wind turbine having at least one rotor blade, comprising a system for determining a parameter indicative of blade deflection. The system comprises an effective source of electro-magnetic (EM) radiation arranged to transmit an EM radiation signal; a receiver arranged to receive the EM radiation signal, the signal having an irradiance, and the receiver being spaced apart from the effective source in a spanwise direction of the blade. The effective source and the receiver are arranged such that deflection of the blade results in a straight-line distance between the effective source and the receiver varying. The system also comprises a processor configured to determine a parameter indicative of blade deflection and/or blade loading on the basis of the irradiance of the EM radiation signal received by the receiver.

The "term effective" source as used herein refers to the apparent position of an electromagnetic (EM) light source with respect to a point of view, such as from the point of view of a receiver. Consequently, the position of an effective source does not always correlate with the position of a physical EM light source. The presence of optical instruments lying in the optical path between a physical EM light source and a receiver often have the effect of shifting the position of the effective source relative to the position of the physical source, as observed by a receiver. This can be understood by considering the example where a focusing lens is introduced in the optical path between a laser and an optical receiver. With respect to the receiver, the position of the effective source is the point of incidence of the laser beam on the focusing lens. In other words, from the point of view of the receiver, it appears as if the received EM radiation is transmitted from the point of incidence of the laser beam on the focusing lens. Similarly, if the receiver is encased within a protective housing, then the effective source is the point of incidence of the laser beam on the protective housing, since it appears to the receiver as if the received EM signal has been transmitted from the point of incidence on the protective housing. In other examples the position of the effective source may correlate with the location of the physical source, for example, the location of a laser emitting an EM signal. This may occur for example, where no optical instruments are placed in the optical path lying between the physical source and the receiver, in which case from the point of view of the receiver the received EM radiation appears as if it had originated from the location of the physical source. This definition of effective source will be adopted in the present description and claims, and it should be appreciated that the position of the effective source may not correlate with the position of the physical source in all hereinafter described embodiments. The previously described arrangement of receiver and effective source ensures that as the blade is deflected, the irradiance measured by the receiver varies as the straight-line distance between the effective source and the receiver varies. The quantitative relationship between measured irradiance and distance of separation is dependent on the type of waves associated with the EM radiation signal. For example, where the EM radiation signal comprises spherical waves, the irradiance is inversely proportional to the square of the distance of separation. In this way, the deflection of the blade may be determined on the basis of the measured irradiance. For example, an angle of deflection may be determined. The receiver may be mounted to the rotor blade. For example, in certain embodiments the receiver may be mounted to the surface of the rotor blade and the effective source may be located on the surface of the rotor blade; or alternatively, inside the rotor blade. Mounting the receiver and positioning the effective source inside the rotor blade protects the apparatus from external environmental factors that could interfere with their correct operation. Furthermore, the amount of background noise is also reduced, further improving the operation of the apparatus.

Preferably, each one of the wind turbine's rotor blades is associated with a receiver and effective source pair. In this way, the blade deflection of each blade is determinable when the wind turbine is in use.

One of the effective source or receiver may be located in the vicinity of the rotor blade tip and the other of the effective source or receiver may be located in the vicinity of the blade root. For example, the effective source may be located in the vicinity of the blade root, and the receiver in the vicinity of the tip of the blade, or vice versa. This configuration improves the accuracy with which the deflection of the blade tip relative to the root is determined.

The system may comprise a transmitter, such as an optical transmitter, located on the rotor blade; a physical source of EM radiation located remotely from the transmitter; and a waveguide arranged to connect the transmitter to the physical source. The physical source may relate to any type of EM radiation source, such as a laser emitting an EM wave having a Gaussian beam profile, such as a diode laser for example. Diode lasers are compact and light, and therefore convenient for use with a wind turbine. The optical transmitter may relate to any one or more optical devices, such as an arrangement of optical lenses, arranged to output an EM radiation signal from the waveguide and to focus it on the receiver, and/or any optical instrument located between the transmitter and the receiver. In the absence of any optical instruments present in the optical path located between the transmitter and receiver, the position of the effective source correlates with the position of the transmitter. The optical transmitter is commonly formed integral to the waveguide. Alternatively, where one or more optical instruments are located in the optical path present between the transmitter and the receiver, the position of the effective source correlates with the point of incidence of the EM radiation emitted by the transmitter on the one or more optical instruments present in the optical path between the transmitter and the receiver. Non-limiting examples of the aforementioned optical instrument comprise any one or more of an optical lens, a translucent medium, and a transparent medium.

