There are several proposals for barrages, as well as tidal lagoons. According to BERR, a barrage would be a very complex project, but the basic concept is well understood and is the application of mature, commercially available technology.
“A tidal barrage has been successfully operated at La Rance, northern France since the 1960s,” it adds.
Why choose the River Severn estuary? Because the Severn has the second highest tidal range in the world, up to 14m (46ft) on a spring tide – second only to the Bay of Fundy in Nova Scotia, home of the Annapolis power generation project. Also, a Severn barrage could save 5.6million tonnes of CO, based on displacing a new gas-fired power plant. Current proposals include a 16.1km (10-mile) barrage with 216 hydro-electric turbines from Brean Down, near Weston-Super-Mare, to Lavernock Point, near Cardiff, on the Welsh side of the Severn estuary, capable of generating 8,640 Megawatts – 4.4% of UK electricity.
In August, 2007, the Welsh Assembly, which owns 377 acres in the Lavernock area of Penarth, agreed to keep the land in state ownership until the outcome of Severn estuary tidal-power studies were known.
Another is the smaller English Stones or “Shoots” proposal, 4.1km (2.5 miles) long near the existing Severn road bridges, north of Avonmouth docks, with 40 turbines capable of generating 1,005MW – 0.7% of UK electricity demand, while a much bigger barrier advocated – perhaps not surprisingly – by potential contractors could stretch from Minehead to Wales.
Dr John Loveless, senior lecturer in civil engineering at the University of Bristol, says a barrage is only one of several systems capable of harvesting tidal energy from the Severn. Energy can be captured kinetically, exploiting water velocity, and another method, as in a barrier, is to capture potential energy from the different height of water at high and low tide.
“If you just put a propeller in the water, while you can collect power, you can only collect a maximum of 60% of the energy in the tidal stream. (The btz number). You cannot design to get any more,” says Loveless.
He and his colleagues have been researching alternatives that involve building caissons on the sea floor to which turbines could be attached.
“The thing with the barrage is that you have to put a huge amount of money in before you get any money back,” he points out.
“You have to invest billions of pounds.”
And if the barrage takes longer to build than expected, economic payback is deferred.
With the caisson-type structure advocated by Loveless, a wave-energy device could be mounted on top and, above that, wind and solar devices could be deployed to capture more energy.
Enthusiasm for the Severn Barrage is by no means overwhelming, and Professor Simon Haslett, head of the department of geography and assistant head of the School of Science and the Environment at Bath Spa University, is one of the doubters.
Haslett spoke at an Atlantic Geoscience Society conference in Halifax, Nova Scotia, earlier this year and found that the Canadians have little enthusiasm for the barrage concept, based on their own experience of the Annapolis Royal barrage in the Bay of Fundy, which was built in 1984.
Although it generates 30million Kilowatt hours of electricity a year, the barrage has been blamed for river-bank erosion.
“For the Canadians, the idea of a barrage is now history and doesn’t even get raised as an option during tidal power debates,” Haslett said in a newspaper interview.
“If the Canadians are right, we shouldn’t even waste any time and millions of pounds of taxpayers’ money considering a barrage. Instead, consideration should be given to tidal-stream turbines.”