FIELD OF THE INVENTION

The present invention relates to a wind energy farm comprising a plurality of wind turbines. At least some of the wind turbines are of a kind in which a hub comprises a blade carrying structure, and in which one or more wind turbine blades are connected to the blade carrying structure via a hinge, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure.

BACKGROUND OF THE INVENTION

Wind turbines are normally controlled in order to provide a desired power output and in order to control loads on the wind turbine. For horizontal axis wind turbines, i.e. wind turbines with a rotor which rotates about a substantially horizontal rotational axis, this may be obtained by controlling a pitch angle of the wind turbine blades. In this case the angle of attack of the wind turbine blades relative to the incoming wind is adjusted by rotating the wind turbine blades about a longitudinal axis.

As an alternative, wind turbines may be provided with wind turbine blades which are connected to a blade carrying structure via hinges, thereby allowing a pivot angle defined between the wind turbine blades and the blade carrying structure to be varied. In such wind turbines the diameter of the rotor of the wind turbine is varied when the pivot angle is varied. Such a type of wind turbine is, e.g., described in US 4,632,637.

In wind energy farms a plurality of wind turbines are positioned within a site of the wind energy farm. If the wind turbines are arranged within a certain distance from each other, the operation of one wind turbine may affect the wind which reaches one or more of the other wind turbines of the wind energy farm, this is referred to as wake effects. For instance, the extractable energy of the wind which reaches a wind turbine which is arranged downstream relative to one or more the other wind turbines, may be less than the extractable energy of the wind which reaches the one or more upstream wind turbines. Accordingly, the power production of wind turbines arranged downstream may be smaller than the power production of wind turbines arranged upstream. Furthermore, various other wake effects may be introduced, such as turbulence, which also affect the operation of downstream wind turbines. Therefore it is necessary to impose a certain distance between the wind turbines, typically at least three times the rotor diameter of the wind turbines, when planning a wind energy farm. This introduces an upper limit to the total energy production per unit area of the wind energy farm.

SUMMARY OF THE INVENTION

It is an object of embodiments of the invention to provide a wind energy farm which allows the wind turbines to be arranged with a smaller mutual distance than prior art wind energy farms.

It is a further object of embodiments of the invention to provide a wind energy farm in which the total power production per unit area is higher than for prior art wind energy farms.

The invention provides a wind energy farm comprising a plurality of wind turbines, wherein at least a group of the wind turbines are of a first type of wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure, and one or more wind turbine blades connected to the blade carrying structure via a hinge, each wind turbine blade being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, wherein the wind turbine blades of each wind turbine defines a rotor area, the rotor area being variable as a function of the pivot angle, and wherein the rotor area comprises a swept part being swept by the wind turbine blades during rotation of the hub, and an un-swept centre part which is not swept by the wind turbine blades during rotation of the hub, wherein the wind turbines of the first type of the wind energy farm are positioned in such a manner that the un-swept part of the rotor area of a given upstream wind turbine, as seen along at least a dominating wind direction, overlaps with the swept part of the rotor area of one or more downstream wind turbines of the plurality of wind turbines.

Thus, the present invention provides a wind energy farm, i.e. a group of wind turbines arranged within a specified site, and which at least partly share various infrastructure, such as access roads, grid connecting cables, substations, etc.

The wind energy farm comprises a plurality of wind turbines which each comprises a tower, a nacelle, a hub mounted rotatably on the nacelle, and one or more wind turbine blades. The nacelle is mounted on the tower via a yaw system, thereby allowing the nacelle to perform yawing movements relative to the tower about a substantially vertical rotation axis, in order to direct the wind turbine blades in accordance with the direction of the incoming wind. The yaw system may be an active yaw system in which the nacelle is rotated actively by means of a yaw drive mechanism, e.g. on the basis of measurements of the wind direction. As an alternative, the yaw system may be a passive yaw system in which the nacelle automatically rotates according to the wind direction without the use of a yaw drive mechanism. As another alternative, the yaw system may be a combination of an active yaw system and a passive yaw system, in the sense that it may operate actively under some circumstances and passively under other circumstances.

