Geothermal Energy

INTRODUCTION: WHAT IS GEOTHERMAL ENERGY?

Geothermal energy is energy created by the heat of the Earth. Under the Earth's crust lies a layer of thick, hot rock with occasional pockets of water. This water sometimes seeps up to the surface in the form of hot springs. Even where the water does not travel naturally to the Earth's surface, it is sometimes possible to reach it by drilling. This hot water can be used as a virtually free source of energy, either directly as hot water, steam, or heat or as a means of generating power. Geothermal energy is nonpolluting, inexpensive, and in most cases renewable, which makes it a promising source of power for the future.

The word geothermal comes from two Latin words, geo, meaning "earth," and thermal, meaning "heat." So the word geothermal means "heat from the earth." In most cases, the geothermal resource that people want is water that has been trapped within the Earth, where it becomes very hot.

Types of geothermal energy

There are two main types of geothermal energy. The energy can be used directly, as heat or hot water, or it can be a means of generating electricity.

Naturally hot water has been recognized as a resource for thousands of years. People have used hot springs for bathing, for medical treatments, and as heating for their buildings. The hot water can also be used in agriculture, aquaculture, industry, and other applications.

Geothermal power can also generate electricity. Geothermally generated electricity is becoming increasingly important. In 1999 over 8,000 megawatts of electricity were produced by about 250 geothermal power plants around the world, located in twenty-two different countries. Most of these power plants are located in developing nations. However, that same year the United States produced nearly 3,000 megawatts of geothermal electricity, more than twice the amount of power generated by wind and solar power. Ten percent of the electricity in Nevada and 6 percent of the electricity in Utah came from geothermal power plants.

Historical overview: notable discoveries and the people who made them

Knowledge of geothermal energy is very old. Ancient Chinese and Japanese people bathed in hot springs and used the water for cooking. Ancient Romans used the water from hot springs as a medicine for skin diseases, and the buildings in ancient Pompeii were heated with hot water that ran under them. Native Americans settled near hot springs more than 10,000 years ago. During the Middle Ages in Europe people traveled to towns in Germany and France that had built spas, or health resorts, around natural hot springs.

Words to Know

Aquaculture

The formal cultivation of fish or other aquatic life forms.

Balneology

The science of baths, especially for therapeutic use.

Core

The center, innermost layer of the Earth.

Crust

The outermost layer of the Earth.

Geothermal reservoir

A pocket of hot water contained within the Earth's mantle.

Lava

Molten rock contained within the Earth that emerges from cracks in the Earth's crust, such as volcanoes.

Magma

Liquid rock within the mantle.

Mantle

The middle layer of the Earth between the inner core and the outer crust.

Turbine

A device that uses the movement of a liquid or gas to spin a machine that produces electricity.

Discoveries of the 1800s

During the 1800s European settlers moved westward across the North American continent. They noted the existence of hot springs and settled near them. In 1807 John Colter (1774–1813) is believed to have found hot springs in what is now Yellowstone National Park. That same year the city of Hot Springs, Arkansas, was founded. By 1830 Asa Thompson of Hot Springs was selling visitors the right to sit in a wooden tub fed by a hot spring; the price was $1 per person. The hot springs area in Arkansas was declared a national park in 1921.

In 1847 William Bell Elliot, a member of John Fremont's California survey group, found a valley full of steaming hot springs that he described as resembling the gates of hell. He named the area "the Geysers" (though it did not actually contain geysers). The region was located north of San Francisco. Within five years the area had been developed into a resort spa that was visited by famous people such as the author Mark Twain and the presidents Theodore Roosevelt and Ulysses S. Grant. Ten years later Sam Brannan built a $500,000 resort southeast of the Geysers called Calistoga, which resembled European resorts with racetracks, bathhouses, a hotel, and a skating pavilion.

Hot Springs Baths

The ancient Greeks and Romans knew of a number of natural hot springs, many of them located near volcanoes. The oldest known hot springs bath still in existence is located in Merano, Italy. People are believed to have used it five thousand years ago. Bath, England, has long been famous for its natural hot springs. The waters at Bath are about 120°F (48°C) and contain numerous minerals, including calcium and magnesium. Ancient Celts are believed to have bathed in the springs as early as 800 BCE (before the common era). The Romans built bath houses around the springs nearly two thousand years ago; the town became a major tourist resort starting around the time of Queen Elizabeth I (1533–1603), who went there often to bathe.

Germany is full of natural hot springs, many of which long ago became the sites of baths. Ancient Romans built baths on these springs. Like Bath in England, Germany's bath towns became extremely popular with the rich in the nineteenth century. Towns such as Bad Cannstatt and Baden-Baden grew rich from their well-to-do visitors who came to bathe, be massaged, drink the waters, and indulge themselves in other entertainments. These baths are still popular today and have been supplemented with modern healing treatments such as shiatsu massage and with trendy shopping facilities. Japanese baths are likewise famous around the world. Hot springs resorts, called onsen, attract millions of visitors who come to soak in the waters. The waters often contain particular minerals that are said to have specific effects on physical and mental health. Some baths have facilities for drinking the water or inhaling the steam.

Americans began experimenting with large-scale geothermal heating in 1864, with the construction of the Hot Lake Hotel in La Grande, Oregon. In 1892 the city of Boise, Idaho, built a geothermal district heating system that piped hot water from a geothermal reservoir to the buildings in town.

Beginning of geothermal electricity

The first geothermal electrical power plant was built in Larderello, in Tuscany, Italy, in 1904. Larderello is a geologically active area that was used in Roman times as a hot springs resort. This made it ideal as a site for experimenting with geothermal energy. The first plant lit up five light bulbs, using the steam that came from cracks in the ground. In 1911 a larger plant opened in the area, and it was the only geothermal power plant in the world until after World War II. The plant at Lardarello was destroyed during World War II, but it was quickly rebuilt. Engineers from New Zealand and other countries went to visit the Larderello plant to learn how it was built and also noted the enthusiasm that the Italian engineers had for their plant. Lardarello's plant still produces enough power for one million households in Italy, nearly ten percent of the total geothermal power produced in the world.

In 1921 John D. Grant drilled a well at the Geysers with the hope of using its steam to generate electricity. The next year he built the first geothermal power plant in the United States. His power plant generated enough power to power the lights at the Geysers resort. However, geothermal power at the time cost more than other sources of power to produce, so this effort was soon abandoned.

Through the 1920s people continued to drill experimental wells in Oregon and California, hoping to take advantage of the heat within the Earth. In 1927 the Pioneer Development Company drilled some wells in Imperial Valley, California. In 1930 gardeners in Boise, Idaho, opened the first geothermally heated commercial greenhouse, using the water from a 1,000-foot (305-meter) well. That year Charlie Lieb of Klamath Falls, Oregon, built the first downhole heat exchanger, which he used to heat his home. In 1940 the Moana neighborhood of Reno, Nevada, began using geothermal heat for residential heating. Eight years later the first groundwater heat pumps went into use in Ohio and Oregon.

The first flashed steam geothermal power plants, which depressurized hot water to produce steam, were built in the late 1940s. In 1960 the United States' first large-scale geothermal power plant began operation, at the same site and with the same name (the Geysers) as the earlier spa. Its first turbine produced 11 megawatts of net power. As of the early 2000s the Geysers was the largest geothermal plant in the world.

Hot Springs Monkeys

Humans are not the only creatures to have noticed and taken advantage of natural hot springs. Japanese macaques, also known as snow monkeys, are large monkeys that live in northern Japan. The Japanese winter is cold and snowy, but the macaques have learned a trick that helps them keep warm: They sit in natural hot springs that come up from the ground. The monkeys got so enthusiastic about hot springs that the prefecture (governmental district) of Nagano decided to build them their own hot springs and feeding stations to keep them away from human hot tubs and spas.

Governmental encouragement

In the 1970s the United States and other nations created several agencies and passed laws to encourage the development of geothermal energy. In 1970 the United States passed the Geothermal Steam Act, which gave the Secretary of the Interior the authority to use public lands for environmentally sound geothermal exploration and development. The Geothermal Resources Council was formed to make it easier to develop geothermal resources worldwide. The Geothermal Energy Association, founded in 1972, was created by several companies around the world to develop geothermal electricity generation and direct heat technology. The 1974 Geothermal Energy Research, Development and Demonstration Act instituted a geothermal loan guaranty program, which gave investment security to companies attempting to create technologies to use geothermal energy. In 1975 the Geo-Heat Center was formed at the Oregon Institute of Technology; the institute began using runoff from its geothermal heating system to heat water used to raise freshwater prawns.

The first geothermal food processing and crop drying plant was opened in Brady Hot Springs, Nevada, in 1978. It received $3.5 million from the Geothermal Loan Guaranty Program. That year the United States Department of Energy opened a facility in Fenton Hill, New Mexico, to test "hot dry rock" energy generation, a process in which water is pumped into an area of hot rock, becomes superheated, and then is pumped back to the surface so that the heat can be siphoned off. This facility managed to generate some electricity two years later.

Between 1979 and 1982 the Department of Energy sponsored development of a geothermal electrical power plant in Imperial Valley, California, as well as research into direct uses of geothermal energy for heating and agriculture. The first flashed steam plant in the United States was built in Brawley, California, in 1980. In 1981 a binary power plant was built in California's Imperial Valley. The plant was so successful that Ormat, the company that built it, paid off its loan within one year. By 1984 there were geothermal power plants in Hawaii, Nevada, and in the Salton Sea in California.

In 1989 the first hybrid geothermal power plant opened in Pleasant Bayou, Louisiana. It used both geothermal heat and methane to create electricity. During the 1990s several geothermal power plants went into operation in the Pacific Northwest, Nevada, and Hawaii. In 1994 the United States Department of Energy created two programs to increase the use of geothermal power generation and heat pumps in an effort to reduce greenhouse gas emissions.

In 2000 the U.S. Department of Energy created its GeoPowering the West initiative, which funded twenty-one partnerships with private companies to develop geothermal energy in the western United States. Several groups in the western states spent the early 2000s working to identify barriers to geothermal development and to create ways to make geothermal energy more commonly used.

How geothermal energy works

Geothermal energy uses the heat of the Earth to produce electricity and heat. This form of power works because the inside of the Earth is much hotter than the surface.

The structure of the Earth

The Earth consists of several layers of matter. The outer layer, called the crust, is the surface where people live and plants grow. It is composed of aluminum, silicon, oxygen, iron, and other minerals. Below the crust is a layer called the mantle, a thick layer of rock and oxides that comprises about 82 percent of the Earth's total volume. It is made up mostly of peridotite, a kind of rock containing iron, magnesium, oxygen, and silicon. The mantle is mostly solid but can also flow like a liquid when it is under pressure. The top layer of the mantle consists of hot liquid rock called magma. The crust floats on top of this liquid rock.