The physical source may relate to a conventional light source emitting light in the visible and/or non-visible spectrum, such as infra-red or ultraviolet light for example.

The physical EM radiation source may be positioned local to the rotor blade, or remotely therefrom. The physical source may be located in the rotor hub of the wind turbine and may be operatively coupled to a transmitter via a waveguide, such as a fibre optic, as described previously. One advantage of this arrangement is that a single EM radiation source may be coupled to the transmitters and receivers associated with each one of the wind turbine's rotor blades. Use of a non-electrically conducting waveguide, such as a fibre optic, reduces the amount of electrically conducting apparatus present in the wind turbine. This reduces the likelihood of the wind turbine being struck by lightning in adverse weather conditions. This configuration also improves accessibility for maintenance purposes, since the physical EM radiation source is directly accessible via the rotor hub for servicing.

Alternatively, each transmitter may be coupled to its own EM radiation source.

The processor may be operatively coupled to the receiver and may be positioned local to the receiver. Alternatively, the processor may be located remotely to the receiver. In the latter case, the processor may be located in the wind turbine rotor hub, and operatively coupled to the receiver associated with each rotor blade, thus further reducing the weight of the system.

The processor may be operatively coupled to the receiver via an electrically conducting guide arranged to conduct an electrical signal associated with the received EM radiation signal measured by the receiver. Alternatively, the processor may be operatively coupled to the receiver via a waveguide, such as an optical fibre, in which case the received EM radiation signal is guided to the processor where it is measured by a sensor arranged local to the processor.

In certain embodiments the system comprises first and second receivers arranged such that deflection of the rotor blade results in the straight-line distance between the effective source and one of the receivers increasing, and in the straight-line distance between the effective source and the other receiver decreasing. The processor is configured to determine a magnitude and/or direction of deflection and/or a magnitude and/or direction of blade loading by comparing the irradiance of the EM radiation signal received by the first receiver with the irradiance of the EM radiation signal received by the second receiver. This configuration is advantageous in that it enables both the direction of deflection and/or blade loading and the associated magnitude of deflection and/or blade loading to be determined. Preferably, the first and second receivers are arranged such that deflection of the blade in a flapwise and/or edgewise direction of the rotor blade results in the straight-line distance between the effective source and one of the receivers increasing, and in the straight-line distance between the effective source and the other receiver decreasing. For example, the first and second receivers may be arranged comprising a straight-line distance of separation having a length component parallel to a thickness of the blade, or in other words, having a component along a line perpendicular to the chordwise direction of the blade. This may be achieved when the first and second receivers are arranged respectively in the vicinity of the suction side and in the vicinity of the pressure side of the rotor blade. This arrangement of receivers enables flapwise blade deflection and/or blade loading, and/or the magnitude of flapwise deflection and/or loading to be determined.

Similarly, the first and second receivers may be arranged comprising a straight-line distance of separation having a length component parallel to the chordwise direction of the blade. For example, the first and second receivers may be arranged respectively in the vicinity of the leading edge and the trailing edge of the rotor blade. This arrangement of receivers enables edgewise blade deflection and/or blade loading, and/or the magnitude of edgewise deflection and/or loading to be determined.

The processor may be arranged to calculate an irradiance ratio on the basis of the irradiances of the EM radiation signals received from the first and second receivers, and to determine direction of magnitude and/or direction of deflection and/or a magnitude and/or direction of blade loading on the basis of the calculated irradiance ratio.

In certain embodiments the system comprises first and second effective sources configured to transmit first and second EM radiation signals respectively. The first and second EM radiation signals being distinguished from one another by a distinguishing characteristic. The first and second effective sources are arranged such that deflection of the rotor blade results in the straight-line distance between the receiver and one of the effective sources increasing, and in the straight-line distance between the receiver and the other effective source decreasing. The receiver is configured to receive the first and second EM radiation signals, and the processor is configured to determine a magnitude and/or direction of deflection and/or a magnitude and/or direction of blade loading by comparing the irradiance and/or the distinguishing characteristic of the first and second EM radiation signals received by the receiver. This alternative configuration also enables both the direction of deflection and/or blade loading and the associated magnitude of deflection and/or blade loading to be determined.

