WITHOUT MOVING PARTS

Faraday knew little about the nature of the electric charges whose flow forms an electric current. Today we know that they are (most often) electrons, particularly the free electrons floating round in a metal "free" because they have left the atoms to which they were originally bound. But there are other materials in which to make electrons free to flow; for example, a gas can be heated to very high temperatures, until its molecules bump so violently against each other that some electrons are kicked out of their atoms. If this hot gas is forced through a "channel" surrounded by a magnetic field, a voltage is generated, which can be tapped by two electrodes. The arrangement is known as an MHD generator; the MHD stands for magnetohydrodynamics.

The hot gas is obtained by burning a fuel, such as coal or gas; the "channel" is a square pipe, two opposite faces of which act as the electrodes, with the other two faces insulating them. Although virtually all present methods of large-scale electric power generation are based on Faraday's Law, MHD is the only one which converts heat to electricity directly, without the intervening medium of a gas (usually steam) to press on turbine blades or other mechanical arrangements to rotate an orthodox generator.

The absence of moving parts is only one of the advantages of MHD. The main advantage is a greatly improved efficiency. In a large conventional fossil-fired plant it can go as high as 41%; a nuclear plant (High Temperature Gas Reactor) can do almost as well (39%). To go still higher in conventional plants would require a higher steam temperature, and that runs into trouble with the highly stressed mooring parts, inaccessible to cooling and subject to close tolerances. But none of these points is a problem to MHD, at least not in principle: Its efficiency can reach 50% with present technology, and probably 60% with further development.

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One way of pushing the efficiency is by a "bottoming cycle." Unlike present power plants, which make steam from water and condense it back to water again, MHD works with a hot gas all the time; when the gas has produced elicit in the channel, it is still hot enough, not just to grow peanuts (see last month's issue), but to run a gas or steam turbine to produce mote electricity.

The ultimate in compactness and efficiency would be nuclear heat, rather than burning fossils, to produce the hot gas. This is not yet possible because at present no commercially acceptable fission reactor produces a sufficiently high temperature.

And there are more fringe benefits.. By a number of benign coincidences, the MHD process would burn coal fairly cleanly. To achieve sufficient thermal ionization (knocking out electrons from atoms) in the gases produced by burning coal or other fossils, the gases must be "seeded" by some metal-bearing compound. This is usually potassium carbonate, which will also capture sulfur, and the potassium sulfates are then easily trapped in the potassium recycling mechanism. That means that burning high-sulfur coal within pollution standards may become possible. The recycling mechanism also acts as a precipitator, ridding the coal smoke of 99% of its particulates. And to top it off, preliminary experience suggests that the rapid cooling on the walls of the channel decomposes the nitrogen oxides to well below EPA's NOx standards.

Nor is this all. MHD power generation promises to be economical, too: Capital and fuel costs lie between those of fossil-fired and nuclear plants, and operating costs should be smaller than either, so that the total costs per kWh could bee ome competitive with nuclear.