NOISE REDUCTION IN A WIND TURBINE WITH HINGED BLADES

Abstract

A method for controlling a wind turbine (1) is disclosed. The wind turbine (1) comprises one or more wind turbine blades (5), each wind turbine blade (5) being connected to a blade carrying structure (4) mounted on a hub (3), via a hinge (6) at a hinge position of the wind turbine blade (5), each wind turbine blade (5) thereby being arranged to perform pivot movements relative to the blade carrying structure (4) between a minimum pivot angle and a maximum pivot angle. A maximum noise level value representing a maximum allowable noise to be generated by the wind turbine (1) is received. An optimal pair of tip speed for the wind turbine (1) and rotational speed of the wind turbine (1) is derived, based on the received maximum noise level value. The pivot angle of the wind turbine blades (5) is then adjusted to a pivot angle which results in the derived optimal pair of tip speed and rotational speed.

Claims

1. A method for controlling a wind turbine, the wind turbine comprising a tower, at least one nacelle mounted on the tower via a yaw system, a hub mounted rotatably on each nacelle, each hub comprising a blade carrying structure, and one or more wind turbine blades, each wind turbine blade being connected to the blade carrying structure via a hinge at a hinge position of the wind turbine blade, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, the method comprising the steps of: receiving a maximum noise level value representing a maximum allowable noise to be generated by the wind turbine, deriving an optimal pair of tip speed for the wind turbine and rotational speed of the wind turbine, based on the received maximum noise level value, and adjusting the pivot angle of the wind turbine blades to a pivot angle which results in the derived optimal pair of tip speed and rotational speed.

2. The method according to claim 1, wherein the step of deriving an optimal pair of tip speed for the wind turbine and rotational speed of the wind turbine comprises the steps of: deriving a tip speed reference for the wind turbine, based on the maximum noise level, and deriving an optimal pair of rotor diameter and rotational speed of the wind turbine which results in a tip speed of the wind turbine which is equal to the derived tip speed reference, and wherein the step of adjusting the pivot angle of the wind turbine blades comprises adjusting the pivot angle of the wind turbine blades to a pivot angle which results in the derived rotor diameter.

3. The method according to claim 2, wherein the step of deriving an optimal pair of rotor diameter and rotational speed of the wind turbine comprises deriving a rotor diameter which results in a tip speed of the wind turbine which is equal to the derived tip speed reference, given that the current rotational speed of the wind turbine is maintained.

4. The method according to claim 1, wherein the step of deriving an optimal pair of tip speed and rotational speed of the wind turbine comprises maximizing a power production of the wind turbine.

5. The method according to claim 1, further comprising the step of applying a biasing force to the wind turbine blades which biases the wind turbine blades towards a position defining a minimum pivot angle, and wherein the step of adjusting the pivot angle of the wind turbine blades comprises adjusting the biasing force applied to the wind turbine blades.

6. The method according to claim 1, wherein the step of adjusting the pivot angle of the wind turbine blades comprises adjusting a force applied to the wind turbine blades which causes the wind turbine blades to move towards a position which increases the pivot angle.

7. The method according to claim 1, further comprising the step of adjusting a generator torque of the wind turbine in order to reach the derived optimal pair of tip speed and rotational speed.

8. The method according to claim 1, wherein the maximum noise level value is received from a central controller.

9. A method for controlling a wind turbine, the wind turbine comprising a tower, at least one nacelle mounted on the tower via a yaw system, a hub mounted rotatably on each nacelle, each hub comprising a blade carrying structure, and one or more wind turbine blades, each wind turbine blade being connected to the blade carrying structure via a hinge at a hinge position of the wind turbine blade, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, the method comprising the steps of: receiving a maximum tip speed value representing a maximum allowable tip speed of the wind turbine, based on a level of leading edge erosion and/or risk of development of leading edge erosion, deriving an optimal pair of rotor diameter and rotational speed of the wind turbine which results in a tip speed of the wind turbine which is equal to or smaller than the maximum tip speed value, and adjusting the pivot angle of the wind turbine blades to a pivot angle which results in the derived rotor diameter.

