Airborne wind energy systems convert wind energy into electricity using tethered flying devices, typically flexible kites or aircraft. Replacing the tower and foundation of conventional wind turbines can substantially reduce the material use and, consequently, the cost of energy, while providing access to wind at higher altitudes. Because the flight operation of tethered devices can be adjusted to a varying wind resource, the energy availability increases in comparison to conventional wind turbines. Ultimately, this represents a rich topic for the study of real-time optimal control strategies that must function robustly in a spatiotemporally varying environment. With all of the opportunities that airborne wind energy systems bring, however, there are also a host of challenges, particularly those relating to robustness in extreme operating conditions and launching/landing the system (especially in the absence of wind). Thus, airborne wind energy systems can be viewed as a control system designer’s paradise or nightmare, depending on one’s perspective. This survey article explores insights from the development and experimental deployment of control systems for airborne wind energy platforms over approximately the past two decades, highlighting both the optimal control approaches that have been used to extract the maximal amount of power from tethered systems and the robust modal control approaches that have been used to achieve reliable launch, landing, and extreme wind operation. This survey will detail several of the many prototypes that have been deployed over the last decade and will discuss future directions of airborne wind energy technology as well as its nascent adoption in other domains, such as ocean energy.

In this study we propose a robust vortex model for timedependent vortex shedding at separation locations and trailing edge. The model, which is able to capture flow separation and reattachment phenomena, aims at improving a previously developed a multiple-wake vortex lattice model [1], which could not describe flow reattachment phenomena on suction and pressure surfaces. Starting frompotential theory the two-dimensional Leading Edge Inflatable (LEI) kite airfoil is discretized by several straight panels with point vortices at quarter chord point of each panel. A constant-strength vortex panel is shed at each separation location and is convected in the next time step as vortex blob without change in its strength for further time steps. The circulation is defined as a closed line integral of the tangential velocity component around the fluid element. 􀀀 ≡ ∮C V · ds Considering a closed line integral around the separation panel, as described in Katz [2], applying the above equation, we get d􀀀S dt = D Dt ∮ Vds = d dt (Vids − Vi+1ds) ∼= 1 2 (Vi 2 − Vi+1 2), and 􀀀Ss, 􀀀Sp are separated wake strengths defined using above formulation on suction and pressure sides respectively. U∞ 2D LEI kite airfoil discretized into straight panels with vorticity placed at quarter chord point. Together, the Np bound vortex strengths 􀀀Sp, as well as 􀀀Ss and 􀀀W, give Np + 3 unknowns. The boundary conditions are no flow penetration through the surface (applied at three-quarter chord point on each panel) and the vorticity shed during the time step at separation locations, along with Kelvin-Helmholtz theorem, form Np + 3 boundary conditions. Circulations obtained from iterative solution scheme are post processed using timedependent Bernoulli’s equation for momentary pressure distribution.