Optimal crosswind towing and power generation with tethered kites

Nonpowered aerial vehicles such as kites can provide a means of transmitting wind energy from higher altitudes to the ground via tethers. Although there have been many proposals for systems to extract wind energy from higher altitudes, this paper focuses on the use of a light lifting body at the end of a tether to generate useful power. Two major configurations are studied: 1) the kite is used to tow a ground vehicle in the crosswind direction and 2) the kite is flown to generate power using a ground generator. In both cases, the useful work done by the kite is transmitted to the ground through the tether. Both, applications require control of the kite trajectories. A simplified system model is used to study the nature of the optimal trajectories of the system for different wind speeds and system parameters. Numerical results illustrate that optimal power generation is most sensitive to the cycle time, tether length, and wind speed. The power output for power generation, as well as the average towing velocity for the towing case, are roughly proportional to the cube of the wind speed.