Geothermal Energy

What is Geothermal Energy?

Geothermal energy is heat energy originating deep in the earth’s molten interior. It is this heat energy that is responsible for tectonic plates, volcanoes and earthquakes. The origin of this heat is from primordial heat (heat generated during the Earth's formation) and heat generated from the decay of radioactive isotopes. The temperature in the earth’s interior is as high as 7000°C, decreasing to 650 - 1200°C at depths of 80 km -100 km (Wright, 1998). Through the deep circulation of groundwater and the intrusion of molten magma into the earth’s crust, to depths of only 1 km-5 km, heat is brought closer to the earth’s surface. The hot molten rock heats the surrounding groundwater, which is forced to the surface in certain areas in the form of hot steam or water (e.g. hot springs and geysers). The heat energy close to, or at the earth’s surface can be utilised as a source of energy, namely geothermal energy.

Figure 1 A Lava Fountain is an example of the amount of heat stored in the Earth

The total geothermal resource is vast. An estimated 100 PWh (1 x 10¹⁷ W) of heat energy is brought to the earth’s surface each year (World Energy Council 1994). However, geothermal energy can only be utilised in regions where it is suitably concentrated. These regions correspond to areas of earthquake and volcanic activity, which occur at the junctions of the tectonic plates that make up the earth’s crust. It is at these junctions that heat energy is conducted most rapidly from the earth’s interior to the surface, often manifesting itself as hot springs or geysers.

Low-grade geothermal resources are more abundant and widespread. They are located in deep sedimentary basins around the world (e.g. along the Gulf Coast of the United States and in Central and Southern Europe), as well as on the edges of tectonic plates. Western Australia’s Coral Bay sits on top of a very low-grade source, which is nonetheless used (among other things) as a source of water for bathing in tourist centres.

The Need for Geothermal Power in a Sustainable Energy system

With the concern over anthropogenic climate change (i.e. man-made climate change), there is a growing awareness that we must utilise energy resources that are sustainable. Geothermal power is one such sustainable resource that has the potential to supplement our energy systems and to displace many conventional fuels such as coal. This is due to the inherent stability of the resource. In contrast to many renewable technologies, such as wind or solar, the geothermal resource can be used 24 hours a day, 7 days a week.

Geothermal Resources

There is currently an estimated 15,000 MW of direct use and over 8,000 MW of generating capacity in geothermal resources worldwide. To put geothermal generation into perspective, this generating capacity is about 0.4% of the world total installed generating capacity. In 2003 there was 8,402 MW of installed geothermal electricity generation capacity worldwide. This total is stabilising after a growth period due to the over exploitation of the Californian fields in the United States which is decreasing output, with investments from other countries making up for this deficit. The US is the largest producer of geothermal electricity, followed by the Philippines, Mexico, Indonesia, Italy, Japan and New Zealand (International Geothermal Association, 2007).

Based on data from over 3,500 boreholes, conservative estimates of the Australian geothermal resource suggest the energy available for electricity generation is 23 million petajoules, or 7,500 years of Australian energy consumption at the current level. Over 80% of this resource is located in the Eromanga (Great Artesian) Basin. About 11% of this energy resource (2.5 million petajoules), or more than 800 times the current annual demand for electricity in Australia, is thought to be in granite rock, which is the most favoured host rock for heat extraction in what is known as Hot Dry Rock (HDR) (Australian National University, 2003).

There are four types of geothermal resources: hydrothermal, geopressured, hot dry rock and magma. Of the four types, only hydrothermal resources are currently commercially exploited.

Hydrothermal
Hydrothermal (or hot water) resources arise when hot water and/or steam is formed in fractured or porous rock at shallow to moderate depths (100 m to 4.5 km) as a result of either the intrusion in the earth’s crust of molten magma from the earth’s interior, or the deep circulation of water through a fault or fracture (World Energy Council 1994) (see Figure 2). High temperature hydrothermal resources (with temperatures from 180°C to over 350°C) are usually heated by hot molten rock. Low temperature resources (with temperatures from 100°C to 180°C) can be produced by either process (Wright, 1998).

Figure 2 Hydrothermal plant in New Zealand.

Hydrothermal resources come in the form of either steam or hot water depending on the temperatures and pressures involved. High-grade resources are usually used for electricity generation, while low grade resources are used in direct heating applications.

Figure 3 Simplified cross section of the essential characteristics of a geothermal site (Image adapted from Boyle, 1998).

