GB2459453A

GB2459453A - Aerodynamic overspeed limitation for wind turbine rotor(s)

Info

Publication number: GB2459453A
Application number: GB0807297A
Authority: GB
Inventors: Barry Robert Marshall
Original assignee: BRM POWER Ltd
Current assignee: BRM POWER LIMITED
Priority date: 2008-04-21
Filing date: 2008-04-21
Publication date: 2009-10-28
Anticipated expiration: 2028-04-21
Also published as: GB2459453B; WO2009130500A2; WO2009130500A3; GB0807297D0

Abstract

A horizontal axis wind turbine blade configuration is disclosed which can 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. 'Auxiliary Aerofoil Section(s)' are connected to a wind turbine rotor hub which under particular operating conditions dissipate some of, and under severe conditions dissipate virtually all of, the energy that the conventional wind turbine blade(s) or blade section(s) extract from the wind. The Auxiliary Aerofoil Section(s) are configured to function as a propeller using aerodynamic lift 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. The 'Auxiliary Aerofoil Sections" may be provided at the tips of the primary aerofoil sections or separate auxiliary and primary blades may be used on the same hub. Alternatively separate auxiliary and primary rotors may be linked on the same axis or on respective axes.

Description

Part 2 -Summary

A horizontal axis wind turbine blade configuration which can absorb some or all of the energy which exceeds the capacity of the energy conversion system, when wind speeds exceed a particular level.

This invention covers the use of'Auxiliary Aerofoil Sections' connected to a wind turbine rotor which dissipate some of the energy that the Primary Aerofoil Sections' extract from the wind under particular conditions. The Auxiliary Aerofoil Sections are configured to function as a propeller using aerodynamic lift as the principle energy dissipation mechanism when the rotor speed exceeds a predetermined level (for a particular pertaining wind speed) set by the design.

Part 3 -Background

a) Basic HAWT Aerodynamics For a horizontal axis wind turbine to extract the maximum amount of energy from the wind at or below a maximum normal operating wind speed the 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 4 major factors which result in the relative wind are: i. The remote (undisturbed) wind ii. The rotation of the rotor.

iii. The resistance of the rotor to the air flow This reduces the speed of the remote wind and results in a local wind speed' at the rotor. The amount of speed reduction is referred to as the axial induction factor and is given the symbol a iv. The reaction of the air on the rotating blade This creates a rotational velocity of the air in the plane of rotation of the rotor which results in an effective rotational speed' of the air at the blade which is greater than the speed of the blade. The amount of speed increase is referred to as the radial induction factor and is given the symbol a'.

These 4 influences are shown in the vector diagram Fig 1.

The relative wind acting on the blade creates lift and drag forces which result in torque and thrust forces on the blade and thus the rotor. The torque and thrust forces act in the plane of rotation of the rotor 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 extracted by an energy conversion system.

The vector diagram Fig 2 (in comparison with Fig 1) illustrates the changes in the relative wind direction that occur when rate of increase in energy extraction by the energy conversion system lags behind an increase in the energy extracted form the wind. This results in an increased rotational speed. As can bee seen the change in the direction of the relative wind with respect to the aerofoil results in a reduction in the angle of attack.

Similar geometric effects occur throughout the length of the blade(s).

b) Energy Conversion System Economics.

To achieve acceptably low lifetime cost for a conventional variable speed horizontal axis wind turbine over a moderate lifetime, the energy conversion system (normally an electric generator) is designed such that it's maximum power output capability corresponds to the energy that the turbine extracts from a moderate to high wind speed which is frequently experienced at the location. When higher wind speeds occur and the energy produced by the rotor exceeds the capacity of the energy conversion system the surplus energy causes the turbine rotor to accelerate to a higher rotational speed.

Without an energy output limiting system, small increases in wind speed cause a significant energy surplus. A potentially disasterous operating regime will occur when the maximum power output capability of the energy conversion system is exceeded. An energy limiting system MUST therefore be used to prevent damage from overspeed of the rotor and/or overload of system components.

