WO2019221600A1

WO2019221600A1 - Airborne wind energy system

Info

Publication number: WO2019221600A1
Application number: PCT/NL2019/050282
Authority: WO
Inventors: Roger Jules COENEN; Gijs Johan BOUMAN; Xander Guy GERRMANN; Nick Michel DEKKERS; Gabriel Andreas KÖNIG; Johannes Otto PESCHEL; Johannes Catherina Marie BREUER
Original assignee: Enevate B.V.
Priority date: 2018-05-14
Filing date: 2019-05-14
Publication date: 2019-11-21
Also published as: NL2020920B1

Abstract

Airborne wind energy system comprising a wind-engaging member 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 (104), 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 (104) and/or winding and unwinding the tether (2).

Description

Title: Airborne wind energy system

The invention concerns an airborne wind energy system comprising a wind-engaging member 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 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, and a system wherein the wind-engaging member can provide wind engaging

characteristics.

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

Advantageously, according to an aspect of the invention, there is provided an airborne wind energy system comprising a wind-engaging member 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 includes a main wing section that extends substantially normally with respect to the tether during operation, wherein the wind engaging member fiirther includes at least one secondary wing section that extends substantially normally with respect to the main wing section, the secondary wing section in particular being configured to provide stability and/or to create lift to reduce an officourse angle of the main wing section.

The secondary wing section can improve flight characteristics of the respective wind-engaging member, thereby in particular providing increased efficiency, in particular during a during a pumping (reel out or energy production) phase of the system. In particular, the secondary wing section can reduce a sideslip of the wind-engaging member (‘aircraft’) during a certain flight path of a power production phase.

In a preferred embodiment, the main wing section is a fixed wing. For example, the main wing section can include an airfoil shaped body.

Also, it is preferred that the system includes a bridle system having separate power-lines that are connected to respective connection points at the main wing section, in particular in a V-line configuration.

The system can at least include e.g. a secondary wing section that protrudes upwardly or downwardly (i.e. laterally) from an upper side of the main wing section, the upper side of the main wing section being faced away from the tether during operation.

Also, advantageously, the secondary wing section can be a fixed wing and/or an airfoil shaped body.

In a further embodiment there is provided a tail section remote from the main wing section, the tail section including at least one of:

-a rudder for yaw control of the wind engaging member; and - an elevator for pitch control of the wind engaging member. Further, another aspect of the invention, which can be combined with the first aspect, is characterized by 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 includes a rigid airfoil main wing section that extends substantially normally with respect to the tether during operation, wherein the bridle system includes at least two separate power-lines that are connected to respective connection points at the main wing section, in particular in a V-line configuration.

In this way, a more stable and efficient airborne wind energy system can be obtained.

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 further embodiment of the airborne wind energy system, in a perspective view;

Figures 4-5 shows the further embodiment in a side and front view, respectively;

Figure 6 shows a perspective, partly opened view of another example of the airborne wind energy system; and

Figure 7 shows the example of Figure 6, including a remote power source.

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 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 another embodiment (see Figures 3-4) a fixed wing type wind engagement member 104 is installed.

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 11, 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 cyde 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 device 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 3-5 show shows a further embodiment, which differs from the example of Figures 1-2 in that the wind-engaging member 104 is of a fixed (rigid) wing airfoil type, having a rigid main wing section 104A that extends substantially normally with respect to the tether during operation.

More particularly, the present main wing section 104A is a fixed wing, provided by an airfoil shaped body. Thus, the main wing section 104A can provide lift during flight, e.g. during at least part of the flight trajectory 10 (see Fig. 3). As in the embodiment of Figures 1-2, the resulting wind force induced traction can be transferred from the wind-engaging member 104 to the tether 2, to be transferred to the energy converting device 6.

In this example, a tail section 152, 153 of the system provides a steering device 119 for example to steer the relative angle of the wind engaging member with respect to tether, and e.g. to provide power- and depower flight modes of the wind-engaging member 104. For example, the remote tail section can include a rudder 153 for yaw control of the wind engaging member 104. In addition, it can include an elevator 152 for pitch control of the wind engaging member 154. Adjusting a position of the rudder 153 can be performed by one or more suitable actuators or servos, e.g. via aa remote control by the control center 8, as will be appreciated by the skilled person. The same holds for adjusting the elevator 152. The skilled person will also appreciate that the steering device can also be configured differently, e.g. having a V-type elevator/rudder-combination. The tail section 119 can be connected to the rigid main wing 104A by a suitable rigid coupling structure 151, e.g. an elongated coupling frame, rod or the-like. Besides, for example, there can be implemented an autopilot system or pitch control system, including for example an angle-of attack sensor device for detecting an angle of attack of the wind-engaging member 104, and for controlling a pitch of the wind-engaging member 104 depending on a detected angle-of attack.

