WO2019190313A2

WO2019190313A2 - Airborne wind energy system

Info

Publication number: WO2019190313A2
Application number: PCT/NL2019/050183
Authority: WO
Inventors: Johannes Catherina Marie BREUER; Johannes Otto PESCHEL
Original assignee: Enevate B.V.
Priority date: 2018-03-27
Filing date: 2019-03-25
Publication date: 2019-10-03
Also published as: NL2020673B1; WO2019190313A3

Abstract

Airborne wind energy system comprising a wind-engaging member with a bridle system (3) connected to a tether (2), a tether storage device for winding and unwinding the tether (2), an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member (4), for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member (4) and/or winding and unwinding the tether (2), wherein the wind engaging member (4) is a kite or kite-like structure, wherein the bridle (3) comprises a number of separate power- lines (11) that are connected to respective connection points located at or near a leading edge of the wind-engaging member (4), and optionally one or more steering lines (16) that are connected to the wind engaging member at respective one or more steering points remote from the connections points, wherein the wind-engaging member includes at least one air passage having an operable closing member.

Description

Title: Airborne wind energy system

The invention concerns an airborne wind energy system comprising a wind-engaging member with a bridle system connected to a tether, a tether storage device for winding and unwind-ing the tether, an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member and/or to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind- engaging member and/or winding and unwinding the tether. The invention also concerns a method for operating such wind energy system and a launch and landing system.

Such a system is known from NL2009528. Summarizing the prior art, airborne wind energy systems are designed to operate at higher altitudes than conventional tower-based wind energy systems. The wind- engaging members are typically rigid wings, flexible wings or aerostats. The wind-engaging member is tethered to a ground station. The ground station comprises a tether storage device, typically a winch, to wind and unwind the tether, and is connected to an energy converting device, typically a generator. During unwinding of the tether, the wind-engaging member is steered along a certain flight trajectory that can depend on the wind direction. Respective cross wind flight manoeuvers generate a high traction force which is transferred by the winch to the generator where it is converted to electricity. When reaching e.g. the maximum tether length the wind engaging member is de-powered. This means that the relative angle of the wind-engaging member with respect to the apparent wind is reduced such that the traction force in the tether is minimized. Using the generator as a motor, the tether will then be wound onto the drum. Since the traction force during winding is substantially lower than during unwinding, the energy consumed is only a fraction of the energy generated during

unwinding. The present invention aims to provide an improved airborne wind energy system. In particular, the invention aims to provide an airborne wind energy system that can be efficiently operated, that can provide good or optimum energy generation as well as ease of use. Also, the invention aims to provide a durable airborne wind energy system.

According to a first aspect of the invention there is provided a system that is characterized by the features of claim 1.

Advantageously, there is provided an airborne wind energy system comprising a wind-engaging member with a bridle system connected to a tether, a tether storage device for winding and unwinding the tether, an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member, for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member and/or winding and unwinding the tether, wherein the wind engaging member is a kite or kite -like structure, wherein the bridle comprises a number of separate power- lines that are connected to

respective connection points located at or near a leading edge of the wind- engaging member, and optionally one or more steering lines that are connected to the wind engaging member at respective one or more steering points remote from the connections points, wherein the wind-engaging member includes at least one air passage having an operable closing member.

It has been found that the air passage can provide improved wind- engaging member control via relatively simple means. For example, the one or more air passages can be opened or closed, depending on the state or position of the wind-engaging member, and/or depending on a trajectory that is to be taken by the wind-engaging member during operation. The opening and respective closing member can be located at various positions of the wind-engaging member, in particular in a position that allows a desired kite control based on the state of the closing member (i.e. closing or releasing the respective air passage).

Further, according to another advantageous aspect of the invention, there is provided an airborne wind energy system, for example a system according to the above-described first aspect, comprising a wind- engaging member with a bridle system connected to a tether, a tether storage device for winding and unwinding the tether, an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member, for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member and/or winding and unwinding the tether, wherein the wind engaging member is a kite or kite-like structure, wherein the bridle comprises a number of separate power- lines that are connected to respective connection points located at or near a leading edge of the wind-engaging member, and optionally one or more steering lines that are connected to the wind engaging member at respective one or more steering points remote from the connections points, wherein the wind-engaging member includes a frame holding a flexible wind-catching structure, e.g. a sheet or a canopy-type sheet, wherein the frame comprises a plurality of interconnected elongated frame members, wherein a number of the frame members are connected by a respective hinge joint.

