A METHOD AND A SYSTEM FOR DETERMING THE WIND SPEED OR THE WIND DIRECTION EXPERIENCED BY A WIND TURBINE

Abstract

A method for determining the wind speed or the wind direction experienced by a wind turbine such that a wind turbine setting such as an inflow angle error may be adjusted based on the determined wind speed or wind direction is defined, wherein a single airflow sensor is used together with a position sensor such as an accelerometer so that the value of the single airflow sensor at a previous position of the rotor may be warped and used together with a value of the single airflow sensor of a current position.

Claims

1. A method for determining the wind speed or the wind direction experienced by a wind turbine such that a wind turbine setting such as an inflow angle error may be adjusted based on the determined wind speed or wind direction, said method comprising: providing said wind turbine having a tower, a nacelle, and a rotor, said rotor including a hub, a number of wind turbine blades mounted to said hub, and a spinner surface for diverting an airflow around said hub, said method further comprising: providing an angular sensor, and determining the rotor position by means of said angular sensor, providing an airflow sensor at said spinner surface, said airflow sensor rotating with said rotor during rotation of said rotor, determining an airflow variable by means of said airflow sensor as a function of rotor positions during a rotation of said rotor, said airflow variable varying with said rotor position when said inflow angle error being different from zero, determining a first airflow value corresponding to the value of said airflow variable at a first rotor position, determining a second airflow value corresponding to the value of said airflow variable at a second rotor position, said second rotor position being substantially ⅙ to 4/6 of a rotation before said first rotor position, and determining the wind speed or the wind direction experienced by said wind turbine as a function of said first airflow value, and said second airflow value.

2. The method according to claim 1, comprising determining a third airflow value corresponding to the value of said airflow variable at a third rotor position, and determining the wind speed or the wind direction experienced by said wind turbine as a function of said first airflow value, said second airflow value, and said third airflow value.

3. The method according to claim 1, said airflow sensor being an air velocity sensor or an air pressure sensor for measuring an air velocity or an air pressure respectively.

4. The method according to claim 1, said airflow variable being an air velocity or an air pressure.

5. The method according to claim 1, said airflow sensor determining said airflow variable at said spinner surface, or said single airflow sensor extending from said spinner surface for determining said airflow variable at a distance from said spinner surface.

6. The method according to claim 1, providing a single air flow sensor.

7. The method according to claim 1, said second rotor position being substantially ⅓ of a rotation before said first rotor position.

8. The method according to claim 1, said third rotor position being substantially 2/6 to ⅚ of a rotation before said first rotor position.

9. The method according to claim 8, said third rotor position being substantially ⅔ of a rotation before said first rotor position.

10. A method for determining the wind speed or the wind direction experienced by a wind turbine such that a wind turbine setting such as an inflow angle error may be adjusted based on the determined wind speed or wind direction, said method comprising: providing said wind turbine having a tower, a nacelle, and a rotor, said rotor including a hub, a number of wind turbine blades mounted to said hub, and a spinner surface for diverting an airflow around said hub, said method further comprising: providing an angular sensor, and determining the rotor position by means of said angular sensor as a function of time during a rotation, said rotor position being determined at least at a first rotor position, providing an airflow sensor at said spinner surface, said airflow sensor rotating with said rotor during rotation of said rotor, determining the values of an airflow variable by means of said airflow sensor as a function of time during a rotation of said rotor, the values of said airflow variable varying with said rotor position when said inflow angle error being different from zero, at a second rotor position: a) determining at which first time point said rotor was at said first rotor position, b) determining the value of said airflow variable at said second rotor position, and determining the value of said airflow variable at said first time point corresponding to when said rotor was at said first rotor position, c) determining the wind speed or the wind direction experienced by said wind tur bine as a function of the value of said airflow variable at said second rotor position and the value of said airflow variable at said first time point corresponding to when said rotor was at said first rotor position.

11. A system for determining the wind speed or the wind direction experienced by a wind turbine having a tower, a nacelle, and a rotor including a hub, a number of wind turbine blades mounted to said hub, and a spinner surface for diverting an airflow around said hub, such that a wind turbine setting such as an inflow angle may be adjusted based on the determined wind speed or wind direction, said system comprising: an angular sensor for determining the rotor position, a single airflow sensor for mounting at said spinner surface, and rotating with said rotor such that an airflow variable may be determined by means of said single airflow sensor as a function of rotor positions during a rotation of said rotor, said airflow variable varying with said rotor position when said inflow angle error being different from zero, a processing unit being configured for determining a first airflow value corresponding to the value of said airflow variable at a first rotor position, and a second airflow value corresponding to the value of said airflow variable at a second rotor position, said second rotor position being substantially ⅙ to 4/6 of a rotation before said first rotor position, said processing unit further being configured for determining the wind speed or the wind direction experienced by said wind turbine as a function of said first airflow value, and said second airflow value.

