Method of operating a wind turbine

Abstract

Method of operating a wind turbine comprising a plurality of blades rotatable along their longitudinal axes using a pitch mechanism, and comprising one or more movable trailing edge surfaces. The method includes predicting, at a first moment in time, a high load for one or more of the blades at a second moment in time. The method further comprises actuating on one or more of the movable trailing edge surfaces of these blades such that the trailing edge surfaces have a wider range of control to counteract the predicted high loads before the second moment in time, and simultaneously pitching the blades such as not to negatively affect the operation of the wind turbine. The method furthermore comprises, at the second moment in time, actuating the one or more movable trailing edge surfaces of the at least one or more blades to counteract the high loads.

Claims

1. A method of operating a wind turbine comprising a plurality of blades, each of the blades being rotatable along its longitudinal axis using a pitch mechanism, and each of the blades comprising one or more movable trailing edge surfaces, the method comprising: at a first moment in time predicting that a first of the blades will experience a high load at a later second moment in time, before the second moment in time, actuating one or more of the movable trailing edge surfaces of the first blade such that the trailing edge surfaces of the first blade are moved from an initial position to a modified position that will provide a wider range of control to counteract the first high load at the second moment in time, and simultaneously pitching the first blade to a position to at least partially compensate for lift changes induced in the first blade by movement of the trailing edge surfaces to the modified position, and at the second moment in time, actuating the one or more movable trailing edge surfaces of the first blade from the modified position to a final position to counteract the first high load.

2. The method according to claim 1, wherein the pitching the first blade comprises pitching the first blade to a position such that a generation of electrical power by the wind turbine is not negatively affected by the modified position of the trailing edge surfaces.

3. The method according to claim 1, wherein the pitching the first blade comprises pitching the first blade to a position such that loads on the first blade remain substantially at the same level as at the first moment in time with the trailing edge surfaces at them modified position.

4. The method according to claim 1, wherein the predicting a high load for the first blade comprises using a LIDAR.

5. The method according to claim 1, wherein the predicting a high load for the first blade comprises measuring loads in the wind turbine.

6. The method according to claim 1, wherein the trailing edge surfaces of each of the blades comprises one or more flaps.

7. The method according to claim 6, wherein each of the blades comprises a plurality of the flaps distributed along the length of the blade, and wherein each of these flaps is adapted to be actuated individually.

8. The method according to claim 1, wherein one or more of the movable trailing edge surfaces of the first blade are moved substantially towards their most negative position at the modified position, in which the lift of the first blade is most reduced, and the first blade is pitched such as to increase its angle of attack.

9. The method according to claim 1, wherein the wind turbine comprises an individual pitch system for each of the blades, such that each of the blades can be pitched individually.

10. The method according to claim 1, wherein the predicting a high load for a first blade at a second moment comprises measuring loads in another wind turbine.

11. The method according to claim 1, wherein one or more of the movable trailing edge surfaces of the first blade are moved substantially towards their most positive position at the modified position, in which the lift of the first blade is most increased, and pitching the first blade such as to decrease its angle of attack.

12. A method of operating a wind turbine comprising a plurality of blades, each of the blades being rotatable along its longitudinal axis using a pitch mechanism, and each of the blades comprising one or more movable trailing edge surfaces, the method comprising: at a first moment in time predicting that a first blade will experience a first high load at a second later moment in time, and predicting that a second blade will experience a second high load at a third moment time later than the second moment in time, before the second moment in time, actuating on one or more of the movable trailing edge surfaces of the first blade from an initial position such that they assume a modified position that will provide a wider range of control to counteract the first blade high load at the second moment in time, before the third moment in time, actuating on one or more of the movable trailing edge surfaces of the second blade from an initial position such that they assume a modified position that will provide a wider range of control to counteract the second blade high load at the third moment in time, before the second moment in time, pitching the first and second blades of the wind turbine to a respective position to at least partially compensate for lift changes induced in the first and second blades by movement of the respective trailing edge surfaces to the modified position, at the second moment in time, actuating the one or more movable trailing edge surfaces of the first blade from the modified position to a final position to counteract the first blade high load, and at the third moment in time, actuating the one or more movable trailing edge surfaces of the second blade from the modified position to a final position to counteract the second blade high load.

13. The method according to claim 12, comprising pitching the first and second blades to a position such that a generation of electrical power by the wind turbine is substantially not negatively affected by the modified position of the respective trailing edge surfaces.

