WO2010034749A2

WO2010034749A2 - Flow control device and method of controlling a fluid boundary layer on a rotating wind turbine blade

Info

Publication number: WO2010034749A2
Application number: PCT/EP2009/062337
Authority: WO
Inventors: Ian Chatting
Original assignee: Vestas Wind Systems A/S
Priority date: 2008-09-26
Filing date: 2009-09-23
Publication date: 2010-04-01
Also published as: WO2010034749A3

Abstract

The invention relates to a flow control device for a wind turbine generator blade comprising at least one flow effector for affecting a fluid boundary layer at said wind turbine generator blade, said at least one flow effector being movable between a retracted position and a projected position. The flow control device further comprises at least one actuator adapted to retract or project said flow effector, said actuator being adapted to communicate with the air pressure on at least one surface of the wind turbine generator blade. The invention also relates to a method of controlling a fluid boundary layer on a rotating wind turbine blade comprising at least one movable flow effector connected with an actuator. Furthermore, the invention relates a wind turbine generator comprising a flow control device according the invention and use of the flow control device.

Description

Flow control device and method of controlling a fluid boundary layer on a rotating wind turbine blade

Technical field The present invention generally relates to a flow control device for a wind turbine generator blade comprising at least one flow effector for affecting a fluid boundary layer at said wind turbine generator blade, and where the at least one flow effector is movable between a retracted position and a projected position. Moreover, the invention relates to a method of controlling a fluid boundary layer on a rotating wind turbine blade comprising at least one movable flow effector. The invention also relates to a wind turbine generator comprising the flow control device and use of the flow control device.

Background of the invention

It is known to improve the performance of wind turbines by using flaps extending from the surfaces of the turbine blades. Depending on which surface of the turbine blade the flaps are located on, the flaps will either increase or decrease the speed of the rotating turbine blades. When the flaps are located on the pressure side of the turbine blade it will increase the speed and when the flaps are located on the suction side of the turbine blade it will decrease the speed .

The flaps function by breaking a boundary layer flowing on the surface of the turbine blade and provides a turbulent flow behind the flaps. Moreover, forming a turbulent flow in a zone on one surface of the turbine blade may serve to improve a laminar flow on the other surface of the turbine blade. Thus, if flaps are located on the pressure side of a wind turbine blade adjacent to the trailing edge, the turbulent flow formed by the flaps, may result in a more stable flow on the suction side of the turbine blade and thereby serve to avoid stall.

The flow is called "attached" when it flows over the surface from the leading edge to the trailing edge. However, when the angle of attack exceeds a certain critical angle, the flow on the suction side of the turbine blade does not reach the trailing edge, but leaves the blade surface at a separation zone. Beyond this zone, the flow direction is reversed, i.e. it flows from the trailing edge backward to the zone of separation. This effect is known as stall. Stall dramatically reduces the lift of the turbine blade, and hence the power produced by the wind turbine, and thereby the economy of the wind turbine.

The use of flaps is one solution to decrease the appearance of stall. Flaps have been installed on wind turbine blades as static flaps or alternatively as movable flaps, that can be moved by use of wires, hinges or other mechanical devices.

US 7,028,954 B2 discloses microfabhcated translational stages for control of aerodynamic loading. The disclosed micro-electro-mechanical translation tabs serves as slidable flaps in a blade, whereby the load on the blade can be adjusted by either extending or extracting the tabs from the surface of the blade. However, the system requires complicated control devices and is still rather costly to install in a wind turbine blade.

Summary of the invention

In view of the above, an objective of the present invention is to provide a flow control device for a wind turbine blade, which is uncomplicated in use and furthermore is easy to manufacture and install in a wind turbine blade.

Thus in a first aspect the invention relates to a flow control device for a wind turbine generator blade comprising at least one flow effector for affecting a fluid boundary layer at said wind turbine generator blade, said flow control device comprising an actuator adapted to actuate said at least one flow effector between a retracted and a projected position as a result of the air pressure on at least one surface of the wind turbine generator blade. The invention provides a flow control device that utilizes the air pressure on at least one side of the wind turbine generator blade (in the following referred to as the blade) to optimize the performance of the wind turbine by affecting the boundary layer on the blade when required. Thus, a flow effecter can be projected from the surface of the blade to affect the boundary layer on the surface of the blade and when the action of the flow effector is no longer required, the flow effector can be retracted entirely into the internal part of the blade and no longer affect the boundary layer on the surface of the blade. Depending on the nature of the boundary layer the flow effector may be fully or partly retracted into the blade. The projection or retraction of the blade is performed by the action of an actuator that may be adapted to utilize the air pressure on the surface of the blade with which the actuator may be adapted to communicate with. Consequently, a simple and cost effective device with few mechanical parts is provided for optimizing the performance of a blade. For the purpose of obtaining a simple design, the flow effector is in one embodiment retractable and projectable in a direction substantially perpendicular to the surface of the blade. In this embodiment the flow control device forms a barhere in the boundary layer that functions very effectively, which at the same time can be manufactured in an uncomplicated manner and subsequently placed in a blade.

