Geothermal Energy

The geothermal energy is the energy coming from the Earth sub-surface.

The heat production's origin :

The energy of the Earth's crust comes from the original accretion of the planet (20%) and the radioactive decay of certain elements (80%), the lifetime of this energy is measured in billions of years (like the sun). The more we get down in depth, the more we get heat.

This thermal energy can be used directly, or it can be converted into electrical power with specific conditions.
Direct use and electricity :