WO2007054098A1

WO2007054098A1 - Wind power plant and method of controlling the blades in order to prevent tower strike

Info

Publication number: WO2007054098A1
Application number: PCT/DK2006/000625
Authority: WO
Inventors: Régis Jacques BENSOUSSAN
Original assignee: Lm Glasfiber A/S
Priority date: 2005-11-14
Filing date: 2006-11-13
Publication date: 2007-05-18

Abstract

A wind power plant of the horizontal-axis type comprising a tower and a number of blades is presented wherein a magnetisable material is arranged at the most distal part of a blade at a distance from the axis of rotation of the blades and likewise at the tower in approximately the same distance from the axis of rotation of the blades. When the two magnetisable materials influence each other, the blade will, during its rotation, be influenced correspondingly by forces that increase the blade's distance to the tower. The magnetisable material comprises permanent magnets, electromagnets and ferro- or paramagnetic material. The magnetisable material at the tower can be arranged between tower and blade or such that the blade travels between it and the tower.

Description

WIND POWER PLANT AND METHOD OF CONTROLLING THE BLADES IN ORDER TO PREVENT TOWER STRIKE

The invention relates to a wind power plant with a number of blades rotating about an approximately horizontal axis and a method of controlling the blades during their passage past the tower of the wind power plant.

Background

Wind power plants utilizing the energy of the wind become increasingly bigger and have increasingly longer blades to increase their effect. However, this means that the absolute deformations of the blades and deflections at the blade tip increase correspondingly and may be quite considerable.

According to one example a blade of 61 m may deflect as much as 10 m at the blade tip in case of a wind speed of 28 m/sec, corresponding to gale force winds. It is a requisite dimensional parameter of a wind power plant that, of course, the blades must not hit the tower during operation.

This can be avoided in various ways. For instance, the rotor can be moved further away from the tower, thereby reducing the risk of a blade colliding with the tower in strong winds or in case of powerful gusts of wind. However, this in undesirable since a larger distance between rotor and tower entails correspondingly longer main shaft and hence larger forces in bearings and gear as well as higher weight of the nacelle, with ensuring higher demands to the turbine foundation. Conversely, it is desirable to be able to move the rotor as close to the tower as possible for the very same reasons.

The deformation of the blades can also be reduced by making the blades more rigid, which, however, may be difficult without also increasing the mass of the blades, whereby the loads in the nacelle will be increased and make higher demands to carrying capacity and rigidity of tower and foundation and the blades due to their self-weight. It is also known to employ pre-bending blades, where, initially and at lower wind speeds, each blade bends forwards into the wind. The risk of collision with the tower is here correspondingly reduced. However, pre-bending blades have a more complex design and the collision problem is not solved; rather it is postponed only until a given wind speed or magnitude of wind gusts.

One latter option is that of narrowing the tower of the wind turbine in front of the blade tips, thereby allowing the blades to bend more before hitting the tower. This is associated with the drawback, however, that the rigidity and carrying capacity of the tower is reduced quite considerably where the tower is narrowed with ensuing strength problems on the tower.

Object and description of the invention

It is the object of the invention to provide a wind turbine for which the above risk of collision between blade and tower is minimised even in case of longer and more flexible blades and at high wind speeds and gusts of wind. Further advantages will be mentioned later on in the specification.

Thus, the present invention relates to a horizontal-axis wind power plant comprising a tower and a number of blades, wherein a magnetisable material is arranged at the most distal part of at least one blade at a distance from the axis of rotation, and at least one further magnetisable material is mounted at the tower at approximately the same distance from axis of rotation of the blades. Hereby the magnetisable materials will influence each other and the at least one blade will, during its rotation, be influenced by forces that increase the distance of the blade to the tower. Hereby the advantageous aspect is provided that it is avoided that the blades hit the tower during their rotation, even at very high wind speeds or powerful gusts of wind. Likewise, the invention enables use of more flexible blades, ie it is possible to save material in the blades and thereby reduce the loads in the hub and the problems of the self-weight of the blades. Thus it is possible to use longer blades with this system compared to a scenario where this system is not available. By placing the magnetic material optimally at tower and at the blade tips and by regulating the magnitude of the magnetic fields, optionally as a function of the current speed of wind, the path of the blades past the tower can also be controlled quite accurately and thereby the force can be maximised. Moreover, further magnetisable material can arranged at the tower, whereby the blade is not only pushed or pulled around the tower, but a positive power impact or a push in its direction of rotation is also imparted to the blade. It also counteracts the tower shadow due to the fact that, when the blades bend forwards, the blades experience it as if the wind speed increases when they travel past the tower.

