For a longer period of time, the construction of wind turbines for generating electrical energy has undergone a huge development. This development has lead to a significant increase in the effect of the wind turbines.

In order to increase the amount of electric energy production, the size of the wind turbines have been increased. Hereby, a wind turbine with a larger production capacity is achieved, which has lead to a considerable improvement in the operational efficiency of the wind turbines.

The development of ever-larger wind turbines leads to the fact that the length of the wings is increased significantly, which again results in a rise in the consumption of material, the costs of transportation and erection of the wind turbines, and which is an increasing problem for the development of the large wind turbines. This problem is particularly present by off shore based wind turbines, due to the very high costs of transportation and erection.

The increased length of the wings also results in that the wings requirement for the stiffness of the wing is increased in order to ensure that the wing is not bent so much that a wing of a rotor might hit the tower of the wind turbine under extreme whether conditions, since the wind turbine is most frequently provided with a rotor, which is arranged upwind with respect to the tower. Moreover, the requirements for strength of the wings are increased, since the enlargement of the wing amongst other things results in an increased dead load.

Taking this background into consideration, it is the object of the present invention to provide a design for a wing for a wind turbine, where the requirements for stiffness

may be significantly reduced and where a reduction in the overall weight of the wing may be achieved.

This object is achieved by a wing for a wind turbine according to the characterising part of claim 1.

It is well known that a wing of a wind turbine will deflect backwards relative to the rotor plane because of the wind load. The centrifugal forces, which are created by the rotation, will attempt to straighten the wing. This stretching force depends on the distribution of the mass of the wing, the cone angle of the rotor and the speed of rotation. By the known designs of wings for wind turbines, the centre of gravity is typically positioned in a distance of about 1/3 of the entire length from the wing root as the thickness of the wing is considerably larger at the root end than at the tip end due to e. g. strength requirements. Thus, the straightening function is quite small due to the relative low mass of the outermost part of the wing.

By the invention, advantage is made of the centrifugal force from the wing itself and from one or more built-in masses with relatively high density built in at the wing tip or in the vicinity thereof and possibly also at one or more other locations in the wing.

Hereby, an increased straightening force is created, which is sufficient in size for the purpose of reducing the deflection of the wing and thereby the flap-wise moment in the wing.

By an increased wind load, the wing will deflect backwards relative to the wind direction. Hereby, the angle relative to the rotation plane is increased, which results in a straightening force perpendicular on the rotor plane, which completely or partially will compensate for the flap-wise moment and results in a simultaneous achievement of the desired geometric shape of the deflection. The masses contribute to stretching the wing, so it remains at least approximately straight. This is of immense importance with respect to the regulation of the angle of attack of the wing, in particular at high pitch velocities. A wing, which is deflected due to the wind load,

will move in an inappropriate and uncontrolled manner by a change in the pitch angle, i. e. the angle of attack. By the invention, a more predictable aerodynamical effect by the pitch regulation may be achieved.

The wing is preferably substantially made of fibre-reinforced material, such as e. g. glass fibre and/or carbon fibre. Hereby, a flexible wing with a high tensile strength may be achieved. Since the wing according to the invention mainly is loaded with an increased tensile tension, the weight of the wing may be reduced as the requirements for flap-wise rigidity is reduced. Within the field of aerodynamics, it is well known that the efficiency of a wing is expressed by the relation between the lift coefficient and the drag coefficient of the wing (CL/CD), the so-called glide ratio or L/D ratio.

One of the ways to reduce the drag coefficient and thereby improve the L/D ratio is by reducing the thickness of the wing. By a wing according to the invention, it is possible to use thinner wing profiles since the requirements to the flap-wise rigidity of the wing may be reduced, due to the active straightening effect by a wing according to the invention.

A wing according to a preferred embodiment of the invention is provided with mass concentrations in the outermost 2/3rd of the wing in order to increase the straightening force.

In a first embodiment, mass concentrations comprise embedded sand, lead or similar material having a high mass density, said mass being arranged at least near the tip of the wing.

By making the mass concentrations as one or more cavities, such as one or more tanks in the wings at the wing tip and/or in a predetermined distance therefrom in the outmost part of the wing, where cavities are completely or partially filled with material with a relative high mass density, a particularly effective mass distribution may be achieved for the creation of the desired wing geometry for the wing, when it is in use.

In another embodiment the tanks are preferably completely or partially filled with material with a flowable material, such as a granular material or a liquid. It is thus possible to make the mass of the at least one tank variable as the wing moreover comprises means for regulating the mass in the tank, said means including controlling the inlet and outlet of flowable material to and from the at least one tank and means for retaining the material in the tank.

The flowable material is preferably selected from a group of materials comprising sand, lead balls and high density granulates. Alternatively, the flowable material may be a liquid selected from a group of materials comprising water, glycols, and other similar liquids or mixtures thereof or mercury. It is to be ensured that this liquid is prevented from freezing, since the ability to regulate the mass concentration otherwise would be blocked, just as there would be a risk of structural damage to the wing construction in the event of a volume increase due to freezing of the liquid.

This risk is naturally eliminated by use of a flowable granular material, however one should ensure when using a granular material that this material cannot change its state of the material when exposed to the rather big temperature changes that may occur in a wind turbine wing.

The wing is preferably dimensioned in such a way that the wing deflection, out of the rotor plane due to the impact of the wind, is completely or partially equalized by a straightening force provided by the centrifugal force of the masses, and which is created when the wing is deflected backwards in the wind by the wing angle relative to the rotor plane without a wind load. The wing may also be mounted with a backwardly directed angle in relation to the rotation plane. By having the masses and their positions correctly adjusted, an increased self-straightening of the wing may be achieved, so that the backward deflection may completely or partially be prevented.