10. A wind turbine comprising a tower, at least one nacelle mounted on the tower via a yaw system, a hub mounted rotatably on each nacelle, each hub comprising a blade carrying structure, and one or more wind turbine blades, each wind turbine blade being connected to the blade carrying structure via a hinge at a hinge position of the wind turbine blade, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, wherein the wind turbine further comprises a mechanism arranged to adjust the pivot angle of the wind turbine blades in response to a maximum noise level value representing a maximum allowable noise to be generated by the wind turbine.

11. The wind turbine according to claim 10, further comprising a biasing mechanism arranged to apply a biasing force to the wind turbine blades which biases the wind turbine blades towards a position defining a minimum pivot angle, and wherein the mechanism arranged to adjust the pivot angle of the wind turbine blades is arranged to adjust the applied biasing force.

12. The wind turbine according to claim 10, wherein the wind turbine is a downwind wind turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The invention will now be described in further detail with reference to the accompanying drawings in which

[0089] FIG. 1 is a front view of a wind turbine according to an embodiment of the invention,

[0090] FIGS. 2 and 3 are side views of the wind turbine of FIG. 1 with the wind turbine blades at two different pivot angles,

[0091] FIGS. 4 and 5 show details of a mechanism for adjusting a pivot angle of wind turbine blades of a wind turbine according to an embodiment of the invention,

[0092] FIG. 6 illustrates a wind turbine according to an embodiment of the invention with the wind turbine blades in three different positions,

[0093] FIGS. 7-9 illustrate various mechanisms for adjusting a pivot angle of wind turbine blades of wind turbines according to embodiments of the invention,

[0094] FIG. 10 is a graph illustrating tip speed as a function wind speed for a traditional wind turbine and for a wind turbine operated in accordance with a method according to an embodiment of the invention, respectively,

[0095] FIG. 11 is a graph illustrating power production as a function of wind speed for a traditional wind turbine and for a wind turbine operated in accordance with a method according to an embodiment of the invention, respectively,

[0096] FIG. 12 is a graph illustrating rotational speed as a function of wind speed for a traditional wind turbine and for a wind turbine operated in accordance with a method according to an embodiment of the invention, respectively,

[0097] FIG. 13 is a graph illustrating generated noise level as a function of rotor diameter for a traditional wind turbine and for a wind turbine operated in accordance with a method according to an embodiment of the invention, respectively, and

[0098] FIGS. 14 and 15 are graphs illustrating tip speed as a function of wind speed for a traditional wind turbine and for a wind turbine operated in accordance with a method according to two alternative embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0099] FIG. 1 is a front view of a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 comprises a tower 2 and a nacelle (not visible) mounted on the tower 2. A hub 3 is mounted rotatably on the nacelle, the hub 3 comprising a blade carrying structure 4 with three arms. A wind turbine blade 5 is connected to each of the arms of the blade carrying structure 4 via a hinge 6. Thus, the wind turbine blades 5 rotate along with the hub 3, relative to the nacelle, and the wind turbine blades 5 can perform pivoting movements relative to the blade carrying structure 4, via the hinges 6.

[0100] Each wind turbine blade 5 defines an aerodynamic profile extending along the length of the wind turbine blade 5 between an inner tip end 5a and an outer tip end 5b. The hinge 6 is arranged at a hinge position of the wind turbine blade 5, the hinge position being at a distance from the inner tip end 5a as well as at a distance from the outer tip end 5b.

