Download Fact sheet 9: Geothermal energy - Australian Institute of Energy

FACT SHEET 9:
GEOTHERMAL ENERGY
Geothermal energy originates deep within the Earth’s molten interior. It is
this heat energy which is responsible for movement of 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 80 –100 km [1].
Heat is brought closer to the Earth’s surface by deeply circulating
groundwater and molten magma rising through the Earth’s crust to depths
of only 1 –5 km. The hot molten rock heats the surrounding groundwater,
which is forced to the surface in the form of hot water and steam (eg. hot
springs and geysers). When this heat energy is close to or at the Earth’s
surface it can be used as a source of energy, namely geothermal energy.
The total geothermal resource is vast. An estimated 100 PWh (1 x 1014
kWh) of heat energy reaches the Earth’s surface from the subterranean
depths each year [2]. Unfortunately geothermal energy can only be used
directly in places where it is concentrated (high grade). These places
usually occur at the junctions of the tectonic plates that make up the Earth’s
crust and are areas of earthquake and volcanic activity. Here the heat
energy travels rapidly from the Earth’s interior to the surface, often
producing hot springs or geysers.
Figure 1.
World High Temperature Geothermal Provinces
(Source: Geothermal Energy, 1998, University of Utah)
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.
1
Background
Geothermal energy has been used by humans for many centuries for such
things as space heating, water heating, cooking, and medicinal bathing.
The first geothermal electricity generation plant was constructed in 1904 in
Larderello, Italy. It had a capacity of 250 kW and used geothermal steam to
generate electricity. The second geothermal power station was built in the
1950s at Wairakei, New Zealand followed by The Geysers in California in
the 1960s [3].
The United States is the largest producer of geothermal electricity,
followed by the Philippines [4].
Table 1.
World wide installed generating capacity by country.
(Courtesy of International Geothermal Association)
Geothermal resources are not strictly renewable because as the heat
escapes from the interior of the planet the Earth cools. This occurs on a
very much longer time scale than human lifetimes. So in our time frame
geothermal energy is classified as 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 vary from a few to over 1,000MW of
thermal power [1]. At present however, all installed generators are
exceeding the recharge rates of the resources. The geothermal resources
are therefore being depleted and are effectively being ‘mined’ in a similar
way to fossil fuels.
2
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 exploited commercially.
HYDROTHERMAL
Hydrothermal, or hot water, resources are found in places where hot water
and/or steam is formed in fractured or porous rock at shallow to moderate
depths (100 m to 4.5 km). The ground water is heated either by molten
magma coming close to the Earth’s crust or by 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. Low temperature hydrothermal resources, with temperatures
from 100°C to 180°C, can be produced by either process [1].
Figure 2.
Hydrothermal plant in New Zealand.
(Image courtesy of Dr Chris Lund, ASEC)
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.
3
Figure 3.
Simplified cross section of the essential characteristics of a geothermal site.
(Image adapted from Boyle, 2002)
Hydrothermal resources require three basic components: a heat source (eg
crystallised granite), an aquifer containing water that can be accessed from
the surface and an impermeable cap rock to seal the aquifer (see Figure 3).
The geothermal energy is usually tapped by drilling into the aquifer and
extracting the hot water or steam.
HOT DRY ROCK
Hot dry rock (HDR) resources are found in areas where the flow of heat
from the interior of the earth to the surface is higher than average but there
is no water because no aquifers or fractures are present. These are
required to conduct water to the surface. It may be necessary to create
these fractures so that water can be pumped down a borehole and allowed
to circulate through cracks in the hot rock several kilometres below the
surface. The hot water is then brought back up to the surface through
another borehole and used to generate electricity. The water can be reused
over and over again.
This resource is very large and is generally more accessible than
hydrothermal resources. The geological structure of Australia indicates that
there is a large potential for hot dry rock technologies to be used for energy
production in the eastern States of Australia. Figure 4 is a false colour map
of Australia showing the relative potential of HDR technologies. Red
indicates greatest potential, blue the least. While worldwide several HDR
test projects are showing promising results, the technology has not yet
been commercially demonstrated.
4
Figure 4.
Hot Dry Rock potential
GEOPRESSURED
A geopressured geothermal resource consists of deeply buried water that
contains dissolved methane. It is found in large, deep aquifers under high
pressure. The water and methane are trapped in sedimentary formations at
a depth of about 3 km to 6 km [2]. The temperature of the water is in the
range of 90°C to 200°C. Three forms of energy can be obtained from
geopressured resources: firstly thermal energy, secondly hydraulic energy
due to the high pressures and thirdly chemical energy by burning the
dissolved methane gas. While technologies are available to tap
geopressured resources, they are not currently economically competitive.
The major region of geopressured reserves discovered to date is in the
northern Gulf of Mexico.
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 range of 700 - 1,200 ° C. Technology is not available to utilise
magma as an energy source.
Using Geothermal Energy
Geothermal energy can be used in two ways as a source of direct heat or
for electricity generation.
DIRECT USE
Hydrothermal resources of low to moderate temperature (20° - 150°C) are
used 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. China uses 38 PJ, Japan 27
5
PJ and the USA and Iceland each use 20 PJ of geothermal energy for
direct heat [9]. Many other countries with hot springs use this resource.
About twice as much geothermal energy is used for direct heat as for
electricity generation.
Direct-use geothermal systems usually consist of a production facility (eg. a
well) to bring the heated water to the surface, a mechanical system (eg.
piping, heat exchanger, pump, controls) to move 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 usually needed when the
geothermal fluid contains salt and other dissolved solids.
Heat pumps are often used to move the heat energy from place to place.
