WO2023138823A1

WO2023138823A1 - Control system for maintaining stall margin of a wind turbine blade with an active aerodynamic device

Info

Publication number: WO2023138823A1
Application number: PCT/EP2022/084065
Authority: WO
Inventors: Dillon Volk; Alejandro Gomez Gonzalez; Scott Johnson
Original assignee: Siemens Gamesa Renewable Energy A/S
Priority date: 2022-01-18
Filing date: 2022-12-01
Publication date: 2023-07-27

Abstract

It is described a control system (900) for maintaining an attached airflow (171) flowing from a leading edge (211) of a rotor blade (120, 520, 920) of a wind turbine (100) to a trailing edge (212) of the rotor blade (120, 520, 920). The control system (900) comprises a detection device (960) configured for detecting a current angle of attack of the rotor blade (120, 520, 920) at a predefined longitudinal section (123) of the rotor blade (120, 520, 920), a determining device (950) configured for determining a stall margin which is a difference of the current angle of attack and an expected stall angle at the predefined longitudinal section (123), wherein the determining device (950) is further configured for determining a balanced state of the air-flow (171), wherein in the balanced state the stall margin is equal to a predefined stall margin, and an aerodynamic device (230, 530, 930, 940) arranged on a surface (224, 225) of the rotor blade (120, 520, 920) at a chordal position (231, 531) between the leading edge (211) and the trailing edge (212), wherein the aerodynamic device (230, 530, 930, 940) is movable between a retracted configuration and an extended configuration, wherein the aerodynamic device (230, 530, 930, 940) is configured for manipulating the stall margin by moving between the retracted configuration and the ex- tended configuration until the balanced state is met. Furthermore, a wind turbine (100) is described comprising an above-described control system (900) and a method (100) for maintaining an attached airflow (171) are described.

Description

DESCRIPTION

Control system for maintaining stall margin of a wind turbine blade with an active aerodynamic device

Field of invention

The present invention relates to the field of stall induced vibrations of a wind turbine blade undergoing an airflow flowing from a leading edge of a rotor blade of a wind tur- bine to a trailing edge of the rotor blade. Particularly, the present invention relates to a control system for maintaining an attached airflow flowing from a leading edge of a rotor blade of a wind turbine to a trailing edge of the rotor blade, a wind turbine comprising the control system, and a method for maintaining an attached airflow flowing from a leading edge of a rotor blade of a wind turbine to a trailing edge of the rotor blade.

Art Background

In the past stall induced vibrations have been a problem at positive stall in stall induced wind turbines, but the intro- duction of the pitch bearing has mitigated this. However, in today's wind turbines comprising longer blade lengths, it is more likely that the outer portion of the blade will face negative stall angles of attack during operation when the pitch bearing is used for speed control.

Alternatively, an aerofoil with a different stall behaviour could be considered to avoid stall induced vibrations. Howev- er, using such aerofoils may sometimes come up with a non- optimal aerodynamic efficiency.

EP 2253 835 Al discloses a blade for a rotor of wind turbine having a profiled contour generating a lift when being im- pacted by an incident airflow. In the radial direction, the profiled contour is divided into a root region with a sub- stantially circular or elliptic profile closest to the hub, an aerofoil region with a lift generating profile further away from the hub, and a transition region between the root region and the aerofoil region. The aerofoil region comprises a first longitudinal segment provided with a number of first flow altering devices arranged so as to adjust the aerodynam- ic properties of the first longitudinal segment to substan- tially meet the target axial induction factor at the design point.

EP 2253 835 Al discloses a wind turbine blade with a plural- ity of longitudinally extending flow guiding device parts at- tached to a profiled contour on a pressure side of the blade. The longitudinally extending flow guiding device parts are grouped together to form a first flow guiding device group in a transition region of the blade. The flow guiding device is permanently attached to the surface of the wind turbine blade, cannot be actively controlled, and is utilised for in- creasing the lift and the energy yield.

EP 2 713 044 Al discloses a wind turbine rotor blade compris- ing a root portion and an aerofoil portion. Further the wind turbine rotor blade comprises an airflow correction arrange- ment arranged on a pressure side of the blade over at least a portion of a thickened zone, which airflow correction ar- rangement comprises a spoiler arranged at a trailing edge and realised to increase blade lift as well as a vortex generator arranged between a leading edge and the trailing edge and re- alised to maintain an attached airflow between the vortex generator and the spoiler.

EP 2 778392 Al discloses a rotor blade for a wind turbine which includes a body extending between a blade root and a blade tip. The body defines a pressure side and a suction side extending between a leading edge and a trailing edge. In addition, the body defines a chord line extending between the leading and the trailing edge. The rotor blade also includes an aerofoil modifier coupled to at least one of the pressure side or the suction side of the body. The aerofoil modifier defines an end surface disposed adjacent to the trailing edge. At least a portion of the end surface extends at a non- perpendicular angle relative to the chord.

WO 2012/146252 Al discloses a method of operating a wind tur- bine comprising at least one trailing edge control surface on at least one rotor blade, comprising operating the wind tur- bine in a first mode, in which a rotor blade angle of attack and a trailing edge control surface deflection are set ac- cording to one or more wind turbine control parameters, and selectively operating the wind turbine in a second, noise re- duced, mode, in which for a given set of wind turbine control parameters, the trailing edge control surface deflection is increased towards the pressure side and the rotor blade angle of attack is decreased with respect to the first mode.

WO 2010/066500 Al discloses a wind turbine blade with a flow guiding device attached to a profiled contour on a pressure side of the blade. The flow guiding device extends along at least a longitudinal part of a transition region of the blade and is arranged so as to generate a separation of airflow along at least a central longitudinal portion of the flow guiding device from the pressure side of the blade at a point between the flow guiding device and a trailing edge of the blade, when the blade is impacted by an incident airflow. The flow guiding device is arranged at a relative choral posi- tion, seen from the leading edge of the blade, lying in an interval between 40% and 92%. The height of the flow guiding device to the profiled contour is at least 10% of a maximum thickness of the profiled contour for each transvers cross section.

