A Candle-Powered iPhone?
If you’ve been reading Kibbles and Bytes over the past few months, you might have noticed I’ve written more than once about power and electricity. A few weeks ago, I wrote about “*generating electricity and how watts work.*”:http://blog.smalldog.com/kibbles/kb867/ In that article, I mentioned that I’ve experimented with thermoelectric electricity generation and I thought this week I’d explain a bit about what that is and how it works. Spoiler alert: it’s pretty cool!
Sometimes it can seem like our iPhones are just electricity black holes. Sure, they last a pretty long time, but they still need to be charged a lot. This problem has spawned a whole line of products to help charge our devices when a wall outlet isn’t available. Most of these that I’ve seen are based on solar. We even carry a number of solar-based charging systems by “*Goal Zero*”:http://www.smalldog.com/category/?mmfg%5B0%5D=Goal+Zero. These are really cool products, and they work really well, but you probably already know the catch: Without sunlight, all they can do is look nice.
With this in mind, I set out to see if I could use thermoelectric generation to generate power on demand. Thermoelectric generation uses something called the “*Seebeck Effect*”:http://en.wikipedia.org/wiki/Thermoelectric_effect. When you have two dissimilar metals joined together in a loop, a temperature difference between the junction points will create a current. You can use any two metals, but modern devices use a P-N semiconductor junction. These tiny semiconductor pieces are small (sometimes only about 1/8″ cubes) so dozens (or more) are linked together to form a thermoelectric module. When you apply heat to one side, and cool the other side, the module generates electricity.
Could you use one of these to charge something like an iPhone? Yes and no. These modules are typically only 5%-8% efficient, so you need to work really hard to get appreciable power from them. In my design, I used a 5-watt module and a tea light candle. 5 watts is enough to charge a smartphone, but to get that power, the hot side needs to be about 300C and the cold side would need to be around 25C. That’s a hard differential to create, and I was never able to do quite that well.
There are things you can do with that low power though, and having it on-demand allows it to be useful in ways solar panels can’t be. One thing you can do is use your own body heat to power an LED flashlight. How is that possible? LEDs don’t need a lot of current, but they do need a few volts. The small voltage generated from the heat of your hand can be boosted to drive the LEDs. Thermoelectric generators also power deep space probes. Out of reach of the sun’s rays, they use heat from “*radioactive isotope decay*”:http://world-nuclear.org/info/Non-Power-Nuclear-Applications/Transport/Nuclear-Reactors-for-Space/ to drive the generators. One final application is in waste heat energy harvesting. Thermoelectric generators are used to capture waste heat energy from industrial processes to improve energy efficiency.
Obviously you can’t use radioactive isotopes to generate your power, but a few companies have developed thermoelectric generators for the consumer market. The most famous is probably the “*Bio-Lite camp stove*”:http://www.biolitestove.com/. They claim it can charge an iPhone, which is probably true, but I expect it takes a very, very long time. Since it’s a stove, you also can’t use it indoors. Another company, Tellurex, has a device you can run with a tea light candle (like my design) called “*t-POD*”:http://buytpod.com/tpod-catalog/enviro-light/. I actually bought that one to try, and I have to say, it works really well. It comes with a bright LED light it powers, but I’ve plugged in other small circuits as well.
For now it looks like we’re mostly stuck with solar when it comes to powering our devices without a wall outlet, but we might see that change in the near future.
