GB2185788A

GB2185788A - Blade for wind turbine

Info

Publication number: GB2185788A
Application number: GB08701540A
Authority: GB
Inventors: William Murray Somerville
Original assignee: Nei International Research & D
Current assignee: Nei International Research & D
Priority date: 1986-01-28
Filing date: 1987-01-23
Publication date: 1987-07-29
Anticipated expiration: 2007-01-23
Also published as: GB8602007D0; GB2185788B; GB8701540D0

Abstract

The starting characteristic of wind turbines in light winds is improved by providing the turbine with blades 10 each having a primary aerofoil 12 on the leading edge of which is mounted a secondary aerofoil 14. The secondary aerofoil 14 is mounted for movement between an operative position in which it is spaced from the primary aerofoil 12 whereby air can flow between the two aerofoils and an inoperative position in which it lies against and substantially conforms to the leading edge of the primary aerofoil 12. The secondary aerofoil 14 is resiliently biased towards its operative position and is movable against the bias towards its inoperative position by centrifugal forces. In use, the flow of air between the two aerofoils creates a low pressure region at the trailing edge of the secondary aerofoil 14 thereby inducing flow over the secondary aerofoil 14 to create aero-dynamic lifting forces which tend to turn the rotor. The secondary aerofoil may be located only between 50% and 100% (and preferably only between 60%-90%) of the radius of the primary aerofoil (measured from the blade root). The blade can form either part of a rotor which rotates about a horizontal axis or part of a rotor which rotates about a vertical axis. If the turbine has two blades, means may be provided for synchronising movement of the secondary aerofoils. <IMAGE>

Description

SPECIFICATION Wind turbine The invention relates to a wind turbine having fixed-pitch blades and, in particular, to such a turbine which is stall regulated.

To function effectively as a prime mover for electrical power generation, a wind turbine rotory should operate at a constant speed of rotation so that a simple fixed-ratio mechanical transmission may be used to turn the generator at a constant speed appropriate to the frequency of alternating current required e.g.

50 or 60Hz.

It is preferable to keep the construction of the turbine rotor simple by using fixed-pitch blades secured, e.g. by being bolted, to the rotor hub. Such constructions apply particularly to smaller wind turbines having rotor diameters of 25mm and below, but an effective technique for controlling the maximum speed of rotation of the rotor would no doubt enable this form of construction to be used on larger machines.

These requirements, which are directed to give reliability and minimum cost of construction and operation, create problems in operation. At present, these problems are solved in part by a compromise in the use of stallregulated blade forms. Stall-regulated blade forms have less twist than would be required to give a constant angle of attack at all blade radii. The angle of attack at any given blade radius is defined as the angle between the pitch angle of the blade-section chord at that radius and the resultant wind vector formed by the vectorial addition of the circumferential blade velocity vector at that radius and the velocity vector of the wind.If the blade twist is such that, at every section the angle of attack gives maximum lift (approximately 12-16") at some nominal wind speed and speed of rotation, then a slight increase in wind speed would stall the whole blade and a dramatic reduction of aerodynamic lift and hence torque and power would occur. By designing the blade such that the blade root section is at a steeper angle of attack than the tip, stall can be made to develop progressively at greater and greater radii as the wind speed increases and thus producing an approximately constant power over a range of wind speeds.

In very high wind speeds, the power begins to increase once more and, since the increase is largely produced by wind pressure reaction on the flat surface of the blades, it is difficult to control except by spoilers, airbrakes or other moveable surfaces.

Also, it is necessary to keep the blade pitch flat to reduce the torque produced by wind pressures.

As a consequence, wind turbines having fixed-pitch blades, particularly stall-regulated blade forms, have poor starting characteristics in light winds.

Although the above description refers specifically to horizontal axis turbines, it will be readily apparent that vertical axis turbines having fixed-pitch blades (whether fixed or variable tilt angle blades) can also have poor starting characteristics in light winds for similar reasons.

It is an object of the present invention to reduce or obviate the aforementioned disadvantage.

According to the present invention, a wind turbine comprises a rotor having at least one fixed-pitch blade comprising a primary aerofoil and a secondary aerofoil mounted on the leading edge of the primary aerofoil for movement between an operative position in which the secondary aerofoil is spaced from the primary aerofoil whereby air can flow between the two aerofoils and an inoperative position in which the secondary aerofoil lies against and substantially conforms to said leading edge, the secondary aerofoil being resiliently biased towards said operative position and being movable against said bias towards said inoperative position by centrifugal force.

Preferably, the rotor is mounted on a horizontai axis.

In that instance, the secondary aerofoil is located between 50%-100% of the radius of the primary aerofoil, typically between 60%-90% of said radius.

The invention also includes a wind turbine blade comprising primary and secondary aerofoils as defined in the preceding paragraphs.

A wind turbine will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 is a cross-section through a wind turbine blade constructed in accordance to the invention, the secondary aerofoil being in its operative position; Figure 2 is a similar cross-section to Fig. 1 but with the secondary aerofoil shown in its inoperative position; and Figure 3 is a view on line Ill-Ill in Fig. 1 of a part of the length of the blade.

