Sea 'windmills' for Cook Strait?

SEA POWER: Artistic rendition of a Voith Hydro Holding OCT marine turbine mounted underwater on a cradle-like structure.

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Twenty commercial-scale marine turbines may be installed on the sea floor almost directly under the main route of the Cook Strait ferries in Tory Channel.

Auckland company Energy Pacifica is preparing its resource consent application for the marine turbines – described by the company's chairman, Dr Anthony Bellve, as underwater windmills – to be installed 20-25 metres below the sea surface.

The company will hold a public presentation and discussion tomorrow night at the Marlborough Convention Centre in Blenheim at 7pm.

Mr Bellve said the company had surveyed three likely sites – French Pass, the Karori rip on the Wellington side of Cook Strait and Tory Channel – and found Tory Channel to be the best suited. That site had a tidal current of 3.6m per second and the best combination of currents and sea depth as well as good access to the national grid.

The public meeting is part of Energy Pacifica's consultation for its proposed resource consent.

Dr Bellve said a marine turbine was essentially an underwater windmill which had twin rotors with a blade at each end – sort of a double-headed windmill.

The turbines proposed for Tory Channel would be installed with the top of the blades about 20-25m below the sea surface to ensure vessels would not be at risk of hitting them. The deepest draught of the Cook Strait ferries is 6m.

The 20 marine turbines, each able to produce up to 1.2MW of electricity, would be secured to the seabed by digging a hole and anchoring a mono-pile, which the turbine would be mounted on.

An alternative would be to mount the turbine on the seabed with a ballast and four smaller piles.

The turbine blades are linked to an underwater generator and generated electricity would be carried by underwater cables buried about a metre below the sea floor to a substation.

The proposal sees power then run through a transformer which increases the voltage before being transmitted to a shore-based substation.

Dr Bellve said there were several advantages with marine turbines. These included:

* Power density generated by tidal current is far greater than wind – and marine turbines are about one-fifth the size of a traditional wind turbine.

* Tidal current is entirely predictable – for every tide there are two highs with a fast rapid movement at the change in tides – four times a day which means optimum power can be produced for about 15 hours a day.

* Unlike wind, the tide is always going in or out so marine turbines are always working.

* A marine turbine is located under the sea so there are no detrimental visual effects. The only thing that would be evident would be a small substation on nearby land.

When asked what the downside of the proposed marine turbines might be, Dr Bellve said "perhaps the sediment might be an issue when the holes for the pylons were being drilled".

"We are going through the consultative process at this point and have talked with National Institute of Water and Atmospheric and Maritime New Zealand regarding maritime safety. This meeting is a further step."

He said if resource consent was approved, Energy Pacifica would start with just a few turbines.

The installation of the 20 turbines could take several years, he said.

- The Marlborough Express