At the center of the Earth is the core, a chunk of extremely hot iron and nickel. The core itself consists of two layers, the outer core, which is liquid, and the inner core, which is solid because of the tremendous pressure it experiences. The center of the core is about 4,000 miles (6,400 kilometers) from the surface of the Earth.

Water heated underground

The Earth's temperature increases about 41.7°F (5.4°C) for every 328 feet (about 100 meters) traveling from the surface to the core. About 10,000 feet (3,048 meters) below the surface, temperatures are hot enough to boil water. The inner core may be over 9,000°F (4,982°C). This heat constantly travels upward toward the surface, heating the mantle, which carries heat toward the crust. Similar to the curved pieces of peeled skin from an orange, Earth's outermost layers are cut and fractured into pieces or sections called plates. Like the inner and outer sides of an orange peel, these plates have distinct sections, an inner side and an outer crust. Each plate (also called a lithospheric plate) moves over a hotter, denser—but in many ways more fluid-like (molten)—region of Earth's interior termed the asthenosphere (a portion of Earth's mantle). The visible continents such as North and South America are actually an outer crust of the lithospheric plates upon which they ride, shifting slowly over time as a result of forces, including differences in temperature, which help move or drive the plates. The theory that describes this motion is perhaps the most important in all of geology (the study of Earth's structure) and is called plate tectonics (the theory of plate structure and movements). Although plates move very slowly (in many cases, just inches per year) they are, of course, very heavy and so their rubbing, sliding, slipping, collisions, and bending causes earthquakes.

Where the edges of plates overlap, volcanoes may form. Depending on the materials that compose them, one plate may drive under another (subduction) or both plates may drive skyward to form mountain chains. Hot magma from Earth's molten inner layers (or from pieces of plate being destroyed during subduction) can carve tunnels, chambers, and channels in the plate and crust and so allow hot magma to reach the surface of the plate (even if it is under the ocean). When magma reaches the surface and flows from a volcano it becomes known as lava. Volcanoes can also form over areas in plates away from the edges (especially thinner areas of plates under the oceans) called "hot spots" where molten material from Earth's mantle pushes upward.

The rock underground is full of cracks and small pockets, and these can fill with water. Water that gets trapped in underground caves will get very hot, even hotter than boiling temperature, but it cannot boil because there is no place for steam to escape into the air. This water sometimes finds its way to the surface in the form of hot springs. Most of the hot water stays underground in pockets called geothermal reservoirs.

Making use of geothermal energy

There are several ways to make use of geothermal energy. The most basic is simply to use the water as hot water when it comes out of the ground. The water can be channeled to different places as heat, for heating homes, or for cooking.

Engineers can drill down into the ground to reach geothermal reservoirs and then use the hot water, steam, or heat to power generators to make electricity. Scientists have developed techniques to find geothermal water. When they find reservoirs, they drill production wells down into them. The hot water or steam travels up the well to the surface, where it can be collected and harnessed for various uses.

The Ring of Fire and other hot spots

The Pacific Ocean is one of the most geologically active areas in the world. The land that borders the Pacific is sometimes known as the Ring of Fire because of the volcanic activity that occurs there. New Zealand, Japan, the Philippines, Hawaii, Alaska, California, and other places in the area experience a great deal of tectonic shifting, as pieces of the Earth's crust move around and crash into one another. All of these areas also have active volcanoes.

There are active volcanoes in many other places. Iceland has so much volcanic activity that it derives much of its power from geothermal sources. Kenya, Turkey, Italy, and Zambia all have enough geothermal energy to make profitable use of it. Because of the nature of current geothermal technology, these geologically active areas are also the main sites of geothermal power.

Current and future technology

Geothermal energy technologies are used in the generation of electricity and in direct uses of the hot water. There is room for development of new technologies in both categories.

Geothermal power plants

One of the most important uses of geothermal energy is to generate electricity. In geothermal power plants, hot water drawn from geothermal reservoirs through production wells spins turbine generators, which produce electricity. The used water is injected back into the reservoir through another well called an injection well. This water gets hot again and helps maintain the pressure within the reservoir. If all the water were removed and not replenished, the reservoir would eventually cool off and run out of water, making it useless. Groundwater must be very hot in order to generate electricity. Water colder than 250°F (121°C) is currently not usable for power.

Geysers, Hot Springs, Mudpots, and Fumaroles

Magma heats water trapped or flowing underground. Hot springs are places where hot water rises up from the Earth on a regular basis. Geysers are explosive hot springs where hot water periodically shoots out of a hole in the ground. Fumaroles are openings near volcanoes that emit steam and sulfurous gases. They can look like holes or cracks in the ground and may stay in the same spot for centuries or come and go within weeks. Mudpots are fumaroles or hot springs that form in areas with small amounts of water. The water bubbles up to the surface and creates a crater filled with boiling mud.

There are three main types of geothermal power plants.

• Flashed power plants have reservoirs with water between 300 and 700°F (148 and 371°C). This water comes up from the well and is flashed (turned quickly) into steam, which powers a turbine.
• Binary power plants have reservoirs with water between 250 and 360°F (121 and 182°C), which is not quite hot enough to generate enough steam to power a turbine. These plants use the heat from the water to heat another liquid with a lower boiling temperature, called a binary liquid. The binary liquid boils and produces steam to spin a turbine.
• Dry steam power plants have reservoirs that produce steam but not water. The steam is piped directly into the plant, where it spins a turbine.

There are also hybrid power plants that combine geothermal heat with other sources of energy, such as methane. All types of geothermal power plant have no emissions and can produce a large amount of power. Geothermal power is especially appealing because it is possible to have power plants of almost any size, from tiny 100 kilowatt plants to much larger 100 megawatt plants that are connected to national power grids. They can operate twenty-four hours a day every day of the year, but they can also vary operation according to demand.

Direct uses of geothermal energy

Hot water is useful in and of itself. Some common uses of geothermal water include:

• Using hot springs for bathing. This is called balneology.
• Growing plants in winter greenhouses.
• Heating the ground in which outdoor crops are growing to prevent it from freezing.
• Growing fish and shellfish for commercial purposes.
• In industry, such as pasteurizing milk or washing wool.
• Heating buildings or cities through underground channels. Reykjavik, Iceland, has the world's largest geothermal district heating system.
• Piping water under streets and sidewalks to keep them from freezing.
• Geothermal heat pumps that use the heat from just a few feet below the Earth's surface instead of heat from geothermal reservoirs. These can heat or cool homes anywhere, not just in areas with geothermic activity.

Every use of geothermal water as hot water saves energy. Heating water takes a great deal of power, and every gallon that does not have to be heated can save oil, coal, wood, or other heating fuels.

Direct uses of geothermal energy provide about 10,000 thermal megawatts of energy in thirty-five countries around the world. This does not include the use of geothermal waters for bathing by individuals who have not developed the resources for commercial use. In the United States in the late 1990s there were eighteen district heating systems, twenty-eight fish farms, thirty-eight greenhouse establishments, twelve factories, and more than two hundred spas using geothermal waters.

Developing technology

Scientists are working to create technology that will make geothermal energy more accessible to people everywhere, not just to those living in areas with shallow geothermal reservoirs. The entire planet has heat beneath its surface, but not all places have hot water. Deeper drilling techniques could make more areas of heat and steam accessible. Scientists would love to take advantage of the heat from magma in the mantle, but there is not yet a workable technology to do this.

Engineers are working to develop technology that would make hot dry rocks (HDR) 3 to 6 miles (5 to 10 kilometers) below the surface usable for power. Techniques include piping water down to the hot rock to create steam. Teams in the United Kingdom, Australia, France, Switzerland, and Germany are working on HDR technology as of the early 2000s. It remains to be seen if they can devise a method of producing power that is worth its cost.

Benefits of geothermal energy

There are many benefits to using geothermal power. It is clean and nonpolluting. It does not require the consumption of fossil fuels, so it reduces dependence on foreign or domestic oil, and it reduces harmful emissions from burning these fuels. Geothermal plants do not destroy large tracts of land. They are efficient: A geothermal plant usually can produce more power than a fossil fuel-burning plant of the same size.

Geothermal plants are also very reliable. Because they do not depend on external fuel sources, they can run twenty-four hours a day, every day of the year. This is not always possible with power plants that burn coal or oil, which must be transported from distant locations. Geothermal plants are not vulnerable to weather, natural disasters, strikes, political disturbances, or other events that can disrupt fuel supplies.

Geothermal plants, on many levels, are flexible. It is possible to build them of modular components and to add or adapt components as the need arises. This is usually not possible with fossil fuel-burning plants. Geothermal power plants are especially valuable in areas with small power grids or in cases where a power grid is in the process of expanding. Flexible geothermal plants can provide backup power while the rest of the grid is installed.

Geothermal energy is generally sustainable and renewable. The Earth generates heat constantly. Rainfall and snowmelt continuously replenish reservoirs, and returning used water to the underground reservoir maintains its pressure and heat so that the reservoir can be used for an indefinite period of time.

Drawbacks of geothermal energy

The major limitation of geothermal power is that it can only be implemented in areas where there is a ready supply of hot water underground. This limits its use to geologically active areas such as California, Iceland, Japan and the rest of the Pacific Rim, and other areas with a thin crust, an active mantle, and pockets of subterranean hot water.

Only the hottest water can be used to generate electricity. Some places have naturally heated groundwater that is not hot enough to produce the steam needed to turn turbines. That water is still usable for other purposes but not as a workable power source.

It is possible to deplete a reservoir. If a geothermal reservoir runs out of water or grows too cool, it ceases to be useful, though this depletion can take decades or even centuries. For this reason some experts claim that geothermal energy is not actually a renewable resource.

In the early 2000s there are few areas with enough readily accessible geothermal water to produce electricity at a price that can compete with other sources of power. This may change as technology improves and other geothermal sources become usable or as the price of fossil fuels increases.

Environmental impact of geothermal energy

Like solar power and wind power, geothermal energy is clean. Geothermal power plants do not have to burn fuels so they do not produce emissions of greenhouse gases or other pollutants, which means they do not contribute to smog or global warming. They do emit very small amounts of carbon dioxide, about four percent of the amount emitted by burning fossil fuels. Binary plants produce no emissions at all. Areas that have geothermal power plants tend to have much better air quality than those with fossil fuel-burning power plants.

A geothermal power plant can be small compared to other types of power plants, so it is not as disruptive to the landscape. It can be built right next to its geothermal well. There is no need to build dams, dig mines, cut down trees, or dispose of wastes, which are necessary with other common forms of power. It is actually possible to build geothermal power plants in the middle of farmland or forests without damaging the surrounding plants and animals.

Even in areas where geothermal energy is not powerful enough to create electricity, people can still make use of local hot water for heating and bathing. This means they do not have to use electricity from other sources to heat their water, which can help save money and fuel.