According to one embodiment of the above-described wind power plant, the magnetisable material at least at the one blade is a permanent magnet, a paramagnetic or a ferromagnetic material or an electromagnet. By using a magnetisable material of one of the former types, it is possible to avoid having to lead current into the blade, which is advantageous since efficient lightning protection of such system is difficult.

According to one embodiment of the above-described wind power plant, the at least one magnetisable material at the tower is a permanent magnet, an electromagnet or a paramagnetic or ferromagnetic material.

The invention further relates to a wind power plant, wherein one or more magnets are mounted around the most part of the tower. Hereby it is accomplished that the blade is influenced in the same manner whether or not the rotor rotates and yaws into the wind following different directions of wind. According to a further embodiment of the wind power plant according to the invention at least the one magnetisable material at the tower is mounted in a fitting that can be turned around the tower. Hereby the fitting and the magnetisable material can be turned around the tower to the same extent as the one in which the rotor yaws into the wind.

According to a further embodiment of the wind power plant according to the invention, the at least one magnetisable material at the tower is displaced away from the tower, whereby the magnet influences the blade prior to it reaching the tower. This is advantageous in that the blade can hereby be influenced earlier, meaning that, even if it has large inertia or amount of movement, it can be influenced to the effect that it moves around the tower.

According to a further embodiment several magnets are arranged in a bearing which is able to rotate about the tower. By adjusting the peripheral speed of each magnetisable material to the peripheral speed of the rotor, each blade will then experience a constant power impact from the tower's magnets throughout the entire path of the blade past the tower. Hereby inconvenient thrusts on the blade are obviated.

One embodiment describes a wind power plant according to one or more of the above-mentioned elements wherein the at least one magnetisable material at the tower is mounted on the other side of the blades away from the tower, whereby, during its rotation, the at least one blade will be influenced by attracting forces there from, increasing the distance of the blade to the tower. This location of the magnetisable material at the tower makes it possible to select a ferro- or paramagnetisable material at the one part of the tower or at the blade and a magnet at the other.

A further embodiment describes a wind power plant according to one or more of the above-mentioned elements, wherein the magnetisable material at least at the one blade is mounted displaced outwards in front of the leading edge of the blade.

The invention further relates to a blade for a wind power plant comprising a magnetisable material arranged at the most distal part of the blade and for use in a wind power plant as described by the above. The advantages of this are as described earlier for the wind power plant.

Moreover the invention relates to a tower for wind power plants comprising a magnetisable material mounted at the tower and for use in a wind power plant as described by the above. The advantages of this are as described earlier for the wind power plant.

Finally the invention relates to a method of controlling blades on a wind power plant during passage past the tower of the wind energy plant, wherein a magnetic field is generated at the most distal part of at least one blade, and that at least one further magnetic field is generated at the tower, whereby, during its rotation, the at least one blade will be influenced by forces that increase the distance of the blade to the tower. The advantages of this method are also as disclosed above and as will otherwise appear from the description.

Brief description of drawings

In the following the invention will be described with reference to the figures, wherein

Figure 1 shows a wind power plant, seen from the side, with magnets in blade tips and on the tower;

Figure 2 shows the passage of a blade past the tower, seen from above; Figure 3 shows an alternative embodiment of the location of magnets on the tower;

Figure 4 shows an embodiment with a number of magnets arranged on the tower; and

Figure 5 shows an embodiment with a magnet arranged on the other side of the blade, away from the tower.

