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ENERGY TRANSITION

Hydrogen

CHINA

April 2023

December 2021

OIES PAPER: ET23

OIES PAPER: ET06

Aliaksei Patonia, Research Fellow, OIES

Rahmatallah Poudineh, Head of Electricity Research, OIES

Anders Hove, Research Associate, OIES

Michal Meidan, Senior Research Fellow, OIES

Philip Andrews-Speed, Senior Research Fellow, OIES

Hydrogen storage for a

net-zero carbon future

The contents of this paper are the authors’ sole responsibility. They do not necessarily represent the views

of the Oxford Institute for Energy Studies or any of its Members.

The contents of this paper are the authors’ sole responsibility. They do not

necessarily represent the views of the Oxford Institute for Energy Studies or any of

its members.

Oxford Institute for Energy Studies

(Registered Charity, No. 286084)

This publication may be reproduced in part for educational or non-profit purposes without special

permission from the copyright holder, provided acknowledgment of the source is made. No use of this

publication may be made for resale or for any other commercial purpose whatsoever without prior

permission in writing from the Oxford Institute for Energy Studies.

ISBN 978-1-78467-199-0

The contents of this paper are the authors’ sole responsibility. They do not necessarily represent the views

of the Oxford Institute for Energy Studies or any of its Members.

Abstract

If a hydrogen economy is to become a reality, along with efficient and decarbonized production and

adequate transportation infrastructure, deployment of suitable hydrogen storage facilities will be crucial.

This is because, due to various technical and economic reasons, there is a serious possibility of an

imbalance between hydrogen supply and demand. Hydrogen storage could also be pivotal in promoting

renewable energy sources and facilitating the decarbonization process by providing long duration

storage options, which other forms of energy storage, such as batteries with capacity limitations or

pumped hydro with geographical limitations, cannot meet. However, hydrogen is not the easiest

substance to store and handle. Under ambient conditions, the extremely low volumetric energy density

of hydrogen does not allow for its efficient and economic storage, which means it needs to be

compressed, liquefied, or converted into other substances that are easier to handle and store. Currently,

there are different hydrogen storage solutions at varying levels of technology, market, and commercial

readiness, with different applications depending on the circumstances. This paper evaluates the relative

merits and techno-economic features of major types of hydrogen storage options: (i) pure hydrogen

storage, (ii) synthetic hydrocarbons, (iii) chemical hydrides, (iv) liquid organic hydrogen carriers, (v)

metal hydrides, and (vi) porous materials. The paper also discusses the main barriers to investment in

hydrogen storage and highlights key features of a viable business model, in particular the policy and

regulatory framework needed to address the primary risks to which potential hydrogen storage investors

are exposed.

