It sounds like a simple task: use ambient energy from your surroundings to do something handy, such as charge your iPod from the vibrations of your morning run. But few of the many energy-harvesting devices introduced over the past hundred years have lived up to the promise of the idea. They just don’t seem able to eke out enough power to do anything useful. That looks set to change.
An energy-harvesting device that uses carbon nanotubes is already hitting power levels higher than any others have ever shown. The nanotubes are spun into a yarn around a twentieth of a millimetre thick, then twisted into coils thinner than the width of a human hair. These coils are then submerged in an electrolyte to complete the device, dubbed a twistron harvester.
Using yarn less than the weight of a mosquito, the team was able to generate enough power to run an LED – just from the ambient activity of ocean waves and body movements. “We’re generating higher amounts of power than any other harvesters, some of which have been around for decades,” says Ray Baughman at the University of Texas at Dallas.
Energy harvesters have modest goals: rather than powering homes, they aim to power remote devices for which changing a battery is a pain, says Yu Jia at the University of Chester, UK. For example, in 2015, Jia used a previous generation of energy harvesters to power sensors on the underside of the Forth Road Bridge in Scotland. “For maintenance and testing, it’s really helpful to have wireless sensors under the bridge,” he says. For similar reasons, the UK company Perpetuum attaches sensors to the bottom of trains.
Useful as these are though, “changing the batteries requires a lot of scaffolding and hassle”, says Jia. What’s more, the energy harvesters only work because a lot of vibrations feed them, either from cars crossing the bridge or trains rumbling along.
What if you want sensors in places where there are no vibrations? “The navy would love to have thousands of buoys dotted around the ocean, measuring pollution levels, temperature, and then relaying that information back to shore,” says Baughman. But when the batteries on their sensors run out, it won’t be possible to go around changing them all – and that’s where energy generated from the motion of waves would be useful.
Baughman and his colleagues demonstrated that their twistron harvesters would provide enough energy for the job when attached to a weight at one end and a buoy at the other. When stretched, the yarn’s ability to hold charge decreases, with the knock-on effect of increasing the associated voltage. As the buoy moves, it stretches the harvester, generating electricity.
Although they produce enough power, the harvesters are too expensive for commercial use at the moment. Producing the yarn isn’t easy and takes a lot of time and money. But Baughman hopes that will change in the future.
“This is the first discovery of a harvester based on a twist process, but it’s only a baby. We’ve still got a lot to learn,” he says.