Drag-Mitigating Dynamic Flight Path Design and Sensitivity Analysis for an Ultra-Long Tether Underwater Kite

Abstract

This paper presents a computational study of an underwater kite operating in environments requiring tethers exceeding a kilometer in length, referred to herein as ultralong tether (ULT) applications. Leveraging a detailed dynamic model of the kite and tether, we study the relationship between path shape and tether drag at varying tether lengths in order to develop meaningful insights as to the operation of systems that require ultralong tethers in order to reach viable flow resources. An initial study of a ULT kite application operating in a uniform flow field is presented, demonstrating that with an appropriately designed flight path, the kite is able to suppress the motion of the majority of the tether in order to achieve an order of magnitude greater power output than can be achieved with a straight tether, which is the typical assumption used in a performance estimation. Tether drag mitigation is characterized through the use of a novel metric termed effective tether length, which characterizes the total length of tether engaged in drag production. It is shown that a high-performance path shape can reduce the effect of tether drag by over 50%. This performance is shown to be comparable to the multi-airborne wind energy system (MAWES) proposed by Leuthold et al. (2017, “Induction in Optimal Control of Multiple-Kite Airborne Wind Energy Systems,” IFAC-PapersOnLine, 50(1), pp. 153–158; 2018, “Operational Regions of a Multi-Kite Awe System,” 2018 European Control Conference (ECC), IEEE, Limassol, Cyprus, June 13–15, pp. 52–57), which suppresses tether motion through mechanical design rather than merely though careful path selection and control. This initial study in a uniform flow field is followed by two sensitivity studies: one that assesses performance in realistic environments where the flow magnitude is a function of altitude above the seabed and a second that assesses the impact of tether drag reduction techniques. It will be shown that by careful selection of path shape, site, and tether design, a single kite in a ULT marine application can achieve performance rivaling that of the MAWES without the extra required mechanical complexity.