Stall induced vibrations have been shown to be a problem for wind turbines. This problem was first investigated for stall regulated wind turbines, where the maximum operating RPM (ro- tations per minute) has been regulated by stalling the out- board section of the blades. However, in modern pitch regu- lated wind turbines the outer blade span can be subjected to negative stall conditions at high wind speed, high pitch an- gle combinations.

Therefore, avoiding stall induced vibrations is crucial from the economic perspective of the wind turbine. Further, an in- crease aerodynamic efficiency is a key factor for the opera- tional costs of the wind turbine.

Summary of the Invention

It may be an objective of the present invention to provide a control system for maintaining an attached airflow from a leading edge of a rotor blade of a wind turbine to a trailing edge of the rotor blade, wherein the control system is able to increase aerodynamic efficiency and to reduce operational costs of a wind turbine.

This objective may be solved by the control system for main- taining an attached airflow flowing from a leading edge of a rotor blade of a wind turbine to a trailing edge of the rotor blade, a wind turbine comprising such a control system, and a method for maintaining an attached airflow from a leading edge of a rotor blade of a wind turbine to a trailing edge according to the independent claims.

According to a first aspect of the present invention, a con- trol system for maintaining an attached airflow flowing from a leading edge of a rotor blade of a wind turbine to a trail- ing edge of the rotor blade is disclosed. The control system comprises (i) a detection device configured for detecting a current angle of attack of the rotor blade at a predefined longitudinal section of the rotor blade, wherein a longitudi- nal direction of the rotor blade extends from a hub region of the rotor blade to a tip region of the rotor blade, (ii) a determining device configured for determining a stall margin which is a difference of the current angle of attack and an expected stall angle at the predefined longitudinal section, wherein the determining device is further configured for de- termining a balanced state of the airflow, wherein in the balanced state the stall margin is equal to a predefined stall margin, and (iii) an aerodynamic device arranged on a surface of the rotor blade at a chordal position between the leading edge and the trailing edge, wherein the aerodynamic device is movable between a retracted configuration having a minimum airflow resistance and an extended configuration hav- ing a maximum airflow resistance, wherein the aerodynamic de- vice is further configured for manipulating the stall margin by moving between the retracted configuration and the extend- ed configuration until the balanced state is met.

The control system according to the present invention is con- figured for delaying a stall behaviour of a rotor blade by using an aerodynamic device mounted to a surface of the rotor blade. Thereby, potential negative stall induced instabili- ties may be avoided and an operation in a more favourable at- tached flow region may be maintained.

Hence, the control system may be based on the idea that a high pitch angle stability behaviour may be directly influ- enced by the stall behaviour of one or more aerodynamic de- vices (aerofoils) positioned outboard, namely at a tip region of the rotor blade.

The control system according to the present invention is based on the idea that moving an aerodynamic device between a retracted configuration having a minimum airflow resistance (drag), and an extended configuration having a maximum air- flow resistance (drag), such that an expected stall angle of a predefined longitudinal section of a rotor blade positioned at an outboard end respectively near the tip region of the rotor blade, may be delayed to a different stall angle value. Thereby, the wind turbine may be operated at a higher RPM (rotations per minute) for a given pith angle. The control system according to the present invention may avoid stall induced vibrations. Hence, the economic viability of the wind turbine may be increased through avoidance of the structural loads induced from the avoided stall induced vi- brations .

The term expected stall angle may particularly denote an an- gle at which stall of the airflow around the rotor blade caused by an incoming wind field occurs. Hence, the angle at which eddies are expected to occur in the airflow, causing a delamination of the airflow around the rotor blade.

The expected stall angle may be predefined and dependent on an operational mode of the wind turbine, such as for example idle or power production mode. Additionally, the expected stall angle may be dependent on environmental constraints, such as for example a low wind period, a high wind period, or a gust of wind.

According to the present invention, the current angle of at- tack and the expected stall angle both denote an angle be- tween an extension of the chord of the rotor blade and the predominant wind direction of an incoming local wind field. Particularly, the local wind field hitting the rotor blade at the leading edge.

The term predefined longitudinal section may particularly de- note a critical spanwise section seen in the longitudinal di- rection of the rotor blade.

The term balanced state of the airflow may particularly de- note a state in which the stall margin which is defined to be a difference of the current angle of attack and the expected stall angle, is equal to the predefined stall margin.

The predefined stall margin may be defined dependent on at least one environmental condition and/or dependent on at least one individual wind turbine operational condition. The predefined stall margin is between 5° and 1°, in particular between 3° and 1°, further in particular 1°. Thereby, a stall induced instabilities may be reliably avoided.

The term aerodynamic device may denote a separate device mounted to the rotor blade and/or a part of the rotor blade configured for changing its outer geometrical shape. Both the separate part and the part of the rotor blade may influence the airflow from the leading edge to the trailing edge. The extended configuration having a maximum airflow resistance may particularly denote the state in which the aerodynamic device is deflected from the outer surface of the rotor blade. The retracted configuration having a minimum airflow resistance may particularly denote the state in which the aerodynamic device is not deflected and therefore the surface of the rotor blade may have its original outer shape.

On the one hand, the aerodynamic device may be configured to change between the retraced configuration and the extended configuration such that the aerodynamic device is either in the retracted configuration or in the extended configuration. On the other hand, the aerodynamic device may be configured to gradually move, in other words deflect, between the re- tracted configuration and the extended configuration such that the aerodynamic device may deflect to an individually adapted certain extent.

If the aerodynamic device is applied to the suction side of the rotor blade, a maximum angle of attack of the rotor blade may be delayed. When the aerodynamic device is in the extend- ed configuration, the rotor blade may see a negative AC_L in- crement but may still reach the same C_Lmax at a new delayed a_max. Additionally, the drag at the rotor blade is also in- creased. C_L may denote the lift coefficient of the rotor blade and a may denote the angle of attack at the leading edge of the rotor blade. Similarly, if the aerodynamic device is applied to the pres- sure side of the rotor blade, a minimum angle of attack of the rotor blade may be delayed. The aerodynamic device may be applied to the operating envelope, in other words may be in the extended configuration, when approaching negative stall, to avoid stall induced vibrations.

