Doing my research on alternative energy sources i came across Tidal Power Plants. so here are the basics from the website About.com
How Tidal Power Plants Work
There are three basic ways we can harness tidal power .
By Mary Bellis, About.com Guide
Tidal Power - Wave Energy
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.
3 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.
1 Wave Energy
Kinetic energy (movement) exists in the moving waves of the ocean. That 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.
2 Tidal Energy
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.
3 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.