The present airborne wind energy system also includes a bridle system 103 for connecting the tether 2 to the fixed main wing 104A. In this case, the bridle system has a relatively simple configuration and includes two (or more) separate power- lines 111 that are connected to two respective (spaced-apart) connection points at the main wing section, in particular in a V-line configuration. The power lines 111 can be provided by a single line (e.g. cord), split in half. In this example, both power lines 111 have the same length, and are connected in a symmetrical configuration to the main wing 104A (i.e. at equal distances v from the longitudinal center of the wing, and e.g. both at the same distance w from a leading edge of the wing).

Optionally, the bridle includes more than two such power lines 111. The resulting configuration provides increased stability of improved flight characteristics of the wind 104A, with the V-line bridle system 111 preventing the wing 104A from rolling, leading to more efficient energy generation.

Another advantageous aspect of the present embodiment concerns the application of a secondary wing section 104B, which can act as a sort of air anchor.

Referring to the drawings, in particular, the wind engaging member 104 further includes a single secondary wing section 104B that extends substantially normally with respect to the main wing section 104A, the secondary wing section 104B preferably being configured to provide stability, and in addition to create lift to reduce an off-course angle of the main wing section. This additional lift is indicated by an arrow U in Figure 3. The additional lift U has a direction that deviates from the lift that is generated by the main wing 104A. In this example, the additional lift U is at least directed in parallel with respect to a main axis of the main wing 104A (i.e. sideways with respect to a general flight direction of the main wing 104A), in order to reduce an off-course angle of the main wing section. In a further embodiment, the secondary wing section 104B may be controllable or e.g. pivotable with respect to the main wing 104A in order to adjust the angle (pitch) of the main wing 104A during its flight. Alternatively, the secondary wing section 104B is immovably fixed to the main wing section 104A. As follows from Fig. 5, the main wing section 104A and secondary wing section (air anchor) 104B can form a T-shaped wind-engaging member, seen in front view. Herein, the secondary wing section 104B that protrudes upwardly from an upper side of the main wing section 104A, the upper side of the main wing section being faced away from the tether 2 during operation. The wind-engaging member can also have various other shapes, e.g. H-shaped (as in Fig. 6) or differently.

In a preferred embodiment, the secondary wing section 104B as such can be a fixed wing. Also, the secondary wing section 104B can be provided by a respective airfoil shaped body.

During operation, the resulting the wind engaging member 104 can be mainly controlled by the elevator 152 for pitch control and the rudder 154 for yaw control, in addition to winding and unwinding of the tether 2, such that the wind engaging member 104 follows a desired or predetermined flight path 10.

The secondary wing section 104B, extending orthogonal to a main lifting surface of the main wing section 104A, can function as a sort of air anchor to increase flight stability, the secondary wing section 104B e.g. cooperating with the tail section (e.g. rudder) 119 of the system.

The wind engaging member 104 is particularly advantageous during a pumping phase (i.e. tether reeling out, energy production phase) of the system. In particular, the secondary wing section 104B -as an anchor in the air- can give stability and can allow yaw steering with the rudder part of the tail section 119.

Secondly, the secondary wing section 104B can create additional lift U to reduce the off-course angle (the angle between heading of the main wing and its course). A resulting low off-course angle allows for more accurate control of the wind engaging member 104.

In a further embodiment, the system can includes one or more propulsion devices for lifting the respective wind-engaging member 104 from a launching level to a higher wind engagement level. Such a propulsion device can e.g. include a driven rotor/propeller (preferable a foldable propeller). Also, activation and deactivation of a propulsion device is preferably controllable by the control center, e.g. via a suitable remote control. Alternatively or additionally, a propulsion device 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 can e.g. be electrically powerable, wherein the wind-engaging member 104 can include a suitable electrical power source for storing energy to power the propulsion device. The propulsion devices can assist in stably launching and/or landing of the wind-engaging member 104.

