Styles of Geothermal Energy
Most geothermal energy used around the world occurs in volcanically active areas where magma at shallow depths in the earth’s crust heats water contained in sedimentary horizons (aquifers). However, in recent years, other forms of geothermal energy have become the target of exploration and development in regions without recent volcanism.
In Australia two main sources of geothermal energy are present: Hot Rocks and Hot Aquifers. These are shown schematically in the Figure below.
Comparison of Hot Rock and Hot Aquifer geothermal resources
Hot Rock geothermal resources are commonly referred to as “Hot Dry Rocks” (HDR) or “Engineered Geothermal Systems” (EGS).
The heat from Hot Rock resources comes from granitic rocks that contain low, but elevated levels of uranium and thorium. As these elements decay over millions of years they generate heat which is trapped within the granite body and surrounding rocks by a thick (typically 2 – 4km) overlying sequence of sedimentary rocks that act as a thermal blanket.
Because Hot Rock resources are typically massive and do not contain interconnected spaces that water can pass through (permeability) the rocks need to be artificially fractured, a process called hydrofracturing, to enable to the circulation of hot water. Hot Rock resources have the potential to generate hundreds of Megawatts (MW) of electricity.
While there are no commercial Hot Rock projects currently in production, pilot plants at Soultz (France) and soon at Innamincka (South Australia) demonstrate the potential of Hot Rock resources.
Hot Aquifer Geothermal Resources are commonly referred to as “Hot Sedimentary Aquifers”, “Hot Wet Geothermal Systems” or “Hydrothermal Systems”.
These resources occur within water-laden sedimentary horizons at shallow to moderate depths (2.5 to 3.5km). Although they tend to be cooler (120 – 180ºC) than Hot Rock resources (up to 250 ºC), they are able to be explored and developed quickly at a much lower capital cost using conventional oil-field and geothermal technology.
Hot Aquifers in Australia may occur where one or more of the following four geological settings are present:
1. where radioactive decay in basement rocks heats water in overlying aquifers;
2. where remnant heat from old volcanic centres preserves an elevated geothermal gradient;
3. where hot water wells up from very deep basins along thermal density and/or pressure gradients; or
4. where rapid tectonic uplift has brought deep hot water and rocks closer to the surface and compressed geothermal gradients.
The Heber field (California) is one example of a producing Hot Aquifer geothermal resource. The Heber plant has an average annual capacity of 42MW, and forms part of a geothermal field which produces 92MW electricity annually from hot acquifers with resource temperatures of about 180ºC.