Carbon Nanotube Yarns Generate Electricity When Stretched & Twisted

©. UT Dallas

In the near future, clothing made with "twistron" yarns could power wearable devices with the motion of the wearer's body.

When compared to relatively mature renewable energy technologies such as solar and wind power, energy-harvesting fabric is still in its infancy, but work on a number of fronts has been yielding enough promising results to continue developing this micro-scale electricity source. And recent research into one approach to converting mechanical energy into electricity suggests that not only could small medical devices and wearables be powered with clothing woven from a special type of yarn, but large-scale applications of the "twistron" yarn could eventually harvest electricity from the motion of ocean waves.

A team of international researchers, led by scientists at The University of Texas at Dallas (UT Dallas) and Hanyang University in South Korea, has developed a special yarn spun from carbon nanotubes that can generate electricity when twisted or stretched, which they say could "provide battery-free power with a small footprint." The carbon nanotubes themselves are incredibly thin, some 10,000 times thinner than a human hair, yet stronger than steel, and while not exactly simple or cheap to make just yet, the twistron yarns made from it show great promise as mechanical energy harvesters. A shirt with twistron in it could power a breathing or heart monitor, or other small-scale medical devices, without needing to incorporate a battery into them, according to the team.

"The easiest way to think of twistron harvesters is, you have a piece of yarn, you stretch it, and out comes electricity." - Dr. Carter Haines, associate research professor in the Alan G. MacDiarmid NanoTech Institute at UT Dallas

The twistron yarn was made with enough twist in it that it coils up on itself "like an over-twisted rubber band," which enables its energy-harvesting qualities. When the "harvester" yarn is stretched out or twisted, it decreases the volume of the carbon nanotubes, thereby increasing their energy due to the electrical charges on the yarn coming closer together, which in turn increases the voltage on the yarn's charge and allows for electricity to be harvested from it. According to Dr. Ray Baughman, the director of the NanoTech Institute and co-author of the study, coiled twistron yarns which are stretched 30 times per second can generate 250 watts of peak power per kilogram, which is said to be "over a hundred times higher electrical power per weight when stretched compared to other weavable fibers reported in the literature."

Twistron harvester yarns require either a coating of, or submersion in, an electrolyte, which can be as simple as table salt and water, and although sea water is a much more complex solution than that, tests of the new material in the ocean were positive, and could lead to larger-scale applications in marine energy. Researchers say that twistron performance can scale upward by increasing the diameter of the yarn and deploying multiple strands of it in parallel, but one hurdle to scaling up the technology at the moment is the cost of producing it. However, large-scale applications are only one aspect of the technology, as sensors, communications, and IoT devices may be one of the most appropriate uses of the material.

"If our twistron harvesters could be made less expensively, they might ultimately be able to harvest the enormous amount of energy available from ocean waves. However, at present these harvesters are most suitable for powering sensors and sensor communications. Based on demonstrated average power output, just 31 milligrams of carbon nanotube yarn harvester could provide the electrical energy needed to transmit a 2-kilobyte packet of data over a 100-meter radius every 10 seconds for the Internet of Things." - Baughman

The team, which has filed a patent on the technology, published its findings in the journal Science under the title "Harvesting electrical energy from carbon nanotube yarn twist."