According to a second aspect of the present invention, a wind turbine is disclosed. The wind turbine comprises a tower, a rotor mounted to the tower, wherein the rotor comprises at least one rotor blade, and the above-described control sys- tem, wherein the control system is connected to the at least one rotor blade.

Hence, the wind turbine may be based on the idea that moving an aerodynamic device between a retracted configuration hav- ing a minimum airflow resistance, and an extended configura- tion having a maximum airflow resistance, such that an ex- pected stall angle of a predefined longitudinal section of a rotor blade positioned at an outboard end respectively near the tip region of the rotor blade, may be delayed to a dif- ferent stall angle. Thereby, the wind turbine may be operated at a higher RPM (rotations per minute) for a given pith an- gle.

The wind turbine according to the present invention may avoid stall induced vibrations. Hence, the economic viability of the wind turbine may be increased through avoidance of the structural loads induced from the avoided stall induced vi- brations .

Wind turbine according to the present invention may particu- larly denote a floating wind turbine.

The control system is connected to the at least one rotor blade may denote that the control system is on the one hand mounted to at least one component of the wind turbine other than the rotor blade, e.g., the tower, the hub and/or the na- celle, and is connected to the rotor blade. On the other hand, the control system may be entirely mounted to the rotor blade.

According to a third aspect of the present invention, a meth- od for maintaining an attached airflow flowing from a leading edge of a rotor blade of a wind turbine to a trailing edge of the rotor blade is disclosed. The method comprises (i) de- tecting a current angle of attack of the rotor blade at a predefined longitudinal section of the rotor blade, wherein a longitudinal direction of the rotor blade extends from a hub region of the rotor blade to a tip region of the rotor blade, (ii) determining a stall margin being a difference of the current angle of attack and a predefined stall angle at the predefined longitudinal section, (iii) determining a balanced state of the airflow, wherein in the balanced state the stall margin is equal to a predefined stall margin, and (iv) moving between a retracted configuration having a minimum airflow resistance, and an extended configuration having a maximum airflow resistance, an aerodynamic device arranged on a sur- face of the rotor blade at a chordal position between the leading edge and the trailing edge until the balanced state is met.

Hence, the method for maintaining an attached airflow flowing from a leading edge to a trailing edge of a rotor blade of a wind turbine may be based on the idea that moving an aerody- namic device between a retracted configuration and an extend- ed configuration such that an expected stall angle of a pre- defined longitudinal section of a rotor blade positioned at an outboard end respectively near the tip region of the rotor blade, may be delayed to a different stall angle value. Thereby, a wind turbine may be operated at a higher RPM (ro- tations per minute) for a given pith angle.

The method for maintaining the attached airflow according to the present invention may avoid stall induced vibrations. Hence, the economic viability of a wind turbine may be in- creased through avoidance of the structural loads induced from the avoided stall induced vibrations.

According to an exemplary embodiment of the present inven- tion, the determining device is further configured for deter- mining if the stall margin is larger than the predefined stall margin, wherein, if the stall margin is larger than the predefined stall margin, the determining device is further configured for determining whether the aerodynamic device is in the extended configuration or in the retracted configura- tion, and wherein if the aerodynamic device is in the extend- ed configuration, the aerodynamic device is configured for moving from the extended configuration to the retracted con- figuration until the balanced state is met.

By moving from the extended configuration to the retracted configuration may denote that the aerodynamic device is deac- tivated. Thereby, the airflow resistance of the aerodynamic device is decreased. Hence, the stall margin may be influ- enced to decrease until the stall margin equals the prede- fined stall margin.

Determining if the stall margin is larger than the predefined stall margin may particularly denote that the determined stall margin is compared to a stored value of the predefined stall margin. The determining device may compare the stored value of the predefined stall margin with the determined val- ue of the stall margin.

Additionally, if the stall margin is larger than the prede- fined stall margin, and the aerodynamic device is in the re- tracted configuration, the aerodynamic device may be further configured for maintaining its present configuration, namely maintaining in the retracted configuration.

The term configured for moving from the extended configura- tion to the retracted configuration may particularly denote that the aerodynamic device may receive a control signal from a controller, which indicates that the aerodynamic device starts to move from the extended configuration to the re- tracted configuration. If the determining device determines that the balanced state is met or if a fully retracted con- figuration is reached, the aerodynamic device stops moving. Thereby, stall induced vibrations may be easily avoided.

Particularly, a reliable control system may be provided which is able to react on wind direction changes of the incoming local wind field. Particularly, if the aerodynamic device should not need to be in the extended configuration due to environmental influences, such as for example the incoming local wind field, the aerodynamic device stays in the re- tracted configuration or is moved from the extended configu- ration into the retracted configuration. Hence, a more at- tached flow from the leading edge to the trailing edge may be ensured and at the same time a power consumption may be de- creased. Hence, a wind turbine operation may be ensured.

According to a further exemplary embodiment of the present invention, the determining device is further configured for determining if the stall margin is smaller than the prede- fined stall margin, wherein, if the stall margin is smaller than the predefined stall margin, the aerodynamic device is configured for moving from the retracted configuration to the extended configuration until the balanced state is met.

The term configured for moving from the retracted configura- tion to the extended configuration may particularly denote that the aerodynamic device may receive a control signal from a controller, which indicates that the aerodynamic device starts to move from the retracted configuration to the ex- tended configuration. If the determining device determines that the balanced state is met or if a fully extended config- uration having a maximum airflow resistance, is reached, the aerodynamic device stops moving. Thereby, the control system may be provided having a reliable and fast reaction such that stall induced flat- ter/instabilities may be inhibited. Hence, the airflow from the leading edge to the trailing edge may be maintained at- tached. Therefore, the wind turbine may be provided having an increased economic viability.

According to a further exemplary embodiment of the present invention, the aerodynamic device comprises a spoiler pivot- ably mounted on the surface of the rotor blade.

