It is also a thermal power plant, but the steam required for power generation is available naturally in some part of the earth below the earth surface. According to various theories earth has a molten core. The fact that volcanic action takes place in many places on the surface of earth supports these theories.

Steam well:

Pipes are embedded at places of fresh volcanic action called steam wells, where the molten internal mass of earth vents to the atmosphere with very high temperatures. By sending water through embedded pipes, steam is raised from the underground steam storage wells to the ground level.

Separator:

The steam is then passed through the separator where most of the dirt and sand carried by steam are removed.

Turbine:

The steam from the separator is passed through steam drug and is used to run the turbine which in turn drives the generator. The exhaust steam from the turbine is condensed. The condensate is pumped into the earth to absorb the ground heat again and to get converted into steam.

Location of plant, installation of equipment like control unit etc., within the source of heat and the cost of drilling deep wells as deep as 15,000 meters are some of the difficulties commonly encountered.