Preferably, the first and second effective sources are arranged such that deflection of the rotor blade in a flapwise and/or edgewise direction results in the straight-line distance between the receiver and one of the effective sources increasing, and in the straight-line distance between the receiver and the other effective source decreasing. For example, the first and second effective sources may be arranged comprising a straight-line distance of separation having a length component parallel to a thickness of the blade, or in other words, having a component along a line perpendicular to the chordwise direction of the blade. This may be achieved when the first and second effective sources are arranged respectively in the vicinity of the suction side and in the vicinity of the pressure side of the rotor blade. This arrangement of receivers also enables flapwise blade deflection and/or blade loading, and/or the magnitude of flapwise deflection and/or loading to be determined.

Similarly, the first and second effective sources may be arranged comprising a straight- line distance of separation having a length component parallel to the chordwise direction of the blade. For example, the first and second effective sources may be arranged respectively in the vicinity of the leading edge and the trailing edge of the rotor blade. This arrangement of receivers also enables edgewise blade deflection and/or blade loading, and/or the magnitude of edgewise deflection and/or loading to be determined.

The distinguishing characteristic of the EM radiation signals is selected from any one of: frequency, wavelength, colour, or polarisation.

In a further embodiment, the system comprises a plurality of receivers or effective sources spaced apart in the spanwise direction of the blade, and the processor is configured to determine a parameter indicative of local blade deflection and/or local blade loading at the associated spanwise location of the receivers or effective sources. This configuration enables a more precise determination of blade deflection and/or blade load to be determined along the spanwise direction of the rotor blade. This is particularly useful where the rotor blade is subject to non-uniform loads and/or non-uniform deflection along its length. The system may comprise a plurality of receivers spaced apart along the spanwise direction of the rotor blade, and the processor is configured to determine local blade deflection and/or local blade loading at the associated spanwise location of the receivers.

Similarly, the system may comprise a plurality of effective sources spaced apart in the spanwise direction of the blade. The processor may be configured to determine local blade deflection and/or local blade loading at the associated spanwise location of the effective sources. As described previously, each one of the EM radiation signals emitted by the plurality of effective sources may comprise a distinguishing characteristic. This enables the different EM radiation signals to be distinguished by the processor. The system may comprise a plurality of pairs of first and second receivers, each pair being spaced apart along the spanwise direction of the rotor blade. The first and second receivers comprised in each pair may be arranged to have an equidistant straight-line distance of separation from the effective source. The first and second receivers comprised in each pair may be located as described previously, respectively in the vicinity of the pressure side of the blade and the suction side of the blade, enabling flapwise deflection and/or load to be determined along the spanwise direction of the blade. Similarly, the first and second receivers comprised in each pair may be located as described previously, respectively in the vicinity of the leading edge and the trailing edge, enabling edgewise deflection and/or load to be determined along the spanwise direction of the blade.

The system may comprise a protective housing comprised of an at least partially translucent material, and the receiver or each receiver is arranged within the protective housing. In this example, the surface of the protective housing is the effective source. Use of a protective housing to encase the receiver or each receiver advantageously protects the receivers from environmental pollutants, such as dust which could result in erroneous results. It also reduces the amount of maintenance required.

A second aspect of the present invention relates to a method for determining deflection of a rotor blade of a wind turbine. The method comprises the steps of: (a) transmitting an EM radiation signal from a transmitting position; (b) receiving the EM radiation signal at a receiving position, the receiving position and the transmitting position being spaced apart in a spanwise direction of the blade, and being arranged such that deflection of the rotor blade results in a straight-line distance between the transmitting position and the receiving position varying; and (c) determining a parameter indicative of blade deflection and/or blade loading on the basis of the irradiance of the EM radiation signal received at the receiving position.

This aspect of the invention and its associated embodiments, summarised below, benefit from the same advantages as described in relation to the first aspect of the invention and its associated embodiments. Step (a) of the method may comprise transmitting the EM radiation signal from the vicinity of the rotor blade tip, and step (b) may comprise receiving the transmitted EM radiation signal in the vicinity of the blade root.

