EP3055556A1

EP3055556A1 - Hinged vortex generator for excess wind load reduction on wind turbine

Info

Publication number: EP3055556A1
Application number: EP13780029.8A
Authority: EP
Inventors: Kristian R. Dixon; Edward A. Mayda
Original assignee: Siemens AG
Current assignee: Siemens Gamesa Renewable Energy AS
Priority date: 2013-10-09
Filing date: 2013-10-09
Publication date: 2016-08-17
Also published as: WO2015053768A1

Abstract

A vortex generator (VG) (42, 64, 80, 82, 84) mounted via a mounting plate (44, 92) to a wind turbine blade (22, 98). The VG airfoil (46) may be pivotally attached to the mounting plate by a spring hinge (48, 85) that causes the VG airfoil to passively deflect downstream in excess wind conditions to reduce lift on the blade above a predetermined wind speed. A blade pitch controller (99) sets a positive angle of attack on an outboard portion of the blade above the predetermined wind speed that compensates for lift reduction from deflection of VGs on an inboard portion of the blade. The spring hinge may provide a pivot axis (57) parallel to the suction side of the blade or normal (85) to it. A pivot limiter (58, 93/94) may hold the VG in an operating position against a spring preload, overcome by the predetermined wind speed.

Description

HINGED VORTEX GENERATOR FOR EXCESS WIND LOAD REDUCTION

ON WIND TURBINE

FIELD OF THE INVENTION

The invention relates to vortex generators on aerodynamic surfaces, and particularly to vortex generators that deflect to feather above a predetermined wind speed for excess wind load reduction on wind turbines.

BACKGROUND OF THE INVENTION

Wind turbines sometimes encounter excess wind speeds beyond structural design capacity or beyond what is needed for maximum rated power output. The "rated wind speed" of a wind turbine is the lowest wind speed at which it produces power at its rated capacity. Damaging blade loads can be sustained above this rated wind speed in what is called the 'post-rated' regime of 12 - 25 m/s or 15 - 20 m/s for example, depending on the turbine. To maximize efficiency over a range of wind speeds, many wind turbines vary the blade pitch depending on torque/power output. Beyond the safe operating wind speed, the blades are feathered, and the rotor may be stopped or idled.

To maintain rated power in the post-rated wind speed regime, the blade is pitched towards feather, and as a result, the outboard parts of the blade may have a near zero or negative angle of attack, and are thus at or near idle or in a negative lift and/or brake state. In such conditions, most or all of the torque needed to meet rated power is generated by inboard parts of the blade. "Relative wind" at each radial position on the blade is the vector sum of the free stream wind, the tangential blade motion, and an axial induction factor calculated as known in the art. The tangential airfoil speed increases with radius, so turbine blades are twisted along their span to provide a higher absolute pitch at the root and a lower absolute pitch at the tip.

Absolute pitch ("pitch" herein) is the angle between the blade chord line and the rotation plane of the rotor (FIG 20). When the outer portions of the blade are pitched to an idle or braking position, the inboard parts may still produce lift without excess rotor speed and excess power beyond the wind turbine's rating.

During operation at rated wind speed or less, the blade pitch is set for an angle of attack that maximizes power production and is associated with high lift levels being generated at all blade spanwise locations as is known in the art. A sudden gust increases the angle of attack to a stall condition that reduces lift temporarily, thus protecting the blade from lift overloads and rapid load changes. However in the post- rated wind configuration with zero or negative angle of attack on parts of the blade, there is little safety margin for gusts, because the outboard blade sections are operating far from stall. In this configuration, a gust initially increases the angle of attack toward maximum lift, causing a large and rapid load change in the outer portion of the blade leading to damaging high stress amplitude fatigue loads.

Vortex generators (VGs) are relatively small airfoils extending in a perpendicular direction from a larger airfoil such as wind turbine blade to induce vortices 27, 29 (FIG 1 ) that entrain momentum from the relative wind flow 24 into the blade boundary layer. This improves aerodynamic performance by preventing or delaying flow separation on the wind turbine blade.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a perspective view of a prior art wind turbine blade with vortex generators.

FIG. 2 is a perspective view of a prior art vortex generator.

