Net-Zero Emissions Energy Systems

28 Pages·2017·1.21 MB·English

RESEARCH
REVIEW SUMMARY ◥ nisms to quickly and cheaply balance large
and uncertain time-varying differences be-
tweendemand and electricity generation;
electrified substitutes for most fuel-using
ENERGY
devices; alternative materials and manu-
facturing processes for structural materials;
Net-zero emissions energy systems
andcarbon-neutralfuelsforthepartsofthe
economy that are not easily electrified. Re-
cycling and removal of
StevenJ.Davis*,NathanS.Lewis*,MatthewShaner,SoniaAggarwal,DougArent, carbon from the atmo- ONOURWEBSITE ◥
InêsL.Azevedo,SallyM.Benson,ThomasBradley,JackBrouwer,Yet-MingChiang, sphere (carbon manage-
Readthefullarticle
ChristopherT.M.Clack,ArmondCohen,StephenDoig,JaeEdmonds,PaulFennell, ment)isalsolikelytobe athttp://dx.doi.
ChristopherB.Field,BryanHannegan,Bri-MathiasHodge,MartinI.Hoffert, an important activity of org/10.1126/
EricIngersoll,PaulinaJaramillo,KlausS.Lackner,KatharineJ.Mach, any net-zero emissions science.aas9793
MichaelMastrandrea,JoanOgden,PerF.Peterson,DanielL.Sanchez, energy system. The spe- ..................................................
DanielSperling,JosephStagner,JessikaE.Trancik,Chi-JenYang,KenCaldeira* cific technologies that will be favored in
futuremarketplacesarelargelyuncertain,
BACKGROUND:Net emissions of CO2 by search,development,demonstration,andde- butonlyafinitenumberoftechnologychoices
human activities—including not only en- ployment. It may take decades to research, existtodayforeachfunctionalrole.Totake
ergyservicesandindustrialproductionbut develop,anddeploythesenewtechnologies. appropriate actions inthenear term, it is
also land use and agriculture—must ap- imperative to clearly identify desired end D
o
proach zero in order to stabilize global ADVANCES:A successful transition to a points.Toachievearobust,reliable,andaf- w
n
mean temperature. Energy services such futurenet-zeroemissionsenergysystem fordable net-zero emissions energy system lo
a
aslight-dutytransportation,heating,cooling, is likely to depend on vast amounts of in- laterthiscentury,effortstoresearch,develop, d
e
and lighting may be relatively straight- expensive, emissions-free electricity; mecha- demonstrate, and deploy those candidate d
forward to decarbonize by elec- technologies must start now. fro
m
trifyingandgeneratingelectricity h
from variable renewable energy OUTLOOK:Combinations of known tech- ttp
sources(suchaswindandsolar) nologies could eliminate emissions related ://s
anddispatchable(“on-demand”) to all essential energy services and pro- cie
n
nonrenewablesources(including cesses, but substantial increases in costs c
e
nuclearenergyandfossilfuelswith areanimmediatebarriertoavoidingemis- .s
c
carboncaptureandstorage).How- sions in each category. In some cases, in- ie
n
ever,otherenergyservicesessential novation and deployment can be expected ce
m
tomoderncivilizationentailemis- toreducecostsandcreatenewoptions.More a
sions that are likely to be more rapid changes may depend on coordinat- g.o
difficulttofullyeliminate.These ingoperationsacrossenergyandindustry rg
difficult-to-decarbonizeenergyser- sectors,whichcouldhelpboostutilization o /
n
vicesincludeaviation,long-distance rates of capital-intensive assets, but this J
u
transport,andshipping;production will require overcoming institutional and ne
ofcarbon-intensivestructuralmate- organizationalchallengesinordertocreate 29
rialssuchassteelandcement;and newmarketsandensurecooperationamong , 2
0
provisionofareliableelectricity regulatorsanddisparate,risk-aversebusi- 1
8
supplythatmeetsvaryingdemand. nesses.Twoparallelandbroadstreamsof
Moreover, demand for such ser- research and development could prove use-
vices and products is projected ful:researchintechnologiesandapproaches
toincreasesubstantiallyoverthis that can decarbonize provision of the most
century.Thelong-livedinfrastruc- difficult-to-decarbonize energy services, and
turebuilttoday,forbetterorworse, research in systems integration that would
willshapethefuture. allowreliablea▪ndcost-effectiveprovisionof
Here,wereviewthespecialchal- theseservices.
lenges associated with an energy
systemthatdoesnotaddanyCO
2
to the atmosphere (a net-zero
Thelistofauthoraffiliationsisavailableinthefullarticleonline.
emissions energy system). We *Correspondingauthor.Email:[email protected](S.J.D.);
discuss prominent technolog- [email protected](N.S.L.);[email protected]
(K.C.)
ical opportunities and barriers Ashowerofmoltenmetalinasteelfoundry.Industrial CitethisarticleasS.J.Davisetal.,Science360,eaas9793
foreliminatingand/ormanaging processessuchassteelmakingwillbeparticularly (2018).DOI:10.1126/science.aas9793
emissionsrelatedtothedifficult- challengingtodecarbonize.Meetingfuturedemandfor
to-decarbonize services; pitfalls suchdifficult-to-decarbonizeenergyservicesandindustrial
in which near-term actions may productswithoutaddingCO totheatmospheremaydepend
2
makeitmoredifficultorcostlyto ontechnologicalcostreductionsviaresearchandinnovation, TOMORROW’SEARTH
achievethenet-zeroemissions aswellascoordinateddeploymentandintegrationof Readmorearticlesonline
goal; and critical areas for re- operationsacrosscurrentlydiscreteenergyindustries. atscim.ag/TomorrowsEarth
Davisetal.,Science360,1419(2018) 29June2018 1of1
RESEARCH
REVIEW ◥ tegratedassessmentmodelsremainschalleng-
ing(4–6).
Here,wereviewthespecialchallengesasso-
ciatedwithanenergysystemthatdoesnotadd
ENERGY
anyCO totheatmosphere(anet-zeroemissions
2
energy system). We discuss prominent techno-
Net-zero emissions energy systems logicalopportunitiesandbarriersforeliminat-
ingand/ormanagingemissionsrelatedtothe
difficult-to-decarbonizeservices;pitfallsinwhich
StevenJ.Davis1,2*,NathanS.Lewis3*,MatthewShaner4,SoniaAggarwal5, near-termactionsmaymakeitmoredifficultor
DougArent6,7,InêsL.Azevedo8,SallyM.Benson9,10,11,ThomasBradley12, costlytoachievethenet-zeroemissionsgoal;
JackBrouwer13,14,Yet-MingChiang15,ChristopherT.M.Clack16,ArmondCohen17, andcriticalareasforresearch,development,
StephenDoig18,JaeEdmonds19,PaulFennell20,21,ChristopherB.Field22, demonstration,anddeployment.Ourscopeis
BryanHannegan23,Bri-MathiasHodge6,24,25,MartinI.Hoffert26,EricIngersoll27, notcomprehensive;wefocusonwhatnowseem
themostpromisingtechnologiesandpathways.
