A group of University of Wisconsin-Madison(UW-M) engineers and a collaborator from China have developed a nanogenerator that harvests energy from a car's rolling tire friction. The nanogenerator could provide automakers with a new way to derive greater efficiency out of their vehicles. The researchers development is reported in the journal Nano Energy. Xudong Wang, the Harvey D. Spangler fellow and an associate professor of materials science and engineering at UW-M, and his PhD student Yanchao Mao have been working on this device for about a year. The nanogenerator relies on the triboelectric effect to harness energy from the changing electric potential between the pavement and a vehicle's wheels. The triboelectric effect is the electric charge that results from the contact or rubbing together of two dissimilar objects.

"The friction between the tire and the ground consumes about 10% of a vehicle's fuel," Wang says. "That energy is wasted. So if we can convert that energy, it could give us very good improvement in fuel efficiency."

The nanogenerator relies on an electrode integrated into a segment of the tire. When this part of the tire surface comes into contact with the ground, the friction between those two surfaces ultimately produces an electrical charge—a type of contact electrification known as the triboelectric effect.

Wang and his colleagues used a toy car during initial trials. Credit: College of Engineering.During initial trials, Wang and his colleagues used a toy car with LED lights to demonstrate the concept. They attached an electrode to the wheels of the car, and as it rolled across the ground, the LED lights flashed on and off. The movement of electrons caused by friction was able to generate enough energy to power the lights, supporting the idea that energy lost to friction can actually be collected and reused.

"Regardless of the energy being wasted, we can reclaim it, and this makes things more efficient," he says.

The researchers also determined that the amount of energy harnessed is directly related to the weight of a car, as well as its speed. Therefore the amount of energy saved can vary depending on the vehicle—but he estimates about a 10% increase in the average vehicle's gas mileage given 50% friction energy conversion efficiency.

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University of Wisconsin-Madison (UW-M)