Benign tides

The tides are moving strongly in favour of marine energy, promising to add a whole new form of renewable power generation.   BY LINDSAY CLARK

Tidal.jpgTwo tidal energy projects, one on each side of Cook Strait, are now under development with another awaiting the go-ahead on the Kaipara Harbour entrance.

All three projects plan to install development stage underwater electricity generators to generate power from strong and fast flowing tidal streams near coastlines.

Marine generation technology is currently about where wind technology was 15 years ago, when the country’s first major wind turbine was erected in the Wellington suburb of Brooklyn in 1993 – and is still generating power.

Just a few kilometres south of the turbine, where the sharp Wellington ridgeline plunges down a cliff into Cook Strait, the first tidal power turbine is planned to bring power ashore.

In April this year Christchurch-based Neptune Power was granted the first resource consent to operate a British-designed, New Zealand-built tidal stream generator as a developmental trial.

Directly across the strait in the narrow Tory Channel entrance to the Marlborough Sounds an Auckland-based company, Energy Pacifica, plans to put 10 tidal turbines into the swift tides that flow through the channel.

The two Cook Strait projects aim to tap into the huge tidal flows that sweep through the strait — some of the strongest tidal currents in the world. The key reason for the strength of these currents is that the main lunar tidal bulge which sweeps anti-clockwise around

New Zealand’s coasts is nearly always out of phase when passing the western and eastern ends of Cook Strait.

This difference in water level drives very fast tidal currents of up to two to 2.5 metres a second (4-5 knots) or greater at times through parts of Cook Strait and into Marlborough Sounds. The difference is compounded at spring tides with a higher tidal range along our west coast than along the east coast.

Neptune Power plans to install a developmental 1MW turbine 4.5 kilometres off Sinclair Head on Wellington’s southwest coast. This is at the southern end of the ‘Karori rip’ where tides accelerate as they turn the south west corner of the North Island. The site has been deliberately chosen in slower tides for testing all aspects of operating a marine energy plant.

A Neptune Power director, applied physicist David Beach, says Cook Strait was capable of generating a tremendous amount of energy, but there’s still a lot to learn about operating a power turbine in the tough marine conditions. Neptune hopes to begin generating power from the single pilot unit by 2010.

Beach says the positively buoyant generator, largely made of fibreglass and carbon fibre, will float about 70 metres below the sea surface and be tethered to a large concrete and steel mass anchor on the seabed a further 30 metres down.

The 10 metre square concrete anchor will be built like an open top box and floated to the site then flooded to sink it. The open top will then be filled with further heavy ballast. The anchor will have steel ‘spades’ sticking out underneath to further prevent the force of the tides dragging the anchor along the seabed.

The floating generator unit will consist of two 15 metre long parallel ducts with turbines inside containing counter-rotating blades to cancel out torque and stop the twisting a single unit might incur.

The 12 metre turbines will be of an open centre design with the blades mounted together on the outer rim and running against sealed magnets on the inside wall of the ducts to generate the power.

The open centre – like a hubless wheel – will allow the tidal stream to flow unobstructed right through the middle of the generator. This will ‘pull’ water across the turbine blades which at the rim should turn at approximately 10m/s at 16-17 rpm.

The twin turbine ducts will have a larger diameter outflow than inflow. This design gives a neat automatic steering mechanism into the current as the wider outflow ducts meet more water resistance than the narrower inlets.

The buoyant tethered design of the turbine unit also allows it to flip end-over-end when the tide turns to always face the tidal stream.

A six to eight kilometre long buried armoured cable will carry 11kV of power just outside the the boundary of the newly proscribed Wellington south coast marine reserve and to feed in to a Wellington city network power transformer in the small coastal suburb of Owhiro Bay.

The cable will also contain a fibre-optic line to provide real-time communication, including underwater video camera links, between the turbine structure and a monitoring base onshore.

Beach says the development project would have a 1MW capacity - enough to supply 800 households. But because the trial location was in a less powerful part rip, he expected it would generate something like 500 kilowatts - or enough for 400 houses.

A commercial turbine farm would need to be built further around the coast where water speeds are 1.5mps or more faster than the experimental unit. Beach says a critical fact about tidal marine energy is that when the current speed is doubled, eight times more power can be produced.

Neptune is discussing partnership with a British turbine manufacturer who had recently successfully tested a 1/6th size prototype, says but declined to disclose the name of the company.

“All of the turbine unit except the cable will be built in New Zealand using boat building skills in this country with fibreglass and carbon fibre. The turbine designer is in fact a New Zealander with a yachtmaking background.”

Across the strait in the Marlborough Sounds the latest operator Energy Pacifica plans to install up to 10 commercial-scale marine turbines near the Cook Strait entrance to the Tory Channel.

Chairman of Energy Pacifica Dr Anthony Bellvé has been a driving force in marine energy developments in New Zealand making the original proposal for generating power at the mouth of the Kaipara Harbour and was with Crest Energy in its first years of developing that project.

Bellvé says he helped form Energy Pacifica in early 2007 to introduce novel methods of renewable and sustainable energy including marine and solar energy. He says his company surveyed three sites with the fastest tidal currents, French Pass in northern Marlborough Sounds, the Karori rip on the Wellington side of the strait and Tory Channel.

“We found Tory Channel to be an optimal site with a tidal current speed of 3.6 metres a second and the best combination of currents, bathymetry and accessibility to the national electricity transmission network”, Bellvé says.

He too emphasised that higher current speeds greatly increased the commercial potential of marine tidal energy. Eight times more tidal stream power is produced during spring tides than at neaps.

Energy Pacifica has just applied for RMA resource consents from the Marlborough District Council. It plans to install 10 commercial-scale marine turbines each able to produce up to 1.2 MW in the 3.6 metres a second in the strong peak currents at the Cook Strait end of Tory Channel.

The 17 kilometre channel which 1.2 million people travel through a year nearly all on the Cook Strait ferries en route to Picton is 500 to 700 metres wide in its narrower stretches and from 40 to 60 metres deep. The tidal generators would be totally invisible to ferry passengers deep below the six metre draft ferries.

Bellvé saiys the higher west coast tidal range is compressed in the funnel-like Queen Charlotte Sound and has a focusing effect in Tory Channel particularly at the Cook Strait end where speeds increase.

Like the tides, the marine energy industry is constantly on the move, and Cook Strait is going to be a key part of future developments.

Energy NZ  No.6  Spring 2008
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