In the vast majority of cases the cost of incorporating an energy conversion system with a maximum capability to match those higher wind speeds cannot be justified as the amount of extra energy that can be extracted will be limited by the time periods when the higher wind speed occurs.

c) Energy Output Limiting Systems Except in the case of very small turbines, conventional horizontal axis wind turbines have functionality which limits the energy output from the rotor to the energy conversion system. Unless a system for limiting the energy output of the rotor is both fitted and operational, when the wind speed rises above a predetermined level set by the design the rotor will in time, (and often extremely rapidly) accelerate to a dangerously high rotational speed.

Systems that are frequently used on horizontal axis wind turbines inlude: I. Rotor blade pitch control ii. Blade tip drag devices iii. Yaw control iv. Furling v. Rotor brakes Etc Rotor blade pitch control turns a part of, or the entire blade about a longitudinal axis such that the aerodynamic lift force and consequently the torque and energy produced by the blade is reduced. This reduction of the lift force can be achieved either by increasing the angle of attack to create stall conditions on (parts of) the blade, or by decreasing the angle of attack.

These systems require a mechanism managed by a control system. The mechanisms can be driven by mechanical hydraulic or electrical actuators or centrifugal devices.

Blade tip drag devices alter the shape of the blade tip by protrusion of a moveable section of, or alteration of the geometry of a blade tip such that aerodynamic drag results. This aerodynamic drag absorbs some of the rotational energy produced by the remainder of the blade.

These systems require a mechanism managed by a control system to alter the shape of the blade. These mechanisms are conceptually similar to the systems used for rotor blade pitch control.

Yaw control alters the alignment of the axis of rotation of the wind turbine rotor from the normal position of alignment with the direction of the undisturbed wind, (to maximise energy extraction) to a misaligned position which has adverse effects on the aerodynamic performance of the rotor and reduces the amount of energy produced.

These systems require a mechanism, managed by a control system, to effect the required misalignment through a variation in the functionality of the system used to achieve the alignment of the rotor axis with the wind direction for normal operation.

Furling of a wind turbine is the rotation of the axis of rotation of the wind turbine rotor such that the rotor no longer faces into the wind either directly (as in normal operation) or misaligned (as in yaw control). It is in effect the most extreme form of yaw control.

These systems require only relatively simple mechanisms but result in the turbine ceasing to produce any worthwhile power output.

Rotor brakes are generally used to stop the turbine rotor not to control it's speed. The use of a conventional brake on an operating wind turbine rotor would generate very large quantities of heat which would be difficult to dispose of.

Part 4 -This Invention a) Fundamental Aspects.

This invention provides a solution to the problem of the dissipation of the surplus energy extracted by a wind turbine rotor in high wind speed. Aerodynamic lift is used as the principle energy absorbing mechanism. There is no requirement for any form of operating mechanism.

Fig 3 shows a simple viable configuration for the use of Primary and Auxiliary Aerofoil Sections on a horizontal axis wind turbine rotor. A similar turbine (without Auxiliary Aerofoil Sections) would require the use of a mechanically actuated energy limiting system of the type listed in 3 c) above. This invention provides the functionality necessary to limit the energy output from the rotor in wind speeds that would otherwise result in an overload of turbine components and/or the energy conversion system.

There is no requirement for any form of operating mechanism for the Auxiliary Aerofoil Sections to be effective. The blades are fixed rigidly to the rotor hub, they do not rotate about any longitudinal axis, there are no changes in the geometry or configuration of any part of the blade, the rotational axis of the rotor remains aligned to the direction of the remote wind.

b) Description of Operation

For a particular, specific, practical wind turbine design the following operating conditions will be possible and normal.

(The particular design and operating conditions in this description are set to allow the functionality of the invention covered by this patent application to be readily understood.

They do not form any constraint on the scope of this patent application.) When the rotor is subject to wind speeds below a particular wind speed (that speed will be referred to as the Auxiliary Aerofoil Zero Lift Speed) the rotor speed and the orientation of the Auxiliary Aerofoil Sections are such that the angle of attack is positive.

The angle of relative wind is greater than the pitch angle (the zero lift angle.) The Auxiliary Aerofoil Sections will produce small amounts of lift which result in additional rotor torque. Vector Diagram Fig 4 shows this condition.