For at least some of the wind turbines forming a group of wind turbines, the hub comprises a blade carrying structure, and the one or more wind turbine blades are connected to the blade carrying structure via a hinge. Thereby each wind turbine blade is arranged to perform pivot movements relative to the blade carrying structure, via the hinge. A pivot angle is defined between each wind turbine blade and the blade carrying structure, depending on the position of the hinge and thereby of the wind turbine blade relative to the blade carrying structure. Accordingly, the pivot angle defines the direction along which a given wind turbine blade extends relative to the blade carrying structure, and thereby relative to the hub. The pivot angle can vary between a minimum pivot angle and a maximum pivot angle.

The hinge may be or comprise a bearing, e.g. in the form of a journal bearing, a roller bearing, or any other suitable kind of bearing.

Thus, at least some of the wind turbines are of a kind which comprises hinged wind turbine blades, and all of the wind turbines of the wind energy farm may be of this kind. However, it is not ruled out that some of the wind turbines of the wind energy farm are of a kind without hinged wind turbine blades.

The wind turbine blades of each wind turbine, which is of a kind having hinged wind turbine blades, defines a rotor area which is variable as a function of the pivot angle in such a manner that the minimum pivot angle defines a maximum rotor area and the maximum pivot angle defines a minimum rotor area. Thus, the rotor area decreases as the pivot angle increases, and vice versa.

At any given pivot angle the rotor area comprises a swept part and an un-swept centre part. The swept part of the rotor area is a part which is actually swept by the wind turbine blades during rotation of the hub. Accordingly, the swept part of the rotor area contributes to the extraction of energy from the wind, and thereby the swept part of the rotor area contributes to changes in the wind caused by operation of the wind turbine, and thereby affecting the operation of any wind turbines arranged downstream as seen along the direction of the incoming wind.

Similarly, the un-swept centre part of the rotor area is not affected by the wind turbine blades during rotation of the hub. Accordingly, this part of the rotor area does not contribute to the extraction of energy from the wind, or contributes significantly less to the extraction of energy from the wind than the swept part of the rotor area. The incoming wind can therefore pass essentially unaffected through this part of the rotor area, or at least significantly less affected than the wind passing the swept part of the rotor area, towards any wind turbines arranged downstream as seen along the direction of the incoming wind. The blade carrying structure may be provided with one or more elements configured to improve aerodynamic properties of a surface of the blade carrying structure. Such elements may typically be arranged in the un-swept centre part of the rotor area, in which case the un-swept centre part of the rotor area will provide some contribution to the energy production of the wind turbine, and therefore to some extent affect the wind passing there through. However, the wind passing through the un-swept centre part of the rotor area will normally be affected significantly less than the wind passing the swept part of the rotor area.

Thus, the rotor area comprises a centre part where the incoming wind can pass the wind turbine essentially unaffected and part arranged circumferentially with respect to the centre part which is swept by the wind turbine blades and thereby contributes to the energy production of the wind turbine.

In the present context the terms 'upstream' and 'downstream' should be interpreted in the following manner. At any given wind direction, the incoming wind reaches a wind turbine which is arranged upstream relative to another wind turbine before it reaches this other wind turbine. Similarly, the incoming wind reaches a wind turbine which is arranged downstream relative to another wind turbine after it has reached this other wind turbine. Accordingly, whether a given wind turbine is regarded an upstream wind turbine or a downstream wind turbine relative to another wind turbine depends on the direction of the incoming wind. Furthermore, at a given wind direction, any given wind turbine may be regarded as 'upstream' relative to one or more wind turbines, and at the same time may be regarded as 'downstream' relative to one or more other wind turbines.

The wind turbines of the wind energy farm are positioned in such a manner that the un-swept part of the rotor area of a given upstream wind turbine, as seen along at least one wind direction, overlaps with the swept part of the rotor area of one or more downstream wind turbines, as seen along at least one wind direction. Thus, at least the upstream wind turbine is of a kind having hinged wind turbine blades, thereby having an un-swept part of the rotor area. Thereby the wind which passes a given upstream wind turbine essentially unaffected, via the un-swept part of the rotor area, reaches the swept part of the rotor area of at least one downstream wind turbine, and thereby the downstream wind turbine(s) can extract energy from this essentially unaffected part of the wind. Accordingly, the wind which reaches the downstream wind turbines is significantly less affected by the passage of the upstream wind turbine than would be the case if the rotor area of the upstream wind turbine had not been provided with an un-swept part. This results in a reduction of wake effects and an increase in available energy in the wind at the positions of the downstream wind turbines. Accordingly, the wind turbines of the wind energy farm can be positioned with a smaller mutual distance without introducing unacceptable wake effects and while maintaining an acceptable power production of the downstream wind turbines. Thereby the total energy production per unit area of the wind energy farm can be increased.