PaulinaJaramillo8,KlausS.Lackner28,KatharineJ.Mach29,MichaelMastrandrea4,
Ourassertionsregardingfeasibilitythroughout
JoanOgden30,PerF.Peterson31,DanielL.Sanchez32,DanielSperling33,
arenottheresultofformal,quantitativeecono-
JosephStagner34,JessikaE.Trancik35,36,Chi-JenYang37,KenCaldeira32*
micmodeling;rather,theyarebasedoncompar-
isonofcurrentandprojectedcosts,withstated
Someenergyservicesandindustrialprocesses—suchaslong-distancefreighttransport,
assumptionsaboutprogressandpolicy.
airtravel,highlyreliableelectricity,andsteelandcementmanufacturing—areparticularly
Amajorconclusionisthatitisvitaltointegrate
difficulttoprovidewithoutaddingcarbondioxide(CO )totheatmosphere.Rapidly
2 currentlydiscreteenergysectorsandindustrial D
growingdemandfortheseservices,combinedwithlongleadtimesfortechnology o
processes.Thisintegrationmayentailinfrastruc- w
dseervveilcoepsmbeontthaensdselnotnigalliafnedtimuregsenotf.eWneeregxyaminfinraesbtraurcriteurrse,amndakoeppdoerctaurnbitoinesizaatsisooncoiaftethdewseith tauctriavleamndaniangsetimtuetniotnoaflctarrabnosnfoinrmthaetioennesr,gaysswysetlelmas. nload
thesedifficult-to-decarbonizeservicesandprocesses,includingpossibletechnological e
d
smoeluettiofuntsuarendderemsaenadrcshfoarntdhdeesevesloeprvmiceenstapnrdioprirtoiecse.sAsersanwgiethoofuetxnisettinagddtietciohnnoolfoCgiOes2tcoould Aanvdiatsihoinp,plionngg-distancetransport, from
theatmosphere,buttheirusemaydependonacombinationofcostreductionsvia h
researchandinnovation,aswellascoordinateddeploymentandintegrationofoperations In2014,medium-andheavy-dutytruckswith ttp
acrosscurrentlydiscreteenergyindustries. meantripdistancesof>160km(>100miles) ://s
accountedfor~270MtCO emissions,or0.8% c
P 2 ie
ofglobalCO emissionsfromfossilfuelcom- n
2 c
eopledonotwantenergyitself,butrather renewablesources(includingnuclearenergy bustion and industry sources [estimated by e
theservicesthatenergyprovidesandthe andfossilfuelswithcarboncaptureandstorage). using(7–9)].Similarlylongtripsinlight-duty .sc
ie
productsthatrelyontheseservices.Even However,otherenergyservicesessentialtomo- vehiclesaccountedforanadditional40MtCO, n
2 c
withsubstantialimprovementsinefficiency, derncivilizationentailemissionsthatarelikely andaviationandothershippingmodes(such em
globaldemandforenergyisprojectedto tobemoredifficulttofullyeliminate.These astrainsandships)emitted830and1060Mt a
g
hiwnuhcmirleeaa,nnseeatmcetmaivrikistseiideoslny—sooivnfeccrlaurtbhdoiisnngcdeinnootxutidroeyn((Cl1y)O.e2M)nfeerraognmy- dapvrifoifaditcuiuoclntti-,otlono-ndogfe-ccdaairrsbbtoaonnn-ciizenetterenannseisrvpgeoysrtstre,uracvntiudcresashliimnpcpaltiuendrgie-; (CrFeOisg2p.,o2rne).sspMibeecleatinvfwoelrhy.~ilAe6,l%tboogotefhthggelloro,bbtaahlleeCsenOes2rogueyrmcdeiessmswiaoennrdes on .org/
andindustrialproduction,butalsolanduseand alssuchassteelandcement;andprovisionof fortransportationandtheratioofheavy-to Ju
agriculture—mustapproachzerotostabilizeglo- areliableelectricitysupplythatmeetsvarying light-dutyvehiclesisexpectedtoincrease(9). ne
balmeantemperature(2,3).Indeed,interna- demand.Totheextentthatcarbonremainsin- Light-dutyvehiclescanbeelectrifiedorrun 29
tionalclimatetargets,suchasavoidingmore volvedintheseservicesinthefuture,net-zero onhydrogenwithoutdrasticchangesinperfor- , 2
0
than2°Cofmeanwarming,arelikelytorequire emissionswillalsoentailactivemanagement manceexceptforrangeand/orrefuelingtime. 1
8
anenergysystemwithnet-zero(ornet-negative) ofcarbon. Bycontrast,general-useairtransportationand
emissionslaterthiscentury(Fig.1)(3). In2014,difficult-to-eliminateemissionsrelated long-distancetransportation,especiallybytrucks
Energyservicessuchaslight-dutytranspor- toaviation,long-distancetransportation,and orships,haveadditionalconstraintsofrevenue
tation,heating,cooling,andlightingmaybe shipping;structuralmaterials;andhighlyreliable cargospaceandpayloadcapacitythatmandate
relatively straightforward todecarbonize by electricitytotaled~9.2GtCO,or27%ofglobal energysources with high volumetric and grav-
2
electrifyingandgeneratingelectricityfromvar- CO emissionsfromallfossilfuelandindustrial imetricdensity(10).Closed-cycleelectrochemical
2
iablerenewableenergysources(suchaswind sources(Fig.2).Yetdespitetheirimportance, batteriesmustcontainalloftheirreactantsand
andsolar)anddispatchable(“on-demand”)non- detailedrepresentationoftheseservicesinin- products.Hence,fuelsthatareoxidizedwith
1DepartmentofEarthSystemScience,UniversityofCalifornia,Irvine,Irvine,CA,USA.2DepartmentofCivilandEnvironmentalEngineering,UniversityofCalifornia,Irvine,Irvine,CA,USA.
3DivisionofChemistryandChemicalEngineering,CaliforniaInstituteofTechnology,Pasadena,CA,USA.4NearZero,CarnegieInstitutionforScience,Stanford,CA,USA.5EnergyInnovation,San
Francisco,CA,USA.6NationalRenewableEnergyLaboratory,Golden,CO,USA.7JointInstituteforStrategicEnergyAnalysis,Golden,CO,USA.8EngineeringandPublicPolicy,CarnegieMellon
University,Pittsburgh,PA,USA.9GlobalClimateandEnergyProject,StanfordUniversity,Stanford,CA,USA.10PrecourtInstituteforEnergy,StanfordUniversity,Stanford,CA,USA.11Department
ofEnergyResourceEngineering,StanfordUniversity,Stanford,CA,USA.12DepartmentofMechanicalEngineering,ColoradoStateUniversity,FortCollins,CO,USA.13DepartmentofMechanical
andAerospaceEngineering,UniversityofCalifornia,Irvine,Irvine,CA,USA.14AdvancedPowerandEnergyProgram,UniversityofCalifornia,Irvine,CA,USA.15DepartmentofMaterialScienceand
Engineering,MassachusettsInstituteofTechnology,Cambridge,MA,USA.16VibrantCleanEnergy,Boulder,CO,USA.17CleanAirTaskForce,Boston,MA,USA.18RockyMountainInstitute,
Boulder,CO,USA.19PacificNationalNorthwesternLaboratory,CollegePark,MD,USA.20DepartmentofChemicalEngineering,SouthKensingtonCampus,ImperialCollegeLondon,London,UK.
