The Econmist - USA (2021-10-09)

The Economist October 9th 2021 Science&technology 77

dicted,trpv1turnedouttobeheatsensi­

tive. When thetemperature rises above

43°C,thechannelthroughitopens,per­

mitting ionsofcalcium andsodiumto

pass. That chemical signal stimulatesa

nerveimpulsewhichtellsthebrainabout

thetemperaturechange.

trpv1turnedouttobeoneofseveral

temperature­sensitiveionchannels,some

ofwhichregisterheat,andsomecold.It

was one ofthe cold­sensitive varieties,

trpm8, which was discovered simulta­

neouslybyDrJuliusandDrPatapoutian.

DrPatapoutianthenwentontolookat

thesensationoftouch.Molecularbiology

havingadvancedintheinterim,hewas

abletoworkwithwholeproteins—or,rath­

er,thegenesforwholeproteins.Heidenti­

fied 72 proteins,expressedina mechani­

callysensitivecellline,thatlookedlikepo­

tentialtouch­sensitiveion channels.He

testedthemoneata time,bysilencingthe

genesthatencodethemandpokingthere­

sultingcells.Thefirst 71 silencingshadno

effect.Butthe72ndprovedtobeofthepro­

teinhewaslookingfor.Hecalledthatpro­

teinpiezo1.

Innature,piezo1isfoundnotinsens­

oryneurons,butratherinorganslikethe

bladderwherepressuresensitivityisim­

portant.ButDrPatapoutiandiscovereda

similarchannel,piezo2, whichis,indeed,

foundinnerveendings.Itisthisthatisre­

sponsiblefortouchandproprioception.

Fascinatingwork,then.Andimportant.

Itisthroughthesenses,andthesenses

alone,thatpeopleareabletoperceivethe

world.Buttosometheawardcameasa

surprise.Ina yearofcovid,theyhadbeen

expectingthehonourstogoelsewhere—

perhapstotheinventorsofmrna­vaccine

technology.LikeGod,however,thevarious

Nobel­prizecommitteesworkinmysteri­

ouswaystheirwonderstoperform.n

Thethermoelectriceffect

Cool thinking

R

efrigerators and air­conditioners

are old and clunky technology, and rep­

resent a field ripe for disruption. They con­

sume a lot of electricity. And they generally

rely  on  chemicals  called  hydrofluorocar­

bons  which,  if  they  leak  into  the  atmo­

sphere,  have  a  potent  greenhouse­warm­

ing  effect.  Buildings’  central­heating  sys­

tems,  meanwhile,  are  often  powered  by

methane in the form of natural gas, which

releases  carbon  dioxide,  another  green­

house gas, when it is burned, and also has a

tendency to leak from the pipes that deliv­

er it—which is unfortunate, because meth­

ane,  too,  is  a  greenhouse  gas,  and  one

much more potent than CO 2.

One potential way of getting around all

this  might  be  to  exploit  what  is  known  as

the thermoelectric effect, a means of carry­

ing  heat  from  place  to  place  as  an  electric

current.  Thermoelectric  circuits  can  be

used either to cool things down, or to heat

them  up.  And  a  firm  called  Phononic,

based in Durham, North Carolina, has de­

veloped a chip which does just that.

The  thermoelectric  effect  was  disco­

vered  in  1834  by  Jean  Charles  Peltier,  a

French physicist. It happens in an electri­

cal  circuit  that  includes  two  materials  of

different conductivity. A flow of electrons

from the more conductive to the less con­

ductive causes cooling. A flow in the other

direction causes heating. 

The reason for this is that electrons are

able  to  vibrate  more  freely  when  pushed

into  a  conductive  material.  They  thereby

transfer  energy  to  their  surroundings,

warming  them.  When  shunted  into  a  less

conductive  one,  electrons’  vibrations  are

constrained, and they absorb energy from

their  surroundings,  cooling  those  sur­

roundings  down.  An  array  of  thermoelec­

tric circuits built with all the high­conduc­

tivity materials facing in one direction and

all  the  low  conductivity  ones  in  the  other

can thus move heat in either direction, by

switching  the  polarity  of  the  current.  For

reasons  buried  in  the  mathematics  of

quantum  physics,  the  heat  thus  flowing

does  so  in  discrete  packages,  called  pho­

nons. Hence the name of the firm.

The  thermoelectric  effect  works  best

whentheconductorsinvolvedareactually

semiconductors,withbismuthandtinbe­

ingcommonchoices.Fancycamerascon­

tainsimplecoolingchipswhichusethese,

as do some scientific instruments. But

Phononic’sboss,TonyAtti,thinksthatis

smallbeer.UsingthegoodofficesofFabri­

net,a chipmakerinThailand,hehasstart­

edmakingmoresophisticatedversionsat

highvolume,usingthesetoftoolsand

techniquesnormallyemployedtoetchin­

formation­processingcircuitsontowafers

madeofsilicon.Inthiscase,though,the

wafersaremadeofbismuth.

Theresultsare,admittedly,stilla long

wayfromsomethingthat couldheat or

coola building.Buttheyarealreadyfind­

inglucrativeemploymentinapplications

wherespaceisata premium.Atthemo­

ment,thefastest­growingmarketiscool­

ingtheinfraredlasersusedtofireinforma­

tion­encodingphotonsthroughfibre­op­

ticcables,forthelong­distancetransmis­

sionof data. They are also beingused,

though,inthe 5 gmobile­phonebasesta­

tionsnowstartingtoblanketstreetcor­

ners,tokeepthebatteriesofelectricvehi­

cles at optimal operating temperatures,

andascomponentsoftheoptical­frequen­

cyradar­likesystemsknownaslidar, that

helpguideautonomousvehicles.

The crucial question from Mr Atti’s

pointofviewiswhethersemiconductor­

basedthermoelectronicscanbreakoutof

these niches and become more main­

stream,in thewaythatsemiconductor­

basedelectronicsandlightinghavedone.

Inparticular,hewouldliketoincorporate

heat­pumpingchipsintobuildings,topro­

videthemwithintegralthermoregulation.

In theircurrent form, thermoelectric

chipsareunlikelytoreplaceconventional

airconditioningandcentralheating be­

causetheycannotmoveheatoverthedis­

tancesrequiredtopumpitinandoutofa

buildinginbulk.Buttheycouldnonethe­

lessbeusedasregulators.Insteadofturn­

inga bigair­conditioningsystemonoroff,

tolowerorraisethetemperaturebythe

smallamountsrequiredtomaintaincom­

fort, with all the cost that entails, thermo­

electric chips might tweak matters by mov­

ing heat around locally. 

Phononic has already run trials of such

local­temperature­tweaking  chips  in  Sin­

gapore, in partnership with Temasek, that

country’s  state­run  investment  fund.  In

2019 spGroup,  Singapore’s  utility  compa­

ny, installed eight of the firm’s heat pumps,

which comprise an array of chips pointed

down  at  people,  pumping  heat  out  of  the

air  above  them,  on  the  boardwalk  on

Clarke  Quay  in  the  city.  Phononic  claims

the  devices  lowered  the  temperature  in

their vicinity by up to 10°C and, as a bonus,

consequentlyreduced humidity by 15%. If

that can bescaledup, it would certainly be

a cool result.n

A new approach to carrying heat aroundasanelectriccurrent

Hot vibes

Thermoelectric circuit

Source:Phononic

Copper

conductors

Semiconductors

Heat

flow

Heatabsorbed→

(coldside)

→ Heat

emitted

(hot side)

→

→

→

→

Electrical

Moreconductive current

Lessconductive

Electrical

insulator

Electrical

insulator

→ → → → → →

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