WO2011150942A1

WO2011150942A1 - An improved wind turbine doppler anemometer

Info

Publication number: WO2011150942A1
Application number: PCT/DK2011/050195
Authority: WO
Inventors: Carsten Hein Westergaard
Original assignee: Vestas Wind Systems A/S
Priority date: 2010-06-04
Filing date: 2011-06-02
Publication date: 2011-12-08
Also published as: GB201013239D0

Abstract

The invention relates to a wind turbine doppler anemometer, and in particular a LIDAR apparatus 10 for Feed Forward Control of a wind turbine. The apparatus 10 is mounted on a wind turbine nacelle 3 by a rotatable mount or bearing 12, such that its line of sight 14 can be steered into the wind independently of the direction in which the nacelle 3 is orientated. The rotatable mount can include a wind vane 40 with fins 46 or vanes that receive a force from the incident wind. Alternatively, the rotatable mount can be controlled based on the signal from separate wind sensors 18 that provide a wind direction output to a control system. In a simpler construction, the LIDAR apparatus is fixed to the nacelle, and the line of sight of the LIDAR apparatus is directed towards a rotatable mirror. For Feed Forward Control, this ensures that the measurement of wind velocity produced by the doppler anemometer is reliable and accurate, and allows accurate wind speed information to be available during extreme weather events, as well as during maintenance or failure of the yaw drive.

Description

AN IMPROVED WIND TURBINE DOPPLER ANEMOMETER

Field of the Invention This invention relates to wind turbines, and more specifically to an improved doppler anemometer for wind turbine control.

Background to the Invention The rotor blades of wind turbines are designed to extract power from the wind by virtue of their aerodynamic shape, and subsequent wind induced rotation. For horizontal axis wind turbines, the blades rotate around a rotor hub attached to a nacelle mounted on a wind turbine tower, and the rotation of the blades turns a drive shaft connected in turn to a generator which produces electricity. For horizontal axis wind turbines to operate efficiently and extract the maximum power from the wind, the wind turbine nacelle and the axis around which the wind turbine blades rotate, is angled into the wind to the greatest extent possible, and the rotational axis of the blades is aligned with the wind direction.

Rotation of the nacelle into the wind is achieved by a yaw or azimuth drive system, mounted on the tower. Such drives typically comprise a bearing that is fully rotatable around an axis colinear with the tower, and one or more electric or hydraulic drives for rotating the bearing relative to the tower. In this way, the nacelle, mounted on the bearing, can be turned through 360 degrees in the horizontal plane. The use of LIDAR to control operation of wind turbines is known, for example, from US 6,320,272 of Lading et al, which teaches the use of a laser wind velocity measurement system such as a LIDAR (Light Detection and Ranging) apparatus mounted on the nacelle. LIDAR operates by emitting a laser beam to measure the conditions at a distance in front of the wind turbine, typically between 0.5 and 3 rotor diameters away from the turbine, and therefore of the order of 50 to 300m for a large modern wind turbine. LIDAR operates in known manner either by detecting air molecules or by detecting aerosols or other particles entrained in the air stream and calculating information about the air flow from these measurements. In Feed Forward Control systems, a LIDAR apparatus is arranged to look directly along the wind turbine blade axis of rotation and measure the incident wind velocity upwind of the wind turbine blades. The wind velocity information can then be used to optimise control of the wind turbine in advance of the wind impinging on the blades. Aspects that are frequently controlled in feed forward systems are the generator speed and the pitch angle of the wind turbine blades. The pitch angle of the blade affects the amount of lift

experienced by the blade at a given wind speed, and is therefore regulated to maximise the power extracted from the wind, while avoiding overloading the blade or generator when the wind is too strong. Similarly, the generator speed is controlled to ensure that the output frequency of the alternating current it produces is within set operational boundaries.

As noted above, LIDAR apparatus are typically aligned with their line of sight pointing along the rotational axis of the wind turbine blades. This makes sense, as it is the speed of the wind impinging on the wind turbine blades that is of interest. We have however appreciated that there is a drawback with this arrangement, in that it assumes that the wind turbine nacelle and the rotational axis of the wind turbines are perfectly aligned with the wind direction, and does not account for the delay in the yaw drive system in correctly aligning the nacelle. We have appreciated that there is a need for an improved LIDAR apparatus.

Summary of the Invention

The invention is defined in the independent claims to which reference should now be made. Advantageous features are set forth in the appendent claims.

