15 December 2013

Converting waste energy into electricity by triboelectric effect

Image shows how power is generated by sliding two materials together and then creating a gap between them. This effect could be used to produce power for portable electronic devices. Credit: Inertia Films

From ocean waves to the action of walking across a floor, energy is all around us. Now, a materials scientist from Georgia Tech has developed a system to capture that “wasted energy” and convert it into electricity.

By using a phenomenon known as the triboelectric effect, a team led by Zhong Lin Wang has been able to create significant amounts of electric power by simply rubbing or touching two different materials together according to a series of reports published across six different journals.

Wang said the discovery could offer a new way to power mobile devices by capturing the mechanical energy from walking, vibration, passing cars or any multitude of sources.

“We are able to deliver small amounts of portable power for today’s mobile and sensor applications,” Wang said. “This opens up a source of energy by harvesting power from activities of all kinds.”

Simply put, a triboelectric generator uses two sheets of materials, one an electron donor and the other an electron acceptor. When the sheets touch, electrons pass from the donor material to the acceptor. After being separated, one sheet retains an electrical charge. If an electrical load is then connected to the two surfaces, a small current will flow to equalize the charges. Repeating the process creates an alternating current.

Another type of triboelectric generator produces a current if two materials are rubbed together before being separated.

“The fact that an electric charge can be produced through triboelectrification is well known,” Wang said. “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 the earliest stages of their research, Wang’s team has boosted the power output density of their generator by a factor of 100,000 meaning a square meter of single-layer material can produce as much as 300 watts. The researchers have also increased their catalog of energy gathering devices from shoe inserts to floor mats to buoys bobbing on ocean waves which produce energy via water flow across a patterned polymer surface.

The team has also learned to apply micron-scale patterns to the polymer sheets increasing the contact area and therefore the effectiveness of the charge transfer.

In addition to using the technology for power generation, Wang said the triboelectric effect could be used as sensors without an external power source. Because the generators create a current when they are physically bothered, they could be used to sense changes in flow rates, sudden movements or 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.”

Wang added his team plans to continue studying the generators and sensors to boost their output and sensitivity.

“Everybody has seen this effect, but we have been able to find practical applications for it,” Wang said. “It’s very simple, and there is much more we can do with this.”

Georgia Institute of Technology

Now read: Redefined thermoelectronic generator efficiency


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