Scottish minister Robert Stirling developed an engine in the 19th Century that used heated air instead of steam as the motive force for a piston engine. Now an acoustical version of the principle has emerged. As described in an article in a recent issue of Applied Physics Letters, a joint team from Los Alamos National Laboratory and Northrop Grumman Space Technology have created TASHE — the “thermoacoustic-Stirling heat engine.”

The work of LANL scientist Scott Backhaus and Emanuel Tward and Mike Petach from Northrop Grumman, TASHE would be used to generate electricity aboard spacecraft, and would be quite a step up from the thermoelectric devices now used, which convert roughly 7 percent of their heat energy into electricity using heat from the decay of a radioactive fuel.

By contrast, TASHE converts up to 18 percent of its heat source energy into usable electricity. The expansion of helium gas inside the engine drives the process, as described in a recent issue of Physics News Update:

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 upshot: the size and power of instruments in deep space probes can be increased, a critical factor given the tight resources available to spacecraft designers. Evaluate TASHE in light of Project Prometheus, a scheme that would create compact reactors to fly on a new generation of robotic space probes, including the Jupiter Icy Moons Orbiter (JIMO), which would depart no earlier than 2012 and rendezvous with Europa, Ganymede, and Callisto.

Sources: Press release from DOE/Los Alamos National Laboratory via EurekAlert 16 Sept. 2004; “Acoustically powered deep-space electric generator,” by Phil Schewe and Ben Stein, in Physics News Update No. 695, August 4, 2004. The original paper is S. Backhaus, E. Tward and M. Petach, “Traveling-wave thermoacoustic electric generator” in Applied Physics Letters, Vol. 85, Issue 6, pp. 1085-87.