By bringing two differently charged surfaces into contact, and then separating them, Zhong Lin Wang and colleagues at the Georgia Institute of Technology have created what is called the triboelectric effect. Akin to what causes static electricity shocks, this process is much more efficient than previous efforts, particularly that of electromagnetic induction and piezelectric generators (which use ceramic crystals to convert pressure into voltage).
They then used the triboelectric nano-generators (TENGs) to help create a cost-effective power generating backpack. This was done by coating one side of plastic cards with aluminum film filled with nano-scale pores. The other side had copper film that had an array of polymer nanowires on its surface. They then arranged the cards in a rhombus, like a collapsible cardboard box.
Each step taken forces the box to collapse on itself in order to make two sides come into contact. During this time, nanowires and pores interlock, increasing the contact area, and in doing so, boosting the charge. Following each collapse, a spring allows the sides to jump back into shape.
Currently, the TENGs are about 50% efficient, as compared to piezoelectric systems, which are known to average 8% efficiency. The test showed that a 2 kilogram backpack can generate of 1 watt of power during walking, a power output that could realistically run 40 LEDs simultaneously. While they still have a long way to go, the size and weight of these backpacks offer a huge upgrade from what came before.
Wang and company envision a future where TENGs are built directly into sensors, phones and wearable computers (think Google Glass). The team has already created a stand-alone generator capable of powering a smartphone, so they are will on their way to developing products that may prove essential in the future.