WO2019141332A2

WO2019141332A2 - Control of a wind energy park comprising airborne wind energy systems

Info

Publication number: WO2019141332A2
Application number: PCT/DK2019/050021
Authority: WO
Inventors: Thomas S. Bjertrup Nielsen; Torben Ladegaard Baun
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2018-01-22
Filing date: 2019-01-21
Publication date: 2019-07-25
Anticipated expiration: 2020-07-22
Also published as: WO2019141332A3

Abstract

The invention relates to a method for controlling the operation of a number of airborne wind energy systems arranged in a wind energy park, where each airborne wind energy system comprises a wind engaging member being coupled to a ground station via a cable, and where the wind engaging members are operated to move along a flight path comprising a power generation phase of a generally upwards movement and a recovery phase of a generally downwards movement. The method comprises determining a position of a wind engaging member of a first airborne wind energy system and its distance to some predetermined reference location, and then controlling the operation of the airborne wind energy system in its power generation phase as a function of the determined distance. The controlling comprises steering the wind engaging member along a flight path of an upwards cyclic pattern having a width in a horizontal plane which increases with the distance. The invention further relates to operating more wind engaging members in the wind energy park along the same direction.

Description

CONTROL OF A WIND ENERGY PARK COMPRISING AIRBORNE WIND ENERGY SYSTEMS

FIELD OF THE INVENTION

The present invention relates to a method for controlling the operation of a number of airborne wind energy systems arranged in a wind energy park and where the airborne wind energy systems comprise a wind engaging member being coupled to a ground station via a cable.

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 wind engaging 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. Compared to e.g. a wind turbine with a rotating rotor, an airborne wind energy system is by some considered disadvantageous because of the relatively more complex movement in the air of the kite or glider of the airborne wind energy system. This movement which by some is considered to be a significant visual disturbance is further increased in a wind energy park of more kites and gliders moving around in systems or patterns which may be difficult for an observer to understand or relate to.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method of controlling a number of airborne wind energy systems in a wind energy park wherein the visual disturbance of the wind engaging members is reduced.

It is a further object of embodiments of the invention to provide a method of controlling a number of airborne wind energy systems in a wind energy park with a view to reduce the environmental impact of the airborne wind energy systems.

According to a first aspect the invention relates to a method for controlling the operation of a number of airborne wind energy systems arranged in a wind energy park, each airborne wind energy system comprising a wind engaging member being coupled to a ground station via a cable and wherein each airborne wind energy system is operated to move the wind engaging member along a flight path comprising a power generation phase of a generally upwards movement and a recovery phase of a generally downwards movement, the method comprising

- determining a position of a wind engaging member of a first airborne wind energy system;

- determining a distance between the wind engaging member of the first airborne wind energy system and a predetermined reference location, and - controlling the operation of the first airborne wind energy system in the power generation phase as a function of the determined distance, comprising steering the wind engaging member along a flight path of an upwards cyclic pattern of a width in a horizontal plane, and wherein the width increases with the distance.

The airborne wind energy systems together form a wind energy park. Each of the airborne wind energy systems controlled by the method according to the invention comprises a wind engaging member such as a kite or a glider or even more kites in a kite train. The wind engaging member is coupled to a ground station via one or more cables. Accordingly, the airborne wind energy system is mechanically attached to the ground station by means of the cable. Further, each of the airborne wind energy systems are operated to move the wind engaging member along a flight path or flight trajectory which comprises a power generation phase of a generally upwards movement and a recovery phase of a generally downwards movement. The moving of the wind engaging member is realised by a combination of the forces of the wind acting on the wind engaging member, the tensioning forces in the cable(s) and in any steering lines connected to the wind engaging members, and optionally by the operation of different lift altering means on the wind engaging means. The wind engaging member is operated to be gradually pulled up by the wind in the power generation part of the flight trajectory consequently unwinding the cable(s) from a winch at the ground station. During the power generation phase, a typical mode of flight is a crosswind flight of upwards spinning circles, ovals and/or figure-of-eights, or the like. After having reached a certain height, the wind engaging member is typically guided at least partly out of the wind and the cable(s) are retracted during the recovery phase. According to the invention, a position of a wind engaging member of a first airborne wind energy system is determined. The position may be an absolute position or a position relative to some reference point. This may be used in determining a distance between the wind engaging member of the first airborne wind energy system and a

predetermined reference location. The reference location may for example be the ground station of the airborne wind energy system in which case the distance can be determined directly as the length of the cable(s). Alternatively, the distance may be estimated by determining the height of the wind engaging member above the ground. The reference location may alternatively be the location of a structure such as a building or a group of houses near the airborne wind energy system. The distance from the wind engaging member to the reference location may be determined based on estimated or measured values.