There are some minor environmental drawbacks to using geothermal resources. Geothermal reservoirs sometimes contain hydrogen sulfide gas, which smells like rotten eggs and can be toxic at high concentrations. Geothermal power plants use scrubbers to remove this gas from emissions. Geothermal water also contains a high concentration of minerals, so geothermal wells must contain several layers of pipe and casings to prevent geothermal water from mixing with ordinary groundwater. Because geothermal power plants re-inject their used geothermal water back into the underground reservoir, in most cases the geothermal water never gets near groundwater and cannot harm aquatic plants and animals.

The areas around geothermal power plants experience increased activity, such as small earthquakes, and there is a danger of landslides. Federal laws in the United States prohibit the construction of geothermal power plants in national parks, such as Yellowstone. However, the environmental problems associated with using geothermal energy are generally far less serious than those caused by using fossil fuels.

Economic impact of geothermal energy

Geothermal power is produced locally, in the same area in which it is used. This means that states or nations do not have to pay other countries for fuel, as most countries do with fossil fuels. All economic benefits from a geothermal power plant remain in the area that produces the energy.

Using geothermal water saves money on other fuels, either to create electricity or to heat water. In the late 1990s, worldwide use of geothermal energy saved the equivalent of 830 million gallons of oil or 4.4 million tons of coal. This amount could be increased considerably if the use of geothermal energy were expanded.

Societal impact of geothermal energy

Much of the world's geothermal energy is used by developing nations that cannot afford to use fossil fuels for power and that may not have other sources of energy. Thailand, Indonesia, the Philippines, and the Azores have all been making use of geothermally generated electricity since the late 1980s or early 1990s.

Geothermal energy is a good, nonpolluting way for developing nations to build their infrastructures without destroying the landscape or polluting their air and water. The power produced by geothermal energy can raise standards of living in remote areas that are too far from other power sources. Because the energy is inexpensive, nations may be able to use the energy generated during off-peak hours for regional development projects, such as pumping water for irrigation. Local communities can be in complete control of their source of power, making them less dependent on their own government or foreign aid.

Many developing nations have created energy policies that emphasize using local resources for power, encouraging local private investment in energy, and expanding power into rural areas. Geothermal energy is very compatible with these goals because it can be locally run, and the resulting power used by the local community.

Barriers to implementation or acceptance

Many areas have geothermal potential that has not been tapped. During the twentieth century fossil fuels were a cheap and established source of power, and few areas have any incentive to spend the money to build geothermal power plants. People have not yet become aware of the many potential uses of geothermal water, so they are not taking full advantage of it. For example, in some areas naturally hot water is used for purposes that ordinary water could fulfill, such as irrigation of crops and municipal water supplies.

Many nations, both developing and advanced, have conducted initial investigations into their own geothermal potential. They have identified numerous geothermal reservoirs that could be used directly or converted into electricity, but they have not pursued the deep drilling needed both to confirm the reservoirs' potential and to exploit them. Geothermal energy does require large capital investments in its initial stages, and this investment comes with some risk. This initial investment deters (holds back) many governments and companies, as does the fact that it can take several years to achieve a return on the investment of building a geothermal power plant. Fossil fuel power plants earn back their investments much more quickly. It is also known, however, that geothermal power plants have long-term economic benefits, including low operating costs and long-term profits.

One major difficulty with developing geothermal power is access to land. Because geothermal power plants can only be built on or near geothermal reservoirs, power companies must be able to buy or lease this land.

In the early 2000s, most cities and buildings have been designed around fossil fuels and other more traditional sources of energy. Oil, gas, and coal companies do not want to see fossil fueled power plants closed because that would cause them to lose customers. Utilities do not want to have to rebuild existing power plants to convert them to geothermal power because that would be very expensive. In many countries the government grants monopolies to utility companies that make it possible for those companies to provide power at all times regardless of price fluctuations, but also make it impossible for alternative energy suppliers to compete in an open market.

AGRICULTURAL APPLICATIONS

Geothermal water is very useful in agriculture. Agricultural applications make direct use of geothermal water, using it to heat and water plants, to warm greenhouses, or to dry crops.

In agriculture, geothermal water is used mainly as a source of heat and moisture. Irrigation pipes can bring hot water to cold ground, making it possible to grow crops that would otherwise die. It can also be piped into greenhouses to keep them warm and to maintain humidity. As with most other uses of geothermal energy, geothermal agriculture is only practical in areas that have geothermal resources. It is possible in agriculture, however, to use geothermal water that is much too cold for power generation or even home heating. Only a few nations have thus far made much use of geothermal heat for agricultural purposes. They include the United States, Kenya, Greece, Guatemala, Israel, and Mexico.

Current uses of geothermal energy in agriculture

The main agricultural uses of geothermal water include heating and watering open fields, warming and humidifying greenhouses, and drying crops.

Open field agriculture

Geothermal water can be used to keep the soil in open fields at a steady warm temperature. Farmers run irrigation pipes under the soil to provide both water and heat to the crops. Cool-weather root crops and rapidly growing trees grow faster and more abundantly if the soil temperature is kept at about 70°F (21°C). Using geothermal water for irrigation extends the growing season and keeps plants from being damaged by low air temperatures.

Geothermal water can also sterilize soil to kill pests, fungus, and diseases that can harm crops. Sterilization requires very hot water so that the steam can be applied directly to the soil. The farmers either heat the soil from pipes underneath it, or they apply the steam above the soil and cover it with a plastic sheet to keep the heat inside.

Greenhouses

Greenhouses are buildings with clear plastic or glass walls and ceilings that trap solar heat to create a controlled atmosphere for growing plants. Greenhouses often benefit from another source of heat during the winter months. Heating greenhouses with geothermal water helps maintain a constant temperature, resulting in a more reliable crop and faster-growing plants. The water in the pipes can be released into the air inside the greenhouse, raising humidity if necessary.

There are several techniques used to heat greenhouses with geothermal water. These include plastic tubes, finned pipes, finned coils, soil heaters, or unit heaters. These parts can be combined according to water temperature and the preferences of the grower and the plants. For example, a grower producing roses would want to create a heating system with good air circulation and low humidity. A grower producing tropical plants could adjust the system to create high humidity and high soil temperatures. Chinese shiitake mushroom growers in Fujian province use geothermal heat in a greenhouse to speed production time.

Two large greenhouses at the La Carrindanga Project in Bahia Blanca, Argentina, have been using geothermal pipes to heat their facilities. These greenhouses have sliding glass side panels that can open and close to regulate humidity and heat, and misting systems to water plants and maintain moisture in the air. The geothermal water runs through pipes buried just beneath the surface of the soil, where the heat from the water easily reaches plant roots. Boxes containing dirt and seeds can sit on top of these pipes so that they receive heat from below. The beds grow vegetables, flowers, and indoor and outdoor plants from seeds and cuttings. Bahia Blanca has an unreliable climate and is not a very good location for outdoor agriculture, but its geothermally heated greenhouses are very productive and reliable.

Drying crops

The heat from geothermal water can also be used to dry crops and timber. For example, since the mid-1980s the Broadlands Lucerne Company in New Zealand has been using geothermal steam to dry alfalfa.

Benefits and drawbacks of agricultural applications

Geothermally heated greenhouses are especially useful in marginal areas where the climate is unreliable. They make plant and vegetable production more efficient, and they reduce the time it takes seeds to germinate and grow to maturity. In addition, they make it possible to grow crops in the off-season, when such plants ordinarily would not grow and when they can be sold for higher prices. Farmers can grow plants under denser and more controlled conditions. They lose fewer plants and can make more precise commitments to buyers for future deliveries of crops.

However, geothermal water is not available everywhere. Not every farming operation can make use of geothermal resources because either there are none in the region or they are too difficult to reach. Installing equipment to pipe geothermal water into a farm can be expensive and time-consuming.

Impact of agricultural applications

Using geothermal water to enhance agriculture causes few environmental problems. It does not pollute the land because only water is emitted, although if the water is contaminated with heavy metals, such as mercury, this could cause a public health concern. The use of geothermal water could potentially result in farms being constructed in areas that would otherwise not be suitable for agriculture, which could destroy natural landscape and animal habitat.

Economically, using geothermal water in agriculture can be quite inexpensive. If geothermal wells already exist, then the farmers need invest only in steel or plastic pipes to transport the steam or hot water to the field, greenhouse, or drying facility. In many places the hot water is quite shallow and inexpensive to reach.

Despite this comparative lack of expense, even this level of equipment is too expensive for many individuals and businesses. There are many regions that have geothermal resources that could be used for agriculture that have not yet been able to take advantage of them. For example, the Oserian Development Company on the shores of Lake Naivasha, Kenya, grows flowers for market. It has considered using hot water from the Olkaria Geothermal field to sterilize the soil. As of the early 2000s this plan had not been implemented because of the cost.

AQUACULTURAL APPLICATIONS

Aquaculture is the raising of fish and other aquatic animals in a controlled environment—basically, it is the farming of fish, shellfish, and other freshwater or marine (saltwater) creatures. Using geothermal water in aquaculture helps keep water temperatures consistent, which increases survival rates and makes the creatures grow faster.

Low-temperature geothermal resources that are not hot enough to produce electricity are very useful to fish farmers. Animals grown in water of the proper temperature grow faster and larger than those in cold water or water with fluctuating temperatures. They are also more resistant to disease and die less frequently.

Fish farmers with access to geothermal water can use it to regulate the temperatures of their fish ponds. Though the mechanism to accomplish this can be complicated, basically what happens is that the fish farmer opens valves to allow geothermal water to flow into the fish ponds until they reach the desired temperature. The valves are then closed to prevent the water from getting too hot. The mechanism is similar to adding hot water to a bathtub to bring the temperature to the desired level.

Water flow can be adjusted throughout the year to account for air temperatures. Most ponds contain some mechanism to circulate the water and keep it all at an even temperature. Aquaculture operations usually have several ponds, which are kept small enough to be heated or cooled easily.

Current uses of aquacultural applications

Geothermal water has played a role in aquaculture for more than thirty years. In the 1970s the Oregon Institute of Technology began using runoff from the school's geothermal heating system to heat water used to raise freshwater prawns. In Arizona, fish farmers use geothermal waters between 80 and 105°F (26 and 41°C) to raise bass, catfish, and tilapia. The Salton Sea and Imperial Valley areas in southern California are home to about fifteen aquaculture operations. These fish farms produce about ten million pounds of fish every year, mostly catfish, striped bass, and tilapia, which are almost all sold in California.

People in other nations have also taken advantage of geothermal water for aquaculture. There are geothermal eel farms in Slovakia. Geothermal fisheries in Iceland grow arctic char, salmon, abalone, and other fish and shellfish. China has over 500 acres of geothermal fish farms, while Japanese fish farms grow eels and alligators. There are also fish farms in France, Greece, Israel, Korea, and New Zealand.

