U.S. patent application number 13/260197 was filed with the patent office on 2012-01-19 for automatic pitch control for horizontal axis wind turbines.
This patent application is currently assigned to kinetic harvest limited. Invention is credited to Ralph-Peter S. Bailey.
Application Number | 20120014794 13/260197 |
Document ID | / |
Family ID | 40639967 |
Filed Date | 2012-01-19 |
United States Patent
Application |
20120014794 |
Kind Code |
A1 |
Bailey; Ralph-Peter S. |
January 19, 2012 |
AUTOMATIC PITCH CONTROL FOR HORIZONTAL AXIS WIND TURBINES
Abstract
A self setting and self powered system for adjusting the blade
angles of a wind turbine such that they have a high angle of attack
when parked to promote early start up, move to their ideal setting
angle for normal running, can respond to gusts and lulls with
appropriate changes, and feathers the blades to limit the rpm and
reduce loads in storm conditions. The system balances the thrust
loads on the turbine against centripetal forces generated by masses
reacting to the rpm, increasing the blade pitch with gusts to
increase torque and hence rotational acceleration and reducing
blade pitch in lulls to conserve rotational momentum, and uses
springs to establish the starting position and the rpm limit
appropriate for the generator mechanism.
Inventors: |
Bailey; Ralph-Peter S.;
(Steyning, GB) |
Assignee: |
kinetic harvest limited
Wiston, Steyning, Sussex
GB
|
Family ID: |
40639967 |
Appl. No.: |
13/260197 |
Filed: |
March 24, 2010 |
PCT Filed: |
March 24, 2010 |
PCT NO: |
PCT/GB2010/050500 |
371 Date: |
September 23, 2011 |
Current U.S.
Class: |
416/52 |
Current CPC
Class: |
F03D 7/0224 20130101;
Y02E 10/72 20130101; Y02E 10/723 20130101; F05B 2260/77
20130101 |
Class at
Publication: |
416/52 |
International
Class: |
F01D 7/02 20060101
F01D007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2009 |
GB |
0904921.4 |
Claims
1. A self powered automatic pitch adjustment system for a turbine
that includes a shaft and at least one blade characterized by a
blade pitch angle, the system comprising: a mechanism for adjusting
said blade pitch angle, said mechanism coupled with the shaft; an
element for generating a centripetal force when the turbine rotates
and for resolving said force axially along the shaft, wherein an
axial wind force on the turbine is balanced against the axially
resolved centripetal force, said element coupled with said
mechanism, such that when the wind force builds faster than the
centripetal force the blade pitch angle is increased and when the
wind force reduces faster than the centripetal force the pitch
blade angle is reduced.
2. The system of claim 1, wherein the mechanism comprises a hub
holding the at least one blade, wherein the hub is displaced alone
the shaft in a rearward direction in response to an increasing wind
force but is displaced along the shaft in a forward direction in
response to an increasing centripetal force so that it results in a
displacement that determines the blade pitch angle.
3. The system of claim 1, wherein the mechanism further comprises a
spring force member adapted to act along the shaft to augment the
wind force so as to return the at least one blades to a high blade
pitch angle when both wind force and speed of rotation of the
turbine are low.
4. The system of claim 2, wherein the mechanism further comprises a
preload device adapted to enable the hub to be displaceable along
the shaft in said rearward direction, when a wind force is above a
defined level, irrespective of the centripetal force.
5. The system of claim 2, wherein: the element for generating a
centripetal force comprises one or more masses mounted on lever
arms to swing radially outward, the mechanism comprises means for
transforming radial movement of the masses into movement in said
forward and rearward directions.
6. The system of claim 1 wherein the centripetal force is generated
by a plurality of masses constrained to move out radially from the
shaft with a means to transform the radial displacement into an
axial displacement along the shaft.
7. The system of claim 2, wherein the mechanism further comprises:
a cam coupled with the at least one blade; and one or more rollers
fixed to the shaft and coupled with the cam; wherein displacement
of the hub causes said cam to be proportionately rotated by said
one or more rollers.
8. The system of claim 1, wherein the centripetal forces are
generated by allowing the blades to move out radially and where
such motion is translated into an axial force to oppose the wind
force.
9. The system of claim 2, wherein the at least one blade comprises
an axle, and wherein the mechanism further comprises a pulley on
said axle and a belt attached to the hub and running around said
pulley so that the net displacement of the hub causes the belt to
rotate the at least one blade about the axles proportionally to
said net displacement.