Description of embodiments

Figure 1 shows a wind power plant 100, seen from the side. The wind turbine comprises a rotor 101 with a number of blades 102 (herein three blades) that rotate about a more or less horizontal axis 103, and wherein the rotor is turned into the wind 104. In the Figure, dotted lines indicate a non-deformed blade 110. In case of high wind speeds or gusts of wind the blades may bend so much 111 due to the wind load that the rotor needs to be arranged with a correspondingly larger distance to the tower 105, which in turn leads to a longer main shaft and larger forces in bearings and hub. In order to enable the opposite: arrangement of the rotor closer to the tower 105 to avoid these problems while simultaneously the clearance 113 between the blades 102 and the tower 105 is ensured even in case of extreme loads, a magnet 106 is provided in or close to the blade tip 107 and an opposite-pole magnet 108 on the tower 105. The magnet 108 on the tower is arranged in approximately the same height as the magnet 106 on the blade, whereby the magnetic forces are exploited maximally. When, during rotation, the blade 102 approaches the tower, the two magnets 106, 108 will influence each other with repellent and repulsive forces, whereby the blade is pushed further away 112, while the blade travels past the tower, and the clearance 113 is increased. This is also illustrated in Figure 2 which shows a section of the tower 105, seen from above and down towards the ground. The dotted line 201 illustrates the movement of a blade past the tower in a scenario where the blade is loaded so much that it will collide with the tower. The fully drawn line 202 illustrates the path of a blade as described by an embodiment of the present invention. A blade profile 203 corresponding to two different positions of the blade in its path is included in the drawing. As is also described in the context of Figure 1 , a magnet 106 is arranged in or at the blade tip and one or more magnets 108 mounted on the tower 105. It is important to the execution of the invention that the magnet on the blade is opposite the magnet(s) on the tower, whereby the path of the blade will be pushed way from the tower. In Figure 2 the magnet 108 sits all the way around the tower as an annular magnet with the north pole in the outermost position and the south pole in the innermost position or the other way around. The advantage of having the magnet all the way around the tower is that the magnetic influence on the blade thereby becomes the same for all settings of the rotor relative to all wind directions 104. According to a further embodiment a number of magnets are arranged side-by-side all the way around the tower, whereby the same independence of the wind direction is accomplished.

The magnet (or magnets) in the blade may, in the embodiments shown, either be a permanent magnet or an electromagnet. The former is advantageous in that the issue of establishing power in the blade is hereby avoided, which could otherwise cause a number of problems relating to efficient lightning protection of the blades. On the tower both permanent magnets and electromagnets may be used. To generate a particularly strong magnetic field it is an option to use super-conductive material in the coil of an electromagnet.

Also conceivably, the repulsive forces between blade tip and tower may also be generated by use of a magnet of one of the above types on the one part (blade or tower) and a super-conductive material on the other part, said material acting by opposing a magnetic field and hence repelling the magnet.

If the magnet 106 in the blade tip is arranged closer to its trailing edge or even behind the trailing edge 204 of the blade, the magnetic forces will also make the most distal part of the blade twist. In that case the magnets can be arranged such that the blade, while being just in front of the tower, is turned slightly into the wind, and the angle 208 between the wind direction 104 and the cord 209 of the blade profile is increased as shown in Figure 2. By regulating the blade in this manner to temporarily be in the area in front of the tower, the wind is used more to advantage, the wind speed thus being reduced slightly in front of the tower. In addition to this a load reduction on the blade occurs. Thus the magnets also serve to even out the power curve of the blade, whereby the small dips that would otherwise inevitably occur each time the blade passes the tower and when the wind speed is slightly lower are evened out.

According to an alternative embodiment of the invention the magnet 108 is mounted in a fitting 301 on the tower 105 as outlined in Figure 3. Herein the magnet 108 and the fitting are arranged tapering at an angle 302, pointing towards the direction the blade comes from. Hereby it is ensured that the blade is influenced by the magnet in time for it to change its rotation path and be pushed away from the tower. At which angle and how far in front of the tower the magnet is to be set depend on the mass and speed of rotation of the blade. In this embodiment the fitting 301 with the magnet 108 can be turned around the tower as shown by the arrow 303 and follow the yawing of the rotor.

