Wind installation comprising a wind turbine and an airborne wind energy system

Abstract

A wind installation comprising a wind turbine (1) and an airborne wind energy system (12, 13), e.g. in the form of a kite (12) or a glider (13) is disclosed. The wind turbine (1) is electrically connected to the power grid via a power transmission line (27). The wind installation further comprises an airborne wind energy system (12, 13), e.g. in the form of a kite (12) or a glider (13), for generating electrical energy. The airborne wind energy system (12, 13) comprising a separate generator is coupled to the wind turbine (1) via a cable (6) and the separate generator is electrically connected to the power transmission line (27).

Claims

1. A wind installation, comprising: a wind turbine comprising a tower placed on a foundation on a wind turbine site, the wind turbine further comprising at least one nacelle mounted on the tower and for each nacelle, a rotor coupled to the nacelle and being rotatable about an axis of rotation, the rotor being connected to a generator for converting energy of the rotating rotor into electrical energy for a power grid, the wind turbine being electrically connected to the power grid via a power transmission line; and an airborne wind energy system comprising a separate generator for generating electrical energy, the airborne wind energy system being coupled to the wind turbine via a cable and the separate generator being electrically connected to the power grid; wherein one end of the cable of the airborne wind energy system is mounted on the nacelle.

2. The wind installation according to claim 1, wherein the separate generator is an airborne generator.

3. The wind installation according to claim 1, wherein the separate generator is positioned in the nacelle.

4. The wind installation according to claim 1, wherein the separate generator is coupled to a converter unit and/or a transformer of the wind turbine.

5. The wind installation according to claim 1, wherein the airborne wind energy system is mounted on the nacelle via a mounting base being rotatably connected to the nacelle.

6. The wind installation according to claim 1, further comprising a control system for controlling an operation of the airborne wind energy system in dependence on an operation of the wind turbine.

7. A wind energy park comprising a number of wind installations wherein at least one wind installation is a wind installation according to claim 1.

8. The wind installation according to claim 1, further comprising a control structure configured to extract and retract the cable to thereby control movement of a part of the airborne wind energy system which is moved to a higher altitude.

9. The wind installation according to claim 8, wherein the control structure is configured to execute a predetermined movement pattern of the cable effecting rotational movement of the airborne wind energy system.

10. The wind installation according to claim 9, wherein the rotor defines a rotational plane, the rotational plane defining a substantially cone-shaped flow area axially aligned along the axis of rotation, and wherein the rotational movement of the airborne wind energy system is outside the cone-shaped flow area.

11. The wind installation according to claim 9, wherein the rotational movement of the airborne wind energy system is substantially circular.

12. A method for controlling operation of a wind installation, comprising: providing the wind installation of claim 1; and controlling operation of the airborne wind energy system in dependence on operation of the wind turbine.

13. The method according to claim 12, wherein the airborne wind energy system is launched when electrical energy production of the wind turbine is below a rated power for the wind turbine.

14. The method according to claim 12, wherein the airborne wind energy system is retracted when electrical energy production of the wind turbine reaches a rated power for the wind turbine.

15. The method according to claim 12, wherein the airborne wind energy system is retracted at wind speeds above a predefined wind speed upper threshold.

16. The method according to claim 12, wherein the operation of the wind turbine is stopped during launch and/or during retraction of the airborne wind energy system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIGS. 1-3 illustrate wind installations according to three embodiments of the invention,

(3) FIGS. 4 and 5 are perspective views of two airborne wind energy systems for use in a wind installation according to an embodiment of the invention,

(4) FIGS. 6-9 illustrate wind installations according to four embodiments of the invention,

(5) FIGS. 10 and 11 illustrate operation of wind installations according to embodiments of the invention,

(6) FIG. 12 is a graph illustrating power output and thrust relating to a wind installation according to an embodiment of the invention,

(7) FIG. 13 illustrates mounting of an airborne wind energy system on a wind turbine according to an embodiment of the invention,

(8) FIGS. 14 and 15 illustrate wind energy parks according to two embodiments of the invention,

