Using deep geothermal drilling to produce supercritical water to be fed into the steam generators of existing coal power plants can help provide clean energy using former fossil fuel power plants. Deep geothermal wells can be drilled almost anywhere in the world, not just where there is volcanic activity. An improvement over conventional drills are gyrotrons that use microwave energy. While the gyrotron can stay at the surface, a waveguide can deposit the energy where it is needed. Existing wells can be used as a starting point to reach initial depths of 500 meters or more, from where the microwaves can extend the wells to the necessary depths of 5 to 20 kilometers.
Exhausted oil formations can be used to harness geothermal energy while providing energy storage much like a battery. The process uses water injected into a deep well, while using a second well to extract highly pressurized, heated water when needed. This can be done for many hours after injection has completed, thus effectively storing energy.
Water passing through iron-rich underground rock formations results in the production of hydrogen, a resource that can be tapped by drilling and then feeding hydrogen motors in a similar manner to current gas wells (so-called “orange hydrogen”).
It is also possible to directly source naturally occuring hydrogen (so-called “white hydrogen”).
Drill rigs can be re-purposed for geothermal energy production (thanks to to a larger temperature gradient of the underwater heat source) and to create CCS (Carbon Capture and Storage) reservoirs.
http://www.forbes.com/sites/williampentland/2014/05/17/submarine-drones-drive-demand-for-subsea-power-plants/
“Submarine Geothermics;Hydrothermal Vents and Electricity Generation” suggests concepts to harness such deep-sea thermal energy sources:
http://www.geothermal-energy.org/pdf/IGAstandard/WGC/2010/3704.pdf
https://www.heise.de/newsticker/meldung/Strom-aus-dem-Vulkan-3660940.html
Seadrill, Transoceon, Diamond Offshore
Nautilus Minerals to mine hydrothermal vents
Fervus Energy uses fracking technology for geothermal energy: https://eartharxiv.org/repository/view/5704/