The Severn Barrage: a lost cause?
One time Welsh Secretary of State Peter Hain recently resigned from the Shadow Cabinet to devote more time to promoting the idea of a Tidal barrage on the Severn estuary. He said that it could potentially be ‘the biggest single source of renewable energy in Europe and one of the biggest in the world.’
It would certainly be a large project. The version proposed by the Severn Tidal Power Group in the early/mid 2000’s would have 8.6 giga watts (GW) of turbine generating capacity mounted in a barrage stretching 11 miles or so across the Severn from Lavernock Point in Wales to Bream Down in Somerset and costing over £30 billion. It could generate around 4.6 % of the UK’s electricity. However, it is not clear how much of this could actually be used. The barrage would trap a head of water at high tide and generate power from it on the ebb for a few hours twice each day. But, the tide cycles shift each day and also vary in height over the lunar cycle, so that at times the barrage would deliver full power when there was no demand for it, but at others, when demand was high, it would produce no power at all. Who needs 8.6 GW in the middle of a summer’s night? It is conceivable that at some point in the future we will have a hydrogen economy, so that large slabs of energy like this could be stored, but for the moment the big barrage just looks too big and inflexible.
In a report in 2007 the Sustainable Development Commission calculated that, assuming the barrage’s output offset power from gas fired plants, once built, it would only avoid around 0.92% of UK total annual carbon emissions. You can get far larger emission reductions at much lower costs from almost any other renewable energy option. Add to that the massive potential for negative local and regional environmental impacts from building a structure that in effect dams up this very large estuary, and it is not surprising that the government, in its last review of the project in 2010, decided that it was not worth supporting at this point as a publicly funded project. However it left the door open to the private sector to come up with proposals.
The rejection of the barrage plan and slow progress on other smaller barrage ideas around the country, has had a positive effect in that commercial attention has now been focused on what is arguably a much better concept- smaller independent tidal turbines mounted on the sea bed, running on horizontal tidal flow rather than trying to create a head of water from the vertical rise and fall of the tides. Marine Current Turbines’ 1.2 MW SeaGen unit has be running very successfully since 2008 in Strangford Narrows Northern Ireland, supplying electricity to the grid. They are now planning an 8MW tidal array near Skye in Scotland and 10 MW tidal array off N. Wales. There are many other tidal current projects. The attraction is that they are modular, flexible and have low environmental impacts. In all over 1GW of projects are planned around the UK - in which case, given that high tides occur at different times around the country, the net output from the tidal turbine network as whole would be more continuous than with a single large barrage.
However the Severn estuary - and the big barrage - still attracts enthusiasm. Hain is backing a new proposal for a Severn Barrage project proposed by private consortium, Corlan Hafren (Welsh for ‘Severn Group’), which includes engineering consultancies Arup and Halcrow. http://www.corlanhafren.co.uk/
It would be at the same location as chosen the STPG, but, it’s claimed, would be cheaper and less invasive. Hain says ‘The private finance is potentially in place through the Corlan Hafren business project and they have assembled the necessary financial interests, sovereign wealth funds and investments’. But he adds ‘We will need to get a rather complex hybrid bill through Parliament that’s got to be a private bill - it will need government support.’
Prof. Roger Falconer of Cardiff University has also backed the idea. He estimated the cost of the new scheme at £26bn. The developers reckon they can improve the economics by generating electricity on both ebb and flow tides, allowing greater flexibility in the way it is operated and reducing the likely environmental impact. The total energy output would, they say, be similar to that from the STPG design, but it would be generated over a longer time, with four production periods, thus improving the match with grid demand. The tidal head it produced would also be less, with a larger number of slower turbines being used, so potentially reducing local environmental impacts.
However, the RSPB, WWF and other environmental/wildlife groups, who strenuously opposed the STPG barrage idea, do not yet seem convinced that this new one is much better. For example they fear that the loss of mud flats, although reduced in the new scheme, could still have a major impact on wading birds. By contrast, the supporters argue that the barrage could actually improve biodiversity in the estuary, since, due to the reduction in tidal flow, silt would drop out so that the water became clearer, supporting more species. The barrage would also offer flood/storm protection for sites up steam.
Cutting across this debate, there are more radical ideas for harvesting the energy of the estuary. Friends of the Earth have always promoted tidal lagoons as less invasive options. These would be free-standing self-bounded and segmented reservoirs in shallow water in the middle of the estuary - so they wouldn’t interfere significantly with the tidal flows. DECCs 2010 study looked at variant, in-shore lagoons impounding some of the coast-line. They were not impressed. Neither was FoE - they would have larger impacts on shore/near shore based species.
A completely different low-impact project concept is the SMEC venturi turbine system proposed by VerdErg. This would have series of small venturi pipes mounted in a permeable ‘tidal fence’, the estuary flow sucking water through attached secondary pipes, with these secondary flows being used to drive generators. VerdErg see this approach being applicable at a range of scales. It has looked at Solway Firth as one possible site for a prototype, and also at smaller river sites. But the Severn would be the big one!
There have been many other proposals for tidal fences, tidal reefs, two-way turbines and the like, with some ideas being reinvented. For example, two-way operation is an old idea, discarded since the gains were seen as small (you can’t then take either ebb or flow generation to full completion) and not enough to offset the costs and maintenance penalties of variable pitch turbines. So far however, despite many projects and plans over the years, this is one big one that seems to have got away! Maybe it is just too big.
Frontier Economics Report for WWF, RSPB et al http://assets.wwf.org.uk/downloads/frontiereconomicsbarrage_repo.pdf
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