When the rotor is subject to a wind speed equal to the Auxiliary Aerofoil Zero Lift Speed the rotor speed and the orientation of the Auxiliary Aerofoil Sections are such that the angle of attack is zero and no lift is produced. Vector Diagram Fig 5 shows this condition When the rotor is subject to a wind speed that is marginally above the Auxiliary Aerofoil Zero Lift Speed the rotor speed and the orientation of the Auxiliary Aerofoil Sections are such that the angle of attack is negative and the sections start to function as a propeller.

This absorbs some of the energy the Primary Aerofoil Sections extract from the wind. In these conditions the maximum capacity of the energy conversion system has not been reached and that system can be used to maintain a balance between Primary Aerofoil Section energy extraction, energy dissipation by the Auxiliary Aerofoil Sections and energy extraction by the energy conversion system. Vector Diagram Fig 6 shows this condition.

Further increases in the wind speed above the Auxiliary Aerofoil Zero Lift Speed result in the rotor speed increasing and the Auxiliary Aerofoil Sections propeller absorbing increased amounts of energy. As this condition progresses, the balance between Primary Aerofoil Section energy extraction, energy extraction by the energy conversion system and energy dissipation by the Auxiliary Aerofoil Sections is controlled more and more by the dissipation of energy by the Auxiliary Aerofoil Sections. With increasing wind and rotor speeds the energy available for extraction by the energy conversion system will be gradually reduced. However the rate at which the energy available for extraction reduces will be dependant upon the particular design and will to some degree be controllable by maximising the energy extraction by the energy conversion system and thus reducing the rotor speed.

This invention allows the final rotor speed to be readily constrained at levels for which mechanical strength and other design criteria can be conveniently and economically met.

This arrangement allows the wind turbine rotor to remain aligned with the direction of the remote wind, in high wind speeds, without the requirement for an excessively high capacity energy conversion system or for the use of a mechanically actuated energy limiting system of the type listed in 3 c) above..

c) Configuration Options The arrangement shown in Fig 3 is probably the simplest configuration by which the 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 control the rotational speed of the rotor(s).

i. The use of separate blades which function as Auxiliary Aerofoils Sections on the same hub as the Primary Aerofoil Sections.

ii. The use of a second (Auxiliary Aerofoil Section) rotor mechanically linked to the first (Primary Aerofoil Section) 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 rotor mechanically linked to the first (Primary Aerofoil Section) 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 Section) rotor mechanically linked to the first (Primary Aerofoil Section) rotor which not only absorbs excess energy produced by the first rotor but also influences the passage of air through the rotor carrying the Primary Aerofoil Sections. The second rotor mechanically linked by a speed increasing, speed decreasing or speed neutral linkage d) Additional Benefits One effect of the Auxiliary Aerofoil Sections acting as a propeller is that the thrust generated (the force normal to the plane of rotation of the wind turbine rotor) opposes the thrust experienced by the Primary Aerofoil Sections. This reduces the loadings which act on the rotor and the support structures in the direction of the remote wind.

The is invention allows a horizontal wind turbine rotor to operate in a greater range of wind speeds without the use of systems that are frequently used for limiting the energy output on wind turbines described in Section 3 c).

This invention will allow wind turbine rotors to be manufactured to common designs for locations with widely differing wind speed versus time profiles through: a) The use of longer Primary Aerofoil Sections to enable greater energy capture from low wind speeds b) The ability to operate higher wind speeds than would otherwise be possible without the need to restrict the rotor operating speed by the use of conventional energy output limiting systems.

Claims

Energy_Output Limiter for Wind Turbine Rotor(s) Claims: I A 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.
2 A number of wind turbine blade(s) on a rotating hub, said blade(s) embodying Auxiliary Aerofoil Section(s) according to Claim 1, 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.
3 A number of wind turbine blades on a rotating hub, some of said blades embodying Auxiliary Aerofoil Section(s) according to Claim 1, 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.
4 A number of wind turbine blade(s) on a rotating hub, said blade(s) embodying Auxiliary Aerofoil Section(s) according to Claim I, 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.
A rotating hub and wind turbine blade(s) according to Claim 4, 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.
6 Wind turbine blade(s) on a rotating hub according to Claim 2, 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.
7 Wind turbine blade(s) on a rotating hub according to Claim 3, 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.