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Tidal energy and grid balancing

by Dave Elliott

There has been a renewed push for a Severn Tidal Barrage, but, as I have reported in earlier posts, many saw it as too big to fund and too invasive to allow. Dr Nicholas Yates from the National Oceanography Centre, who has carried out the research with a team at Liverpool University, has backed smaller barrages, which he suggested could supply 15% of UK electricity. He  told the BBC: ‘I think it’s unfortunate that attention for tidal range has tended to focus on the Severn, it’s the wrong place to start, it’s too big. Start small, it’s what the Danes did with wind – start small, learn quick and build up.’

It’s not clear if the very big 8.6 GW Severn Barrage would ever be viable economically or environmentally, but a series of smaller barrages, perhaps leading up to larger ones, along with tidal current turbines and possibly tidal lagoons, could be. That seems to be the conclusion that South Korea has already reached- they now have a 240MW barrage, and more of similar size are planned, along with several tidal current turbine projects.

In the UK context, in a review for the Royal Society, Dr Nicholas Yates and colleagues, claim that, together, tidal range and tidal stream options, might be able to deliver 20% of UK electricity Moreover, given that peak tide times lag around the coast, they could collectively provide power on a continuous basis, this facility being further enhanced by operating barrages, and possibly also lagoons, with pumped storage. Yates et al say that pumping water behind barrages could help to ‘enable a degree of power balancing (pumped storage) across multi-scheme systems’.

They looked specifically at small barrages projects on Solway Firth, Morecambe Bay, Mersey and the Dee  Adding in a barrage on the Severn, they concluded that ‘the pulses in electrical output from the northwest and the Severn in ebb-mode are out of phase so that they operate in complementary fashion and extend the daily generation window to nearly 20 hours’.  If barrage schemes on the east coast of the UK, including the Wash, Humber, Thames, and potentially the Forth and Tay, were also included, then, they say ‘partially by reason of their tidal phase lags’, a further extension of the daily tidal energy generation ‘window’ might be made. And, going one step further, a ‘combination of tidal stream farms and tidal barrages has the potential to provide continuous base load electricity generation’.

Tidal current turbines of course don’t offer pumped storage opportunities, adding pumping to barrages and lagoons could further improve the system. It could not only provide continuous power but also help to deal with variable outputs from renewables like wind- storing excess outputs, when demand was low, and balancing the grid when there was a shortfall in wind.

So what is the overall tidal potential? Yates et al concluded that ‘a potential future contribution amounting to 20% of he UK’s present electricity demand from tidal energy was possible’ with ‘a combination of barrages/lagoons (approx. 15%) and present estimates for tidal stream devices (approx. 5%)’. http://rsta.royalsocietypublishing.org/site/2013/1985.xhtml

That is more than has sometimes been claimed in the past, although the tidal current/stream turbine contribution seem surprisingly small.  But Yates et al report that studies had shown that tidal current projects ‘offer only a modest energy capture potential compared with the tidal range (barrage) solutions within the Irish Sea’. However surely that depends on how many tidal current projects you go for? Then again, in terms of balancing overall tidal generation, as I noted in my last post, if you go for very large project e.g. in Pentland Firth, where there is a huge resource, that would dominate. This may be fine in total energy terms (the more the merrier), but it would not add much extra to the potential balancing amongst geographical scattered sites from tidal time delay. Ideally you want lots well spread out, not one big site. It is the same problem with the large Severn Barrage. Only a small part of its output could be matched, at other times, by other sites

However, a distributed network of tidal current turbines does sound possible, although that may be some way off.  Apart from the 1.2MW Seagen, what is available at present still mostly at the demonstration stage. As Prof. AbuBakr Bahaj from Southampton University put it his review for the Royal Society, ‘unlike wind energy, there are currently various designs being promoted, with no single device design emerging as a winner so far. In addition, there are many aspects of the technology requiring various optimization and design improvements, as well as in situ experience in operation and maintenance.’  He noted that ‘small scale research and development studies are underway that are geared to inform optimized site array designs, including consideration of wakes, the influences of turbulence and their impact on device spacing’.

Nevertheless, the potential is there for expansion, when the technology has been developed fully. The UK has some of the best sites for tidal current projects. Scotland has 25% of the available European tidal potential – Pentland Firth and Orkney Waters contain 6 of the top 10 UK tidal site and recent studies have suggested that the potential UK resource may actually have been understated by perhaps a factor of 10. It is now thought to be over 100 TWh, nearly 30% of UK electricity requirements: http://www.offshorevaluation.org

Moreover, the UK Carbon Trust has said that, with continued targeted innovation, by 2025, ‘the UK’s best marine energy sites could generate electricity at costs comparable with nuclear and onshore wind’ : http://www.carbontrust.co.uk/news/news/press-centre/2011/Pages/MEA.aspx

Note I tend to call them ‘tidal current turbines’, some others call them ‘tidal stream turbines’. It’s the same thing! But we need a different term to cover non-tidal ocean currents/streams! Which are ultimately solar driven. And you also have to watch out for the use of the phrase  ‘tidal waves’: which are neither tide or wave related. They are rapid flows in seas due to seabed earthquakes, nowadays, tragically after some awful disasters, more usually called tsunamis. Needless to say, they have no potential as energy sources.

* While tidal power is much in the news, sadly, Wavegen’s Isle of Lewis wave energy causeway Oscillating Water Column project was halted due to financial concerns, Irelands Wave Bob project has also gone bust and E.ON has withdrawn from the Pelamis Orkney project. But there are plenty of other wave energy projects under development., like Ecotricity’s bicycle pump-like ‘Searaser’ piston device, which, in one version would  pump water up to a reservoir on-land  ready for generation when power is needed: so it includes an energy storage option.

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