In one embodiment of the flow control device, the flow effector is located in the surface of the blade opposite to the surface of the blade with which the actuator communicates. Thus, if the flow effector is located in the surface of the blade constituting the pressure side, then it will be the air pressure on the surface constituting the suction side of the blade that affects the air pressure mechanism. This particular embodiment may advantageously be utilized to minimize the risk of stall.

In an alternative embodiment of the flow control device, the flow effector is located in the surface of the blade corresponding to the surface of the blade with which the actuator communicates. Such an embodiment may e.g. be used to regulate the speed of rotating blades in a wind turbine.

If desired it is possible to combine the two embodiments in a blade to obtain stall and/or speed regulation properties in the blade. Generally, under normal conditions, it will not be desirable to introduce a turbulent flow on the surfaces of a blade and, consequently it is generally preferred that the flow control device is located near or at the trailing edge of the blade. The turbulent flow created by the flow effector will then appear near or just after the trailing edge and may serve to improve an attached flow on a blade on the surface opposite to the surface with the flow effector.

To obtain a flow control device, which is capable of forming an effective barhere to the flow along a surface of a blade it is preferred that the flow effector can be projected over the surface over the blade with an projection corresponding to 0-10% of the chord of the blade. When the projection is 0% the flow effector is fully extracted and does not affect the airflow over the blade. With a projection of the flow effector in a range of e.g. 1 -10% of the chord of the blade a barrier is formed for the airflow and an effective turbulent flow can be formed when the blade is in use in a wind turbine. The chord of the blade refers to the distance between the leading edge and trailing edge of the blade measured in the direction of the normal airflow.

It is desirable to provide a flow control device that is easy to manufacture and at the same time is effective in use, consequently, it is preferred that the flow effector has a simple geometrical shape, preferably the shape of a rectangular plate. Naturally other shapes may be used e.g. a curved or a partly curved shape. A rectangular plate will simply function as a flap when mounted in a blade and, furthermore, the flow effector will be very simple to manufacture. When mounted in a blade the longest extension of the plate should be placed in a direction from the hub to the tip of the blade and preferably with the longest extension being substantially parallel with the trailing edge of the blade. In this manner the flow effector may form a long unbroken barrier on the surface of a blade. Preferably the flow effector is manufactured from a metallic material, such as aluminum or stainless steel. However, the flow effector may also be manufactured from a polymer material, corresponding to the polymer material of the blade.

The actuator is preferably operated by air pressure. Although the actuator may be based on a flexible membrane, bellow, balloon or similar, in an embodiment of the flow control device, the actuator comprises a piston in a housing. To obtain an air pressure driven actuator it is desirable that the rim of the piston is in airtight connection with the walls of the housing and that the piston is slidable along the walls of the housing. In this embodiment the air pressure driven actuator can, in principle, be seen as a simple piston pump, and it will function in a manner similar to a piston pump. Although the housing may be cylindrical or have other desired shapes, it is preferred that the housing has the shape of a box, that makes it easy to fit in a recess in a blade. The housing may be manufactured from various materials, e.g. metallic materials like aluminum or stainless steel or polymer materials. The piston may be manufactured in similar materials.

The piston is connected to the flow effector at one side. The other side of the piston is communicating with a chamber formed by the housing and the piston. When the flow control device is mounted in a blade, the chamber has a connection, e.g. a duct to one surface of the blade, so that the air pressure in the chamber can be adjusted, optionally by use of a valve, to correspond to the air pressure on the surface of the blade. When the air pressure in the chamber is different from the air pressure on the side of the piston connected to the flow effector, the piston will move to equalize the difference in the air pressures and by that movement the piston will move the flow effector to a projected or a retracted position. Generally, when the air pressure in the chamber is lower, the flow effector will be in a retracted position and when the air pressure in the chamber is higher, the flow effector will be in a projected position.

