Knotty problems for sea windmill

The challenges facing Britain’s bid to build the first under- water ‘windmill’ to generate electricity from tidal currents are daunting. The sea is not kind to big electrical machinery, gearboxes and turbine blades. Cavitation, corrosion, high torque and massive bending forces are all potential headaches. Yet engineers at Hampshire-based renewable energy consultancy IT Power believe […]

The challenges facing Britain’s bid to build the first under- water ‘windmill’ to generate electricity from tidal currents are daunting.

The sea is not kind to big electrical machinery, gearboxes and turbine blades. Cavitation, corrosion, high torque and massive bending forces are all potential headaches.

Yet engineers at Hampshire-based renewable energy consultancy IT Power believe they will be generating power from the sea by 2001.

IT Power is leading a European consortium in a £1.2m project backed by the EU’s Joule research programme to design and build the first underwater sea current turbine to supply electricity to the national grid. It aims to produce 300kW of power at 6p per kWhr, about the same price as wind energy.

The idea is to mimic a wind turbine to capture electricity from underwater tidal currents. The marine current kinetic energy in the oceans and seas is enormous. A 5 8 knot sea current (9 15km/h) represents an energy source equivalent to 390 km/h wind speed. A 20m diameter marine current turbine running in a 5 knot tide would produce 1MW, the same as a 60m diameter windmill in winds of 40 knots.

Tidal flows around Europe’s coastline reach 2 4 knots. And an EU study has listed 106 potential tidal current sites in Europe equivalent to 12,500MW of electrical power capacity. Some 42 of these sites are around the UK, with 20 in the south west.

IT Power plans to build a 15m diameter rotor and mount it on a tower sunk into the sea bed 25 30m below the sea surface, somewhere off the Devon or Cornwall coast. Catching the tide twice a day, it will spin at about 10rpm to produce around 300kW of power to be fed by cable into a national grid line.

The first challenge is configuring the turbine blades. The main consideration is the ‘solidity factor’ says Peter Fraenkel, IT Power technical director. ‘This is the fraction of the swept area occupied by the blades.’ The proposed marine turbine will need a solidity of 50%, hence half the swept area will be metal, says Fraenkel, making the blades broad and heavy. Wind turbines, for example, have a solidity of just 5 10% and have thin, light blades.

Although weight is not a problem under water, the extra material pushes up the cost of the blades, says Fraenkel. However, their size will be an advantage in resisting the massive bending forces they will experience. And they can be made of steel, which lowers the cost, he says.

Another factor is cavitation. Metal blades erode if they travel too fast through water. The problem is worse in shallow waters. Since the tops of the turbine blades will be about 4 5m below the surface, cavitation limits the speed of blade tips to 10m/s, hence a 10rpm limit for the 15m diameter turbine.

The next challenge is to mount the windmill on the seabed. IT Power has teamed up with offshore piling company Seacore based in Gweek, Cornwall. Seacore is a world leader in underwater rock socket drilling. It can drill holes over 2m diameter in solid granite 30m below the surface ‘without anyone getting their feet wet’, says Fraenkel.

The technique will allow a single, large-diameter prefabricated pile to be grouted into the hole to support the turbine tower which will slew vertically around the pile. This is a structurally more efficient way of building underwater foundations than sinking caissons or driving in multiple piles, claims Seacore.

Although the turbine will operate below water, the slewing mechanism, electrical controls and the transformers needed to link to the grid will be mounted at the top of the tower above sea level to avoid having to waterproof everything at depth.

The challenge for third consortium member ITT Flygt of Sweden, a submersible pump and generator specialist, will be to design the turbine. ‘There are few existing underwater windmills available,’ says Fraenkel. Fourth member, the University of Kassel in Germany, will design the electronics and electrical connections to withstand at least five years at sea without maintenance.

The low rotor speed means torque will be large, says Fraenkel, and will demand a two-stage gear box. He says the simplest way to programme the rotor to point into the tide will be to use tide tables and a clock. At other times the blades will be parked at right angles to the current.

But the biggest challenge is finding a suitable site to build the turbine. The Crown Estate own the seabed; Trinity House controls seaways; the local authority grants permission to bring a cable ashore and the regional electricity company makes the connection to the grid.

And despite a £675,000 EU grant, the consortium needs more money to complete the project.