10. The system of claim 2, wherein the at least one blade comprises
an axle, and wherein the mechanism further comprises a crank
attached to said axle and a push rods running from a fixed point on
the hub to said cranks so that the net displacement of the hub
causes the push rods to rotate the blades about the axles
proportionally to said net displacement.
11. The system of claim 5, wherein the means for transforming
includes a belt or a chain attached to the hub.
12. The system of claim 5, wherein the means for transforming
includes: a cam attached to the hub, the cam having a profile; and
a roller attached to the element for generating a centripetal force
and adapted to act against the profile when the element undergoes
said radial movement.
13. The system of claim 5, wherein the means for transforming has
at least one characteristic that is selected to optimize a balance
between the axial wind force and the axially resolved centripetal
force.
14. The system of claim 9, wherein at least one end of the belt is
attached to the hub via a spring force member.
15. The system of claim 14, wherein the spring force member is
adapted to act along the shaft to augment the wind force so as to
return the at least one blade to a high blade pitch angle when both
wind force and speed of rotation of the turbine are low.
16. A wind turbine comprising: a shaft; a hub rotatably mounted on
the shaft and moveable along the shaft; at least two blades mounted
on the hub, wherein a wind force on the blades causes the blades to
rotate about the shaft and to be forced in a first direction along
the shaft; a centripetal element rotatable with the blades around
the shaft, generating a centripetal force towards the shaft; a
coupling mechanism for balancing the centripetal force with a force
on the hub in a second direction along the shaft opposite to the
first direction; and a mechanism for adjusting a pitch angle for
each blade in response to movement of the hub in the first and
second directions.
17. The turbine of claim 16, wherein the mechanism adjusts the
pitch angle to a relatively steep pitch angle with movement in the
first direction and a relatively shallow pitch angle with movement
in the second direction.
18. The turbine of claim 16, wherein the centripetal element
comprises a plurality of centripetal masses and wherein the
coupling mechanism comprises lever arms that on which the
centripetal masses are constrained to move out radially from the
shaft whereby the lever arms transform radial displacement into
axial displacement.
Description
[0001] The present application pertains to a novel means by which
the blade pitch angle of a wind turbine can be automatically set
for optimum power output in changing wind conditions.
FIELD OF THE INVENTION
[0002] The present invention relates to a mechanism that can be
employed by Horizontal Axis Wind Turbines (HAWT) to set the optimum
blade pitch angle for the differing conditions they are likely to
encounter in order to optimize their power yield.
[0003] Whereas large turbines can justify complex servo driven
pitch adjustment systems supplied with wind condition data from
sensors, these are costly and power hungry. Small scale turbines
consequently primarily use simple fixed pitch systems.
[0004] There have been proposed a number of compromise solutions
that are self powered from the centripetal force on the turbine
blades, but these are often poor compromises, not adequately
optimizing the start up condition and the overload condition. Also
none of them can react quickly to best utilize the transient
changes in wind speed such as gusts and lulls.
BACKGROUND OF THE INVENTION
[0005] All large scale turbines currently use a variable pitch
adjustment system to set the best blade angle of attack to match
the prevailing wind conditions. This ideal setting angle for
optimum lift/drag performance of the selected aerofoil is added to
the angle defined by the Tip Speed Ratio (TSR) which is the blade
tip speed divided by the wind speed. Studies published by Mutschler
and Hoffman titled "Comparison of Wind Turbines Regarding their
Energy Generation" indicate that variable pitch can deliver around
20% more power than fixed pitch systems and up to 38% for small
high TSR systems. Gains are most pronounced where the average wind
speed is quite low and turbulence high--as with smaller turbines on
lower towers.
[0006] It can be shown that a small turbine operating at an average
wind speed of 6 m/s would need to adjust its angle of attack by
around 7 degrees faced with a gust or lull of 3 m/s to stay at its
optimum performance. Failure to do so in a lull would reduce its
coefficient of lift from .about.2 to .about.1.3 and in a gust its
drag coefficient would increase from .about.0.03 to .about.0.075
with only a small increase in lift. If the fluctuation was 6 m/s,
in a lull the turbine would act as a fan and waste its angular
momentum accelerating the air and in a gust the list coefficient
would drop to .about.1.8 with drag climbing hugely to
.about.0.15
[0007] Large systems use transducer data on wind speed and turbine
rpm to instruct electrically powered servos to adjust the blade
pitch proportionately. It is worth noting though that the high
blade inertia and the relatively slow speed of servos does not
enable them to optimize on as quick a wind speed variation as may
be experienced. Mutschler uses a simulation with an average 20%
turbulence deviation and changes that occur between their limits in
.about.10 seconds. Actual wind data suggests these changes are much
faster as FIGS. 1 and 2 that show a typical pattern over a 10
second period in both normal and turbulent conditions. With energy
being proportional to the cube of the wind speed, the fluctuations
equate to a doubling or tripling of energy within a few seconds.
