WO2009130500A2

WO2009130500A2 - Energy output limiter for wind turbine rotor(s)

Info

Publication number: WO2009130500A2
Application number: PCT/GB2009/050398
Authority: WO
Inventors: Barry Robert Marshall
Original assignee: Brm Power Limited
Priority date: 2008-04-21
Filing date: 2009-04-21
Publication date: 2009-10-29
Also published as: GB2459453B; WO2009130500A3; GB0807297D0; GB2459453A

Abstract

A horizontal axis wind turbine blade configuration is provided which is adapted to dissipate the energy which would otherwise exceed the capacity of the energy conversion system, when the wind speed and/or the wind turbine rotational speed exceed(s) levels established by the particular design. Aerofoil section(s) (A) connected to a HAWT rotor hub dissipate some of, and under severe operating conditions can dissipate virtually all of, the energy that the conventional wind turbine blade(s) or blade section(s) extract from the wind. The aerofoil Section(s) (A) are configured to use aerodynamic lift in a similar manner to a propeller or lift rotor as the principle energy dissipation mechanism when the rotor speed exceeds a certain value for a particular pertaining wind speed, that rotor speed being an outcome of the overall design.

Description

Energy Output Limiter for Wind Turbine Rotor(s)

The present invention relates to an energy output limiter for wind turbine rotor(s).

For a horizontal axis wind turbine to extract the maximum amount of energy from the wind the (line of) axis of rotation of the wind turbine rotor must be aligned with the direction of the remote (undisturbed) wind. The rotor blades experience a 'relative wind' which has a different velocity from the remote wind.

The relative wind results from the interactions between the remote (undisturbed) wind, the resistance of the rotor to the air flow and the rotating blade(s).

Figure 1 is a Blade Element Momentum (BEM) vector diagram showing the wind and turbine blade vectors, and the resulting force vectors, for a wind turbine blade element in normal operation. The following is a key to the symbols used in Figure 1 :

u Remote (undisturbed) wind; Vector.

Ω r Rotor blade element; Vector. p Rotor plane; Line of. a Rotor axis; Line of. z Zero lift; Line of.

W Local wind; Vector.

E Effective rotational (speed); Vector.

R Relative wind; Vector. L Lift force; Vector.

D Drag force: Vector.

Q Torque force Vector.

T Thrust Force vector. θ Pitch Angle. α Angle of attack.

C Conventional Wind Turbine Blade element.

The relative wind (R) acting on the blade aerofoil (C) creates lift (L) and drag (D) forces which result in torque (Q) and thrust (T) forces on the blade and thus the rotor. The torque and thrust forces act in the plane of rotation of the rotor (p) and normal to the plane of rotation respectively. The torque force coupled with the rotation of the rotor results in the rotor producing power which is delivered to an external application by an energy conversion system.

Figure 2, in comparison with Figure 1 , is a BEM vector diagram showing the changes in the relative wind vector that occur when the energy conversion system has passed the limit of its capacity in a rising wind speed. The increasing availability of energy in the wind leads to an increase in energy extraction by the rotor blades, but as the energy conversion system has passed its limit a disproportionate increase in the rotational speed occurs. A dangerous potential for uncontrolled overspeed of the rotor can develop with only moderate increases in the remote wind speed.

Except in the case of extremely small turbines, conventional horizontal axis wind turbines require functionality which limits the rotor speed and/or energy output from the rotor to the energy conversion system. The following are frequently used systems, and these would be well known to those skilled in the art:

I. Rotor blade pitch control ii. Load induced stall control iii. Blade tip drag devices iv. Yaw control

V. Furling vi. Rotor brakes

It is desirable to provide an alternative solution to the above-identified problem of rotor overspeed.

According to a first aspect of the present invention there is provided a rotor arrangement for a horizontal axis wind turbine, the rotor arrangement comprising a main aerofoil adapted and arranged to generate aerodynamic lift to extract energy from oncoming wind and to convert that energy into rotational energy, and an auxiliary aerofoil arranged to be driven at least to some extent by the main aerofoil and adapted and arranged to generate aerodynamic lift in the manner of a propeller so as to dissipate some or all of the energy extracted by the main aerofoil as a measure of the rotational speed of the main aerofoil speed rises. The presence of the auxiliary aerofoil is intended to assist at least to some extent in the prevention of overspeed of the main aerofoil.

