The world's first electricity-generating tidal lagoons are to be built in six locations across Britain, with at least one already in the planning stage. Here's how it'll work in the first location, Swansea Bay. A sea wall eight kilometres long will be built to encircle a large area of water three kilometres out from the coast, isolating it from the sea like a lagoon. Turbines will be set in the wall, which will then generate energy from the difference in water height four times a day as the tide rises and falls. One of the benefits of tidal power, compared to solar or wind, is that it's totally predictable. We know how much energy such a scheme would generate decades into the future, making it much easier to integrate with existing electrical infrastructure. It'll also last a long time -- the firm estimates about 120 years.