Geothermal Power Station
Geothermal Power
Stations
Lucien Y. Bronicki, Chairman
ORMAT Group
1
or liquid dominated (hot water). These are the
only commercially used resources at the present
time.
Organic Rankine cycle (ORC): A cycle using an
organic liquid as motive fluid (instead of water)
in a Rankine cycle.
Renewable energy: Energy, which is not exhausted
by use with time. Renewable energies include
direct solar energy, geothermal sources, wind,
hydroelectric plants, etc.
I.
INTRODUCTION
A. Source of Geothermal Energy
I..
II.
III.
IV.
V.
Introduction
Current Geothermal Energy Utilization
Current Uses and Commercial Status
The Ultimate Potential
Ecological and Enviornmental
Considerations
Geothermal energy is renewable heat energy from
deep in the earth. It originates from the earth’s
molten interior and the decay of radioactive
FIGURE 1. A representative geothermal reservoir.
[From Nemzer, M. (2000). Geothermal Education
Office, web site http://geothermal.marin.org.]
GLOSSARY
Binary geothermal power plant: A power plant in
which the geothermal fluid provides the heat
required by the organic working fluid.
Direct heat use: Utilization of low- and moderatetemperature geothermal resources for space and
water heating, for industrial processes and
agricultural applications.
Energy conversion: Conversion of one type of
energy to another such as the heat of a
geothermal resource to electricity, etc.
Geothermal combined cycle: Combined use of
geothermal steam and brine for power generation
by using a back-pressure steam turbine and
organic turbines.
Geothermal energy: Totally or partially renewable
heat energy from deep in the earth. It originates
from the earth’s molten interior and the decay of
radioactive materials. It is brought near to the
surface by deep circulation of ground water.
Geothermal heat pump (GMP): Application using
the earth as a heat source for heating or as a heat
sink for cooling
Geothermal resources: The four types of
geothermal
resources
are
hydrothermal,
geopressured, hot dry rock (HDR) and magma.
All are suitable for heat extraction and electric
power generation.
Hydrothermal resources: Geothermal resources
containing hot water and/or steam dropped in
fractured
porous rocks
at shallow
to moderate
Areas
whereorGeothermal
Projects
are in operation
depths. Categorized as vapor-dominated (steam)
materials; heat is brought near to the surface by deep
circulation of groundwater and by intrusion into the
earth’s crust of molten magma originating from great
depth (see Figure 1). In some places this heat comes
to the surface in natural streams of hot steam or
water, which have been used since prehistoric times
for bathing and cooking. By drilling wells this heat
can be tapped from the underground reservoirs to
supply pools, homes, greenhouses, and power plants.
The quantity of this heat energy is enormous; it
has been estimated that over the course of a year, the
equivalent of more than 100 million GWh of heat
energy is conducted from the earth’s interior to the
surface. But geothermal energy tends to be relatively
diffuse, a phenomenon which makes it difficult to
tap. If it were not for the fact that the earth itself
concentrates geothermal heat in certain regions
(typically regions associated with the boundaries of
tectonic plates in the earth’s crust, see Figure 2),
geothermal energy would be essentially useless as a
heat source or a source of electricity using today’s
technology.
2Geothermal Power Station
FIGURE 2. World map showing lithospheric plate
boundaries. [From Nemzer, M. (2000). Geothermal
Education
Office,
web
site
http://geothermal.marin.org.]
There is some ambiguity on the issue of
geothermal energy being a “renewable” resource.
Some geothermal sites may be developed in such a
manner that the heat withdrawn equals the heat being
replaced naturally, thus making the energy source
renewable for a long period of time. At other sites,
the resource lifetime may be limited to some decades.
In any case, even if it is not technically a renewable
resource, potential global geothermal resources
represent such a huge amount of energy that,
practically speaking, the issue is not the finite size of
the resource but availability of technologies that can
tap the resource in an economically acceptable
manner.