Sometimes it may desirable to regulate the movement of the piston, either to facilitate or restrict the movement of the piston in response to a change in pressure and, therefore, in one embodiment the piston is connected with one or more pressure regulating devices. The pressure regulating device may be pneumatical, hydraulical, elastic or mechanical. In one embodiment the pressure regulating device is mechanical and preferably a spring. This embodiment is a very simple and cost effective way to provide a pressure regulating device.

To facilitate the mounting and maintenance of the flow control device in a blade, the housing that comprises the piston connected with the flow effector and optionally the pressure regulating device forms a replaceable unit. Such a replaceable unit is easy to mount in e.g. a pre-prepared recess in a blade. It is also easy to replace the unit in the blade when the flow control device has a malfunction or is worn out.

In one embodiment of the flow control device according to the invention the flow control device comprises several replaceable units. This particular embodiment provides the option to have several flow effectors placed in a blade. The several flow effectors may be adjusted to function in the same way or adjusted individually to function in different ways, e.g. having different extension in response to a specific air pressure.

The flow control device according to the invention, may comprise a control device for controlling the pressure in the air pressure driven actuator. The control device may e.g. be a computer connected with one or more sensors, e.g. conventional pressure sensors, MEM's (micro-electromechanical sensors) or surface mounted pressure pads, and one or more valves. The control device may then regulate the pressure in the actuator e.g. in response to a change in the air pressure on one surface of the blade or the speed of the rotating blade. The control device may also comprise means to fix the flow effector in a desired position, e.g. an retracted position. In a second aspect, the invention relates to a method of controlling a fluid boundary layer on a rotating wind turbine blade comprising at least one movable flow effector connected with an actuator, said method comprises the step of: allowing the actuator to move the movable flow effector between a retracted and a projected position in accordance with the air pressure on the at least one surface of the blade.

The method utilizes the air pressure on the rotating blade of a wind turbine to control the flow on the surface of the blade by letting the air pressure on a surface of the blade control the movement of the actuator which moves the movable flow effector. It is thus clear that the method requires only a few and relatively simple parts to function and control the flow on a surface of a rotating blade. The actuator is preferably pressure driven and may communicate with the surface of the blade via a duct, which is relatively simple to establish within the blade. The method is easy to perform and at the same time cost-effective.

To obtain an even more effective method for controlling the fluid boundary on a wind turbine blade, the method preferably comprises the further steps of establishing communication between at least one surface of the wind turbine blade and the actuator, and allowing the pressure on the at least one surface of the wind turbine blade to affect the actuator. Thereby, the actuator will move the flow effector between a retracted and a projected position in accordance with the pressure on the surface of the blade.

To improve the method an embodiment of the method comprises the further step of controlling the air pressure that affects the pressure driven mechanism. The air pressure may be controlled by a computer connected with sensors and valves. The valves may be located in the duct that establishes the communication between the pressure driven actuator and the surface of the blade.

For the purpose of obtaining a highly effective flow control it is preferred that the flow effector is retracted or projected in a direction substantially perpendicular to the surface of the blade. Such an embodiment provides for the establishment of an effective barrier for the flow and formation of a turbulent flow.

In one embodiment of the method it is preferred that the flow effector has the shape of a rectangular plate, and preferably the major surfaces of the plate are substantially parallel with the trailing edge of the blade. This embodiment provides for a high impact on the flow on the surface of the blade and thereby an effective control of the flow when the flow effector is in a projected position.

According to the method the actuator may comprise a piston in a housing and functions corresponding to a piston pump as previously described.

Although that the flow effector may be located on both the suction side and the pressure side of the blade, according to a preferred embodiment the flow effector is located on the pressure side of the blade. The flow effector, may thus be located near the trailing edge and thereby serve to improve the flow on the suction side of the blade.

In a further preferred embodiment of the method the actuator communicates with the suction side of the blade. Thereby it is the air pressure on the suction side that determines the projection or retraction of the flow effector. Accordingly the flow effector is moved to a retracted position when the pressure on the suction side of the blade is high, and similarly the flow effector is moved to a projected position when the pressure on the suction side is low. Consequently, the flow on the suction side can be improved.

In one preferred embodiment of the method the movement to a projected position is affected by one or more pressure regulating devices. The pressure regulating devices may facilitate or inhibit the movement to the projected position. The pressure-regulating device is preferably mounted in connection with the pressure driven mechanism.