A horizontal axis wind turbine (not shown) has a rotor (not shown) connected through a transmission (not shown) to a generator (not shown).

The rotor of the turbine has a hub (not shown) to which are bolted three equiangularly-disposed blades 10. Each blade 10 has a primary aerofoil 12 and a secondary aerofoil 14 located along the leading edge of the primary aerofoil 12.

The primary aerofoils 12 are preferably twisted such that the turbine is stall regulated.

Each primary aerofoil 12 has a recess 16 for accomodating the secondary aerofoil 14 when it is in its inoperative position (see Fig. 2).

The recess 16 extends from the region of the tip of the primary aerofoil 12 radially inwardly therealong, typically between 90%-60% of the radius of the primary aerofoil 12.

Each primary aerofoil 12 has a plurality of additional recesses 18 (see Fig. 3) for accommodating, in the inoperative position of the secondary aerofoil 14, hinged mounting plates 20 and springs 22. The mounting plates 20 mount the secondary aerofoil 14 on the leading edge of the primary aerofoil 12 such that it can move between its operative and inoperative positions.

The springs 22 bias the secondary aerofoil 14 towards its operative position. The strength of the springs 22 is selected such that the secondary aerofoil 14 will have moved to its inoperative position under the influence of centrifugal force when the rotor has a rotational speed of 50%-60% of the optimum working speed of the turbine, for example.

When the turbine is at rest, the relative positions of the primary and secondary aerofoils 12, 14 are as shown in Fig. 1. As the wind speed increases from zero, the wind flows over the aerofoils 12, 14 as indicated by the arrows in Fig. 1. The air stream flows between the spaced-apart aerofoils 12, 14. As the air flow passes from between the aerofoils 12, 14, it conforms to the surface of the primary aerofoil 12 owing to the Coanda effect.

Consequently, the air flow between the aerofoils 12, 14 creates a low pressure region at the trailing edge of the secondary aerofoils 14 thereby inducing flow over the secondary aerofoils 14 to create aerodynamic lifting forces which tend to turn the rotor.

Once the wind speed is sufficient to produce say approximately 10% of the rated power of turbine, the rotor begins to turn and the generator produces power.

A conventional turbine would not start until the wind speed is sufficient to produce about 50% of the rated power of the turbine.

Once the rotor has started to turn, the rotor accelerates as energy is extracted from the wind. As the rotor accelerates, the centrifugal forces acting on the secondary aerofoils 14 eventually overcome the bias of the springs 22 and the secondary aerofoils 14 move to their inoperative positions in which they lie against the substantially conform to the respective leading edges of the primary aerofoils 12. Typically, the secondary aerofoils 14 have moved completely into their inoperative position when the rotor has reached 50%-60% but preferably 75% of its working speed.

Once the rotor speed is about 50% of its working speed, the primary aerofoils 12 are capable of accelerating the rotor up to the working speed.

Modifications are possible within the scope of the invention. For example, the movement of the secondary aerofoils 14 to their inoperative positions can be synchronised. Such synchronisation can be achieved by mounting a plate centrally on a torsion spring and connecting the plate to the secondary aerofoils 14 by wires. In this instance, a proportion of the force biasing the secondary aerofoils 14 into their respective operative positions is provided by the torsion spring. Thus, when one secondary aerofoil 14 starts to move to its inoperative position, it rotates the plate against the action of the torsion spring thereby lessening the tension in the wires to the other secondary aerofoils 14 which will then also start to move to their respective inoperative positions. The invention is also applicable to untwisted primary aerofoils.

As previously mentioned, the invention is applicable to vertical axis turbines also. In that instance, the secondary aerofoil may extend the full length of the primary aerofoil. Alternatively, when the secondary aerofoil does not extend the full length of the primary aerofoil, the secondary aerofoil can be placed centrally of the primary aerofoil, or, alternatively, the secondary aerofoil can be divided into two (or more) sections and disposed at each end of the primary aerofoil.

Claims

1. A wind turbine comprising a rotor having at least one fixed-pitch blade comprising a primary aerofoil and a secondary aerofoil mounted on the leading edge of the primary aerofoil for movement between an operative position in which the secondary aerofoil is spaced from the primary aerofoil whereby air can flow between the two aerofoils and an inoperative position in which the secondary aerofoil lies against and substantially conforms to said leading edge, the secondary aerofoil being resiliently biased towards said operative position and being movable against said bias towards said inoperative position by centrifugal force.

2. A wind turbine according to claim 1, in which the rotor is mounted on a horizontal axis.

3. A wind turbine according to claim 2, in which the secondary aerofoil is located only between 50%-100% of the radius of the primary aerofoil as measured from the root of the blade.

4. A wind turbine according to claim 2, in which the secondary aerofoil is located only between 60%-90% of the radius of the primary aerofoil as measured from the root of the blade.

5. A wind turbine according to claim 1 in which the rotor is mounted on a vertical axis.

6. A wind turbine according to any one of the preceding claims, in which, when the turbine has at least two blades, movement of the secondary aerofoils, at least to the respective inoperative positions, is synchronised.

7. A wind turbine according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

8. A wind turbine blade comprising primary and secondary aerofoils as defined in any one of the preceding claims.