FIELD OF THE INVENTION

The invention relates to sensors for detecting turbulence on wind turbine blades.

BACKGROUND OF THE INVENTION

In order to optimise the aerodynamic energy efficiency of wind turbines the angle of attack of the blades can be adjusted to maximise the aerodynamic lift forces acting on the blades. As the angle of attack is increased the risk for blade stall also increases. When the blade stalls due to a change of laminar air flow into turbulent air flow over the blade the aerodynamic energy efficiency decreases significantly. The stall may also induce high loads and vibrations on the blades which may shorten the lifetime of the blades. Accordingly, there is a need for avoiding that blades of a wind turbine stalls.

US 8,152,440 discloses pressure sensors to be applied in predetermined patterns to airfoil structures, such as wind turbine blades without impacting the blade structure and fluid dynamic characteristics. The pressure sensors measure blade performance with high fidelity. The pressure measurements are transmitted to processing to determine blade characteristics and environment including flow separation, stagnation point, angle of attack, lift and drag and wind speed.

SUMMARY OF THE INVENTION

It is an object of the invention to improve wind turbines in relation to avoiding stall of the wind turbine blades.

It is a further object of the invention to provide a sensor capable of detecting conditions for turbulence on wind turbine blades.

In a first aspect of the invention there is provided a turbulence sensor configured to be connected to a wind turbine blade, the sensor comprises

- an aerodynamic element configured to promote turbulence of air flowing over an upper surface of the blade at a predetermined location of the element, - a detector configured to detect conditions for air turbulence, wherein the detector is located at the predetermined location.

Advantageously, the aerodynamic element is configured to promote turbulence so that turbulence is induced at the location of the element before other locations of the wind turbine blade. Accordingly, based on a sensor output from the turbulence sensor, actions may be taken to prevent that turbulence develops over larger parts of the wind turbine blade. For example, based on a detected condition for air turbulence the pitch of the blade may be adjusted until the sensor no longer shows conditions for air turbulence.

The aerodynamic element may be configured to promote turbulence by means of its location at or near the trailing edge of the wind turbine blade, by means of a shaping of the aerodynamic element to provoke creation of turbulence.

Accordingly, the turbulence sensor could be configured so that it comprises

- an aerodynamic element configured to be connected to a trailing edge of the blade so that the element forms an extension of the blade in a chordwise direction so as to promote turbulence of air flowing over an upper surface of the blade at a predetermined location of the element,

- a detector configured to detect conditions for air turbulence, wherein the detector is located at the predetermined location.

Alternatively or additionally, the turbulence sensor could be configured to be connected to the upper surface of the blade so that an orientation direction of the aerodynamic element is parallel with a chord direction of the blade, where the sensor comprises

- an aerodynamic element configured to promote turbulence of air flowing over an upper surface of the blade at a predetermined location of the element, wherein the aerodynamic element comprises a first section wherein a height of the aerodynamic element gradually increases along the orientation direction, and a second section wherein the height of the aerodynamic element gradually decreases along the orientation direction, wherein the second section comprises the predetermined location, and - a detector configured to detect conditions for air turbulence, wherein the detector is located at the predetermined location.

Thus, according to an embodiment the aerodynamic element is configured to be connected to a trailing edge of the blade so that the element forms an extension of the blade in a chordwise direction.

Since turbulence has a tendency to start at blade locations located farthest downstream of the blade, turbulence is more likely to start at the extension than elsewhere on the blade. Accordingly, a detector located at the extension may detect turbulence before turbulence starts elsewhere on the blade.

According to an embodiment the predetermined location is located on a part of the aerodynamic element which forms the extension.

According to another embodiment, the sensor is configured to be connected to the upper surface of the blade so that an orientation direction of the aerodynamic element is parallel with a chord direction of the blade, and wherein the

aerodynamic element comprises

- a first section wherein a height of the aerodynamic element gradually increases along the orientation direction, and

- a second section wherein the height of the aerodynamic element gradually decreases along the orientation direction, wherein the second section comprises the predetermined location.

