WO2011026495A2

WO2011026495A2 - Wind turbine rotor blade

Info

Publication number: WO2011026495A2
Application number: PCT/DK2010/050226
Authority: WO
Inventors: Tim Behrens; Carsten Hein Westergaard
Original assignee: Vestas Wind Systems A/S
Priority date: 2009-09-04
Filing date: 2010-09-01
Publication date: 2011-03-10
Also published as: WO2011026495A3

Abstract

A wind turbine rotor blade comprising: a flap for modifying the aerodynamic surface of the rotor blade; a chamber disposed in the rotor blade and configured such that movement of the flap causes a property change in the chamber, the property change being a change of air pressure within the chamber and/or a change of the volume of air in the chamber; a nozzle disposed on the aerodynamic surface of the rotor blade and in fluid communication with the chamber; wherein the chamber and the first nozzle are configured such that the property change in the chamber generates an air mass flow through the nozzle.

Description

Wind Turbine Rotor Blade

The present invention relates to a wind turbine rotor blade. In particular it relates to wind turbine blades having devices and active flow control for modifying the aerodynamic surface and camber of the blade in order to alleviate loads acting on the wind turbine rotor.

Modern wind turbines are controlled during operation in order to optimise the performance of the wind turbine in different operating conditions. The different operating conditions can arise from changes in wind speed and wind gusts which are local fast variations in wind speed. It is well known to regulate the speed of rotation of the rotor of a horizontal axis wind turbine by pitching the blades of the rotor. This is typically achieved by turning the blades about their longitudinal axis to influence the aerodynamic angle of attack of the rotor blades, this is the method used in pitch controlled wind turbine and active stall controlled wind turbines.

Wind turbines are subjected to loads of a highly variable nature due to the wind conditions. In modern wind turbines, as the rotor is typically able to control its pitch angle, the pitch can be used not only for controlling the speed of the rotor, but also for reducing the variations in load on the blades. However, due to the large length of modern wind turbine blades and the associated high inertia of the masses to be rotated about a pitch axis, the blade pitch mechanisms are not ideal for reacting rapidly to variations in wind speed which occur over a short time frame. In addition the length of wind turbine blades is increasing with new technology and the blades are becoming more flexible due to their greater length. Consequently, with the length of wind turbine blades increasing, when the blades are pitched there is a longer time lag for the pitch to change at the tip where the main loads are on the blades. Furthermore, controlling the loads on the blades with the use of a pitch system can be problematic as the blade pitch bearings become damaged with constant use.

It is possible to regulate the loads acting on the blades of a wind turbine rotor with devices which modify the aerodynamic surface or shape of the blades such as by deformable trailing edges or adjustable flaps which can include trailing edge flaps, ailerons, spoilers, slats and gurney flaps. Such aerodynamic devices are advantageous because they allow a faster response time due to their relatively low inertia as they are small compared to the size of the entire wind turbine blade. One such example of a wind turbine blade which has a deformable trailing edge is described in WO2008/132235.

It is an aim of the present invention to alleviate the loads on a wind turbine blade with a combination of active flow control and aerodynamic optimisation. According to the present invention there is provided a wind turbine rotor blade comprising:

a flap for modifying the aerodynamic surface of the rotor blade;

a chamber disposed in the rotor blade and configured such that movement of the flap causes a property change in the chamber, the property change being a change of air pressure within the chamber and/or a change of the volume of air in the chamber;

a nozzle disposed on the aerodynamic surface of the rotor blade and in fluid communication with the chamber; wherein

the chamber and the nozzle are configured such that the property change in the chamber generates an air mass flow through the nozzle.

According to the invention, an air mass flow through a nozzle on the surface of the rotor blade is caused to occur by the deflection of a flap. This air mass flow provides a means of controlling the boundary layer on the surface of the rotor blade. It is desirable to keep a boundary layer on an airfoil section attached to the airfoil section for as long as possible, and the use of an air mass flow through a nozzle will allow this to be achieved.