In a first example, the invention provides a wind turbine comprising: a nacelle with a rotor hub on which one or more wind turbine blades are mounted; a doppler anemometer for detecting wind velocity along a line of sight; a rotatable mount for ensuring that the doppler anemometer line of sight is always aligned with the incident wind direction. Thus, any differences between the line of sight of the doppler anemometer and the wind direction are automatically taken into consideration, and the accuracy of the anemometer is greatly improved. In one embodiment, the rotatable mount is a wind vane operable to turn under the influence of the incident wind and adopt a fixed orientation relative to the incident wind direction, and the doppler anemometer is mounted on the wind vane. In an alternative embodiment, the doppler anemometer is a LIDAR apparatus mounted on the nacelle, and a mirror is mounted on the wind vane for angling the LIDAR line of sight towards the incident wind direction. In a further embodiment, the wind turbine comprises: drive means for rotating the rotatable mount; a control system for operating the drive means; and at least one wind sensor for determining the incident wind direction and for giving an output to the control system; wherein the control system is operable to cause the rotatable mount to rotate based on the output from the wind sensor. In this embodiment, the doppler anemometer can be a LIDAR apparatus mounted on the rotatable mount, or a LIDAR apparatus mounted on the nacelle, with a mirror is mounted on the rotatable mount for angling the LIDAR line of sight towards the incident wind direction. In a further embodiment, the wind turbine comprises: drive means for rotating the rotatable mount; a control system for operating the drive means; and wherein the control system is operable to: vary the orientation of the rotatable mount, and therefore the line of sight of the doppler apparatus, about the expected incident wind direction; determine the direction of maximum indicated wind velocity; adjust the rotatable mount, such that the line of sight of the doppler anemometer is directed towards the direction of maximum indicated wind velocity.

In a further embodiment, the invention provides a wind turbine comprising: a nacelle with a rotor hub on which one or more wind turbine blades are mounted; a doppler anemometer for detecting wind velocity along a line of sight, and giving an output; a wind sensor for indicating the direction of the incident wind, and giving an output; a controller for receiving the outputs from the doppler anemometer and the wind sensor, and for applying a correction to the doppler anemometer output to give a corrected output, such that the corrected output indicates the wind velocity along the incident wind direction.

The doppler anemometer apparatus can advantageously be incorporated as part of a feed forward control system for the wind turbine.

Corresponding methods are also provided.

Brief Description of the Drawings

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

Figure 1 is a front elevation view of a horizontal axis wind turbine; Figure 2 is a schematic illustration of a first example of the invention; Figure 3 is a schematic illustration of a control system for use with the invention; Figure 4 is a schematic illustration of a second example of the invention. Detailed Description of the Preferred Embodiments

The invention relates to a wind turbine doppler anemometer, and in particular a LIDAR apparatus for Feed Forward Control of a wind turbine. The apparatus is mounted on a wind turbine nacelle or rotor hub by a rotatable mount or bearing, such that its line of sight can be steered into the wind independently of the direction in which the nacelle is orientated. The rotatable mount is therefore provided in addition and separately to any yaw drive for adjusting the orientation of the nacelle. The rotatable mount can include a wind vane with fins or vanes that receive a force from the incident wind. Alternatively, the rotatable mount can be controlled based on the signal from separate wind sensors that provide a wind direction output to a control system. In a simpler construction, the LIDAR apparatus is fixed to the nacelle, and the line of sight of the LIDAR apparatus is directed towards a rotatable mirror. For Feed Forward Control, this ensures that the measurement of wind velocity produced by the doppler anemometer is reliable and accurate, and allows accurate wind speed information to be available during extreme weather events, as well as during maintenance or failure of the yaw drive.

Figure 1 illustrates a wind turbine 1 , comprising a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade 5 is mounted on a hub 6. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in Figure 1 may be a small model intended for domestic or light utility usage, or for example may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm. In the latter case, the diameter of the rotor may be as large as 150 metres or more.

Figure 2 illustrates a first example of the invention. A LIDAR apparatus 10 is mounted on a rotatable bearing 12 on the top of the nacelle 3. The rotatable bearing is shown in Figure 2 as standing proud of the nacelle surface 15 for clarity, although in alternative embodiments it may be flush with the surface to avoid undue disruption to the aerodynamic properties of the surface 15. The rotatable bearing 12 and the LIDAR 10 can be rotated through 360 degrees of motion independently of the nacelle 3 and the direction in which the nacelle is pointed. An electric or hydraulic drive system (not shown) is provided to turn the rotatable bearing 12 in accordance with signals from a control system which will be described with reference to Figure 3.

