Dynamic Simulation Techniques for Airborne Wind Energy Systems

Airborne wind energy (AWE) systems harness wind power using devices that fly in controlled patterns. Two main concepts exist, where the kite is performing crosswind maneuvers. In the ’drag mode’ concept, power is generated onboard, whereas power is generated on the ground in the ’pumping cycle concept’. Quasi-steady state modeling (QSM) is a robust technique for simulating the behavior of a soft kite in a pumping cycle AWE system due to its ease of use – requiring minimal tuning of modeling coefficients – and its ability to rapidly deliver results with reasonable accuracy. QSM effectively predicts parameters like tether force and kite velocity for smaller AWE systems. However, its limitations become apparent with increasing kite mass, where the neglect of inertial forces leads to notable inaccuracies in tether tension and unresolved phase differences between acceleration and velocity. The threshold kite mass-to-surface ratio at which these errors become significant remains undefined, as well as a consistent definition of quasi-steadiness in the context of AWE. This research aims to present such definition and to quantify the validity limits of assuming quasi steadiness. Dynamic equations of motion tailored to AWE applications are derived, and various parameterization techniques to simulate dynamic motion along a predetermined flight path are presented. The solution space of three modeling approaches – steady, quasi-steady, and dynamic – are compared to assess the significance of different components of inertial forces, such as those caused by Coriolis, centrifugal, Euler, or relative accelerations. The study shows that if only a time-averaged quantity is of interest (e.g. average traction power), the quasi-steady model is reasonably accurate, provided a sufficient distance to the feasibility boundaries of the solution space. However, the quasi-steady model fails to accurately predict other aspects of the solution, such as amplitude or phase. The limitations of this analysis stem from the use of a steady aerodynamic model with constant lift and drag coefficients, and the assumption of a rigid, inertia-free tether. Consequently, the presented dynamic model still leaves various unsteady real-world effects unresolved, introducing potential sources of error. This research seeks to serve as a foundational step for future research, enabling more in-depth investigation of the many unsteady phenomena present in AWE systems.