The plastic gizmo in Zhong Lin Wang’s hand doesn’t look like tomorrow’s solution to our looming energy crisis. It’s about the size and shape of a small grapefruit, but smooth and translucent. As he shakes it, a smaller ball inside bounces around freely.
“If you’re out of power, you’re out of everything,” says Wang, speaking in a fierce whisper that demands listeners lean in. He stands perfectly still, but the shaking makes the interior ball clatter around like a frustrated piece of popcorn. In his other hand, Wang holds a small circuit board with a blinking LED light in the middle. A wire connects the plastic sphere to the light. The more he shakes, the louder the clatter, and the faster the white light blinks on and off.
We're in a windowless basement room on Georgia Tech’s Atlanta campus. A trio of fresh-faced researchers stand nearby in white lab coats, watching and smiling. One holds a keyboard, and another a piece of red and yellow fabric.
“In our environment, everything is moving, everything is changing,” Wang says, still shaking. “It’s all energy, and so much is wasted.” He wants to do something about that. For the last decade and a half, Wang, an electrical engineer and nanotechnologist, has sought ways to scavenge energy from the movements of ordinary life.
His timing couldn’t be better. The energy problem is big: We need power in large doses to keep our cities lighted and cars running, and we need electricity in small doses — lots of them — to recharge batteries in our phones, fitness trackers and tablets. Those demands have a cost. Last year in the United States, about two-thirds of the total energy demand required burning fossil fuels like coal and natural gas, a process that releases carbon dioxide and other greenhouse gases into the atmosphere, where they’re reshaping the climate.
Renewable power sources, including sun, wind and water, provided another 17 percent or so of total energy demand. But harnessing the forces of nature involves challenges that are formidable — and currently unsolved. Even the bike lights and elliptical machines that convert exercise into electricity need a lot of OOMPH to work.
Instead, Wang is pioneering an engineering effort to generate electricity with a small oomph. Like from footsteps. Or raindrops hitting a car. Or the effort required to press keys on a keyboard. Or the small vibrations of a shirt, worn through the day. These ordinary motions, and others, could charge our devices and light our homes.
Built into that plastic sphere in Wang’s hand is a kind of generator that uses cheap, readily available materials to produce a current. The concept is simple, but it’s the kind of engineering simplicity that nevertheless requires decades of research and trial and error, and error, and error, and error. Such a generator, Wang says, can enable that keyboard to harvest energy from keystrokes, or turn clothing into a mini power plant.
For the last decade and a half, Wang, an electrical engineer and nanotechnologist, has sought ways to scavenge energy from the movements of ordinary life.
Wang’s idea is new in the sense that researchers have only begun to explore and understand it, but in another sense, it’s quite old. He uses what is called the triboelectric effect. You already know about triboelectricity, if not necessarily by name. It’s how we explain why clothes stick together after tumbling inside a dryer, or why unexpected shocks zap us in the winter.
Triboelectricity’s more common name is static electricity.
The “triboelectric effect” describes what happens when two dissimilar materials rub against each other and exchange charges, leaving one more positive and the other more negative. (Tribo- comes from the Greek word for “to rub.”) It’s the spark that flies from your fingertip to the doorknob after you shuffle across the carpet in socks on a cold, dry day.
“The idea is to harvest those sparks,” says micro-engineer Jürgen Brugger of the École Polytechnique Fédérale de Lausanne, in Switzerland. He began researching energy-harvesting schemes using triboelectric materials about two years ago, after hearing about Wang’s work.
The ancient Greeks observed that after rubbing a piece of amber with animal fur, the hardened tree sap would attract dust and other small particles. The word electric, coined by Elizabethan scientist William Gilbert, speaks to these origins: It traces back to elektron, Greek for amber. Schoolteachers use the same amber-on-fur demonstration to introduce the fundamentals of electricity, showing that two rubbed amber rods will repel each other. Bored kids at birthday parties rub their heads with balloons to make their hair stand up, and to get the balloons to stick to walls.
