WO2019120403A1

WO2019120403A1 - An airborne wind energy system with a safety system

Info

Publication number: WO2019120403A1
Application number: PCT/DK2018/050322
Authority: WO
Inventors: Torben Ladegaard Baun; Thomas S. Bjertrup Nielsen; Niels Vinther Voigt
Original assignee: Vestas Wind Systems A/S
Priority date: 2017-12-22
Filing date: 2018-12-04
Publication date: 2019-06-27

Abstract

An airborne wind energy system (100) and a method for controlling such an airborne wind energy system (100), which comprises a kite (102) connected by a number of steering lines (301) to a control box (300) and coupled via a cable (105) to a winch system on a ground station (104). The control method comprises detecting an error signal from a component of the airborne wind energy system (100), wherein the error signal is set by a detection of a breaking of the cable (105) and/or a fault on the winch system controlling the extraction and retraction of the cable (105). The control method further comprises controlling the movement of the control box (300) according to a landing mode, and activating a safety device connected to the control box (300).

Description

l

AN AIRBORNE WIND ENERGY SYSTEM WITH A SAFETY SYSTEM.

FIELD OF THE INVENTION

The present invention relates to a method for controlling an airborne wind energy system comprising a kite connected by a number of steering lines to a control box and coupled via a cable to a winch system on a ground station, and wherein the control box is configured for controlling the movement of the kite via the steering lines. The invention further relates to an airborne wind energy system.

BACKGROUND OF THE INVENTION Various airborne wind energy systems, being capable of capturing wind energy at a higher altitude than traditional wind turbines, are known. Common to these systems is that a part of the system is launched to a high altitude, where energy of the wind is harvested. The harvested energy is transferred to a ground station, either in the form of mechanical energy or in the form of electrical energy. In the case that the transferred energy is in the form of mechanical energy, a generator will normally be arranged at the ground station in order to convert the mechanical energy into electrical energy. In the case that the transferred energy is in the form of electrical energy, the airborne wind energy system comprises an airborne generator, i.e. the part of the system which is launched to a high altitude includes a generator. The part of the airborne wind energy system being launched to a high altitude may, e.g., include a kite or a glider.

A number of airborne wind energy systems are described in Cherubini, et al., 'Airborne Wind Energy Systems: A review of the technologies', Renewable and Sustainable Energy Reviews, 51 (2015) 1461-1476. Two or more airborne wind energy system can with benefits be placed in wind energy parks both onshore and offshore, meaning that more than one airborne wind energy system can be in operation at the same time and with a ground/sea level unit placed relative close together within a specific site area. Autonomous kites can operate at altitudes of hundreds or even thousands of meters. Both the kite itself and the cables connecting them to the ground may pose a variety of safety risks to neighbouring structures and especially to persons on the ground near a wind energy park.

DESCRIPTION OF THE INVENTION It is an object of embodiments of the invention to provide a method for controlling of an airborne wind energy system with a higher safety and reduced risk for persons near the airborne wind energy system.

It is a further object of embodiments of the invention to provide an airborne wind energy system and a method of controlling the airborne wind energy system wherein more parts of the system may be reused even after severe faults in the system.

According to a first aspect, the invention relates to a method for controlling an airborne wind energy system, wherein the airborne wind energy system comprises a kite connected by a number of steering lines to a control box and coupled via a cable to a winch system on a ground station, and wherein the control box is configured for controlling the movement of the kite via the steering lines. The control method comprises:

- detecting an error signal from a component of the airborne wind energy system, wherein the error signal is set by a detection of a breaking of the cable and/or a fault on the winch system controlling the extraction and retraction of the cable,

- controlling the movement of the control box according to a landing mode, and - activating a safety device connected to the control box.

The airborne wind energy system controlled by the method according to the invention comprises a kite as a wind engaging member. The kite is coupled to a control box via steering lines the operation and movements of which the control box controls thereby controlling the movement of the kite. The kite and the control box are connected to a winch system in a ground station via a cable. Accordingly, the airborne wind energy system is mechanically attached to the ground station by means of the cable. The winch system controls an extraction and retraction of the cable and thereby the height and in part the position and movement of the kite. If an error signal from a component of the airborne wind energy system is detected, the control mode of the control box is changed to a so-called landing mode with a view to land the control box as safely as possible on the ground with minimal risk of damage to itself or other objects. The landing mode may involve controlling the movement of the kite as an isolated system with or without any cable connection to the ground and such as to steer and paraglide the kite with the control box down to the ground. Alternatively or additionally this may involve releasing the kite from the control box and guiding the control box by other means such as a parachute or an airbag.

