The tides are an enormous and consistent renewable energy resource, but taking advantage of them has proven frustrating. Newfangled minimal-impact in-stream devices have been slow to demonstrate their commercial viability; earlier “barrage” projects – dams on estuaries, essentially – have delivered energy as promised, but at the cost of degraded ecosystems.

Now a potential solution to the puzzle – called tidal lagoon power, a sort of middle ground between in-stream and barrage – seems to be gaining momentum in the United Kingdom.

The big insurer Prudential said this week that it would invest in Tidal Lagoon Swansea Bay, a proposed project on the coast of Wales that is forecast to produce some 400 gigawatt-hours of power a year, 9 percent of Wales’ annual domestic electricity use.

Media reports put Prudential’s investment at up to £100 million, more than a tenth of the cost of the privately financed project. Mark Shorrock, CEO of Tidal Lagoon Power, the project developer, called the investment a “clear endorsement of our vision to introduce tidal lagoon infrastructure into the UK’s low carbon energy mix.”

The vision at Swansea is of a seawall extending about 1.5 kilometers from shore and loops around to cordon off 11.5 square kilometers of the bay, where the tidal range is an impressive 8.5 meters. One section of the seawall is open – that’s where 16 turbines are housed. As the developer explains:

To generate electricity, as the sea starts to rise (flood tide) from low tide level, water is prevented from entering the Lagoon for an average of 2.5 hours, which creates a difference in water levels known as ‘head’. Once sufficient head has been reached, the water is allowed to flow into the Lagoon through the turbines, turning the runner (like a propeller) and generating electricity. This process is repeated on the ebb tide, where the water is prevented from leaving the Lagoon until there is sufficient head to start the process again.

The environmental community that was so skeptical about a Severn Estuary barrage project farther up the Bristol Channel – ultimately nixed by the government – is warmer to tidal lagoon power. The group Friends of the Earth in Wales has tentatively endorsed the Swansea proposal, believing it has the potential to largely avoid an issue deadly to barrages – the impounding of vast biologically rich (and sensitive) inter-tidal areas and tributary rivers.

Tidal Lagoon Power has done a masterful job of packaging its project, as well. Every big renewable energy project talks about clean energy and economic development. But in text and renderings, the Swansea seawall is alive with sauntering, recreating and touring citizens. Tidal Lagoon Power says the power plant can be “a unique venue for opportunities in the arts, culture, education, recreation,” and “a foundation venue for local and national sports use.”

It can also be an expensive way to produce electricity – although if it’s not a one-shot deal, that picture could change.

To encourage development of low-carbon energy, the UK sets “strike prices” – minimum guaranteed prices, essentially – for power produced using various technologies. An analysis by the engineering firm Pöyry [PDF] said Swansea Bay would need a strike price of around £168/MWh to be viable; at that level, it beats the pants of the in-stream tidal power that’s being pursued, where the strike price is £305/MWh.

But the real competition is with offshore wind, where the current £155/MWh strike price is set to drop to £140/MWh by 2017/18.

By that measure, Swansea Bay looks like a hard sell, but Pöyry said that two subsequent proposed tidal lagoon projects will actually be much less expensive – needing strike prices around £130/MWh for the second and  £92/MWh for the third. The lower costs aren’t a product of expected construction or technological progress, but instead come simply from building at a bigger scale – each lagoon would offer more than a gigawatt in capacity.

Why not build bigger from the start? Well, this is a first go at this technology. Tidal Lagoon needs to prove itself before truly massive projects are undertaken. But viewing the lagoons as a portfolio, “The volume-weighted average central strike price for the three lagoons is around £111/MWh, offering the potential for renewable electricity considerably lower than the strike prices for offshore wind, quickly and at scale,” Pöyry said.

If this alone doesn’t inspire the government to support the project with adequate pricing, there are more intrinsic if untested arguments in favor of the tides over the winds.

Tidal lagoons, like their distant cousin big hydro, could offer extremely long lifespans – 120 years, compared to the 25 years from offshore wind. In other words, you’d have to build that offshore wind power plant five times over to match a tidal lagoon. Also, while tidal lagoon power isn’t constant, it is very, very predictable, and a geographically diverse lagoon portfolio with differing high and low tides could smooth the generation curve.

Ultimately, Tidal Lagoon Power hopes to build six lagoons, which could supply eight percent of the UK’s electricity. But if it’s going to happen, it needs to start at Swansea.