Dubbed “twistron” energy harvesters, the yarns could potentially be used in a variety of possible applications, say the researchers, including harvesting energy from ocean waves or from temperature fluctuations. In addition, they could be used in smart fabrics and powering Internet of Things (IoT) devices.
To make the twistron harvesters, the researchers first took hollow cylinders of carbon 10,000 times smaller in diameter than a human hair and twist-spun them into high-strength, lightweight elastic yarns. The yarns were then submerged in or coated with an ionically conducting material, or electrolyte.
“Fundamentally, these yarns are supercapacitors,” says Dr. Na Li, a research scientist at the NanoTech Institute and co-lead author of a paper on the research. “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.”
As described by Dr. Carter Haines, associate research professor in the Alan G. MacDiarmid NanoTech Institute at UT Dallas and co-lead author of the paper, when a harvester yarn is twisted or stretched, the volume of the carbon nanotube yarn decreases, bringing the electric charges on the yarn closer together and increasing their energy. This increases the voltage associated with the charge stored in the yarn, enabling the harvesting of electricity.
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, says Dr. Ray Baughman, director of the NanoTech Institute and a corresponding author of the study. “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.”
In experiments, the researchers showed that a twistron yarn “weighing less than a housefly” could light a small LED each time the yarn was stretched. A twistron harvester was also shown to be able to convert mechanical energy generated by a polymer artificial 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,” says Li. “Twistron technology might be exploited for such applications where changing batteries is impractical.”
The yarn was also sewn into a shirt to form a self-powered breathing monitor. Normal breathing stretched the yarn, which generated an electrical signal.
“Electronic textiles are of major commercial interest, but how are you going to power them?” says Baughman. “Harvesting electrical energy from human motion is one strategy for eliminating the need for batteries. Our yarns produced over a hundred times higher electrical power per weight when stretched compared to other weavable fibers reported in the literature.”
In a proof-of-concept demonstration, the researchers also showed that the yarn would work in ocean water by deploying a 10 centimeter-long yarn, weighing only 1 milligram, between a balloon and a sinker that rested on the seabed. The balloon would rise every time an ocean wave arrived, stretching the yarn up to 25 percent and thereby generating measured electricity.
“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,” says Baughman. “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.”
The researchers have filed a patent on their technology. For more, see “Harvesting electrical energy from carbon nanotube yarn twist.”
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