Tidal Power

Tidal power (tidal energy), is a form of hydropower that exploits the rise and fall in sea levels due to the tides, or the movement of water caused by the tidal flow.   Because the tidal forces are caused by interaction between the gravity of the Earth, Moon and Sun, tidal power is essentially inexhaustible and classified as a renewable energy source.   Although not yet widely used, tidal power has great potential for future electricity generation and is more predictable than wind energy and solar power.   In Europe, tide mills have been used for nearly a thousand years, mainly for grinding grains. ^{Wiki n.p.}

Tidal power can be classified into two types, stream and barrages:   Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines similar to underwater wind turbines.   This method is gaining in popularity because of the lower ecological impact compared to the second type of system, the barrage.   Tidal barrages make use of the potential energy from the difference in height (or head) between high and low tides, and their use is better established.   These suffer from the dual problems of very high civil infrastructure costs and environmental issues.   Modern advance in turbine technology may eventually see large amounts of power generated from the oceans using the tidal stream designs.   Arrayed in high velocity areas where natural flows are concentrated such as the west coast of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in south east Asia and Australia.   Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated. ^{Wiki n.p.}

Barrage tidal power involves building a barrage and creating a tidal lagoon.   The barrage traps a water level inside a basin.   Head (a height of water pressure) is created when the water level outside of the basin or lagoon changes relative to the water level inside.   The head is used to drive turbines.   The largest such installation has been working on the Rance river, France, since 1967 with an installed peak power of 240 MW, and an annual production of 600 GWh (about 68 MW average power)   The basic elements of a barrage are caissons, embankments, sluices, turbines and ship locks.   Sluices, turbines and ship locks are housed in caisson.   Embankments seal a basin where it is not sealed by caissons. ^{Wiki n.p.}

Operation:^{Wiki n.p.}

• The basin is filled through the sluices until high tide.   Then the sluice gates are closed.   The turbine gates are kept closed until the sea level falls to create sufficient head across the barrage, and then are opened so that the turbines generate until the head is again low.   Then the sluices are opened, turbines disconnected and the basin is filled again.   The cycle repeats itself. Ebb generation takes its name because generation occurs as the tide ebbs.
• The basin is filled through the turbines, which generate at tide flood.   This is generally much less efficient than ebb generation, because the volume contained in the upper half of the basin, which is where ebb generation operates, is greater than the volume of the lower half, thus making the difference in levels between the basin side and the sea side of the barrage, and therefore the available potential energy less than it would otherwise be.
• Turbines are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation).   This energy is more than returned during generation, because power output is strongly related to the head.

The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the fish. A tidal current turbine will have a much lower impact.   Barrage systems are sometimes affected by problems of high civil infrastructure costs associated with what is in effect a dam being placed across two estuarine systems, and the environmental problems associated with changing a large ecosystem.   Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea.   This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.   As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. "Tidal Lagoons" do not suffer from this problem.   Estuaries often have high volume of sediments moving through them, from the rivers to the sea.   The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.   Again, as a result of reduced volume, the pollutants accumulating in the basin may be less efficiently dispersed, so their concentrations may increase.   For biodegradable pollutants, such as sewage, an increase in concentration is likely to lead to increased bacteria growth in the basin, having impacts on the health of the human community and the ecosystem.   Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them.   Also, some fish will be unable to escape the water speed near a turbine and will be sucked through.   Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15% from pressure drop, contact with blades, cavitation, etc. This can be acceptable for a spawning run, but is devastating for local fish who pass in and out of the basin on a daily basis.   Alternative passage technologies (fish ladders, fish lifts, etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing. ^{Wiki n.p.}

Tidal barrage power schemes have a high capital cost and a very low running cost.   As a result, a tidal power scheme may not produce returns for years, and investors are thus reluctant to participate in such projects.   Governments may be able to finance tidal barrage power, but many are unwilling to do so also due to the lag time before investment return and the high irreversible commitment. ^{Wiki n.p.}

A relatively new technology tidal stream generators draw energy from currents in much the same way as wind turbines.   The higher density of water, some 832 times the density of air, means that a single generator can provide significant power.   Even more so than with wind power, selection of location is critical for a tidal stream power generator.   Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses.   Tidal power schemes do not produce energy all day.   A conventional design, in any mode of operation, would produce power for 6 to 12 hours in every 24 and will not produce power at other times.   As the tidal cycle is based on the rotation of the Earth with respect to the moon (24.8 hours), and the demand for electricity is based on the period of rotation of the earth (24 hours), the energy production cycle will not always be in phase with the demand cycle.   However, the tides are relatively reliable and more predictable than other alternative energy sources, such as wind.   Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity,[citation needed] which is efficient compared to other energy resources such as solar power or fossil fuel power plants. ^{Wiki n.p.}

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