The Earth's internal heat naturally flows to the surface by conduction at a rate of 44.2 terawatts, (TW,) and is replenished by radioactive decay of minerals at a rate of 30 TW. These power rates are more than double humanity’s current energy consumption from all primary sources, but most of it is not recoverable. In addition to heat emanating from deep within the Earth, the top ten meters of the ground accumulates solar energy (warms up) during the summer, and releases that energy (cools down) during the winter.Beneath the seasonal variations, the geothermal gradient of temperatures through the crust is 25–30 °C per kilometre (km) of depth in most of the world. The conductive heat flux is approximately 0.1 MW/km2 on average. These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation or a combination of these.
A geothermal heat pump can extract enough heat from shallow ground anywhere in the world to provide home heating, but industrial applications need the higher temperatures of deep resources. The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. The more demanding applications receive the greatest benefit from a high natural heat flux, ideally from using a hot spring. If no hot spring is available, the next best option is to drill a well into a hot aquifer. If no adequate aquifer is available, an artificial one may be built by injecting water to hydraulically fracture the bedrock. This last approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America. Much greater potential may be available from this approach than from conventional tapping of natural aquifers.Estimates of the electricity generating potential of geothermal energy/geothermal power plant vary from 35 to 2000 GW depending on the scale of investments. Upper estimates of geothermal power plant resources assume enhanced geothermal wells as deep as 10 kilometres (6 mi), whereas existing geothermal wells are rarely more than 3 kilometres (2 mi) deep. Drilling at this depth is now possible in the petroleum industry, although it is an expensive process. The deepest research well in the world, the Kola superdeep borehole, is 12 kilometres (7 mi) deep. This record has recently been imitated by commercial oil wells, such as Exxon's Z-12 well in the Chayvo field, Sakhalin.