PhotoGasoline may be relatively cheap now but that doesn't mean we shouldn't be looking for ways to make transportation more efficient.

A group of University of Wisconsin-Madison (UW-Madison) engineers and a collaborator from China are doing just that. They report success in developing a nanogenerator that harvests energy from a car's rolling tire friction – literally, where the rubber meets the road.

A nanogenerator is a system that can convert mechanical or thermal energy created by small-scale physical change into electricity. Using one, the team believes automakers will be able to increase the efficiency of the vehicles they produce, reducing consumers' fuel consumption.

Triboelectric effect

When two different objects – like a rubber tire and an asphalt highway – rub against one another it produces what is called the triboelectric effect. It's an electric charge that no one really notices in the case of a vehicle because it is allowed to dissipate.

Xudong Wang, an associate professor at UW-Madison, says the nanogenerator provides an excellent way to take advantage of friction-created energy that is usually lost.

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

Integrated into the tire

A nanogenerator would take the form of an electrode integrated into a segment of a tire. Every time this part of the tire surface comes into contact with the pavement, the friction created by those 2 surfaces produces an electrical charge – the triboelectric effect. The electrode captures it.

How do they know it works? It was relatively simple. When they tested their theory Wang and his colleagues used a toy car outfitted with LED lights to demonstrate the concept.

The team placed the nanogenerator onto the wheels of the car. When they rolled the car 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 theory that energy created by friction can actually be collected and reused.

Reclaiming lost energy

"Regardless of the energy being wasted, we can reclaim it, and this makes things more efficient," Wang said. "I think that's the most exciting part of this, and is something I'm always looking for, how to save the energy from consumption."

The amount of energy created and collected from this friction is determined largely by two factors – how much the vehicle weighs and how fast it goes. Both are directly related to friction.

Still, using typical weights and average speed, Wang estimates a nanogenerator could reduce a vehicle's fuel consumption by 10%, based on 50% friction energy conversion efficiency.

"There's big potential with this type of energy," Wang said. "I think the impact could be huge."

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