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Definition: Tidal power from DK Eyewitness Books: Oil

Tides move huge volumes of water up and down river estuaries twice daily. To exploit this, a barrage can be built across the estuary and equipped with turbines that turn in both directions. These harness the flow of water when the tide is coming in and going out. But there are concerns that interfering with tidal flows could be harmful to the wildlife of estuary habitats.

Summary Article: Tidal Power from Green Energy: An A-to-Z Guide

Tides are the cyclic rising and falling of the ocean surface caused by the gravitational attraction of the moon and the sun acting on the ocean waters of the rotating earth. Tides cause changes in the depth of marine and estuarine water bodies and produce oscillating currents known as tidal streams. Moreover, tidal power can be used to mean the generation of electricity using the mechanical force created by the rise and fall of ocean surge. It is a renewable, largely abundant, nondepleting, and clean source of energy. Tides are very powerful, with the sea moving very quickly, producing an immense force of moving water that can be adequately harnessed for gainful work. Tidal energy is produced by using special energy generators purposely designed to convert the mechanical power of ocean currents to electricity. These generators are generally called tidal “stream” generators. They contain turbines mounted on gearbox shafts and operate in a manner similar to wind turbines, but they produce more electrical energy at low tidal velocities compared with the same amount of wind speeds for wind turbines. The generators are placed underwater in places or areas where high tides are common and recurrent. They are designed to capture the kinetic energy of moving water and subsequently convert it to electrical energy. A key feature of most tidal stream turbines is that the turbines use technology already developed from the wind industry. The only difference is the support structure.

Apart from the use of tidal stream generators, another approach to creating tidal power is the use of dams or barrage systems. The dams are based on using embankments at a bay or estuary with a large tidal range. The tidal range must usually be above 5 meters. Power is generated at ebb tides as the barrage system creates a significant head of water. When the tide rises, the barrage opens and allows water to flow inward; it closes when the tide stops. Water is then trapped within the barrage basin and thereby creates a hydrostatic head. Because of this hydrostatic head, the water flows through the passage leading through turbines, driving the turbines and consequently generating electricity as it flows in and out. Tidal barrage systems work more like hydropower systems. The disadvantages of using the barrage system include the very high civil infrastructural cost, the inadequacy of suitable sites for construction, and the daily intermittent nature of the ocean tides.

Tidal variation is a common phenomenon. It is caused by the moon's movement, its interaction with the sun, and the gravitational pull between the moon and Earth. Tides may either be diurnal (producing one tidal cycle of high and low water levels per day) or semidiurnal (having two high and two low water levels per day). This diurnal difference produces daily inequality in the water gradients, which change with time and are generally low when the moon is over the equator. The history of tide discovery and its relation to the sun's and moon's activities has come a long way. For example, the link between tides and the moon was first theorized by Seleucus of Seleucia in the second century. He wrote that the rising and falling of tides were the result of the attraction of the moon and that the height of the tides depends on the location of the moon relative to the sun. In Europe around 730 C.E., the Venerable Bede described how the rise of the tide on one coast of the British Isles coincided with the fall on the other side and described the progression in times of the same high water along the Northumbrian coast.

The use of tidal power provides some advantages. Once a tidal power plant has been built, its operation and maintenance is relatively cheap. It does not produce greenhouse gases, as no burning of fuels is involved. It only uses natural energy from flowing water and tidal currents to power turbine installations. It produces no air pollutants and provides clean and reliable energy capable of powering industries and communities. It is renewable because the tides will continue to ebb and flow, and the energy harvest can keep going on and on (because as long as the Earth and moon continue to move and interact with each other and with the sun, and the ocean exists on the surface of the earth, there will always be tidal movements and variations, and so tidal power will always be available). The tides, though intermittent, are predictable and thus encourage energy planning. The use of tidal power also removes foreign exchange expenditures that would have been channeled to purchasing petroleum fuels and creates employment opportunities. It can adequately be relied on where high tides are harvestable when adequate energy storage capacities are provided or alternatives are available during the absence of tides. A major disadvantage of tidal power stations is that they can only generate power when the tide is flowing in or out, meaning that the power plants can only generate electrical energy for about 10 to 12 hours per day. However, tides are totally predictable, so other power stations can generate power at those times when the tidal station is out of action. As for the environmental impact assessment of tidal power plants, the barrage plants have been found to affect the salinity and circulation of the seawater in which they are located. Tidal stream generators also can affect fish—the fast-moving turbine blades of these generators can kill fish that flow through the systems.

See Also:

Hydroelectric Power, Renewable Energies, Wind Power, Wind Turbine

Further Readings
  • Alternative Energy News. “Tidal Power: News and information About Tidal Energy Technology.” (Accessed January 2009).
  • Renewable Energy Center. “Wave and Tidal Power.” (Accessed January 2009).
  • Olanrewaju, Ajayi Oluseyi
    Covenant University, Nigeria
    Copyright © 2010 by SAGE Publications, Inc.

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