Alternatively, step (a) of the method may comprise transmitting the EM radiation signal from the vicinity of the blade root, and step (b) comprises receiving the transmitted EM radiation signal in the vicinity of the rotor blade tip. Step (b) may comprise receiving the EM radiation signal at a first receiving position and at a second receiving position. The first and second receiving positions being arranged such that deflection of the rotor blade results in the straight-line distance between the transmitting position and one of the first and second receiving positions increasing, and in the straight-line distance between the transmitting position and the other one of the first and second receiving positions decreasing; and step (c) comprises determining a magnitude and/or direction of deflection and/or magnitude and/or direction of blade loading by comparing the irradiance of the EM radiation signal received at the second receiving position. The first receiving position and the second receiving position may be arranged such that deflection of the blade in the flapwise and/or edgewise direction results in the straight- line distance between the transmitting position and one of the first and second receiving positions increasing, and in the straight-line distance between the transmitting position and the other one of the first and second receiving positions decreasing. Step (c) may comprise determining a magnitude of flapwise and/or edgewise bending and/or a magnitude of flapwise and/or edgewise loading.

Step (a) may comprise transmitting a first EM radiation signal from a first transmitting position and transmitting a second EM radiation signal from a second transmitting position. The first and second EM radiation signals being distinguishable from one another by a distinguishing characteristic, and the first and second transmitting positions being arranged such that deflection of the blade results in the straight-line distance between one of the first and second transmitting positions and the receiving position increasing, and in the straight-line distance between the other one of the first and second transmitting position and the receiving position decreasing. Step (c) may comprise determining a magnitude and/or direction of deflection and/or magnitude and/or direction of blade loading by comparing the irradiance and/or the distinguishing characteristic of the first and second EM radiation signals received at the receiving position.

The first transmitting position and the second transmitting position may be arranged such that deflection of the blade in a flapwise and/or edgewise direction results in the straight- line distance between one of the first and second transmitting positions and the receiving position increasing, and in the straight-line distance between the other one of the first and second transmitting positions and the receiving position decreasing. Step (c) may comprise determining a magnitude of flapwise and/or edgewise bending and/or a magnitude of flapwise and/or edgewise loading.

As mentioned in relation to the first aspect, the distinguishing characteristic may be selected from any one of frequency, wavelength, colour and polarisation. Step (c) may then comprise determining a magnitude and/or direction of deflection and/or magnitude and/or direction of blade loading by comparing any one of frequency, wavelength, colour and polarisation of the first and second EM radiation signals received at the receiving position.

Step (a) may comprise transmitting a plurality of EM radiation signals each one from a different one of a plurality of transmitting positions spaced apart in a spanwise direction of the blade. Step (b) may comprise receiving the plurality of EM radiation signals; and step (c) may comprise determining a parameter indicative of local blade deflection and/or local blade loading at the associated spanwise location of the plurality of transmitting positions.

Step (b) may comprise receiving an EM radiation signal at a plurality of different receiving positions spaced apart in a spanwise direction of the blade; and step (c) may comprise determining a parameter indicative of local blade deflection and/or local blade loading at the associated spanwise location of the plurality of receiving positions.

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;

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

Figure 4a is a schematic illustration of a front view of a rotor-hub assembly of a wind turbine, wherein each blade is configured with a single transmitter and receiver in accordance with an embodiment of the invention; whilst Figure 4b illustrates a rotor hub assembly wherein each blade is configured with a single transmitter and two receivers in accordance with an alternative embodiment of the invention;

Figure 5a is a schematic illustration showing how both receivers of Figure 4b measure the same irradiance when the blade is undeflected; whilst Figure 5b illustrates how the relative distance of the two receivers with respect to the transmitter varies as the blade is deflected by an angle a, resulting in different irradiances being measured at each receiver;

Figure 6a is a schematic illustration showing an undeflected turbine blade configured with two transmitters and a single receiver in accordance with an alternative embodiment of the invention; whilst Figure 6b illustrates how the relative distance between the transmitters and the receiver changes as the blade is deflected, and

Figure 7a is a schematic illustration showing an alternative arrangement, wherein two optical receivers are located within a protective housing; and Figure 7b is a magnified plan view of the housing showing how the position of the effective source on the housing varies as the blade is deflected. 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, in addition to Figure 1. 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. The pressure side 17 of the blade 10 is the side where a high pressure region is formed as air flows across the airfoil profile of the blade. The suction side 19 is the side where a low pressure region is formed as air flow across the airfoil profile of the blade. The difference in pressure formed at the pressure side 17 and the suction side 19 generates lift. When the blade is mounted on a rotatable rotor, the lift acts on the pressure side 17 and causes the blade to rotate in the direction of the lift. The straight line £ (Figure 1) through the blade's thickness, intersects the pressure side 17 and the suction side 19, and is perpendicular to the chordline D.