The LIDAR apparatus 10 uses a laser pointing along a line of sight 14 into the wind. The laser illuminates aerosols or other particles carried in the wind, which results in a small fraction of the light being backscattered into a receiver 16. Motion of the target along the beam direction leads to a change, δν, in the light's frequency via the Doppler shift, given by:

|δν| = f (2V_LOS) / c = (2V_LOS) / λ where c is the speed of light (3 x 10⁸ m/s), V_Los is the component of target speed along the line of sight (i.e., the beam direction), and f and λ are respectively the laser frequency and wavelength. This frequency shift is accurately measured by mixing the return signal with a portion of the original beam and picking up the beats on a photodetector at the difference frequency. Wind sensors 18 for determining the wind direction such as wind vanes, and/or directional anemometers are also provided on the nacelle 3 to provide an indication of the wind direction, and optionally the speed, to a control system. Two wind sensors 18 are illustrated in Figure 2, as this allows the wind direction information from both sensors to be combined or averaged to give a more accurate reading. It will be appreciated that in practice, only a single sensor 18 or indeed any number of sensors 18 could be used.

Referring now to Figure 3, the control system shall now be described in more detail. The control system comprises a controller 20, which may be a dedicated controller for the LIDAR apparatus, or part of the SCADA (supervisory control and data acquisition) system common to most large scale networked wind turbines. The controller 20 receives the output from the wind sensors 18, which as noted above, includes at least the direction of the incident wind at the nacelle 3. Based on the detected wind direction, the controller 20 instructs the drive system (here indicated as 22) for the rotatable bearing 12 to turn so that the line of sight 14 of the LIDAR apparatus 10 is coincident with the wind direction. It will be appreciated here that the wind direction and the line of sight direction of the LIDAR are parallel and ideally exactly opposite in direction. In this example, the accuracy of the wind velocity measured by LIDAR apparatus 10 can be improved, as the control system ensures that the line of sight of the LIDAR apparatus 10 is in fact pointing in the correct direction for the measurement to be taken. An alternative example of the invention is illustrated in Figure 4. In this example LIDAR apparatus 10 is mounted upon a rotatable wind vane 40. The wind vane 40 has a plate 42 for supporting the LIDAR apparatus 10, supported on a rotational mounting 44. Vanes 46 attached to the plate 40 ensure that the LIDAR apparatus 10 naturally adopts the correction orientation with regard to the incident wind.

Both of these examples of the invention allow the LIDAR to fully track the wind, even in extreme weather events such as wind gusts or squalls resulting in rapidly changing wind directions, and during maintenance or failure of the yaw drive system when the nacelle can no longer be correctly orientated.

The examples discussed above are for illustrative purposes only, and a number of variations will occur to the skilled person. In a particular example of the invention, for example, the LIDAR apparatus 10 is fixed to the nacelle surface with its line of sight directed towards a mirror or other optical device, which is in turn mounted on the rotating drive means 12 or wind vane 40. By rotating only the mirror so that the beam from the LIDAR is directed into the wind direction, a far simpler device can be constructed. In this arrangement, it can be advantageous to angle the LIDAR beam upwards, and use a 45 degrees mirror mounted on a rotatable periscope like arrangement for angling the beam into the wind.

It will be appreciated that the LIDAR apparatus need not be mounted for full 360 degree rotatability for the examples of the invention described above to provide advantages. A range of rotation of 90 degrees (45 degrees to either side of the forward direction of the nacelle) or 180 degrees (90 degrees to either side of the forward direction of the nacelle) would still likely allow the LIDAR apparatus to track the more sensitive changes in the wind direction that are too fast for the nacelle under the control of the yaw drive to track.

It will be appreciated that in an alternative example, relative motion of the LIDAR with respect to the nacelle 3 can be avoided, by correcting the LIDAR measurement to take into account differences between its line of sight direction 14 and the wind direction. In this arrangement, the direction of the wind indicated by wind sensors 18 is fed into controller 20, along with the reading of the LIDAR apparatus 10. If the LIDAR apparatus is fixed to the nacelle 3, then the difference in angle between the line of sight direction and the wind direction is essentially the angle Θ of the wind direction indicated by the wind sensor 18. The angle can then be used as a scaling factor, such as 1/cos Θ for example, to adjust the indicated wind speed of the LIDAR apparatus 10.

In a further example of the invention, the wind sensors 18 can be omitted, and the rotatable mount 12 under the control of the drive means 22 operated to turn the LIDAR apparatus into and out of the expected incident wind direction while recording the detected wind speed. Assuming that this operation is carried out more quickly than any local changes in wind speed or direction, the direction of the incident wind will be indicated by the direction in which the maximum incident wind speed was detected. The controller 20 can analyse the recorded wind speeds, and determine the maximum value, and the corresponding direction, and subsequently angle the LIDAR apparatus 10 appropriately. This process would need to be repeated at regular intervals.

