When groundwater comes in contact with very hot rock below the surface of the Earth, the heated water may rise back to the surface in the form of hydrothermal structures. The hot water can blast out of the Earth as the steam and hot water of a geyser. It may escape through a small crack in the Earth as a steam vent or fumarole. Or it can bubble out gently in a hot spring.

Hot springs have been used for cooking and bathing for thousands of years. This type of energy is called geothermal energy, energy that comes from the Earth. Starting at the end of the 19th century, people found ways to use the energy from the hot water and steam to heat homes and make electricity. These uses have grown significantly in the last few decades.

Geothermal energy provides a reliable and fully renewable source of electricity and heat.  It is always available, unlike solar or wind energy. It is renewable: rainfall and the return of cooled water replace the hot water and steam removed from the ground. Geothermal energy does not completely solve the CO2 emissions problem, though. The pumps that bring water out of the ground use electricity. However, a geothermal energy system significantly reduces reliance on fossil fuels.

What Is Geothermal Energy?

Geothermal energy comes from the core of the Earth. Not directly, of course. The molten core of the Earth contains a lot of energy, in the form of heat. This heat moves through the interior of the Earth toward the surface. It melts the rock along the way, forming magma. If the magma reaches the surface, it becomes lava and a volcano forms.

	Click for animation. Geothermal district heating Geothermal district heating pumps hot water from deep below the Earth’s surface to heat homes and businesses. The hot water is pumped up and passes by a heat exchanging fluid. This fluid transmits the heat to a loop of hot water, which goes to houses and offices.

In some parts of the world, the magma rises to points very close to the Earth’s surface, particularly at the plate boundaries of the Earth’s crust or in scattered hot spots. In these locations magma may be found at depths of a few kilometers (miles) or less. The rocks above these magma chambers become very hot. If groundwater or rainwater reaches these rocks the water becomes heated as well. Water temperatures can be as high as 275 degC (527 degF) while under pressure. The groundwater forms hot springs, geysers, and fumaroles when it reaches the Earth’s surface.

Now that we know the source of geothermal energy, how does it provide heat or electricity? There are two methods: direct geothermal heating and geothermal power plants.

Direct Geothermal Heating

Direct geothermal heating uses water from hot springs or hot groundwater to provide heat to homes and offices and for businesses such as snow melting, greenhouse production, and fish farming. Hot water reservoirs with temperatures ranging from 10 to 149 degC (50 to 300 degF) are best suited for this use. In 2005, 72 countries reported utilizing geothermal energy for direct uses, providing more than 16,000 megawatts of energy. Countries that use direct geothermal heating include France, Germany, Iceland, the United States, and China.

Hot springs have long attracted people for cooking and bathing and as meeting places. The earliest North American Indians left traces of their visits to hot springs. The Japanese have basked in hot springs for centuries. Ancient Romans built community baths fed by natural hot springs. The hot water was believed to boost a person’s health. In fact the modern health spa developed from the belief in the health benefits of the ancient baths.

	Click for animation. Geothermal energy—Geysers plant At a geothermal power plant such as the Geysers, heated steam and water from deep below the surface is tapped to turn turbines. The turbines are attached to generators, which then make electricity.

The first evidence of direct geothermal heating comes from Pompeii, Italy, where water from hot springs heated some of the buildings. A 14th-century French village also did the same thing. When Iceland was settled, the numerous hot springs were used for bathing and washing. The idea of using them for more began in 1755-56 when trial hot water wells were dug. However, nothing developed from these wells.  In Iceland, the next attempt at large-scale geothermal energy began in 1928. New hot water wells were drilled, and a school became the first building to use geothermal heat, in 1930. Today, direct geothermal heating warms most of Iceland’s houses.

The first modern direct geothermal district heating system started in the United States, in the western city of Boise, Idaho. Drilling for geothermal wells got under way in 1890. By 1892 Boise had geothermal district heating for its buildings.

There are two types of systems that use geothermally heated water to warm buildings. The first system that was developed pumps hot water from a geothermal reservoir and pipes it directly to the buildings that use it for heat. Most of that water is put back into the ground after it cools, to make its way back to the groundwater reservoir. Newer systems pump the hot water to a heat exchanger, which contains a fluid that absorbs the heat from the water. This heating fluid, which may be water, is contained in a separate loop that goes around to the buildings in the district. A system that uses a heat exchanger keeps the hot water and the heating fluid separate. The reason for this separation: the hot groundwater may contain salts and minerals that can clog the district heating system. Most of the hot water pumped out of the ground gets put back into the ground to prevent the reservoir from being depleted.

Direct geothermal systems have a minimal impact on the environment. Older systems that use the heated water directly must return that water to the groundwater reservoir to maintain the system. In newer systems, the hot water only reaches the heat exchanger, so it quickly returns to the groundwater reservoir. Direct systems rely on water pumped from the ground, and the systems must use energy generated by fossil fuels to power the pumps. Thus direct geothermal systems indirectly add to CO2 emissions. However, these systems significantly reduce the reliance on fossil fuels for heat.

