An international team of researchers led by researchers at the University of Texas (UT) at Dallas—where they have been working on making carbon nanotube-based yarns for well over a decade—has devised a way to make these carbon nanotube yarns into devices that can harvest energy from stretching or twisting them.
In research described in the journal Science, the initial results show promise for immediate use in powering small sensor nodes in Internet of Things (IoT) applications. The team says its nanotube yarns could produce larger amounts of energy by flexing and twisting in response to the movement of ocean waves.
While it appears as though these nanotube yarns are exploiting a piezoelectric effect, in which a material can generate an electric charge in response to applied mechanical stress, the yarn’s behavior makes it more closely tied to so-called electroactive polymers (EAPs), which are a kind of artificial muscle.
“Basically what's happening is when we stretch the yarn, we're getting a change in capacitance of the yarn. It’s that change that allows us to get energy out,” explains Carter Haines, associate research professor at UT Dallas and co-lead author of the paper describing the research, in an interview with IEEE Spectrum.
This makes it similar in many ways to other types of energy harvesters. For instance, in other research, it has been demonstrated—with sheets of rubber coated with electrodes on both sides—that you can increase the capacitance of a material when you stretch it and it becomes thinner. As a result, if you have charge on that capacitor, you can change the voltage associated with that charge.
“We're more or less exploiting the same effect but what we're doing differently is we're using an electric chemical cell to do this,” says Haines. “So we're not changing double layer capacitance in normal parallel plate capacitors. But we're actually changing the electric chemical capacitance on the surface of a super capacitor yarn.”
While there are other capacitance-based energy harvesters, those other devices require extremely high voltages to work because they're using parallel plate capacitors, according to Haines.
“Even if you try to make them very thin, you're still normally talking about hundreds of thousands of volts. Then you need to convert [an external bias voltage] down to something on the order of a few volts to use,” says Haines. “In our case, we don't have to do these very high voltages. Instead, the very process of putting the yarn inside an electrolyte charges it. And so in this case you don't need to go and have an external bias voltage and all the complicated circuitry associated with it.”
In order for the harvester yarn—which the researchers have dubbed “twistron” yarns—to generate energy, the carbon nanotubes have to be coiled, but not too tightly. In this way, when you twist the yarn you compress the nanotubes together.
“It's kind of like when you wring out a wet towel, you're squeezing out that volume of liquid that's trapped inside,” adds Haines. “So, when we twist the yarn, we fundamentally get this harvesting effect.”
If you want to make a yarn from which you can get energy when you stretch it, you have to somehow make that stretch cause a twist, according to Haines. To induce that phenomenon, the yarns have to be twisted under the right conditions so that you get the right amount of tension on the yarn to form coils.
The researchers calculated that by stretching the fiber at a frequency of 12 hertz, the yarn could generate 56 watts per kilogram of its own weight. On that basis, 31 milligrams of yarn is roughly what it would take to transmit a two-kilobyte data package every 10 seconds for a 100 meter radius.
While that may be a fairly convoluted metric, Haines says that this means that the material would prove effective for powering sensor nodes in IoT applications. “You can downscale these energy harvesters to very small sizes where it's almost impossible to make motors and electromagnetic generators [that would otherwise be necessary] to generate these types of energies,” adds Haines.
Haines concedes that for large-scale energy harvesting, electromagnetic generators are still are still king. But one of the advantages of the yarns is that they can be scaled to either very large or very small yarns.
At the larger end of the scale is where Haines and his colleagues would like to see the technology progress. This accounts in part for why they tested the yarn in the ocean to see how effective it would be at producing energy from waves. While a one centimeter long piece of yarn did prove effective at generating power, scaling this up to larger pieces remains a hurdle.
“The problem is that our yarns are very expensive to make,” says Haines. “The process we used to make them is fairly impractical for producing a kilogram of yarn, whereas it’s really easy to make very small yarns that are one-tenth the diameter of a human hair and are still capable of generating power.”
Haines adds: “We hope this bring this technology to the point where we can use low-cost materials, enabling us to make a giant rope of some cheap fiber, throw it out in the ocean, attach it to some buoys at the top, and harvest huge amounts of energy.”