According to the invention, the first airborne wind energy system is controlled during all or a part of the power generation phase as a function of the

determined distance, which controlling comprises a steering of the wind engaging member along a flight path of an upwards cyclic pattern of a width in a horizontal plane, and wherein the width increases with the distance. The upwards cyclic pattern may comprise spinning circles, ovals and/or figures-of- eight or other figures repeated cyclically or near cyclically but at increasing heights or circling gradually upwards. In this way, if the wind engaging member is moved in upwards spinning circles or ovals, the diameters of the circles or ovals are increased as the kite or glider is moved upwards. If the wind engaging member is moved in a pattern of upwards spinning figure-of-eight or similar, the width of the pattern in a horizontal plane may be taken as the distance between the outmost points of the two circulars portions of the figure-of-eight, or alternatively as the width of one of the circulars portions. As the width of the upwards cyclic pattern is increased, the flight path in at least a part of the power generating phase thereby may form a part of a cone or a funnel.

By the controlling of the wind engaging member to flow along a flight path of an upwards cyclic pattern of increasing widths is obtained a flight path with a visually more harmonic and less disturbing appearance. When observed from the reference location, the wind engaging member will appear to move in a cyclic pattern of a more uniform size in that the wind engaging member when further away moves in a larger cyclic movement than when closer to the observer at the reference location earlier in its power generation phase.

Traditionally, the kite or glider of an airborne wind energy system is controlled along a flight path to maximize the power produced in the power generation phase and to minimize the energy consumed in the recovery phase which controlling results in the wind engaging member being operated upwards in a cyclic pattern of constant or near- constant diameter or width. Such flight path may however be seen from a position on the ground as the kite or glider moving in smaller and smaller cyclic patterns and with a higher risk of being perceived as disturbing the eye. With the flight path according to the invention this risk is considerably reduced and the visual and aesthetic impact on the landscape by the wind energy park reduced.

In an embodiment, the method further comprises determining a position of the wind engaging member of each of the number of airborne wind energy systems and a distance between each of the wind engaging members and one or more predetermined reference positions, and controlling the operation of each of the number of airborne wind energy systems in their power generation phase according to the same function of the determined distance.

Hereby each of the number of airborne wind energy system in the wind energy park may be operated in the same way, i.e. each with their wind engaging member controlled to move along a flight path of an upwards cyclic pattern of an increasing width. The width increases may then be determined as a function of the distance from each wind engaging member to the same, all different, or partly different reference locations (such as to each ground station or to one or more structures in or near the wind energy park). Hereby, the visual impact and disturbance of the wind energy park is further reduced for observers. The visual disturbance is considerably reduced not only for an observer looking directly at the wind energy park but also for an observer looking at one or more of the kites or gliders out of a corner of an eye.

In an embodiment, the reference location corresponds to the position of the ground station of the first airborne wind energy system or a position of a structure in or near the wind energy park, such as a building or a road. When the reference location is the position of the ground station, the distance to the wind engaging member can be determined simply by measuring the length of the cable extending between the ground station and the wind engaging member. Hereby is obtained a flight path with a width which can be easily determined yet which appears more harmonic to the eye from most positions around and inside the wind energy park than with the traditional flight paths of a constant or near- constant width.

By a reference point corresponding to a position of a structure in or near the wind energy park, such as a building or a road, the flight path is visually adopted to be more harmonic and less disturbing when seen and observed from a particular location of choice. This may be advantageous in operating the wind energy park to appear less disturbing when seen from specific positions where for example people live or come more often or in higher numbers. The reference point may be taken as an average or middle position of a larger area such as group of houses. In an embodiment, the reference location may be a road and the distance may then be determined as the shortest distance to the road or some average or middle position along the road.

In an embodiment the position of the wind engaging member is determined by means of measurements with a GPS and/or an optical camera and/or by the use of three dimensional accelerometers, from the length of the cable from the ground station and up to the wind engaging member and the angle of the cable extending from the ground station or from combinations hereof.

In an embodiment, the steering of the wind engaging member along a flight path is performed by controlling an extraction and/or retraction of the cable, by controlling a number of steering lines attached to the wind engaging member, and/or controlling the operation of lift altering members of the wind engaging member. Kites or gliders are typically constructed to yield a forward force and thereby movement when oriented in certain angles relative to the relative wind direction. A kite or glider may typically be steered by changing its pitch angle by the controlling or operation of steering lines extending from the wind engaging member and/or by the operation or movement of lift altering members such as panels or flaps. Some airborne wind systems comprises more than one cable between the wind engaging member and the ground station and the kite or glider can then be steered by operation of the cables. When changing the pitch angle of the kite or glider, the angle relative to the direction of the relative wind speed changes. Changes to the pitch angle of the entire wind engaging member changes the speed whereas changes to the pitch angle in one side of the wind engaging member makes it turn.

