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One of the most common and powerful examples in the natural world could be thunder and lightning - the friction of cumulonimbus clouds create static electricity.
"In our daily lives there is tribo-electrification everywhere - when we walk, speak or shake our body, we create power. When it rains, when a car wheel rotates, when the tide pushes and pulls, all of these generate power," Wang explains.
However, the energy source had been considered interesting but impractical, because it produces very weak electric current despite high voltage.
"For example, the voltage created by a man walking on synthetic carpets can reach thousands of volts - several times home power systems' voltage," Wang says. Yet he will not feel an electrical charge because the electric current generated in the friction is too low.
Wang and his team started testing a film material that enhances electric currents created by the process back in 2011, when he was working for the Georgia Institute of Technology.
The first triboelectric generator was tested successfully in January 2012, with an output power of 3.67 milliwatts per square meter - enough to power up a heart pacemaker.
Since then the team had been striving for output of more power. They published their findings in a series of theses in international journals, including Nature Communication and Nano Energy.
Their latest research report appeared in the American Chemical Society's ACS Nano in February, in which the team managed to generate power from noise. When a sound wave passes through the air, the friction can be converted into power, Wang explains.
For instance, a person speaking into the air generates 50 volts (about the voltage of wire telephones), and 70- to 110-decibel noises (equivalent to the volume of a speaker attached to a desktop computer) can be transferred to light up 20 LED lights at the same time.
"I believe it is time to industrialize this technology," says Wang, who believes the costs are low enough to make it practical.
