Mark Aull Vice President of Aerospace
Windlift LLC 10410 Globe Rd suite 116 Morrisville, NC 27560 USA mark@windlift.com www.windlift.com
Airborne Wind Energy System Optimizer (AWESOpt)
for Fly-gen Analysis and Optimization
Mark Aull, Andy Stough Windlift LLC
A performance analysis tool fast enough to be feasible for system design optimization is important for fly-gen AWE technology to mature. Design trade-offs are difficult to analyze with traditional tools; using simulations to iterate through parameters for design optimization is undesir-able. The analysis tool developed is better suited for this task. It analyzes AWE performance as an ‘inverse prob-lem’ to controls-focused models: the path, velocity, and lift of the aircraft are input as Fourier series coefficients, guaranteeing a closed, steady state cycle (rather than iter-ating to converge to a steady state solution). The gravita-tional & inertial forces for the trajectory and aerodynamic forces for the aircraft & tether are calculated, the roll an-gle to balance lateral force is determined, then tether ten-sion and rotor force are calculated to balance the lateral forces, allowing power to be calculated. No controller de-sign or tuning is required for each set of system parame-ters, and there are no deviations from the proscribed tra-jectory or instabilities due to a controller.
Constraints can be applied to tension, rotor force, roll an-gle, roll rate, and wingtip angle of attack, etc. to ensure that the trajectory is realizable. Analyzing one system on one trajectory requires no iteration, though ensuring constraints are met may. Comparing different system de-signs is relatively computationally inexpensive, therefore iterating over system designs to minimize a cost function is feasible. Other analyses have optimized flight paths, however fly-gen system performance requires optimiza-tion of turbine drag or flight speed and requires account-ing for mass (another analysis concluded that mass is an
important parameter in ground-gen performance[1] and fly-gen aircraft generally have higher mass than compa-rable ground-gen aircraft).
Power transfer calculated and simulated
A validation against a high fidelity simulation matches well, despite simplifications such as a straight rigid tether, constant lift and drag coefficients, and a simplified rotor model. A design optimization focused on tether parame-ters is presented. The tether plays a central role in system performance. In a simplified model, increasing allow-able tether tension, power transmission capability, and length increase rated power. However, the associated in-crease in tether weight and drag reduce output through-out the power curve. The AWESOpt tool determined op-timal tether length and conductor and strength member sizes in order to maximize system-level capacity factor.
References:
[1] R. van der Vlugt, A. Bley, M. Noom, R. Schmehl: Quasi-Steady Model of a Pumping Kite Power System. Renewable Energy, Vol. 131, pp. 83-99, 2019. doi:10.1016/j.renene.2018.07.023