By "flap" is meant any device that may be used to alter the aerodynamic profile of an airfoil section of the rotor blade. For example, the flap may be a trailing edge flap, a leading edge flap, a slat or a spoiler or a deformable trailing edge.

The air mass flow through the nozzle may comprise an outflow of air being expelled from the chamber to the outside of the rotor blade; or an inflow of air being sucked from the outside of the rotor blade into the chamber. A jet of air being expelled through the nozzle to the outside of the rotor blade, or suction of air through the nozzle to the inside of the rotor blade can both help to control the boundary layer and delay separation of the boundary layer.

During movement of the flap, the shape of the chamber may change such that the volume of the chamber changes. For example, the chamber may be formed as a reservoir in a rubber element making up the flap, so that when the flap deflects, the volume of the chamber will also change. If the volume of the chamber is reduced in size, this will consequently cause a build up of air pressure within the chamber or cause air within the chamber to be expelled. If the volume of the chamber is increased in size, this will consequently cause a reduction of air pressure within the chamber or cause air to be sucked into the chamber.

During movement of the flap, a piston may move within the chamber such that the volume of air in the chamber changes. A piston may be integrally connected to the flap so that any movement of the flap will cause the piston to move in the chamber, thus changing the volume of the chamber.

Preferably, the wind turbine rotor blade further comprises a second chamber disposed in the rotor blade and a second nozzle disposed on the aerodynamic surface of the rotor blade; wherein; the first chamber is in fluid communication with the first nozzle disposed on a suction surface of the rotor blade and the second chamber is in fluid communication with the second nozzle disposed on a pressure surface of the rotor blade. By providing two chambers, each in fluid communication with a nozzle, allows a change of air pressure within the first chamber and/or a change of the volume of air in the first chamber to be reciprocated in the second chamber. For example, when there is an increase in air pressure or a decrease in volume of the first chamber, there is a corresponding decrease in air pressure or an increase in volume of the second chamber. Therefore, each time the flap is deflected, energy may be provided to two different chambers and an air mass flow may be provided to two different nozzles. This results in a more efficient way of controlling the boundary layer and hence helping to prevent or delay separation of the boundary layer.

The terms "suction surface" and "pressure surface" have their ordinary meaning within the field of aerodynamics.

Movement of the flap in a direction towards the pressure surface of the rotor blade may cause an outflow of air to be expelled from the first chamber to the outside of the rotor blade, through the first nozzle disposed on the suction surface of the rotor blade. When the flap is moved towards the pressure surface of the rotor blade this will result in an increase of lift for the airfoil section of the rotor blade. The camber of the airfoil section will have been increased and there is an increased likelihood that the boundary layer may separate from the suction surface of the rotor blade. Accordingly, by expelling a jet of air through the nozzle on the suction surface will help to keep the boundary layer attached to the suction surface.

Movement of the flap in a direction towards the suction surface of the rotor blade may cause an outflow of air to be expelled from the second chamber to the outside of the rotor blade, through the second nozzle disposed on the pressure surface of the rotor blade. When the flap is moved towards the suction surface of the rotor blade this will result in a decrease of lift for the airfoil section of the rotor blade. A wind turbine rotor blade airfoil section will have a positive camber and so movement of the flap towards the suction surface will result in a decrease in the camber of the airfoil section and there is an increased likelihood that the boundary layer may separate from the pressure surface of the rotor blade. Accordingly, by expelling a jet of air through the nozzle on the pressure surface will help to keep the boundary layer attached to the pressure surface.

Movement of the flap in a direction towards the pressure surface of the rotor blade may cause an inflow of air to be sucked from the outside of the rotor blade into the first chamber, through the first nozzle disposed on the suction surface of the rotor blade. When the flap is moved towards the pressure surface of the rotor blade this will result in an increase of lift for the airfoil section of the rotor blade. The camber of the airfoil section will have been increased and there is an increased likelihood that the boundary layer may separate from the suction surface of the rotor blade. Accordingly, by sucking air into the rotor blade through the nozzle on the suction surface will help to keep the boundary layer attached to the suction surface.

