Harnessing the power of the sea

While marine energy technologies are not yet commercially competitive with existing technologies, the pace of development is fast. The Aotearoa Wave and Tidal Energy Association says the first commercial wave farm will be operational in Portugal within a year and LINDSAY CLARK finds that there is plenty of activity in New Zealand too.

Tidal.jpgGenerating power from the sea in New Zealand is closer than you might think.

A “blue energy” project extracting electricity from powerful tidal currents at the mouth of the large Kaipara Harbour, just north of Auckland, is the most advanced marine energy project in the country.

The 200 MW project being developed by independent Auckland company Crest Energy is one of the largest tidal power projects under development around the world.

The other region where there is a large potential for tidal energy in New Zealand is Cook Strait. An almost unlimited potential for tidal energy exists in the Cook Strait area.

As well as a wider need for more renewable sources of energy, what has turned attention to tidal energy has been the invention of electricity generators which can be moored to the seabed to generate power from the moving tidal streams.

The rapid development over the past decade of wind-powered electricity has turned engineers’ thoughts to also generating power from strong tidal movements.

With water being more than 800 times more dense than air, tides only need to move at around 2.5 metres per second to generate power with the latest turbines. A typical water flow speed contains eight to 10 times the kinetic energy of air.

One of Crest Energy’s directors, Anthony Hopkins, says that the company had looked at a number of harbours around the country including the other large west coast harbours such as Manukau, Hokianga and Kawhia.

But Kaipara Harbour, 75 kilometres northwest of Auckland, has the biggest tidal flows in New Zealand and is among the largest harbours in the world.

The Kaipara has more than double the tidal flow of Manukau and at the highest (spring) tide stores 20 times as much water as do the Wellington or Lyttelton harbours. Importantly it has the highest average current speed of any New Zealand harbour.

Hopkins says the Kaipara Harbour mouth is a hostile place with large shifting sandbars which have claimed 100 shipwrecks.

This might seem a strange place to think of generating electricity. But Hopkins says his company has carried out detailed bathymetry and hydrological studies of the area. Charts going back 150 years show that the deepwater main channel where Crest proposes to place its turbines has changed little over that time.

He says the dangerous sandbars outside the harbour mouth do change position frequently with big waves from the Tasman Sea breaking over the bar as they roll in. Inside, in the calmer main channel, waves reach only about 1.5 metres high.

Additional attractions of the Kaipara for Crest are the lack of major commercial shipping (thanks to the five metre depth entrance over the sandbar), proximity to Auckland and Northland and low leisure usage of the harbour mouth.

Crest plans to place up to 200 turbines deeper than 30 metres within a 10 kilometre-long section of the main channel.

“This will be a world first for the sheer scale of a tidal stream project,” Hopkins says.

The company is looking at potential turbines still in their development stage. They are looking at fixed one megawatt machines with turbine blades designed to generate power when the tide is both flowing in (flood) and out (ebb tide). The approximately 10 metre diameter blades would be enclosed in a wide-mouth “venturi” housing which increases water speed and energy output.

The generators would be hooked up to a 30 kilometre DC submarine cable (similar to the Cook Strait cables) to carry power to the eastern side of the Kaipara. The cable would be laid in deeper harbour channels and hook up with the present transmission lines south of Wellsford.

Being underwater the whole power station will be invisible even from the closest beach, and completely silent. Only the sound of breaking surf will be heard by anyone strolling down the nearby beach.

The Kaipara project would cost $500-600 million and for economic reasons, would have to be scaled up rapidly to near full capacity.

Hopkins says that from costing the company has done, tidal power from the project seems economically similar to wind power on short and long term marginal costs.

The tides would generate power on the inward and outward tides four times a day with no output on the slack tides between. The project would be capable of producing power 50-60 percent of the time.

Because tides depended on the monthly lunar cycle the blocks of generation would gradually change each day. Hopkins says Crest would be able to meet at least one peak electricity demand period each day.

Unlike wind, generation times would be completely predictable into the future,  Hopkins says.

Crest applied last year for resource consent for its project.

When it comes to looking outside harbours for likely tidal energy sites Cook Strait is the usual area of focus.

Dr Craig Stevens, a marine physicist at the NIWA research institute in Wellington,  who has been looking at the question of harnessing electricity from tides in New Zealand says the amount of tidal energy available is phenomenal. “The only question is how much of it can you harness?”

Tidal ranges in New Zealand are not that great with the largest tides reaching around four metres in range, whereas tides reach 11 metres off the northwest shelf of Australia and 17 metres in the Bay of Fundy in Canada.

Stevens says that the shape of New Zealand’s land mass results in a relatively small tidal range driving quite large flows in both Cook and Foveaux Straits. Tidal sequences circulate the country and at Cook Strait the east and west coast tides are almost 180 degrees out of phase, creating substantial flows.

“There is more than enough flow for viable energy production as flows can reach speeds well in excess of the 2.5 metres a second threshold.

“We know the basic tides can reach two metres a second in places and measured currents in Cook Strait have sometimes been almost double the expected tidal flow.”

This suggests at times and in specific locations there will be ample resource.

But Stevens warns against regarding tidal energy as “totally predictable” for power generation — particularly in open water like Cook Strait.

The tidal streams there are known for being unreliable due to wind-driven currents, changes in ocean circulation and headland eddies. There have been reported cases of tides running for 18 hours in one direction.

Cook Strait is also deep — mostly over 200 metres, but with some shoals 80 metres deep. Some early NIWA research indicates that current speeds are higher over these pinnacles and shoals than in nearby waters. More longer-term monitoring will be needed to find the best energy potential sites.

It is known that tidal speeds of up to seven knots (13 kilometres per hour) are frequently experienced on the Wellington side of the strait in the vicinity of Karori Rock and Cape Terawhiti. But as a rule this does not last over an hour.

This area of strongest tides is very close to New Zealand’s “electricity highway” — the Cook Strait cable where it comes ashore. It is also just offshore from the biggest wind power project planned to date, Meridian Energy’s 210 MW West Wind.

One of the companies which is in the early stages of planning an array of generators in Cook Strait, Neptune Power of Christchurch, has been examining the technical and legal issues involved.

One of the two Neptune partners, physicist David Beach, says marine current  generation technology is still very much under development. The initial plan is to float the turbines 40 metres below the surface on the Wellington side of the strait.

Neptune is looking at the TiDel turbine made in Newcastle, England which looks like a stubby wind turbine and which flips over to face whichever way the tide was flowing from. Two 500 MW turbines are mounted in pairs and chained to the ocean floor.

Beach says Neptune is also talking to Tidal Generation of Bristol about a different designed turbine. English engineers are currently at the forefront of the new tide generation developments.

John Huckerby, the executive officer of the Wellington-based Aotearoa Wave and Tidal Energy Association (AWATEA), says there are also a number of other companies involved in both tidal and wave energy projects and research, mostly at early stages.

Established in April 2006, the aim of the Aotearoa Wave and Tidal Energy Association (AWATEA) is to promote marine energy in New Zealand. Its members comprise marine energy project developers, major electricity and generators, marine industry sector participants, research organizations and interested individuals.

AWATEA has a work programme to develop a number of initiatives, including a developing dedicated marine energy strategy.

 In a press release late last year Huckerby says that whilst overseas marine technology developments are more advanced than technologies being developed in New Zealand, about half of the current domestic projects are developing new technologies. 

 

Energy NZ  Vol.1 No.1  Winter 2007
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