B. Nature of the Geothermal Energy Resource
On average, the temperature of the earth increases by
about 3°C for every 100 meters in depth. This means
that at a depth of 2 km, the temperature of the earth is
about 70°C, increasing to 100°C at a depth of 3 km,
and so on. However, in some places, tectonic activity
allows hot or molten rock to approach the earth’s
surface, thus creating pockets of higher temperature
resources at easily accessible depths (World Energy
Council, 1994).
The extraction and practical utilization of this
heat requires a carrier which will transfer the heat
towards the heat-extraction system. This carrier is
provided by geothermal fluids forming hot aquifers
inside permeable formations. These aquifers or
reservoirs are the hydrothermal fields. Hydrothermal
sources are distributed widely but unevenly across
the earth. High-enthalpy geothermal fields occur
within well-defined belts of geologic activity, often
manifested as earthquakes, recent volcanism, hot
springs, geysers and fumaroles. The geothermal belts
are associated with the margins of the earth’s major
tectonic or crustal plates and are located mainly in
regions of recent volcanic activity or where a
thinning of the earth’s crust has taken place. One of
these belts rings the entire Pacific Ocean, including
Kamchatka, Japan, the Philippines, Indonesia, the
western part of South America running through
Argentina, Peru, Ecuador, Central America, and
Western North America. An extension also penetrates
across Asia into the Mediterranean area. Hot crustal
material also occurs at mid-ocean ridges (e.g.,
Iceland and the Azores) and interior continental rifts
(e.g., the East African rift, Kenya and Ethiopia).
Low-enthalpy resources are more abundant and
more widely distributed than high-enthalpy
resources. They are located in many of the world’s
deep sedimentary basins, for example, along the Gulf
Coast of the United States, Western Canada, in
Western Siberia, and in areas of Central and Southern
Europe, as well as at the fringes of high-enthalpy
resources.
There are four types of geothermal resources:
hydrothermal, geopressured, hot dry rock, and
magma. Although they have different physical
characteristics, all forms of the resource are
potentially suitable for electric power generation if
sufficient heat can be obtained for economical
operation.
1.
Hydrothermal Resources
These are the only commercially used resources at
the present time. They contain hot water and/or steam
trapped in fractured or porous rock at shallow to
moderate depths (from approximately 100 to 4,500
m). Hydrothermal resources are categorized as vapordominated (steam) or liquid-dominated (hot water)
according to the predominant fluid phase.
Temperatures of hydrothermal reserves used for
electricity generation range from 90°C to over 350°C,
but roughly two thirds are estimated to be in the
moderate temperature range (150-200°C). The
highest quality reserves contain steam with little or
no entrained fluids, but only two sizeable, highquality dry steam reserves have been located to date
at Larderello in Italy and The Geysers field in the
United States.
Recoverable resources available for power
generation far exceed the developments to
date. Many countries are believed to have
potential in excess of 10,000 MWe which
would fulfil a considerable portion of their
electricity requirements for many years (e.g.
the Philippines, Indonesia and the US).
Important
low-enthalpy
hydrothermal
resources are not necessarily associated with
young volcanic activity. They are found in
sedimentary rocks of high permeability
which are isolated from relatively cooler
near-surface ground water by impermeable
strata. The water in sedimentary basins is
heated by regional conductive heat flow.
These baasins (e.g. the Pannonian Basin) are
commonly hundreds of kilometers in
diameter at temperatures of 20-100’C. They
are exploited in direct thermal uses or with
heat pump technology.
Geothermal Power Station
FIGURE 3. The first geothermal power plant at
Larderello, Italy (1904). [From Nemzer, M. (2000).
Geothermal
Education
Office,
web
site
http://geothermal.marin.org.]
Of the geothermal resource types, hydrothermal
energy is the most widely applied and most costcompetitive and the only one presently used
commercially. The uses of magma, geopressured and
hot dry rock systems are still at experimental stages,
although the latter two types have been technically
demonstrated successfully and energy extraction has
been experimentally verified.
B. Present State-of-the-Art
For geothermal energy utilization, a number of
technological solutions have been introduced. Several
of these are still under development while some are
in commercial use but still undergoing continuous
improvement. The following is an overview of the
technology solutions and their developmental status,
thus establishing a basis for subsequent discussion.
1.
