Did you know? The average person radiates about 100 W of heat from their body. The amount of energy used for residential and commercial lighting in the US is 170 W per person.
A Canadian teenager has won a $25,000 scholarship at the Google Science Fair for inventing a flashlight powered by the heat from your hand. If body heat is such a convenient energy source, then why don’t we power everything that way? After all, heat is one of the most abundant forms of energy. Unfortunately, it’s also a form of energy that’s very hard to put to work.
You probably already knew that heat is a form of energy. Everything around you—anything with a temperature above absolute zero (-273 degrees Celsius)—has some heat energy.
Unfortunately, all that heat energy doesn’t do you any good (besides, you know, keeping you from freezing to death) because you normally can’t get at any of it. When an object (say, a chair) is at the same temperature as its surroundings (such as the air in the room), there’s no way to capture its heat energy and put it to work.
Peltier tiles explained
A Peltier tile consists of two ceramic tiles placed on top of each other. In between, pieces of conductor are arranged so that when one ceramic tile is hotter than the other, the thermopower of the pieces of conductor all add together to maximize the amount of energy produced. It's a little like a sandwich, where the ceramic tiles are slices of bread and the pieces of conductor are the meat. Click on image to enlarge (Eric Mills
To get work out of heat, there needs to be a temperature difference. More precisely, you can get work out of heat flow. As heat energy moves from one object to another, some of it can be siphoned off to do your bidding.
Did you know? The thermopower effect was discovered in 1821. However, electrons weren't discovered until the late 19th century, and the effect wasn't fully understood until the mid 20th century.
This is essentially what Ann Makosinski, a 15-year-old from Victoria, BC, did when she built a body heat-powered flashlight. Because your hand will be warmer than the surrounding air on all but the hottest days, there’s usually heat flow between the two.
An object’s ability to produce electricity from heat flow is called thermopower. Electric current flows when electrons move in one direction through a conductor. When a conductor is hot, the electrons tend to spread out. When a conductor is cool, the electrons tend to move closer together, or condense. So if one end of the conductor is hot, and the other end is cool, the electrons will move away from the hot end and toward the cool end, producing a current.
A cleverly-designed circuit will keep the electrons moving for as long as there is heat flow. In the case of Makosinski’s flashlight, she used a device called a Peltier tile to convert heat flow into electricity. Even so, the heat flow from your hand to the air doesn’t provide much power.
So Makosinski used a transformer to boost the voltage and very efficient LED lights to make the most of what power she could wring out of the Peltier tiles.
The amount of electricity you can produce by harnessing the flow of heat from your body to the air around you may not be very large, but it could be enough to charge iPods, cell phones, and various types of wearable computers that are currently being developed. Who wouldn’t want to live in a future where you never have to plug in your phone?
Scholarly publications and textbooks
- Ashcroft NW, and Mermin ND. 1976. Solid State Physics. Thomson Learning.
- Knight RD. 2004. Physics for Scientists and Engineers: A Strategic Approach. Pearson Education.