Geothermal Energy Systems

What is Geothermal Energy?

Geothermal energy is heat energy originating deep in the earth’s molten interior. It is this heat energy which is responsible for tectonic plates, volcanoes and earthquakes. The temperature in the earth’s interior is as high as 7000 °C, decreasing to 650 - 1200 °C at depths of 80km-100km [1]. Through the deep circulation of groundwater and the intrusion of molten magma into the earth’s crust to depths of only 1km-5km, 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, eg 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.

The total geothermal resource is vast. An estimated 100PWh (1 x 1017 W) of heat energy is brought to the earth’s surface each year [2]. 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 (eg along the Gulf Coast of the United States and in Central and Southern Europe), as well as on the edges of the tectonic plates.

Background

Geothermal energy has been used by humans for many centuries in applications such as space and water heating, cooking, and medicinal bathing. The first geothermal power generation plant was constructed in 1904 in Larderello, Italy. This had a capacity of 250kW and used geothermal steam to generate electricity. The second geothermal power station built was in the 1950s at Wairakei, New Zealand. This was followed by The Geysers in California in the 1960s [3].


Figure 1 Geothermal power plant at Wairakei , New Zealand.
(Image courtesy of International Geothermal Association)


Figure 2 The Geysers geothermal power plant in California, USA.
(Image courtesy of International Geothermal Association)

There is currently (1998) 8,240MW of installed geothermal electricity generation capacity worldwide. The United States is the largest producer of geothermal electricity, followed by the Philippines [4].


Table 1 World wide geothermal capacity by country.
(Courtesy of International Geothermal Association)

Geothermal resources are also used extensively in non-electrical, direct heat applications, such as space heating and in a range of agricultural and industrial processes. Nearly 10,000MW of thermal energy is sourced worldwide from geothermal energy in a number of countries, including Japan, China, and Iceland [1].

Geothermal resources are not strictly renewable in the sense that geothermal activity occurs on a much larger time scale than human lifetimes. However, they are renewable if the rate of extraction of the energy is less than the rate of replenishment of the resource. The natural recharge rates of geothermal reserves varies from a few to over 1,000MW of thermal energy [1]. However, in current practice, all installations are exceeding the recharge rates of the resources. The geothermal resources are therefore being used in a non-sustainable manner, and are effectively being ‘mined’ as fossil fuels are.

Geothermal resources

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 (100m to 4.5km) 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 [2]. High temperature hydrothermal resources, with temperatures from 180 ° C to over 350 ° C, are usually heated by hot molten rock. While low temperature resources, with temperatures from 100° C to 180° C, can be produced by either process [1].


Figure 3 Hydrothermal plant in New Zealand.
(Image courtesy of Dr Chris Lund, ACRE)

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 4 Simplfied cross section of the essential characteristics of a gethermal site.
(Image adapted from Boyle, 1998)

Hydrothermal resources require three basic components (see Figure 4) a heat source (eg 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.

Geopressured

Geopressured geothermal resources consist of 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 3km-6km [2]. 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. This resource is virtually limitless and is more accessible than hydrothermal resources. The geological profile of Australia is such that there is a large potential for hot dry rock technologies to be used for energy production in the eastern states of Australia. Figure six is a false colour map of Australia showing the relative potential of HDR techologies. Red indicates greatest potential, blue the least.


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


Figure 6 The potential for HDR technologies in Australia.
(Image courtesy of the Hot Rock Energy program, Australian National University
adapated from ERDC report 243, 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 which ranges from 700 - 1,200 °C. The resource has not been well explored to date.

Utilisation of Geothermal Energy

Geothermal energy can be utilised in two ways: direct heat or electricity generation.

Direct Use

Hydrothermal resources of low to moderate temperature (20° -150 ° C) are utilised to provide direct heating for a range of applications in the residential, commercial and industrial sectors. These applications include space heating, water heating, greenhouse heating, heating for aquaculture, food dehydration, laundries, and textile processes. Such applications are common in Iceland, the United States, Japan and France, as well as in many other countries. In Iceland, most of the residential houses are heated by geothermal district heating systems [1].

Direct-use geothermal systems usually consist of a production facility (eg a well) to convey the heated water to the surface, a mechanical system (eg piping, heat exchanger, pump, controls) to convey the heat energy to where it is required, and a disposal system (eg injection well or storage pond) to receive the cooled fluid. Heat exchangers are generally required due the salt and solids content of the geothermal fluid.

