Sound Waves Power Space Travel

A way has been found to provide power for deep space mission. A University of California scientist working at Los Alamos has developed a way to generate electrical power for deep-space travel using sound waves. The traveling-wave thermoacoustic electric generator could power space probes to the furthest reaches of the universe.

Scott Backhaus, working with colleagues from Northrop Grumman, designed a thermoacoustic system that is twice as efficient as similar thermoelectric generators on spacecraft and which uses heat from the decay of a radioactive fuel to generate electricity.

Current devices convert only 7 per cent of the heat source energy into electricity. The traveling-wave engine converts 18 per cent of the heat source energy into electricity.

The attraction of the device, now with the efficiency needed for space travel, is the fact that the only moving component in the device besides the helium gas itself is an ambient temperature piston. This gives it the reliability needed for long-distance space probes.

Traveling wave thermoacoustic heat engines convert high-temperature heat to acoustic power with high efficiency, without moving parts. Electrodynamic linear alternators and compressors have shown high acoustic-to-electricity transduction efficiency along with long maintenance-free lifetimes. Optimising a small traveling wave thermoacoustic engine for use with an electrodynamic linear alternator gave the Backhus team a traveling-wave thermoacoustic electric generator. It is a power conversion system, good for demanding applications such as electricity generation aboard spacecraft.

Thermoacoustics is the thermodynamic interaction of acoustics with solid surfaces that posses a temperature gradient. The oscillations of acoustic pressure generate heat transfers to and from solid surfaces while acoustic displacement oscillations cause the heat transfers to happen at spatially separate locations.

Time phasing of the pressure and displacement oscillations, the sign and magnitude of the temperature gradient in the solid and the location of that temperature gradient in the acoustic wave can all be used to create a variety of devices. These include standing-wave and traveling-wave heat engines and refrigerators. The gas undergoing the acoustic motion is the only moving component. The absence of moving parts lets you tailor the device geometry to a particular application and gives you the reliability.

The new device integrates a traveling wave thermoacoustic heat engine with a linear alternator to generate electricity from high-temperature heat. They used a flexure-bearing-supported linear alternator. It is composed of a stack of several spiral-cut circular metallic plates with a piston attached to its center. The stack forms an extremely stiff bearing in the radial direction, soft in the axial direction. This lets the piston move in its cylinder with a radial clearance as small as 10 micrometers. The stiff flexure bearing keeps the piston from touching the cylinder and the small clearance effectively forms a nonwearing seal that requires no lubrication.

A coil of copper wire attached to the piston oscillates with it. As the coil moves through a magnetic field generated by permanent magnets, the linear motion of the piston is transformed into electricity.

Converting high temperature heat to acoustic power is good where the acoustic power can be used directly, such as powering a traveling-wave thermoacoustic refrigerator. When it is converted to electricity through a linear alternator, interface requirements and size restrictions put extra demands on the engine that can significantly change its design and optimisation.

This made optimisation harder but still doable; you minimise mass and volume, because it is on a spacecraft, while maximising electric power output. They modified the thermoelectric engine to minimise the peak-to-peak stroke while increasing acoustic power output.

The traveling-wave engine is a modern adaptation of the 18th 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. It works by sending helium gas through a stack of 322 stainless-steel wire mesh discs called a regenerator. The regenerator is connected to a heat source and heat sink that causes the helium to expand and contract. This creates powerful oscillating sound waves that drive the piston of a linear alternator to generate electricity.

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