WIND ENERGY PLANT

Description

[0001] The present invention relates to a wind turbine having an aerodynamic rotor with at least one rotor blade, wherein the rotor blade has an active flow control device which is designed to actively influence a flow over the rotor blade.

[0002] Vortex generators, used as a passive flow control measure for wind turbine rotor blades, are well known. Their primary purpose is to delay flow separation across the rotor blade profile, or more precisely, to shift it to larger angles of attack. The use of vortex generators allows the rotor blade to operate over a wider angle of attack range without stalling. The wide angle of attack range is advantageous because a wind turbine is exposed to a wide variety of operating conditions, which may even deviate significantly from the design point.

[0003] In order to ensure that the turbine yield is as little fluctuating as possible and at the same time optimised, it is therefore necessary to ensure that, if possible, no flow separation occurs on the rotor blade at any operating point, which could otherwise lead to considerable performance losses of the wind turbine.

[0004] Known operating conditions that can lead to a critical increase in the angle of attack include, for example, rotor blades wetted by rain or otherwise contaminated, high turbulence intensity in the incoming flow, high vertical shear of the ground boundary layer in the incoming flow, high horizontal shear in the incoming flow when the turbine is partially in the wake of an obstacle, or air densities that are lower than the design air density. To counteract all of these influencing factors, vortex generators have established themselves as a cost-effective and effective passive flow control device. Vortex generators are usually mounted on the suction side of the rotor blade up to a relative rotor radius of r / R=0.2-0.7; in individual cases, they are also mounted on the pressure side and, increasingly, in areas on the suction side up to the blade tip at r / R=0.95-1.

[0005] However, the installation of vortex generators is not without its critical impact on performance and noise. Vortex generators generate longitudinal vortices in the boundary layer flow on the rotor blade, which increase the near-wall flow velocities in the blade boundary layer, thereby reducing the risk of boundary layer separation. At the same time, frictional drag naturally increases, which can lead to a considerable drop in the glide ratio. It is known that the influence of the glide ratio on turbine performance increases significantly with increasing approach to the blade tip.

[0006] A disadvantage of conventional vortex generators is that they have a power-reducing effect even in operating ranges or operating points where their stabilizing effect is not needed, for example, because it is not raining. Due to the stationary, i.e. permanently installed, vortex generators, the yield is therefore lower than it could be at operating points where the angle of attack applied to the rotor blade is sufficiently small and no flow separation is to be expected.

[0007] The German Patent and Trademark Office has searched the following prior art in the priority application for the present application: DE 10 2010 032 676 A1, US 2010 / 0143123 A1, US 2011 / 0110777 A1, US 2015 / 0132130 A1, WO 2010 / 133594 A1.

[0008] US 2010 / 0076614 A1 relates to a method for operating a wind turbine including a flow control system. The method includes operating the wind turbine in a first mode, operating the wind turbine in a second mode that differs from the first mode, collecting operating data of the wind turbine during at least the second mode, determining an effectiveness of the flow control system using the collected operating data, and performing an action based on the effectiveness of the flow control system.

[0009] WO 2009 / 025549 A1 discloses a wind turbine comprising a rotor with a number of rotor blades, each having a leading edge and a trailing edge. At least one rotor blade is provided with a series of openings located near the trailing edge. The rotor blade has fluid displacement means that, during use, alternately force fluid out of and into the openings. This creates vortices at the trailing edge, which reduce the noise generated.

[0010] US 2011 / 0103950 A1 discloses a wind turbine rotor blade system having a blade rotatably mounted on a rotor of a wind turbine. The system further comprises a controller and one or more openings arranged along at least one surface of the blade, and a fluid movement device arranged to provide a fluid to or from the one or more openings. A controlled amount of the fluid is supplied to the one or more openings. The amount of fluid is determined by the controller. A wind turbine and a method for operating a wind turbine are also disclosed. A prior art solution is also disclosed in US 9,279,412 B2.

