Learning to Harvest Wasted Power

Wednesday, December 18, 2013 @ 04:12 PM gHale

With the stomp of a foot, one thousand LED bulbs lit up with no batteries or power cord.

The current comes from 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. Georgia Institute of Technology Professor Zhong Lin Wang and his research team have learned to harvest this power and put it to work.

Wang is using 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 things like walking, the wind blowing, vibration, ocean waves or even cars driving by.

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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,” 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 separate, one sheet holds an electrical charge isolated by the gap between them. If an electrical load then connects 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 end up produced. In a variation of the technique, the materials – most commonly inexpensive flexible polymers – produce current if they rub together before separating. Researchers are also building generators producing DC current.

“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 their first publication on the research, Wang and his research team increased the power output density of their triboelectric generator by a factor of 100,000, reporting a square meter of single-layer material can now produce as much as 300 watts. They have found the volume power density reaches more than 400 kilowatts per cubic meter at an efficiency of more than 50 percent. The researchers 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 phenomenon. The output from one piezoelectric device was much larger than expected, and the cause of the higher output traced back to incorrect assembly that allowed two polymer surfaces to rub together.

“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 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 consist of 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 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.”

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