Hydrothermal resources require three basic components (see Figure 3) a heat source (e.g. crystallised magma), an aquifer containing accessible water, and an impermeable cap rock to seal the aquifer. The geothermal energy is usually tapped by drilling into the aquifer, and extracting the hot water or steam.

In Australia there is a large hydrothermal potential in the Great Artesian Basin from central South Australia through most of western Queensland to the Gulf of Carpentaria (see Figure 4).

Figure 4 Hydrothermal resources in Australia (courtesy of the SA Dept. of Transport, Energy and Infrastructure).

Geopressured
Geopressured geothermal resources consist of a hot brine, saturated with methane, found in large, deep aquifers under high pressure. The water and methane is trapped in sedimentary formations at a depth of about 3 km - 6 km (World Energy Council 1994). The temperature of the water is in the range of 90°C - 200°C. Three forms of energy can be obtained from geopressured resources: thermal energy, hydraulic energy from the high pressure, and chemical energy from burning the dissolved methane gas. The major region of geopressured reservoirs discovered to date is in the northern Gulf of Mexico.

Hot Dry Rock

Hot dry rock (HDR) is a heated geological formation formed in the same way as hydrothermal resources, but containing no water, as the aquifers or fractures required to conduct water to the surface are not present (see Figure 5). This resource is virtually limitless and is more widely accessible than hydrothermal resources.

Figure 5 Hot Dry Rock Technology (Image courtesy of the Hot Rock Energy program, Australian National University).

The geological profile of Australia is such that there is a large potential for HDR technologies to be used for energy production in the eastern states of Australia. Figure 6 is a false colour map of Australia, showing the temperature of Earth's crust at a depth of 5 km.

Figure 6 The potential for HDR technologies in Australia are closely related to the temperature of Earth (courtesy of the Hot Rock Energy program, Australian National University

There may be locations in the northern Perth Basin that might have a high enough temperature gradient for HDR applications. The potential resource of the region is estimated to be 49 EJ, which may be sufficient to supply the electricity requirements of the South West Integrated System (SWIS) for in excess of 100 years. The estimated break-even price for electricity from an HDR project proposed by Geodynamics Limited for the Cooper Basin was 6.2 c/kWh initially, at demonstration plant stage, falling to around 4.0 c/kWh at full-scale production. These figures are yet to be demonstrated, but there seems to be increasing reasons to consider further investment in HDR opportunities within Western Australia. (ERDC, 1994).

Magma
Magma, the largest geothermal resource, is molten rock found at depths of 3km -10km and deeper, and therefore not easily accessible. It has a temperature that ranges from 700°C - 1,200°C. The magma resource has not been well explored to date.

The Future for Geothermal Energy

In the short term, it is likely that hydrothermal resources will remain the only geothermal resource that is commercially viable. However this resource alone represents an immense source of energy. It is estimated that 80 GW of geothermal electricity could be generated in the short to medium term from known hydrothermal resources worldwide (Wright, 1998).

In the medium to longer term, technological developments will see the utilisation of the geothermal energy in hot dry rocks and geopressured reservoirs. Usable geothermal resources will no longer be limited to the shallow hydrothermal reservoirs. These resources represent a virtually limitless source of energy, and are the future of sustainable geothermal energy.

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

Geothermal Technology - RISE Information Portal

Geothermal Education Office

US DOE Office of Geothermal Technologies

CADDET Geothermal Register

CREST

Chevron Thermal Energy

Hot Dry Rock research at UNSW (pdf)

Hot Dry Rock research at ANU

International Geothermal Association

References

Australian National University, 2003. “Hot Rock Energy Website” (Online) http://hotrock.anu.edu.au/resource.htm (Accessed 16 February 2007).

Burns, K.L., Weber, C., Perry, J. and H. J Harrington, (2000). “Status of the Geothermal Industry in Australia.” (Online), http://iga.igg.cnr.it/pdf/WGC/2000/R0559.PDF (Accessed 16 February 2007).

Energy Research and Development Corporation (ERDC), 1994."Hot Rock Feasibility Study, Report 243", The Australian Commonwealth Government, Canberra.

International Geothermal Association, 2007. Website (Online) http://iga.igg.cnr.it/index.php (Accessed 16 February 2007).

World Energy Council, 1994. New renewable energy resources, Kogan Page, London.

Wright, P.M., 1998. "The earth gives up its heat", Renewable Energy World, vol.1, no.3, pp.21-25.