Thereby, a mechanically simple and reliable aerodynamic de- vice may be provided.

Pivotably mounted to the surface according to the present in- vention may denote that the spoiler is attached to the rotor blade on one edge and may be movable between the retracted configuration and the extended configuration. In other words, in the retracted configuration, the spoiler may abut against the surface of the rotor blade and form one common flat plane without protrusions.

In particular, the spoiler may be a micro spoiler or a macro spoiler. The terms micro respectively macro is used to define the size of the spoiler in relation to the size of the rotor blade.

On the one hand, an influence of the micro spoiler may advan- tageously be restricted to a small surface area. Also due to its small size, a micro spoiler may be mechanically robust and less error prone.

A macro spoiler may advantageously influence the airflow over a large longitudinal extension of the rotor blade. Thereby, the macro spoiler may advantageously maintain an attached airflow over a large longitudinal extension of the rotor blade. Hence, the macro spoiler may cover the longitudinal extension of more than one predefined longitudinal section. Additionally, a macro spoiler may be easily manufactured and therefore cost efficient.

According to a further exemplary embodiment of the present invention, the aerodynamic device comprises a flap mounted to the trailing edge, wherein the flap is extendable.

The flap being extendable may particularly denote, that the flap mounted to the trailing edge may increase a total sur- face of the rotor blade. Hence, a chordal length between the leading edge and the trailing edge may be increased and therefore, the current stall margin may be increased. There- fore, stall induced instabilities may be avoided.

According to a further exemplary embodiment of the present invention, the aerodynamic device may be a trailing edge flap and/or an aileron. In particular, the trailing edge flap and/or the aileron would seek to alter the minimum angle of attack.

According to a further exemplary embodiment of the present invention, the aerodynamic device comprises a deformable sur- face area, in particular, a mechanically deformable surface area or a pneumatically deformable surface area.

A deformable surface area according to the present invention may denote an area of the surface of the rotor blade coming in contact with the air flow of an incoming wind field. This area may be built such that its shape may be adapted.

In the retracted configuration, the deformable surface area may correspond to the outer shape of the operating envelope of the rotor blade over the entire surface.

In the extended configuration, the deformable surface area may protrude over the operating envelope of the rotor blade. Hence, a smooth transition between the outer surface of the rotor blade and the deformable surface area may be provida- ble. Additionally, as all of the movable activating compo- nents of the deformable surface are positioned inside the ro- tor blade, a reliability of the aerodynamic device and there- fore of the rotor blade is increased.

A mechanically deformable surface area may advantageously comprise an easy build-up. Hence, the mechanically deformable surface area may be reliable. A pneumatically deformable sur- face area may be quickly adapted to changes in the incoming wind field. Hence, the pneumatically deformable surface area may provide a short response time.

A deformation of the deformable surface area may be electri- cally actuated. Further, the deformation may be initiated by detecting a piezoelectric signal on the rotor blade, particu- larly on the deformable surface area. Additionally, during deformation, the deformation of the deformable surface may be monitored by a piezoelectric device providing piezoelectric signals dependent on a performed deformation.

According to a further exemplary embodiment of the present invention, the aerodynamic device is arranged at a pressure side of the rotor blade.

Thereby, vibrations induced by negative stall may be avoided.

By arranging the aerodynamic device at the pressure side of the rotor blade, the current angle of attack may be deter- mined for the predefined longitudinal section of the rotor blade over the operating envelope. When the current angle of attack is approaching negative stall, a pressure side aerody- namic device deflection angle may be increased by moving the aerodynamic device from the retracted configuration towards the extended configuration. Further, pitch may be adjusted accordingly to hold an optimal rotational speed of the rotor. Negative stall will then be avoided and large vibration am- plitudes which could result from the negative stall.

For example, if a predefined stall margin of 1° is defined, the aerodynamic device arranged on the pressure side, may be moved from the retracted configuration towards the extended configuration, when the current angle of attack is within 1° of negative stall. As the blade pitches out, the aerodynamic device will be further moved towards the extended configura- tion, to maintain the predefined stall margin of 1° between the expected stall angle and the current angle of attack. In other words, if a stall margin of 1 degree is defined. Incre- ment the pressure side spoiler deflection when the precalcu- lated tip angle of attack is within 1 degree of negative stall. As the blade pitches out the spoiler deflection will be increased to maintain a 1 degree margin between the nega- tive stall angle of attack and the operating angles of attack of the wind turbine.

According to a further exemplary embodiment of the present invention, the chordal position between the leading edge and the trailing edge is positioned at 0.5 to 1.0 of a total chord length of the rotor blade, wherein the total chord length is defined between the leading edge and the trailing edge.

In particular, the chordal position is positioned at 0.7 to 1.0 of the total chord length, further in particular at 0.9 of the total chord length.

By positioning the aerodynamic device within the above- defined chordal length, the airflow may be influenced at a chordal length before eddies occur. Hence, the aerodynamic device may influence the airflow over the total chord length.

According to a further exemplary embodiment of the present invention, the determining device is further configured for determining a stall margin size of a difference of the stall margin and the predefined stall margin, wherein the aerody- namic device is further configured for adjusting an activa- tion speed of the aerodynamic device based on the determined stall margin size.

If the stall margin size is small, the stall margin is near the predefined stall margin. Hence, the airflow is near the balanced state. Therefore, the activation speed of the aero- dynamic device may be slow because solely a small angular distance must be overcome to meet the balanced state.

If the stall margin size is large, the stall margin is far away from the predefined stall margin. Hence, the airflow is in a state different from the balanced state. Therefore, the activation speed of the aerodynamic device must be fast be- cause a large angular distance must be overcome to meet the balanced state. Additionally, increasing the activation speed may advantageously avoid stall induced instabilities. There- fore, a control system with an increased reliability may be provided .

