Acoustically Powered Deep-Space Electric Generator

Space is a new frontier for an acoustical version of a 19th-century mechanical device. For future deep space missions to the outer planets and beyond, space agencies would like their probes to have a lighter, smaller, and more efficient source of electricity. With this need in mind, a Los Alamos-Northrop Grumman team (Scott Backhaus, backhaus@lanl.gov) has built a device that uses sound waves to produce 60 watts of electricity.

The core of this device is called TASHE, short for "thermoacoustic-Stirling heat engine." An acoustical version of a 19th-century engine design (named after Scottish minister Robert Stirling, who invented it), the TASHE is a looped contraption made from pipes and heat-exchanging devices.

In the TASHE system, intense, spontaneously generated sound waves (in the place of mechanical pistons in the 19th-century design) shuttle parcels of helium gas between a cold end and hot end. The hot and cold end temperatures are generated by connecting the engine to a high-temperature heat source and an ambient-temperature heat sink through the heat exchangers. Thermally driven expansion and contraction of the gas, in concert with pressure oscillations (induced by the temperature difference), intensify the power of the initial sound waves which become strong enough to drive a piston connected to the device. The motion of the piston vibrates a coil of copper wire that produces electricity as it moves relative to a permanent magnet.

The acoustic device has 18% efficiency, compared to 7% for thermoelectrics, the current electrical-generation technology in spacecrafts in which a temperature difference across a material is converted into electric power. (In both designs, small amounts of radioactive material provide the high-temperature heat needed for operation.)

The new device can produce a projected 8.1 watts of electricity per kilogram, as opposed to 5.2 watts/kg for thermoelectrics. These properties allow for a potential increase in the size and power of science instruments in future space probes.This is the latest application of the TASHE, which is also being developed to liquefy remote reserves of natural gas for a more economical transport of this fossil fuel resource to market than previously possible. (Backhaus, Tward, and Petach, Applied Physics Letters, 9 August 2004.)