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. 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 information on the operation of one or more of the airborne wind energy systems is used to improve the operation of other airborne wind energy systems in the park.

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 optimize the power output and reduce any interference of the wind engaging members with each other.

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. The method comprises:

- determining over a time interval a number of positions of the wind engaging member of a first airborne wind energy system;

- determining a wind speed and a wind direction;

- determining a wake corridor of the wind engaging member of the first airborne wind energy system based on the determined positions, the wind speed, the wind direction and a predetermined wake angle, and

- controlling the operation of a second airborne wind energy system positioned in a downwind direction relative to the first airborne wind energy system, the controlling comprising steering the wind engaging member of the second airborne wind energy system into a position outside at least a first part of the wake corridor of the wind engaging member of the first airborne wind energy system.

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 a cable. Accordingly, the airborne wind energy system is

mechanically attached to the ground station by means of the cable. According to the method, a wind speed and a wind direction is determined along with a number of positions of the wind engaging member of a first airborne wind energy system determined over a time interval.

The wind direction and/or wind speed may be determined at some specific height above the ground or may alternatively or additionally be determined as the free wind direction and free wind speed at or near the height of the wind engaging member of the first airborne wind energy system. In this way any changes in a wind direction at different heights may be taken into account and the wake corridor determined more accurately. The wind direction and wind speed at or near the height and position of the wind engaging member can be measured directly or indirectly by means of one or more anemometers and wind direction sensors connected to the wind engaging member of the first airborne wind energy system and/or attached to a control unit and/or steering lines. These parameters can also be determined in-directly by the knowledge of the position of the wind engaging member compared to the position of the ground unit, a pitch angle and left/right control of the kite, a cable angle, line force and/or a winch moment and rotational speed.

A position of the wind engaging member may be 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. The position may be determined as an absolute position or as a position relative to some reference point. The positions of the wind engaging member is determined over a time interval or a time period such as some predetermined time period, or over the time of a power production phase and/or a recovery phase i.e. over a part of or an entire flight cycle. By use of the determined positions, the wind speed, the wind direction and a predetermined wake angle a wake corridor of the wind engaging member of the first airborne wind energy system is determined. In contrast to the

determination of wakes behind a rotor of a wind turbine, the wind engaging member of an airborne wind energy system moves around as well as up and down considerably. This leads to the wake generated by the kite or glider moving correspondingly. As the wake corridor determination is based on a number of positions of the wind engaging member, the movement of the wake generated by the moving of the wind engaging member is taken into account effectively.

The wind engaging member of one airborne wind energy system will generate a wake downwind of the wind engaging member wherein the wake comprises a reduced mean wind speed and increased turbulence compared to the free wind stream. The wake will expand in a wake corridor of a generally conical shape extending in the direction of the wind. In estimating the wake expansion, a wake corridor with a pre-set or predetermined wake angle can be used. The wake angle can as an example be determined from experiments or estimated based on numerical simulations. The wake influence is the strongest closest to the wind engaging member. Further downwind the influence of the wind engaging member reduces and a less strong or clear wake is seen. I.e. the wake dissipates gradually over a distance behind the wind engaging member.

Therefore the number of positions of the kite or glider over a time interval can advantageously be used in determining the part of the wake where the wake influence is the strongest.

The method then further comprises controlling the operation of a second airborne wind energy system positioned in a downwind direction relative to the first airborne wind energy system in such a way as to at least partly avoid the wake corridor of the first airborne wind energy system. The controlling

comprises steering the wind engaging member of the second airborne wind energy system into a position outside at least a first part of the wake corridor of the wind engaging member of the first airborne wind energy system. By avoiding at least a first part of the determined wake corridor, the strongest part of the wake corridor is avoided.

The second airborne wind energy system is positioned in a downwind direction relative to the first airborne wind energy system such as positioned in a direction directly behind the first airborne wind energy system or positioned partly behind the first airborne wind energy system.

The steering of the wind engaging member may be 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. The steering the wind engaging member into a position outside the wake corridor may comprise steering the kite or glider out in a sideways direction relative to the wind direction, steering the kite or glider up to an increased above the wake corridor, steering the kite or glider down to a lower position, or combinations hereof.

