Geothermal energy is defined as heat from the Earth and it is considered a renewable resource. It is also recognised as a ‘green’ energy source as it emits negligible amounts of greenhouse gases.
Commercial production of geothermal energy is not new with the first geothermal electricity-generating plant beginning operation in Italy in 1904; the plant is still in operation. The current annual global capacity of geothermal electricity is 10,715 megawatts (MW). The USA is the largest producer of geothermal electricity with a total annual capacity of 2687MW. Australia’s current capacity is an estimated 0.2MW with Queensland’s contribution (from a single power station in Birdsville) being 0.08MW (80 kilowatts).
The majority of geothermal activities in Queensland are exploration with the only geothermal production activity occurring at the Birdsville geothermal power station. The Birdsville geothermal power station is owned and operated by Ergon Energy and draws on a 1.28km deep free-flowing bore into the Great Artesian Basin that has existed for more than 75 years.
There are 5 types of geothermal sources of energy available however only types 1 (hot water) and 2 (hot dry rock) are expected to be assessed for geothermal energy potential in Queensland.
Hot water geothermal energy
This process involves sourcing energy from a natural spring or man-made bore where hot water (which is below the 100°C boiling point) is extracted from an underground reservoir. The heat is transferred to another liquid which has a boiling point lower than water. The other liquid then boils turning a turbine. This is the type of geothermal energy accessed by the Birdsville Power Station.
Hot dry rock geothermal energy
This is the newest source of geothermal energy which is now available due to the technology of hydraulic fracturing. It is also the main source of geothermal energy being explored in Australia. The fracturing opens up the rock allowing water to be injected down a well into the rock formation. A second well is drilled to allow the steam to vent up into the power station.
Vapour geothermal energy
These are steam vents one would traditionally describe as geothermal surface activity (e.g. Old Faithful—Yellowstone National Park USA). In simple terms the vent is capped and a turbine installed.
Geopressure geothermal energy
This type of energy source is like a hot rock oven; a hot, dry underground cavern that when tapped and filled with water will produce steam. This activity can often be used in empty oil fields as the heat which helped produce the oil in the first place continues to emit long after the oil has been extracted.
Magma geothermal energy
This is the main geothermal energy source that is harnessed by the USA, Philippines, Iceland and New Zealand. Access is made into fault lines where the intense heat from the magma travels up the access vent heating the water within a power station system.
There are environmental considerations associated with underground geothermal activities which are strictly managed under the conditions of an environmental authority issued under the Environmental Protection Act 1994.
Environmental considerations of underground geothermal activities (e.g. drilling wells, hydraulic fracturing) include:
- Water Injection and hydraulic fracturing—can result in groundwater impacts and aquifer interconnectivity.
- Subsidence—can cause a shift in the surrounding earth form. This impact is more likely found as a result of vapour geothermal activity and is therefore unlikely to occur in Queensland.
- Seismicity—can induce micro-seismic activity (of magnitude 2–3 which is generally not felt on the surface).
- Geothermal surface features—can impact nearby natural surface features (e.g. hot springs) as a result of drawdown or temperature reduction.
Geothermal versus unconventional gas
While geothermal activities require hydraulic fraccing and water injection, the correlation with unconventional gas (such as coal seam gas activities) ends there. There is no requirement for linear infrastructure such as water and gas lines, LNG facilities or water treatment facilities.