This is a photo of the Atlantis AK1000, a 130 ton, 74-foot-tall tidal turbine that will be installed underwater off the cost of Scotland. It is designed to supply electrical power for 1,000 households.
Sea water, which is 832 times denser than air, gives a 5 knot ocean current more kinetic energy than a 350 km/h wind; therefore ocean currents have a very high energy density. Hence a smaller device is required to harness tidal current energy than to harness wind energy.
Tidal current energy takes the kinetic energy available in currents and converts it into renewable electricity. As oceans cover over 70% of Earth's surface, ocean energy (including wave power, tidal current power and ocean thermal energy conversion) represents a vast source of energy, estimated at between 2,000 and 4,000 TWh per year, enough energy to continuously light between 2 and 4 billion 11W low-energy light bulbs.
Both the U.S. and the U.K., for example, have enough ocean power potential to meet around 15% of their total power needs.