[0011] Against this background, it was an object of the present invention to provide a wind turbine with improved yield. In particular, it was an object of the present invention to provide a wind turbine that only activates the stabilizing effect of vortex generators when there is a risk of flow separation, while in all other cases, disruptive interventions in the boundary layer flow are avoided.

[0012] According to the invention, this object is achieved by a wind turbine with an aerodynamic rotor having at least one rotor blade according to claim 1. The rotor blade has an active flow control device designed to actively influence a flow over the rotor blade. The flow control device comprises an opening on a rotor blade surface, called a rotor blade surface opening, wherein the flow control device is configured to extract and / or expel air with a controllable air flow through the rotor blade surface opening.The wind turbine has a controller that is configured to control an amount of the controllable air flow through the rotor blade surface opening according to at least one of the following rules: a) when a speed threshold value of a speed of the rotor is exceeded, successively increasing the maximum controllable air flow with increasing speed, and / or b) when a torque threshold value of a torque of the rotor is exceeded, successively increasing the maximum controllable air flow with increasing torque.

[0013] According to the invention, the yield of rotor blades is further increased. In principle, the boundary-layer flow stabilizing effect of the vortex generators will only be used when there is a risk of flow separation, which is what the active flow control device is designed for. If this risk does not exist, ideally all interventions that disrupt the boundary-layer flow should be avoided.

[0014] By introducing momentum and / or energy, the active flow control device has a similar stabilizing effect on the boundary layer flow as stationary vortex generators. The active flow control device preferably operates by injecting or extracting air, either in a steady or unsteady manner, through openings on the rotor blade surface. The injection or extraction process generates longitudinal vortices at the openings, which increase the near-wall flow velocities and thus stabilize the boundary layer flow against flow separation.

[0015] The advantage of this active flow control device is that the air blowout or suction can be switched on or off as needed. This eliminates a major disadvantage of state-of-the-art passive vortex generators: stabilizing the flow against separation is only performed when necessary due to the prevailing boundary conditions and is otherwise omitted, thus avoiding the performance-reducing effects of the control measure.

[0016] The inventive control of the active flow control device, namely, when a rotor speed threshold is exceeded, the maximum controllable air flow is gradually increased with increasing speed, and / or when a rotor torque threshold is exceeded, the maximum controllable air flow is gradually increased with increasing torque, ensures that the flow is actually stabilized against separation precisely when and only when there is a risk of separation. This approach can thus further increase the overall system yield.

[0017] In one embodiment, the maximum controllable airflow can be determined or expressed as the magnitude of a maximum wall-normal velocity across the opening(s) of the rotor blade surface. This is particularly advantageous for openings that are open perpendicular to the surface of the rotor blade. In other cases, for example, when the airflow is expelled or extracted at an angle to the normal of the rotor blade surface, it is particularly advantageous to determine or control the magnitude of the airflow as a volume flow.

[0018] Within certain limits, it can usually be assumed that the increasing stabilizing effect, i.e. the prevention of flow separation on the rotor blade, increases with the amount of controllable air flow.

[0019] One aspect of the present invention is therefore based on limiting the maximum airflow. This maximum airflow does not necessarily have to be constant across the entire opening or the entire rotor blade and used for control purposes; rather, the control system according to the invention offers the possibility of providing different outflows in the longitudinal direction of the rotor blade, with the maximum airflow being limited across the entire rotor blade.

[0020] According to the invention, when the speed threshold or the torque threshold is exceeded, the maximum controllable air flow is gradually increased with increasing speed or increasing torque. Particularly preferably, the maximum controllable air flow is gradually increased up to a second speed threshold or a second torque threshold, which are greater than the speed or torque threshold, and is no longer increased when the second speed threshold or the second torque threshold is exceeded, even though the speed or torque continues to increase.