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 6 _b i _{ade tip} 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. For the purposes of the present invention, the total blade tip angle 6 _b i _{ade tip} may be considered to be the sum of the blade's pitch angle e _roo t defined at its root and the twist angle Θ. Blade bending may occur in a so-called 'flapwise' direction or in a so-called 'edgewise' direction. The flapwise direction is perpendicular to the plane containing the longitudinal axis L and the chordwise direction D of the blade, i.e. in the direction £ indicated in Figure 1 , and perpendicular to the L-x plane. Bending in the flapwise direction typically occurs when the wind turbine blade is subjected to large external loads perpendicular to the blade's longitudinal axis L. The edgewise direction is generally in the chordwise direction D (i.e. perpendicular to the L-y plane), and edgewise bending is typically caused by the weight of the blade as the blade rotates. Blade bending may result in significant displacement of the blade tip from the straight longitudinal axis L. Blade tip bending is further explained 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 U 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.

It will be appreciated that the flapwise direction is perpendicular to the edgewise direction, and Figures 3a and 3b schematically illustrate either flapwise or edgewise bending depending upon the orientation of the blade. For example, for flapwise bending the blade deflection is in a direction comprised in the L-y plane (see Figure 1), in the direction of the straight line £, and Figures 3a and 3b show a side perspective view of the wind turbine blade 10, 10' taken in the L-y plane. Similarly, for edgewise bending the blade deflection is in the chordwise direction of the blade in the direction of straight line D (see Figure 1), and Figures 3a and 3b show a side perspective view of the wind turbine blade 10, 10' taken in the L-x plane.

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 4a 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 airfoil 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 (i.e. substantially in the direction of the x-axis). Each blade 24a, 24b, 24c of the rotor-hub 22 assembly is configured with at least one optical transmitter 30, which serves as the effective source in this example, 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 L of the blade 24a, 24b, 24c. This may interchangeably be referred to as the spanwise direction of the blade. 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 or at the blade tip. The bending angle or may not be constant throughout the blade's length, and therefore positioning the optical receiver 32 substantially in the vicinity of the blade tip ensures that the determined bending angle or is an accurate reflection of the position of the blade tip. The at least one optical transmitter 30 is located substantially in the vicinity of the root 12 of the blade 24a, 24b, 24c. This also improves the accuracy with which the bending angle or is determined. In use the root 12 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 is configured to emit an eletro-magnetic (EM) signal 24, such as a light signal, which is subsequently received by the optical receiver 32, which is arranged to measure an irradiance value of the received EM-signal 24. Within the present context the term irradiance is used to indicate intensity. For example, the irradiance of a light signal is its optical intensity. The bending angle or is determined on the basis of the measured irradiance value.

Preferably, the optical transmitter 30, and the optical receiver 32 are located inside the shell of the turbine blade 24a, 24b, 24c. An example of how the blade tip bending angle or may be determined from the measured irradiance of the received EM-signal is now described.

The irradiance value of a received EM-signal, measured by the receiver 32, is compared with a reference irradiance value, and the bending angle or is determined on the basis of this comparison. The reference irradiance value is preferably pre-measured when the blade 24a, 24b, 24c has a known bending angle a. For example, the reference irradiance value may be determined when the bending angle a is zero (i.e. when the blade is undeflected / not under load). This measurement defines a reference irradiance value that may subsequently be used for comparison purposes in order to determine the bending angle a of the blade tip. For example, where the transmitter 30 emits spherical EM-waves, then the measured irradiance / is proportional to the inverse square of the relative radial distance r between the receiver and the transmitter 30 eq.1 As the blade 24a, 24b, 24c is deflected through the bending angle or, the position of the optical receiver 32 is displaced relative to the transmitter 30. This displacement results in a variance of the radial distance r (i.e. the straight line distance) between the transmitter 30 and receiver 32, which in turn is associated with a variance in the measured irradiance value. By comparing the measured irradiance value with the reference irradiance value, the change in radial distance between the optical receiver 32 and the transmitter 30 is determined, and the bending angle o of the blade tip is determined. The accuracy of the determined bending angle o of the blade tip is improved by positioning the transmitter 30 in closer proximity to the blade tip.

