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: 2013 in focus

Night steam at a Geodynamics facility

Night steam at a Geodynamics facility

The geothermal sector in Australia is still in the early stages of development, accounting for around 0.001% of the country's total clean energy generation. We currently have two operational plants – a 0.12 MW facility at Birdsville in Queensland and Geodynamics' new 1 MW Habanero Pilot Plant at Innamincka in South Australia.

The Habanero Pilot Plant is Australia's first working EGS system and one of only a handful worldwide. Running from 30 April to 7 October 2013, the pilot trial produced results that exceeded expectations and will be of significant value to the developing geothermal industry. The Habanero plant is being maintained for further trials or as part of future commercial development of the technology.

There are more geothermal projects currently under development in South Australia and Queensland.

Outlook

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.

Sources:

  • Geothermal energy: clean and sustainable energy for the future, CSIRO, 2012
  • Clean Energy Council Renewable Energy Database
  • Clean Energy Australia Report 2013