In this way, a sturdy, durable wind-engaging member can be provided that can be powered by relatively high wind loads without failure or rupture. In particular, the one or more hinge joints can provide good force transmission between various parts of the wind-engaging member, and can reduce local load-induced stresses to avoid breaking of (rather rigid) frame members or reduced changes of breakage.

According to a third aspect of the invention, which can be combined with one or more of the above-mentioned aspects, there is provided an airborne wind energy system comprising a wind-engaging member with a bridle system connected to a tether, a tether storage device for winding and unwinding the tether, an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member, for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member (and/or winding and unwinding the tether, wherein the wind engaging member is a kite or kite-like structure, wherein the bridle comprises at least two separate power- lines that are connected to respective connection points located at or near a leading edge of the wind-engaging member, and optionally one or more steering lines that are connected to the wind engaging member at respective one or more steering points remote from the connections points, wherein at least one of the power lines, preferably two, is split into at least two lines that are connected to the leading edge of the wind-engaging member via a driven pulley.

It has been found that in this way, better control of the wind- engaging member can be achieved, allowing improved flight paths and therefore improved, more efficient energy generating functioning. In particular, the driven pulley can be operated to change orientation of the wind-engaging member in a straight-forward manner, allowing a proper or desired wind-engaging member positioning during flight to efficiently and swiftly achieve a desired flight path.

According to a fourth aspect of the invention, which can be combined with one or more of the above-mentioned aspects, there is provided an airborne wind energy system comprising a wind-engaging member with a bridle system connected to a tether, a tether storage device for winding and unwinding the tether, an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member, for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member and/or winding and unwinding the tether, wherein the wind engaging member is a kite or kite-like structure, wherein the bridle comprises a number of separate power- lines that are connected to respective connection points located at or near a leading edge of the wind-engaging member, and optionally one or more steering lines that are connected to the wind engaging member at respective one or more steering points remote from the connections points, wherein the system includes one or more propulsion devices for lifting the wind-engaging member from a launching level to a higher wind engagement level.

In this way, the wind wind-engaging member can be launched swiftly and reliably, e.g. independent of wind (if any), to achieve a desired flight level that is suitable for energy generation. The propulsion device can e.g. be a driven propeller or similar propulsion device.

Further advantageous embodiments follow from the dependent claims. The invention will now be elucidated referring to the drawings of a non-limiting embodiment. Therein shows:

Figure 1 an embodiment of an airborne wind energy system according in perspective view;

Figure 2 the airborne wind energy system of Fig. 1 in unwinding respectively winding modes;

Figure 3 schematically shows a side view of part of a further embodiment of the airborne wind energy system;

Figure 4 is a cross-section over line IV-IV of Fig. 3;

Figure 5 shows a detail of a leading edge of a further embodiment of the airborne wind system, in top view;

Figure 6 shows part of another further embodiment of the airborne wind energy system of Fig. 1; Figure 7 schematically shows a side view of part of an embodiment of the airborne wind energy system having additional propulsion means;

Figure 8 schematically shows a side view of a launching mast of an airborne wind energy system;

Figure 9 is similar to Fig. 8 and schematically shows an example of a ground launching system;

Figure 10 is similar to Fig. 8 and schematically shows an example of a launching system wherein the wind engaging member is held at a certain distance above a ground level;

Figures 11A, 11B show a further embodiment of the system of Fig. 10, wherein the wind engaging member is transported by a launching vehicle; and

Figures 12A, 12B show a further example of a system for launching the wind engaging member.

Corresponding or similar features are denoted by corresponding or similar reference signs in this patent application.