[0021] Preferably, a positive value corresponds to blowout, i.e., air flow through the opening to the outside, and a negative value to suction. It is particularly preferred that both blowout and suction occur in different operating ranges and / or at different positions on the rotor blade.

[0022] Preferably, the flow control device is configured to vary the amount of air flow through the rotor blade surface opening along a rotor blade length of the rotor blade.

[0023] This is particularly advantageous when implemented at a constant system speed.

[0024] The opening can comprise a single, elongated opening extending substantially in the longitudinal direction of the rotor blade, or a plurality of partial openings, such as slots or holes, arranged at different positions in the longitudinal direction of the rotor blade. The plurality of partial openings can be formed at the same position in the profile chord direction or at different positions in the profile chord direction. For example, a plurality of rows of partial openings can also be formed at different positions in the profile chord direction, each extending substantially in the longitudinal direction of the rotor blade.

[0025] Particularly preferably, an opening is provided in a region surrounding the leading edge of the rotor blade, for example, in a range from 5% to 35% of the local profile depth, preferably in a range from 15% to 35%, and particularly preferably in a range from 20% to 35% of the local profile depth. Alternatively or additionally, a further opening can be provided in the region of the rotor blade trailing edge, for example, in a range from 30% to 100% of the local profile depth, particularly preferably from 80% to 100%. In both cases, the opening can be configured either as a single elongated opening or as several partial openings.

[0026] Preferably, the controller is configured to control the amount of controllable air flow through the rotor blade surface opening such that the amount of air flow decreases over the rotor blade length.

[0027] Preferably, the controller is configured to control the amount of controllable air flow through the rotor blade surface opening as a function of an air density, in particular such that the decrease in the amount over the rotor blade length decreases with lower air density.

[0028] Preferably, the controller is configured to control the amount of controllable air flow through the rotor blade surface opening as a function of a turbulence intensity, such that the decrease in the amount over the rotor blade length increases with lower turbulence intensity.

[0029] This design is based on the realization that lower turbulence intensity requires blowing or suction only in an area closer to the rotor blade root. Combining this design with air density-dependent control is particularly advantageous, for example, in winter.

[0030] Preferably, the controller is configured to vary the amount of air flow through the rotor blade surface opening depending on an azimuth angle of the rotor.

[0031] In this case, effects of vertical and / or horizontal shear can be taken into account, particularly at constant system speed.

[0032] Preferably, the controller is configured to cyclically control the amount of air flow through the rotor blade surface opening over the azimuth angle such that at an azimuth position of the rotor blade with maximum ground clearance, called the 12 o'clock position, a greater amount of air flow exits through the rotor blade surface opening than at the azimuth position with the smallest ground clearance, called the 6 o'clock position.

[0033] This control is particularly advantageous in vertical shear with positive shear coefficients, ie when wind speeds increase with altitude, which is usually the case.

[0034] For situations where the wind speed decreases with increasing height above ground, i.e., where the wind speed is lower at the 12 o'clock position than at the 6 o'clock position, the vertical shear exhibits a negative shear coefficient. In this case, the controller is particularly preferably configured to cyclically control the amount of airflow through the rotor blade surface opening across the azimuth angle such that a smaller amount of airflow exits the rotor blade surface opening at the 12 o'clock position than at the 6 o'clock position.

[0035] This makes it particularly possible to counteract vertical wind shear, i.e. a variation in wind speed and / or wind direction over height.

[0036] Similarly, additionally or alternatively, horizontal wind shear, for example due to the wake of an obstacle, can be taken into account by different controls on both sides in the horizontal direction, called 3 o'clock position and 9 o'clock position.

[0037] Preferably, the opening of the flow control device is designed such that an air flow exits or flows in substantially parallel to the rotor blade surface.

[0038] This allows for particularly low-resistance blowout / extraction. Alternatively, the opening can be at least partially and / or sectionally inclined or parallel to the rotor blade surface, for example, with a projection, a slope, and / or an edge. This allows for airflows of virtually any magnitude and direction to be defined.

