Sensitivity Analysis of Airborne Wind Turbine Design Variables
Using trajectory optimization
T. van Alsenoy (TU Delft - Aerospace Engineering)
G.J.W. van Bussel – Mentor (TU Delft - Wind Energy)
R. Schmehl – Mentor (TU Delft - Wind Energy)
M. Voskuijl – Graduation committee member (TU Delft - Flight Performance and Propulsion)
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Abstract
The objective of this research is to determine the impact of design variables on power output of the Makani Power Wing7 airborne wind turbine. This system consists of a rigid wing connected to the ground using a using a number of bridles and a tether. The tethered wing flies crosswind trajectories and generates power using on-board turbines. Sensitivities of power output to design variables are required for various design trade-offs. A simulation and optimization approach is presented which allows analysis of a fairly complex model with realistic controls and constraints. Using trajectory optimization removes the assumption of a predefined trajectory and allows analysis of the impact of the flightpath itself. The wing is modeled as a rigid body with six degrees of freedom. Aerodynamic forces are determined using a response surface on results of a vortex lattice code. A time-varying position of the bridle attachment point couples the wing to the tether. This tether is implemented as a straight elastic element with analytic approximations for the effects of mass and aerodynamic drag. The power system model consists of a turbine response surface and DC-equivalent generator model. Optimal trajectories are determined using the direct collocation method resulting in a capacity factor of 48.1% for a site with uniform wind field and annual mean wind speed of 7.5 m/s. The relative impact of aerodynamic lift and drag on long term averaged power output are found to be an order magnitude more important than wing mass. Topology and circularity of the flight path are found to have a low impact on power.