As follows from Figures 1, 2, an airborne wind energy system generally includes a ground station 1, a tether 2, a bridle system 3 and a wind engaging member 4. The ground station 1 comprises a tether storage device 5, an energy converting device 6, a battery/power electronics module 7 and a control center 8 (the a battery/power electronics module 7 can e.g, be a separate component). In place of using a rechargeable battery for storing electrical energy, a mechanical energy storage device may be employed. The tether storage device is typically a drum. The energy converting device 6 may for instance be a generator connected to the drum. The battery/power electronics module is configured to store energy, and may e.g. supply energy to an energy distribution network or power grid. As the electric power is intermittently produced the battery or other storage device (for instance one or more appropriate capacitors) is applied to balance the electric energy over the pumping cycle of the system. It stores the energy generated during unwinding of the tether and will release a small fraction of this energy for winding the tether, as hereinafter will be explained in more detail.

Moreover, the battery will ensure a nominal electricity output also during periods in which the system is not generating energy. It is re- marked that the storage capacity of the battery (or other storage device) can remain limited when simultaneously several airborne wind energy systems in accordance with the invention are applied that connect to such

battery/storage device. The control center 8 may comprise several interconnected computers hosting different software components required for operating the airborne wind energy system 1. In addition, the control center 8 may comprise wireless modems to connect remote sensors, remote actuators and a steering device 19. The function of the steering device 19 is further explained hereinafter with particular reference to figure 3.

The tether 2 transfers the traction force generated by the wind- engaging member 4 to the tether storage device 5. The tether 2 is typically made of a strong and lightweight plastic fiber and is connected to the bridle system 3 of the wind- engaging member 4. The connection of the tether 2 with the bridle system 3 preferably includes additional safety features such as a metal-based weak link, which ruptures at a predefined maximum traction load, and a cable release system e.g. a (pyrotechnic) cable cutter. Further a two-stage fabric-based shock absorber is provided as part of the safety mechanism that connects the tether (before any of the controlled rupture points) with the kite itself, thus bypassing the bridle system. The connection may also include a (not shown) sensor to measure tether force.

The wind-engaging member 4 as shown in figure 1 can e.g. be a kite or kite-like structure, of the wing type. For example, the wing type kite can include an inflatable membrane. Such inflatable membrane wing kite is robust and still sufficiently flexible to be optimal steerable.

In figure 2 the principle of power generation by the airborne wind energy system is shown. The system is operated in periodic pumping cycles, alternating between unwinding and winding of the tether 2. In a non limiting example, during unwinding, the wind-engaging member 4 is steered along a certain flight trajectory 10 transverse to the wind in order to optimize the traction force in the tether 2. In an embodiment, the flight trajectory will be a figure-eight manoeuver. When reaching e.g. a maximum tether length, the wind-engaging member 4 is de-powered. In an

embodiment, the wind- engaging member is de-powered by rotating the wind-engaging member 4 relative to the tether 2 by means of actuators in the steering device. The wind-engaging member 4 is then aligned with the apparent wind direction WD, i.e. the wind direction that the wing

experiences during flight. The tether storage device 5 will start to retract the tether 2 and accordingly will bring the wind-engaging member 4 to its initial position. From there a new pumping cycle may start. A de-powering by rotating the wind-engaging member (or alternatively, stopping following the figure-eight manoeuver trajectory) reduces the traction force during winding considerably and therefore the energy consumption during winding is only a fraction of the energy generated during unwinding. Optimization of the power output requires an optimal synchronization of winding/unwinding and flight dynamics of the wind-engaging member, as will be appreciated by the skilled person..

For transmission of kite (i.e. wind) force to the tether 2, the bridle 3 of the system includes a number of separate power- lines 11 that are connected to respective connection points located at or near a leading edge LE of the wind-engaging member 4. Optionally, the bridle 3 can include one or more steering lines 16 that are connected to the wind engaging member at respective one or more steering points remote from the connections points. The bridle 3 can be configured such that its power-lines 11 take up and transmit most of the wind traction force, during unwinding, to the tether 2, wherein the steering lines 16 transmit only a small (i.e.

significantly smaller than the force transmitted by the power lines) amount of wind force from the wind-engaging member 4 to the tether 2 (during unwinding). In such an embodiment, thus, a clear distinction can be made between the function of the powerlines and the steering lines.

In an alternative embodiment (not shown), power-lines can also serve as steering lines and vice-versa, wherein all those bridle lines can be controlled for steering the kite.

