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Juice Architects/Tidal Lagoon Swansea Bay Plc Juice Architects/Tidal Lagoon Swansea Bay Plc

Just outside the Welsh city of Swansea, the U.K. is planning one of the most innovative power plants ever constructed. It’s not the plant’s size that is striking, though it could ultimately provide power to 155,000 homes for 120 years. It’s the source of its power that breaks ground: tides channeled into an artificially constructed lagoon.

Granted full planning permission this June, the Swansea Bay Tidal Lagoon will be the world’s first ever plant to generate electricity using this method. Should it prove successful, the plant’s template could be adopted worldwide as a way of generating green power while simultaneously providing sea wall protection to coastal communities.

Tidal electricity generation in itself is nothing new, of course. Tidal power stations have been producing electricity since France built the world’s first in 1966. Swansea’s tidal lagoon model nonetheless takes the technology to a new stage, making it more adaptable and softening its environmental impact.

Tidal power plants built so far work on the tidal barrage model, where embedded turbines within a dam are strung across a site that has naturally strong and reliable tidal flow, typically an estuary. As the tide flows through, the barrage’s blockage creates a difference in water levels, the resulting pressure pushing water through the turbines to generate electricity. The key limitation of this model is that it only works in sites where tides pass through some form of easily bridgeable gap.

The tidal lagoon model gets round this problem by actually altering the coastline to create the correct conditions. Instead of bridging a river mouth, it requires the construction of what is effectively an artificial harbor, a stretch of water enclosed by a semicircular, rock-clad barrier built up from the seabed. A harbor mouth is created within this barrier, where the inflow and outflow of water powers a set of turbines. Swansea’s barrier will be 5.9 miles long when constructed, with a mouth fitted with up to 26 turbines, a final result outlined in this video.

Constructing the correct conditions in this way—rather than looking for places where they occur naturally—greatly increases the number of potential sites for tidal power stations. As a byproduct, it can also create a new public amenity. Swansea’s barrage will host a walkway, fishing spots, an aquatic community farm and a visitors’ center, while its protected lagoon could also host watersports events.

The model also opens up the possibility of future barrages that have a double function. They could be built as offshore sea walls designed to protect vulnerable coastal communities, and simultaneously provide these communities with a green, renewable source of energy. Once construction is complete, these seawalls-cum-power plants could theoretically be almost carbon neutral.

Whether that means they are entirely environmentally friendly is another matter. So far, tidal barrages’ record in this area has been mixed. Research at France’s Rance Tidal Power Station found that its construction had profoundly affected the environment of the Rance River’s estuary. Even with turbine gaps wide enough for fish to swim through (and supplementary sluices opened outside high and low tide periods), fish migration suffered. The salinity of the estuary inside the barrier also dropped, precipitating changes in flora and fauna.

Tidal lagoon power plants should sidestep many of these problems. As they don’t span a river mouth, lagoons are far less likely to disrupt water, sediment and marine wildlife flows. In fact, by reducing turbidity they may increase plant and animal life, as reducing choppiness enables marine plants to grow faster and thus attracts more species that feed on them. This lesser impact has led the environmental organization Friends of the Earth to cautiously endorse the lagoon model as an alternative to the barrage.

A rendering of a planned recreation walkway to be included in the Swansea Bay Tidal Lagoon. Courtesy Tidal Lagoon Swansea Bay Plc

There’s still a major sticking point: cost. Swansea’s tidal lagoon will cost £1 billion (roughly $1.5 billion) to construct. Over a 120-year lifespan, the plant could recoup its investment, but the cost of the electricity it produces will have to be subsidized by government for up to 35 years. This would grant the lagoon a higher level of subsidy than anything currently available in the U.K. for wind or solar power—a big ask. Exactly how much this subsidy will have to be is currently being thrashed out, and while the government agreed on planning permission for the lagoon in June, they have delayed the start of construction until at least 2017.

There’s still momentum behind the project, however. In fact, plans are already being drawn up for two more tidal power plants elsewhere along the Severn Estuary, at Cardiff and Newport. It’s possible that the company developing the tidal lagoon might settle for a lower subsidy in return for being given the go-ahead on these other sites. Elsewhere in Britain, longstanding plans are also afoot for a tidal power plant (working on something closer to the wind farm model) on the seabed at Scotland’s Pentland Firth. Taken together, these projects make the U.K. one of the most interesting experimentation grounds for renewable energy right now—beneath the water, at least.