A wing according to the invention may be mounted with a fixed cone angle relative to the plane of rotation, preferably between 1° and 60°. Hereby, the straightening

flap-wise force will be increased, and hereby contribute to furnish the wing with the desired aerodynamic deflection geometry. By choosing a fixed cone angle, components for adjustment of the cone angle may be eliminated, as it is otherwise known from US-A-5,584,655, since the variable masses in the wings, combined with a fixed cone angle, exceed the effect which may occur by varying the cone angle combined with a known wing without extra masses built-in near the wing tip.

In a preferred embodiment of the invention, the wing is mounted in a rotor, which is arranged in the lee side relative to the tower of the wind turbine.

Alternatively, it is realised that the wing may be mounted in a rotor, which is arranged in the wind side relative to the tower of the wind turbine.

In the following, the invention is described in more detail under reference to the accompanying drawings, in which: fig. 1 is a schematic illustration of a wind turbine with a rotor having wings according to a preferred embodiment of the invention; fig. 2 shows a use of a wing according to the invention in another type of wind turbine; and fig. 3 is a schematic illustration of a wing with a built-in adjustable mass distribution according to the invention.

As shown in fig. 1, the wind turbine comprise a tower 1, which on the top is provided with a nacelle 2, which is connected to a rotor having a number of wings 3 arranged in the wind side with respect to the tower 1.

It is the aim to have the distance between the wings 3 and the tower 1 by a wind turbine of the front-rotor type relatively large in order to avoid that the wings 3 collide with the tower 1 because of deflection backwards towards the tower 1 due to the wind load. The wings for a wind turbine are traditionally designed with relative

high rigidity, which results in a relatively large weight. This relatively large weight of the rotor must be absorbed in the nacelle on the top of the tower, which results in a large tower weight in order to absorb the large forces and torques that derive from the rotor and the wind load.

By a wing 3 according to the invention, a mass concentration 4 is built in at least in the tip of the wings. The mass concentration 4 may consist of an embedding of one or more of the previously mentioned materials. The wind load on the wings 3, as indicated by V, results in a backward deflection of the wing 3. However, the centrifugal force from the discrete mass 4 and potentially other masses and the dead load of the wing will by rotation of the wing, provide a centrifugal force C, which will create a straightening force F which will prevent the deflection deriving from the wind load.

By the invention, it is realised that the requirements for flap-wise rigidity of the wing may be reduced considerably by a wing and a wind turbine construction as shown in the figures. Each of the wings 3 in the rotor is provided with a mass concentration at the tips and possibly one or more mass concentrations in a predetermined distance therefrom. The wing 3 itself may be more flexibly dimensioned, since the wing may be dimensioned with a focus on considerations concerning the increased tension forces rather than with a focus on achieving a high rigidity. The requirements for flap-wise rigidity may be reduced significantly by a wing according to the invention.

The flap-wise rigidity is achieved by exploiting the centrifugal force C from the wing and the built-in mass 4 in the wing tip for a complete or partial elimination of the flap-wise moment. The choice of material for the wing 3 may be similar to the known wing constructions, e. g. a glass fibre and/or carbon fibre reinforced wing construction. By the invention, it is realised that the wing may also be constructed in other materials and combinations thereof.

The mass concentrations may be provided by embedding of a material with high density, e. g. lead on predetermined discrete points in the wing. Alternatively or as a

supplement to this, the mass concentrations may be created by inserting one or more wing tanks 5 or similar cavities in which a flowable material 6 may be accumulated, as shown in figure 3.

Through a long period of time, there has been a tradition of designing wind turbines as a so-called front rotor, i. e. with the rotor positioned up-wind relative to the tower.

Contrary to this, wind turbines of the rear rotor type are also known, i. e. where the rotor is positioned down wind of the tower 1, such as shown in fig. 2. The reason for the traditional choice of a front rotor rather than a rear rotor is that the position of the tower in the wind slows down the wind and thus results in a disturbance of the flow, including a drop in the flow velocity (a large drop out) on a wing 3 that passes the tower 1. By a front rotor, this problem is not so significant, but even in front of the tower the wind flow will be influenced by the tower.

The drop out caused by the tower 1 decrease by the distance from the tower 1 to the wing 3. By the invention, it is realised that a wing according to the invention is suitable for use in a rotor which is placed in the lee side with respect to the tower, as shown in fig. 2.

The wings 3 in the rotor are mounted via a hub in the nacelle 2 with a backwardly projecting angle of e. g. 20°. Besides in the wing tip masses may as mentioned earlier be built in either"directly"embedded or inserted e. g. in wing tanks 5. For instance, as shown in fig. 3, a mass 4 may be embedded in the wing 3 just as a wing tank 5 may be arranged which is connected to regulation means 7 for adjusting the content of flowable material 6, which is provided in the wing tank 5. The flowable material may be a liquid, e. g. mercury, water, glycol or the like. It must however be ensured that the liquid is prevented from freezing with respect to the temperature range that the wing may be exposed to during operation. Alternatively, the material may be flowable granulates, such as sand, small metal balls or the like. The means 7 for regulation of the content in the tanks 5 may e. g. comprise one or more pipe or tube connections, which via pump means and at least one stop valve, are in connection

with a reservoir of flowable weight material. Hereby, the mass in the tank 5 may be changed and the wing may thereby be adjusted to the desired deflection profile by different wind speeds.

By the invention, it is naturally realised that there may be more tanks 5 with regulateable contents of flowable mass in each wing, even though figure 3 only shows a single tank. It is furthermore realised that the regulateable mass in the wings may be established in other ways than the manner shown in fig. 3 without departing from the scope of the invention as set forth in the accompanying claims.