Geothermal Power Plants

Geothermally generated electricity is on the increase. According to a 2005 report from the Italian energy provider ENEL, geothermal power plants were supplying 8,900 megawatts to 24 countries worldwide. The United States makes more geothermal electricity than any other country, with about 32% of the world total.
The first geothermal power plant was built in Larderello, Italy, in 1904. A group led by Prince Piero Ginori Conti developed a way to use the steam from local fumaroles to turn turbines and power a generator. This plant remains in operation today. In the 1950s the government of New Zealand began studying the possibility of using the Wairakei geothermal field to generate power. The field included geysers, fumaroles, hot springs, and mud pools. The Wairakei geothermal power plant, the second in the world, opened in 1958. The largest plant making geothermal electricity is The Geysers, near Santa Rosa, California. This plant opened in 1960. Although no geysers actually exist in that location, steam vents dot the region. The Geysers produces about 750 megawatts of power—enough for a city the size of San Francisco.

Direct steam plant schematic

Flash steam plant schematic

Binary cycle plant schematic Drawings courtesy of U.S. Department of Energy

Since 2000, geothermal power generation has tripled in France, Russia, and Kenya. Countries as diverse as the Philippines, Iceland, and El Salvador produce an average of 25% of their electricity from geothermal sources, while Tibet meets 30% of its energy needs this way.

Geothermal plants use one of three different processes to generate electricity. Dry steam or direct steam plants are built in locations where the main hydrothermal features are steam vents. A production well captures pressurized steam escaping from the ground and pipes it to a turbine. The turbine consists of a series of angled blades mounted to a central shaft. Pressurized steam passes through the turbine, causing it to spin on its central axis. The spinning turbine then powers a generator. The water cools and goes back into the ground. Larderello and The Geysers are examples of direct steam plants.

A flash steam plant uses water at temperatures above 180 degC (360 degF) to make steam. The flash technique takes deep, high-pressure hot water and sprays it into lower-pressure tanks. The water rapidly turns to vapor, “in a flash,” creating something called “flashed steam.” This high-pressure steam turns the turbines, which power the generator and make electricity. The cooled water gets injected back into the ground.

A binary cycle plant makes use of moderately hot geothermal water, at 107 to 182 degC (225 to 360 degF). The geothermal water goes into a heat exchanger, where it passes by a secondary fluid with a much lower boiling point than water. The geothermal heat causes the secondary fluid to “flash” to vapor, turning the turbines. The geothermal water never directly reaches the turbine; it is injected back into the ground from the heat exchanger. Most geothermal resources fall into the moderate temperature category; thus binary plants are the most likely to be built in the future. 

Geothermal Heat Pumps

As we all know, the temperature at the surface of the Earth varies widely, depending on location, elevation, season, and current weather. Underground, the story changes. At 30.5 to 122 m (100 to 400 ft) below the surface of the Earth, the temperature stabilizes at a range between 7 and 21 degC (45 and 70 degF), depending on latitude. The geothermal heat pump takes this underground heat and brings it to the surface, where it can be used to heat a building in cool weather. The system can be reversed, so that in warm weather heat is removed from the house and pumped underground to cool. Some of the heat removed in warm weather can be used to heat water.

The concept is an old one. British mathematician and physicist Lord Kelvin worked out the idea of a pump to take heat out of the ground in 1852 but never developed the concept any further. The first modern geothermal heat pump system was installed in a home in Indianapolis, Indiana, in 1945. Interest in this method of heating remained low until oil prices climbed in the 1970s. By then, there was interest in these systems in Europe as well. Geothermal heat pumps continue to be used mostly in North America and Europe.

Most geothermal heat pumps use a closed loop system. A series of pipes is installed underground. These pipes can be straight or looped, horizontal or vertical, and can be in the soil or in groundwater or under a pond, if there is one near by. A heat exchanging fluid fills the pipes. The fluid picks up the underground heat and brings it to the building. Inside the building is a heat exchanger, which takes the heat from underground and transfers it to a compression system. The compressed heat is then forced throughout the building through heating ducts. The heat can also be used to provide hot water.

Drawings courtesy of U.S. Department of Energy

The other type of geothermal heat pump is an open loop system. This system uses groundwater as the heat exchanging fluid. Two wells are drilled: one to bring up warmer groundwater, and one to inject it back into the reservoir after the heat is removed. This type of system requires easily accessible and very clean groundwater.

The indoor heat exchanger and pump run on electricity, so the geothermal heat pump does not completely replace fossil fuels. However, it makes a building more energy efficient, reducing the reliance on oil, natural gas, or electricity for heat.

Issues

Geothermal power plants cause most of the concerns about geothermal energy. One issue: the sinking of the ground as the water or steam is initially drawn out. This can be a serious problem. At Wairakei, the ground dropped as much as 13 m (42.7 ft) when the plant began operations. This remains an issue at Wairakei. At newer plants, water is quickly reinjected to maintain the water pressure and underground reservoir level.

Binary geothermal power plants do not cause emissions of any gas. However, dry steam and flash steam plants emit relatively small amounts of CO2, depending on the content of the water. Some hydrogen sulfide is emitted as well, but not in quantities significant enough to contribute to acid rain. Because of dissolved sulfur compounds in the groundwater, the plants produce a sulfur smell that people dislike. In the United States, geothermal power plants must remove the hydrogen sulfide, either by burning it or by converting it to sulfur dioxide. The sulfur dioxide can then be dissolved or be converted to sulfuric acid and sold. Salts and minerals cleaned from the water are injected back into the groundwater reservoir. Some sludge also gets produced; sludge is now being processed to remove valuable minerals.

Down the Road

Researchers are focusing on tapping the Earth’s heat by drilling to warmer layers deep inside the Earth—for example, close to the mantle. However, this requires the ability to drill much deeper than current capabilities.  

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