21JointBioenergyInstitute,5885HollisStreet,Emeryville,CA,USA.22WoodsInstitutefortheEnvironment,StanfordUniversity,Stanford,CA,USA.23HolyCrossEnergy,GlenwoodSprings,CO,
USA.24DepartmentofElectrical,Computer,andEnergyEngineering,UniversityofColoradoBoulder,Boulder,CO,USA.25DepartmentofChemicalandBiologicalEngineering,ColoradoSchoolof
Mines,Golden,CO,USA.26DepartmentofPhysics,NewYorkUniversity,NewYork,NY,USA.27LucidStrategy,Cambridge,MA,USA.28TheCenterforNegativeCarbonEmissions,ArizonaState
University,Tempe,AZ,USA.29DepartmentofEarthSystemScience,StanfordUniversity,Stanford,CA,USA.30EnvironmentalScienceandPolicy,UniversityofCalifornia,Davis,Davis,CA,USA.
31DepartmentofNuclearEngineering,UniversityofCalifornia,Berkeley,Berkeley,CA,USA.32DepartmentofGlobalEcology,CarnegieInstitutionforScience,Stanford,CA,USA.33Instituteof
TransportationStudies,UniversityofCalifornia,Davis,Davis,CA,USA.34DepartmentofSustainabilityandEnergyManagement,StanfordUniversity,Stanford,CA,USA.35InstituteforData,
Systems,andSociety,MassachusettsInstituteofTechnology,Cambridge,MA,USA.36SantaFeInstitute,SantaFe,NM,USA.37Independentresearcher.
*Correspondingauthors:Email:[email protected](S.J.D.);[email protected](N.S.L.);[email protected](K.C.)
Davisetal.,Science360,eaas9793(2018) 29June2018 1of9
RESEARCH | REVIEW
ambientairandthenventtheirexhausttothe range, heavy-duty trucks powered by current andvolumetricenergydensitylikelypreclude
atmospherehaveasubstantialchemicaladvan- lithium-ionbatteriesandelectricmotorscancar- battery-orhydrogen-poweredaircraftforlong-
tageingravimetricenergydensity. ry ~40% less goods than can trucks powered distancecargoorpassengerservice(12).Auto-
Battery-andhydrogen-poweredtrucksarenow bydiesel-fueled,internalcombustionengines. nomoustrucksanddistributedmanufacturing
usedinshort-distancetrucking(11),butatequal The same physical constraints of gravimetric mayfundamentallyaltertheenergydemandsof
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Fig.1.Schematicofanintegratedsystemthatcanprovide mission;blue,hydrogenproductionandtransport;purple,
essentialenergyserviceswithoutaddinganyCO2totheatmo- hydrocarbonproductionandtransport;orange,ammoniaproduction
sphere.(AtoS)Colorsindicatethedominantroleofspecific andtransport;red,carbonmanagement;andblack,endusesof
technologiesandprocesses.Green,electricitygenerationandtrans- energyandmaterials.
Davisetal.,Science360,eaas9793(2018) 29June2018 2of9
RESEARCH | REVIEW
Table1.Keyenergycarriersandtheprocessesforinterconversion.Processeslistedineachcellconverttherowenergycarriertothecolumnenergy
carrier.Furtherdetailsaboutcostsandefficienciesoftheseinterconversionsareavailableinthesupplementarymaterials.
To
From e– H2 CxOyHz NH3
............................................................................................................................................................................................................................................................................................................................................
e– Electrolysis($5to6/kgH ) Electrolysis+methanation Electrolysis+Haber-Bosch
....................................................................................................................................2....................................................................................................................................................................................
Electrolysis+Fischer-Tropsch
............................................................................................................................................................................................................................................................................................................................................
H Combustion Methanation Haber-Bosch($0.50to
2
............................................................................................................................................................................(..$...0.....0...7....t..o....0.....5..7.../..m....3...C....H...4..)..................................................0.....6..0.../...k..g....N....H...3..)..............
Oxidationviafuelcell Fischer-Tropsch($4.40
to$15.00/gallonof
gasoline-equivalent)
............................................................................................................................................................................................................................................................................................................................................
CxOyHz Combustion Steamreforming Steamreforming+
($1.29to1.50/kgH) Haber-Bosch
............................................................................................................................2............................................................................................................................................................................................
Biomassgasification
($4.80to5.40/kgH )
................................................................2............................................................................................................................................................................................................................................................................
NH3 Combustion Metalcatalysts Metalcatalysts+methanation/
(~$3/kgH ) Fischer-Tropsch
....................................................................................................................2.................................................................................................................................................................................................... D
Sodiumamide o
............................................................................................................................................................................................................................................................................................................................................ w
n
lo
a
d
e
thefreightindustry,butifavailable,energy-dense may bedirectly used in anengine ormay be portationfuelsatcostsroughlycompetitivewith d
liquidfuelsarelikelytoremainthepreferred cracked toproduce hydrogen. Its thermolysis gasoline(forexample,U.S.$19/GJorU.S.$1.51/ fro
energysourceforlong-distancetransportation mustbecarefullycontrolledsoastominimize gallonofethanol)(22).Astechnologymatures hm
services(13). productionofhighlyoxidizedproductssuchas andoveralldecarbonizationeffortsoftheenergy ttp
Optionsforsuchenergy-denseliquidfuelsin- NOx (17). Furthermore, like hydrogen, ammo- systemproceed,biofuelsmaybeabletolargely ://s
cludethehydrocarbonswenowuse,aswellas nia’sgravimetricenergydensityisconsiderably avoidfossilfuelinputssuchasthoserelatedto c
ie
hydrogen,ammonia,andalcoholsandethers. lowerthanthatofhydrocarbonssuchasdiesel on-farmprocessesandtransport,aswellasemis- n
c
In each case, there are optionsfor producing (Fig.3A). sions associated with induced land-use change e.s
cbaerbinotne-gnreautterdaltooralonwet--czaerrboonemfuiseslsiotnhsatencoerugldy Biofuels (li2q3u,i2d4f)u.eTlhseinexatefunttutroewnheti-czherboioemmaissssiownilslesunperpglyy cien
c
