Optimal Cyclic Control of an Ocean Kite System in a Spatiotemporally Varying Flow Environment

This paper presents a technique for maximizing the power production of a tethered marine energy-harvesting kite performing cross-current figure-eight flight in a 3D spatiotemporally varying flow environment. To generate a net positive power output, the kite employs a cyclic spooling method, where the kite is spooled out while flying in high-tension cross-current figure-eight flight, then spooled in radially towards the base-station under low tension. The present work focuses on two key contributions. First, we present a transition controller for robustly switching between power-generating spool-out operation and low-tension (power-consuming) spool-in operation under variable flow conditions. Second, we present an optimization that selects spool-out speeds and mean elevation angles for the kite during the spool-out phase such that the net power produced per spooling cycle is maximized. Simulation results for a six-degree-of-freedom kite model in a 3D spatiotemporally varying flow environment (which superimposes high-frequency turbulence onto a low-frequency flow field) are shown for a baseline case as well as an optimized case. These simulation results show robust transition, along with a 9.3 percent cycle-averaged power output increase as a result of the optimization.