Tidal power harnesses the power of the tides to turn a rotor or generate electricity. Mankind has been harnessing this power since at least Roman times, when tidal mills were made to grind grain. Tidal mills work by being set up next to a small dam near a small tidal inlet or estuary. As the tide rises, the dam is left open, and the water rises through it, building up on the other side. Then, as the tide reaches its maximum extent, the door of the dam is brought down and the water is trapped. The tide falls, and when it reaches a suitable level, a small channel in the dam is opened up and the water flows out, through a waterwheel, which can then be used to grind grain.
Modern tidal power devices operate on a much larger scale, though they have only found limited use in electricity generation so far. Still, tidal power is much more reliable than wind energy and solar power, and more recently there have been several new design and deployment initiatives of tidal power generators.
Tidal power gets its energy from the gravitational influence of the Moon on the oceans of the Earth. As the Moon orbits the Earth, it pulls the oceans towards it slightly, leading to tides with ranges between a few meters (~10 ft) to 16 meters (53 ft) in extreme cases, like the Bay of Fundy at the eastern coast of Canada. This energy can be exploited by tidal power generators in a number of ways.
There are two main types of tidal power generators: tidal stream systems, which exploit the kinetic energy of tides, using water to power turbines in a similar way that windmills use air to power turbines. The other main type are barrages, which operate on a principle similar to tidal mills, operating as a wide dam across an inlet or estuary. Barrages suffer from high cost, environmental issues, and a worldwide limit of viable sites, while tidal stream systems avoid these issues. A lower-impact variation on the barrage is a tidal lagoon, which only covers a portion of an inlet. Another advantage is that tidal lagoons can be configured to generate power continuously while barrages cannot.
Tidal stream generators, the lowest cost-to-entry and most viable tidal power technology, was only developed recently, so it makes sense to say that this new power generation method has just begun to be exploited, and may have quite a future ahead of it.