system(Fig.1),andeachcanalsobeintercon- Conversionofbiomasscurrently provides the systemthusdependsonadvancesinconversion em
vertedthroughexistingthermochemicalprocesses mostcost-effectivepathwaytononfossil,carbon- technology,competingdemandsforbioenergy ag
(Table1). containingliquidfuels.Liquidbiofuelsatpresent andland,thefeasibilityofothersourcesofcarbon- .o
Hydrogenandammoniafuels rceopnrseusemnetd~4b.2yEthJeoftrthanesrpoourgthlsye1c0to0rEwJoorfldenweirdgey. ntieountrwailtfhueoltsh,earndobijnetcetgivreastio(2n5)o.fbiomassproduc- on rg/
Thelowvolumetricenergydensityofhydrogen Currently,themainliquidbiofuelsareethanol Ju
favorstransportandstorageatlowtemperatures fromgrainandsugarcaneandbiodieselandre- Synthetichydrocarbons ne
(–253°Cforliquidhydrogenatatmosphericpres- newable diesel from oil seeds and waste oils. Liquidhydrocarbonscanalsobesynthesized 29
sure)and/orhighpressures(350 to700bar), Theyareassociatedwithsubstantialchallenges through industrialhydrogenationoffeedstock , 2
0
thusrequiringheavyandbulkystoragecontain- relatedtotheirlife-cyclecarbonemissions,cost, carbon,suchasthereactionofcarbonmonoxide 1
8
ers(14).Tocontainthesametotalenergyasa andscalability(18). andhydrogenbytheFischer-Tropschprocess
dieselfuelstoragesystem,aliquidhydrogen Photosynthesisconverts<5%ofincidentra- (26).Ifthecarboncontainedinthefeedstock
storagesystemwouldweighroughlysixtimes diationtochemicalenergy,andonlyafraction istakenfromtheatmosphereandnofossilen-
moreandbeabouteighttimeslarger(Fig.3A). ofthatchemicalenergyremainsinbiomass(19). ergyisusedfortheproduction,processing,and
However,hydrogenfuelcellorhybridhydrogen- Conversionofbiomasstofuelalsorequiresen- transport of feedstocks and synthesized fuels,
batterytruckscanbemoreenergyefficientthan ergyforprocessingandtransportation.Land theresultinghydrocarbonswouldbecarbon-
thosewithinternalcombustiondieselengines usedtoproducebiofuelsmusthavewater,nu- neutral(Fig.1).Forexample,emissions-freeelec-
(15),requiringlessonboardenergystorageto trient,soil,andclimatecharacteristicssuitable tricitycouldbeusedtoproducedihydrogen(H)
2
achievethesametravelingrange.Toyotahas foragriculture,thusputtingbiofuelsincompeti- bymeansofelectrolysisofwater,whichwould
recentlyintroducedaheavy-duty(36,000kg), tionwithotherlanduses.Thishasimplications bereactedwithCO removedfromtheatmo-
2
500-kWfuelcell/batteryhybridtruckdesigned forfoodsecurity,sustainableruraleconomies,and sphereeitherthroughdirectaircaptureorphoto-
totravel200milesonliquidhydrogenandstored theprotectionofnatureandecosystemservices synthesis(whichinthelattercasecouldinclude
electricity,andNikolahasannouncedasimilar (20).Potentialland-usecompetitionisheightened CO capturedfromtheexhaustofbiomassor
2
battery/fuelcellheavy-dutytruckwithaclaimed byincreasinginterestinbioenergywithcarbon biogascombustion)(27,28).
rangeof1300to1900km,whichiscomparable captureandstorage(BECCS)asasourceofnega- Atpresent,thecostofelectrolysisisamajor
with today’s long-haul diesel trucks (16). If hy- tiveemissions(thatis,carbondioxideremoval), barrier.Thiscostincludesboththecapitalcosts
drogencanbeproducedaffordablywithoutCO whichbiofuelscanprovide(21). ofelectrolyzersandthecostofemissions-free
2
emissions,itsuseinthetransportsectorcould Advancedbiofueleffortsincludeprocessesthat electricity;60to70%ofcurrentelectrolytichy-
ultimatelybebolsteredbythefuel’simportance seektoovercometherecalcitranceofcelluloseto drogencostiselectricity(Fig.3C)(28,29).The
inprovidingotherenergyservices. allowuseofdifferentfeedstocks(suchaswoody cheapestandmostmatureelectrolysistechnology
Ammoniaisanothertechnologicallyviable crops,agriculturalresidues,andwastes)inorder availabletodayusesalkalineelectrolytes[suchas
alternative fuel that contains no carbon and toachievelarge-scaleproductionofliquidtrans- potassiumhydroxide(KOH)orsodiumhydroxide
Davisetal.,Science360,eaas9793(2018) 29June2018 3of9
RESEARCH | REVIEW
Fig.2.Difficult-to-eliminate
emissionsincurrentcontext.
(AandB)EstimatesofCO
2
emissionsrelatedtodifferent
energyservices,highlighting
[forexample,bylongerpie
piecesin(A)]thoseservices
thatwillbethemostdifficult
todecarbonize,andthe
magnitudeof2014emissions
fromthosedifficult-to-
eliminateemissions.The
sharesandemissionsshown
herereflectaglobalenergy
systemthatstillrelies
primarilyonfossilfuelsand
thatservesmanydeveloping
regions.Both(A)theshares
and(B)thelevelofemissions
relatedtothesedifficult-to-
decarbonizeservicesare
D
likelytoincreaseinthefuture. o
w
Totalsandsectoralbreak- n
downsshownarebased loa
primarilyondatafromthe de
d
IanntderEnDatGioAnRal4E.n3edrgaytaAbagseenscy from
(8,38).Thehighlightedironandsteelandcementemissionsarethoserelated commercialemissionsarethoseproduceddirectlybybusinessesand h
tothedominantindustrialprocessesonly;fossil-energyinputstothose households,and“Electricity,”“Combinedheat&electricity,”and“Heat” ttp
sectorsthataremoreeasilydecarbonizedareincludedwithdirectemissions representemissionsfromtheenergysector.Furtherdetailsareprovidedin ://s
c
fromotherindustriesinthe“Otherindustry”category.Residentialand thesupplementarymaterials. ie
n
c
(NaOH)]togetherwithmetalcatalyststopro- of the relative simplicity of large, long-term frastructure.Between2000and2015,cementand e
.s
ducehydrogenatanefficiencyof50to60%and storageofchemicalfuels.Hence,usingemissions- steelusepersistentlyaveraged50and21tonsper c
ie
acostof~U.S.$5.50/kgH (assumingindustrial freeelectricitytomakefuelsrepresentsacritical milliondollarsofglobalGDP,respectively(~1kg n
2 c
electricitycostsofU.S.$0.07/kWhand75%uti- opportunityforintegratingelectricityandtrans- perpersonperdayindevelopedcountries)(4). em
lizationrates)(29,30).Atthiscostofhydrogen, portationsystemsinordertosupplyapersistent Globally,~1320and1740MtCO emissionsem- a
2 g
theminimumpriceofsynthesizedhydrocarbons demandforcarbon-neutralfuelswhileboosting anatedfromchemicalreactionsinvolvedwiththe .o
w[ooru$ld5.5b0et$o1.$560.5t0o/$ga1.l7lo0n/liatenrdo$f4d2ietsoel$e5q0upivearleGnJt, utilizationratesofsystemassets. m(Faign.u2f)ac(t8u,r3e8o,f3c9e)m;aelntotgaentdhesrt,etehl,isreesqpueactteivsetloy on rg/
assumingcarbonfeedstockcostsof$0to100per Directsolarfuels ~9% of global CO2 emissions in 2014 (Fig. 1, Jun
tlfirotonemroofsrtCe$aO1m2.5ar0nefpdoervrmeGriynJgl(o2ow8f)fpo].rsosFicolerCssHcoc4moisnpttsaoroiCfsOo$n20,a.0Hn5d/2 Pptohhoopttrooocedaleutacctleyrosftcushedelmitrheiccrtaolluycgsehplllsiatorwtrifapitcaeirratlibcpyuhluaosttieno/gsmysonultenhcleuigslihasrt, po(4fu0cr,po4nl1e)s,tasrntuedcetlbidoluenmec)a.onAudlltdihsbopeurogsjhuebcmtsetadatnetortiigaarllolsywinrbetdyenu3sc.3iet%dy e 29, 20
H currentlycosts$1.30to1.50perkg(Fig.3D, withouttheland-useconstraintsassociatedwith peryearto2.4billiontonsin2025(42),andce- 1
2 8
red line) (29, 31). Thus, the feasibility of syn- biomass(35).Hydrogenproductionefficiencies mentproductionisprojectedtogrowby0.8to
thesizinghydrocarbonsfromelectrolyticH may canbehigh,butcosts,capacityfactors,andlife- 1.2%peryearto3.7billionto4.4billiontonsin
2
dependondemonstratingvaluablecross-sector timesneedtobeimprovedinordertoobtainan 2050(43,44),continuinghistoricalpatternsof
benefits,suchasbalancingvariabilityofrenew- integrated,cost-advantagedapproachtocarbon- infrastructureaccumulationandmaterialsuseseen
ableelectricitygeneration,orelseapolicy-imposed neutralfuelproduction(36).Short-livedlabora- inregionssuchasChina,India,andAfrica(4).
