Tidal+energy+-+Audrey

=**Tidal Energy**=

//What is Tidal energy?//
Tidal energy is the use of gravitational forces of the sun and moon, as the form as tides, converting them to mainly electrical energy. Tidal energy is eco-friendly, reliable and a predictable form of energy conversion. The concept of harnessing tidal energy is similar to the windmill, but unlike harnessing wind energy, tidal energy is constant and predictable.

[[image:renewable-energy-tidal-2.gif width="395" height="395" caption="The workings of tidal energy"]]
Footnotes: tidal energy is one of the oldest form of energy, evident from tide mills found in UK, Spain and France, even before 1100AD.

//Methods of Harnessing://
There are 2 ways to exploit tidal energy: =//Examples://= The first tidal power station was the [|Rance tidal power plant] built over a period of 6 years from 1960 to 1966 at [|La Rance], France. It has 240 MW installed capacity. The plant produces 0.012% of the power consumed by France, with a peak rating of 240 [|Megawatts] for its 24 [|turbines]. Annual output is about 600 million kWh, or about 68MW average power. The barrage is 750 [|metres] long, from Brebis point in the west to Briantais point in the east. It is located south of [|Dinard] and [|Saint Malo], at the mouth of the river. The power plant portion of the dam is 332.5 metres long. The tidal basin measures 22.5 km².
 * By building semi-permeable barrages across estuaries with a high tidal range
 * By harnessing offshore tidal streams.

Turbines
The plant's turbines utilise the energy of the tides and the river current in turn; and as such are bi-directional.

=//**Energy conversion**//= Tidal energy conversion is as such: gravitational energy --> kinetic energy --> kinetic energy --> electrical energy (from the pull of the (the ebbing of (movement of (formed by the moon to the earth) the wave) the turbine) movement of the turbine)

where: The factor half is due to the fact, that as the basin flows empty through the turbines, the [|hydraulic head] over the dam reduces. The maximum head is only available at the moment of low water, assuming the high water level is still present in the basin.
 * //h// is the vertical [|tidal range],
 * //A// is the horizontal area of the barrage basin,
 * //ρ// is the [|density] of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg per cubic meter) and
 * //g// is the acceleration due to the [|Earth's gravity] = 9.81 meters per second squared.

Tidal power schemes being considered
In the table, "-" indicates missing information, "?" indicates information which has not been decided
 * **Country** || **Place** || **Mean tidal range (m)** || **Area of basin (km²)** || **Maximum capacity (MW)** ||
 * United Kingdom and Channel Islands || [|River Severn] || 7.8 || 450 || [|8640] ||
 * Russia || [|Penzhinskaya Bay] || 6.0 || 20,500 || 87,000 ||

=//**Environmental impacts**//= Studies show that the placement of a barrage could have an impact on the ecosystem and water inside the basin. Many governments have been reluctant in recent times to grant approval for tidal barrages. Through research conducted on tidal plants, it has been found that tidal barrages constructed at the mouths of estuaries pose similar environmental threats as large dams. The construction of large tidal plants alters the flow of saltwater in and out of estuaries, which changes the hydrology and salinity and possibly negatively affects the marine mammals that use the estuaries as their habitat. The La Rance plant, off the Brittany coast of northern France, was the first and largest tidal barrage plant in the world. It is also the only site where a full-scale evaluation of the ecological impact of a tidal power system, operating for 20 years, has been made. French researchers found that the isolation of the estuary during the construction phases of the tidal barrage was detrimental to flora and fauna, however; after ten years, there has been a “variable degree of biological adjustment to the new environmental conditions” Some species lost their habitat due to La Rance’s construction, but other species colonized the abandoned space, which caused a shift in diversity. Also as a result of the construction, sandbanks disappeared, the beach of St. Servan was badly damaged and high-speed currents have developed near sluices, which are water channels controlled by gates

=//**Advantages and Disadvantages**//=

Advantage

 * Once you've built it, tidal power is free.
 * It produces no greenhouse gases or other waste.
 * It needs no fuel.
 * It produces electricity reliably.
 * Not expensive to maintain.
 * Tides are totally predictable.
 * Offshore turbines and vertical-axis turbines are not ruinously expensive to build and do not have a large environmental impact.

Disadvantage

 * A barrage across an estuary is very expensive to build, and affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. There are few suitable sites for tidal barrages.
 * Only provides power for around 10 hours each day, when the tide is actually moving in or out.