FIG. 3 is a top view of a pair of diverging prior art vortex generators.

FIG. 4 is a perspective suction side view of a vortex generator showing aspects of an embodiment of the invention.

FIG 5 shows a sectional back view taken along line 5-5 of FIG 4.

FIG 6 shows the vortex generator of FIG 5 in a deflected position.

FIG 7 is a sectional view of a hinge barrel with a frictional damping sleeve.

FIG 8 is a sectional view of an embodiment with magnets.

FIG 9 is a conceptual graph of VG deflection at a predetermined wind speed.

FIG 10 is a conceptual graph of VG deflection with magnets.

FIG 1 1 is a sectional view of an embodiment with a mounting plate positioned upstream of the VG airfoil. FIG 12 is a sectional view of the embodiment of FIG 1 1 in a deflected position. FIG 13 is an exploded perspective view of an embodiment with a vertical pivot axis.

FIG 14 is an assembled view of embodiment FIG 13.

FIG 15 is a top view of a vortex generator in an operating position with a vertical pivot axis in a front half of the VG airfoil.

FIG 16 shows the vortex generator of FIG 16 feathered in alignment with the relative wind.

FIG 17 is a top view of a vortex generator in an operating position with a vertical pivot axis in a back half of the VG airfoil.

FIG 18 shows the vortex generator of FIG 17 pivoted by wind to a stopping point broadside to the wind.

FIG 19 is a suction side view of a wind turbine blade with a row of vortex

FIG 20 is a schematic sectional view of a prior art wind turbine blade

DETAILED DESCRIPTION OF THE INVENTION

The inventors recognized that it would be beneficial to reduce post-rated blade loads without reducing efficiency in winds up to the rated wind speed. They have accomplished this in an embodiment of the invention by mounting vortex generators with spring hinges onto the inboard sections of the turbine blades, such that these vortex generators passively deflect to feather, thereby reducing the vortex generator's stall delay effect and subsequently reducing the power produced by the inboard sections. The blades could then be pitched to maintain a more positive angle of attack on outer portions of the blades in order to recover the power lost on the inboard sections. This has the benefit of facilitating easier stalling of the outer portions of the blades in gusts at post-rated wind speeds. Because the outer portions of the blades are operated at a higher angle of attack during high wind speeds, the load changes experienced during a gust are limited by stalling of those portions of the blades, thereby reducing the amplitude of the fatigue cycles on the blades.

FIG 1 shows a prior art wind turbine blade 20 with a suction side aerodynamic surface 22. Vortex generators 26, 28 are mounted on the surface 22. Airflow 24 relative to the turbine blade generates vortices 27, 29 that entrain energy from the airflow 24 into the boundary layer of the surface 22, which delays or prevents flow separation from the surface 22.

FIG 2 shows a prior art vortex generator (VG) 26, which is a small airfoil extending from the larger aerodynamic surface 22 of the wind turbine blade 20. It has a pressure side 30 (hidden), suction side 32, leading edge 34, trailing edge 36, a root portion 38 attached to the larger aerodynamic surface, and a distal portion or tip 40. Such foils are commonly triangular or delta-wing-shaped plates as shown, and have a high leading edge sweep angle Λ, such as 50-80 degrees.

FIG 3 shows a top view of two diverging vortex generators 26, 28, separated by a distance P. Each VG is a foil with a length L and an angle of incidence Φ to the relative airflow 24. A high incidence angle Φ, such as 10-40 degrees, creates a high pressure difference between the pressure and suction sides of the VG. The exemplary incidence angle Φ shown in the drawing is 15 degrees. The combination of high incidence angle Φ and high sweep angle Λ (FIG 2) promotes leakage of high pressure flow from the pressure side 30 to the suction side 32. As the local flow 40 wraps around the VG leading edge 34, it forms a shear layer that rolls into a vortex 29.

