GEOTHERMAL GRADIENTS

Although this is probably not a source that can soon make a major contribution, it is a very promising one, and extracting energy from Hot Dry Rock (HDR) is being intensively studied by the Los Alamos Scientific Labs on their test site in the Jemez Mountains in New Mexico. As we first reported in 1974, the method is to form a cavity by hydraulic fracturing, and to pump water into this hot reservoir, where it will turn into steam and return to the surface by a separate exit pipe. This has been achieved; a reservoir at about 185 degrees C has been formed and tested for 75 days at an extraction rate of about 5 MW thermal, and plans are now underway to enlarge this to 50 MW electrical.

Such a reservoir should last for 30 years; the hope is that as it cools, thermal stresses will make the fractures self-propagating, so that this energy facility would be of indefinite duration.

The thermal gradient of hot rock usually lies between 20 and 30 degrees C per km. Though the method is as yet uneconomical, beset with technical difficulties and not yet fully tried, its potential is staggering: The HDR energy contained in the first 10 km below the US is some 13 million quads! (1 quad = 1 quadrillion BTU; the US now uses about 76 quads per year.) And right above this 160,000 years' worth of US energy supply march the pundits who assure us that energy is running out and that we must get used to living in the age of shortages and diminished expectations.

Not among this sorry crowd is Sid Altschuler, an engineer from Richland, Wash., who gave an interesting paper on "Geothermal power from salt domes" at the 13^th Intersociety Energy Conversion Conference in San Diego in August 1978.

He proposes to use the heat in salt domes extending from 15,000 to 40,000 feet below the surface as an energy source, with the cold water (or other fluid) running down a well pipe one foot in diameter, and the steam returning through a concentric central pipe. There would be no contamination problems, since the same clean liquid circulates through this double-pipe heat exchanger. The attraction of this system is that at 200 degrees C salt begins to become plastic, and at 350 degrees C it is completely plastic, so that a weighted pipe with a pointed end would sink to the bottom of the salt dome under its own weight without any drilling; an insulated pipe with an open end would then be inserted to form the return path for the steam.

Altschuler's calculations indicate an average yield of 2 MW electric. After running for 100 years, the well would still produce 75% of its original power.

[More: R.G.Cummings a.o., "Mining earth's heat," Technology Review, Feb. 1979. Altschuler's paper is not yet published.]