Movement of the flap in a direction towards the suction surface of the rotor blade may cause an inflow of air being sucked from the outside of the rotor blade into the second chamber, through the second nozzle disposed on the pressure surface of the rotor blade. When the flap is moved towards the suction surface of the rotor blade this will result in an decrease of lift for the airfoil section of the rotor blade. The camber of the airfoil section will have been decreased and there is an increased likelihood that the boundary layer may separate from the pressure surface of the rotor blade. Accordingly, by sucking air into the rotor blade through the nozzle on the pressure surface will help to keep the boundary layer attached to the pressure surface.

The wind turbine rotor blade may further comprise: a pressure accumulator in fluid communication with the chamber, and disposed between the chamber and the nozzle; a control valve connecting the pressure accumulator to the nozzle; the pressure accumulator being configured such that movement of the flap creates an increase of air pressure within the pressure accumulator; and means for regulating the control valve to expel air from the pressure accumulator through the nozzle to the outside of the rotor blade. By providing a pressure accumulator allows air pressure to be stored. A jet of air can then be expelled into the boundary layer at a desired time to control the boundary layer. This desired time could be when there is particularly large camber on the airfoil profile on the rotor blade, for instance.

The wind turbine rotor blade may further comprise a plurality of flaps each having an associated chamber. The plurality flaps may extend in the spanwise direction of the rotor blade and because each flap has its own chamber, a greater control of the boundary layer is provided along the length of the rotor blade.

The flap and the nozzle may be located at different spanwise positions on the rotor blade. For instance, a flap a the tip end of the rotor blade may store pressure in an associated chamber, and a jet of air can be expelled into the boundary layer near the root end of the rotor blade. This is advantageous because it allows the boundary layer to be controlled in regions of the rotor blade that are remote from the flap itself.

A wind turbine generator may be provided having at least one wind turbine rotor blade according to the invention. The wind turbine may be a three bladed horizontal axis wind turbine.

The invention will now be described by way of examples with reference to the following Figures in which: Figure 1 illustrates a plan view of a wind turbine blade according to the present invention.

Figure 2 illustrates a partial cross section of a first example of a wind turbine blade along the line ll-ll in Figure 1.

Figures 3a and 3b illustrate a cross section of a second example of a wind turbine blade along the line ll-ll in Figure 1.

Figure 4 illustrates a cross section of a third example of a wind turbine blade along the line ll-ll in Figure 1.

Figure 5 illustrates a plan view of a wind turbine blade according to the present invention.

Figure 1 shows a rotor blade 10 for a horizontal axis wind turbine (not shown). The rotor blade 10 comprises a leading edge 1 1 , a trailing edge 12, a root end 13 and a tip end 14. The root end 13 is adapted to be mounted on a hub of a wind turbine as is well known in the art. The rotor blade 10 is also provided with two trailing edge flaps 15 and 16 which form part of the trailing edge 12. The trailing edge flaps 15 and 16 are typically deflected in order to reduce the loads experienced on the rotor blade 10 when it is in use and rotating about the hub. However, the trailing edge flaps may also be used when the rotor is at a standstill in a parked condition, in order to alleviate the loads on the wind turbine. The trailing edge flaps are each deflected by an actuator (not shown) which may be an electrical, mechanical or pneumatic actuator. As can be seen in Figure 1 , each flap 15 and 16 has a chord length in the direction between the leading edge and the trailing edge and a span length in the direction between the root end and the tip end. Figure 1 shows two flaps, however the invention is not limited to the number of flaps present.

Figure 2 shows a cross section of the flap 15 along the line ll-ll in Figure 1 . The flap 15 is connected to a blade body 17 about which it rotates as shown by the double headed arrow 18 - the actuation means is not shown in Figure 2. A hinge is provided at the spanwise edges of the flap 15 to connect the flap 15 to the blade body; Figure 2 illustrates a cross section of the mid-span of the flap 15 and thus does not show the hinge connections.