FIG 4 shows a suction side view of vortex generator in an embodiment 42 of the invention. A mounting plate 44 is attached to the blade surface 22 by adhesive or other means. A VG airfoil 46 is attached to the mounting plate 44 by a spring hinge 48. The spring hinge may take any of several forms, exemplified here by one or more helical torsion springs 50 on a hinge pin 52 that is journaled in hinge barrel portions 54, 56 of the mounting plate 44 and the VG airfoil 46, and providing a VG airfoil pivot axis 57 parallel to the suction side 22. A pivot stop 58 is provided by a contact interface between the VG airfoil 46 and the mounting plate 44. This allows the torsion spring(s) to be preloaded with sufficient torsion 60 to hold the VG airfoil 46 upright against dynamic pressure from all winds up to a predetermined wind speed, thus maintaining full efficiency of the vortex generator until lift reduction is needed. Above the

predetermined wind speed, the vortex generator is deflected downstream, reducing lift. The spring rate and preload may be designed using known spring design methods to result in deflection only at and above the predetermined wind speed. Spring parameters include the material, wire diameter, coil diameter, number of coils, and number of springs in the hinge. Other embodiments may utilize an elastomer in lieu of or along with a spring.

FIG 5 is a sectional back view taken along line 5-5 of FIG 4. FIG 6 shows the vortex generator in a deflected position. The VG airfoil 46 may or may not lie completely flat against the base plate 44, depending on the wind speed, spring rate, and torsion preload.

FIG 7 is a sectional view of a hinge barrel portion 54 of the base plate with a frictional damping sleeve 62 to prevent flutter of the VG airfoil 46. This sleeve may be a polymer or other damping material.

FIG 8 shows an embodiment 64 with magnets 66, 68 attached to the mounting plate 44 and the VG airfoil 46 respectively on opposite sides of the pivot stop interface 58. These magnets collaborate with the torsion spring(s) to hold the VG airfoil upright until the predetermined wind speed is reached. Then the magnets release the VG airfoil to deflect downstream, reducing lift. This arrangement allows the torsion spring to be weaker, since the magnets help the spring hold up the VG airfoil. A weaker spring in turn allows the VG airfoil to deflect more fully. The torsion spring in this embodiment may be designed to raise the VG airfoil to an angle at which the magnets are close enough to pull together when the wind speed falls. The blades may be temporarily stalled by pitch control after an excess wind condition has passed in order to reset these vortex generators.

FIG 9 is a conceptual graph of VG deflection / dynamic wind pressure in the form of a stress/strain curve, showing aspects of configuring the invention. The spring is designed to stay within its elastic limits and cycle fatigue limits. A torsion preload 72 may be provided by twisting the spring before installation so that the torsion balances the dynamic pressure torque at a predetermined relative wind speed 74. Deflection 70 does not start until the predetermined relative wind speed is reached. A lower preload allows more deflection, while a higher preload maintains the upright VG position in higher winds. The invention provides control of these tradeoffs at the design stage in a passive mechanism that does not necessarily require power or actuators. VG deflection provides additional wind handling capability within the stress/fatigue limits of the blade, because it reduces the large and rapid load changes on the outer portion of the blade that could lead to damaging high stress amplitude fatigue loads unless the VG effect is reduced or eliminated in such winds. This may provide higher availability and capacity factors for the wind turbine, because the wind turbine could remain operational at higher wind speeds than previously possible, and because the length of time between maintenance outages may be increased since the invention can reduce damaging fatigue loads. "Capacity factor" herein means the ratio of the actual output of a power plant over a period of time compared to its potential output if it were possible for it to operate at its rated capacity during the entire period. "Availability factor" herein means the ratio of the time that the turbine is available to produce power in a period compared to the length of that period.

FIG 10 conceptually illustrates another design configuration of the invention using a magnetic embodiment such as shown in FIG 8 and a weaker spring than in FIG 9. In this example, the preload 72 plus the magnets are designed to hold the VG upright against the stop 58 until the rated wind speed 76 is passed. Then the magnets separate at 78. The VG is fully upright throughout normal operating range of wind speeds. It deflects at a predetermined wind speed 74 above the rated wind speed. It can deflect more than in the configuration of FIG 9 because the spring is weaker, while still maintaining efficiency during normal winds since the VGs are held fully upright via the magnets. Again, this increases the availability and capacity factors of the wind turbine, because it provides additional wind handling capability in the post-rated wind regime.