According to a further exemplary embodiment of the present invention, detecting the current angle of attack of the rotor blade at the predefined longitudinal section of the rotor blade comprises determining the current angle of attack based on at least one operational parameter of the wind turbine, wherein the operational parameter comprises at least one of a group consisting of a wind speed of an incoming wind field, a pitch angle of the rotor blade, a rotational speed of a rotor to which the rotor blade is attached to, a deflection angle of the rotor blade, a deflection angle of the aerodynamic de- vice, and a drag coefficient.

Thereby, the current angle of attack may reliably be deter- mined independent of environmental influences which could disturb an accurate direct measurement of the angle of attack at the rotor blade. It should be emphasized that determining the current angle of attack based on one single of the above-mentioned operational parameters of the wind turbine. Thereby, the current angle of attack may be efficiently and reliably determined.

Additionally, the current angle of attack may be determined based on another one two or more of the above-mentioned oper- ational parameters. Hence, the angle of attack may be deter- mined precisely.

According to a further exemplary embodiment of the present invention, the determining device further comprises an aero- dynamic measurement device configured for determining aerody- namic measurement data of the rotor blade, wherein in partic- ular the aerodynamic measurement device comprises at least one of a group consisting of a piezoelectric pressure belt sensor, a light detecting and ranging (LIDAR) device, and a pressure tap, wherein determining the current angle of attack at the predefined longitudinal section comprises determining the angle of attack based on the aerodynamic measurement da- ta.

Thereby, the angle of attack may be determined based on meas- urement data instead of using a precalculated angle of at- tack. Additionally, the aerodynamic measurement device may be a piezoelectric pressure belt sensor, a light detecting and ranging (LIDAR) device, and/or a pressure tap which are standard components and are mass-produced. Thereby, a cost- efficient way for determining the angle of attack may be pro- vided. Furthermore, the aerodynamic measurement device may be arranged at the predefined longitudinal section such that the angle of attack may be precisely determined.

According to a further exemplary embodiment of the present invention, the predefined longitudinal section is arranged at 0.8 to 1.0 of a total longitudinal length of the rotor blade, wherein the total longitudinal length is defined between the hub region and the tip region of the rotor blade. Arranged at 0.8 to 1.0 of a total longitudinal length accord- ing to the present invention denotes that the predefined lon- gitudinal section comprises a longitudinal extension which extends entirely in the range of 0.8 to 1.0 of the total lon- gitudinal length. At the above-defined range the airflow may delaminate first. Hence, at the above-defined longitudinal length stall may first occur. Therefore, the airflow may re- liably be maintained attached before stall induced vibrations occur to a large extent.

According to a further exemplary embodiment of the present invention, the control system further comprises an active stall detecting device configured for detecting a rotor blade stall at the predefined longitudinal section, in particular based on data from an aircraft stall warning sensor, wherein the active stall detecting device is further configured for activating a movement of the aerodynamic device between the retracted configuration and the extended configuration.

Thereby, the control system may provide a fast and reliable reaction on abrupt occurring stall.

Activating a movement of the aerodynamic device between the retracted configuration and the extended configuration ac- cording to the present invention may denote that if stall is detected by the active stall detecting device, the aerodynam- ic device starts moving between the retracted configuration and the extended configuration.

A first reaction on an occurring stall may be providable in- dependently of determining the stall margin. Additionally, the determining device is further configured for determining if the aerodynamic device is in the retracted configuration or in the extended configuration. If the aerodynamic device is in the retracted configuration, the active stall detecting device activates a movement from the retracted configuration towards the extended configuration. If the aerodynamic device is in the extended configuration, the active stall detecting device activates a movement from the extended configuration towards the retracted configuration.

Subsequently to activating the movement between the retracted configuration and the extended configuration, the detection device detects the current angle of attack of the rotor blade.

It has to be noted that embodiments of the invention have been described with reference to different subject-matters. In particular, some embodiments have been described with ref- erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belong- ing to one type of subject matter also any combination be- tween features relating to different subject matters, in par- ticular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

Brief Description of the Drawings

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited.

Fig. 1 shows a wind turbine according to an exemplary embodi- ment of the invention.

Fig. 2 shows an aerodynamic device on a pressure side in an extended configuration according to an exemplary embodiment of the invention. Fig. 3 shows two graphs showing a lift coefficient dependent on an angle of attack according to an exemplary embodiment of the invention.

Fig. 4 shows two graphs showing a drag coefficient dependent on an angle of attack according to an exemplary embodiment of the invention.

Fig. 5 shows an aerodynamic device on a suction side in an extended configuration according to an exemplary embodiment of the invention.

Fig. 6 shows two graphs showing a lift coefficient dependent on an angle of attack according to an exemplary embodiment of the invention.

Fig. 7 shows two graphs showing a drag coefficient dependent on an angle of attack according to an exemplary embodiment of the invention.

Fig. 8 shows a method for maintaining an attached airflow ac- cording to an exemplary embodiment.

Fig. 9 shows a rotor blade according to an exemplary embodi- ment.

Fig. 10 shows the aerodynamic device of Fig. 2 in a retracted configuration according to an exemplary embodiment of the in- vention.

Fig. 11 shows an active stall detecting device according to an exemplary embodiment of the invention. Detailed Description

The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele- ments are provided with the same reference signs.

