Harnessing Geothermal Energy

Arenal Volcano, Costa Rica

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 What is Geothermal Energy? An overview

How does Harnessing Geothermal Energy Work?

U.S. Geothermal Energy Potential

Applications of Geothermal Energy

Costs of Geothermal Energy

Other Benefits of Geothermal Energy

Works Cited

What is Geothermal Energy? An Overview


The term geothermal comes from the Greek geo meaning earth and therine meaning heat thus geothermal energy is energy derived from the natural heat of the earth. The heat that flows from the Earth's hot interior due to crustal plate movements, zones of high heat flow, may be located close to the surface where convective circulation plays a signifcant role in bringing the heat close to the surface (World Bank Group, 2004).

earth coreThe Earth's crust, on which we live and depend, is in large part the product of millions of once-active volcanoes and tremendous volumes of magma that did not erupt but instead cooled below the surface. Such persistent and widespread volcanism has resulted in many valuable natural resources throughout the world. ... Groundwater heated by large, still-hot magma bodies can be tapped for geothermal energy. -- Excerpt from: Brantley, 1994, Volcanoes of the United States, USGS General Interest Publication

Deep circulation of groundwater along fracture zones will bring heat to shallower levels, collecting the heat flow from a broad area and concentrating it into shallow reservoirs or discharging as hot springs. These reservoirs may contain hot water and/or stream. By drilling into these reservoirs, the hot water and/or steam is piped to the surface where it is used for direct use applications, or the high pressure steam is separated to drive turbines for power generation. The low energy waste water form such power generaiton is then usually re-injected back into the reservoir, or further utilised for direct heat applications. This technology enables it to be utilised to generate electricity and provide domestic and industrial heat. Geothermal energy has proven to be reliable, economic, environmentally friendly and renewable (World Bank Group, 2004).

In general there are two main categories, (1) the high temperature resources and (2) the moderate/low temperature resources. The high temperature geothermal resources - 220 degrees Celsius and up - are predominantly found in volcanic regions and island chains. The moderate to low temperature resources are found on all continents. The high temperature are almost always used for power production while most of the low temperature resources are used for direct heating purposes or agriculture and aquaculture.

How does Harnessing Geothermal Energy Work?

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.
geothermal schematic

There are 3 types of geothermal power plants in use today, and they are:
Dry Steam SchematicDry 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. An example of a dry steam generation operation is at the Geysers in North California, shown at right (Green Jobs, 2002).

flash steam schematic
Flash Steam Plants
which use high pressure hot water to produce steam when thepressure is reduced. Flash steam power plants use hot water (>360 ºF, or >182 ºC) from the geothermal reservoir. 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 (Green Jobs, 2002).

binary cycle schematicBinary Cycle Plants
which use moderate-temperature water (225 to 360 ºF, or 107 to 182 ºC) 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 (Green Jobs, 2002). The Mammoth Pacific binary geothermal power plant, located at the Casa Diablo geothermal field, is pictured at right (Idaho National Engineering and Environmental Laboratory, 2004).

horizontal heat pumpheat pumpThere is also another method, called geothermal heat pumps. The earth’s surface layer remains at an almost constant temperature between 10 to 16C (50 to 50F). In this method, 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. These heat pumps function as both air-conditioning and heating systems in one (Green Jobs, 2002)

verticle heat pump

U.S. Geothermal Energy Potential

energy potential

In 1999, geothermal energy provided 0.4% of U.S. electricity generation (14.3 billion kWh), enough to supply electricity to over 1,400,000 average U.S. homes. U.S. geothermal capacity grew only slightly from 1990 to 1998, by 2.7% from 2,775 MW to 2,850 MW. Worldwide, geothermal capacity in 1999 was 8.24 million kW, or 0.26% of the 3,180 million kW of total world installed electrical generating capacity. Worldwide geothermal capacity grew much more rapidly than the U.S. over the last decade, by over 40% from 5,867 MW in 1990 to 8,240 MW in 1998. However, the United States still accounted for 35% of worldwide installed geothermal capacity in 1998 (Geothermal Education Office, 2003).

Puna Geothermal, the only geothermal power plant in Hawaii, can generate a net of 25 MW of electrical energy (or 24% of Hawaii's electrical energy) from the geothermal fluids produced from the Puna geothermal field (USGS, 2004).

