| Tidal-power is the power achieved by
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| | caissons, embankments, sluices, turbines
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| capturing the energy contained in moving
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| | and ship locks. Sluices, turbines and
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| water mass due to tides. Two types of
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| | ship locks are housed in caisson (very
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| tidal energy can be extracted: kinetic
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| | large concrete blocks). Embankments seal
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| energy of currents between ebbing and
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| | a basin where it is not sealed by
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| surging tides and potential energy from
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| | caissons.
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| the difference in height (or head)
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| | The sluice gates applicable to tidal
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| between high and low tides. The former
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| | power are the flap gate, vertical rising
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| method - generating energy from tidal
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| | gate, radial gate and rising sector.
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| currents - is considered much more
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| | Modes of operation
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| feasible today than building ocean-based
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| | Ebb generation
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| dams or barrages, and many coastal sites
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| | The basin is filled through the sluices
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| worldwide are being examined for their
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| | and freewheeling turbines until high
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| suitability to produce tidal (current)
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| | tide. Then the sluice gates and turbine
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| energy.
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| | gates are closed. They are kept closed
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| Tidal power is classified as a renewable
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| | until the sea level falls to create
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| energy source, because tides are caused
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| | sufficient head across the barrage and
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| by the orbital mechanics of the solar
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| | the turbines generate until the head is
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| system and are considered inexhaustible
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| | again low. Then the sluices are opened,
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| within a human timeframe. The root source
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| | turbines disconnected and the basin is
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| of the energy comes from the slow
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| | filled again. The cycle repeats itself.
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| deceleration of the Earth's rotation. The
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| | Ebb generation (also known as outflow
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| Moon gains energy from this interaction
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| | generation) takes its name because
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| and is slowly receding from the Earth.
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| | generation occurs as the tide ebbs.
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| Tidal power has great potential for
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| | Flood generation
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| future power and electricity generation
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| | The basin is filled through the sluices
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| because of the total amount of energy
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| | and turbines generate at tide flood. This
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| contained in this rotation. Tidal power
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| | is generally much less efficient than ebb
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| is reliably predictable (unlike wind
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| | generation, because the volume contained
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| energy and solar power). In Europe, Tide
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| | in the upper half of the basin (which is
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| Mills have been used for nearly 1,000
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| | where ebb generation operates) is greater
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| years, mainly for grinding grains.
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| | than the volume of the lower half (the
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| The efficiency of tidal power generation
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| | domain of flood generation). This is
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| in ocean dams largely depends on the
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| | compounded by the fact that there is
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| amplitude of the tidal swell, which can
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| | usually a river flowing into the basin,
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| be up to 10 m (33 ft) where the periodic
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| | filling the basin as the tide rises and
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| tidal waves funnel into rivers and
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| | making the difference in levels between
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| fjords. Amplitudes of up to 17 m (56 ft)
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| | the basin side and the sea side of the
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| occur for example in the Bay of Fundy,
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| | barrage (and therefore the available
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| where tidal resonance amplifies the tidal
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| | potential energy) less than it would
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| waves.
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| | otherwise be. This is not a problem with
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| As with wind power, selection of location
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| | the lagoon model: the reason being that
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| is critical for a tidal power generator.
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| | there is no current from a river to slow
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| The potential energy contained in a
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| | the flooding current from the sea.
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| volume of water is
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| | Pumping
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| E = xMg
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| | Turbines can be powered in reverse by
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| where x is the height of the tide, M is
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| | excess energy in the grid to increase the
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| the mass of water and g is the
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| | water level in the basin at high tide
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| acceleration due to gravity. Therefore, a
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| | (for ebb generation and two-way
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| tidal energy generator must be placed in
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| | generation). This energy is returned
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| a location with very high-amplitude
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| | during generation.
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| tides. Suitable locations are found in
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| | Two-basin schemes
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| the former USSR, USA, Canada, Australia,
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| | With two basins, one is filled at high
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| Korea, the UK and other countries (see
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| | tide and the other is emptied at low
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| below).
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| | tide. Turbines are placed between the
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| Several smaller tidal power plants have
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| | basins. Two-basin schemes offer
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| recently started generating electricity
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| | advantages over normal schemes in that
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| in Norway. They all exploit the strong
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| | generation time can be adjusted with high
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| periodic tidal currents in narrow fjords
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| | flexibility and it is also possible to
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| using sub-surface water turbines.
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| | generate almost continuously. In normal
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| One method of extracting tidal energy
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| | estuarine situations, however, two-basin
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| involves building a barrage and creating
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| | schemes are very expensive to construct
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| a tidal lagoon. The barrage traps a water
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| | due to the cost of the extra length of
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| level inside a basin. Head is created
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| | barrage. There are some favourable
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| when the water level outside of the basin
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| | geographies, however, which are well
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| or lagoon changes relative to the water
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| | suited to this type of scheme.
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| level inside. The head is used to drive
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| | Tidal "wind farms"
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| turbines. In any design this leads to a
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| | A new scheme plans to use turbines
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| decrease of tidal range inside the basin
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| | similar to those found in wind farms to
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| or lagoon, implying a reduced transfer of
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| | generate electricity via large current
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| water between the basin and the sea. This
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| | areas such as Cook Strait in New Zealand.
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| reduced transfer of water accounts for
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| | There are two operational devices known
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| the energy produced by the scheme. The
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| | worldwide, one developed by Hammerfest
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| largest such installation has been
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| | Strom in Norway, the other by Marine
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| working on the Rance river (France) since
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| | Current Turbines in the Severn Estuary,
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| 1967 with an installed (peak) power of
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| | UK. Other device developers include
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| 240 MW, and an annual production of 600
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| | Swanturbines, Lunar Energy and Open
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| million kWh (about 68 MW average power).
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| | Hydro.
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| The basic elements of a barrage are
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