"It is a well-known fact that the interior portions of the globe are very hot, the temperature rising, as observations show, with the approach to the center at the rate of approximately 1— C. for every hundred feet of depth." — The Problem of Increasing Human Energy, 1901

"All that is necessary to open up unlimited resources of power throughout the world is to find some economic and speedy way of sinking deep shafts." — Our Future Motive Power, 1931

The arrangement of one of the great terrestrial-heat power plants of the future.  Water is circulated to the bottom of the shaft, returning as steam to drive the turbine, and then returned to liquid form in the condenser, in an unending cycle. . . . The internal heat of the earth is great and practically inexhaustible. . . .

Nikola Tesla's development of a system for the generation, distribution and utilization of electricity caused him to invest considerable thought into potential sources of mechanical energy to support the process.  In 1931 he wrote an article for Everyday Science and Mechanics entitled "Our Future Motive Power" which analyzed some alternatives to the combustion of fossil fuels for obtaining electrical power.  A closely related piece also appeared in the New York Times. The following observations on hot dry rock geothermal energy technology for AC power generation are derived from these two pieces.
 

Our Future Motive Power
The material as well as intellectual progress of Man is becoming ever more dependent on the natural forces and energies he is putting to his service.  While not exactly a true measure of well being and enlightenment, the amount of power used is a reliable indication of the degree of safety, comfort and convenience, without which the human race would be subject to increasing suffering and want and civilization might perish. . . . 

It is noteworthy that . . . in 1852 Lord Kelvin called attention to natural heat as a source of power available to Man.  But, contrary to his habit of going to the bottom of every subject of his investigations, he contented himself with the mere suggestion.  Later, when the laws of thermo-dynamics became well understood, the prospects of utilizing temperature differences in the ocean, solid earth or the atmosphere, have been often examined. . . . 

In this regard, the utilization of temperature differences in the solid earth presents several important advantages.  It would make it unnecessary to go down to the tropics where power is of smaller value. Indeed, the colder the climate the better.  A shaft could be sunk in the midst of a densely populated district and a great saving effected in the cost of distribution.  The shaft would be costly, of course, but the apparatus cheap, simple and efficient.  The [above] drawing illustrates its essential parts comprising a boiler, a condenser, cooled by river or other water available, on the ground, a turbine coupled to a generator, and a motor-driven high vacuum pump.  The steam or vapor generated in the boiler is conveyed to the turbine and condenser through an insulated central pipe while the condensed water flows by gravity through another pipe reaching to depth at which the temperature of the ground exceeds that of the condensate.  By circulating the steam in great volume through the turbine and condenser I am able to maintain a considerable temperature difference between the ground and the interior of the shaft, so that a very great quantity of heat flows into the same continually; to be transformed into mechanical work.  The only requisite is a sufficient volume of cooling water.

By this method it is practicable to supply all the power which a small community may require from a shaft of moderate depth, certainly less than a mile.  And for isolated dwellings a few hundred feet depth would be ample, particularly if such a fluid such as ether is employed for running the turbine. [New York Times, Nov. 8, 1931] All that is necessary to open up unlimited resources of power throughout the world is to find some economic and speedy way of sinking deep shafts.