Further, a safety device connected to the control box is activated. Prior to activation, the safety device connected to the control box may be provided inside the control box or anywhere on the outside of the control box.

Alternatively, the safety device may be provided on the steering lines, or on the cable. The safety device is typically compactly folded prior to activation such that it does not affect aerodynamics of the airborne wind energy system. Once the error signal is detected, the safety device is activated, typically, by unfolding, inflating, or automatic assembling such that it creates a protection for the control box or ensure safe landing on the ground.

The error signal may be set by a central control unit of the airborne wind energy system, and/or by the control box upon receiving signals from one or more sensors from which the control unit and/or control box can determine that the cable between the ground station and the control box is broken or near broken, or that there is a fault in the winch system. Such fault may for example be a jamming of the cable or a damage to the main bearing or main shaft in the winch system. The detection of the error may be made based on a cable-break detection algorithm. The input signals used in the determination may for example come from a strain gauge on the cable, based on images from one or more cameras, or power output signal, torque at the winch, strain in the cable, speed and/or acceleration of the control box and/or cable, position of the control box and/or cable (relative to expected position), cable angle, sudden death of signals through the cable, light through the cable and including a combination of one or more of mentioned input signals.

The method may further include releasing a part of or any remaining cable from the control box. In an embodiment, the cable may be released from the control box. This may be performed by means of some releasing or snap mechanism and may be activated upon the detection of the cable error signal. If the cable is broken somewhere along its length thereby causing the cable error signal to be set, the remaining part of the cable hanging from the control box may hereby be released. If the winch system is still operating, the other end of the cable extending from the ground may be retracted or pulled in by the winch system thereby reducing the amount of cable falling from the sky. This may be advantageous if the length of the remaining cable is so long as to be a

disadvantage for example because of its end dragging over the ground or being in risk of hitting other objects, or if the weight of the cable is disadvantageous for the controlled landing of the control box.

Alternatively, the entire cable or any remaining cable may remain attached to the control box during the landing and guiding down of the control box. This may in some situations aid in guiding the control box down to the ground in a controlled manner and for example prevent the control box from gliding too far away by the wind. Hereby is obtained a control method where the system reacts quickly and autonomously to a cable error and in a way to reduce the risks and increase the safety of both the control box itself and of the surroundings. In an embodiment of the invention, the controlling of the movement of the control box according to the landing mode may comprise steering the kite as a paraglider towards the ground. The steering may be obtained by the controlling and the moving of the steering lines up to the kite. As the cable is disconnected from the control box, the movement of the kite when a steering line is pulled or loosened has changed and the control box may therefore control the kite in the same or similar way as a paraglider, whereby the kite with the still attached control box can glide relatively slowly to the ground and in a well-controlled manner.

In an embodiment according to the above, the method may further comprise steering the kite towards a predetermined landing zone on the ground. The positions of a number of well-suited landing zones may be pre-programmed into the control box and the kite may be directed thereto by means of a GPS in the control box. In this way the kite may be directed to a landing zone which is well- suited, for example an open field, a lake or the like. This furthermore increases the possibilities of retrieving and re-using the control box.

In an embodiment, the controlling of the movement of the control box according to a landing mode may comprise operating a turbine connected to the control box. The turbine may be connected to the control box such as to act as a propeller and yield an airstream in a controllable direction. Alternatively, the turbine may be mounted to give small bursts of thrust in a fixed direction when operated.

In an embodiment, the method may further comprise releasing the steering lines to the kite from the control box. Hereby the control box is relieved of the relatively large kite which may prove difficult to control, for example, at high wind speeds or turbulent wind conditions. A well-controlled gliding or descent to the ground of the control box may then be obtained by the one or more safety devices.

In an embodiment, the activation of the safety device may comprise one or more of inflating an airbag attached to the control box, releasing a parachute attached to the control box, and releasing a flotation device attached to the control box. These different safety devices may be used alone or in combinations to act as both steering and landing gear to the control box. The airbag may be mounted such as to fold around and shield at least a part of the control box when inflated. The flotation device may be activated upon impact and/or contact with water and may for example comprise an inflated member attached by a line to the control box.

