With one stomp of his foot, Zhong Lin Wang illuminates a thousand LED bulbs – with no batteries or power cord.
The current comes from essentially the same source as that tiny spark that jumps from a fingertip to a doorknob when you walk across carpet on a cold, dry day. Wang and his research team have learned to harvest this power and put it to work.
A professor at the Georgia Institute of Technology, Wang is using what’s technically known as the triboelectric effect to create surprising amounts of electric power by rubbing or touching two different materials together. He believes the discovery can provide a new way to power mobile devices such as sensors and smartphones by capturing the otherwise wasted mechanical energy from such sources as walking, the wind blowing, vibration, ocean waves or even cars driving by.
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. The research has been supported by a variety of sponsors, including the National Science Foundation; U.S. Department of Energy; MANA, part of the National Institute for Materials in Japan; Korean corporation Samsung and the Chinese Academy of Sciences. The research has been reported in journals including ACS Nano,Advanced Materials, Angewandte Chemie, Energy and Environmental Sciences, Nano Energy and Nano Letters.
“We are able to deliver small amounts of portable power for today’s mobile and sensor applications,” said Wang, a Regents professor in Georgia Tech’s School of Materials Science and Engineering. “This opens up a source of energy by harvesting power from activities of all kinds.”
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.
By continuously repeating the process, an alternating current can be produced. 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 explained. “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.”
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.
Wang and his team accidentally discovered the power generating potential of the triboelectric effect while working on piezoelectric generators, which use a different technology. The output from one piezoelectric device was much larger than expected, and the cause of the higher output was traced to incorrect assembly that allowed two polymer surfaces to rub together. Six months of development led to the first journal paper on the triboelectric generator in 2012.
“When two materials are in physical contact, the triboelectrification occurs,” said Wang, who holds the Hightower Chair in the Georgia Tech School of Materials Science and Engineering. “When they are moved apart, there is a gap distance created. To equalize the local charge, electrons have to flow. We are getting surprisingly high voltage and current flow from this. As of now, we have discovered four basic modes of triboelectric generators.”
Since their initial realization of the possibilities for this effect, Wang’s team has expanded applications. 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 in November, 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 even falling raindrops.
“If a mechanical force is applied to these generators, they will produce an electrical current and voltage,” he said. “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 generators 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,” said Wang. “It’s very simple, and there is much more we can do with this.”
The Latest on: Triboelectric Generators
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The Latest on: Triboelectric Generators
- Proposed standards for triboelectric nanogenerators could facilitate comparisonson September 24, 2019 at 5:00 pm
"For triboelectric generators, because the mechanical input is varied, you have different kinds of measurements to evaluate the performance," said Zi. "These figures of merit are considerably more ...
- Energy Scavengers: Static Electricity Could Power the Worldon August 26, 2019 at 5:00 pm
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” ...
- Transcutaneous ultrasound energy harvesting using capacitive triboelectric technologyon August 1, 2019 at 12:45 pm
Hinchet et al. describe a triboelectric generator that can convert externally applied ultrasound into an internal electricity source capable of delivering sufficient energy to recharge a battery on a ...
- Flexible Generators Could Power Next-Gen Wearable Technologyon June 24, 2019 at 7:01 am
The triboelectric effect creates a charge when two materials ... Walking with the flip-flop generates electricity with repeated contact between the generator and the wearer’s skin. Stanford wired the ...
- Flexible Generators Turn Movement into Energyon May 31, 2019 at 8:32 am
For that, thank the triboelectric effect, by which materials gather a charge ... Walking with the flip-flop generates electricity with repeated contact between the generator and the wearer's skin.
- A Next-Generation Triboelectric Nanogenerator (TENG) to Realize Constant Current from Electrostatic Breakdownon April 16, 2019 at 6:52 am
Mechanical energy harvesting has attracted much attention as explored through the techniques of electromagnetic generators (EMGs), piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators ...
- Best in snow: New scientific device creates electricity from snowfallon April 14, 2019 at 5:00 pm
A triboelectric nanogenerator ... self-extinguishing motion sensor and power generator, which could be embedded in shoes or clothing worn by firefighters and others who work in harsh environments.
- Scientists create fire-retardant sensors for safety gear in harsh environmentson March 18, 2019 at 3:41 pm
“Using triboelectric charging, the carbon aerogel functions as a motion sensor and a power generator,” said Abdelsalam Ahmed, the study’s first author, a visiting scholar at McMaster University.
- Triboelectric Generators to Power Implantable and Wearable Medical Deviceson March 18, 2019 at 4:42 am
Triboelectric generators that convert mechanical energy into electricity are an ideal solution, but their efficiency and other limitations have prevented their adoption in the medical space.
- How to power 1,000 LED bulbs by stomping one footon December 17, 2018 at 4:00 pm
“This opens up a source of energy by harvesting power from activities of all kinds.” In its simplest form, the triboelectric generator uses two sheets of dissimilar materials, one an electron donor, ...
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