The marvel of static electricity once seemed a promising way forward in the great electrification of the world. In 1663, Prussian scientist Otto von Guericke, who was also the mayor of Magdeburg, generated eerie yellow sparks by rubbing a spinning sulfur ball with his hands. His invention is often recognized as the first electrostatic generator, and some Magdeburgians reportedly believed their mayor capable of magic. In the following centuries, people used electrostatic generators for a wide variety of sometimes dubious applications, from “electric baths” as medical treatment for movement disorders and lead poisoning, to electrifying — some might say electrocuting — plants.
Triboelectricity’s glow eventually faded. In 1831, British physicist Michael Faraday unveiled the first electromagnetic generator, which uses a moving magnet to induce an electric current in a coiled wire. That changed everything. Today, the generators in coal plants, wind turbines, nuclear power plants and hydroelectric dams — basically anything that works by converting physical movement into electricity — has an electromagnetic generator at its heart.
Only photocopiers still make use of static electricity, in the form of distributed charges to direct ink on paper. For the most part, it’s been punted to the status of an everyday nuisance that falls somewhere between mildly annoying and extremely dangerous. We go down plastic slides and get shocked on the dismount; we’re told not to use cellphones or sit in cars when pumping gas because stray charges can spark fumes. Lightning, the most violent display of static electricity, kills dozens of people every year in the U.S.
Until 2010, Wang barely gave a second thought to static electricity. He never meant to spark an energy revolution. But what he calls a happy accident in the lab revealed that triboelectric materials could produce big voltages, setting the scientist on a path to harvest them.
Early in his career, Wang was motivated by the allure of discovering new materials and new phenomena, “regardless of if they had an application,” he says. But that outlook changed in the late 1980s, when he started working at Oak Ridge National Laboratory in Tennessee and saw scientists using new materials to solve real-world problems. By the time he moved to Georgia Tech in 1995, where he’s been ever since, his work had a clear purpose. “I only wanted to study materials that really had a benefit,” he says. His new projects always begin with the same question: What can we use this for?
In 2005, Wang focused his lab on designing devices that could power themselves. He worked with piezoelectric crystals, which generate sparks when they’re bent, compressed or otherwise deformed. They were first identified by Marie Curie’s husband more than 100 years ago, but the materials tend to be brittle and hard to work with.
Eight years ago, Wang and his graduate students were testing a device, a sort of electric sandwich made of thin slices of piezoelectric materials. The engineers were having trouble removing all the air gaps between the layers, which they assumed would hamper the electric flow of the device. When they tested the design, however, they recorded a higher voltage — three to five times higher — than they expected.
“We thought it had to be an artifact of the testing,” Wang says, referring to experimental error. It turned out some air gaps remained, which meant that something other than the piezoelectric effect was responsible. The team realized the voltage must result from charges exchanged when the materials rubbed together: static electricity. That realization was a defining event in Wang’s research.
It Doesn’t Take Much
By 2012, Wang’s group had developed the first triboelectric nanogenerator (TENG). Despite the diminutive-sounding name, the generators range in size from a few millimeters up to a meter; the “nano” refers to the scale of the charges. Since then, Wang’s lab has designed and tested dozens of potential applications for these energy-harvesting devices. He’s also motivated multiple groups and thousands of researchers around the world to build their own applications. Ideas for workable TENGs range from paper-based audio speakers that charge while folded up and tucked in a shoe, to generators that convert the mechanical rise and fall of a breath to power a pacemaker.
A TENG relies on the same principle as static electricity: When two different materials come into contact, electric charges can accumulate on one, leaving the other with the opposite charge. In the case of that plastic sphere in Wang’s hand, charges accumulate when the interior and exterior balls touch and separate, over and over. Attach electrodes and wires to the oppositely charged materials, and current flows to correct the imbalance. It won’t be a big current, but many applications don’t need much.
Most researchers agree that triboelectric generators have the most potential when it comes to powering small devices, like phones and watches, but Wang wants to go big. His team recently took a few dozen of those plastic spheres to a neighborhood swimming pool — after hours — and set them loose to oscillate in the ripples. Even the slightest bobbing produced enough energy to power small lights or devices. Their calculations suggest that a grid of 1,000 spheres, floating freely in the ocean, should generate enough power for a standard lightbulb. A grid measuring about a third of a square mile could power a small town.
