A consortium of UK engineering firms is proposing a barrage across the Severn estuary that could, it is projected, harness tidal energy to generate double the output of a nuclear power station.
When Tony Blair signalled last month that he would approve a new generation of nuclear power stations it was seen by many as a kick in the teeth for renewable power. Why bother, some asked, with the incremental benefits offered by wind turbines and solar cells when a few new nuclear power stations can supply all our needs and more? But while the ability of nuclear power to fulfil our spiralling demands is undisputed, the renewables ‘death knell’ is both premature and inaccurate. A consortium of UK engineering companies is proposing a multi-billion pound renewable project that, it claims, could generate 17TWh a year, meeting five per cent of our annual electricity demands.
The Severn Tidal Power Group (STPG), a consortium of engineers from Sir Robert McAlpine, Balfour Beatty, Taylor Woodrow and Alstom, is pushing for a 10-mile concrete barrage or dam that will stretch across the Severn estuary from Lavernock Point near Cardiff to Brean Down near Weston-super-Mare in Somerset. Claimed to be capable of double the output of a nuclear power station, the proposal is a fine example of how renewable power could compete in scale with nuclear. The group claims that the structure will exploit the unique tidal conditions of the Severn estuary, where a difference of 14m between high and low tides gives it the second highest tidal range in the world.
It’s such a big project that the government has to carry it forward. Projects like this have to have very strong political support, otherwise the planning process just doesn’t work
Roger Hull, Severn Tidal Power Group
The barrage would create a basin that will fill on the rising tide, impounding the water as the tide falls and then letting it rush out through a series of power-generating turbines. The engineering plans for the barrage actually date back to 1989, when the project was shelved around the time that the electricity industry was privatised. However, STPG spokesman and Sir Robert McAlpine engineer Roger Hull told The Engineer that the time is now right for the scheme to be put back on the table. STPG’s proposal recently won the backing of both Rhodri Morgan on the Welsh Assembly and Peter Hain, secretary of state for Wales. Following a presentation to DTI energy minister Malcolm Wicks, the consortium is waiting to find out if the government’s impending energy review will help complete the scheme’s resurrection. Hull said public support is essential for the proposal to get off the ground. ‘It’s such a big project that the government has to carry it forward. Projects like this have to have very strong political support, otherwise the planning process just doesn’t work,’ he said.
The barrage will be made from a series of huge caissons (concrete blocks that are common in underwater construction). Some of those could contain turbines to let water out and generate electricity, some would contain sluices to let the water in and some would just be blank. Hull said the caissons could be constructed in the UK’s old oil rig construction yards and towed to the site, where they would be winched into position, flooded and then sunk. Clearly, with the Severn estuary carrying a fair amount of shipping, provision must be made for the large boats that travel in and out of Bristol docks. This would require the construction of huge locks, similar to those used on routes like the Panama Canal. Hull conceded that mariners might not be happy about negotiating these locks, but claimed ships inside the barrage will enjoy considerably higher low tides, significantly reducing the risk of grounding. Construction of the dam is expected to take around 10 years and cost £12-15bn, and while the project would represent a huge feat of engineering, it would be underpinned by proven technology.
Most comparable is a smaller tidal barrage that has been operating in the Rance estuary near St Malo, France for 40 years. Water would surge through the barrage on the rising tide. The sluices would then be shut at high tide and the water level outside the barrage would drop as the tide goes out. When a suitable differential in water level is achieved, water would be released from the basin through the turbines, reducing the tide within the barrage from around 14m to 7m. The scheme has provoked some much publicised criticism from environmentalists who fear that it would destroy the habitat of many of the rare wading birds that live on the mudflats upstream of the barrage. The STPG accepts this. The low tide within the basin would, Hull explained, be about the same as the current mid-tide and around half the mudflats upstream of the barrage would be permanently lost. But while some habitat would be lost, the barrage would, he claimed, create some benefits for the Severn’s ecosystem.
The estuary is currently a very harsh environment, with the fast-moving tides scouring the bed to such a degree that there’s an enormous amount of sediment in suspension and relatively little marine life. Hull argued that a barrage would create a more attractive environment for marine organisms: ‘There will be less sediment in suspension. Therefore more sunlight will penetrate and more marine organisms will grow, creating a better food chain leading to more life there.’ With the scheme also offering a degree of flood protection for the areas upstream of the barrage, Hull claimed that the case for the project is hard to dismiss. ‘The benefits seem to outweigh the disadvantage — which is that a few birds would have to move from this area to that area. On the face of it, it’s a project with many more benefits than disadvantages,’ he said. But not everyone agrees. Peter Ullman, chief executive of UK tidal energy company Tidal Electric, claimed that the environmental impact of a barrage would not be limited to its effect on the local bird population.
