Geothermal energy uses the earth's natural internal heat to generate electricity and heating. Geothermal energy may 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 energy generation in the Philippines and Kenya.
Types of geothermal energy
The most common source of geothermal energy around the world is hot springs associated with volcanic activity.
Other types of geothermal energy are:
- Hot sedimentary aquifers (HSA), which 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', which exploit the heat stored in rocks deep beneath the earth by fracturing the rock to create permeable reservoirs.
- Direct use systems, which utilise shallow underground reservoirs where there is a slight temperature difference between the surface and groundwater.
Although Australia has no volcanic structures, there is significant potential for geothermal energy to be extracted using hydrothermal and hot fractured rock processes.
How an enhanced geothermal system (EGS) works
Getting energy from 'hot rocks' relies on techniques established by the oil and gas industries.
Wells are drilled to a depth of 3–5 kilometres below the surface to find heat-producing granites. Water is pumped into the wells and through cracks in the rocks, where it becomes heated to a temperature of up to 300°C.
This extremely hot water is then pushed back to the surface, where the heat is used to drive a turbine and produce electricity. The water is recycled and the process can begin again.
Geothermal energy in Australia: 2014 in focus
Night steam at a Geodynamics facility
The geothermal sector in Australia is still in the early stages of development, accounting for around 0.002% of the country's total clean energy generation.
The successful Habanero Pilot Plant run by Geodynamics in South Australia's Cooper Basin in 2013 provided some cause for optimism, but the remote nature of the site has made it challenging to find customers for the resources. Geodynamics is now looking to diversity into other types of renewable energy while it continues to develop its geothermal technology.
Only one small commercial power plant is currently operating - a 0.12 MW facility run by Ergon Energy at Birdsville in Queensland.
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, projects such as the Habanero Pilot Plant are continuing to advance the field.
- Geothermal energy: clean and sustainable energy for the future, CSIRO, 2012
- Clean Energy Council Renewable Energy Database
- Clean Energy Australia Report 2014