The downstream wind turbine may also be of a kind having hinged wind turbine blades, i.e. of a kind which has a rotor area with a swept part as well as an un swept part. Alternatively, the downstream wind turbine may be of a kind without hinged wind turbine blades, e.g. a pitch controlled wind turbine. In this case the rotor area of the downstream wind turbine will normally not comprise an un swept part, and the swept part of the rotor area of the downstream wind turbine constitutes the entire rotor area.

It should be noted that, in the present context, the term 'overlap' should be interpreted to mean an overlap defined at the position of the downstream wind turbine between the part of the wind which has previously passed through the un-swept centre part of the rotor area of the upstream wind turbine and the swept part of the rotor area of the downstream wind turbine. The overlap needs not be complete, in the sense that the entire swept part of the rotor area of the downstream wind turbine receives a part of the wind which has passed through the un-swept part of the rotor area of the upstream wind turbine, or in the sense that all of the part of the wind which has passed through the un-swept part of the rotor area of the upstream wind turbine reaches the swept part of the rotor area of the downstream wind turbine, as long as a portion of this part of the wind reaches a portion of the swept part of the rotor area of the downstream wind turbine. However, it is desirable to position the wind turbines in such a manner that the overlap is optimized. One or more of the wind turbines may have a rotor axis which extends along a direction which defines an angle relative to horizontal. Such an angle is sometimes referred to as a tilt angle. In wind turbines defining a tilt angle, the rotor plane is arranged at an angle with respect to vertical. Tilt angles are sometimes introduced in order to prevent collisions between the wind turbine blades and the tower, and this is particularly relevant for upwind wind turbines, where the incoming wind tends to push the wind turbine blades towards the tower. However, tilt angles could also be introduced in downwind wind turbines.

In the case that an upstream wind turbine of a downwind type defines a tilt angle, and thereby an angle between the rotor plane and vertical, the wake of the upstream wind turbine will follow a downwardly inclined direction behind the upstream wind turbine and towards the downstream wind turbine(s). Accordingly, the overlap between the un-swept centre part of the rotor area of the upstream wind turbine and the swept part of the rotor area of the downstream wind turbine, as defined above, will be shifted downwards relative to the actual horizontal position of the un-swept centre part of the rotor area of the upstream wind turbine. Thus, when planning the wind energy farm in such a manner that overlaps as defined above occur, possible tilt angles of the wind turbines should be taken into account.

In the case that one or more of the wind turbines have blade carrying structures being provided with one or more elements configured to improve aerodynamic properties of a surface of the blade carrying structure, as described above, these elements may be selected in accordance with the position of the individual wind turbines within the wind energy farm. For instance, upstream wind turbines may be provided with elements which reduce thrust and/or drag to the greatest possible extent, or no elements at all may be provided on the upstream wind turbines. On the other hand, downstream wind turbines may be provided with elements which increase power production to the greatest possible extent. Thereby the wind passes the un-swept centre part of the rotor area of the upstream wind turbines as unaffected as possible, while the downstream wind turbines are allowed to extract energy from the wind to the greatest possible extent. The pivot angle may be adjusted automatically in response to the wind speed, during operation of the wind turbine. For instance, the centre of mass of each wind turbine blade may be positioned along the longitudinal direction of the wind turbine blade in such a manner that centrifugal forces acting on the wind turbine blades will tend to move them. In this case, an increase in wind speed, causing an increase in the rotational speed of the hub, increases the centrifugal forces acting on the wind turbine blades, thereby moving the wind turbine blades towards larger pivot angles. This causes a decrease in the rotor area and thereby a decrease in the energy extracted from the wind. This will tend to decrease the rotational speed of the hub. Accordingly, at any given wind speed, an equilibrium pivot angle is obtained, which balances the rotational speed of the hub and the rotor area.

Furthermore, aerodynamic forces acting on the aerodynamic profiles of the wind turbine blades may cause the wind turbine blades to pivot in such a manner that the rotor area is reduced as the wind speed increases.