(9) FIG. 16 illustrates electrical connection of wind installations according to an embodiment of the invention to a power grid,

(10) FIG. 17 illustrates operation of a wind turbine and an airborne wind energy system according to six embodiments of the invention,

(11) FIGS. 18 and 19 illustrate wind installations comprising multirotor wind turbines according to two embodiments of the invention,

(12) FIGS. 20 and 21 illustrate operation of wind installations comprising multirotor wind turbines according to embodiments of the invention,

(13) FIG. 22 is a flow chart illustrating a method for controlling the operation of a wind installation according to an embodiment of the invention,

(14) FIG. 23 illustrates a wind installation according to an embodiment of the invention, and

(15) FIG. 24 illustrates operation of the wind installation illustrated in FIG. 23.

DETAILED DESCRIPTION OF THE DRAWINGS

(16) FIG. 1 illustrates a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted on the tower 2. A rotor 4 is coupled to the nacelle 3 in a manner which allows the rotor 4 to rotate relative to the nacelle 3 when wind is acting on wind turbine blades (not shown) mounted on the rotor 4.

(17) The rotor 4 is connected to a main shaft 5, and rotating movements of the rotor 4 are thereby transferred to the main shaft 5. The main shaft 5 is, in turn, coupled to a generator (not shown) via a gear system (not shown). Thereby rotating movements of the main shaft 5 are transformed into electrical energy by means of the generator.

(18) An airborne wind energy system (not shown) is coupled to the wind turbine 1 via a cable 6. The cable 6 is mechanically coupled to the main shaft 5 by the cable 6 being wound around an element 7 being arranged around the main shaft 5. Thereby extracting or retrieving the cable 6 causes the element 7 to rotate. This rotation can be transferred to the main shaft 5, thereby increasing the rotational speed of the main shaft 5 and accordingly increasing the energy production of the generator. This allows the capacity of a power transmission line connecting the generator to a power grid to be utilised to a greater extent, in particular in the case that the energy production of the wind turbine 1 is low, e.g. due to low wind speeds.

(19) The cable 6 may be extracted and retrieved by means of movements of the airborne wind energy system, which could in this case be in the form of a kite. This will be described in further detail below. The energy generated by the airborne wind energy system is, according to this embodiment, transferred to the wind turbine 1 in the form of mechanical energy.

(20) FIG. 2 illustrates a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 is similar to the wind turbine 1 of FIG. 1, and it will therefore not be described in detail here. In FIG. 2 the gear system 8 and the generator 9 of the wind turbine 1 are shown.

(21) In the embodiment of FIG. 2, the cable 6 is wound around an element 7 which is coupled to the gear system 8 via a rotating shaft 10. Thereby rotational movements of the element 7, caused by extracting or retrieving the cable 6, are transferred to the gear system 8, thereby increasing the rotational speed of the input shaft of the generator 9. Accordingly, the energy production of the generator 9 is increased, similar to the situation described above with reference to FIG. 1. Accordingly, in the embodiment of FIG. 2 the energy generated by the airborne wind energy system is also transferred to the wind turbine 1 in the form of mechanical energy.

(22) FIG. 3 illustrates a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 is similar to the wind turbines 1 of FIGS. 1 and 2, and it will therefore not be described in detail here.

(23) In the embodiment of FIG. 3 the cable 6 is electrically connected to a transformer 11 of the wind turbine 1. The transformer 11 is also connected to the generator (not shown) of the wind turbine 1. Thus, the energy generated by the airborne wind energy system is transferred to the wind turbine 1 in the form of electrical energy, and the cable 6 therefore needs to be electrically conducting.

(24) Thus, also in this embodiment, the capacity of the power transmission line connecting the wind turbine 1 to the power grid can be utilised to a greater extent.