[0039] Preferably, the air flow through the opening of the flow control device is controlled via a flow velocity and / or a blow-out or suction rate.

[0040] Preferably, the blow-out or suction rate is determined as a product of a surface porosity and a ratio of the average blow-out or suction velocity across the opening and the relative inflow velocity of the undisturbed flow.

[0041] Here, the surface porosity is preferably defined as a ratio of open area, ie area formed by the opening or openings, to the total rotor blade surface.

[0042] Preferably, the opening comprises several partial openings distributed along the length of the rotor blade. The partial openings can be designed as slots or as differently shaped openings, for example, with a rectangular, round, oval, or any other shaped cross-section.

[0043] Further advantages and preferred embodiments are described below with reference to the attached figures. Herein: Fig. 1 schematically and exemplarily shows a wind turbine, Fig. 2 schematically and exemplarily shows a rotor blade, Fig. 3 schematically and exemplarily shows a result of an implementation in the control system, Fig. 4 schematically and exemplarily shows a result of an implementation in the control system and Fig. 5 schematically and exemplarily shows a result of an implementation in the control system.

[0044] Fig. 1shows a schematic representation of a wind turbine according to the invention. The wind turbine 100 has a tower 102 and a nacelle 104 on the tower 102. An aerodynamic rotor 106 with three rotor blades 108 and a spinner 110 is provided on the nacelle 104. During operation of the wind turbine, the aerodynamic rotor 106 is set into rotation by the wind and thus also rotates an electrodynamic rotor or rotor of a generator, which is directly or indirectly coupled to the aerodynamic rotor 106. The electrical generator is arranged in the nacelle 104 and generates electrical energy. The operation of the wind turbine 100 is controlled by a controller 112. The controller 112 can be implemented entirely or partially within the nacelle 104, the tower 102, or even remotely from the wind turbine 100, for example, in a central wind farm control system of a wind farm.

[0045] Fig. 2shows a schematic representation of a rotor blade 108 of the wind turbine 100 of the Fig. 1 The rotor blade 108 extends from a rotor blade root 114 to a rotor blade tip 116 in the rotor blade longitudinal direction L. The length of the rotor blade 108 in the rotor blade longitudinal direction is also referred to as radius R due to its use in the rotor 106.

[0046] In the area of ​​the rotor blade root 114, a blade connection 118 is provided for connecting the rotor blade 108 to a rotor blade hub of the rotor 106. The pitch angles of the rotor blades 108 can be changed by pitch motors, for example, at the rotor blade roots 114 of the respective rotor blades 108.

[0047] Along the rotor blade longitudinal direction L, the rotor blade 108 has different aerodynamic profiles at radius positions r. Each aerodynamic profile extends from a point on the leading edge 120 to a point on the trailing edge 122, which are connected via a suction side 124 and an opposite pressure side 126. At the leading edge 120, the air flow splits into two partial flows via the suction side 124 and the pressure side 126.

[0048] The trailing edge 122 has a blunt trailing edge portion 122a in a region near the hub and a pointed trailing edge portion 122b in a region near the rotor blade tip 116. The thickness of the trailing edge 122 substantially decreases in the longitudinal direction L of the rotor blade.

[0049] In Fig. 2Also shown is a flow control device 128, which is designed to actively influence a flow over the rotor blade 108. The flow control device 128 comprises a plurality of rotor blade surface openings 130 configured as openings, which in this example are arranged at different radius positions in the rotor blade longitudinal direction L and have a substantially rectangular cross-section. The flow control device is configured to suck in and / or blow out air with a controllable air flow through the rotor blade surface opening 130.

[0050] The rotor blade surface openings 130 can have any shape and be arranged at any position in any number on the suction side 124 and / or the pressure side 126. Combinations of different shapes or a single rotor blade surface opening 130 are also conceivable.

