What types of tidal power exist?
There are three main mechanisms for harnessing tidal energy:
Turbines are installed under water in a tidal stream, taking advantage of a fast-flowing body of water created by tides. These turbines have been designed to take advantage of the dual directional flow of the tidal current. They are most effective in shallow water and machines are large and disruptive.
The larger established tidal power projects are rather similar to hydroelectric schemes, except that the dams are substantially bigger. A huge barrage is built across a river estuary for example, so when the tide goes in and out, the water flows through tunnels in the dam. The ebb and flow of the tides can be used to turn a turbine, or it can be used to push air through a pipe, which then turns a turbine. Large lock gates, like the ones used on canals, can be opened and closed to allow ships to pass. Because tidal barrages completely block the normal flow of water, they can be very damaging to marine life.
These are different from tidal barrages in that they do not form a barrier all the way across the estuary. They can be sited offshore, and consist of a (normally circular) perimeter embankment impounding water. It is thought that lagoons are therefore less ecologically damaging than barrages. Energy production works in two ways: at high tide, a barrier goes up and water rushes into the lagoon through turbines, generating kinetic energy. At low tide, water that has built up inside the lagoon does the same on its way out.