The flap 15 is designed such that when it is deflected a piston 20 moves inside a reservoir 21 . The reservoir 21 comprises a curved cylinder extending from a suction surface 22 to a pressure surface 23 of the rotor blade 10. The reservoir 21 may be circular in cross section or have some other suitable shape.

As can be seen in Figure 2, the reservoir 21 is split in two by the piston 20 into an upper section 21 a that communicates with the suction surface 22 via nozzle 24; and a lower section 21 b that communicates with the pressure surface 23 via nozzle 25. When the flap 15 is actuated the movement will simultaneously move the piston 20 which is integral with the flap 15 inside the reservoir 21 and compress or expand the air in upper section 21 a or lower section 21 b. For example, if the flap 15 is actuated so that it deflects towards the pressure surface 23, that is the flap moves in a downwards direction according to Figure 2, the piston 20 will moves upwards thus forcing air out of the upper section 21 a through the nozzle 24. Simultaneously, air will flow from the pressure surface 23 into the lower section 21 b.

When a i r is forced out of nozzle 24 i nto the airflow passing along the aerodynamic surface of the blade it will help the boundary layer to stay attached because it provides momentum to the boundary layer as is well known in the field of aerodynamics. By helping to keep the boundary layer attached to the suction surface will increase the lift-to-drag ratio of the airfoil section because separation of the boundary layer is delayed further toward the trailing edge 12. It is advantageous to keep the boundary layer attached to the surface because if the boundary layer separates from the aerodynamic surface of the rotor blade there is a large increase in drag. In this example, when the flap 15 deflects downwards, the pressure surface 23 air will be sucked into the lower section 21 b and this will reduce the aerodynamic friction resistance on the pressure surface 23 by sucking away at least a portion of the air forming the boundary layer along the pressure surface 23.

If the flap 15 is deflected toward the suction surface 22, air will be expelled from nozzle 25 and sucked into nozzle 24.

In this example, it has been shown how the reservoir is divided into two sections, each with a corresponding nozzle. However, the invention can also be implemented with a single nozzle on either the suction surface 22 or the pressure surface 23.

When the flap 15 is deflected it will create a high curvature or camber in the airfoil section which will normally increase the likelihood of the boundary layer separating from the aerodynamic surface. Therefore the use of the active flow control to help the boundary layer to stay attached is particularly desirable when the flap 15 is deflected.

The nozzles 24 and 25 are shown to exit onto the aerodynamic surface at the front edge, in the upwind direction, of the flap 1 5. However, the nozzles could be provided elsewhere on the aerodynamic surface, such as toward the leading edge 1 1 of the rotor blade 10 with a suitable conduit connecting the reservoir to the nozzles. The position of the nozzle will be a matter of design choice based on the aerodynamic properties of the rotor blade.

The mechanism shown in Figure 2 has been described so that when the flap is moved towards the suction surface 22, air is expelled through nozzle 25 on the pressure surface, and when the flap is moved towards the pressure surface 23 air is expelled through nozzle 24 on the suction surface. However, boundary layer control can also be achieved through suction, so the mechanism of Figure 2 could be arranged such that when the flap is moved towards the suction surface 22, air is sucked through nozzle 25 on the pressure surface into the lower chamber section 21 b, and when the flap is moved towards the pressure surface 23 air is sucked through nozzle 24 on the suction surface into upper chamber section 21 a. This can be achieved by connecting the piston 20 to the flap 15 through a gear wheel such that when the flap moves in a downwards direction, the piston will move in an upwards direction and vice-versa.

