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Binary geothermal power plant A power plant in which the geothermal fluid provides the heat required by the organic working fluid Geothermal.

Direct heat use Utilization of low- and moderate-temperature 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, and is brought near the surface by deep circulation of ground water.

Geothermal heat pumps (GHP) Application using the earth as a heat source for heating or as a heat sink for cooling

Geothermal resource Sources of geothermal energy: hydrothermal; geopressured; hot, dry rock (HDR); and magma. All are suitable for heat extraction and electric power generation.

Hydrothermal resource Geothermal resource containing hot water and/or steam dropped in fractured or porous rocks at shallow to moderate depths. Categorized as vapor dominated (steam) or liquid dominated (hot water). These are the only commercially used resources at the present time geothermal company.

Organic Rankine cycle (ORC) A cycle using an organic liquid as motive fluid (instead of water) in a Rankine cycle.

Renewable energy Energy source which is not exhausted by use with time. Renewable energies include direct solar energy, energy from geothermal sources, wind, hydroelectric plants, etc.

Geothermal Power Stations

Lucien Y. Bronicki
ORMAT Industries, Ltd.
P.O.Box 68, Yavne, Israel


A. Source of Geothermal Energy
Geothermal energy is renewable heat energy from deep in the earth. It originates from the earth’s molten interior and the decay of radioactive materials; heat is brought near the surface by deep circulation of groundwater and by intrusion into the earth’s crust of molten magma originating from great depth (see Fig. 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 one year, the equivalent of more than 100 million GWhr of heat energy is conducted from the earth’s interior to the surface. But geothermal energy tends to be relatively diffuse, 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 Fig. 2), geothermal energy would be essentially useless as a heat source or a source of electricity using today’s technology.

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.

Figure 1. A representative geothermal reservoir

B. Nature of the Geothermal Energy Resource
On average, the temperature of the earth increases by about 3°C for every 100 m 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 toward 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, and Ecuador, Central America, and western North America. An extension also penetrates across Asia into the Mediterranean area. Hot crustal material also occurs at midocean 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, western Siberia, and 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.

Figure 2. World map showing ithospheric plate boundaries

1. Hydrothermal Resources
These are the only commercially used resources at the present time (Kestin et al., 1980). They contain hot water and/or steam trapped in fractured or porous rock at shallow to moderate depths (from approximately 100 to 4500 m). Hydrothermal resources are categorized as vapor-dominated (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 is 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, high-quality dry steam reserves have been located, 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 satisfy a considerable portion of their electricity requirements for many years (e.g., the Philippines, Indonesia, and the United States).

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 basins (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.

2. Geopressured Resources
Geopressured geothermal resources are hot water aquifers containing geothermal plant dissolved methane trapped under high pressure in sedimentary formations at a depth of approximately 3 – 6 km. Temperatures range from 90 to 200°C, although the reservoirs explored to date seldom exceed 150°C. The extent of geopressured reserves is not yet well known world-wide, and the only major resource area identified is in the northern Gulf of Mexico region, where large reserves are believed to cover an area of 160,000 km2. This resource is potentially very promising because three types of energy can be extracted from the wells, thermal energy from the heated fluids, hydraulic energy from the high pressures involved, and chemical energy from burning the dissolved methane gas (world Energy Council, 1994).

3. Hot, Dry Rock Resources
These resources are accessible geologic formations that are abnormally hot but contain little or no water. The hot, dry rock (HDR) potential is 200 GW in the United States (GeothermEx, 1998) and 60 GW in Europe (Baria et al., 1998). The basic concept in HDR technology is to form a geothermal reservoir by drilling deep wells (400 – 5000 m) into high-temperature, low-permeability rock and then forming a large heat-exchange system by hydraulic or explosive fracturing. Injection and production wells are joined to form a circulating loop through the reservoir, and water is then circulated through the fracture system (Baria et al., 1998; Grassiani and Krieger, 1999).


A. Brief History
Geothermal energy has had a long history of use in applications such as therapeutic hot baths, space and water heating, and agriculture (Nemzer, 2000). It was not until 1904 that the power of natural geothermal steam was first harnessed to produce electricity by Prince Piero Ginori Conti at Larderello, Italy (see Fig. 3). During these early decades, there was little growth of this application due to cheap competing sources of electric power, and it was 1958 before the next large-scale geothermal power station was commissioned, at Wairakei, New Zealand. Only sources which were easy to exploit such as The Geysers in California, and some liquid-dominated geothermal resources in Japan and New Zealand were developed to the commercial power stage before 1970.Renewable energy

Figure 3. The first geothermal power plant at Larderello, Italy (1904)

Of the geothermal resource types, hydrothermal energy is the most widely applied and most cost competitive 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, characterize their conditions, and optimize the locations of wells. geothermal technology Drilling technology used for geothermal development derives historically from the petroleum industry. Certain critical components such as drilling muds were modified to work in high-temperature 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. remote power unitsIt 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 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 Fig. 4).

Figure 4. A district heating plant

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 high-temperature 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 United States, 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 all or part of the function of a separate hot-water heater. Because electricity is used only to transfer heat and not to produce it, the GHP will deliver 3 – 4 times more energy than it consumes. It can be used effectively over a wide range of earth temperatures. Current growth rates for GHP systems run as high as 20% per year in the United States and the outlook for continued growth at double-digit rates is good. The U.S. Department of Energy Information Administration (EIA) has projected that GHPs in the United States 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).geothermal resource

C. Plant Options for Power Generation
There are several types of energy-conversion processes for generating electricity from hydrothermal resources (see Fig. 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).