Wang doesn’t want to stop there; he sees the potential for a wealth of untested possibilities. Imagine a matrix of these spheres covering an area of the ocean equal to the state of Georgia and extending about 30 feet down. That’s about a quadrillion spheres.
“If we use this,” he says, in his demanding, fierce whisper, “the power generated is for the whole world.”
The Triboelectric Wave
Research on TriboElectric NanoGenerators (TENGs), which exploit everyday static electricity to power devices, extends beyond the lab of Zhong Lin Wang.
“A lot of research groups worldwide, from academics and industry, are rushing to TENG research for self-powered internet-of-things sensors, electronics and healthcare applications,” says electrical engineer Sang-Woo Kim, a professor at South Korea’s Sungkyunkwan University.
In response to Wang’s initial research, Kim’s group was the next to start pursuing TENGs. In 2015, they introduced a material that uses triboelectric threads — clothing made from this material can charge a smart watch after only a few hours of being worn. In 2017, they followed up with a stretchable TENG-based fabric. The paper, published in ACS Nano, discussed the relative power-generating merits of knitted and woven textiles.
Ramakrishna Podila of Clemson University has been developing these technologies for four years. He recently unveiled a TENG-based wireless energy generation system that uses PLA, a common biodegradable polymer, as one of its electrodes. In lab tests, they found that it can charge another device through the air up to 16 feet away.
Micro-engineer Jürgen Brugger’s group, in Switzerland, has been developing hybrid generators that combine triboelectric and piezoelectric materials. (Piezoelectric materials generate current when bent or deformed.) “If one wants to get the maximum energy out of any piece of a device, one should combine these different harvesting mechanisms,” he says.
Nelson Sepúlveda at Michigan State University shares Wang’s vision of the world as being rich with wasted, harvestable energy. In late 2016, he took the idea further by designing a FENG — a ferroelectret nanogenerator. It works basically the same way as the TENG, except you wouldn’t need to do anything to create a charge; the materials could already have electric charges built in. When the charged materials press together, the electric charges shift around, creating an imbalance, which produces a current.
Sepúlveda’s group has used FENGs to create a Michigan State flag that harvests energy by flapping in the wind — it can then double as a loudspeaker that plays the school’s fight song. It could also work in the other direction, as a microphone. Like Wang’s group, they’ve also designed a keyboard that harvests the energy of keystrokes using static electricity.
Triboelectricity suggests a clear way to solve existing energy challenges with materials. “If you don’t need a new material, why invent one?” muses Ramakrishna Podila, a physicist at Clemson University in South Carolina. And that solution could soon come to a gadget near you.
In China, Wang’s startup company, NairTENG, is already selling triboelectric-powered air filters, with plans to release TENG-based shoes — with ports to charge your devices — in the next two years. Soon, it’ll be possible to recharge your phone’s battery with a gentle stroll. Triboelectric devices could show up in the U.S. within five years, Wang predicts.
Like many new technologies, however, the success or failure of triboelectrics as a major energy source depends on how well its applications can scale up and endure conditions messier than a pristine lab. Wang’s plastic spheres would need to be durable enough to withstand the elements, and be specially designed not to interfere with marine life. Plus, it’s not clear they could be produced in the massive numbers Wang’s dreams require.
Some researchers aren’t even convinced there’s much of a future for triboelectrics beyond portable devices. But perhaps the biggest open question hanging over TENGs is why they work at all. High school physics teachers and college professors tell students that the materials exchange charges, citing terms like electron affinity. But in reality, says Podila, scientists don’t really understand why those charges move. Some physicists think individual charged particles like electrons jump from one material to another; others argue that entire charged molecules, called ions, do the jumping. Still others suggest that tiny fragments of one material break off on one another, taking their charges with them.
“The fundamental science is largely unknown,” says Podila. While not a problem now, a failure to understand the basics could hamper scientists’ efforts to make more efficient energy harvesters and contribute a solution to the world’s energy crisis.
Wang agrees that understanding why static electricity works is a critical step in producing the technology, but he thinks that’s a surmountable obstacle. He has no doubts about its potential.
The world has spent nearly 200 years developing electric tools that exploit Faraday’s ideas about electromagnetism, turning motion into electricity. For Wang, triboelectricity as an energy source is a newborn: “This is just the beginning."