Exploration and Extraction
Hydrothermal development begins with exploration
to locate and confirm the existence of a reservoir with
economically exploitable temperature, volume and
accessibility. The geosciences (geology, geophysics
and geochemistry) are used to locate reservoirs, to
characterize their conditions and to optimize the
locations of wells. Drilling technology used for
geothermal development derives historically from the
petroleum industry. Certain critical components such
as drilling muds were modified to work in hightemperature environments but proved to be only
marginally adequate. Materials and equipment
capable of dealing not only with increasing
temperatures but also with hard, fractured rock
formations and saline, chemically reactive fluids
were needed. As a result, a specialized part of the
drilling industry devoted to geothermal development
evolved.
Hydrothermal fluids may be produced from
wells by artesian flow (i.e. fluid forced to the surface
by ambient pressure differences) or by pumping. In
the former case, the fluid may flash into two phases
(steam and liquid), whereas under pumping the fluid
remains in the liquid phase. The choice between these
two production modes depends on the characteristics
of the fluid and the design of the energy-conversion
system.
Geothermal fields generally lend themselves to
“staged” development, whereby a modestly sized
plant can be installed at an early stage of field
assessment. It may be small enough to be operated
with confidence on the basis of what is known of the
field. Its operation provides the opportunity for
3
obtaining reservoir information which may lead to
the installation of additional stages.
2.
Direct Heat Use
The abundant low- and moderate-temperature
hydrothermal fluids may be used as direct heat
sources for space and water heating, for industrial
processes, and for agricultural applications. The
major uses include balneology, space heating and hot
water supplies for public institutions. District-heating
systems for groups of buildings are the predominant
other uses (see Figure 4).
FIGURE 4. A district heating plant. [From Nemzer,
M. (2000). Geothermal Education Office, web site
http://geothermal.marin.org.]
Other applications are greenhouse
heating, warming fish ponds in aquaculture,
crop drying, and various washing and drying
applications in the food, chemical, and
textile industries. In regions where hightemperature resources occur, combination of
electricity production with these uses (e.g.,
in Iceland) is possible.
In the direct-use of geothermal systems,
fluids are generally pumped through a heat
exchanger to heat air or a liquid, although
the resource may be used directly if the salt
and solids contents are low. These systems
exemplify the simplest applications using
conventional off-the-shelf components.
For most of the specified uses, the
hydrothermal source is at about 40°C. With
heat pump technology, a hydrothermal
source of 20°C or less can be used as a heat
source, as is done, for example, in the USA,
Canada, France, Sweden, and other
countries. The heat pump operates on the
same principle as the home refrigerator,
which is actually a one-way pump. The
geothermal heat pump (GHP) can move heat
in either direction. In the winter, heat is
removed from the earth and delivered to the
home or building (heating mode). In the
summer, heat is removed from the home or
building and delivered for storage to the
earth (air-conditioning mode). In either
cycle, water is heated and stored, supplying
4Geothermal Power Station
all or part of the function of a separate hotwater heater. Because electricity is used
only to transfer heat and not to produce it,
FIGURE 5.
plant
Schematic of a geothermal
over a wide range of earth temperatures.
Current growth rates for GHP systems run
as high as 20% per year in the USA and the
outlook for continued growth at double-digit
rates is good. The US Department of Energy
Information Administration (EIA) has
projected that GHPs in the USA could
provide up to 68 Mtoe (mega-tons of oil
equivalent) of energy for heating, cooling
and water heating by 2030 (Lund and
Freeston, 2000).
C. Plant Options for Power Generation
There are several types of energy-conversion
processes
for
generating
electricity
from
hydrothermal resources (see Figure 5). These include
dry steam and flash steam systems, which are
traditional processes, and binary cycle and total flow
systems, which are newer processes with significant
advantages (World Energy Council, 1994).
the GHP will deliver 3-4 times more energy
than it consumes. It can be used effectively
to a steam turbine. This is a well-developed,
commercially available technology, with typical unit
sizes in the 35-120 MWe capacity range. Recently, in
some places, a new trend of installing modular
standard generating units of 20 MWe has been
adopted. In Italy, smaller units in the 15 to 20 MWe
range have been introduced.
