Twisting electricity generation with nanotube technology

Today I came across a fascinating piece of research which has the potential to be a real game-changer – an international research team led by scientists at The University of Texas and Hanyang University in South Korea has developed high-tech yarns that generate electricity when they are stretched or twisted. In their study, published in Science, they describe “twistron” yarns and their possible applications, including harvesting energy from the motion of ocean waves or from temperature fluctuations.

“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, Alan G. MacDiarmid NanoTech Institute at UT Dallas.

What exactly is a carbon nanotube?

Carbon nanotubes are allotropes of carbon with a cylindrical structure, which have exceptional strength and stiffness. They have extraordinary thermal conductivity, mechanical, and electrical properties.

Nanotubes are members of the fullerene structural family. Their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene are rolled at specific and discrete (“chiral”) angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Individual nanotubes naturally align themselves into “ropes” held together by van der Waals forces.

How the yarns generate electricity

To create the twistrons, the researchers twist-spun the nanotubes into high-strength, lightweight yarns, twisting until the yarns coiled like an over-twisted rubber band. To enable the yarns to generate electricity, they were coated in an ionically conducting material, or electrolyte, such as salt-water.

“Fundamentally, these yarns are supercapacitors. 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,”
– Dr. Na Li, research scientist at the NanoTech Institute.

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 yarns 30 times a second generated 250 watts per kilogram of peak electrical power when normalised to the yarn’s weight.

Potential applications

In the lab 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, the team connected a twistron yarn to an artificial muscle that contracts and expands when heated and cooled. The yarn was able to convert the mechanical energy generated by the muscle into electrical energy. The researchers also sewed the yarns into a shirt and demonstrated that normal breathing stretched the yarn to generate an electrical signal, indicating potential use as a respiration monitor.

“Electronic textiles are of major commercial interest, but how are you going to power them? 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 fibres reported in the literature,”
– Dr. Ray Baughman, director of the NanoTech Institute

The team also demonstrated the ability of the yarns to generate electricity from wave motion in an experiment in the sea off the east coast of South Korea. They attached a 10 centimetre-long yarn, weighing only 1 milligram, between a balloon and a sinker that rested on the seabed. Every time a wave arrived, the balloon rose, stretching the yarn up to 25% and thereby generated electricity. The technique could be scaled by operating many such 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. 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.

And therein lies the rub…production of these yarns is very expensive.

Many discoveries arrive in a blaze of hopeful publicity, only to fizzle out when attempts are made to economically reproduce them at scale, and twistrons may face the same fate. However, if an economic mode of production can be developed, the potential applications would only be limited by our imaginations. But that’s a big if….