NL2036700B1 - Method of controlling an airborne wind energy system - Google Patents

Abstract

A method for controlling an Airborne Wind Energy System the method comprising the steps of controlling a wind engaging member, in a first flight mode, wherein the method comprises controlling a steering angle of the power kite by balancing the steering lines; and/or controlling an angle of attack of the power kite by pulling and releasing the back line and/or the front line such that the kite is steerable in a crosswind direction to generate power by unwinding the tether; and, in a second flight mode, controlling a relative distance between both wingtips of the kite and the central part of the leading edge such that an effective lift generating surface area of the power kite is reduced. Figure 2B

Description

Method of controlling an airborne wind energy system

Field of Invention

The field of the invention relates to a method for controlling an Airborne Wind Energy

System. Particular embodiments relate to a computer program product comprising a computer- executable program of instructions for performing the method.

Background

An airborne wind energy system, abbreviated as AWES, typically comprises 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 bridle control 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 the tether, one or more control units for steering the wind-engaging member and/or winding and unwinding the tether.

Such a system is known from for example NL2009528. 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 acrostats. 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 suitable flight trajectory that can depend on the wind direction. Respective crosswind flight manoeuvres generate a high traction force which is transferred by the winch to the generator where itis converted to electricity. When approaching 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.

Controlling an airborne wind energy system, in particular in a landing or take-off phase can be difficult. Depowering the wind-engaging member by reducing the relative angle of the wind- engaging member with respect to the apparent wind has the tendency to overshoot the ground station of the AWES. This overshoot can cause dangerous situations or even flip the ground station on its side.

Summary

The object of embodiments of the present invention is to provide a method for effectively and safely controlling the flight of a wind engaging member in every phase of its respective flight.

According to a first aspect of the present invention there is provided a method for controlling an Airborne Wind Energy System, abbreviated as AWES. The AWES comprises a wind- engaging member comprising a kite having a leading edge and a trailing edge. The AWES further comprises a bridle system and a bridle control device. The bridle system is configured to connect the kite to the bridle control device and comprises at least two steering lines being connected to respective zones including wingtips of the kite, a back line connected to a back zone of the kite comprising a central part of the trailing edge, and, optionally, a front line connected to a front zone of the kite including a central part of the leading edge. The AWES further comprises a tether connected to the bridle control device. The AWES further comprises a ground unit comprising a tether storage device for winding and unwinding the tether, an energy converting device connected to the tether storage device, and a controller for controlling the bridle control device and/or winding and unwinding of the tether. The controller is configured to control the bridle control device such that the kite is operable in at least a first flight mode and a second flight mode. In the first flight mode the method comprising the following steps controlling a steering angle of the power kite by balancing the steering lines. Optionally, an angle of attack of the power kite is controlled by pulling and releasing the back line and/or the front line such that the kite is steerable in a crosswind direction to generate power by unwinding the tether. In the second flight mode, the method comprises controlling a relative distance between both the wingtips and the central part of the leading edge. In this way an effective lift generating surface area of the power kite is reduced.

Also, the effective lift vector is controlled in such a way that it reduces the required steering input.

In this way a flying kite with reduced power and improved roll stability is obtained. The above- described method of controlling an AWES offers several technical advantages: firstly the method enables the AWES to operate in two distinct flight modes - the first flight mode and the second flight mode. This flexibility allows the system to adapt its behaviour based on prevailing wind conditions and energy requirements as well as the respective flight phase the wind-engaging member is situated in. In the first flight mode, the system provides precise control over the power kite's steering angle by balancing the steering lines. This level of control ensures stable and efficient flight, allowing the AWES to optimize power generation as it steers across the wind direction. The method further allows for controlling the angle of attack of the power kite by manipulating the back line and/or the front line. This dynamic adjustment of the kite's angle of attack ensures better aerodynamic efficiency, contributing to increased power generation during the first flight mode. By unwinding the tether during the first flight mode, the AWES can effectively convert the kite's motion into electrical energy. The controlled steering and angle of attack adjustments enable the system to maximize power generation and harvest wind energy efficiently.

