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Hidden powers

Renewable energy Tidal power

Clean, green renewable and, best of all, invisible. David Taylor reports on north Devon's tidal power experiment.

All too often wind energy schemes never get off the ground, beaten by planners and fierce opposition from local residents. This was the case in North Devon only last year, where two wind farm projects failed to get planning consent.

But even as the local campaigners were celebrating their hard-fought victory, another renewables project was getting the go-ahead right under their noses.

The successful scheme is a lot smaller than the two defeated wind farm projects - indeed, it is not a full scale power project at all but an experimental one. It will be located in the Bristol Channel, 1km off the coast at Lynmouth, and it will be underwater.

The organisation behind this novel scheme is Marine Current Turbines (MCT), a Bristol-based company set up by renewables consultancy IT Power with backing from the European Commission, the UK government and a consortium of industrial partners. The idea is the brainchild of MCT technical director Peter Fraenkel who saw the power of tidal streams as a possible solution to the technical and practical shortcomings of both wind turbine and tidal barrage power generation.

MCT's idea is to use the relentless power of tidal currents in much the same way as wind turbines use air currents.

The rotors, which are long and slender like their wind-powered cousins, will be mounted on large diameter monopiles on the seabed and will be turned by the out-going and incoming tides: the rotor blades will be reversible to exploit flow in both directions.

'The potential rewards are much greater than with wind power', says Fraenkel. 'For a start, the energy intensity of a marine current is at least four times that of wind. An efficient offshore wind turbine can expect to achieve up to 1.5MW/h per square metre of rotor blade per year, whereas we expect to get about 6MW/h from our marine turbine'.

Another benefit of using marine currents is that tides are predictable and reliable; wind is not. 'Tides also run in only two directions whereas wind often changes direction, ' says Fraenkel. This has a practical impact on the siting of adjacent turbines. Because of the down-wind turbulence caused by the revolving rotors, and because the wind can blow from any direction, wind turbines have to be spaced well apart to prevent them interfering with each other's efficiency.

But MCT's underwater turbines can be sited directly side by side across the current and never interfere with each other.

In fact, says Fraenkel, the closer together they are, the more efficiently they use the tidal flow.

The £2.3M trial project off Lynmouth is phase one of a larger scheme. It will comprise a single turbine, with two 5.5m fixed blades which will operate only on the outgoing tide. 'The main purpose of this phase is to get hands-on experience of a full-scale installation, ' Fraenkel says.

'One of the difficulties we have to overcome is the variation in static pressure acting on the rotor blades. With a diameter of 11m there's a difference in water pressure of 0.5 bar from the top to the bottom of the rotation and that imposes a large fatigue load on the blades.' The composite carbon fibre and GRP rotor blades have to withstand not only fluctuating static water pressures, but also a peak thrust of 27t on an outgoing tide.

The 2.1m diameter pile absorbing all this force is made by Cornish marine piling specialist and 40% MCT stakeholder, Seacore. 'The seabed is basically loose rock and cobbles. We thought we'd find bedrock after about 1.5m, but we took core samples and found it's pretty poor quality. We'll have to use a sleeved pile, which rather surprised us, ' says Fraenkel.

The pile will have to be driven up to 18m into the bed. The main challenge for Seacore will be the enormous 9.8m tidal range in that part of the Bristol Channel: piling work will only be possible at neap tides, and will require good weather.

INFOPLUS www. marineturbines. com

The rotor and turbine assembly is designed so that installation and maintenance can all be done from the surface. The whole turbine assembly will be fixed to a sleeve that slides up and down the pile, the top of which projects a few metres above the high tide mark. This will house a hydraulic system with which to raise and lower the turbine. Installation is scheduled for completion early next spring. If successful, the plan is to construct a twinrotor 1MW commercial plant on the same site.

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