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Lucien Y. Bronicki, Chairman

I.. Introduction
II. Current Geothermal Energy Utilization
III. Current Uses and Commercial Status
IV. The Ultimate Potential
V. Ecological and Enviornmental Considerations


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 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. 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 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.
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.

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 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 – A representative geothermal reservoir. [From Nemzer, M. (2000). Geothermal Education Office, web site]). 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 – World map showing lithospheric plate boundaries. [From Nemzer, M. (2000). Geothermal Education Office, web site, 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.

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 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 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, high-quality 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 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 dissolved methane trapped under high pressure in sedimentary formations at a depth of approximately 3 to 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 to date 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 viz., 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 USA(4) and 60 GW in Europe(5). The basic concept in HDR technology is to form a man-made geothermal reservoir by drilling deep wells (4,00-5,000 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 man-made reservoir, and water is then circulated through the fracture system (Baria et al., 1998; Grassiani and Krieger, 1999).

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