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ENERGYTRANSITIONENERGYTRANSITIONHydrogenCHINAApril2023December2021OIESPAPER:ET23OIESPAPER:ET06AliakseiPatonia,ResearchFellow,OIESRahmatallahPoudineh,HeadofElectricityResearch,OIESAndersHove,ResearchAssociate,OIESMichalMeidan,SeniorResearchFellow,OIESHydrogenstorageforanet-zerocarbonfutureThecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.iThecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsmembers.Copyright©2023OxfordInstituteforEnergyStudies(RegisteredCharity,No.286084)Thispublicationmaybereproducedinpartforeducationalornon-profitpurposeswithoutspecialpermissionfromthecopyrightholder,providedacknowledgmentofthesourceismade.NouseofthispublicationmaybemadeforresaleorforanyothercommercialpurposewhatsoeverwithoutpriorpermissioninwritingfromtheOxfordInstituteforEnergyStudies.ISBN978-1-78467-199-0Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.iiAbstractIfahydrogeneconomyistobecomeareality,alongwithefficientanddecarbonizedproductionandadequatetransportationinfrastructure,deploymentofsuitablehydrogenstoragefacilitieswillbecrucial.Thisisbecause,duetovarioustechnicalandeconomicreasons,thereisaseriouspossibilityofanimbalancebetweenhydrogensupplyanddemand.Hydrogenstoragecouldalsobepivotalinpromotingrenewableenergysourcesandfacilitatingthedecarbonizationprocessbyprovidinglongdurationstorageoptions,whichotherformsofenergystorage,suchasbatterieswithcapacitylimitationsorpumpedhydrowithgeographicallimitations,cannotmeet.However,hydrogenisnottheeasiestsubstancetostoreandhandle.Underambientconditions,theextremelylowvolumetricenergydensityofhydrogendoesnotallowforitsefficientandeconomicstorage,whichmeansitneedstobecompressed,liquefied,orconvertedintoothersubstancesthatareeasiertohandleandstore.Currently,therearedifferenthydrogenstoragesolutionsatvaryinglevelsoftechnology,market,andcommercialreadiness,withdifferentapplicationsdependingonthecircumstances.Thispaperevaluatestherelativemeritsandtechno-economicfeaturesofmajortypesofhydrogenstorageoptions:(i)purehydrogenstorage,(ii)synthetichydrocarbons,(iii)chemicalhydrides,(iv)liquidorganichydrogencarriers,(v)metalhydrides,and(vi)porousmaterials.Thepaperalsodiscussesthemainbarrierstoinvestmentinhydrogenstorageandhighlightskeyfeaturesofaviablebusinessmodel,inparticularthepolicyandregulatoryframeworkneededtoaddresstheprimaryriskstowhichpotentialhydrogenstorageinvestorsareexposed.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.iiiContentsAbstract.................................................................................................................................................iiContents...............................................................................................................................................iiiFigures..................................................................................................................................................iiiTables...................................................................................................................................................iv1.Introduction.......................................................................................................................................12.Overviewofthemaintypesofhydrogenstorage.........................................................................22.1.Purehydrogenstorage.................................................................................................................32.1.1.Compressedhydrogen..........................................................................................32.1.2.Liquefiedhydrogen................................................................................................52.2.Synthetichydrocarbons................................................................................................................52.2.1.Compressedandliquefiedsyntheticnaturalgas(SNG)........................................52.2.2.Syntheticgasoline(petrol)anddiesel....................................................................62.3.Chemicalhydrides........................................................................................................................72.3.1Ammoniaandmethanol.........................................................................................72.3.2.Formicacidandisopropanol.................................................................................82.4.Liquidorganichydrogencarriers(LOHCs)...................................................................................92.5.Metalhydrides..............................................................................................................................92.5.1.Elementalmetalhydrides......................................................................................92.5.2.Intermetallichydrides..........................................................................................102.5.3Complexmetalhydrides.......................................................................................102.6.Porousmaterials........................................................................................................................112.6.1.Carbon-basedmaterials......................................................................................112.6.2.Metal-organicframeworks(MOFs)......................................................................112.7.Overallevaluation......................................................................................................................123.Factorstoconsiderforinvestmentinhydrogenstorage..........................................................123.1.Furthertechnicalandtechnologyissuesandchallenges..........................................................143.2.Otherfactors,uncertainties,andbarriersforinvestmentinhydrogenstorage.........................164.Businessmodelsandpoliciesforhydrogenstorage.................................................................214.1.Rangeofpossiblebusinessmodels..........................................................................................224.2.1.Addressingthepricerisk.....................................................................................234.2.2.Addressingthedemandrisk................................................................................244.2.3.Choosinganoptimumbusinessmodelforhydrogenstorage..............................244.3.Furtherchallengesandquestions.............................................................................................255.Conclusion......................................................................................................................................26References..........................................................................................................................................29FiguresFigure1:Globalhydrogenconsumptionbyindustry...............................................................................1Figure2:Volumetrichydrogendensityandgravimetrichydrogencontentofbestperformingsubstancesforeachtypeofmajorhydrogenstorageoptions...............................................................12Figure3:Approximatevolumeandtotalweightcontaining100kgofH2ofbestperformingsubstancesforeachtypeofmajorhydrogenstorageoptions..................................................................................14Figure4:Estimatesofhydrogenstorageneedby2050vs.potential...................................................19Figure5:Rangeofpossiblebusinessmodels.......................................................................................23Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.ivTablesTable1:Keycharacteristicsofsomemajorhydrogenstorageoptions.................................................4Table2:Approximateindicatorsofhydrogenation/dehydrogenation,storagecapacityandtechnology,marketandcommercialreadinesslevelsfortheviewedhydrogenstorageoptions............................13Table3:Keyadvantagesanddisadvantagesofmajorhydrogenstorageoptions..............................15Table4:Approximatecapital,operationandmaintenancecostsofstoringpurehydrogenindifferentforms(USD2017/kWh)............................................................................................................................17Table5:StoragevolumeneededtoaccommodateEurope’s2-weekpeakenergydemandof326TWh17Table6:Somekeycharacteristicsofthemaingeologicaloptionsforundergroundhydrogenstorage...18Table7:TRL,MRL,CRLofvarioushydrogenstoragealternativesandtheiralignedfundingoptions...20Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.11.IntroductionHydrogen(H2)–themostabundantelementintheuniverse–iswidelyviewedasacrucialelementinthedecarbonizationstrategiesofmanycountriesinrecentyears(USEnergyInformationAdministration,2022).Thisissoprimarilybecauseofitsversatilepotentialapplicability(e.g.itcanbeusedasafuel,feedstock,andmeansofenergystorage)combinedwiththefactthatitdoesnotproducecarbondioxide(CO2)whencombusted1(AirLiquide,2022).Moreover,incontrasttofossilfuelswheredepositsaregeographicallylimitedtospecificgeologicalconditions,greenhydrogen–theH2generatedfromwaterwithrenewablepowerthroughtheelectrolysisprocess–couldpotentiallybeproducedanywhereintheworld,thoughnotwiththesamecostefficiency(PatoniaandPoudineh,2022).Thatiswhyanincreasingnumberofcountrieshavebeenenthusiasticallysettinggreenhydrogenproductiontargetsthatwouldsupposedlyhelptheminreachingtheirdecarbonisationtargetsorgenerateexportrevenueswhentheyhaveabundantlow-costrenewables(Power,2021).Atthesametime,becauseofvaryingeconomicconditionsaswellasdifferingcompetitiveadvantageinproducinglow-costdecarbonizedhydrogen,manycountriesalsorecognizedtheneedtoimporthydrogeninordertoachievetheirnet-zerocarbonaspirationsontime2.Forthispurpose,somehavesignedagreementsandmemorandaofunderstandingtoexplorethepossibilityoffuturehydrogensupplies(Landsvirkjun,2020,RWE,2021,dw,2022).Others,forexampleJapan,havegonefurtherandpilotedfirstlong-distanceshipmentsofhydrogenanditsderivatives(suchasliquidorganichydrogencarriersandammonia)fromremotelocationslikeBruneiandSaudiArabiatotheirshores(PatoniaandPoudineh,2022).Thatiswhy,overall,inanticipationoftheadventofaglobalhydrogeneconomy,bothhydrogenproductionandtransportaspectsofthehydrogenvaluechainhavealreadybeenfocusedonbyscholars,policymakers,andenergypractitioners.Onecrucialelementofthisvaluechainthathasbeeninsufficientlyexploreduptothispointisstorage.Theimportanceofhydrogenstoragecannotbeoverstated.Noviablebusinessmodelforhydrogenasaninternationallyorevenlocallytradedcommoditycouldpossiblyomitthefactthatthissubstanceinmostcaseswillhavetobestoredatleastrightafteritsproductionandbeforeitsdeliverytotheenduser.Obviously,withthecurrentmodelofhydrogenuseforindustrialpurposes(mostlyoilrefiningandammoniafertilizerproduction)3(Figure1),H2hasbeenconsumedprimarilyclosetoitsgenerationpointsothatbothstorageandtransportationofthissubstancehavenotplayedadecisiveroleinitsvaluechain(IEA,2019).Atthesametime,ifahydrogeneconomyisevertobecreated,thisapproachwillnolongerbethedominantformofthehydrogenvaluechain.Infact,increasingthetradabilityofhydrogenwillrequireconsideringthepeculiaritiesandchallengesassociatedwithpreservingH2indifferentquantitiesforvariousperiodsoftime.Figure1:GlobalhydrogenconsumptionbyindustrySource:WHA(2021)WiththegrowingdemandandacceleratedmanufacturingofH2aroundtheworld,theneedforhydrogenstorageislikelytoriseproportionally.Themaindriverofdemandforhydrogenstorageislikelytobetheeventualimbalancebetweentheproductionandconsumptionofhydrogen.Forinstance,inthefuture,1Althoughhydrogenisviewedasasubstancethatcouldbeusedforvariousdecarbonisationpurposes,itismostoftenregardedasanelementthatcouldpotentiallyreplacefossilfuelsassourcesofenergy(Qazi,2022).Inthisconnection,ifthisreplacementtakesplaceinsomeform,hydrogenwillnotbeaprimaryenergysource(becauseitwillhavetobeproduced)butwillratherservethepurposeofenergystorageinachemicalform(Mohammadi-Ivatloo,Mohammadpour,andAnvari-Moghaddam,2021).2Forinstance,inhisaddresstotheEuropeanParliament,FransTimmermans,theEuropeanCommissionerforClimateAction,admittedthat‘Europe[was]nevergoingtobecapabletoproduceitsownhydrogeninsufficientquantities’(Recharge,2022a).3Forinstance,forammoniaproduction,hydrogenisnormallygeneratedfromthemainfeedstock(usually,naturalgas)asapartoftheproductionprocessandisconsumedonsite(PatoniaandPoudineh,2020).55%25%10%10%Ammonia(fertiliser)productionOilrefiningMethanolproductionOtherThecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.2manufacturingrateofblueandturquoiseH2–hydrogengeneratedfromfossilfuelswithcarboncapture,utilization,andstorage(CCUS)–willbereasonablyflat(linear)toprovidethemaximumefficiency(Megiaetal,2021).Atthesametime,thisproductionpatternisunlikelytocoincidewithhydrogendemandallthetime,whichcouldbeaddressedthroughstorage.Indeed,storageisalreadyakeycomponentofexistingfossilfuelsupplychains.Moresignificantly,ifhydrogenisproducedthroughelectrolysisusingsolarandwindpower,thegenerationprocesswillbeintermittent.Here,apartfromdailyandseasonalvariabilityassociatedwithlower/highermagnitudeofwindspeedandsolarirradiationatdifferenttimeofthedayandyear,weatherconditionscausingtheso-called‘Dunkelflaute’events–anticyclonicgloomywindlessdayswhenlittleornoenergycanbegeneratedbywindandsolar(Matsuoetal,2020)–willsignificantlyimpacthydrogenproduction.Asaresult,H2manufacturingmaynotcoincidewiththetimeofitspeakconsumption.Hereagain,hydrogenstoragewillbeofcriticalusetomeetmakeboththesupplyanddemand,sincethedemandsidemaynotbeextremelyresponsivetooverproductionorunderproductionofhydrogen4.Apartfromthat,withthevariablenatureofwindandsolar–theenergysourcesthataregoingtoplayanevengreaterroleinadecarbonizedenergysystemofthefuture–hydrogenstoragecouldbecomeameansofgridbalancingwhenoverproductionandunderproductionissuesoccur.Currently,whenrenewablegenerationoftenhastobecurtailedduetoovergenerationorlocalnetworksissues,convertingelectronsintomoleculesandbackthroughthepower-to-Xtechnologiescouldhelptoavoidtheseissues(ITMPower,2022).Asaresult,H2storagewouldplayanextremelyimportantroleintheentiredecarbonizationprocess,sinceitwouldfacilitatefurtherspreadofrenewableenergysourcesthroughofferingbothbackupandseasonalstorageoptionswherebatteriesandpumpedhydrohavesignificantcapacityorgeographicallimitations5.Thisissobecausenaturalgas,whichismostoftenstoredtocontributetomeetingseasonalenergydemand,isnotacarbon-freesolutionandthusshouldbesubstitutedwithamoresustainableone.Inthiscontext,anincreasingnumberofresearchersviewhydrogenanditsderivativesassuchsubstitutes(Guerraetal,2020).Ontheotherhand,despitealltheadvantagesofhydrogenstorageaswellastheopportunitiesthatitmaybring,keepingandpreservingH2forlatertransportationorconsumptiondoesnotappeartobetheeasiesttask.Infact,duetotheverynatureofthissimplestofallelementsthatiseasilylostintotheatmosphere,hydrogenstorageisgenerallyachallengingundertaking6.Additionally,becauseofitsextremelylowvolumetricenergydensity,pureH2needstobeeithercompressedorliquefied.Withbothprocessesbeingenergyintensiveandexpensive7,itmaynotalwaysbeeconomictostorehydrogeningaseousorliquidforms.Inthesesituations,convertinghydrogenintoothersubstancesthatareeasiertohandleandstorecanbeadvantageous.Atthemoment,however,thereisnoclearstanceonwhichhydrogenstorageoptionistheonethatwilloffermoreadvantagesthantherest.Asaresult,itisstilluncertainiftherewillbeasinglepreferredvariantthatwillbeadoptedbymoststakeholdersintheto-be-createdhydrogeneconomy.Infact,differenthydrogenstoragesolutionsarelikelytobepreferreddependingoncircumstances.Thispaperthusevaluatestherelativemeritsofalternativesindifferentapplications.Theoutlineofthepaperisasfollows.Section2providesanoverviewandcomparisonofmajortypesofhydrogenstorageoptionsthatarecurrentlybeingexploredandconsideredbyscholars,policymakersandbusinesses.FactorstoconsiderforinvestmentinhydrogenstorageanddiscussiononmajorbarriersforsuchaninvestmentarepresentedinSection3.Section4,inturn,elaboratesonthebusinessmodelsandpolicyoptionsthatwouldfacilitateinvestmentinH2storagefacilities.Finally,theSection5providesconcludingremarks.2.OverviewofthemaintypesofhydrogenstorageAlthoughhydrogenisconsideredtobeoneofthekeyelementsintheglobaldecarbonisationdebate,itsstorageismorecomplicatedthanstorageofmanyotherenergysourcesthatwearemuchmoreusedto,suchascoal,oil,andevennaturalgas.ThisissoprimarilybecausetheH2moleculeisthesmallestandlightestin4Theneedforstoragewouldbelessacuteincaseofveryresponsivedemand.Inthiscontext,incentivesfordemand-sideflexibilitycouldpotentiallybeusedasacompetitivealternativetomoreshort-durationstorage.5Atthemoment,chemicalenergystorageisgenerallymoreadvantageoustoelectrochemical(batteries)solutionsandhasfewerlimitationsifcomparedwithmechanical(pumpedhydro)options(PatoniaandPoudineh,2020).6Forinstance,beinghighlyflammableperse,whendispersedintotheair,hydrogenbecomesexplosive.7Forexample,liquefactionthatallowsforreachinghydrogen’sgreaterdensitypresupposescoolingitdownto-252.9oCandconsumesmorethan30percentoftheenergycontentofthehydrogen(USDepartmentofEnergy,2022).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.3theentireuniverse(AriLiquide,2022).This,combinedwiththefactthathydrogenisagasunderambientconditions,makesits‘taming’andpreservationparticularlychallenging.Tomakehydrogenstoragelessproblematicandmoreeconomicallyfeasible,itsdensityshouldbeimprovedsothatthevesselwhereitispreservedcouldcarrytheamountofenergygreaterthanoratleastcomparabletothatofmorecommonlyusedfossilfuels.Ingeneral,however,undernormaltemperatureandpressure,H2hasextremelylowvolumetricdensity(around0.0812kg/m3)(Mølleretal,2017).Thisindicatorgraduallyriseswhenhydrogeniscompressedorliquefied.Forinstance,under700bar,H2’svolumetricdensityisalready42kg/m3and,asaliquid,itnears70.8kg/m3(ibid).Nevertheless,thiswouldstillnotturnitintotheenergycarryingchampion,asothersubstancesmaybearmoreenergywithinthesameunitofvolume(Table1).Thatiswhy,althoughhydrogenhasthehighestgravimetricenergydensityofallknownsubstances(120-142MJ/kg),itfallsshortwhencomparedtoconventionalfossilfuelswithrespecttovolumetricenergydensity(GENH2,2022).Insimpleterms,fora1-kgweightlimit,hydrogenwouldcontainthemostenergy,whilefora1-litertankvolumelimit,otherfuelswouldbecapableofcarryingmoreenergy(ibid).Inthisconnection,althoughhydrogenoftenrepresentsthefocalelementinenergytransitionanddecarbonisationdebates,improvinghydrogen‘storability’mayrequirethiselementtobeconvertedintoothersubstancesthatwouldcontainH2butwouldsimultaneouslybeeasiertodealwithandwouldhavehighervolumetrichydrogendensity.Nevertheless,thepreferredpreservationoptionwoulddependnotonlyontheH2densityandcontentcharacteristicsbutalsoonmanyotherfactorsincludingstoragevolume,duration,safetyaswellasthepurposeforwhichthestoredhydrogenwouldultimatelybeused.Ingeneral,H2anditsderivativescouldbestoredinthreeaggregatestates(gaseous,liquid,andsolid)(Table1).Thehydrogenstorageoptionsthatarecurrentlyattractingthegreatestattention,inturn,couldbedividedintosixmajorgroups(types):(i)purehydrogenstorage,(ii)synthetichydrocarbons,(iii)chemicalhydrides,(iv)liquidorganichydrogencarriers,(v)metalhydrides,and(vi)porousmaterials(ibid).2.1.PurehydrogenstorageAtthemoment,purehydrogenstoragecanbedoneintwoways:H2canbeeithercompressedorliquefied(USDepartmentofEnergy,2021).Hence,hydrogencanbepreservedphysicallyaseitheragasoraliquid.Althoughstoringhydrogeninapureformmaynotalwaysbethemosteconomicallyadvantageousoption,therearestillseveralbenefitsthatthistypeofstoragemaybring.Firstandforemost,physicalstoragerepresentsthemostmaturehydrogenstoragetechnologyavailableatthemoment(USDepartmentofEnergy,2020).ThisisalsothetechnologythatallowsusingpureH2directlyafteritspreservation–thatis,withoutanyconversion.Nevertheless,bothcompressedandliquefiedhydrogenstoragehavesignificantdisadvantagesoverotherstorageoptionsthatneedtobeconsideredaswell.2.1.1.CompressedhydrogenHydrogenstorageinaformofgasnormallyrequireshigh-pressuretanks(350-700bar)(USDepartmentofEnergy,2021).AlthoughthismeansthatcompressingH2willbeassociatedwithenergyuse(around6kWh/kgforcompressionto700bar)andthusfurtherexpenses,noadditionalstepswouldneedtobetakenincomparisontothepreservationmethodsnormallyassociatedwithhydrogenderivatives(e.g.,hydrogenation/adsorption,etc.)(Folkson,2014).Apartfromthat,providedallthetechnologicalrequirementsforthestoragevesselsandfacilitiesaremet,theveryprocessofstoringcompressedhydrogenaswellasextractingitfromthestoredvesselwouldnotrequiresignificantenergyuse(Table1).This,incombinationwiththeabilitytokeephighpurityofthepreservedH2aswellasrelativelyhighspeedofinjectioninto/withdrawalfromthestoragevessel,makescompressedhydrogenstoragetheoptionthatiscurrentlypreferredforusageinfuelcellvehicles(Elberryetal,2021).Infact,boththefuellingtime(normally,under3minutesforastandardtankofaround0.2m3)aswelltheenergyneededtocoverthedrivingrangeofover500kmarecomparabletothoseofconventionalgasoline-anddiesel-propelledcombustioncars(Sloth,2013).Additionally,compressedhydrogenstorageisalsotheonlymajorH2preservationoptionforlarge-scale(e.g.,country-scale)storagepurposes–thatis,itisthealternativethatisoftenexpectedtoreplaceundergroundnaturalgasstorage8(Elberryetal,2021).8Although,theoretically,syntheticnaturalgas(SNG)couldbeusedforthesamepurposes,itsgenerationatascalecomparabletotheoneofnaturalgasextractionisyettobeachieved(ifatall)duetotechnologyandfinancialconstraints(see2.2.Synthetichydrocarbons).4Table1:Keycharacteristicsofsomemajorhydrogenstorageoptions9Source:AdaptedfromRyabov(1976),AlyeaandKeane(1993),Eliazetal(2000),Becheretal(2003),Tzimasetal(2003),Graetzetal(2006),Hodoshimaetal(2006),Eigenetal(2007),AMF(2010),BanerjeeandTyagi(2011),Foudakis(2011),Subrahmanyametal(2011),MazloomiandGomez(2012),TozziniandPellegrini(2012),Gaoetal(2013),Nielsenetal(2013),Tianetal(2013),FrancoandCasarosa(2014),Balakhonov,Vatsadze,andChuragulov(2015),LototskyyandYartys(2015),Nishiharaetal(2017),Sivasubramanianetal(2017),EscolaEuropea(2018),EuropeanCommission(2018),Garcia-Holleyetal(2018),Haoetal(2018),Mustafaetal(2018),Romanosetal(2018),Schollenbergeretal(2018),Younisetal(2018),Wangetal(2018),Zhongetal(2018),AnderssonandGrönkvist(2019),Lietal(2019),Modishaetal(2019),Potekhin(2019),Wijayantaetal(2019),Dingetal(2020),Gattia,Jangir,andJain(2020),HimMax(2020),Huynhetal(2020),Kazakov,Bodikov,andBlinov(2020),RaoandYoon(2020),Asifetal(2021),Jalid,Khan,andHaider(2021),Sultana,Saha,andReza(2021),Thomasetal(2021),Camposetal(2022),CompositesWorld(2022),Thijs,Ronge,andMartens(2022),Valentini,Marrocchi,andVaccaro(2022),VatsaandPadhi(2022),Weietal(2022),Wilkleretal(2022).9Valuesrepresentedinthetableareapproximateandbasedonthedatafromavailableintheliterature.Thatiswhytheycanvaryforspecificcasesandshouldnotbetakenasprecisenumbers.10Energyusedforstoragedependsonsuchfactorsastank/storagevesselinsulation,efficiencyofBOGreliquefication(forliquidstorageofgaseoussubstances),etc.-production,-compression,-liquefaction/hydrogenation/sorptionStateStorageformsChemicalformula/exampleMolarmass(g/mol)Gravi-metricenergydensity(MJ/kg)VolumetricH2density(kg/m3)Gravi-metricH2density(wt%)TypicalconditionsforH2(ad-/physi-)sorption/hydrogenation/productionTypicalconditionsforstorageTypicalconditionsforH2desorption/de-hydrogenation/releaseTempe-rapture(oC)Pressure(bar)Energy(kJ/mol)Tempe-rature(oC)Pressure(bar)Energy(kJ/molperday)10Tempe-rature(oC)Pressure(bar)Energy(kJ/mol)GasCompressedhydrogen(700bar)H22.016120-14242100Ambient700~9.798Ambient700<10n/aSynthetichydro-carbons(e-fuels)Compressedsyntheticmethane/naturalgas(SNG)(250bar)CH416.04353.6-55.6~32.2~25.13250-35030-40250~206>1.8200-250>1.9700-10003-25~165LiquidLiquefiedSNG~101.78>68~19.008~-1610.3-1619.06-34.7Syntheticgasoline(petrol)C8H1860-15044-46.4~119.816400-500>20048.5-61AmbientAmbientn/a>5001-4<40SyntheticdieselC12H23198-20245.4-45.6~119.114700-1500200-700~80~80048.5-61LiquidhydrogenH22.016120-142~70.8100-252.8Ambient>25.66-252.8>28.3n/aChemicalhydridesLiquidammoniaNH317.03121.18-22.5107.7-12017.65300-500140-250~92.4~0.9-33~1350-9001-1030.6-46Methanol(MeOH)CH3OH32.0420.1-22.495.04-9912.1200-30010-70>41.2Ambientn/a250-90025-50>70FormicacidCH2O246.03~4.58~534.390-1406-10~34.7150-225~Ambient~29.81Isopropanol(i-PrOH)C3H8O60.1~34.1~25.93.320-6560-20040-4870-1950.5-1.5~61.4LiquidorganichydrogencarriersToluene/Methylcyclohexane(MCH)C7H8/C7H1498.186~7.3547.1-47.46.16>350Ambient10.5-18.4~3501-9~68Naphtalene/decalinC10H8/C10H18138.25~42.97~65.47.29~280>100~16.3~240~3563.9-68.3Benzene/cyclohexaneC6H6/C6H1284.16~3.9~55.97.2070-150<20~119.5~4001-889-138Dibenzyltoluene(DBT)/perhydro-dibenzyltoluene(PDBT)C21H20/С21H33290.54~12.9~646.20>15015-50~171300-390<4~65.4SolidMetalhydridesElementalmetalhydridesMagnesiumhydrideMgH226.329-10.886-1096-7.6260-42530-300~70.6Ambient-40n/a-0.6250-400~Ambient74.7-118AluminiumhydrideAlH329.99>36.68~148~10.1~6001-350~10485-14075-135~20Inter-metallichydridesAB5-typeLaNi5/LaNi5H6432/438.440-60~1051-1.520-801.5-2.512.27-40~Ambient1.6~54.3AB2-typeZrMn2/ZrMn2H201/202.1~1002.15-3.820-5030-60>20Ambient-2001-250>29.9AB-typeTiFe/TiFeH104/104.7~90<5.4300-40010-6510-28.1Ambient-401-2510-28ComplexmetalhydridesAlanatesNaAlH454~65~543.5-5.4~1006-1257.4-11885-2606-6679-92BorohydridesLiBH421.78~121~18.5600-700100-20056.37-88300-450>330-59AmidesLiNH222.96<544.5-5.2~150>20~55.2285-500~Ambient40.4-73.6PorousmaterialsCarbon-basedCarbonfibresCn(C3H3N)n12.01(carbon)0.8-2~18<5.44~-196-ambient1-406-11~-196-ambient<2504.4-12160-50056.5-238Carbonnanotubes5-10135-325ActivatedcarbonCH2O2~0.065516.70.1-7.5<59GrapheneC70H30~0.916-171-7.7<100CarbonaerogelV2O5•nH2O0.014-0.023<4.8<65TemplatedcarbonC45H6O2~0.3<175.5-7.3100-340Metal-organicframeworks(MOFs)Cr3F(H2O)2O(BDC)3~709.4~0.57~11.5<10~100~51.215-8060-85~5~78.7Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.5Ontheotherhand,althoughhydrogencompression(to700bar)isalmostthreetimeslessenergyintensivethanitsliquefaction,theamountofenergythatisrequiredisstillquiteimpressive–itiscomparableto13-18percentofthelowerheatingvalue(alsoknownasnetcalorificvalue)(Jensen,Vestbø,andBjerrum,2007