Tidal energy exploits the rise and fall of tides caused by interaction of the gravitational fields of the Moon and Sun. Tidal movements are both periodic and predictable, and were already used before 1100 ad in tide mills in the United Kingdom and France to grind grain. Their modern version, the tidal power plant, which takes advantage of the difference in water levels between low and high tide, operates on the same principle as an ordinary hydroelectric plant. Behind a dam constructed across a bay or estuary, water flowing through sluices is collected during high tide and later released at low tide through turbines to generate electricity. In order to be exploitable for electricity generation however, differences between high and low tides must be significant, and only a few locations worldwide, generally in river estuaries, are suitable. Today, the only large-sized tidal power station with 240-MW generating capacity is situated at the estuary of the Rance River, near Saint Malo, France. It was built in the 1960s and has now completed more than 30 years of successful operation. Since then, the only other somewhat significant tidal power plants constructed have been a 18-MW project in Annapolis, Bay of Fundy, Canada, a 3.2-MWdevice in China, and a very small 0.4-MWunit near Murmansk, Russia . In the European community, the technically exploitable resource is estimated at 105.4 TWh per year, with only around 50 TWh per year economically viable, and the majority of sites (90%) being located in France and the United Kingdom . Beyond the EU, Canada, the states of the Former Soviet Union, Argentina, Western Australia and Korea have potential sites that have been investigated. Taking advantage of this resource, South Korea has recently announced its plans to construct the world's largest tidal plant (260 MW capacity). Tidal energy per se is a mature technology, but it requires high capital expenditure, long construction times, and has low load factors, leading to long payback periods and high electricity generating costs. Thus, governments are likely to remain the only entities to undertake large-sized projects in this field. One significant factor for the limited use of tidal power raised by ecologists has been the potential environmental impact of such projects. Studies conducted in the United Kingdom, however, concluded that this technology does not necessarily cause major environmental changes; rather, economic considerations are the real barrier to further development of tidal power. Nevertheless, even if all the potentially technically exploitable sites for tidal energy were to be developed, this alternative source of energy would still only represent a very small fraction of our energy needs.
Tides also drive marine currents. Although energy in marine currents is generally diffuse, it is concentrated in some locations near the coast where sea flows are channeled through constrained topographies such as straits and islands. Tidal turbines which resemble wind turbines but are placed under water can be used to generate electricity from these currents. Since the density of water is about 1000 times that of air, the power density of these currents are appreciably higher than that of wind, and therefore much smaller turbines are required. In contrast to wind, marine currents are also highly predictable, and they would also have considerably less environmental impact than the construction of a tidal dam.
Tidal turbines, however, are still only in the research and development phase and very few prototypes are currently in operation in the United Kingdom and Norway . Tidal turbines with a total generating capacity of 200 kW were also recently installed in New York City .
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.