priceof~$400pertonofCO emitted(which torydemonstrations have also produced liquid Decarbonizingtheprovisionofcementand
2
wouldalsoraisefossildieselpricesby~$1.00/liter carbon-containingfuelsbyusingconcentrated steel will require major changes in manufac-
or~$4.00/gallon). CO streams(Fig.1H)(37),insomecasesby turingprocesses,useofalternativematerials
2
Intheabsenceofpoliciesorcross-sectorcoor- usingbacteriaascatalysts. thatdonotemitCO duringmanufacture,or
2
dination,hydrogencostsof$2.00/kg(approaching carboncaptureandstorage(CCS)technologies
thecostoffossil-derivedhydrogenandsynthe- Outlook tominimizethereleaseofprocess-relatedCO
2
sizeddieselof~$0.79/literor$3.00/gallon)could Large-scale production of carbon-neutral and totheatmosphere(Fig.1B)(45).
beachieved,forexample,ifelectricitycostswere energy-denseliquidfuelsmaybecriticaltoachiev-
$0.03/kWhandcurrentelectrolyzercostswere inganet-zeroemissionsenergysystem.Suchfuels Steel
reducedby60to80%(Fig.3B)(29).Suchreduc- couldprovideahighlyadvantageousbridgebe- Duringsteelmaking,carbon(cokefromcoking
tionsmaybepossible(32)butmayrequirecentral- tweenthestationaryandtransportationenergypro- coal)isusedtoreduceironoxideoreinblast
izedelectrolysis(33)andusinglessmaturebut ductionsectorsandmaythereforedeservespecial furnaces, producing 1.6 to 3.1 tons of process
promisingtechnologies,suchashigh-temperature priorityinenergyresearchanddevelopmentefforts. CO pertonofcrudesteelproduced(39).This
2
solidoxideormoltencarbonatefuelcells,or isinadditiontoCO emissionsfromfossilfuels
Structuralmaterials 2
thermochemicalwatersplitting(30,34).Fuel burnedtogeneratethenecessaryhightemper-
marketsarevastlymoreflexiblethaninstan- Economicdevelopmentandindustrialization atures(1100to1500°C).ReductionsinCO emis-
2
taneouslybalancedelectricitymarketsbecause arehistoricallylinkedtotheconstructionofin- sionspertonofcrudesteelarepossiblethrough
Davisetal.,Science360,eaas9793(2018) 29June2018 4of9
RESEARCH | REVIEW
theuseofelectricarcfurnace(EAF)“minimills” are high (40 to 50% and 35% by volume,re- assteelandcement,orclosesubstitutes,without
thatoperatebyusingemissions-freeelectricity, spectively)(Fig.1,GandE)(51,52). addingCO totheatmosphere.Althoughalter-
2
efficiencyimprovements(suchastopgasrecovery), nativeprocessesmightavoidliberationanduse
newprocessmethods(suchas“ultra-lowCO Cement ofcarbon,thecementandsteelindustriesare
2
directreduction,”ULCORED),processheatfuel- About40%oftheCO emissionsduringcement especiallyaversetotheriskofcompromisingthe
2
switching, and decreased demand via better productionarefromfossilenergyinputs,withthe mechanical properties of produced materials.
engineering. For example, a global switch to remainingCO emissionsarisingfromthecalcina- Demonstrationandtestingofsuchalternatives
2
ultrahigh-strengthsteelforvehicleswouldavoid tionofcalciumcarbonate(CaCO)(typicallylime- atscaleisthereforepotentiallyvaluable.Unless
3
~160MtCO annually.Theavailabilityofscrap stone)(53).Eliminatingtheprocessemissions anduntilsuchalternativesareproven,eliminating
2
steel feedstocks currently constrains EAF pro- requiresfundamentalchangestothecement- emissionsrelatedtosteelandcementwillde-
ductionto~30%ofglobaldemand(46,47),and makingprocessandcementmaterialsand/or pendonCCS.
theotherimprovementsreduce—butdonot installationofcarbon-capturetechnology(Fig.1G)
Highlyreliableelectricity
eliminate—emissions. (54).CO concentrationsaretypically~30%by
2
Prominentalternativereductantsincludechar- volumeincementplantfluegas[comparedwith Moderneconomiesdemandhighlyreliableelec-
coal(biomass-derivedcarbon)andhydrogen. ~10to15%inpowerplantfluegas(54)],improv- tricity;forexample,demandmustbemet>99.9%
Charcoalwasuseduntilthe18thcentury,andthe ingtheviabilityofpost-combustioncarboncap- ofthetime(Fig.1A).Thisrequiresinvestmentin
Braziliansteelsectorhasincreasinglysubstituted ture.FiringthekilnwithoxygenandrecycledCO energygenerationorstorageassetsthatwillbe
2
charcoalforcoalinordertoreducefossilCO isanotheroption(55),butitmaybechallenging usedasmallpercentageofthetime,whendemand
2
emissions (48). However, the ~0.6 tons of char- tomanagethecompositionofgasesinexisting ishighrelativetovariableorbaseloadgeneration.