Further, in an embodiment, the activation of the safety device may comprise emitting a warning signal and/or a tracking signal comprising one or more of a light, sound, and radio signal. Hereby persons or vehicles or airplanes may be adequately warned when the control box is descending to the ground attracting the attention of any persons in the area. This is especially advantages as the control box with a kite or parachute may glide a significant distance with the wind before reaching the ground. Furthermore the warning signals may prove sufficient to avoid person injury whereby a barrier or fence around an entire wind energy park can be avoided.

The activation of the safety device may comprise emitting a tracking signal comprising one or more of a light, sound, and radio signal. The tracking signals may in principal be the same as the warning signals and may potentially be emitted from the same warning system. However, the tracking signals

advantageously need not be as loud or powerful as the warning signals.

The invention in a further aspect relates to an airborne wind energy system comprising a kite connected by a number of steering lines to a control box and coupled via a cable to a winch system on a ground station, and wherein the control box is configured for controlling the movement of the kite via the steering lines, the control box further comprising a safety device configured to be activated by the control box upon the detection of an error signal. The advantages hereof and of the following embodiments are as described in the previous in relation to the control method. In an embodiment, the error signal may be set by a detection of a breaking of the cable and/or a fault on the winch system controlling the extraction and retraction of the cable.

In a further embodiment, the airborne wind energy system may further comprise a cable releasing mechanism operatable by the control box and configured for releasing the cable from the control box.

In a further embodiment, the airborne wind energy system may further comprise a releasing mechanism operatable by the control box and configured for releasing the steering lines to the kite from the control box. In an embodiment, the safety device may comprise one or more of: a parachute attached to the control box, an airbag attached to the control box and

configured for at least partly shielding the control box when inflated, and a turbine attached to the control box and configured for influencing the movement of the control box when operated. In a further embodiment, the safety device may comprise a warning system for emitting one or more of light, sound, and radio signals.

The safety device may further comprise a tracking system for emitting one or more of light, sound, and radio signals.

In an embodiment, the safety device may comprise a flotation element attached to the control box and configured for releasing upon contact with water.

In an embodiment, the control box may further comprise a battery configured for the supply of energy to the safety system when activated and/or operated.

In an embodiment of the invention, the control box may further comprise a GPS tracker. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view of an airborne wind energy system for use in a wind energy park according to an embodiment of the invention,

Figures 2A and B illustrate the power generation phase and the recovery phase of an airborne wind energy system according to embodiments of the invention,

Figs. 3-4 illustrate a wind energy park with a number of airborne wind energy systems according to embodiments of the invention, and

Figs. 5-8 illustrate a control box comprising different types of safety systems according to various embodiments of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of an airborne wind energy system 100 for use in a wind energy park according to an embodiment of the invention. The airborne wind energy systems 100 comprises a wind engaging member 101 catching wind and which is moved by the wind and is connected to a ground station 104 via one or more cables 105. The wind engaging member 101 is in the form of a kite 102 connected to a control box 300 via steering lines 301 and to a winch system (not shown) in the ground station 104 typically via a single cable 105. The operation of the kite 102 can be fully or partly controlled by the operation of the steering lines 301 by the control box 300 and in addition to the extraction and retraction of the cable 105 controlled from the winch system.

The extraction of the cable 105 from the winch system generates mechanical energy which is transferred via the winch system to a generator positioned on the ground station 104. The generator is in turn electrically coupled to a power transmission line and to a power grid and/or power storage optionally via a converter and/or transformer. Figures 2A and 2B illustrate the operation of the kite 102 and with typical flight trajectories 400 indicated. Typically, the kite operation comprises a power generation phase of upwards movement 410 of the kite where the kite 102 may extract the cable 105. Here, the wind acting on the kite 102 and the tensioning forces in the cable 105 and in the steering lines 301 cause the kite 102 to move along a flight trajectory having the shape of an upwards spinning figure eight 401 or circular pattern 501. Subsequently, the kite 102 is retracted while moving along a substantially linear path 420. During this recovery phase wherein the kite 102 is retracted, energy may be consumed. However, the energy consumed is expected to be less than the energy being generated during the upwards spinning movement of the kite 102. Upon reaching a minimum height, the kite 102 is operated to enter a new power generation phase.

Typically, the kite 102 may be extracted by the wind to a maximum height in the range of 600-1000 m depending on the type of kite 102, and is retracted to a minimum height in the range of 50-150 m. Typically, the recovery phase takes up in the order of 10-30% of the time of a total cycle of a power generation phase followed by a recovery phase.

