Retraction phase analysis of a pumping kite wind generator

Airborne wind energy systems have developed very fast in the past five years. One of the most promising systems is the so called pumping kite wind generator, which is based on a cycle of two phases: the traction or generation phase and the retraction or consumption phase. An optimal balance between both phases is crucial in order to obtain an economically viable system. This work is devoted to the investigation of the retraction phase, i.e. the reel-in phase of a pumping kite wind generator, from the theoretical point of view. The most common approaches for the implementation of the retraction phase in the literature are studied from the point of view of the energy as well as time consumption. The first step of this work is the modeling of the dynamic behavior of the system during the tether reel-in process including the aerodynamic coefficients of a ram-air kite and by performing computational simulations. Perfect control is supposed. Hence, assumed that the control system shows its best performance, results of performed simulation experiments confirm that the behavior of the retraction phase is ruled by the system dynamics. The net energy gain of the complete cycle particularly depends on the efficiency of the retraction phase.