The World Needs More Gigantic Sci-Fi Sea Dams

Imagine this. A structure, 24 times longer than the Hoover Dam, stretching out into the sea. Its 9-kilometer wall curves towards the horizon before returning to rejoin the coast, creating a giant artificial lagoon.

Under the water line, a channel fitted with 16 turbines connects the lagoon to the ocean. As the tide goes in and out, the lagoon fills and drains, spinning the turbines to generate more than 530 gigawatt-hours of clean electricity each year—enough to power 155,000 homes. If this sounds like an engineering challenge too far, it’s not.

The Swansea Bay tidal lagoon in South Wales might have taken as little as three years to start generating power if approved. Yet it was never built. The issue? Money.

The UK government turned down the £1. 3 billion ($1. 46 billion) project in 2018 on the grounds that it was too expensive.

Tidal power has a high price tag. But if government support got behind it, it could follow wind and solar energy to become a cheap, clean, and essential energy source. With global energy demand expected to double by 2050, consistent clean power sources like tidal will increasingly be needed to underpin renewables like wind and solar that fluctuate with the weather.

It’s either that or a return to fossil fuels. “From an energy-security point of view, adding tidal stream turbines to the renewable mix is a really interesting proposition,” says Danny Coles, a research fellow at the University of Plymouth. Interesting, yes.

But after decades of work, major questions remain over how best to harness the power of the waves. In the UK, the favored way of generating electricity from the sea is to use tidal stream technology—turbines fixed to the seabed that capture energy from the fast, horizontal flow of water near the coast. But the tidal stream industry is in its infancy, and investment is high-risk.

To lure in more investors, the industry needs to show that turbines can generate power over long periods without excessive maintenance and that costs will continue to fall, says Coles. That’s easier said than done. In a review published last year , Coles and his colleagues estimated that the levelized cost of energy (LCOE) for UK tidal stream power—that is, the price that the energy needs to be sold at to cover the lifetime costs of producing it—is currently £240 ($271) per megawatt-hour.

In comparison, the LCOE for offshore wind in the UK is estimated at £121 per MWh in 2020, and it could be half that by 2025. Government subsidies would help. Between 2013 and 2020, subsidies for solar power in the UK helped reduce its cost by 60 percent and increase output six-fold.

Since 2014, the government has auctioned off contracts where it buys green energy for a fixed price to make entering the renewables sector economically viable. But state support in tidal energy—in the UK and beyond—has been spasmodic at best. Only in the most recent auction round did the UK government offer contracts to tidal projects, and the commitment was small: £20 million for tidal power a year out of a total subsidy budget of £285 million.

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.

But tidal projects haven’t just fallen through due to lack of funding—there are environmental concerns too. The UK’s most ambitious tidal proposal—a £30 billion project harnessing the enormous tidal power of the Severn Estuary, which would link the English and Welsh coasts with a barrage—was abandoned in 2010, partly to avoid disrupting birds that feed and winter in the area. (The project has been back on the agenda since March 2022, however, when a coalition of local authorities, businesses, and scientists set up an independent commission to reconsider it.

) Fears that turbine blades can injure marine animals have also been a dampener. In 2021, a 37-year-old tidal power plant in Canada was shut down partly because the Department of Fisheries and Oceans had found that the plant’s turbine was killing fish . Turbines can also disrupt the mixing of water between the seabed and the surface, which is important for cycling nutrients in the sea and sustaining the food web.

But research suggests such environmental costs are, on balance, worth it: In a 2018 study , Michela De Dominicis and her colleagues at the UK’s National Oceanography Center showed that even if some 19,000 turbines were installed in Scottish waters and water mixing was disrupted, this would still have a net positive environmental impact because of the clean energy generated. “We are perturbing the environment by putting many turbines in the water, but at the same time, it’s something that is going to reduce climate change,” De Dominicis says. Yes, tidal power remains expensive, but then so were solar and wind power just a few years ago.

Then along came subsidies, up went investment and adoption, and the rest is history. And unlike so many other renewable energy sources, tidal has one big advantage: The sea never stops churning. “Tidal energy can complement wind and solar to provide benefits to the energy system as a whole,” says Coles.

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