The proposed park control method is advantageous in that the airborne wind energy systems are hereby controlled in such a way that the wind engaging members do not experience wakes of each other or at least in such a way that wakes from other wind engaging members only have a minor influence on the power production for downwind kites or gliders. Hereby the power production is optimized and further is prevented that the wind engaging member operates in wakes with increased turbulence levels results in increased dynamic loads on the kite or glider and thereby increased wear. The avoidance of the increased turbulence levels in the wake corridor further provides for an improved controlling of the movement of the kite or glider.

A further advantage of the control method is that the ground stations of the airborne wind energy systems can hereby be positioned closer to each and with more airborne wind energy systems on the same ground area without or with only minimal reduction of the power generation of the wind energy park. A reduced distance between the ground stations of a wind energy park is furthermore beneficial in reducing the costs for the infrastructure on the site as well as reducing the electrical losses in the cables on or in the ground between the ground structures.

In an embodiment of the invention, the method further comprises determining a position and a velocity of the wind engaging member of the second airborne wind energy system, and wherein the steering is determined as a function of the determined position and velocity. Hereby the wind engaging member of the second airborne wind energy system is improved and the kite or glider may be steered to a position outside the wake corridor faster and more accurately.

In an embodiment the length of the time intervals is set as a predetermined parameter and/or is set to be inversely proportional to the determined wind speed. As the strength or intensity of the wake corridor is seen to decrease with the distance from the object causing the wake, the wake corridor of the first wind engaging member may advantageously be based on determined positions from a shorter time interval the higher the wind speed. Alternatively the length of the time interval may simply be set as a predetermined parameter such as in the range of 1-20 seconds, 1-10 seconds, or 1-5 seconds.

In an embodiment, the wake corridor of the wind engaging member of the first airborne wind energy system is determined based on an average and/or mean value of the determined positions of the wind engaging member of the first airborne wind energy system . Hereby the wake corridor is determined in a simpler and presumably faster way yet with a reduced accuracy which in some situations may be considered sufficiently accurate.

In an embodiment, the first part of the wake corridor lies within a distance from the wind engaging member of the first airborne wind energy system, wherein the distance is set as a function of a main length of the wind engaging member of the first airborne wind energy system. The main length of a wind engaging member may be determined as a largest cross sectional width of the wind engaging member. Hereby only the part of the wake corridor yielding the highest influence and disturbance on the wind flow is determined, and the restrictions of the steering of the kite or glider of the second airborne wind energy system are less severe.

In an embodiment according to the above mentioned, the distance is determined as a parameter times the main length of the wind engaging member of the first airborne wind energy system, the parameter being in the range of 1-20, such as in the range of 3-10. Hereby the controlling reflects experiments showing that the wake will be the strongest at a distance of 0 - 3*L behind the kite or glider, where L is the main length of the wind engaging member. Further, the wake will be medium strong from 3* L - 6 *L behind the kite or glider, and relatively weak at distances further out than 10 *L behind the kite or glider.

In an embodiment, the control method further comprises continuously re- determining the number of positions and the wake corridor of the wind engaging member of the first airborne wind energy system, and updating the steering of the wind engaging member of the second airborne wind energy system based on the re-determined wake corridor. Hereby the second wind engaging member is continuously and repeatedly steered outside any wake from the upwind airborne wind energy system.

In an embodiment, the operation controlling the second airborne wind energy system comprises determining a flight path at least partly outside the first part of the wake corridor and steering the wind engaging member of the second airborne wind energy system to follow the determined flight path. By the determination of a flight path at least partly outside the first part of the wake corridor, the kite or glider of the second airborne wind energy system may be steered more continuously for longer periods of time while still avoiding the wake of the upstream kite or glider.

In an embodiment of the invention, the wind energy park comprises a third airborne wind energy system positioned in a direction upwind to the first and second airborne wind energy systems. The method here further comprises

- determining over the time interval a number of positions of the wind engaging member of a third airborne wind energy system;

- determining a wake corridor of the wind engaging member of the third airborne wind energy system based on the determined positions, the wind speed, the wind direction and the predetermined wake angle; and

- steering the wind engaging member of the second airborne wind energy system into a position outside at least a first part of the wake corridor of the wind engaging member of the first airborne wind energy system and outside at least a first part of the wake corridor of the wind engaging member of the third airborne wind energy system.

