Tidal energy is a form of marine renewable energy that depends on the flow of tidal waters. Mechanical mills driven by tidal movements in estuaries were used by the Egyptians, Greeks and Romans to grind grain. More recently, tidal waters are being used to generate electricity. Energy can be harnessed from the tidal range and tidal stream.
Tidal range refers to the vertical difference in water level between the times of high tide and low tide. The most common approach in tidal range electrical generation is to build dams or barrages across an estuary to store water in a reservoir being brought in by the tide. The difference in water level between the reservoir and the falling sea level on the other side of the dam creates potential energy that can then be released through turbines located in the barrage converting some of the potential energy of the stored water into electricity, similar to hydroelectricity stations constructed on river systems. In tidal range electrical generation this can be done only on the ebb tide (one-way generation), or on both the rising and falling tides (two-way generation). Building a barrage across an estuary (or river) has a number of undesirable environmental consequences and, for this reason, is a less popular solution these days. However, creating a lagoon or shore-connected impoundment in an estuary or bay with high tidal range could provide some of the advantages of tidal range generation with some fewer environmental concerns.
Tidal stream refers to the motion of the flowing water (kinetic energy). The force of the flowing water can drive a turbine, just like wind drives wind turbines. Since kinetic energy is a function of mass and speed, places in the world where large amounts of water are moving at high speeds (>1 m/s) are candidates for tidal stream energy extraction. The Minas Passage in the Bay of Fundy is arguably the most desirable location in the world for tidal stream energy extraction. There are many different technologies that will convert some of the kinetic energy of the fast flowing water into electricity.
The Community & Business Toolkit for Tidal Energy Development is a great resource for those wishing to know more about tidal energy.