(25) FIG. 4 is a perspective view of an airborne wind energy system in the form of a kite 12, for use in a wind installation according to an embodiment of the invention. The kite 12 catches the wind and is thereby moved. This causes a cable 6 attached to the kite 12 to be extracted or retrieved, thereby generating mechanical energy. This mechanical energy is transferred to a wind turbine in a suitable manner. For instance, the mechanical energy may be transferred to the drivetrain of the wind turbine, e.g. to a main shaft of to a gear system as described above with reference to FIGS. 1 and 2. Alternatively, the mechanical energy may be transferred to a separate generator, which is in turn electrically coupled to an electrical component of the wind turbine, e.g. to a transformer, as described above with reference to FIG. 3, or to a converter unit.

(26) FIG. 5 is a perspective view of an alternative airborne wind energy system in the form of a glider 13, also sometimes referred to as a Makani, for use in a wind installation according to an embodiment of the invention. The glider 13 is provided with five rotors 14, each being capable of extracting energy from the wind and generating electrical energy. The generated electrical energy is transferred to a wind turbine by means of an electrically conducting cable (not shown), e.g. in the manner described above with reference to FIG. 3.

(27) FIG. 6 illustrates operation of the kite 12 of FIG. 4. It can be seen that the wind acts on the kite 12 and causes it to move along a movement pattern. For instance, the kite 12 may be extracted along a substantially linear path and subsequently retracted while moving along a movement pattern having the shape of the figure eight, as indicated by the dotted line. During the linear movement of the kite 12, mechanical energy may be transferred to an element arranged at the attachment point 15, thereby causing electrical energy to be generated, e.g. in the manner described above with reference to FIGS. 1-3. During the subsequent retraction of the kite 12, energy may be consumed. However, the energy consumed is expected to be less than the energy being generated during the linear movement of the kite 12.

(28) It should be noted that, even though FIG. 6 shows the kite 12 being connected directly to a foundation 16, it might alternatively be connected to a wind turbine, e.g. in the manner illustrated in any of FIGS. 1-3.

(29) FIG. 7 illustrates operation of the glider 13 of FIG. 5. It can be seen that the wind acts on the glider 13 and causes it to move along a substantially circular movement pattern, as indicated by the dotted line. This movement of the glider 13 causes rotation of the rotors 14, and thereby electrical energy is generated.

(30) The electrical energy is transferred to a suitable electrical component, e.g. a transformer or a converter unit, arranged at the attachment point 15, via electrically conductive cable 6.

(31) It should be noted that, even though FIG. 7 shows the glider 13 being connected directly to a foundation 16, it might alternatively be connected to a wind turbine, e.g. in the manner illustrated in any of FIGS. 1-3.

(32) FIG. 8 illustrates a wind installation according to an embodiment of the invention. FIG. 8a is a side view of the wind turbine 1 and FIG. 8b is a cross sectional view of the wind turbine 1 along the line B-B. The wind turbine 1 comprises a tower 2, a nacelle 3 mounted on the tower 2 and a rotor 4, carrying a set of wind turbine blades 17, coupled rotatably to the nacelle 3.

(33) An airborne wind energy system in the form of a kite 12 is coupled to the tower 2 of the wind turbine 1 via cables 6 and via a bearing system 18 arranged circumferentially with respect to the tower 2. The allows the attachment point 19 between the cables 6 and the bearing system 18 to be rotated about the centre axis of the tower 2, thereby positioning the kite 12 relative to the wind turbine 1 in accordance with the direction of the wind. Accordingly, the bearing system 18 provides a separate yawing system for the airborne wind energy system, which operates independently of the yawing system of the wind turbine 1.

(34) FIG. 9 illustrates a wind installation according to an embodiment of the invention. FIG. 9a is a side view of the wind turbine 1 and FIG. 9b is a cross sectional view of the wind turbine 1 along the line B-B. The wind turbine 1 of FIG. 9 is very similar to the wind turbine of FIG. 8, and it will therefore not be described in detail here. However, in the embodiment of FIG. 9 the airborne wind energy system is in the form of a glider 13, and the cable 6 is electrically conductive.

