Hot dry rocks – a form of geothermal energy

‘Geothermal’ means heat stored in rock. The best evidence of geothermal activity can be seen in regions close to the boundaries of tectonic plates – such as Japan and New Zealand – where hot springs, volcanoes and geysers are plentiful. These resources are already being used in some countries for heating and electricity generation.

The words ‘Australia’ and ‘geothermal’ are not often closely associated. Australia doesn’t have any active volcanoes and relatively few hot springs or geysers. Yet, according to some Australian scientists, we have some of the best reserves of hot dry rocks in the world, offering prospects for a plentiful supply of energy.

Australia’s hot dry rock resources are found in granite rock layers buried up to several kilometres underground, beneath layers of sedimentary rock. They are hot – up to 300ºC – because of what is known as the radiogenic decay of minerals, in which trace elements in the granite slowly break down, releasing heat as they do.

Australian hot dry rock resources are unusually well suited to extraction because of a combination of three factors:

• Heat is being generated in the crust at more than twice the global average.

• The ‘blankets’ of sedimentary rock above the granite provide excellent insulation but are also of an optimal thickness for heat extraction.

• The hot dry rocks are oriented horizontally, providing good (and relatively cheap) drilling access.

The process of extracting the heat is quite simple. Water is pumped down into the hot granite through a bore-hole that may be several kilometres deep. This helps to open up existing tiny cracks in the granite, increasing the permeability of the rock. The water is converted to steam by the heat and is channelled to the surface through another bore-hole, where it can be used to drive a turbine and thereby generate electricity.

Energy from hot dry rocks is not strictly renewable because the granite mass will eventually cool down. Nevertheless, it produces no greenhouse gases or other pollutants and has a very small ‘footprint’ on the landscape (unlike coal mining, hot dry rock energy requires no large-scale excavations). Some scientists say that Australia has enough hot dry rock resources – particularly in the Hunter Valley near Newcastle and the Eromanga Basin near the South Australia/Queensland border – to provide all our energy needs for centuries. A pilot project in the Hunter Valley is now underway.