Energy Generation from a Variable Tethered Kite Moving in a Horizontal Flightpath

Tethered-wing power systems contain a gliding wing anchored to the ground by a cable and are capable of generating energy from wind power. They are a viable possibility for collecting energy from stronger, more consistent winds found in the upper regions of the atmosphere where conventional wind turbines are incapable of reaching. We examine the dynamics and performance of a novel system where the tether is oriented both upwind and downwind of the ground attachment point during normal operation of the device, instead of entirely downwind as previously studied systems. Some suggest that tethered-wing prototypes built by Makani Power and Ampyx Power have motions analogous to those found on the blade tips of conventional horizontal-axis wind turbines. If this analogy is correct, then the system we examine has motions that are analogous to the motions of the blades on conventional vertical-axis wind turbines. The system we examine has a ground-based generator (i.e. winch with generator) which is mechanically coupled to the aircraft. Energy is generated on the reel-out phase of each cycle while a smaller amount of energy is consumed during the reel-in phase of each cycle.  Research is directed towards determining system viability and the existence of stable, periodic motions. We develop a simple 2D model which captures the dominant dynamics of this system and show, via simulation, that for some parameter sets, the proposed system is capable of stable periodic motions with a simple open loop controller. To determine aerodynamic forces in the model, lift and drag coefficients are used, therefore, we assume the flow is fully developed around the aircraft. Initial results show that the periodic motions are abundant and easily found, but many of the solutions violate the constraint of positive tether tension. Some of these periodic solutions produce net positive power. Solutions with a consistently taut tether are found when the aircraft mass increases, and further study will determine the feasibility of an increased aircraft mass. The small scale system we examine, where parameter optimization was not performed, predicts an average cycle power of more than 500 Watts in a 10 m/s wind. Continued analysis will predict if a physical system is viable, but initial results indicate that stable periodic motions of the more complex system may be possible.