According to a further embodiment the entire tower with mounted fitting and magnet or magnets can be turned. Figure 4 shows an embodiment of the invention wherein a number of magnets 108 are mounted on the tower 105. Herein the magnets are arranged such that the blade can be controlled more accurately past the tower in the desired path. Simultaneously the last magnet sensed by the blade during its passage past the tower may also be oriented such relative to to the magnet on the blade that it pushes the blade further on in its rotation. Figure 4 also outlines an embodiment where the magnet in the blade is arranged on a rod or the like 401 a distance in front of the blade, whereby the blade detects the repulsive and regulating forces from the magnets on the tower earlier on in its rotation path.

Figure 5 shows the lowermost part of a wind power plant with the lowermost part of the tower 105 and a by-passing blade 102, seen from the side. In this embodiment the magnet 108 on the tower 105 is arranged on the other side of the blade 102 compared to the tower by means of a fitting 303 which is, in this case, taken right below the path of the blade. In order to ensure the clearance between the blade 102 and the tower 105 herein, the magnet 108 is to attract the blade. Therefore a permanent magnet 106 is included in the blade which is, as opposed to the previous examples, oriented such that it has the same direction as the magnet 108 of the tower thereby causing the blade to bend further away from the tower. Alternatively a ferromagnetic or paramagnetic material 501 may be arranged in the blade which is not permanently magnetised, but becomes magnetised in the magnet field of the tower magnet and is attracted thereto, and this means again further away from the tower. Under one heading these types of materials are designated magnetisable materials.

It is a further advantage of the present invention that it is possible to control and regulate on a continuous basis how much the blade is to bend away from the tower, quite simply by regulating the force of the magnetic field from either the tower or the blade or both. It will be understood that the invention as taught in the present specification and figures can be modified or changed while continuing to be comprised by the protective scope conferred by the following claims.

Claims

C l a i m s

1. A wind power plant of the horizontal-axis type comprising a tower and a number of blades, characterised in that a magnetisable material is arranged at the most distal part of at least one blade at a distance from the axis of rotation of the blade; and that at least one further magnetisable material is mounted at the tower in approximately the same distance from the axis of rotation of the blades, thereby causing the magnetisable materials to influence each other and the at least one blade, during its rotation, to be influenced by forces that increase the distance of the blade to the tower.

2. A wind power plant according to claim 1, characterised in that the magnetisable material at least at the one blade is a permanent magnet.

3. A wind power plant according to one or more of claims 1-2, characterised in that the magnetisable material at least at the one blade is a paramagnetic or ferromagnetic material.

4. A wind power plant according to one or more of claims 1-3, characterised in that at least the one magnetisable material at the tower is a permanent magnet.

5. A wind power plant according to one or more of claims 1-4, wherein at least the one magnetisable material at the tower is an electromagnet.

6. A wind power plant according to one or more of claims 1-5, characterised in that at least the one magnetisable material at the tower is a paramagnetic or ferromagnetic material.

7. A wind power plant according to one or more of claims 1-6, characterised in that one or more magnets is/are mounted around the most part of the tower.

8. A wind plant according to one or more of claims 1-7, characterised in that the at least one magnetisable material at the tower is mounted in a fitting that can be rotated around the tower.

9. A wind power plant according to one or more of claims 1-8, characterised in that the at least one magnetisable material at the tower is mounted displaced outwards from the tower, whereby the magnet influences the blade prior to it reaching the tower.

10. A wind power plant according to one or more of claims 1-9, characterised in that the at least one magnetisable material at the tower is mounted on the other side of the blades away from the tower, whereby the at least one blade will, during its rotation, be influenced by attracting forces there from, increasing the distance of the blade to the tower.

11. A wind power plant according to one or more of claims 1-10, characterised in that the magnetisable material at least at the one blade is mounted displaced outwards in front of the leading edge of the blade.

12. A blade for a wind power plant, characterised in comprising a magnetisable material arranged at the most distal part of the blade and for use in a wind power plant as described by claims 1-11.

13. A tower for wind power plant, characterised in comprising a magnetisable material mounted at the tower and for use in a wind power plant as described by claims 1-11.

14. A method of controlling blades on a wind power plant during their passage past the tower of the wind power plant, characterised in that a magnetic field is generated in the most distal part of at least one blade; and that at least on further magnetic field is generated at the tower, whereby the at least one blade will, during its rotation, be influenced by forces that increase the distance of the blade to the tower.