WO2011000628A2

WO2011000628A2 - Improved wind turbine blade control

Info

Publication number: WO2011000628A2
Application number: PCT/EP2010/056852
Authority: WO
Inventors: Jonas Romblad
Original assignee: Vestas Wind Systems A/S
Priority date: 2009-06-30
Filing date: 2010-05-19
Publication date: 2011-01-06
Also published as: WO2011000628A3

Abstract

The invention relates to a wind turbine blade (5) comprising at least one flow modifying device (51) adapted to modify the airflow so as to provide a control force (CF), located downwind of a shear centre (SC) of the blade (5), so as for the blade (5) to twist to provide a change (dAF) of a lift force (AF1, AF2) on the blade (5), the lift force (AF1, AF2) including the control force (CF), the change (dAF) being positive in a direction opposite to the direction of the control force (CF).

Description

IMPROVED WIND TURBINE BLADE CONTROL TECHNICAL FIELD

The invention relates to a wind turbine blade, and providing an improved manner of handling loads on the blade.

BACKGROUND

In the wind power industry there is an increased focus on turbines with increased sizes and power outputs, which entails issues regarding the handling of turbine loads. Using control surfaces, such as flaps or ailerons, on the wind turbine blades, to decrease the loading, has been discussed in depth. A presentation of a number of different solutions can be found in "State of the art and prospectives of smart rotor control for wind turbines", T K Barlas and G A M van Kuik, Delft University Wind Energy Research Institute, Journal of Physics:

Conference Series 75 (2007) 012080.

WO2005064156 describes a wind turbine having segmented blades which are individually movable around an axle. The segments can be positioned to define different blade twists along the blade. According to the embodiment the trailing edge can be fitted with a flap, which when deflected will change the pitch of its segment. However, such a segmented blade requires quite a complicated structure, entailing problems with weight, wear and cost.

Handling loads by twisting the blades has also been discussed. WO2008052677 describes a wind rotor blade having a central spar capable of twisting around a central axis. According to one embodiment the spar has an initial twist which is "untwisted" when fully loaded.

However, active control of loads can not be accomplished with this solution. US6769873 describes a wind turbine assembly adapted to twist the turbine blades dynamically according to load conditions. The blades use flexible smart materials, such as piezoelectric material. This is a rather costly solution. US4364708 relates to a windmill having blades with variable pitch and variable spanwise twist. A linkage connected to the hub is used to vary the pitch and spanwise twist according to wind conditions. However, this solution entails problems with weight, wear and cost. WO2008002809 describes the use of weight masses and the centrifugal force to change the blade twist. This also results in problems with weight, wear and cost.

US2009074573 describes the use of a trailing edge flap to directly control the load of a blade. DE29723460U1 describes the use of separate control surfaces or flaps as a means to turn a blade which can rotate around a bearing or a blade built up of sections which can rotate around bearings. SUMMARY

An object of the invention is to provide a manner to handle loads on wind turbine blades in a reliable and weight effective way. Another object of the invention is to improve wind turbine blade aerodynamic performance and load control in a simple manner.

These objects are reached with a wind turbine blade according to claim 1. Thus the invention provides a wind turbine blade comprising at least one flow modifying device adapted to modify the airflow so as to provide a control force, located downwind of a shear centre of the blade, so as for the blade to twist to provide a change of a lift force on the blade, the lift force including the control force, the change being positive in a direction opposite to the direction of the control force. Preferably, the flow modifying device is adapted to modify the airflow so as to provide the control force so as for the blade to twist elastically.

Thus, the control force is at least partly directed perpendicular to a local chordline of the blade. Here "a local chordline" refers to a chordline at a spanwise position of the control force. As an explanatory remark it is pointed out that in accordance with traditional mechanics the shear centre provides on each cross-section transverse to the longitudinal direction of the blade an imaginary point, through which a force can be applied without inducing any torsion of the blade. In the spanwise direction, the shear centre provides what is here referred to as a torsion axis, downwind of which the control force is located, thus causing a torsion or twist of the blade. For clarity, here blade twist means, in the spanwise direction, a continuously increasing departure from the blade geometry in an unloaded state.

The blade, being a wind turbine blade, presents a pressure side and a suction side. As understood, in this presentation, the lift force on the blade includes the contribution of the control force. The lift force is normal to the direction of the resultant air flow, i.e. the air flow resulting from the wind and the velocity of the blade while rotating.

