Power smoothing by kite tether force control for megawatt-scale airborne wind energy systems

Airborne wind energy is an emerging technology that uses tethered flying devices to capture stronger and more steady winds at higher altitudes. Compared to smaller systems, megawatt-scale systems are substantially affected by gravity during flight operation, resulting in power fluctuations. MegAWES, a 3 MW reference model, experiences power fluctuations between -5.8 MW and +20.5 MW every 12.5 seconds during the traction phase when using its baseline controller at a wind speed of 22 m/s. The baseline controller does not have a power limit, leading to high peak power, and aims to keep the tether force constant, causing it to consume power when the kite is flying upwards. In this paper, we implement an optimal torque controller in the MegAWES framework and show that this eliminates the power consumption during the traction phase. Furthermore, we propose a kite tether force controller that allows setting a power limit when combined with the 2-phase reeling strategy, which decreases the peak power. Our new architecture reduces the power output range by 75% to between +3.7 MW and +9.4 MW in strong wind conditions.