Geothermal energy is energy which is generated and stored within the Earth in the form of heat. The sources of this thermal energy in the deep earth are the heat from when the planet formed and accreted, which has not yet been lost. Heat produced by the decay of radioactive elements, as well as frictional heating caused by the denser core material sinking to the centre of the planet. Geothermal energy can be exploited as a reliable and constant source of low-carbon, renewable heat that is not dependent on weather conditions.
Source:
https://www.thinkgeoenergy.com/geothermal/what-is-geothermal-energy/
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