Hydrokinetic energy harvesting using tethered undersea kites

Abstract

In this work an emerging hydrokinetic energy technology, Tethered UnderSea Kites (TUSK), is studied. One TUSK concept uses an axial-flow turbine mounted on a rigid underwater kite to extract power from an ocean current or tidal flow. A second concept removes the turbine from the kite, and instead generates power by transmitting hydrodynamic forces on the kite through the flexible underwater tether to a generator on a floating buoy. TUSK systems have potential advantages, mainly the TUSK systems should be able to extract more power from an ocean current or tidal flow than a same-sized fixed marine turbine. This is possible because TUSK kites can move in cross-current motions at velocities significantly higher than the current velocity to increase power output compared to same sized marine turbines. Maximum theoretical power output is estimated for TUSK systems, and detailed comparisons of key performance parameters between TUSK and conventional marine turbines are made. Initial design considerations for TUSK system components are discussed including the underwater kite, buoyancy systems, the floating buoy and mooring system, underwater kite tether, the mounted turbine, and required control systems. Governing equations of motion to study the dynamics of the kite and tether in a TUSK system are developed, and a baseline simulation is studied to estimate kite trajectories, kite pitch, roll and yaw dynamics, power output, kite aerodynamic forces, and tether tensions. The issue of cavitation in TUSK systems at turbine blade tips and on the kite airfoil is studied. Standard cavitation theory is applied to TUSK systems to identify critical cavitation curves as a function of kite operation depth, kite lift-to-drag ratio, and turbine airfoil minimum pressure coefficient.