The bending angle or may be determined from a reference table configured to associate measured irradiance values with bending angles. Such a reference table may be generated during a calibration / testing process, where the accurate measurements of bending angle a and measured irradiance values may be carried out in a controlled environment. For example, the irradiance values for a myriad of different bending angles may be measured and populated in the reference table, in order to obtain a range of different bending angles associated with a range of different irradiance values.

Furthermore, the range of different bending angles may be generated by applying different loads to the rotor blade. In this way, a known load value is correlated with each bending angle and measured irradiance value.

Alternatively, the bending angle or may be calculated from first principles using known trigonometric identities, which are within the common general knowledge of the reader skilled in the art, and for this reason are not discussed herein.

Figure 4b schematically illustrates an alternative example of a rotor-hub 22 assembly. In contrast with the example illustrated in Figure 4a, each blade 24a, 24b, 24c is configured with at least two optical receivers 32a, 32b. Each blade 24a, 24b, 24c is also configured with an optical transmitter 30, which in this example serves as the effective optical source. A first one of the optical receivers 32a is arranged on the pressure side 17 of the blade 24a, 24b, 24c, and a second one of the optical receivers 30b is arranged on the suction side 19 of the blade 24a, 24b, 24c. The first and second optical receivers 32a, 32b are separated along a thickness of the blade 24a, 24b, 24c in a direction of the straight line £, perpendicular to the chordwise direction of the blade 24a, 24b, 24c, and substantially perpendicular to the longitudinal axis L of the blade. The optical receivers 32a, 32b are located substantially in the vicinity of the tip of the blade 24a, 24b, 24c, to enable accurate determination of the flapwise bending angle or at the blade tip in the L-y plane. It is to be appreciated that the present embodiment functions equally well for any linear separation of the optical receivers 32a, 32b, which comprises a separation component in the direction of the straight line E. In other words, the straight line distance of separation between the optical receivers 32a, 32b need not necessarily be parallel to the straight line E. Accordingly, in the present context and going forward separation along the thickness of the blade 24a, 24b, 24c means any separation of the first and second optical receivers 32a, 32b having a separation component in the direction of the straight line E.

In order to enable accurate determination of the edgwise bending angle a in the L-x plane, a third optical receiver (not shown) may be arranged on the leading edge 17 of the blade 24a, 24b, 24c, and a fourth optical receiver (not shown) may be arranged on the trailing edge 20 of the blade 24a, 24b, 24c, in the chordwise direction of the blade (i.e. in the direction of the straight line D). Alternatively, it is sufficient that the straight line separation of the third and fourth optical receivers comprise a separation component that is parallel to the chordwise direction (i.e. parallel to the straight line D). Going forward, separation in the chordwise direction means any separation of optical receivers having a separation component parallel to the chordwise direction.

Figures 5a and 5b schematically illustrate how the relative radial distances r _A , r _B

between the optical receivers 32a, 32b and the effective source, which is represented by the transmitter 30 in this example, vary as the blade 10 bends in the L-y plane (i.e.

flapwise bending).

Figure 5a schematically illustrates a blade 10 when it is straight (i.e. not under load). The contour lines Cj, C ₂ , C ₃ trace lines of equal irradiance. At any point along a single contour line the irradiance measured from the EM-signal emitted by the transmitter 30 is constant. The difference in irradiance measured between the contour lines is inversely proportional to the square of the difference in radial distance between the contour lines, in accordance with equation 1. Preferably, the transmitter 30 is arranged equidistant (e.g.