[0051] The invention lies in the control and regulation of the air flow that is blown out and / or sucked out through the rotor blade surface opening 130 by means of the flow control device 128. The blowing in or sucking out creates longitudinal vortices at the rotor blade surface openings 130, which lead to an increase in the near-wall flow velocities and thus to a stabilization of the boundary layer flow against flow separation. The advantage of these active flow control measures is that the blowing out or sucking out of the air can be switched on or off, i.e., it takes place as needed. The stabilization of the flow against separation is therefore only carried out when necessary due to the prevailing boundary conditions. In other cases, this is omitted in order to avoid any performance-reducing effects of the control measure.This approach can further increase the overall plant yield.

[0052] In the following, some examples of the control of the flow control device 128 by the controller 112 are given with reference to the Figures 3 to 5explained, which lead to an optimization of the system yield. The effectiveness of the active control measure by means of the flow control device 128 is characterized here by an amount of the maximum controllable air flow through the rotor blade surface opening 130 or by an amount of the maximum wall-normal speed above the rotor blade surface opening 130. A positive value corresponds to a maximum air flow / wall-normal speed of the blow-out, a negative value to a maximum extraction rate / extraction speed that would occur in the rotor blade surface opening 130. In general, it can be assumed, up to certain limits, that with an increasing amount of the maximum air flow or speed in the rotor blade surface opening 130, an increasing stabilizing effect also occurs, thus flow separation on the rotor blade can be prevented.

[0053] For rotor blade surface openings 130 that allow for a blowout normal to the surface, the airflow can easily be considered as the maximum wall-normal velocity. Differences arise in particular when, due to the geometry of the rotor blade surface opening 130, a significant tangential flow component arises through the rotor blade surface opening 130. Especially for these cases, the embodiments described below as examples for the maximum wall-normal velocity can be generalized accordingly to the maximum controllable airflow.

[0054] Fig. 3 shows schematically and exemplarily a preferred implementation in the controller 112, wherein a course of the maximum wall-normal velocity | max | is plotted vertically against the system speed n or the system torque M. The magnitude of the maximum wall-normal velocity | max | increases with increasing speed n, since in a partial load range 302, flow separation is often not to be expected, intervention of a control measure is not necessary, the rotor blade 108 can deliver the maximum power coefficient and yield. With increasing wind speed and speed n, the risk of significantly power-reducing flow separation at the rotor blade increases and thus the need for flow control. Therefore, the maximum wall-normal speed is successively increased starting from a threshold value 304 of the speed n with increasing speed n up to the maximum speed value n max raised.

[0055] Depending on other operating parameters or environmental parameters, the course of the increase in the maximum wall-normal velocity | max| from the threshold value 304 of the system speed n and can, for example, have a linearly increasing curve 306, a limited curve 308 or an exponential curve 310.

[0056] Fig. 4 shows schematically and exemplarily the course of the maximum wall-normal velocity | max | above the rotor radius rjR normalized with the maximum radius R for each of two air densities ρ 1 , ρ 2 or two turbulence intensities TI 1 , TI 2 of the inflow at constant system speed n. A curve 402 belongs to the air density ρ 1 and the turbulence intensity TI 1 . A gradient 404 belongs to the air density ρ 2 and the turbulence intensity TI 2 .

[0057] In this example, as mentioned at constant speed n, ρ 1 > ρ 2 and / or TI 2 > TI1 . This means that the smaller the air density or the greater the turbulence intensity of the incoming flow, the further out, ie at the larger r / R, the flow around the rotor blade must be controlled, ie the amount of the maximum wall-normal speed | max | in the rotor blade surface opening 130 assumes a value greater than zero. For the curve 402, the magnitude of the maximum wall-normal velocity | max | up to a radius position 406 greater than zero, for the curve 404 accordingly up to a radius position 408 which is closer to the rotor blade tip than the radius position 406. The magnitude of the maximum wall-normal speed | max | decreases accordingly from a global maximum in the area of ​​the rotor blade root for the curve 404 more slowly than for the curve 402. Although a linear curve 404, 402 is shown, other decreasing curves with the radius position are also possible.

