Technology news

31 December 2013

Collapsing backpack charges gadgets as you walk

WEARABLE computers are on their way and soon you’ll be able to power them yourself. A new type of nano-generator converts movement from walking into electricity to keep your gadgets going.

Wearable generators often use electromagnetic induction, which is efficient but requires bulky, heavy magnets. Smaller, lighter piezoelectric generators use ceramic crystal to convert pressure into voltage, but they are expensive and a lot less efficient.

Now Zhong Lin Wang and colleagues at the Georgia Institute of Technology, Atlanta, have captured the electricity generated from bringing two differently charged surfaces into contact, then separating them. This is called the triboelectric effect, the same process that causes static electricity shocks.

To use triboelectric nano-generators (TENGs) to create a power-generating backpack, the team coated one side of plastic cards with aluminium 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 (see diagram).

Have backpack, will charge batteries

Every step you take makes the box collapse in on itself so the two sides of the cards come into contact. Nanowires and pores interlock, increasing the contact area and, correspondingly, the amount of charge that builds up. After each collapse, a spring makes the sides jump back into shape, separating the cards and creating a potential difference that drives current through a circuit. The TENGs are about 50 per cent efficient, comparing well to piezoelectric systems, which struggle to get beyond 8 per cent.

In tests the 2 kilogram backpack generated over 1 watt of power during walking, enough to run 40 LEDs simultaneously (ACS Nano, Existing backpack generators based on electromagnetic induction produce 5 to 20 watts, but weigh 10 times as much.

“The 2 kilogram backpack generated 1 watt of power during walking, enough electricity to run 40 LEDs”

A separate experiment used the same method to charge a lithium-ion battery (ACS Nano, Wang envisions TENGs built directly into sensors, phones and wearable computers. His team recently built a stand-alone generator capable of powering a smartphone.