E. Flashed Steam Plants
More complex cycles are used to produce energy
from liquid-dominated reservoirs which are
sufficiently hot (typically above 160°C) to flash a
large proportion of the liquid to steam. As shown in
Figure 7, single-flash systems evaporate hot
geothermal fluids to steam by reducing the pressure
of the entering liquid and direct it through a turbine.
In dual-flash systems, steam is flashed from the
remaining hot fluid of the first stage, separated and
fed into a dual-inlet turbine or into two separate
turbines. In both cases, the condensate may be used
for cooling while the brine is re-injected into the
reservoir. This technology is economically
competitive at many locations and is being developed
using turbogenerators with capacities of 10-55 MWe.
A modular approach, using standardized units of 20
MWe, is being implemented in the Philippines and
Mexico.
D. Dry Steam Plants
Conventional steam-cycle plants are used to produce
energy from vapor-dominated reservoirs. As is shown
in Figure 6, steam is extracted from the wells,
cleaned to remove entrained solids and piped directly
FIGURE 6. General Electric Co. dry steam plant at The
Geysers, California. [From Nemzer, M. (2000). Geothermal
Education Office, web site http://geothermal.marin.org.]
FIGURE 7.
The Mitsubishi flash steam plant in
Beowawe,
Nevada.
[Courtesy
of
Mitsubishi
Heavy
Industries]
FIGURE 8 An air-cooled binary plant
2. Moderate Enthalpy Resources (160 to
190°C)
Geothermal Power Station
5
3. Total Flow Turbines
For moderate enthalpy, two-phase resources with
steam quality between 10 and 30%, binary plants are
efficient and cost-effective. Furthermore, when the
geothermal fluid has a high non-condensible gas
(NCG) content, even higher efficiency can be
obtained than with condensing steam turbines.
This binary two-phase configuration is
used in the São Miguel power plant in the
Azores Islands (see Figure 9). Separated
steam containing NCG is introduced in the
vaporizer heat exchanger to vaporize the
organic fluid. The geothermal condensate at
the vaporizer exit is then mixed with the hot
separated brine to provide the preheating
medium for the organic fluid. Since the
onset of silica precipitation is related to its
concentration in the brine, dilution of the
brine with the condensate effectively lowers
the precipitation temperature at which silica
crystallizes. This lower temperature added
3.5 MW of heat to the cycle, representing
20% of the total heat input. The additional
heat is utilized at the same thermal
efficiency as the remaining heat in the
combined steam-brine cycle. Since the cycle
efficiency is about 17%, the lowtemperature heat
FIGURE 9 The ORMAT two-phase binary
geothermal power plant in São Miguel,
Azores Islands
produces about 600 additional kW. The
main advantage of the geothermal combined
cycle plant over conventional steam plants
lies in the efficiency of the power plant
when using both steam and brine in the
conversion process. It provides sustainable
power and does not deplete the geothermal
reservoir since all fluids are reinjected. This
feature contributes to the environmental
acceptability of the plant since it operates
without emissions and no abatement of noncondensable gases (NCG) is needed. The
air-cooled condensers contribute to the low
physical profile of the plant and there is no
plume.
This is an experimental process, based on using
concurrently steam, hot water, and the pressure of
geothermal resources (i.e. the total resource), thereby
eliminating energy losses associated with the
conventional method of flashing and steam
separation. These systems usually channel a mixture
of steam and hot water into a rotating conversion
system and capture the kinetic energy of the mixture
to power an electric generator.
III. CURRENT
STATUS
USES
AND
COMMERCIAL
Electricity from geothermal energy has been
generated in Italy for more than 90 years.
Until 1974, the total installed capacity for
converting
geothermal
energy
into
electricity was only about 770 MWe (in
Italy, Japan, New Zealand, the United
States, and Mexico). Following the second
oil shock, the world-wide installed capacity
achieved its highest growth of 17.2% per
year. The number of geothermal powerproducing countries increased from 10 to 17.
