Tidal stream, barrage and lagoon: the three approaches
Three technologies take energy from the tide: stream turbines in a fast current, a barrage that dams an estuary, and a lagoon that encloses part of the coast. They differ so much in footprint that treating them as one subject causes most of the confusion.
Tidal stream
Turbines sit in a fast current, driven by the horizontal movement of water. This is where nearly all current development is concentrated: small footprint, invisible from shore, and buildable one machine at a time. The drawback is access. The largest arrays are in Scottish waters, where the Pentland Firth concentrates flow between the mainland and Orkney.
Tidal barrage
A dam across an estuary holds water back and releases it through turbines, using the height difference. It is the oldest approach: La Rance in Brittany has run since the 1960s, and the largest by capacity is at Sihwa Lake in South Korea. A barrage produces a lot of power and permanently changes the estuary behind it, which is why so few exist.
Tidal lagoon
A lagoon is a barrage that does not block the whole estuary: a wall is built out from the coast with turbines in it, keeping most of the output while leaving the estuary open. The sticking point has been cost, since a lagoon is a large civil project whose bill lands entirely up front.
A useful test when reading about tidal power
When someone says it is proven, they usually mean a barrage. When someone says it is expensive and early, they usually mean tidal stream. Both are true about different things.