The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form ofheatfrom the core to the surface.

Historically has been limited to areas neartectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications such as home heating, opening a potential for widespread exploitation.

Geothermal electric plants were traditionally built exclusively on the edges of tectonic plates where high temperature geothermal resources are available near the surface. Thethermal efficiencyof geothermal electric plants is low, around 10-23%, because geothermal fluids do not reach the high temperatures of steam from boilers.

In the geothermal industry,low temperaturemeans temperatures of 300 °F (149 °C) or less. Low-temperature geothermal resources are typically used in direct-use applications, such as district heating,greenhouses,fisheries, mineral recovery, andindustrial process heating. However, some low-temperature resources can generate electricity using binary cycle electricity generating technology.

Direct heating is far more efficient than electricity generation and places less demanding temperature requirements on the heat resource.

Source: Wikipedia.com