In the example of Figure 2, the flap 15 may be provided as a fibre glass shell mounted on supporting spars and ribs. However, as shown in Figure 3a and 3b, the flap 15 may also be formed from an elastomeric material. In Figure 3a and 3b, the flap is substantially a solid member attached to the blade body 17 which is caused to deflect by an actuator (not shown). As shown in Figure 3b, the flap has been deflected in a direction towards the pressure surface 23. Embedded in the flap 15 are two reservoirs 30 and 31 . Upper reservoir 30 is situated near the suction surface 22 and lower reservoir 31 is situated near the pressure surface 23. When the flap 15 deflects, the shape of the reservoirs 30 and 31 also change since they are formed as chambers in the elastomeric material; this in turn causes the volume of each reservoir 30 and 31 to change such that they will either expel air or suck in air.

Upper reservoir 30 is attached to a nozzle 34 via means of a conduit 33 and the nozzle 34 is located on the pressure surface 23. Lower reservoir 31 is attached to a nozzle 36 via means of a conduit 35 and the nozzle 36 is located on the suction surface 22.

When the flap 15 deflects downwards towards the pressure side 23, lower reservoir 31 is compressed forcing air to be expelled through nozzle 36 which will help the boundary layer to stay attached and prevent or delay separation of the boundary layer. At the same time, upper reservoir 30 will expand which will suck air through nozzle 34 and into the upper reservoir 30, and this suction on the pressure surface will help the boundary layer to stay attached to the airfoil profile.

When the flap 15 deflects upwards towards the suction side, upper reservoir 30 is compressed forcing air to be expelled through nozzle 34 which will help the boundary layer to stay attached and prevent or delay separation of the boundary layer. At the same time, lower reservoir 31 will expand which will suck air through nozzle 36 and into the lower reservoir 31 , and this suction on the suction surface will help the boundary layer to stay attached to the airfoil profile.

The mechanism shown in Figure 3a and 3b has been described so that when the flap is moved towards the suction surface 22, air is expelled through nozzle 34 on the pressure surface, and when the flap is moved towards the pressure surface 23 air is expelled through nozzle 36 on the suction surface. However, boundary layer control can also be achieved through suction, so the mechanism of Figure 3a and 3b could be arranged such that when the flap is moved towards the suction surface 22, air is sucked through nozzle 34 on the pressure surface into the lower reservoir 31 , and when the flap is moved towards the pressure surface 23 air is sucked through nozzle 36 on the suction surface into upper reservoir 30. This can be achieved by simply connecting the nozzle 26 on the suction surface to the upper reservoir 30 and connecting the nozzle 34 on the pressure surface to the lower reservoir 31.

Figure 4 shows a further example of the invention in which an additional reservoir is provided that can be filled with pressurised air. As shown in Figure 4, the flap 15 is configured with a reservoir 40 and a piston 41 that are similar to the reservoir and piston arrangement in Figure 2. In use, when the flap 15 is deflected, a storage reservoir 42 is filled with pressurised air, and the storage reservoir 42 acts as pressure accumulator. The pressurised air can then be expelled through a nozzle 43 provided on the suction surface 22 in order to control the boundary layer. The pressurised air is provided to the nozzle 43 through a conduit 44 which is regulated by a control valve 45. The control valve 45 is connected to a control unit (not shown) that may be provided in the blade or in the wind turbine nacelle and which determines when the control valve 45 should be opened or closed in order to control the boundary layer. The reservoir 40 is divided in two parts by the piston 41 and each part of the reservoir 40 is connected through a conduit 46a, 46b to the storage reservoir 42 through check valves 47a and 47b. A conduit 48 connecting both check valves 47a and 47b is provided to the storage reservoir 42. The check vales 47a and 47b are configured so that they only let air pass in the direction from the reservoir 40 to the storage reservoir 42. A reed valve 49 is provided at the points where the reservoir 40 interfaces with the suction surface 22 and the pressure surface 23. The reed valves 49 are configured such that they only let air pass into the reservoir 40 from outside the rotor blade 10, i.e. air cannot pass from the reservoir 40 to the exterior of the rotor blade 10.

As can be seen in Figure 4, when the flap 15 is deflected upwards toward the suction surface 22, the air in the lower part of the reservoir 40 is compressed and this will cause the valve 47b to open and this pressurised air will pass into the storage reservoir 42. Every time the flap 15 is deflected, and in any direction, pressurised air is provided to the storage unit 42. The control unit (not shown) will then control the valve 45 to determine when this pressurised air should be expelled through the nozzle 43 in order to control the boundary layer.