The main species raised in geothermal waters are catfish, bass, trout, tilapia, sturgeon, giant freshwater prawns, alligators, snails, coral, and tropical fish. The warmth of geothermal water makes it possible to raise tropical marine (saltwater) species in cold, land-locked places such as Idaho.

Some creatures have a range of temperatures in which they thrive. For example, catfish and shrimp grow at about 50 percent of optimum rate at temperatures between 68 and 79°F (20 and 26°C) and grow fastest at about 90°F (32°C), but they decline at temperatures higher than that. Trout thrive at around 60°F (15.5°C) but dislike lower or higher temperatures.

Scientists are investigating using geothermal aquaculture to grow plants that humans and animals could eat. Possible crops include kelp, duckweed, algae, and water hyacinth. As of the early 2000s, the technology was not yet good enough to allow economically worthwhile harvesting and processing.

Benefits and drawbacks of aquacultural applications

Like other direct uses of geothermal water, aquaculture allows an area to make use of groundwater that may not be hot enough to generate electricity but is still hot enough to be useful as hot water. Arizona, for example, has a great deal of geothermal water that is under 300°F (149°C), which cannot generate electricity but is very useful in aquaculture.

The fish grown in geothermal fisheries are healthier and stronger than fish grown in unheated fish ponds. Fish farmers can regulate temperature throughout the year to make sure the fish grow to a consistent size year-round.

However, fish farmers must be careful to regulate water temperature. The water in and near the pipes bringing in the hot groundwater can get very hot, creating pockets that are too hot for fish. For aquaculture to work well, there must be a source of cool water in addition to the hot water. Some geothermal fisheries collect geothermal water in holding ponds and let it cool in order to regulate pond temperatures. If the water does not circulate evenly there can also be cold spots. This can make the fish crowd into areas where the temperature is at the right level. The hot pipes also can be dangerous to human workers who must wade into the pools for repairs, feeding, and harvesting.

Impact of aquacultural applications

For the most part, using hot groundwater to heat fish ponds is good for the environment. A farm that uses geothermal water is not burning fossil fuels or other sources of heat to regulate water temperature and is therefore not emitting pollutants. Many geothermal aquaculture operations use water that has already been used by geothermal power plants or heating systems. The water has lost most of its heat but is still hot enough to raise the temperature of the fish ponds, so it can be put to a second use before disposal.

Aquaculture itself has both good and bad aspects for the environment. It takes pressure off wild fisheries, many of which have been severely overfished. In some areas, however, it contributes to water pollution.

Economically, using geothermal energy to heat water for aquaculture can have many benefits. Places that use water that has already been used for heating or electricity generation can heat their fish ponds essentially for no cost. They can also enjoy the economic benefit of selling the fish or prawns that they produce. Fish grown in geothermally heated water grow faster than fish in unheated water, so some fish farmers can grow extra fish crops for sale. Heated water makes it possible to grow fish in winter when it ordinarily would not be possible. Selling tropical fish for the pet store market can be quite profitable. Developing nations can export their fish produce for good prices, bringing foreign capital into the country.

GEOTHERMAL POWER PLANTS

One very promising use of geothermal power is the generation of electricity. Areas with hot geothermal reservoirs can use this heat and steam to create electricity without having to spend money for fuel and without polluting the atmosphere or ground.

All types of geothermal power plants use geothermal steam to turn a turbine. The turbine is attached to a generator that creates the electricity. The electricity is then fed into a grid, which is attached to individual users. There are three main types of geothermal power plant: binary, dry steam, and flashed steam. There are also hybrid power plants that combine geothermal energy with other energy sources. The type of plant built in a given area depends on what sort of geothermal resource is available, either steam or liquid and either high or low temperature.

Binary power plants

Binary power plants use a two-step process to extract power from geothermal water that is not quite hot enough to spin a turbine by itself. The hot water is pumped up through the ground and passed through a heat exchanger that contains a fluid with a much lower boiling point than water. The heat from the geothermal water causes this "binary" fluid to flash into vapor. That vapor spins the turbine, which powers the generator. The geothermal water is injected back into the reservoir. The binary fluid stays inside the tank, where it is used over and over again. Nothing is released into the atmosphere.

Many areas have geothermal reservoirs with water that is below 400°F (204°C). Moderate-temperature geothermal water is much more common than high-temperature water. The United States Department of Energy predicts that most geothermal power plants built in the future will be binary power plants that can take advantage of this slightly cooler water.

Dry steam plants

Dry steam plants use the steam that comes up from a geothermal reservoir to power turbines that power generators. The liquid and steam is then injected back into the reservoir to regain its heat and maintain the reservoir's pressure. Dry steam was the first technology used to build geothermal power plants. The plant built in Lardarello, Italy, in 1904 used dry steam technology. The Geysers in northern California uses dry steam to produce power. Dry steam is still the largest source of geothermal power in the world.

Flashed steam plants

Flashed steam plants are the most common type of geothermal power plant. These plants use geothermal water that is over 360°F (182°C). The fluid is pumped up at high pressure and then sprayed into a tank that is at lower pressure than the water. This causes the geothermal water to "flash," or turn into steam instantly. The steam spins a turbine, which powers a generator. Fluid left in the first tank is then pumped into another tank to be flashed again. After the water has been used, it is injected back into the reservoir to regain its heat.

Hybrid power plants

Some areas do not have enough geothermal energy to run a full power plant. These places can be ideal sites for hybrid power plants that combine different types of power generation. They can combine different types of geothermal energy generation or combine geothermal energy with other energy sources, even fossil fuels.

Benefits and drawbacks of geothermal power plants

Geothermal power plants are usually built with modular designs, which makes them very flexible. It is easy to start with a small plant and then add additional units if the demand for electricity increases. Geothermal plants can also use some of their water, either freshly pumped or after being used for electricity, for other direct purposes, such as heating or aquaculture.

However, not every location can use geothermal power. Geothermal power plants must be located near a geothermal reservoir that has water of at least 250°F (121°C) and preferably 300°F (148°C). Not all reservoirs have water this hot. An ideal geothermal reservoir is hot with low mineral content, has shallow aquifers nearby to make it easy to re-inject used water, is on private land in order to make it easier to get permits, is near existing electrical transmission lines, and has a supply of cooler water for cooling. It also needs a high enough volume of water to keep flowing steadily. In the United States, only the western states and Hawaii have these resources.

Environmental impact of geothermal power plants

Geothermal power plants are generally environmentally clean. They do not burn fossil fuels, so they help conserve those fuels for other purposes. They produce no emissions to contribute to air pollution, the greenhouse effect, or global warming. There is no smoke surrounding geothermal power plants. Dry steam and flashed steam plants emit excess steam and small amounts of gases, while binary plants emit nothing at all because all the fluids are contained within the system and recycled. Areas that use geothermal power have some of the best air quality readings in the world. Lake County, California, which has five geothermal power plants, is the only county in the United States that has met the strictest governmental air quality standards since the mid-1990s.

Geothermal plants do not need space to store fuels, and they do not create large piles of ash that must be cleared or oil spills that damage oceans. They also do not pollute groundwater, unless the geothermal water has a high concentration of minerals or metals.

Unlike most other power plant types, geothermal plants do not require large amounts of space to function. They can be built right on top of geothermal reservoirs. The pumps that bring water up from geothermal reservoirs are small, especially compared to those used by coal mines or oil wells. They do not tear up large plots of land or destroy forests. There is no need to build major highways, railroads, or pipelines in order to transport fuel to geothermal power plants because their source of power is directly below them. It is actually possible to build geothermal power plants in the midst of farmland or forests, where they can coexist with livestock and wildlife.

Economic impact

Geothermal power plants require an initial investment in finding reservoirs, digging wells, and building a plant with turbines. This initial investment can be quite heavy, between $3,000 and $5,000 per kilowatt. Once a plant is built, however, it can be more economical than producing power with fossil fuels. Fuel does not have to be purchased to run the plant, which saves money and makes operations more predictable, as the plant is not affected by fluctuations in the price of oil, gas, or coal.

Issues, challenges, and obstacles of geothermal power plants

In developed nations the existing utility companies have a large investment in their currently functioning power plants. These plants usually run on fossil fuels, though there are some nuclear and hydroelectric power plants. The utilities themselves and the oil and gas companies that supply their fuel have an interest in maintaining things as they are. There is little incentive for them to give up their source of income in favor of geothermal power.

Geothermal power plants can only be built on or near geothermal reservoirs. These reservoirs are often on private land or land that is already being used for some other purpose. A company that wants to build a geothermal power plant must first get access to the land over the reservoir, which can be difficult, expensive, and time-consuming. There needs to be a great deal more research and development before geothermal power generation becomes practical around the world.

GEOTHERMAL HEATING APPLICATIONS

One obvious use of geothermal energy is for heat. Many cities and homes use naturally hot water to keep them warm in winter. There are two main ways to use geothermal water for heating. The older method is using the water directly. Newer technology involves using a geothermal heat pump.

Direct heating

Direct heating pumps the water from the geothermal reservoir in the ground and passes it through pipes running through buildings. The heat from the water moves from the pipes through the walls into the air inside the building. This system can also be used to heat water.

For direct heating, the best geothermal water temperature is under 212°F (100°C). In fact, water with a temperature as low as 95°F (35°C) can be used for direct heating. In some areas, such as Iceland, the geothermal water is pure enough that it can be pumped directly through radiators. In most places, however, chemicals in the water make it necessary to filter the water through heat exchangers that extract the heat from the water.

Oregon's Geothermal Zone

Klamath Falls, Oregon, has used geothermal heating for homes since 1900. In the early 2000s, more than 550 geothermal wells were in use, heating homes, pools, schools, and businesses. Geothermal pipes run under the sidewalks and highways to keep them clear of snow. In 1982 the city built a geothermal district heating system that heats the entire eastern part of the city. Two wells east of downtown pump water that is about 210°F (98°C) from underground reservoirs to the central mechanical room at the County Museum. This water is treated and then delivered to customers. It is about 180°F (82°C) when it reaches the seven hundred homes and buildings that use geothermal heat. When it returns to the mechanical room, it has lost about 40°F (4°C) of temperature. It is then injected back into the reservoir to be recycled.

Geothermal heat pumps

Newer technology uses geothermal water to run a heat pump, similar to an electric heat pump. A geothermal heat pump forces heat in a direction it would not ordinarily go. Most heat pumps can function as both heating and cooling units. In winter they heat air and pump it through the house. In summer they absorb hot air and pump it into the ground. Geothermal heat pumps are particularly efficient because they start with air or water that is already hot and thus do not have to heat it as much as ordinary heat pumps, which start with cold outside air. Geothermal heat pumps use 30 to 60 percent less electricity than traditional heat pumps because they do not have to create their own heat, just move it from place to place.