In an embodiment, the width increases proportionally to the distance i.e. as a linearly increasing function of the distance. Hereby is obtained an increase of the width of the upwards cyclic patterns which corresponds the eye of sight of an observer at the reference location.

In an embodiment the method according to the previous further comprises for the first and/or each of the airborne wind energy systems determining a velocity of the wind engaging member, and further controlling the velocity in dependence of the width of the upwards cyclic pattern. Hereby the wind engaging members are further controlled in their power generation phase to move with a velocity tuned to match the width of the upwards cyclic pattern. Hereby the wind engaging members can be moved faster in a later and higher part of the flight path than a lower and earlier part of the flight path where the widths and the cycling distances are smaller.

The velocity may be controlled such that a time of two or more periods in the upwards cyclic pattern remains essentially constant. Hereby the one or more flight engaging members are operated to make each of the cyclic movements in the upwards cyclic pattern take approximately the same time. This further reduces the disordered or sometimes even chaotic appearance of the flight paths of the wind engaging members and makes the perception of the wind energy park more harmonic and calm.

In an embodiment the method according to any of the above further comprises determining a direction of movement of the wind engaging member of the first airborne wind energy system, and further steering a wind engaging member of a second airborne wind energy system into a position in a second flight path comprising the same direction of movement. The direction of movement may be determined from the determined positions of the wind engaging member taken over time or during a time interval. The second airborne wind energy system may be any other of the airborne wind energy systems of the wind energy park. The second flight path may be in part or completely identical in shape or correspond to the flight path of the first airborne wind energy system. By steering the wind engaging member into a position comprising the same direction of movement, the two wind engaging members fly more or less synchronously at least for a time period. Hereby the visual appearance of wind engaging members as they move around has been harmonized and simplified even further reducing the visual impact on the surrounding correspondingly.

In an embodiment, the control method is performed only in daylight hours and/or within a predetermined time interval. The airborne wind energy systems may then be controlled to maximize the power generation outside the daylight hours where the visual appearance of the kites or glider flying around in the air is of no or little concern. The airborne wind energy systems may alternatively or additionally be operated according to the invention within a predetermined time interval where the visual appearance of the wind energy park is of particular interest. The control method may likewise be performed within a predetermined time interval even when partially or completely dark in that the airborne wind energy systems may be required to fly with flight lights over some periods of time. Hereby the decrease of power generated by the wind energy park because of the alterations and potentially non-optimal control of the wind engaging members to increase the visual appearance and perception of the wind energy park is reduced to a minimum.

According to a second aspect the invention relates to a method for controlling the operation of a number of airborne wind energy systems arranged in a wind energy park, wherein at least a first and a second airborne wind energy system of the number of airborne wind energy systems each comprise a wind engaging member in the form of a glider and being coupled to a ground station via a cable and wherein the gliders are both operated to be moved along a cyclic closed flight path, the method comprising for each of the first and second airborne wind energy systems

- determining a distance between the ground station and a predetermined reference location, and - controlling the operation of the airborne wind energy systems as a function of the distance, comprising steering the wind engaging member along a cyclic closed flight path of a width in a horizontal plane and at a height from the ground, wherein the width and the height both increases with the distance. In an embodiment the width and the height are linearly proportional to the distance.

The airborne wind energy systems comprising gliders are normally operated so that the gliders cycle at a constant or near constant height above the ground. By the proposed control method, the flight path of each glider is adjusted

corresponding to the distance to the reference location. Hereby a glider further away is operated in a cyclic closed flight path of a larger width or diameter and a greater height than a glider of an airborne wind energy system positioned closer to the reference locations.

The advantages hereof are as for control method according to the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which Figs. 1-2 are perspective views of airborne wind energy systems for use in a wind energy park according to an embodiment of the invention,

Figs. 3A and 3B illustrate the power generation phase and the recovery phase of an airborne wind energy system according to prior art,

Fig. 4 illustrate a wind energy park with a number of airborne wind energy systems according to embodiments of the invention, Figs. 5A-C illustrate the power generation phase and the recovery phase of an airborne wind energy system according embodiments of the invention, and

Fig. 6-8 illustrate a wind energy park with a number of airborne wind energy systems according to embodiments of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Figures 1 and 2 are perspective views of two airborne wind energy systems 100 for use in a wind energy park operated according to embodiments of the invention. The airborne wind energy systems 100 all comprise a wind engaging member 101 catching and moved by the wind and connected to a ground station 104 via one or more cables 105.

