System and method for airborne wind energy production

The invention relates to a system for electric power production from wind. The invention further relates to a method for operation of a system for electric power production from wind. The invention also relates to a glider for a system for production power from wind and to the use of a glider for production of electric power from wind.

The production of electric power from wind is generally accomplished by airfoils or structures with an aerodynamic profile, which produce lift forces upon exposure to wind. Thereby, energy is extracted from the wind, which can be converted into electricity, for example by exploring said lift forces to drive an electrical generator. Well known wind turbines for instance comprise a rotor with aerodynamically profiled rotor blades, wherein the lift forces of the rotor blades cause the rotor to turn. The rotor is mounted to an electrical generator, which is for example located on top of a tower, for production of electricity.

In order to explore wind energy resources at altitudes above few hundred meters over ground, where the average wind is stronger and steadier due to less disturbing interaction with the earth's surface, it has been proposed to use airborne airfoils. These concepts are often referred to as airborne wind energy or airborne wind energy production.

WO 2007/139 412 A1 discloses a system destined to harness wind resources comprising a ground station group and a lighter-than-air airborne group mutually connected by a cable group. Further, a control group is foreseen to significantly alter the resultant force vector of aerodynamic forces acting on the airborne group and tensioning the cable in order to create a work phase and a recovery phase, respectively.

US 6 254 034 B1 discloses a tethered kite travelling a closed cycle. While the kite travels downwind, it is driven by the wind and the tether is pulled from a windlass drum that is rotationally connected to the rotor of an electrical generator. The rotor of the generator spins and produces energy. The cycle is completed by travelling the kite upwind back to the starting point. Net energy is gained when the energy generated during downwind travel of the kite exceeds the energy required for the upwind return.

One option for a low consumption of energy at the upwind return is to cause the wind to blow the kite to a high altitude at the end of the downwind travel. The kite then returns in an upwind descent. However, this method suffers from a large dead time, because no energy is generated during rise and return of the kite. Therefore, the integrated energy yield is relatively poor.

Another option is to alter the angle of attack or the equivalent supporting plane surface of the kite at the end of the downwind travel, thereby reducing the pull of the kite on the tether. The kite is now pulled back by switching the generator to motor operation, wherein due to the reduced pull less energy is consumed than has been produced during the preceding downwind travel. Here, a fraction of the generated energy has to be spent in the interest of safety, because controllable flight of the kite requires a certain amount of pull on the tether.

The underlying problem of the invention is to provide for electric power production from wind using an airborne airfoil, wherein in particular the integrated energy yield and/or the operational safety are to be improved with respect to the prior art described above.

According to the invention, this problem is solved by a system for electric power production from wind comprising a glider, said glider having an airfoil, on-board steering means with control surfaces to aerodynamically exert torque on the glider for pitching, rolling and yawing the glider when airborne, a flight control means for operation of the steering means, and a connection means for a tether, the system further comprising a ground station, said ground station comprising a reel for the tether, a rotating electrical machine connected to the reel, and a ground station control means for operation of the reel and the rotating electrical machine, and the system further comprising a master control means for operation of the system in at least two alternative operation modes, wherein a first operation mode of the system is provided for electric power production with the rotating electrical machine from a rotation of the reel induced by reeling out the tether using a lift force generated upon exposure of the airfoil of the airborne glider to wind, and wherein a second operation mode of the system is provided for system recovery by driving the reel with the rotating electrical machine, thereby reeling in the tether onto the reel.

A glider or sailplane in terms of the invention in particular is a fixed wing aircraft, wherein on-board steering means allow for full flight manoeuvrability of the glider around its longitudinal axis, its lateral axis and its vertical axis. In terms of the invention, these three principle axes form a Cartesian coordinate system, wherein the origin of said coordinate system is defined to be at the centre of gravity of the glider.