).Inaddition,whenH2iscompressedto700bar,itsvolumetricenergydensitywillbemuchlower(5.6MJ/l)thanthatofsuchfuelsasgasoline(32.0MJ/l)(Elberryetal,2021).Althoughthischallengecouldpartiallybeaddressedthroughacombinationofahigh-pressurecylinder/tankandasolid-statematerialcontaininghydrogen(see2.5.Metalhydridesand2.6.Porousmaterials)(Mølleretal,2016),ineconomicterms,preservingcompressedhydrogeninstoragevesselsofgreatervolume(e.g.,tanksorspherestransportablebymarinevessels)maynotseemtobeequallyasattractiveasstoringliquidH2oritsderivatives.Besides,storinghighlycompressedgasesrequiresadditionaladjustmentstothedesignandimplementationofthestoragecylindersbecauseofsafetyconcerns,whetherperceivedorreal(Gupta,Basile,andVeziroğlu,2015).2.1.2.LiquefiedhydrogenOfferinghighervolumetricdensity(70.8kg/m3),whichisalmosttwiceofthatassociatedwithhydrogenunder700bar(42kg/m3),liquefiedH2canthuspreserveagreateramountofhydrogeninthesameunitofvolume(Viswanathan,2016).Inaddition,withfewerpotentialrisksincomparisontothosethatcompressedgasesnormallyhaveaswellassimilarbenefitsofhighH2purityandrelativelyquickrefuelling,thishigherdensityadvantageofliquidhydrogenstoragewastakenintoaccountbyNASAwhentheyoptedforthismodeofhydrogenstoragefortheirSpaceShuttleProgram(NASA,2018).ThisrationalealsoseemstoliebehindthelogicofKawasakiHeavyIndustries’projectthatbuilttheworld’sfirstliquefiedH2carrierthatdeliveredtheworld’sfirstliquefiedhydrogencargofromAustraliatoJapaninFebruary2022(Recharge,2022b).Atthesametime,themainchallengeassociatedwithstoringliquefiedhydrogenistheneedforcryogenictemperatures(-252.0oC)thatcankeeptheH2inliquidform,which,inturn,isassociatedwithsignificantenergyuse.Infact,althoughtheminimumtheoreticalenergytoliquefyH2fromambientconditionsis3.3-3.9kWh/kgLH2,theactualliquefactionenergyrequirementsaresubstantiallyhigher–atleast10-13kWh/kgLH2,dependingonthesizeoftheliquefactionoperation,theoriginaltemperatureofhydrogenandotherfactors(USDepartmentofEnergy,2009).Asaresult,althoughwithnovelliquefactionmethods,suchasanactivemagneticregenerativeliquefier,lessenergywillberequiredintheprocess,liquefactionwithtoday’sfullycommercializedprocessesneed30-40percentofthelowerheatingvalue(ibid).Storinghydrogenasaliquidcooledtocryogenictemperaturesalsorequirestakingadditionalmeasurestopreventandminimizeboil-offgas(BOG)challengesthatwouldresultinhydrogenloss,unlessitisreliquefiedorutilizedinanefficientmanner(Viswanathan,2016).SincebothH2’sproductionandliquefactionarehighlyenergyintensive,BOGmanagementplaysanimportantroleinpreservingtheliquidhydrogenproduced.Thatiswhyvesselsstoringliquidhydrogenrequiresophisticatedinsulationtechniquestominimizeunavoidableheattransferleadingtohydrogenloss(ibid).2.2.SynthetichydrocarbonsLimitationsassociatedwithhydrogen’slowvolumetricdensityaswellashighenergyintensityofcompressionandliquefactionandtheresultingsafetyandcostchallengesmadescholarsandenergycompaniesconsideralternativewaysofpreservingthissubstance.Inthiscontext,convertinghydrogenintosynthetichydrocarbonsandbackappearstobeanoptionthatisgraduallygainingattention.Here,thelogicisquitesimple–throughcombiningsustainablyproducedhydrogenwithcapturedcarbon(eitherfromairorindustrialprocesses),itwouldbepossibletosynthesizeoneofthefuelsthatarealoteasiertostoreandtransportthanH2itself:forexample,methane,gasolineordiesel(Lee,Seidl,andMeyer,2021).Intheend,whenhydrogenneedstobeextracted,separationofcarbonanditsultimatedeliverybacktothepointofgenerationofsynthetichydrocarbonswouldtheoretically‘close’theloopandmaketheentireprocesscarbon-neutral11.This,incombinationwithalreadywell-developedinfrastructurethatwasoriginallytailoredforhydrocarbons,makesthisoptionveryattractive.2.2.1.Compressedandliquefiedsyntheticnaturalgas(SNG)Theterm‘syntheticnaturalgas’(SNG)generallyrelatestoavarietyofnaturalgasalternativesthatareascloseaspossibleincompositionandpropertiestonaturalgas(MANEnergySolutions,2022).AlthoughSNG,inprinciple,canbederivedfromvarioussources(incl.coal,biomassandwaste),themostcommonconceptbehindtheproductionofcarbon-neutralSNGatscalethatiscurrentlybeing11Here,theround-tripefficiencyremainsquestionablethough.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.6debatedrestsontheprocessthatproducesmethaneandwaterfromareactionofhydrogenwithcarbondioxide–theSabatierreaction:CO2+4H2→CH4+2H20Here,anidealscenariowouldbetocouplegreenhydrogenproductionwiththecaptureofindustrially-generatedCO2ortheonedirectlyfromtheairsothatSNGismanufacturedspecificallyforstorageandtransportationpurposesandthenisdehydrogenated–hydrogenis‘extracted’fromit–beforethefinaluse(Navajasetal,2022).Withanalreadywell-developedstorageanddeliveryinfrastructurefornaturalgas,preservingandtransportingSNGwouldbenodifferentandthusnoinfrastructureadjustmentswouldbeneeded,whichwouldalsomeannorelatedcostswouldbeincurred(ibid)12.While,intheory,theSabatierreactioncouldplayanimportantroleinsolvingtheclimatechangeproblemthroughcreatingcleansyntheticnaturalgasandhydrogenfuelfromthecapturedCO2andwater,thedeploymentofthisreactionatanindustrialscalehassofarbeenlimited.Someofthekeyreasonsfortherestrainedapplicabilityofthismethanationrelatetoelevatedtemperatures(250-350oC)andpressures(around30-30bar)aswellastheneedtouseanickel,rutheniumoralumina(aluminiumoxide)catalyst,whichresultsinhighenergydemand,significantcostsandgenerallyloweconomicefficiency(Waietal,2020).Although,foranet-zerocarbonscenario,someofthesechallengescouldbemitigatedviabiologicalmethanation,sinceitreplacesmetalliccatalystswithbiocatalysts(methanogenicmicroorganisms)andworksundermuchlowertemperatures(35-70oC)andpressures(1-15bar),theindustrialapplicationofbiomethanationhassofaralsobeenlimitedmostlybecausemicroorganismsrequirealotmorespaceandtimetoachievethesameproductionasacomparablyratedSabatierprocess(Ferrari,2020).ThatisperhapswhytheInternationalSpaceStationseemstobetheonlyplacewheremethanation(throughtheSabatierprocess)hasbeencontinuouslyused,thoughnotforthepurposeofmethanesynthesis13(NASA,2011).SinceSNG’sstructureisbroadlysimilartothatofnaturalgas,it’spreservationwouldhavethesameadvantagesifcomparedtohydrogenstorage,suchaslowerenergyuseforcompression(normally,to200-250barforsyntheticCH4(methane)insteadof350-700barforH2)andthuslowercostsanddurabilityrequirementsforstoragevesselsandfacilities(Navajasetal,2022).However,asseenfromTable1,evenifcompressedto250bar,SNG’svolumetrichydrogendensitywouldbequitelow(around32.2kg/m3),which,inpartwithits‘medium’gravimetrichydrogencontent(around25.13wt%)maynotmaketheenergy-intensiveandthusexpensivesynthesisprocessquiteworththeeffort.Thisisespeciallythecaseforsmall-scalepreservationofhydrogen,giventhatasimilaramountofenergywouldthenbeneededtodehydrogenateSNGfortheultimateuseofH2.Insuchcircumstances,evenwithadditionalenergyconsumption,liquefiedSNGmaybecomeabetteroptionformedium-scalestorage14,asitgreatlyincreasesthevolumetrichydrogendensity(toaround101.78kg/m3)(Table1).Inanycase,althoughusingSNGinbothcompressedandliquefiedformswouldrequirenomajorinvestmentinitsstorageanddeliveryinfrastructure,makingthecarbonmanagementa‘closedloop’wouldneedthecreationofstorageandtransportationfacilitiesfortheCO2involved.This,inturn,wouldresultinadditionalcosts.2.2.2.Syntheticgasoline(petrol)anddieselTofurthersimplifythestorageprocess,syntheticfuelsthatareliquidunderambientconditionscouldbeused.Here,bothsyntheticgasoline(C8H18)anddiesel(C12H23)thatdonotneedtoincludeany‘liquefaction’intheirvaluechainsalsohavesignificantlyhighervolumetrichydrogendensity(around119.8kg/m3and119.1kg/m3forsyntheticgasolineanddiesel,respectively)(Table1).However,despiteofferingtheadvantagesofwell-developedinfrastructureandnear-zerostoragecosts,thesefuelshaveanumberofsignificantdrawbacksthatmaypreventthemfrombeingconsideredasfeasibleoptionsforhydrogenpreservation.Infact,thediscussedcarbon-neutralprocessofmanufacturingsyntheticgasolineanddieselisbasedontheFischer-Tropschprocess,whichrepresentsacollectionofchemicalreactionsconvertingamixtureofcarbonmonoxideandhydrogenintoliquidhydrocarbons:12ThisistheideacurrentlypursuedbysuchcompaniesasTreeEnergySolutions(TES)aimingtocompleteanew‘greengas’terminalattheGermanportofWilhelmshaventhatissupposedtobeabletostorethedeliveriesofits‘carbon-neutral’liquefiede-methanebeforethewinterof2025(TES,2022).13TheInternationalSpaceStationusestheSabatiersystemtoproducewaterfromcarbondioxidegeneratedbycrewmetabolismandhydrogensynthesisedwhilegeneratingstationoxygen(NASA,2011).Whilewateristhenretainedforrecyclingprocesses,themethaneisventedoutsideofthespacestation(ibid).14Naturalgasisusuallystoredatalargescaleinundergroundstoragefacilitiesasagascompressedtoaround200bar(INES,2022).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.7(2n+1)H2+nCO→CnH2n+2+nH2OTypically,thesereactionsneedhightemperatures(400-1500oC)andpressures(over200bar)aswellastheuseofcobalt,ironorrutheniumcatalysts(Hööketal,2014)(Table1).Thesehightemperatures,pressures,andthusenergyrequirementsgenerallymakenaturalgas,coal,andbiomassthemostcommontypesoffeedstock,thoughalotmoreattentionhasbeenrecentlypaidtothedevelopmentofeconomicallysoundprojectsfore-fuelproductionfromwater,air,andelectricityastherawmaterialsneeded(LeeEnterprisesConsulting,2021)15.Onamoregeneralnote,whileitisstillperceivedbymanythatmanufacturingsyntheticfuelsthroughtheFischer-Tropschprocesswouldonlybeeconomicallyfeasibleatveryhighoilpricesand/orwhenheavilysubsidised(Wangetal,2017),evenwithimprovedefficiencyanddramaticallyloweredcost,bothsyntheticdieselandgasolinewillremainhydrocarbonsandthussubstancesthatwouldneedtobedehydrogenatedifH2needstobeused.Inthisconnection,justasinthecaseofe-methane,dehydrogenationwouldalsonecessitatepropercarbonmanagementandmostlikelythecreationofcarbonstorageanddeliveryinfrastructurefromscratch16.2.3.ChemicalhydridesChemicalhydridesrepresentanothergroupofsubstancesthatcouldpotentiallybeusedforhydrogenpreservation.Althoughthiscategoryiscomprisedofmorediversechemicalsthansuchassyntheticfuels(e.g.,theycanbemanufacturedvianon-relatedprocessesinaverydifferentway),theirstorageisgenerallysimilartothatofsynthetichydrocarbonswithallthemainadvantagesofminimumenergycosts,noneedforinfrastructuredevelopment,etc.inplace.Apartfromthat,whiletheirvolumetrichydrogendensityandgravimetrichydrogencontentcharacteristicsaresimilartothoseofsyntheticfuels,theirproductionanddehydrogenationismostlylessenergyintensiveandthuscheaper.2.3.1AmmoniaandmethanolAmmoniaandmethanolrepresentthemostprominentchemicalsinthisgroup,astheyareoftenviewedassomeoftheclosestrivalstohydrogenitselfforH2storageanddeliverypurposes(Aziz,Wijayanta,andNadiyanto,2020).Thisissoprimarilyduetoadecentcombinationoftheirrelativelyhighvolumetrichydrogendensity(107.7-120kg/m3forliquidammoniaand95.04-99kg/m3formethanol)andhighgravimetrichydrogencontent(17.65wt%forliquidammoniaand12.1wt%formethanol)(Table1).Here,althoughammoniaisgaseousunderambientconditionsandthusneedsliquefactiontofacilitateitsstorage,incontrasttohydrogenandSNGitneedsalotlessenergyforthat,sinceitonlyneedstobecooleddownto-33oC,whereasSNGandH2needtobecooleddownto-161and-252.8oC,respectively(ibid).Methanol,ontheotherhand,isalreadyliquidundernormalatmosphericpressureandtemperatureandthusdoesnotneedanyadditionaltransformations.Whenitcomestousingahydrogenderivativethatwouldbecompletelycarbonneutral,ammonia(NH3)wouldrepresentthewinner,since,incontrasttomethanol(CH3OH),itdoesnotcontaincarbon(C)atomsinitsmoleculeandthuswouldnotneedcarbonmanagementafteritiscracked.Besides,althoughitsmostcommonproductionmethodrestsonnaturalgasasthemainfeedstock,power-to-ammoniatechnologieshavealreadybeenappliedatscalefordecadeswithsuchcountriesasEgypt,Iceland,India,Norway,andPeruwhichconstructedcost-competitivelarge-scalerenewableammoniaplantsafter1945(Krishnanetal,2020).Infact,thelargestrenewableammoniaplanttodatewasbuiltinthe1960s17,inAswan,Egyptasaresponsetotheabsenceofnaturalgasandinordertoprovidefoodsecurityforthecountry,astheammoniawouldthenbeusedtoproducefertilizers(ibid).Atthesametime,althoughtheproduction,storage,andtransportationofammoniahasbeenwidelyused(primarilyduetothefertiliserindustry),ammoniacracking–i.e.‘separation’ofH2fromNH3–mayrepresentalesswell-developedstagethatneverthelessneedstobeincludedinthehydrogenvaluechain,ifammoniaischosenasthestorageoptionforhydrogen.Thisstage,however,being,inessence,similartosteammethanereforming,alsorequireshightemperatures(350-900oC)andelevatedpressure(upto10bar)andtakesplaceinthepresenceofanickelcatalyst(JohnsonMatthey,2022).15Here,Audi’se-gasolineande-dieselproductioninitiativethatissupposedtobeindustriallypilotedinthecompany’snewplantinSwitzerlandseemstobethemostscaleduponeatthemoment(Audi,2018).Thisinitiative,however,stilldoesnotcompletelyeliminateCO2emissions–itisreportedtoonlyreduceitbyaround80percent(ibid).16Althoughtheproductionanduseofe-fuelsmaypotentiallymakesensetolowerthecarbonimpactoftransport,withallthecomplexitiesandquestionableeconomicrationalemaketheiruseforhydrogenstorageandtransporthighlydebatable.Here,itmaybemorecost-efficientandreasonabletoimplementcarboncapture,utilisationandstoragetechnologies–i.e.completelyomittingtheuseofhydrogen–tomaketheentireprocesscarbon-free.17Withaproductionof400-500tonnesofNH3perday,theammoniaplantinAswanwasthelargestrenewableammoniaplanteverbuilt(Krishnanetal,2020).ItwasoriginallyoperatedwithalkalineelectrolysersfromDeNora,buttheywerelaterreplacedwiththeonesfromBrownBoveri(ibid).