The pressure regulating device may be pneumatical, hydraulical, elastic or mechanical. In one embodiment the pressure regulating device is mechanical and preferably a spring. This embodiment is a very simple and cost effective way to provide a pressure regulating device.

In a further aspect the invention relates to a wind turbine generator comprising a flow control device as described above. In yet another aspect the invention relates to use of a flow control device as described above for controlling the air flow on a rotating rotor blade in a wind turbine plant.

Brief description of the drawings The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

Fig 1 diagrammatically illustrates a wind turbine generator blade;

Fig 2 diagrammatically illustrates a perspective partly view of a wind turbine generator blade;

Fig 3 diagrammatically illustrates a cross section of a wind generator blade comprising a pressure driven mechanism;

Detailed description of preferred embodiments

Generally a wind turbine blade is defined as having two surfaces, the suction side, where the pressure becomes lower when the blade rotates, and the pressure side where the pressure becomes higher when the blade rotates. Moreover, the blade has two edges denoted the leading edge (first edge in the direction of rotation) and the trailing edge.

Fig 1 illustrates a wind turbine generator blade 1 having a leading edge 2, a trailing edge 3, a suction side 4 and a pressure side 7 (not shown in the figure). The wind turbine generator blade 1 comprises an air pressure communication system 14 e.g. comprising a duct system. The duct system communicates with an opening 14a positioned on the suction side 4 of the blade near the leading edge 2. The duct system 14 communicates with pressure-operated actuators (not visible in Fig 1 ) within the blade. These pressure-operated actuators also communicate with the pressure side of the blade. In this manner the pressure difference between the suction side 4 and the pressure side 7 of the wind turbine generator blade can be utilized to operate the actuators, and thereby project or retract a flow control device e.g. a flap.

Fig 2 illustrates part of a wind turbine generator blade 1 with a flow effector 5 positioned in a recess in the pressure side 7 of the blade. The wind turbine generator blade 1 comprises a flow effector 5 positioned in a housing 8 having a pressure inlet 9. The flow effector 5 has a T-profiled cross section that has an extension in the wind turbine generator blade 1 in the pressure side 7. The T-profiled cross section extends in a substantially perpendicular direction of the surface of the pressure side 7. Furthermore the flow effector 5 elongates substantially parallel to at least part of the trailing edge 3. The T- shaped profile 5 and the housing 8 together forms an actuator, which can move the flow effector 5 between a retracted and a projected position. Fig 2 illustrates the flow effector 5 in a partly projected position. Fig 3A and 3B illustrate a cross section of a wind generator blade 1 with an actuator. The actuator comprises a housing 8 with a pressure inlet 9 and a piston 10. A flow effector 5 is connected with the piston 10 and also positioned in the housing 8. The piston 10 is connected with the flow effector 5 such that the rim of the piston 10 is in airtight connection with the walls of the housing 8. The piston 10 is, thus, able to move the flow effector 5 between a retracted position shown in Figure 3A and a projected position shown in Figure 3B. When the flow effector 5 is moved to a projected position the rectangular part of the T-profile projects out of the pressure side 7 forming a barrier for airflow. In general the maximum projected position of the flow effector 5 corresponds to approximately 10% of the chord of the wind turbine generator blade and the flow effector 5 may be projectable in a range of 0- 10% of the chord of the blade. When the projection is 0% the flow effector 5 is in an extracted position and does not affect the airflow over the blade 1. However, when the projection of the flow effector 5 is in a range of e.g. 1-10% of the chord of the blade a barhere is formed for the airflow and an effective turbulent flow is formed. In a preferred embodiment of the invention the maximum projected position is 45 mm and, thus, the barrier formed by the flow effector 5 will have a height of 45 mm in relation to the surface of the pressure side 7 of the blade 1.

The pressure inlet 9 of the housing 8 is in a mounted position in a wind tower generator connected to a pressure measuring system such that a change in the pressure difference between the suction side 4 and the pressure side 7 may cause a pressure change in a pressure chamber 13. The pressure chamber is made up of the housing 8 and one surface of the piston

