On modeling and control of the retraction phase for airborne wind energy systems

The retraction phase of a ground-based airborne wind energy system is the second part of a two phase power generation cycle. In the first phase, energy is produced by exploiting the aerodynamic lift exerted by a wing tethered to the ground and controlled to fly crosswind paths. Power is produced by unreeling the tether, wound around drums connected to generators, under high traction force. In the retraction phase, the tether is reeled-in after its maximum length has been reached. In this paper, a new dynamical model of a tethered wing for the retraction phase is derived from first principles, and a flight controller based on this model, which is straightforward to implement and tune, is proposed. Simulation results comparing this new strategy to an existing approach are presented. Besides a better performance, the main advantage of the new approach is that it uses readily available measurements for feedback control, hence resulting in a more robust and reliable solution.