The tidal energy is resulting from the movement of the Eau created by the Marée S, caused by the combined effect of the forces of gravitation of the the Moon and the Sun. It is used either in the form of potential energy - the rise in the sea level, or in the form of kinetic energy - the currents of tide.

The tidal energy is not new: the first mills with tide were built with the Middle Ages in Brittany.

Principles

See also: Tide

The phenomenon of tide is due to the differential of time of rotation between the Earth (24 hours) and the Moon (28 days) which is thus relatively fixed compared to this one. It follows that the terrestrial sphere turns inside a sea water sphere lengthened in the two directions by lunar attraction. One can use this energy of rotation, which causes (in proportions negligible, although final) to slow down the Earth and to move away the Moon for reasons from conservation of the kinetic Moment from the unit.

Energy known as tidal thus constitutes a recuperation of the kinetic energy of rotation of the Earth.

Corresponding energy can be collected in two forms:

• potential energy (by exploiting the variations of the sea level): it is the technique used in the Power plant tidal of Rancid the

• kinetic energy (by exploiting the currents of tide, which can be collected by turbines, or Hydrolienne S).

The sites adapted to the collecting of the tidal energy are very few; they concentrate in the areas where, of the fact in particular of the hydrodynamic conditions, the amplitude of the tidal wave (lower than the meter far from the coasts) is amplified: they are in particular case in France in Bay of Mount Saint Michel (where the factory of Rancid is), with Canada in Bay of Fundy (where marling exceeds 10 meters, which generates intense currents of tide being able to exceed 5 nodes, that is to say approximately 10 km/h).

The optimal exploitation of the potential energy requires important installations, which notably modify ecological balances in generally fragile zones; it is probable that this way will hardly any more be exploited in the future, and that the factory of Rancid will remain an isolated experiment.

The collecting of the kinetic energy of the currents of tide is currently prospected; to be exploitable, the currents must exceed 3 nodes over notable durations.

Therefore, to summarize, orsque the tide goes up, one preserves a side of the stopping and orsque the tide goes down, one releases the water which serat higher than the opposite side and by fesant that, one activates élices and these same élices produce électricitée.

Potential

The order of magnitude of the energy naturally dissipated annually by the tides is estimated at 22.000 TWh is the equivalent of the combustion of less than 2 Gtep (gigatonnes equivalent Pétrole). This figure is to be compared with consumption energy of humanity, about 10 Gtep.

Only a fraction of this energy being recoverable, the tidal energy will be able to contribute for the future only for one weak share to the satisfaction of the world needs.

Compared to the majority of other natural energies (not exactly renewable, to see low: from which energy comes…), the tidal energy has the advantage of being perfectly prédictible: in a given point, the supplied energy depends only on the relative position of the stars and the Earth; moreover, the propagation of the tidal wave is not instantaneous (for example several hours ago of shift between the passage of this wave in Brest and in Pas-de-Calais): this contributes overall to " étaler" production, and to erase the passages to zero periodicals of the production in a point.

History

The first tidal energy utilization goes up with the Années 1120, with the construction of mills to tides, using the bar of the Adour.

Tidal power plant of the Rancid one

See also: tidal Power plant of Rancid the

The first installation of electrical production using the tidal energy is the tidal power plant of the Rance in France. It was installed on a site which, with tides whose amplitude can reach 13 meters, had already known in the history of many “mills with tide”. Work of the stopping started in 1961, and the power site was definitively completed in 1966.

Since its connection with the network in 1967, the factory of Rancid has 24 “bulbs groups” having each one an alternator of 10 MW, that is to say a total working installed capacity of 240 megawatt. The factory produces 500 to 600 million kWh per annum, that is to say between 2000 and 2500 hours per annum of operation in equivalent full power.

New technologies

Currently, of more decentralized systems are under development and seem very promising. They use either the rise in the sea level (potential energy), or currents of tide.

One can in particular quote projects such as “ Mighty Whale ”, “ AWS ” (project of demonstration of 2 MW to the Portugal), “ LIMPET ”, “ DAVIS ” (Blue Energy), “ Sea Snail ”, etc

An interesting realization is with Hammerfest, a city in the north of the Norway. Hammerfest Strøm is first underwater tidal power plant. This factory resembles a windmill whose blades turn thanks to the flow and to the backward flow of the tides and delivers 300 kilowatts (in comparison, the tidal power plant of Rancid provides 240 megawatts).

