A team of engineers at the University of Wisconsin-Madison working with colleagues in China has developed a way for the turning wheels of a car to generate power.
Their method coats small sections of each wheel with a flexible thin-film polymer engineered to make it good at grabbing electrons. That's useful because most road surfaces are good donators of these particles. Putting the two together effectively creates a circuit allowing a charge to flow – forming the basis of a power generator, actually what's called a triboelectric generator.
Triboelectrics sounds exotic but it's really very familiar, especially when you know that “tribo” means “to rub”. Run a comb through your hair and hear a crackle – that's triboelectric charging. Vigorously rubbing a balloon on a woolly jumper and then sticking it to the wall exploits the same effect.
The combing or rubbing motion transfers the charge between the two materials and was first noticed in ancient Greece. Scholars back then realised that the effect was most pronounced when they used amber – tellingly, the Greek for amber is elektron.
Flash forward
The key to getting the tyre-power generator working is the structure of the flexible polymer. More formally it is known as a triboelectric nano-generator because of the micro-engineering that has gone into understanding how it works.
Most triboelectric nano-generators operate by combining two materials that can swap electrons. That does not occur with car tyres because the weight of the vehicle crushes the two together and swiftly destroys their power-generating properties. But there is plenty of energy at that interface between tyre and Tarmac being wasted, says Professor Xudong Wang from UW-Madison, who led the research. About 10 per cent of a vehicle's fuel is consumed in overcoming the friction where rubber meets the road.
Initial trials of the generator successfully powered six LEDs fitted to a toy car. Every time the lengths of polymer touched the road surface the small, low-power lights flashed. Bigger vehicles travelling at higher speeds should be able to recover more energy, the research suggests.
“Based on the amount of friction energy consumed by cars and the triboelectric energy conversion efficiency, ideally harvesting the friction energy from tyre rotation could improve fuel efficiency by about five per cent,” says Prof Wang.
That's a significant amount and could add to the other methods used by electric and hybrid cars to recover power. Some of those techniques, such as regenerative braking, work best if the brakes are applied slowly and gently, while the wheel-based power generator works more consistently.
High and low
However, Prof Wang admits that the research is at an early stage and has yet to be proven with a bigger vehicle. “I developed the concept in my lab, but how to integrate the generator component in a real tyre is a completely different story,” he says. “It requires designing and integrating electrode materials inside the tyre and also circuit development.”
Prof Wang adds that with appropriate funding the problems should be straight-forward to solve. He and his colleagues are continuing to refine their work, improve its efficiency and scale it up.
Many other research groups around the world are also looking at using triboelectric generators to harvest or scavenge energy that would otherwise be lost. That work has swiftly improved the energy-grabbing efficiency of numerous potential materials. In just two years one leading group at Georgia Institute of Technology in the US has raised the output power density of candidate materials from just 300 milliwatts per square meter to 500 watts.
Challenges ahead
But not everyone is convinced that materials that can be rubbed and squeezed to generate power will be all conquering. “There's just not that much energy to harvest,” says Dr Paul Mitcheson, a reader in electrical energy conversion at Imperial College London – an expert in these scavenging substances.
He thinks there are fundamental limits on how well some of the proposed systems will work, because they often only involve a tiny deformation or movement induced by vibration. There's an upper limit to how much power that can give rise to.
“Even if the materials are good in isolation, they end up being part of a whole system and that might drain away some of that efficiency.” he says.
Early applications for these materials have only generated enough power to run small sensors. However, that means more data is gathered which could well aid attempts to make bigger things, be they cars or phones, do more with less power.
“That extra efficiency does not just come from the energy the harvester generates,” he says. “It also comes from the additional information you have about the way it works."
