By Dave Elliott
At a meeting of the House of Commons Liaison Committee, which brings together the chairs of select committees, PM David Cameron in effect provided an overview of his take on key aspects of UK energy policy. It was quite revealing, with justifications being offered for the extensive cut-backs in support for most low-carbon projects, in order ‘to deliver low carbon at the lowest cost’. Very little seems to have survived unscathed. (more…)
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.’
The effects of the gravitational pull of the moon on the seas, coupled to a lesser extent with that of the sun, provide a significant potential source of renewable energy. There is a race underway to tap into it.
Tidal power extraction technology comes in various shapes and forms. Barrages across estuaries, trapping high tides to create a head of water, may be the most familiar, but free-standing tidal current turbines, working on the horizontal flows rather than the vertical tidal range, are a less environmentally invasive and easier to install option. There are now reputed to be 150 or so tidal current turbine projects of various types and scales underway in the UK and elsewhere, with the most developed being MCT’s 1.2MW Seagen, now installed in Strangford Narrows in Northern Ireland.
Other UK projects include the nicely named ‘Lunar energy’ seabed-mounted ducted rotor, the ‘Pulse Tidal’ oscillating hydroplane system being tested in the Humber, and the ‘Tidal Delay’ system, which feeds power to a heat store, so that power can be generated continuously. There is also a proposal for a permeable ‘tidal fence’ across the Severn estuary, housing a series of tidal turbines, as an alternative to a solid ‘tidal range’ barrage, which would in effect dam the estuary.
There are many more tidal current projects and devices at various stages of development- there is very much a flurry of innovation going on, with tidal technology being seen as basically simpler than wave energy technology. That’s hardly surprising since, with wave energy, we are trying to tap into complex chaotic wave motions in an interface between air and water, whereas tidal flows, running smoothly below the surface, are by comparison much more linear.
Although the UK still leads in this field, challenges are emerging from overseas. For example, Singapore-based Atlantis is planning to install some of their ducted rotor units in a 30MW project in Pentland Firth in Scotland. And Irelands Open Hydro is planning to install a series of 1MW versions of its novel Open Centre turbine in France, Canada- and Alderney. In addition, Voith Siemens have developed a novel gearless 1MW propellor turbine design, which is to be used in a 100 MW array in the Wando project in South Korea. Canada and the USA are also pushing ahead with a range of systems- the US Dept of Energy has allocated its first Marine Energy Grants- $7.3m in all, to 14 projects, with more now being planned.
Cleary tidal power has gone international and there is a race to be first in what could be a very large global market. In the end what will probably decide the winners is economics There is talk of some tidal current devices getting down to 2-3p/kWh in time, so the prospects look good, while there are dark mutterings amongst some of the tidal current enthusiasts about the cost of the main rival approach in the UK- the proposed Severn Tidal Barrage, put by some at 9p/kWh.
The race is on
Although it’s often seen as the front-runner in the UK, the large Cardiff to Weston Barrage isn’t the only tidal range option. There have been proposals for even larger barrages further down the Severn estuary. Alternatively, smaller barrages on the Severn and elsewhere (e.g. the Mersey Solway Firth, Humber etc) might be less invasive, and offshore tidal lagoons even less so.
There are also various different operational options. In terms of increasing the continuity of power output, there is the option of having segmented lagoons, so that some degree of storage might be possible, and it is also possible to pump water uphill behind barrages or lagoons, using excess off-peak power from the grid. Barrage operation on the incoming tidal flow is also an option, although that means using two-way turbines, which are more complex, expensive and prone to excessive wear and breakdown.
Although pumped storage is sometimes included as an option, most new barrage designs use conventional one-way turbines. The problem then is that they will only fire off twice roughly every twenty-four hours, with large pulses of energy for a couple of hours, which may not be matched to electricity demand. So, for example, although the 8.6GW Severn Barrage might be able to generate 4.6% of UK’s electricity, only some of that could actually be used effectively in practice- unless we also spent money on major electricity storage facilities. According to the generally pro-barrage Sustainable Development Commission, by some time after 2020, when it was working, the barrage would only reduce UK emissions by about 0.92% – not very much for £20 billion, the expected construction cost.
By contrast, a network of smaller tidal turbines around the coast could deliver more continuous output, since peak tide occurs progressively later in time at each site. Tidal turbines can also be designed to swivel around to run on the flow and the ebb i.e. four times per 24 hour period. And they can be installed on a modular basis relatively quickly.
All in all, with most UK environmental groups strongly opposed to large estuary-wide barrages like that proposed for the Severn, the tidal current option looks like to one the watch. But who will win in that race is far from clear. The UK may be ahead technologically, but for example, S. Korea has plans for installing a total of around 500MW of tidal current projects.