Patent Application
20210363965
The invention relates to a method for operation of a system for airborne wind energy production, said system comprising a ground station, an airborne glider with an airfoil, and a tether connecting said glider with said ground station, said ground station comprising a rotatable reel for storing excess length of said tether and an electrical rotary machine in effective connection to said reel, wherein said system is operated in a regular operation mode with a repeated operation cycle, said operation cycle comprising a production phase with increasing free length of tether including flying said glider away from said ground station and producing energy by driving said electrical rotary machine via the tether using lift generated by said airfoil of said glider exposed to wind, and said operation cycle further comprising a reel-in phase with decreasing free length of tether including flying said glider towards said ground station.
The invention further relates to a respective system for airborne wind energy production.
With such a system, which for instance is known from EP 2 631 468 A1, electric power usually is produced by steering the glider to follow a high-lift flight pattern during the first operating phase, which results in high load on the tether, which can be used to drive an electrical machine at the ground station. During the second operating phase, the glider usually is steered to follow a low-lift flight pattern with the electrical machine at the ground station reeling in excess length of the tether, thereby consuming much less electricity than generated during the first operating phase.
Likewise traditional wind turbines, systems for airborne wind energy production usually are intended for automated operation, requiring a high level of operational safety. These systems also need to be operable in a large range of wind conditions, with optimized efficiency and with few down-time for economic reasons.
It is an object of the invention to provide for a method for operation of a system for airborne wind energy production ensuring both operational safety and economic viability.
This object is achieved by a method for operation of a system for airborne wind energy production, said system comprising a ground station, an airborne glider with an airfoil, and a tether connecting said glider with said ground station, said ground station comprising a rotatable reel for storing excess length of said tether and an electrical rotary machine in effective connection to said reel, wherein said system is operated in a regular operation mode with a repeated operation cycle, said operation cycle comprising a production phase with increasing free length of tether including flying said glider away from said ground station and producing energy by driving said electrical rotary machine via the tether using lift generated by said airfoil of said glider exposed to wind, and said operation cycle further comprising a reel-in phase with decreasing free length of tether including flying said glider towards said ground station, wherein the method according to the invention is characterized in that wind conditions are monitored and operation of said system is changed to a low wind operation mode when monitored wind conditions drop below a predetermined lower wind condition threshold and/or to a high wind operation mode when monitored wind conditions raise above a predetermined upper wind condition threshold.
Here, the term wind conditions in particular refers to one or more parameters appropriate to characterize a wind condition. These parameters may include but are not limited to wind speed, wind direction, or frequency, duration, and peak wind speeds of gusts.
A glider or sailplane in terms of the invention in particular is a fixed wing, heavier-than-air aircraft, wherein on-board steering means allow for full flight maneuverability 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 center of gravity of the glider.
It is an aspect of the invention to provide distinct operation modes for regular operation, where priorities lie in maximum energy production, and low and/or high wind operations, where priorities lie in risk mitigation to assure safety. Thus, the invention allows for separately optimizing operation during these operation modes, which in particular is beneficial when implementing automated operation routines.
In a preferred embodiment of the invention, said operation cycle of said regular operation mode comprises a first transitional phase between a production phase and the consecutive reel-in phase and/or wherein said operation cycle of said regular operation mode comprises a second transitional phase between a reel-in phase and the consecutive production phase.
Having a first transitional phase enhances operational safety, for instance because termination of the production phase is enabled at any time without being constrained by boundary conditions for starting the reel-in phase. The second transitional phase enables to smoothly transfer flight operation of the glider into optimal conditions for starting the next production phase without being constrained by operation of the reel and/or the electrical rotary machine.
It is further beneficial when operation modes are changed during said first transitional phase and/or said second transitional phase. Most stable system operation is expected when operation modes are changed during said first transitional phase.
Maximum energy yield is expected when during said production phase, flight of said glider is controlled for maximum lift and a tension of said tether is controlled for maximum power output, in particular via torque control by said electrical rotary machine. In particular, the term power output refers to the instantaneous power transferred to electricity or electric energy, respectively, by means of the electrical rotary machine.
In order to avoid system overload or to mitigate other hazards to system structure and/or operation, it is further preferred that power output of said system is reduced by temporarily reducing the efficiency of said system for power production.
Here, efficiency refers to the fraction of energy present in wind, which is actually harvested and converted into electricity by the system.
