Wake Forest’s Power Felt a hot new use for body heat

Keeping cellphones charged longer is one possible use for Wake Forest’s Power Felt

CorrespondentAugust 26, 2012 

The annoyance and panic caused by a dead cellphone may soon be less of a headache, thanks to a swatch of felt and your hot body.

You don’t have be a physical specimen. All you’d have to do is hold – or even sit on – a piece of Power Felt to keep your phone charged 10 percent to 20 percent longer per day via your body heat. It’s one of many potential applications for the new thermoelectric device, developed by researchers at Wake Forest University’s Center for Nanotechnology and Molecular Materials.

It converts wasted body heat into electric current.

Power Felt could be used for lining automobile seats to improve battery power, or for insulating pipes or collecting heat under roof tiles to reduce gas or electric bills. It could also have monitoring applications: lining clothing or sports equipment to help determine performance, or wrapping IV or wound sites for tracking patients’ medical needs. Even the toy industry could see a big impact.

Perhaps most important, David Carroll, director for the Wake Forest center at Winston-Salem and leader of the Power Felt research, trumpets some lifesaving possibilities: “You could keep it with an emergency kit for powering a cell phone, a flashlight, a weather radio. This could be crucial during power outages or after accidents.”

How it works

Power Felt is made up of tiny carbon nanotubes locked in flexible plastic fibers and made to look like fabric. It creates a charge by using temperature differences, such as room temperature versus body temperature. This principle is known as thermoelectrics – currently the subject of extensive research within the scientific community.

Carroll gives an example. “If you grab one end of a bar of metal, the electrons that heat your hand become warm. As they warm, they seek out the cold spots, which would be the other end of the bar. So (the electrons go) rushing down to the other end of the bar.

“So I have an excess of electrons on one side, and a depletion of excess electrons underneath my hands, so I have a voltage between the two which is called the thermal voltage. And that’s what Power Felt generates.”

But what if there is no significant temperature difference? Not a problem, Carroll says: “Power Felt also has the ability to collect power from mechanical noise, any vibration that’s around it – the motion of the body or the vibration of a car.”

Because of the way Power Felt’s material is put together, he says, “current goes in only one direction. ... So random noise gives you a voltage and a current in one direction, which normal piezoactive materials don’t do – like quartz, where voltage changes when you change the direction of motion.”

Boosting the power

Graduate student Corey Hewitt, a key researcher on the Power Felt team, is trying to determine ways to add more nanotube layers and make them thinner to increase power output. This is especially important in Power Felt’s potential uses in clothing.

“When I started the project, I didn’t realize that it would have such practical applications down the road,” Hewitt says. “As I started looking into the materials and what we could use them for and how we could potentially incorporate them into clothing ... I started realizing how we could make the material lighter and more powerful.

“Even though they’re not as good as the best thermoelectrics, they have other qualities where they could be incorporated into the clothing that’s actually exciting that makes them really interesting materials.”

Among the possibilities: a jacket with a completely thermoelectric inside liner that is warmed by body heat – as the exterior remains cold from outside temperature.

The cost hurdle

Hewitt plans to continue with Power Felt research for at least a couple more years. Beyond that, his future with the project hinges on how quickly the product is able to be developed and produced commercially.

Much of the timetable for him and for Power Felt is tied to affordability. Standard thermoelectric devices use a more efficient compound called bismuth telluride to turn heat into power in products such as mobile refrigerators and CPU coolers, but with a cost of up to $1,000 per kilogram. Researchers theorize that with increasing demand for and use of Power Felt – made more inexpensively than devices with bismuth telluride – costs would come down.

“Let’s take the example of my car,” Carroll says. “It’s true that the thermoelectric fabric that we make has a ZT (a measure of thermodynamic efficiency) of around 0.2 or 0.3 that is really bad, but I can make it 100 times bigger than the bismuth telluride. So the effective ZT that I have is close to 3.

“In fact, if you manufacture what I make now, because it’s so inexpensive, because it is a fabric, the total cost of the power that is generated is roughly a dollar per watt. The reason that’s an important number is, that’s what the solar industry uses to say whether or not they’re producing power efficiently. I’m dong that with a ZT that’s below 3.

“It’s kind of bringing the business world to the physics world and having them shake hands,” says Carroll, adding that Wake Forest has already filed provisional patents. He says it may cost consumers only an extra dollar to add Power Felt to a cellphone cover.

Fun for toy lovers

Wake Forest researchers often take their findings to local elementary schools for hands-on interaction and learning. Carroll says young students’ input on Power Felt has contributed to its development, and the project has rekindled some of the researchers’ inner child.

“The first place you’re ever going to see (Power Felt) is in toys,” Carroll says. “Kids absolutely love this stuff because you can put it all places and get power. You run little motors and stuff. ... You can make little toys run. You can make little cars run around from a pad.

“Those are the fun things, because kids begin to imagine really wild places to start using this. ... they’re interested in how it works, but they’re more interested in what can they do with it. It seems to be a property of children: ‘Can I put it here, or can I make it do that?’ They kind of want to fiddle with it, and that’s great.

“There’s loads and loads of science behind this, but that’s not the fun part of Power Felt. The fun part of Power Felt is playing with it.”

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