Geothermal Energy

I

Introduction

Geothermal Energy, energy contained in intense heat that continually flows outward from deep within Earth. This heat originates primarily in the core. Some heat is generated in the crust, the planet’s outer layer, by the decay of radioactive elements that are in all rocks. The crust, which is about 5 to 75 km (about 3 to 47 mi) thick, insulates the surface from the hot interior, which at the core may reach temperatures from 4000° to 7000° C (7200° to 12,600° F). Where the heat is concentrated near the surface, it can be used as a source of energy.

II

Geothermal Geology

The distance from Earth’s surface to its center is about 6,500 km (about 4,000 mi). From Earth’s surface down through the crust, the normal temperature gradient (the increase of temperature with increase of depth) is 10° to 30° C per km (29° to 87°F per mi). Underlying the crust is the mantle, which is made of partially molten rock. Temperatures in the mantle may reach 3700° C (6700° F).

The convective (circulating) motion of this mantle rock drives plate tectonics—the 'drift' of Earth's crustal plates that occurs at a rate of 1 to 5 cm (0.4 to 2 in) per year. Where plates spread apart, molten rock (magma) rises up into the rift (opening), solidifying to form new crust. Where plates collide, one plate is generally forced (subducted) beneath the other. As the subducted plate slides slowly downward into the mantle’s ever-increasing heat, it melts, forming new magma. Plumes of this magma can rise and intrude into the crust, bringing vast quantities of heat relatively close to the surface. If the magma reaches the surface it forms volcanoes, but most of the molten rock stays underground, creating huge subterranean regions of hot rock.

III

Geothermal Reservoirs

In certain areas, water seeping down through cracks and fissures in the crust comes in contact with this hot rock and is heated to high temperatures. Some of this heated water circulates back to the surface and appears as hot springs and geysers. However, the rising hot water may remain underground in areas of permeable hot rock, forming geothermal reservoirs. Geothermal reservoirs, which may reach temperatures of more than 350° C (700° F), can provide a powerful source of energy.

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IV

Geothermal Power Plants

Geothermal reservoirs within about 5 km (about 3 mi) of Earth’s surface can be reached by drilling a well. The hot water or steam from wells can be used to turn turbine generators to produce electricity. A power plant that uses this natural source of hot water or steam is called a geothermal power plant.

At the beginning of the 21st century, there were some 380 geothermal power plants in 22 countries around the world with a combined installed capacity of about 8,000 megawatts. Geothermal energy provided 1.6 percent of the world’s total electricity, serving the electricity needs of about 60 million people, mostly in developing countries. About 2.5 percent of the electricity produced in the United States came from geothermal power plants. The electricity produced from geothermal power in the United States represented about 37 percent of the world’s output of electricity from geothermal power. The United States, the Philippines, Italy, Mexico, Indonesia, Japan, New Zealand, and Iceland are the largest producers of geothermal energy.

There are three types of geothermal power plants: flash steam plants, dry steam plants, and binary plants.

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