CA1155062A

CA1155062A - Wind turbine with yaw trimming

Info

Publication number: CA1155062A
Application number: CA000376965A
Authority: CA
Inventors: Robert E. Gustafson
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1980-05-19
Filing date: 1981-05-06
Publication date: 1983-10-11
Also published as: ZA813056B; JPS5716267A; AR224689A1; NL8102370A; NO811653L; IT1136605B; IL62819A0; SE8103049L; DE3119738A1; JPH0211747B2; IT8121791A0; SE455115B; GB2076070B; AU7069081A; FR2484552A1; GB2076070A; FI811522L; BR8103027A; ES8203465A1; ES502288A0

Abstract

Abstract of the Disclosure The disclosure teaches a wind turbine. The wind turbine includes a rotatable hub and at least one airfoil blade mounted at a root portion thereof on the rotatable hub and pivotally movable about the root portion into and out of the wind in response to vertical wind velocity gradients acting on the blade. The wind turbine is characterized by the blade being connected to the hub in such manner that pivoting movement into and out of the wind by the blade adjusts blade pitch relative to the direction of the wind to a degree sufficient to counteract any tendency of the turbine to precess away from an optimum yaw heading thereby minimizing yaw imbalance of the hub due to the influence of the vertical wind velocity gradients on the blade.

Description

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Description Wind Turbine with Yaw Trimming Technical Field This invention relates to wind turbines and more particularly to wind turbines designed for optimum per-formance when maintained in a particular orientation relative to wind direction.

Background Art Wind turbines or windmills of the type having a hub - 10 or rotor with a plurality of airfoil blades mounted thereon and adapted to rotate about a horizontal axis, generally operate at peak efficiency when the rotor and blades are oriented into the wind or within a degree or two of that direction. To allow the hub to position itself into the wind, the hub and a shaft connecting the hub to the turbine load are generally pivotable about a vertical yaw axis.
Both active and passive means have been employed to trim the turbine in yaw for maintaining a desired orienta-tion of the wind turbine with the wind. The active means generally employ a wind direction sensor which, through a suitable control system, activate means to power the hub in yaw to effect a dispostion of the hub into the wind and means to maintain such disposition for as long as the wind direction remains constant. The passive apparatus generally rely on a "weather vane" effect wherein side loading of the hub and collateral structure by the wind maintains the alignment of the turbine with the wind.
While the active means may effectively position and hold the turbine into the wind, such means generally involve complex apparatus and therefore, tend to lower the economic efficiency of the turbine, thus raising the cost of the power provided by the turbine.
The passive or weather vane mechanism of trimming the !
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- 2 - ~S50~z turbine in yaw has proven to be relatively effective when applied to wind turbines having relatively short and stiff blades. However, in modern, large wind turbines, having blades of 125 feet or more in length, to achieve a minimi-zation of weight, the blades are sometimes of a hollow, composite construction of substantial inherent elasticity.
Such wind turbine blades if rigidly mounted to the hub and exposed to vertical wind velocity gradients and gravitation-al forces during normal operation, tend to cyclically bend or "flapH, militating against maintenance of the orient-ation of the turbine into the wind. If, for purposes of accommodating the vertical wind velocity gradients, the blades are pivotally mounted on the hub in pivotal relation to a "teeter" axis transverse to the axis of rotation of the hub and shaft and the longitudinal axes of the blades, rotation of the blades so mounted eliminates such elastic flapping but nevertheless results in a horizontal precess-ion of the hub and blades about the teeter axis. Such precession is the result of the combined rotation and teetering of the blades under the influence of vertical wind velocity gradients and gravity and causes the turbine to angularly displace itself from the proper orientation with respect to the wind by pivotal movement about the yaw axis.
Accordingly, it is an object of the present invention to provide a wind turbine with improved means for trimming the turbine in yaw for setting and maintain-ing the orientation of the turbine directly into the wind.
It is another object of the present invention to provide such a wind turbine wherein the trimming means are passive in nature.
It is another object of the present invention to provide such a wind turbine wherein the trimming means are economical, making no substantial contribution to the cost of the turbine or the energy produced thereby.

