Geothermal energy is produced by extracting the natural, internal heat of the earth to create electricity and heat. Geothermal energy can be stored in granite rocks often called 'hot rocks' or trapped in liquids such as water and brine (hydrothermal process).
Many countries generate significant amounts of electricity from geothermal energy. Iceland sources 25 per cent of its total electricity generation from geothermal sources, while geothermal energy represents around 17 per cent of the energy generation in the Philippines and Kenya.
This geothermal energy is sourced from hot springs associated with volcanic activity, which is the conventional source globally.
Australia has no volcanic structures but there is significant potential for geothermal energy from hydrothermal and hot fractured rock processes.
Types of geothermal energy
Aside from the conventional energy, geothermal energy can be:
- Hot Sedimentary Aquifers (HSA) that exploit naturally occurring reservoirs that have been heated by proximate hot rocks or crustal heat flow
- Enhanced Geothermal Systems (EGS), commonly referred to as 'hot rocks', that exploit the heat stored in rocks deep beneath the earth by fracturing the rock to create permeable reservoirs
- direct use systems that utilise shallow underground reservoirs where there is a slight temperature difference between the surface and groundwater.
How energy is extracted
Getting the energy from the hot rocks relies on techniques established by the oil and gas industry. Wells are drilled to a depth of 3-5 kilometres below the surface to find the heat-producing granites. Water is then pumped down in the wells and through the cracks in the rocks. The water is heated to a temperature of up to 300 °C and pushed back to the surface where the heat is used to drive a turbine and produce electricity. The water used is recycled.
A year in focus
In Australia, the sector is still in the early stages of development. There is one operational plant at Birdsville in Queensland. In total, 0.002 per cent of Australia's electricity generation comes from geothermal energy.
In May 2013, Geodynamics commissioned its 1 MW pilot plant at Innamincka in South Australia, making it the most advanced hot rock geothermal project in the country. The Habanero plant is one of only three EGS plants operating worldwide. The pilot facility is centred upon the Habanero-4 well, which has been drilled to a depth of 4204 m with temperatures up to 241°C. Pilot trials included extended reservoir and plant testing to determine performance and characteristics, and feasibility studies for small-scale projects.
There are also project activities underway in South Australia and Western Australia.
With its promise of emission-free power generation 24 hours a day, geothermal energy has attracted a lot of interest. Although developing the technology to harness Australia's hot rock resources has not progressed at the pace early investors were hoping for, advancement continues to be made.
- Geothermal energy: clean and sustainable energy for the future, CSIRO, 2012
- Clean Energy Council Renewable Energy Database