Tethered Undersea Kites systems (TUSK) is a new hydrokinetic energy technology in which a tethered, rigid-winged hydro-kite is submerged in an ocean or tidal current and controlled to move in high-speed cross-current motions. A turbine is mounted on the hydro-kite in one TUSK concept, or the flexible unwinding tether transmits generated hydrodynamic forces to a power generation system in another concept. The Tethered Undersea Kite (TUSK) was first conceived by Magnus Landberg a researcher in Sweden, in 2007. Underwater kites look to be feasible to build using commercial available technology. According to economic analyses conducted by various research teams TUSK systems may be able to produce electricity at about half the current cost for fixed hydrokinetic turbines, and a bit below the cost of the power produced by offshore wind turbines. Those researchers attribute the lower costs to improved power-to-weight ratios derived from replacing the inner blades and support tower of a traditional turbine with a lightweight, low-cost tether. The main benefits of TUSK is that the hydro-kite can move in high-speed cross-current motions (much like a kite in air) over large swept areas to greatly increase power output. The kites’ ability to move in figure-eight motions–which causes it to zip through the water several time faster than the current itself–will amplify the water’s energy output.

Introduction

The combustion of fossil fuels is currently used to meet the majority of global energy needs. Since fossil fuels contribute significantly to climate change, renewable energy technologies will need to be further developed. It is clear that energy extraction from the world’s ocean and tidal currents using hydrokinetic energy systems will play a major role. For decades, researchers have been pursuing the concept of tapping hydrokinetic energy from ocean or tidal currents to generate electric power. Still, ocean-current (or hydrokinetic) energy conversion remains a largely unfulfilled promise. Virtually all of the existing generation units use fixed turbines mounted on the seafloor. These stationary generation units depend on the currents to spin their blades and crank their generators. The amount of generated power is proportional to the cube of the water velocity flowing through the turbine increase this velocity by 26 percent and the power doubles. The velocity and the generated power can be increased even more dramatically if the turbine, instead of standing still, can be put in motion so it can actively move through the current. In TUSK system a tethered undersea kite is connected by a flexible tether to a support structure on the ocean surface or floor. TUSK systems have potential advantages over conventional marine turbines, mainly that TUSK systems will be able to generate cost-effective energy with; 1) smaller, less costly systems, and 2) at more locations within ocean currents and tidal flows where current speeds are too low to make marine turbines feasible. . A TUSK system with wing area of 30 m2 operating in that current will produce about 300 kW of power. A school of 100,000 such wings operating in the world’s ocean currents would generate 30 GW of power.

Tidal Turbine

Tidal turbines are very much like underwater windmills except the rotors are driven by consistent, fast-moving currents. The submerged rotors harness the power of the marine currents to drive generators, which in turn produce electricity. Water is 832 times denser than air and consequently tidal turbine rotors are much smaller than wind turbine rotors and therefore can be deployed much closer together and still generate equivalent amounts of electricity.

Tidal turbines are fixed to seabed. They are connected to the grid via an armoured power export cable. Marine or tidal currents, unlike many other forms of renewable energy, are a consistent source of kinetic energy caused by regular tidal cycles influenced by the phases of the moon. Tidal turbines are installed on the seabed at locations with high tidal current velocities, or strong continuous ocean currents where they extract energy from the flowing water. Sitting stationary on the seabed, they typically require currents of 2.5m/s or faster to produce electricity cost-effectively.

Offshore Wind Turbine

Offshore wind power or offshore wind energy is the use of wind farms constructed in bodies of water, usually in the ocean on the continental shelf, to harvest wind energy to generate electricity. Higher wind speeds are available offshore compared to on land so offshore wind power’s electricity generation is higher per amount of capacity installed. The cost of offshore wind power is higher than that of onshore wind generation.

Components

The power plant consists of a wing which carries a turbine directly coupled to a generator in a nacelle. Rudder and the servo control system steers the kites in the predetermined trajectory. The struts are via a top joint connected to the tether which is connected to the bottom joint at the sea bed foundation. The tether accommodates the tether rope and cables for communication and power distribution.