(35) FIG. 10 illustrates operation of wind installations according to an embodiment of the invention. Three wind turbines 1 are shown in FIG. 10, each comprising a tower 2, a nacelle 3 and a rotor 4 carrying a set of wind turbine blades 17. Each wind installation further comprises an airborne wind energy system in the form of a kite 12 coupled to the nacelle 3. Thereby the kites 12 rotate along with the respective nacelles 3 as these perform yawing movements relative to the respective towers 2, in order to direct the wind turbine blades 17 into the incoming wind. Thereby it is ensured that the kites 12 are launched in a direction pointing away from the wind turbine blades 17 of the wind turbine 1 onto which they are coupled. This reduces the risk of collisions between the cables 6 and the wind turbine blades 17.

(36) Furthermore, the kites 12 are launched in such a manner that they are arranged above neighbouring wind turbines 1, thereby reducing the risk of collisions between the kites 12 and neighbouring wind turbines 1.

(37) It can be seen that the kites 12 are launched to an altitude which is well above the wake created by the wind turbines 1. Furthermore, the wind speeds prevailing at this altitude are expected to be generally higher than the wind speeds prevailing at the altitude of the rotors 4 of the wind turbines 1. This provides a good utilisation of the available wind at the site of the wind turbines 1, and the total energy production of the site can thereby be increased as compared to a situation where airborne wind energy systems are not coupled to the wind turbines 1.

(38) The kites 12 are able to move along specified movement paths, e.g. as described above with reference to FIG. 6. Thereby mechanical energy is generated and transferred to the respective wind turbines 1. Here the mechanical energy may be transferred to the drive trains of the wind turbines 1, e.g. as described above with reference to FIGS. 1 and 2. Alternatively, the mechanical energy may be supplied to a separate generator arranged in the nacelle 3, and the electrical energy generated by the separate generator may be supplied to a suitable electrical component of the wind turbine 1, such as a transformer or a converter unit, e.g. in the manner described above with reference to FIG. 3.

(39) FIG. 11 illustrates operation of wind installations according to an embodiment of the invention. The wind turbines 1 of FIG. 11 are very similar to the wind turbines of FIG. 10, and they will therefore not be described in further detail here.

(40) However, in the wind installations of FIG. 11 the airborne wind energy systems are in the form of gliders 13. The gliders 13 are able to move along specified movement paths, e.g. as described above with reference to FIG. 7. Thereby the rotors 14 of the gliders 13 generate electrical energy, and the generated electrical energy is transferred to the respective nacelles 3 via electrically conducting cables 6. Here the electrical energy is supplied to a suitable electrical component of the wind turbine 1, such as a transformer or a converter unit, e.g. in the manner described above with reference to FIG. 3.

(41) FIG. 12 is a graph illustrating power output and thrust relating to a wind installation according to an embodiment of the invention. The dashed line 20 represents power output (P) from the wind turbine as a function of wind speed (v). The solid line 21 represents thrust (T) on the wind turbine as a function of wind speed (v). At wind speeds within zone 22 it is possible to increase the total power output from the wind installation without increasing costs or mechanical wear on the wind turbine, by coupling an airborne wind energy system to the wind turbine. At wind speeds within zone 23 it is also possible to increase the total power output from the wind installation by coupling an airborne wind energy system to the wind turbine. However, in this case the costs of the electrical parts of the wind turbine are increased. In zone 23 the wind turbine and/or the airborne wind energy system may be derated in order to limit the total power production to a certain maximum level. For instance, the wind turbine may be derated while the power production of the airborne wind energy system is increased, in order to decrease the loads on the wind turbine.

(42) FIG. 13 illustrates mounting of an airborne wind energy system on a wind turbine 1 according to an embodiment of the invention. FIG. 13a is a side view of the wind turbine 1 and FIG. 13b is a top view of the wind turbine 1. The airborne wind energy system is mounted on the nacelle 3 of the wind turbine 1, via a cable 6. Thereby the airborne wind energy system is in general rotated along with the nacelle 3 as it performs yawing movements. However, the cable 6 is attached to a mounting base 24 being rotatably connected to the nacelle 3. Accordingly, the attachment point of the cable 6 is allowed to rotate slightly relative to the nacelle 3. This may, e.g., be required when the airborne wind energy system moves along a movement pattern, e.g. as described above with reference to FIGS. 6 and 7.

