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.
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.