- 3 - 1 ~ ~S 0 G2 In accordance with a particular embodiment of the invention there is provided a wind turbine. The turbine includes a rotatable hub and at least one airfoil blade mounted at a root portion thereof on the rotatable hub and pivotally movable about the root portion into and out of the wind in response to vertical wind velocity gradients acting on the blade. The wind turbine is characterize~ by the blade being connected to the hub in such manner that pivoting movement into and out of the wind by the blade adjusts blade pitch relative to the direction of the wind to a degree sufficient to counteract any tendency of the turbine to precess away from an optimwm yaw heading thereby minimizing yaw imbalance of the hub due to the influence of the vertical wind velocity gradients on the blade.
In accordance with the present invention a wind turbine is provided with passive means for trimming the wind turbine in yaw, that is, for maintaining the orient-ation of the wind turbine generally into the wind. Such yaw trimming means comprise a mounting of the blades at root portions thereof, to the hub such that teetering of the blades into and out of the wind under the influence of vertical wind velocity gradients effects an adjustment in blade pitch relative to wind direction. Such pitch adjustment decreases the lift on those blades experiencing higher wind velocities and angle of attack while increas-ing lift on those blades experiencing lower wind velocity and angle of attack due to the gradient. This equalizing of lift over the turbine blades minimizes`any horizontal hub precession or yaw imbalance resulting from such teeter-ing thereby assuring maintenance of correct trimming of the turbine with the wind.
Further, in accordance with the invention, such a pitch adjustment is attained by pivotally mounting the blade to the hub such that the blade pivots, under the . :, - 3a - 1~5~2 influence of the velocity gradient about an axis oblique to the longitudinal axis of the blade. In an alternate embodiment the pitch adjustment is attained by a mount-ing of the blades such that the blades are pivotable about their longitudinal axes and linking the blades at outer portions thereof to the hub or end portion of the main turbine shaft so that flapping or teetering of the blade effects a desired pivotal displacement of the blade about its longitudinal axis to attain the pitch setting required to minimize horizontal hub precession or yaw imbalance.
The foregoing, and other features and advantages of the present invention, will become more apparent in the light of the following description and accompanying drawing.
Fig. 1 is a front elevation view of the wind turbine of the present invention:

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Fig. 2 is an enlarged, partially sectioned, isometric view of the interior of the turbine hub, portions of the hub being broken away to show details of construction;
Fig. 3 is a side elevation view of the wind turbine of the present invention;
Fig. 4 is a top plan view of the wind turbine of the present invention;
Fig. 5 is a sectional view of the upper blade illus-trated in Fig. 3, this view being taken along line 5-5 of Fig. 3 and illustrating the lift and drag forces on that blade;
Fig. 6 is a sectional view of the lower blade illus-trated in Fig. 3 taken along line 6-6 of Fig. 3 and illustrates the lift and drag forces acting on that blade;
Fig. 7 is a top plan view of a prior art hinged wind turbine construction illustrating a cocking of the net wind thrust vector from the axis of rotation of the hub in response to flapping or teetering of the blades;
Fig. 8 is a view similar to Fig. 7, but illustrating the yaw misalignment of the prior art wind turbine with the wind direction due to the angular offset or cocking of the thrust vector with respect to the yaw axis;
Fig. 9 is a graphical representation of the relation-ship between yaw acceleration and yaw angle for a pair of typical large wind turbines constructed in accordance with the prior art as shown in Figs. 7 and 8, one of the tur-bines being provided with a teetered connection between the blades and hub and the other provided with a rigid con-nection, and Fig. 9 also graphically illustrating such a relationship for a large wind turbine constructed in accordance with the present invention;
Fig. 10 is a graphical representation of the rela- -tionships between power ratio and yaw angle and between thrust ratio and yaw angle-for a typical large wind tur-bine such as that of the present invention; and Fig. ll is a view similar to Fig. 2, but illustratingan alternate embodiment of the present invention.