Recent emphasis has been placed on power
production using the liquid hydrothermal
resource since power production with dry
steam has been commercially viable for
several decades. In the year 2000, a total of
over 8,000 MWe were produced from
geothermal resources in more than 20
countries. Substantial market penetration has
thus far occurred only with hydrothermal
technology.
Table 1 shows selected countries and
their installed power plants. From 1978 to
1985, the world-wide installed electrical
capacity grew at an average annual rate of
about 17.2%. The causes of the growth
surge were the two oil shocks (1973 and
1979) and expectations of further oil-price
rises. Many of the known profitable
resources were exploited and much work
was devoted to exploration for new
hydrothermal resources. After the oil price
collapse in 1990, the growth rate fell to
about 4% per year. Since most of the
subsidies for renewable energy and
especially those for geothermal energy
6Geothermal Power Station
TABLE I. World-wide geothermal installed capacity in the year
2000 in MWe.
United States
The Philippines
Mexico
Italy
Indonesia
Japan
New Zealand
Iceland
El Salvador
Costa Rica
Nicaragua
2,228
1,909
855
785
589
547
437
170
161
142
70
Kenya
Guatemala
China
Russia
Turkey
Portugal (Azores)
Ethiopia
France (Guadalupe)
Thailand
Australia
Total
57
33
29
23
20
16
9
4
0.3
0.17
8154
were almost completely stopped, this growth
rate is not negligible.
During the past 25 years, geothermal technology
(mainly hydrothermal) has changed from mainly
balneological uses to widespread industrial,
agricultural and district-heating usage, and from the
use of dry steam resources to power production from
a wide spectrum of resources. The energy-conversion
technology has become a mature and commercially
viable technique. Binary and geothermal combined
cycle power plants, which reached maturity with
more than 600 MWe of commercially installed
capacity, operate as closed-loop geothermal power
plants with almost zero pollutants and no water
consumption. Plants of a few hundred kilowatts up to
tens of megawatts may be installed in a period of a
few months and provide
indigenous energy sources.
clean,
sustainable
The second approach to better resource
utilization involves the use of a regenerative
cycle through the addition of a recuperator
heat exchanger between the organic turbine
and the air-cooled condenser, since the
organic vapor tends to superheat when the
vapor is expanded through the turbine. In
this case, the recuperator reduces the amount
of heat that must be added to the cycle from
the external source, thereby reducing the
required brine-flow rate. This procedure
results in a reduction of about 7% of the
total heat input to produce the design power
output.
A. Geothermal Combined Cycle Plants
For efficient use of a steam-dominated resource, a
geothermal combined cycle is applied. The steam
first flows through a back-pressure steam turbine and
is then condensed in the organic turbine vaporizer
(see Figure 10). The condensate and brine are used to
preheat the organic fluid as in the two-phase binary
confiiguration. This concept was first used in 1989 to
repower a back-pressure steam plant in Iceland.
FIGURE 10 The ORMAT geothermal combined-cycle power plant in Puna, Hawaii
Subsequent uses were with a 30 MW plant in Hawaii in 1992, followed by a 125 MW plant in the Philippines and a
60 MW plant in New Zealand. (Bronicki, 1998).
B. Direct Applications of Hydrothermal Energy
Direct applications of geothermal energy involve a wide variety of end uses, such as space heating and cooling,
industrial heat, greenhouses, fish farming, and health spas. Existing technology and straightforward engineering are
involved. The technology, reliability, economics, and evironmental acceptability of the direct use of geothermal
energy has been demonstrated throughout the world. Space heating is the dominant application (37%), while other
common uses are bathing/swimming/ balneology (22%), heat pumps (14%) for air cooling and heating, greenhouses
(12%), fish farming (7%), and industrial processes (7%).
The relative share of Asia has increased in recent years for direct energy production. It is
estimated to be 44% of total at present mainly because of rapid expansion in China. The
European share has decreased to 37%, while that of the Americas has grown to 14% due to
increased uses of heat pumps in the USA.
Direct applications use both high- and low-temperature geothermal resources and are
therefore much more widespread in the world than electricity production. Direct applications are,
however, site specific for the market, as steam and hot water are rarely transported long distances
from geothermal sites.