They indicate that if pitch change could be effected quickly enough
further power gains are possible.
[0008] The compromise self powered solution that uses centripetal
force to adjust the pitch angle cannot react quickly as the rotors
need time to accelerate. Its effect is to reduce the pitch and
hence increase the TSR in high winds. This does not assist
significantly with start up and can only react to overload by
putting the blades into a stall which while reducing rpm does so at
the cost of high axial thrust loads and so puts extra strain on the
tower.
[0009] Various other means have been proposed such as allowing the
blades to pivot at a skew angle such that when gusts blow them back
they increase the blades' angle of attack. This response is
difficult to gear to optimum setting levels and while helping
marginally with start up does not provide an overload solution.
[0010] Most small systems turn away from excess wind by using a
rudder to skew themselves sideways where they become deliberately
inefficient and turbulent, or have other means to spill the excess
power. Although the generator may be protected from spinning too
fast, the windmill structure still has to bare the increased wind
load. The result of overload protection is often a reduction in
power output just when the energy density is the greatest.
OBJECTS OF THE INVENTION
[0011] A principal object of this invention, therefore, is to
provide a means to automatically set the blade pitch to a low TSR
in order to deliver maximum torque at zero rotational speed for
early turbine start up.
[0012] A further object of this invention is to increase the TSR as
the turbine accelerates and hold it stable at the ideal continuous
running condition.
[0013] A further object of this invention is to quickly react to
gusts or transient increases in wind speed by reducing the TSR to
increase the torque and hence rate of rotational acceleration.
[0014] A further object of this invention is to quickly react to
lulls or transient decreases in wind speed by increasing the TSR to
reduce the torque and hence maintain rotational momentum.
[0015] A further object of this invention is to react to excessive
wind speed in order not to exceed the safe rpm limit by reducing
the TSR in these conditions--akin to feathering the blades where
they generate less drag.
[0016] A further object of this invention is to provide for all of
the above with a self powered and relatively simple mechanism that
can be built at low cost.
[0017] Other and further objects will be explained hereinafter and
more particularly delineated in the appended claims.
SUMMARY OF THE INVENTION
[0018] In summary, the invention proposes to utilize both the axial
thrust on the rotor and the centripetal forces related to its
rotational velocity to achieve an optimum solution for all
conditions.
[0019] The basic concept is that axial thrust pushes the turbine
back along its shaft causing the pitch to be increased and TSR
lowered, while rpm related centripetal force pulls the turbine
forward causing the TSR to be increased.
[0020] The result of this in gusty conditions is that when the wind
speed increases faster than the turbine can accelerate, the force
imbalance causes the turbine to be displaced backwards and thereby
reduces the TSR to avoid stalling and so to better utilize the
available power as increased torque.
[0021] Equally when there is a lull, the imbalance moves the
turbine forward increasing the TSR and thereby reducing the torque
and hence helping it conserve its rotational momentum.
[0022] To cause the turbine to move to a low TSR for early start up
in low wind, a spring is provided to push the turbine back,
effectively cooperating with any wind.
[0023] To cause the turbine to move towards a low TSR in order to
limit the rpm in overload conditions and track the maximum safe
power even as wind continues to increase in speed, a further spring
device is employed. This overload spring acts between the
centripetal forcing element and the turbine and thereby enables the
turbine to move back in high winds irrespective of the amount of
centripetal force being generated. It is set to a preload such that
it begins to permit displacement after the known thrust generated
at the rpm limit is exceeded and at a spring rate appropriate to
maintain that limit as the TSR is proportionately reduced.
[0024] Rather than utilize the blades to generate the balancing
centripetal force, in a preferred embodiment separate masses are
used. The advantage of decoupling the centripetal force from the
blades is that blade forces could become excessively high and the
increase in tip diameter as they move out radially could be a
problem where the blades run in a confined duct as in a system
where a diffuser is used to accelerate the wind through the
turbine.
[0025] Masses can be arranged to fly out axially, perhaps even
surrounding the blade shaft, but a preferred embodiment is for them
to swing up on arms much like what occurs with a speed
governor.
