B. Geothermal Resources
Geothermal energy resources result from complex geologic processes that lead to heat concentration at accessible depths.(7) The different forms of geothermal energy resources--hydrothermal, hot dry rock, geopressured, magma, and earth heat--all result from this concentration of Earth's heat in discrete regions of the subsurface. Temperature within the Earth increases with increasing depth (Figure 19). Highly viscous or partially molten rock(8) at temperatures between 1,200 and 2,200°F (650 to 1,200°C) is postulated to exist everywhere beneath the Earth's surface at depths of 50 to 60 miles (80 to 100 kilometers), and the temperature at the Earth's center, nearly 4,000 miles (6,400 kilometers) deep, is estimated to be 7,200°F (4,000°C) or higher. Heat flows constantly from its sources within the Earth to the surface.
Three sources of internal heat are most important: (1) heat released from decay of naturally radioactive elements; (2) heat of impact and compression released during the original formation of the Earth by accretion of in-falling meteorites; and (3)
heat released from the sinking of abundant heavy metals (iron, nickel, copper) as they descended to form the Earth's core.An estimated 45 to 85 percent of the heat escaping from the Earth originates from radioactive decay of elements concentrated
in the crust.(9),(10) The remainder results from slow cooling of the Earth, with heat being brought up from the core by convection in the viscous mantle.(11)
The different forms of geothermal resources have different characteristics that are important to geothermal energy development:
- Hydrothermal resources are steam or hot water reservoirs that can be tapped by drilling to deliver heat to the surface for thermal use or generation of electricity. Technologies to tap hydrothermal resources are proven commercial processes. Dry steam resources are relatively rare.
- Hot dry rock resources are defined as heat stored in largely impermeable rocks (Figure 20). Access to these resources involves fracturing rock injecting cold water own one well, circulating it through the hot fractured rock, and drawing off the now hot water from another well. Since the current technologies are entering a development phase, this is not a commercial process at this time.
- Geopressured resources consist of deeply buried brines at moderate temperature that contain dissolved methane. Three sources of energy are available: thermal, mechanical, and chemical (methane gas). While technologies are available to tap geopressured brines, they are not currently economically competitive. No funds are currently being directed toward accessing these resources.
- Magma (molten rock) resources offer extremely high-temperature geothermal opportunities, but existing technology does not allow recovery of heat from these resources.
- Earth heat itself can be used as the source and/or sink of heat for the operation of geothermal heat pumps, a proven technology.
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