The auxiliary aerofoil may be arranged to dissipate some or all of the energy extracted by the main aerofoil by generating a counter-rotational lift component.

The measure may be a ratio of the speed of the tip of a blade comprising the main aerofoil to the oncoming wind speed. This is otherwise known as the tip speed ratio.

The auxiliary aerofoil may be arranged in a first range of the measure to generate aerodynamic lift to extract energy from the oncoming wind and to convert that energy into additional rotational energy to drive the main aerofoil, and in a second, higher, range of the measure to generate the aerodynamic lift in the manner of a propeller so as to dissipate energy.

The lower end of the second range may be below a normal operating level for the measure. Beyond that level for the measure, it may be that the effectiveness of the main aerofoil to extract energy from the wind will diminish and the effectiveness of the auxiliary aerofoil to dissipate energy will increase, a balance thus developing between the energy dissipation by the auxiliary aerofoil and the energy extraction by the main aerofoil and other miscellaneous forces at work in the system, and the measure reaching a stable level.

The rotor arrangement may comprise a blade formed of both the main aerofoil and the auxiliary aerofoil. With such an arrangement, a profile of the aerofoil pitch angle along the length of the blade may be arranged to exhibit a discontinuity or marked transition between the main aerofoil part and the auxiliary aerofoil part. The discontinuity or transition may be abrupt or gradually implemented.

The main aerofoil and auxiliary aerofoil may be arranged in series along the length of the blade, with the auxiliary aerofoil arranged towards an outer end of the blade and the main aerofoil arranged towards an inner end of the blade.

The blade formed of both the main aerofoil and the auxiliary aerofoil may have a substantially continuous surface. The rotor arrangement may comprise a blade formed of the main aerofoil and another blade formed of the auxiliary aerofoil.

The rotor arrangement may comprise a first rotor having a blade formed of the main aerofoil and a second rotor having a blade formed of the auxiliary aerofoil.

The first rotor may have an axis of rotation different to that for the second rotor.

The first rotor may have an axis of rotation substantially the same as that for the second rotor.

The second rotor may be arranged to influence oncoming wind flowing through first one rotor.

The first and second rotors may be mechanically linked by a speed increasing, speed decreasing or speed neutral linkage.

The auxiliary aerofoil may be arranged selectively to be driven at least to some extent by the main aerofoil. In this way, the auxiliary aerofoil can be linked in or out as required. For example, the auxiliary aerofoil may be operating independently of the main aerofoil until the main aerofoil reaches a point where it is considered desirable to start to introduce a braking influence on the main aerofoil.

The auxiliary aerofoil may act as a safety system component to limit or to assist in the limitation of rotor overspeed, for example for a wind turbine which would normally run at a fixed speed.

The auxiliary aerofoil may be substantially fixed relative to the main aerofoil.

There may be a plurality of such main and auxiliary aerofoils.

According to a second aspect of the present invention there is provided a horizontal axis wind turbine comprising a rotor arrangement according to the first aspect of the present invention. According to a third aspect of the present invention there is provided wind turbine blade(s) incorporating 'Auxiliary Aerofoil Section(s)' configured for the dissipation of some or all of the energy extracted by conventional wind turbine blade(s) or blade section(s) when the wind speed and/or the wind turbine rotational speed rise(s) beyond levels set by the particular design, said Auxiliary Aerofoil Section(s) generating aerodynamic lift in the mode of a propeller which forms the principle method of energy dissipation.

According to a fourth aspect of the present invention there is provided a number of wind turbine blade(s) on a rotating hub, said blade(s) embodying Auxiliary Aerofoil Section(s) according to the third aspect of the present invention, said blades also embodying conventional wind turbine blade section(s) configured to extract energy from the oncoming wind and to convert that energy into rotational energy.