The at least one wind direction include a dominating wind direction at a site of the wind energy farm. In the present context the term 'dominating wind direction' should be interpreted to mean a wind direction, or a limited range of wind directions, which occurs more frequently than other wind turbines at the site of the wind energy farm. Thus, according to this embodiment, the wind turbines of the wind energy farm are positioned in such a manner that it is ensured that the downstream wind turbines receive essentially unaffected wind, as described above, when the most frequent wind direction occurs. For instance, in the case that the wind energy farm comprises wind turbines with hinged wind turbine blades as well as wind turbines without hinged wind turbine blades, the wind turbines may be positioned in such a manner that the wind turbines without hinged wind turbine blades are positioned downstream relative to at least one wind turbine with hinged wind turbine blades, when the dominating wind direction occurs. The dominating wind direction may be determined as the most dominant direction of the wind rose of the site of the wind energy farm. A first wind turbine of the plurality of wind turbines may have a first hub height, and a second wind turbine of the group of wind turbines may have a second hub height, where the first hub height differs from the second hub height, and wherein the first wind turbine is arranged downstream relative to the second wind turbine, as seen along the dominating wind direction.

According to this embodiment, hub heights of at least two wind turbines which are arranged one behind the other, i.e. one being downstream relative to the other, have different hub heights. In the present context, the term 'hub height' should be interpreted to mean a vertical distance from a base level, e.g. the ground, a wind turbine foundation, a water surface, etc., to the position where the hub is mounted on the nacelle. Thus, the hub height of a wind turbine determines the vertical level at which the hub of the wind turbine is positioned. Thereby the hub height also determines the vertical position of the un-swept centre part of the rotor area.

Accordingly, since the hub heights of the two wind turbines differ, the vertical positions of the un-swept centre parts of their rotor areas also differ. Thereby the un-swept centre part of the rotor area of the upstream wind turbine is displaced relative to the un-swept centre part of the rotor area of the downstream wind turbine, and the un-swept centre parts are therefore not completely overlapping. This results in an overlap between the un-swept part of the rotor area of the upstream wind turbine and the swept part of the rotor area of the downstream wind turbine. Thus, according to this embodiment, this overlap is provided by the differing hub heights. Furthermore, the differing hub heights displace the swept part of the rotor area of the upstream wind turbine relative to the swept part of the rotor area of the downstream wind turbine, thereby allowing a larger part of the incoming wind to reach the swept part of the rotor area of the downstream wind turbine essentially without being affected by the operation of the upstream wind turbine.

As an example, the first/second hub height could be within the range of 115 m to 135 m, such as within the range of 120 m to 130 m, such as approximately 126 m, and the second/first hub height could be within the range of 180 m to 220 m, such as within the range of 190 m to 210 m, such as approximately 203 m.

Alternatively or additionally, the overlap between the un-swept centre part of the rotor area of the upstream wind turbine and the swept part of the rotor area of the downstream wind turbine may be provided in a number of alternative ways. For instance, the overlap may be provided appropriately designing the un swept centre parts and swept parts of the rotor areas of the respective wind turbines.

The size of the un-swept part of the rotor area of each of the wind turbines may be variable as a function of the pivot angle. For instance, the wind turbine blades may comprise an inner tip and an outer tip defining the extremities of the wind turbine blades along a longitudinal direction. In the present context the term 'inner tip' should be interpreted to mean the part of the wind turbine blade which is closest to the hub, and the term 'outer tip' should be interpreted to mean the part of the wind turbine blade which is furthest away from the hub.

The hinge which connects a blade to the blade carrying structure may be positioned at a distance from the inner tip and at a distance from the outer tip.

In this case the wind turbine blade has an inner part extending between the hinge and the inner tip and an outer part extending between the hinge and the outer tip. During pivoting movements of the wind turbine blade, the inner tip as well as the outer tip will move. In particular, the inner tip will move in such a manner that, when the pivot angle is increased, the inner part of the wind turbine blade is moved away from the un-swept centre part of the rotor area, thereby increasing the size of the un-swept centre part, and vice versa.

