Cool Vibrations

If you think about it, all of engineering is a question of how energy is transformed from one form to another. Science can be said to occupy a slightly different (but overlapping) inquisitive geography, covering a lot of area to explain how anything works at all, but engineering is, at it's heart, how to make stuff do other stuff. In the land of mechanics, the goal is often to trick nature into converting some kind of time-domain phenomenon into the frequency domain, to transform natural forces into some reciprocating or rotating device to mimic the rhythmic thudding and pounding of our own bodies. Gasoline is exploded to turn driveshafts, water's toppled over a dam to make alternating current, uranium is pounded with neutrons to do either.

At its best, this sort of engineering can be elegant. In 1824, Nicolas Léonard Sadi Carnot published a famous thought experiment that reduced a heat engine, such as the steam engines that had been tinkered with for a century, to its very essence. In his minimalist picture of energy conservation, heat is transferred from one constant-temperature thermal reservoir to another, transforming some of itself into useful work in the process. (The devil's in the details of course--probably in a pissing match down there with Maxwell's demons--but Carnot had the insight that even when all of the heat operations are completely reversible, it's still impossible to get one hundred percent of that heat turned into mechanical work. This would be later reflected in the formal theorem and the textbook thermodynamic cycle that bears his name, and from there proceed to bore ten generations of engineering students to tears.) The opposite of a heat engine is a heat pump. In this picture, work is added to the device to move heat from hot to cold--you need energy to move it uphill. Carnot's engine outlines the minimum legal moves, assuming each step occurs on level ground, but since heat can flow either way at those constant-temperature steps, turnign a Carnot prime mover into a heat pump is easy. They work in the real case too, and end up subject to the same theoretically mandated inefficiencies.

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That's heat, but what about work? I mean, it's mechanical energy, but it's not all that useful taken straight from the tap. Human-sized work, power on-demand, always consists of something clumsily stroking back and forth or ungracefully spinning round and round. It doesn't really do it for me. There are some nice-looking machines out there, to be sure, but to me, elegance implies some level of minimalism, a sort of Platonic expression of beauty that is only as complicated as it needs to be. I find it hard to grant such a standard to some chortling, vibrating motor, wires and vacuum lines dangling like dreadlocks, that insists on screeching its gears and clearing its pipes at me to clamor for the center of attention.

Turning heat into reciprocating work has been inelegant for centuries. About the time that Carnot was monkeying with proto-thermodynamics, a minister in Scotland was building something like the Frenchman's pure heat engine. Stirling's device had gas moving back and forth between a hot and cold cylinder, each one equipped with a piston and timed so that the gas would be in the right place to push the hot cylinder open as it expanded and pull the cold one closed as it contracted. Visualizing the actual timing is kind of like those spatial IQ tests, and isn't overly interesting really, (you can find out more than you want to know here, as well as a hundred other places), however it gets more interesting once you start making analogies. In 1979, Peter Ceperley realized that the time lag between the pressure and motion cycles of a gas inside a Stirling engine was the same as in a traveling acoustic wave. Pieces of this had actually been known for a long time: glassblowers, for instance, realized that their tubes would start to hum when the hot ends were being worked, and pulse tube refrigerators, which work on similar principles, had been around for awhile too, but it's safe to say that Ceperley and German researcher Nikolaus Rott started the modern study of thermoacoustics. Turning heat directly into sound, it turns out, can be both efficient and useful, able to accomplish the same things that a mechanical heat engine or heat pump could do without all the grubby rigamarole. Thermoacoustics is elegant.

Consider an acoustic wave: unlike a wave on a string or on water, it doesn't move the medium up and down, but squeezes it in and out as the sound energy passes through. If you take the point of view of a tiny chunk of matter that's caught along in the disturbance, it will also move back and forth a little bit as it compresses and expands. If you could furthermore put a tiny thermometer in that element, you'd find that, just as with the gases you learned about in high school chemistry, there's a tiny increase in temperature as it's compressed and a tiny cooling effect as it expands. Normally, this amounts to not a goddamn thing (except for maybe a way for intense sound waves to dissipate energy), but if you can harvest those parcels of heat--remove the heat of compression when the gas is over here, add some heat to the cooler gas when it's over there, then you've used acoustic work to carry heat over a small distance. Start counting up the infinitude of those tiny elements, and you've got a heat pump on a useful scale, and it only consists of an acoustic source (a speaker, basically), a tube, and a surface (a porous material with lots of surface area) to take care of that heat transfer.

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Your home air conditioner or refrigerator has a great big compressor that condenses a gas to a liquid, pumps it through some plumbing where it evaporates, absorbing heat, and then carries it back to the beginning to be dropped off before the fluid is condensed again. Like an AC circuit with jiggling electrons, an acoustic chiller doesn't actually have to pump material all the way along the whole loop, but is content to rob the energy from those short-distance oscillations. Your normal compressor, the heavy bastard in your fridge say, doesn't shrink down well, it doesn't run at variable speed (nothing like hearing it switching on and off all night long), it's got sliding seals that leak and age, and it's full of environmentally nasty refrigerants. Most of the desire to develop alternative refrigeration strategies like thermoacoustics has arisen from the desire to eliminate ozone-depleting CFCs and HCFCs. (Thermoacoustic devices typically use helium. Check out what Ben and Jerry are doing with it.) Thermoacoustic refrigerators don't outperform the old models yet (about 20% of the ideal efficiency as opposed to 35%), but they should at some point. Some versions of these devices have been predicted to approach Carnot's limiting performance.

We can go more elegant than this even, and build a device with no moving parts whatsoever. You can build a device that is both a heat engine and a heat pump. One end generates an acoustic wave from a heat source, while the other end consumes the sound energy to perform cooling, there is no driver. Heat-driven coolers have a few niche applications. In one, Praxair is working on liquefaction of natural gas using enormous, barn-sized thermoacoustic coolers, gas-fired on one end, with liquid propane dripping off the other. I'm more enthusiastic about the impacts this sort of technology could have in the developing world, where centralized power production is unreliable. English researchers just got 4 million dollar grant to develop portable heat-driven coolers (the figure is cribbed from October's Popular Science). One end would run off a cooking fire, the other would preserve your food. Siphon off some more of that energy, and you could even generate electrical power in between. I want one.

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It's cool stuff (so to speak) and in case you wondered why I'd been going on for months about thermodynamics, it's because I've been dabbling.

Additional links for free further reading:
Los Alamos National Labs do some of the best work in this field. (Greg Swift is one of the biggest names. Check 'em out.)