A characteristic of the above embodiments is that they do not start to deflect until a predetermined wind speed 74 is reached. This maintains full effectiveness of the vortex generator during normal wind speeds. In another embodiment, the pivot stop 58 and the torsion preload are eliminated from embodiment 42 of FIG 4. In such embodiment, the springs hold the airfoil upright at zero spring force and zero wind speed. The VG airfoil deflects at all wind speeds, and thus presents a decreasing profile with wind speed.

FIG 1 1 is a sectional view of an embodiment 82 with a mounting plate 44B extending upstream of the VG airfoil 46. A bore for an end 50B of the spring may be provided in the mounting plate 44B. This bore may be accommodated by additional thickness 44C over part or all of the mounting plate 44B. As shown, the additional thickness is formed as a mound over the bore for the spring end 50B. FIG 12 shows embodiment 82 folded flat against the blade surface 22. This embodiment may also have magnets (not shown) on the pivot stop 58B and the mounting plate 44B.

FIG 13 is an exploded perspective suction side view of an embodiment 84 providing a spring hinge with a pivot axis 85 normal to the blade surface 22 via a pivot shaft 86 and a pivot shaft end plate 87 that rotate in journals 89, 90 in the cover plate 91 and the base plate 92 respectively. The pivot shaft end plate 87 may have a radially extending stop tab 93 that limits rotation to a range set by a rotation limiting chamber 94. This chamber has a first stop 94A that contacts the tab 93 when the VG airfoil is positioned at a desired angle of incidence for vortex production, and an opposed stop 94B that contacts the tab when the VG airfoil is feathered downstream in alignment with the relative wind. The base plate 92 houses a spring 95 in a spring chamber 96. The spring chamber may be circular as shown, to hold a single-layer circular or spiral spring, thus keeping the base plate thin. The spring may have additional coils in a flat spiral if needed. A first fixed end 95A of the spring may be retained in a radial side channel 96A of the spring chamber 96. The second working end 95B of the spring extends through a curved slot 97 in the cover plate 91 to contact the VG airfoil 46. Two ends 97A, 97B of the slot 97 may serve as spring range limiters to stop the travel of the spring end 95B at the same rotational limits as the shaft pivot limiters 94A, 94B respectively, thus distributing and balancing the load of limiting VG rotation.

FIG 14 is a perspective suction side assembled view of embodiment 84 of FIG 13. The spring end 95B urges the VG airfoil to the pivot limit provide by the first stop 94A at an angle of incidence such as 20 degrees with respect to the relative wind flow 24. When the wind dynamic pressure exceeds the spring preload, it pivots the VG airfoil toward a feathered position aligned with the relative wind flow.

FIG 15 is a top view of a vortex generator 84 in an operating position with a vertical pivot axis 85 in a front half of the VG airfoil. FIG 16 shows the vortex generator of FIG 16 feathered in alignment with the relative wind 24. FIG 17 is a top view of a vortex generator 84B in an operating position with a vertical pivot axis 85 in a back half of the VG airfoil, for example at or behind the highest part of the VG airfoil. FIG 18 shows the vortex generator of FIG 17 pivoted by excess wind to a stopping point broadside to the wind 24, where it becomes a spoiler that initiates separated flow, resulting in the loss of lift and increased drag, slowing the wind turbine rotor. Resetting of this embodiment may be done by temporarily stalling the blade via high pitch after an excess wind condition has passed.

FIG 19 is a suction side 22 view of a wind turbine blade 98 with a row of vortex generators 42 on an inboard portion P1 of the suction side 22, for example along at least part of an inboard 2/3 thereof. A pitch controller 99 may operate as described herein automatically and dynamically by control logic in response to input of sensors and/or other inputs not shown. The outboard portion P2, for example the outboard 1/3 of the blade, may be without vortex generators, since stalling is desirable on the outboard portion in a gust during the excess wind condition as described herein.

If the vortex generators 42 on a blade 98 all have the same preloads, they will tend to "trip" in succession from the radially outermost VG inward, due to the increase in relative wind speed with radial position. This inherently avoids all the VGs tripping at once. To control the tripping sequence, the VG torsion preloads may be varied. Some VGs on a blade may have weaker springs than others, so they trip in a particular order. This may be used to enhance the radially outer-to-inner sequence, adding time between successive deflections of adjacent VGs.