One way to temporarily reduce system efficiency according to the invention is by retaining tension of said tether above a predetermined tension threshold, wherein said tension threshold in particular is a function of wind conditions and/or of system design parameters and/or of system state parameters. This is for instance possible by adjusting counter-torque of the electrical rotary machine, which in particular is or can be torque-controlled. Increasing tension of the tether at low wind conditions can increases airspeed of the glider at the cost of power output, which in particular is beneficial to ensure above-critical airspeed of the glider.
Another way to temporarily reduce system efficiency according to the invention is by retaining lift of said glider below a predetermined lift threshold, wherein said lift threshold in particular is a function of wind conditions and/or of system design parameters and/or of system state parameters. This is for instance possible by reducing the angle of attack of the glider in flight. If foreseen by glider design, lift can also be reduced by altering the effective aerodynamic profile of the wing, for instance by means of flaps, if available. Retaining lift below threshold enables to avoid critical loads on the glider structure. Also, over-powering the generated is effectively avoidable.
An alternative to decreasing lift is increasing drag of the glider, for instance by means of air brakes, if available.
Yet another way to temporarily reduce system efficiency according to the invention is by increasing an elevation and/or a size of a flight pattern for said glider. This changes the angle of the wind with respect to at least parts of the flight path of the glider, potentially reducing the theoretically maximum amount of energy in the wind accessible for extraction. Often, raising the elevation makes system operation, in particular flight control, more robust against gusts. Another aspect of increased pattern size is reduced turning radii, which makes safe flight operation less demanding.
It is further preferred when said low wind operation mode includes a repeated operation cycle, said operation cycle comprising a first phase with increasing free length of tether including flying said glider away from said ground station, and said operation cycle further comprising a second phase with decreasing free length of tether including flying said glider towards said ground station, wherein said glider is pulled towards said ground station via said tether during at least a part of said second operating phase, thereby increasing velocity of said glider, wherein additional velocity is used to raise altitude of said glider during the following second operating phase.
Thus, the invention enables the glider to stay airborne when wind conditions are insufficient to generate at least the lift necessary to support the glider's own weight. This avoids landing the glider, which is a risky operation requiring complex technical measures and/or manual intervention by a human operator. Another aspect of having the glider airborne is that regular operation can resume as soon as wind conditions are sufficient, avoiding the need to launch the glider beforehand.
Another preferred embodiment of the invention is characterized in that said high wind operation mode includes a repeated operation cycle, said operation cycle comprising a production phase with increasing free length of tether including raising altitude of said glider, thereby producing energy by driving said electrical rotary machine via the tether using lift generated by said airfoil of said glider exposed to wind, and said operation cycle further comprising a reel-in phase with decreasing free length of tether including lowering altitude of said glider, wherein apart from altitude variations said glider remains essentially stationary.
This way, the invention enables energy production even at wind conditions which are prohibitive for the high loads occurring in cross wind flight in the regular operation mode of the system.
For further risk mitigation, said high wind operation mode preferably comprises controlling flight of said glider to hover stationary, in particular at wind conditions above a predetermined critical wind condition threshold, wherein in particular said critical wind condition threshold is higher than said upper wind condition threshold.
Benefits of keeping the glider airborne have been presented already. However, being airborne at highest wind conditions still is potentially hazardous. Thus, wind conditions rare preferably monitored continuously, wherein said glider is landed upon detection or forecast of potentially hazardous conditions.
The object of the invention as discussed in the beginning is also achieved by a system for airborne wind energy production comprising a ground station, an airworthy glider with an airfoil, and a tether for connecting said glider with said ground station, said ground station comprising a rotatable reel for storing excess length of said tether and an electrical rotary machine in effective connection to said reel, said system further comprising a control mechanism for operation of said system, wherein said system is characterized in that said control mechanism is constructed and designed for operation of said system in accordance with one embodiment of the method according to the invention.
The invention is described below, without restricting the general intent of the invention, based on exemplary embodiments with reference to the drawings. The drawings show in:
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.
The airworthy or airborne part of the system comprises a glider 10, which in the embodiment depicted in
The main wing 14 can for instance be constructed from a single wing, as in the embodiment depicted in
In flight, the glider 10 is maneuvered by control surfaces, which in the exemplary embodiment comprise ailerons 20 at either side of the main wing 14, 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 center 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 an electrical rotary machine 46. The electrical rotary machine 46 is for instance 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 46.
Regular operation of the system shown in
In the production phase, 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 used to drive the electrical rotary machine 46 as a generator in order to produce electricity.
As long as the tether 44 is reeled out, the glider 10 flies away from the ground station 40. The production phase thus is limited by the overall length of the tether 44.
During the reel-in phase, i.e. for reeling in the tether 44 onto the reel 42, the electrical rotary machine 46 is operated as a motor, while at the same time the glider 10 is steered along a low lift flight pattern 54 in order to minimize pull on the tether 44.