14. The method according to claim 12, comprising pitching the first blade to position such that loads on the first blade remain substantially at the same level as at the first moment in time with the respective trailing edge surfaces at the modified position, and pitching the second blade to a position such that loads on the second blade remain substantially at the same level as at the first moment in time with the respective trailing edge surfaces at the modified position.

15. The method according to claim 14, wherein one or more of the movable trailing edge surfaces of the first blade are moved towards a substantially neutral position at the modified position.

16. The method according to claim 15, wherein the first blade is pitched such as to increase the angle of attack of the first blade.

17. The method according to claim 15, wherein pitching the first blade is pitched such as to decrease the angle of attack of the first blade.

18. The method according to claim 14, wherein the predicting a first high load for a first blade at a second moment in time, and the predicting a second high load for a second blade at a third moment in time comprises predicting the same high load for the first and second blades.

19. The method according to claim 14, wherein the predicting a high load for a first blade at a second moment in time, and the predicting a high load for a second blade at a third moment time comprises measuring loads in the wind turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which:

(2) FIG. 1a illustrates a wind turbine;

(3) FIG. 1b illustrates a wind turbine blade comprising a plurality of flaps along its span;

(4) FIG. 1c illustrates a possibility of predicting high loads that may be used in embodiments of the present invention

(5) FIGS. 2a-2c illustrate the effect of a flap on the lift coefficient and drag coefficient of an aerodynamic profile for different angles of attack and different flap positions;

(6) FIG. 3 illustrates various flap mechanisms that may be used in embodiments of the present invention;

(7) FIG. 4 illustrates a typical power curve for a variable speed wind turbine;

(8) FIGS. 5a-5c and 5e illustrate various methods of operating a wind turbine according to different embodiments of the present invention; and

(9) FIG. 5d illustrates a plurality of “Mexican hat” wind gusts.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) FIG. 1a illustrates a wind turbine comprising a tower 9, and a rotor comprising a first blade 1, a second blade 2 and a third blade 3. The swept area of the rotor is indicated using reference sign 4. In accordance with embodiments of the invention, each of the blades may be rotated substantially along their longitudinal axes, using a collective pitch mechanism or three individual pitch mechanisms.

(11) FIG. 1b illustrates a top view of a wind turbine blade 1. The wind turbine blade extends from the blade root 5 to a blade tip 6. In the displayed embodiment, three flaps 41, 42, 43 have been provided along the trailing edge, substantially in an outer portion of the blade span. In other embodiments, more or less flaps may be provided. Additionally, in other embodiments, flaps may only be provided along an inner portion of the blade span, or alternatively flaps may be provided substantially along the entire blade span.

(12) FIG. 1c illustrates the use of a LIDAR 51 on a wind turbine. LIDAR stands for Light Detection and Ranging and involves an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light. LIDARs and specifically Doppler LIDARs are well known in the field of wind energy for measuring wind speeds ahead of a wind turbine and they will not be further described herein. LIDARs may be used to e.g. measure wind speeds, wind shear (as indicated with reference sign 52 in FIG. 1c), and local turbulences, e.g. the area indicated with reference sign 7 in FIG. 1a.

(13) FIGS. 2a-2c illustrate the effect of a flap on the lift coefficient and drag coefficient of an aerodynamic profile for different angles of attack and different flap positions. FIG. 2a illustrates a profile 10 comprising a flap in three different positions: a neutral position 11, a positive position 12 (in which the flap has a positive contribution to the lift) and a negative position 13 (in which the flap has a negative contribution to the lift).

(14) FIG. 2b illustrates a typical curve 21 of the lift coefficient of a profile vs. angle of attack. The angle of attack a has been illustrated in FIG. 2a and may be defined as the angle between a reference line on a lifting body (in this case, the chord line of an airfoil 28) and the vector 29 representing the relative motion between the blade and the air through which it is moving. In the case of a wind turbine, the vector representing the relative motion between the blade and the air may be composed of two components: the wind velocity v.sub.w and the rotational velocity of the local portion of the blade v.sub.r. The rotational velocity of the local portion of the blade may be expressed as

(15) $v_r=2\pi .Math.r.Math.\frac{n}{60},$
wherein r is the radius of the local portion of the blade and n is the number of revolutions per minute. The total wind velocity experienced by the blade portion may thus be expressed as v=√{square root over (v.sub.r.sup.2+v.sub.w.sup.2)}, and the angle of attack may be determined in accordance with:
tan α=v.sub.w/v.sub.r

(16) The angle of attack of a portion of the blade may thus be affected by the prevailing wind speed, the rotational speed of the rotor, its position along the blade span, the design twist of the airfoil and the pitch angle of the blade.