In the second flight mode, the method allows for controlling the relative distance between the wingtips and the central part of the leading edge.

This results in the reduction of the effective lift-generating surface area of the power kite, which is beneficial in situations where excessive lift needs to be avoided, such as during strong winds, during landing or take-off or when the energy output is not required at maximum capacity.

The advantage of controlling the relative distance in the second flight mode lies in the ability to enhance efficiency, stability, safety, and adaptability in specific wind conditions or energy requirements.

By reducing the effective lift generating surface area, the power kite generates less lift during the second flight mode.

As a result, the airborne wind energy system can convert a higher proportion of the wind's kinetic energy into electrical power, leading to increased overall energy conversion efficiency in particular because less energy is consumed when reeling in the kite between power generating phases.

Moreover, during strong wind conditions or gusts, excessive lift can strain the system and affect its stability.

By controlling the relative distance to reduce the effective lift generating surface area, the system can mitigate the risks associated with overpowering the tether and ground unit.

This increased stability helps to prevent potential damage to the equipment and ensures safe and reliable operation.

The ability to adjust the effective lift generating surface area allows the airborne wind energy system to adapt to varying wind speeds and energy demands.

In situations where maximum power output is not required, such as during low energy demand periods or when wind speeds are too high, reducing the surface area helps the system maintain optimal performance without excessive strain.

The reduction in lift and aerodynamic forces during the second flight mode can lead to reduced wear and tear on the power kite and bridle system components.

Consequently, this extends the overall lifespan of these components, reducing maintenance costs and improving the long-term reliability of the airborne wind energy system.

By fine-tuning the effective lift generating surface area, the system gains better control over its flight characteristics.

This flexibility allows the AWES to operate efficiently across a broader range of wind speeds and conditions, ensuring continuous energy generation in varying weather patterns.

Incorporating a controlled reduction in the lift-generating surface area provides a safety margin that can be crucial in emergency situations or unexpected changes in wind conditions.

The system can maintain stable flight even during sudden wind gusts or turbulent weather, reducing the risk of operational disturbances or accidents.

In conclusion, controlling the relative distance to reduce the effective lift generating surface area offers multiple advantages, including improved efficiency, enhanced stability, adaptability to varying wind conditions, extended component lifespan, wind range flexibility, and increased safety.

Moreover, as previously mentioned, the lift vector of the kite is changed in the second mode.

This is used to passively stabilize the kite.

The advantage hereof is based on the insight that the effective lift vector of a kite can influence roll stability by creating a torque or twisting force on the kite.

When the lift vector is not aligned with the kite's centre of gravity, it can create a moment of force that tends to rotate the kite around its axis. This twisting force can cause the kite to roll or tilt to one side or the other. By controlling a relative distance between both the wingtips and the central part of the leading edge a kite design that is well-balanced and stable in flight is obtained. reducing the amount of steering that is required and reducing the amount of power that the kite can generate. These benefits contribute to the overall performance, reliability, and economic viability of the airborne wind energy system. In the second flight mode tether management can also be improved. The ground unit with the tether storage device allows efficient tether winding and unwinding. This enhanced tether management ensures the kite's movement is well-controlled and avoids tangling or entanglement issues, enhancing the safety and reliability of the AWES. In summary, the described method combines the dual flight modes to provide dynamic adjustments to the kite's steering and power generation ensuring optimal power generation, while maintaining system stability and safety during phases where power generation is not essential. Preferably, the controlling in the second flight mode, comprises controlling the relative distance such that the effective lift generating surface area is reduced by at least 10%, preferably 20%, preferably at least 30%, more preferably at least 40%.

Preferably, the step of the controlling the relative distance comprises pulling the at least two steering lines such that the relative distance between the wingtips and the central part is increased. Controlling the relative distance in the second flight mode by pulling the at least two steering lines to increase the distance between the wingtips and the central part has several advantages. Increasing the relative distance between the wingtips and the central part of the kite reduces the overall wing surface area exposed to the wind. This reduction in exposed or projected wing surface area leads to a decrease in lift and drag forces experienced by the kite. Adjusting the relative distance through the steering lines allows for better control over the kite's flight dynamics.