[0101] FIG. 2 is a side view of the wind turbine 1 of FIG. 1 with the wind turbine blades 5 positioned at a minimum pivot angle, i.e. at a pivot angle which results in a maximum rotor diameter of the wind turbine 1. In FIG. 2 the nacelle 7 can be seen. The wind turbine blades 5 are biased towards this position by means of a wire attached to the inner part of the wind turbine blades 5, i.e. at a position between the hinge 6 and the inner tip end 5a. This will be described in further detail below with reference to FIGS. 4 and 5.

[0102] FIG. 3 is a side view of the wind turbine 1 of FIGS. 1 and 2. In FIG. 3 the wind turbine blades 5 are positioned at a larger pivot angle than the minimum pivot angle of FIG. 2. Thereby the rotor diameter of the wind turbine 1 is smaller in the situation illustrated in FIG. 3 than in the situation illustrated in FIG. 2. Assuming that the rotational speed of the wind turbine 1 is the same in the two situations, the tip speed will be lower in the situation illustrated in FIG. 3 than in the situation illustrated in FIG. 2. Since the noise generated by the wind turbine 1 depends strongly on the tip speed, the noise generated by the wind turbine 1 is thereby lower in the situation illustrated in FIG. 3 than in the situation illustrated in FIG. 2. In FIG. 3 a portion of the wires 8 pulling the wind turbine blades 5 towards the minimum pivot angle position can be seen.

[0103] The wind turbine 1 of FIGS. 1-3 may be operated in the following manner. Initially the wind turbine 1 is operated in an ordinary manner, extracting as much energy as possible from the wind, without considering the noise generated by the wind turbine 1. The wires 8 bias the wind turbine blades 5 towards the minimum pivot angle position, while centrifugal forces acting on the wind turbine blades 5 and possibly aerodynamic forces acting on the wind turbine blades 5 attempt to move the wind turbine blades 5 towards larger pivot angles in such a manner that the higher the rotational speed of the wind turbine 1, the larger the combined centrifugal and aerodynamic force will be. Thus, for a given wind speed, and thereby a given rotational speed of the wind turbine 1, an equilibrium is obtained which positions the wind turbine blades 5 at a certain pivot angle.

[0104] At a certain point in time, a maximum noise level value is received. The maximum noise level value represents a maximum allowable noise to be generated by the wind turbine 1. Accordingly, the maximum noise level value indicates an upper limit for the noise which the wind turbine 1 is allowed to generate under the given circumstances. The maximum noise level value may be a fixed value, representing a noise level which should not be exceeded at any time. Alternatively, the maximum noise level may be a dynamic value which varies according to prevailing conditions, such as time of day, time of year, wind speed, wind direction, etc.

[0105] Based on the received maximum noise level value, an optimal pair of tip speed for the wind turbine 1 and rotational speed of the wind turbine 1 is derived.

[0106] Thus, operating the wind turbine 1 at the tip speed and rotational speed of the derived optimal pair results in a noise being generated by the wind turbine 1 which is below the maximum noise level value. Furthermore, the derived pair of tip speed and rotational speed is optimal in the sense that other considerations are taken into account, such as power production of the wind turbine 1, loads on the wind turbine 1, etc. The optimal pair of tip speed and rotational speed could, e.g., be derived by deriving a tip speed reference ensuring that the maximum noise level value is not exceeded, and deriving an optimal pair of rotor diameter and rotational speed of the wind turbine which results in a tip speed of the wind turbine 1 which is equal to the derived tip speed reference. The optimal pair of tip speed and rotational speed is then the tip speed reference and the rotational speed of the optimal pair of rotor diameter and rotational speed.

[0107] Then the pivot angle of the wind turbine blades 5 is adjusted to a pivot angle which results in the derived optimal pair of tip speed and rotational speed. Operating the wind turbine 1 with the wind turbine blades 5 arranged at this pivot angle will, accordingly, has the consequence that the maximum noise level value is not exceeded.