Figures 3 and 4 illustrate the operation of airborne wind energy systems 100 in a wind energy park 500 according to an embodiment of the invention and as seen from a side and in a top view, respectively. A number of airborne wind energy systems 100 are shown in the figures, each comprising a wind engaging member

101 in the form of a kite 102 and each connected to a ground station 104 via a cable 105. The wind engaging members 101 are here shown as all being kites

102 of the same type. However, in an embodiment, an energy park 500 may be equipped with different types of airborne wind energy systems 100 such as for example a kite next to a glider, etc. The airborne wind energy systems 100 may be directly or indirectly connected optionally via one or more central control units (not shown) which in part or completely may contribute to the controlling of the airborne wind energy systems.

The kites 102 are able to move along specified movement paths or flight trajectories generating mechanical energy, e.g. as described above with reference to figure 2AB. It can be seen that the kites 102, or gliders, i.e., the wind engaging members 101 are in different positions along their movement patterns or flight trajectories and thereby need not stand precisely in the wind direction (indicated by the arrow 501). Thus, the kites and/or gliders 101 need not to operate in a synchronous manner. It should also be noted that the direction of the wind 501 at the positions of the wind engaging members 101 may be the same or may vary for one reason because of the height variations between the kites and/or gliders 101 at a specific time.

According to an embodiment of the invention, if a fault either on the cable 105 or on the winch system controlling the extraction and retraction of the cable 105 is detected, any remaining piece of the cable 105 attached to the control box 300 may be released, and the movement of the control box 300 is controlled according to a landing mode. Further, a safety device connected to the control box 300 is released.

This is illustrated in figure 4 showing a wind energy park 500 comprising a number of airborne wind energy systems 100 and as seen from above. Here, the cable 105 of a first airborne wind energy system 700 has broken whereby an error signal is set. The remaining cable 105a hanging from the control box 300 is then about to be released and the movement of the control box 300 of the faulty airborne wind energy system 700 is operated according a landing mode and a safety device connected to the control box 300 activated. The wind engaging member 101 of the faulty airborne wind energy system 700 is thereby allowed to fall, glide or to be steered to the ground with safety precautions taken to reduce the risk of impacting or colliding with other equipment or persons and to increase the chance of recovery and re-use of the control box 300.

Additionally, figure 4 further illustrates how a belt or a clearance may be made for the wind engaging member 101 of the faulty airborne wind energy system 700 with the broken cable 105 to land. Some of the airborne wind energy systems 900 in the downwind direction 501 have been retracted 1001, and some of their neighbouring airborne wind energy systems 910 on both sides are operated to steer their wind engaging members in directions (illustrated by arrows 901) away from the landing path of the control box of the faulty airborne wind energy system 700. In this way the airspace is cleared for the control box and the wind engaging member of the first airborne wind energy system 700 to be taken or guided down even if the wind engaging member drifts by the wind over a considerable distance. The airborne wind energy systems marked with Vs are continued to be operated according to a normal control mode.

The controlled movement of the control box 300 according to a landing mode may in an embodiment comprise steering the kite 102 as a paraglider towards the ground. This is illustrated in figure 5 showing a sketch of an airborne wind energy system 100 where the cable has been broken and/or released from the control box 300 by a release mechanism (not shown) and where the kite 102 is then steered as a paraglider by a controlled operation of some or all of the steering lines 301 by the control box 300. The control box 300 further comprises a turbine 302 which here is positioned on an end of the control box 300 and is operated as a propeller to further aid in the steering of the control box 300. The control box 300 may comprise a battery (not shown) electrically connected and configured to supply the energy necessary to operate the steering lines 301 as well the propeller 302 and/to activate and drive another safety device connected to the control box 300. The safety device of the control box 300 and/or the kite 102 may further comprise emitting a warning signal from the flying unit with the aim of warning people, and ground or air traffic near the falling or gliding control box 300. The warning signals from the warning system 303 are indicated by the waves 304. The signals may comprise light, sound and radio signals or combinations thereof. Similarly, the safety device may comprise a tracking system (not shown) emitting tracking signals. These are emitted with the aim of facilitating the tracking, retrieval, and re-use of the control box 300 from where it has landed. The tracking signals may likewise comprise light, sound and radio signals or combinations thereof. Figures 6 and 7 illustrate the safety device comprising an airbag 600 attached to the control box 300. During descent a hatch 601 is automatically opened from which the airbag 600 is inflated upon or shortly before impacting the ground. When inflated, the airbag 600 shields at least partially the control box 300 both reducing the impact on the control box 300 itself and on any object being hit by the control box 300. The airbag 600 may also act as a floatation device preventing the control box 300 from sinking if landing in water. In the