In the airborne wind energy system 100 of figure 1, the wind engaging member 101 is in the form of a kite 102 connected to a winch system (not shown) in the ground station 104 via two cables 105. The operation of the kite 102 can thereby be fully or partly controlled by operation of the cables and the winch system.

In the airborne wind energy system 100 of figure 2, the wind engaging member 101 is also in the form of a kite 102. In this system, the kite is connected to a control unit 300 via steering lines 301 and to a winch system (not shown) in the ground station 103 via a cable 105. The operation of the kite 102 can thereby be fully or partly controlled by the operation of the steering lines 301 by the control unit in addition to the extraction and retraction of the cable 105 controlled from the winch system.

The wind engaging members 101 of the airborne wind energy systems 100 may further comprise different lift changing members (not shown) such as flaps or panels which may be operated to steer the kite 102.

For both airborne wind energy systems of figure 1 and 2, the extraction of the one or more cables 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 3A and B illustrate the operation of the kite 102 and with typical flight paths 400 indicated according to prior art. Typically, the kite operation

comprises a power generation phase of upwards cyclic movement 410 of the kite where the kite 102 may extract the cable(s) 105. Here, the wind acting on the kite 102 and the tensioning forces in the cable(s) 105 and any steering lines 301 causes it to move along a flight trajectory having the shape of an upwards spinning figure eight 401 or circle or oval 501. Such typical flight path during power generation is also indicated in figure 1 by the dotted line. During the power generation phase, the kite is operated to optimize the power generation. 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 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, 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.

Figure 4 illustrates 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. A number of airborne wind energy systems 100 are shown in the figure, each comprising a wind engaging member 101 connected to ground stations 104 via cables 105. The wind engaging members are here shown as all being kites 102 of the same type. However, in an embodiment, an energy park may be equipped with different types of airborne wind energy systems such as for example a kite next to a glider, all gliders 200 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 contribute to the controlling of the airborne wind energy systems.

The wind engaging members 101 are able to move along specified movement paths or flight trajectories generating mechanical energy, e.g. as described above with reference to Fig. 3A and B.

It can be seen that the kites or gliders 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 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 at a specific time.

Figures 5A, 5B, and 4C illustrate the operation of a first airborne wind energy system 100 in a wind energy park according to an embodiment of the invention. Each airborne wind energy system 100 in of the number of airborne wind energy systems comprises a wind engaging member 101 coupled to a ground station 104 via a cable 105 and each airborne wind energy system is operated to move the wind engaging member 101 along a flight path 400 comprising a power generation phase 410 of a generally upwards movement and a recovery phase 420 of a generally downwards movement. In difference to the flight paths of the wind engaging members as known in the art and as illustrated in figures 3A and 3B, the operation of the first airborne wind energy system 100 in the power generation phase 410 is here according to embodiments of the invention controlled as a function of a distance determined between the wind engaging member 101 and a predetermined reference location 500. The reference location is in figures 5A-C a position relatively close to the ground station 104. The reference location can as an example be a position of a structure such as a building or a part of a road from where it is desirable that the visual appearance of the airborne wind energy system is optimized. As illustrated in figures 5A-5C, the wind engaging member 101 is steering along a flight path 400 of an upwards cyclic pattern 410 and of a width 501 in a horizontal plane increasing with the determined distance between the kite and the reference location.

In figure 5A, the power generation phase 410 of the flight path 400 comprises an upwards cyclic pattern of figures-of-eight of widths increasing in the direction away from the reference location 500. In figures 5B and C the flight path comprises circles or ovals of a diameter increasing upwards and away from the reference location. The flight paths during the power generation phase as illustrated in figures 5A-C attains a generally conical shape or funnel-like shape.

Figures 6-8 illustrate a wind energy park 500 with a number of airborne wind energy systems 100. The wind engaging members 101 of each airborne wind energy systems 100 are generally in different positions in their respective flight trajectories or paths, some of them in the power generation phase and others being retracted in their recovery phase.

Both figure 6 and 7 illustrate the flight path of each of the number of kites 101 according to an embodiment in which here the flight paths of each of the kites are of the same overall shape with the flight path of the power generation phase 410 forming a cone or of a funnel-like shape. Here the width of the upwards cyclic pattern increases as a function of the distance from the kite down to the ground unit 104 of each of the airborne wind energy systems. Further, the kites 101 are here steered along a flight part which in the power generation phase comprises an upwards cyclic pattern of an increasing width corresponding to the flight path described in figure 5A.