[0108] The pivot angle of the wind turbine blades 5 may be adjusted in the following manner. As described above, the wires 8 pull the wind turbine blades 5 towards a position defining a minimum pivot angle, and thereby a maximum rotor diameter of the wind turbine 1. In the case that it is necessary to reduce the tip speed in order to decrease the noise level, the pulling force applied to the wind turbine blades 5 by the wires 8 is reduced. This allows the wind turbine blades 5 to more easily move towards a larger pivot angle, and thereby towards a smaller rotor diameter. Therefore a new equilibrium position for the pivot angle at a given rotational speed is obtained, at a larger pivot angle. Accordingly, the wind turbine 1 will be operated with a smaller rotor diameter, and thereby with a lower tip speed. This reduces the noise generated by the wind turbine 1.

[0109] The wind turbine 1 illustrated in FIG. 2 is operated at maximum rotor diameter, e.g. with a maximum force applied to the wind turbine blades 5 by the wires 8. In the wind turbine 1 of FIG. 3, the force applied to the wind turbine blades 5 by the wires 8 has been decreased, resulting in an increased pivot angle, a decreased rotor diameter, a reduced tip speed and thereby a reduced noise level generated by the wind turbine 1.

[0110] FIGS. 4 and 5 show details of a mechanism for adjusting a pivot angle of wind turbine blades 5 of a wind turbine according to an embodiment of the invention. The wind turbine could, e.g., be the wind turbine 1 of FIGS. 1-3.

[0111] FIG. 4 shows a portion of a blade carrying structure 4 and a portion of a wind turbine blade 5. The wind turbine blade 5 is pivotally mounted on the blade carrying structure 4 via a hinge (not shown). A wire 8 is connected to the wind turbine blade 5 at a position between an inner tip end 5a of the wind turbine blade 5 and the position of the hinge. The wire 8 extends from the connecting position at the wind turbine blade 5, via a pulley 9 and along the blade carrying structure 4 towards a hub (not shown).

[0112] A pulling force applied by means of the wire 8 pulls the wind turbine blade 5 towards a position defining a minimum pivot angle. In FIG. 4 the wind turbine blade is arranged at the minimum pivot angle. Reducing the pulling force applied by means of the wire 8 will allow the wind turbine blade 5 to more easily pivot towards larger pivot angles, in the manner described above with reference to FIGS. 1-3.

[0113] FIG. 5 is a cross sectional view of part of a hub 3 and part of a nacelle 7. Arms of a blade carrying structure 4 are mounted on the hub 3. The wires 8 which are also illustrated in FIG. 4 are connected to winch mechanisms 10 arranged in the hub 3. Thereby the pulling force applied by means of the wires 8 can be adjusted by rotating the winch mechanisms 10, thereby adjusting the length of the wires 8.

[0114] FIG. 6 illustrates a wind turbine 1 according to an embodiment of the invention with the wind turbine blades 5 arranged at three different pivot angles. The wind turbine 1 could, e.g., be the wind turbine of FIGS. 1-3.

[0115] The left most drawing shows the wind turbine 1 with the wind turbine blades 5 positioned at a minimum pivot angle, and thereby with a maximum rotor diameter.

[0116] The middle drawing shows the wind turbine 1 with the wind turbine blades 5 positioned at a pivot angle which is larger than the pivot angle of the left most drawing. Accordingly, the rotor diameter of the wind turbine 1 of the middle drawing is smaller than the rotor diameter of the wind turbine 1 of the left most drawing. Thereby the tip speed of the wind turbine 1 of the middle drawing is lower than the tip speed of the wind turbine 1 of the left most drawing, leading to a lower noise generation of the wind turbine.

[0117] The right most drawing shows the wind turbine 1 with the wind turbine blades 5 positioned at an even larger pivot angle, resulting in a very small rotor diameter, an even lower tip speed and thereby an even lower noise generation of the wind turbine 1. It can be seen that the wind turbine blades 5 are arranged substantially parallel to a rotational axis of the hub 3. This position is sometimes referred to as ‘barrel mode’.

