Renew On Line (UK) 29

Extracts from the Nov-Dec 2000 edition of Renew
These extracts only represent about 25% of it

Time for Tide?

The Severn Tidal Barrage idea seems to be back on the agenda- or at least that's the hope of the Severn Tidal Power Group, the industrial consortium who have been trying to promote it for years. With the nearby Hinkley Point MAGNOX reactor set to be closed (see later), it has been argued in the local press that opportunity exists for new generation capacity in the West- and the STPG has pointed out that a 8.6GW capacity barrage would create a lot of jobs in the area, as well as meeting 60% of the UK's target of obtaining 10% of its electricity from renewables by by 2010.

With the deployment of on-land wind power slowed, offshore wind still stalled and energy crop projects only growing slowly, it certainly might be tempting to think big and go for the Barrage. As we reported in Renew 127, the Royal Commission on Environmental Pollution included the barrage in its scenarios for the longer term future. It saw the barrage as supplying an average power equivalent of 2.2 GW. But for the moment the main problem is the capital cost, around £10 billion. That's what stalled the idea last time it was looked at seriously - in the late 1980's- just at the point when the electricity industry was about to be privatised. Whereas it stood some chance as a publicly funded project, private companies would find it hard to fund a major project like this- the pay back time is too long.

And then there's the likely opposition of environmental groups to the potential local eco-system impacts - something we explored in detail at the time. While wildlife interests were implacably opposed, in fact not all the impacts would necessarily be adverse. For example, the reduction in tidal range behind the barrage would have an impact on some wading birds, since the area of regularly exposed mud flats would be reduced, thus reducing their feeding grounds. But at the same time, the reduced tidal movement would allow suspended silt to drop out of the water, so that light would penetrate more deeply and increase the estuaries biological productivity: other species might therefore thrive. The key point is that the balance amongst species would probably shift - and, for all but the 100% preservationist, it’s hard to judge whether that's good or bad.

Even so, most environmentalists shudder at the idea of building a giant invasive dam across the Severn estuary, and certainly, one way or another, these days giant projects are out of favour, whereas small dispersed locally embedded projects are all the rage. A network of smaller scale barrages - or, more likely, modular offshore tidal stream technology (see our Technology section) - looks like a much better bet than the giant 12 mile long Severn scheme. After all, in the absence of large scale energy storage capacity, if we are seeking to obtain as nearly continuous output as possible from devices using intermittent or cyclic energy sources, then it's better to rely on a range of small geographically dispersed generators rather than a single large generator. What use is 8.6GW from the Severn in the middle of the night? And, at other parts of the lunar phase (e.g. neap tides), no power at peak demand times?

Of course, as and when large scale energy storage capacity is developed, then the Severn scheme might come into its own - one longer term option being to use the barrage output to power the generation of hydrogen gas by electrolysis, and then feed the hydrogen to the gas grid. However, the environmental factors would probably still tip the balance away from Barrages of whatever size, in favour of tidal current devices - the overall resource potential for barrages and tidal currents is roughly the same in the UK. But then, if the Climate Change problem is a bad as some people think, we might need both.

Contact: STPG, McAlpine Ltd, Eaton Court, Maylands Avenue, Hemel Hemstead, Herts HP2 7TR

* The US company Tidal Electric have announced plans for two bounded reservoir tidal schemes in Wales, with enclosed basins constructed in the sea to trap tides. See Renew 117. If funding can be found the aim is to build a 30MW scheme off the coast at Swansea and a 400MW scheme off the coast at Rhyl. More details in Renew 129. Meanwhile see http://www.tidalelectric.com