Heat pumps are often used to deliver the heat energy. Geothermal heat pumps are devices which operate on the same principal as the refrigerator, but can move heat in either direction. The devices can therefore take advantage of the relatively constant temperature of the earth's interior, using it as a source and sink of heat for both heating and cooling. In summer, heat is extracted from the building being cooled and dissipated into the earth. In winter, heat is removed from the earth and pumped into the building. Such systems are used widely in Switzerland and the Scandinavian countries. Through the use of geothermal heat pumps, marginal geothermal resources with temperatures as low as 20°C can be utilised.

The direct use of geothermal resources is a proven, mature technology and is commercially viable for many applications. The utilisation of the resource can result in a net saving in energy costs for consumers in homes and commercial operations.

Electricity generation

High temperature geothermal resources can be used for electricity production. There is currently over 8GW of installed geothermal electricity generation capacity worldwide (see Table 1). There are a number of energy conversion technologies, which use the geothermal resource. These include dry steam, flash steam and binary cycle systems.

Geothermal electricity can be used for base load power, as well as for peak load demand as required. In many parts of world, geothermal electricity is competitive with conventional energy sources.

Dry Steam Power Plant

The dry steam power plant (Figure 7) is suitable where the geothermal steam is not mixed with water. Production wells are drilled down to the aquifer and the superheated, pressurised steam (180 ° - 350 ° C) is brought to the surface at high speeds, and passed through a steam turbine to generate electricity. In simple power plants, the low pressure steam output from the turbine is vented to the atmosphere, but more commonly, the steam is passed through a condenser to convert it to water . This improves the efficiency of the turbine and avoids the environmental problems associated with the direct release of steam into the atmosphere. The waste water is then reinjected into the field via reinjection wells.


Figure 7 Schematic of a dry Steam Power Plant
(Image adapted from Boyle, 1998)

The waste heat is vented through cooling towers in common with conventional fossil fuel plants. In common also with conventional power stations, the energy conversion efficiencies are low, around 30% [3]. The efficiency and economics of dry steam plants are affected by the presence of non-condensable gases such as carbon dioxide and hydrogen sulphide. The pressure of these gases reduces the efficiency of the turbines, and in addition, the removal of the gases on environmental grounds adds to the cost of operation.

Dry steam power plants are the simplest and most economical technology, and therefore are widespread. The technology is well developed and commercially available, with units typically in the 35MW-120MW range [2]. The United States and Italy have the largest dry steam geothermal resources, but these resources are also found in Indonesia, Japan and Mexico. The Geysers field in California is a dry steam field. It is the largest geothermal power source in the world, with an installed capacity of about 1,100MW.

Flash Steam Power Plant

Single flash steam technology is used (Figure 8) where the hydrothermal resource is in a liquid form. The fluid is sprayed into a flash tank, which is held at a much lower pressure than the fluid, causing it to vaporise (or flash) rapidly to steam. The steam is then passed through a turbine coupled to a generator as for dry steam plants. To prevent the geothermal fluid flashing inside the well, the well is kept under high pressure.


Figure 8 Single Flash Steam Power Plant
(Image adapted from Boyle, 1998)

The majority of the geothermal fluid does not flash, and this fluid is reinjected into the reservoir or used in a local direct heat application. Alternatively, if the fluid remaining in the tank has a sufficiently high temperature, it can be passed into a second tank, where a pressure drop induces further flashing to steam. This steam, together with the exhaust from the principal turbine, is used to drive a second turbine or the second stage of the principal turbine to generate additional electricity. Typically, a 20 - 25% increase in power output is achieved, with a 5% increase in plant costs [3].

Flash steam plant generators range in size from 10MW to 55MW, but a standard size of 20MW is used in some countries, including the Philippines and Mexico [2].

Binary Cycle Power Plant

Binary cycle power plants (Figure 9) are used where the geothermal resource is insufficiently hot to efficiently produce steam, or where the resource contains too many chemical impurities to allow flashing. In addition, the fluid remaining in the tank of flash steam plants can be utilised in binary cycle plants (eg Kawerau, New Zealand).

In the binary cycle process, the geothermal fluid is passed through a heat exchanger. The secondary fluid, which has a lower boiling point than water (eg isobutane or pentane), is vaporised, and expanded through a turbine to generate electricity. The working fluid is condensed and recycled for another cycle. All of the geothermal fluid is reinjected into the ground in a closed-cycle system.


Figure 9 Binary Cycle Power Plant
(Image adapted from Boyle, 1998)

Binary cycle power plants can achieve higher efficiencies than flash steam plants, and they allow the utilisation of lower temperature resources. In addition, corrosion problems are avoided. However, binary cycle plants are more expensive, and large pumps are required which consume a significant percentage of the power output of the plants. The unit sizes are typically in the range of 1MW to 3MW, and these are used in a modular arrangement.