FIG 20 schematically illustrates an airfoil sectional profile of a prior art wind turbine blade 20 with a pressure side 100 and a suction side 22. A straight chord line Ch spans between the leading edge LE and the trailing edge TE. The length of the chord line Ch is the airfoil chord length. Vector Vw represents the wind velocity outside the influence of the rotor. An axial free-stream vector Va represents the axial component of the air inflow at the blade 20 after reduction of the wind velocity Vw by an axial induction factor as known in the art. Combining Va with a tangential velocity component Vt gives a relative wind vector 24 at an angle Φ relative to the rotation plane 102 of the wind turbine rotor. The angle of attack AoA is the angle between the relative wind vector 24 and the chord line Ch. The absolute pitch angle Θ is the angle between the chord line Ch and the rotation plane 102. This may also be called the twist angle at a given radial position. The lift vector L is perpendicular to the relative inflow vector 24. The drag vector D is directed aft parallel to the relative wind vector 24.

The embodiments described utilize a vortex generator that reacts passively to wind pressure and that requires no power to operate. Other embodiments may be envisioned where a vortex generator is actively controlled, such as by an actuator that controls a position of the vortex generator about its pivot axis. The actuator may be controlled by logic having wind speed or pressure as an input, with the deflection 70 of the vortex generator being controlled or permitted as a function of the wind speed or pressure, similar to the function illustrated in FIGs. 9 and 10. In another embodiment, sensors may be located along the length of a blade to provide local measurements of stress/strain/torque, and such measurements may be processed to calculate a desired position of vortex generators along the length of the blade in order to optimize the performance of the blade under varying wind conditions.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.

Claims

CLAIMS The invention claimed is:

1 . Apparatus for excess wind load reduction on a wind turbine comprising: a plurality of vortex generators on a suction side of a wind turbine blade;

wherein each of the vortex generators is designed to deflect in response to wind pressures that occur above a predetermined wind speed, wherein the deflection reduces lift on the wind turbine blade above the predetermined wind speed.

2. The apparatus of claim 1 , wherein the plurality of vortex generators is mounted along at least a portion of an inboard 2/3 of the wind turbine blade.

3. The apparatus of claim 1 , wherein the plurality of vortex generators is mounted only within an inboard 2/3 of the wind turbine blade.

4. The apparatus of claim 2, further comprising a blade pitch controller that dynamically sets a positive angle of attack on an outboard 1 /3 of the wind turbine blade at wind speeds above a rated wind speed of the wind turbine, wherein the positive angle of attack causes the wind turbine blade to generate additional lift equal to the lift reduction on the inboard 2/3 of the wind turbine blade due to the deflecting of the vortex generators.

5. The apparatus of claim 1 , wherein each of the vortex generators comprises a vortex generator airfoil mounted on the suction side of the wind turbine blade via a spring hinge.

6. The apparatus of claim 5, further comprising a blade pitch controller that dynamically sets a pitch of the wind turbine blade to an angle that generates an amount of lift over the blade that compensates for the lift reduction due to the deflecting of the vortex generators effective to maintain a rated power output of the wind turbine.

7. The apparatus of claim 5, wherein the spring hinge comprises a torsion preload on the vortex generator airfoil against a pivot stop that holds the vortex generator airfoil in an upright position relative to the suction side of the wind turbine blade, wherein the torsion preload is designed to oppose wind pressures against the vortex generator airfoil up to the predetermined wind speed, and to release the vortex generator airfoil to deflect downstream at and above the predetermined wind speed.

8. The apparatus of claim 5, wherein the vortex generators are mounted to the suction side by a mounting plate, and the spring hinge comprises:

a torsion preload that holds the vortex generator airfoil upright relative to the suction side via a pivot stop interface between the vortex generator airfoil and the mounting plate; and

a first magnet on the vortex generator airfoil and a second magnet on the mounting plate;

wherein the torsion preload and the magnets in combination are designed to hold the vortex generator airfoil in an upright position relative to the suction side of the wind turbine blade, and to release the vortex generator airfoil to deflect downstream at and above the predetermined wind speed.