According to a fifth aspect of the present invention there is provided a number of wind turbine blades on a rotating hub, some of said blades embodying Auxiliary Aerofoil Section(s) according to the third aspect of the present invention, some of said blades embodying conventional wind turbine blade(s) or blade section(s) configured to extract energy from the oncoming wind and to convert that energy into rotational energy.

According to a sixth aspect of the present invention there is provided a number of wind turbine blade(s) on a rotating hub, said blade(s) embodying Auxiliary Aerofoil Section(s) according to the third aspect of the present invention, said rotating hub being mechanically linked to other rotor(s), said other rotors embodying conventional wind turbine blade(s) or blade section(s) configured to extract energy from the oncoming wind and to convert that energy into rotational energy.

According to a seventh aspect of the present invention there is provided a rotating hub and wind turbine blade(s) according to the sixth aspect of the present invention, said rotating hub positioned to influence the oncoming wind flowing through other rotor(s) said other rotors embodying conventional wind turbine blade(s) or blade section(s) configured to extract energy from the oncoming wind and to convert that energy into rotational energy.

According to an eighth aspect of the present invention there is provided wind turbine blade(s) on a rotating hub according to the fourth aspect of the present invention, said Auxiliary Aerofoil Section(s) working as a safety system component(s) to limit or to assist in the limitation of rotor overspeed on a wind turbine rotor which normally runs at a fixed speed.

Reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 , discussed hereinbefore, is a BEM vector diagram for a wind turbine blade element in normal operation;

Figure 2, also discussed hereinbefore, is a BEM vector diagram for a wind turbine blade element outside of normal operation;

Figure 3 is a schematic illustration of a horizontal axis wind turbine rotor according to an embodiment of the present invention;

Figure 4 is a BEM vector diagram showing an Auxiliary Aerofoil Section in an embodiment of the present invention operating at a tip speed ratio below a threshold value;

Figure 5 is a BEM vector diagram showing an Auxiliary Aerofoil Section in an embodiment of the present invention operating at a tip speed ratio at the threshold value;

Figure 6 is a BEM vector diagram showing an Auxiliary Aerofoil Section in an embodiment of the present invention operating at a tip speed ratio above the threshold value;

Figure 7 is a graph illustrating how the chord length and pitch angle varies with BEM element number for a horizontal axis wind turbine blade according to one possible implementation of an embodiment of the present invention; and

Figure 8 provides the data upon which the graph of Figure 7 is based.

Figure 3 shows a simple viable configuration for the use of Conventional and Auxiliary Aerofoil Sections on a horizontal axis wind turbine rotor embodying the present invention, with each blade having a Conventional Aerofoil Section C and an Auxiliary Aerofoil Section A.

The use of an aerofoil section connected to a rotating HAWT hub to create lift through the application of torque to the aerofoil with the intention of consuming energy available from the rotating hub is not a feature of known turbines.

An embodiment of the present invention provides a solution to the need for the dissipation of surplus energy extracted from the wind by a wind turbine rotor in a high wind speed. Aerodynamic lift is the principle energy dissipation mechanism.

An embodiment of the present invention requires that there exists a discontinuity in design intent between the Conventional Aerofoil Sections, which are primarily configured to extract energy from the wind, and the Auxiliary Aerofoil Sections, which are configured to dissipate energy extracted from the wind by the Conventional Aerofoil Sections. This is not a feature of known turbines.

An embodiment of the present invention has no fundamental requirement for any form of control or operating mechanism or for changes in use of the geometry or configuration of any part of the blade, although such systems may offer advantages for certain designs applications and sizes of wind turbines. The Auxiliary Aerofoil Sections can be fixed rigidly to a rotor hub, to the ends of conventional wind turbine blades or to passive extensions.

An embodiment of the present invention allows the rotor axis to remain aligned with the direction of the remote (undisturbed) wind and allows continued operation in high wind speeds, without the requirement for the use of one of the energy limiting system of the types listed above.

The above-mentioned discontinuity in design intent can be realized using an abrupt or a gradually implemented discontinuity.

