SHORT RUNS AND SHORT CIRCUITS

The main reason why MHD has so far worked only for short runs is not the high temperature of the gas (some 5,000 degreesF) as such, but rather the degradation of the resistivity of the insulating channel walls. This is brought about by seed compounds dissolved in water: As electricity is tapped from the flow of the seeded hot gas, this solution penetrates into the insulator, provides an electrical path, and a short gradually develops between the electrodes.

There are several possible countermeasures, for example, adding a small amount of refractory to the fuel (A refractory is a material that does not significantly deform or chemically change under high temperatures.) The refractory will coat the walls, and if its temperature is above the dew point of the seed, it will prevent the seed from condensing on the insulating wall and building a short circuit as it absorbs water vapor.

More difficult problems have been solved. The cooling of the channel walls, for example, would - ideally require a material that is a good conductor of electricity, yet a bad conductor of heat but there is no such material. AVCO solved the problem by a composite structure: water-cooled metal plates with refractory insulation between them. Either this month or next, NASA's Lewis Research Center will publish a comprehensive Energy Conversion Alternatives Study, commissioned by NASA, ERDA and NSF, which examines alternative methods of generating power from coal. Not surprisingly, it finds MHD the most promising of all methods studied not excluding advanced turbine cycles or fuel cells.

Will the report wake up the country to the need for MHD?

No; but perhaps a band of blackmailing sheiks will.

[More: Work on MHD is widely scattered over the technical literature; there are also annual symposia. A good place to start is MHD Power Generation by A. Kantrowits and R.J. Rosa (two top experts) in The Physics Teacher, November 1975. For some short brochures on MHD, write PR Depth AVCO Everett Res. Lab., 2385 Revere Beach Pkwy., Everett, MA 02149]