Description

[0087] The invention will now be explained in more detail below by means of examples with reference to the accompanying drawings, in which

[0088] FIG. 1 shows a wind turbine.

[0089] FIG. 2 shows a magnification of the part of the wind turbine included in the circle of FIG. 1.

[0090] FIG. 3 shows the air velocity as a function of time.

[0091] FIG. 4 shows the rotor position as a function of time.

[0092] The invention may, however, be embodied in different forms than depicted below, and should not be construed as limited to any examples set forth herein. Rather, any examples are provided so that the disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. For the same reason reference numerals refer to the same elements throughout the description. For the same reason elements will, thus, not be described in detail with respect to the description of each figure.

[0093] FIG. 1 shows a wind turbine 1 of a three-bladed horizontal-axis tower design comprising a tower 2, a nacelle 6, and a rotor with the wind turbine blades being upwind of the tower and designated the reference numerals 4a, 4b, 4c.

[0094] The rotor includes a hub, to which the three wind turbine blades are mounted. The rotor rotates around a rotor axis—the rotor axis goes through the center of the rotor, and is substantially in a horizontal plane as the name of the wind turbine design indicates (in practice there may be an inclination of about 5°).

[0095] A spinner 8 covers the hub and has a spinner surface with a curvature so that the airflow may be diverted smoothly around the center of the rotor. The spinner surface is illustrated as a semi-sphere, but it may also have a planar front face or another shape.

[0096] FIG. 2 shows a magnification of the part of the wind turbine included in the circle of FIG. 1—specifically the center of the rotor, which includes the hub covered by the spinner surface.

[0097] An airflow sensor 10 is shown at said spinner surface. The airflow sensor is mounted with an offset with respect to the center of the rotor. With respect to the rotor axis, the airflow sensor is mounted with an offset about β=35°, i.e. the airflow sensor is mounted with a sensor position angle with respect to the rotor axis (rotor shaft axis), the sensor position angle may be in the range 10°-65°.

[0098] For a flat spinner surface with the airflow sensor mounted on the planar part of the spinner surface, the offset may be chosen so that the distance to the shaft axis is in the range 0.5 m-2 m.

[0099] The airflow sensor may be a one-dimensional sonic sensor comprising a sensor body (housing), which may house the electronics and signal processing means.

[0100] The 1D sonic sensor may further comprise a bent rod, and two sensor heads attached to the bent rod opposite to each other (a proximal sensor head 12a, and a distal sensor head 12b—the proximal sensor head being closer to the point where the bent rod protrudes from the spinner surface than the distal sensor head).

[0101] Alternatively, the sensor may have two rods such that the proximal sensor head may be attached to a second rod, which extends from the spinner surface along side the first rod—the second rod extending a shorter distance than the first rod.

[0102] The airflow sensor may measure the air velocity (airflow variable) in a direction, which has a (measurement) angle to the tangential airflow flowing over the spinner surface at the location of the sensor, i.e. the velocity of the air flowing from the distal sensor head to the proximal sensor head may be measured.

[0103] The measurement angle may be about 35° or in general in the range of 0°-90°.

[0104] The sensor body is mounted to the inside of the spinner and the bent rod and sonic sensor heads protrude through a hole in the spinner. In this way, the sensor unit can easily be exchanged from the inside of the spinner, by detaching the sensor body from the spinner and retracting the bent rod and sonic sensor heads through the hole in the spinner.

[0105] The arrangement of the airflow sensor has the additional advantage that the distal sensor head does not disrupt the airflow through the sensor. This results in a more accurate reading of the airflow over the spinner surface.

[0106] Since the airflow sensor is mounted with an offset, the airflow sensor moves along a circular path as the rotor rotates.

[0107] A sound wave may be sent from the first sensor head to the second sensor head. The second sensor head receives the sound wave, and a second sound wave is sent from the second sensor head to the first sensor head.