(43) FIG. 14 shows a wind energy park according to an embodiment of the invention. Thus, the wind energy park comprises a number of wind installations, nine of which are shown from above. Each wind installation comprises a wind turbine and an airborne wind energy system in the form of a kite 12 attached to the nacelle 3 of the wind turbine 1 by means of a cable 6. The direction of the incoming wind is indicated by arrow 25. It can be seen that the nacelles 3 of the wind turbines 1 have all been yawed to a position where the rotors 4 are directed towards the incoming wind 25. It can also be seen that all of the kites 12 are launched in a direction away from the respective wind turbines 1 along the direction of the incoming wind 25. It can also be seen that the kites 12 are in different positions along their movement patterns. Thus, the kites 12 need not to operate in a synchronous manner.

(44) FIG. 15 shows a wind energy park according to an embodiment of the invention. The wind energy park of FIG. 15 is very similar to the wind energy park of FIG. 14, and it will therefore not be described in detail here. However, in the wind energy park of FIG. 15 the airborne wind energy systems are in the form of gliders 13.

(45) FIG. 16 illustrates electrical connection of wind installations according to an embodiment of the invention to a power grid. FIG. 16 shows four wind installations according to an embodiment of the invention, each comprising a wind turbine 1 and an airborne wind energy system in the form of a kite 12. The wind turbines 1 are arranged in a wind energy park, which also comprises a number of wind turbines 1a, four of which are shown, without an airborne wind energy system coupled hereto.

(46) The wind turbines 1, 1a are all connected to a substation 26 via respective power transmission lines 27. The maximum capacity of each power transmission line is 3400 kVa. Under some wind conditions, the wind turbines 1, 1a are not capable of maintaining an energy production which utilises the maximum capacity of their power transmission lines 27. Under these circumstances the wind installations may launch their kites 12, thereby increasing the total energy production of the wind installation. Thereby the capacities of the power transmission lines 27 are utilised to a greater extent, and the total energy production of the wind energy park is increased.

(47) It should be noted that the airborne wind energy system of one or more of the wind installations could be in the form of a glider instead of in the form of a kite.

(48) FIG. 17 illustrates operation of a wind turbine and an airborne wind energy system according to six embodiments of the invention. The graphs show power production as a function of wind speed. The solid lines 28 represent power production of a wind turbine, and the dashed lines 29 represent total power production of the wind turbine and an airborne wind energy system. The area 30 between the curves 28, 29 represents the contribution to the total power production provided by the airborne wind energy system.

(49) FIG. 17a illustrates a situation where an airborne wind energy system in the form of a kite is mounted on the wind turbine. The airborne wind energy system is launched at low wind speeds, where the power production of the wind turbine is below rated power. Accordingly, the total power production is increased at these wind speeds. However, when the power production of the wind turbine reaches rated power, the airborne wind energy system is retracted, and the total power production corresponds to the power production of the wind turbine. It can be seen from FIG. 17a that the kite is able to produce power at wind speeds which are below the cut-in wind speed for the wind turbine.

(50) FIG. 17b illustrates a situation similar to the situation illustrated by FIG. 17a. However, in FIG. 17b the airborne wind energy system is in the form of a glider. It can be seen from FIG. 17b that contribution to the total power production provided by the glider is somewhat lower than the contribution provided by the kite of FIG. 17a. Furthermore, the cut-in wind speed of the glider is substantially identical to the cut-in wind speed of the wind turbine.

(51) FIG. 17c illustrates a situation where the airborne wind energy system is in the form of a kite, similar to the situation illustrated in FIG. 17a. The operation at low wind speeds is essentially as described above with reference to FIG. 17a. However, in this case, when the power production of the wind turbine reaches rated power, the airborne wind energy system remains in the launched state, and thereby the airborne wind energy system continues to contribute to the total power production, until a cut-out wind speed for the airborne wind energy system is reached. Thus, in the situation illustrated in FIG. 17c, the total power production exceeds the rated power of the wind turbine within a large wind speed range. This requires that the power transmission line connecting the wind turbine to the power grid is designed to handle this increased power production.

