Where Geothermal Energy Comes From






Nesjavellir Geothermal Power Plant in Þingvellir, Iceland.

Nesjavellir Geothermal Power Plant in Þingvellir, Iceland.

According to contemporary scientific theory, at the formation of the universe an immense amount of energy was released, and this same source of energy can still be found inside the earth. Harnessing that energy is the process that provides what is known as geothermal power.

The earth’s interior is in constant flux, with plate tectonics creating volcano eruptions, earthquakes and other acts of nature. The constant volatility of geothermal energy makes it difficult to harness safely, but new methods are being developed to compensate for that volatility and make geothermal a more attractive option as a source of energy.

Origins of Geothermal Energy

The word geothermal is based on the Greek words geo, which means earth, and therme, meaning heat. Geothermal energy is heat stored in the interior of the Earth. The core of the Earth, made from fiery, radioactive molten rock left over from Earth’s creation millions of years ago, is over 11,000 degrees which is even hotter than the surface of the sun. The heat from the Earth’s core produces geothermal energy from the decay of the radioactive materials in the core and layers of rock surrounding it.

Geothermal energy escapes from the Earth’s core through natural occurring manifestations such as volcanoes, geysers and hot springs. Another way that geothermal energy is released from the Earth’s core is through plate tectonics. Plate tectonics are the cracks, boundaries, and faults in the Earth’s crust where seismic activity allows magma to ascend to or near the surface.

Another form of geothermal energy is the surface heat produced from sunlight. When the sun shines, its rays warm the Earth’s surface. This solar energy is absorbed by the surface and moves deeper into the crust. Although some of the solar energy is lost during the darkness, enough heat is generated to provide an ongoing energy source.

Diagram showing the origin and methods of use of geothermal energy.

Diagram showing the origin and methods of use of geothermal energy.

Natural Examples of Geothermal Energy

Grand prismatic hot spring in Yellowstone National Park. Image by US National Park Service.

Grand prismatic hot spring in Yellowstone National Park. Image by US National Park Service.

Structurally speaking, the earth is made up of an inner core, an outer core, a mantle, and a crust. The interaction between the outer two layers provides the source of geothermal power. The mantle is completely liquid and comprised mostly of magnesium and iron. It is hot enough to melt rocks. The crust traps that heat and causes energy to build up.

The crust varies in its width, in some areas it is only a few miles wide. In these locations especially, known as hot spots, the heat of the mantle can break through to the surface often in the form of volcanic activity. In some cases, underground sources of water can become superheated from contact with the mantle, placing intense pressure on the crust. The resulting release of energy is called a geyser; Old Faithful in Yellowstone National Park, is a well-known example in the United States.

Another naturally occurring example of the release of geothermal energy is when cracks in the earth’s crust permit a release of the built up super heated water and steam. These hot spots are known as hot springs, and are the result of subsurface water being continuously heated by the mantle. Hot springs can be found all over the world.

Harnessing Geothermal Power

All this heat energy can be harnessed and converted to electricity in several different ways. Dry steam reservoirs tap into the energy found in high pressure hot spots that cause geysers; wet steam reservoirs utilize the heat of steam from hot springs and geothermal wells; and hot water reservoirs harness underground water sources to heat residences and commercial buildings.

Dry Steam Reservoirs

Dry steam reservoirs capitalize on geothermal energy released in the form of steam. Dry steam reservoir geothermal plants trap steam in pipes, and that steam is then used to turn turbines which generate electricity. Dry steam reservoirs have actually been used for many years; an Italian reservoir providing power to electric railroads, for example, has been in use for over a century. One drawback is that dry steam reservoirs don’t always refill themselves with water and steam in a predictable fashion, and thus cannot be counted on as a consistent source of energy.

Diagram of a dry steam geothermal power plant.

Diagram of a dry steam geothermal power plant.

Wet Steam Reservoirs

Wet steam reservoirs address the issue of refilling the reservoirs by recycling the water released as steam. Once the steam has been used to turn turbines and has cooled it is pumped back into the subsurface pool to maintain water content in the hot spot. Wet steam reservoirs increase the consistency of energy output from hot spots.

Diagram of a flash steam geothermal power plant.

Diagram of a flash steam geothermal power plant.

Hot Water Reservoirs

Hot water reservoirs are the most common reservoir plant type. In a hot water reservoir the water isn’t hot enough to become steam, but it still warmer then the surface temperature. Heated water is piped into the walls of buildings and the heat radiates into building interiors. That same water is returned to the reservoir to be heated once again, which makes these systems very cheap to operate once they are built.

Diagram of a geothermal heat pump system for heating buildings. Image by Fred the Oyster. License: CC BY-SA 3.0

Diagram of a geothermal heat pump system for heating buildings. Image by Fred the Oyster. License: CC BY-SA 3.0

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