It's happened to all of us before. We drag our feet across the carpet and reach to open the door, only to be zapped by the doorknob. Sometimes there's even a visible spark.
A team of Clemson University physicists at the Clemson Nanomaterials Institute (CNI) tackled this idea when they devised the ultra-simple triboelectric nanogenerator, or U-TENG. The device is designed to take mechanical motion — like the waves in the ocean, the tap of a foot or the clap of a hand — and transform it into electricity, which can then power lights or electronic devices, such as calculators, among other things.
The device takes a new approach on existing TENGs, which are constructed from materials new to the nanotechnology field, such as graphene, carbon nanotubes and a type of silicone known as polydimethylsiloxane, or PDMS. The use of novel materials seems inventive; however, the newness of these materials provides a challenge when transferring the TENG from the stage of research and development to mass production.
"The U-TENG is made of materials that are commercially available," said graduate student Sai Sunil Mallineni, the first author on a recently published article on U-TENGs in the journal Nano Energy. "We used the plastic from which drinking water bottles (polyethylene terephthalate, or PET) are made, and high-temperature tape (polyimide, or Kapton) to assemble a U-TENG."
"You take these two materials, make them electrically conducting by adding a nanometer-thick layer of indium tin oxide so that current can flow through and put a spacer between them. Then, you tape them together. That's it," said Ramakrishna Podila, a co-author of the study and an assistant professor of physics at Clemson. "That's the U-TENG."
PET plastic and Kapton tape make one of the more efficient combinations of materials in the triboelectric series, which is a ranking of materials based on their tendency to give off positive or negative charges. The farther apart two materials are in the series, the greater their static attraction when brought into contact.
CNI director Apparao Rao explained that when the tape and the plastic are repetitively pressed together, the tape has a natural tendency to pull electrons from the plastic.
The U-TENG takes five minutes to assemble at a cost of only 60 cents. It packs a punch by delivering a peak voltage of 500 Volts - enough energy to power more than 300 light-emitting diodes (LEDs). It can endure temperatures up to 140 degrees Fahrenheit. And since the materials are readily available, the U-TENG is amenable for production on a large scale.
"You need not go back to the drawing board or reinvent the wheel in order to harvest energy; you just have to find a technology, such as the U-TENG, and scale it," Podila said.
Once the U-TENG is assembled, it becomes a matter of supplying repetitive motion to bring the two materials into contact, transforming mechanical energy into electricity. When you tap your foot, for instance, the plastic comes in contact with the tape. When you lift your foot, the force is released and the electrons in the plastic and the tape redistribute between the two materials; however, the electrons don't distribute evenly.
The plastic becomes more positive than it initially was and the tape more negative, thus generating voltage. A power conversion circuit harvests the electrical current, which can be stored in a capacitor until needed.
"There's no battery in this setup," Rao said. "The U-TENG provides the power and its energy comes from converting mechanical motion into electricity."
"You can put it under walkways, and as people walk past, you can harvest energy," said Yongchang Dong, another graduate student co-author.
"You can also monitor the traffic flow. Every time a car drives over a U-TENG, a voltage is generated," said Sriparna Bhattacharya, a faculty researcher involved with the U-TENG project at CNI.
"You can even integrate this into a T-shirt, so that as you're walking, it's rubbing against your skin, there's wind blowing against you and you can use that energy to light up LEDs," Podila said.
"You can put this in your shoes and they'll light up if you're running in the dark," Rao said.
"Or it can be integrated into those flags that people put on their cars for tailgating, so that at night, only Clemson flags light up!" Podila said.
The technology seems too good to be true, but it gets even better. Herbert Behlow, a co-author of the study, emphasized that the U-TENG can withstand the test of time.
"The U-TENGs did not show any loss in performance up to 20,000 cycles - and in many cases its peak voltage actually increased with prolonged use," Behlow said.
The reason for this is the U-TENG becomes rougher with usage, thereby increasing its contacting surface area. The U-TENG's performance improves as the surface area increases.
The team at CNI is continuing its research to make a wireless TENG that can affect even more areas of the world's energy landscape. They're also applying for a patent to protect their invention before they begin mass-producing the wireless TENG.
"With the way that science is going, we are gaining so much technology, but that doesn't necessarily mean it's overlapping with the solutions we seek for the set of problems we have in society," Podila explained. "The problems are the same, but now we have more technology that is not directly relevant. In light of that, we took two technologies - PET and Kapton tape - that are already scalable, already existing and created a U-TENG for energy harvesting."
The team's paper, detailing the advent of the U-TENG, was published in the journal Nano Energy in March 2017. Startup funds for the study were provided by Clemson University under grant number X-1460320 and by the University's Watt Family Innovation Center (grant number 2301812.) The researchers at the Clemson Nanomaterials Institute are wholly responsible for the content of this study, of which the funders had no input.