The figures might not add up now, but the amount of untapped energy available ought to be hard to ignore. Coles and his colleagues found that tidal stream power has the potential to meet 11 percent of the UK’s current annual electricity demand, or 11.5 gigawatts. But according to the trade association Renewable UK, only six projects are fully or partially operational, producing a total of 10.6 megawatts—or less than 0.1 percent of what’s said to be possible. Further projects with a capacity of 370 megawatts have been approved, but even if built, the UK would still only be capturing roughly 3 percent of the energy reportedly available.

Yet the UK is ideally placed to show the world the potential of tidal energy. It also has some of the largest tidal ranges in the world, which are ripe to be exploited in a different way. “At the moment, the government is not considering tidal range. And that, in my view, is a very big mistake,” says Roger Falconer, emeritus professor of water and environmental engineering at Cardiff University in Wales.

Where the difference between high tide and low tide is great, barrages—large dam-like structures that stretch across a bay or river estuary—could capture the tide’s energy as it goes in and out. Unlike with the tidal stream approach, which places turbines in open water, with a barrage the turbines are built into the structure, which forces the water to pass through the turbines when the tide goes out. Because the turbines are protected in a sturdy concrete structure, they are easier to maintain and would only need to be replaced every 40 years or so, compared with exposed tidal stream turbines like those in the UK’s current tidal projects, which last about 20 years.

But these megastructures aren’t cheap. A power station at the mouth of the Rance River in Brittany, France, uses a barrage. Built in 1966, it’s still in operation, and for a long time it was the world’s largest tidal barrage power plant: Its 24 turbines produce enough electricity for 225,000 people. It cost $115 million (around $1 billion in today’s money). In 2011 it was surpassed by a plant on Sihwa Lake, an enclosed body of water created by a tidal barrage in South Korea, which produces 10 percent more electricity each year than La Rance. That plant cost $560 million—or $739 million in today’s money.

If the location isn’t right for a barrage, it’s also theoretically possible to build a semicircular seawall out into the sea to create a lagoon that traps water—what’s essentially a gigantic, sci-fi sea dam. As the tide recedes, a difference in water level builds between the lagoon and the surrounding water. Once the difference is large enough, sluice gates open so that the water rushes through the gaps and sets underwater turbines in motion. The proposed Swansea project would have done this, albeit on a smaller scale.

Though that lagoon failed to get funding, Falconer is helping to develop another tidal lagoon in the Bristol Channel that could generate 6.5 terawatt-hours of electricity per year. That’s a lot less than the two new nuclear reactors being built up the coast at Hinkley Point, which will generate 25 terawatt-hours a year. But the Hinkley reactors are much more expensive: They’ll cost £26 billion ($29 billion) and last 60 years, whereas the tidal lagoon would cost £8.5 billion and last at least twice as long, Falconer says. Harnessing the power of tidal ranges might be expensive, but the up-front cost could still come in well below other consistent means of energy production, like nuclear.

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