10 such that a pressure change leads to a change in the volume of the pressure chamber 13, whereby the pressure change is equalized and the flow effector 5 is moved to either a projected position or a retracted position. The piston 10 is connected with a number of pressure regulating devices 11 in form of springs. In this way a pressure change in the chamber 13 leads to either a contraction or an extension of the springs such that the piston 10 is moved slidable along the walls of the housing 8. In a preferred embodiment the housing 8 is mounted by re-moveable mounting means 15 e.g. screws such that the housing 8 easily can be replaced. The fluid boundary layer around the wind generator blade 1 can have different forms depending on the wind speed and the position of the flow effector 5. When the flow effector 5 is in the retracted position no particular changes of the fluid boundary layer takes place. In a preferred embodiment this is the optimum position when the wind speed is above approximately 8m/s, whereas when the wind speed is below approximately 8m/s, e.g. approximately 4-8m/s, the optimum position of the flow effector is in a projected position to avoid stall. The wind speed is generally measured at the top of the nacelle and the speed of the rotating blades is a function of the wind speed.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. A flow control device for a wind turbine generator blade comprising 5 at least one flow effector for affecting a fluid boundary layer at said wind turbine generator blade, said flow control device comprising an actuator adapted to actuate said at least one flow effector between a retracted and a projected position as a result of the air pressure on at least one surface of the wind turbine generator blade. 0

2. A flow control device according to claim 1 , wherein the flow effector can be retracted and projected in a direction substantially perpendicular to the surface of the blade.

3 A flow control device according to claim 1 or 2, wherein the flow effector is located in the surface of the blade opposite to the surface of the5 blade with which the actuator communicates.

4. A flow control device according to claim 1 or 2, wherein the flow effector is located in the surface of the blade corresponding to the surface of the blade with which the actuator communicates.

5. A flow control device according to anyone of the preceding claims, o wherein the flow control device is located at the trailing edge of the blade.

6. A flow control device according to anyone of the preceding claims, wherein the flow effector can be extended with an extension over the surface over the blade corresponding to 0-10% of the chord of the blade.

7. A flow control device according to anyone of the preceding claims, 5 wherein the flow effector has the shape of a plate and where the plate is substantially parallel with the trailing edge of the blade.

8. A flow control device according to claim 7, wherein the plate has a substantially rectangular shape.

9. A flow control device according to anyone of the preceding claims, 0 wherein the actuator comprises a piston in a housing and where the rim of the piston is in airtight connection with the walls of the housing and the piston is slidable along said walls.

10. A flow control device according to claim 9, wherein the piston is connected to the flow effector. 5

11. A flow control device according to claim 9 or 10, wherein the piston is connected with one or more pressure regulating devices.

12. A flow control device according to anyone of the claims 9 to 11 , wherein said housing comprises the piston and the pressure regulating device, and said housing forms a replaceable unit.

13. A flow control device according claim 12, comprising several replaceable units.

14. A flow control device according to anyone of the preceding claims, comprising a control device for controlling the pressure affecting the actuator.

15. A method of controlling a fluid boundary layer on a rotating wind turbine blade comprising at least one movable flow effector connected with an actuator, said method comprises the step of: allowing the actuator to move the movable flow effector between a retracted and a projected position in accordance with the air pressure on the at least one surface of the blade.

16. A method according to claim 15, comprising the further step of establishing communication between at least one surface of the wind turbine blade and the actuator.

17. A method according to claim 15 or 16, comprising the further step of allowing the pressure on the at least one surface of the wind turbine blade to affect the actuator.

18. A method according to anyone of claim 15 to 17, wherein the actuator is operated by air pressure.

19. A method according to claim 18, comprising the step of controlling the air pressure that affects the actuator.

20. A method according to anyone of the claims 15 or 19, wherein the flow effector is retracted or projected in a direction substantially perpendicular to the surface of the blade.

21. A method according to anyone of the claims 15-20, wherein the flow effector has the shape of a plate.

22. A method according to anyone of the claims 15-21 , wherein the actuator comprises a piston in a housing.

23. A method according to anyone of the claims 15-22, wherein the flow effector is located on the pressure side of the blade.

24. A method according to anyone of the claims 15-23, wherein the actuator communicates with the suction side of the blade.

25. A method according to anyone of the claims 15-24, wherein the flow effector is moved to a retracted position when the air pressure on the suction side of the blade is high.

26. A method according to anyone of the claims 15-24, wherein the flow effector is moved to a projected position when the air pressure on the suction side is low.

27. A method according to anyone of the claims 15-26, wherein the movement to a retracted or projected position is affected by one or more pressure regulating devices.

28. A wind turbine generator comprising a flow control device according to anyone of the claims 1 -14.

29. Use of a flow control device according to anyone of the claims 1 -14 for controlling the air flow on a rotating rotor blade in a wind turbine plant.