Image Courtesy of Geothermal-Energy.org
Geothermal energy is present everywhere beneath the earth’s surface, although the highest temperature, and thus the most desirable, resources globally are concentrated in regions of active or geologically young volcanoes. These are usually associated with plate tectonic boundaries, and more locally along the boundaries with “hot spots” or thermal plumes where the narrowly focused upward flow of extremely hot mantle material nears the earth’s surface. Magma, which originates from the mantle, rises and feeds volcanoes. Much of this intruding magma remains pooled in the crust beneath volcanoes however, and constitutes an intense, high-temperature geothermal heat source for periods of thousands to millions of years. Additionally, tectonic plates grind against one another and fracture rock, thus allowing water to circulate at depth and transport heat toward the earth’s surface. Together, the rise of magma from depth and the circulation of hot water (hydrothermal convection) maintain the high heat flow prevalent along tectonic plate boundaries.
Accordingly, these plate boundaries with associated hot spot zones are prime target areas for the discovery and development of high-temperature hydrothermal-convection systems capable of producing steam that can drive turbines to generate electricity.
Geothermal resources are similar to many mineral resources. A mineral deposit is generally evaluated in terms of the quality or purity (grade) of the ore and the amount of this ore (tonnage) that can be mined profitably. Such grade and size criteria can also be applied to the evaluation of geothermal potential. Grade would be roughly analogous to temperature, and size would correspond to the volume of heat-containing material that can be tapped.
Thermal energy is hosted in a broad range of geothermal environments, and these commonly are classified by temperature and the amount of fluid: water and/or steam available for carrying the energy to the earth’s surface
Determining whether or not heat can be extracted from a particular geothermal environment is critically dependent upon depth. The pertinent question is whether the geothermal target is within economically drillable depths, roughly 4km maximum with current technologies. Each of the possible geothermal environments can occur over a range of depths. For example, most magma bodies are estimated to lie between 5km to 10km depth. Exploitable geothermal energy in hydrothermal systems with temperatures approximating 250 C may be discovered at depths from one to several kilometres. For optimum exploitation, the challenge is to locate geothermal environments of high temperature, high porosity and permeability, and shallow depth.
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