In general terms, with reference to straight and level flight, the longitudinal axis relates to the direction of motion, the vertical axis relates to the direction of lift and the lateral axis is essential horizontal to complete a Cartesian coordinate system.

The glider for instance comprises a fuselage and a main wing, wherein the main wing constitutes or comprises an airfoil. In this configuration, the longitudinal axis is essential parallel to the fuselage, the lateral axis is essential parallel to the main wing and the vertical axis is perpendicular to both the longitudinal and the lateral axis. Those skilled in the art will appreciate that the glider can have another airplane configuration, for instance an all-wing aircraft, with appropriate definitions of the principle axes.

In terms of the invention, rolling refers to a rotation of the glider around its longitudinal axis, pitching refers to a rotation of the glider around its lateral axis and yawing refers to a rotation of the glider around its vertical axis.

Steering means of the glider comprise control surfaces to aerodynamically exert torque on the glider around one or more of the glider's principle axes. These control surfaces for instance comprise so called ailerons to mainly induce rolling, so called elevators to mainly induce pitching, and a so called rudder to mainly induce yawing. However, those skilled in the art will appreciate that other control surfaces known in aviation technology are also appropriate steering means in terms of the invention. In particular, a control surface can induce a rotation around an arbitrary axis which does not correspond to one of the principle axes of the glider.

Besides control surfaces, the steering means of the glider for instance further comprise actuators, such as electric motors or hydraulic systems with pumps and cylinders, for moving the control surfaces. These actuators are for instance powered by an on-board power source, such as a battery.

A glider provides the advantage of low aerodynamic resistance or drag and a high aerodynamic lift due to the fixed wing with rigid aerodynamic profile or airfoil, respectively. This is in particular beneficial, because the energy effectively extracted from the wind strongly depends on lift and drag, in particular on the so called lift-over-drag-ratio.

Since both the glider and the tether contribute to the overall drag, it is further preferred when the tether has a drag-optimized shape or structure. This can for instance be a helical structure, which turned out to have a lower resistance or drag than a tether with a circular cross section.

Another advantage of a glider is that the flight of the glider is stable even without any load on the tether, while a kite requires a certain pull on the tether for stable flight. Therefore, the invention allows for zero tether load and correspondingly minimum energy consumption during system recovery in the second operation mode.

The flight of the glider is controllable and stable by itself, in particular without any connection to the ground. Even in case of system failures, for example when the generator load on the reel is lost or the tether breaks, the glider can be landed safely. The system according to the invention thus provides for particularly effective and safe production of electric power from wind energy.

It is preferred that the flight control means provides for a first flight control mode for automated flight operation and for a second flight control mode for manual operation, in particular via a remote control unit with wired or wireless connection to the flight control means. For example, the first flight control mode provides for automated operation with optimised energy yield, wherein the second flight control mode allows for manual intervention during maintenance and testing of the system as well as for emergency interventions in the case of failures. This way, the operation is simplified and the safety in operation is further enhanced.

Another preferred embodiment of the invention is characterized in that the system, in particular the glider, further comprises an air speed sensor for determining an air speed of the glider. In terms of the invention, air speed refers to the speed of movement or velocity, respectively, of the glider with respect to the surrounding air. In particular due to the presence of wind, the air speed in general differs from the ground speed of the glider, i.e. the velocity of the glider relative to the ground. The air speed sensor preferably is a directional sensor providing both the magnitude and the direction of the glider's air speed.

Knowledge about the air speed of the glider is in particular beneficial for optimized flight control, in particular for maximising the lift force and the averaged energy yield. For best precision of the air speed measurement, the air speed sensor is preferably arranged at the glider. Alternatively, the air speed sensor can also be positioned at the tether, wherein the position of the air speed sensor at the tether is preferably close to the connection between the tether and the glider.

