kite power systems in automatic operation

Photo 1: Simultaneaous operation of two kite power systems: EnerKite in the foreground, TU Delft in the background and a commercial airliner cruising at very high altitude 

Decreasing the generation costs of renewable energy is the major objective of Airborne Wind Energy. The various technologies essentially use tethered flying devices to extract wind energy from the atmosphere and the low environmental footprint of these systems is attractive for many deployment scenarios. More than forty different teams worldwide are presently involved in the fast-paced development of this innovative technology sector (photo 2).

Photo 2: Selection of institutions active in Airborne Wind Energy in 2012
Photo 3: Converting the traction power of a kite by means of periodic pumping cycles
Photo 4: Instantaneous traction power measured over five consecutive pumping cycles 

On 22 May 2012, the team of EnerKite, a high-tech enterprise from Berlin, and the kite power research group of Delft University of Technology presented their current technology demonstrators during a joint test at the former naval airbase Valkenburg. Test objective was automatic operation of the systems which was demonstrated successfully. Both systems use periodic pumping cycles (photo 3) to convert the traction power of an inflatable membrane kite into electrical energy (photo 4). The major differences are in the type of kite that is used and in the implementation of steering and de-powering (de-powering denotes a controlled pitching of the wing to decrease its angle of attack and thus the traction force and energy consumption during reel-in of the tether).

Photo 5: The 21m2 ram-air wing of the EnerKite system during flight with clearly visible air inlets at the leading edge of the wing
Photo 6: The 21m2 ram-air wing of the EnerKite system with airliner cruising at very high altitude
Photo 7: TU Delft researchers and an engineer of EnerKite observing the operation of the kites next to the EnerKite ground station
Photo 8: Roland Schmehl (TU Delft), Sören Sieberling (Ampyx-Power) and Christian Gebhard (EnerKite),  from left to right 

The “EK30” system of EnerKite uses a ram-air wing which is inflated by the impinging flow penetrating through openings in the leading edge (photo 5 and photo 6). Originally developed for parachute applications, this type of wing has a good aerodynamic performance and can be scaled to larger sizes. Three separate tethers are used to connect the wing to the ground station which incorporates three drum-generator modules mounted on a swivel platform on the back of a truck (photo 7 and photo 8). This ground station combines all required functionality: synchronized reel-out and reel-in of the tethers as well as steering and de-powering of the wing. Angular sensors are used to measure the direction of the central tether which is used as input data for the autopilot system.

Photo 9: The 25m2 LEI tube kite of TU Delft during flight
Photo 10: Remote-controlled steering of the TU Delft kite high above Valkenburg (Photo: Max Dereta)
Photo 11: Remote-controlled steering of the TU Delft kite high above Valkenburg (Photo: Max Dereta)Photo 12: Cross-wind operation of the TU Delft kite observed from the ground station

The 20kW system of TU Delft uses a Leading Edge Inflatable (LEI) tube kite. Very popular among kite boarders, this type of wing has good de-powering characteristics and can be pre-inflated to a well-defined, waterproof shape (photo 9). Steering and de-powering is implemented by an airborne control unit which is suspended below the wing (photo 10 and photo 11). This tele-operated cable robot is connected to the drum-generator module at the ground by a single tether which has the advantage of minimizing aerodynamic losses during cross-wind operation of the kite (photo 12, photo 13 and photo 14). Three redundant wireless links ensure a robust data transfer between kite control unit and ground station, even when operating at high altitude. As components of a research platform, the kite and its control unit are equipped with several sensors, including a Pitot tube for measuring air speed, as well as a Global Positioning System (GPS), an Inertial Measurement Unit (IMU) and a cable force sensor (photo 15).

Photo 13: The 20kW ground station of the TU Delft with the tower of Valkenburg airfield in the background
Photo 14: The drum-generator module of the ground station
Photo 15: Jonathan Ramirez mounting the GPS/IMU to the central strut of the kite assisted by Anna Plätzer and John van den Heuvel
Photo 16: Tracking results of the autopilot during reel-out

For the tracking control of the figure-of-eight flight manoeuvres during reel out, the TU Delft team successfully used the 2-Loop Adaptive Autopilot (2LAP). The control algorithm had just been developed by Claudius Jehle, an Erasmus exchange student from TU Munich, as part of his MSc research (photo 16, announcement graduation pdf). The joint test of EnerKite and TU Delft was special because of the coinciding heavy flight traffic to Amsterdam international airport. Large commercial airliners were overflying Valkenburg on their landing approach while the two energy kites operated simultaneously in the assigned lower airspace up to 350m altitude. This experience essentially confirmed that air traffic and airborne wind energy are compatible and in fact only a question of a suitable regulatory framework.

For more information contact:
Dr.-Ing. Roland Schmehl
Associate Professor
Delft University of Technology
Faculty of Aerospace Engineering / ASSET chair
Kluyverweg 1, 2629 HS Delft, The Netherlands
P: +31 15 278 5318
M: +31 61 495 6025
F: +31 15 278 3661
E: r.schmehl@tudelft.nl W www.kitepower.eu
T: www.twitter.com/kite_power

Broadcast video clip on Kite Power Technology
The short video clip on Kite Power technology recorded during the test on 22 May 2012, will be shown during the Dutch TV show "Watt Nu?" (http://wattnu.nl) on the following dates:

• 10 June @ 21:00 on RTL 7
• 11 June @ 10:20 on RTL Z
• 12 June @ 12:20 on RTL Z
• 13 June @ 14:20 on RTL Z
• 14 June @ 16:20 on RTL Z
• 15 June @ 21:20 on RTL Z
  
  Video
  
https://vimeo.com/44253714
This promo video was produced by TUD startup Frank & Frens [cc] and very nicely shows how our students are integrated in the project.