Geothermal energy refers to thermal or electrical power produced from the thermal energy contained in the Earth. The use of geothermal energy is based on the temperature difference between a mass of buried rock and water and a mass of water (or air) at the Earth's surface. The temperature difference thermodynamically allows the production of thermal energy which is converted directly or indirectly to mechanical or electrical energy.
Geothermal gradients in the Earth generally range between 10 to 30 degrees Centigrade/kilometer. The source of these gradients is from conductive heat flow from the deep crust and mantle and from radioactive decay of uranium, thorium, and potassium isotopes. Some granitic rocks in the upper crust contain abnormally high concentrations of radioactive elements resulting in enhanced heat flow towards the Earth's surface. Advective heat flow, associated with the movement of magma and hot water in the subsurface, can be superimposed on the regional conductive-type of heat flow resulting in very high temperatures near the Earth's surface. These areas are the primary targets for geothermal exploration and development.
Although geothermal energy is present everywhere beneath the Earth's surface, this energy must be concentrated to be an effective power source. Conversion of geothermal energy into power is possible only when: (1) located at shallow drilling depths, usually less then 3 kilometers, but possibly as deep as 6 to 7 km, (2) economics are improved at shallow depths where drilling costs are lowered, (3) porosity and fracture permeability are sufficiently high to produce large quantities of thermal waters, and (4) the hot geothermal fluids can be efficiently transported (typically less than a few tens of kilometers) to a power generating facility.
The accessible geothermal resource base in the United States at depths less than 7 km is estimated to be 17,200,000 x 1018 joules although this estimate reflects the large volume of rock involved rather than actual recoverable resources. The U.S. Geological Survey has estimated identified and undiscovered convection hydrothermal resources in the United States to be 2,400 x 1018 joules which is the energy equivalent to 430 x 109 barrels of oil. Additionally, resource estimates for geothermal-geopressured parts of the northern Gulf of Mexico range from 270 x 1018 to 2,800 x 1018 joules. The use of geothermal energy may increase in other countries as other sources of energy are depleted and become more expensive and as additional information is collected on the geometry of individual geothermal systems
For more information contact:
Joel Renner, Chair
Phone: (208) 526-9824
Email