Design of Optimal Airfoils for Crosswind Kite Power Systems

This paper introduces a novel framework for designing optimal airfoils for crosswind kites which are tethered flying systems used to harness high-altitude wind energy. The improved geometric parameter method, a state-of-the-art airfoil design approach, is employed here for the first time in the context of airborne wind energy airfoil design. The nondominated sorting genetic algorithm is used as the optimization method, and XFOIL is adopted to obtain aerodynamic lift and drag coefficients of the airfoils. Pareto-optimal fronts and the corresponding optimal airfoil profiles at various maximum thickness ratios are obtained for a baseline system that neglects three-dimensional flow effects and tether drag. For the first time in the literature, the effects of induced drag due to finite aspect ratio kites on the optimal airfoils are examined. Additionally, the effects of including the tether drag on the optimal solutions are explored. It is found that when the induced drag is included optimal airfoils feature a cusped trailing edge. On the other hand, when the tether drag is considered, the optimal airfoils are found in shape to be reminiscent of flapped airfoils, suggesting a multi-element airfoil design. Finally, unlike most studies in the literature, the present work conducts post-optimization Reynolds-averaged Navier–Stokes flow simulations to gain deeper insights into the aerodynamic performance of the optimized airfoils and to provide comparisons with XFOIL results.