Figures 6-7 show an example of a wind engaging member of a system, including a number of propulsion devices 250 for lifting (and optionally landing) the respective wind-engaging member 204. The present example differs from the embodiments shown in Figures 1-5 in that the wind-engaging member 204 includes a main wing section 204A and a plurality of secondary wing section 204B, extending substantially normally with respect to the main wing section. In figure 6, the main wing section 204A as such is composed of a number (three, in this example) sub-sections that are connected together to from the main wing section.

This example also includes a remote tail section having a rudder 253 for yaw control of the wind engaging member, and an elevator 252 for pitch control of the wind engaging member, wherein the respective tail section 219 is be connected to the rigid main wing 204A by a suitable rigid coupling structure 251.

In the example, the propulsion devices include respective, motor driven propellers 250 (wherein one or more motors and e.g. a suitable motor- propeller transmission can be implemented for driving the propellers 250).

The propellers 250 can e.g. be electrically powered (respective propeller drives receiving e.g. power from a battery that may be integrated with or located in the wind-engaging member 204), and can be connected to/in several positions of the wind-engaging member 204.

Alternatively, as is shown in Figure 7, electrical power for powering the one or more propulsion devices 250 can be provided from a power source 275 that is remote (i.e. separate) from the wind engaging member 204. Such a remote power source can e.g. be located on/at ground level, for example being part of or associated with the ground station 1, and/or from an energy storage device associated with the system.

Alternatively, such a remote electric power source 275 can be connected to the tether 1 of the system (to be lifted by the tether 1 from a ground level) at a relatively large distance from the wind engaging member 204, for example a distance of more than 10 meter, in particular a distance of more than 50 meter and preferable a distance of at least 100 meter. Thus, the remote power source 275 becomes airborne during a launching phase of the wind engaging member 204, but preferably does not have to follow the same trajectory as the wind engaging member 204 during a wind power energy generating phase thereof. In this way, application of a bulky, relatively heavy integrated electrical power source in the wind engaging member 204 can be avoided, whereas the system can still provide sufficient electrical energy to the propulsion device(s) 205 during launch and/or landing of the wind engaging member 204. (which can require a relatively large amount of electrical energy depending e.g. on a flight level to be achieved and a weight of the wind engaging member 204).

The system can include an electrical conductor for transferring the power from the remote power source 275 to the wind engaging member 204 (in particular to power the one or more propulsion devices 250). For example, the tether 2 can be configured for conducting electric energy from the remote power source 275 to the wind engaging member 204, wherein the electrical conductor can integrated with a respective (upper) part 1EC of the tether 2. Alternatively, the system can include a dedicated, separate electrical conductor line (having an integrated electrical conductor) extending from the remote power source 275 to the wind engaging member 204 to provide the electric power. According to a further embodiment, energy can be transferred from the remote power source 275 to the electrical conductor via a respective electrical contact 276, e.g. through a sliding contact, induction and/or a plug, in order to power the wind engaging member 204 via that conductor. In both cases, one or more power lines 111 (connecting an upper section of the tether 2 to the wind engaging member 205) can be configured for electrically linking the conductor to the wind engaging member (for transmitting the electrical power thereto), or a dedicated conductor line can be available, separate from the power lines 111 to provide the electrical power transmission.

Besides, optionally, the remote power source 275 can be detachably connected to the tether, via a controllable detachable connection, wherein the power source 275 can be detached (e.g. automatically or upon remote control) e.g. after a certain amount of electrical power has been fed to the wind engaging member 204, and/or when a state of charge of the remote power source 275 has dropped to a certain (relatively low) threshold level. Optionally, the remote power source 275 can be provided with a dedicated, integrated landing means, for example an automatically deployable parachute, to safely land the power source 275.

Referring to the drawing (see Fig. 6), in the example, the propellers 250 are positioned symmetrically with respect to each other and the wind-engaging member 104, allowing stable take-off. For example, propeller rotation axes 250A can extend substantially in parallel with respect to a forward flight direction D of the wind-engaging member 204.

In this example, the propellers are connected to the secondary wing sections 204B, in particular to distal sides of those section s204B.

Alternatively or additionally, one or more propellers can be connected to suitable location so the main wing section 204A.

It is preferred that each of the propellers 250 can be adjusted from a wing propelling state, wherein the propeller rotates to lift the system, to a idle propeller state wherein it not powered. In an idle propeller state, preferably, the propeller is automatically retracted or folded-in to a shape wherein it applies no or significantly no aerodynamic drag to the wind- engaging member 204.