Generally, the system includes one or more steering devices 19, e.g. incorporated in the bridle, to generate the steered movement of the wind engaging member 4. The steering device 19 can include e.g. one or more actuators (more particularly one or more drums) for winding and

unwinding one or more of the steering lines 16, to adjust the orientation of the wind engaging member 4 in order to follow a predetermined or desired flight path (as mentioned above).

Figures 3-4 show a further embodiment, which differs from the example of Figures 1-2 in that the wind-engaging member 104 includes at least one air passage AP having an operable closing member F. In this example, the closing member is a pivotable closing flap F (see Fig. 4), e.g. being pivotable with respect to a pivot axis or pivot point f_p. Alternatively, e.g., the closing member F can be configured to translate or linearly move between respective or desired operating positions.

In particular, as follows from Figure 3, the including wind- engaging member 104 can include a plurality of spaced-apart air passages AP and respective closing members F.

Each of the flaps F can e.g. be adjustable between a wind-engaging state, closing the respective air passage AP of the wind-engaging member 104 and a wind-releasing state, substantially releasing the respective air passage AP for wind-disengagement. When the flap F has been closed, wind/air can substantially not pass the respective air passage AP of the wind-engaging member 104, allowing the wind engagement member 104 to operate as if that particular air passage is not present. On the other hand, when the flap F has been moved to the opened state (see Fig. 4), air can pass the opening AP (indicated by arrow and dashed line X). This can lead to a depowering mode of the wind-engaging member 104. Moreover, in a further embodiment, various flaps F can be independently controllable such that one ore more flaps F can be moved to a wind-releasing state for opening the respective openings (allowing air to pass the openings), wherein remaining flaps can remain in closing states. In this manner the flaps F can be used as steering devices, e.g. to control or assist controlling left or right

movement/turning etc. of the wind-engaging member.

The skilled person will appreciate that various control means or structures can be available for controlling the position of the closing member (e.g. flap) F. For example, each closing member can be provided with a remote control and can be connected to a respective local actuator, motor or servo for controlling a respective state of the closing member F. Also, spring means can be provided for inducing or counteracting movement of a closing member by spring force. Besides, one or more dedicated control lines and a respective transmission system can be integrated in the wind-engaging member 104 for changing a position of a closing member, as will be appreciated by the skilled person. Furthermore, the system can be arranged for controlling or setting the position of the closing member via or under control of the control center 8.

Figure 5 show part of another further embodiment of the airborne wind energy system, which differs from the examples of Figures 1-4 in that the wind-engaging member includes a frame 150, 153 holding a flexible wind-catching structure WS, e.g. a sheet or a canopy-type sheet consisting of one or more layers of flexible material, wherein the frame comprises a plurality of interconnected elongated frame members 150, 153 wherein a number of the frame members are connected by a respective hinge joint 151. In particular, in this case, the leading edge of the wind-engaging member 104 includes an array of elongated frame members 150,

interconnected by hinge joints 151.

In particular, the frame includes a plurality of elongated frame members 153 extending transversally with respect to a leading edge LE of the wind engaging member, proximal ends of the transversal frame members in particular being connected to leading edge frame members via respective T-connectors 152.

The frame elongated members 150, 153 can e.g. be straight or slightly curved elements. These frame members can be made of relatively rigid material, e.g. fiber-reinforced plastic or aluminum or the—like. They can be tubular elements or solid (e.g. non-hollow) elements). The present wind-engaging member has a semi-rigid construction.

Each of the hinge connections 151 can be configured to provide various rotational degrees of freedom of the linked frame members 150. Any rotational degree of freedom of the hinge connection 151 can e.g. be limited to certain ranges, e.g. using one or more stops, dampers and/or springs. For example, in this embodiment, a hinge connection 151 is configured to allow axial rotational freedom of movement, that is, over to axial center lines of those frame members 150 (as is schematically indicated by arrows Q). This can allow swinging or pivotal displacements of the transversal frame parts 153 (and T-connectors 152) with respect to each other, providing improved durability and force transmission.

It should be observed that in the present example, the T-connectors 152 and hinge joints are shown to be separate parts, however, these features can also be integrated with each other as will be appreciated by the skilled person. For example, a connection point between a said transversal frame member 153 and a leading edge frame member 150 can be configured to provide a rotational degree of freedom between those members, e.g. to allow a certain swinging or pivotal movement of the said transversal frame member 153 with respect to the leading edge frame member 150.

