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What is Geothermal Energy?

Geothermal energy is the energy harnessed from steam and hot water enclosed in the Earth’s crust. The word geothermal is a hybrid word that comes from “geo”, a Greek word meaning “Earth,” and “therme” meaning “heat.” The term geothermal energy denotes the combined thermal energy below the considerably cooler and thinner surface of the Earth, constituting about 260 billion cubic miles (1012 cubic kilometers) of rock at melting temperatures.

Geothermal energy is generated from the formation of the planet, decay of long-lived radioactive isotopes (radiogenic heat), volcanic activity and solar energy absorbed at the Earth’s surface. Most of geothermal energy is generated by radiogenic heat— when radioactive isotopes of Thorium (232Th), Uranium (235U, 238U) and Potassium (40K) decay to a more stable state.

    Most of geothermal energy is generated by decay of radioactive isotopes -- Thorium, Uranium and Potassium.

Most of geothermal energy is generated by decay of radioactive isotopes — Thorium, Uranium and Potassium.

The geothermal energy of the Earth is enormous. For each mile of depth below the Earth’s surface, the temperature increases around 80 °F (26.6 °C). The difference in temperature between the core of the Earth and the surface is known as geothermal gradient. The geothermal gradient forces a permanent transfer of thermal energy in the form of heat from the Earth’s core to the surface.

The Earth is made up of 3 circular layers: the core (inner and outer), the mantle, and the crust (Picture 1). At the center of Earth is a solid inner core with an estimated temperature of about 8,000 °F (4426.6 °C). The heat emitted in the Earth’s core is strong enough to melt rock into a molten fluid of extremely high-temperature also known as magma. The molten fluid or magma forms the liquid outer core of the Earth. The core, consisting largely of iron and smaller quantities of nickel, takes up 16% of the Earth’s volume. The layer that surrounds the core is called the mantle (Picture 1) and comprises 83% of the Earth’s volume.

    Picture 1: The Earth's crust, mantle, and core

Picture 1: The Earth’s crust, mantle, and core

Magma is fluid-like and less dense than the nearby rock from which it was derived. Thus, it tends to rise and moves slowly toward the Earth’s crust. The temperature of magma is between 600°F (315.6°C) and 2,400 °F (1,316 °C). The tendency of a warmer, less dense material to rise and cooler, more dense material to sink is referred to as convection and results in the transfer of heat. Geothermal energy is being perpetually generated in the Earth’s mantle as magma travels upward while surface groundwater escapes down to the Earth’s interior. The hot magma heats the water and forces it back up through Earth’s cracks. In its liquid form, geothermal energy is also referred to as hydrothermal energy.

In most of the world, the geothermal gradient through the crust is 124–138 °F per mile (25–30°C per kilometer) of depth. The conductive heat flux (the rate of heat energy transfer) is about 0.1 MW/km2 on average. Near tectonic plate boundaries, where the Earth’s crust is thinner, these values are much higher.

Geothermal energy powers natural hot springs, volcanoes and geysers. Volcanoes, in particular, are perhaps the most eminent geothermal occurrences. Volcanism most frequently occurs at spreading ridges where tectonic plates diverge or alongside subduction zones where tectonic plates converge. It is not surprising that volcanic areas have the best potential for geothermal energy given the function of magma in geothermal energy generation. The hot magma typically remains well below the Earth’s surface to heat rocks as well as water originating from rainfall and snowmelt that is sinking deep into the Earth. When a volcano erupts, however, hot magma from below ground shoots through the Earth’s crust, flowing onto the surface. When magma erupts onto the Earth’s surface it is called lava.

The heat for most hot springs and geysers originates from the underground water that is heated by warm, volcanic rock. A hot spring is a geothermal manifestation in which the water temperature is higher than the average human body temperature. The word “geysir” is of Icelandic origin meaning to “rush forth”. Geyser is a hot spring that irregularly emits hot water and steam with great force. Well-known geysers are found in Iceland, New Zealand and Wyoming. Due to their highly mineralized content, the waters of many hot springs are believed to have therapeutic properties.

    Lava is a molten rock (magma) ejected by a volcano. A solidified rock is also referred to as lava.

Lava is a molten rock (magma) ejected by a volcano. A solidified rock is also referred to as lava.

Most of the hot geothermal water remains trapped deep underground in cracks and porous rocks. This natural underground water is called a geothermal reservoir.

Geothermal resources are classified into three main categories depending on phases and forms: Convective hydrothermal resource (vapor or hot-water-dominated), other hydrothermal resources (sedimentary, geopressed and radiogenic) and hot rock resources (magma and solidified).

What is Geothermal Energy: History

The use of geothermal energy began before history was even chronicled. Throughout the past, people across the world utilized geothermal energy in the simplest of ways. Hot springs, in particular, have been utilized for bathing since the Paleolithic period. A stone pool, built in the 3rd century BC, on China’s Lisan mountain is known as the world’s oldest use of geothermal energy. Ancient Romans used geothermal energy through the water it heated for cooking, bathing, and occasionally for heating homes. These uses were dependent on proximity to places like the region around the Mt. Vesuvius (such as the buried city of Pompeii) where magma was near the Earth’s surface. In the 1stcentury AD, Romans invaded Aquae Sulis, now Bath (located in Somerset, England). They used the large natural hot springs to feed public baths and under-floor heating.

Chaudes-Aigues, France, is considered the world’s oldest geothermal district heating system that has been in operation since the 14th century. In the early 19th century the geothermal fluids were being exploited for their energy content. A chemical industry was set up during that period in Larderello, Italy to extract boric acid (H3BO3) from the naturally emerging hot waters.

