Curran A. Crawford Associate Professor University of Vicoria Sustainable Systems Design Lab Institute for Integrated Energy Systems Department of Mechanical Engineering
PO Box 1700 STN CSC Victoria, BC Canada V8W 2Y2
curranc@uvic.ca www.ssdl.uvic.ca
To Fly a Kite, Glider or Prop Wing?
Aakash Rao, Curran A. Crawford
Department of Mechanical Engineering, University of Victoria Airborne wind energy is still in its infancy, as evidenced
by the variety of technology concepts being pursued by companies and research groups. Many considerations apply when choosing a concept to adopt, including not just operational energy harvesting efficiency and costs, but also take-off/landing operations, noise generation, design elements affecting social acceptance and power output characteristics affecting grid integration. Some concepts place conventional rotors on aerostats (or use the aerostat itself as a rotor) which fundamentally limits power capture as they cannot take advantage of cross-wind operation. Autogiro type concepts use a rotor for both lift and energy capture, so that the rotor must op-erate at significant yaw angles again reducing efficiency. The remaining concepts can be grouped into either ro-tor wing or pumping-mode subsets and are the subject of this paper. Rotor wing concepts fly at a constant tether length and use rotors as turbines to extract energy in the generation mode and as propellers for takeoff/landing. Pumping-mode devices use kites or rigid gliders to pull out a tether from a ground-based generator in generation mode, and then use the same electrical machine to reel in the tether during a shorter duration retraction phase. A dynamic model of these three classes of concepts has been implemented using a Lagrangian dynamics formu-lation to capture kite and tether inertial effects. Kite at-titude dynamics are not included; rather the angle-of-attack and roll angles are used as system control vari-ables, with tether azimuth and zenith angles describing wing/kite position. Aerodynamics models for all three
based on 3D wing theory force the system. The kite is as-sumed to be rigid, while the rotor forces and power out-put on the rotor wing are comout-puted using look-up tables from blade element momentum theory pre-computed solutions indexed with tip speed ratio λ. A logarithmic boundary layer profile is used to define ambient wind speeds. Comparing results to previous studies shows the model to have reasonable accuracy for the intended sys-tem study.
The dynamic equations governing the systems are im-plemented in an optimization framework (Matlab pack-age GPOPS) which allows for simultaneous optimization of the flight paths, rotor control variables (flight time of cycle, angle-of-attack rate, roll rate, tether payout accel-eration for pumping-mode, λ for rotor wing) and system sizing variables (tether diameter, wingspan, rotor diame-ter). Average power output is used as the objective func-tion with system mass computed from required tether di-ameter to sustain the loading. A parametric optimization study is carried out to compare concepts at a nominal rated power of 1 MW. Tether lengths around 1500 m lead to flight altitudes around 500 m in the assumed Class 5 wind speed condition (10 m/s @ 10 m height with 0.1 power law exponent). Multiple initial flight paths are used to demonstrate the presence of local minima. System performance results are used to discuss the relative per-formance of the concepts and inform future optimization work to include more detailed system mass and cost es-timates in a multi-point objective function formulation with multiple flight conditions.