Fig. 1 shows a wind turbine 100 according to the present in- vention. The wind turbine 100 comprises a tower 111. At one end of the tower 111, in particular the upper end of the tow- er 111, there is mounted a nacelle 112. The wind turbine 100 may be a floating wind turbine or a wind turbine mounted to a ground. The nacelle 112 is usually mounted rotatable with re- gard to the tower 111. The nacelle 112 usually accommodates the generator of the wind turbine 100 and the gear box (if the wind turbine is a geared wind turbine). Furthermore, the wind turbine 100 comprises a hub 114 which is rotatable about a rotor axis Y. The hub 114 is often described as being a part of a wind turbine rotor 113, wherein the wind turbine rotor 113 is capable to rotate about the rotor axis Y and to transfer the rotational energy to an electrical generator (not shown). The wind turbine 100 further comprises at least one blade 120 (in the embodiment of Fig. 1, the rotor 113 comprises three blades 120, of which only two blades 120 are visible) mounted on the hub 114. The blades 120 extend sub- stantially radially with respect to the rotational axis Y. Each rotor blade 120 is usually mounted pivotable to the hub 114, in order to be pitched about respective pitch axes X. This improves the control of the wind turbine 100 and in particular of the rotor blades 120 by the possibility of mod- ifying the direction at which the wind is hitting the rotor blades 120. Each rotor blade 120 is mounted to the hub 114 at its hub region 121. The hub region 121 is opposed to a tip region 122 of the rotor blade 120. Additionally, a predefined longitudinal section 123 of the rotor blade 120 is shown, wherein a longitudinal direction extends from the hub re- gion 121 to the tip region 122 of the rotor blade 120. Fur- thermore, an airflow 171 is shown before hitting the rotor blades 120 of the wind turbine 100. Fig. 2 shows an aerodynamic device 230 on a pressure side 224 of the rotor blade 120 in an extended configuration according to an exemplary embodiment of the invention. The aerodynamic device 230 is formed as a spoiler 230 pivotably mounted to the pressure side 224 of the rotor blade 120 at a chordal po- sition 231 between a leading edge 211 and a trailing edge 212. As may be seen in Fig. 2, the side opposite to the pressure side 224 is a suction side 225 of the rotor blade 120. An airflow 171 of an incoming (local) wind field hits the rotor blade 120 at the leading edge 211 and passes the rotor blade 120 on the pressure side 224 respectively on the suction side 225.

Further, the spoiler 230 is in the extended configuration having a maximum airflow resistance. The chordal position 231 is positioned at 0.5 to 1.0 of a total chord length 235, wherein the total chord length 235 is measured from the lead- ing edge 211 to the trailing edge 212. Thereby, the spoil- er 230 influences the airflow 171 on the entire pressure side 224.

Fig. 3 shows two graphs 341, 342 showing a lift coefficient dependent on an angle of attack according to an exemplary em- bodiment of the invention. A first graph 341 and a second graph 342 are plotted each showing the lift coefficient on the ordinate 332 and the angle of attack on the abscissa 331. The first graph 341 shows the lift coefficient dependent on the angle of attack of the aerodynamic device 230 in a re- tracted configuration (shown in Fig. 10). Furthermore, the second graph 342 shows the lift coefficient dependent on the angle of attack of the aerodynamic device 230 in an extended configuration (shown in Fig. 2). As illustrated in Fig. 3, when deflecting the spoiler 230 from the retracted configura- tion to the extended configuration, the minimum angle of at- tack is delayed. Particularly, a first minimum angle of at- tack 351 (in the retracted configuration) is delayed to a smaller second minimum angle of attack 352 (in the extended configuration) . Thereby, stall induced vibrations may be avoided.

Fig. 4 shows two graphs 443, 444 showing a drag coefficient dependent on an angle of attack according to an exemplary em- bodiment of the invention. A third graph 443 and a fourth graph 444 are plotted each showing the drag coefficient on the ordinate 432 and the angle of attack on the abscissa 331. The third graph 443 shows the drag coefficient dependent on the angle of attack of the aerodynamic device 230 in a re- tracted configuration (shown in Fig. 10). Furthermore, the fourth graph 444 shows the drag coefficient dependent on the angle of attack of the aerodynamic device 230 in an extended configuration (shown in Fig. 2). As illustrated in Fig. 4, when deflecting the spoiler 230 from the retracted configura- tion to the extended configuration, the drag coefficient may be increased.

Fig. 5 shows an aerodynamic device 530 on a suction side 225 in an extended configuration according to an exemplary embod- iment of the invention. The aerodynamic device 530 is embod- ied as a spoiler 530 pivotably mounted to the suction side 225 of the rotor blade 520 at a chordal position 531 be- tween the leading edge 211 and the trailing edge 212. Oppo- site to the suction side 225 the rotor blade 520 comprises the pressure side 224. By including the spoiler 530, the max- imum angle of attack of the rotor blade 520 may be delayed. When the spoiler 530 is used, the rotor blade 520 sees a neg- ative ACL increment, but it will still reach the same CL_max at a new, delayed a_max. The drag of the rotor blade 520 may also be increased.

Fig. 6 shows two graphs 645, 646 showing a lift coefficient dependent on an angle of attack according to an exemplary em- bodiment of the invention. A fifth graph 645 and a sixth graph 646 are plotted each showing the lift coefficient on the ordinate 332 and the angle of attack on the abscissa 331. The fifth graph 645 shows the lift coefficient dependent on the angle of attack of the aerodynamic device 530 in a re- tracted configuration. Furthermore, the sixth graph 646 shows the lift coefficient dependent on the angle of attack of the aerodynamic device 530 in an extended configuration (shown in Fig. 5). As illustrated in Fig. 6, when deflecting the spoil- er 530 from the retracted configuration to the extended con- figuration, the maximum angle of attack 655, 656 is delayed. Particularly, a first maximum angle of attack 655 (in the re- tracted configuration) is delayed to a larger second maximum angle of attack 656 (in the extended configuration). Thereby, a stall behaviour of the rotor blade 520 may be delayed and therefore avoided.

Fig. 7 shows two graphs 747, 748 showing a drag coefficient dependent on an angle of attack according to an exemplary em- bodiment of the invention. A seventh graph 747 and an eighth graph 748 are plotted each showing the drag coefficient on the ordinate 432 and the angle of attack on the abscissa 331. The seventh graph 747 shows the drag coefficient dependent on the angle of attack of the aerodynamic device 530 in a re- tracted configuration. Furthermore, the eighth graph 748 shows the drag coefficient dependent on the angle of attack of the aerodynamic device 530 in an extended configuration (shown in Fig. 5). As illustrated in Fig. 7, when deflecting the spoiler 530 from the retracted configuration to the ex- tended configuration, the drag coefficient may be increased.