Figure 6 shows a further non-limiting example of the system, which differs from the embodiments shown in Figures 1-5 in that two of the powerlines 11 of the bridle 3’ have been split into two respective lines 11A, 11B that are both connected (at spaced-apart connection points) to the leading edge LE of the wind-engaging member 4, via pulleys 15 .For example, the two distal lines 11A, 1 IB of a said power line 11 can be part of a single line that runs through a respective pulley 15. According to an aspect the pulley is a driven pulley 15 so that the lengths of the distal lines 11A, 11B can be actively adjusted (wherein shortening of one of the two sections 11 A, 11B leads to lengthening of the other section 11B, 11A respectively, and vice-versa). Each pulley 15 can e.g. include an actuator or motor, for driving the pulley 15 and for adjusting the length of the respective distal power line sections 11 A, 11B. Control of the driven pulley can include remote control. Also, the system can be arranged for controlling or setting a length of a distal power line section 11 A, 11B, via the driven pulley, via or under control of the control center 8. A said driven pulley 15 can have a first operating state wherein the pulley is powered for adjusting a length of a said distal power line section 11A, 11B and/or for holding the power line section 11A, 11B at a certain length. In addition, the pulley 15 can have a second non-operating state wherein the pulley is not powered, e.g. leaving the power line sections 11 A, 11B free to adjust respective lengths e.g. based on wind loading of the wind-engaging member. By actively controlling (driving) the pulleys, improved control of flight characteristics of the wind-engaging member can be achieved. For example, the pulleys 15 can be driven to actively adjust the length of the distal power line sections 11A, 11B to adjust a shape of the wind-engaging member, e.g. to increase or decrease an angle of attack or to adjust other wind engaging properties of the wind-engaging member. Figure 7 show a further alternative embodiment. It differs from the examples shown in any of figures 1-6 in that the system includes one or more propulsion devices M for lifting the respective wind-engaging member 204 from a launching level to a higher wind engagement level. Figure 8 schematically shows a launching level of a said wind engaging member 4, namely a low, close to the ground level S wherein the wind engaging member 4 is e.g. being suspended from a launching mast K (a top of the mast K e.g. having a rigid suspension member protruding outwardly kl, for holding a leading edge LE of the wind engaging member 4).

In order to reliably move the respective wind-engaging member from a low level to an operating, high, level, a respective propulsion device can be temporarily activated such that it pulls the wind-engaging member upwardly. During the launch, the respective ground station 1 can unwind the tether 2. In a further embodiment, the mast K can be arranged for guiding a respective lower section of the tether upwardly as well. Moreover or alternatively, the system can include a further flexible connection line 42 (e.g. cord) that can be connected to the tether 2 and the leading edge LE of the respective wind-engaging member 4. Such a further connection line can be used to lift the wind engaging member 4 from a ground level to a higher launching level upwards along the mast K. In particular, as follows from Fig. 8, the lifting line 42 can be guided along (e.g. through) the mast K and via the suspension member kl, and can be detachably connected to the respective wind-engaging member 4 (e.g. via the tether 2).

The example of Figure 7 includes two spaced-apart propulsion devices M, connected to spaced-apart sections of the respective wind- engaging member 204. For example, one of the propulsion devices M can be connected to the leading edge LE of the wind-engaging member 204, e.g. to a rigid leading edge frame member 250 (if available). Another propulsion device M can be connected to a distal section or trailing edge, e.g. to or near an end of a frame member 253 (if available) of the wind-engaging member 204. Each propulsion device M can e.g. include a driven rotor. Also, activation and deactivation of a propulsion device M is preferably

controllable by the control center, e.g. via a suitable remote control.

Alternatively or additionally, a propulsion device M can have an automatic control, e.g. for automatically deactivating the propulsion device depending on one or more sensor data or parameters, e.g. on a sensor detected altitude or temperature or other parameter. A propulsion device M can e.g. be electrically powerable, wherein the wind-engaging member 204 can include a suitable electrical power source for storing energy to power the propulsion device M. The propulsion devices can assist in stably launching and/or landing of the wind-engaging member 204.

