GB2471060A - Automatic pitch control for horizontal axis wind turbines - Google Patents

Abstract

An automatic blade pitch adjustment system balances the axial thrust loads on a wind turbine. against centripetal forces caused by rotation. If the thrust loads build faster than the centripetal loads the Tip Speed Ratio (TSR) is reduced to increase torque and if the thrust loads reduce faster than the centripetal force the TSR is increased to conserve rotational momentum. Springs (fig 3,33,37) can establish the starting position and the rpm limit. Centripetal force can be exerted by masses 3 that swing on arms, causing a cam (fig 2, 17) to oppose the wind thrust and movement of the turbine along its rotation axis. Centripetal forces can alternatively be generated by radial movement of the blades. The system is self powered and adjusts the blade angles to have a high angle of attack when parked to promote start up: move to their ideal setting angle for normal running; responds to gusts and lulls with appropriate changes, and feathers the blades to limit the rpm and reduce loads in storm conditions.

Description

AUTOMATIC PITCH CONTROL FOR HAWT WIND TURBINES

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

The present invention r&ates 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 sc&e 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.

BACKGROUND OF THE INVENTION

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 smafl 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.

t can be shown that a sma turbine operating at an average wind speed of 6 rn/s would need to adjust its angle of attack by around 7 degrees faced with a gust or of 3 m/s to stay at its optimum performance. Faure to do so in a lu wou'd reduce its coefficient of ift from 2 to 1.3 and in a gust its drag coefficient wou'd increase from 0.03 to 0.075 with on'y a sma'l increase in lift. f the fluctuation was 6 mis, in a uU the turbine wou'd act as a fan and waste its angu'ar momentum acc&erating the air and in a gust the ist coefficient wou'd drop to 1.8 with drag dimbing hugely to 0.15 Large systems use transducer data on wind speed and turbine rpm to instruct eectricaHy powered servos to adjust the b'ade pitch proportionate'y. R is worth noting though that the high b'ade inertia and the relativeiy stow 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% turbu'ence deviation and changes that occur between their imits in 10 seconds. Actual wind data suggests these changes are much faster as in the foowing two graphs that show a typica' pattern over a 10 second period in both norma' and turbu'ent conditions. With energy being proportiona' to the cube of the wind speed, the fluctuations equate to a doub'ing or tripling of energy within a few seconds. They indicate that if pitch change could be effected quick'y enough further power gains are possible.

The compromise se'f powered solution that uses centripeta force to adjust the pitch angie cannot react quick'y as the rotors need time to accelerate. ts effect is to reduce the pitch and hence increase the TSR in high winds. This does not assist significant'y with start up and can on'y 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 b'ades 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 marginay with start up does not provide an overload so'ution.

Most smaU 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

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 acc&eration.

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 aU 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.

SUMMARY OF THE NVENTON

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 so'ution for a conditions.

The basic concept is that axial thrust pushes the turbine back along its shaft causing the pitch to be increased and TSR lowered, white 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 comfortably accelerate, the force imbalance causes the turbine to be displaced backwards and thereby reduces the TSR to avoid staUing 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 eady 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 cou'd 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 acce'erate the wind through the turbine.

S

Masses can be arranged to fly out axiay, 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 roUer 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 shaftth.

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 move this force balance point, and as quick as the wind thrust load changes.

With gusts the wind powered backwards disp'acement of the turbine provides the power to turn the b'ades to a sower TSR enabUng them to acce'erate faster instead of tending towards stafl. With MIs the stored rotational inertia of the turbine pulls it forwards and so reduces 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 w now be described.

DRAWINGS

The present invention can best be understood in conjunction with the accompanying drawings, in which: Fig. 1 Shows two isometric views of the mechanism. The left hand view shows tin 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 b'ades suitable for a high TSR.

Fig. 2 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 imit. The right hand view shows the hub pulled forward, moving the CAM to its anti-clockwise limit.

Fig. 3 Shows a side view section through the centre of the mechanism, shown in its start up condition.

Fig. 4 Shows a similar side view section, but with the mechanism at its maximum rpm condition.

Fig. 5 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.

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 1 The mechanism is show with ts 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 roVer (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.

In FIG 2 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).

In FIG 3 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 proffled 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 rol'er (35) to rise to its top position.

In FIG4 The sectioned view now shows the mechanism in its maximum forward position where the roUer (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 rol'ers 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).

In FIGS 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.

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.

Claims (7)

1. CLAIMSWhat is claimed is: 1. A self powered automatic pitch adjustment system where the axial thrust loads on a turbine are balanced against axiaUy resolved centripetal forces caused by the turbines rotation such that if the thrust loads bud faster than the centripetal loads the Tp Speed Ratio (TSR) is reduced in order to increase turbine torque and if the thrust loads reduce faster than the centripetal loads the TSR is increased in order to conserve rotational momentum.
2. 2. The system of claim 1 where a spring is included to act axially alongside the wind thrust so as to return the turbine to a low TSR parking position in ow wind.
3. 3. A system as in claims 1 and 2 where the action of the centripetal forces is limited by a spring preload device such that above a defined force lev& the spdng compliance enables the turbne to move axially and so proportionately reduce the TSR and thereby effectively limit the maximum rpm.
4. 4. A system as in claims 1,2 or 3 where the centripetal loads are provided by masses swinging out on lever arms causing an axial force reaction in a CAM to oppose the wind thrust force on the turbine.
5. 5. A system as in claims 1, 2 or 3 where the centripetal forces are provided by masses constrained to move out radially with a means to transform the radial displacement into an axial displacement.
6. 6. A system as in claims 1, 2 or 3 where the displacement of the turbine along its axis of rotation causes CAMs attached to the turbine blade axles to be proportionately rotated by rollers otherwise fixed to the axially stiff generator shaft.
7. 7. A system as in daims 1, 2 or 3 where the centripeta forces are generated by aowing the b'ades to move out radiay and where such motion is transformed nto an axia' force to oppose the thrust force.