coalneededpertonofsteelproducedrequire cement kilnsthatare not gas-tight,operate at Astheshareofrenewableelectricityhasgrown
0.1 to0.3haofBrazilianeucalyptusplantation veryhightemperatures(~1500°C),androtate(56). intheUnitedStates,naturalgas-firedgenerators
(48, 49). Hundreds of millions of hectares of A substantialfraction of process CO2 emis- haveincreasinglybeenusedtoprovidegenerat- D
o
highlyproductivelandwouldthusbenecessary sionsfromcementproductionisreabsorbedon ingflexibilitybecauseoftheirrelativelylowfixed w
tomeetexpectedcharcoaldemandsofthesteel a time scale of 50 years through natural car- costs(Fig.3B),theirabilitytorampupanddown nlo
industry,andassociatedlandusechangeemis- bonationofcementmaterials(57).Hence,capture quickly(58),andtheaffordabilityofnaturalgas ad
sionscouldoutweighavoidedfossilfuelemissions, ofemissionsassociatedwithcementmanufacture (59).Inothercountries,otherfossil-fuelsources ed
ashashappenedinBrazil(48).Hydrogenmight mightresultinoverallnet-negativeemissions orhydroelectricityareusedtoprovideflexibility. fro
alsobeusedasareductant,butqualitycouldbe asaresultofthecarbonationofproducedcement. WeestimatethatCO emissionsfromsuch“load- m
compromisedbecausecarbonimpartsstrength Ifcompletecarbonationisensured,capturedpro- following”electricity2were~4000MtCO2in2014 http
andotherdesirablepropertiestosteel(50). cessemissionscouldprovideanalternativefeed- (~12%of global fossil-fueland industry emis- ://s
Costnotwithstanding,captureandstorageof stockforcarbon-neutralsyntheticliquidfuels. sions), based loosely on the proportion of elec- c
ie
processCO emissionshasbeendemonstrated tricitydemandinexcessofminimumdemand n
andmaybe2feasible,particularlyindesignssuch Outlook (Fig.2)(60). ce
.s
astopgasrecyclingblastfurnaces,wherecon- Afuturenet-zeroemissionsenergysystemmust Thecentralchallengeofahighlyreliablenet- c
ie
centrationsandpartialpressuresofCOandCO provideawaytosupplystructuralmaterialssuch zeroemissionselectricitysystemisthustoachieve n
2 c
e
m
Fig.3.Comparisonsofenergysourcesand a
g
technologies.A)Theenergydensityofenergy .o
rg
sourcesfortransportation,includinghydrocar- o /
bons(purple),ammonia(orange),hydrogen n
J
(blue),andcurrentlithiumionbatteries(green). u
n
(B)Relationshipsbetweenfixedcapitalversus e
2
variableoperatingcostsofnewgeneration 9
resourcesintheUnitedStates,withshaded , 2
0
rangesofregionalandtaxcreditvariationand 1
8
contoursoftotallevelizedcostofelectricity,
assumingaveragecapacityfactorsandequip-
mentlifetimes.NGcc,naturalgascombined
cycle.(113).(C)Therelationshipofcapitalcost
(electrolyzercost)andelectricitypriceonthe
costofproducedhydrogen(thesimplestpossi-
bleelectricity-to-fuelconversion)assuminga
25-yearlifetime,80%capacityfactor,65%
operatingefficiency,2-yearconstructiontime,
andstraight-linedepreciationover10yearswith
$0salvagevalue(29).Forcomparison,hydrogen
iscurrentlyproducedbysteammethanerefor-
mationatcostsof~$1.50/kgH (~$10/GJ;red
2
line).(D)Comparisonofthelevelizedcostsof
dischargedelectricityasafunctionofcycles
peryear,assumingconstantpowercapacity,
20-yearservicelife,andfulldischargeover
8hoursfordailycyclingor121daysforyearly
cycling.Dashedlinesforhydrogenandlithium-
ionreflectaspirationaltargets.Furtherdetails
areprovidedinthesupplementarymaterials.
Davisetal.,Science360,eaas9793(2018) 29June2018 5of9
RESEARCH | REVIEW
theflexibility,scalability,andlowcapitalcosts Energystorage stationarybatteries.NotshowninFig.3D,less-
ofelectricitythatcancurrentlybeprovidedby efficient(forexample,70%round-trip)batteries
Reliableelectricitycouldalsobeachievedthrough
naturalgas–firedgenerators—butwithoutemit- energystoragetechnologies.Thevalueoftoday’s basedonabundantmaterialssuchassulfurmight
tingfossilCO2.Thismightbeaccomplishedbya energystorageiscurrentlygreatestwhenfrequent reducecapitalcostperunitenergycapacityto
mixofflexiblegeneration,energystorage,and $8/kWh(withapowercapacitycostof$150/kW),
cyclingisrequired,suchasforminute-to-minute
demandmanagement. frequencyregulationorpricearbitrage(72).Cost- leadingtoalevelizedcostofdischargedelectri-
effectivelystoringanddischargingmuchlarger cityforthegrid-scaleusecaseintherangeof
Flexiblegeneration $0.06to0.09/kWh($17to25perGJ),assuming
quantitiesofenergyoverconsecutivedaysandless
Evenwhenspanninglargegeographicalareas, 20to100cyclesperyearover20years(81).
frequent cycling may favor a different set of
asysteminwhichvariableenergyfromwind Utilizationratesmightbeincreasedifelec-
innovativetechnologies,policies,andvaluation
andsolararemajorsourcesofelectricity will (72,73). tricvehiclebatterieswereusedtosupportthe
haveoccasionalbutsubstantialandlong-term electricalgrid[vehicle-to-grid(V2G)],presuming
mismatchesbetweensupplyanddemand.For Chemicalbonds thatthedisruptiontovehicleownersfromdim-
example, such gaps in the United States are Chemicalstorageofenergyingasorliquidfuels inishedbatterychargewouldbelesscostlythan
commonlytensofpetajoules(40PJ=10.8TWh= isakeyoptionforachievinganintegratednet- anoutagewouldbetoelectricityconsumers(82).