The ruins of Pompeii, located near Naples, Italy. Pompeii was destroyed during the eruption of Mt. Vesuvius.

The ruins of Pompeii, located near Naples, Italy. Pompeii was destroyed during the eruption of Mt. Vesuvius.

About the same time, the utilization of the natural steam for its mechanical energy began. The steam was used to raise liquids in primitive gas lifts. Later, it was used in reciprocating and centrifugal winches and pumps in drilling as well as in the local industry of boric acid.  Between 1910 and 1940 the low pressure steam in Tuscany, Italy was used to heat the buildings and greenhouses. The geothermal heat pump was invented 1852 by Lord Kelvin, a British mathematician and physicist. It’s fist successful implementation waited until the late 1940s.

Understanding of geothermal energy began to advance toward the end of the 19th century as countries began developing their geothermal resources on an industrial scale. In the US, the first geothermal district heating system began operations in Boise, Idaho (USA) in 1892. Iceland is yet another pioneer in the utilization of geothermal energy. It began utilizing geothermal energy in 1928 for domestic heating purposes.

In the early 20th century century, the increased need for electricity led to the exploration of geothermal power as a generating source. In 1904, Prince Piero Ginori Conti, an Italian businessman and politician, successfully tested the geothermal power generator in Larderello, Italy, lighting four light bulbs. The existence of large quantities of hot, dry steam made the formation at Larderello relatively simple to exploit for power production. In 1911, Larderello, Italy became home to the world’s first commercial geothermal power plant.

    Mount Ngaruahoe, a stratovolcano located in New Zealand. This active volcano erupted 45 times during the 20th century.

Mount Ngaruahoe, a stratovolcano located in New Zealand.
This active volcano erupted 45 times during the 20th century.

The first geothermal wells were drilled in Beppu, Japan in 1919 while in 1921, geothermal wells were drilled at the Geysers, USA. Italy was the world’s single industrial producer of geothermal electricity until 1958 when New Zealand built a small geothermal plant. In 1959, a first geothermal plant was opened in the Pathe field, Mexico and in 1960 the fist commercial geothermal power plant opened at the Geysers, USA. In 1967, the first world binary cycle power plant was constructed in Kamchatka, Russia. Since that time, geothermal energy has been put to many other uses.

What is Geothermal Energy: Uses

Today, the most common use of geothermal energy is for heating. Geothermal energy is used for heating of homes, greenhouses, offices, factories, fish farms, mines and pools. Geothermal energy is also used for the production of electricity.

The site of Roman Baths, located in Bath, England. The city of Bath was established by Romans in the AD 60 and was known as Aquae Sulis, meaning "the waters of Sulis".

The site of Roman Baths, located in Bath, England. The city of Bath was established by Romans in the AD 60 and was known as Aquae Sulis, meaning “the waters of Sulis”.

Since geothermal energy is concentrated in underground reservoirs in the forms of steam, hot water and hot rocks, tapping into this source entails drilling through the Earth’s crust. In certain cases, due to high pressure, the hydrothermal fluids are simply being spewed out. Owing to these conditions, the pumping systems are not needed to extract the thermal fluids to the Earth’s surface.

Geothermal energy is used in three main ways. The first way is the use of hot water at or close to the Earth’s surface, known as direct geothermal energy. The second way utilizes water that is below the ground, but not too deep, through geothermal heat pumps. The third way is using hot water and steam from deep underground to generate electricity via geothermal power plants.

What is Geothermal Energy: Advantages

Geothermal energy is attracting widespread public interest due to its many advantages, namely, reliability, renewability and cleanliness. A greatest benefit of geothermal energy is that it represents a renewable energy source. With rainfall and snowmelt constantly maintaining the supply of water to geothermal reservoirs, production from geothermal fields can be sustained indefinitely. As research efforts aimed at developing alternative energy sources continue to evolve, geothermal energy may become even more advantageous source of power to meet human needs in an environmentally friendly way.

Bibliography

DeGunther, Rick. Alternative Energy for Dummies. Hoboken, NJ: Wiley, 2009.

Dickson, Mary H., and Fanelli, Mario. What Is Geothermal Energy. Publication. Pisa, Italy: Istituto Di Geoscienze E Georisorse, CNR, 2004.

Geothermal Energy Association. Available at http://www.geo-energy.org.

International Geothermal Association. IGA International Geothermal Association. Available at http://www.geothermal-energy.org.

Lee, Sunggyu, Speight, James G. and Loyalka, S. K.. Handbook of Alternative Fuel Technologies. Boca Raton: CRC, 2007.

Monroe, James S., Wicander, Reed and Hazlett, Richard. Physical Geology: Exploring the Earth. Belmont, CA: Thomson Brooks/Cole, 2007.

Pierce, Val. Introduction to Geothermal Power. New Delhi: World Technologies, 2011.

Povarov, Oleg, and Nikolskiy, Alexander. “Experience in Running Geothermal Power Plants in Severe Climate Conditions in Russia.” Workshop Abstracts. Proc. of Electricity Generation from Enhanced Geothermal Systems, Hôtel Régent, 14-16 September 2006, Strasbourg. Sept. 2006.

Tabak, John. Solar and Geothermal Energy. New York: Facts On File, 2009.

Wachtel, Alan and Voege, Debra. Geothermal Energy. New York, NY: Chelsea Clubhouse, 2010.

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