The above-mentioned discontinuity in design intent can be realized by a discontinuity in the angle of attack between the Conventional and the Auxiliary Aerofoil Sections. In an embodiment of the present invention a discontinuity in the angle of attack is a consequence of the above-mentioned discontinuity in design intent. The above-mentioned discontinuity in design intent can be realized by the use of similar angle of attack, lift and other design criteria but a discontinuity in the tip speed ratios used for the design of the Conventional and the Auxiliary Aerofoil Sections. In an embodiment of the present invention a discontinuity in the design tip speed ratio is a consequence of the above-mentioned discontinuity in design intent.

Figure 7 shows the pitch angle and chord profile for a HAWT blade on a three blade rotor designed using BEM to generate approximately 1 kW(e) when operating at a tip speed ratio of approximately 4.5 in a wind speed of 7.5 m/s. The data upon which Figure 7 is based is included in the table of Figure 8. The conventional aerofoil used for this design is cambered with a zero lift angle of attack of -3.5 degrees and a maximum C_L of 1.37 at an angle of attack of 16 degrees. The auxiliary aerofoil is symmetrical with a maximum C_L of 1 .20 at an angle of attack of 16 degrees. This information is included herein to provide an illustration of the potential that an embodiment of the present invention offers. It shall not be taken as a properly evaluated design, and it should be appreciated that it is merely one example configuration of many possible configurations embodying the present invention.

The operating conditions in this description are set to allow the functionality of an embodiment of the present invention to be readily understood. The symmetrical aerofoil section shown in Figures 4, 5 and 6 is chosen merely to provide a clear illustration of the change from positive to negative angle of attack which can occur during operation.

The symbols used in Figures 4, 5 and 6 are the same as those used in Figures 1 and 2.

The description below assumes that other options to limit the energy output from the rotor are not employed. Except where explicitly mentioned, drag effects are ignored.

For a particular, specific, practical wind turbine design configured as shown in Figure 3 the Auxiliary Aerofoil Sections have (as for any aerofoil) three modes of operation dependant on the relative wind. A particular mode may exist for only a small period of time and only at small sections of the blade if the turbine is operating in unsteady wind conditions. BEM Vector Diagram Figure 4 shows the Auxiliary Aerofoil Sections operating at a tip speed ratio below a specific value. They produce lift resulting in rotor torque which operates in the same direction as the torque produced by the Conventional Aerofoil Sections.

BEM Vector Diagram Figure 5 shows the Auxiliary Aerofoil Sections operating at a tip speed ratio at a specific value which results in the angle of attack on the Auxiliary Aerofoil Sections being zero. The Auxiliary Aerofoil Sections produce no lift.

BEM Vector Diagram Figure 6 shows the Auxiliary Aerofoil Sections operating at a tip speed ratio above a specific value which results in the angle of attack on the Auxiliary Aerofoil Sections being negative. They produce lift resulting in rotor torque which operates in the opposite direction to the torque produced by the Conventional Aerofoil Sections.

With the turbine subject to an increasing remote wind speed there will be a point where the energy conversion system reaches the limit of its capacity and application of additional load can no longer be used to control the tip speed ratio. With a rising tip speed ratio the angle of attack on the Conventional Aerofoil Sections will fall, reducing their capability to extract energy from the wind and the angle of attack on the Auxiliary Aerofoil Sections will become more negative, increasing their capability to dissipate energy. A balance will thus develop between the energy dissipation by the Auxiliary Aerofoil Sections, the energy extraction from the wind by the Conventional Aerofoil Sections and the power delivery by the energy conversion system.

Provided mechanical integrity is maintained a turbine with appropriately configured Auxiliary Aerofoil Sections will achieve the balance described above regardless of the level of power delivery from the energy conversion system. It will therefore be possible to operate an appropriately designed and configured turbine with no additional equipment to protect against the effects of load disconnection or energy conversion system failure.

