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Tidal power – and energy storage

By Dave Elliott

Tidal energy is developing relatively slowly, but has a significant potential. Although two large tidal barrages now exists  (with 240 MW units in France and South Korea), the emphasis is on free-standing tidal current turbines, which  harvest the horizontal flow of the tides, rather than trapping tide rises behind dams, as with barrages. So the environmental impact is likely to be much less. They can also be installed relatively quickly, one by one, so reducing project finance problems. The 6th Tidal Summit, organised by TidalToday in Nov 2012, reviewed the scene, with a key issue being the need to get costs down. DECC said they has to get to down below £100/MWh by around 2025.

Some good progress has been made, with the UK still well in the lead. Around 11MW is installed or in the planning/consent phase in UK waters. Next up were the 9 MW Siemens/MCT projects planned for Kyle Rhea (Scotland) and Skerries (Wales).  But it was a relatively slow business, with many devices still at the test and demonstration stage.

The European Renewable Energy Action Plan target was 2.1 GW by 2020, delivering 6 TWh, p.a, with 1.3 GW in the UK, 380 MW in France, 250 MW in Portugal,100 MW in Ireland, 75 MW in Spain and 3 MW in Italy. At present site for around 1 GW have been agreed in the Pentland Firth area, but the Carbon Trust had claimed that the total UK Tidal Current resource could be 8 GW delivering 21 GWh/y, an estimate backed by at the Summit by Vattenfall. Whereas at the moment ministers are talking of only 200–300 MW being installed by 2020.

The UK and the EU are not the only locations where tidal current devices have significant prospects. The tidal resource in S. Korea is said to be very large, and there are major projects going ahead, including one backed by Voith. Kawasaki Heavy Industries also unveiled their new 1MW device at the Summit. The USA is also taking tidal stream power seriously. For example , as reported at the Summit,  the US Dept. of Energy has invested $10m in Ocean Renewable Power Company’s novel horizontal axis TidGen unit. In Sept 2012 it delivered power from its test site in the Bay of Fundy. Next a series of units are to be installed, with a total capacity of 3MW, over the next 4 years. With many other projects also underway e.g. in New Zealand, and Open Hydro’s 10 MW French project, 2013 looks like being a good year for tidal power. www.tidaltoday.com/

Interestingly there was no mention of tidal barrages: the tidal stream option is evidently seen as far preferable, less invasive and less costly. You could of course use tidal barrages or tidal lagoons for pumped storage, which might give them an edge, but if you wanted to use a tidal barrage or lagoon for pumped storage, you would have to make its containment walls higher (and stronger) to carry the extra height of water at full tide, or else not use it for storage at high tide. Nevertheless, it has been seen as an option, possibly using multi-segmented reservoirs: see Prof David Mackay’s 2007 paper ‘Enhancing Electrical Supply by Pumped Storage in Tidal Lagoons’: www.inference.phy.cam.ac.uk/sustainable/book/tex/Lagoons.pdf

Lagoons build in the sea, rather than in estuaries, have also been proposed, but storing wind-derived energy. For example, the Danish Green Power Island concept would store power produced by offshore wind turbines. The island pumped hydro storage system could take excess energy from the equivalent of 15 hours production of 30 5MW wind turbines. The retaining embankment could also be used for biomass production, with possibly marine biomass (algae) also being produced in the lagoon water. It has also been suggested that it could have floating PV arrays. But no tidal energy. www.greenpowerisland.dk

One advantage of reservoirs like this is that, unlike tidal barrages or tidal lagoons in estuaries, the locations of which are defined by geography, there are many coastal sites where you could have sea filled lagoons, as well as many sites where reservoirs could be built onshore on elevated land, filled from the sea, lakes or rivers. Pumped storage is of course well established for convention power use. One of the largest schemes is the 1.87GW Ludington plant on the shore of lake Michigan www.consumersenergy.com/content.aspx?id=1830

However, some wave power devices also make use of pumped storage, directly pumping sea water up hill to a reservoir on shore, for subsequent power generation when needed, and some proposed tidal current turbine systems have also adopted this idea e.g. the Hales turbine.

Then again, storage is only one way to deal with the variability of tidal and other renewable sources. The debate over intermittency has usually focused on wind, but although tides are cyclic, varying with the lunar cycles, since high tides occur at different times around the coast of the UK, it might be possible to get more nearly continuous power from a series of tidal plants sited appropriately.

A 1994 study looked at possible mutual balancing of outputs from UK tidal barrages. With 52 mainly small barrages, operating in ebb generation mode, it found that the overall scheme had a peak power availability in excess of 6GW, but the firm power, i.e. continuously available, output produced by the integration would be approximate 1GW at springs, falling to less than 20% of this level at neap tide. However, a scheme based on 36 barrages, including on the Severn and the Wash, could achieve a peak power of 12GW with a firm power capability of not less than 4GW at springs, provided that operation of the barrages included the flood, two-way and variable head systems of generation. Although less at neaps. W. Watson ‘Firm Power’NATTA/OU 1994.

A more recent study looks just at West Coast sites: www.sciencedirect.com/science/article/pii/S014111870900090X  I will be covering that work in my next post.

What about tidal current turbines? There have been several studies of the timing of tidal stream sites: e.g. G. Sinden ‘Variability of UK marine resources’. Carbon Trust 2005: www.carbontrust.com/media/174017/eci-variability-uk-marine-energy-resources.pdf Sankaran Iyer, et al ‘Phasing of tidal current energy around the UK and potential contribution to electricity generation’, Supergen report. For a good short overview by Simon Waldman, looking at EU sites as well, see : www.firecloud.org.uk/linkedin_pubs/110925_diss_summary.pdf

The latter concludes that UK tidal stream sites could provide a continuous output of approximately 27% of their maximum output.  The main problem seems to be that the very large potential of the Pentland Firth dominates, so out of phase infill from other smaller sites can’t compensate. It was almost the same with barrages- the Severn dominated. Nevertheless, a network with tidal and also wave sources feeding in, could help compensate for the variability of wind e.g. waves are in effect stored wind, persisting for some while after the wind has died down. A 2009 study by Redpoint for the BWEA found that, for a  notional 25GW programme, a mix of 60% wind and 40% wave/tidal current and tidal range reduced the need for fossil backup by 2.3GW. With 70% wind, and 30% tidal current, the reduction was 2.15GW.

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