[0108] The air velocity, in the direction between the tips, can be determined by the difference in the time it takes for the two sound waves to travel the distance between the two sensor tips. These sensors have no moving parts and are therefore very robust. They can also be heated in order to prevent ice build-up in cold climates.

[0109] As an alternative to a 1D sonic anemometer, a pitot tube, a savonious rotor, a propeller anemometer, or a cup anemometer may be used.

[0110] FIG. 3 shows the air velocity as measured by the airflow sensor for a little more than three rotations of the rotor as a function g of time.

[0111] The graph exhibits a sinusoid shaped curve with a relative small amount of noise (the high frequency fluctuations), which is indicative of a fairly small amount of turbulence/non laminar airflow during the tree rotations.

[0112] The sinusoid stems from the fact that the airflow sensor moves along a circular path as the rotor rotates.

[0113] The amplitude of the sinusoid increases with the inflow angle error, i.e. if the wind turbine is completely aligned to the direction of the wind, the inflow angle is zero and the measured air velocity as a function of the rotor rotating is constant.

[0114] More turbulent wind conditions may result in an air velocity which does not appear as sinusoid as the curve in FIG. 3 due to the high frequency fluctuations being greater.

[0115] Not shown in FIG. 1 is an angular sensor for determining the rotor position—the rotor position is to be understood as the rotation angle of the rotor. The angular sensor such as an accelerometer may be located inside the sensor body of the airflow sensor.

[0116] The angular sensor may be used to determine the airflow variable (air velocity V) as a function ƒ of rotor position x such that a given rotor position x may be mapped to an airflow value V=ƒ(x) by the function ƒ. The rotor position is defined as x=0°=360° when the airflow sensor is at its highest position.

[0117] The two sensors may sample such that for every measured/determined rotor position value there is exactly one measured airflow value—these two values define an ordered pair.

[0118] For a full rotation two ordered pairs of airflow value and rotor position may be collected/measured (x.sub.1, V.sub.1) and (x.sub.2, V.sub.2).

[0119] Alternatively, three ordered pairs of airflow value and rotor position may be collected/measured per rotation of the rotor—the sampling frequency of the two sensors (airflow sensor and angular sensor) may be even higher such that they sample up to 10 samples per second.

[0120] FIG. 4 shows the rotor position from 0° to 360° as a function h of time for a little more than three rotations—the rotor position being measured by the angular sensor.

[0121] The airflow variable as a function of rotor position is then ƒ=g(h(⋅)).

[0122] The measurement of the air velocity at the spinner surface may be used to determine the wind speed as well as the wind direction experienced by the wind turbine (free wind speed U, angle of attack relative to the rotor axis θ and azimuth angle φ on the spinner) by the following three relationships:

V.sub.1=V(360)=U(K.sub.1 cos θ+K.sub.2 sin θ cos φ)

V.sub.2=V(∂.sub.1)=U(K.sub.1 cos θ+K.sub.2 sin θ cos(φ+∂.sub.1))

V.sub.3=V(∂.sub.2)=U(K.sub.1 cos θ+K.sub.2 sin θ cos(φ+∂.sub.2))

[0123] V.sub.1, V.sub.2 and V.sub.3 are measured within one rotation of the rotor by the airflow sensor.

[0124] K.sub.1, K.sub.2 are constants. The two constants can be checked by measurements of wind speed and wind direction from a free mast (K.sub.1) and measurements during yawing of the wind turbine (K.sub.2), i.e. K.sub.1 relates to a nacelle transfer function (the transfer function between the free wind speed and the nacelle wind speed (wind speed measured behind the rotor). The free wind speed may be measured by a sensor on a mast 2.5 rotor diameters from the wind turbine for example.

[0125] A spinner transfer function is the transfer function between the free wind speed and the wind speed at the center of the rotor just in front of the rotor, i.e. typically at the spinner surface.

[0126] K.sub.2 tells about the shape of the spinner surface. The constant K.sub.α=K.sub.1/K.sub.2 is chosen such that the same wind speed is measured during the yawing.

[0127] During calibration for determining the constants, there may be measured over a period in order to find the average best constants.

[0128] Alternatively, the two constants may be determined by numerical simulations (computational fluid dynamic) with Navier-Stokes equations as basis for the simulation/model.

[0129] Typical values for the constants are K.sub.1=0.75211 and K.sub.2=0.92183.

