Underwater turbines capture the energy of currents caused by changing water levels. The tides themselves result from gravitational pull caused by the sun and moon, so they have a distinctive advantage as an energy source: tides are predictable, permanent forces that are completely independent from external factors, unlike solar and wind energy.

 

Originally, tidal power plants were set on a bay protected by a dam which separates them from the open sea. In this model the dam contains turbines attached to the seabed facing both directions. During high tide the water flows from the ocean through the turbines into the dam, and the kinetic energy of the water flow is turned into electricity. At low tide the tidal range, i.e. the difference between high tide and low tide, is used to allow the water held back by the dam to stream back into the sea, powering turbines in the process.

A new technological approach is to place turbines directly on the seabed to capture powerful currents, instead of using the tidal range. Near coasts these currents are especially powerful, providing ideal locations for tidal stream energy power plants. Another advantage of this technology is that it protects the environment in costal regions and is unobtrusive. Turbines are "invisible" on the seabed and their relatively slow revolution speed has little effect on marine life.

 

Experts estimate that up to 800 TWh each year of energy could be extracted from ocean currents, about the same as the capacity of 70 large nuclear power plants.