24hoursofmeanU.S.electricitydemandin2015) zeroemissionsenergysystem(Table1).Stored Forexample,ifallofthe~150millionlight-duty
andspanmultipledays,orevenweeks(61).Thus, electrolytichydrogencanbeconvertedbackto vehiclesintheUnitedStateswereelectrified,
evenwithcontinental-scaleorglobalelectricity electricityeitherinfuelcellsorthroughcom- 10%ofeachbattery’s100kWhchargewould
interconnections(61–63),highlyreliableelectricity bustioningasturbines[power-to-gas-to-power provide1.5TWh,whichiscommensuratewith
in such a system will require either verysub- (P2G2P)](Figs.1,FandP,and3D,redcurve); ~3hoursofthecountry’saverage~0.5TWpower
stantialamountsofdispatchableelectricitysources commercial-scaleP2G2Psystemscurrentlyexhibit demand. It is also not yet clear how owners
(eithergeneratorsorstoredenergy)thatoperate around-tripefficiency(energyoutdividedby wouldbecompensatedforthelong-termimpacts D
less than 20% of the time or corresponding energyin)of>30%(74).Regenerativefuelcells, ontheirvehicles’batterycyclelife;whetherpe- ow
amountsofdemandmanagement.Similarchal- inwhichthesameassetsareusedtointerconvert riodsofhighelectricitydemandwouldbeco- nlo
lenges apply if most electricity were produced electricityandhydrogen,couldboostcapacity incidentwithperiodsofhightransportation ad
e
bynucleargeneratorsorcoal-firedpowerplants factorsbutwouldbenefitfromimprovements demand;whethertheubiquitouscharginginfras- d
equippedwithcarboncaptureandstorage,sug- inround-tripefficiency(now40to50%inproton- tructureentailedwouldbecost-effective;whether fro
gesting an important role for generators with exchangemembranedesigns)andchemicalsub- thescaleandtimingoftheconsent,control,and m
h
higher variable cost,suchasgasturbinesthat stitutesforexpensivepreciousmetalcatalysts payment transactions would be manageable at ttp
u(Fsieg.sy1nPt)h(e6t4ic).hydrocarbonsorhydrogenasfuel (75H,y7d6)r.ogencanalsoeitherbecombinedwith gpreird-1r5elmeviannptesrciaolde)s;(o~r3h0omwiellmioenrgtirnagnstaeccthionnos- ://sc
ie
Equippingdispatchablenaturalgas,biomass, nonfossilCO viamethanationtocreaterenew- logiesandsocialnorms(suchassharedauton- n
2 c
orsyngasgeneratorswithCCScouldallowcon- ablemethaneorcanbemixedinlowconcen- omousvehicles)mightaffectV2Gfeasibility. e
.s
tinuedsystemreliabilitywithdrasticallyreduced trations(<10%)withnaturalgasorbiogasfor c
CO emissions.Whenfueledbysyngasorbio- combustioninexistingpowerplants.Existing Potentialandkineticenergy ien
2 c
masscontainingcarboncapturedfromtheat- naturalgaspipelines,turbines,andend-useequip- Waterpumpedintosuperposedreservoirsfor em
mosphere,suchCCSoffersanopportunityfor mentcouldberetrofittedovertimeforusewith laterreleasethroughhydroelectricgenerators a
g
negativeemissions.However,thecapitalcosts purehydrogenorricherhydrogenblends(77,78), isacost-effectiveandtechnologicallymature .o
ofCCS-equippedgeneratorsarecurrentlyconsi- althoughtheremaybedifficulttrade-offsofcost optionforstoringlargequantitiesofenergywith o rg/
derablyhigherthanforgeneratorswithoutCCS andsafetyduringsuchatransition. highround-tripefficiency(>80%).Althoughcap- n
(Fig.3B).Moreover,CCStechnologiesdesigned Currentmass-marketrechargeablebatteries italcostsofsuchpumpedstoragearesubstantial, Ju
n
for generators that operate a large fraction of servehigh-valueconsumermarketsthatprize whencycledatleastweekly,levelizedcostsof e
thetime(withhigh“capacityfactors”),suchas round-tripefficiency,energydensity,andhigh dischargedelectricityarecompetitive(Fig.3D). 29
coal-burningplants,maybelessefficientand charge/dischargerates.Althoughthesebatteries Majorbarriersaretheavailabilityofwaterand , 2
0
effectivewhengeneratorsoperateatlowercapa- canprovidevaluableshort-durationancillary suitablereservoirs,socialandenvironmentalop- 1
8
city factors (65). Use of CCS-equipped gener- services(suchasfrequencyregulationandback- position,andconstraintsonthetimingofwater
atorstoflexiblyproduceback-upelectricityand uppower),theircapitalcostperenergycapacity releases by nonenergy considerations such as
hydrogenforfuelsynthesiscouldhelpalleviate andpowercapacitymakesthemexpensivefor floodprotection,recreation,andthestorageand
temporalmismatchesbetweenelectricitygener- grid-scaleapplicationsthatstorelargequantities deliveryofwaterforagriculture(83).Under-
ationanddemand. ofenergyandcycleinfrequently.Foranexample groundandunderseadesigns,aswellasweight-
Nuclearfissionplantscanoperateflexiblyto grid-scale use case with an electricity cost of basedsystemsthatdonotusewater,mightexpand
followloadsifadjustmentsaremadetocoolant $0.035/kWh (Fig. 3D), the estimated cost of thenumberofpossiblesites,avoidnonenergy
flowrateandcirculation,controlandfuelrod dischargedelectricitybyusingcurrentlithium- conflicts,andallaysomesocialandenvironmental
positions,and/ordumpingsteam(66–68).Inthe ionbatteriesisroughly$0.14/kWh($39/GJ)if concerns(84–86).
UnitedStates,thedesignandhighcapitalcosts cycleddailybutrisesto$0.50/kWh($139/GJ) Electricitymayalsobestoredbycompressing
ofnuclearplantshavehistoricallyobligatedtheir forweeklycycling.Assumingthattargetsfor airinundergroundgeologicformations,under-
near-continuous“baseload”operation,oftenat halvingtheenergycapacitycostsoflithium-ion watercontainers,orabove-groundpressureves-
capacityfactors>90%.Ifcapitalcostscouldbe batteriesarereached(forexample,~$130/kWh sels.Electricityisthenrecoveredwithturbines
reducedsufficiently,nuclearpowermightalso ofcapacity)(73,79,80),thelevelizedcostofdis- whenairissubsequentlyreleasedtotheatmo-
becomeacost-competitivesourceofload-following chargedelectricitywouldfallto~$0.29/kWh sphere.Diabaticdesignsventheatgenerated
power,butcostsmayhaveincreasedovertimein ($81/GJ)forweeklycycling.Costestimatesfor duringcompressionandthusrequireanexternal
someplaces(69–71).SimilartoCCS-equipped currentvanadiumredoxflowbatteriesareeven (emissions-free) source of heat when the air is
gasgenerators,theeconomicfeasibilityofnext- higherthanforcurrentlithium-ionbatteries,but released,reducinground-tripefficiencyto<50%.
generationadvancednuclearplantsmaydepend lowercostflowchemistriesareindevelopment Adiabaticandisothermaldesignsachievehigher
on flexibly producing multiple energy products (81).Efficiency,physicalsize,charge/discharge efficiencies (>75%) by storingboth compressed
suchaselectricity,high-temperatureheat,and/or rates,andoperatingcostscouldinprinciplebe airandheat,andsimilarlyefficientunderwater
hydrogen. sacrificedtoreducetheenergycapacitycostsof systemshavebeenproposed(84).
Davisetal.,Science360,eaas9793(2018) 29June2018 6of9
RESEARCH | REVIEW
Thermalenergy Captureandstoragewillbedistinctcarbon cooperationamongregulatorsanddisparate,risk-
managementservicesinanet-zeroemissions aversebusinesses.Wethussuggesttwoparallel
Thermalstoragesystemsarebasedonsensible
energy system (for example, Fig. 1, E and J). broad streams of R&D effort: (i) research in
heat(suchasinwatertanks,buildingenvelopes,
moltensalt,orsolidmaterialssuchasbricksand Carboncapturedfromtheambientaircouldbe technologiesandprocessesthatcanprovidethese
used to synthesize carbon-neutral hydrocarbon difficult-to-decarbonizeenergyservices,and(ii)
gravel),latentheat(suchassolid-solidorsolid-
fuels or sequestered to produce negative emis- researchinsystemsintegrationthatwouldallow
liquidtransformationsofphase-changematerials),
orthermochemicalreactions.Sensibleheatstorage sions.Carboncapturedfromcombustionofbio- fortheprovisionoftheseservicesandproducts
mass or synthesized hydrocarbons could be inareliableandcost-effectiveway.