Herein reference is made to the change of the lift force being positive. Thereby, the positive change is such that if the lift force decreases, the positive change is directed opposite to the un-changed lift force, and if the lift force increases, the positive change is in the same direction as the un-changed lift force. It should be noted that the direction of this positive change being opposite to that of the control force, does not necessarily mean that they are separated by 180°, but merely that one of the aerodynamic force change and the control force is in a direction towards which the suction side is facing, and the other is directed in a direction towards which the pressure side is facing. This aspect of the invention implies that the positive change of the lift force, due to the twisting of the blade, is larger than the control force itself.

Thus, the invention provides an aero servo elastic control of the twist of wind turbine blade. Control of the flow modifying device can be accomplished in any suitable way, for example, via a suitable actuator, by control signals from a control unit based on input signals related to loads on the blade, e.g. from strain gauges, etc. As would be understood, a relatively low torsion stiffness of the blade will accomplish said blade twist upon the appearance of the control force. Since the desired change of the blade lift properties are provided by the blade twist, and not by the flow modifying device, an effective active load control can be

accomplished with relatively low activation loads for the flow modifying device, which reduces wear, and/or makes it possible to provide lighter control devices.

Also, the invention makes it possible, by using twist control through a control system that is far less complicated than known suggested solution mentioned above, to adjust the twist of the blade to have the desired shape in a range of wind speeds. In other words, by a simple control system, the blade twist can be adjusted to optimise aerodynamic performance and load control through a wide variety of wind conditions.

Further, the twist control provided by the invention can be used to move the load distribution toward the root of the blade, whereby the deflection at the blade tip will be reduced for the same total load. This may allow using less material in the blade and/or reduce coning and tilt of the blade, e.g. to avoid tower strikes for upwind rotor turbines.

Preferably, the flow modifying device is a deflectable aerodynamic control surface. For example, the flow modifying device could be a trailing edge flap at a trailing edge of the blade. Thereby, the flap being deflected in one direction will cause the blade to twist in the opposite direction. This result, accomplished by a suitable torsion stiffness, means that loads can be controlled with a smaller actuating force than if the whole load change was performed by the flap itself, as in prior art wind turbine blade trailing edge flap solutions. This reduces loads in flap actuation devices, whether they include mechanical control rods or any other means, so as to reduce wear and/or weight of such devices.

Preferably, the trailing edge flap chord length at any spanwise position of the trailing edge flap is 10-30% of the local blade chord length at the respective spanwise position. Too small a flap will not be able to twist the blade and too large a flap may be less effective since the additional forces on the flap get close to the elastic axis of the blade.

In alternative embodiments, the flow modifying device can be located downwind of a trailing edge of a main body of the blade and be fixedly connected to the main body. Thereby, a control force created by the flow modifying device would act on the blade via the fixed connection, so as to twist the main body to provide a positive change of the lift force on the blade in a direction opposite to that of the control force. Preferably, at least one of the at least one flow modifying device is at least partly located radially outside of a position corresponding to 60%, preferably 80%, of the radius of a rotor comprising the blade. This ensures a twisting forces of the entire blade or a major part thereof. The objects are also reached with a wind turbine blade comprising at least one flow modifying device adapted to modify the airflow so as to provide a control force, located upwind of the shear centre of the blade, so as for the blade to twist to provide a change of a lift force on the blade, the lift force including the control force, the change being positive in the same direction as the direction of the control force. Preferably, the flow modifying device is adapted to modify the airflow so as to provide the control force so as for the blade to twist elastically.

The objects are also reached with a wind turbine comprising a blade according to any one of the claims 1-9. DESCRIPTION OF THE FIGURES

Below, embodiments of the invention will be described with reference to the drawings, in which fig. 1 shows a front view of a wind turbine, fig. 2 shows a perspective view of a blade on the wind turbine in fig. 1, fig. 3 shows a view of a blade lateral cross-section oriented as indicated with the lines III-III in fig. 2, fig. 4 shows a perspective view of a blade in an alternative embodiment of the invention, fig. 5 shows a lateral cross-section of a blade in a further embodiment of the invention, and fig. 6 shows a perspective view of a blade in yet another embodiment of the invention. DETAILED DESCRIPTION

Fig. 1 shows a wind turbine 1, with a tower 2, a nacelle 3, and a rotor including a hub 4 and three blades 5. The longitudinal direction of the blades 5 is herein also referred to as the spanwise direction of the blades. As can be seen in fig. 2, each blade 5 is provided, at a trailing edge 54 thereof, with a flow modifying device in the form of a trailing edge flap 51, which is located closer to the tip 52 than the root 53 of the blade 5. It should be noted that alternatively, a plurality of flaps 51 can be distributed in the spanwise direction of the blade 5.