In a further example shown in Figure 5, eight separate flaps 15 to 22 are provided at the trailing edge 12 of the rotor blade 10. Each flap 15 to 22 has its own reservoir system as described above in relation to Figures 2 or 3 and is connected to a common conduit 50 that is connected to a storage reservoir 51 that stores pressurised air from the reservoir system of each flap 15 to 22. A control valve and check valves (not shown) are also provided in a similar configuration as described in relation to Figure 4. The storage reservoir 51 is located near to the root end 13 of the rotor blade 10 where a high lift airfoil profile with a large chord in used. By placing a nozzle or nozzles in this region, air can be expelled as shown by the arrows 52 in an area where it is particularly advantageous to control the boundary layer.

In each of the above examples, a single nozzle or an array of nozzles may be provided in both the spanwise direction and in the chordwise direction, to suck air into the airfoil or expel air from the airfoil section.

Claims

1. A wind turbine rotor blade comprising:

a flap for modifying the aerodynamic surface of the rotor blade;

2. A wind turbine according to claim 1 , wherein the air mass flow through the nozzle comprises:

an outflow of air being expelled from the chamber to the outside of the rotor blade; or an inflow of air being sucked from the outside of the rotor blade into the chamber.

3. A wind turbine rotor blade according to claim 1 or 2, wherein during movement of the flap, the shape of the chamber changes such that the volume of the chamber changes.

4. A wind turbine rotor blade according to claim 1 or 2, wherein during movement of the flap, a piston moves within the chamber such that the volume of air in the chamber changes.

5. A wind turbine rotor blade according to any one of the preceding claims, further comprising a second chamber disposed in the rotor blade and a second nozzle disposed on the aerodynamic surface of the rotor blade; wherein;

the first chamber is in fluid communication with the first nozzle disposed on a suction surface of the rotor blade and the second chamber is in fluid communication with the second nozzle disposed on a pressure surface of the rotor blade.

6. A wind turbine rotor blade according to claim 5, wherein movement of the flap in a direction towards the pressure surface of the rotor blade causes an outflow of air to be expelled from the first chamber to the outside of the rotor blade, through the first nozzle disposed on the suction surface of the rotor blade.

7. A wind turbine rotor blade according to claim 5 or 6, wherein movement of the flap in a direction towards the suction surface of the rotor blade causes an outflow of air to be expelled from the second chamber to the outside of the rotor blade, through the second nozzle disposed on the pressure surface of the rotor blade.

8. A wind turbine rotor blade according to claim 5, wherein movement of the flap in a direction towards the pressure surface of the rotor blade causes an inflow of air to be sucked from the outside of the rotor blade into the first chamber, through the first nozzle disposed on the suction surface of the rotor blade.

9. A wind turbine rotor blade according to claim 5 or 8, wherein movement of the flap in a direction towards the suction surface of the rotor blade causes an inflow of air being sucked from the outside of the rotor blade into the second chamber, through the second nozzle disposed on the pressure surface of the rotor blade.

10. A wind turbine rotor blade according to claim 1 , further comprising:

a pressure accumulator in fluid communication with the chamber, and disposed between the chamber and the nozzle;

a control valve connecting the pressure accumulator to the nozzle;

the pressure accumulator being configured such that movement of the flap creates an increase of air pressure within the pressure accumulator; and further comprising:

means for regulating the control valve to expel air from the pressure accumulator through the nozzle to the outside of the rotor blade.

1 1. A wind turbine rotor blade according to any one of the preceding claims, further comprising a plurality of flaps each having an associated chamber.

12. A wind turbine rotor blade according to any one of the preceding claims, wherein the flap and the nozzle are located at different spanwise positions on the rotor blade.

13. A wind turbine generator having at least one wind turbine rotor blade according to any of the preceding claims.