This energy is given by the heat of the deep layers of Earth crust.
In fact, the average temperature of Earth increases of about 3 C every 100 m of depth, so that at 4000 m, below ground level, in a NOT volcanic region, the temperature of rocks is about 120 C more than that on the surface.
In the practice, it's very difficult to exploit the heat at these depths, because still too expensive.
In the volcanic regions, instead, rocks are very hot also close to the surface, heated by trapped lava masses also in zones where the superficial volcanic activity is going to extinction.

The technologies exploit the hot water sources coming from these superficial hot rocks that can spring out from the surface as hot vapour at temperatures of more than 200 C. This water vapour is trapped and diverted to the power plants to generate electric power.
Here, this vapour is filtered to eliminate all its salts in the aerosol, mainly BORAX, (Na2B4O7), SiO2, sulphates and many others that could encrust the pipelines. Then, the vapour goes in heat exchangers to turn in steam other water that makes move the turbines.

This energy source is today exploited too little; only in the highly volcanic Iceland, that get from it nearly all its energy needs; then, in some places, like, for ex., in California and in Italy, at Larderello, South Tuscany, where the water condensed from vapour contains high amounts of borax, extracted and sold.

But there's also the possibility of not exploiting only natural vapour sources from the ground, by pumping water through drills down to the hot rock layers (not too deep, for not multiplying too much the costs) and let it come up again as hot steam for turbines.

Also geothermic energy, like the solar light, the wind and the bio-gas will need to be massively used to replace as soon as possible oil, coal and nuclear energy that are too pollutant, limited and dangerous energy sources.

Learn more about this author, Aldo Bonincontro.
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