Electricity Generation from
Geothermal Energy in Australia
A geothermal system consists of three elements: a heat source, permeable
rock, and a fluid to transport heat to the surface. The heat source is the
only fundamental requirement because permeable rock reservoirs can be
artificially created and fluids can be introduced. There are two main types
of geothermal systems that can be used to generate electricity.
Hydrothermal systems have fluids circulating through rock pores or
fractures (as a liquid or vapour) in areas where high heat-flow is present.
These systems are often found near active tectonic plate boundaries where
volcanic activity has occurred, such as in Iceland, New Zealand and the
Philippines. Hydrothermal systems can also form above areas of hot
basement rocks (see Figure 1), and it is this type of system that is found
in Australia. High-temperature hydrothermal systems are often exploited
for electricity generation, while low-temperature hydrothermal systems are
more suited to direct-use applications (see Geoscience Australia’s Factsheet
“Direct-use of Geothermal Energy: Opportunities for Australia”).
Hot Rock systems do not have fluids naturally circulating through the
rock and in most cases, the rock needs to be fractured to achieve the
fluid flow required for heat transfer. Hot Rock systems are normally
associated with granites that contain anomalously high concentrations
of the naturally radioactive elements uranium (U), thorium (Th) and
potassium (K). Although enriched in these elements compared to other
rocks, element concentrations are still relatively low (commonly ~0.002%
U, ~0.01% Th and ~4% K). The radioactive decay of these elements
over millions of years generates heat, which is trapped when the granites
become buried by insulating sediments. The thicker the insulating layer,
the hotter the temperatures, for example, granites at 3 kilometres depth
overlain by insulating sediments can be hotter than 200°C [i].
Hot rocks at these depths g