[Headline] We have been developing a kite for the generation of electric, mechanical, or thermal energy since December 2019. 

## Summary / Abstract/ Idea
Wind pushes a high flying, autonomous kite therby extending a dynamical anchor line connected to a ground-base generator, ultimately converting mechanical energy. The long anchor line allows to harvest stronger and more steady winds at much higher altitudes than land-based wind generators generally can tap into. Combined with a much lower construction cost due to less material required, our system is predicted to have a lower overall life-time energy generation costs than current ground-based wind-energy generators.

Our current prototype (September 2020) works as follows:

## Hardware

The system consists of a kite, a ground station, and a line connecting both.

The purpose of the ground station is to convert mechanical energy into electrical energy (or rather any respective output energy type desired) and to keep the line tight for controlled kite operations. The ground stations contains a winch (a generator/motor connected to a spool), electronics to operate the generator as a motor (an electronic speed controller), power electronics to convert the output of the generator to the desired voltage (e.g. rectifier + dc-dc-converter), sensors for determining the force on the line and the speed of the line, and a processor for switching between the two modes of power generation and line tightening (reeling?).

The sole task of the kite is to pull the line as strongly as possible for as long as possible (equations: see below). An on-board processor and sensors enable it to operate full autonomously throughout the three major phases of starting/initial ascent, energy generation by pumping motions, and landing/final descent. A propeller provides thrust when necessary, e.g. for starting and landing. Our prototype is equipped with a single, front-mounted electrical motor allowing for an additional mode of flight: a drone-like hovering motion with the nose pointing up. The kite is operated via three controls: one propeller for thrust generation when necessary, an elevator, and a rudder. Rather than using ailerons to control the roll of the aircraft we have chosen an aerodynamically roll-stable design both with respect to the gravitational force and with respect to the force exerted by the line. This spares us from using a GPS-like sensor and dedicated wind sensors or cameras, as the windflow stabilizes the roll of the kite and pushes the kite such that the line points away from wind. From this wind induced orientation we can deduce the wind direction among other parameters.


## Principal flight modes

The kite has three basic modes of flight:
1. Aircraft-like gliding, i.e. flying like a powered aircraft or like a glider. Used for: idle mode, height changes
2. Kite-like pulling, i.e flying like a kite pushed by wind. Used for: energy production
3. Drone-like hovering, i.e. hovering nose-up. Used for: starting/landing, height changes

(?) In the first mode, during gliding, the force exerted by the line is relatively small in comparison to the gravitational force.
(?) In the second mode, when the kite is pulling on and extending the line, it is the other way around.
Technical notes on principal flight modes
In both the first and second mode, efficiency of the aircraft is increased by variation of tail-length and by dynamical longitudinal placement of the line-to-kite attachement (the center of gravity). This is made possible by a D-controller on the elevator, also resulting in keeping the kite compact(?) and the control surfaces well in the airflow from the propeller. [Hinweis: dies ist mein Versuch deinen urpspruenglichen Satz in zwei kurzere Saetze zu teilen]

In kite mode we use a six-axis inertial measurement unit (gyroscope + accelerometer) and a height sensor (barometer) to determine the relative direction of the gravitational force and the distance to the ground. On this basis alone the processor controls the kite.

The hover mode uses PD-controllers for elevator and rudder to keep the kite level (nose up). The roll is stabilised by the wind or left varying in a calm. The height is controlled using a PD-controller based on the barometer.

[Notiz] Sollte nicht noch irgendwo ein Satz stehen, der sagt, dass der Kite zwar alle drei Flight-Modi kann, er aber aerodynamisch und sonstwei auf "kite-like pulling" optimiert ist?

## Flight modes by weather conditions

Via its on-board-procession the kite is programmed for the following flight operations dependent on the current and the predicted wind/weather situation as classified in five conditions:

1. Lowest wind speeds not even supporting gliding
2. Low wind speeds supporting gliding but not energy production
3. Medium to high wind speeds supporting energy production
4. Higher wind speeds (or unfavourable weather) unsuitable for energy production but still safe for gliding 
5. Highest wind speeds (or un-yielding weather) unsuitable for any airborn mode

## Detailled flight mode operations

At lowest wind speeds (predicted or observed for all reachable heights), the kite operates in glider mode aka idle mode. Here, only being loosely attached to the ground station with the nose pointing up the incoming wind makes it always escape upwards (gliding).

With wind speed picking up, and as soon as there is enough wind at any reachable height or flight level for energy production (determined by weather forecast data or by dedicated sensory equipment), the kite enters hover mode and ascends up to a height that has enough wind. During hover mode the neutral position of the elevator is set such that the line is held tight even in a calm, i.e. about 10 degrees upward (in local coordinates) pitch. Note: it might be more energy efficient to cover some of this difference in height by flying a spiral in powered aircraft mode, or - in the less likely event of predicted wind at low height only - the target height will be reached via gliding mode only. Once the desired height has been reached, the existence of sufficient wind can be determined by reading the P-term (or derived directly from the pitch angle) of the elecator (sp?). Having no I-term on the elevator and a line attachment in front of the center of pressure in hover mode pitches the kite into the wind.
Given sufficient wind for energy production the propeller is turned off, transitioning from hover mode to energy production mode. The kite is now controlled by the rudder to fly figure eights or circles in order to pull on the line.