In an especially preferred embodiment of the invention, the glider comprises a control unit incorporating both the flight control means of the glider and the master control means of the system. This way, the connection between the flight control means and the master control means is particularly short in a physical sense, making it fast and robust against distortion and failure. In particular, fast feedback between the flight operations, which is rather complex and influenced by eventually fast changing wind conditions, and the operation of the system as a whole is or can be established.

Another preferred embodiment of the invention is characterized in that the system, in particular the ground station, further comprises a tension sensor for determining a tension of the tether. The tension of the tether is a good indicator for the overall system load and can for instance be used as an input parameter for controlled reeling out of the tether. The tension sensor is for example arranged at the ground station or integrated into the tether, wherein the tension sensor preferably is or can be connected to the ground station control means.

Said ground station control means is preferably designed for retaining a predetermined target tension of the tether, in particular during reeling out the tether. By this measure, negative impact of the load, which is exerted on the glider via the tether, on the glider flight behaviour can be minimized.

The ground station control means preferably is designed for retaining a predetermined reel speed, in particular during reeling in the tether. In terms of the invention, reel speed refers to the length of tether being reeled in or reeled out in a given amount of time. Thus, the reel speed is in particular correlated with the rotational velocity of the rotating reel.

Retaining a predetermined reel speed, which is for instance accomplished by retaining a predetermined target rotational velocity of the reel, is very effective in reducing sagging of the tether when the lift force or load exerted on the tether is low.

It is further preferred that the tether comprises a power transmission line between the glider and the ground station. This way, the various systems of the glider, such as electronics of the flight control means or actuators of the flight control means, are powered from the ground. For emergency cases, where the power connection between the ground and the glider may be broken, the glider, however, can comprise a relatively low capacity power source allowing for control and safe landing of the glider.

The tether preferably comprises a data transmission line between the glider and the ground station, for instance for communication between the flight control means, the ground station control means and/or the main control means. Additional, alternative or redundant communication channels can for instance be realized by wireless transmission.

The problem of the invention is further solved by a method for operation of a system for electric power production from wind according to the invention, said system comprising a glider connected to a tether and a ground station with a reel for the tether, wherein said system is operated alternately in a first operation mode for power production and a second operation mode for system recovery, wherein the first operation mode comprises

• steering the glider to follow a first flight pattern, thereby generating a lift force by means of an airfoil of the glider being exposed to the wind,
• pulling the tether by means of said lift force and reeling out the tether, thereby inducing a rotation of the reel, and
• converting the rotation of the reel into electric power, in particular by means of a rotating electrical machine connected to the reel,

wherein the second operation mode comprises

• steering the glider to follow a second flight pattern for reducing the pull on the tether and
• reeling in the tether onto the reel by driving the reel, in particular by means of said rotating electrical machine connected to the reel.

The integrated energy yield, i.e. the amount of electric power produced at given wind conditions in certain amount of time, is particularly optimized when the lift force generated by the airfoil is maximized during operation of the system in the first operation mode and minimized during operation in the second operation mode. The energy yield is further increased when the time required for reeling in the tether, that is the minimum duration for the system to be operated in the second operation mode, where no electric power is produced, is as short as possible.

Both aspects are optimized by the invention due to the beneficial features of the glider mentioned already above. In particular, the manoeuvrability of a glider allows for a first flight pattern, which is in particular a high lift flight pattern, where the glider for instance flies downwind of the ground station across the wind along a flight path, which for instance resembles a circular or eight-shaped figure. Highest lift forces are generally achieved in fast cross-wind flight of the glider.

In the second flight pattern, which is in particular a low lift flight pattern, the glider for instance is brought in a descent towards the ground station. Here, the pull on the tether is reduced and eventually brought to zero. This way, minimum energy is consumed for spinning the reel, while no energy is required to pull the glider towards the ground station. At the same time, the speed of the glider towards the ground station is maximized and the dead time, i.e. the time required for system recovery, is minimized.

