One day it may be possible to harvest the otherwise wasted energy of your footsteps or of cars driving by to power your smartphone.
That’s the concept researchers at the Georgia Institute of Technology are developing using what’s technically known as the triboelectric effect to create surprising amounts of electric power by rubbing or touching two different materials together.
Beyond generating power, the technology could also provide a new type of self-powered sensor, allowing detection of vibrations, motion, water leaks, explosions—or even rain falling.
“We are able to deliver small amounts of portable power for today’s mobile and sensor applications,” says Zhong Lin Wang, a professor in the School of Materials Science and Engineering. “This opens up a source of energy by harvesting power from activities of all kinds.”
How It Works
In its simplest form, the triboelectric generator uses two sheets of dissimilar materials, one an electron donor, the other an electron acceptor. When the materials are in contact, electrons flow from one material to the other.
If the sheets are then separated, one sheet holds an electrical charge isolated by the gap between them. If an electrical load is then connected to two electrodes placed at the outer edges of the two surfaces, a small current will flow to equalize the charges.
In a variation of the technique, the materials—most commonly inexpensive flexible polymers—produce current if they are rubbed together before being separated. Generators producing DC current have also been built.
“The fact that an electric charge can be produced through triboelectrification is well known,” Wang explains. “What we have introduced is a gap separation technique that produces a voltage drop, which leads to a current flow in the external load, allowing the charge to be used.
“This generator can convert random mechanical energy from our environment into electric energy.”
Power Shirts and Shoe Inserts
Since their first publication on the research, Wang and his research team have increased the power output density of their triboelectric generator by a factor of 100,000—reporting that a square meter of single-layer material can now produce as much as 300 watts. They have found that the volume power density reaches more than 400 kilowatts per cubic meter at an efficiency of more than 50 percent.
The researchers have expanded the range of energy-gathering techniques from “power shirts” containing pockets of the generating material to shoe inserts, whistles, foot pedals, floor mats, backpacks, and floats bobbing on ocean waves.
They have learned to increase the power output by applying micron-scale patterns to the polymer sheets. The patterning effectively increases the contact area and thereby increases the effectiveness of the charge transfer.
They can now produce current from contact between water—sea water, tap water, and even distilled water—and a patterned polymer surface. Their latest paper, published in the journal ACS Nano, described harvesting energy from the touch pad of a laptop computer.
They are now using a wide range of materials, including polymers, fabrics, and even papers. The materials are inexpensive, and can include such sources as recycled drink bottles. The generators can be made from nearly transparent polymers, allowing their use in touch pads and screens.
Beyond its use as a power source, Wang is also using the triboelectric effect for sensing without an external power source. Because the generators produce current when they are perturbed, they could be used to measure changes in flow rates, sudden movement, or falling raindrops.
“If a mechanical force is applied to these generators, they will produce an electrical current and voltage,” he says. “We can measure that current and voltage as electrical signals to determine the extent of the mechanical agitation. Such sensors could be used for monitoring in traffic, security, environmental science, health care, and infrastructure applications.”
For the future, Wang and his research team plan to continue studying the nanogenerators and sensors to improve their output and sensitivity. The size of the material can be scaled up, and multiple layers can boost power output.
“Everybody has seen this effect, but we have been able to find practical applications for it,” says Wang. “It’s very simple, and there is much more we can do with this.”
The US Department of Energy, National Science Foundation, National Institute for Materials Science in Japan, Samsung, and Chinese Academy of Sciences supported the work.