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Tidal Energy is produced by the centrifugal force as an effect of the Earth spinning move and of the gravitational attraction of the Moon (as primary source) and of the Sun (as secondary source) (fig.3 a, b, c).
That makes tides to be reliably predicted for years to centuries ahead. As a physical principle tidal power facilities harness the energy from the rise and fall of the tides- as vertical motion, and from the double-sense daily horizontal water currents flow determined by tides (flood currents- moving in the direction of the coast- and ebb currents, the currents receding from the coast).
So, the existing two high and respectively two low tides, are related to the daily Earth rotation and positioning toward the Moon, while the so called Spring and Neap tides are related to a more complex relationship established among the force fields of the Sun, Moon and Earth.
The highest one, the Spring tide, occurs when the Sun and the Moon line up with the Earth – either on the same side or on opposite sides (fig.4); consequently, the lowest tide, the Neap, occurs when the Sun and the Moon are at 90 deg related to the Earth (fig.4).The construction principle of a tidal system is rather simple, as long as either a dam or a section of an estuary are going to run as a reservoir in the action area of the tide.
There are two moments of the process: the flooding one- when the highest level tide floods and fill the reservoir having only one –way access gate; and as soon as the water withdraws at the lowest tide level the existing water from the reservoir is released toward a water wheel.
For harvesting this type of energy, the ideal sites are located on estuary next narrow channels, which experience significant differences between high and low tides.
Using the present achievements of Technology and Materials Science typical energy conversion installations consists of tidal dams and underwater turbines grid structure. The global potential of such energy harvesting systems (only from tides) is about 1800 TWh/year, and it is meant to remain a renewable resource as long as Water, Moon, Sun and Earth will exist.
On almost all significant shore areas there are archaeological clues about the existence of different techniques for the use of tide power, from water mills to irrigations. For the European Middle Ages mills were very familiar on each type of landscape, although these were powered by animal force, wind or water.
In those areas of the Earth where the tides were high enough (fig. 5) tide mills represents an effective alternative to the river water mills. Several installations, raised on the Middle Ages, are still in place in England (Woodbridge Tide Mill) and in France (Rance estuary).
During centuries the technical refinements on tidal energy harvesting have been focused on design and materials. Only the technological achievements of the last decades allowed a systematic study of the tidal phenomenon at global scale, mapping, modelling, simulating and predicting their effects. The tidal systems emerged as a result, using the most adequate solution for the entire tidal power harvest through more efficient installations.
It is obvious that significantly high tides are on very few shore areas of the Earth (fig.5) so, in order to capture tide energy having at least 50% efficiency of the conversion, some other kinematic sources should be envisaged. Flood and ebb currents are such additional sources having the advantage of a predictable and relative stabile flow direction.
Undersea turbines, even less efficient than wind turbines (mainly due to the different densities of the activated fluids -air and water) can put in place more compact grids than windmills farm, thus increasing their effectiveness. There are also other types of devices for the waves and currents harvesting from seas and oceans (fig.2), but the existing demonstrators didn’t reached yet either the energy efficiency or the economical efficiency threshold for large scale use.