According to this embodiment, the pivot angle of the upstream wind turbine may be adjusted to increase the size of the un-swept centre part of the rotor area, in order to maximize the overlap between the un-swept part of the rotor area of the upstream wind turbine and the swept part of the rotor area of the downstream wind turbine. Each of the wind turbines may define a minimum size of the un-swept part of the rotor area. In the example above, the minimum size of the un-swept part of the rotor area is obtained at minimum pivot angle. Since, according to this embodiment, a minimum size of the un-swept part of the rotor area is defined for each of the wind turbines, it is ensured that none of the wind turbines, being of the kind having hinged wind turbine blades, can have its wind turbine blades pivoted to a position where there is no un-swept part of the rotor area, i.e. a position where essentially the entire rotor area is swept. Thereby it is ensured that, regardless of the pivot angle, a part of the incoming wind will always be allowed to pass the wind turbine, via the un-swept part of the rotor area, and reach the rotors of any downstream wind turbine essentially unaffected by the operation of the upstream wind turbine.

The minimum size of the un-swept part of the rotor areas of the respective wind turbines may differ according to the positions of the wind turbines within the wind energy farm. This could, e.g., be obtained by using different types of wind turbines. For instance, the design of the blade carrying structure may be different from one wind turbine to another, for instance positioning the connecting points with the wind turbine blades at various radial distances from the hub. The larger the radial distance, the larger the minimum size of the un- swept part of the rotor area will be. For instance, wind turbines arranged upstream when a dominating wind direction occurs may be designed with larger un-swept parts of their rotor areas than downstream wind turbines.

As an alternative, the size of the un-swept part of the rotor area may be constant, i.e. independent of the pivot angle. This could, e.g., be the case if the hinges of the wind turbine blades are positioned at the inner tips of the wind turbine blades.

Alternatively or additionally, the differing minimum sizes of the un-swept parts of the rotor areas may be obtained by operating the wind turbines differently. For instance, the wind turbines may comprise a biasing mechanism which biases the wind turbine blades towards minimum pivot angle. In this case the wind turbines may be operated in such a manner that the applied biasing force differs from one wind turbine to another, depending on the positions of the wind turbines within the wind energy farm. For instance, the upstream wind turbines may be operated with a smaller biasing force than the downstream wind turbines.

As described above, the size of the un-swept part of the rotor area may decrease when the pivot angle decreases, and the size of the un-swept part of the rotor area may increase when the pivot angle increases.

Alternatively or additionally, the size of the swept part of the rotor area of each wind turbine may be variable as a function of the pivot angle. For instance, the size of the swept part of the rotor area may be at a maximum at minimum pivot angle and at a minimum at maximum pivot angle.

The pivot angle of each wind turbine may be adjustable in accordance with a wind direction. As described above, whether or not a given wind turbine is regarded as an upstream wind turbine or as a downwind turbine with respect to another wind turbine depends on the direction of the incoming wind. It may be desirable to ensure that the downstream wind turbines are provided with 'essentially unaffected' wind to a sufficient degree. It may therefore be desirable to decrease the rotor areas of the upstream wind turbines. Since the rotor area is variable as a function of the pivot angle, this decrease in rotor area can be obtained by appropriately adjusting the pivot angle. Furthermore, in the case that the size of the un-swept part of the rotor area is also variable as a function of pivot angle, the pivot angle of an upstream wind turbine may advantageously be adjusted to a position which ensures that the size of the un-swept part of the rotor area is larger than or equal to a minimum size.

When the direction of the incoming wind changes, thereby potentially changing the upstream/downstream status of one or more wind turbines, appropriate adjustments of the pivot angles may be performed to take this into account.

Each of the wind turbines may comprise a biasing mechanism arranged to bias the wind turbine blades towards a minimum pivot angle, and the pivot angle may be adjustable by adjusting a biasing force provided by the biasing mechanism. In the case that the wind turbines are operated in such a manner that the pivot angle is automatically adjusted in response to the wind speed, as described above, the biasing force provided by the biasing means determines the equilibrium pivot angle for a given wind speed. Thus, by reducing the biasing force, the equilibrium pivot angle in increased, and by increasing the biasing force, the equilibrium pivot angle is decreased.

Thus, according to this embodiment, for a given wind direction, the upstream wind turbines may be operated at a lower biasing force than the downstream wind turbines, resulting in a larger un-swept part of the rotor area of the upstream wind turbine, thereby maximizing the overlap between the un-swept part of the rotor area of a given upstream wind turbine and the swept part of the rotor area of a corresponding downstream wind turbine.

