Tidal Energy

هندسة الطاقة الجديدة والمتجددة
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Tidal Power

The tides are formed due to the gravitational force of attraction between the earth, sun and moon. During high tides sea water is trapped in a reservoir, and released later to drive turbines, which in turn produces electricity.
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Tidal power traditionally involves erecting a dam across the opening to a tidal basin. The dam includes a sluice that is opened to allow the tide to flow into the basin; the sluice is then closed, and as the sea level drops, traditional hydropower technologies can be used to generate electricity from the elevated water in the basin. Some researchers are also trying to extract energy directly from tidal flow streams.

The energy potential of tidal basins is large — the largest facility, the La Rance station in France, generates 240 megawatts of power. Currently, France is the only country that successfully uses this power source. French engineers have noted that if the use of tidal power on a global level was brought to high enough levels, the Earth would slow its rotation by 24 hours every 2,000 years.
Tidal energy systems can have environmental impacts on tidal basins because of reduced tidal flow and silt buildup.
Ways of Using the Tidal Power of the Ocean

There are three basic ways to tap the ocean for its energy. We can use the ocean’s waves, we can use the ocean’s high and low tides, or we can use temperature differences in the water.
Wave Energy
Kinetic energy (movement) exists in the moving waves of the ocean. T

hat energy can be used to power a turbine. In this simple example, (illustrated to the right) the wave rises into a chamber. The rising water forces the air out of the chamber. The moving air spins a turbine which can turn a generator.

When the wave goes down, air flows through the turbine and back into the chamber through doors that are normally closed.
This is only one type of wave-energy system. Others actually use the up and down motion of the wave to power a piston that moves up and down inside a cylinder. That piston can also turn a generator.
Most wave-energy systems are very small. But, they can be used to power a warning buoy or a small light house.
Tidal Energy
[URL=“http://powershift.anetce.com/tidal_dam1.gif”][URL=“http://powershift.anetce.com/tidal_dam1.gif”]

Another form of ocean energy is called tidal energy. When tides comes into the shore, they can be trapped in reservoirs behind dams. Then when the tide drops, the water behind the dam can be let out just like in a regular hydroelectric power plant.

In order for this to work well, you need large increases in tides. An increase of at least 16 feet between low tide to high tide is needed. There are only a few places where this tide change occurs around the earth. Some power plants are already operating using this idea. One plant in France makes enough energy from tides to power 240,000 homes.
Ocean Thermal Energy

The final ocean energy idea uses temperature differences in the ocean. If you ever went swimming in the ocean and dove deep below the surface, you would have noticed that the water gets colder the deeper you go. It’s warmer on the surface because sunlight warms the water. But below the surface, the ocean gets very cold. That’s why scuba divers wear wet suits when they dive down deep. Their wet suits trapped their body heat to keep them warm.

Power plants can be built that use this difference in temperature to make energy. A difference of at least 38 degrees Fahrenheit is needed between the warmer surface water and the colder deep ocean water.

Using this type of energy source is called Ocean Thermal Energy Conversion or OTEC. It is being used in both Japan

and in Hawaii in some demonstration projects

Tidal power plant

Plant that harnesses tidal power (the motion of the rising and falling tides) to produce electric power
gate

Movable vertical panel that controls the rate of flow of the water between the sea and the basin.
substation

The devices (such as transformers and changeover switches) that increase the voltage of the electricity and carry it to the network.

basin

Area in which water is stored at high tide; the basin empties out through the penstocks at low tide.
inactive dike

Part of the dam made up mainly of rocky material; it is built between the plant and the operating dam to separate the basin from the sea.
administrative building

power plant
[URL=“http://media.web.britannica.com/eb-media/49/121649-004-16E15661.gif”][URL=“http://media.web.britannica.com/eb-media/49/121649-004-16E15661.gif”]

Part of the dam housing bulb units that are powered by the rise and fall of the sea to produce electricity.
lock

Structure with doors and gates that is built between the sea and the basin; it allows boats to pass from one level to the other
[URL=“http://visual.merriam-webster.com/images/energy/hydroelectricity/tidal-power-plant/tidal-power-plant.jpg”]](http://2.bp.blogspot.com/_nBaWI2hPVZ4/TRlLkv_s43I/AAAAAAAAA-8/6olGWtxN_yw/s1600/merry-christmas-graphic-animation1.gif)
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Category of generation
Ebb generation

[FONT=TimesNewRomanPSMT][SIZE=3]The basin is filled through the sluices and freewheeling turbines until high tide. Then
the sluice gates and turbine gates are closed. They are kept closed until the sea level
falls to create sufficient head across the barrage and 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 (also known as outflow
generation) takes its name because generation occurs as the tide ebbs.

[/size][/font][FONT=TimesNewRomanPSMT][SIZE=4]

Flood generation

[/size][/font]The basin is emptied through the sluices and turbines 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 (the domain of Flood generation).
[FONT=TimesNewRomanPSMT][SIZE=4]Two-way generation

[/size][/font]Generation occurs both as the tide ebbs and floods. This mode is only comparable to
Ebb generation at spring tides, and in general is less efficient. Turbines designed to
operate in both directions are less efficient.
Pumping
Turbines can be powered in reverse by excess energy in the grid to increase the water
level in the basin at high tide (for Ebb generation and two-way generation). This
energy is returned during generation.
[FONT=TimesNewRomanPSMT][SIZE=4]Two-basin schemes

[/size][/font]With two basins, one is filled at high tide and the other is emptied at low tide.
Turbines are placed between the basins. Two-basin schemes offer advantages over
normal schemes in that generation time can be adjusted with high flexibility and it is
also possible to generate almost continuously. In normal estuarine situations, however,
two-basin schemes are very expensive to construct due to the cost of the extra length
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