AsianScientist (Aug. 28, 2017) – An international team of researchers from the US and South Korea have developed yarns that generate electricity when they are stretched or twisted. They published their findings in the Science.
‘Twistron yarns’ are constructed from carbon nanotubes, which are hollow cylinders of carbon 10,000 times smaller in diameter than a human hair. To generate electricity, the yarns must be either submerged in or coated with an ionically conducting material, such as ordinary table salt in water.
In this study, researchers first twist-spun the nanotubes into high-strength, lightweight yarns. To make the yarns highly elastic, they introduced so much twist that the yarns coiled like an over-twisted rubber band. When the harvester yarn was twisted or stretched, the volume of the carbon nanotube yarn decreased, bringing the electric charges on the yarn closer together and increasing their energy. This increased the voltage associated with the charge stored in the yarn, enabling the harvesting of electricity.
“The easiest way to think of twistron harvesters is, you have a piece of yarn, you stretch it, and out comes electricity,” said Dr. Carter Haines, associate research professor in the Alan G. MacDiarmid NanoTech Institute at UT Dallas and co-lead author of the article.
“Fundamentally, these yarns are supercapacitors,” said Dr. Li Na, a research scientist at the NanoTech Institute and co-lead author of the study. “In a normal capacitor, you use energy — like from a battery—to add charges to the capacitor. But in our case, when you insert the carbon nanotube yarn into an electrolyte bath, the yarns are charged by the electrolyte itself. No external battery, or voltage, is needed.”
Stretching the coiled twistron yarns 30 times a second generated 250 watts per kilogram of peak electrical power when normalized to the harvester’s weight.
“Although numerous alternative harvesters have been investigated for many decades, no other reported harvester provides such high electrical power or energy output per cycle as ours for stretching rates between a few cycles per second and 600 cycles per second,” said Dr. Ray Baughman, director of the NanoTech Institute and a corresponding author of the study.
The researchers also showed that a twistron yarn weighing less than a housefly could power a small LED, which lit up each time the yarn was stretched. The researchers then sewed twistron harvesters into a shirt. When worn, the wearer’s breathing motion stretched the shirt and generated an electrical signal, demonstrating its potential as a self-powered respiration sensor.
To further demonstrate that twistrons can harvest waste thermal energy from the environment, Li connected a twistron yarn to a polymer artificial muscle that contracts and expands when heated and cooled. The twistron harvester converted the mechanical energy generated by the polymer muscle to electrical energy.
“There is a lot of interest in using waste energy to power the Internet of Things, such as arrays of distributed sensors,” said Li. “Twistron technology might be exploited for such applications where changing batteries is impractical.”
In another proof-of-concept experiment, co-lead author Dr. Shi Hyeong Kim, a postdoctoral researcher at the NanoTech Institute, deployed a coiled twistron in the sea. He attached a ten-centimeter-long yarn, weighing only one milligram, between a balloon and a sinker that rested on the seabed. Every time an ocean wave arrived, the balloon would rise, stretching the yarn up to 25 percent, thereby generating electricity.
Although the investigators used very small amounts of twistron yarn in the current study, they have shown that harvester performance is scalable, both by increasing twistron diameter and by operating many yarns in parallel.
“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,” Baughman said. “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 two-kilobyte packet of data over a 100 meter radius every ten seconds for the Internet of Things.”
The article can be found at: Kim et al. (2017) Harvesting Electrical Energy from Carbon Nanotube Yarn Twist.
Source: University of Texas at Dallas; Photo: Pexels.
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