Further the flap 51 is located downstream of a torsion axis of the blade indicated in fig. 2 with a broken line SC. As exemplified in fig. 3 with broken lines, the flap 51 is adapted to be deflected in both directions from a neutral position. A lift force without flap deflection is exemplified with an arrow AFl. The flap 51 provides upon deflection a control force CF, in this example in the same direction as the un-changed lift force AFl, so as for the blade to twist, as illustrated by the arrows TM, in a direction opposite to the flap deflection. This provides a change dAF of the lift force on the blade, the change dAF being positive in a direction opposite to that of the control force CF, resulting in a decreased lift force AF2 including the contribution of the control force CF.

It is understood that if the flap is deflected in the opposite direction, assuming that the lift force AFl without flap deflection is directed as indicated in fig. 3, the lift force AFl would of increase. In an alternative embodiments, as illustrated in fig. 4 and fig. 5, the flow modifying device 51 is located downwind of a trailing edge 54 of a main body 50 of the blade 5 and is fixedly connected to the main body 50. In fig. 4, the flow modifying device is a deflectable wing 51 connected to a rod 511 in a rotatable manner so that it can be deflected in relation to the rod 511 similarly to the flap 51 described above. The rod 511 is fixedly connected to the main body 50 and protrudes from the trailing edge 54 thereof in the downwind direction. In fig. 5, the flow modifying device is a flap 51 which is deflectable in relation to a non-deflectable wing 512, which is connected to a rod 511 in turn connected to the main body 50.

A further alternative is shown in fig. 6. The flow modifying device, provided as a wing 51, is located upwind of a leading edge 55 of the main body 50, and therefore adapted to provide a control force located upwind of the shear centre of the blade 5. Thereby, the blade 5 will twist to provide a change of the lift force on the blade 5, the change being positive in the same direction as that of the control force.

Claims

1. A wind turbine blade (5) comprising at least one flow modifying device (51) adapted to modify the airflow so as to provide a control force (CF), located downwind of a shear centre (SC) of the blade (5), so as for the blade (5) to twist to provide a change (dAF) of a lift force (AFl, AF2) on the blade (5), the lift force (AFl, AF2) including the control force (CF), the change (dAF) being positive in a direction opposite to the direction of the control force (CF).

2. A wind turbine blade according to claim 1, wherein the flow modifying device (51) is adapted to modify the airflow so as to provide the control force (CF) so as for the blade (5) to twist elastically.

3. A wind turbine blade according to any one of the preceding claims, wherein the flow modifying device (51) is a deflectable aerodynamic control surface (51).

4. A wind turbine blade according to any one of the preceding claims, wherein the flow modifying device (51) is a trailing edge flap (51) at a trailing edge (54) of the blade.

5. A wind turbine blade according to claim 4, wherein the trailing edge flap chord length at any spanwise position of the trailing edge flap (51) is 10-30% of the local blade chord length at the respective spanwise position.

6. A wind turbine blade according to any one of the claims 1 or 2, wherein the blade comprises a main body (50), and the flow modifying device (51) is located downwind of a trailing edge (54) of the main body (50) and is fixedly connected to the main body (50).

7. A wind turbine blade according to any one of the preceding claims, wherein at least one of the at least one flow modifying device (51) is at least partly located radially outside of a position corresponding to 60%, preferably 80%, of the radius of a rotor comprising the blade (5).

8. A wind turbine blade (5) comprising at least one flow modifying device (51) adapted to modify the airflow so as to provide a control force (CF), located upwind of the shear centre (SC) of the blade (5), so as for the blade (5) to twist to provide a change

(dAF) of a lift force (AFl, AF2) on the blade (5), the lift force (AFl, AF2) including the control force (CF), the change (dAF) being positive in the same direction as the direction of the control force (CF).

9. A wind turbine blade according to claim 8, wherein the flow modifying device (51) is adapted to modify the airflow so as to provide the control force (CF) so as for the blade (5) to twist elastically.

10. A wind turbine comprising a blade according to any one of the claims 1-9.