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Best Mode for Carrying Out the Invention Referring to Figs. 1-4, the yaw stabilized wind tur-bine of the present invention comprises a pair of airfoil blades 6 and 7 mounted on a hub 9 and extending forwardly and radially outwardly therefrom. The hub is rotatable about an axis of rotation 12 and connected to the wind turbine load, i.e. an electrical generator or alternator ~not shown) by main shaft 15 (Fig. 2), the axes of rota-tion of the hub and shaft being coincident. The load and any gearing (not shown~ required to step up the rotational speed of shaft 15 to the load are disposed in nacelle 18, in normal operation, the nacelle being disposed immediately up~ind of the blades and hub. However, it will be under-stood that the present invention is not limited to such up~ind nacelle disposition. The nacelle and hub-blade assembly are pivotable about a yaw axis 21 which may coincide with tower or supportins structure 24, which pivotably supports the wind turbine on yaw bearing 27.
As best seen in Figs. l and 4, yaw axis 21 is generally coplanar with Cintersects) axis of hub rotation 12.
Referring to Fig. 2, the hub comprises an end portion of shaft lS which is received within hollow blade root or stub shaft portion 3Q. The blades are connected to the hub by an oblique pintle or hinge pin 33 received within aligned bores in the stub shaft or root portion and shaft 15. As the blades pivot or teeter into and out of the wind about the hinge pin due to vertical wind velocity gradients, the o~lique disposition of the hinge pin effects an adjustment in blade pitch relative to the direction of the wind to equalize the lift on the turbine blades, for a minimization of yaw imbalance.
As is generally known, winds often exhibit vertical velocity gradients. That is, wind speed proximal the sur-face of the earth is typically significantly lower in magnitude than wind speed measured at points distal the earth's surface, i.e. two or three hundred feet therefrom.
Accordingly, assuming the blades are of e~ual pitch, as ~50~2 the blades rotate, at any single point in time the upper-most blade is exposed to winds of higher velocity and angle of attack than is the lowermost blade. Referring to Figs. 5 and 6, upper blade 6 at any axial location thereon defined by radius r measured from the hub axis of rotation, is acted upon by air of a resultant velocity comprising the vector sum of the velocity of the wind at radius r (Vw) and the wind velocity Qr experienced by the blade due only to its own rotation. The resultant defines with the chord of blade 6, an angle of attack ~1 - Likewise, the resultant velocity of the wind acting on blade 7, the lowermost blade, is the vector sum of wind velocity Vw' measured at radius r and the velocity Qr experienced by blade 7 due to its own rotation. This resultant, due to the magnitude of Vw' defines with the chord of blade 7, an angle of attack ~2' substantially less than angle ~1 Since the lift associated with each of the blades 6 and 7 is proportional to the angle of attack, the lift on the uppermost blade is, as illustrated, substantially greater than the lift on the lower blade.
As the blades rotate, each blade periodically assumes upper and lower positions and therefore, where the blades are rigidly mounted to the rotor, the variation in lift acting on each blade as it periodically assumes upper and lower positions causes a periodic bending or "flapping"
of the blade. Such flapping is not only potentially - injurious to the blade, but causes the turbine to ya~ off its proper heading due in part to yaw disturbing moments resulting directly from blade bending, and in part to an angular shift in the vector resultant of thrust acting on the blades.
In the prior art, the periodic bending or flapping is sometimes eliminated by a connection between the blades and hub wherein the blades pivot about an axis generally transverse to the hub or shaft axis of rotation and the longitudinal blade axes without any accompanying cyclic pitch change. With this prior art, "hingedl' or "teetered"