(52) FIG. 17d illustrates a situation similar to the situation illustrated in FIG. 17c. However, in this case the airborne wind energy system is in the form of a glider. It can be seen that the glider is able to continue producing power at wind speeds above the cut-out wind speed of the wind turbine. This increases the wind speed range in which power is produced by the system.

(53) FIG. 17e illustrates a situation where the airborne wind energy system is in the form of a kite, similar to the situations illustrated in FIGS. 17a and 17c. The operation at low wind speeds is essentially as described above with reference to FIG. 17a. However, in this case, when the wind speed approaches the wind speed at which the wind turbine is able to produce rated power, the wind turbine is derated, i.e. it is deliberately operated to provide a power production which is lower than the rated power. Instead, the airborne wind energy system remains launched, and it is controlled in such a manner that the total power production of the wind turbine and the airborne wind energy system corresponds to the rated power of the wind turbine. This continues until the cut-out wind speed of the airborne wind energy system is reached, where the airborne wind energy system is retracted and the wind turbine is controlled to produce the rated power. Thus, in this case the total power production does not exceed the rated power of the wind turbine at any time, but the loads on the wind turbine are reduced because a substantial part of the total power production is provided by the airborne wind energy system within a large wind speed range.

(54) FIG. 17f illustrates a situation similar to the situation illustrated in FIG. 17e. However, in this case the airborne wind energy system is in the form of a glider. As described above with reference to FIG. 17d, it can be seen that the glider is able to produce power at high wind speeds, and therefore the wind turbine remains derated until the cut-out wind speed for the wind turbine is reached.

(55) FIG. 18 illustrates a wind installation according to an embodiment of the invention. The wind turbine 1 comprises four rotors 4, each mounted on an arm 31 mounted on the tower 2. Thus, the wind turbine 1 of FIG. 18 is a multirotor wind turbine.

(56) An airborne wind energy system in the form of a kite 12 is mounted on the wind turbine 1 at the top of the tower 2, by means of a cable 6. Since the rotors 4 are mounted on the arms 31, at a distance from the tower 2, the wind turbine blades 17 are well clear of the mounting position of the kite 12. Accordingly, the risk of collisions between the wind turbine blades 17 and the kite 12 or the cable 6 is very low. Thus, the wind turbine 1 of FIG. 18 is very suitable for having an airborne wind energy system mounted thereon.

(57) The total power production of the wind installation comprises contributions from each of the rotors 4 and from the kite 12. A given power production level can be reached by appropriately controlling the power production of each of these components 4, 12. For instance, one of the rotors 4 may be completely stopped while the kite 12 produces maximum power.

(58) It should be noted that, even though the wind turbine 1 of FIG. 18 comprises four rotors 4, a multirotor wind turbine having a different number of rotors would also fall within the scope of protection of the present invention. In particular, a multirotor wind turbine having two rotors 4 arranged substantially at the same vertical level would be very suitable.

(59) FIG. 19 illustrates a wind installation according to an embodiment of the invention. The wind turbine 1 of FIG. 19 is very similar to the wind turbine 1 of FIG. 18, and it will therefore not be described in detail here. However, in the wind turbine 1 of FIG. 19, the airborne wind energy system is in the form of a glider 13.

(60) FIG. 20 illustrates operation of wind installations according to an embodiment of the invention. FIG. 20 is very similar to FIG. 10, and it will therefore not be described in detail here. However, in FIG. 20 the wind turbines 1 are of the kind illustrated in FIG. 18.

(61) FIG. 21 illustrates operation of wind installations according to an embodiment of the invention. FIG. 21 is very similar to FIG. 11, and it will therefore not be described in detail here. However, in FIG. 21 the wind turbines 1 are of the kind illustrated in FIG. 19.

(62) FIG. 22 is a flow chart illustrating a method for controlling a wind installation according to an embodiment of the invention. The process is started at step 32. At step 33 it is investigated whether or not the power production of the wind turbine is below the rated power for the wind turbine. If this is not the case, normal operation of the wind turbine is continued, and the process is returned to step 33 for continued monitoring of the power production of the wind turbine.