Preferably, reeling out the tether is controlled to retain a target tension of the tether, which in particular is predetermined as a function of an air speed of the glider. The air speed of the glider in particular is the velocity of the airfoil relative to the air, and thus is an indication for the lift force generated by the airfoil. The target tension does not have to be constant with time. For instance, the air speed, which is a vector quantity and has a magnitude as well as a directional component, will change over the course of the flight pattern with varying angle between direction of flight of the glider and the wind direction. Also, changes in the wind conditions result in air speed differences.

It is also preferred that reeling in the tether is controlled to retain a target reel speed, which in particular is predetermined as a function of an air speed of the glider. However, the target reel speed can also be predetermined from other observables, for instance a ground speed of the glider.

The underlying problem of the invention is further solved by a glider for a system for electric power production from wind energy according to the invention comprising an airfoil for generating a lift force upon exposure of the airfoil to wind, on-board steering means with control surfaces to aerodynamically exert torque on the glider for pitching, rolling and yawing the glider when airborne, a flight control means for operation of the steering means and a connection means for a tether, wherein the glider comprises an on-board control unit incorporating the flight control means and a master control means for the operation of the system in at least two alternative operation modes.

The object of the invention is also solved by use of a glider for production of electric power from wind energy by means of a system according to the invention and/or by executing a method according to the invention, said glider comprising an airfoil for generating a lift force upon exposure of the airfoil to wind, on-board steering means with control surfaces to aerodynamically exert torque on the glider for pitching, rolling and yawing the glider when airborne, a flight control means for operation of the steering means and a connection means for a tether, wherein said glider in particular is a glider as described above.

Further characteristics of the invention will become apparent from the description of the embodiments according to the invention together with the claims and the included drawings. Embodiments according to the invention can fulfill individual characteristics or a combination of several characteristics.

According to the invention, with respect to a feature in particular or particularly means that the feature is an optional feature.

The invention is described below, without restricting the general intent of the invention, based on exemplary embodiments, wherein reference is made expressly to the drawings with regard to the disclosure of all details according to the invention that are not explained in greater detail in the text. The drawings show in:

Fig. 1

schematically a system according to the invention;

Fig. 2a

schematically the operation of a system according to the invention in a first operation mode;

Fig. 2b

schematically the operation of a system according to the invention in a second operation mode; and

Fig. 3

a block diagram for illustrating the control of a system according to the invention.

In the drawings, the same or similar types of elements or respectively corresponding parts are provided with the same reference numbers in order to prevent the elements from needing to be reintroduced.

Fig. 1 shows an exemplary embodiment of a system for electric power production from wind according to the invention.

The airborne or potentially airborne part of the system comprises a glider 10, which in the embodiment depicted in fig. 1 is designed to be a fixed wing aircraft. The glider 10 comprises a fuselage 12, a main wing 14, a tailplane 16 and control surfaces 20, 22, 24. Also shown are the longitudinal axis 32, the lateral axis 34 and the vertical axis 36, which meet at the centre of gravity 30 of the glider and which constitute the intrinsic coordinate system of the glider.

In the example shown, the fuselage 12 comprises a tube constructed from fiber reinforced composite material as mechanical backbone between the main wing 14 and the tailplane 16 and a nacelle for electronics, power source, sensors and the like, which is mounted in front of the main wing 14.

The main wing 14 can for instance be constructed from a single wing, as in the embodiment depicted in fig. 1. However, alternative designs, for instance with a separate main wing 14 on either side of the fuselage 12 are within the scope of the invention.

In flight, the glider 10 is manoeuvred by control surfaces, which in the exemplary embodiment comprise ailerons 20 at either side of the main wing 12, as well as elevators 22 and a rudder 24 at the tailplane 16. The control surfaces 20, 22, 24 for instance are hinged surfaces used to induce torque around the principle axes 32, 34, 36 of the glider 10 by aerodynamic means.