3 1S50~2 construction, the hereinabove noted periodic blade flapping is replaced by a cyclic pivoting of the blades on the hub about the pivot (teeter) axis. Accordingly, as the blades rotate under the influence of the prevailing wind, they will cyclically move into (with) and away from (against) the wind by cyclic teetering on hinge pin 33.
This teetering about the hinge pin as the blades rotate, causes by precession, a pivoting of the hub and blades about the teeter axis that is of greatest magnitude when the teeter axis is oriented vertically. While the magnitude of this precessional teetering will depend upon the wind speed, wind gradient, blade configuration and other aspects of turbine design and operating conditions, such a precessional teetering angularly displaces the hub and blades a degree or two from the ~ind direction. Refer-ring to Fig. 7, the angular displacement of the hub and blades out of alignment with the wind affects a similar cocking or angular displacement of the vector resultant of the net thrust acting on the blades, this thrust vector being defined as extending normal to a line intersecting the blade tips. The cocking of the thrust vector angularly displaces the vector from colinear orientation with the yaw axis. Therefore, the offset thrust vector applies a yaw moment to the turbine resulting in an exaggerated yaw displacement from the desired wind direction as shown in Fig. 8.
Referring to Fig. 9, the effects of the offset of the thrust vector coupled with the resulting hub yaw dis-placement are shown for typical hinged (teetered blade connection) and hingeless (rigid blade connection) large wind turbine rotors at a wind speed of 25 meters per second. As shown in these curves, both prior art hinged and hingeless wind turbine rotors allowed to freely pivot about a yaw axis will displace themselves in yaw signifi-cantly from the desired 0 heading (in-flow angle)~ Thus, the hinged rotor if started at 0 in-flow angle will yaw off heading approximately 15 while the hingeless rotor :, 5(~2 if set at 0 could yaw -33, -22, or approximately 55 off the des~red heading before reaching equilibrium headings (zero yawing acceleration). Both turbines are stabilized in yaw at these offset yaw headings due to a balancing of the thrust moment by aerodynamic forces on the blades.
As illustrated in Fig. lQ, both thrust and power ratios are optimized by maintaining the heading of the turbine substantially dire~tly into the wind. The power ratio is a measure of the output power of the turbine divided by the available power of the wind stream inter-cepted by the turbine and the thrust ratio is a measure of the thrust on the turbine blades divided by the net available thrust from the column of wind intercepted by the turbine blades. Accordingly, as shown in Fig. 10, any substantial displacement from the desired ao yaw angle heading will severely detract from the power generating capabilities of the turbine.
To overcome the deficiencies in yaw stabilization associated with prior art wind turbines, in the present invention, the blades are p~votally mounted about the root portions thereof in such manner that pivoting move-ment or teetering of the blades about hinge pin 33 under the influence of a vertical wind velocity gradient effects a cyclic adjustment of the pitch of the blades relative to the wind direction~ Thus, where the blades are verti-cally oriented as shown in Fig. 3, the upper blade 6 will pivot or teeter with the wind about hinge pin 33 whereby the leading edge of the blade is turned into the wind to - reduce the lift on that blade. Similarly, lower blade 7 will teeter into (against~ the wind turning the leading edge of that blade slightly away from the wind thereby increasing the lift on the lower blade to a value corres-ponding generally to that on the upper blade. Thus, the lift on both blades is essentially equalized thereby minimizing the horizontal precession of the rotor out of alignment with the wind, ~1550~Z
g The amount of pitch adjustment effected by a particu-]ar amplitude of teetering will, of course, depend upon the angle which hinge pin 33 makes with the longitudinal axes ' of the blades. The value of this angle depends upon the prevailing wind conditions at the site of the turbine and the geometry of the turbine itself. However, it has been found that angular offsets of hinge pin 33 from the blade axes of 40-70 have proven satisfactory for use with large turbines, i.e. those having a blade span in the neighbor-hood of 200 or more feet.
Referring again to Fig. 9, the performance of a yawstabilized turbine in accordance with the present invention is shown in the uppermost plot of yaw acceleration vs.
in-flow (yaw) angle. As this curve shows, the yaw acceleration is zero at a zero in-flow angle (orientation of the turbine substantially directly into the wind~. Thus, when set at such a heading by mechanical means or a weather vane effect, the wind turbine of the present inven-tion w,ill maintain that heading for optimum power output.
Referring to Fig. 11, an alternate embodiment of the present invention is shown. In this embodiment, blades 6 and 7 are supported on stub shaft 36-such that the blades may pivot about their longitudinal axes. Accordingly, the blades will be mounted on suitable bearings ~not shown) disposed between the blades and the stub shaft. The bl~de also pivots into and out of the wind about an axis 39 generally transverse t,o the blade and shaft axes. As shown in Fig. 11, axis 39 is defined by a hinge pin 42 extending through stub shaft 36 and main shaft 15. The blade is 3Q linked at an outer portion thereof to shaft 15 by a link 45 pivotally connected at one end thereof to the blade at clevis or mount 48 and at the other end thereof to the main shaft at mount 51. Should a wind turbine having the hub configuration of Fig. 11 encounter vertical wind velocity gradients, the blades will initially teeter about axis 3~ in the manner described hereinabove. However, ~so~z such teetering~ due to the connection the blade with the shaft by link 45, causes the blades to pivot about their own longitudinal axes thereby effecting an adjustment in blade pitch to equalize lift across the span of the blades in the manner described hereinabove.
While the wind turbine of the present invention has been described in connection with turbines having two blades, it will be appreciated that this invention may be employed with turbines having any number of hinged blades.
l~ Where qreater than two blades are employed, the blades will be connected to the hub by an arrangement of gimballed bearings rather than a single hinge. Accordingly, it will be appreciated that although the invention has been shown and described with respect to detailed embodiments thereof, various changes and omissions in form and detail may be made therein without departing from the spirit and the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A wind turbine comprising:
a rotatable hub and at least one airfoil blade mounted at a root portion thereof on said rotatable hub and pivotally mov-able about said root portion into and out of the wind in response to vertical wind velocity gradients acting on said blade said wind turbine being characterized by said blade being connected to said hub in such manner that pivoting movement into and out of said wind by said blade adjusts blade pitch relative to the direction of said wind to a degree sufficient to counteract any tendency of the turbine to precess away from an optimum yaw heading thereby minimizing yaw imbalance of said hub due to the influence of said vertical wind velocity gradients on said blade.

2. A wind turbine according to claim 1 wherein said blade pivots into and out of said wind about an axis oblique to the longitudinal axis of said blade.

3. A wind turbine according to claim 2 wherein said oblique axis is offset from the longitudinal axis of said blade by an angle of from 40 to 70°.

4. A wind turbine according to claim 2 wherein said hub comprises a main shaft, said blade at said root portion thereof being pivotally connected to said main shaft by a pintle disposed along said pivot axis.

5. A wind turbine according to claim 4 wherein said blade at said root portion thereof receives said main shaft therewithin said pintle extending through aligned bores in said main shaft and root portion.

6. A wind turbine according to claim 1 wherein said blade pivotally moves into and out of said wind about an axis generally transverse to the longitudinal axis of said blade, said blade being pivotable about said longitudinal axis and linked at an outer portion thereof to said hub whereby movement of said blade into and out of said wind effects a pivoting of said blade about the longitudinal axis thereof, to effect said pitch adjustment.

7. A wind turbine according to claim 6 wherein said hub comprises a main shaft received within said blade root portion and connected thereto by a pintle received within aligned bores on said main shaft and root portion and extending along said transverse axis.