Triboelectrics offer step-change in energy generation
Zhong Lin Wang, a professor at the Georgia Institute of Technology, is using the triboelectric effect to create electrical power by rubbing or touching two different materials together.
He believes the research 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.
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 said in a statement. ‘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 metre at an efficiency of more than 50 per cent. 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, and backpacks.
They have found that they can 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.
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 or sudden movement.