The positions 600, 700 of each of the wind engaging members 101 at some given time are marked with crosses. In the embodiment sketched in figure 6, the kites are in different positions in their respective flight paths, whereas two of the three kites in the wind energy park of figure 7 are controlled such as to be in the same or near the same positions 700 in two different cycles of the power generation phase and moving in the same direction 701. The two kites thereby appear to fly synchronously and thereby yield a more harmonic and calm visual impression of the wind energy park. Figure 8 illustrate a wind energy park 500 controlled according to an embodiment of the invention and wherein the flight path 400 of the two kites 101 shown are not the same. Rather the kites are each controlled to follow a flight path with a power generation phase comprising an upwards cyclic pattern 410 and of a width 501 in a horizontal plane determined or set as a function of the distance to a common reference location 800. The common reference location is here the location of a house from which location the visual expression of the wind energy park is desirably optimized. It can be seen from figure 8 that the kite the closest to the reference location 800 is moving in an upwards cyclic pattern of smaller width s or diameter. In an embodiment the rotational speed the kites during the power generation phase is adapted to be the same. This is obtained by varying the velocity the kite corresponding to the width of the flight path.

The dotted lines 801mark the visual sight of an observer at the reference location 800 of one period of the upwards cyclic pattern and with the two kites 101 at different distances to the observer. From the figure can be seen that the width 501 of the cyclic pattern is the largest for the kite the furthest away.

Claims

1. A method for controlling the operation of a number of airborne wind energy systems arranged in a wind energy park, each airborne wind energy system comprising a wind engaging member being coupled to a ground station via a cable and wherein each airborne wind energy system is operated to move the wind engaging member along a flight path comprising a power generation phase of a generally upwards movement and a recovery phase of a generally

downwards movement, the method comprising

- determining a distance between the wind engaging member of the first airborne wind energy system and a predetermined reference location, and

- controlling the operation of the first airborne wind energy system in the power generation phase as a function of the determined distance, comprising steering the wind engaging member along a flight path of an upwards cyclic pattern of a width in a horizontal plane, and wherein the width increases with the distance.

2. A method according to claim 1, wherein the reference location corresponds to the position of the ground station of the first airborne wind energy system or position of a structure in or near the wind energy park, such as a building or a road.

3. A method according to any of the preceding claims wherein the position of the wind engaging member is determined by means of measurements with a GPS and/or an optical camera.

4. A method according to any of claims 2-3, wherein the distance between the wind engaging member of the first airborne wind energy system and the ground station of the wind engaging member is determined by measuring the length of the cable.

5. A method according to any of the preceding claims further comprising determining a position of the wind engaging member of each of the number of airborne wind energy systems and a distance between each of the wind engaging members and one or more predetermined reference positions, and controlling the operation of each of the number of airborne wind energy systems in their power generation phase according to the same function of the

determined distance.

6. A method according to any of the preceding claims, wherein the width increases proportionally to the distance.

7. A method according to any of the preceding claims further comprising for the first and/or each of the airborne wind energy systems determining a velocity of the wind engaging member, and further controlling the velocity in dependence of the width of the upwards cyclic pattern.

8. A method according to claim 7, wherein the velocity is controlled such that a time of two or more periods in the upwards cyclic pattern remains essentially constant.

9. A method according to any of the preceding claims further comprising determining a direction of movement of the wind engaging member of the first airborne wind energy system, and further steering a wind engaging member of a second airborne wind energy system into a position in a second flight path comprising the same direction of movement.

10. A method according to any of the preceding claims, wherein the control method is performed only in daylight hours and/or within a predetermined time interval.

11. A method according to any of the preceding claims wherein the steering of the wind engaging member along a flight path is performed by controlling an extraction and/or retraction of the cable, by controlling a number of steering lines attached to the wind engaging member, and/or controlling the operation of lift altering members of the wind engaging member.

12. A method for controlling the operation of a number of airborne wind energy systems arranged in a wind energy park, wherein at least a first and a second airborne wind energy system of the number of airborne wind energy systems each comprise a wind engaging member in the form of a glider and being coupled to a ground station via a cable and wherein the gliders are both operated to be moved along a cyclic closed flight path, the method comprising for each of the first and second airborne wind energy systems

- determining a distance between the ground station and a predetermined reference location, and

- controlling the operation of the airborne wind energy systems as a function of the distance, comprising steering the wind engaging member along a cyclic closed flight path of a width in a horizontal plane and at a height from the ground, wherein the width and the height both increases with the distance.

13. A method according to claim 12, wherein the width and the height are linearly proportional to the distance