Hot rocks under NZers' feet could provide Kyoto-friendly energy

Some of New Zealanders' energy problems could be solved by the hot rocks under their feet, geologists say.

The Government's science funding agency, the Foundation for Research Science and Technology, has allocated $2.6 million in funding over the next three years to investigate using low temperature geothermal energy.

The money is aimed at learning how to tap major reserves of hot rocks and water which heat fluids to less than 150degC, and, in some cases, to temperatures below 80degC.

Project leader Brian Carey, of GNS Science, said natural sources of heat include springs and borehole fluids, shallow aquifers, water and steam discharges from thermal power plants, warm water associated with oil and gas wells, and flooded underground mines.

Even dry rock can be used to power a heat-exchanger at the bottom of a bore-hole, and in some cases, heat from the earth's surface may be able to be stored in the ground for later re-use.

"Low temperature geothermal resources are widespread throughout New Zealand and there is significant potential to increase their use," said Mr Carey.

"They are capable of providing long-term energy and heat supply with low carbon emissions".

According to a GNS geologist, Agnes Reyes, about half of New Zealand's 360 abandoned on-shore oil and gas wells are hot enough to generate electricity, with temperatures reaching nearly 180degC in wells nearly 5km deep.

And the rest have enough heat for direct industrial applications and geothermal heat pumps, which operate efficiently at temperatures between 10degC and 30degC. Ground-source heat pumps are already used by one householder in Hamilton, and one has been installed at Rimutaka Prison.

Abandoned oil and gas wells and sedimentary basins are mostly outside the traditional geothermal areas. One oil and gas well used commercially to produce geothermal energy is the Bonithon-1 well in New Plymouth, drilled in 1908, which provides hot water for the Taranaki Mineral Pools.

In Taranaki alone there were almost 50 other abandoned wells with bottom temperatures higher than 80degC, which could be used for binary cycle power production.

In this process hot water and a second fluid with low boiling point pass through a heat exchanger. The second fluid is boiled to steam, which drives a turbine, and the fluid is re-circulated.

Rural businesses could use some of the dozens of lower-temperature abandoned oil and gas wells to run heat pumps, dry grain, pasteurise milk, and heat glasshouses, and homes.

Combined, the abandoned wells with bottom temperatures over 80degC would generate about 160 megawatts of electricity.

GNS Science will work with specialists from Auckland University and Coal Research Ltd, to draw up a nationwide inventory of low-heat energy sources, and to study how different types of rock around New Zealand store and transfer heat.

Mr Carey said pumice soils were a good insulator, and in some faultline areas, fractured rock might allow hot fluid to percolate freely.

There will also be an analysis of "socio-economic factors", energy and tax policies that might influence the uptake of the technology.

Technical and scientific areas that will need work to exploit such energy sources are likely to include techniques for locating and better characterising low temperature sources, and development of specific pieces of technology.

In 2011 the programme will recommend ways to increase the use of low temperature geothermal resources.

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