Wednesday, February 18, 2009

Don't get fooled again

Tidal Choices

Don’t get fooled again

We need tidal power- but not this way

There are two basic approaches to harvesting tidal energy. Barrages across a tidal estuary can be used to trap high tides, with the head of water then being let out through hydro turbines. This is how the 240MW barrage on the Rance estuary in France works and it is the basis of the proposed 8.6GW Severn Tidal Barrage. By contrast, with tidal current turbines, the energy in the horizontal tidal flows is harvested - in effect they are underwater wind mills. Proponents of the latter argue that, as small free-standing structures with relatively slowly rotating blades, they should have much less environmental impact than tidal range barrages, which block off entire estuaries, and they can be installed on a piecemeal, modular basis. The same is true for wave energy systems.

Unsurprisingly the Severn Barrage is strongly opposed by most environmental group, who see it as a major ecological threat. Moreover a study produced for WWF/ RSPB et al , claimed that its electricity would cost around two times more than that from most other renewable projects.

There are also some key technical issue. A large single barrage on the Severn would only deliver power for a couple of hours twice in every (roughly) twenty four hour tidal cycle, and given that the lunar cycle shifts continually, this power output would not often be matched well to peak power demands. As a result, at least in the absence of major energy storage facilities, although it could generate 4.6 % of the UK’s electricity on average over a year, not all of it could be used. A study by the generally pro-barrage Sustainable Development Commission, concluded that, by the time it was built, the Severn Barrage would only displace about 0.92 % of UK emission from gas fired plants- not much for the estimated £15billion capital cost.

By contrast it has been argued that a distributed network of several smaller tidal barrages or lagoons and /or large numbers of tidal current projects, located at different points around the coast, would be much more efficient and flexible, since the arrival time of the tides is delayed by several hours at each point.

However there is much industry support for the large 8.6GW Severn Barrage, since it uses known (hydro) technology. Moreover, it also has strong political support. It would clearly be a very visible commitment for a government keen to be seen to be supporting renewables.

The government has launched a new study of tidal energy options for the Severn Estuary, looking at large tidal barrages across the estuary and large tidal lagoons- bounded reservoirs. One of the issues is how such schemes might be funded, with some form of public support being a possibility.

However, apart from a 1.3 GW ‘tidal fence’ concept, with tidal current turbines mounted in a non-invasive permeable causeway, tidal current turbine projects are not included in the study- and even tidal fence has been dropped from an interim shortlist. Tidal current projects are evidently seen as being best developed by private finance within the context of the Renewables Obligation, with some initial extra support via Marine Renewables Deployment Fund. But as noted in an earlier Blog, that’s not happening yet- none of the dozens of projects being developed in the UK have proved to be eligible for the funding. Meanwhile the Severn Tidal Review won’t even report finally, after further rounds of consultation, until 2010. It’s all so painfully slow…

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