The Economist - UK (2022-05-07)

76 Science & technology The Economist May 7th 2022

Sustainablepower

Inertial frames

N

o gooddeed, anoldsayinghasit,goes

unpunished.Thatiscertainlytrueof

theintroductionofgreenenergy.Theun­

reliabilityofsolarandwindpowercom­

paredwiththatgeneratedbyfossilfuelsis

wellknown—andwithittheconcomitant

needforstoragefacilitiessuchaslargebat­

terypackstosmooththingsover.

Butgreenenergybringsanother,more

subtle,problem. Modernelectricalgrids

operateonalternatingcurrents(ac),and

theseneedtobeofa fixedandreliablefre­

quency(usuallyeither 50 hz or 60 hz).This

frequency’s stability is maintained by a

phenomenoncalledgridinertia,whichre­

sultsfromthereal,physicalinertia(asde­

scribedbyIsaacNewton’sfirstlawofmo­

tion)embodied inthepower­generating

turbinesoffossil­fuel(and also nuclear

andhydroelectric)powerstations.

Goingfora spin

Theseturbinesactasmassive,inertia­stor­

ingflywheels.Aslongastheiroutputsare

insynchrony(andoneimportantpartof

grid management is to keep them that

way),theresistancetochangewhichtheir

inertiaprovidesstabilisesthewholegrid.

Thefewerthenumberoftheseturbines(as

opposedto windturbines, whichrotate

outofsyncwiththegrid,andsolarpanels,

whichdonotrotateatall),thelessinertiaa

gridhas.Andinsomeparticularlygreen

countriesthisisgettingtobea problem,to

theextentthatnon­power­generatingfly­

wheelsarebeingaddedtothesystemto

providethemissinginertia.

OnesuchplaceisBritain,whichgener­

atesabout30%ofitselectricalpowerfrom

windandsunlight.OnMarch17th,forex­

ample,NationalGrideso—thefirmthat,as

itsnamesuggests,operatesthecountry’s

electricitygrid—cuttheopeningribbonon

a plantbuiltnearKeith,innorthernScot­

land,byStatkraft,a Norwegianrenewable­

energyfirm.The inertiain thisplantis

storedbya pairofsteelflywheels(seepic­

tureoftheroadtrainrequiredtodeliver

them).Eachoftheseflywheelsweighs 194

tonnesandrotatesatupto 500 revolutions

perminute(rpm).

A secondStatkraftplantshouldopenin

the autumn, near Liverpool. Instead of

largemassesrotatingrelativelyslowly,this

will relyon smaller onesspinning fast

(1,500rpm). Both approaches embody

aboutthesameamountofinertia,andin

combinationthepairwillstorearound2%

oftheinertiacurrentlyrequiredto support

Britain’sgrid.Thatisequivalentto the in­

ertialcontributionofa conventional coal­

fired station.Moreover,laterin  the  year

NationalGridesoplanstoaddtwo  more

systems,builtbySiemens,toincrease  its

inertia­storingpotentialstillfurther.

There is, though, an alternative  to

buildingnewflywheels,andthat  is  to  re­

purposeoldones—inotherwords, to rede­

signexistingfossil­fuelstationssimply to

storeinertia,ratherthangenerating  elec­

tricity. NationalGridesois testing  that

idea,too,ina formergas­firedstation  in

northWales.Thishasbeenopen for busi­

nessasaninertiastoresince2021. 

Thefirmhopes,aswellasallthis, to de­

velopwaysofstabilisingthenetwork with­

outspinninglumpsofmetalfortheir own

sake.Thatwillinvolvetheuseof what are

knownasgrid­forminginverters.

Bothsolarpower,whichisa direct cur­

rent(dc) whenit comesoutofthe generat­

ingpanel,andwindpower,which is acbut

stillneedstobetweakedbeforebeing  fed

intoa grid,arefirstprocessedbysemicon­

ductor­baseddevicescalledinverters. This

isalsotrueofthedcdrawnfrom  storage

devicessuchasbatteries,which  are  em­

ployedtosmoothoutirregularities in solar

andwindpower.

Existinginvertersaredescribed as “grid

following”.Thismeanstheymonitor  and

fitinwiththeestablishedfrequency of the

grid  they  are  feeding  into.  That  suits  grid

managers  well  enough  when  solar  and

wind contribute only a small fraction of a

grid’s total power, but is progressively less

suitable  as  that  contribution  rises.  How­

ever, inverters can be designed to be “grid

forming”  instead—meaning  the  current

they  put  out  mimics  the  external  stabilis­

ing  effect  of  mechanical  inertia.  Using

grid­forming  inverters  rather  than  grid­

following  ones  should  allow  much  more

wind and solar power to be integrated easi­

ly into a grid. 

Until  recently,  grid­forming  inverters

had been tested only at small scale. In Jan­

uary,  however,  Britain’s  energy  regulator,

Ofgem, signed off on a technical standard

acceptable to both manufacturers and ser­

vice providers. That will permit their large­

scale  deployment,  and  Julian  Leslie,  Na­

tional Grid eso’s chief engineer, says he ex­

pects big grid­forming inverters to be pro­

viding inertia within two years.

Grid lock

Being an island, Britain has a more or less

self­contained electricity grid. This makes

it  a  good  place  to  try  such  an  experiment.

Success  would  encourage  other  island

grids,  both  real  (Australia’s  and  Ireland’s,

for  example)  and  metaphorical  (such  as

Texas’s,  which  has  few  links  with  the  rest

of  North  America)  to  try.  Larger  grids  in

North  America  and  Europe  will  no  doubt

be watching from the wings. 

The quest for grid inertia, then, is an ex­

ample  of  the  nitty­gritty  adjustments

needed to accommodate the shift in energy

production  and  use  that  is  now  going  on.

Other  technologies,  from  electric  cars  to

hydrogen­gas  supplies,  may  have  higher

profiles.  But  what  is  happeningdown  in

the  engine  room  of  the  green  economy  is

just as important—if not more so.n

Green electricity needs more than just solar panels and wind turbines

Inertia, embodied