The nanogenerator harnesses energy from the changing electric potential between the pavement and a vehicle's wheels. To do so it relies on the triboelectric effect , a type of contact electrification in which an electric charge is produced from the contact or rubbing together of two unlike objects or materials.
An electrode integrated into a segment of a tire is used to capture the resulting charge. The researchers see this as a promising way to reclaim some of the energy that is usually lost due to friction.
"The friction between the tire and the ground consumes about 10% of a vehicle's fuel," says Xudong Wang, an associate professor of materials science and engineering at UW-Madison. "That energy is wasted. So if we can convert that energy, it could give us very good improvement in fuel efficiency."
In their initial trials, the researchers used a toy car with six attached LEDs to demonstrate the concept. Multiple electrodes were attached to the tires of the toy vehicle, and, as the car rolled across the ground, the LEDs flashed. Measurements showed the maximum instantaneous power obtained was 1.79 mW at a load resistance of 10 MΩ - corresponding to an energy conversion efficiency of 10.4%.
The researchers also determined that the energy harnessed increases with the weight and speed of a vehicle. Wang estimates about a 10% increase in the average vehicle's gas mileage given a 50% friction energy conversion efficiency.
For more, see the paper at the journal Nano Energy : Single-electrode triboelectric nanogenerator for scavenging friction energy from rolling tires .