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 in automatic operation during a joint test at the former naval airbase Valkenburg.

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.

Test objective was automatic operation of the systems which was demonstrated successfully. Both systems use periodic pumping cycles to convert the traction power of an inflatable membrane kite into electrical energy. 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).

The “EK30” system of EnerKite uses a ram-air wing which is inflated by the impinging flow penetrating through openings in the leading edge. 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. 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.

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. Steering and de-powering is implemented by an airborne control unit which is suspended below the wing. 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. 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.

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 (invitation to the graduation colloquium).

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.
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