[0130] The two parameters ∂.sub.1 and ∂.sub.2 are a first phase and a second phase respectively, and correspond to the angular positions where the airflow sensor takes measurements. It may be chosen that the parameter ∂.sub.2=2∂.sub.1, and may take on the values ∂.sub.1=120° and ∂.sub.2=240° respectively, i.e. the airflow sensor may measure air velocity at three equidistant angles (rotor positions). However, the rotor positions need not be equidistant.

[0131] Thus, V.sub.2 is an air velocity also measured by the airflow sensor, but at a different rotor position than where the air velocity V.sub.1 was measured, i.e. the value of the output of the airflow sensor at a previous position of the rotor may be warped and used together with the value of the output of the airflow sensor at a current position in order to determine the wind speed experienced by the wind turbine.

[0132] V.sub.3 is an air velocity also measured by the airflow sensor, but at a different rotor position than where the air velocities V.sub.1 and V.sub.2 were measured.

[0133] For example, the airflow sensor may measure at rotor positions of 120°, 240°, and 360°. This is illustrated by the three vertical striped lines extending from the graph in FIG. 4 to the graph in FIG. 3, i.e. as the rotor rotates the airflow sensor measures air velocity and the rotor position is also measured which means that for a given rotor position, the respective air velocity is also known.

[0134] The three above-mentioned relationships may then be solved for the three unknown variables (free wind speed U, angle of attack relative to the rotor axis θ and azimuth angle φ on the spinner). This may be done either analytically or numerically.

[0135] The wind speed experienced by the wind turbine as a function of the first airflow value, the second airflow value and the third airflow value may be determined as:

[00001]$U=\left(V_1,V_2,V_3\right)=\frac{V_1+V_2+V_3}{3K_1\cos \theta }$$\mathrm{where}$$\theta =\arctan \frac{K_1\left(V_2-V_1\right)}{K_2\left(V_1\cos \left(\phi +2\pi /3\right)-V_2\cos \phi \right)}$$\mathrm{and}$$\phi =\arctan \frac{\sqrt{3}\left(V_2-V_3\right)}{V_2+V_3-2V_1}$

[0136] This solves the problem that for more turbulent wind conditions there will be an inconsistency when going back in time to what amounts to an average of ⅓ the time of a rotation and use the airflow measurement at that point in time, i.e. it is not certain that ⅓ the time of a rotation on average corresponds to ⅓ of a rotation. When the airflow is purely laminar there may not be an inconsistency.

[0137] As an alternative, only two ordered pairs may be used for determining the free wind speed U, angle of attack relative to the rotor axis θ and azimuth angle φ on the spinner.

[0138] The starting point is that according to irrational flow theory, the tangential airspeed of the airflow around a sphere at a certain point can be written as:

[00002]$V_x=U\sin \left(\beta \right)\left(1+\frac{R^3}{2{\left(R+d\right)}^3}\right)$

[0139] where R is the radius of the spinner (provided the spinner surface is spherically shaped), and d is distance or height above the spinner surface in which the sensor measures.

[0140] For more complex shapes of the spinner surface there may not be closed form analytical expressions, and a numerical calculation may be needed instead.

[0141] In the xy-plane the azimuth angle φ may be determined by measuring V.sub.1=ƒ(270) and determining V.sub.2=ƒ(90), which then constitute the two ordered pairs. The ratio between the two is termed:

[00003]$F=\frac{V_1}{V_2}$

[0142] From this ratio, the azimuth angle φ may be determined from the following inverse trigonometric function:

[00004]$\phi =\arctan \left(\frac{1-F}{1+F}\tan \beta \right)$

[0143] The free wind speed U may be determined as:

[00005]$U=\frac{V_1}{\sin \left(\beta -\phi \right)\left(1+\frac{1}{2}{\left(\frac{R}{R+d}\right)}^3\right)}$

[0144] For the determination of the angle of attack, the two ordered pairs are V.sub.1=ƒ(180) and V.sub.2=ƒ(0) used for the ratio, which enters the inverse trigonometric function defined above.

[0145] Below is a list of reference signs used in the detailed description of the invention and in the drawings referred to in the detailed description of the invention. [0146] 1 Wind turbine [0147] 2 Tower [0148] 4a First wind turbine blade [0149] 4b Second wind turbine blade [0150] 4c Third wind turbine blade [0151] 6 Nacelle [0152] 8 Spinner surface [0153] 10 Airflow sensor [0154] 12a Proximal sensor head [0155] 12b Distal sensor head [0156] β Offset [0157] α rotor axis [0158] x rotor position