Spin­ning a yarn proves pow­er­fully in­ter­est­ing

The Press and Journal (Aberdeen) - - ENERGY - Jeremy Cress­well

A re­mark­able “en­ergy” fi­bre ca­pa­ble of gen­er­at­ing elec­tric­ity when stretched or twisted has been de­vel­oped by US and South Korean en­ergy re­searchers.

The in­ter­na­tional re­search team, led by sci­en­tists at The Uni­ver­sity of Texas (UoT) at Dal­las and Hanyang Uni­ver­sity in South Ko­rea, say there are mul­ti­ple po­ten­tial fu­ture ap­pli­ca­tions for the nan­otech­nol­ogy-based “twistron”.

They have al­ready demon­strated that twistron can be utilised for the har­vest­ing en­ergy from the mo­tion of ocean waves or from tem­per­a­ture fluc­tu­a­tions.

In a proof-of-con­cept demon­stra­tion, Dr Shi Hyeong Kim, a post­doc­toral re­searcher at the NanoTech In­sti­tute, UoT, waded into the frigid surf off the east coast of South Ko­rea to de­ploy a coiled twistron in the sea.

He at­tached a 10cm-long yarn, weigh­ing only 1 mil­ligram (about the weight of a mos­quito), 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%, thereby gen­er­at­ing mea­sured elec­tric­ity.

When sewn into a shirt, th­ese yarns have also served as a self-pow­ered breath­ing mon­i­tor.

And in the lab, the re­searchers showed that a twistron yarn weigh­ing less than a house­fly could power a small LED, which lit up each time the yarn was stretched.

There is a lot of in­ter­est in us­ing waste en­ergy to power the In­ter­net of Things, such as ar­rays of dis­trib­uted sen­sors and it is thought this fi­bre tech­nol­ogy might be ex­ploited for such ap­pli­ca­tions where chang­ing bat­ter­ies is im­prac­ti­cal.

The team have 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.

Even though the re­searchers have so far used only very small amounts of twistron yarn in the cur­rent study, they have shown that har­vester per­for­mance is scal­able, both by in­creas­ing di­am­e­ter and by op­er­at­ing many yarns in par­al­lel.

Imag­ine if it be­came a se­ri­ous con­tender in the har­vest­ing of wave en­ergy where many tech­nolo­gies have been de­vel­oped but none have so far made the grade as com­mer­cial propo­si­tions.

“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.

Twistron yarns are con­structed from car­bon nan­otubes, which are hol­low cylin­ders of car­bon 10,000 times smaller in di­am­e­ter than a hu­man hair.

The 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.

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.

Stretch­ing the coiled twistron yarns 30 times a sec­ond gen­er­ated 250 watts per kilo­gram of peak elec­tri­cal power, dur­ing tri­als.

Although nu­mer­ous al­ter­na­tive har­vesters have been in­ves­ti­gated for many decades, no other re­ported har­vester has so far pro­vided such high power out­puts.

Twistron is said to be ca­pa­ble of gen­er­at­ing over a hun­dred times higher elec­tri­cal power per weight when stretched com­pared to other weav­able har­vester fi­bres so far de­vel­oped.

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

“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.

“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.”

From left, Dr Carter Haines, Dr Shi Hyeong Kim and Dr Nai Li of the Alan G MacDiarmid NanoTech in­sti­tute at UT Dal­las are lead au­thors of a study that de­scribes car­bon nan­otube yarns that gen­er­ate elec­tric­ity when they are stretched or twisted

The gen­er­at­ing ca­pa­bil­i­ties of the Twistron has many ap­pli­ca­tions, from en­ergy pro­duc­tion to health

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