Tidal movement where the sea meets the land is caused through a complex relationship by the gravitational pull the moon (and to a small extent the sun) has over the earth. It is because of this exertion of power that sea levels on shore rise (to ‘high tide’) and fall (‘low tide’). This process, the movement of the water rising up and then receding back, happens twice a day and consists of an incredibly enormous movement of the seas. It is a regular cause of disaster at sea, both in causing chaos with ships and boats, and with swimmers, surfers, divers and other enjoyers of the sea, who are unaware of the timing and/or power of the tides movements.
As you can imagine, tides contain an incredible amount of energy, the forces at play here are huge, and a lot of potential energy is going to waste, literally washing up on the beach. This energy could be harnessed all over the world. Tidal dams can take advantage of all this energy with high tide pushing through the turbine and turning it, and at low tide the water flowing out which also turns it. To some tidal energy provides a fantastic opportunity, but others are not so enamoured with it.
Classic dam design
For proponents of Tidal energy it is seen as a perfectly inexhaustible energy source which must be harnessed. The tides will always be there as the moon will always be there. If either of these things somehow stop the human race won’t be around for much longer afterwords anyways. This process does not produce any of the harmful and poisonous greenhouse gasses that are so problematic for the environment, which makes them a perfect candidate to replace the non renewable and climate hurting energy sources we use now. Tides are predictable, we know their cycles and their patterns, unlike wind, sun and oil reserves.
Critics say point out that the cost of building a tidal power plant is very high (though the cost of running it is quite low) and that there are not that many locations suited to their construction, and these locations are obviously limited to coastal regions. There is also fear that building tidal energy stations may have detrimental effects on sea life. The key problem currently is that these stations are not commercially viable and more research and development must be undertaken before they will be economically sensical to build.
Overall this last point may be true for many reasons, developments in technology are happening all the time and now seems like potentially a bad time to build, as the designs and equipment that are used may well be outdated within a year. There is a lot to be said for tidal energy though, and research and development must continue. It could well be that in a few years we can go ahead and have high functioning viable tidal plants opening and starting to supply energy to all at just the time that we need them most.