Advantageously, by means of the configuration the height of the aerodynamic element modifies the thickness of the wind turbine blade by an initial increasing thickness and a subsequent decreasing thickness. The subsequent decreasing thickness may improve conditions for creating turbulence so that the sensor may be able to detect turbulence before turbulence starts elsewhere on the blade.

In an embodiment the aerodynamic element further comprises

- a third section wherein the height of the aerodynamic element gradually decreases along the orientation direction, wherein an average slope of the decrease of the height in the third section is larger than an average slope of the decrease of the height in the second section, and wherein either the second or the third section comprises the predetermined location and the detector.

Advantageously, the different average slopes of the sections with decreasing heights may enable different degrees of how much creation of turbulence is promoted. Thus, the third section may be provided with a turbulence detector to enable early detection of turbulence. Alternatively, both the second and third sections may be provided with a turbulence detector to enable detection of how or how fast turbulence propagates. Thus, according to an embodiment, the second section comprises a first predetermined location and the third section comprises a second predetermined location, wherein first and second detectors are located at the first and second predetermined locations. According to an embodiment the sensor is configured to be connected to a trailing edge of the blade so that the element forms an extension of the blade in a chordwise direction. It is noted that the upper surface comprises the trailing edge so that mounting the sensor on or near the trailing edge for creating a local blade extension may be seen as a special case of mounting the sensor on the upper surface.

According to an embodiment the detector is configured to detect pressure variations. A second aspect of the invention relates to a wind turbine blade comprising at least one turbulence sensor according to the first aspect.

A third aspect of the invention relates to a wind turbine comprising at least one wind turbine blade according to the second aspect.

A fourth aspect of the invention relates to a method for manufacturing a wind turbine blade, where the method comprises

- providing a wind turbine blade,

- attaching at least one turbulence sensor according to the first aspect to the turbine blade. In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which Fig. 1 shows a wind turbine,

Figs. 2A-B illustrates progression of turbulence on a blade,

Figs. 3A-D illustrates different types of turbulence sensors, and

Figs. 4A-D illustrates different shapes of the aerodynamic element of a turbulence sensor.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a wind turbine 100 (WTG) comprising a tower 101 and a rotor 102 with at least one rotor blade 103, such as three blades. The rotor is connected to a nacelle 104 which is mounted on top of the tower 101 and being adapted to drive a generator situated inside the nacelle. The rotor 102 is rotatable by action of the wind. The wind induced rotational energy of the rotor blades 103 is transferred via a shaft to the generator. Thus, the wind turbine 100 is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator.

The blades 103 can be pitched in order to alter the aerodynamic properties of the blades, e.g. in order to maximise uptake of the wind energy. The pitch is normally adjusted so that the airstream incident on the blades flows smoothly over the blade surfaces. In order to maximise uptake of the wind energy, i.e. by

maximising the lift forces on the blades, it may be desired to adjust the pitch to relative small angles, i.e. a relative large angle of attack. However, at a certain angle of attack the blade may enter into stall wherein the airflow separates from the surface of the blade. Downstream of the airflow separation where the airflow is no longer attached to the blade surface, but is separated, the airflow is turbulent. At the areas with airflow separation the lift is reduced and, therefore, the total aerodynamic efficiency reducing as the blades enter into stall. Fig. 2A shows a top view of a wind turbine blade 200 comprising a leading edge 201, a trailing edge 202, a root section 203 and a blade tip 204. The incident wind impinges along the leading edge 201. Normally, the stall of the blade, i.e. the airflow separation, develops from the trailing edge 202 toward the leading edge 201 and from the root section 203 towards the blade tip 204. Fig. 2 illustrates a stall situation wherein a first part of the blade 211 is in a stall condition and a second part 212 wherein the airflow is still attached (laminar airflow). The first and second parts 211, 212 are separated by a stall front line 213.

By leading edge is meant the edge where incident air impinges the moving blade and by trailing edge is meant the edge of the blade where a laminar flow of air leaves the blade.