By increasing the distance, the kite becomes more stable and less prone to sudden changes in direction or movements caused by gusts or turbulent winds. Improved stability ensures smoother and more consistent power generation, enhancing the overall reliability and lifetime of the airborne wind energy system. The ability to adjust the relative distance between the wingtips and the central part enables the system to adapt to different wind conditions. In strong winds, reducing the wing surface area minimizes the risk of overpowering the kite, while in light winds, increasing the surface area ensures sufficient lift generation. This adaptability broadens the system's operational wind range, allowing it to generate energy effectively in various wind speeds. By controlling the relative distance through the steering lines, the system can fine-tune its aerodynamic profile for maximum power generation. The adjustment helps the kite maintain an ideal angle of attack. resulting in improved lift-to-drag ratios and overall energy conversion efficiency. The increased stability achieved by controlling the relative distance through the steering lines provides additional safety margin while facing adverse weather conditions. The system can better withstand strong gusts or turbulence, reducing the risk of loss of control and potential damage to the equipment.

Controlling the relative distance by pulling the steering lines also allows for precise adjustments to 5 the kite's configuration. This level of control enables the system to respond quickly to changing wind conditions and energy demands. ensuring optimal performance and reliable power generation.

By reducing the lift and aerodynamic forces, the kite and related components experience less stress during operation. This reduction in mechanical strain can lead to a longer lifespan for these components, reducing maintenance costs and increasing the overall system's durability.

Preferably, the pulling comprises simultaneously pulling the at least two steering lines.

Simultaneously pulling the at least two steering lines to increase the relative distance between the wingtips and the central part allows for synchronized and balanced adjustments to the kite's configuration. This coordinated control ensures that the kite maintains its stability and remains in a controlled flight path during the second flight mode. By pulling the at least two steering lines simultaneously. the system achieves precise and controlled changes to the kite's wing geometry.

Pulling the at least two steering lines to increase the relative distance between the wingtips and the central part can be performed in a synchronized manner. The synchronized adjustment ensures that the relative distance is changed evenly, preventing any uneven stress or strain on the kite structure.

Simultaneously pulling the steering lines to adjust the relative distance results in quicker response times compared to individual line adjustments. This enhanced responsiveness allows the airborne wind energy system to adapt swiftly to changing wind conditions, optimizing power generation efficiency.

Preferably, the step of the controlling the relative distance further comprises maintaining the length of the back line or the front line. The step of maintaining the length of either the back line or the front line simplifies the control process. Instead of adjusting all lines independently, the system focuses on two lines, streamlining the control mechanism and reducing the risk of error.

Maintaining the length of the back line or the front line while adjusting the relative distance allows the system to retain the appropriate tension required for efficient power generation. This ensures that the kite maintains its optimal angle of attack during the second flight mode, minimizing energy losses and maximizing power output. The controlled and synchronized adjustment of the steering lines ensures that the kite's flight remains within safe operational parameters. The maintained length of either the back line or the front line provides a safety buffer, preventing potential overextension or failure of the kite during the second flight mode. The synchronized control of the at least two steering lines, combined with maintaining the length of either the back line or the front line, enhances the overall reliability of the system. The balanced and precise adjustments reduce the likelihood of unintended manoeuvres or abrupt changes that could lead to system instability or damage.

Preferably, the step of the controlling the relative distance comprises releasing the front line and/or back line. This step can be performed as an alternative to the pulling of the steering lines and can also be performed in conjunction with the pulling of the steering lines. Introducing the option to release the front line provides greater flexibility in controlling the relative distance.