At least some of the wind turbines may be downwind wind turbines. According to this embodiment, the rotors of the wind turbines face away from the incoming wind, i.e. the wind reaches the wind turbine blades after having passed the nacelle. Downwind wind turbines are very suitable for applying passive yaw systems, i.e. yaw systems which automatically direct the rotor of the wind turbine in accordance with the incoming wind without the use of yaw drives and control systems. Furthermore, in downwind wind turbines a passive cooling system can be arranged upstream with respect to the rotor, thereby enabling improved cooling of various wind turbine components.

Alternatively or additionally, at least some of the wind turbines may be upwind wind turbines, in which case the rotor faces the incoming wind.

At least one of the wind turbines may be provided with stay cables, each stay cable being connected at one end to the tower of said at least one wind turbine and at the other end to a stay cable foundation, and at least one of the stay cable foundations may also be a foundation of the tower of an adjacent wind turbine.

Stay cables are applied to wind turbine towers in order to provide stability to the towers without increasing the thickness of the tower wall to an unacceptable level. This is particularly relevant for high towers. The end of a stay cable which is not connected to the tower needs to be anchored to the ground. This is normally done by means of a stay cable foundation. By allowing a foundation of one of the other wind turbines to also function as the stay cable foundation, the establishment of a separate stay cable foundation is avoided. Thereby the costs of erecting the wind energy farm can be reduced.

Since the wind turbines of the wind energy farm according to the invention are arranged with a smaller mutual distance than the wind turbines of prior art wind farms, the wind turbines are sufficiently close to each other to allow a stay cable to be anchored to a foundation of an adjacent wind turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a side view of four wind turbines of a wind energy farm according to a first embodiment of the invention,

Fig. 2 illustrates rotor areas of two of the wind turbines of Fig. 1 at two different wind speeds,

Fig. 3 is a side view of three wind turbines of a wind energy farm according to a second embodiment of the invention,

Fig. 4 is a side view of four wind turbines of a wind energy farm according to a third embodiment of the invention,

Fig. 5 is a side view of two wind turbines of a wind energy farm according to a fourth embodiment of the invention, and

Fig. 6 is a side view of two wind turbines of a wind energy farm according to a fifth embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of four wind turbines 1 (la, lb). The wind turbines 1 form part of a wind energy farm 2 according to a first embodiment of the invention. The wind turbines 1 are all downwind wind turbines. The wind energy farm 2 may comprise further wind turbines in addition to the four wind turbines 1 illustrated in Fig. 1.

Each wind turbine 1 comprises a tower 3 and a nacelle 4 carrying a hub 5. The hub 5 comprises a blade carrying structure 6, and three wind turbine blades 7, two of which are shown, are connected to the blade carrying structure 6 via a hinge 8. Thereby the wind turbine blades 7 are able to perform pivot movement relative to the blade carrying structure 6, between a minimum pivot angle where the wind turbine blades 7 are arranged with a largest possible angle to the rotational axis of the hub 5, and a maximum pivot angle where the wind turbine blades 7 are arranged substantially parallel to the rotational axis of the hub 5. The latter position may be referred to as 'barrel mode'. In Fig. 1 the wind turbine blades 7 are shown at a pivot angle which is close to, but larger than the minimum pivot angle. The direction of the incoming wind is indicated by arrow 9.

The wind turbine blades 7 define a rotor area being the area within the outer tips of the wind turbine blades 7. The rotor area varies as a function of pivot angle in such a manner that the rotor area is at a maximum when the pivot angle is at a minimum, and at a minimum when the pivot angle is at a maximum.

The rotor area comprises a swept part and an un-swept centre part. The swept part is the part of the rotor area which is actually swept by the wind turbine blades 7 when the hub 5 rotates, and it is therefore the part of the rotor area which contributes to the power production of the wind turbine 1. The un-swept part is the part of the rotor area which is arranged within the inner tips of the wind turbine blades 7. Accordingly, this part of the rotor area is not swept by the wind turbine blades 7 when the hub 5 rotates, and the incoming wind therefore passes this area essentially unaffected by the wind turbine blades 7. Two of the wind turbines la have a hub height which is lower than the hub height of the other two wind turbines lb. Each of the wind turbines la with a low hub height is positioned upstream relative to one of the wind turbines lb with a high hub height. This has the consequence that the un-swept part of the rotor area of each of the upstream wind turbines la overlaps with the swept part of the rotor area of one of the downstream wind turbines lb. Accordingly, the wind which passes essentially unaffectedly through the un-swept part of the rotor area of the upstream wind turbine la reaches the swept part of the rotor area of the downstream wind turbine lb. This reduces the wake effects experienced at the downstream wind turbine lb and increases the amount of energy available in the wind for extraction by the downstream wind turbine lb.

