Friday, February 02, 2018

Ask-a-Physicist: Pulling "Juice" Out of Thin Air

This week, Andrew from Quincy, WA wrote in to ask:

I'm writing a book, and trying to think of small-scale power sources—I want the ideas to be at least theoretically possible. Is it theoretically possible to slightly compress an atom to cause the electrons to vibrate? Also could that cause heat as well, and could you harness either of those to produce electricity?

Hi Andrew,
I love answering questions like this one—so often, it seems like the best way to learn about our own world is to invent new ones and try to make them run smoothly.

I can think of two things right off the bat that might fit the bill for you: Piezoelectrics and Peltier (pel-tee-yay) elements! "Piezoelectric" sounds fancy, but if you've ever used an electric/grill lighter (i.e. a click-button lighter, rather than one with a spark wheel) you've used a piezoelectric crystal!

When you push down the button on a grill lighter, you're really pulling back a tiny hammer inside the device; when it clicks, the hammer is being released and striking a piece of quartz crystal (or other piezoelectric material) that generates a short burst of very high voltage when compressed—they literally turn compression straight into electricity!

Although it takes thousands of volts to produce a spark this long, the extreme voltage exists just for a fleeting moment, meaning there's not as much energy in that spark as you might think.
Piezoelectric materials owe this unique property to the atomic structure of their crystals; they need to be asymmetrical in a certain way such that, when the atoms of the crystalline lattice are squeezed together, the concentration of electrons ends up higher at one end. Probably the neatest part, though, is that the reaction goes both ways! Applying a voltage to either side of a piezoelectric crystal will cause it to change shape a little; this is how quartz watches "transduce" an electrical signal into a mechanical one, and how many machines that use ultrasonic vibrations work.

Acoustic levitation is one of the many places piezoelectrics find applications.

In reality, the properties of crystals like that are much better for transducing relatively small signals; it'd be hard to get enough juice out of quartz to power something like a phone—and we know that for a fact, because the US military tried to develop the technology by putting piezo elements in soldiers' boots. Unfortunately, there's "no such thing as a free electron"—if you're pulling juice out of a troop's footsteps, it's making it harder for them to walk. That's not always a project-killer—I've been in plenty of circumstances where I'd have traded sore legs for a full cell phone battery—but in this case, it seems it was.

Fortunately for you, though, you're writing a story, so you can have whatever super-efficient metamaterials or "flexo-electrics" it takes to make this work, as long as there's a kernel of hard science at the center. That said, I'd advise you to stay within the bounds of thermodynamics, i.e. make sure the total energy going into your system is roughly equivalent to the amount coming out.

For example, a standard cell phone battery holds about ten watt-hours of energy, meaning it could theoretically power something that requires ten watts for an hour—or a standard sixty-watt lightbulb for about ten minutes—before running down. To generate that much power, you'd have to do about the amount of work it takes to climb twelve stories of stairs; we walk through the math on all this in our previous post on "People Power", which might be useful in helping you get a sense for these things. You can't have a drone flying on harvested acoustic energy, and if a laser is going to burn a hole in something, it's going to need an energy input roughly on par with what you'd expect it to take to drill or punch that same hole.

Peltier elements are another material with fascinating electromechanical properties—they turn a heat differential into electricity, or vice versa. They're generally flat sheets, and when you put a voltage across the faces, the heat migrates rapidly to one side—one face gets hot, the other gets cold. Just like with the piezoelectrics, this reaction works both ways—a Peltier element with one side against a person's skin and the other facing out into a cold atmosphere would generate a nice voltage difference between its wires—literally pulling electricity out of thin air! It's important to remember that heat itself isn't what generates the voltage, though, it's the heat difference between the faces.

The "Lumen" flashlight was a kickstarter project that claimed to be powered by body heat, using thermoelectric generators, but we're skeptical that it could generate a useful output—and the technology obviously hasn't caught on.
Image Credit: Lumen
On top of these, there's things like triboelectricity—better known as static electricity—which might be harvestable with an unobtainium coating on your boots, as well as more exciting new developments that take advantage of quantum effects to produce something similar to the piezoelectricity mentioned above.

There are a million and one creative ways to power small devices (keep an eye out for our upcoming piece on hygrobots) but hopefully this is more than enough to get you started!

Thanks for writing in!

—Stephen Skolnick

P.S. Inquiring minds can submit their own questions at this link. We don't have time to answer all of them, but if you're lucky, you'll see your answer featured here!

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