[0118] FIG. 7 is a schematic view illustrating a wind turbine 1 according to a second embodiment of the invention. The wind turbine 1 of FIG. 7 is very similar to the wind turbine 1 of FIGS. 1-3, and it will therefore not be described in detail here.

[0119] The wind turbine 1 of FIG. 7 is not provided with the wires illustrated in FIGS. 1-3. Instead the wind turbine blades 5 are biased towards a position defining a minimum pivot angle, and thereby a maximum rotor diameter, by means of a hydraulic mechanism 11 connected between the blade carrying structure 4 and the wind turbine blade 5, at a position between the inner tip end 5a of the wind turbine blade 5 and the hinge 6. The hydraulic mechanism 11 applies a biasing force to the wind turbine blades 5 which pulls the wind turbine blades 5 towards the position defining a minimum pivot angle. The applied biasing force can be adjusted by adjusting a pressure of the hydraulic mechanism 11.

[0120] FIG. 8 is a schematic view illustrating a wind turbine 1 according to a third embodiment of the invention. The wind turbine 1 of FIG. 8 is very similar to the wind turbines 1 of FIGS. 1-3 and 7, and it will therefore not be described in detail here.

[0121] In the wind turbine 1 of FIG. 8 the wind turbine blades 5 are connected to the blade carrying structure 4 via a hinge 6 at the inner tip end 5a of the wind turbine blade 5. Furthermore, the wind turbine 1 of FIG. 8 is not provided with biasing means biasing the wind turbine blades 5 towards a position defining a minimum pivot angle, and thereby a maximum rotor diameter. Instead a hydraulic mechanism 12 is connected between the blade carrying structure 4 and the wind turbine blade 5, and the wind turbine blades 5 can be pulled towards a position defining maximum pivot angle, and thereby minimum rotor diameter by means of the hydraulic mechanism 12. Accordingly, the hydraulic mechanism 12 applies a force to the wind turbine blades 5 which causes them to move in this direction.

[0122] In the case that an adjustment of the pivot angle of the wind turbine blades 5 is required, this can be obtained by adjusting the force applied to the wind turbine blades 5. In the wind turbine 1 of FIG. 8 this can be obtained by adjusting the pressure of the hydraulic mechanism 12.

[0123] FIG. 9 is a schematic view illustrating a wind turbine 1 according to a fourth embodiment of the invention. The wind turbine 1 of FIG. 9 is very similar to the wind turbines of FIGS. 1-3, 7 and 8, and it will therefore not be described in detail here.

[0124] Similarly to the wind turbine 1 of FIG. 8, the wind turbine blades 5 of the wind turbine 1 of FIG. 9 are connected to the blade carrying structure 4 via a hinge 6 at the inner tip end 5a of the wind turbine blades 5. However, in the wind turbine 1 of FIG. 9 the force applied to the wind turbine blades 5 causing them to move towards a position defining maximum pivot angle, and thereby minimum rotor diameter, is provided by means of wires 13 connected to winches 14 mounted on the blade carrying structure 4. In the case that an adjustment of the pivot angle of the wind turbine blades 5 is required, this can be obtained by operating the winches 14, thereby adjusting the length of the wires 13 and accordingly the applied pulling force.

[0125] FIG. 10 is a graph illustrating tip speed as a function of wind speed at the level of the hub of a wind turbine. The unmarked lines 15, 16 represent a wind turbine being controlled in accordance with a prior art method, while the lines marked with ‘X’ 17 and ‘X+’ 18 represent a wind turbine being controlled in accordance with a method according to a first embodiment of the invention.

[0126] Line 15 represents the prior art control method during operation without noise constraints, line 16 represents the prior art control method during operation with noise constraints, line 17 represents the control method according to the first embodiment of the invention during operation without noise constraints, and line 18 represents the control method according to the first embodiment of the invention during operation with noise constraints.