Geothermal Power Station
7
China has geothermal water in almost every province. The direct utilization is expanding at a
rate of about 10% per year, mainly in the space heating (replacing coal), bathing, and fishfarming sectors. Japan is also blessed with very extensive geothermal resources which so far
have mainly (80%) been used for bathing, recreation and tourism and, to a lesser extent, for
electricity production. This development has improved the quality of life of people significantly,
but only a fraction of the available geothermal energy is actually used. Turkey has greatly
increased the direct use of geothermal resources in recent years. Mexico is the first country in
the tropics to report significant direct use of geothermal energy. Switzerland and Sweden have
recently joined the top league through extensive use of ground-source heat pumps.
C. Heat-Pump Applications
Geothermal energy has until recently had a considerable economic potential only in areas where thermal water or
steam is found concentrated at depths of less than 3 km in restricted volumes, analogous to oil in commercial oil
reservoirs. This status has changed with developments in the application of ground-source heat pumps using the
earth as a heat source for heating or as a heat sink for cooling, depending on the season. As the result, all countries
may use the heat of the earth for heating and/or cooling, as appropriate. It should be stressed that heat pumps can be
used everywhere.
During the last decade, a number of countries have encouraged individual house owners to
install ground-source heat pumps to heat their houses in the winter and (as needed) cool them in
the summer. Financial incentive schemes have been set up, commonly funded by the
governments and electric utilities, as the heat pumps reduce the need for peak power and thus
replace new electric generating capacity. The USA leads the way with about 400,000 heat-pump
units (about 4,800 MWt) and energy production of 3,300 GWh/y in 1999. The annual increase is
about 10%. Other leading countries are Switzerland, Sweden, Germany, Austria, and Canada.
8Geothermal Power Station
TABLE II Direct Geothermal Use during the year 2000
Region
Africa
America
Asia
Europe
Oceana
Total
Installed
Capacity
MWt
121
5,954
5,151
5,568
318
17,112
a
Yearly
Production
GWh/a
492
7,266
22,532
18,546
2,049
50,885
a
From Lund, J.W., and Freeston, D.H. (2000). “World direct uses of geothermal energy 2000.” In “Proceedings, World
Geothermal Congress,” pp. 1-21.
Switzerland, a country not known for hot springs or geysers, provides an example of the
impact this development can have on geothermal applications in what previously would have
been called non-geothermal countries. The energy extracted out of the ground with heat pumps
in Switzerland amounts to 434 GWh/y. The annual growth rate is 12%.
TABLE III The World’s Top 15 Direct Use Countries for Geothermal Energy
China
Japan
USA
Iceland
Turkey
New Zealand
Georgia
Russia
France
Hungary
Sweden
Mexico
Italy
Romania
Switzerland
Installed
Capacity
MWt
2814
1159
5366
1469
820
308
250
307
326
391
377
164
326
152
547
a
Yearly
Production
GWh/a
8724
7500
5640
5603
4377
1967
1752
1703
1360
1328
1147
1089
1048
797
663
a
From Lund, J.W., and Freeston, D.H. (2000). “World direct uses of geothermal energy 2000.” In “Proceedings, World
Geothermal Congress,” pp. 1-21.
Prior to 2000, the total installed capacity for the direct use of geothermal energy world-wide was about 17,000 MWt
(see Table 2). The top fifteen primary users of direct geothermal heat and the year 2000 capacity are listed in Table
3. The total installed capacity in 1975 was only about 3,100 MWt (excluding balneology).
IV. THE ULTIMATE POTENTIAL
The growth rate of the geothermal energy market is not limited by the lack of resources. During the early oil crises,
intensive investigations led to the discoveries of many geothermal reservoirs for electricity generation, some of
which are in operation while about 11,000 MWe of proven resources are not yet tapped. In the near future, the
growth rate will most probably be 3-4% annually, as has been the case during the past few years.
However, if environmental impacts of energy use are internalized, then the real value of
geothermal technology including its superior environmental characteristics and local resource
features will be taken into account and the geothermal market will become more profitable. As
Geothermal Power Station
9
the result, there will be enhanced geothermal exploration and R&D. The growth rate should then
reach 6-7% and more. This outlook should encourage the development of other geothermal
resources. Hot dry rock and geopressured technologies may reach maturity around 2010.