Furthermore, the un-swept part of the rotor area of one of the wind turbines lb with a high hub height overlaps with the swept part of the rotor area of the wind turbine la with low hub height arranged immediately downstream thereof. The remarks set forth above are therefore equally applicable for these two wind turbines la, lb.

The overlap between un-swept and swept parts of rotor areas described above allows the wind turbines 1 to be positioned within the wind energy farm 2 with a smaller mutual distance. Accordingly, the number of wind turbines 1 per unit area, and thereby the total energy production per unit area, of the wind energy farm 2 can be increased.

Fig. 2 illustrates rotor areas 10a, 10b of two of the wind turbines la, lb of Fig. 1 at two different wind speeds. The left part of Fig. 2 shows the rotor areas 10a, 10b at approximately 4 m/s, and the right part of Fig. 2 shows the rotor areas 10a, 10b at approximately 8 m/s. Each rotor area 10 comprises a swept part 11 (11a, lib) and an un-swept centre part 12 (12a, 12b). The swept part 11 is the part of the rotor area 10 which is actually swept by the wind turbine blades when the hub rotates. The un-swept centre part 12 is the central part of the rotor area 10, and this part is not swept by the wind turbine blades when the hub rotates. Thus, the swept part 11 contributes to the power production of the wind turbine, whereas the un-swept centre part 12 does not contribute to the power production of the wind turbine. Incoming wind pass the wind turbine essentially unaffected via the un-swept centre part 12 of the rotor area 10.

It can be seen that the rotor area 10 of each wind turbine is larger when the wind speed is approximately 8 m/s than when the wind speed is approximately 4 m/s. This is because the wind turbine blades are designed in such a manner that when the hub rotates, centrifugal forces and/or aerodynamic forces acting on the wind turbine blades cause the wind turbine blades to pivot towards larger pivot angles. This causes the rotor area 10 as well as the swept part 11 of the rotor area 10 to decrease. Thereby the ability of the wind turbine to extract energy from the wind decreases, thereby causing a decrease in the rotational speed of the hub, which decreases the centrifugal and/or aerodynamic forces which are pushing the wind turbine blades towards larger pivot angles. Accordingly, at any given wind speed, the wind turbine blades will find an equilibrium pivot angle which balances the various forces acting on the wind turbine blades. The higher the wind speed, the larger the equilibrium pivot angle will be. This is reflected in Fig. 2, illustrating that the pivot angle is larger, resulting in a smaller rotor area 10, when the wind speed is 8 m/s than when the wind speed is 4 m/s.

It can be seen that the hub heights of the two wind turbines differ from each other, thereby displacing the centres of the rotor areas 10a, 10b relative to each other. Accordingly, the un-swept parts 12a, 12b of the rotor areas 10a, 10b are also displaced relative to each other. As can be seen, this has the consequence, that the un-swept part 12a of the rotor area 10a of the upstream wind turbine overlaps with the swept part lib of the rotor area 10b of the downstream wind turbine. This applies at low wind speeds as well as at high wind speeds. Accordingly, wind which passes the upstream wind turbine essentially unaffected via the un-swept part 12a of the rotor area 10a thereof, reaches the swept part lib of the rotor area 10b of the downstream wind turbine. Thereby the power production of the downstream wind turbine is improved.

Fig. 3 is a side view of three wind turbines 1 of a wind energy farm 2 according to a second embodiment of the invention. The wind turbines 1 illustrated in Fig. 3 are similar to the wind turbines 1 of Fig. 1, and they will therefore not be described in detail here.