[0058] A plant is exposed to potentially strongly fluctuating turbulence intensity at many locations TI and / or due to seasonal fluctuations in mean temperature, also exposed to fluctuating air densities p. Depending on the ambient conditions, the control measure for separation control is activated radially on the rotor blade 108 only as far towards the blade tip as necessary.

[0059] With identical turbulence intensity TI For example, in winter, the control will only be activated up to smaller radius positions r / R than in summer, where the air densities p are reduced compared to winter. The same applies to the influence of turbulence intensity. TI. Many locations experience a day-night cycle during the day, i.e. within 24 hours, with negligible fluctuations in air density p, in which a higher turbulence intensity occurs during the day when sunlight is present TIin the flow for the wind turbine 100 than at night, when the turbulence intensity TI This daily cycle would be counteracted by the control system 112 in such a way that during the day, when turbulence intensity is high, YOU, the flow control is activated towards larger radius positions r / R than at night, when the turbulence intensity TI These are just two examples of how the factors air density p and turbulence intensity TI influence the control of an active flow control measure 128 on the rotor blade 108 in an advantageous manner in order to prevent separation of the flow around the rotor blade as far as possible.

[0060] Fig. 5 finally shows schematically and exemplarily the magnitude of the maximum wall-normal velocity | max | in the rotor blade surface opening 130 above the rotor azimuth ωat constant speed n for different vertical shears η 1 , η 2 of the bottom boundary layer of the incoming flow.

[0061] Different shears η of the soil boundary layer often occur again during the day at a wind turbine 100. At high positive shear, in the example of the Fig. 5 at η 2, the wind speed at greater distances from the ground may be significantly higher than near the ground. Fig. 5 a curve of the maximum wall-normal velocity | max | for comparatively lower shear η 1 as a curve 502, while a curve 504 for a larger shear η 2 , η 2 > η 1 is shown.

[0062] So if the rotor blade is in the so-called 12 o'clock position, ie the position where the blade tip has the greatest ground clearance, in other words at or at 0° rotor azimuth ω , then the rotor blade 108 in this position usually experiences higher wind speeds VW than at the 6 o'clock position, or at 180° rotor azimuth, when the blade tip is at its closest ground clearance. This is indicated by a low point of the maximum wall-normal velocity | max | for both courses 502, 504 in one area 506.

[0063] This results in higher angles of attack at the 12 o'clock position than at the 6 o'clock position. It is therefore preferable to activate the control measure at the 12 o'clock position, while this is not necessary or less necessary at the 6 o'clock position. This procedure is more important the higher the vertical shear.

[0064] There is also the case where negative vertical shear occurs, meaning the wind speed decreases substantially with altitude. In this case, the control measure can be activated at the 6 o'clock position, while this is not necessary or less necessary at the 12 o'clock position. In the case of negative vertical shear, the greater the magnitude of the shear, the more pronounced the procedure should be.

[0065] This approach is also advantageously applicable in the case of horizontal shear. Pronounced horizontal shear can occur, for example, when a wind turbine 100 is positioned with part of its rotor disk in the wake of another wind turbine or other obstacle. The part of the rotor disk that is in the wake then experiences lower wind speeds than the part of the rotor disk that is subject to undisturbed wind flow.

[0066] In principle, a procedure as in Fig. 5 shown for vertical shear, with the difference that during the course of the maximum wall-normal velocity there is a phase shift of 90° to the 3 or 9 o'clock position.

[0067] The field of application of the invention is, in particular, rotor blades 108 on wind turbines 100 that have an active flow control system, for example, the flow control device 128, in which air is blown into the boundary layer flow on the rotor blade through openings in the rotor blade surface, either stationary or unsteady, or is drawn out through the openings. The system is capable of varying the speed of the blown-out or drawn-out air in terms of amplitude and frequency.