An alternative illustration of exemplary system operation during the production phase is shown in
Here, the glider 10 flies along a production flight path 51 downwind of the ground station 40. The production flight path 51 resembles an repeated, essentially figure-eight shaped loop. Elevation, which can be expressed as ratio of altitude of the flight path 51 over distance to the ground station 40, is relatively low, allowing for a small angle between the average tether direction and the wind 50.
Dashed line 120 indicates the rated power of the generator at the ground station 40.
The achievable level of power output 111 depends on wind conditions, in particular on wind speed.
Indicated with arrows are characteristic thresholds for wind speeds.
Below lower threshold 131, wind conditions are insufficient for regular flight of the glider 10, even without any power production. In other words, energy available for extraction from wind 50 is not even enough to keep the glider 10 airborne.
For such low wind conditions, the invention provides for a low wind operation mode, which is illustrated in
The holding flight pattern 51′ resembles a figure-eight shaped closed loop. Distributed along the flight path are reel-out phases, where excess length of tether 44 is increased and reel-in phases, where excess lengths of tether 44 is decreased.
According to the invention, a pulling force is exerted on the tether 44 during at least a part of at least one of the reel-in phases, thereby pulling the glider 10 towards the ground station 40. This increases velocity of the glider 10, which can in turn be used for gain in altitude during the next reel-out phase. In other words, the tether 44 is used to increase kinetic energy of the glider 10, which then is transformed to potential energy and helps keeping the glider 10 aloft.
The invention even allows to fly the glider 10 in the absence of wind 50.
Alternatively, the glider 10 can be landed when wind conditions drop below lower threshold 131. The eventual choice should estimate the expected duration of a low wind period and be based on both economic considerations and risk assessment. In general, there will be a trade-off between power consumption and maintenance costs of keeping the glider 10 aloft versus higher risk of landing.
Further shown in
Regular operation slightly differs for different ranges of wind conditions, wherein these ranges are indicated by A, B, C, and D, respectively, in
At wind conditions within range A, the glider 10 is generally controlled to fly for maximum lift, while torque of the generator at the ground station 40 is optimized for maximum energy yield. Within wind condition range A, both tension of the tether 44 and reel-out speed increase with increasing wind speed, resulting in a cubic increase of average power output 110 with increasing wind speed.
At the transition between range A and range B, tension of the tether 44 reaches its design maximum, so that generator torque cannot be increased any further without compromising operational safety of the system.
Therefore, for wind conditions within range B, the generator torque is controlled to maximum tether tension, while flight of the glider 10 is still controlled for maximum lift. Within range B, the reel-out speed increases linearly with increasing wind speed resulting in a linear increase in power output.
The power output 111 shown in
The power output 111C for exemplary wind conditions within range C is shown in
The situation for exemplary wind conditions in range D is depicted in
One approach is to decrease lift and/or increase drag of the glider 10 as described before. However, this will in general result in unnecessarily high loads on the structure of the glider 10, in particular wing and steering surfaces together with the respective hinges and actuators.
In a preferred embodiment of the invention, the elevation of the flight path 51 is increased, which lowers the maximum power output 115 towards optimized power output 116, shown as dotted line. Starting from there, system efficiency is further reduced by decreasing lift or increasing drag of the glider 10, as described before. As a result, the actual power output 111D is constant with time at level of the rated generator power 120.
At particularly gusty wind conditions, it is an option to reduce the targeted power output 111D below the rated generator power 120 in order to increase the safety margin of the system for appropriately reacting to unforeseeable wind gusts without compromising operational or structural safety.
With reference to
At even higher wind speeds above critical threshold 133, power generation is terminated completely and the system is controlled solely with the target to minimize risks. Safest option always is to land the glider 10 and to secure it at the ground. When proper risk assessment permits, it is also possible within the scope of the invention to control the glider 10 to hover stationary with flight being controlled to minimum structural load on the glider 10, the tether 44 and the ground station equipment.
Those skilled in the art will appreciate that the production flight path 51 and the holding flight path 51′ are both exemplary embodiments. Other principle shapes such as circular or oval shapes are also meant to be covered by the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 012 490.3 | Oct 2016 | DE | national |
The present application is a continuation of U.S. application Ser. No. 16/342,549, filed Apr. 17, 2019, which application is U.S. National Stage of PCT/EP2017/025311, filed Oct. 18, 2017, and claims priority to DE 10 2016 012 490.3, filed Oct. 19, 2016.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16342549 | Apr 2019 | US |
| Child | 17397469 | US |
