In research reported in a recent issue of the journal Applied Physics Letters, Laboratory scientist Scott Backhaus and his Northrop Grumman colleagues, Emanuel Tward and Mike Petach, describe the design of a thermoacoustic system for the generation of electricity aboard spacecraft.

The traveling-wave engine/linear alternator system is similar to the current thermoelectric generators in that it uses heat from the decay of a radioactive fuel to generate electricity, but is more than twice as efficient.

The new design is an improvement over current thermoelectric devices used for the generation of electricity aboard spacecraft. Such devices convert only 7 percent of the heat source energy into electricity.

The traveling-wave engine converts 18 percent of the heat source energy into electricity. Since the only moving component in the device besides the helium gas itself is an ambient temperature piston, the device possesses the kind of high-reliability required of deep space probes.

The traveling-wave engine is a modern-day adaptation of the 19th century thermodynamic invention of Robert Stirling - the Stirling engine - which is similar to a steam engine, but uses heated air instead of steam to drive a piston.

Instead of high-pressure steam, temperature difference could drive his engine with impressive efficiency. Stirling's invention was novel because it had no potentially explosive boiler, but instead relied on cyclical gas expansion fueled by a heat exchanger.

What Stirling called an 'economiser' was a porous solid that stored some heat between cycles so that it could be returned to the gas during its heated phase.

The traveling-wave engine works by sending helium gas through a stack of 322 stainless-steel wire-mesh discs called a regenerator (akin to Stirling's porous economiser).

The regenerator is connected to a heat source and a heat sink that causes the helium to expand and contract. This expansion and contraction creates powerful sound waves - in much the same way that lightning in the atmosphere causes the thermal expansion that produces thunder.

These oscillating sound waves in the traveling-wave engine drive the piston of a linear alternator that generates electricity.

Backhaus told American Scientist magazine that "Faced with such [efficiency] losses say, from the resistance of the wires in a transmission line electrical engineers long ago found an easy solution: Increase the voltage and diminish the current so that their product (which equals the power transferred) remains constant."

"So we reasoned that if the oscillatory pressure could be made very large and the flow velocity made very small, in a way that preserved their product, we could boost the efficiency of the regenerator without reducing the power it could produce."

The use of traveling waves allows the engine to substitute wave action for prototypical piston and crank portions - a design sometimes called 'a pistonless Stirling engine'.

Standing waves of sound, move helium back and forth. Backous noted that "The device that was needed had to reproduce some of the attributes of a standing wave (high pressure and small flow velocity) while also having some of the attributes of a traveling wave (pressure had to rise and fall in phase with velocity, not with displacement)."

So if one imagined blowing across a long tube filled with helium, then the result might have some of the same characteristics of a high-pitched loudspeaker, but in this case resonating on a single main frequency and extracting work from each oscillation.

Backhous concluded: "In 2099, the National Academy of Engineering probably will again convene an expert panel to select the outstanding technological achievements of the 21st century."

"We hope the machines that our unborn grandchildren see on that list will include thermoacoustic devices, which promise to improve everyone's standard of living while helping to protect the planet."

"We and a small band of interested physicists and engineers have been working hard over the past two decades to make acoustic engines and refrigerators part of that future. The latest achievements are certainly encouraging, but there is still much left to be done."

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