Geothermal Energy

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Geothermal energy technologies rely on tapping the heat within the earth itself. In this, these technologies differ from most renewable energy technologies which somehow, directly or indirectly, capture solar energy and also that the heat energy taken from the earth is not apparently replenished. Modern thermal models of the earth, in fact, must take into account the radiogenic heat continually generated by the decay of the long-lived radioactive isotopes of uranium (U 238 , U 235 ), thorium (Th 232 ) and potassium (K 40 ), which are present in the Earth. Added to radiogenic heat, are other potential sources of heat such as the primordial energy of planetary accretion. Realistic theories on this were not available until the 1980s, when it was demonstrated that there is no equilibrium between the radiogenic heat generated in the Earth's interior and the heat dissipated into space from the Earth, and that our planet is slowly cooling down - albeit very slowly - less than 350C drop in the core temperature of about 4000C in 3 billion years!. The punch line is therefore that geothermal energy technologies are not truly renewable but instead harness energy which otherwise would be uselessly dissipated and the effect on the earth's temperature is so infinitesimal that the technologies can realistically be regarded as renewable.

Milestones in the Development of Geothermal Energy

Early 1900's First geothermal electricity commercialization Conversion of high-grade hydrothermal resources to electricity began in Italy in the early 1900s.
1960 U.S. commercialization The first commercial-scale development tools were placed at The Geysers in California, a 10-megawatt unit owned by Pacific Gas & Electric.


Reinjection of geothermal fluids Injection of spent geothermal fluids back into the production zone began as a means to dispose of waste water and maintain reservoir life.
1972 Deep well drilling Technology improvements led to deeper reservoir drilling and access to more resources.
1977 Hot dry rock demonstrated In 1977, scientists developed the first hot dry rock reservoir at Fenton Hill, New Mexico.
1978 Federal research and development (R&D) funding exceeds $100 million U.S. Department of Energy (DOE) funding for geothermal research and development was $106.2 million (1995 dollars) in fiscal year 1978, marking the first time the funding level surpassed $100 million. It remained above $100 million until fiscal year 1982, when it was reduced to $56.4 million (1995 dollars). Currently, the budget is in the $30 million to $40 million range. 1978 Public Utility Regulatory Policies Act (PURPA) enacted PURPA mandated the purchase of electricity from qualifying facilities (QFs) meeting certain technical standards regarding energy source and efficiency. PURPA also exempted QFs from both State and Federal regulation under the Federal Power Act and the Public Utility Holding Company Act.
1980 First commercial binary system The first commercial-scale binary plant in the United States, installed in Southern California's Imperial Valley, began operation in 1980.
1980's California Standard Offer Contracts California's Standard Offer Contract system for PURPA QFs provided renewable electric energy systems a relatively firm and stable market for output, allowing the financing of such capital-intensive technologies as geothermal energy facilities.
1982 Hydrothermal generating capacity of 1,000 megawatts Geothermal (hydrothermal) electric generating capacity, primarily utility-owned, reached a new high level of 1,000 megawatts.
1989 Geopressured power plant demonstrated In 1989, DOE and the Electric Power Research Institute operated a 1-megawatt demonstration plant in Texas, extracting methane and heat from brine liquids.
1990 Drop in Federal funding for geothermal R&D to $15 million DOE funding for geothermal energy research and development declined throughout the 1980s, reaching its low point in fiscal year 1990.
1991 Magma drilling project reaches a depth of 7,588 feet The world's first magma exploratory well was drilled in the Sierra Nevada Mountains to a depth of 7,588 feet. It did not encounter magma at that depth inside the caldera.
1994 Industry consolidates and looks at new markets California Energy became the world's largest geothermal company through its acquisition of Magma Power. Near-term international markets gained the interest of U.S. geothermal developers.
1985-95 U.S. geothermal developers have added nearly 1,000 megawatts of geothermal electric generating capacity outside The Geysers
1995 Worldwide geothermal capacity of 6,000 megawatts in 20 countries.

Source: US Energy Information Administration

Geothermal resources range from shallow ground to hot water and rock several miles below Earth's surface, and even farther down to the extremely high temperatures of molten rock called magma. The uses to which the earth's energy can be put depend on the temperature of the hot fluid extracted from it as illustrated below.

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Temperature C
Saturated Steam
Conventional Power production; evaporation of highly concentrated solutions; refrigeration by amonia absorption; digestion in paper pulp, kraft.
Conventional Power production; heavy water via hydrogen sulfide process; drying of diatomaceous earth.
Conventional Power production; drying of fish meal; drying of timber.
Conventional Power production; alumina via Bayer's process.
Conventional Power production; drying farm products at high rates; canning of food.
Conventional Power production;evaporation in sugar refining; extraction of salts by evaporation and crystallization.
Fresh water by distillation; most multiple-effect evaporations; concentration of saline solutions.
Drying and curing of light aggregate cements
Drying of organic materials (seaweed, grass, vegetables etc.)


