The adjective geothermal is a combination of the Greek “geo” = earth and “therme” = heat and stands for the heat taken from the earth.
Definition according to VDI 1998: (translated from the original German version)
Geothermal energy is energy stored as heat under the solid surface of the earth.
The overall amount of geothermal energy is made up in range of 30 percents by residual heat. Kinetic energy from the agglomeration of matter 5 billion years ago, when the planet was created, was stored as heat energy within the rocky mass of the earth. Due to the low heat conductivity still today residual heat from this genesis process is present under the earth’s surface.
Approximately 70 % of the geothermal energy is supplied by the radioactive decay of the isotopes potassium 40, uranium 235, uranium 238 and thorium 232. This process continuously generates heat that is stored in the rocks.
Close to the earth’s surface the mean temperature is 10°C (50°F) and increases towards the core by 3°C (37.4°F) each 100 meters.
As the residual heat and the heat from decay process are finite, geothermal energy is not a renewable energy. However, its potential will prospectively ensure reliable energy supply for the next millions of years. Therefore geothermal energy can be considered to be a renewable energy on the human scale.
The essential advantage of energy taken from the earth’s inside in comparison to other renewable energies is its ability to supply baseload energy. Geothermal heat is unaffected by daily, seasonal and annual changes and therefore constantly and consistently available. It can be extracted from its subsurface reservoir by various proven technologies. As virtually no carbon is emitted by using geothermal energy it is regarded to be outstandingly climate friendly. The proportion of extracted primary energy to useful heat is very advantageous.