As an alternative to conventional wind turbines, this study considered kites with onboard wind turbines driven by a high airspeed due to crosswind flight (“drag power”). The hypothesis of this study was, that if the kite's lift coefficient is maximized, then the power, energy yield, allowed costs and profit margin are also maximized. This hypothesis was confirmed based on a kite power system model extended from Loyd's model. The performance of small-scale and utility-scale kites in monoplane and biplane configurations were examined for increasing lift coefficients. Moreover, several parameters of the utility-scale system were optimized with a genetic algorithm. With an optimal lift coefficient of 4.5, the biplane outperformed the monoplane. A 40 m wing span kite was expected to achieve a rated power of about 4.1 MW with a power density of about 52 kW/m2. A parameter sensitivity analysis of the optimized design was performed. Moreover, to demonstrate the feasibility of very high lift coefficients and the validity of a utilized simplified airfoil polar model, CFDs of a proposed high-lift multi-element airfoil were performed and the airfoil polars were recorded. Finally, a planform design of a biplane kite was proposed.@en

This study considers kites with onboard wind turbines driven by a high airspeed due to crosswind flight (“drag powerž [1, 2]). An optimal power curve and an optimal overall power plant design with requirements for a detailed kite design are derived. For that, the model of [3], which extents Loyd’s model by an airfoil polar model, a 3D wing model, a tether drag model, a wind field model and an economics model, is further extended by a model for the electrical cables of the tether and their sizing, an actuator disk model for the rotors for crosswind flight in turbine and propeller mode aswell as for hovering, and a drivetrain model (efficiencies, masses, costs). A biplane kite with a very high lift multi-element airfoil is considered, as it is found as optimal in [3]. The power curve with all meaningful regions and required actuations (rotor drag coefficient/induction factor, lift coefficient, actuated drag via air brakes or sideslipping) is derived. With a genetic algorithm, all free design parameters are optimized and numerous parameter studies are performed. One result is that a 40mwingspan biplane kite with a wing area of 80m2, a lift coefficient of 4 and a tether length of 370machieves a nominal electrical power of 7 MW, i.e. it has a power density of 90kW/ 2. Moreover, the kite power plant has a maximum allowed cost of 5.5 Mio.USD to achieve a LCOE of 0.05 USD/kWh and the kite has a maximum allowed wing mass density of 140 kg/m2. A biplane kite is expected to be superior to a monoplane kite with respect to its ability to sustain the very high wing loading of 1600 kg/m2 caused by the high lift coefficient. First simple component verifications have been conducted, but further verifications are planned for both, component level and systemlevel. In this talk, the derivation and underlying assumptions of the kite model are presented and key results of the parameter studies are discussed. @en