Drying of stockfish; intense de-icing operations.
Space heating; greenhouse space heating.
Refrigeration (lower temperature limit)
Animal husbandry; greenhouse combined space and hot bed heating.
Mushroom growing; baineological baths.
Soil warming
Swimming pools; biodegradation; fermentation; warm water for year-round mining in cold climates; de-icing.
Hatching of fish; fish farming
Source: J.S. Rinehart,Geysers and Geothermal Energy (NewYork, NY, Springer-Verlag,1980)

Heat Pumps

The earth's surface layer remains at an almost constant temperature between 10 to 16C (50 to 50F). Geothermal heat pumps use a system of buried pipes linked to a heat exchanger and ductwork into buildings. In winter the relatively warm earth transfers heat into the buildings and in summer the buildings transfer heat to the ground or uses some of it to heat water. Heating and air conditioning accomplished with one system!







Geothermal heat pump equipment in a Beijing apartment building

Source: NREL National Photographic Information Exchange

Direct Use

This relies on access to naturally occurring hot water which is most common in the earthquake zones such as the Pacific "Ring of Fire". In the US most geothermal reservoirs are located in the western states, Alaska and Hawaii. The hot water can be used directly to heat buildings - residential, commercial and agricultural, and to assist in processes such as fish farming and vegetable dehydration.

US Geothermal Resources
Geothermal heat melts ice on the sidewalk
Source: NREL National Photographic Information Exchange

Electricity Production

Deep wells, a mile or more deep, can tap reservoirs of steam or very hot water that can be used to drive turbines which power electricity generators.


Schematic Geothermal Power Plant
Source: Greenjobs

Geyser Dry Steam Geothermal Field, California
Source: NREL Library


There are three type of geothermal power plants in use today and they are,


Dry Steam Plants which use geothermal steam directly. Dry steam power plants use very hot (>455 °F, or >235 °C) steam and little water from the geothermal reservoir. The steam goes directly through a pipe to a turbine to spin a generator that produces electricity. This type of geothermal power plant is the oldest, first being used at Lardarello, Italy, in 1904. Flash Steam Plants which use high pressure hot water to produce steam when the pressure is reduced. Flash steam power plants use hot water (>360 °F, or >182 °C) from the geothermal reservoir.15 When the water is pumped to the generator, it is released from the pressure of the deep reservoir. The sudden drop in pressure causes some of the water to vaporize to steam, which spins a turbine to generate electricity. Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur, but generally 50 times less than traditional fossil-fuel power plants.16 Hot water not flashed into steam is returned to the geothermal reservoir through injection wells. Figure 3 is a schematic of a typical flash steam power plant.
Source: Greenjobs
Source: Greenjobs
Binary Cycle Plants which use moderate-temperature water (225 to 360 °F, or 107 to 182 °C) from the geothermal reservoir. In binary systems, hot geothermal fluids are passed through one side of a heat exchanger to heat a working fluid in a separate adjacent pipe. The working fluid, usually an organic compound with a low boiling point such as Iso-butane or Iso-pentane, is vaporized and passed through a turbine to generate electricity. An ammonia-water working fluid is also used in what is known as the Kalina Cycle. Makers claim that the Kalina Cycle system boosts geothermal plant efficiency by 20-40% and reduces plant construction costs by 20-30%, thereby lowering the cost of geothermal power generation.
Source: Greenjobs

Historical Growth

Power generated from geothermal sources increased from increased by an average of about 3.5% per year between 1990 and 2000 (see figure below). In the decond half of the same decade, the energy produce from direct use of geothermal sources increased by over 13% per year!

Worldwide Geothermal Power Generation and Direct Use

Source: International Geothermal Association


The adjacent table shows that the major producers are the USA and the Philippines, followed by Italy, Mexico, Indonesia, Japan, and Nicaragua. The others are very small by comparison. Over the decade, the biggest growth was seen in the Philippines (1GWe) while the USA actually lost over 500mWe of capacity.


Installed Generating Capacity (MWe)



0.67 0.67 0
0 0.17 0.17
19.2 28.78 29.17

Costa Rica

0 55 142.5

El Salvador

95 105 161
0 0 8.52

France (Guadeloupe)

4.2 4.2 4.2
0 33.4 33.4
44.6 50 170
144.75 309.75 589.5
545 631.7 785
214.6 413.71 546.9
45 45 45
700 753 755
New Zealand
283.2 286 437
35 70 70
891 1227 1909
Portugal (The Azores)
3 5 16
Russia (Kamchatka)
11 11 23
0.3 0.3 0.3
20.6 20.4 20.4
2774.6 2816.7 2228
5831.72 6833.38 7974.06
Source: International Geothermal Association

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Associated Jobs

In 1996, the U.S. geothermal energy industry as a whole provided approximately 12,300 direct jobs in the United States, and an additional 27,700 indirect jobs in the United States. The electric generation part of the industry employed about 10,000 people to install and operate geothermal power plants in the United States and abroad, including power plant construction and related activities such as exploration and drilling; indirect employment was about 20,000.

