A tidal lagoon such as the one proposed for Swansea Bay generates electricity by closing a wall around part of the sea. The tide piles up or drains away outside it, then gates open so the height difference drives water through turbines. It does this twice in a single tide: once as the sea rises, once as it falls.
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The lagoon is a man-made enclosure, built with a barrier wall much like a harbour or a marina. Inside, the water is cut off from the sea’s ordinary rise and fall. Outside the wall, the tide keeps moving. That gap between the two levels is what does the work.
How the trapping works
As the tide comes in, gates in the sea wall stay shut and water builds up outside. Once the outside level is high enough above the water held inside, the gates open and the sea rushes through turbines set into the wall, turning them as it goes. The lagoon fills up while this happens, and the flow slows and stops once the levels equalise.
Then the process reverses. The tide outside starts to fall while the lagoon holds its higher level. At low tide, the gates open again and water rushes out of the lagoon, back through the turbines, generating a second burst of electricity from the same tide.
Why it runs for most of the day, not just at the turn of the tide
The four tidal flows a day at a site such as Swansea Bay, in Wales, mean the sequence of building up and releasing repeats twice for the incoming tide and twice for the outgoing one. Engineers behind the proposed Swansea Bay lagoon said their turbines would generate power for about 14 hours out of every 24, not in one continuous run but in the pattern the tide sets.
Because the tide flows in a predictable pattern, a lagoon’s output is reliable and does not need gas or coal stations held on standby.
Why a lagoon rather than a barrage across a whole estuary
A tidal barrage works on the same basic principle, but it dams the full width of an estuary rather than enclosing a smaller area next to the shore. The Severn Barrage, once proposed across the Severn Estuary between England and Wales, would have flooded intertidal mudflats and drawn opposition from conservation groups worried about the effect on wading birds. A parliamentary committee called the case for it “unproven”.
A lagoon takes in a much smaller area, so campaigners have argued it disturbs less of the tidal flow than a barrage spanning an entire estuary. That view is not universal. The RSPB gave a cautious welcome to the Swansea Bay lagoon proposal while saying it would need to see the details before backing it in full.
What it would take to build one
Six lagoons were once proposed for the UK: four around Wales, one in Somerset and one in Cumbria, and engineers behind the plans said together they could supply about 8% of the country’s electricity. The Swansea Bay scheme alone would have needed six miles of sea wall.
None of that came cheap. The company behind the Swansea Bay proposal put the construction cost at £750m to £850m for that single lagoon, and estimated all five original UK sites together would have cost about £12bn to build. To be financially viable, the project needed a subsidy of £180 per megawatt-hour of electricity produced, above the support then given to offshore wind.
The company argued that the subsidy could fall quickly as the technology developed and more projects came online.