Fig. 8 shows a method 800 for maintaining an attached airflow according to an exemplary embodiment. The method 800 for maintaining an attached airflow 171 flowing from a leading edge 211 of a rotor blade 120 of a wind turbine 100 to a trailing edge 212 of the rotor blade 120. In a first step 810, the method 800 comprises detecting 810 a current angle of attack of the rotor blade 120 at a predefined longi- tudinal section 123 of the rotor blade 120, wherein a longi- tudinal direction of the rotor blade 120 extends from a hub region 121 of the rotor blade 120 to a tip region 122 of the rotor blade 120. In a second step 820, the method comprises determining 820 a stall margin being a difference of the cur- rent angle of attack and an expected stall angle at the pre- defined longitudinal section 123. Additionally, a balanced state of the airflow 171 is determined, wherein in the bal- anced state the stall margin is equal to a predefined stall margin.

Furthermore, the method 800 comprises determining 830 if the stall margin is larger than the predefined stall margin. If the stall margin is larger than the predefined stall margin, the method 800 comprises determining 840 whether the aerody- namic device 230 is in the extended configuration (shown in Fig. 2) or in the retracted configuration (shown in Fig. 10). If the aerodynamic device 230 is in the retracted configura- tion, the method comprises remaining 841 in the retracted configuration. In other words, the aerodynamic device 120 is not moved if it is detected that the aerodynamic device 230 is in the retracted configuration. If the aerodynamic de- vice 230 is in the extended configuration, the method com- prises moving 842 to the retracted configuration until the balanced state is met.

The method 800 further comprises determining 850 if the stall margin is smaller than the predefined stall margin. If the stall margin is smaller than the predefined stall margin, the method 800 comprises moving 851 the spoiler 230 from the re- tracted configuration (shown in Fig. 10) to the extended con- figuration (shown in Fig. 2) until the balanced state is met. In other words, moving 851 the spoiler 230 until an adequate stall margin is obtained.

Fig. 9 shows a rotor blade 920 according to an exemplary em- bodiment. The rotor blade 920 comprises a total longitudinal length 926 measured from the hub region 121 to the tip re- gion 122, wherein the predefined longitudinal section 123 is positioned at 0.8 to 1.0 of the total longitudinal length 926. It may be understood that the predefined longitu- dinal section 123 may also extend over the tip region 122 of the rotor blade 920. A determining device 950 is mounted near the hub region 121 of the rotor blade 920 configured for de- termining a stall margin which is the difference between the current angle of attack and the expected stall angle at the predefined longitudinal section 123. It may be understood that the determining device 950 may alternatively be mounted to another component of the wind turbine 100 such as for ex- ample the tower 111, the nacelle 112 or the hub 114 (shown in Fig. 1). Furthermore, the detection device 960 is mounted on the rotor blade 920 adjacent to the predefined longitudinal section 123, wherein the detection device 960 is configured for detecting the current angle of attack of the rotor blade 920 at the predefined longitudinal section 123. It may be understood that the detection device 960 may be mounted to another component of the wind turbine 100. For example, when the detection device 960 is configured for determining the current angle of attack based on the rotational speed of the rotor 113, the detection device 960 may be mounted to the hub 114.

The rotor blade 920 further comprises in the predefined lon- gitudinal section 123, a deformable surface area 940 which may be mechanically or pneumatically deformable between the retracted configuration and the extended configuration, and a flap 930 mounted to the trailing edge 212, wherein the flap 930 is extendable and is shown in the retracted configu- ration in Fig. 9. Additionally, the determining device 950 comprises the aerodynamic measurement device 951 arranged in the predefined longitudinal section 123 and configured for determining aerodynamic measurement data of the rotor blade 920. The determined aerodynamic measurement data are sent to the determining device 950 and the determining de- vice 950 is configured for determining the current angle of attack at the predefined longitudinal section 123 based on the determined aerodynamic measurement data. Furthermore, an active stall detecting sensor 970 is arranged at the prede- fined longitudinal section 123. The active stall detecting sensor 970 is configured for detecting a rotor blade stall at the predefined longitudinal section 123 and for activating a movement of the flap 930 and/or the deformable surface ar- ea 940 between the retracted configuration and the extended configuration .

Fig. 10 shows the aerodynamic device 230 of Fig. 2 in a re- tracted configuration according to an exemplary embodiment of the invention. The aerodynamic device 230 is formed as a spoiler 230 wherein an edge of the spoiler 230 is coplanar with the pressure side 224 of the rotor blade 120. The re- tracted configuration may have a minimum drag (airflow re- sistance).

Fig. 11 shows an active stall detecting device 970 according to an exemplary embodiment of the invention. The active stall detecting device 970 shown in Fig. 11 is embodied as an air- craft stall warning sensor which is commonly used in aero- nautics, and which is mounted to the leading edge 211 of the rotor blade 120.

It should be noted that the term "comprising" does not ex- clude other elements or steps and "a" or "an" does not ex- clude a plurality. Also, elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con- strued as limiting the scope of the claims.

Claims

1. Control system (900) for maintaining an attached air- flow (171) flowing from a leading edge (211) of a rotor blade (120, 520, 920) of a wind turbine (100) to a trailing edge (212) of the rotor blade (120, 520, 920), the control system (900) comprising a detection device (960) configured for detecting a cur- rent angle of attack of the rotor blade (120, 520, 920) at a predefined longitudinal section (123) of the rotor blade (120, 520, 920), wherein a longitudinal direction of the rotor blade (120, 520, 920) extends from a hub re- gion (121) of the rotor blade (120, 520, 920) to a tip re- gion (122) of the rotor blade (120, 520, 920), a determining device (950) configured for determining a stall margin which is a difference of the current angle of attack and an expected stall angle at the predefined longitu- dinal section (123), wherein the determining device (950) is further configured for determining a balanced state of the airflow (171), where- in in the balanced state the stall margin is equal to a pre- defined stall margin, an aerodynamic device (230, 530, 930, 940) arranged on a surface (224, 225) of the rotor blade (120, 520) at a chordal position (231, 531) between the leading edge (211) and the trailing edge (212), wherein the aerodynamic device (230, 530, 930, 940) is mova- ble between a retracted configuration having a minimum air- flow resistance and an extended configuration having a maxi- mum airflow resistance, wherein the aerodynamic device (230, 530, 930, 940) is con- figured for manipulating the stall margin by moving between the retracted configuration and the extended configuration until the balanced state is met.