During launch, the propellers (multi-prop) can be activated (e.g. powered by a remote power source 275) for stably lifting the wind-engaging member 204 to a desired flight level. Optionally, the propellers can be used for accelerates the aircraft 104 into crosswind flight. Next, the propellers 250 can be brought to idle propeller states, e.g. including folding the propellers backward/inwards to low-drag propeller states, after which the wind-engaging member 204 can be operated for power generation (as described above). Further, optionally, after power generation the propellers can be activated again for stably and safely landing the device (thereby providing e.g. a Vertical Take Off And Landing -VTOL- apparatus).

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.

Furthermore, as will be appreciated by the skilled person, the main wing section 104A can extend substantially perpendicular with respect to the tether during operation, in particular with respect to an upper part of the tether and during a during a pumping (reel out or energy production) phase of the system. As is mentioned above, it is preferred that the main wing section 104A is allowed to perform certain pitch movements during flight, e.g. controlled by the elevator of the tail section 119, which can lead to variations in the orientation of the main wing section 104A with respect to a nearby upper part of the tether 2.

Further, according to a preferred embodiment, an orientation of the secondary wing section 104B with respect to the main wing section 104A can be adjustable (e.g. as a rudder, such as: pivotal with respect to a pivot axis that extends normally with respect to the main wing section 104A), for providing additional steering control of the a wind-engaging member.

Similarly, the secondary wing section 104B as such can include a rudder section that is adjustable (e.g. pivotal with respect to a pivot axis extending normally with respect to the main wing section 104A).

Besides, the wind engaging member 104 can include a single secondary wing section 104B, or two or more (spaced-apart) secondary wing sections, for providing stability and/or to create lift to reduce an off-course angle of the main wing section.

Also, an emergency system can be provided for slow-speed landing the wind-engaging member, for example an emergency system including an automatically deployable parachute.

Claims

1. Airborne wind energy system comprising a wind-engaging member 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 (104), for example to steer the relative angle of the wind engaging member with respect to the tether, one or more control units for steering the wind-engaging member (104) and/or winding and unwinding the tether (2), wherein the wind engaging member (104) includes a main wing section (104A) that extends substantially normally with respect to the tether during operation, wherein the wind engaging member further includes at least one secondary wing section (104B) that extends

substantially normally with respect to the main wing section, the secondary wing section in particular being configured to provide stability and/or to create lift to reduce an off-course angle of the main wing section.

2. Airborne wind energy system according to claim 1, wherein the main wing section (104A) is a fixed wing.

3. Airborne wind energy system according to claim 1 or 2, wherein the main wing section includes an airfoil shaped body.

4. Airborne wind energy system according to any of the preceding claims, including a bridle system having two or more separate power-lines (111) that are connected to respective connection points at the main wing section (104A), in particular in a V-line configuration.

5. Airborne wind energy system according to any of the preceding claims, at least including a secondary wing section (104B) that protrudes upwardly or downwardly from an upper side of the main wing section, the upper side of the main wing section being faced away from the tether (2) during operation.

6. Airborne wind energy system according to any of the preceding claims, wherein the secondary wing section is a fixed wing.

7. Airborne wind energy system according to any of the preceding claims, wherein the secondary wing section includes an airfoil shaped body.

8. Airborne wind energy system according to any of the preceding claims, including a tail section remote from the main wing section, the tail section including at least one of:

-a rudder for yaw control of the wind engaging member (104); and

- an elevator for pitch control of the wind engaging member (104).

9. Airborne wind energy system, for example a system according to any of the preceding claims, comprising a wind-engaging member with a bridle system (103) 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 (104), 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 (104) and/or winding and unwinding the tether (2), wherein the wind engaging member (104) includes a rigid airfoil main wing section that extends substantially normally with respect to the tether during operation,

characterized in that the bridle system includes separate power- lines (111) that are connected to respective connection points at the main wing section, in particular in a V-line configuration.

10. Airborne wind energy system according to any of the preceding claims, including one or more propulsion devices for lifting the respective wind-engaging member (104, 204) from a launching level to a higher wind engagement level.

11. Airborne wind energy system according to claim 10, including a power source for powering the one or more propulsion devices, wherein the power source is remote from the wind engaging member (204).