systemsarecharacterizedbylowenergydensities
recycledtoproducemorefuels(98).Storageof Wehavefocusedonprovisionofenergyser-
[36to180kJ/kgor10to50watt-hourthermal
(Wh )/kg]andhighcosts(84,87,88).Future capturedCO2(forexample,underground)will vices without adding CO2 to the atmosphere.
th
costtargetsare<$15/kWh (89).Thermalstor- berequiredtotheextentthatusesoffossilcar- However,manyofthechallengesdiscussedhere
th
bonpersistand/orthatnegativeemissionsare couldbereducedbymoderatingdemand,such
ageiswellsuitedtowithin-dayshiftingofheat-
needed(20). asthroughsubstantialimprovementsinenergy
ing and cooling loads, whereas low efficiency,
ForindustrialCO capture,researchandde- andmaterialsefficiency.Particularlycrucialare
heatlosses,andphysicalsizearekeybarriersto 2
fillingweek-long,large-scale(forexample,30%of velopmentareneededtoreducethecapitalcosts the rate and intensity of economic growth in
andcostsrelatedtoenergyforgasseparation developingcountriesandthedegreetowhich
dailydemand)shortfallsinelectricitygeneration.
and compression (99). Future constraints on suchgrowthcanavoidfossil-fuelenergywhile
Demandmanagement land,water,andfoodresourcesmaylimitbio- prioritizinghumandevelopment,environmental
Technologiesthatallowelectricitydemandtobe logicallymediatedcapture(20).Themainchal- protection, sustainability, and social equity
shiftedintime(load-shiftingorload-shaping)or lenges to direct air capture include costs to (4,107,108).Furthermore,manyenergyservices
curtailedtobettercorrelatewithsupplywould manufacturesorbentsandstructures,energize relyonlong-livedinfrastructureandsystemsso
improveoverallsystemreliabilitywhilereducing theprocess,andhandleandtransportthecap- thatcurrentinvestmentdecisionsmaylockin D
theneedforunderused,flexibleback-upgenerators turedCO2(100,101).Despitemultipledemon- patterns of energy supply and demand (and ow
(90,91).Smartchargingofelectricvehicles,shifted strations at scale [~15 Mt CO2/year are now thereby the cost of emissions reductions) for nlo
heatingandcoolingcycles,andschedulingof being injected underground (99)], financing halfacenturytocome(109).Thecollectiveand ad
e
appliancescouldcost-effectivelyreducepeak carbon storage projects with high perceived reinforcinginertiaofexistingtechnologies,pol- d
loads in the United States by ~6% and thus risks and long-term liability for discharge re- icies, institutions, and behavioral norms may fro
avoid77GWofotherwiseneededgenerating mainsamajorchallenge(102). activelyinhibitinnovationofemissions-freetech- m
capacity (~7% of U.S. generating capacity in Discussion nologies(110).EmissionsofCO2andotherra- http
2017)(92).Managinglargerquantitiesofenergy diativelyactivegasesandaerosols(111),fromland ://s
demand for longer times (for example, tens of We have estimated that difficult-to-eliminate useandland-usechangecouldalsocausesub- c
petajoulesoverweeks)wouldinvolveidlinglarge emissionsrelatedtoaviation,long-distancetrans- stantialwarming(112). ien
c
industrialusesofelectricity—thusunderutilizing portationandshipping,structuralmaterials,and e
othervaluablecapital—oreffectivelycurtailing highlyreliableelectricityrepresentedmorethan Conclusion .sc
ie
service.Exploringanddevelopingnewtechnol- aquarterofglobalfossilfuelandindustryCO Wehaveenumeratedhereenergyservicesthat n
2 c
ogiesthatcanmanageweeklyorseasonalgaps emissions in 2014(Fig. 2). But economic and mustbeservedbyanyfuturenet-zeroemissions em
in electricity supply is an important area for human development goals,trends ininterna- energysystemandhaveexploredthetechnolo- a
g
furtherresearch(93). tionaltradeandtravel,therapidlygrowingshare gicalandeconomicconstraintsofeach.Asuccess- .o
Outlook oscfavlaereialebclteriefniceartgioynsooufroctehse(r10se3c),toarnsdatlhlseulgagrgeest- fsuylsttermansiistiolinketloyatofutduerpeennedt-zoenrothemeiasvsiaoilnasbielniteyrgoyf on rg/
Nonemittingelectricitysources,energy-storage thatdemandfortheenergyservicesandpro- vastamountsofinexpensive,emissions-freeelec- Ju
n
technologies,anddemandmanagementoptions cessesassociatedwithdifficult-to-eliminateemis- tricity;mechanismstoquicklyandcheaplybal- e
thatarenowavailableandcapableofaccom- sionswillincreasesubstantiallyinthefuture.For ancelargeanduncertaintime-varyingdifferences 29
modatinglarge,multidaymismatchesinelec- example, in some of the Shared Socioeconomic betweendemandandelectricitygeneration;elec- , 2
0
tricitysupplyanddemandarecharacterizedby Pathwaysthatwererecentlydevelopedbythe trifiedsubstitutesformostfuel-usingdevices; 1
8
high capital costs compared with the current climatechangeresearchcommunityinorderto alternative materials and manufacturing pro-
costsofsomevariableelectricitysourcesorna- frameanalysisoffutureclimateimpacts,global cessesincludingCCSforstructuralmaterials;and
turalgas–firedgenerators.Achievingaffordable, finalenergydemandmorethandoublesby2100 carbon-neutralfuelsforthepartsoftheeconomy
reliable, and net-zero emissions electricity sys- (104);hence,themagnitudeofthesedifficult-to- thatarenoteasilyelectrified.Thespecifictech-
temsmaythusdependonsubstantiallyreducing eliminateemissionscouldinthefuturebecom- nologiesthatwillbefavoredinfuturemarket-
suchcapitalcostsviacontinuedinnovationand parablewiththeleveloftotalcurrentemissions. places are largely uncertain, but only a finite
deployment, emphasizing systems that can be Combinationsofknowntechnologiescould number of technology choices exist today for
operatedtoprovidemultipleenergyservices. eliminateemissionsrelatedtoallessentialen- eachfunctionalrole.Totakeappropriateactions
ergy services and processes (Fig. 1), but sub- inthenear-term,itisimperativetoclearlyiden-
Carbonmanagement
stantial increases in costs are an immediate tify desired endpoints. If we want to achieve a
Recyclingandremovalofcarbonfromtheatmo- barriertoavoidingemissionsineachcategory. robust,reliable,affordable,net-zeroemissions
sphere(carbonmanagement)islikelytobeanim- Insomecases,innovationanddeploymentcan energy system later this century, we must be
portantactivityofanynet-zeroemissionsenergy beexpectedtoreducecostsandcreatenewop- researching,developing,demonstrating,andde-
system.Forexample,synthesizedhydrocarbons tions(32,73,105,106).Morerapidchangesmay ployingthosecandidatetechnologiesnow.
thatcontaincarboncapturedfromtheatmosphere dependoncoordinatingoperationsacrossenergy
willnotincreaseatmosphericCO whenoxidized. andindustrysectors,whichcouldhelpboost
2
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