In Fig. 2A line 281 illustrates a chord line and direction 281 indicates the direction of the incident wind.

Fig. 2B shows sectional view A-A of the blade and airflow at cutting line A-A of Fig. 2A. The cross sectional view shows that the attached airflow separates from the upper surface 251 (the suction side) at front line location 214 and that the airflow becomes turbulent after the front line location 214. The airflow along the lower surface 252 (pressure side) may remain laminar.

Fig. 3A shows a wind turbine blade 300 which is similar to the blade 200 in Fig. 2A. The blade 300 comprises one or more turbulence sensors 311, 312 which have been attached to the upper surface 251 of an existing blade e.g. by use of an adhesive. The turbulence sensors 311, 312 comprises an aerodynamic element configured to promote turbulence of air flowing over the blade at a predetermined location of the element. Accordingly, the aerodynamic element is configured to induce local separation of the airflow so that turbulence is induced at the predetermined location of the element before other locations of the blade. The aerodynamic element is provided with a detector configured to detect conditions for air turbulence such as pressure variations. The detector is preferably located at the predetermined location.

The blade 300 is illustratively shown with different types of sensors 311, 312. In general a blade may be provided with one or more sensors 311, 312 of a single type, or a plurality of sensors of different types.

In general, the aerodynamic element of the sensor types 311, 312 are configured, e.g. by means of shaping and/or location on the blade, to modify the aerodynamic properties of the blade 300 locally, i.e. at the location of the aerodynamic element, so as to promote turbulence or separation of the air flowing over the element at a predetermined location of the element.

Fig. 3B shows cross sectional view B-B of the blade 300 and sensor 312 at cutting line B-B shown in Fig. 3A. The aerodynamic element 321 of the sensor 312 is configured to promote turbulence of air flowing over the blade by means of its configuration enabling it to be connected to a trailing edge of the blade so that the element forms an extension of the blade in a chordwise direction, i.e. in a direction of the chord 281. Due to the extension of the element beyond the trailing edge 202, turbulence may start over an upper surface of the extended portion before turbulence starts elsewhere on the upper surface 251 of the blade.

The predetermined location of the element where turbulence is expected to start is located at the part which extend beyond the trailing edge 202. Accordingly, the detector 322 is located at the predetermined location, i.e. at a location at the extension to the trailing edge; in other words is located on a part of the

aerodynamic element which forms the extension.

As shown, the aerodynamic element 321 may be plate-shaped. For example, the aerodynamic element 321 may be configured as a shell which extends the curvature of the blade 300 beyond the trailing edge 202.

The detector may be embedded in the aerodynamic element 321, e.g. in a hole of element so that the detector is able to measure the conditions for air turbulence, e.g. pressure variations. Preferably, the detector is embedded so that the surface of the detector levels the surface of the aerodynamic element. For example, the sensor may be an optical sensor, e.g. the optical vibration sensor described in WO2011/110179, capable of measuring pressure variations and, thereby, conditions for turbulence in the form of oscillating pressures. Other useable detectors include traditional microphones, mechanical sensor such as a simple flap, near-field LIDAR devices or similar laser based devices.

Fig. 3C shows cross sectional view C-C of the blade 300 and sensor 311 at cutting line C-C shown in Fig. 3A. The aerodynamic element 331 of the sensor 311 is configured to promote turbulence of air flowing over the blade by means of its shape. Due to the shape of the element, turbulence will start over an upper surface of the element 331 before turbulence starts elsewhere on the upper surface 251 of the blade. Fig. 3D shows a sensor 313 with an alternative configuration of an aerodynamic element 341 which is configured both to form an extension of the blade similar to the aerodynamic element 321 and by its shape to promote turbulence at a predetermined location of the element similar to the aerodynamic element 331. The sensor 313 is provide with two detectors 322. In general a single sensor 311- 313 may be provided with one or more detectors 322 located at the

predetermined location, i.e. in an area of the aerodynamic element 321-323 where turbulence is expected to be promoted most rapidly. Figs. 4A-C show different shapes (in cross sectional views) of the aerodynamic elements 331,341 of the sensors 311,313 in Figs. 3C-D, i.e. shapes which provoke early separation of the air stream at the predetermined location of the

aerodynamic elements compared to other locations of the blade 300 which is not provided with sensors.