A score of factories of this type will be installed in 2004, and will feed approximately 1.000 dwellings. The main difficulty which this type of installation presents (in addition to corrosion) is maintenance, the temperature of water hardly exceeding a few degrees.

The systems currently being studied and using the energy of the tides at sea have a cost comparable with wind energy. They could thus experience a fast development.

Underwater tidal power station

During the summer 2002, the first power station tidal which uses the underwater currents was tested with the the United Kingdom. There exists more than 40 sites in this country rich in coast where such an experiment is possible. In theory, there is enough energy while running of tides to generate more than one quarter of the electricity of the country.

The United Kingdom chose miser on the underwater currents more regular than the currents of tides of surface or the swell. All depends on local topography. The ocean comprises, in fact, of the channels where ascending or downward water masses are tightened in a reduced space. The British decided to check if the use of this energy drawn from the marine currents is exploitable in order to retard throttle to greenhouse effect. For that, they engaged of the colossal expenses to build a prototype of tidal power station which can produce up to 1580 kw of electricity. The machine is installed in the the Shetland.

Two “Hydroplane S” of 15 meters assembled on a base will oscillate with the tide in order to activate a hydraulic engine which will generate electricity. Hydraulic pistons control the angle by which the hydroplanes of the Stingray company must face the current of the tide to obtain a maximum of water. As for a wing of plane, their angle of attack changes to create a phenomenon “of elevator” which pushes the hydroplane upwards and downwards. While moving, the hydroplanes make move an arm which actuates a pump to make assemble oil high pressure through a hydraulic engine which makes turn an electric generator.

The structure makes 35 tons, it is with 20 meters above sea-bed and will function in currents going from 2 to 3 meters a second. Essence is manufactured out of steel, the hydroplane is reinforced by plasticized glass. The Stingray company works only for the tides which move in a single direction. The other companies will try to manufacture hydroplanes able to work on the 4 tides so that they will produce electricity the three quarters of time.

The economic experts estimate that technically they do not doubt any more feasibility of the project. On the other hand, they call into question the cost associated with this kind with production. It is estimated that one will produce electricity between 4,7 and 12 pence per kWh. It is thus more expensive than the wind nuclear energy or .

Origin of the energy of the tides

The water of the oceans, because of the lunar addition of attraction on the side of the the Moon and of its lower value on the side opposed to this one has, out of cut, a pace of ellipse, whose main roads are directed towards the direction the Ground-Moon. The Ground turning of 24 hours (roughly) while the Moon does it only in 28 days (roughly), the rotation movement of the Earth in this fixed water mass produces the phenomenon of the tides.

When one retains this water by a stopping, one slows down the movement of it, and thus at the same time - in an infinitesimal way - the Earth. That has, because of the law of the action and the reaction, an effect on the Moon, which it would be possible to study by a method finite elements, but an easy way to do it more quickly exists: since there is conservation of the moment of inertia and that the Earth slows down, the effect of braking is thus that the Moon moves away (in an infinitesimal way it also compared to its distance).

This deceleration exists in any event: at the end of the 19th century, the year made 365,242196 days and today 365,242190 days (one will notice that the difference relates already to the 9th decimal, whereas the astronomers work more readily with 16. An effect of eight orders of magnitude in lower part is thus noticed by them if it cumulates on more than one ten years, which is the case for the Rancid one).

The tidal power plants do nothing but increase a little braking. They thus use in fine the kinetic energy of rotation of the Earth, materializing in a new way an old dream expressed by Gaston de Pawlowski (of the Institute) and of Alphonse Allais which was to install a wheel toothed on the equator to recover the terrestrial rotation movement .

The possible climatic effects of a fall - even light - number of revolutions of the Earth to date do not seem to be studied. Qualitatively, it is known that a significant deceleration:

• would decrease the speed of the terrestrial thermal cycle (hot and night periods diurnal cold)
• would increase the variations in temperature between the days and the nights, with for consequence an increase in the atmospheric movements (storms…)

One can however observe that it is a question of collecting an energy which is dissipated currently completely by friction in the ocean, primarily in the vicinity of the coasts.

Reference

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