If the tip speed ratio rises to a sufficiently high level in a high wind speed the effects of drag on the outer extremities of the blade will add to the power dissipation performance of the Auxiliary Aerofoil Sections. Considering again the example described above with reference to Figures 7 and 8, the blade design has the conventional aerofoil designed with an angle of attack of 3.5 degrees at a turbine TSR of 6. The auxiliary aerofoil is designed with an angle of attack of zero at a turbine TSR of 4. The TSR which results in the highest power output is a result of the influence of both aerofoil sections and with the turbine generating approximately 1 kW at a TSR of 4.5 the auxiliary aerofoils are already dissipating some of the energy extracted (they are travelling faster than the zero lift design point).

If the design is such that the turbine TSR reaches 6 (the conventional aerofoil design point) before the auxiliary aerofoils start to dissipate energy (i.e. if the zero lift of the auxiliary aerofoils is at a TSR of 6), a balance may not be achieved. The rate of rise of energy (from a high starting point) that the conventional aerofoils are extracting may not be overtaken by the rate of rise of the energy (from zero as a starting point) that the auxiliary aerofoils can dissipate, before the turbine TSR reaches a dangerously high level.

The blade design illustrated in Figures 7 and 8 reaches the point where the conventional and auxiliary aerofoils balance one another, without the generator taking any load, at a TSR of about 6.9.

If it were not for drag effects, at a TSR of 6.9 and a wind speed of 53 m/s the blade tips would be travelling at well above the speed of sound. This doesn't happen in practice as drag starts to drain more of the energy than the conventional aerofoils are extracting as the tips approach sonic velocity. If a turbine embodying the present invention is to operate safely without other devices to limit the rotor speed and/or energy output, the maximum TSR must be constrained to values which do not result in the blade tips closely approaching sonic velocity.

The arrangement shown in Figure 3 is perhaps the simplest configuration by which an embodiment of the present invention can be incorporated into a horizontal axis wind turbine.

The following list of other possible configurations; though physically different all use the same principle to limit the power output of the rotor(s): i. The use of separate blades which function as Auxiliary Aerofoil Sections on the same rotor hub as the Conventional Aerofoil Sections.

ii. The use of a second (Auxiliary Aerofoil Sections) rotor mechanically linked to the first (Conventional Aerofoil Sections) rotor with the same axis of rotation. The second rotor mechanically linked by a speed increasing, speed decreasing or speed neutral linkage.

iii. The use of a second (Auxiliary Aerofoil Sections) rotor mechanically linked to the first (Conventional Aerofoil Sections) rotor with a different axis of rotation. The second rotor mechanically linked by a speed increasing, speed decreasing or speed neutral linkage.

iv. The use of a second (Auxiliary Aerofoil Sections) rotor mechanically linked to the first (Conventional Aerofoil Sections) rotor which not only dissipates excess energy produced by the first rotor but also influences the passage of air through the first rotor. The second rotor mechanically linked by a speed increasing, speed decreasing or speed neutral linkage.

v. The use of Auxiliary Aerofoil Sections on blade(s) of a wind turbine that would normally operate at a fixed speed but which under fault or extreme operating conditions use the energy dissipation capability of the Auxiliary Aerofoil Section(s) to limit or to assist in limiting the rotor speed.

One effect of the Auxiliary Aerofoil Sections acting in a similar manner to a propeller is that the thrust generated opposes the thrust experienced by the Conventional Aerofoil Sections. This reduces the loadings which act in the direction of the remote wind on the rotor and the support structures.

An embodiment of the present invention will allow horizontal axis wind turbine rotors to be manufactured to common designs for use in locations with widely differing wind speed profiles through:

a) The use of longer Conventional Aerofoil Sections to enable greater energy capture from low wind speeds b) The ability to operate in higher wind speeds than would otherwise be possible

without the need for systems that are frequently used for limiting the energy output on wind turbines.

It will be appreciated that the terms "Auxiliary Aerofoil Sections" and "Conventional Aerofoil Sections" are used herein only to differentiate the (auxiliary) aerofoil sections which act to limit the power output of the rotor from the (conventional) aerofoil sections that are, at the same time, extracting power from the wind. No significance relating to the effectiveness, functionality or other properties of the idea shall be attributed to this terminology. The "conventional" and "auxiliary" aerofoil sections can alternatively be denoted as "main" and auxiliary" aerofoils respectively.