Figure 5. Schematic of a geothermal plant

D. Dry Steam Plants
Conventional steam-cycle plants are used to produce energy from vapor-dominated reservoirs. As is shown in Fig. 6, steam is extracted from the wells, cleaned to remove entrained solids, and piped directly to a steam turbine. This is a well-developed, commercially available technology, with typical unit sizes in the capacity range 35 – 120 MWe. 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 – 20 MWe range have been introduced. Green Energy

Figure 6. General Electric Co. dry steam plant at The Geysers, California

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 Fig. 7, single-flash systems evaporate hot geothermal fluids to steam by reducing the pressure of the entering liquid and directing 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 reinjected 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.

Figure 7. The Mitsubishi flash steam plant in Beowawe, Nevada

F. Binary-Cycle Plants

1. Low-Enthalpy Resources (100 – 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 Fig. 8). 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, (UNITAR/UNDP, 1989).

During the early 1980s, in order to increase the power output from a given brine resource by increasing the thermal cycle efficiency, a supercritical cycle using isobutane was developed as well as a cascade concept. The supercritical 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.

Figure 8. An air-cooled binary plant geothermal electricity

2. Moderate Enthalpy Resources (160 – 190°C)

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 noncondensible gas (NCG) content, even higher efficiency can be obtained than with condensing steam turbines (Bronicki, 1998).

This binary two-phase configuration is used in the São Miguel power plant in the Azores Islands (see Fig. 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 low-temperature heat produces about additional 600 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 noncondensible gases (NCG) is needed. The air-cooled condensers contribute to the low physical profile of the plant and there is no plume.geothermal power generation

3. Total Flow Turbines
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.


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 worldwide installed capacity achieved its highest growth of 17.2% per year. The number of geothermal power-producing 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 8000 MWe was produced from geothermal resources in more than 20 countries. Substantial market penetration has thus far occurred only with hydrothermal technology.

Table I shows selected countries and their installed power plants. From 1978 to 1985, the worldwide 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 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 clean, sustainable indigenous energy sources.

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. geothermal activity

Figure 9. The ORMAT two-phase binary geothermal power plant in São Miguel, Azores Islands

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 backpressure steam turbine and is then condensed in the organic turbine vaporizer (see Fig. 10). The condensate and brine are used to preheat the organic fluid as in the two-phase binary configuration. This concept was first used in 1989 to repower a backpressure steam plant in Iceland. 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).

Figure 10. The ORMAT geothermal combined cycle power plant in Puna, Hawaii

TABLE 1. Worldwide geothermal installed capacity in the year 2000 in MWe.

United States 2228 Kenya 57
The Philippines 1909 Guatemala 33
Mexico 855 China 29
Italy 785 Russia 23
Indonesia 589 Turkey 20
Japan 547 Portugal (Azores) 16
New Zealand 437 Ethiopia 9
Iceland 170 France (Guadalupe) 4
El Salvador 161 Thailand 0.3
Costa Rica 142 Australia 0.17
Nicaragua 70 Total 8154

B. Direct Applications of Hydrothermal Energy Geothermal power plants
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 environmental 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 United States.

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.

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 fish-farming 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 a 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 United States leads the way with about 400,000 heat-pump units (about 4800 MWt) and energy production of 3300 GWhr/year in 1999. The annual increase is about 10%. Other leading countries are Switzerland, Sweden, Germany, Austria, and Canada.

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 nongeothermal countries. The energy extracted out of the ground with heat pumps in Switzerland amounts to 434 GWhr/year. The annual growth rate is 12%.

Prior to 2000, the total installed capacity for the direct use of geothermal energy world-wide was about 17,000 MWt (see Table II). The top 15 primary users of direct geothermal heat and the year 2000 capacity are listed in Table III. The total installed capacity in 1975 was only about 3100 MWt (excluding balneology).

Table II. Direct Geothermal Use During The Year 2000a

Region Installed capacity
(MWt) Yearly Production
Africa 121 492
America 5,954 7,266
Asia 5,151 22,532
Europe 5,568 18,546
Oceana 318 2,049
Total 17,112 50,885

a From Lund, K.W., and Freeston, D.H. (2000)
“World direct uses of geothermal energy 2000.” In “Pro-
ceedings, World Geothermal Congress,” pp. 1-21.

TABLE III. The World’s Top 15 Direct Use Countries for Geothermal Energya

Installed capacity
(MWt) Yearly production
China 2814 8724
Japan 1159 7500
United States 5366 5640
Iceland 1469 5603
Turkey 820 4377
New Zealand 308 1967
Georgia 250 1752
Russia 307 1703
France 326 1360
Hungary 391 1328
Sweden 377 1147
Mexico 164 1089
Italy 326 1048
Romania 152 797
Switzerland 547 663

a From Lund, K.W., and Freeston, D.H. (2000), “World direct uses of geothermal energy 2000.” In “Proceedings, World Geothermal Congress,” pp. 1-21.

IV. THE ULTIMATE POTENTIAL geothermal electricity

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 discovery of many geothermal reservoirs for electricity generation, some of which are in operation, while about 11,000 MWe of proven resources is 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 a 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 1.2×1013 GWhr or 1.03×109 Mtoe. It therefore appears that geothermal energy is an abundant resource (Naki?enovi? 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 nonnuclear energy source in the United States, 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 includes geothermal reservoirs with a minimum temperature of 80°C at a maximum depth of 6 km, except for geopressured resources which are included to 7 km . Also included are low-temperature resources in the 40 – 80°C range to a depth of 2 3 km.

kg CO2 per kWh

Figure 11. Relative CO2 emissions for different energy resources. Renewable energy


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, as illustrated in Table IV.

TABLE IV. Land Area Occupied for Different Energy Technologies

Technology Land area
(m2 per GWhr/year
for 30 years)
Coal (including coal mining) 3,642
Solar thermal 3,561
Photovoltaics 3,237
Wind (land with turbines and roads) 1,335
Geothermal 404


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