Engineers have eyed the Severn estuary as a potential source of energy for more than 150 years. With the second highest tidal range in the world, the 40-mile-long stretch of water, which runs from the second Severn crossing, in South Gloucestershire out into the Bristol Channel, is home to the lion’s share of the UK’s tidal energy resource.
Over the past century, several studies have looked in depth at the possibility of generating electricity from the estuary, but each one has concluded that a project would either be too expensive or too environmentally destructive. Research shows that building a barrage – a huge concrete dam – across the estuary could reduce the amount of unique intertidal habitat available to wildlife by up to 65 per cent, with a serious impact on biodiversity. The area is designated as both a Special Protection Area under the EC Birds Directive and a Special Area of Conservation.
In 2006, the Government asked the Sustainable Development Commission (SDC) to look again at the problem. The SDC reported back in 2007, saying that it was possible to generate energy from the Severn estuary, but with strict conditions. These included finding or creating compensatory habitat to replace that lost as a result of a barrage project, making sure that EU directives on habitat and bird-life were met, and a recommendation that the project be publicly run – not a private-sector initiative – to avoid short-termism.
Dizzy with the thought of being able to hit its EU renewable energy targets, the Government announced in July this year that it would plough ahead and examine 10 different proposals for harnessing the power of the estuary, with a shortlist to be drawn up in the coming months. Over these pages, we examine some of the more promising and potentially less environmentally destructive alternatives to the favoured Cardiff-Weston barrage.
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Cardiff-Weston barrage
The most well-studied option and often referred to simply as ‘the Severn barrage’, this proposal would see nearly 10 miles of concrete and rock infill stretch between Lavernock Point, Penarth, west of Cardiff, and Brean Down, near Weston-super-Mare. The barrage would allow the tide to flow up the estuary through sluice gates, then shut the gates and wait for a water level difference of up to six metres. It would then release the water through 216 turbines, generating more electricity than any other single power source in the UK.
That power would come at a cost, however: the barrage would flood valuable intertidal habitat upstream and reduce the tidal range of the estuary overall by 11 per cent, affecting the 65,000 birds that overwinter there. Sediment would be churned up and deposited upstream, fish could be killed by the turbines and ships would have to use vast locks built into the structure to travel up-river. It is opposed by most of the major environmental groups.
For more information, see
www.sd-commission.org.uk
• Terawatt hours of electricity produced each year = 17
• Output as a percentage of UK electricity = 4.2 per cent
• Cost per kilowatt hour = 3.6-22.3 pence/kWh
• Size = 15.9km in length
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Tidal fence
Unlike a barrage, the fence would not ‘dam’ the estuary, but instead use a string of underwater turbines to generate electricity from the natural flow of the tide. There would be much less of an effect on the estuary’s tidal range than with a barrage, and both ships and fish could pass easily. The fence might also generate electricity at more useful times of day than a barrage, though it would produce a fraction of the energy of other proposals.
For more information, see
www.senergyworld.com/
• Terawatt hours of electricity produced each year = 4
• Output as a percentage of UK electricity = 1 per cent
• Cost per kilowatt hour = (uncosted)
• Size = 9km in length
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Tidal reef
Halfway between a barrage and a fence, the tidal reef would see an array of floating turbines connected to concrete bases and strung between Aberthaw, Wales, and Minehead in Somerset. The brainchild of veteran hydro engineer Rupert Armstrong-Evans, the reef is estimated to require 10 million tonnes less rock infill than a barrage, and, because it would operate with a water-level difference of just two metres, is predicted to have a smaller impact on the estuary’s tidal range. Fish should be able to navigate the slow-turning turbines, and ships may be able to sail through ‘doors’ in the reef, rather than using slow and expensive locks. Low-tech components could make the project cheaper than the barrage, and the electrical output would be smoother and more constant.
For more information, see
www.evans-engineering.co.uk
• Terawatt hours of electricity produced each year = 20
• Output as a percentage of UK electricity = 5 per cent
• Cost = (uncosted)
• Size = 20 km in length
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Tidal lagoons
Lagoons are artificial enclosures constructed in the estuary with turbines built into the walls. They operate on both the ebb and flow of the tide, as the water fills, and then empties from, the lagoon enclosure. Because the lagoons wouldn’t block the estuary in the same way as a barrage, they would allow the passage of fish and ships without the need for ladders or locks.
Having lagoons may also reduce the problems associated with a build-up of sediment. There are fears that the ‘channel’ created between the three proposed lagoons would lead to faster currents in the estuary, however, which could bring other problems.
The lagoons would also require much greater quantities of aggregate ‘filler’ to build than the barrage. The proposal is supported by Friends of the Earth but opposed by RSPB.
For more information, see www.tidalelectric.com
• Terawatt hours of electricity produced each year = 6.5-24, depending on scale
• Output as a percentage of UK
electricity = 1.6-6 per cent
• Cost per kilowatt hour = 2.0-26.72 pence/kWh
• Size = a minimum of 82.2km of embankments
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Shoots barrage
A small barrage just downstream from the current M4 Severn crossing. A barrage in this location was originally proposed in 1933 by Lord Brabazon, and has been revived as an opportunity to generate electricity as well as to provide a high-speed rail link from England to Wales.
Less than half the size of the Cardiff-Weston Barrage, the Shoots barrage would be less environmentally destructive, could be built in five years and would be compatible with other technologies, such as tidal lagoons.
It would harness only a tiny amount of the estuary’s available power, however, and would still be far from environmentally benign: projections show that the barrage could lead to increased concentrations of certain metals upstream of the barrage, high levels of siltation and lower levels of dissolved oxygen in the water, which could affect wildlife.
For more information, see www.sd-commission.org.uk
• Terawatt hours of electricity produced each year = 2.75
• Output as a percentage of UK electricity = 0.7 per cent
• Cost per kilowatt hour = 3.03-15.4 pence/kWh
• Size = 6.5km in length
This article first appeared in the Ecologist September 2008