[0026] This action can be arranged to cause a roller on the mass
lever to act against a cam whose profile presents a local ramp
angle which gears the amount of axial balancing force generated to
the optimum for any given rpm.
[0027] The scheme requires that the blade shaft is rotated in
proportion to the axial displacement of the hub. A preferred means
of achieving this is by employing two rollers attached to the fixed
generator shaft acting on double sided cams attached to the blade
shafts. The cam angle progression can again be selected to vary the
gearing so the relationship need not be linear. As the turbine is
displaced, the cams are obliged to rotate as the rollers are in a
fixed position with respect to the generator shaft.
[0028] A further embodiment uses a belt firstly attached to the
fixed part of the hub, then wrapped around the blade axles attached
to the axially displaceable part of the hub and then returned to
the fixed part via a spring. As the blade set is displaced axially,
the belts cause the blades to rotate on their shafts with the slack
taken up by the spring or belt lengthened by the spring.
[0029] In this embodiment the springs act as a preload device in
not just maintaining belt tension, but also in trying to bull the
belt in they move the blades set such that it increases their pitch
angle and becomes more feathered as would be appropriate in a start
up condition. In doing so it also holds the centripetal masses in
their closed position.
[0030] After start up, it is desirable for the TSR to rapidly
increase to its best running mode, and yet the start up spring
holds in place the masses that generate the centripetal force
necessary to cause this. The centripetal forces must therefore
increase quicker than the thrust forces in order for them to become
dominant and start to move the hub forwards. The displacement will
continue as the turbine accelerates until the centripetal forces
are once again balanced by the growing thrust loads augmented by
the spring.
[0031] The balance point is reached at the point where the turbine
torque has decreased with increasing TSR such that it can no longer
accelerate. This is helped by the start up spring which provides
its axial force in proportion to the axial turbine displacement
that sets the TSR. As such, the balance point reached tends to
increase it's TSR a little as the wind speed increases (and the
spring has a relatively smaller forcing effect). This is helpful in
providing more torque at lower speed to better match the
characteristics of the attached generator.
[0032] To tailor this balance point to the ideal TSR the
centripetal forces are geared by a varying ramp angle on the CAM
against which they act as previously explained.
[0033] It can now been seen that wind speed fluctuations will act
to move this force balance point, and as quick as the wind thrust
load changes.
[0034] With gusts the wind thrust displaces the turbine backwards
and thereby provides the power to turn the blades to a lower TSR
enabling them to accelerate faster instead of tending towards
stall. With lulls the centripetal masses using the stored
rotational inertia of the turbine now have less balancing wind
thrust so pull the hub forwards and so reduce the blade pitch and
hence the torque with a higher TSR instead of tending towards
turning into a fan where rotational momentum is quickly lost.
[0035] The charm of these reactions is that they can occur quickly,
being powered from large wind thrust and momentum power
reserves.
[0036] Best mode and preferred designs and techniques will now be
described.
DRAWINGS
[0037] The present invention can best be understood in conjunction
with the accompanying drawings, in which:
[0038] FIG. 1 shows a first data illustration;
[0039] FIG. 2 shows a second data illustration;
[0040] FIG. 3 shows a force balance relationship;
[0041] FIG. 4 shows a tip speed ratio relationship;
[0042] FIG. 5 Shows two isometric views of the mechanism. The left
hand view shows it in start up condition where the blades are set
to provide maximum torque. The right hand view shows it in maximum
rpm condition where the weights have swung out, pulling the hub
forward and setting the blades suitable for a high TSR.
[0043] FIG. 6 Shows two top views with the upper blade system
sectioned through its driving CAM. The left hand view shows it in
start up condition with the blade CAM at its backwards limit and
where it's offset between the CAM rollers has caused it to rotate
to its clockwise limit. The right hand view shows the hub pulled
forward, moving the CAM to its anti-clockwise limit.
[0044] FIG. 7 Shows a side view section through the centre of the
mechanism, shown in its start up condition.
[0045] FIG. 8 Shows a similar side view section, but with the
mechanism at its maximum rpm condition.
[0046] FIG. 9 Shows a similar side view section, but in its
overload condition where the overload springs have compressed and
allowed the hub to move back despite the high centripetal force
opposing it.
[0047] FIG. 10 Shows the embodiment where a belt is used to rotate
the blades axles. In this view the belts have been pulled back by
their springs causing the blades to move to a high pitch feathered
position.
