A sound way to generate electricity.

A sound way to generate electricity

Converting heat energy into electricity is nothing new. That's what happens in a coal-fired power plant, for instance. But transforming heat energy first into sound energy and then into electrical power is somewhat unconventional. This latter process is the basis for a liquid-metal acoustic heat engine now being developed at the Los Alamos (N.M.) National Laboratory (LANL).

Although the acoustic engine isn't as efficient as other heat engines, such as steam turbines, it has the advantage of having no moving mechanical parts and hence a high reliability, says LANL's Greg Swift. Such engines could be used to provide electrical power for satellites and other applications where reliability is extremely important.

The prototype liquid-metal acoustic heat engine consists of a 1-meter-long metal tube, closed at both ends and filled with liquid sodium. A stack of thin parallel plates, made from molybdenum, sits inside the cylinder. Two sets of tiny tubes carry fluids that keep one end of the plates at 125|C and the other at 700|C.

"Sound is generated by the temperature difference across that stack,' says Swift, who with Al Migliori designed the engine. "It appears spontaneously,' he says. "The liquid sodium starts to sing.' In other words, the liquid begins to oscillate. These oscillations in the presence of a magnetic field at the tube's midpoint allow an electrical current to be generated.

So far, in two separate projects, the researchers have confirmed that they can, indeed, produce sound from heat and that their magnetohydrodynamics generator does convert acoustic waves into electrical energy. Eventually, the two steps will be combined into a single working generator. "Nothing we have learned has discouraged us yet,' says Swift.

The sodium engine is just one extension of earlier work on acoustic heat engines (SN: 12/4/82, p. 358). Tom Hofler, a graduate student from the University of California at San Diego and now working at LANL, is developing a loudspeaker-driven refrigerator. In this case, electrical energy is converted into acoustic waves (as in a loudspeaker) and these sound waves produce a cooling effect.