"Tidal energy is produced by the surge of ocean waters during the rise and fall of tides. Tidal energy is a renewable source of energy.
During the 20th century, engineers developed ways to use tidal movement to generate electricity in areas where there is a significant tidal range—the difference in area between high tide and low tide. All methods use specialgenerators to convert tidal energy into electricity."
"Tidal Energy Generators
There are currently three different ways to get tidal energy: tidal streams, barrages, and tidal lagoons.
For most tidal energy generators, turbines are placed in tidal streams. A tidal stream is a fast-flowing body of water created by tides. A turbine is a machine that takes energy from a flow of fluid. That fluid can be air (wind) or liquid (water). Because water is much more dense than air, tidal energy is more powerful than wind energy. Unlike wind, tides are predictable and stable. Where tidal generators are used, they produce a steady, reliable stream of electricity.
Placing turbines in tidal streams is complex, because the machines are large and disrupt the tide they are trying toharness. The environmental impact could be severe, depending on the size of the turbine and the site of the tidal stream. Turbines are most effective in shallow water. This produces more energy and allows ships to navigatearound the turbines. A tidal generator's turbine blades also turn slowly, which helps marine life avoid getting caught in the system."
Here are some key components from: http://www.environmentalleader.com/2013/04/10/the-present-and-future-of-tidal-power/