Geothermal heat pumps work by pumping water or a mix of water and antifreeze through the ground next to a house or building. The ground temperature remains relatively constant throughout the year, generally between 45 and 55°F (7 and 12°C). In winter, underground pipes absorb heat from the Earth. This heated water circulates into the heat pump, where it is concentrated so that it will increase to the desired room temperature. The heat pump then pumps the hot air through the ducts in the building, heating the rooms. In summer the process is reversed; the hot air is sucked from the building and dispersed into the ground. The geothermal heat pump system uses ordinary ductwork, so there is no need to modify existing ducts.

Geothermal heat pumps usually can at least partially heat water for the home. This is not necessarily possible all year round. During the summer the heat pump can use excess heat to warm domestic hot water, but during the winter there is not as much heat available to warm the water. A home with a geothermal heat pump must usually have an alternate source of heat for water, but even so, using excess heat for even part of the year results in an energy savings. New technology is improving this situation; because geothermal heat pumps are so much more efficient than other forms of water heating, some manufacturers are now selling geothermal heat pumps that heat water separately, thereby providing hot water year-round.

Geothermal heat pumps, like all heat pumps, produce slightly warmer air than fossil fuel furnaces. Geothermal heat pumps generally produce hot air between 95 and 103°F (35 and 39°C), as opposed to conventional heat pumps, which produce hot air between 90 and 95°F (32 and 35°C). Geothermal heat pumps require more open ductwork for air flow than fossil fuel furnaces, which can be a problem when converting older houses to geothermal heat.

Current uses of geothermal heating applications

People have used geothermal water to heat buildings for hundreds of years. People in Paris, France, heated buildings with geothermal water six hundred years ago. Boise, Idaho, began using geothermal heating in 1892. This system is still in use there, where four district heating systems heat over five million square feet (152 million square meters) of space.

Starting in the 1960s, other cities began to take notice of the potential benefits of geothermal energy. By the early 2000s geothermal direct heating was common in Iceland, Hungary, Poland, China, Argentina, Croatia, France, and Turkey. Reykjavik, Iceland, has the world's largest geothermal heating system, with about two hundred miles (320 kilometers) of pipes running throughout the city. The city is almost entirely heated by geothermal heat.

Geothermal heat pumps are gradually gaining popularity as people learn about them. Geothermal heat pumps in the early 2000s were considered much more efficient than the ones made in 1990. Experts foresee some continuing improvements but believe they will be small compared to improvements already made.

Benefits and drawbacks of geothermal heating applications

Geothermal direct heat is inexpensive and nonpolluting. Places that have sufficient geothermal resources can heat entire cities for just the cost of running the pipes. The heat is always available and does not depend on fuel supplies. However, geothermal direct heat is only possible in areas with substantial geothermal resources. That means it cannot be a worldwide solution to the heating problem.

On the other hand, geothermal heat pumps can be used almost anywhere in the world because they do not require the presence of geothermal reservoirs. They make it possible to use geothermal resources that were formerly considered unusable. They can be used for summer cooling in addition to winter heating, and sometimes they can supply hot water as well. These pumps are easiest to install in new buildings; it is difficult to convert existing homes to geothermal heat pumps, and they cost more than electric heat pumps.

Impact of geothermal heating applications

Geothermal heating has many obvious environmental benefits. It does not pollute the air at all because geothermal heating involves no combustion and therefore no emissions. Geothermal heat pumps pose few environmental problems. They use an antifreeze substance, but it is usually a nontoxic chemical called propylene glycol or small amounts of methanol, both of which are commonly used in windshield washing solutions.

Economically, geothermal heat can be much less expensive than other sources of heat, such as fossil fuels or wood, but that cost depends on several factors. The initial installation costs can be high, but if the heating system works well it can pay for itself quickly. Geothermal heat works especially well in areas that already have wells dug into geothermal reservoirs. If there are already wells in place, a district or institution only needs to buy pipelines, heat exchangers, and pumps. A heating system is more expensive to install if there is not already a good geothermal reservoir in use.

Geothermal heat pumps currently cost more than conventional ones, but once they are installed the cost of running them is less than that of any other conventional form of heat, including natural gas. This savings depends on the cost of fossil fuels; as fossil fuels get more expensive, geothermal heat may become more economical. It is estimated that geothermal heat pumps can reduce the power used to heat or cool a house by one to five kilowatts of generating capacity at peak time, which can result in major savings on residential heating and cooling costs.

Issues, challenges, and obstacles of geothermal heating applications

Experts estimate that almost three hundred communities in the western United States are close enough to geothermal heat sources to use them as a district heating system. Many other countries have the potential to use more geothermal energy for district heating. People are gradually taking more interest in geothermal resources as fossil fuels become more expensive and the dangers of air pollution become more apparent.

Implementing a geothermal heating system is a major investment. It requires money, labor, and a willingness to take the risk that it may not work. The technology is still new and does not have a long track record, nor are there many people who are experts in installing geothermal heating systems. There have been unsuccessful attempts to use geothermal heat.

In the early 2000s, there were about 500,000 geothermal heat pumps in use in the United States. Switzerland and several other countries were implementing programs to increase geothermal heat pump usage. There is plenty of potential for expansion. People do not use them mainly because they are not widely available, and few people know that they exist. There is also the problem of persuading people to buy geothermal heat pumps when they cost more than conventional climate control systems.

INDUSTRIAL APPLICATIONS

Many industries need steam or hot water for their operations. Geothermal water is an excellent low-cost source of this basic item. Industries generally need very hot water, hotter than the water used in agriculture or aquaculture, though there is much variation. Plants can be built right next to geothermal reservoirs and pipe the water or steam straight into the operation.

Current uses of industrial applications

Geothermal water is useful in any industry that requires steam or hot water. Some uses include:

• Timber processing
• Pulp and paper processing
• Washing wool
• Dyeing cloth
• Drying diatomaceous earth (a light, abrasive soil used as a filtering material and insecticide)
• Drying fish meal and stock fish
• Canning food
• Drying cement

• Drying organic materials such as vegetables, seaweed, and grass
• Refrigeration

Benefits and drawbacks of industrial applications

Using geothermal water and steam saves companies the cost of heating water and saves the environment some of the pollution that would be caused by heating the water. However, geothermal water is only available in a few places, so most industries cannot use it.

Issues, challenges, and obstacles

The use of geothermal water in industry is still very new, and as of the early 2000s, not many industries are taking advantage of it. Few people know whether or not geothermal energy is available or how to use it if it is. Implementing geothermal energy requires installing equipment such as pipes, which can be expensive or difficult in an existing plant.

Geothermal Dye Works

In most cases an industry uses geothermal water because it is a cheap source of heat and/or water. In a few cases, however, industries take advantage of the unique mineral properties of geothermal water. In Iwate Prefecture, Japan, there is a new geothermal dye factory that uses the minerals in geothermal water as a mordant, a substance that makes dye pigments stick to cloth, and also as a substance that can remove dye from cloth. The factory uses a method of folding and tying the cloth with string, soaking it in dye made with geothermal water, and then rinsing it and unfolding it. The combination of steam, heat, and the hydrogen sulfide in the geothermal water leaves beautiful and unique patterns on the cloth.

For More Information

Books

Cataldi, Raffaele, ed. Stories from a Heated Earth: Our Geothermal Heritage. Davis, CA: Geothermal Resources Council, 1999.

Dickson, Mary H., and Mario Fanelli, eds. Geothermal Energy: Utilization and Technology. London: Earthscan Publications, 2005.

Geothermal Development in the Pacific Rim. Davis, CA: Geothermal Resources Council, 1996.

Graham, Ian. Geothermal and Bio-Energy. Fort Bragg, CA: Raintree, 1999.

Wohltez, Kenneth, and Grant Keiken. Volcanology and Geothermal Energy. Berkeley: University of California Press, 1992.

Periodicals

Anderson, Heidi. "Environmental Drawbacks of Renewable Energy: Are They Real or Exaggerated?" Environmental Science and Engineering (January 2001).

Web sites

"Geo-Heat Center." Oregon Institute of Technology. http://geoheat.oit.edu. (accessed on July 19, 2005).

"Geothermal Energy." World Bank. http://www.worldbank.org/html/fpd/energy/geothermal. (accessed on July 19, 2005).

Geothermal Energy Association. http://www.geo-energy.org. (accessed on August 4, 2005).

Geothermal Resources Council. http://www.geothermal.org. (accessed on August 4, 2005).

"Geothermal Technologies Program." U.S. Department of Energy: Energy Efficiency and Renewable Energy. http://www.eere.energy.gov/geothermal. (accessed on July 22, 2005).

Other sources

World Spaceflight News. 21st Century Complete Guide to Geothermal Energy (CD-ROM). Progressive Management, 2004.

GEOTHERMAL ENERGY

Geothermal energy is heat energy that originates within Earth itself. The temperature at the core of our planet is 4,200°C (7,592°F), and heat flows outward to the cooler surface, where it can produce dramatic displays such as volcanoes, geysers, and hot springs, or be used to heat buildings, generate electricity, or perform other useful functions. This outward flow of heat is continually being maintained from within by the decay of radioactive elements such as uranium, thorium, and radium, which occur naturally in Earth. Because of its origin in radioactivity, geothermal energy can actually be thought of as being a form of natural nuclear energy.

The U.S. Department of Energy has estimated that the total usable geothermal energy resource in Earth's crust to a depth of 10 kilometers is about 100 million exajoules, which is 300,000 times the world's annual energy consumption. Unfortunately, only a tiny fraction of this energy is extractable at a price that is competitive in today's energy market.

In most areas of the world, geothermal energy is very diffuse—the average rate of geothermal heat transfer to Earth's surface is only about 0.06 watt per square meter. This is very small compared to, say, the solar radiation absorbed at the surface, which provides a global average of 110 watts per square meter. Geothermal energy can be readily exploited in regions where the rate of heat transfer to the surface is much higher than average, usually in seismic zones at continental-plate boundaries where plates are colliding or drifting apart. For example, the heat flux at the Wairakei thermal field in New Zealand is approximately 30 watts per square meter.

A related aspect of geothermal energy is the thermal gradient, which is the increase of temperature with depth below Earth's surface. The average thermal gradient is about 30°C (54°F) per kilometer, but it can be much higher at specific locations—for instance, in Iceland, where the increase is greater than 100°C (180°F) per kilometer in places.

TYPES OF GEOTHERMAL SOURCES

Geothermal sources are categorized into various types: hydrothermal reservoirs, geopressurized zones, hot dry rock, normal geothermal gradient, and magma.

Hydrothermal Reservoirs

Groundwater can seep down along faults in Earth's crust and become heated through contact with hot rocks below. Sometimes this hot water accumulates in an interconnected system of fractures and becomes a hydrothermal reservoir. The water might remain underground or might rise by convection through fractures to the surface, producing geysers and hot springs.

