A very, very small part of this energy is already used in the form of geothermal waters. It is mainly used by the spa and some smaller towns are heated using such waters. Probably Iceland is leading here, where geothermal energy is used to produce electricity. The water has a temperature of about 356 K. But it is a negligible amount of energy in relation to all the energy produced by humanity.
Dear Grzesik,
Actually, we can use It. But there are some practical issue.
This is not economycal when compound to solar power and wind powder for the tropical countries.
Other issue is initial cost of investigation and installation is high and change location to location.
But this may have future for countries having poor solar radiation in some seasons
Piyal Aravinna
Present, little more in the future.
For large scale energy production, geothermal is limited to favorable natural occurrences: you must sit over a volcano with own water circulation (Iceland, Larderello, California). Wherever it was feasible (profitable), it has been done.
The proportions in those two cases are not small, Larderello outputs about 1 GigaWatt, at very low cost.
But there are few of lucky places like those. Maybe we can still install some large electric plant on the oceanic ridges?
I have read that home-scale geothermal heating can be done wherever you want, but i'm not so sure.