(63) In the case that step 33 reveals that the power production of the wind turbine is below the rated power for the wind turbine, this is an indication that the capacity of a power transmission line connecting the wind turbine to a power grid is not utilised fully. Therefore the process is forwarded to step 34, where an airborne wind energy system coupled to the wind turbine is launched. Prior to initiating the launch of the airborne wind energy system the operation of the wind turbine is stopped in order to avoid collisions between the launching airborne wind energy system and moving wind turbine blades of the wind turbine.

(64) At step 35 it is investigated whether or not the launch of the airborne wind energy system has been completed. If this is not yet the case, operation of the wind turbine remains stopped and the process is returned to step 35 for continued monitoring of the launching process.

(65) In the case that step 35 reveals that the launch of the airborne wind energy system has been completed, it is considered safe to restart operation of the wind turbine. The process is therefore forwarded to step 36, where the wind turbine is started. Accordingly, the total power production of the wind installation includes the power production of the wind turbine itself as well as the power production of the airborne wind energy system. Accordingly, the total power production of the wind installation is increased, and the capacity of the power transmission line can be utilised to a greater extent.

(66) At step 37 it is investigated whether or not the power production of the wind installation has reached the rated power for the wind turbine. If this is not the case, operation of the wind turbine as well as operation of the airborne wind energy system is continued, and the process is returned to step 37 for continued monitoring of the power production of the wind installation.

(67) In the case that step 37 reveals that the power production of the wind installation has reached the rated power for the wind turbine, it may be assumed that the power production of the wind turbine itself is now sufficient to fully utilise the capacity of the power transmission line. The additional power production provided by the airborne wind energy system is therefore no longer required. Accordingly, the process is forwarded to step 38, where retraction of the airborne wind energy system is initiated. During the retraction of the airborne wind energy system, operation of the wind turbine is stopped in order to avoid collisions between the airborne wind energy system and rotating wind turbine blades of the wind turbine.

(68) At step 39 it is investigated whether or not the retraction of the airborne wind energy system has been completed. If this is not yet the case, operation of the wind turbine remains stopped and the process is returned to step 39 for continued monitoring of the retraction process.

(69) In the case that step 39 reveals that the retraction of the airborne wind energy system has been completed, the process is forwarded to step 40, where operation of the wind turbine is started.

(70) Finally, the process is returned to step 32 in order to monitor the power production of the wind turbine.

(71) FIG. 23 illustrates a wind installation according to an embodiment of the invention. The wind installation comprises a wind turbine 1 and an airborne wind energy system 13. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted on the tower 2. A rotor 4 is coupled to the nacelle 3 in a manner which allows the rotor 4 to rotate relative to the nacelle 3 when wind is acting on wind turbine blades 17 mounted on the rotor 4. The airborne wind energy system 13 is coupled to the wind turbine 1 via a cable 6.

(72) The wind installation comprises a control structure (not shown) which is configured to control movement of the part of the airborne wind energy system 13 which is launched to a higher altitude.

(73) The control structure is configured to execute a predetermined movement pattern effecting rotational movement of the airborne wind energy system 13, i.e. a 360 degrees movement about the axis of rotation.

(74) The rotor 4 defines a rotational plane 41; i.e. the plane in which the blades 17 rotate. The rotational plane 41 defines a substantially cone shaped flow area 42 axially along the axis of rotation, where the outer periphery of the cone shaped flow area is defined by the wind turbine blade tips 43. The movement of the airborne wind energy system 13 is controlled so that the rotational movement is outside flow area 42. Thereby the energy production of the airborne wind energy system 13 can be increased due to specific flow conditions caused by the blades 17. This is schematically illustrated by V1 and V4, where V1 is the air velocity in front of the blades 17 and V4 is the air velocity behind the blades 17, where V4 is larger than V1.

(75) FIG. 24 illustrates operation of the wind installation illustrated in FIG. 23, where movement of the airborne wind energy system 13 is controlled so that the rotational movement hereof is outside the flow area 42 (illustrated by the dotted line).