Hydrothermal reservoirs are the only geothermal sources that have been used for commercial energy production. Because of the high pressure deep below Earth's surface, the water in these sources can become heated well above the usual boiling temperature of 100°C (212°F). As the superheated water makes its way to the surface, either by convection or because a geothermal well has been drilled, some or all of the water will vaporize to become steam because of the lower pressures encountered. The most desirable geothermal sources have very high temperatures—above 300°C (572°F)—and all of the water vaporizes to produce dry steam (containing no liquid water), which can be used directly in steam-electric turbines.

Wet steam reservoirs are much more common than the simple dry type. Again, the field is full of very hot water, under such high pressure that it cannot boil. When a lower-pressure escape route is provided by drilling, some of the water suddenly evaporates (flashes) to steam, and it is a steam-water mixture that reaches the surface. The steam can be used to drive a turbine. The hot water also can be used to drive a second turbine in a binary cycle, described in the section "Electricity Generation" in this article.

Many geothermal reservoirs contain hot water at a temperature too low for electricity generation. However, the water can be used to heat buildings such as homes, greenhouses, and fish hatcheries. This heating can be either direct or through the use of heat pumps.

Geopressurized Zones

Geopressurized zones are regions where water from an ancient ocean or lake is trapped in place by impermeable layers of rock. The water is heated to temperatures between 100°C (212°F) and 200°C (392°C) by the normal flow of heat from Earth's core, and because of the overlying rock, the water is held under very high pressure as well. Thus energy is contained in the water because of both the temperature and the pressure, and can be used to generate electricity. Many geopressurized zones also contain additional energy in the form of methane from the decay of organic material that once lived in the water. The U.S. Geological Survey has estimated that about one third of the energy from geopressurized zones in the United States is available as methane.

Hot Dry Rock

In many regions of the world, hot rocks lie near Earth's surface, but there is little surrounding water. Attempts have been made to fracture such rocks and then pump water into them to extract the thermal energy, but the technical difficulties in fracturing the rocks have proven to be much more troublesome than anticipated, and there has been a problem with water losses. Consequently, progress in extracting energy from hot, dry rocks has been slow. However, experiments are ongoing in the United States, Japan, and Europe because the amount of energy available from hot, dry rocks is much greater than that from hydrothermal resources. The U.S. Department of Energy estimates that the total energy available from high-quality hot, dry rock areas is about 6,000 times the annual U.S. energy use.

Normal Geothermal Gradient

In principle the normal geothermal gradient produces a useful temperature difference anywhere on the globe. If a hole is drilled to a depth of 6 kilometers (which is feasible), a temperature difference of about 180°C (324°F) is available, but no technology has been developed to take advantage of this resource. At this depth, water is unlikely, and the problems of extracting the energy are similar to the difficulties encountered with hot, dry rocks near the surface.

Magma

Magma is subterranean molten rock, and although the potential thermal resource represented by magma pools and volcanoes is extremely large, it also presents an immense technological challenge. The high temperatures produce obvious problems with melting and deformation of equipment. The most promising candidates for heat extraction from magma are young volcanic calderas (less than a few million years old) that have magma relatively close to the surface. There have been some preliminary test wells drilled at the Long Valley caldera, located about 400 kilometers north of Los Angeles. The hope is that heat can be extracted in this area by drilling a well down to the magma level and pumping water into the well to solidify the magma at the bottom of the well. Then more water could be pumped into the well, become heated by contact with the solidified magma, and then returned to the surface to generate steam for electricity production.

ELECTRICITY GENERATION

There are two general ways by which geothermal energy can be utilized: generation of electricity, and space heating. Production of electricity from a geothermal source was pioneered in an experimental program at the Larderello thermal field in Italy in 1904, and a 205-kilowatt generator began operation at this site in 1913. By 1998 the world's geothermal electrical generating capacity was about 8,240 megawatts, which represents only a small portion of the total electricity capacity of 2 million megawatts from all sources. However, the geothermal capacity has been growing steadily, and the 1998 amount represents a 40 percent increase above the 1990 value of 5,870 megawatts. The amount of electrical energy generated geothermally worldwide in 1998 was about 44 terawatt-hours, or 0.16 exajoule, representing approximately 0.4 percent of global electricity generation.

The United States is the world leader in geothermal electricity production, with about 2850 megawatts of capacity. As shown in Table 1, nine other countries each had more than 100 megawatts of electrical capacity in 1998, with this group being led by the Philippines, producing 1,850 megawatts of geothermal electrical power.

The world's most developed geothermal source is at the Geysers plant in California's Mayacamas Mountains, about ninety miles north of San Francisco. Electricity has been generated at this site since 1960, and as of 1999 the total installed generating capacity there was 1,224 megawatts. It has been demonstrated at the Geysers and at other geothermal electric plants that electricity from geothermal resources can be cost-competitive with other sources.

To produce electricity from a geothermal resource, wells are drilled into the reservoir, and as the hot, high-pressure water travels to Earth's surface, some of it vaporizes into steam as the pressure decreases. The hotter the original source, the greater the amount of dry steam produced. For dry-steam sources, the technology is basically the same as for electric plants that use the burning of fossil fuels to produce steam, except that the temperature and pressure of the geothermal steam are much lower. Dry-steam fields are being used in the United States, Italy, and Japan. In the Geysers geothermal area in California, the steam temperature is about 200°C (392°F) and the pressure about 700 kilopascals (7 atmospheres). Because geothermal steam is cooler than fossil-fuel steam, the efficiency of conversion of thermal energy to electricity is less than at a fossil-fuel plant; at the Geysers it is only 15 to 20 percent, compared to 40 percent for fossil fuels. The generating units are also smaller, ranging in size from 55 to 110 megawatts at the Geysers.

For geothermal reservoirs that are at lower temperatures, and hence produce less dry steam, working plants usually employ multiple-vaporization systems. The first vaporization ("flash") is conducted under some pressure, and the remaining pressurized hot water from the ground, along with the hot residual water from the turbine, can be flashed again to lower pressure, providing steam for a second turbine. Flashed power production is used in many countries, including the United States, the Philippines, Mexico, Italy, Japan, and New Zealand.

If the temperature of the original hot water is too low for effective flashing, the water can still be used to generate electricity in what is referred to as binary cycle (or organic cycle) electricity generation. The hot water is pumped to the surface under pressure to prevent evaporation, which would decrease the temperature, and its heat is transferred to an organic fluid such as isobutane, isopentane, Freon, or hexane, all of which have a boiling temperature lower than that of water. The fluid is vaporized by the heat from the water and acts as the working fluid in a turbine. It has been estimated that geothermal reservoirs with temperatures suitable for binary cycle generation are about fifty times as abundant as sources that provide pure dry steam.

Some geothermal power plants use a combination of flash and binary cycles to increase the efficiency of electricity production. An initial flash creates steam that drives a turbine; then the binary cycle is run, using either the hot water remaining after the initial flash or the hot exhaust from the turbine.

Geopressurized zones, discussed earlier, also are areas where electricity could be generated geothermally,

not only from the hot water but also from the associated methane. Three geopressurized well sites in the United States have been developed experimentally for electricity production, but the cost of the electricity generated is considerably higher than that from conventional energy sources.

DIRECT GEOTHERMAL ENERGY USE

Geothermal sources at too low a temperature for electricity generation can be utilized for space heating, bathing, and other uses. Hot fluid from the reservoir is piped to the end-site location through insulated pipes to minimize loss of heat energy. Pumps are installed either in the geothermal well itself (if the temperature is low enough) or at the surface to drive the fluid through the piping system. To operate, these pumps require energy, usually supplied by electricity. The hot fluid can be used itself to provide the heating, or it can be pass through a heat exchanger where another working fluid is heated.

Direct geothermal energy is used for space heating of homes, greenhouses, livestock barns, and fish-farm ponds. As well, it is employed as a heat source in some industrial processes, such as paper production in New Zealand and drying diatomite in Iceland. Since the industrial applications usually require higher

temperatures than, say, space heating, it is advantageous to cascade the geothermal fluid, using it first at high temperature in industry and then afterward at lower temperature for another use.

Another way by which heat from the ground can be utilized is through the use of heat pumps. Pipes containing a fluid are buried in the ground, and heat can be extracted from the ground in the winter to heat a building, and dissipated in the ground in the summer to provide air conditioning. Essentially a heat pump acts like a refrigerator, extracting thermal energy from one area and moving it to another area. In the winter, the ground is cooled and the building is heated, and in the summer the heat pump runs in reverse, to cool the building and heat the ground.

An extensive summary of the various direct uses of geothermal energy has been compiled. Worldwide the most common use is for space heating (33%), followed by bathing, swimming, and therapeutic use of baths (19%). Table 2 shows the worldwide percentages for other uses, as well as percentages for the top four direct-use countries. As seen in this table, the specific uses of geothermal energy vary greatly from country to country.

The geothermal direct-use power capacity worldwide in 1997 was close to 10,000 megawatts, with China and the United States leading the way with 1910 megawatts each. Iceland and Japan also had more than 1,000 megawatts each, as shown in Table 3, and more than thirty countries in total were using geothermal heat. Total geothermal direct-use energy in 1997 was about 37 terawatt-hours, or 0.13 exajoule.

One of the countries that makes extensive use of direct geothermal heat is Iceland. This might seem surprising for a country with "ice" as part of its name, but Iceland lies in a region of continental-plate activity. Virtually all the homes and other buildings in Iceland are heated geothermally, and there is also a small geothermal electric plant. In Iceland's capital city, Reykjavik, geothermally heated water has been used for space heating since 1930, and the cost of this heating is less than half the cost if oil were used.

Other important examples of geothermal heating are in France, both near Paris and in the Southwest. During oil-exploration drilling in the 1950s, hot water was discovered in the Paris region, but exploitation did not begin until the 1970s as a result of rapidly increasing oil prices. In France, the equivalent of 200,000 homes are being provided with space heating and water heating from geothermal sources. One interesting feature of the French geothermal sources is that they do not occur in regions of elevated thermal gradient.

ENVIRONMENTAL EFFECTS OF GEOTHERMAL ENERGY

The most important potential environmental impacts of geothermal energy are water and air pollution. At the largest geothermal plants, thousands of tons of hot water and/or steam are extracted per hour, and these fluids contain a variety of dissolved pollutants. The hot geothermal water dissolves salts from surrounding rocks, and this salt produces severe corrosion and scale deposits in equipment. To prevent contamination of surface water, the geothermal brine must either be returned to its source or discarded carefully in another area. In addition to salts, the geothermal waters sometimes contain high concentrations of toxic elements such as arsenic, boron, lead,

and mercury. The 180-megawatt Wairakei geothermal electrical plant in New Zealand dumps arsenic and mercury into a neighbouring river at a rate four times as high as the rate from natural sources nearby.