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.8Hence,itisenergy-intensiveandwouldresultinfurthercosts,aswouldthepropernitrogenmanagementafterthecrackingprocess(2NH3⇌N2+3H2).Similaristrueformethanol,whichshouldalsobedehydrogenatedbeforeH2couldbeused18(AlbericoandNielsen,2015):CH3OH⇌CO+2H2;CO+H2O→CO2+H2Justasinthecaseofammonia,dehydrogenationofCH3OHisanendothermicreactionanditrequireshightemperatures(250-900oC),elevatedpressure(25-50bar)aswellashomogenousprecious-metal-basedcatalysts(Wakizakaetal,2016).Asaresult,althoughtheelectrificationoftheprocessispossible,theexhaustivemethanoldehydrogenationiscurrentlyrealisedprimarilythroughsteamreformingrunmostlyonfossilfuels(AlbericoandNielsen,2015).Theconditionsthatareneededfortheproductionofmethanolarequitesimilartothoserequiredforthesynthesisofammoniaandthatiswhyitmostoftenusesnaturalgasasafeedstock,butcouldalsobegeneratedthroughthecombinationofgreenhydrogenandcapturedcarbon(HaldorTopsoe,2022):CO+2H2⇌CH3OH;CO2+3H2⇌CH3OH+H2OIncontrasttogreenammonia,however,theworld’sfirstindustrial-scalee-methanolproductionprojectisyettobefinishedbyEuropeanEnergy(EuropeanEnergy,2022).Intheseconditions,whennolarge-scaleproductionofgreenhydrogen-basedmethanolisavailable,relyingonthissubstanceforthefuturepreservationoflargequantitiesofH2fortheto-be-createdhydrogeneconomyisquestionable.2.3.2.FormicacidandisopropanolApartfromammoniaandmethanol,formicacid(CH2O2)andisopropanol(C3H8O)areoftenviewedastwootheralternativesforthepreservationofhydrogen.Here,thesamelogicapplies:theirvolumetrichydrogendensity(around53kg/m3forformicacidand25.9kg/m3forisopropanol)aswellasgravimetrichydrogencontent(4.3and3.3wt%forformicacidandisopropanol,respectively)aremakingthemsuitableforbeingusedforhydrogenstorage(Table1).Inadditiontothat,althoughtheircharacteristicsarelessimpressivethanthoseof,forexample,,ammonia,bothareliquidundernormalatmospherictemperatureandpressure.Atthesametime,sincebotharecurrentlymostlyproducedfromotherfossilfuels-derivedchemicals,theattentiontothemhasbeenlesssignificantincomparisontomanyotheralternativesforH2storage.Atthemoment,mostoftheformicacidcurrentlyproducedatanindustrialscaleismadefromcarbonmonoxide,eitherbyheatingitwithsodiumhydroxidetoproducesodiumformate,whichisthenacidified(EppingerandHuang,2017):NaOH+CO→HCOONa→HCOOHAlternatively,itisgeneratedviathebase-catalysedreactionofcarbonmonoxide(CO)andmethanoltomakemethylformate,whichisthenhydrolysedtotheacid(ibid):CH3OH→HCHO→HCOOHFormicacidalsohappenstobeamajorby-productofaceticacidproduction.Inthiscase,however,asinthecaseofothermajorformicacidproductionpathways,alltherawmaterialsarenormallyderivedfrompetroleum(incl.methanol)andthusmakingthisprocesscompletelycarbon-neutralwouldrelyoneitherimplementationofCCUSorpotentialelectrificationofquestionableefficiency(ACS,2022).Similarly,atthemoment,theindustrialproductionofisopropanolisalmostentirelyreliantonfossilfuels.Thisissobecausethischemicalhasbeenlargelymanufacturedfrompropyleneviatwomajorcommercialroutes–indirecthydrationofrefinery-gradepropyleneanddirecthydrationofchemical-gradepropylene(PanjapakkulandEl-Halwagi,2018):C3H6+H2O→(CH3)2CHOH;CH3COCH3+H2→CH3CH(OH)CH3Here,giventhatpropyleneisproducedprimarilyasaby-productofpetroleumrefiningandofethyleneproductionbysteamcrackingofhydrocarbonfeedstocks(ibid),makingthisprocesscompletelycarbon-neutralislikelytobeextremelycomplicated.Inaddition,sincetheverymoleculesofbothisopropanolandformicacidcontaincarbon,dehydrogenationoftheseelementswillalsoraisetheneedforpropercarbonmanagement,ifvaluechainofhydrogenderivedfromthemneedstobeconsideredcarbon-neutral.18AndtheCO2willpresumablyhavetobecapturedandstored.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.92.4.Liquidorganichydrogencarriers(LOHCs)Liquidorganichydrogencarriers(LOHCs)representorganiccompoundsthatcanabsorbandreleasehydrogenviachemicalreactions(Modishaetal,2019).Intheprocess,withthetransformation(i.e.hydrogenation)ofthebasicsubstanceintotheonecarryinghydrogen(i.e.hydrogenation)andback(i.e.dehydrogenation),thebasicsubstanceremainsthesame,whichcouldallowforitscontinuoususeincycles.Apartfromofferingthepossibilitytopotentiallycreatea‘closeloop’forcarbon,LOHCsremainliquidunderambienttemperatureandpressure,whichsignificantlysimplifiesandfacilitatestheirstoragewithrespecttobothpreservationconditionsandinfrastructureneeded(ibid).Atthemoment,someofthemostpromisingLOHCsthatareattractingthegreatestattentionofresearchersaretoluene/methylcyclohexane(MCH)(C7H8/C7H14),naphthalene/decalin(C10H8/C10H18),benzene/cyclohexane(C6H6/C6H12),anddibenzyltoluene(DBT)/perhydro-dibenzyltoluene(PDBT)(C21H20/С21H33)(Table1).WhilenoneoftheseLOHCsrequiresspecificadjustmentsforhydrogenstorageandthusnoadditionalenergyandfinancialcostswouldbeincurred,eachofthesechemicalshasdifferentrequirementsforhydrogenationanddehydrogenation.Thoughelevatedtemperaturesandpressurewouldbeneededforeach,energydemandforthetoluene/MCHandnaphthalene/decalincoupleswouldnormallybelowerthanforbenzene/cyclohexaneandDBT/PDBT,whichwouldultimatelymakethemslightlycheaperalternativesforuseofH2storage(Table1).Onadifferentnote,whileeachofthesesubstancescontainscarbon,noneofthemisgeneratedinacarbon-neutralwayeither19.AlthoughthecarbonthateachoftheseLOHCscontainscouldpotentiallybereusedandthusnotreleasedintotheatmosphere,theirgravimetrichydrogencontentvaryingbetween6.19and7.29wt%(Table1)maynotnecessarilybeattractiveenoughinallthecases,forexample,whenalltheemissionsneedtobeabated.Thisisparticularlysoifothermore‘hydrogen-heavy’alternativeswithsimilarstoragecharacteristicsandavailablepower-to-Xproductionpathways(e.g.ammoniaormethanol)areonthelisttochoosefrom.Asaresult,theuseoftheseLOHCsforthepurposeofpreservinghydrogenmaybelimitedtosomenicheapplicationswhenotherconsiderations(e.g.availabilityorcost)areprevailing.2.5.MetalhydridesMetalhydridesrepresentabroadgroupofmaterialsthatcouldbeusedforstablepreservationofhydrogeninaconcentratedsolidform.TheybondthestoredH2tometalormetalloidelementsandalloysandallowforsomeofthegreatestvolumetrichydrogendensitiesamongallthestorageoptions(Azzaro-Pantel,2018)(Table1).Atthesametime,althoughtheycankeephydrogeninaverycompactform,theirgravimetrichydrogencontentinmostcasesislessimpressive,whichgenerallymeansthatthisstoragesolutionnormallyoffersheavierpreservationalternativesperunitofH2storedthantherest(ibid).Althoughthisfactormaylimitthetransportabilityofthestoragesubstance,metalhydridesgenerallyofferalow-riskandastablewayofH2preservationthatiscoupledwithitshighpuritywhenitisreleased(Colbeetal,2019).Asaresult,ifthesecharacteristicsareprioritized,thishydrogenstorageoptionmaybefavouredovertheremainingones.Whileofferinghydrogenstorageinsolidformunderambientconditionsorthosethatareclosetoambient,metalhydridesarecurrentlyconsideredmostlyfortankstorage.Asaresult,thevolumeoftheH2theypreserveislimitedtosmall-andmedium-scale.This,alongwithotherchallenges,stillneedstobeaddressedwithfurtherresearchsothatmetalhydridescouldbecomeawell-spreadhydrogenstoragesolution.2.5.1.ElementalmetalhydridesElementalmetalhydridessuchasmagnesiumhydride(MgH2)andaluminiumhydride(AlH3)havebeensomeofthefirstsolidmaterialsexploredfortheirhydrogenstoragepotentialsincethelate1960s(Yartysetal,2019).ThisissobecauseofseveralreasonswithacombinationofgoodH2capacity(upto7.6wt%forMgH2and10.1wt%forAlH3)andlowcostbeing,perhaps,themainones(WangandWang,2017).Thisisalsowhytheirleveloftechnologyreadinessforhydrogenpreservationatthemomentappearstobemoreadvancedthanthatofothermetalhydrides(Table2).Nevertheless,itdoesnotmeanthattheyareflawless.19Inparticular,tolueneisproducedduringoilrefining,directlyasaby-productofstyrenemanufactureandindirectlyasaby-productofcoke-ovenoperations(Lietal,2021).Naphthalene,inturn,isusuallymanufacturedfromeithercoaltarviaitsdistillationandfractionationorpetroleumbydealkylationofmethylnaphtalenesinthepresenceofhydrogen(Prasad,Vithanage,andKapley,2019).Benzene,again,ispredominantlysynthesizedfrompetroleumandcoalviacatalyticreforming,steamcrackingandtoluenedisproportionationprocesses,aswellascoalprocessing(Mengetal,2021).Finally,DBTisnormallyproducedfrombenzylchloride,which,itself,ispreparedindustriallybythegas-phasephotochemicalreactionoftoluenewithchlorine(Wunsch,Berg,andPfeifer,2020).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.10Ingeneral,duetotheabundanceofcheapmagnesiumandthepossibilityofgeneratingalotofinexpensivehydrogen,MgH2couldbelabelledthearchetypemetalhydride(Matar,2010).Ithaslightweightandhighstability(ibid).However,itsdesorptiontemperatureishigh(250-400oC)whiletheabsorptionkineticsisveryslow(Table1).Asaresult,althoughmagnesiumhydridemeetsalmostallthekeycriteriaforpracticalapplication,itsmaindrawbacks–poorkinetics,severethermalmanagement,andhighstability–thatis,factorsmakingitshydrogenationanddehydrogenationslow,energy-andcost-intensivewhichstillhinderitsfull-scalecommercialusageandsuccessfulimplementationatanindustrialscale(WangandWang,2017).Aluminiumhydrideaddressessomeofthesechallengeswhilealsoofferingoneofthebestcombinationsofvolumetricandgravimetrichydrogencontentsoutofallthehydridesviewed:148kgH2/m3and10.1wt%,respectively(Table1).Incontrasttomagnesiumhydride,AlH3caneasilyreleaseH2whenheatedanddesorptionrequireslessertemperatures(85-140oC)(Suetal,2021).Ontheotherhand,aluminiumhydrideisgenerallyformedbyreactingAlwithH2atextremelyhighhydrogenpressureandtemperatures(Table1),whichsimultaneouslymakesthisprocesslengthyandenergydemandingandthusalsolimitsitsdevelopmentandcommercialisationsuccess(Jiang,Wang,andZhu,2021).2.5.2.IntermetallichydridesTocircumventsomeofthekeydrawbacksofelementalmetalhydridesandinvestigateotheradvantagesthatsolid-stateH2preservationcouldoffer,intermetallichydrideswereexplored.Thoughitbroadlydependsontheirspecifictype,ingeneral,theyrequirelowertemperaturesandpressuresforhydrogenation/dehydrogenation(Table1).Ontheotherhand,justlikeothersolidstructuresviewedforH2preservation,intermetalliccompounds(here,AB5,AB2,andABtypesbeingthemainones20)thatareknowntobeabletostorereversiblysignificantamountsofhydrogenhavestillnotbeenfullycommercialized(SikoraandKuna,2007).AB5-typeintermetalliccompoundsandtheirderivativeshaverichchemistryandsuitablepropertiestobesuccessfullyusedforhydrogenstorage.Here,thederivativesofLaNi5(lanthanumpenta-nickel)(e.g.,LaNi5H6andsimilarsystems)areknowninparticularasprototypesofhydrogenabsorbingandhydrideformingintermetalliccompounds(Joubertetal,2021).However,despitesuchadvantagesasstoragestabilityandrelativelylowtemperatures,pressuresandthusenergyneededforthehydrogenation/dehydrogenationprocesses,themaindeficiencyoftheseoptionsrelatestotheamountofhydrogenthattheycanstorerelativetotheamountofmetalalloy(e.g.,1-1.5wt%forLaNi5H6)(Table1).Inordertoovercomethischallenge,othermetalalloyswithsimilarcrystalstructuresbutwhichallowforhigherH2massdensitiessuchasMg-basedcompounds,AB2-typehydrides,havebeenexplored(Naccarella,2017).TheirmainadvantageincomparisontoAB5-typealloysisthattheamountofhydrogenthattheycanstoreistwo-threetimeshigher(Table1).However,thesesystemsstillappeartobemoredemandingintermsofpressureneededforhydrogenationandextractionofhydrogen,whichislessquickthanthatofAB5-typecompounds(ibid).Finally,apartfromhavinghighH2weightcapacities(upto5.4wt%),AB-typecompoundshavethelightestmolarmassoutoftheviewedintermetallichydrides(Lysetal,2020).Inthiscontext,specifically,titanium-iron(TiFe)compoundsaresomeoneofthemostpromisinghydrogenstoragealloysbecauseoftheireconomicmeritbasedontheabundanceandlowcostoftheirconstitutingelements(ibid).Ontheotherhand,TiFerequirelaborioustreatmentaftersynthesistopromotethefirsthydrogenabsorption,sincetheydonotreadilyabsorbhydrogenunderambientconditionsbecauseofanativepassivatinglayer(Dematteisetal,2021).Infact,TiFealloysappeartobesensitivetoairmoistureandmightreactwithit,whichwillresultintheformationofoxidesandhydroxidesandthushinderedreactionwithhydrogen(ibid).Asaresult,asuccessfulapplicationanduseofthistypeofmetalhydridesneedsfurtherresearchandimprovement.2.5.3ComplexmetalhydridesFinally,complexhydridesarethegroupofmetalhydridesthatareparticularlyinterestingduetotheirrelativelylowweight(Züttel,2004).Theirmolarmass(e.g.,21.78g/molforlithiumborohydride(LiBH4)and22.96g/molforlithiumamide(LiNH2))(Table1),isgenerallylowerthanthemolarmassofthemostcommonintermetallichydridesdiscussedabove(201.1-432.37g/mol)aswellasmanyelementalmetalhydrides(26.32-29.99g/mol)(Table1).Here,borohydrides,ingeneral,andLiBH4,inparticular,representthecompoundwiththehighestgravimetrichydrogendensityatroomtemperatureknowntoday(around18.5wt%)(ibid).Thatiswhytheyareoftenseenas‘ideal’storagematerialformobile20Here,A=rare-earthatoms,B=transitionmetal(SikoraandKuna,2007).