How an In­dian-ori­gin sci­en­tist is har­ness­ing dark­ness to cre­ate light

The Times of India (New Delhi edition) - - Times Sport - Rebecca Boyle

Aaswath Ra­man was driv­ing through a vil­lage in Sierra Leone in 2013 when an idea came to him as sud­denly as, per­haps, a light bulb switch­ing on.

The vil­lage was not equipped with elec­tric­ity, and Ra­man, an elec­tri­cal en­gi­neer at the Univer­sity of Cal­i­for­nia, Los An­ge­les, was un­aware he was in a vil­lage un­til he heard the voices of shad­owed hu­man fig­ures.

“It took us about five min­utes to re­alise we were pass­ing through a town, be­cause it was com­pletely dark,” Ra­man said.

Ra­man won­dered whether he could use all that dark­ness to make some­thing to light it up, not unlike the way that so­lar pan­els gen­er­ate elec­tric­ity from the sun’s heat and light.

He did. In new re­search pub­lished on Thurs­day in the jour­nal Joule, Ra­man demon­strated a way to har­ness a dark night sky to power a light bulb.

His pro­to­type de­vice em­ploys ra­dia­tive cool­ing, the phe­nom­e­non that makes build­ings and parks feel cooler than the sur­round­ing air after sun­set. As Ra­man’s de­vice re­leases heat, it does so un­evenly, the top side cool­ing more than the bot­tom. It then con­verts the dif­fer­ence in heat into elec­tric­ity. In the pa­per, Ra­man de­scribed how the de­vice, when con­nected to a volt­age con­verter, was able to power a white LED.

Jef­frey C Grossman, a ma­te­ri­als sci­en­tist at the Mass­a­chu­setts In­sti­tute of Tech­nol­ogy, said the work was “quite ex­cit­ing” and showed prom­ise for de­vel­op­ment of low­power ap­pli­ca­tions at night.

“But there is def­i­nitely a long way to go if they want to use it as an al­ter­na­tive to adding bat­tery stor­age for so­lar cells,” Grossman adds.

Ev­ery­thing emits heat, ac­cord­ing to the laws of ther­mo­dy­nam­ics. At night, when one side of Earth turns away from the sun, its build­ings, streets and jacket-less peo­ple cool off. If no clouds are present to trap warmth, ob­jects on the Earth can lose so much heat that they reach a lower

Aaswath Ra­man’s

de­vice em­ploys ra­dia­tive cool­ing, a phe­nom­e­non that makes build­ings and parks feel cooler than the sur­round­ing air after sun­set. His pro­to­type uses a poly­styrene disk coated in black paint and cov­ered with a wind shield. At its heart is

a ther­mo­elec­tric gen­er­a­tor

How the de­vice works

tem­per­a­ture than the air sur­round­ing them. The cloud­less at­mos­phere be­comes a port­hole to the void, through which warmth flows.

Hu­mans have taken ad­van­tage of this ef­fect for mil­len­ni­ums. Six thou­sand years ago, peo­ple in what are now Iran and Afghanista­n con­structed enor­mous bee­hive-shaped struc­tures called yakhchal, which used this pas­sive cool­ing ef­fect to cre­ate and store ice in the desert.

Mod­ern sci­en­tists have stud­ied how to har­ness en­ergy from Earth’s day-night swings in tem­per­a­ture, but that work has mostly re­mained the­o­ret­i­cal. In 2014, re­searchers led by Fed­erico Ca­passo, an elec­tri­cal en­gi­neer­ing pro­fes­sor at Har­vard, cal­cu­lated that at best only about 4 watts of en­ergy can be ex­tracted from a square me­tre of cold space. By conHu­mans have taken ad­van­tage of this ef­fect for mil­len­nia. Six thou­sand years ago, peo­ple con­structed enor­mous bee­hive-shaped struc­tures called to cre­ate and store ice in the desert

yakhchal

trast, a so­lar panel gen­er­ates about 200 watts per square me­tre in di­rect sun­light.

Nonethe­less, a de­vice that could pro­duce any amount of elec­tric­ity at night would be valu­able; after the sun sets, so­lar cells don’t work and winds of­ten die down, even as de­mand for light­ing peaks.

The pro­to­type built by Ra­man re­sem­bles a hockey puck set in­side a chaf­ing dish. The puck is a poly­styrene disk coated in black paint and cov­ered with a wind shield. At its heart is an off-the shelf gad­get called a ther­mo­elec­tric gen­er­a­tor, which uses the dif­fer­ence in tem­per­a­ture be­tween op­po­site sides of the de­vice to gen­er­ate a cur­rent. A sim­i­lar de­vice pow­ers Nasa’s Cu­rios­ity rover on Mars.

Usu­ally, the tem­per­a­ture dif­fer­ence in these gen­er­a­tors is stark, and they are care­fully en­gi­neered to sep­a­rate hot and cold. Ra­man’s de­vice in­stead uses the at­mos­phere’s am­bi­ent tem­per­a­ture as the heat source. The shift from warm to cool is very slight, mean­ing the de­vice can’t pro­duce much power.

His de­vice is el­e­vated on alu­minium legs, en­abling air to flow around it. As the dark puck loses warmth to the night sky, the side fac­ing the stars grows colder than the side fac­ing the air-warmed tabletop. This slight dif­fer­ence in tem­per­a­ture gen­er­ates a flow of elec­tric­ity.

When paired with a volt­age con­verter, the pro­to­type pro­duced 25 mil­li­watts of power per square me­tre. That is about three orders of mag­ni­tude lower than what a typ­i­cal so­lar panel pro­duces, and well short of even the roughly 4-watt max­i­mum ef­fi­ciency for such de­vices. Still, sev­eral ex­perts said the pro­to­type was an im­por­tant con­tri­bu­tion to a new and rel­a­tively un­usual space in the re­new­able en­ergy sec­tor.

“This is a neat com­bi­na­tion of ra­dia­tive cool­ing with ther­mo­elec­tric ma­te­ri­als,” said Ellen D Wil­liams, a physics pro­fes­sor at the Univer­sity of Mary­land. “Both tech­nolo­gies are proven and prac­ti­cal, but I haven’t seen them com­bined like this. They did this with in­ex­pen­sive ma­te­ri­als, sug­gest­ing it could be made into use­ful prod­ucts for the de­vel­op­ing world.”

One chal­lenge will be im­prov­ing the de­vice’s ef­fi­ciency with­out rais­ing its costs, said Lance Wheeler, a ma­te­ri­als sci­en­tist at the US Na­tional Re­new­able En­ergy Lab­o­ra­tory. Al­though ther­mo­elec­tric de­vices are less ef­fi­cient and more ex­pen­sive than pho­to­voltaic cells, they can be more durable. Con­ceiv­ably, Ra­man said, ther­mo­elec­tric de­vices could com­ple­ment so­lar­pow­ered lights in ar­eas where chang­ing bat­ter­ies is a chal­lenge, like on street lamps or in re­mote ar­eas.

“I fig­ured the amount of elec­tric­ity we could get would be pretty small, and it was,” he said. “But walk­ing around in Sierra Leone, I re­alised light­ing re­mains a big prob­lem, so it’s an op­por­tu­nity as well.”

Ra­dia­tive cooler

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