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NZ: Chance to turn the tide of power supply

The anticipated power crisis in Auckland can be circumvented by careful, innovative planning and the deployment of tidal-current turbines.

Rather than importing electricity from the south using a $2 billion upgrade of transmission lines, as proposed by Transpower, the focus should be on generating more power locally.

This would avoid the costs, dependencies and inefficiencies inherent in long-distance transmission.

Local power generation could be achieved by installing tidal turbines in Auckland's regional harbours. Two harbours are superbly suited for this purpose, and tidal-current power generation is cost-effective and environmentally friendly.

This country has a long-standing record of generating power from renewable resources - hydro-electricity (61 per cent), geothermal (5 per cent), windmills (1 per cent) and biomass (0.3 per cent).

The prevalent use of renewable energy underpins our clean green image.

Clear policy mandates for further harnessing renewable resources would make New Zealand an international model as well as providing clean blue energy throughout the 21st century.

The rest of our power is derived from non-renewable sources - gas (23 per cent), coal (8 per cent) and oil (0.5 per cent).

Meeting increases in electricity demand from such fossil fuels has three major disadvantages.

First, the combustion of them, particularly coal, releases unwanted greenhouse gases.

Dumping these gases deep underground would increase gas and coal-fired electricity costs by 60 per cent for gas and 90 per cent for coal.

Second, our natural gas supplies are near depletion. Maui's reserves have plunged to 14 per cent and those of Kupe are limited - despite "reassessing" old data.

Future gas combustion may mean importing LPG from Australia.

Third, fossil fuels have increased substantially in price.

Coal has soared from US$52 to US$125 ($71 to $171) a tonne in recent months and is likely to increase further.

China, the United States and India anticipate consuming another 900 million tonnes a year to fuel the 843 coal-fired plants to be built by 2012.

Oceans produce vast quantities of renewable energy. Power generation from tides and currents is a clean and viable option.

The Antarctic current south of New Zealand, powered by the Earth's rotation and prevailing winds, moves 130 to 190 million cubic metres of water a second.

The tides, forced by the moon and sun's gravitation, engender huge amounts of sustainable energy.

Their timing and height can be predicted with precision.

Geography, wind and barometric pressure influence tidal events, but only modestly.

Several seaside countries harness marine energy. The potential is considerable.

England and Ireland's 19,717km coastline has features capable of generating 6000 to 10,000MW of power.

A tidal-turbine-fence transport bridge proposed for the 4km-wide Dalupiri Passage in the Philippines has a design capacity of 2.2GW, equivalent to two nuclear power plants.

Smaller tidal turbines are operating in Canada, England and Norway.

With a coastline of 17,200km, New Zealand is well positioned to generate energy from tidal currents. Sailors are aware that 7-knot (3.6 m/sec) currents race through Tory Strait and French Pass in the Marlborough Sounds.

There are also substantial currents in the North Island's Manukau, Kaipara and Hokianga Harbours.

These large harbours produce 5 to 6-knot currents and tidal flows of 100,000 cu m a second four times a day from the flood and ebb tides.

Their tidal volumes are 12 times greater than the flow in our largest rivers.

Auckland's proximity to these large tidal harbours favours local power generation and distribution.

The Kaipara is the largest harbour in New Zealand. The entrance is 5.6km wide and its gorge is 40m deep with a scoured rocky floor. Its flood and ebb tides generate currents of 2 to 3 m/sec. Given these characteristics, it is very suitable for tidal current turbines.

Other New Zealand cities are near harbours suitable for tidal-current generation.

Wellington Harbour and Cook Strait offer enormous potential close to considerable consumer demand.

Importantly, tidal turbines could be situated throughout the country to iron-out the peaks and troughs in generation which would arise from the disparity in the times at which tides occur along the coast.

Tidal current turbines have important logistic, economic and environmental advantages:

* They do not require large initial capital investment. Units can be added as population growth, commercial expansion and industrial development increases demand.

* They are sited inside harbour entrances and along coastlines in depths of 25 to 70m. Sites are chosen where currents (2 to 3-m/sec) are optimal for achieving efficient generation.

* Horizontal axis turbines are equipped with single or twin axial rotors 16 to 20m in diameter and installed on concrete pylons 4m in diameter. The rotor blades pitch through 180 degrees to harness flood and ebb currents. The units can be placed 5 to 10m apart across or along the current.

* Vertical axis turbines, equipped with highly efficient hydrofoil blades. are constructed in caissons extending to depths of 70m, perhaps in conjunction with an overhead transportation bridge.

* The turbines are installed so as not to interfere with the passage of small or large vessels. Their rotors swing slowly (10 to 20 rpm), enabling safe movement of marine life.

They are out of sight and hearing, which negates two detrimental features of windmill turbines.

* Service vessels hoist turbines to the surface for routine inspection and maintenance. Sediments do not damage the rotors, nor do sediments accumulate around the installations.

* Generation capacity extends over 14 hours each day and 30 per cent of the total tidal energy can be extracted without harmful downstream effects.

The Auckland region used 1510 to 1525MW of electricity last year and it is anticipated that demand will increase to 2.5 per cent (38MW) a year. Fifty turbines capable of generating 75MW would be able to meet such an increase.

Furthermore, they would generate electricity at a kilowatt-cost equivalent to that of a new hydro-electric dam.

Tidal current turbines in the Hokianga, Kaipara and Manukau could be connected by transmission cables laid down the west coast seabed, and east through the Manukau Harbour to Auckland substations.

Alternatively, underground cables could be laid from the Kaipara Harbour directly southeast through Henderson and into Auckland.

Such local power generation would delay if not obviate Transpower's proposed upgrading of transmission lines.

Clean blue tidal power would circumvent increasing EMF-related health hazards while preserving our landscapes for posterity.

These objectives are achievable with foresight, skilful planning and political will.

--

Dr Anthony Bellve, a retired professor of Harvard University and Columbia University, lives at Whitford.

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