Contents I. Introduction
Wind power and solar thermal energy are currently the economically most viable forms of renewable energy. For the conversion of the kinetic energy of wind into electrical energy, ground-based windmills with horizontal axis are generally employed. The generator, driven by the rotor blades via a gearbox, and the connected power electronics converter for coupling to the grid are housed in a nacelle at the top of a tower. The tower height is dimensioned according to the length of the rotor blades and/or the power of the windmill since according to Betz [1], the maximum power that can be extracted from the wind is given by P_{{\rm T},{\rm i}}={8\over 27}\rho A_{{\rm T}}v_{{\rm W}}^{3} \eqno{\hbox{(1)}} (cf. Appendix A1), where is the area swept by the rotor blades, the density of air, and the wind speed. Windmills of high power hence require high towers and overall a very large fraction of mechanically supporting parts at high cost. For example, even a very small windmill with 100 kW output already involves an overall weight of the tower of 18t, whereby the weight of the nacelle is an additional 4.4t and that of the rotor blades 2.3t (3-blade rotor, , dimensioned for ). This fundamental limitation of conventional wind turbines and the lower ground friction and hence increasing wind speed and constancy with increasing altitude given by v_{{\rm W}}(h)=v_{{\rm W}}^{\ast}({h\over h^{\ast }})^{\alpha_{\rm H}} \eqno{\hbox{(2)}} ( and are a reference height and speed, and is the Hellmann's exponential, depending on the ground surface and vertical temperature gradients) have led to the suggestion of radically new concepts for wind energy exploitation, based on initial considerations by Loyd [2]. The basic idea here consists of implementing only the blades of the windmill in the form of a power kite flying at high speed perpendicular to the wind, thus avoiding the entire mechanical support structure of conventional windmills. The ideas go as far as exploitation of the wind energy in the jetstream at an altitude of 10'000 m with wind speeds of up to 50 m/s (compared to typically 10m/s near the ground) and/or a 125-fold higher power density (W/m2) according to (1) compared for example to . However, also an increase in by only 25% already results in a doubling of the power density (cf. Fig. 1). Now the technical challenge of this fascinating concept consists in transmitting the wind power to the ground. For this purpose two possible methods are discussed: on the one hand, the power absorbed by the power kite could be converted via a tether into torque on a tether drum situated on the ground, which drives a generator. To suppress twisting in the tether, the power kite is flown in a figure-of-eight trajectory and the tether is unrolled by the pull of the kite; in a recovery phase the kite is subsequently turned out of the wind, lowering the force acting upon it, and the tether rolled up again. The cycle is then periodically repeated. The versions of this concept, generally known as a pumping power kite, range from direct conversion (SwissKitePower, [3]) to carrousel-like structures with several kites (KiteGen, Univ. of Torino, [4], [5]) and Laddermill structures [6], whereby a significant challenge is caused not only by the construction, but also by the optimal flight control of the kite to assure maximum power gain [7], [8]. Dependency of the maximum output power of a wind turbine on the area swept by the rotor blades and the wind speed (cf. (1) and (65), parameter ). Demonstrator of an Airborne Wind Turbine (AWT) system of Joby Energy [9]. Conceptual drawing of aerofoil and turbines of a 100 kW AWT system of Joby Energy [9]; length / width: 11m/1m. Comparison of the physical size of a conventional ground-based 100 kW wind turbine and an AWT of equal power output. For the calculation of the given numerical values please see Appendix A5. AWT basic electric system structure.