Torque around the longitudinal axis 32 is induced by means of the ailerons 20, which can be or are operated simultaneously and in opposite directions. Here, opposite directions means that when the left aileron is moved upwards with respect to the main wing 14, the right aileron is moved downwards. By this, lift is enhanced on the right side of the main wing 14 and reduced on the left side of the main wing 14, causing a torque around the longitudinal axis 32. The resulting movement of the glider 10, a rotation around its longitudinal axis 32, is referred to as rolling.

A rotation of the glider 10 around its lateral axis 34, which is referred to as pitching, is achieved by the elevators 22, which are used to increase or decrease the lift at the tailplane, thereby inducing a torque around the lateral axis 34.

Rotation of the glider 10 around its vertical axis 36, which is referred to as yawing, is induced by the rudder 24.

The glider 10 is connected to the ground station 40 via a tether 44, which is attached to or connected with the glider 10 at a connection means which is preferably arranged close to the centre of gravity 30 of the glider 10. This way, varying loads on the tether 44 do not significantly impair the balance of the glider 10 in flight.

At the ground station 40, excess length of the tether 44 is stored on a reel 42, which is connected to a rotating electrical machine 46. The rotation electrical machine 46 is connected to an electricity storage and/or distribution system (not shown) such as a power grid, a transformer station or a large scale energy reservoir. Those skilled in the art will appreciate that the power storage and/or distribution system can be any device or system capable of receiving electricity from and delivering electricity to the rotating electrical machine.

The system shown in fig. 1 is alternately operated in a first operation mode for production of electric power, illustrated in fig. 2a, and a second operation mode for system recovery, illustrated in fig. 2b.

In the first operation mode, which in particular is an energy production operation mode, the glider 10 is steered to follow a high lift flight pattern indicated by line 55 downwind of the ground station 40. The direction of the wind is indicated by arrow 50. During cross-wind flight, in particular fast cross-wind flight, the airfoil or the main wing 14, respectively, of the glider 10 generates a lift force much larger than required to keep the glider 10 at a given altitude. As a consequence, the glider exerts a pull on the tether 44, which is correlated to the excess lift force.

The pull on the tether 44 is used for reeling out the tether 44 from the reel 42 in direction of arrow R, thereby inducing a rotation of the reel 42. The resulting torque, which in particular depends on the diameter of the reel 42 and the force with which the tether 44 is pulled, is transmitted to the rotating electrical machine 46, where the mechanical energy is transformed to the electric power. Optionally, a gear box is arranged between the reel 42 and the rotating electrical machine 46, which is not shown in the figures for reasons of simplicity.

By controlling and influencing the rotational speed of the reel 42, for instance by an adjustable counter torque of the rotating electrical machine, the load on the tether 44 and on the glider 10, respectively, can be influenced.

As long as the tether 44 is reeled out, the glider 10 flies away from the ground station 40. Keeping the system in the first operation mode thus is limited by the overall length of the tether 44.

According to the invention, there is thus provided for a second operation mode for system recovery, in particular for recovery of the tether. This second operation mode is illustrated in fig. 2b.

For recovery of the tether 44, i.e. for reeling in the tether 44 onto the reel 42, the rotating electrical machine 46 is operated as a motor rather than as a generator. The necessary power for instance is provided or delivered by the electricity storage and/or distribution system.

In order to minimize the power consumption during system recovery, the pull on the tether 44 is reduced by steering the glider 10 to follow a low lift flight pattern indicated by dashed line 54. This low lift flight pattern 54 for instance is a descent or a fast dive of the glider 10 against the wind 50 towards the ground station 40. The low lift flight pattern 54 can also be an approach of the glider 10 towards the ground station 40 without loss in altitude, including a slight gain in altitude.

As the glider 10 approaches the ground station 40, the free length of the tether 44 is shortened and the tether 44 is reeled in onto the reel 42 as indicated by arrow R'.