Further aspects of the invention, which optionally can be combined with one or more of the above-described aspects, are depicted in Figures 9- 12.

Figure 9 schematically shows a further example of a system for launching the wind engaging member 4. Generally speaking, the idea for launching is to safely hold the wind engaging member 4 (e.g. on the ground, on an afore -mentioned mast or with another airborne device) first, and to subsequently‘load’ the wind engaging member 4 such that it can fly, wherein a respective wind engaging member 4 releasing mechanism releases a locking system to free the wind engaging member 4.

For example, in Figure 9, the launching system includes an automated release platform and a number of ground pins 500 that together hold the wind engaging member 4 on the ground before launch. Usually, two lines (launching lines) 501 connect a leading edge of the wind engaging member 4 to the automated release platform via the ground pins 500. The automated release platform holds both lines by passing the line loop around a rotary pin which is eventually locked with e.g. a magnet. In this situation, the main tether 2 can be safely tensioned which consequently loads the different lines of the wind engaging member 4, including the launching lines. To launch the wind engaging member 4, a signal is send to the automated release platform which deactivates the magnetic field and releases the respective magnet. Thus, the pins 500 release the lines.

Another possibility is to make use of a mast and two launching lines 601 to hold the wind engaging member 4 (Fig 3). One end of the line is attached to the mast whereas the other end of the line features an

electromagnet 603 which can be deactivated remotely. In order to connect the wind engaging member 4 to the launching lines, ferromagnetic elements (e.g. plates) 604 can be provided in/on the wind engaging member 4, to be engaged by the electromagnets 603. The configuration of this setup is preferably such that the wind engaging member’s 4 trailing edge is not touching the ground and e.g. therefore can adapt itself to a certain the power setting (e.g. given by a wind engaging member control unit 19). To launch the wind engaging member 4, a signal is send to the magnets 603 which are then deactivated and thus free the wind engaging member 4. The launching lines with the magnet fall along the mast while the ferromagnetic elements can remain part of the wind engaging member 4 and fly with it.

This launching system can be combined with a mean of

transportation, as is depicted in Figures 11A, 11B. For instance, a launching mast can be part of a vehicle LV, e.g. being mounted on the back of a pick-up car or the-like, with the wind engaging member 4 (e.g. deflated or inflated) already connected to the mast. Thus, the wind engaging member 4 can be easily transported to a launching site, to be attached to the main tether,

(e.g. inflated and deployed) and loaded. The pick-up car is an example but can also be replaced by any kind of driven autonomous vehicle.

With the above described ground launching system (of Figures 11A, 11B), a landing procedure can include of first reeling-in of the tether 2 and then slowly approaching (by the vehicle LV) the wind engaging member 4 towards the ground, on one side of a wind window until the respective steering device 19 touches the ground. Once the steering device 19 touches the ground, the wind engaging member 4 e.g. glides over the steering device 19 and lands smoothly with its leading edge on the ground.

Further, in order to launch the wind engaging member 4 with the help of an airborne device, several possibilities are imagined. BA non limiting example is depicted in Figures 12A, 12B. For instance, a balloon BN filled up with helium and/or hot air can be used to lift up the deflated wind engaging member 4 (with steering device 19) in the air (Fig 6). The main tether 2 can e.g. be passed through a ring or other connection device 700, that is connected to the balloon BN. In such a way, the wind engaging member 4 can be brought high enough where the wind is sufficient for the wind engaging member 4 to fly. At this altitude, in case of an inflatable wind engaging member, the wind engaging member 4 can be inflated (from an initially non-infiated launching condition) to an inflated flying-shape, e.g. using an optional on board pump system. Once the wind engaging member 4 flies, the balloon BN can be reeled in towards the ground and e.g. deflated. The ring or respective connection device 700 slides down the ground as well in such a way it does not interfere with the main tether 2. For landing, the process can be reversed. The wind engaging member 4 can e.g. be parked at zenith. The balloon BN can be inflated and reeled out from a low altitude to a higher altitude, namely that of the wind engaging member 4. The wind engaging member 4 can then be deflated, to falls behind the balloon BN (i.e. being lifted again by the balloon BN). Next, the balloon and suspended wind engaging member 4 can be brought toward the ground by reeling in both balloon and wind engaging member.