Applications of Geothermal Energy

direct uses1. Space/District Heating: Schemes utilizing geothermal heat provide over 80% of the central heating needs of Reykjavik city in Iceland and are employed in many towns in USA, Poland and Hungary. The World Bank is currently supporting a program in Poland for using hot water from unsuccessful oil wells to displace the use of coal for district heating (World Bank Group, 2004). 

2. Agriculture and Aquaculture: In temperate and colder climates, greatly improved plant and fish growth can be achieved by heating soils, greenhouses and fish ponds using geothermal heat. One example of this is the largely successful Osearian Farm, Kenya (World Flowers, 2005).

3. Power Generation: With over 8000 MW of installed capacity, geothermal electric power generation is a well-proven technology that has been especially successful in countries and islands that have a high reliance on imported fossil fuels (World Bank Group, 2004).

Costs of Geothermal Energy

Costs for geothermal electricity generation are 4.5-7 cents per kilowatt-hour. This is competitive with some fossil fuel facilities, but one must keep in mind the drastic reduction of pollution. Delivered costs depend on ownership arrangements, financing, transmission, the quality of the resource, and the size of the project.

Geothermal plants are relatively capital-intensive, with low variable costs and no fuel costs. Usually financing is structured so that the project pays back its capital costs in the first 15 years, delivering power at 5-10¢/kWh. Costs then fall by 50-70 percent, to cover just operations and maintenance for the remaining 15-30 years that the facility operates (World Bank Group, 2004).

Other Benefits of Geothermal Energy

1. Minimize air pollution: Current geothermal fields produce only about one-sixth of the carbon dioxide that a natural gas fueled electrical generating power plant produces and none of the nitrous oxide (NOx) or sulfur bearing (SOx) gases. New state of the art geothermal binary and combined cycle plants produce virtually no air emissions. Each 1,000 MW of new geothermal power will offset 1.9 million pounds per year of noxious and toxic air pollution emissions in Western skies and offset about 7.8 billion pounds per year of climate affecting CO2 emissions from gas fired plants or much larger amounts from coal fired plants (USGS, 2004).

2. Renewable energy source: All types of geothermal energy are renewable as long as the rate of heat extraction from the earth does not exceed the rate at which the thermal reservoir it depends upon is recharged by the earthâs heat. A geothermal reservoir that has been used for electricity generation may take several hundred years to recharge after it has been completely depleted. District heating system reservoir recovery may take 100-200 years, and geothermal heat pump reservoir recovery may take30 years or so.

3. Reduces dependence on imported energy: Geothermal energy is generated with indigenous resouces, thereby reducing trade deficits. Reducting 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 displaces with domestic energy resources (Green Jobs, 2004).

    Although geothermal energy has been around for over a century, we are just now realizing its potential for replacing and/or supplementing current energy resources.  In the U.S., most geothermal sources are in the western states, including Alaska and Hawaii.  This limited usage seems to be geothermal energy's greatest downfall, despite being clean, efficient, and cheap.  When compared to other alternative energies, such as solar, geothermal energy only outputs a small fraction of that of solar energy dispersed atop the earth's surface.  We should consider its margin for improvement.  Although geothermal energy units can be expanded as needed, its methods and its technologies have been virtually stagnant.  Also, as we explore alternative energy sources, we should consider its appropriateness for various points on the globe.

osearian farm

Works Cited

nGeothermal Education Office. “About Geothermal Energy.” 07 April 2003. http://geothermal.marin.org/GEOpresentation/sld108.html

Green Jobs, "Introduction to Geothermal Energy." 18 August, 2002. http://www.greenjobs.com/Public/info/industry_background.aspx?id=11

Idaho National Engineering and Environmental Laboratory. “Geothermal Energy Research.” 11 September 2004. http://geothermal.id.doe.gov/
Kilpenin, Malia. “Renewable Energy.” 20 May 1997. http://community.hei.com/altenergy/geo.ad.html
U.S. Geological Survey. “The Plus Side of Volcanoes – Geothermal Energy.” 16 December 2004. http://vulcan.wr.usgs.gov/LivingWith/PlusSide/geothermal.html
World Bank Group. “Geothermal Energy.” 18 August 2004. http://www.worldbank.org/html/fpd/energy/geothermal/
World Flowers. “Technical: The Harnessing of Geothermal Energy.” 27 February
2005. http://www.world-flowers.co.uk/09technical/geothermal.html