[0127] It can be seen that for low wind speeds, the tip speed in the four cases described above is identical. Accordingly, at low wind speeds the wind turbine is operated to obtain the same tip speed, regardless of whether the prior art control method or the method according to the invention is applied, and regardless of whether or not noise constraints are applying.

[0128] For high wind speeds, the wind turbine being controlled in accordance with the prior art method is operated with a significantly lower tip speed when noise constraints are applying, illustrated by line 16, than when there are no noise constraints, illustrated by line 15. As previously described, this is because the noise generated by the wind turbine depends strongly on the tip speed.

[0129] However, the wind turbine being controlled in accordance with a method according to the first embodiment of the invention is operated at almost identical tip speeds when there are no noise constraints, illustrated by line 17, and when noise constraints are applying, illustrated by line 18. It should be noted that in the case the method was applied in order to reduce erosion, the tip speed would be reduced to a greater extent, i.e. line 18 would be arranged at a lower tip speed and the difference between lines 17 and 18 would be larger.

[0130] FIG. 11 is a graph illustrating power production as a function of wind speed. The solid line 19 and the dashed line 20 represent a wind turbine being controlled in accordance with a prior art method, while the lines marked with ‘X’ 21 and ‘X+’ 22 represent a wind turbine being controlled in accordance with a method according to an embodiment of the invention.

[0131] Line 19 represents the prior art control method during operation without noise constraints, line 20 represents the prior art control method during operation with noise constraints, line 21 represents the control method according to the invention during operation without noise constraints, and line 22 represents the control method according to the invention during operation with noise constraints.

[0132] It can be seen that for low wind speeds, the power production in the four cases described above is identical. Accordingly, at low wind speeds the power production of the wind turbine is the same, regardless of whether the prior art control method or the method according to the invention is applied, and regardless of whether or not noise constraints are applying.

[0133] For high wind speeds, the power production of the wind turbine being controlled in accordance with the prior art control method is significantly reduced when noise constraints are applying, illustrated by line 20, as compared to when there are no noise constraints, illustrated by line 19. Thus, in this case a noise reduction is obtained, but the consequence is a reduction in the power produced by the wind turbine. This is because the noise reduction is obtained by reducing the tip speed, and the tip speed can only be reduced by also reducing the rotational speed of the wind turbine, and thereby the power production.

[0134] However, as can be seen from lines 21 and 22, the power production of the wind turbine being controlled in accordance with the method according to the invention is maintained almost at the level of the prior art control method without noise constraints, illustrated by line 19, regardless of whether or not noise constraints are applying. The power production is only reduced slightly in a small region around the nominal wind speed. This is because the tip speed can be reduced, thereby reducing the noise generated by the wind turbine, without reducing the rotational speed, and thereby the power production of the wind turbine, because the rotor diameter is adjustable.

[0135] Thus, operating the wind turbine in accordance with a method according to the invention, it is possible to reduce the noise generated by the wind turbine, essentially without reducing the power production of the wind turbine.

[0136] FIG. 12 is a graph illustrating rotational speed as a function of wind speed. The solid line 23 and the dashed line 24 represent a wind turbine being controlled in accordance with a prior art method, while the lines marked with ‘X’ 25 and ‘X+’ 26 represent a wind turbine being controlled in accordance with a method according to an embodiment of the invention.

[0137] Line 23 represents the prior art control method during operation without noise constraints, line 24 represents the prior art control method during operation with noise constraints, line 25 represents the control method according to the invention during operation without noise constraints, and line 26 represents the control method according to the invention during operation with noise constraints.

[0138] It can be seen that for low wind speeds, the rotational speed in the four cases described above is identical. Accordingly, at low wind speeds the rotational speed of the wind turbine is the same, regardless of whether the prior art control method or the method according to the invention is applied, and regardless of whether or not noise constraints are applying.