The secondary airborne device can also be a lifter kite, a zeppelin, a drone or any autonomous flying device.

Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that particular features, structures, or

characteristics of one or more embodiments may be combined in any suitable manner to form new, not explicitly described embodiments.

Claims

1. Airborne wind energy system comprising a wind-engaging member with a bridle system (3) connected to a tether (2), a tether storage device for winding and unwinding the tether (2), an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member (4), for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member (4) and/or winding and unwinding the tether (2), wherein the wind engaging member (4) is a kite or kite-like structure, wherein the bridle (3) comprises a number of separate power- lines (11) that are connected to respective connection points located at or near a leading edge of the wind-engaging member (4), and optionally one or more steering lines (16) that are connected to the wind engaging member at respective one or more steering points remote from the connections points,

characterized in that the wind-engaging member includes at least one air passage having an operable closing member.

2. Airborne wind energy system according to claim 1, wherein the closing member is a pivotable closing flap.

3. Airborne wind energy system according to any of the preceding claims, including a plurality of spaced-apart air passages and respective closing members.

4. Airborne wind energy system according to any of the preceding claims, wherein the closing member is adjustable between a wind-engaging state, closing the respective air passage of the wind-engaging member and a wind-releasing state, substantially releasing the respective air passage for wind-disengagement.

5. Airborne wind energy system, for example a system according to any of the preceding claims, comprising a wind-engaging member with a bridle system (3) connected to a tether (2), a tether storage device for winding and unwinding the tether (2), an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member (4), for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind- engaging member (4) and/or winding and unwinding the tether (2), wherein the wind engaging member (4) is a kite or kite-like structure, wherein the bridle (3) comprises a number of separate power- lines (11) that are connected to respective connection points located at or near a leading edge of the wind-engaging member (4), and optionally one or more steering lines (16) that are connected to the wind engaging member at respective one or more steering points remote from the connections points,

characterized in that the wind-engaging member includes a frame holding a flexible wind-catching structure, e.g. a sheet or a canopy-type sheet, wherein the frame comprises a plurality of interconnected elongated frame members, wherein a number of the frame members are connected by a respective hinge joint.

6. System according to claim 5, wherein a leading edge of the wind- engaging member includes an array of elongated frame members, interconnected by hinge joints.

7. System according to claim 5 or 6, wherein the frame includes a plurality of elongated frame members extending transversally with respect to a leading edge of the wind engaging member, proximal ends of the transversal frame members in particular being connected to leading edge frame members via respective T-connectors.

8. Airborne wind energy system comprising a wind-engaging member with a bridle system (3) connected to a tether (2), a tether storage device for winding and unwinding the tether (2), an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member (4), for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind- engaging member (4) and/or winding and unwinding the tether (2), wherein the wind engaging member (4) is a kite or kite-like structure, wherein the bridle (3) comprises at least two separate power- lines (11) that are connected to respective connection points located at or near a leading edge of the wind-engaging member (4), and optionally one or more steering lines (16) that are connected to the wind engaging member at respective one or more steering points remote from the connections points,

characterized at least one of the power lines, preferably two, is split into at least two lines that are connected to the leading edge of the wind-engaging member (4) via a driven pulley.

9. Airborne wind energy system comprising a wind-engaging member with a bridle system (3) connected to a tether (2), a tether storage device for winding and unwinding the tether (2), an energy converting device connected to the tether storage device, a steering device to generate a steered movement of the wind engaging member (4), for example to steer the relative angle of the wind engaging member with respect to tether, one or more control units for steering the wind-engaging member (4) and/or winding and unwinding the tether (2), wherein the wind engaging member (4) is a kite or kite-like structure, wherein the bridle (3) comprises a number of separate power- lines (11) that are connected to respective connection points located at or near a leading edge of the wind-engaging member (4), and optionally one or more steering lines (16) that are connected to the wind engaging member at respective one or more steering points remote from the connections points, characterized in that the system includes one or more propulsion devices for lifting the wind-engaging member from a launching level to a higher wind engagement level.

10. Airborne wind energy system according to claim 9, including a driven rotor connected to a leading edge of the wind-engaging member, and preferably also a rotor connected to a trailing edge of the wind-engaging member.