9. The apparatus of claim 7, further comprising a blade pitch controller operable to dynamically set a pitch of the wind turbine blade to an angle that generates an amount of lift over the blade that compensates for the lift reduction due to the deflecting of the vortex generators effective to maintain the wind turbine at a rated capacity above the predetermined wind speed.

10. The apparatus of claim 1 , further comprising a blade pitch controller that dynamically reduces a pitch of the blade from an optimum lift angle of attack at the rated wind speed to a lesser angle of attack above the rated wind speed, but maintains a positive angle of attack on an outboard 1/3 of the wind turbine blade above the predetermined wind speed that compensates for the lift reduction due to the deflection of the vortex generators.

1 1 . The apparatus of claim 1 further comprising a blade pitch controller that dynamically sets a positive angle of attack on an outboard 1/3 of the wind turbine blade, at and above the predetermined wind speed, wherein the positive angle of attack generates an amount of lift over the wind turbine blade equal to the lift reduction due to the deflecting of the vortex generators.

12. The apparatus of claim 1 , wherein each of the vortex generators comprises a vortex generator airfoil mounted on the suction side of the wind turbine blade via a spring hinge comprising a helical torsion spring surrounding a hinge pin journaled in at least two hinge barrel portions of the vortex generator airfoil and at least two hinge barrel portions of a mounting plate attached to the suction side of the wind turbine blade.

13. The apparatus of claim 1 , wherein each of the vortex generators comprises a vortex generator airfoil mounted on the suction side of the wind turbine blade via a pivot shaft with an axis normal to the suction side, wherein the pivot shaft is journaled in a mounting plate comprising a flat torsion spring oriented in a plane parallel to the suction side of the wind turbine blade, wherein the spring urges the pivot shaft toward a pivot limit at an operating position of the vortex generator airfoil.

14. The apparatus of claim 13, wherein the pivot axis is located in a forward half of the vortex generator airfoil, causing the vortex generator airfoil to deflect toward alignment with a relative wind direction.

15. The apparatus of claim 13, wherein the pivot axis is located in a back half of the vortex generator airfoil, causing the vortex generator airfoil to deflect to a limit at a position broadside to a relative wind direction.

16. Apparatus for excess wind load reduction on a wind turbine comprising: a plurality of vortex generators, each vortex generator attached via a mounting plate to a suction side of a wind turbine blade;

each of the vortex generators comprising a vortex generator airfoil pivotally attached to the mounting plate by a spring hinge that causes the vortex generator airfoil to passively deflect downstream in response to a wind condition above a predetermined wind speed, wherein the passive deflection reduces lift on the wind turbine blade above the predetermined wind speed; and

a blade pitch controller that sets a positive angle of attack on an outboard portion of the wind turbine blade above the predetermined wind speed that compensates for the lift reduction due to the deflection of the vortex generators.

17. The apparatus of claim 16, wherein the spring hinge comprises a helical torsion spring surrounding a hinge pin journaled in at least two hinge barrel portions of the vortex generator airfoil and at least two hinge barrel portions of the mounting plate; and a stop comprising a first contact surface on the vortex generator airfoil and a second contact surface on the mounting plate that stops the vortex generator airfoil in an upright position against a preload in the torsion spring.

18. The apparatus of claim 16, wherein the spring hinge comprises a flat torsion spring in the mounting plate, a vortex generator airfoil pivot shaft with a pivot axis normal to the suction side, and a pivot range limiter comprising first and second contact surfaces in the mounting plate and corresponding first and second contact surfaces on an end plate of the pivot shaft, wherein the spring urges the pivot shaft toward a pivot limit at an operating position of the vortex generator airfoil with a spring preload.

19. The apparatus of claim 18, wherein the pivot axis is located in a forward half of the vortex generator airfoil, causing the vortex generator airfoil to deflect toward alignment with a relative wind direction above the predetermined wind speed.

20. The apparatus of claim 18, wherein the pivot axis is located in a back half of the vortex generator airfoil, causing the vortex generator airfoil to deflect broadside to a relative wind direction above the predetermined wind speed.