This paper presents a computational study of an underwater kite operating in environments requiring tethers exceeding a kilometer in length, referred to herein as an ultra-long 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 to develop meaningful insights as to the operation of systems that require ultra-long tethers in order to reach viable flow resources. An initial study 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. This performance is further shown to be comparable to the multi-airborne wind energy system (MAWES) proposed by Leuthold et al. [1, 2], 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.

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