Tidal stream-Ocean current

Tidal stream turbines harness the flow of ocean currents in the same way that wind turbines harness the flow of wind. Tidal stream turbines can be mounted directly on the seabed, or floating and moored to the seabed. Technologies are approaching commercialisation, with the testing of full-scale devices in real-sea conditions, led by European companies. The design of tidal stream turbines is approaching design convergence. Converging designs generally comprise two- to three-bladed horizontal-axis turbines. Alternative designs include: vertical axis turbines, which work under the same principles as horizontal axis turbines, except the rotor turns on a vertical axis; oscillating hydrofoils, that have a hydrofoil attached to an oscillating arm, which is lifted by the tidal stream to generate power; enclosed tips, or Venturi Effect devices increase the velocity of the tidal stream by funnelling it through a duct; tidal kites, which are tethered to the seabed with a turbine attached below its 'wing', and 'flies' in a figure-of-eight path to exaggerate the speed of the waterflow through the turbine; and Archimedes screws, a helical corkscrew device which draws power from the tidal stream as the water flows up the spiral, turning the turbine.
Tidal stream has reached a Technology Readiness Level (TRL) of between 6 and 8, depending on device type. Devices and their auxiliary technology are expected to reach commercialisation following around ten years (estimated) of further research, development and real-sea experience. The rated power of existing tidal technology ranges between smaller-scale devices of 0.1-0.25 MW, and larger scale of 1 and 2 MW, with scope to increase by 50% or more in coming years. The progress of tidal stream in recent years is demonstrated by the operating hours accumulated, capacity deployed and electricity generated, with companies operating at all scales active in Europe and globally. Since 2010, more than 26.8 MW of tidal stream has been deployed in Europe, and more globally. 11.9 MW of this is currently operating, and 14.9 MW has now been decommissioned.