(17) The typical curve 21 shows that with increasing angle of attack, the lift coefficient of the profile increases, until “stall”. If the angle of attack is increased further, the lift coefficient is reduced.

(18) If a flap is put in its positive position, the lift of the airfoil is increased. A curve 22 that is substantially parallel to curve 21 may be the result: for any angle of attack, the lift coefficient is slightly higher. Similarly, a curve 23 that is substantially parallel to curve 21 corresponds to the same profile, but with the flap in a negative position. It will be clear that in practice, the curves of the lift coefficient may be different for any profile employed on a blade.

(19) FIG. 2c illustrates curves for the same three positions of the flap of the lift coefficient vs the drag coefficient. Curve 31 illustrates the profile with the flap in the neutral position, curve 32 illustrates the profile with the flap in the positive position and curve 33 illustrates the profile with the flap in the negative position. The C.sub.L-C.sub.D curve may comprise a portion in which the drag hardly increases with increasing lift. The curve may also comprise a portion (closer to stall) along which the drag increases more rapidly than the lift.

(20) The effect of the positive flap position is generally that the drag increases and the effect of a negative flap position is generally that the drag decreases.

(21) It may be deducted from the illustrated curves that if loads are to be decreased, i.e. if high wind loads are imminent, one option is to reduce the angle of attack of a blade. This may be accomplished using a pitch mechanism. However, a disadvantage of the pitch mechanism is that it may be too slow to react to a prediction of a high load. Alternatively, the flap may be activated towards its negative position, so that the lift and drag may be reduced at the same angle of attack.

(22) It will be clear that the lift curves (C.sub.L vs α) and drag curves may vary if a different airfoil is used. Furthermore, it should be noted that three-dimensional aspects may affect the lift and drag curves of a blade as well. These three-dimensional aspects are herein ignored, since the general teaching (flap towards a positive position increases the lift, and flap towards a negative position decreases the lift, representing a jump to a different lift curve) will approximately hold true for a blade as well.

(23) FIG. 3 illustrates some of the different flaps that may be used in embodiments of the present invention. Profile 15 comprises a “plain flap” illustrated in its neutral position 11, its positive position 12, and its negative position 13. Profile 16 illustrates a “split flap” in its neutral position, and its positive position. Such a split flap does not have a negative position.

(24) Profile 17 comprises a “slotted flap” in its neutral position 11, positive position 12 and negative position 13. With a slotted flap, the air may flow partially through the slot in between the flap and the remainder of the profile. Finally, profile 18 comprises a “Fowler flap” in its neutral position, its positive position 12 and its negative position 13. A peculiar property of the Fowler flap is that the chord of the profile in its neutral position is longer than in its negative position.

(25) Other movable trailing edge surfaces than the ones illustrated may be used in embodiments of the present invention. For example, also a Continuously Deformable Trailing Edge (CDTE) may also be used. In some embodiments, a blade may comprise one or more portions that are continuously deformable along its span.

(26) Depending on which kind of flap or movable surface is used, the influence on the curves of the lift coefficient and drag coefficient may vary. In general, actuating the flap towards its positive position increases the curvature of an airfoil, which increases the lift. It further increases the drag, but to a lesser extent. As such, the loads on a blade increase, but this may be acceptable, in accordance with circumstances. The increase of the lift increases the aerodynamic torque of the rotor.

(27) Actuating a flap towards its negative position decreases the lift and may increase the drag, but generally not to the same extent. The extent to which the drag increases (and it may actually decrease) also depends on the angle of attack. The decrease in lift reduces the aerodynamic torque and the loads on the blade in general. Moving the flap to its negative position may thus be used to counteract e.g. temporary high loads on a blade.

(28) FIG. 4 illustrates a prior art control method and shows respectively the pitch angle (β), rotor speed (ω.sub.rotor), electrical power (P) and aerodynamic torque (M) at varying wind speeds.

(29) As previously described, the pitch angle is generally not changed until nominal wind speed is reached, e.g. at 11 m/s. At a slightly lower wind speed, e.g. around 8.5 m/s, nominal rotor speed may be reached. At wind speeds above nominal wind speed, the pitch angle may be varied such as to maintain the aerodynamic torque substantially constant. The rotor speed, generator torque and electrical power generated may also be maintained substantially constant. This may be maintained from nominal wind speed to cut-out wind speed.

(30) The pitch mechanism is particularly suitable for adapting the wind turbine blade to a varying wind speed. However, the pitch mechanism may not be suitable to react to a sudden wind gust. Actuation of the pitch mechanism in these circumstances may be too slow.