) from the optical receivers 32a, 32b, such that when the blade is straight, the irradiance l _A measured at the first receiver 32a, is equal to the irradiance l _B measured at the second receiver 32b (e.g. / _Λ =/ _β ). Figure 5b schematically illustrates flapwise bending of the blade 10' by a bending angle a. The radial distances r _A ', r _B ' between each receiver 32a', 32b' and the transmitter 30 are no longer equal, and the receivers 32a', 32b' are no longer equidistant from the transmitter 30. As a result, both receivers 32a', 32b' are now located on different intensity contour lines C _a -, C _b -. The second receiver 32b' is now closer to the transmitter 30 than the first receiver 32b' (e.g. _A > B ), and the irradiance Ι _Β · measured at the second receiver 32b' is greater than the irradiance Ι _Α · measured at the first receiver 32a' (e.g. /Β'> Λ-)- Using equation 1 the intensity ratio may be defined ^ κ φ eq.2

Equation 2 shows that the ratio of intensities is proportional to the ratio of the squares of the radial distances. As described in relation to the preceding example, a reference table may be consulted correlating irradiance ratios, and/or radial distance ratios, to bending angles. The reference table is preferably generated in a controlled environment, as described previously. Alternatively, the bending angle or may be defined, as mentioned previously, from first principles using known trigonometric functions.

Figures 6a and 6b schematically illustrate an alternative example comprising a single receiver 38, and two transmitters 36a, 36b. The receiver 38 is arranged equidistant from the two transmitters 36a, 36b, such that the radial distance r _36a of the first transmitter 36a relative to the receiver 38, and the radial distance r _36b of the second transmitter 36b relative to the receiver 38 are equal (e.g. r _36a = r _36b ). Each one of the transmitters 36a, 36b, serves as a different effective source in this example. The receiver 38 is located outboard relative to the two transmitters 36a, 36b in the vicinity of the blade tip. As the position of the receiver 38 changes with respect to the two transmitters 36a, 36b, the relative radial distances change. This is schematically illustrated in Figure 6b. The radial distance r _36a - of the first transmitter 36a relative to the displaced receiver 38' is now greater than the radial distance r _36b - of the second transmitter 36b relative to the displaced receiver 38'. Accordingly, the irradiance l _36b - measured at the displaced receiver 38' due to the EM-signal transmitted from the second receiver 36b will be greater than the irradiance / _36a' due to the EM-signal transmitted from the first receiver 36a (e.g. I _36b > l _36a . Different wavelengths/frequencies may be used to enable the receiver to distinguish between the EM-signals transmitted from each transmitter 36a, 36b. Similarly, polarisation may be used to enable the receiver to distinguish between the two emitted EM-signals. For example, the first transmitter 36a may be configured to transmit a circularly polarised EM-signal, whereas the second transmitter 36b may be configured to transmit a linearly polarised EM-signal. The receiver may be provided with a filter enabling the two received EM-signals to be distinguished on the basis of their polarisation, and/or wavelength/frequency. Alternatively, the two transmitters may transmit different colours of the visible light spectrum. A relative increase in irradiance of one colour over the other indicates the direction of bending.

The blade may be provided with a plurality of pairs of receivers or transmitters, to enable the bending angle o of the blade to be determined along its length. This is convenient where the bending angle a is not constant along the blade's length. Each pair of receivers or transmitters is spaced apart along the blade's length at predetermined intervals, which may be arbitrarily selected, depending on the required accuracy. Each pair of receivers or transmitters enables the bending angle a to be determined at their respective position along the blade's length, in the same way as previously described. The positions of the transmitters and receivers in the previously described examples may be interchanged. In other words, the one or more receivers may be positioned in the vicinity of the blade's root 12, and the one or more transmitters positioned outboard relative to the receivers. In such examples, the bending angle a is determined in the same way as previously described.

Figures 7a and 7b illustrate a further example of the present invention. Figure 7a illustrates a blade 10 when it is straight (i.e. not under load), in which the optical receivers 32a, 32b are enclosed in a protective housing 34. The protective housing 34 is preferably comprised of an at least partially translucent material, which enables at least a portion of the light incident on the housing 34 to pass through it. Light which passes through the housing is subsequently measured by the optical receivers 32a, 32b substantially as described in previous embodiments.

Within the present context a translucent material is considered to be a material which allows incident light to pass through it, and wherein some of the light may be scattered on exiting the translucent material. A transparent material is considered to be a special type of translucent material. A transparent material allows light incident on it to pass through it, and does not scatter the incident light when it exits the transparent material. Accordingly, in certain embodiments the protective housing 34 may be comprised of an at least partially transparent material, and in the ensuing description it is to be

understood that reference to a translucent material may also relate to an at least partially transparent material.