In 1978, the Electric Power Research Institute (EPRI) published a report on the ultimate
potential for geothermal energy on a global basis. The accessible global total is very much
greater than today’s usage. Most of the total resource is contained in hot dry rocks.
The geothermal resource base underlying the continental land masses of the world to a depth of 3 km and at
temperatures higher than 15°C was calculated to be 1.2 x 10 13 GWh or 1.03 x 109 Mtoe. It therefore appears that
geothermal energy is an abundant resource (Nakićenović et al., 1998). If we are able to exploit only 1% of this
energy we will have enough energy for several hundred years. In order to tap most of this energy resource, we need
to invest money to improve the existing technologies, especially extraction of energy from hot dry rocks (World
Energy Council 2000).
The authors of a study for the US DOE’s Office of Renewable Energy (US DOE, 1998) argued that geothermal
energy is by far the most abundant non-nuclear energy source in the US, accounting for nearly 40% of the total
energy resource base. The total resource base is defined as concentration of naturally occurring solid, liquid, or
gaseous materials in or on the Earth’s crust in such a form that economic extraction of the commodity is currently or
potentially feasible. In this study, the resource base
FUEL TYPE
kg CO2 per kilowatt hour
FIGURE 11. Relative CO2 emissions for different energy resources
includes geothermal reservoirs with a minimum temperature of 80°C at a maximum depth of 6 kilometers, except
for geopressured resources which are included to seven kilometers. Also included are low-temperature resources in
the 40-80°C range to a depth of 2-3 kilometers.
V. ECOLOGICAL AND ENVIRONMENTAL CONSIDERATIONS
The successful implementation of the Kyoto targets, introducing internationally agreed vehicles to mitigate
greenhouse gas emissions, will enhance the use of non-fossil-fuel systems, including geothermal energy (Fig. 11).
There are other environmental advantages to geothermal energy, as power plants using it require far less land area
than other energy sources. This last is illustrated in Table IV.
TABLE IV. Land Area Occupied for Different Energy Technologies
Technology
Coal (including coal mining)
Solar thermal
Photovoltaics
Wind (land with turbines and
roads)
Geothermal
Land area
(m2 per
GWhr/year
for 30 years)
3,642
3,561
3,237
1,335
404
Oil
Coal
TYPE
mal:
r Combined Cycle
Geothermal Power Station
10
mal:
Gas –
bine
0
0.25
0.5
0.75
1.0
1.25
1.5
kg CO2 per kiloWatt-hour
BIBLIOGRAPHY
Co
al
Allegrini, G., et al. (1991) “Growth Forecast in Geothermoelectric Capacity in the World to the year 2020.
Baria, R., Baumgartner, J., and Gerard, A. (1998) “International Conference-4th HDR Forum, Strasbourg, France”,
SOCOMINE, EHDRA.
Barnea, J. (1981) “The Future of Small Energy Sources,” UNITAR, New York.
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F. Binary-Cycle Plants
1.
Low-Enthalpy Resources (100 to 160°C)
For low-enthalpy resources, binary plants based on the use of organic Rankine cycles (ORC) are utilized to
convert the resource heat to electrical power (see Figure 9). The hot brine or geothermal steam is used as
the heat source for a secondary (organic) fluid, which is the working fluid of the Rankine cycle.
During the early 80’s, in order to increase the power output from a given brine
resource by increasing the thermal cycle efficiency, a super-critical cycle using isobutane
was developed as well as a cascade concept. The super-critical cycle may be slightly
more efficient than the cascading cycle, but the cascading system has the advantage of
lower operating pressures and lower parasitic loads in the cycle pumps. For example, at a
power plant in Southern California, a three-level arrangement was employed and resulted
in increased efficiency or power output gain of about 10% over that achievable with a
simple ORC.
For all of the configurations and systems, a modular approach was employed so that high plant
availability factors of 98% and above were achievable.
Encyclopedia of Physica Science and Tecnology, Third Edition, Volume 6
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