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Run Silent, Run Swift


Article # : 10564 

Section : NATURAL SCIENCE
Issue Date : 1 / 1993  2,446 Words
Author : Marsha Freeman

       The silent submarine in Tom Clancy's Hunt for Red October was only fiction, but last June the Japanese ship Yamato 1 made its sea Trials, becoming the first MHD-propelled silent ship in the world. The magnetohydrodynamic, or MHD, thruster has no moving parts, which accounts for the quiet. As a scientific principle, MHD has been known for over 150 years. But the Yamato 1 is the first commercial prototype of this promising technology.
       
       The same characteristic of MHD technology that makes it silent--no moving parts--makes it an efficient way to propel a ship, or to produce electricity. The basic principle involves the interaction of magnetic and electrical fields with an electrically conducting fluid. In the case of ship propulsion, an electric current is applied perpendicular to a magnetic field. Perpendicular to both is the flow of the water. The seawater, which is electrically conducting because of the natural salts, is accelerated as it moves through the thruster tube, and as the faster-moving water exists the rear of the ship, the ship is pushed forward.
       
       But the application of MHD to electric power generation is where it could prove its real worth. The electrically conducting moving fluid can be the hot gas produced by burning coal, oil, or natural gas. The working fluid can also be a liquid metal that is heated by solar energy or nuclear fission. When thermonuclear fusion is available, the fluid will be the charged atomic particles, or plasma, from the fusion reaction directly. The working fluid flows through an MHD channel that is surrounded by large magnets. An electrical potential is produced perpendicular to the magnetic fields and the direction of the fluid flow, and electricity is extracted from electrodes along the sides of the channel.
       
       Today most electricity is produced when the heat of combusted fuel produces steam that is used to turn turbines. But nearly two-thirds of the energy in the coal, oil, or gas is lost in the process. As a direct conversion process with no moving parts, MHD can double that efficiency. For every ton of fuel used, twice as much electricity could be produced.
       
       Correspondingly less energy is wasted as heat, producing less thermal pollution, and less water is needed for cooling. Because the fuel is burned in oxygen enriched air in an MHD generator, it burns at a higher temperature and more completely, so there is much less carbon pollution produced. In order to increase the electrical conductivity of the coal gas, potassium seed is added. The potassium combines with the sulfur in the fuel pollutants. The amount of nitrogen oxide emitted from the plant can also be reduced by increasing the amount of oxygen, as opposed to air, in the preburn mixture. No scrubbers or other costly pollution control equipment will be needed to meet environmental regulations with MHD power plants.
       
       The technical challenges
       
       Even though experiments in MHD have been going on since the 1960s, and the first patent for MHD ship propulsion was granted in the United States in 1961, it has been only recently that some of the technical challenges have been met that will bring MHD into realization.
       
       It is not surprising that the world's first MHD ship was built in Japan, because the government there has a research and development
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