Automatic Retraction and Full Cycle Operation for a Class of Airborne Wind Energy Generators
This work provides practical control solutions for the retraction phase, a known bottleneck in airborne wind energy systems, enabling full-cycle autonomous operation.
The paper addresses the retraction phase control problem for ground-based airborne wind energy systems, proposing two control strategies. Experimental results demonstrate complete autonomous power generation cycles, validating the approaches against first-principle models.
Airborne wind energy systems aim to harvest the power of winds blowing at altitudes higher than what conventional wind turbines reach. They employ a tethered flying structure, usually a wing, and exploit the aerodynamic lift to produce electrical power. In the case of ground-based systems, where the traction force on the tether is used to drive a generator on the ground, a two phase power cycle is carried out: one phase to produce power, where the tether is reeled out under high traction force, and a second phase where the tether is recoiled under minimal load. The problem of controlling a tethered wing in this second phase, the retraction phase, is addressed here, by proposing two possible control strategies. Theoretical analyses, numerical simulations, and experimental results are presented to show the performance of the two approaches. Finally, the experimental results of complete autonomous power generation cycles are reported and compared with first-principle models.