[0068] The purpose of the invention is to optimize the yield of wind turbines by activating and actively controlling the flow control measure in the rotor blade boundary layer only when its stabilizing effect is necessary to prevent flow separation, which significantly reduces power. If such an intervention is not necessary, the control measure is deactivated and has little to no detrimental effect on the turbine yield. The control of the control measure preferably takes into account the wind field impinging on the wind turbine 100 and the parameters characterizing the wind field, such as turbulence intensity, vertical and horizontal shear, and air density, in order to determine optimal operating parameters for the flow control system, with the goal of preventing flow separation on the rotor blade 108.

Claims

1. A wind power installation (100) having an aerodynamic rotor (106) with horizontal axis with at least one rotor blade (108), wherein the rotor blade (108) has an active flow control device, which is designed to actively influence a flow over the rotor blade, wherein the flow control device (128) comprises an opening in a rotor blade surface, referred to as a rotor blade surface opening (130), wherein the flow control device (128) is configured to draw off and / or blow out air through the rotor blade surface opening (130) by way of a controllable air flow, wherein the wind power installation (100) has a controller (112) which is configured to control an amount of the controllable air flow through the rotor blade surface opening (130) according to at least one of the following rules: - if a rotational speed threshold value of a rotational speed (n) of the rotor is exceeded, increasing the maximum controllable airflow successively with increasing rotational speed, - if a torque threshold value of a torque (M) of the rotor is exceeded, increasing the maximum controllable air flow successively with increasing torque (M).

2. The wind power installation (100) as claimed in claim 1, wherein the flow control device (128) is configured to vary the amount of the air flow through the rotor blade surface opening along a rotor blade length of the rotor blade.

3. The wind power installation (100) as claimed in claim 2, wherein the controller (112) is configured to control the amount of the controllable air flow through the rotor blade surface opening (130) such that the amount of the air flow decreases over the rotor blade length.

4. The wind power installation (100) as claimed in claim 3, wherein the controller (112) is configured to control the amount of the controllable air flow through the rotor blade surface opening (130) in a manner dependent on an air density, in particular such that the decrease in the amount over the rotor blade length decreases with lower air density.

5. The wind power installation (100) as claimed in claim 3 or 4, wherein the controller is configured to control the amount of the controllable air flow through the rotor blade surface opening (130) in a manner dependent on a turbulence intensity, such that the decrease in the amount over the rotor blade length increases with lower turbulence intensity.

6. The wind power installation (100) as claimed in one of the preceding claims, wherein the controller is configured to vary the amount of the airflow through the rotor blade surface opening in a manner dependent on an azimuth angle of the rotor.

7. The wind power installation (100) as claimed in claim 6, wherein the controller is configured to control the amount of the air flow through the rotor blade surface opening (130) cyclically over the azimuth angle such that an amount of the air flow through the rotor blade surface opening (130) is greater at an azimuth position of the rotor blade with maximum distance from the bottom, referred to as the 12 o'clock position, than at the azimuth position with minimum distance from the bottom, referred to as the 6 o'clock position.

8. The wind power installation (100) as claimed in one of the preceding claims, wherein the rotor blade surface opening (130) of the flow control device (128) is formed such that an air flow exits or flows in substantially parallel to the rotor blade surface.

9. The wind power installation (100) as claimed in one of the preceding claims, wherein the air flow through the rotor blade surface opening (130) of the flow control device is controlled via a flow speed and / or a blowing-out or drawing-off rate.

10. The wind power installation (100) as claimed in claim 9, wherein the blowing-out or drawing-off rate is determined as a product of a surface porosity and a quotient of mean blowing-out or drawing-off speed over the opening (130) and the relative incident flow speed of the undisturbed flow.

11. The wind power installation (100) as claimed in one of the preceding claims, wherein the opening (130) consists of multiple sub-openings (130) distributed over the rotor blade length.