Figures 1 , 2, 4, 5 and 6 and related text use the simple BEM representation of the action of the wind on an element of a single conventional wind turbine blade, the resulting forces and the blade movement. This is recognised to be a simplification of the true aerodynamic effects.

The above description refers almost exclusively to Auxiliary Aerofoil Sections and Conventional Aerofoil Sections. It is recognised that other blade features/sections provide important functionality. This use of other blade features is not precluded by the use of Auxiliary Aerofoil Sections.

In the above description the meaning of the term "tip speed ratio" is the ratio of "ultimate tip" speed to the remote wind speed. The "ultimate tip" is the physical extremity of the blade not the extremity of any section.

In the above description a positive angle of attack on both the Conventional and the Auxiliary Aerofoil Sections produces torque which acts to turn the rotor in the normal operating direction.

Claims

CLAIMS:

1. A rotor arrangement for a horizontal axis wind turbine, the rotor arrangement comprising a main aerofoil adapted and arranged to generate aerodynamic lift to extract energy from oncoming wind and to convert that energy into rotational energy, and an auxiliary aerofoil arranged to be driven at least to some extent by the main aerofoil and adapted and arranged to generate aerodynamic lift in the manner of a propeller so as to dissipate some or all of the energy extracted by the main aerofoil as a measure of the rotational speed of the main aerofoil speed rises, thereby assisting to prevent overspeed of the main aerofoil.

2. A rotor arrangement as claimed in claim 1 , wherein the measure is a ratio of the speed of the tip of a blade comprising the main aerofoil to the oncoming wind speed.

3. A rotor arrangement as claimed in claim 1 or 2, wherein the auxiliary aerofoil is arranged in a first range of the measure to generate aerodynamic lift to extract energy from the oncoming wind and to convert that energy into additional rotational energy to drive the main aerofoil, and in a second, higher, range of the measure to generate the aerodynamic lift in the manner of a propeller to dissipate energy.

4. A rotor arrangement as claimed in claim 3, wherein the lower end of the second range is below a normal operating level for the measure.

5. A rotor arrangement as claimed in any preceding claim, comprising a blade formed of both the main aerofoil and the auxiliary aerofoil.

6. A rotor arrangement as claimed in any preceding claim, comprising a blade formed of the main aerofoil and another blade formed of the auxiliary aerofoil.

7. A rotor arrangement as claimed in any preceding claim, comprising a first rotor having a blade formed of the main aerofoil and a second rotor having a blade formed of the auxiliary aerofoil.

8. A rotor arrangement as claimed in claim 7, wherein the first rotor has an axis of rotation different to that for the second rotor.

9. A rotor arrangement as claimed in claim 7, wherein the first rotor has an axis of rotation substantially the same as that for the second rotor.

10. A rotor arrangement as claimed in claim 9, wherein the second rotor is arranged to influence oncoming wind flowing through first one rotor.

1 1. A rotor arrangement as claimed in any one of claims 7 to 10, wherein the first and second rotors are mechanically linked by a speed increasing, speed decreasing or speed neutral linkage.

12. A rotor arrangement as claimed in any preceding claim, wherein the auxiliary aerofoil is arranged selectively to be driven at least to some extent by the main aerofoil.

13. A rotor arrangement as claimed in any preceding claim, wherein the auxiliary aerofoil acts as a safety system component to limit or to assist in the limitation of rotor overspeed, for example for a wind turbine which would normally run at a fixed speed.

14. A rotor arrangement as claimed in any preceding claim, wherein the auxiliary aerofoil is substantially fixed relative to the main aerofoil.

15. A rotor arrangement as claimed in claim 14, when dependent on claim 5, wherein the blade formed of both the main aerofoil and the auxiliary aerofoil has a substantially continuous surface.

16. A horizontal axis wind turbine comprising a rotor arrangement as claimed in any preceding claim.