Researchers have developed yarns that generate electricity when they are stretched or twisted. The “twistron” yarns are constructed from carbon nanotubes — hollow cylinders of carbon 10,000 times smaller in diameter than a human hair. The nanotubes are twist-spun into high-strength, lightweight yarns; to make the yarns highly elastic, they are twisted so the yarns coil like an over-twisted rubber band.
In order to generate electricity, the yarns must be either submerged in or coated with an ionically conducting material, or electrolyte, which can be as simple as a mixture of ordinary table salt and water. Fundamentally, the yarns are supercapacitors. In a normal capacitor, energy — like from a battery — is used to add charges to the capacitor. In the new method, when the carbon nanotube yarn is inserted into an electrolyte bath, the yarns are charged by the electrolyte itself; no external battery or voltage is needed.
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.
Tests showed that a twistron yarn weighing less than a housefly could power a small LED, which lit up each time the yarn was stretched. To show that twistrons can harvest waste thermal energy from the environment, a twistron yarn was connected 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. The researchers also sewed twistron harvesters into a shirt. Normal breathing stretched the yarn and generated an electrical signal, demonstrating its potential as a self-powered respiration sensor.
A coiled twistron also was deployed in the sea. A 10-centimeter-long yarn, weighing 1 milligram (about the weight of a mosquito), was attached 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 measured electricity. Even though very small amounts of twistron yarn were used in the current study, the harvester performance was shown to be scalable, both by increasing twistron diameter and by operating many yarns in parallel.
Based on demonstrated average power output, just 31 milligrams of carbon nanotube yarn harvester could provide the electrical energy needed to transmit a 2-kilo-byte packet of data over a 100-meter radius every 10 seconds for uses such as the Internet of Things.