Fig. 4D shows a top-view of the sensors 311,313 illustrated in Figs. 4A-C. Fig. 4D illustrates the sensor which is connected to the upper surface 251 of the blade so that an orientation direction 491 of the aerodynamic element 331,341 or sensor 311,313 is parallel or substantial parallel with the chord direction 281 of the blade. In other words the sensor is configured to be connected to the blade 300 so that the orientation direction 491 is parallel or substantially parallel with the cord direction 281 or flow direction of air streaming over the blade when the blade is in operation on a wind turbine 100.

5 Fig. 4A shows an example of a possible shape of the aerodynamic element 331, 341 which comprises a first section 411 wherein a height 421 gradually increases along the orientation direction 491 and a second section 412 wherein the height 421 gradually decreases along the orientation direction. Due to the change from a positive slope of the first section 411 to a negative slope of the second section 10 412, the second section may provoke turbulence and, therefore, comprises the predetermined location. The sensor 311, 313 is configured to be mounted on the wind turbine blade 300 so that the second section 412 is located downstream of the first section 411 relative to air flowing in the chordwise direction 281.

15 The height 421 is a height measured from a bottom surface of the aerodynamic element, i.e. the bottom surface which is connected to the upper surface 251 of the blade, to an upper surface of the aerodynamic element, i.e. the surface which receives the stream of air. Accordingly, the height 421 defines a thickness of the aerodynamic element, e.g. the cross-sectional thickness shown in Fig. 4A.

20

Fig. 4B shows another example of a possible shape of the aerodynamic element 331, 341 which further comprises a third section 413 wherein the height 421 of the aerodynamic element gradually decreases along the orientation direction 491, and wherein the average slope of the decrease of the height 421 in the third

25 section 413 is larger than the average slope of the decrease of the height 421 in the second section 412. The third section 413 is located downstream of the second section 412 relative to air flowing in the chordwise direction. The third section 413 may generate turbulence before the second section or vice versa. Accordingly, either the second or the third section comprises the predetermined

30 location where turbulence is expected to start and the detector 322.

Alternatively, as shown in Fig. 4B both the second and third sections 412, 413 may be provided with a detector 322. Advantageously, one of the detectors may detect turbulence before the other so as to obtain a gradual and possibly more 35 robust detection of turbulence. Accordingly, the second and third sections 412, 413 may comprise respective first and second predetermined locations arranged for promoting air separation, and first and second detectors 422 may be located at the first and second predetermined locations. The first, second and third sections 411, 412, 413 may have a linear shape, i.e. a constant or substantially constant slope, within each of the sections. Alternatively, as shown in Fig. 4C a sensor 311, 313 may be configured with a second section

414 having a curved shape, i.e. a non-constant slope along the orientation direction 491, and one or more detectors 322 located on the second section.

Alternatively or additionally, also the first section 411 may have a curved shaped.

Any of the sensors described in Figs. 4A-C may be configured for mounting on an upper surface of the blade either on a surface located between the leading and trailing edges 201, 202, alternatively at the trailing edge or on on a surface near the trailing edge so that the element, or part of the element, forms an extension of the blade in a chordwise direction. Accordingly, the sensors in Figs. 4A-C may be configured as sensors 311, 313 shown in Figs. 3C-D.

The output signal from any of the sensors 311-313, e.g. a signal indicating presence or non-presence of turbulence may be used in a pitch control system comprised by the wind turbine 100. When one or more output signals from corresponding one or more sensors 311-313 indicate presence of turbulence, the pitch control system may adjust the pitch angle of the blades until one or more output signals previously indicating turbulence starts indicating non-presence of turbulence. For example, the pitch control system may be configured to ensure that a predetermined maximal number of sensors 311-313 (mounted on a blade) indicating presence of turbulence is not exceeded.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.