The system can now adjust the kite's wing geometry both by pulling the steering lines and by releasing the front line, offering different approaches to achieving the desired reduction in lift generating surface area. Releasing the front line allows for fine-tuned adjustments to the kite's configuration. It provides the ability to control the relative distance with precision, making smaller or more subtle changes when necessary, which may be particularly beneficial in response to varying wind conditions. Combining both pulling the steering lines and releasing the front line allows for dynamic control of the kite's flight characteristics. The system can adjust the kite's wing geometry in real-time, responding to changes in wind speed or energy requirements, thereby optimizing power generation throughout different flight conditions. Releasing the front line provides an additional safety measure, allowing the system to avoid excessive lift generation, which could occur during strong winds. This safety feature helps prevent overloading the tether and ground unit, ensuring the stability and integrity of the airborne wind energy system. The option to release the front line, either independently or in conjunction with pulling the steering lines, improves the system's adaptability to a wide range of wind conditions. This adaptability is crucial for maintaining stable and efficient operation across varying weather scenarios. By incorporating multiple methods for controlling the relative distance, the system gains redundancy in its flight control mechanisms. H one approach faces technical issues or constraints, the AWES can rely on the alternative method, ensuring a robust and reliable operation. In summary, including the step of releasing the front line in controlling the relative distance adds flexibility, and capability for fine- tuned adjustments, energy conservation, dynamic control, safety measures, operational adaptability, and redundancy to the airborne wind energy system. This comprehensive approach enhances the system's overall efficiency, stability, and reliability, making it better equipped to harness wind energy effectively and sustainably.

Alternatively, the step of the controlling the relative distance can comprise releasing the at least two steering lines while at least maintaining the length of the back line or the front line such that the relative distance between the wingtips and the central part is decreased. Releasing the at least two steering lines allows the system to decrease the relative distance between the wingtips and the central part of the kite. In this way an alternative method for reducing the effective lift surface is provided. Decreasing the relative distance between the wingtips and the central part results in a reduction in the wing surface area exposed to the wind. This reduction in wing area decreases the lift generated by the kite, preventing excessive forces that may occur during strong winds and enhancing the system's stability. This energy-conserving measure is beneficial when the system needs to operate at a lower power output or during periods of low energy demand. By independently releasing the steering lines, the system gains precise control over the kite's flight dynamics. This fine-tuned adjustment allows the kite to maintain a specific flight path and angle.

Decreasing the relative distance provides an additional safety measure by reducing the lift generated by the kite. This can prevent overloading of the tether and ground unit, mitigating potential risks during turbulent weather or sudden wind changes. Including the option to release the steering lines offers adaptability to varying wind conditions. The AWES can adjust the relative distance as needed, ensuring optimal performance across a broad range of wind speeds and directions. By offering multiple methods for controlling the relative distance, the system gains redundancy in its flight control mechanisms. If one approach encounters technical issues, the alternative method can be used to maintain stable and efficient operation.

According to yet another aspect of the present invention, there is provided a computer program product comprising a computer-executable program of instructions for performing, when executed on a computer, the steps of the method of any one of the method embodiments described above.

It will be understood by the skilled person that the features and advantages disclosed hereinabove with respect to embodiments of the method may also apply, mutatis mutandis, to embodiments of the computer program product.

According to yet another aspect of the present invention, there is provided a digital storage medium encoding a computer-executable program of instructions to perform, when executed on a computer, the steps of the method of any one of the method embodiments described above.

It will be understood by the skilled person that the features and advantages disclosed hereinabove with respect to embodiments of the method may also apply, mutatis mutandis, to embodiments of the digital storage medium.

According to yet another aspect of the present invention, there is provided a bridle control device programmed to perform a method comprising the steps of any one of the methods of the method embodiments described above.

It will be understood by the skilled person that the features and advantages disclosed hereinabove with respect to embodiments of the method may also apply, mutatis mutandis, to embodiments of the bridle control device.

Further aspects of the present invention are described by the dependent claims. The features from the dependent claims, features of any of the independent claims and any features of other dependent claims may be combined as considered appropriate to the person of ordinary skill in the art, and not only in the particular combinations as defined by the claims.

Brief description of the figures

The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of devices of the present invention. The above and other advantages of the features and objects of the present invention will become more apparent, and the present invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

Figure 1 schematically illustrates an embodiment of an airborne wind energy system:

Figure 2A schematically illustrates a kite being operated in the first mode according to an exemplary embodiment ;

Figures 2B-2D schematically illustrate an embodiment of a kite being operated in the second mode according to an exemplary embodiment.

