COMPARED with solar and wind energy, which are booming, tidal power is an also-ran in the clean-energy stakes. But if you did want to build a tidal power station, there are few better sites than the estuary of the River Severn, in Britain. Its tidal range, the difference in depth between high and low tides, of around 15 metres is among the largest in the world.

Engineers and governments have been toying with the idea since at least 1925. But none of the proposed projects has materialised. Price is one objection. A study by Britain’s National Infrastructure Commission, published last year, reckoned that tidal energy might cost between £216 and £368 ($306-521) per MWh of electricity by 2025, compared with £58-75 for seagoing wind turbines and £55-76 for solar panels. Environmentalists also worry that any plant would alter the tides it was harnessing, making life harder for wildlife.

As he describes in a paper just published in the Proceedings of the Royal Society, though, an engineer called Rod Rainey thinks he has a way around both problems. He proposes to replace the conventional turbines of previous planned schemes with a much older technology. Specifically, he plans to span the estuary with a line of breast-shot water wheels. This is a design that dates back to the early days of the Industrial Revolution. Examples can be found fixed to the sides of picturesque old watermills.

But there would be nothing old-fashioned about Mr Rainey’s wheels. Thirty metres high and 60 wide, they would be made, in shipyards, from ordinary steel. Two hundred and fifty of them, along with the necessary supporting structures, would be floated into place and secured to the seabed, creating a line 15km long. Together, they could supply power at an average rate of 4GW. That is about as much as two biggish nuclear power stations would manage. Substituting one of the wheels with a set of locks would provide a shipping channel about twice the width of that through the Isthmus of Panama, permitting upstream ports such as Avonmouth and Cardiff to continue operating.

On paper, at least, Mr Rainey’s scheme looks attractive. Some of its advantages are environmental. The “breast” in a breast-shot water wheel is a structure on the riverbed (or, in this case, the seabed) that forms a near-watertight seal with the vanes on the bottom of the wheel. This means that if a motor is used to reverse the direction in which the wheel is turning, it will act as a pump instead of a generator. By pumping at the right points in the tidal cycle, such a system could minimise the impact on water levels behind it, helping preserve wetlands and other desirable habitats. Conventional turbines turn quickly, mincing any fish that come too close. Mr Rainey’s water wheels, by contrast, would revolve at a comparatively stately three metres a second. This is slow enough, he reckons, to permit fish to swim through easily.

There could be financial benefits, too. Existing tidal-power schemes, such as the one on the Rance estuary in France, funnel their water through tapered concrete channels, because the turbines used in them work best when water is flowing fast. Water wheels are quite happy with slower currents, eliminating the need to build expensive channels. Also, steel is cheaper than the concrete used in other schemes. Comparing the amount of steel in his water wheels with the amount used in offshore wind turbines (for which it is a significant cost) has convinced Mr Rainey that his scheme could be competitive. One of his wheels would contain about half as much steel again as does a modern wind turbine, but it would produce five times more power.

So far, so hard-headed. The final advantage of Mr Rainey’s scheme, though, is pure poetry. Water wheels powered the early stages of the Industrial Revolution, but were eventually replaced by coal. There is a pleasing symmetry to the idea of fighting the planet-heating side-effects of fossil fuels with the help of an energy source they had supposedly made obsolete.