EMBODIMENTS

(76) The invention may e.g. be covered by the following embodiments:

(77) Embodiment 1. A wind turbine (1) comprising a tower (2) placed on a foundation on a wind turbine site, the wind turbine (1) further comprising at least one nacelle (3) mounted on the tower (2) and a rotor (4) coupled to each nacelle (3) generating electrical energy for a power grid, the wind turbine (1) being electrically connected to the power grid via a power transmission line (27), the wind turbine (1) further comprising an airborne wind energy system (12, 13) for generating electrical energy, the airborne wind energy system (12, 13) being coupled to the wind turbine (1) via a cable (6) and electrically connected to the power transmission line (27).

(78) Embodiment 2. A wind turbine (1) according to embodiment 1, wherein the airborne wind energy system (12, 13) is mechanically coupled to a drivetrain of the wind turbine (1).

(79) Embodiment 3. A wind turbine (1) according to embodiment 1 or 2, wherein the airborne wind energy system (12, 13) comprises at least one separate generator.

(80) Embodiment 4. A wind turbine (1) according to embodiment 3, wherein the airborne wind energy system (13) comprises at least one airborne generator.

(81) Embodiment 5. A wind turbine (1) according to embodiment 3, wherein the airborne wind energy system (12, 13) comprises at least one generator positioned in the nacelle (3).

(82) Embodiment 6. A wind turbine (1) according to any of embodiments 3-5, wherein the separate generator is coupled to a converter unit and/or a transformer (11) of the wind turbine (1).

(83) Embodiment 7. A wind turbine (1) according to any of the preceding embodiments, wherein one end of the cable (6) of the airborne wind energy system (12, 13) is mounted on the nacelle (3).

(84) Embodiment 8. A wind turbine (1) according to embodiment 7, wherein the airborne wind energy system (12, 13) is mounted on the nacelle (3) via a mounting base (24) being rotatably connected to the nacelle (3).

(85) Embodiment 9. A wind turbine (1) according to any of embodiments 1-6, wherein one end of the cable (6) of the airborne wind energy system (12, 13) is mounted to the foundation or the tower (2) of the wind turbine (1).

(86) Embodiment 10. A wind turbine (1) according to embodiment 9, wherein the cable (6) is mounted via a bearing system (18) arranged circumferentially with respect to the tower (2).

(87) Embodiment 11. A wind turbine (1) according to embodiment 10, wherein the bearing system (18) is arranged at some height and/or near the ground.

(88) Embodiment 12. A wind turbine (1) according to any of the preceding embodiments, wherein the wind turbine (1) comprises a control system for controlling the operation of the airborne wind energy system (12, 13) in dependence on the wind turbine operation.

(89) Embodiment 13. A wind energy park comprising a number of wind turbines (1) wherein at least one wind turbine (1) is a wind turbine (1) according to any of the preceding embodiments.

(90) Embodiment 14. A method for controlling the operation of a wind turbine (1), the wind turbine (1) comprising a tower (2) placed on a foundation, the wind turbine (1) further comprising at least one nacelle (3) mounted on the tower (2) via a yaw bearing and a rotor (4) coupled to each nacelle (3) generating electrical energy for a power grid, the wind turbine (1) further comprising an airborne wind energy system (12, 13) for generating electrical energy, the method comprising controlling the operation of the airborne wind energy system (12, 13) in dependence on the wind turbine operation.

(91) Embodiment 15. A method according to embodiment 14 wherein the airborne wind energy system (12, 13) is launched when the power production of the wind turbine (1) is below a rated power for the wind turbine (1).

(92) Embodiment 16. A method according to embodiment 14 or 15, wherein the airborne wind energy system (12, 13) is retracted when the power production of the wind turbine (1) reaches a rated power for the wind turbine (1).

(93) Embodiment 17. A method according to any of embodiments 14-16, wherein the airborne wind energy system (12, 13) is retracted at wind speeds above a predefined wind speed upper threshold.

(94) Embodiment 18. A method according to any of embodiments 14-17, wherein operation of the wind turbine (1) is stopped during launch and/or retraction of the airborne wind energy system (12, 13).