Description of embodiments

A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g.. digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The program storage devices may be resident program storage devices or may be removable program storage devices, such as smart cards. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods.

The description and drawings merely illustrate the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the present invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the present invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the present invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

The functions of the various elements shown in the figures, including any functional blocks labelled as “processors”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.

Figure 1 schematically illustrates an embodiment of an airborne wind energy system. As follows from Figure 1, an airborne wind energy system generally includes a ground station 200, a tether 140, a bridle system 120 and a wind engaging member 100. The ground station 200 comprises a tether storage device 210, an energy converting device 220, and a controller 230. The tether 140 is connected to a bridle control device 130. The bridle system 120 is configured to connect the wind engaging member 100 to the bridle control device 130. Optionally, a battery/power electronics module 240 may be provided. Any kind of energy storage means can be used to store energy, for example an electrical storage means such as a battery or capacitor. In place of using a rechargeable battery for storing electrical energy, a mechanical energy storage device may be employed or the generated power may be used to run a mechanical implement such as a pump etc. The tether storage device 210 is typically a drum, rotating around a respective rotation axis during operation. The energy converting device 220 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 airbome wind energy systems in accordance with the invention are applied that connect to such battery/storage device or when the grid is sufficiently robust to deal with fluctuations. The controller 230 may comprise several interconnected computers hosting different software components required for operating the airborne wind energy system 1. In addition, the controller 230 may comprise wireless modems to connect remote sensors, remote actuators and a bridle control device 19. The function of the bridle control device 19 is further explained hereinafter with particular reference to figure 3.

The tether 140 transfers the traction force generated by the wind-engaging member 4 to the tether storage device 210. The tether 140 is typically made of a strong and lightweight plastic fiber, cord, rope or rod, and is connected to the bridle system 120 of the wind- engaging member 4. The connection of the tether 140 with the bridle system 120 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. The connection may also include a (not shown) sensor to measure tether force.

The wind engaging member 100 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 optimally steerable. In another embodiment a fixed-wing type wind-engagement member can be installed.

For example, instead of the kite there can be provided a wind-engaging member in the form of a fixed (rigid) wing airfoil type, having e.g. a rigid main wing section that extends substantially normally with respect to the tether during operation. The bridle system comprises at least two steering lines 121, 122 being connected to respective zones including wingtips of the kite. The bridle system may further comprise at least one of a back line connected to a back zone of the kite comprising a central part of the trailing edge and a front line 123 connected to a front zone of the kite including a central part of the leading edge. The back line is not shown in figure 1.

The AWES includes one or more bridle control device 130, e.g. incorporated in the bridle, to generate the steered movement of the wind engaging member 100. The bridle control device 130 can include e.g. one or more actuators for winding and unwinding one or more of the steering lines 122, 123, to adjust the orientation of the wind engaging member 100 in order to follow a predetermined or desired flight path. The skilled person will understand that other ways configured for pulling or releasing the lines can be provided, for example a means which does not necessarily wind the lines on a roll but allows a portion of the lines to pass through said means such that their relative steering lengths are shortened, can also be used.

Figures 2A-2D schematically illustrate the first and second modes of the AWES system. In the figures 2A-2D a kite 110 is illustrated having a bridle system with two steering lines 121, 122 and a front line 123. The two steering lines 121, 122 being connected to respective zones including wingtips of the kite 110. Circles indicate the connection points of the lines to the kite body. It will be clear to the skilled person that more steering lines can be provided, or that alternate configurations of lines, for example steering lines which are bundled, can be provided. The front line 123 is connected to a front zone of the kite 110 including a central part of the leading edge. It is not essential that the front line is connected to the exact center of the kite. It will be clear that multiple front lines may be provided or that the front line or front lines can be configured in a different manner such as a bundle of front lines .