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.11applications.Ontheotherhand,thescientificunderstandingofthemechanismofthehydrogendesorptionfromborohydridesaswellasabsorptionbythemstillremainsachallengeandfurtherresearchisneeded(ibid).Alanates(e.g.,lithiumaluminiumhydride(LiAlH4)andsodiumaluminiumhydride(NaAlH4)),inturn,haveattractedextensiveattentionduetotheirdecentH2storagecapacity(3.5-5.4wt%)thatiscombinedwithrelativelylowcostofrawmaterials(Zhaoetal,2021).However,justlikemostothercomplexhydrides,theyalsohavehighdesorptiontemperatureandsluggishkinetics(Walker,2008)(Table1).Atthemoment,thisstillsignificantlyrestrictstheirpracticalapplicationand,asaresult,commercialization(Zhaoetal,2021).Ingeneral,thermaldecompositionofamidesaloneusuallygivesoffammoniaratherthanhydrogen(Wang,Li,andChen,2013).Perhaps,thisisthemainreasonwhytheyhadnotbeenconsideredascandidatesforH2storageuntiltheLiNH2-LiHcompositewasreportedin2002toreversiblystorealargeamountofhydrogen(ibid).However,theirde/re-hydrogenationkineticsarealsosluggishandleadtounfavourableoperatingtemperatures,whichalsofurtherhampertheirusage(Caoetal,2012).2.6.PorousmaterialsAlthoughthisreviewofhydrogenstoragetechnologiesisnotexhaustive,porousmaterialsrepresentthefinalgroupofH2preservationoptionsdiscussedinthispaper.Thisissobecauseofthespecialattentionpaidtotheirdevelopmentbyleadingresearchersaswellasbusinessinitiatives.Thisattentionowesprimarilytothefactthatsolid-stateporousmaterialscanpotentiallystorecomparableamountsofhydrogeninasaferandmoreefficientmannerrelativetothetechnologiesthatarecurrentlybeingusedformostapplications(suchase.g.,fuelcellautomobiles)(Chenetal,2022).Whileotherhydrogenstoragealternativesarelikelytobeinvestigatedaswell,porousstructuresasagroupofdiversesolutionsappeartobethelastmajorarrayofoptionsthatarecurrentlybeingexploredmostactively.Here,carbon-basedhydrogenstorageoptionsandmetal-organicframeworksrepresenttwoofthemostimportantfamiliesofporousmaterialspraisedfortheirsignificanthydrogenstoragepotential.Althoughalotofefforthasbeenmadetonearasuccessfulmarketentryofthesesolutions,noneofthemhasalreadyapproachedthetechnologyreadinesslevelthatisneededforfully-fledgedcommercialisation.Furthermore,noneoftheoptionshasgonebeyondapplicationforsmall-scalestorage.2.6.1.Carbon-basedmaterialsCarbon-basedhydrogenstoragesolutionscurrentlyincludeanumberofoptionswithcarbonfibres21,nanotubes,aerogel,templatedandactivatedcarbonaswellasgraphenebeingsomeofthemostpromisingonesforpotentialcommercializationintheforeseeablefuture(ibid).Here,incontrasttoe.g.,metal-organicframeworks(seebelow),thesematerialsaremoreresistanttooxidation,havebetterreversibilityandcyclicabilityandmoderatethermodynamicstability(Fan,Wang,andZheng,2022).Whileactivatedcarbon,carbonfibresandnanotubesappeartobeatahigherleveloftechnologyreadinessincontrasttotheremainingcarbon-basedhydrogenstorageoptions(Table2),theystillneedfurtherresearchanddevelopmenteffortssothattheirdeficienciesaresuccessfullyaddressed.Mostnotably,manycarbon-basedmaterialsarecharacterizedbyalowhydrogenationlevelunderambientconditions.Thismeansthat,foralotofthem,hydrogenuptakesignificantlyincreasesundercryogenictemperaturesandsignificantlydiminishes(downto1wt%)atroomtemperatures(Xia,Yang,andZhu,2013).Thischallengecoupledwithhighpotentialenergyconsumptionneededfortheirquickdecarbonizationsubstantiallylimitstheirapplicability.2.6.2.Metal-organicframeworks(MOFs)Metalorganicframeworks(MOFs)representtheothermostpromisingfamilywithinthecategoryofporoushydrogenstoragematerials.Ingeneral,theyareaclassoforganic-inorganichybridandcrystallineporousmaterials,whoseframeworkstructures,poreenvironment,andfunctionalitycanbeadjustedforspecificconditionsofhydrogenstorage(Caietal,2021).Mostofthemconsistofasimplecubicframeworkthatprovidesthemwithahighsurfaceareaandlargepores(Froudakis,2011).Inaddition,theirgravimetrichydrogencontentismoresignificant(upto10wt%)(Table1).ThesefactorsmakeMOFssuperiortocarbon-basedmaterialsintermsofsuitabilityforincreasinghydrogenstoragecapacity.21Carbonfibresinaformofcarbonfibre-reinforcedpolymer(CFRP)compositeshavealreadybeenusedtoreinforcehydrogenstoragetanksforthepreservationofcompressedH2(Gardiner,2022).Atthesametime,carbonfibresalsoshowsomeuniqueadvantagestobeusedasasolid-statehydrogenstorageoptionduetoitslowgas-solidinteraction,tunabletexture,highporevolumeandexcellentchemicalandthermalstability,amongothercharacteristics(Fan,Wang,andZheng,2022).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.12Atthesametime,mostMOFsarestillverysensitivetohumidity(ibid).Besides,theypossesslesserstructuralstabilitytoawiderangeofprocessingconditionsandrequirespecifictemperatureandpressureprerequisitesforstoredhydrogen22(Caietal,2021).Thatiswhy,althoughmetal-organicframeworksarecurrentlysomeofthemostintensively-researchedhydrogenadsorbents,testingtheirapplicabilitybeyondtheboundariesofsmall-scaleH2preservationisyettocome(Ahmedetal,2019).2.7.OverallevaluationAsseen,eachofthemaintypesofthehydrogenstoragetechnologiesrepresentedinthisstudyhasitsownadvantagesanddrawbacks.Forinstance,whilestoringpurehydrogenineithercompressedorliquefiedformwillguaranteeitsmaximumgravimetriccontent,theseoptionsarenottheleadingoneswhenitcomestoprovidingthemostcompetitivevolumetricH2density(Figure2).Atthesametime,metalhydrides,ingeneral,andaluminiumhydride(AlH3),inparticular,providethehighestvolumetricH2densitybutfallshortwhenitcomestodeliveringthehighestgravimetrichydrogencontent.Figure2:VolumetrichydrogendensityandgravimetrichydrogencontentofbestperformingsubstancesforeachtypeofmajorhydrogenstorageoptionsSource:VisualizationbasedontheinformationfromTable1WhilesuchoptionsascompressedandliquefiedSNG/e-fuelsseemtoofferthenextbestsolutionintermsofvolumetricandgravimetrichydrogencontentcorrelation,theenergyusedfortheirgenerationanddehydrogenationisquitesignificant.Thisisalsotrueformostoftheremainingnon-directhydrogenstoragealternatives,suchaschemicalandmetalhydrides,LOHCs,etc.Therefore,fromatechnicalperspective,thechoiceofoptimumhydrogenstorageentailstradeoffsbetweensomeofthekeytechnicalparameterssuchasvolumetricandgravimetrichydrogendensity,temperature,pressureandenergyusedforconversionandextractionofhydrogen,amongothers.However,theseparametersarenottheonlydeterminantshere,asthereareotherfactorsthatarealsolikelytoinfluencetheultimatepreferenceforonespecifictechnologyoveranother.Amongthose,suchcharacteristicsasmaximumstoragecapacitysafety,availabilityofinfrastructure,technology,market,andcommercialreadinesslevelswouldmostlikelyplaykeyroles.3.Factorstoconsiderforinvestmentinhydrogenstorage22Unfortunately,atthemoment,nosignificanthydrogenstoragecapacityhasbeenachievedinMOFsatambientconditions(ZelenakandSaidan,2021).050100150kg/m3Porousmaterials(carbonfibresandnanotubes)Metalhydrides(AlH3)LOHCs(decalin)Chemicalhydrides(liquidammonia)LiquidhydrogenLiquide-fuels(syntheticgasoline)CompressedSNG(250bar)Compressedhydrogen(700bar)050100wt%Porousmaterials(carbonfibres/nanotubesandMOFs)Metalhydrides(borohydrides)LOHCs(decalin)Chemicalhydrides(liquidammonia)LiquidhydrogenLiquide-fuels(liquefiedSNG)CompressedSNG(250bar)Compressedhydrogen(700bar)Volumetrichydrogendensity(kg/m3)Gravimetrichydrogendensity(wt%)13Table2:Approximateindicatorsofhydrogenation/dehydrogenation,storagecapacityandtechnology,marketandcommercialreadinesslevelsfortheviewedhydrogenstorageoptions23StateStorageformsApprox.volumecontaining100kgofH2(m3)Approx.totalweightcontaining100kgofH2(kg)MostpopulartypesofstoragevesselscurrentlyavailableGeometricalvolumeofmaximumstoragecurrentlyavailable(m3)Approx.weightofhydrogenstoredinthemaximumavailablevolume(tonnes)Readinesslevelsfortheentirecycleofhydrogenuse:hydrogenation/sorption–storage–dehydrogenation/desorption24Technologyreadinesslevel(TRL)Marketreadinesslevel(MRL)Commercialreadinesslevel(CRL)GasCompressedhydrogen700bar~2.38100Cylinders/containers/tanks~2625~1.18-95-62-6350barSaltcaverns~906,03026~23,650Synthetichydro-carbonsCompressedsyntheticmethane/naturalgas(SNG)(250bar)~3.11~397.93Salt&rockcaverns/aquifers/fields<50.15million271.6million6-94-63-61-31-5~1Cylinders/containers/tanks~9,500~305.9LiquidLiquefiedSNG~0.98Tanks/Hortonspheres~270,000~27,480Syntheticgasoline(petrol)~0.83~625Cylinders/containers/tanks~100,000~11,980Syntheticdiesel~0.84~714.29~11,910Liquidhydrogen~1.41~100Tanks/Hortonspheres3,800(operated)~269.046-93-61-510,000(planned)~708ChemicalhydridesLiquidammonia0.83-0.93~566.57Cylinders/containers/tanks~50,0005,390-6,0007-96-74-63-41-5~1Methanol(MeOH)1.01-1.05~832.64~100,0009,500-9,900Formicacid28~1.892,325.58<5,000<2657-93-54-61-21-5~1Isopropanol(i-PrOH)3.863,030.30~30,000~777LiquidorganichydrogencarriersToluene/Methylcyclohexane(MCH)2.11-2.121,623.38~100,0004,710-4,7404-71-4~1Naphtalene/decalin1.531,371.74~6,540Benzene/cyclohexane1.791,388.89~5,590Dibenzyltoluene(DBT)/perhydro-dibenzyltoluene(PDBT)1.561,612.90~6,400SolidMetalhydridesElementalmetalhydridesMagnesiumhydride0.92-1.161,666.67-1,315.79~4.5<=0.267-94-61-5Aluminiumhydride~0.68~990.10Inter-metallichydridesAB5-type0.956,666.67-10,0005-72-4~1AB2-type~1.02,631.58-4,651.16AB-type~1.11<1,851.85Complex-metalhydridesAlanates~1.851,851.85-2,857.144-61-3~1Borohydrides~0.83~540.54Amides~1.851,923.08-2,222.22PorousmaterialsCarbonfibres~5.56<1,838.24~0.2~0.0677-84-51-2Carbonnanotubes1000-2000Activatedcarbon~5.991,333.33-100,000Graphene5.88-6.251,298.70-10,0005-62-3~1Carbonaerogel<2,083.332-4~1~1Templatedcarbon<5.881,369.86-1,818.18Metal-organicframeworks(MOFs)<8,70<1,000Source:AdaptedfromUSDepartmentofCALEDON(1990),Newberry(2006),Energy(2012),MethanolInstitute(2013),Fendtetal(2015),Pérez-Fortesetal(2016),CORDIS(2017),EuropeanCommission(2019),Kawasaki(2020),Mouchahametal(2020),RoyalSociety(2020),Zhangetal(2020),Devaraj,Syron,andDonnellan(2021),GlobalTimes(2021),GlobalTimes(2022),IEA(2021),OffshoreTechnology(2021),Puharetal(2021),BNamericas(2022),DEMACO(2022),EuroTankWorks(2022),GKNHydrogen(2022),TechnodyneInternationalLimited(2022),Uniper(2022a).23Valuesrepresentedinthetableareapproximateandbasedonthedatafromavailableintheliterature.24Therepresentedlevelsareapproximate,sinceMRLandCRLdependnotonlyonTRLbutalsoonthepolicydefiningthepriceofCO2avoidance.25Althoughgreaterstoragevolumecouldpotentiallybeachievedforcompressedhydrogen,thevolumeindicatedinthetableisthebiggestonecurrentlyusedforstoringandtransportingcompressedhydrogen(HydrogenEurope,2021).26In2017,AirLiquidecommissionedtheworld’slargestundergroundhydrogenstoragefacilityinSpindletopDome,Texas(Djizanneetal,2022).27TotalcapacityoftheZhongyuangasstorageclusterinNorthChinacommissionedbySinopec(OffshoreTechnology,2021).28Duetothefactthatacidsarecorrosiveinnature,formicacidshouldnotbestoredinoxidizingmaterialssuchase.g.,metalcontainers/tanks.Instead,polyethyleneandpolypropylenecanbeusedtostoreformicacid(CALEDON,1990).-Conventional/thermochemical/biochemicalproductionpathways,-power-to-Xproductionpathway.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.14Asmentionedinthepreviouspart,althoughvolumetricandgravimetrichydrogendensitiesaswellastemperature,pressureandenergyrequirementsforthehydrogenation/dehydrogenationandstoragephasesappeartobesomeofthekeycharacteristicsaffectingthechoiceofaspecifichydrogenpreservationoption,thereareadditionalfactorsthatshouldbeincludedinthedecision-makingprocess.Here,mostnotably,thevolumeandweightofthestoredsubstancesundereachspecificstoragemodeaswellasthescaleandtypeofthestoragevesselsareimportant.Inadditiontothat,thetoxicity,corrosivenessandflammabilityofthestoredmatteritselfarealsofeaturesthatcannotbeignoredwhenthinkingaboutwaysofH2preservation.Finally,whenplanninginvestmentinhydrogenstorage,oneshouldtakeintoaccounttechnology,market,andcommercialreadinesslevels(TRL,MRL,andCRL)–i.e.theindicatorsthatwoulddescribeeachofthestudiedtechnologies’progressintermsoftechnologicalmaturity,preparednessformarketintroduction,aswellasstageofoffering‘success’asacommercialproduct(EuropeanCommission,2019).Apartfromtechnicalandtechnology-relatedchallengesthatwouldinfluencetheultimatecostandthusbusinessattractivenessofaspecificH2storageoption,thereisarangeofbarriersanduncertaintiesassociatedwithhydrogenpreservationand,onamoregeneralnote,withtheentirehydrogenvaluechainthatshouldbetakenintoaccountbythedecision-makers.Here,suchaspectsasuncertaintiesassociatedwiththedemandforhydrogen,itsregulationaswellasthehighpotentialcostandscaleofstoragearejustafewthatcouldbementioned.Inthisrespect,analyzingkeyissueswouldhelptohighlightthemainchallengesthatasoundbusinessmodelforthedevelopmentofhydrogenstoragewouldface.3.1.FurthertechnicalandtechnologyissuesandchallengesEachofthemainhydrogenstoragealternativesreviewedinthispaperhasitsowndistinctvolumetricandgravimetricH2densities(seeTable1).Theircombinationresultsinsignificantdifferencesinthevolumeandweightthattheseoptionswouldoccupytopreservethesameamountofhydrogen(seeTable2).Forinstance,whilepreserving100kgofH2inaformofaluminiumhydride(AlH3),onlyaround0.68m3ofspacewouldbeneededtoaccommodateit(comparedtoforexample,around2.38m3forpurehydrogenat700bar).However,itwillweighalmostatonne–around990.1kg(comparedtoe.g.566.57kgofliquidammonia).Figure3:Approximatevolumeandtotalweightcontaining100kgofH2ofbestperformingsubstancesforeachtypeofmajorhydrogenstorageoptions29Source:VisualizationbasedontheinformationfromTable2.Therefore,ifonlythesevolumeandweightcharacteristicsarecompared(Figure3),noneoftherepresentedoptionscouldbeviewedasthemostadvantageousthatcouldbeusedastheultimate,‘best’hydrogenstorageoption.Atthesametime,severalsubstanceswouldbequiteclosetothe‘goldenmiddle’–thatis,offeradecentcombinationofvolumetricandgravimetricdensitiesallowingthemtooccupyneitherexcessivespacenorweight.Inthiscontext,liquidhydrogenitself,liquide-fuels29ThesedatashouldbeviewedasapproximatesincemostoftherepresentedsubstancesstillhavelowTRLlevel.0246m3Porousmaterials(carbonfibres/nanotubes)Metalhydrides(AlH3)LOHCs(decalin)Chemicalhydrides(liquidammonia)LiquidhydrogenLiquide-fuels(syntheticgasoline/petrol)CompressedSNG(250bar)Compressedhydrogen(700bar)050010001500kgPorousmaterials(MOFs)Metalhydrides(borohydrides)LOHCs(decalin)Chemicalhydrides(liquidammonia)LiquidhydrogenLiquide-fuels(syntheticgasoline/petrol)CompressedSNG(250bar)Compressedhydrogen(700bar)Approx.volume(m3)Approx.totalweight(kg)Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.15(syntheticgasoline/petrol),liquidammoniaandevensomemetalhydrides(specificallyaluminiumhydrideandborohydrides)woulddemonstratemediumindicators,whichcouldtheoreticallyallowthemfornotonlystationarypreservationbutalsostoragewiththefollow-updeliverywithoutanytransformations(whichwillobviouslydependontheenduse).Table3:Keyadvantagesanddisadvantagesofmajorhydrogenstorageoptions30StateStorageformsCharacteristicsVolumetrichydrogendensityGravimetrichydrogencontentStoragevolume/scaleavailableEnergyuseforstorageSpeedofinjectionto/withdrawalfromstoragevesselNeedfordehydrogenation/desorptionNeedforcarbonmanagementafterdehydrogenationTechnologyreadinessDevelopmentlevelofstorageinfrastructureCorrosivenessToxicityFlammabilitySmallMediumLargeGasCompressedhydrogenMediumHighYesNoYesHighHighNoHighMediumYesNoYesSynthetichydrocarbons(e-fuels)Compressedsyntheticmethane/naturalgas(SNG)LowMediumYesNoMediumYesLow-mediumHighNoLiquidLiquefiedSNGHighHighSyntheticgasoline(petrol)Low/noneYesSyntheticdieselLiquidhydrogenMediumHighHighMediumNoMedium-highMediumYesNoChemicalhydridesLiquidammoniaHighMediumMediumYesNoMediumHighYesMethanol(MeOH)Low/noneHighYesFormicacidMediumLowLowIsopropanol(i-PrOH)LowLiquidorganichydrogencarriersToluene/Methylcyclo-hexane(MCH)Low-mediumLow-mediumNoNaphtalene/decalinMediumBenzene/cyclohexaneDibenzyltoluene(DBT)/perhydro-dibenzyltoluene(PDBT)SolidMetalhydridesElementalmetalhydridesHighLow-mediumNoLowNoMedium-highLowNoNoNoInter-metallichydridesLowMediumMediumComplex-metalhydridesMedium-highLow-mediumLow-mediumPorousmaterialsCarbonfibresLowMediumHighMedium-highCarbonnanotubesActivatedcarbonLowGrapheneMediumCarbonaerogelLowTemplatedcarbonMetal-organicframeworks(MOFs)Low-mediumSource:AdaptedfromCALEDON(1990),Newberry(2006),USDepartmentofEnergy(2012),MethanolInstitute(2013),Fendtetal(2015),Pérez-Fortesetal(2016),CORDIS(2017),EuropeanCommission(2019),Kawasaki(2020),Mouchahametal(2020),RoyalSociety(2020),Zhangetal(2020),Devaraj,Syron,andDonnellan(2021),GlobalTimes(2021),GlobalTimes30InTable3,‘favourable’characteristicsarehighlightedwithgreen,‘lessfavourable’withyellow/amber,and‘unfavourable’withred.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.16(2022),IEA(2021),OffshoreTechnology(2021),Puharetal(2021),BNamericas(2022),DEMACO(2022),EuroTankWorks(2022),GKNHydrogen(2022),TechnodyneInternationalLimited(2022),Uniper(2022a).Atthesametime,ifinvestorsneedtoconsiderthescaleofhydrogenstorage,thechoiceofpreferredoptionsmaybesignificantlyaltered.Inparticular,atthemoment,country-scaleH2storageinlargequantitiessimilartothatofnaturalgascouldonlybeofferedbycompressedhydrogen,whichwouldthenbepumpedunderground(Table2).31Porousmaterials,inturn,wouldonlybecurrentlyexperimentallyusedincombinationwithpurehydrogenstorageassorbentsinsmall-scaletanksthatwouldlowertherequirementsforpressureandtemperature(Chenetal,2022).Similarly,duetotheirrelativelynovelnature,metalhydrideswouldbeusedastank‘fillers’tofacilitateH2preservationinsaferandmorestableconditionsandwouldthusnotbeamedium-andlarge-scalehydrogenstorageoptionthatwouldpotentiallycompetewithbatteries(Ferreira-AparicioandChaparro,2019).Hence,atthemoment,onlyliquidoptions(i.e.liquidsyntheticfuels,chemicalhydrides,andLOHCs)32seemtobeabletoofferbothsmall-andmedium-scalehydrogenstorage.Inaddition,notalltheviewedpotentialhydrogenstoragealternativesareequallymatureintermsoftechnology,market,andcommercialreadiness.Specifically,withcompressedandliquefiedhydrogenbeingthe‘ripest’options,syntheticfuels,chemicalandelementalmetalhydridesaswellassomecarbon-basedporousmaterials(fibres,nanotubesandactivatedcarbon)aretherunners-up(Table2).Nevertheless,thelistoftheseoptionswillhavetoshrinktocompressed/liquefiedH2,elementalhydrides,methanolandammoniaaswellasactivatedcarbonwithcarbonfibresandnanotubesifonlyzero-carbonprocessesneedtobeconsidered.Thislist,however,willbereducedfurthertocompressed/liquefiedhydrogen,methanolandammoniaifbothsmall-andmedium-scalestorageneedtobeprovided.Ontheotherhand,ifsafetyandstabilityofstorageiscounted,suchaspectsascorrosiveness,toxicity,andflammabilityofthestoredsubstanceshouldbetakenintoaccountaswell.Inthiscase,elementalmetalhydrideswillremaintheonlytechnologicallyadvancedoptionthatwouldguaranteesafepreservationofhydrogen(Table3).This,however,meansthat,onlyrelativelysmallamountsofH2couldbeconsideredforstoragewithmaximumsafety.Atthesametime,thisalsomeansthatallotherviewedsmall-,medium-,andlarge-scaleoptionsreadyfororclosetocommercializationposesomesafetyconcerns.Ingeneral,asdemonstratedinTable3,noneofthehydrogenstoragealternativesperformsbestacrossallthecategoriesthatareimportantforcreatingasoundbusinesscaseforhydrogenstorage.ThissuggeststhattherewilllikelynotbeasinglepreferredoptionforallthestakeholdersdealingwithH2.Itisthusmorelikelythattheultimatestoragesolutionwillbechosenbasedonthecombinationofthefinaluseofhydrogenwiththecharacteristicsrepresentedabove.Inaddition,thedecision-makingprocesswillmostlikelyalsoincludemoregeneralaspectsofhydrogenstorage.333.2.Otherfactors,uncertainties,andbarriersforinvestmentinhydrogenstorageCostisamongthemostimportantaspectstoconsiderwhenchoosingaspecifichydrogenstorageoptiontoinvestin.Here,however,estimatingthespecificexpensesthataninvestorwouldincurwhenchoosingoneoptionoveranotherishardbecauseofdifferentTRL/MRL/CRLaswellasproject-specificcharacteristics.Whenitcomestopreservingpurehydrogen,theArgonneNationalLaboratoryprovidesthefollowingbriefestimatesofcapital,operationandmaintenancecoststhatareassociatedwiththismodeofhydrogenstorage(Table4).Here,asseen,adjustingtheundergroundfacilitiesthatarecurrentlybeingusedfornaturalgasseemstobetheleastexpensiveoptionduetotheoverallsimilarityofthestorageprocesses.Ontheotherhand,thedownsideofthiswouldbehigheroperationandmaintenancecostsaswellaslackofflexibilityintermsofstoragescale,sinceitwouldonlybeeconomicallyreasonabletousethesefacilitiesforlarge-scaleinitiatives.PreservingH2incompressedorliquefiedforms,inturn,wouldresultinmediumoperationandmaintenancecostsbuthighercapitalexpenses(ifcomparedtothemostmaturesolidstoragealternatives).31Thisisespeciallyimportantwhenconsideringinternationaltrade,wherelargescaleisalmostcertainlyneededtoobtaineconomiesofscale(e.g.forhydrogenpipelines).32Sinceitisnoteconomicaltoliquefyhydrogenforsmall-scalestorage,itiscurrentlymostlydoneatamediumscale.33Aswellasitsfinaluse.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.17Table4:Approximatecapital,operationandmaintenancecostsofstoringpurehydrogenindifferentforms(USD2017/kWh)StorageformCosts34Otherpotentialconsiderations(costs)AvailablestoragescaleCapitalOperationandmaintenanceCompressedgasCylinders/containers/tanks20.40.037CompressorSmall/mediumUndergroundaquifers/reservoirs/caverns0.21-0.520.11PipingLargeLiquid15.40.062LiquefierMediumSolidMetalhydrides15.6-225.20.02n/aSmall/mediumPoroussystems13.7-17.9Source:AdaptedfromGandaandMaronati(2018)Whilesmall-andmedium-scalehydrogenstoragecouldbeprovidedbyanumberofoptions(incl.thoseofferedbyhydrogenderivatives)andhencetheinvestorwillbeabletochoosethealternativethatappearstobethemostsuitableforaspecificbusinesscase,large-(country-)scalestorageofhydrogenisstilltechnologicallylimitedtoH2compressionandinjectionunderground.Inadditiontothat,giventhecalorificcontentdifferencesbetweencompressedhydrogenandnaturalgas,storingthesameamountofenergyintheformofH2compressedto350bar(themaximumoperatingpressureforundergroundstoragefacilities)wouldrequirealmostthreetimesasmuchvolumeasitwouldfornaturalgaskeptat200bar(Table5):Table5:StoragevolumeneededtoaccommodateEurope’s2-weekpeakenergydemandof326TWh35CharacteristicsUnitofmeasurementNaturalgas/SNGHydrogen200barLiquefied700bar350barVolumetricdensitykg/m3~180430-470~42~26.1GravimetricenergydensityMJ/kg53.6-55.6120-142kWh/kg14.89-15.4433.33-39.44Amountneededtocover326TWhMilliontonnes21.11-21.898.27-9.78Storagevolumeneededtocover326TWhMillionm3117.28-121.6144.91-50.91196,9-234,86316.86-374.71Keycalculations:1.1.Gravimetricenergydensityofnaturalgas/SNG:53.6-55.6MJ/kg=14.89-15.44kWh/kg1.2.Gravimetricenergydensityofhydrogen:120-142MJ/kg=33.33-39.44kWh/kg2.1.Amountofnaturalgas/SNGneededtocover2-weekpeakdemand:326,000,000,000kWh:(14.89-15.44kWh/kg)=21.11-21.89milliontonnes2.2.Amountofhydrogenneededtocover2-weekpeakdemand:326,000,000,000kWh:(33.33-39.44kWh/kg)=8.27-9.78milliontonnes3.1.Volumetricdensityofcompressednaturalgas/SNFat200bar=~180kg/m33.2.Volumetricdensityofliquefiednaturalgas/SNF=430-470kg/m33.3.Volumetricdensityofcompressedhydrogenat700bar=~42kg/m33.4.Volumetricdensityofcompressedhydrogenat350bar=~26.1kg/m34.1.Storagevolumeofcompressednaturalgas/SNG(200bar)neededtoaccommodatea2-weekpeakdemand:21.11-21.89milliontonnes:(180kg/m3:1000)=117.28-121.61millionm34.2.Storagevolumeofliquefiednaturalgas/SNGneededtoaccommodatea2-weekpeakdemand:21.11-21.89milliontonnes:(430-470kg/m3:1000)=44.91-50.91millionm34.3.Storagevolumeofcompressedhydrogen(700bar)neededtoaccommodatea2-weekpeakdemand:8.27-9,78billiontonnes:(42kg/m3:1000)=196.9-234.86billionm34.4.Storagevolumeofcompressedhydrogen(350bar)neededtoaccommodatea2-weekpeakdemand:8.27-9,78billiontonnes:(26.1kg/m3:1000)=316.86-374.71billionm334Capitalcostswouldmostlyrelatetotheadjustmentofthealreadyexistingstorageinfrastructure(e.g.thatpreviouslyusedfornaturalgas)totherequirementsofhydrogenstorageintherespectiveformaswellasthecreationofnewstoragefacilities(e.g.compressedstoragetankswithporoussystems/metalhydrides)(GandaandMaronati,2018).Operationandmaintenancecosts,inturn,wouldrelatetothehydrogenation(absorption)/dehydrogenation(desorption)processesandrelatedoperations(ibid).35AccordingtoCihlar,Mavins,andvanderLeun(2021),in2019,atwo-weekpeakdemandinEU-27andtheUKthatwascoveredbynaturalgaswas326TWh.1TWh=1,000,000,000kWh-calculatedfornaturalgas/SNGcompressedto200bar,liquidnaturalgas/SNGandhydrogencompressedto700and350barforillustrativepurposes.Undergroundhydrogenstoragenormallypresupposesoperatingpressureof15-315bar(Cihlar,Mavins,andvanderLeun,2021).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.18Foratwo-weekpeakdemandinEurope-27andtheUKthatisnormallycoveredbynaturalgas,whichconstitutesaround367TWh(Cihlar,Mavins,andvanderLeun,2021),around117.28-121.61millioncubicmetresofnaturalgasstoredat200barwouldbeneeded.Atthesametime,ifthisenergyissupposedtobeprovidedbyhydrogenthatisstoredunder350bar,thevolumeneededwouldbesignificantlybigger:316.86-374.71millioncubicmetres.Whilethismaynotlooklikeasignificantchallengeperse,itturnsoutthatoutofthefouroptionsthatarecurrentlybeingviewedforlarge-scaleundergroundhydrogenstorage(saltandrockcaverns,depletedfields/reservoirs,andaquifers),onlysaltcavernshavebeensuccessfullytestedintermsofsuitabilityforthispurpose–thatis,theyhavethehighestTRLof8andtheremainingoptionsarestillfarbehind(Table6):Table6:SomekeycharacteristicsofthemaingeologicaloptionsforundergroundhydrogenstorageCharacteristicsCavernsDepletedfields/reservoirsAquifersSaltLinedrockGeneralsuitabilityforhydrogenstorageHighSite-specificPotentialtypeofoperation36PeakingandseasonalSeasonalPotentialmaximumnumberofcyclesperyear37101-2Estimatedfacilityworkinggascapacity(TWhH2)380.01-4.120.040.03-14.290.05-3.23Workinggascapacity/Totalgascapacity(%)70>7050-6020-50Depth(m)300-1,800~1,000300-2,700400-2,300Operatingpressure(bar)35-21020-20015-28530-315Largestexpenses(newdevelopment)•Formationofthecavern•Disposalofthebrine•Cushiongas•Compression•Blastingofthecavern•Steellining•Cushiongas•Compression•Well•Infrastructure•Cushiongas•Compression•Explorationanddeterminationofgeology•Wellinfrastructure•Cushiongas•CompressionRelativecostofdevelopment/investmentLowHighLowRelativecostofoperationModerateAverageinjection/withdrawalrate(site-specific)HighModerateGeneralsuitabilityforhydrogenstorageProvenFirsthydrogenstorageindevelopment•Hydrogen-methaneblending(upto10-50%H2proven)proven.