A magnified view of the housing 34 is illustrated in Figure 7b, in which the position of the effective optical source and the illuminating ray is shown at two different times, as the blade is deflected through a bending angle a. A first ray of light 36 emitted from an optical source 30 is incident on the housing 34, when the blade is straight, at a point 38, where the ray 36 passes through the at least partially translucent material of the housing 34. As the first ray of light 36 passes through the surface of the housing 34, each optical receiver 32a, 32b measures an optical irradiance. With respect to each optical receiver 32a, 32b, the point of incidence 38 of the incident optical ray 36 appears as an effective source - i.e. with respect to each optical receiver 32a, 32b it appears as if the optical light source 30 is located at the point of incidence 38.

The irradiance measured by each one of the optical receivers 32a, 32b is dependent on the radial distances r _A , r _B of each one of the receivers 32a, 32b with respect to the position of the effective optical source - in other words, with respect to the point of incidence 38.

As the blade tip is deflected through the bending angle a the position of the housing 34 relative to the physical optical source 30 changes, which in turn results in a displacement of the effective optical source position 38' relative to the optical receivers 32a, 32b. A second optical ray 36', which is emitted at a different time to the first optical ray 36, is incident on the housing 34 at a different point of incidence 38', and with respect to the optical receivers 32a, 32b it appears as if the position of the effective optical source has now moved to the point of incidence 38'. In this way, and as described in relation to previous embodiments, as the blade tip bends, the position of the effective optical source relative to the optical receivers 32a, 32b varies, resulting in a variation in the distance r _A , ¾, r _A ; r _B - between the effective optical source and each one of the optical receivers 32a, 32b. As a result of these varying distances, each optical receiver 32a, 32b measures a different optical irradiance as the blade tip bends. The blade tip bending angle a may then be determined as described previously on the basis of the measured optical irradiances.

It is also to be noted that as the blade tip bends relative to the optical source 30, the straight line distance between each optical receiver 32a, 32b and the optical source 30 also changes. In short, the principle of operation of the present embodiment is analogous to the principle of operation of preceding embodiments.

Use of the protective housing 34 is advantageous in that it protects the optical receivers 32a, 32b from environmental pollutants, such as dust which could result in erroneous results. For example, any dust present on the protective housing 34 will have a substantially uniform impact on the measurements of both optical receivers 32a, 32b, and will not adversely affect results. In contrast, where the optical receivers 32a, 32b are not placed in the same protective housing 34, environmental pollutants such as dust present on one of the optical receivers may have an effect on the irradiances measured by the affected optical receiver, which could result in erroneous results, and may lead to inaccurately calculated blade tip bending angles a. Placing the optical receivers 32a, 32b within the same protective housing reduces the effect environmental pollutants have on the calculated blade tip bending angles or, and also reduces the amount of maintenance required to ensure accurate results.

The housing 34 may be constructed of any material which is at least partially translucent, such that at least a portion of any incident light passes through the housing. For example, the housing may be constructed of a transparent plastic such as Perspex, or of glass.

The housing 32 may be constructed of a scattering material, which is arranged in use to scatter light incident on it through a range of different angles as the incident light propagates through the material. This helps to ensure a more uniform distribution of light within the housing, and is particularly useful where a laser is used as the light source.

The skilled reader will appreciate that reference to optical rays in describing the function and benefit of the protective housing 34 is for illustrative purposes only, and to facilitate the reader's understanding of how this embodiment functions. Whilst Figure 7b was described in terms of a single incident light ray being incident on the housing at any one time, the reader skilled in optics will appreciate that in reality a plurality of rays will be incident on the protective housing 34 at any one time, at a plurality of different incident positions. Nonetheless, the principle of operation does not change, and as the blade tip bends the distances between the effective optical source and the optical receivers 32a, 32b changes resulting in different optical irradiances being measured, wherefrom the blade tip bending angle a may be determined. Incidentally, in the illustrated example the distances between the physical optical source and the optical receivers 32a, 32b also changes.

The present method may be used to calculate the blade bending angle for a wind turbine comprising any number of turbine blades.

Whilst the herein described embodiments relate to a wind turbine comprising three blades, this is non-limiting for illustrative purposes only. All herein provided embodiments are described 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.

Furthermore, all herein described embodiments may work with any type of EM radiation source, having any type of propagation wave, including localised Gaussian waves, such as provided by lasers, or alternatively circular waves, as provided by an LED for example.