embodiment illustrated in figures 6 and 7 a part of the cable 105 is still attached to the control box 300. It may in another embodiment be released from the control box 300 at some point before or during the guiding down of the control box 300. This may be advantageous if the length of the remaining cable 105 is so long as to be a disadvantage for example because of its length dragging over the ground or being in risk of hitting other objects, or if the weight of the cable is disadvantageous for the controlled landing of the control box 300. In figure 8 is sketched a control box 300 during its landing mode. Here first the kite (not shown) is released from the control box 300 by releasing the steering lines, and subsequently a parachute 800 attached to the control box 300 is released. As the parachute 800 unfolds, the descent of the control box 300 slows considerably. Figure 8 further illustrates the inflation of an airbag 600 just before impacting the ground. Then the parachute 800 is released or cut loose from the control box 300 and the airbag 600 may be deflated whereby the control box 300 rests on the ground without being dragged over the ground.

Claims

1. A method for controlling an airborne wind energy system (100), wherein the airborne wind energy system (100) comprises a kite (101, 102) connected by a number of steering lines (301) to a control box (300) and coupled via a cable (105) to a winch system on a ground station (104), and wherein the control box

(300) is configured for controlling the movement of the kite (101, 102) via the steering lines (301), the control method comprising :

- detecting an error signal from a component of the airborne wind energy system (100), wherein the error signal is set by a detection of a breaking of the cable (105) and/or a fault on the winch system controlling the extraction and retraction of the cable (105),

- controlling the movement of the control box (300) according to a landing mode, and

- activating a safety device connected to the control box (300). 2. A method according to claim 1, further including releasing a part of or any remaining cable (105) from the control box (300).

3. A method according to any of the preceding claims, wherein the controlling of the movement of the control box (300) according to the landing mode comprises steering the kite (101, 102) as a paraglider towards the ground. 4. A method according to claim 3 further comprising steering the kite (101, 102) towards a predetermined landing zone on the ground.

5. A method according to any of the preceding claims, wherein the controlling of the movement of the control box (300) according to a landing mode comprises operating a turbine connected to the control box (300).

6. A method according to any of the preceding claims further comprising releasing the steering lines (301) to the kite (101, 102) from the control box (300).

7. A method according to any of the preceding claims, wherein the activation of the safety device comprises one or more of inflating an airbag (600) attached to the control box (300), releasing a parachute (800) attached to the control box (300), and releasing a flotation device attached to the control box (300).

8. A method according to any of the preceding claims, wherein the activation of the safety device comprises emitting a warning signal comprising one or more of a light, sound, and radio signal.

9. A method according to any of the preceding claims, wherein the activation of the safety device comprises emitting a tracking signal comprising one or more of a light, sound, and radio signal.

10. An airborne wind energy system (100) comprising a kite (101, 102) connected by a number of steering lines to a control box (300) and coupled via a cable (105) to a winch system on a ground station (104), and wherein the control box (300) is configured for controlling the movement of the kite (102) via the steering lines (301), the control box (300) further comprising a safety device configured to be activated by the control box (300) upon the detection of an error signal.

11. An airborne wind energy system (100) according to claim 10, wherein the error signal is set by a detection of a breaking of the cable (105) and/or a fault on the winch system controlling the extraction and retraction of the cable (105).

12. An airborne wind energy system (100) according to any of claims 10-11, wherein the airborne wind energy system further comprises a cable releasing mechanism operatable by the control box (300) and configured for releasing the cable (105) from the control box (300).

13. An airborne wind energy system (100) according to any of claims 10-12, wherein the safety device comprises one or more of: an airbag (600) attached to the control box (300) and configured for at least partly shielding the control box (300) when inflated, a parachute (800) attached to the control box (300), a turbine (302) attached to the control box (300) and configured for influencing the movement of the control box (300) when operated, and a flotation element attached to the control box (300) and configured for releasing upon contact with water.

14. An airborne wind energy system (100) according to any of claims 10-13, wherein the control box (300) further comprises a battery configured for the supply of energy to the safety system when activated and/or operated.

15. An airborne wind energy system (100) according to any of claims 10-14, wherein the control box (300) further comprises a GPS tracker.