Spare energy is all around us, from the pressure exerted by every footfall to the heat given off by heavy machinery. In some cases, like regenerative braking in cars, it's easy to harvest, and the equipment needed to do so is simple and economic. In many others, however, we're not there yet.

It's not that we don't have the materials to do so. Piezoelectric generators can harvest stresses and strains, while triboelectric generators can harvest friction, to give two examples. The problem is that their efficiency is low and the cost of the materials is currently high, making them bad fits for any applications.

But a study in today's issue of Science describes a "yarn" made of carbon nanotubes that can produce electricity when stretched. Its developers go on to demonstrate its use in everything from wearable fabrics to ocean-based wave power generators. Given that the raw material for carbon nanotubes is cheap and there are lots of people trying to bring their price down, this seems to have the potential to find some economic applications.

Spinning yarns

The idea behind the new material is simple. The authors started with a collection of carbon nanotubes and spun them into a thread, much as you would with wool. There are several ways of spinning threads, and the authors chose one that created an internal structure that distributed stress evenly among the nanotubes. They then twisted the thread until it formed a coil similar to the ones you'd see on the headset cord of old land-line telephones (remember those?).

When this coil is stretched, the internal strain and friction liberates charges from the carbon nanotubes. Which, of course, isn't particularly useful unless you can harvest them. To do so, the team dunked the whole thing in water with dissolved ions (they used hydrochloric acid but tested other salts). These would ferry the charges to nearby electrodes.

The most impressive thing is how many charges were liberated. During periods of peak strain, the yarn pumped out 250 watts per kilogram. For comparison, a professional bicyclist can only do peak exertions that are about 10 percent of that. And the yarn could sustain this when subjected to 30 stretch/relax cycles a second. Across the full cycle, the yarn could generate more than 40 joules of energy, although it was distributed unevenly, as the stretching and relaxation created a sine wave of alternating current.

One neat thing about this is that the researchers could also spin the yarn in the opposite direction. These strands produced currents at different points of the stretch-relaxation cycle. By mixing the two types of yarn in a single device, it would even out the current production (although it eliminated the smooth alternating current of a homogeneous device).

There’s an app(lication) for that

Having to keep the yarn submerged in an ionic solution the whole time it is operating is obviously a bit of an inconvenience. Unless, of course, the place you're looking to harvest energy from is one giant ionic liquid. The authors made a salt solution that mimicked the concentration found in the ocean. The setup worked just fine, and it had a peak power output of more than 90 watts/kg. So, they put a length of yarn between a weight and a float, and they dumped it into the ocean off South Korea. As waves rolled through, the device generated electricity, though it required a platinum electrode, given the corrosive nature of seawater.

In one of the weirder applications, the researchers hooked up some of the yarn to what they call an artificial muscle: a polymer that contracts when heated. The yarn produced electricity with every heating/cooling cycle.

But the most challenging application was incorporating the yarn into fabrics. Since the yarn needs an electrolyte and an external electrode to work, this isn't as simple as the word "yarn" implies. To manage this, the team put an electrolyte in a gel and used a conducting (but uncoiled) nanotube yarn as the electrode. All of these were bundled together to make a flexible material that could be incorporated into fabrics. When placed in a shirt, the device produced electricity every time someone wearing it breathed.

Overall, the material isn't exceptionally efficient at converting mechanical energy to electricity. But it is quite efficient when the incredibly light weight of the yarn is taken into account. It's also rare in that it can scale anywhere between individual fibers in clothing up to full-scale wave power generators. There seems to be a good chance that somewhere in that range is an effective, economical application—provided the price of carbon nanotubes keeps dropping.

Science, 2017. DOI: 10.1126/science.aam8771 (About DOIs).