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Ancient effect harnessed to get electricity from waste heat

March 30, 2005
Courtesy of the American Chemical Society
and World Science staff

A phe­nom­e­non not­ed by an an­cient Greek phi­los­o­pher has be­come the ba­sis for a de­vice de­signed to gen­er­ate elec­tri­city from the im­mense amounts of en­er­gy wast­ed as heat eve­ry year. 

The “pyroe­lec­tric nano­gen­er­a­tor” is the top­ic of a re­port in the re­search jour­nal Nano Let­ters, pub­lished by the Amer­i­can Chem­i­cal So­ci­e­ty.

In the pa­per, Zhong Lin Wang and col­leagues at the Geor­gia In­sti­tute of Tech­nol­o­gy ex­plain that more than half the en­er­gy gen­er­ated in the U.S. each year is wast­ed. Much of it drifts away as heat re­leased by eve­rything from com­put­ers to cars to elec­tric trans­mis­sion lines.

But heat can be con­vert­ed back to elec­tri­city us­ing some­thing called the pyroe­lec­tric ef­fect. This was first de­scribed by the Greek phi­los­o­pher The­o­phras­tus in 314 B.C., when he no­ticed the gem­stone tour­ma­line pro­duced stat­ic elec­tri­city and at­tracted bits of straw when heat­ed. 

Heat­ing and cool­ing re­ar­range the mo­lec­u­lar struc­ture of cer­tain ma­te­ri­als, in­clud­ing tour­ma­line, and cre­ate an im­bal­ance of elec­trons, or elec­tric­ally charged sub­a­tom­ic par­t­i­cles. As na­ture at­tempts to cor­rect the im­bal­ance, elec­trons start to flow, mak­ing an elec­tric cur­rent.

Wang’s group wanted to apply the prin­ci­ple to make a gen­er­a­tor that could take ad­van­tage of heat changes in the mod­ern world. The re­search­ers made mo­lec­u­lar-scale wires, or nanowires, out of zinc ox­ide, a com­pound added to paints, plas­tics, elec­tron­ics and even food. Us­ing an ar­ray of short lengths of nanowire stand­ing on end, they found a de­vice that pro­duces elec­tri­city when heat­ed or cooled. 

The sci­en­tists sug­gest the nanogen­er­a­tors could even pro­duce pow­er as tem­per­a­tures fluc­tu­ate from day to night. It “can be the ba­sis for self-pow­ered nan­otech­nol­ogy that har­vests ther­mal [heat] en­er­gy from the time-dependent tem­per­a­ture fluctua­t­ion in our en­vi­ron­ment for ap­plica­t­ions such as wire­less sen­sors, tem­per­a­ture im­ag­ing, med­i­cal di­ag­nos­tics and per­son­al microe­lec­tron­ics,” they wrote.

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A phenomenon noted by an ancient Greek philosopher has become the basis for a device designed to generate electricity from the immense amounts of energy wasted as heat every year. The “pyroelectric nanogenerator” is the topic of a report in the research journal Nano Letters, published by the American Chemical Society. In the paper, Zhong Lin Wang and colleagues at the Georgia Institute of Technology explain that more than half the energy generated in the U.S. each year is wasted, much of it as heat released by everything from computers to cars to electric transmission lines. Heat can be converted to electricity using something called the pyroelectric effect, first described by the Greek philosopher Theophrastus in 314 B.C., when he noticed the gemstone tourmaline produced static electricity and attracted bits of straw when heated. Heating and cooling rearrange the molecular structure of certain materials, including tourmaline, and create an imbalance of electrons, or electrically charged subatomic particles. As nature attempts to correct the imbalance, electrons start to flow, making an electric current. Wang’s group wanted to apply the principle to make a generator that could take advantage of heat changes in the modern world. The researchers made molecular-scale wires, or nanowires, out of zinc oxide, a compound added to paints, plastics, electronics and even food. Using an array of short lengths of nanowire standing on end, they found a device that produces electricity when heated or cooled. Thje scientists suggest the nanogenerators could even produce power as temperatures fluctuate from day to night. It “can be the basis for self-powered nanotechnology that harvests thermal [heat] energy from the time-dependent temperature fluctuation in our environment for applications such as wireless sensors, temperature imaging, medical diagnostics and personal microelectronics,” the authors wrote.