[0048] FIG. 11 Shows the same embodiment as FIG. 10 but now in a
running mode at a higher rpm where the centripetal masses have
swung out and pulled the blade set forward, thereby stretching the
springs that hold the belts and consequently causing the belts to
rotate the blade's axles.
[0049] In the drawings, preferred embodiments of the invention are
illustrated by way of example, it being expressly understood that
the description and drawings are only for the purpose of
illustration and preferred designs, and are not intended as a
definition of the limits of the invention.
PREFERRED EMBODIMENT OF THE INVENTION
In FIG. 5
[0050] The mechanism is show with its frame (1) comprising two
profile cut sheets with lengths of tubing welded in a radial
pattern to support the blade axes. This embodiment supports five
blades, but it can be seem that other numbers are possible. The hub
is pushed to the back of its shaft by the spring (2), holding in
the centripetal masses (3) that swing on arms to drive the roller
(4) against the CAM (5) to progressively displace the hub forward
as the weights swing out as shown by (12).
[0051] CAM roller supports (10) retain rollers (6) which act on the
blade setting CAMs (7). It can be seen that the blades (8) rotate
on their axes into position (11) as the weights force the hub
forwards.
In FIG. 6
[0052] The sectioned mechanism in the left hand view shows the back
flange (14) that supports the CAM roller retainers (19) with their
rollers (20) and (22) acting on the CAM (17) to set its degree of
rotation about the blade axis. The roller (23) acts against the
narrow wedge plate (24) and transmits the drive torque from the
frame (13) to the flange (14). This device enables it to be
adjusted to a degree of bearing preload in the rollers consistent
with stiff control. The masses (15) are in their park position.
[0053] In the right hand view the masses (16) have swing out
pulling the hub and frame (25) forwards away from the flange (27)
causing the CAM (18) to rotate the blades from their start up
position (25) to their high rpm position (26).
In FIG. 7
[0054] The sectioned view shows the flange (28) rigidly connected
to the generator shaft and the shaft extension. Connected to the
flange are the CAM roller retainers (32) holding the rollers e.g.
(34) against the CAM (38).
[0055] The frame (30) includes a bore that permits it to slide
along the shaft as required to achieve the force balance axial
offset position. The frame is held up tight against a further
stepped tube (29) by high force springs (33) riding on a radial
array of shafts with end stops (31) trapping one of the frame's
profiled plates between it and the springs. As will be seen this
constitutes the overload preload device.
[0056] The weight is show fully retracted by the action of the
spring (37) displacing the CAM (36) and frame (29 & 30) as far
back as possible causing the roller (35) to rise to its top
position.
In FIG. 8
[0057] The sectioned view now shows the mechanism in its maximum
forward position where the roller (46) in swinging upwards has
pulled the CAM (48) forward, dragging the stepped tube (45) and
frame (39) with it--as may be found at the rpm limit. The start up
preload spring (47) is now fully compressed.
[0058] The blade rotation has been set for maximum rpm as the CAM
(42) has been rotated by the action of the rollers e.g. (40)
retained by the part (43) against the flange that is part of the
shaft extension (44).
[0059] The overload springs (38) once again trap the frame up tight
against the stepped tube (45).
In FIG. 9
[0060] The same sectioned view is shown as in FIG. 4, but now the
wind thrust has overwhelmed the overload springs (38) such that the
frame (39) can slide back with respect to the stepped tube (45)
which retains the centripetal CAM (48). In so doing they can change
the blade angle such that the rpm limit is not exceeded. In
hurricane force conditions this may be as far back as the start up
position where the blades will be essentially feathered for minimum
drag.
In FIG. 10
[0061] The embodiment where belts e.g. (49) are used acting around
the pulleys e.g. (51) tensioned by springs e.g. (50) such that in
this case the springs pull in the axially displaceable part of the
hub that holds the blades (57) which in turn causes the centripetal
masses (55) to be held in their closed position.
In FIG. 11
[0062] The same embodiment as FIG. 6 is shown, but now the blade
set (58) has been forced forward by the centripetal masses (56)
swinging out on their lever arms as a result of the hub's rotation.
The springs e.g. (52) have now been stretched effectively
lengthening the belts e.g. (53) which rotate the pulleys e.g. (54)
and consequently adjust the blade pitch angle in this case reducing
it.
[0063] Further modifications of the invention will also occur to
persons skilled in the art, and all such are deemed to fall within
the spirit and scope of the invention as defined by the appended
claims.
* * * * *