What is more, geothermal, like other renewable energy industries, actually provides more jobs per MW of energy production than conventional (natural gas) power production as revealed in the table below

Employment Rates by Energy Technology

Power Source
Factor Increase
over Natural Gas
for 500MW




Solar Electric
Solar Thermal
Landfill methane/digester gas
Natural gas
Source: Electric Power Research Institute, prepared for the California Energy Commission (CEC),
California Renewable Technology Market and Benefits Assessment, November 2001.

In the developing world, labor costs are much lower than in the USA, resulting in a correspondingly higher number of jobs per unit of energy production. If we assume the average jobs creation ratio in the developing world is twice that in the USA, it suggests that the geothermal power generation industry supported about 38,00 direct and 86,000 indirect jobs worldwide in 1996. By the year 2000, these figures had increased to approximately 48,000 direct and 128,000 indirect jobs respectively. These numbers of course neglect the significant contribution of jobs involved in heat pumps and direct use industries.

The Future

One of the most important economic aspects of geothermal energy is that it is generated with indigenous resources, reducing a nation's dependence on imported energy, thereby reducing trade deficits. Reducing trade deficits keeps wealth at home and promotes healthier economies. Nearly half of the U.S. annual trade deficit would be erased if imported oil were displaced with domestic energy resources.

Nearly half of all developing countries have rich geothermal resources, which could prove to be an important source of power and revenue. Geothermal projects can reduce the economic pressure of fuel imports and can offer local infrastructure development and employment. For example, the Philippines has exploited local geothermal resources to reduce dependence on imported oil, with installed geothermal capacity and power generation to become second in the world after the United States. In the late 1970s, the Philippine government instituted a comprehensive energy plan, under which hydropower, geothermal energy, coal, and other indigenous resources were developed and substituted for fuel oil, reducing their petroleum dependence from 95% in the early 1970s to 50% by the mid-1980s. Developing countries will likely require increasing amounts of power in the coming years and there is a high probability they will use an increasing mix of renewables, including geothermal energy where possible. We have used the existing figures for Direct Use and Power Generation to calculate the combined geothermal energy production for 1995 and 2000. Countries have been grouped into either the developed (e.g. USA, Australia, UK) or developing world (e.g. China, Indonesia, Philippines). Possible growth into the future has been estimated conservatively using observed growth rates but setting anomalously fast growing countries to the average rate for the group. The results shown below show that total geothermal energy production could approach 100GW by 2010 with about one third in the developing world. Given the strategic attractions of renewables for the developing world, the relative comparison may be do a disservice to the developing world. Please note that the figures shown ignore completely any contribution made by geothermal heat pumps.

Source: International Geothermal Association

Within the USA, in California alone the CEC has estimated that planned geothermal developments could contribute over 4000 job-years in construction and over 2000 operating jobs by 2010. We have estimated the potential for job creation in the Developed and Developing worlds using the jobs/MW ratios observed in the USA; we have applied these asan average for the Developed world and doubled them for the Developing world. The resulting estimates shown in the figure below reveal the potential for the creation of about 1.3 million geothermal jobs by 2010.

Source: Greenjobs

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Education and TraIning:

Geothermal Education Office
Education-related Web site including worldwide geothermal resources map, an energy scrapbook, and classroom materials

The Renewable Alternate Energy Laboratory (RAEL) focuses on designing, testing, and disseminating renewable and appropriate energy

"Research of the Building and Environmental Thermal Systems Research Group includes building heat transfer, HVAC systems modeling, building energy simulation, hydronic heating systems, geothermal heat pump systems and ground loop heat exchanger technology."l OSU is also the home of the International Ground Source Heat Pump Association, whose goal is to "promote the growth and advancement of the geothermal industry while ensuring the quality, safety, and reliability of installed systems."

Development of geothermal energy resources. Although they are not a research funding source they do provide technical assistance in related technology development or applications. OIT also provides a clearinghouse service for geothermal development grants. Research at the Geo-Heat Center is supported in part by the US Department of Energy.

Development of reservoir engineering techniques for efficient production of geothermal resources. The primary focus is to investigate reinjection into vapor dominated reservoirs such as The Geysers.

Location and optimization of geothermal energy resources. The Center's team of scientists specializes in geochemistry, hydrogeology, geophysics, thermodynamics, remote sensing, seismology and structural geology (geologic mapping).

The Regional Geophysics Laboratory in the Department of Geological Sciences provides information on terrestrial heat flow and practical applications of low-temperature geothermal energy. The geothermal energy database includes temperature data from hundreds of temperature and other geophysical logs, rock thermal conductivity, and heat flow values from New Jersey to Georgia.

Research, develops tools, and disseminates information to enable people to make informed decisions about energy. The program has been affiliated with the University since 1996. Previously it was part of the state energy office.

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References and Useful Links:

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