If the site isn’t suitable for a dam, it’s also theoretically possible to build a semi-circular seawall in the sea to create a water-holding lagoon – essentially a giant, sci-fi sea dam. As the tide recedes, the difference in water level between the lagoon and the surrounding water increases. Once the difference is large enough, the floodgates are opened, allowing water to flow through the gaps and drive underwater turbines. The proposed Swansea project would do this, albeit on a smaller scale.

Although this lagoon did not receive funding, Falconer is helping to design another tidal lagoon in the Bristol Channel, which can produce 6.5 terawatt hours of electricity per year. That’s a lot less than the two new nuclear reactors being built off the coast of Hinckley Point, which will generate 25 terawatt hours a year. But the Hinckley reactors are much more expensive: they will cost 26 billion pounds ($29 billion) and last 60 years, while the tidal lagoon will cost 8.5 billion pounds and last at least twice as long, Falconer says. Harnessing the power of tidal ranges can be expensive, but the initial costs can still be much lower than other sequential methods of power generation, such as nuclear.

But tidal projects have failed not only because of a lack of funding — there are also environmental issues. Britain’s most ambitious tidal proposal – a £30 billion project harnessing the enormous tidal power of the North Estuary, which would have linked the English and Welsh coasts with a barrier – was rejected in 2010, partly to avoid disturbing feeding and wintering birds in the region. (The project is back on the agenda from March 2022, however, when a a coalition of local authorities, business and academics to create an independent commission to review it.)

Concerns that turbine blades could injure marine animals have also eased. In 2021, a 37-year-old tidal power plant in Canada was shut down in part because the Department of Fisheries and Oceans found that the plant’s turbine killed fish. Turbines can also disrupt the mixing of water between the seabed and the surface, which is important for the cycling of nutrients in the sea and maintaining the food web.

But studies show that such environmental costs are generally worth it: in a 2018 study, Michela De Dominicis and her colleagues from the UK’s National Oceanography Center showed that even if around 19,000 turbines were installed in Scottish waters and water mixing was disrupted, there would still be a net positive impact on the environment due to the generation of clean energy. “We’re disrupting the environment by putting a lot of turbines in the water, but at the same time it’s something that will reduce climate change,” says De Dominicis.

Yes, tidal power remains expensive, but solar and wind power were expensive just a few years ago. Then came subsidies, increased investment and adoption, and the rest is history. And unlike many other renewable energy sources, tides have one big advantage: the sea never stops. “Tidal energy can complement wind and solar energy to benefit the energy system as a whole,” Coles says.