Patent Application
20120014794
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.
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.
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.
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.
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.
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 ˜2 to ˜1.3 and in a gust its drag coefficient would increase from ˜0.03 to ˜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 ˜1.8 with drag climbing hugely to ˜0.15
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 ˜10 seconds. Actual wind data suggests these changes are much faster as
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.
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.
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.
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.
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.
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.
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.
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.
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.
Other and further objects will be explained hereinafter and more particularly delineated in the appended claims.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The charm of these reactions is that they can occur quickly, being powered from large wind thrust and momentum power reserves.
Best mode and preferred designs and techniques will now be described.
The present invention can best be understood in conjunction with the accompanying drawings, in which:
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.
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).
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.
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.
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).
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).
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.
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.
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.
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).
The overload springs (38) once again trap the frame up tight against the stepped tube (45).
The same sectioned view is shown as in
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.
The same embodiment as
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0904921.4 | Mar 2009 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2010/050500 | 3/24/2010 | WO | 00 | 9/23/2011 |
