Geothermal Energy

Geothermal energy is the heat produced deep in the Earth's core. Geothermal energy is a clean, renewable resource that can be harnessed for use as heat and electricity.

Below the Earth’s crust, is the core. A small portion of the core’s heat comes from the friction and gravitational pull formed when Earth was created more than 4 billion years ago. However, most of the Earth’s heat is constantly generated by the decay of radioactive isotopes.

As these isotope decay, its nucleus changes, emitting enormous amounts of energy. Radioactive decay is a continual process in the core. Temperatures there rise to more than 5000 ⁰C. Heat from the core is constantly radiating outward and warming rocks, water, gas, and other geological material.

If underground rock formations are heated to about 1300 ⁰C, they can become magma. Magma is molten rock permeated by gas and gas bubbles. Magma exists in the mantle and lower crust, and sometimes bubbles to the surface as lava.  Magma heats nearby rocks and underground aquifers. Hot water can be released through geysers, hot springs, steam vents, underwater hydrothermal vents, and mud pots.  These are all sources of geothermal energy. Their heat can be captured and used directly for heat, or their steam can be used to generate electricity.

Figure 1: a geothermal power plant in Iceland.

Most of the Earth’s geothermal energy does not bubble out as magma, water, or steam. It remains in the mantle, emanating outward at a slow pace and collecting as pockets of high heat. This dry geothermal heat can be accessed by drilling and enhanced with injected water to create steam.

Many countries have developed methods of tapping into geothermal energy. Different types of geothermal energy are available in different parts of the world. In Iceland, abundant sources of hot, easily accessible underground water make it possible for most people to rely on geothermal sources as a safe, dependable, and inexpensive source of energy.

Almost anywhere in the world, geothermal heat can be accessed and used immediately as a source of heat. This heat energy is called low-temperature geothermal energy. Low-temperature geothermal energy is obtained from pockets of heat about 150° C.  Most pockets of low-temperature geothermal energy are found just a few meters below ground.  Low-temperature geothermal energy can be used for heating greenhouses, homes, fisheries, and industrial processes.

Geothermal heat pumps (GHPs) take advantage of the Earth’s heat and can be used anywhere in the world. GHPs are drilled about 3 to 90 meters deep, much shallower than most oil and natural gas wells. GHPs do not require fracturing bedrock to reach their energy source.

A pipe connected to a GHP is arranged in a continuous loop that circles underground and above ground, usually throughout a building. The loop can also be contained entirely underground, to heat a parking lot or landscaped area.

Flash-steam power plants use naturally occurring sources of underground hot water and steam. Water that is hotter than 180 ⁰Cvis pumped into a low-pressure area. Some of the water “flashes,” or evaporates rapidly into steam, and is funnelled out to power a turbine and generate electricity. Any remaining water can be flashed in a separate tank to extract more energy.

Flash-steam power plants are the most common type of geothermal power plants. The volcanically active island nation of Iceland supplies nearly all its electrical needs through a series of flash-steam geothermal power plants. The steam and excess warm water produced by the flash-steam process heat icy sidewalks and parking lots in the frigid Arctic winter.

Geothermal energy is a renewable resource. The Earth has been emitting heat for about 4.5 billion years and will continue to emit heat for billions of years into the future because of the ongoing radioactive decay in the Earth’s core.   However, most wells that extract the heat will eventually cool, especially if heat is extracted more quickly than it is given time to replenish. Geothermal systems do not require enormous amounts of freshwater. Because water is only used as a heating agent and is not exposed or evaporated. it can be recycled, used for other purposes, or released into the atmosphere as non-toxic steam.

Geothermal energy exists in different forms all over the Earth (by steam vents, lava, geysers, or simply dry heat), and there are different possibilities for extracting and using this heat.