Sounding Off to Deep Space
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."
Scott Backhaus earned his doctorate in physics from the University of California, Berkeley in 1997. He is currently the Reines Fellow at Los Alamos National Laboratory.