Hereby the wind engaging member of the second airborne wind energy system is operated to avoid the wake of both the upwind airborne wind energy systems.

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 and 4 are perspective views of three 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 embodiments of the invention, Fig. 5 shows the wake corridor generated by a kite as seen from the side,

Figs. 6-7 illustrate a wind energy park with a number of airborne wind energy systems and operated according to embodiments of the invention, and

Fig. 8 illustrates the wake corridor of a wind engaging member steered along an upwards spherical flight path.

DETAILED DESCRIPTION OF THE DRAWINGS

Figures 1, 2, and 5 are perspective views of three 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 altering 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 trajectories 400 indicated. Typically, the kite operation comprises a power generation phase of upwards movement 410 of the kite where the kite 102 may extract the cable(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 cyclic pattern of some cyclic shape such as a figure-of-eight 401 or circle or oval 501. Such typical flight path during power generation is also indicated in figure 1 by the dotted line. 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.

In the airborne wind energy system 100 of figure 4, the wind engaging member 101 is in the form of a glider 200, also sometimes referred to as a Makani. The glider 200 is provided with a number of rotors 201, each being capable of extracting energy from the wind and generating electrical energy. The glider 200 is connected to the ground by means of one or more cables 105. Figure 4 further illustrates the operation of the glider 200. It can be seen that the wind acts on the glider 200 and causes it to move along a substantially circular movement pattern, as indicated by the dotted line. This movement of the glider 200 causes rotation of the rotors 201 and thereby electrical energy is generated. The electrical energy is transferred to a suitable electrical component, e.g. a transformer or a converter unit, arranged at the ground station 104 via the cable 105 which comprises an electrically conductive cable 202.

Figure 5 shows a win engaging member lOland its wake in the wind 501 as seen in a side view. The conical lines 333 indicate the expansion and

development of the wake downwind from the current position of the wind engaging member 101 and over time. The wake attains

If the wind engaging member 101 is been held in the same position for period of time and during unchanging wind conditions, the wake corridor 335 attains a conical shape as also illustrated by the lines 333 and with corridor angle indicated as 330. The figure further illustrates that the wake dissipates with the distance from the wind engaging member. The strength or influence of the wake has been seen to decreases as a function of a main length of the wind engaging member L, 107 as indicated in the figures 1, 2, and 4.

Figure 6-7 illustrate an operation of airborne wind energy systems 100 in a wind energy park 500 according to an embodiment of the invention and as seen from a side and in an end view, respectively. A number of airborne wind energy systems 100 are shown in the figures, each comprising a wind engaging member 101 connected to ground stations 104 via cables. The wind engaging members are here shown as all being gliders 200 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 kites 102 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 kites or gliders 101 are able to move along specified movement paths or flight trajectories generating mechanical energy, e.g. as described above with reference to Fig. 3AB and 4.

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 6 and 7 illustrates an embodiment of the invention wherein the position of the upwind wind engaging members 101 are determined during a time interval and their wake corridors 335 extending downwind and determined. The wind engaging member 101 of one or more of the downwind airborne wind energy systems are then steered into positions outside the wake corridors 335 generated by any upwind airborne wind energy systems 100.

Figure 7 illustrates the wake corridors 335 of four different wind engaging members 101 in a wind energy park and as seen in a front view. The circles 700 indicate the sizes of the wake downstream in a certain cross section. The uppermost kite or glider 101 with the smallest circle is thus the furthest away and has generated a wake with a smaller propagation relative to the other kites or gliders lOlin the shown cross section. It can further be seen that all the wind engaging members are controlled to be in positions outside the wakes of other wind engaging members.

The wind engaging members in the figures 6 and 7 are all held more or less stationary in the wind and their wake corridors therefore are relatively simple to determine and remain correspondingly more or less constant. Figure 8 shows a simulation of a wake corridor 335 of a wind engaging member 101 (which here for the illustration purposes has no width) and resulting from the wind engaging member having moved along the upwards spherical flight path 801. As can be seen from the figure, the wake corridor moves around, up and down, and in and out of the plane of the paper because of the more complex shape of the flight path. The horizontal lines within the wake corridor indicate the expansion of the wake at fixed time steps. Where the horizontal lines are closer, the wake moves more in a sideways direction (in and out of the plane of the paper). The wake intensity reduces downwind (not shown).