Another form of ocean energy is called tidal energy. When tides comes into the shore, they can be trapped in reservoirs behind dams. Then when the tide drops, the water behind the dam can be let out just like in a regular hydroelectric power plant. Tidal energy has been used since about the 11th Century, when small dams were built along ocean estuaries and small streams. the tidal water behind these dams was used to turn water wheels to mill grains.
In order for tidal energy to work well, you need relateivel large increases in tides. An increase of at least 16 feet between low tide to high tide is generally needed. There are only a few places where this tide change occurs around the earth. Some power plants are already operating using this idea.
According to the European Union:
"Ninty percent of today's worldwide ocean energy production is represented by a single site: the La Rance Tidal Power Plant (240 MW) that was commissioned in 1966. This type of installation has remained unique in the world and has only been reproduced at much smaller capacities in Canada (20 MW), China (5 MW) and Russia (0.4 MW).
"This type of project was abandoned for many years because of very high initial investment costs as well as the strong local impact that results from it. However, the present economic situation has encouraged South Korea to build a 260 MW dam closing off Sihwa Lake, which is set to be commissioned in 2009. Lighter new techniques, like hydro turbines, are being developed today to harness ocean currents. The leader in this field, the British company, Marine Current Turbine (MCT), should install 1.2MW in Northern Ireland following its 300 kW pilot project in Bristol Bay."
