Geothermal is literally earth (geo from the Greek) heat (therme from the Greek). Heat is a form of energy. Geothermal becomes an energy resource when we can use this heat to our advantage. Most of the Earth’s heat is deep inside the Earth, beyond the reach of technology to extract the heat. 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 loses 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 a couple of meters (about 7 feet) 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 - see link 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 (see link How Does It Form? ). This difference may be a few degrees, or even a few degrees above winter surface temperature for some direct use applications, to a few hundred degrees 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.