Geothermal means, literally, earth-heat (Greek: geo + therme). Heat is a form of energy. Geothermal heat becomes an energy resource when we can use the heat to our advantage. It is an excellent resource that is sustainable, works 24/7, and has a minimal carbon footprint. Most of the Earth’s heat is deep inside the Earth, beyond the reach of technology to extract. At relatively shallow depths, depending on the temperature, the heat may be economically extracted and used.
At shallow depths the Earth may be used as a heat reservoir. In terms of magnitude, the Earth receives more than a thousand times more energy from the Sun than is lost from its internal reserves. However, the solar energy is lost back to space on a daily and seasonal basis. Except for small microclimates around hot springs and active volcanoes, the Sun controls the temperature of Earth’s surface and this temperature generally decreases with latitude from the equator to the poles and with elevation. Soil and rocks are poor conductors of heat and below around seven feet (2 m) below the surface, the annual and seasonal variations in surface temperature are damped out and the temperature is steady at approximately the mean annual ground surface temperature. Although this temperature is defined by the solar energy balance, it is soil and rock properties that make this zone good for use as a heat reservoir. Ground-source heat pumps (or geoexchange heat pumps) use this zone for heat storage and retrieval.
Where subsurface temperatures are significantly hotter than the surface temperatures heat may be extracted for surface use. This situation would occur where the geothermal gradient increases the temperature above the surface temperature. This difference may be only a few degrees, or even a few degrees above winter surface temperature for some direct use applications, to a few hundred degrees Celsius for geothermal electricity generation.
A further requirement for “elevated” subsurface temperatures to be a resource is that there must a mechanism by which the heat can be brought to the surface. For some resources the mechanism may be natural, such as hot springs or artesian (naturally flowing) wells. Other sites may require the drilling of a well and pumping. At many sites high subsurface temperatures are found but the rocks lack sufficient permeability (pathways for fluid flow). These rocks may require artificial fracturing or down-hole heat-exchangers to extract the heat. Research on new technologies to extract heat from potential geothermal reservoirs is continuing.
Areas in Colorado that are prime for new geothermal exploration include the Rico Dome structure in southwest Colorado, Mount Princeton Hot Springs, Waunita Hot Springs, and the San Luis Valley. These exploration targets represent potential sites with high heat flow. There are currently no geothermal electrical power generating facilities in Colorado.
How does it form?
The interior of the Earth is hot. Volcanoes are a dramatic reminder that there is heat in the Earth. Miners who work in deep mines know that the deeper the level, the higher the temperatures. Oil-well drillers also know that the drill pipes are hot when they are pulled from a deep well and the oil is hot as it rises to the surface. What is the source of this heat?
An early theory was that all of this heat was primordial, or remained from the formation of the Earth. However, with the discovery of radioactivity an additional source of heat was found. Unstable isotopes of uranium (235, 238U), thorium (232Th), and potassium (40K) exist in sufficient quantities in most rocks to supply a significant fraction of the heat that is lost from the modern Earth. The total, present-day rate of heat loss from the Earth is estimated to be 46 TW (terawatts or million billion watts), or the equivalent of approximately 69,000 average-sized US coal-fired power plants (average power generation capacity 667 MW).
The Earth is simmering in geologic time. These heat sources are not sufficiently concentrated enough to form a volcano or a geothermal resource directly, but are like a burner on low on a range top. Given enough time, they can bring soup to a simmer. The result is movement of the tectonic plates, a solid crust on the simmering pot of the Earth, broken into pieces that move relative to each other along their boundaries. Most geologic interactions, volcanoes, earthquakes, mountain building, occur close to these boundaries, although there are some important exceptions. Isolated volcanic centers, such as Yellowstone and the Hawaiian Islands pierce the plates far from their edges, sedimentary basins continue to develop long after they have an association with a plate boundary.
Plate tectonics is an important process for geothermal resources in a number of ways. Most volcanoes are associated with plate boundaries and high-temperature geothermal resources are usually found close to active volcanoes. Mountain belts are generally formed in association with plate tectonics. Sometimes the association is obvious, such as the collision of the Indian subcontinent with Asian to form the Himalaya. Sometimes the association is less clear such as the origin of the current elevation of the Colorado Rocky Mountains. However, topographic variations and young faults often allow water to circulate deep in the earth (a few km or a couple of miles) and rise to the surface as a hot spring/thermal resource. Finally, mineral resources can become concentrated in association with melting and recycling of the crust during the plate tectonic cycle. The minerals include those that contain the heat-producing isotopes and some rocks in the continental crust, particularly granitic rocks, contain significantly more heat production than other rocks. These high heat production rocks can produce local warm spots in the crust.
The Colorado Geothermal Development Strategic Plan –- prepared by the State Geothermal Working Group and the Governor’s Energy Office in 2007.
Geothermal Education Office — Clearinghouse for a wide variety of geothermal information.
Geothermal Rising (formerly Geothermal resources Council) — Advocacy organization for geothermal energy use, with information on research.
National Oceanographic and Atmospheric Administration – National Centers for Environmental Information – Thermal Springs in the US — Spring locations with with associated temperatures.
Think Geoenergy — Supplies business-to-business news on the international geothermal energy market.
U.S. Department of Energy – Office of Energy Efficiency and Renewable Energy – National Renewable Energy Laboratory – Geothermal Resource Data, Tools, and Maps — Wide range of online geospatial tools and downloadable U.S. maps and data sets.