Geothermal heat pumps are devices which operate on the same principle
as the refrigerator but can move heat in either direction. They take
advantage of the relatively constant temperature of the earth's interior,
using it as a source of heat for heating and as a heat sink for cooling. In
summer, heat is extracted from the building being cooled and dumped 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
used.
The direct use of geothermal resources is a proven, mature technology and
is commercially viable for many applications. The use of this resource,
where available, 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 8 GW of installed geothermal electricity
generation capacity worldwide (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 to supply base load power, as well as
for peak load demand as required. Where the resource is in good supply,
geothermal electricity can be competitive with conventional energy sources.
Benefits of Geothermal Energy
• Using modern emission controls, geothermal energy is one of the least
polluting sources of energy. The World Energy Council shows emissions of
carbon dioxide from geothermal power generation to be less than one tenth
of that from gas and emissions of sulphur dioxide less than one tenth of
that from coal or oil.
• Geothermal power stations are very reliable with a high availability and
capacity factor. Geothermal power plants are designed to run 24 hours a
day, and operation is independent of the weather or fuel delivery.
• Geothermal power generation technologies are modular in design and
highly flexible. The output of a geothermal plant can be expanded as
required, avoiding the need for a high initial capital outlay.
6
• Geothermal resources are a local energy supply. This means there is
energy supply security, a reduced need for fuel imports and an improved
balance of payments. These issues are particularly important in developing
countries, where geothermal resources can reduce the economic pressures
of importing fuels and can provide local technical infrastructure and
employment.
• Geothermal energy has an inherent energy storage capability and there is
no need for other technology to store the energy as, for example, batteries
are needed with solar cells to store the electrical energy.
• Geothermal power stations do not need a great deal of land.
Constraints to
Geothermal Energy Use
High grade geothermal resources are limited to small areas of the world.
• Geothermal fluids (steam or hot water) usually contain gases such as
carbon dioxide (CO2), hydrogen sulphide (H2S), ammonia (NH3) and
methane (CH4). These gases, if released, can not only add to greenhouse
warming but can also be toxic and smelly. Geothermal fluids also usually
contain dissolved chemicals, which commonly include sodium and
potassium chlorides, arsenic or mercury. These fluids would be a source of
pollution if discharged into the environment. Modern emission control
techniques and re-injection of contaminated fluids back into the ground is
needed to minimise the impacts of these pollutants.
• The waste waters from geothermal plants have a higher temperature and
even if pure need to be cooled before discharge.
• Geothermal production may cause ground subsidence. This is rare in dry
steam resources but possible in liquid dominated fields (eg Wairakai, New
Zealand). The technique of injecting the geothermal fluid back into the
ground can effectively remove this risk.
• Geothermal energy production has been associated with increased
seismic activity. This is a debatable issue as most geothermal fields are
located in regions that are already prone to earthquakes. Seismic activity
has not increased significantly close to production plants where the
geothermal fluid is injected back into the ground and maintains reservoir
pressures, (United Nations Environment Programme).
• Geothermal energy resources will be depleted if used beyond their natural
recharge rate.
• Geothermal plants produce noise pollution during construction (eg drilling
of wells, and the escape of high pressure steam during testing) but this is
not significant once the plants are operating.
7
The Future for Geothermal Energy
The short to medium term future of geothermal energy is encouraging.
Plans already drafted by Indonesia, Philippines and Mexico at the end of
the 1990s, but partly delayed, aim at an additional 2 000 MWe before 2010.
In the direct use sector, China has the most ambitious target: the
substitution of 13 million tonnes of coal by geothermal energy [10].
In the longer term, technological developments will see greater utilisation of
the geothermal energy in hot dry rocks and geopressured reservoirs.
Usable geothermal resources will no longer be limited to shallow
hydrothermal reservoirs. Hot dry rock and geopressured resources
represent a virtually limitless source of energy, and are the future of
sustainable geothermal energy.
Abbreviations
MWe- Megawatts electricity
GW- Gigawatts
PJ-
Petajoules
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Wright, P.M. 1998, "The earth gives up its heat", Renewable Energy
World, vol.1, no.3, pp.21-25.
World Energy Council 1994, New renewable energy resources,
Kogan Page, London.
Boyle, G. (2002). Renewable Energy Power for a Sustainable Future.
Oxford, Oxford University Press in association with the Open
University. (Brown, G. 1996, "Geothermal energy")
IGA (International Geothermal Association) 1998a, "Welcome to our
page with data for the United States ?" (Online), Available World Wide
Web.
Unocal 1998, "Philippine geothermal" (Online), Available World Wide
Web.
Geothermal Education Office 1997, "Geothermal energy worldwide"
(Online), Available World Wide Web
IGA 1998b, "Welcome to our page with data for Indonesia"
http://iga.igg.cnr.it/geoworld/geoworld.php?sub=map&region=asia&co
untry=indonesia
Hinrichs, R.A. 1996, Energy, its use and the environment, 2nd edn,
Saunders College Publishing, Fort Worth.
International Geothermal Association http://iga.igg.cnr.it/index.php
World Energy Council 2003 Survey of Energy Resources, Geothermal
http://www.worldenergy.org/wecgeis/publications/reports/ser/geo/geo.
asp
8
Further Information
For further information on “Geothermal Resources” and the different types
of power plants that use geothermal energy to produce electricity visit the
RE-Files
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)
International Geothermal Association
Unocal Geothermal Energy Operations, Philippines
Geothermal prospects, Indonesia
Indonesian Association of Geologists
Acknowledgments
This information was developed by Serena Fletcher, Katrina Lyon, and
Mark Rayner with assistance from Philip Jennings (Murdoch University) in
June 1999. It was reworked by Christine Creagh (2004, Murdoch University)
for the Australian Institute of Energy
9