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Tapping Into Hot Rocks

Date: 25 May 2004

Tapping Into Hot Rocks

Concern about New Zealand's future electricity supply has prompted a Waikato University scientist to call for a Government-funded study into generating power by using hot rocks lying beneath the Southern Alps on the West Coast.

Associate Professor Earl Bardsley, from the earth sciences department, says this method of generating electricity is currently being trialed in France and New Zealand should also investigate it.

"The Southern Alps are growing in height by about eight millimeters a year. This creates very high temperatures amongst rocks deep underground, heat which isn't generally visible or accessible," says Assoc Prof Bardsley.

"However, it's possible the West Coast may be suitable for a novel solution currently being trialed in France. This requires drilling down to hot rocks and then cracking them by pumping in water at very high pressure.

"Once the cracking process has been completed, cold water is pumped down to pass through the crack system where it heats up to the rock temperature. This hot water eventually emerges in another nearby well as high-pressure steam which can be used to power a turbine. The water is then circulated back down the first well in a closed water cycle."

At the trial site in eastern France, it is estimated that this "hot dry rock" technology will yield a total of about 25 megawatts per well over 20 years.

Assoc Prof Bardsley says hot dry rock resources are non-renewable for practical purposes because the rate of replenishment of heat from the Earth's interior is very small. However, the power source may still last for a number of years provided there is a sufficiently large volume of hot rock available.

"A critical economic factor is the depth that has to be drilled to reach the hot rocks, because expenses increase with increased drilling depth. The trial site in France required drilling five kilometers to obtain rock temperatures greater than 200 ºC. But the most favourable parts of Westland should be better than that, with published research indicating temperatures in excess of 200 ºC about three kilometers below ground surface along a narrow zone near the Alpine Fault, bounded south and north by the Haast and Arthur's pass roads," says Assoc Prof Bardsley.

"The amount of power that could be extracted from this zone might be as much as 4000 MW over a 40 year lifetime. However, a site trial drilling experiment similar to that in France would be essential to establish the viability of the resource. I think funding such a trial would be a worthwhile investment on the part of Government."

Assoc Prof Bardsley says hot dry rock power systems are environmentally friendly in that they are comprised of scattered small stations rather than the single large stations seen in normal geothermal field development. This low visibility could allow development in conjunction with West Coast tourist facilities. "For example, waste heat from a hot dry rock power station at Franz Joseph township could provide domestic heating as well as allowing the creation of hot pools as a tourist attraction."


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