Hot Dry Rock Technology

The concept for utilising the geothermal energy in hot dry rocks is to create an artificial geothermal reservoir by drilling deep twin wells into the rock, and then forming a large heat exchange system by hydraulic or explosive fracturing. Water is circulated down the injection well through the created reservoir (which heats the water), and up the production well. While there is much potential in this technology, it has not yet been commercially demonstrated.

Benefits of Geothermal Energy

Constraints to Geothermal Energy Use

Geothermal Energy in Australia and New Zealand

Only limited amounts of geothermal energy are used in Australia, in stark contrast to New Zealand which produces 75% of its total energy requirements from gerothermal sources. Table 2 identifies Australian geothermal projects and the project status.

Project

Technology

Status
Garden East Apartments, South Australia Direct Use Heating and Cooling Geothermal technology installed and operational 1994
Hot Rock Energy Hot Dry Rock Technology Pilot Plant in the Hunter Valley has received funding
Mulka Cattle Station,
South Australia
20kW binary cycle and flash steam power plants have been constructed Operating since 1987

Geothermal Energy in Asia

The Philippines, Indonesia and Thailand use geothermal energy for electricity production (see Table 1). China and Taiwan have direct use geothermal applications, and to a lesser extent electricity production.

The Philippines is the second largest producer of geothermal electricity in the world, with an installed capacity of 1,848MW (see Table 1). The geothermal resources are extensive in the Philippines due to its location on the edge of the Philippine and Eurasian plates. The first geothermal plant commenced operation in 1979. The two largest fields are Mak Ban (426MW) and Tiwi (330MW), and they supply 16% of the electricity supply on the Philippines’ most populated island, Luzon (Unocal 1998). There is active development of new fields in the Philippines, which may soon make it the largest producer of geothermal electricity in the world. Geothermal energy is also used directly for fish processing, salt production and drying coconuts and fruit (Geothermal Education Office 1997).

Indonesia currently produces about 589MW of geothermal electricity (see Table 1). The Indonesian islands are located on the boundary between the Eurasian and Australian plates, resulting in a very good geothermal resource. The first geothermal development was the dry steam resource at Kamojang in the 1920s, which now produces 140MW of electricity. Currently, the largest field is Gunung Salak which has an installed capacity of 330MW. In addition to large projects, 10 MW of small (35kW-1MW) "mini-geo" plants for isolated villages are scheduled for development in 1999. It is predicted that by the end of 1999, the installed capacity in Indonesia will have increased to 1,079MW, largely supported by the World Bank (IGA 1998b). Geothermal steam and hot water are also used directly for cooking and bathing.

The Future for Geothermal Energy

In the short to medium term, it is likely that hydrothermal resources will remain the only geothermal resource that is commercially viable . This resource alone represents an immense source of energy. It is estimated that 80GW of geothermal electricity could be generated in the short to medium term from known hydrothermal resources worldwide [1].

In the 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

Geothermal Education Office
US DOE Office of Geothermal Technologies
Geothermal Resources Council
Geothermal Energy Association
CADDET Geothermal Register
CREST
The Geysers
Unocal Geothermal Energy Operations
Hot Dry Rock research, Australia (UNSW site, ANU Site)

Asia Pacific Region
International Geothermal Association
Unocal Geothermal Energy Operations, Philippines
Geothermal prospects, Indonesia
Indonesian Association of Geologists

References

1. Wright, P.M. 1998, "The earth gives up its heat", Renewable Energy World, vol.1, no.3, pp.21-25.
2. World Energy Council 1994, New renewable energy resources, Kogan Page, London.
3. Brown, G. 1996, "Geothermal energy", in Renewable energy- power for a sustainable future, ed. G. Boyle, Oxford University Press, Oxford.
4. IGA (International Geothermal Association) 1998a, "Welcome to our page with data for the United States ?" (Online), Available World Wide Web.
5. Unocal 1998, "Philippine geothermal" (Online), Available World Wide Web.
6. Geothermal Education Office 1997, "Geothermal energy worldwide" (Online), Available World Wide Web
7. IGA 1998b, "Welcome to our page with data for Indonesia" (Online), Available World Wide Web.
8. Hinrichs, R.A. 1996, Energy, its use and the environment, 2nd edn, Saunders College Publishing, Fort Worth.

Acknowledgments

This information was developed by Serena Fletcher, Katrina O'Mara, and Mark Rayner with assistance from Philip Jennings (Murdoch University) in June 1999.

The RE -Files were made possible with financial support from the Alternative Energy Development Board of WA