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Tidal assessment

I have reported in previous blogs on some of dozens of tidal current devices of various types and scale under development in the UK and elsewhere. Some of the projects are now well established, having been fully tested at sea, and some have been deployed at full scale- notably Marine Current Turbines 1.2MW SeaGen. However, most are still at relatively early stages of development, with the claims about potential energy outputs and generation capacities being still being unproven, and some are just speculative design concepts and proposals.

When looking at novel proposals care clearly has to be taken to assess the credibility of the claims being made. For example, generation capacities are sometimes claimed which could only be achieved, given the size of the device, at very high water speeds: the energy output is proportional to the square of the turbine radius (which defines the swept area), and the cube of the water speed. At the Tidal Today.com Tidal Summit last year Peter Fraenkel from MCT showed a chart comparing some existing devices on the basis of swept area. Not surprisingly, MCT devices, being well developed at full scale, came out on top, but some of the other rankings were interesting.

While SeaGen obviously won overall, amongst what might be considered to be the other, albeit less developed, front runners, the double rotor Atlantis AK1000 came out best, followed by Tidal Generations device, while Open Hydro didn’t do well. The now abandoned Stingray oscillating hydrofoil did quite well, as did its, in effect, follow up, the double hydrofoil, Pulse Tidal see-saw design . Hammerfest Strom’s turbine did even better. But then the chart didn’t cover any of the ducted turbine designs, or Voith’s new turbine, rated at 1MW, or the new multi-turbine Hydra Morild II) which is rated at 1.5 MW, which has just been installed for testing off Norway. That will have the largest rated capacity so far.

Projected capacity or claimed energy outputs are of course only part of the issue. What really matters is whether the devices are viable in engineering and economic terms. On the former all we have to go on is their success to date- and perhaps inevitably in some cases there have been problems. For example, the Atlantis AK1000 suffered blade failure and is having to be re-installed at EMEC on the Orkneys, and Open Hydro’s test device, installed in the Bay of Fundy, lost all its blades.

You’d expect problems during development- that’s how technology improves- but some device concepts may be more fundamentally limited. For example, in the new NATTA DVD on Tidal Energy, Peter Fraenkel argues that propeller type devices will always be best, as the wind and indeed hydro industry have clearly shown, and he was fairly dismissive of other approaches. Pulse tidal has claimed that its hydroplane will be better in shallow water, since its swept area is larger than a propeller of similar scale. But then it will only extract energy efficiently in the middle of its up/down traverses, much less at the end of each cycle, so the output may be lower. There could also be problems with multi-turbine designs due to wake interaction between the rotors and rotor efficiency losses with ducted designs.

Clearly there is still some way to go before all the engineering issues have been resolved (by testing at sea) so that we can have a more solid basis for cost comparisons. But MCT is doing very well, and the tidal field is certainly an exciting one - bursting with innovative ideas. Let’s hope for some more successes.

In addition to operational and economic issues, if tidal current devices are to be used on a wide scale then a key issue will be their environmental impacts. Most studies of tidal current turbines so far have suggested that impacts will be low. Even large arrays will not impede flows significantly and the rotor blades will turn slowly, slower than wind turbines, and much slower than the turbines in tidal barrages and lagoons or hydro plants, and so should not present a hazard to marine life- fish will be unaffected. Certainly experience with MCT’s SeaGen has not indicated any problems. For example, no seal deaths have been attributed to the turbines since their installation in 2008. A sonar system has been used to detect the approach of any marine mammals, and shut the turbines down. However all structures put in the sea will have some impact, and this needs to be, and already is, carefully assessed when considering possible locations.

There can also be interactions between tidal projects if they are located near each other. The Energy Technologies Institute (ETI) developing a model of the UK’s tidal energy resources to improve understanding of these interactions. The Tidal Modelling Project will investigate the interaction between tidal energy extraction systems located at different positions around the UK, and how energy extraction at one site might affect the energy available and nature of the tidal energy resources at other sites.

Of course if we have tidal projects sensibly sited at various points around the coast then there can also be positive interactions in terms of overall power availability, since high tide and , maximum tidal flow, will occur at a different times at each site.

The Tidal Energy DVD can be obtained from NATTA: www.natta-renew.org. A short taster is at www.youtube.com/watch?v=nsntWXR63Sc

Posted by Dave Elliott on April 23, 2011 12:58 PM | Permalink