[0139] At high wind speeds, rotor speed of the wind turbine being controlled in accordance with the prior art method is significantly reduced when noise constraints are applying, illustrated by line 24, as compared to when there are no noise constraints, illustrated by line 23.

[0140] For the wind turbine being controlled in accordance with a method according to the invention, the rotational speed is also reduced when noise constraints are applying, illustrated by line 26, as compared to when there are no noise constraints, illustrated by line 25. However, the reduction in rotational speed is smaller than the reduction for the wind turbine being controlled in accordance with the prior art method. Furthermore, the rotational speed of the wind turbine being controlled in accordance with a method according to the invention is higher than the rotational speed of the wind turbine being controlled according to the prior art method with no noise constraints, also when noise constraints are applying.

[0141] FIG. 13 is a graph illustrating generated noise levels as a function of rotor diameters for a number of wind turbines with different rotor diameters being controlled according to a prior art method, illustrated by line 27, and for a wind turbine being controlled in accordance with a method according to the invention, illustrated by point 28, where the maximum rotor diameter corresponding to the minimum pivot angle is used. It can be seen that for a given maximum rotor diameter the noise level generated by a wind turbine being operated in accordance with a method according to the invention is lower as compared to the prior art, which is a significant benefit.

[0142] FIGS. 14 and 15 are graphs illustrating tip speed as a function of wind speed at the level of the hub of a wind turbine, similar to FIG. 10. However, FIGS. 14 and 15 illustrate two alternative embodiments of the invention.

[0143] In FIG. 14 the unmarked lines 15, 16 represent a wind turbine being controlled in accordance with a prior art method, as described above with reference to FIG. 10. The lines marked with ‘X’ 29 and ‘X+’ 30 represent a wind turbine being controlled in accordance with a method according to a second embodiment of the invention. Line 29 represents the control method according to the second embodiment of the invention during operation without noise constraints, and line 30 represents the control method according to the second embodiment of the invention during operation with noise constraints.

[0144] It can be seen that for low wind speeds, the tip speed when no noise constraints apply, represented by line 29, and the tip speed when noise constraints apply, represented by line 30, are identical. It can further be seen that the tip speed, in both cases 29, 30, increases as a function of increasing wind speed until a certain wind speed, where a maximum tip speed occurs. The maximum tip speed is lower when noise constraints apply, represented by line 30, than when no noise constraints apply, represented by line 29. At higher wind speeds, the tip speed is reduced. In particular, the tip speed at higher wind speeds is significantly lower than the tip speeds in the prior art scenario, represented by lines 15 and 16.

[0145] Thus, in the embodiment illustrated in FIG. 14, the tip speed, and thereby the noise level, of the wind turbine is significantly lower than the tip speed, and thereby the noise level, of the prior art wind turbine, represented by lines 15 and 16. Furthermore, the amount of time where the wind turbine is operated at the maximum tip speed is minimised. This results in reduced leading edge erosion of the wind turbine blades.

[0146] In FIG. 15 the unmarked lines 15, 16 also represent a wind turbine being controlled in accordance with a prior art method, as described above with reference to FIG. 10. The lines marked with ‘X’ 31 and ‘X+’ 32 represent a wind turbine being controlled in accordance with a method according to a third embodiment of the invention. Line 31 represents the control method according to the third embodiment of the invention during operation without noise constraints, and line 32 represents the control method according to the third embodiment of the invention during operation with noise constraints.

[0147] Similarly to the second embodiment illustrated in FIG. 14, the tip speed when no noise constraints apply, represented by line 31, and the tip speed when noise constraints apply, represented by line 32, are identical at low wind speeds, and the tip speed, in both cases 31, 32, increases as a function of increasing wind speed until a certain wind speed, where a maximum tip speed occurs.

[0148] At higher wind speeds, the tip speed is reduced in such a manner that the tip speed is reduced more when noise constraints apply, represented by line 32, than when no noise constraints apply, represented by line 31.