Figure 2A schematically illustrates a kite 110 being operated in the first mode according to an exemplary embodiment. The first mode can also be called a power generation mode. In the first mode a steering angle of the power kite is controlled by balancing the steering lines 121, 122. To enable precise steering control, the steering lines 121, 122 are preferably connected as far as possible from the center of the kite, being at the wingtips. Generating power using an AWES involves controlling the kite's 100 flight and steering angle to efficiently harness wind energy. The process includes two main aspects: controlling the steering angle of the power kite 100 by balancing the steering lines and controlling the angle of attack of the power kite 100 by pulling and releasing the back line and/or the front line 123. In the first flight mode of the AWES, the method controls the steering angle of the power kite 100 by balancing the steering lines 121, 122. The kite 100 is connected to the bridle control device (as shown in figure 1) through at least two steering lines, which are attached to the wingtips of the kite. By adjusting the tension in these steering lines, the system can change the kite's orientation and angle relative to the wind direction.

When the wind blows, the kite flies in a crosswind direction, forming a figure-eight-like or looping path known as a "laddermill” motion. By balancing the tension in the steering lines, the kite 100 maintains stable flight and generates lift as it moves through the wind. The lift generated by the power kite 100 is indeed a result of the effective lift generating surface area. To clarify, the effective lift generating surface area S refers to the area of the kite's wing that interacts with the wind to produce lift. It is determined by the kite's shape, size, and the angle of attack relative to the wind. In the context of an AWES, controlling the kite's angle of attack and steering angle allows the system to adjust the effective lift generating surface area S. By increasing the angle of attack more of the kite's wing surface interacts with the wind, resulting in higher lift. This lift force is then converted into tension in the tether, which unwinds from the tether storage device and drives the energy converting device to generate electrical or mechanical power,

The lift force thus pulls on the tether 140 connected to the ground station 200. causing the tether to unwind from the tether storage device. In addition to controlling the steering angle, the system can further influence the power generation by controlling the angle of attack of the power kite 100.

This is achieved by manipulating the back line and/or the front line of the bridle system. To increase the kite's angle of attack and generate more lift, the system pulls the back line and/or releases the front line 123, effectively adjusting the kite's pitch angle. As the angle of attack increases, the kite generates more lift force, resulting in higher tension in the tether and increased power output. Conversely, to reduce the kite's 100 angle of attack and generate less lift, the system releases tension from the back line and/or pulls the front line 123. This action decreases the angle at which the kite faces the wind, reducing the litt force and subsequently lowering the tension in the tether.

By dynamically adjusting the angle of attack, the AWES can optimize its power generation according to the wind conditions and energy requirements. During strong winds or high energy demand, the kite can increase its angle of attack for higher power output. Conversely, during low wind conditions or lower energy demand, the kite can reduce its angle of attack to conserve energy and maintain stable flight.

Figures 2B-2D schematically illustrate an embodiment of a kite being operated in the second mode. In all of figures 2B-2D, the effective lift generating surface is illustrated with the brackets indicated by the letter *S’. In the second flight mode, the method allows for controlling the relative distance Ad between the wingtips and the central part of the leading edge. This results in the reduction of the effective lift-generating surface area S of the power kite, which is beneficial in situations where excessive lift needs to be avoided, such as during strong winds, during landing or take-off or when the energy output is not required at maximum capacity. The advantage of controlling the relative distance Ad in the second flight mode lies in the ability to enhance efficiency, safety, and adaptability in specific wind conditions or energy requirements. By reducing the effective lift generating surface area, the power kite generates less lift during the second flight mode. This reduction in lift minimizes aerodynamic drag, enabling the system to operate more efficiently. As a result, the airborne wind energy system can convert a higher proportion of the wind's kinetic energy into electrical power, leading to increased overall energy conversion efficiency in particular because less energy is consumed when reeling in the kite between power generating phases. Moreover, during strong wind conditions or gusts, excessive lift can strain the system and affect its stability. By controlling the relative distance Ad to reduce the effective lift generating surface area S, the system can mitigate the risks associated with overpowering the tether and ground unit. Preferably, the controlling, in the second flight mode, comprises controlling the relative distance Ad such that the effective lift generating surface area S is reduced by at least 20%, preferably at least 30%, more preferably at least 40%.