2. The control system (900) according to claim 1, wherein the determining device (950) is further configured for determining if the stall margin is larger than the prede- fined stall margin, wherein, if the stall margin is larger than the predefined stall margin, the determining device (950) is further config- ured for determining whether the aerodynamic device (230, 530, 930, 940) is in the extended configuration or in the re- tracted configuration, and wherein if the aerodynamic device (230, 530, 930, 940) is in the extended configuration, the aerodynamic device (230, 530, 930, 940) is configured for moving from the extended configu- ration to the retracted configuration until the balanced state is met.

3. The control system (900) according to claim 1 or 2, wherein the determining device (950) is further configured for determining if the stall margin is smaller than the pre- defined stall margin, wherein, if the stall margin is smaller than the predefined stall margin, the aerodynamic device (230, 530, 930, 940) is configured for moving from the retracted configuration to the extended configuration until the balanced state is met.

4. The control system (900) according to any one of the claims 1 to 3, wherein the aerodynamic device (230, 530, 930, 940) comprises a spoiler (230, 530) pivotably mounted to the surface (224, 225) of the rotor blade (120).

5. The control system (900) according to any one of the claims 1 to 4, wherein the aerodynamic device (230, 530, 930, 940) comprises a flap (930) mounted to the trailing edge (212), wherein the flap (930) is extendable.

6. The control system (900) according to any one of the claims 1 to 5, wherein the aerodynamic device (230, 530, 930, 940) comprises a deformable surface area (940), in particular a mechanically deformable surface area or a pneumatically deformable surface area.

7. The control system (900) according to any one of the claims 1 to 6, wherein the aerodynamic device (230, 530, 930, 940) is ar- ranged at a pressure side (224) of the rotor blade (120, 520, 920).

8. The control system (900) according to any one of the claims 1 to 7, wherein the chordal position (231, 531) of the rotor blade (120, 520, 920) is positioned at 0.5 to 1.0 of a total chord length (235) of the rotor blade (120, 520, 920), where- in the total chord length (235) is defined between the lead- ing edge (211) and the trailing edge (212).

9. The control system (900) according to any one of the claims 1 to 8, wherein the determining device (950) is further configured for determining a size of a difference of the stall margin and the predefined stall margin, and wherein the aerodynamic device (230, 530, 930, 940) is fur- ther configured for adjusting an activation speed of the aer- odynamic device (230, 530, 930, 940) based on the determined size.

10. The control system (900) according to any one of the claims 1 to 9, wherein detecting the current angle of attack of the rotor blade (120, 520, 920) at the predefined longitudinal sec- tion (123) of the rotor blade (120, 520, 920) comprises de- termining the current angle of attack based on at least one operational parameter of the wind turbine (100), wherein the operational parameter comprises at least one of a group consisting of a wind speed of an incoming wind field, a pitch angle of the rotor blade (120, 520, 920), a rotational speed of a rotor (113) to which the rotor blade (120, 520, 920) is attached to, a deflection angle of the rotor blade (120, 520, 920), a deflection angle of the aerodynamic device (230, 530, 930, 940), and a drag coefficient.

11. The control system (900) according to any one of the claims 1 to 10, wherein the determining device (950) further comprises an aerodynamic measurement device (951) configured for determin- ing aerodynamic measurement data of the rotor blade (120, 520, 920), wherein in particular the aerodynamic measurement de- vice (951) comprises at least one of a group consisting of a piezoelectric pressure belt sensor, a light detection and ranging (LIDAR) device, and a pressure tap, wherein determining the current angle of attack at the prede- fined longitudinal section (123) comprises determining the angle of attack based on the aerodynamic measurement data.

12. The control system (900) according to any one of the claims 1 to 11, wherein the predefined longitudinal section (123) is arranged at 0.8 to 1.0 of a total longitudinal length (926) of the ro- tor blade (120, 520, 920), wherein the total longitudinal length (926) is defined be- tween the hub region (121) and the tip region (122) of the rotor blade (120, 520).

13. The control system (900) according to any one of the claims 1 to 12, further comprising an active stall detecting device (970) configured for detecting a rotor blade stall at the predefined longitudinal section (123), in particular based on data from an aircraft stall warning sensor (970), wherein the active stall detecting device (970) is further configured for activating a movement of the aerodynamic de- vice (230, 530, 930, 940) between the retracted configuration and the extended configuration.

14. Wind turbine (100) comprising a tower (111), a rotor (113) mounted to the tower (111), wherein the rotor (113) comprises at least one rotor blade (120, 520, 920), a control system (900) according to any one of the claims 1 to 13, wherein the control system (900) is mounted to the at least one rotor blade (120, 520, 920).

15. Method (800) for maintaining an attached airflow flowing from a leading edge (211) of a rotor blade (120, 520, 920) of a wind turbine (100) to a trailing edge (212) of the rotor blade (120, 520, 920), the method (800) comprising detecting (810) a current angle of attack of the rotor blade (120, 520, 920) at a predefined longitudinal sec- tion (123) of the rotor blade (120, 520, 920), wherein a longitudinal direction of the rotor blade (120, 520, 920) extends from a hub region (121) of the rotor blade (120, 520, 920) to a tip region (122) of the rotor blade (120, 520, 920), determining (820) a stall margin being a difference of the current angle of attack and an expected stall angle at the predefined longitudinal section (123), determining (820) a balanced state of the airflow (171), wherein in the balanced state the stall margin is equal to a predefined stall margin, moving (842, 851) between a retracted configuration hav- ing a minimum airflow resistance and an extended configura- tion having a maximum airflow resistance, an aerodynamic de- vice (230, 530, 930, 940) arranged on a surface (224, 225) of the rotor blade (120, 520, 920) at a chordal position (231, 531) between the leading edge (211) and the trailing edge (212) until the balanced state is met.