(31) Further, individual pitching may be used to compensate e.g. for wind shear, illustrated in FIG. 1c. A blade that passes along the bottom of the swept area 4 will experience lower lift, then a blade that passes along a top portion of the swept area. The angle of attack of the blades may be varied along each cycle of rotation to adapt to changing wind speed.

(32) FIGS. 5a-5c and 5e illustrate various methods of operation in accordance with the present invention. In FIG. 5a, the previously shown lift curves 21, 22, and 23 of a typical airfoil are shown. At a first moment in time, “a”, the actual situation of the blade may be that the flap is in a negative position, on curve 24, but not in its most negative position. At this moment, high loads may be predicted for a second moment in time. This may be done e.g. using a LIDAR system. Alternatively, it may be that a high load is predicted e.g. because a neighbouring wind turbine has experienced such a high load. Any other alternative method for predicting loads may also be used, such as e.g. methods involving artificial neural networks or methods involving deterministic algorithms.

(33) In order to prepare for such a high load, the flap may be moved towards its most positive position, so that a wide range of control is available for the flap when the high load occurs. If the flap were only moved towards its most positive position, the lift of the blade would increase and the total loads as well. To counteract this phenomenon, at the same time, the blade may be pitched such that the total lift (and the total loads) remains substantially constant, point “b”. It may be deduced from FIG. 5a that at point “b”, compared to point “a”, the angle of attack is reduced and the lift is maintained substantially constant. An effect of this is that when the high load occurs, the flap may be moved quickly towards its most negative position, point “c”, so as to reduce the loads. When the high load has passed, the flap may be returned towards its most positive position to regain the lift of the blade and the aerodynamic torque of the rotor.

(34) With the move form point “a” to point “b” in FIG. 5a, the lift is maintained substantially constant. The aerodynamic torque of the rotor as well, and therewith the electrical power that may be generated. The operation of the wind turbine may thus be substantially unaffected by the manoeuvres in anticipation of a high load.

(35) FIG. 5b illustrates another similar method. At a first moment in time, point “a”, a high load may be predicted for a first blade at a second moment in time, and a high load may be predicted for a second blade at a third moment in time. In preparation of the high loads, the flap of the first blade may be moved towards a more positive position in accordance with FIG. 5a. At the same time, the blade may be pitched to compensate for this change. However, there may only be little time available before the high load will occur for the first blade. So, at point b1, when the high load occurs, the flap is quickly moved towards point c1.

(36) However, for the second blade, there may be more time available before a high load occurs. This may be the case e.g. for a local turbulence 7, as illustrated in FIG. 1a. With a clockwise rotation, blade 3 will experience the high load before blade 2. The local turbulence may have disappeared before blade 1 could experience the turbulence.

(37) FIG. 5c illustrates another method of operating a wind turbine. At a first moment in time, the blade may be in a configuration corresponding to point a. The flap may be in a substantially positive position. If a wind gust is predicted at this point in time, contrary to what was shown before, the flap may be moved towards its neutral position. At the same time, the blade may be pitched so as to increase the angle of attack.

(38) When the wind gust occurs, the blade will thus be in a configuration in which it has a wider operational range to react to the wind gust. In case of e.g. a Mexican hat wind gust, illustrated in FIG. 5d, the wind speed first decreases below the original wind speed, then quickly increases, then decreases again below the original speed, to finally stabilize again. The flap may thus be moved between points “c” and “d” to adapt to the varying wind speed during the wind gust.

(39) FIG. 5e illustrates another method. In the methods illustrated so far, between the moment of predicting a load, and the moment of the load actually occurring, it was attempted to substantially maintain the lift constant. A constant lift means that the torque generated by the blade is substantially constant as well. The electrical power may thus be maintained at the same level as well. Depending on the particular situation however, the drag may increase, and the total loads as well.

(40) In the method illustrated in FIG. 5e, the lift is however slightly decreased between points “a” and “b”. This may be acceptable under circumstances as well. An advantage is that the total loads may be kept more constant, while at the same time the operational range of the flap to counteract an impending load is increased.

(41) Criteria other than the generation of electrical power, or limit loads may be important during the operation of a wind turbine. For example, a wind turbine may be in a shutdown procedure when the high load is predicted. In this case, the flaps (or other movable surface) may be actuated upon so that they assume a position in which their range of control to counteract the high load is increased. At the same time, the blade(s) may be pitched such as to continue slowing down the wind turbine. In this case, the priority may not be to maximize the generation of electricity or the loads on the blades, but instead may be to continue reducing the speed of the wind turbine.

(42) Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.