Figure 2B illustrates an exemplary embodiment where the step of the controlling the relative Ad distance comprises pulling the at least two steering lines 121, 122 such that the relative distance Ad between the wingtips and the central part is increased. In this way the effective lift generating surface area S is reduced as increasing the relative distance Ad between the wingtips and the central part of the kite reduces the effective wing surface area exposed to the wind. This reduction in wing surface area leads to a decrease in lift and drag forces experienced by the kite.

Adjusting the relative distance through the steering lines allows for better control over the kite's flight dynamics, thus avoiding an overshoot for example when landing or taking off. By increasing the distance. the kite becomes more stable and less prone to sudden changes in direction or movements caused by gusts or turbulent winds. The kite 100 comprises a better roll stability in this manger. The ability to adjust the relative distance between the wingtips and the central part enables the system to adapt to different wind conditions. In strong winds, reducing the wing surface area minimizes the risk of overpowering the kite, while in light winds, increasing the surface area ensures sufficient lift generation. This adaptability broadens the system's operational wind range, allowing it to generate energy effectively in various wind speeds. By controlling the relative distance through the steering lines 121, 122, the system can fine-tune its aerodynamic profile for maximum power generation within system limits. In the exemplary embodiment of figure 2B, the pulling can comprise simultaneously pulling the at least two steering lines. Simultaneously pulling the at least two steering lines 121, 122 to increase the relative distance between the wingtips and the central part allows for synchronized and balanced adjustments to the kite's configuration. This coordinated control ensures that the kite maintains its stability and remains in a controlled flight path during the second flight mode. By pulling the at least two steering lines 121, 122 simultaneously, the system achieves precise and controlled changes to the kite's wing geometry.

The synchronized adjustment ensures that the relative distance is changed evenly, preventing any uneven stress or strain on the kite structure. Moreover, in the example of figure 2B the step of the controlling the relative distance can further comprise maintaining the length of the back line or the front line 123. Again it is noted that only the front line 123 is shown in the figures. The step of maintaining the length of either the back line or the front line simplifies the control process. Instead of adjusting all lines independently, the system focuses on two lines, streamlining the control mechanism and reducing the risk of error. Additionally, or alternatively the step of the controlling the relative distance Ad comprises releasing the front line. This step can be performed as an alternative to the pulling of the steering lines and can also be performed in conjunction with the pulling of the steering lines 121, 122. This provides greater flexibility in controlling the relative distance. The system can now adjust the kite's 100 wing geometry both by pulling the steering lines and by releasing the front line, offering different approaches to achieving the desired reduction in lift generating surface area. Releasing the front line 123 allows for fine-tuned adjustments to the kite's 100 configuration. It provides the ability to control the relative distance Ad with precision,

making smaller or more subtle changes when necessary, which may be particularly beneficial in response to varying wind conditions.

Figures 2C and 2D illustrate alternatives exemplary embodiments to figure 2B. The figures 2C and 2D illustrate the step of the controlling the relative distance which can comprise releasing the at least two steering lines 121, 122 while at least maintaining the length of the back line or the front line 123 such that the relative distance between the wingtips and the central part is decreased. In this way a bird like, horseshoe-like or V-like shape 1s created. Releasing the at least two steering lines allows the system to decrease the relative distance Ad between the wingtips and the central part of the kite. In this way an alternative method for reducing the effective lift surface is provided.

Decreasing the relative distance between the wingtips and the central part results in a reduction in the wing surface area exposed to the wind. A kite with a dihedral shape is a type of power kite that features a distinct V-shaped configuration when viewed from the front or back. The dihedral shape is characterized by the wings angling upwards, forming a V or delta shape. The dihedral shape contributes to the kite's stability during flight. The V-shaped wings create a natural dihedral effect, which tends to level the kite and keep it oriented into the wind. This inherent stability helps maintain consistent flight patterns and minimizes the risk of the kite stalling or losing control. Due to their stable design, kites with a dihedral shape are often easier to launch compared to other kite configurations. When properly handled and released into the wind, they tend to catch the air and lift off from the ground more smoothly. The dihedral shape can improve the kite's aerodynamic efficiency. The upward-angled wings create lift as the wind flows over them, allowing the kite to efficiently convert wind energy into mechanical power. Kites with a dihedral shape are generally well-suited for crosswind steering, allowing them to perform figure-eight or looping patterns in the sky. This capability is advantageous in airborne wind energy systems, where the kite's motion can be harnessed to generate power on the ground using a tether. Despite their inherent stability, dihedral-shaped kites can be designed with a degree of manoeuvrability, enabling controlled flight adjustments as needed. This feature is valuable for optimizing power generation and adapting to changing wind conditions.

