Geothermal Energy in Australia
Geothermal energy is heat contained within the earth. The heat can be used directly (see Fact Sheet Direct-use of Geothermal Energy: Opportunities for Australia), or to generate electricity. Geothermal energy is a baseload renewable resource, which means that it can operate 24 hours a day, 365 days a year, with few CO2 emissions. Geoscience Australia has calculated that there is sufficient energy contained within the Australian crust that if only one per cent of the resource were to be utilised, it would provide 26 000 years worth of electricity (see Fact Sheet Electricity Generation from Geothermal Energy in Australia).
A geothermal system consists of three elements: a heat source covered by insulating rock to trap the heat, and a fluid to transport heat to the surface. 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 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 in sedimentary rocks 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.
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 (see Fact Sheet Induced seismicity and geothermal power development in Australia). 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 sedimentary rocks. The thicker the insulating layer, the hotter the temperatures, for example, granites at three kilometres depth overlain by insulating sediments can be hotter than 200°C.
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