Plans for a barrage across the Severn estuary emerged in the 1970s. The estuary has the second highest tidal range in the world and could generate 17TWh of electricity per year - around 6% of demand in England and Wales.
Interest grew during the 1980s when the Chernobyl disaster raised concerns about the UK's reliance on nuclear power (ENDS Report 139, pp 9-11). But the Government kicked the project into touch in 1989 after a study by its Energy Technology Support Unit highlighted high capital costs and long construction period.
The Severn Tidal Power Group - a consortium of McAlpine, Balfour Beatty, Alsthom, Rolls Royce, Taylor Woodrow and Carillion - pressed for a reappraisal in 1999, but the then Energy Minister, John Battle, declined.
The following year, the Royal Commission on Environmental Pollution recommended that tidal barrages be kept under consideration as an option for reducing carbon dioxide emissions (ENDS Report 305, pp 19-22 ). In response, ETSU commissioned a report from the developers.1The report says that the 8.6GW barrage would avoid the release of 4.6 million tonnes of carbon per year - about 3% of current UK emissions - on the assumption that it displaces coal-fired generation.
The developers claim that the scheme's viability has improved since 1989 due to the introduction of carbon markets, the potential value of the barrage for flood defence and a reduction in the cost of capital. They are hoping that the imminent energy White Paper will back the project, and are calling on the Government to fund a £500,000 reappraisal of the scheme.
The consortium claims that the barrage could be operational by 2014. It puts the capital cost at £10.1-14.0 billion, giving a "reasonable prospect" of achieving generating costs of £60/MWh in 2001 prices for an equity return of 15%. If the developers met only half of the grid costs the figure would fall to £55/MWh.
The cost remains significantly above that of some other large-scale, low-carbon technologies. Last year, the Government's Policy and Innovation Unit put the cost of offshore wind and nuclear in 2020 at £20-30/MWh and £30-40/MWh, respectively.
However, the consortium argues that the barrage could be financed by the private sector provided the Government recognises the value of "non-energy benefits". In addition to earning £20/MWh from electricity sales, plus a premium of £7/MWh to reflect the predictability of the barrage's output, it is looking to two further sources of income:
Roger Hull, McAlpine's development manager, stressed that the report is only a definition study. He hopes that a subsequent report will explore project finance options in detail. Use of the barrage as a rail or road link could also boost the economic case.
The Severn estuary is a Site of Special Scientific Interest, a Special Protection Area under the EU Directive on bird conservation, and is also designated under the international Ramsar convention on wetlands. The developers are currently fighting a proposal to designate the estuary as a Special Area of Conservation under the EU Directive on habitats.
The Countryside Council for Wales is "unlikely" to support the barrage, according to head of maritime and earth science Maggie Hill. She said that under both Directives the barrage could only be built if there was an overriding public interest - and even then, the developers would have to prepare mitigation plans, including creating new habitats to compensate for those lost or damaged.
Ms Hill pointed out that creating compensatory habitats had proved difficult even for relatively small projects. A new habitat created at Gwent Levels to compensate for the destruction of an SSSI by the Cardiff Bay barrage was of a different type - wet grassland and lagoons rather than tidal mudflats.
"I don't see how habitat compensation can be done on that scale," Ms Hill said. "There's a real practical difficulty which hasn't been addressed in the report at all."
The consortium's report acknowledges that the project may cause some habitats to be "changed". But it claims that the barrage may protect others which would otherwise be destroyed by increased coastal erosion driven by climate change. It also proposes that predictions of habitat loss from climate change should form the baseline of any environmental assessment - although these may be extremely difficult to make in practice.
The technology is being developed by Tidal Electric, working with engineers WS Atkins. It involves constructing an enclosed lagoon in a shallow tidal flat.
Tidal Electric hopes to build a 30MW station in Swansea Bay by 2006, followed by a 432MW plant at Rhyl Sands within two years, according to company secretary John Lyon. The firm puts the UK potential for tidal lagoons at some 8GW.
The credibility of tidal lagoons was boosted last year by a report from AEA Technology. This concluded that they could compete on price with conventional generation technologies and yield high rates of return under the renewables obligation.
FoE believes that a small number of large lagoons in the Severn estuary could approach the output of the proposed barrage - but would cover about half the area and avoid the most ecologically sensitive sites.