Geothermal

What is geothermal energy?

Geothermal energy harnesses the heat stored in the Earth's crust.

This heat comes mostly from naturally occurring radioactive decay of rocks in the Earth's outer crust, but can also come from friction caused by shifting or movement of the Earth's crust deep underground.

This heat is trapped underground by overlaying thick sedimentary or other rock deposits that act like a blanket, keeping temperatures very high in the 'basement' in such areas.

Some granites contain quite high levels of radioactive materials, and can store additional heat if they are well insulated by 'blanketing' layers of rock above them. 

The greater the thickness of the covering rock, the higher the temperature the heat-bearing rocks can reach. For example, for rocks to reach temperatures greater than 200 degrees Celsius, they need to be buried at about 3 kilometres deep.

There are four basic types of geothermal energy:

1. Hydrothermal - hot water and steam at depths between 100 metres and 4.5 kilometres
2. Geopressured - hot water aquifers under high pressure at depths of 3 to 6 kilometres
3. Hot dry rock (HDR) - abnormally hot rock formations with little or no water
4. Magma - molten rock at temperatures of 700-1200 degrees Celsius.

Although geothermal energy is not a major contributor to world energy sources, its use is increasing. Significant hydrothermal geothermal electricity and direct heat production occurs in the USA, the Philippines, New Zealand, Iceland, Mexico, Japan, Italy and Indonesia - all areas known for volcanic activity.

Australia lacks areas of volcanic activity, but does have hot aquifers and areas of crustal heat that can be used for electricity generation.

For example, the Birdsville Geothermal Power Station, owned and operated by Ergon Energy, uses wet geothermal energy to generate 80 kilowatts of electricity for the town of Birdsville. This provides around a quarter of the town's electricity requirements.

Attention has recently been focused on hot dry rocks in basement granites beneath sedimentary basins. Queensland is thought to have significant deposits of suitable granites, so this is a renewable energy source with great potential.

Initial estimates indicate that hot dry rocks beneath the Eromanga Basin could meet all of Australia's energy needs for 800 years.

Downloads

Download the geothermal energy fact sheet (PDF - 75KB).