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Batteries not included: How small-scale energy harvesting will power the future

Canned heat: Thermoelectric energy harvesting

If harnessing energy from radio waves and vibrations seems a might far-fetched – did the Jetsons ever do that? – try this notion on for size: Power from body heat.

Yet that’s precisely what they’re working on at Wake Forest University’s Centre for Nanotechnology and Molecular Materials. The technology is called “thermoelectric,” and the Wake Forest approach to thermoelectric is labelled “Power Felt.” Seems that merely by touching this mysterious fabric, body heat is converted into electric current.

Next they’ll tell us the moon’s not made of cheese.

So, what exactly is this Power Felt? For starters, it really does look like fabric. Futuristic fabric, sure, but fabric nonetheless. Made of carbon nanotubes locked in flexible plastic fibers, it can seemingly be “wrapped” around virtually anything. The folks at Wake Forest call it “wearable power.” And then they jump in their spaceships and fly off to their home galaxy.

Power Felt inventor David Carroll thermoelectric energy harvestingWe’ll let PhD-bearer David Carroll, Professor of Physics at Wake Forest and inventor of Power Felt, detail the intricacies.

“Our materials work the way any thermoelectric module works. Imagine that you hold in your hand a metal bar. You grasp it tightly at one end of the bar while the other end is free. Now the electrons that make the metal up are free to move, and your hand is heating them. So under your hand the electrons move more rapidly than say the electrons at the other end of the bar. This means that these electrons will spread out quickly and move away from the heat source. By moving to the cold end, they leave behind a deficit of electron in the hot end. They create a surplus of electrons in the cold end. This establishes a voltage, called the thermoelectric voltage, and as long as there is a difference in temperature, this voltages exists.”

Picture a rechargeable battery that depends on “regular” recharging but is potentially also topped up by an amalgamation of energy harvesting alternatives, and you begin to get an idea what the future might hold”Trouble is that eventually the other end heats up because of the thermoconductivity of the metal. By using lots of nanofibers in a plastic matrix, instead of the metal bar, the electrons can still move down the metallic pathways of the fibers, but the heat is blocked because it is not transported across the junctions from fiber to fiber. This is how we have made our fabrics. Our ‘metal’ fibers are carbon nanotubes.  And within the fabrics are layers upon layers of electronic nanofibers, allowing for electrons and holes to flow freely.”

“Imagine,” says Carroll, “reducing the charging time for your hybrid car because the heat reclamation comes from the cooling engine, or the passengers inside.”

Carroll extols the virtues of his invention but he’s just as realistic. It will not, he says, replace batteries. Not yet anyway. Nor will it work unless “large areas of temperature gradient exist.” The human body is a workable spot. So too is the hood of a car or the seats in an airplane.

“It will make the use of less expensive batteries more attractive from a market perspective. Generally, for market insertion, you don’t want to change too much too quickly, so it will be coupled with existing battery technologies. Your cell phones will last longer on one charge. A passenger jet may be able to use smaller internal generators, saving weight and money.”

grad student corey hewitt with power felt thermoelectric energy harvestingThough Power Felt will never run an electric car or energy-hungry appliances like refrigerators (Carroll tells us a square centimeter produces “nanowatts to tenths of microwatts, depending on thickness”), it will, apparently, be quite capable of augmenting current power structures in such applications. “Imagine,” says Carroll, “reducing the charging time for your hybrid car because the heat reclamation comes from the cooling engine, or the passengers inside.” 

Carroll also looks to house construction, saying Power Felt could conceivably take the place of Tyvek house-wrapping to “generate as much power an inexpensive solar array.”

Carroll makes a case for Power Felt in the mobile world too, suggesting a swatch of it could be included in the covering of batteries at the point of manufacture. By merely placing said batteries on “something warm,” they would partially self-charge. But once again, thus far anyway, it’s a matter of augmenting rather than supplanting regular battery charging. Carroll asks, “Ever been at the airport and had your phone run out of power? Wouldn’t it be nice to make that one last call for someone to pick you up? The power from your body’s heat could do this.” 

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