A potential way to harvest some of the ‘lost’ frictional energy from rolling car tires has been proposed by a group of US and Chinese researchers

The triboelectric effect is a type of contact electrification that occurs as a result of friction between two dissimilar materials. Since 2012, there have been considerable research efforts to harvest this effect, transforming friction into electricity, which can be used elsewhere. Generally based on a pair of thin film materials with different electron affinity, triboelectric nanogenerators (TENGs) have been suggested for use in everything from energy generating shoes to self-powered sensors.

In this paper [DOI:.1016/j.nanoen.2015.04.026], Xudong Wang and his team used a simple, single electrode TENG to scavenge friction energy from the surface of a simulated tyre. Although all wheel-based systems rely on friction to operate (think about how a car behaves on a low-friction icy road), it is also a major source of energy loss. According to the US Department of Energy, between 5–7% of the energy in a vehicle’s fuel is lost to so-called rolling resistance between the tires and road surface.

To operate one surface in a TENG must donate electrons, and the other accept them. According to Wang, most materials used in road construction, such as silica, cement, and asphalt are electron-donating. When put into contact with an electron-accepting material, such as a polymer like Polydimethylsiloxane (PDMS), opposite charges can be induced on both surfaces and then tapped off using electrodes. A rough PDMS surface was produced to simulate the high-friction surface of a road tire, with a copper foil added to the film to act as the device’s single electrode.

As a demonstrator, six of these TENGs were then attached to the rubber wheels of a toy car, which was driven on a road surface. Their maximum power output was found to be 1.79 mW, corresponding to a conversion efficiency of 10.4%. Combined, these TENGs instantaneously powered 6 commercial green LEDs while the vehicle was moving on the ground. The electric output of the TENG-array-on-wheel was found to increase as the driving speed increased. The addition of extra weight to the toy vehicle also improved the power output, as the heavier load resulted in better contact between the road and the rough PDMS surface.

While this work is still at the earliest stages, these results suggest that it may be possible to scavenge some of the ‘lost’ friction energy from rolling vehicle tires. If implemented, this could even improve fuel efficiency. The next stage of the work is to investigate energy storage options, along with optimizing the device conversion efficiency.

Mao, Y. et al, Nano Energy (2015) 15, 227–234 “Single-electrode triboelectric nanogenerators for scavenging friction energy from rolling tires.” DOI:.1016/j.nanoen.2015.04.026