Geothermal water often contains dissolved gases as well as salts and toxic elements. One of the gases that is often found in association with geothermal water and steam is hydrogen sulfide, which has an unpleasant odor—like rotten eggs—and is toxic in high concentrations. At the Geysers plant in California, attempts have been made to capture the hydrogen sulfide chemically, but this job has proven to be surprisingly difficult; a working hydrogen sulfide extractor is now in place, but it was expensive to develop and install, and its useful life under demanding operating conditions is questionable. Geothermal brines also are a source of the greenhouse gas carbon dioxide, which contributes to global warming. However, a typical geothermal electrical plant produces only about 5 percent of the carbon dioxide emitted by a fossil-fuel-fired plant generating the same amount of electricity. Many modern geothermal plants can capture the carbon dioxide (as well as the hydrogen sulfide) and reinject it into the geothermal source along with the used geothermal fluids. At these facilities, the carbon dioxide that escapes into the atmosphere is less than 0.1 percent of the emissions from a coal- or oil-fired plant of the same capacity.

Another potential problem, particularly if geothermal water is not returned to its source, is land subsidence. For example, there has been significant subsidence at the Wanaker field in New Zealand. Finally, an annoying difficulty with geothermal heat has been the noise produced by escaping steam and water. The shriek of the high-pressure fluids is intolerable, and is usually dissipated in towers in which the fluids are forced to swirl around and lose their kinetic energy to friction. However, the towers provide only partial relief, and the plants are still noisy.

GEOTHERMAL ENERGY—RENEWABLE OR NOT?

Most people tend to think of geothermal energy as being renewable, but in fact one of the major problems in choosing a geothermal energy site lies in estimating how long the energy can usefully be extracted. If heat is withdrawn from a geothermal source too rapidly for natural replenishment, then the temperature and pressure can drop so low that the source becomes unproductive. The Geysers plants have not been working at full capacity because the useful steam would be depleted too quickly. Since it is expensive to drill geothermal wells and construct power plants, a source should produce energy for at least thirty years to be an economically sound venture, and it is not an easy task to estimate the working lifetime beforehand.

THE FUTURE OF GEOTHERMAL ENERGY

The main advantage of geothermal energy is that it can be exploited easily and inexpensively in regions where it is abundantly available in hydrothermal reservoirs, whether it is used for electricity production or for direct-use heat. Geothermally produced electricity from dry-steam sources is very cheap, second only tohydroelectric power in cost. Electricity from liquid-dominated hydrothermal sources is cost-competitive with other types of electrical generation at only a few sites. It is unlikely that other types of geothermal sources, such as hot, dry rock and the normal geothermal gradient, will soon become economical. Hence, geothermal energy will provide only a small fraction of the world's energy in the foreseeable future.

An important feature of geothermal energy is that it has to be used locally, because steam or hot water cannot be piped great distances without excessive energy loss. Even if electricity is generated, losses also are incurred in its transmission over long distances. As a result, geothermal energy use has geographical limitations.

The future development of geothermal energy resources will depend on a number of factors, such as cost relative to other energy sources, environmental concerns, and government funding for energy replacements for fossil fuels. An important concern that many members of the public have about future developments is centered around whether resources such as hot springs and geysers should be exploited at all. Many of these sources—such as those in Yellowstone National Park in Wyoming—are unique natural phenomena that many people feel are important to protect for future generations.

Ernest L. McFarland

See also: Biofuels; Diesel Fuel; District Heating and Cooling; Economically Efficient Energy Choices; Electric Power, Generation of; Environmental Problems and Energy Use; Fossil Fuels; Heat and Heating; Heat Pumps; Heat Transfer; Hydroelectric Energy; Reserves and Resources; Seismic Energy; Thermal Energy; Thermal Energy, Historical Evolution of the Use of; Thermodynamics; Water Heating.

BIBLIOGRAPHY

Atomic Energy of Canada Limited. (1999). Nuclear Sector Focus: A Summary of Energy, Electricity, and Nuclear Data. Mississauga, Ont.: Author.

Fridleifsson, I. B. (1998). "Direct Use of Geothermal Energy Around the World." Geo-Heat Center Quarterly Bulletin 19(4):4–9.

Golub, R., and Brus, E. (1994). The Almanac of Renewable Energy: The Complete Guide to Emerging Energy Technologies. New York: Henry Holt.

Howes, R. (1991). "Geothermal Energy." In The Energy Sourcebook, ed. R. Howes and A. Fainberg. New York: American Institute of Physics.

Lund, J. W. (1996). "Lectures on Direct Utilization of Geothermal Energy." United Nations University Geothermal Training Programme Report 1996-1. Reykjavik, Iceland.

McFarland, E. L.; Hunt, J. L.; and Campbell, J. L. (1997). Energy, Physics and the Environment, 2nd ed. Guelph, Ont.: University of Guelph.

Geothermal energy

Geothermal energy is obtained from hot rocks beneath the earth's surface. The planet's core, which may generate temperatures as high as 8,000°F (4,500°C), heats its interior, whose temperature increases, on an average, by about 1°C (2°F) for every 60 ft (18 m) nearer the core. Some heat is also generated in the mantle and crust as a result of the radioactive decay of uranium and other elements.

In some parts of the earth, rocks in excess of 212°F (100°C) are found only a few miles beneath the surface. Water that comes into contact with the rock will be heated above its boiling point. Under some conditions, the water becomes super-heated, that is, is prevented from boiling even though its temperature is greater than 212°F (100°C). Regions of this kind are known as wet steam fields. In other situations the water is able to boil normally, producing steam. These regions are known as dry steam fields.

Humans have long been aware of geothermal energy. Geysers and fumaroles are obvious indications of water heated by underground rock. The Maoris of New Zealand, for example, have traditionally used hot water from geysers to cook their food. Natural hot spring baths and spas are a common feature of many cultures where geothermal energy is readily available.

The first geothermal well was apparently opened accidentally by a drilling crew in Hungary in 1867. Eventually, hot water from such wells was used to heat homes in some parts of Budapest. Geothermal heat is still an important energy source in some parts of the world. More than 99% of the buildings in Reykjavik, the capital of Iceland, are heated with geothermal energy.

The most important application of geothermal energy today is in the generation of electricity. In general, hot steam or super-heated water is pumped to the planet surface where it is used to drive a turbine. Cool water leaving the generator is then pumped back underground. Some water is lost by evaporation during this process, so the energy that comes from geothermal wells is actually non-renewable. However, most zones of heated water and steam are large enough to allow a geothermal mine to operate for a few hundred years.

A dry steam well is the easiest and least expensive geothermal well to drill. A pipe carries steam directly from the heated underground rock to a turbine. As steam drives the turbine, the turbine drives an electrical generator. The spent steam is then passed through a condenser where much of it is converted to water and returned to the earth.

Dry steam fields are relatively uncommon. One, near Larderello, Italy, has been used to produce electricity since 1904. The geysers and fumaroles in the region are said to have inspired Dante's Inferno. The Larderello plant is a major source of electricity for Italy's electric railway system. Other major dry steam fields are located near Matsukawa, Japan, and at Geysers, California. The first electrical generating plant at the Geysers was installed in 1960. It and companion plants now provide about 5% of all the electricity produced in California.

Wet steam fields are more common, but the cost of using them as sources of geothermal energy is greater. The temperature of the water in a wet steam field may be anywhere from 360–660°F (180–250°C). When a pipe is sunk into such a reserve, some water immediately begins to boil, changing into very hot steam. The remaining water is carried out of the reserve with the steam.

At the surface, a separator is used to remove the steam from the hot water. The steam is used to drive a turbine and a generator, as in a dry steam well, before being condensed to a liquid. The water is then mixed with the hot water (now also cooled) before being returned to the earth.

The largest existing geothermal well using wet steam is in Wairakei, New Zealand. Other plants have been built in Russia, Japan, and Mexico. In the United States, pilot plants have been constructed in California and New Mexico. The technology used in these plants is not yet adequate, however, to allow them to compete economically with fossil-fueled power plants .

Hot water (in contrast to steam) from underground reserves can also be used to generate electricity. Plants of this type make use of a binary (two-step) process. Hot water is piped from underground into a heat exchanger at the surface. The heat exchanger contains some low-boiling point liquid (the "working fluid"), such as a freon or isobutane. Heat from the hot water causes the working fluid to evaporate. The vapor then produced is used to drive the turbine and generator. The hot water is further cooled and then returned to the rock reservoir from which it came.

In addition to dry and wet steam fields, a third kind of geothermal reserve exists: pressurized hot water fields located deep under the ocean floors. These reserves contain natural gas mixed with very hot water. Some experts believed that these geopressurized zones are potentially rich energy sources although no technology currently exists for tapping them.

Another technique for the capture of geothermal energy makes use of a process known as hydrofracturing. In hydrofracturing, water is pumped from the surface into a layer of heated dry rock at pressures of about 7,000 lb/in² (500 kg/cm²). The pressurized water creates cracks over a large area in the rock layer. Then, some material such as sand or plastic beads is also injected into the cracked rock. This material is used to help keep the cracks open.

Subsequently, additional cold water can be pumped into the layer of hot rock, where it is heated just as natural groundwater is heated in a wet or dry steam field. The heated water is then pumped back out of the earth and into a turbine-generator system. After cooling, the water can be re-injected into the ground for another cycle. Since water is continually re-used in this process and the earth's heat is essentially infinite, the hydrofracturing system can be regarded as a renewable source of energy.

Considerable enthusiasm was expressed for the hydrofracturing approach during the 1970s and a few experimental plants were constructed. But, as oil prices dropped and interest in alternative energy sources decreased in the 1980s, these experiments were terminated.

Geothermal energy clearly has some important advantages as a power source. The raw material—heated water and steam—is free and readily available, albeit in only certain limited areas. The technology for extracting hot water and steam is well developed from petroleum-drilling experiences, and its cost is relatively modest. Geothermal mining, in addition, produces almost no air pollution and seems to have little effect on the land where it occurs.

On the other hand, geothermal mining does have its disadvantages. One is that it can be achieved in only limited parts of the world. Another is that it results in the release of gases, such as hydrogen sulfide, sulfur dioxide , and ammonia, that have offensive odors and are mildly irritating. Some environmentalists also object that geothermal mining is visually offensive, especially in some areas that are otherwise aesthetically attractive. Pollution of water by runoff from a geothermal well and the large volume of cooling water needed in such plant are also cited as disadvantages.

At their most optimistic, proponents of geothermal energy claim that up to 15% of the United States' power needs can be met from this source. Lagging interest and research in this area over the past decade have made this goal unreachable. Today, no more than 0.1% of the nation's electricity comes from geothermal sources. Only in California is geothermal energy a significant power source. As an example, GeoProducts Corporation, of Moraga, California, has constructed a $60 million geothermal plant near Lassen National Park that generates 30 megawatts of power.

Until the government and the general public becomes more concerned about the potential of various types of alternative energy sources, however, geothermal is likely to remain a minor energy source in the country as a whole.