The V-shaped design of dihedral kites provides a strong and robust structure. This enables them to withstand various wind conditions and resist deformation, contributing to their durability and reliability in challenging environments. Kites with a dihedral shape can be designed to operate effectively across a broad range of wind speeds. The design allows for efficient flight at both high and low wind speeds, making them adaptable to varying weather conditions. The dihedral shape may also help reduce aerodynamic drag, particularly when compared to some flat-winged designs.

Lower drag improves overall efficiency and contributes to higher power generation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the present invention and that those skilled in the art will be able to design alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps not listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The present invention can be implemented by means of hardware comprising several distinct elements and by means of a suitably programmed computer. In claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The usage of the words “first”, “second”, “third”, etc. does not indicate any ordering or priority. These words are to be interpreted as names used for convenience.

In the present invention, expressions such as “comprise”, “include”, “have”, “may comprise”, “may include”, or “may have” indicate existence of corresponding features but do not exclude existence of additional features.

Whilst the principles of the present invention have been set out above in connection with specific embodiments, it is to be understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

Claims (9)

Conclusions 1. A method of controlling an airborne wind energy system, AWES, the AWES comprising a wind catching member (100) having a kite (110) having a leading edge and a trailing edge, a harness system (120) and a harness control device (130), the harness system adapted to connect the kite to the harness control device, the harness system comprising at least two control lines (121, 122) connected to respective zones of the kite comprising wingtips, optionally the harness system comprising a trailing line connected to a trailing zone of the kite comprising a central portion of the trailing edge and a leading line (123) connected to a leading zone of the kite comprising a central portion of the leading edge; a tether rope (110) connected to the harness control device (130); the AWES further comprises a base unit (200) comprising a line storage means (210) for winding and unwinding the tether rope (100), an energy conversion device (220) connected to the line storage device; and a control device (230) for controlling the rigging control device and/or the winding and unwinding of the tether rope, the control device being adapted to control the rigging control device such that the kite can be operated in at least a first flight mode and a second flight mode; the method comprises the steps of: - in the first flight mode, controlling a steering angle of the kite by balancing the steering lines; and/or optionally controlling an angle of attack of the kite by pulling and releasing the back line and/or the front line so that the kite is steerable in a crosswind direction to generate force by unwinding the tether line; - in the second flight mode, controlling a relative distance between the wing tips and the central part of the leading edge. 2. The method of claim 1, wherein the control device, in the second flight mode, comprises controlling the relative distance such that the effective lift generating area is reduced by at least 10%, preferably by at least 20%, preferably by at least 30%, preferably by at least 40%. 3. The method of any preceding claim, wherein the step of controlling the relative distance comprises pulling on at least one of the at least two control lines such that the relative distance between the wing tips and the central portion is increased. 4. The method according to the preceding claim, wherein the pulling comprises pulling simultaneously on the at least two control lines, 5. The method of any of the preceding claims 3-4, wherein the step of controlling the relative distance further comprises maintaining the length of the back line or the front line. 6. The method of any preceding claim, wherein the step of adjusting the relative distance comprises releasing one of the leading and trailing lines. 7. The method of claim 1 or 2, wherein the step of controlling the relative distance comprises releasing at least one of the at least two control lines while maintaining at least one of the trailing line and the leading line such that the relative distance between the wing tips and the central portion is reduced. 8. The method according to the preceding claim, wherein the step of controlling the relative distance comprises pulling on the back line and/or the front line while releasing the at least two steering lines. 9. A computer program product comprising a computer executable program with instructions for, when executed on a computer, performing the steps of the method according to any one of the preceding claims.