It is preferred that the pull on the tether 44 is as low as possible in order to minimize power consumption for reeling in the tether 44 and as fast as possible in order to minimize the dead time, i.e. the period of time where the system does not produce electric power. These goals are preferably met by controlling reeling in the tether 44 to retain a target reel speed, which in particular depends on the velocity of the glider 10 approaching the ground station 40, which for instance can be derived from an air speed of the glider 10.

Fig. 3 shows a block diagram of an exemplary control scheme for the system described above.

The control scheme provides for a modular design with a flight control means 64, a ground station control means 66 and a master control means 62.

The flight control means 64 is designed to control and/or regulate the devices and actuators associated to flight operation of the glider 10. For example, these include but are not limited to ailerons 20, elevators 22 and rudder 24 of the glider 10.

The flight control means 64 for instance comprises algorithms and feedback loops for automated flight operation of the glider 10. It will be appreciated by those skilled in the art that there will be appropriate sensors connected to the flight control means 64 for determining and monitoring the flight conditions, wherein an air speed sensor 18 is shown in fig. 3 as one example.

The ground station control means 66 has the purpose of controlling and/or regulating the components of the ground station 40, in particular the reel 42 and the rotating electrical machine 46. It will be appreciated by those skilled in the art that the conversion of mechanic energy to electric power, i.e. the operation of the rotating electrical machine 46 as a generator, as well as reeling in the tether 44, i.e. the operation of the rotating electrical machine 46 as a motor, is also controlled and/or regulated by the ground station control means 66.

Both the flight control means 64 and the ground station control means 66 in particular provide for different behaviour associated with different operation modes of the system according to the invention. The operation mode of the system itself, in particular a first operation mode for production of electric power and a second operation mode for system recovery, is controlled and/or regulated by a master control means 62. In particular, the master control means 62 preferably comprises for automated decision making and switching between one operation mode and the other.

As indicated by the arrows in fig. 3, the flight control means 64, the ground station control means 66 and the master control means 62 are connected to each other via bi-directional communication channels, wherein in particular the flight control means 64 and the ground station control means 66 communicate status information about the glider 10 or the ground station 40, respectively, to the master control means 62. In reverse, the master control means 62 communicates the current operation mode of the system to both the flight control means 64 and the ground station control means 66, wherein control and/or regulation of the glider 10 and the ground station 40 is performed independently by the flight control means 64 and the ground station control means 66, respectively.

The modular design of the control scheme shown in fig. 3 has the advantage that operational safety of the glider 10 and the ground station 40, which are normally separated by several hundred meters, is guaranteed individually even in the case of signal delays, signal distortions and communication channel failures.

It does not contradict the modular approach when the master control means 62 is arranged in close proximity to either the flight control means 64 or the ground station control means 66. In particular, the master control means 62 and the flight control means 64 are preferably both arranged on board the glider 10 and can in particular be combined into a single control unit.

While the ground station 40 and the ground station control means 66, which preferably are physically located at or close to the ground station 40, are easily accessible for maintenance and testing, the airborne glider 10 is not. It is therefore desirable to have the possibility to intervene in the control, in particular the flight control, of the glider 10. For this purpose, the flight control means 64 allow for external access to the flight control via a remote control unit 68, which is preferably connected to the flight control means 64 via a wireless communication channel.

All named characteristics, including those taken from the drawings alone, and individual characteristics, which are disclosed in combination with other characteristics, are considered alone and in combination as important to the invention. Embodiments according to the invention can be fulfilled through individual characteristics or a combination of several characteristics.

List of References

10

glider

12

fuselage

14

main wing

16

tailplane

18

air speed sensor

20

aileron

22

elevator

24

rudder

30

centre of gravity

32

longitudinal axis

34

lateral axis

36

vertical axis

40

ground station

42

reel

44

tether

46

rotating electrical machine

48

tension sensor

50

wind

52

high lift flight pattern

54

low lift flight pattern

62

master control means

64

flight control means

66

ground station control means

68

remote control unit