The tide is a direct effect of the relative positions of the earth and the moon and, to a lesser extent, the earth and the sun. These powers are evident in how the gravitational forces of these celestial bodies move large bodies of water here on earth. Although affecting the entire globe, these shifts are particularly apparent in places of shallow coastlines and sea floors. The UK, Korea and China are some of these places.
Tidal power refers to converted energy from the tide into electricity or other power sources that are usable by humans. It has as a matter of fact been used as early as in roman times to power mills, though it was not until the 20th century that it was investigated as a potential source of electricity. Following a period of initially tentative developments the first ever tidal power plant was opened in 1966 in La Rance, France, but it failed to gain popularity owing to the high cost of power plant components and the perceived shortage of sites in which power plants could be built.
However interest in tidal power resurfaced towards the end of the 21th century, as technological developments and improvements in design and turbine technology were found to have worked around the aforementioned tradeoffs. Cheaper construction of power plant components and placement of tidal generators in locations where previously none were conceivable suddenly makes tidal power a hot alternative to fossil fuel-based energy sources. After all it fits in well with the environmental awareness of the 21th century in how it is next to infinitely renewable, not to mention how the power generation process omits an absolute minimum of pollutants. Moreover it is actually a force to be reckoned with, being more predictable by far than the highly intermittent solar energy and wind energy alternatives. This predictability translates to easier implementation into existing power grids than the latter two options, never mind that it has yet to be as extensively implemented.
Currently two types of tidal power plants can be found in the world, with a theoretical third variant currently being subject to development. These are tidal stream generators, tidal barrage generators and dynamical tidal power.
The tidal barrage is the pricy and hard to place option that has given tidal power its bad rap. It functions the same way as a river dam, making use of height differences between high and low tides. Indeed, the power plant itself is really just a dam that has been built to seal off the entirety of a tidal estuary. It is these estuaries that experts have deemed impossible to find in the past, whereas it is the dam structure that comes at unjustifiable costs. On top of these issues, although pertinent in their own right, are the various threats that tidal barrages pose to marine life and the like.
Tidal stream generators instead use the moving water of the tide, functioning much like, say, a water mill would in a stream. While the energy outlet is potentially less than it is with tidal barrages, it comes at a low cost and an equally low degree of ecological impact. For these reasons it is becoming the first choice of tidal generators in many parts of the world.
The theoretical dynamic tidal power instead suggests long dams that go straight out into the ocean without enclosing any parts of the coast. These would generate power the same way as the tidal barrage, but without posing as palpable a danger to the surrounding ecosystem. However the cost effectiveness of this type of tidal power plant is debatable and it is doubtful that it could be universally employed - only along the coasts of shallow seas would such construction projects be feasible.