This Power-Generating Shoe Isn’t Ready for Prime Time Yet, but This Kid’s Project is Still Pretty Cool

This is a video by a Angelo Casimiro, a 15-year-old Filipino participating in this year’s Google science fair. And he has seriously tweaked his shoes to do something cool: they spark. And I don’t mean spark like  those kids’ shoes that have stripes that dimly light as you walk that you really wanted to try but evidently you couldn’t wear because none of them could support your ankles (okay, that last part may not have applied to everyone else…). Angelo’s new shoes actually generate a little bit of electricity each time he takes a step. This is incredibly cool.

But just because I can, I’m going to bury the lede for a bit, because I want to contextualize this. Angelo did this as a test to see if it could work AT ALL, and he says he’s nowhere near a final product that you might buy. So before dreams of daily jogging to power your iPhone and laptop dance in your head, we need to look at the electricity we can create and how much we actually use.

Duracell’s basic alkaline non-renewable AA battery has a charge of about 2500-3000 miliampere-hours (mAh), which I estimated based on multiplying the number of hours it was used by the constant currents applied in the graphs on the first page here. The two basic rechargeable NiMH AA batteries have charges of 1700 and 2450 mAh. The battery in my Android smartphone has a charge of 1750 mAh, based on dividing the energy (6.48 watt-hours) by its operating voltage (3.7 V). Based on Angelo’s best reported current of 11 mA on his Google science fair page, it would take 159 hours to fully charge my phone. That’s nearly a week of non-stop running! (Literally! There’s only 168 hours in a week. You could only spend 9 hours doing anything besides running that week if you wanted to charge the phone, or replace one of the two AA batteries it takes to power my digital camera) However, I might be overestimating based on his averages. At around the 3:50 mark in the video, an annotation says that Angelo was able to charge a 400 mAh battery after 8 hours of jogging. That would translate to about 33 hours of jogging to charge my cell phone. No one I know would want to do that, but that is significantly less than jogging non-stop for almost 7 days.

But as Angelo points out, while you not be able to power your phone with his shoe, lots of sensors and gadgets that could go into smart clothes could be powered by this. In the video, he says he was able to power an Arduino board. An Arduino is a common mini-CPU board with extras people often use to make nifty devices, from how Peter Parker locks his room door in The Amazing Spider-Man movie to laser harps you can play by touching beams of light (note that the Arduino isn’t necessarily powering all the other components it is controlling in these cases), so you could potentially control smart clothes that respond to your moving.  A study by MIT’s Media Lab also looked at putting piezoelectric material in shoes and found they could power an RFID transmitter, which can be used to broadcast information to either devices. So perhaps your gym shoes could also act as your gym ID. The 400 mAh battery Angelo mentions is pretty close to the charge of batteries in small blood sugar monitors and over double the charge of some smaller hearing aid batteries.

But in relation to another recent science fair controversy, let’s put Angelo in context. No, he did not “invent” a new way to “charge your phone with your shoes“. Angelo himself points out that his work is more like a proof of concept than anything close to a product, and his numbers show you really won’t want to charge energy heavy devices with it. And MIT and DARPA, that branch of the US Department of Defense that funds crazy research schemes, have both looked at similar systems. (DARPA has looked at piezo-boots that could help power soldiers’ electronics.) Angelo and DARPA also both realize the limits of this: with our current materials, there’s only so much you can stuff into footwear before you run out of room or make it harder to walk. So instead, people have shifted to different goals for piezoelectricity: instead of having the material move with a single person who has to provide all the energy, we can place it where we know lots of people will walk and split the work. In Europe, high foot traffic areas have been covered with piezoelectric sidewalks to power lights, and in Japan, commuters walking through turnstiles in Tokyo and Shibuya stations help power ticket readers and the signboards that guide them to their trains.

Two distinct images. The left image shows a turnstile for ticketing. There is a black strip of material running through it. The right image shows a figure with an explanation in Japanese describing the power-generating nature of the strip.

Piezoelectric strip in ticket turnstile in Japanese subway station, from 2008

But none of this means that Angelo hasn’t done good technical work. It’s just that his effort falls more on the engineering side than the science side. Which is perfectly fine, because Google has categories for electronics and inventions and that other big science fair everyone talks about is technically a science AND engineering fair. Angelo’s shoe modification is posted on instructables and is something you could do in your home with consumer materials. The MIT Media Lab study still worked with custom-made piezoelectrics from colleagues in another lab. So the fact that Angelo could still manage to charge a battery in a reasonable (if you don’t need power right away) amount of time is incredibly impressive. And he also seems quite skilled at designing the circuits he used. As a 15 year old, he easily seems to know more about the various aspects of his circuit he needs to consider than I did through most of my time in college (granted, you didn’t need to know any particularly complicated circuity to be a physic majors). He’s definitely on to a great start if he wants to study engineering or science in college.

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What Happens When You Literally Hang Something Out to Dry?

I got a question today!  A good friend from high school asked:

Hey! So I have a sciencey question for you. But don’t laugh at me! It might seem kinda silly at first, but bear with me. Ok, how does water evaporate without heat? Like a towel is wet, so we put it in the sun to dry (tada heat!) but if its a kitchen or a bathroom towel that doesn’t see any particular increase in temp? How does the towel dry? What happens to the water? Does it evaporate but in a more mild version of the cycle of thinking?
It’s actually a really good question, and the answer depends on some statistical physics and thermodynamics. You know water is turning into water vapor all the time around you, but you can also see that these things clearly aren’t boiling away.