The length of the line can be determined using the height of the kite and the orientation while flying sideways (because roll is controlled aerodynamically and by the direction of the line force).
The angle of attack of the kite can be controlled by varying the neutral position of the elevator. In strong winds it might be necessary to reduce the angle of attack. The strength of the wind can be deduced from the speed of the kite which in turn can be measured as the derivative of the height while flying in vertical direction.

Furthermore when the entire line is rolled out or the kite decides to descend, the angle of attack is reduced sufficiently to allow a descent in glide mode towards the ground station.

Pulling the line in kite mode and descending in glide mode is done periodically without landing in between the phases and with the propeller turned off all along.
Just before the weather ceases to support flying or landing the kite safely or before the battery runs out, the kite descends to the ground station, hovering the last meters.

## Safety

The kite is designed such that sensors, processor, elevator and rudder never fail, e.g. through redundancy. That way, even in case of failure of a component, a controlled landing in glider mode is always possible. In many cases even safer landing via hover mode will be possible still. Larger kite-systems in the future operating inhabitated areas will dispose of an independent emergency parachute for the kite.

[Notiz:] Langfristig wuerde ich minimal noch einen lauten Alarmpiepton einbauen fuer den Fall eines Absturzes/unkonrollierten Landens; dies kann helfen, Leute intuitiv aus in der Crash-/Landezone zu vertreiben. Bei groesseren Modellen, die nicht ausschließlich in voellig unbewohntem Gebiet arbeiten, wuerde ich die Idee eines zusaetzlich unabhaengig ausloesenden Notfallfallschirms umsetzen.

## Equations

P	Power generated
F	Force on the line 
v_l	speed of the line
v_k speed of the kite
v_w wind speed
c_l coefficient of lift (ca. 1.2)
ld	lift to drag ratio
d	density of air
A	wing area

The wind speed is split into the speed of the line and the relative wind speed as seen by the kite. There is an optimum of this split for maximum power generation. The kite flies ld times faster than it "decends" relative to the wind.
v_k = ld * (v_w - v_l)

The lift force is linear in the wing area and quadratic in the kite speed
F = v_k^2 * A * d * c_l * 0.5

Power is force times speed
P = F * v_l

(c_l can be decreased by using elevator)
(v_l or F can be bound by the ground station)

## Ecological considerations: Availability of raw materials

Copper:
- global production per year: 
- necessary for world power supply by kites (... TW): 
- necessary for meaningful negative emissions (CO2 -> C): 

Nickel:

## Economics

Costs for a 2 meter wingspan kite:
(price in Euros)

35	Motor
35	ESC (electronic speed controller)
25	LiPo battery
6	2 servo motors
6	ESP32 (processor)
2	MPU6050 (6-axis motion sensor)
2	BMP280	(barometer)

?	plane (including hull and wings)

150	generator motor (6 kW) (motor in BMW i3 3x cheaper per kW, motor in Tesla?)
?	rectifier
?	DC-DC converter
6	ESP32

Total cost of electronics for a 2 meter kite: 111 Euros
Cost of plane: 100 - 1000 Euros? (cost of raw material extremely low and structure relatively simple)
Total cost of 5kW winch: ~400 Euros? (currently rapid progress in power electronics)

Estimated cost of a 5kW system in 2022 produced in large volume: 500 - 1000 Euro.

[Neu: eine Uebersetzung in Zahlen mit denen ein durschnittlicher Buerger direkt etwas anfangen kann:] Resulting yearly electricity production in regions with median yearly wind speed of 8 m/s at 200 m height is ??? kWh or ?? % of the average national household electricity needs. Depending on operational costs, we aim at an average life-time electricity generation cost for our 5 kW system of ??? ct /kWh. Future larger systems will reduce this price to ?? ct/kWh.

## Online Ressources

 - global wind atlas: https://globalwindatlas.info/
 
 - wikipedia on Airborn Wind Energy: en.wikipedia.org/wiki/Crosswind_kite_power
  
 - magazine: airborne wind energy
	-- https://airbornewindeurope.org/
	-- https://www.windkraft-journal.de/
	-- Blog: https://forum.awesystems.info/
  
 - Other airborne wind energy projects, mostly commercial:
	--KiteMill: www.kps.energy
	--EnerKite: enerkite.de
	--TwingTec: twingtec.ch/de/product/
	--KitePower: https://kitepower.nl/
 
 - research:
	-- Fraunhofer Studie zur vorhergesagten Entwicklung der Windenergiekosten in Europa ueber 2030 hinweg: "LEVELIZED COST OF ELECTRICITY  RENEWABLE ENERGY TECHNOLOGIES"(2018) https://www.ise.fraunhofer.de/content/dam/ise/en/documents/publications/studies/EN2018_Fraunhofer-ISE_LCOE_Renewable_Energy_Technologies.pdf
	--"Airborne Wind Energy Resource Analysis" https://kitepower.nl/resources/1808.07718.pdf
	--"Airborne Wind EnergyTechnology Review and Feasibility in Germany" (2017)https://kitepower.nl/resources/Airborne_Wind_Energy_-_Technology_Review_And_Feasibility_In_Germany.pdf 
	 -- "Cost of Wind Energy Review" (2018): www.nrel.gov/docs/fy20osti/74598.pdf
	(reports estimated target LCOE for 2030: Land-based LCOE (2015$/MWh) 23$/MWh)
 - Blogs, Forums:
	Analysis by KiteKraft of the Failure of Makani:
	https://medium.com/kitekraft/taking-over-the-baton-from-makani-23318d88b7b0