Geothermal energy means heat from the earth. "Geothermal" comes from the Greek words geo (earth) and therme (heat). Geothermal power plants like Bottle Rock tap into naturally occurring heat that is stored in the Earth's crust. How hot? The Earth's temperature gradually increases with depth - reaching more than 4200° C (7600° F) at its core. This heat flows outward from the Earth's core where sometimes it reaches the surface as lava. Usually, however, hot magma remains well below the Earth's crust, where it heats nearby rock and water, sometimes to levels as hot as 700° F. This trapped hot water and/or steam is called a geothermal reservoir. 

When the rising hot water and steam is trapped in permeable and porous rocks under a layer of impermeable rock, it can form a geothermal reservoir.

Wells are drilled approximately two miles deep to tap into these geothermal reservoirs. The steam piped out of the earth and into a power plant, where it powers the turbine that drives a generator to produce electricity. After the energy has been extracted from the geothermal fluids (steam and water), the fluids are injected back into the earth in order to maintain the geothermal reservoir.

Natural steam from the production wells power the turbine generator. The steam is condensed by evaporation in the cooling tower and pumped down an injection well to sustain production.

Like all steam turbine generators, the force of steam is used to spin the trubine blades which spin the generator, prducing electricity. But with geothermal energy, no fuels are burned.   In dry steam power plants, the steam (and no water) shoots up the wells and is passed through a rock catcher (not shown) and then directly into the turbine. Dry steam fields are rare.

Geothermal energy is clean and renewable energy, powered by heat from the Earth - not fossil fuels - and the Earth's heat is virtually limitless. And it's available 24 hours a day which makes it more valuable to the utility than intermittent sources of generation. Producing electricity with geothermal energy also avoids releasing harmful emissions associated with traditional power plants. In fact geothermal power plants release less than 1 percent of the carbon dioxide emissions of a fossil fuel plant.


Illustrations courtesy of the Geothermal Education Office, Tiburon, CA