I’ve said before that temperature and heat are kind of weird, even though we talk about them all the time:

It’s not the same thing as energy, but it is related to that.  And in scientific contexts, temperature is not the same as heat.  Heat is defined as the transfer of energy between bodies by some thermal process, like radiation (basically how old light bulbs work), conduction (touching), or convection (heat transfer by a fluid moving, like the way you might see soup churn on a stove).  So as a kind of approximate definition, we can think of temperature as a measure of how much energy something could give away as heat.
The other key point is that temperature is only an average measure of energy, as the molecules are all moving at different speeds (we touched on this at the end of this post on “negative temperature”). This turns out to be crucial, because this helps explain the distinction between boiling and evaporating a liquid. Boiling is when you heat a liquid to its boiling point, at which point it overcomes the attractive forces holding the molecules together in a liquid. In evaporation, it’s only the random molecules that happen to be moving fast enough to overcome those forces that leave.
We can better represent this with a graph showing the probabilities of each molecule having a particular velocity or energy. (Here we’re using the Maxwell-Boltzmann distribution, which is technically meant for ideal gases, but works as a rough approximation for liquids.) That bar on the right marks out an energy of interest, so here we’ll say it’s the energy needed for a molecule to escape the liquid (vaporization energy). At every temperature, there will always be some molecules that happen to have enough energy to leave the liquid. Because the more energetic molecules  leave first, this is also why evaporating liquids cool things off.
A graph with x-axis labelled

Maxwell-Boltzmann distributions of the energy of molecules in a gas at various temperatures. From http://ibchem.com/IB/ibnotes/full/sta_htm/Maxwell_Boltzmann.htm

You might wonder that if say, your glass of water or a drenched towel is technically cooling off from evaporation, why will it completely evaporate over time? Because the water will keep warming up to room temperature and atomic collisions will keep bringing up the remaining molecules back to a similar Boltzmann distribution.
My friend also picks up on a good observation comparing putting the towel out in the sun versus hanging it in a bathroom. Infrared light from the sun will heat up the towel compared to one hanging around in your house, and you can see that at the hotter temperatures, more molecules exceed the vaporization energy, so evaporation will be faster. (In cooking, this is also why you raise the heat but don’t need to boil a liquid to make a reduction.)

There’s another factor that’s really important in evaporation compared to boiling. You can only have so much water in a region of air before it starts condensing back into a liquid (when you see dew or fog, there’s basically so much water vapor it starts re-accumulating into drops faster than they can evaporate). So if it’s really humid, this process goes slower. This is also why people can get so hot in a sauna. Because the air is almost completely steam, their sweat can’t evaporate to cool them off.

If Only Billy Mays Were Still Around

There’s been a bit of a buzz in battery research lately as chemists have made great strides in truly powering life by the “air you breathe“.  What on earth does that mean aside from being a pointless reference to infomercials I’m obsessed with?  (Aside:  This is actually a problem, I once watched the full half-hour Magic Bullet infomercial because I was bored).  While my previous post talked about researchers redoing a battery idea of Edison’s, this team at the University of Southern California was tinkering with a more  recent design:  “breathing batteries”.  Breathing batteries are basically powered by the rusting of iron by oxygen, though it seems the “breathing” is a bit of a misnomer since the journal article mentions the chemical reactions occurring in liquid (although a lot of literature still uses the term “air”).

Iron rusting actually produces a lot of energy.  If you’ve ever had one of those disposable hand warmers, odds are it was mostly filled with just iron filings and a few other chemicals to speed up the reaction.  But all the heat is coming from the iron corroding REALLY fast.  Iron-air batteries have been around for decades and became very popular during the 1970s energy crisis.  But like the Edison batteries, they fell out of favor when other battery chemistries proved to be more efficient.  Aside from oxygen rusting the iron, there’s a second reaction in the battery that takes charging current and produces hydrogen, and this could take up to half of the energy.  They’ve come back into vogue for similar reasons to the iron-nickel batteries:  the materials are abundant (and cheaper) and safe for both people and the environment.  The Department of Energy hopes improving their efficiency could make for reasonable energy storage in a shift to a renewable energy power grid.

Fine iron particles in the USC battery. Everything looks pretty under electron microscopy.

 

So what made the USC batteries so much better than before?  Pepto-Bismol.  Seriously.  The active ingredient of Pepto-Bismol, bismuth sulfide, was added to the iron electrode. The bismuth prevented hydrogen formation, and reduced the energy loss to only 4%.  It also helped improve how much energy the battery could hold and how quickly the energy could be released, both of which are important factors for storing energy meant for the power grid.

Thomas Edison Strikes Again

Chemists at Stanford have helped bring a battery designed by Thomas Edison into the modern age.  Like us, Edison was also interested in electric cars, and in 1901 he developed a iron-nickel battery.  In a case of buzzwords being right for a reason, the Stanford team used the same elements as Edison, but structured them on the nanoscale.  Edison’s original design sounds like it was essentially just one alloy of iron and carbon for one electrode and one of nickel and carbon for the other electrode.  The new battery consisted of small iron pieces grown on top of graphene (that wonderful form of carbon we’ve talked about before) for the first electrode and small nickel regions grown on top of “tubes” of carbon (which probably means nanotubes).

The new battery is 1000 times more efficient than traditional nickel-iron batteries, but the improvement means it only now is about equal to the energy storage and discharge abilities of our modern lithium ion batteries.  Although there’s lot of research being done on improving our lithium ion batteries, there are some unique advantages to the nickel-iron batteries.  For one, there’s a lot more iron and nickel than lithium, meaning the batteries could be cheaper.  Nickel-iron batteries also don’t contain any flammable materials, while lithium batteries are capable of exploding.  While the nickel-iron batteries might not appear everywhere, their inability to explode could be a boon to electric car manufacturers.