See also Alternative fuels; Fossil fuels; Renewable resources; Water pollution

[David E. Newton ]

RESOURCES

BOOKS

Moran, J. M., M. D. Morgan, and J. H. Wiersma. Environmental Science. Dubuque, IA: W. C. Brown, 1993.

National Academy of Sciences. Geothermal Energy Technology. Washington, DC: National Academy Press, 1988.

Rickard, G. Geothermal Energy. Milwaukee, WI: Gareth Stevens, 1991.

U.S. Department of Energy. Geothermal Energy and Our Environment. Washington, DC: U.S. Government Printing Office, 1980.

PERIODICALS

Fishman, D. J. "Hot Rocks." Discover 12 (July 1991): 22–23.

Geothermal Energy

The word "geothermal" is derived from the Greek words geo (earth) and therme (heat), and "energy" is defined as usable power, such as heat or electricity. The temperature at the Earth's core (6,437 kilometers or 4,000 miles deep) may exceed 4,980°C (9,000°F). Energy from the Earth's interior, in the form of heat, constantly flows outward. Upwelling of hot mantle material to shallower depths and lower pressure produces melting, forming a magma (molten rock) that is less dense than the mantle material from which it was derived. This less dense magma rises slowly through the mantle to the Earth's crust. If the magma reaches the Earth's surface, it is called lava.

If magma remains beneath the Earth's surface within the crust, the heat energy is released slowly over time to the local and even regional area. This residual heat energy transfers to adjacent rock and groundwater , heating them to temperatures exceeding 370°C (700°F). Most of this heat energy remains stored in the Earth's depths, creating geothermal reservoirs. Some heat energy from the geothermal reservoirs transfers to deep-circulating groundwater, and the superheated water travels to the Earth's surface via faults and fractures, creating geysers and hot springs .

High-temperature geothermal resources (temperatures greater than 150°C, or 302°F) are primarily found in volcanic regions and island chains, whereas low-temperature geothermal resources (temperatures less than 100°C, or 212°F) are found throughout the world and on all continents. Worldwide, high-temperature resources usually are found in tectonic settings associated with volcanism , such as the Pacific Rim (the so-called "Ring of Fire")^* and in Iceland. In the United States, energy use associated with geothermal reservoirs takes place primarily in the western states and Hawaii, which are areas of geologically recent volcanism.

Using Geothermal Energy

From prehistoric times, humans have utilized the geothermal energy that flows freely from underground reservoirs to the Earth's surface. The Romans used geothermal water to treat various diseases as well as to heat the city of Pompeii. Native North Americans used geothermal water for cooking and medicinal purposes for more than 10,000 years, and the Maoris of New Zealand have cooked geothermally for centuries. Ancient records in Iceland mention the use of geothermal springs for washing and bathing.

Today, geothermal energy is utilized in three technology categories:

• Heating and cooling buildings via geothermal heat pumps that utilize shallow sources;
• Heating structures with direct-use applications; and
• Generating electricity through indirect use.

Heat Pumps.

Most areas of the world are suitable for geothermal heat pumps; hence, there is a great potential for future worldwide use of heat-pump technology. Geothermal heat pumps take advantage of the fact that a majority of the Earth's upper 3 meters (less than 10 feet) maintain temperatures ranging from 10 to 15.5°C (50 to 60°F).

A geothermal pump system consists of pipes buried horizontally in the upper 3 meters adjacent to a building and then tied into the structure's ventilation system by simply using the subsurface as a heat exchanger. A liquid passed through the pipes picks up residual underground heat in winter, then delivers it to the building, thus warming it. In summer, the liquid transfers heat from the building to the ground, thus cooling the building.

Direct Use.

Direct-use applications utilize groundwater that has been heated to generally less than 100°C (212°F). Direct use of geothermal energy can supply heat for industrial processes, and can boost agricultural and aquaculture production in cold climates by heating greenhouses, soils, and aquaculture ponds. Geothermal uses also include hot spring and spa bathing, as well as residential and regional (district) heating.

Direct uses annually provide 11,000 thermal megawatts of power worldwide. In Reykjavik, Iceland, more than 95 percent of the buildings are heated with geothermal water pumped from deep wells. The city of Klamath Falls, Oregon (USA) heats 35 percent of all residential dwellings and also heats most downtown sidewalks in the winter.

Electricity Generation.

High-temperature geothermal resources almost always are used for power production. Although geothermal energy has been used throughout history, the indirect generation of electricity is relatively new. Wells are drilled into known geothermal reservoirs where temperatures often exceed 360°C (680°F). Steam or super-heated water is brought to the surface under its own pressure where the energy, in the form of steam, is utilized to turn the turbines of an electrical generator.

Three types of geothermal power plants are operating today:

• Dry steam plants, which directly use geothermal steam to turn turbines;
• Flash steam plants, which pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines; and
• Binary-cycle plants, which pass (in separate piping) moderately hot geothermal water by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to steam, which then drives the turbines.

Geothermal electric power generation is a proven technology that is significant in nations with no indigenous fossil fuel resources. Moreover, it is a renewable energy source and one that is considered cleaner than fossil fuel (e.g., geothermal power plants emit less carbon dioxide). Worldwide, geothermal energy produced 7,500 megawatts in 21 countries (mostly developing) in 2000, serving 60 million people. The United States produced 3,000 megawatts from geothermal power plants, supplying about 4 million people.

Environmental Considerations.

Comparatively speaking, geothermal energy is more "environmentally friendly" than fossil fuels or nuclear fission. However, emissions of carbon dioxide gas and smaller amounts of hydrogen sulfide accompany the geothermal fluids extracted from wells. Moreover, hot brine liberated by drilling must be carefully managed to prevent environmental damage and potential human health impacts.

To prevent ground subsidence (sinking) caused by the extraction of geothermal fluids from their underground reserves, waste brines often are reinjected into the well. Reinjection must be done carefully to avoid contaminating potable (drinkable) groundwater supplies. Above-ground disposal of waste brine also must be managed to avoid contamination of seas and waterways.

As more geothermal resources are exploited for their electricity-producing capabilities, it will become increasingly important to control carbon dioxide emissions and properly manage waste brines and other byproducts of mining and power production.

see also Earth's Interior, Water in the; Fresh Water, Natural Composition of; Fresh Water, Physics and Chemistry of; Groundwater; Hot Springs and Geysers; Mineral Water and Spas; Plate Tectonics; Springs; Volcanoes and Water.

Rick G. Graff

Bibliography

Putnam, William Clement. Putnam's Geology, 5th ed. New York: Oxford University Press, 1997.

Duffield, W. A., J. H. Sass, and M. L. Sorey. Tapping the Earth's Natural Heat. U.S. Geological Survey Circular 1125 (1994).

Harsh, Gupta K. Geothermal Resources: An Energy Alternative. New York: Elsevier, 1980.

Internet Resources

Geo-Heat Center. Oregon Institute of Technology. <http://geoheat.oit.edu>.

Geothermal Education Office. <http://www.geothermal.marin.org>.

Geothermal Energy. The World Bank Group. <http://www.worldbank.org/html/fpd/energy/geothermal>.

Geothermal Resources in Iceland. District Energy Library, University of Rochester. <http://www.energy.rochester.edu/is/reyk>.

Soulsby, David. North Atlantic Hot Spot: A Geological History of Iceland. Kibo Productions Ltd. <http://www.kibo-productions.co.uk/nahscript.htm>.

FIRST GEOTHERMAL PLANTS

The first power plant to utilize a geothermal well was brought online in Larderello, Italy in 1904. The first geothermal plant in the United States was at The Geysers in Northern California in 1962.

These two geothermal fields are unusual because they emit dry steam, which is superheated steam with no associated fluids. This kind of steam is preferred over wet steam (the more common kind of steam found in geothermal fields) because it contains less corroding elements and less waste to deal with.

^* See "Volcanoes, Submarine" for an illustration showing the Ring of Fire's location.

Geothermal energy

The discovery that the temperature in deep mines exceeded the surface temperature implied the existence of a source of deep geothermal energy. Within the continental crusts, the temperature differential gradient averages about one micro calorie per square centimeter (equivalent to an increase of about 95°F per mile or 33°C per kilometer of increasing depth).

In some areas geothermal energy is a viable economic alternative to conventional energy generation. Commercially viable geothermal fields have the same basic structure. The source of heat is generally a magmatic intrusion into Earth's crust . The magma intrusion generally measures 1110–1650°F (600–900°C), at a depth of 4.3–9.3 mi (7–15 km). The bedrock containing the intrusion conducts heat to overlying aquifers (i.e., layers of porous rock such as sandstone that contain significant amounts of water ) covered by a dome-shaped layer of impermeable rock such as shale or by an over-lying fault thrust that contains the heated water and/or steam. A productive geothermal generally produces about 20 tons (18.1 metric tons) of steam, or several hundred tons of hot water, per hour. Historically, some heavily exploited geothermal fields have had decreasing yields due to a lack of replenishing water in the aquifer , rather than to cooling of the bedrock.

There are three general types of geothermal fields: hot water, wet steam, and dry steam. Hot water fields contain reservoirs of water with temperatures between 140–212°F (60–100°C), and are most suitable for space heating and agricultural applications. For hot water fields to be commercially viable, they must contain a large amount of water with a temperature of at least 140°F (60°C) and lie within 2,000 meters of the surface.

Wet steam fields contain water under pressure and usually measure 212°F (100°C). These are the most common commercially exploitable fields. When the water is brought to the surface, some of the water flashes into steam, and the steam may drive turbines that can produce electrical power.

Dry steam fields are geologically similar to wet steam fields, except that superheated steam is extracted from the aquifer. Dry steam fields are relatively uncommon.

Because superheated water explosively transforms into steam when exposed to the atmosphere, it is much safer and generally more economical to use geothermal energy to generate electricity , which is much more easily transported. Because of the relatively low temperature of the steam/water, geothermal energy may be converted into electricity with an efficiency of 10–15%, as opposed to 20–25% for coal or oil fired generated electricity.

To be commercially viable, geothermal electrical generation plants must be located near a large source of easily accessible geothermal energy. A further complication in the practical utilization of geothermal energy derives from the corrosive properties of most groundwater and steam. In fact, prior to 1950, metallurgy was not advanced enough to enable the manufacture of steam turbine blades resistant to corrosion . Geothermal energy sources for space heating and agriculture have been used extensively in Iceland, and to some degree Japan, New Zealand, and the former Soviet Union. Other applications include paper manufacturing and water desalination .

While geothermal energy is generally presented as nonpolluting energy source, water from geothermal fields often contains large amounts of hydrogen sulfide and dissolved metals , making its disposal difficult.

See also Earth, interior structure; Energy transformations; Geothermal deep ocean vents; Geothermal gradient; Temperature and temperature scales