Although an example of the invention has been described using a LIDAR system it will be appreciated that any wind sensor system that detects the incident wind at an appropriate position or distance in front of the wind turbine could be used. Other such examples are other doppler anemometer apparatus such as SODAR and RADAR. LIDAR apparatus are however preferred as they can be easily installed for each individual wind turbine and offer a quick and reliable sensing method. Furthermore, the invention is not limited to use of LIDAR's in Feed Forward Control systems, and could be used in with any LIDAR system where the LIDAR should be angled directly into the wind to ensure accuracy. Further, although the wind sensor discussed herein has been described as mounted on the nacelle, it could in alternative examples of the invention by provided as a met mast proximate to the wind turbine.

Claims

1 . A wind turbine comprising:

a nacelle with a rotor hub on which one or more wind turbine blades are mounted; a doppler anemometer for detecting wind velocity along a line of sight;

a rotatable mount, situated on either the nacelle or the rotor hub, for ensuring that the doppler anemometer line of sight is aligned with the incident wind direction.

2. The wind turbine of claim 1 , wherein the rotatable mount is a wind vane operable to turn under the influence of the incident wind and adopt a fixed orientation relative to the incident wind direction,

wherein the doppler anemometer is mounted on the wind vane.

3. The wind turbine of claim 1 , wherein the rotatable mount is a wind vane operable to turn under the influence of the incident wind and adopt a fixed orientation relative to the incident wind direction,

wherein the doppler anemometer is a LIDAR apparatus mounted on the nacelle, and a mirror is mounted on the wind vane for angling the LIDAR line of sight towards the incident wind direction.

4. The wind turbine of claim 1 , wherein the wind turbine comprises:

drive means for rotating the rotatable mount;

a control system for operating the drive means;

at least one wind sensor for determining the incident wind direction and for giving an output to the control system; wherein the control system is operable to cause the rotatable mount to rotate based on the output from the wind sensor.

5. The wind turbine of any preceding claim, wherein the doppler anemometer is a LIDAR apparatus mounted on the rotatable mount.

6. The wind turbine of any preceding claims, wherein the doppler anemometer is a LIDAR apparatus mounted on the nacelle, and a mirror is mounted on the rotatable mount for angling the LIDAR line of sight towards the incident wind direction.

7. The wind turbine of claim 1 , wherein the wind turbine comprises:

drive means for rotating the rotatable mount; and

a control system for operating the drive means; and wherein the control system is operable to:

vary the orientation of the rotatable mount, and therefore the line of sight of the doppler apparatus, about the expected incident wind direction;

determine the direction of maximum indicated wind velocity; adjust the rotatable mount, such that the line of sight of the doppler anemometer is directed towards the direction of maximum indicated wind velocity.

8. A wind turbine comprising:

a nacelle with a rotor hub on which one or more wind turbine blades are mounted; a doppler anemometer for detecting wind velocity along a line of sight, and giving an output;

a wind sensor for indicating the direction of the incident wind, and giving an output; a controller for receiving the outputs from the doppler anemometer and the wind sensor, and for applying a correction to the doppler anemometer output to give a corrected output, such that the corrected output indicates the wind velocity along the incident wind direction.

9. The wind turbine of any preceding claim, wherein the doppler anemometer apparatus is part of a feed forward control system for the wind turbine.

10. A method of calculating the incident wind velocity at a wind turbine having a doppler anemometer with a line of sight for detection of incident wind speed, a rotatable mount a rotatable mount situated on either the nacelle or the rotor hub of the wind turbine, and drive means for rotating the rotatable mount, the method comprising:

determining the incident wind direction and giving an output; and

causing the rotatable mount to rotate based on the output from the wind sensor, such that the line of sight of the doppler anemometer is directed towards the incident wind direction.

1 1 . A method of determining the incident wind velocity at a wind turbine having a doppler anemometer with a line of sight for detection of incident wind speed, a rotatable mount, and drive means for rotating the rotatable mount, the method comprising:

determining the incident wind direction and giving an output; and

determining the wind velocity indicated by the doppler anemometer, and giving an output;

correcting the wind velocity indicated by the doppler anemometer based on the determination of the incident wind direction.

12. A method of determining the incident wind velocity at a wind turbine having a doppler anemometer with a line of sight for detection of incident wind speed, a rotatable mount , a rotatable mount, situated on either the nacelle or the rotor hub of the wind turbine, for ensuring that the line of sight of the doppler anemometer is directed towards the incident wind direction, and drive means for rotating the rotatable mount, the method comprising:

varying the orientation of the rotatable mount, and therefore the line of sight of the doppler apparatus, about the expected incident wind direction;

determining the direction of maximum indicated wind velocity;

adjusting the rotatable mount, such that the line of sight of the doppler anemometer is directed towards the direction of maximum indicated wind velocity;

outputting the wind velocity indicated by the doppler anemometer.