Ullman’s company, which has gained the support of Friends of The Earth, is championing the development of so-called tidal lagoons, offshore structures made from natural materials and installed with conventional low head hydro generating equipment that creates power when the lagoon fills and empties at high and low tides. Ullman suggested that STPG is deliberately trivialising the likely environmental impact of a barrage across the Severn. ‘What you see in the press is that it’s all down to birds and surfers,’ he said, referring to claims that a barrage would spell an end to the world-famous Severn Bore. ‘It’s true that you eliminate the mud and some habitats, but some much more significant things go on. You change it from salt water to sweet water — and every single fish, bird or plant in it either adjusts or dies.’ He also claimed that the barrage will prevent waste from around 18 upstream sewage outfalls from finding its way into the ocean, thus creating ‘the smelliest body of water in the UK’. STPG’s Hull refuted this suggestion: ‘The barrage will not trap any water for more than a few hours. Sea water will enter and leave the barrage basin twice a day, so any waste or sewage will be swept out to sea, as happens now.’ Ullman also takes issue with STPG’s claim that a barrage would help with flood control.
While it would certainly help prevent flooding upstream of the barrage, he said that the water not making it through the barrage on the flood tide would create an increased flood danger downstream. ‘If you increase the amount of water every tide you dramatically increase the coastal erosion throughout the whole area downstream of the barrage. STPG knows this, but the strategy is to trivialise the environmental issues,’ he claimed. Ullman’s group is working on a project to construct a 60MW tidal lagoon one mile offshore in Swansea bay.
According to an independent study carried out by engineering consultancy WS Atkins, this 5km2 structure would be able to generate electricity at 3.4p per KWh. The company is undertaking the scheme with renewables consultant West Coast Energy and Associated British Ports (ABP) which has been helping with sediment transport monitoring. According to Ullman, this is a key environmental issue for any ocean technology. ‘Every time you put something in the ocean you change it and two things could go wrong. Because you’ve changed the currents you either put sediments where you don’t want them or take them away from where you do want them — either scouring beach sands or dumping sediment into shipping channels.’ He said that he expects the Swansea bay application to avoid these problems thanks to its carefully chosen position in the stillest area of the bay. However, Ullman claimed that the primary environmental impact of a tidal lagoon will be that it will add habitat and enhance biodiversity. ‘Within a very short time you’ll have a new wildlife habitat,’ he said.
With the scoping study complete and the project awaiting the go-ahead from the DTI and DEFRA, Ullman is eyeing a number of other likely UK locations for his technology, and among these few are more promising than the Severn estuary. ‘The Severn barrage impounds 185m2 of the estuary,’ he said. ‘To create the same output it would take about 46m2 using a tidal lagoon — the efficiency is much higher because we can generate in two directions. Economically it’s a major advantage.’ Plus, while a tidal barrage would be a public works project, Ullman said that lagoons could be built privately. ‘We have had no public money and don’t expect or require any. The Severn barrage the UK taxpayer will pay for, Tidal lagoons, I’m going to pay for.’
But STPG’s Roger Hull doesn’t agree: ‘Tidal lagoons cannot generate more power than the barrage. It’s a very sound concept but it doesn’t scale up — the amount of power you can generate depends upon the tidal range and how much water you get through your turbines. So if you build something in the Severn estuary, to get the same power as the Severn barrage you would have to trap the same amount of water . The Severn barrage would do that with a 10-mile barrage making use of about 140 miles of coastline to complete the basin. ‘To generate the same power a tidal lagoon would have to provide the whole perimeter of the basin, so it seems quite expensive. They rely on the fact that their lagoons would be built almost entirely in what would be very shallow water at low tide so the mudflats are unaffected — I don’t believe there’s that much shallow water.’
Hull also disputed Ullman’s claim that using both the ebb and flood tides generates more power: ‘Generating in both directions does not produce more power. This is a fact, not an opinion. The barrage could generate on the flood tide as well as the ebb tide, but studies show that more power is generated by only generating on the ebb tide.’ The reason for this, he explained, is that the maximum area of water falling through the maximum height is achieved by trapping the high tide and releasing it to low water level. However, even though ebb and flood generation produces less power overall, Hull didn’t rule out the possibility of looking at whether it would be better suited to the UK electricity market, where power is sold in half-hour slots, and the timing of the power significantly affects its value on the market.
Yet despite the differences in opinion, barrage and tidal lagoons are not mutually exclusive and Hull believes there could be a place for both. ‘It would theoretically be possible to have both schemes in the Severn estuary,’ he said. Ullman concurred, and suggested that while lagoons could be competitive with a barrage, their small-scale deployment would actually be more in tune with his company’s aspirations: ‘We’re not proposing enormous installations, but one-by-one incremental tidal lagoon deployment that will be carefully studied and carefully planned,’ he said.