NOT BY METALS ALONE

Electricity is generated by moving an electrical conductor through a magnetic field (Faraday's Law); but a conductor need not be a metal, or even solid it can be a liquid or a gas. An electrical conductor is something that has free electrons, which are the actual carriers of electricity. A flow of electrons (negative electric charges) is what constitutes an electric current. These free electrons were once attached to positive nuclei in their home atoms, but left them for various reasons. In a metal, the nucleus holds the electrons in the outer shells only very weakly, and they leave their home atoms quite easily, floating around as unattached free electrons in the metal, which is what makes metals such good electric conductors.

On the other hand, in most gases electrons are bound strongly to their atoms and can be knocked out of them only by certain forces such as violent collisions with other atoms. That can be done by imparting a strong thermal motion to these atoms, i.e., by raising the temperature of the gas to a high value.

MHD uses a very hot gas (a gas with free electrons and ions, or a plasma) as the conductor. This is forced through a nozzle at very high velocities and traverses a strong magnetic Eleld in a chamber called "the channel." By Faraday's Law, a voltage will be induced, or more specifically, the electric charges in the gas will be forced sideways (left or right, depending on the polarity) from the original direction of the jet, and this can be tapped by electrodes perpendicular to both the jet and the magnetic field.

There are several advantages. All the rigid parts of the system are at rest; the only thing that moves is the gas. This is, of courses expected to be far more reliable than a turbo-generator, in which masses by the hundreds of tons move at high velocities through clearances with tiny tolerances.

Equally or more important, MHD has a far higher efficiency than conventional thermal plants (whether nuclear or fossil fueled), because it runs at a far higher temperature (about 3,000 degreesC), and Wonderboy Lovins' preachings notwithstanding, higher temperatures mean higher efficiency. On leaving the channel, the gas is still hot enough to heat water and run a conventional steam generating plant. The overall efficiency of such a combined power plant could reach values of 50 to 60%, compared with only 30 to 40% for nuclear and coal fired plants.

The US effort in MHD, headed by AVCO Labs in Everett, Mass., is based on coal, or rather on the hot gases formed in its combustion. To enhance the conductivity of these gases, they are "seeded" with (and shock ionized by) potassium, which also has the fringe benefit of chemically combining with the sulfur in the coal, so that MHD almost automatically produces cleaner emissions into the atmosphere than conventional methods of burning coal. The Soviets burn natural gas, rather than coal, in their MHD generators.

But MHD is also beset with difficulties, such as development of a satisfactory material for the channel walls, which are heavily stressed thermally (3 ,000 degreesC), electrically (3 kV/m), and mechanically (the gas flows at nearly the velocity of sound). The required electrical and thermal properties of the ideal material are almost contradictory, but they can be approximately achieved by refractory segments insulated from each other electrically. The development of these materials, incidentally, was a direct spin off from the experience gained in rocket programs.

Deposits on the walls of the channel soon short out the electrodes (which are another component under heavy and many-sided stress), and MHD generators have often been judged by the length of time for which they could be kept running - for example, a joint US Soviet venture in which the US supplied a 40-ton superconducting magnet for the Soviet U 25 MHD facility made a run of 127 hours at slightly above 20 MW electrical output last May.

This year, AVCO also ran one of its Mark VI channels at 220 kW output power for 250 hours. It was a resounding success as a milestone, but of course not the advent of MHD as a commercial way of generating electric power.

That is not expected before the end of the century.

[More: "MHD: A Report on the state of the art," IEEE Spectrum, N1ay 1978; " New lease of life for MHD," Electrical Review International, 5/5/78.]