In the wind energy farm 2 of Fig. 3, a first wind turbine la is arranged upstream relative to a second wind turbine lb, and the first wind turbine la has a lower hub height than the second wind turbine lb. This is similar to the situation described above with reference to Figs. 1 and 2. Furthermore, a third wind turbine lc is arranged downstream relative to the second wind turbine lb, the third wind turbine lc having a hub height which is identical to the hub height of the second wind turbine lb.

In the wind energy farm 2 of Fig. 3, the wind turbines 1 are provided with a biasing mechanism which biases the wind turbine blades 7 towards minimum pivot angle. This could, e.g., be in the form of wires or the like which are used for pulling the wind turbine blades 7 in this direction by applying a certain wire tension. Thus, when the pivot angle is increased during operation of the wind turbines 1, as described above with reference to Fig. 2, the centrifugal and/or aerodynamic forces acting on the wind turbine blades 7 must work against the biasing force. Accordingly, the size of the biasing force, and thereby the size of the wire tension, determines the equilibrium pivot angle at a given wind speed. The higher the wire tension, the smaller the equilibrium pivot angle will be.

In the wind energy farm 2 of Fig. 3, the first wind turbine la and the second wind turbine lb are operated with substantially identical, low wire tensions. However, the third wind turbine lc is operated with a somewhat higher wire tension, resulting in a smaller pivot angle.

This has the consequence that the swept part of the rotor area of the third wind turbine lc is larger than the swept parts of the rotor areas of each of the first la and second lb wind turbines. This allows the power production of the third wind turbine lc to be maintained at a relatively high level, even though the third wind turbine lc is arranged downstream relative to the first wind turbine la as well as relative to the second wind turbine lb. Thus, by adjusting the wire tension of the wind turbines 1 in accordance with the wind direction, in such a manner that a higher wire tension is applied to downstream wind turbines 1 than to upstream wind turbines 1, the total power production of the wind energy farm 2 can be increased.

Fig. 4 is a side view of four wind turbines 1 of a wind energy farm 2 according to a third embodiment of the invention. The wind turbines 1 of Fig. 4 are very similar to the wind turbines 1 of Figs. 1 and 3, and they will therefore not be described in detail here.

In the wind energy farm 2 of Fig. 4, one of the wind turbines Id is provided with stay cables 13, two of which are shown, connected at one end to the tower 3 of the wind turbine Id, and at an opposite end to foundations of respective adjacent wind turbines la. The stay cables 13 provide stability to the tower. Since the stay cables 13 are anchored to the foundations of the adjacent wind turbines la, rather than to separate stay cable foundations, the total costs of erecting the wind energy farm 2 can be reduced.

It is possible to use the foundations of the adjacent wind turbines la for anchoring the stay cables 13 because the wind turbines 1 are arranged closer to each other than the wind turbines of prior art wind energy farms. As described above, this is due to the fact that the un-swept parts of the rotor areas of the upstream wind turbines 1 overlap with the swept parts of the rotor areas of the downstream wind turbines 1.

Fig. 5 is a side view of two wind turbines 1 of a wind energy farm 2 according to a fourth embodiment of the invention. One of the wind turbines la is of the kind described above with reference to Fig. 1. The other wind turbine le is a traditional upwind wind turbine, i.e. the wind turbine blades 14 of wind turbine le are not hinged. Accordingly, the entire rotor area of this wind turbine le is swept by the wind turbine blades 14 during rotation of the hub 5, and thereby the swept part of the rotor area constitutes the entire rotor area.

The traditional wind turbine le is arranged downstream relative to the wind turbine la with hinged wind turbine blades 7. It can be seen that the part of the wind which passes through the un-swept centre part of the rotor area of the upstream wind turbine la reaches the rotor area of the downstream wind turbine le.

Fig. 6 is a side view of two wind turbines 1 of a wind energy farm 2 according to a fifth embodiment of the invention. The embodiment of Fig. 6 is very similar to the embodiment of Fig. 5, and it will therefore not be described in detail here. However, in Fig. 6 the upstream wind turbine If defines a tilt angle. It can be seen that this has the consequence that the wake of the upstream wind turbine If follows a downwardly inclined direction behind the upstream wind turbine la and towards the downstream wind turbine le. This has been taken into account when planning the wind energy farm 2 by positioning the downstream wind turbine le in such a manner that there is an overlap between the part of the wake pattern which has passed through the un-swept centre part of the upstream wind turbine If and the rotor area of the downstream wind turbine le.