•Purehydrogenstorageunderstudy•Hydrogen-methaneblending(upto10%H2)proven•PurehydrogenstorageunderstudyEstimatedtechnologyreadinesslevel(TRL)forhydrogenstorage~85-63-6~3FurtherR&DneededPrecisioninthetimingofinjectionsandwithdrawalsCompatibilityofliningmaterialswithhydrogen•Effectsofresidualnaturalgas•In-situbacteriareactions•In-situbacteriareactions•TightnessofrocksSource:AdaptedfromCihlar,Mavins,andvanderLeun(2021)andEpelleetal(2022).36Seasonalstoragesitestypicallytakemonthstocompleteafullinjection/withdrawalcycle,sotheyarenormallyusedtomeetseasonalvariationsindemand(Cihlar,Mavins,andvanderLeun,2021).Peakingstoragetypicallycompletesfullinjection/withdrawalcyclesindays/weekssotheyarenormallyusedtomeethourly,daily,andweeklydemandvariations(ibid).Optimizedcluster/hubstorageoperationscanbeusedtoprovideshort-termorpeakservicesindependentlyfromspecificgeologicalcharacteristics(ibid).37Estimatesarebasedoncurrentusefornaturalgas.38Rangeoftotalworkinggascapacityperfacility.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.19Inthesecircumstances,giventhatsaltcavernsarenotdistributedequallyacrossallgeographies,preservinglargequantitiesofhydrogenundergroundmaynotbeavailableeverywhere.Forinstance,withtheambitioustargetsforH2generationandimport,Europewillneedtobeabletoaccommodatethesequantitiesofhydrogenrightafteritsproductionorimportorbeforeitsfinaluse.However,asseenfromFigure4,thiswillmostlikelynotbepossibletoperformifonlysaltcavernsareavailable.Figure4:Estimatesofhydrogenstorageneedby2050vs.potentialSource:Cihlar,Mavins,andvanderLeun(2021).Ontheotherhand,itislikelythatmostsignificanthydrogenconsumptioninEuropewillnotoccurbefore2040andwillnothavethesamemagnitudethroughoutthecontinent(Barnes,2023).Hence,itisunlikelythatH2storagewillbecomeaEurope-wideissuebytheendofthenextdecade,sincemanycountrieswillonlymakeitanimportantcomponentoftheirenergysystemsthen.Asaresult,theywillhavetimetodevelopvarioushydrogenstoragealternatives.Thisalsodemonstratesthat,ifhydrogenistoplayakeyroleindecarbonizationandisusedacrossindustriesandsectors,small-andmedium-scaleH2preservationoptionsthatincludenotonlypurehydrogenstoragebutalsoitsderivativeswillmostlikelyhavetocomplementthelarge-scaleones.Inthiscase,giventheuncertaincostsoftheseundertakingsthatwouldincludeconversionexpenses(e.g.,fromH2toNH3andback),investorsmayhesitatetoengageinsuchprojectsinpreferenceforwell-knownandmostlikelycheaperenergystoragevariants–here,fossilfuelssuchasnaturalgas.Sincehydrogenstorageintheformofitsderivativescouldbethesameasthepreservationofhydrocarbonsorsuchchemicalsasammoniaandmethanol,bothcapitalandoperationandmaintenanceadjustmentsforlaunchingandrunningthesestoragefacilitiesarelikelytobelesssignificantthanforthoseaimedatpurehydrogenstorage.This,however,doesnotincludetheexpensesassociatedwiththeadditionalpre-andpost-storagetreatmentthatwouldtransformhydrogenintoitsderivativeandback.Asaresult,whilestorage-relatedcostsforsuchoptionsmaybelowerthanthosefordifferentalternativestopureH2preservation,these‘transformation’expensesaswellasthosethatareassociatedwithhydrogenpurificationandcarbonmanagementwillstillgotowardsthefinalcostofH2attheendofitsvaluechain.Intheend,thesecost-relateduncertaintiesaswellastheround-tripefficiencycoulddeterminewhatspecifichydrogenstorageoptionwouldultimatelybepreferred.Onamoregeneralnote,thesechallengeswouldcloselyalignwithpolicyandregulation.Inparticular,giventhathydrogenproduction,storage,anddelivery–themainactivitiesinthehydrogenvaluechain–appeartobemoreexpensivethanthesameelementsforhydrocarbons,lackofregulatoryandpolicysupportwillmostlikelydissuadeinvestmentinhydrogenstorage.Inthisrespect,thestartingpointofanH2-supportiveregulationorpolicyshouldbetodealwiththeextracostsofproducingitcarbonfree.Thisisbecauseitisuncertainifexpectedfutureprofitisabletocovercostsandrisksassociatedwithproducingandtransportingzerocarbonhydrogen.Theseuncertaintiesaboutcommercialarrangementswouldalsoequallyapplytohydrogenstorage.Here,clarityshouldbeachievedonsuchissuesasstorageownershipandusage(i.e.whoownsthestoragefacilityandwhouses/operatesit),accesstostorageinfrastructure,andpriceofutilizingit,amongothers.020406080100120TWhHydrogenStorageNeed2050SaltCavernPotentialTotalStoragePotentialThecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.20Table7:TRL,MRL,CRLofvarioushydrogenstoragealternativesandtheiralignedfundingoptionsSource:AdaptedfromUSDepartmentofEnergy(2012),Buchneretal(2019),EuropeanCommission(2019),RoyalSociety(2019),EcosVC(2020),Müller,Skeledzic,andWasserscheid(2021).-Conventional/thermochemical/biochemicalproductionpathways,-power-to-Xproductionpathway.However,themostcrucialuncertaintythatmaypreventbusinessesfromallocatingfundsforH2storageprojectsisthedoubtaboutsufficientdemandforhydrogen.Itmayemanatefromsuchfactorsas,forexample,absenceofstrongpolicysupportorunclearprogressintheindustries’conversionfromhydrocarbonstohydrogen.Morefundamentally,however,itmaybebasedonageneraldisbeliefinthepossibilityofcreatingaviablehydrogenvaluechainandrejectionoftheveryideathathydrogencanpotentiallyreplacefossilfuelsinsomeofmoderneconomy’skeyindustrialprocesses.Here,toovercomethesechallengesanduncertainties,respectivepoliciesandregulationswillhavetobecreatedtosupportdemonstrationprojects,stimulatedemand,incentivizeinvestmentininfrastructureandencourageproducersandindustriestoswitchtolow-carbonhydrogenwhenitmakessense.3939Thereisalsoariskthatpolicydecidestofocusononetechnologywhenalternativenet-zerocarbonoptionscouldpotentiallybesuperiorinsomeenduses.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.21AsTRL/MRL/CRLcharacteristicsoftheexistinghydrogenstorageoptionsdiffersignificantly40(Table7),thepreferredsupportmechanismsforeachofthespecificstorageoptionswillalsovary.Thefundingsourcealsoverymuchdependsonthestageofthemarket,technologyandcommercialreadinesslevelsofspecifichydrogenstoragetechnologies.Forexample,grantfundinginaformofprivateorgovernmentgrantswouldbemostapplicableforstoragetechnologiessuchasgraphene,templatecarbon,carbonaerogel,andMOFs,whileoverallgovernmentsupportwouldalsorelatetosyntheticfuels,isopropanolandformicacidobtainedviapower-to-Xpathways,LOHCs,intermetallicandcomplexhydrides(Table7).Overall,governmentfundingisoftenusedtosupporttheearly-stageresearchanddevelopmentofnascenttechnologiesthatmaybetooriskyforprivateinvestorstofinance.Itisalsousedtoprovidecriticalsupporttoenablethedevelopmentofatechnologyfromthelaboratorytotheprototypestage.Also,governmentfundingcanbeusedtosupportthedeploymentofclosetomaturetechnologiesthatarestrategicallyimportantfordevelopmentofahydrogeneconomy.Othersourcesoffunding,suchasventurecapital,corporatefunding,andcrowdfunding,aretypicallyusedtosupportthecommercializationoftechnologiesthathavealreadyprogressedbeyondtheearlyresearchanddevelopmentphase.Privateinvestorsmaybemorewillingtoinvestinatechnologythathasalreadybeendemonstratedtobeviableandhasaclearpathtocommercialization,asthisreducesthelevelofriskinvolved.Thenextsectiondiscussesapproachestogovernmentsupportedbusinessmodelsforlargescaleandclosetomaturehydrogenstoragetechnologies.Althoughbothsmall-andlarge-scalehydrogenstorageoptionsarelikelytobeimportantinthefuture,governmentsupportforlargescalestoragecanbejustifiedforatleasttworeasons.First,isthatlarge-scalestoragefacilitiesarelikelytobemorecost-efficientthansmallerones,astheycanbenefitfromeconomiesofscaleintermsofequipment,infrastructure,andoperationcosts.Second,large-scalehydrogenstoragefacilitiescanhelptoacceleratethedevelopmentandcommercializationofhydrogentechnologiesbycreatingamarketforhydrogenstorageandstimulatingprivatesectorinvestmentinthetechnology.Thepresenceofahydrogenstoragemarketwillbenefitsmallerandlessmaturetechnologiestoooverthelongrun.414.BusinessmodelsandpoliciesforhydrogenstorageThefundamentalvalueofenergystoragecomesfromthepossibilityofsupplyanddemandimbalanceatdifferenttimescalesandindifferentperiodsoftime.Inessence,theverypurposeofhydrogenenergystorageistotransferenergyacrosstime.Whenthecostofthistransfer(productionandstorage)islowerthanthecostofmeetingdemandfrominstantaneousproductioninthenextperiod,energystoragerepresentsaneconomicvalue.Ontheotherhand,thisdoesnotmeanthatalltheenergysystemscurrentlyinoperationincludeawell-developedstoragecomponent.Infact,therearesystemswherelarge-scaleenergystorageisstilltechnologicallychallengingorimmature.Forinstance,theelectricitysectorinitscurrentformisdesignedwithanassumptionoflittleenergystorageinthesystem(USDepartmentofEnergy,2016).Asaresult,thesystemisdimensionedtomeetpeakdemandwithsignificantsparecapacityacrossthewholesupplychainfromgenerationtodistributionnetworks(ibid).If,inthefuture,largescalestorageofelectricitybecomestechnologicallymatureandeconomicallyefficient,theentirepowersystemwouldbemoreefficient.Atthesametime,thereareenergysystemsinwhichlargescaleenergystorageistechnologicallymaturebutnotnecessarilyeconomicunderallconditions.Forinstance,inthecaseofnaturalgas,manystoragefacilitiesinEuropelosttheireconomicattractivenessovertimeandsomehadtoclose(suchasRoughstorageintheUK)whentheywereoperatingunderlowgaspricesandthewinter/summerspreadcollapsed(Guardian,2017).Fromtheperspectiveofinvestorsingasstorage,sincenewfacilitiestakearoundhalfadecadetodevelopandneedtorunforabouttwodecadestopaybacktheir40WhilesomeoftheH2preservationsolutionsarenearingfullcommercializationandareabouttosuccessfullyenterthemarket,othersarestilleitherataveryearlyresearchanddevelopmentorlaboratorytestingstage.41Here,itmightbeusefultodistinguishbetweenthetwokeyusesofhydrogenstorage–forelectricitystorageorfortransportationofhydrogenitself.Forelectricitystorage,giventhatthereisalreadyanascentforwardmarketforpoweraswellastermcontracts,ContractsforDifference(CfDs)andcapacitymarketsunderactiveconsideration,thecommercialregulatoryframeworkseemstobemoreadvanced.Forstorageofhydrogenfordeliverytoconsumers,theframeworkappearstobelessmatureandthuswillrequiregovernmentsupportintheconsumingmarket.Inthisconnection,standalonestorageprojectsareonlylikelytoappearasthemarketmaturesandaggregationbecomespossible.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.22investments,operationofsuchstorageinstallationsissubjecttosignificanteconomicrisks(GazSystem,2022).Although,inprinciple,hydrogenstoragecouldfacethesamefateasgasstorageinEurope,theeconomicandtechnicalfeaturesofthesetwoindustriesarenotexactlysimilar.Iftheroleofhydrogeninoureconomyincreasestotheextentitisexpected,therisingdemandforthissubstancewillhavetobemetbysufficientmagnitudeofsupply.Thiswillmostlikelyinvolvetheneedforhydrogenstorageinfrastructure.Thisissobecausethelowlevelofsupplydiversity(asleastatanearlystageofthesector’sbuild-up)increasestheprobabilityofsupplyanddemandimbalances.Specifically,becauseofthetechno-economiccomplexityofhydrogentransfer,itisunlikelythatthenumberoflarge-scalehydrogensupplierswillbegreaterthansuppliersofnaturalgas.Apartfromthis,greenhydrogengeneratedbywindandsolarenergysourceswillhaveunstable(variable)productionpatterns–theywillbeintermittent(ArmijoandPhilibert,2020).Infact,evenCCUS-enabledH2productionmaynotmatchthedemandprofileincertaincircumstancesandundercertainconditions(Cloete,Pozo,andÁlvaro,2022).Thatiswhy,withthisdemand-supplyimbalanceinplace,hydrogenstorageislikelytohaveapositiveeconomicvaluefromasocialwelfareperspectivedespitethecostsassociatedwithit.Aquestionthatariseshereisthat,ifhydrogenstoragedoeshaveaneconomicvalue,whyisnoonecurrentlytryingtocaptureit?42Here,thesimpleanswerwouldbe‘becauseofriskconsiderations’.Inparticular,therearetwoprimaryrisksassociatedwithinvestmentinhydrogenstorage.Thefirstoneistheriskofinsufficientdemandforsuchservices–thatis,theriskthatafterinvestmenthappensthereisnostrongneedforH2storage.43Theotheroneistheriskoflowpricetoutilizehydrogenstoragecapacity.Inotherwords,thepriceofhydrogenstorageaccessmightnotbehighenoughtojustifytheinvestment,whichcanhappenforvariousreasons,includingcompetitionamongvariousmodesofhydrogenstorageaswellastechnologydevelopment.Thesetworisksleadtorevenueuncertainty.Thoughthisuncertaintymaydecreaseovertimeasahydrogeneconomydevelops,itneedstobemitigatedthroughefficientriskallocationbetweenthegovernmentandaprivateparty(IISD,2015).Theseriskswillbeallocatedthroughapplyingaspecificbusinessmodel.Todesignabusinessmodelforhydrogenstorageinaproperway,twocriticalquestionshavetobeasked.ThefirstquestioniswhichH2storagetechnologyshouldbeincentivized?Forinstance,ifthegovernmentneedstocreateawell-functioninghydrogeneconomy,itneedstosupportthedeploymentoflarge-scaleH2storage(TankStorage,2022).Inthiscase,asseenfromPart3.2.,itwillhavetosupportsystem-widehydrogenstoragethatiscurrentlyavailableonlyinaformofcompressedH2keptunderground.Here,thecostofrunningsuchastoragesystemcouldbereducedduetotheeconomiesofscaleandsynergyofnaturalgasandhydrogenstoragetechnologies,assomestoragefacilitiescouldpotentiallybeconvertedfromnaturalgastohydrogen(Uniper,2022b).44Thesecondquestioninthisrespectiswhoshouldbeincentivized?Shouldthegovernmentsupporttheuserofthestoragefacilityoritsprovider?Preferringoneovertheotherwillhaveimplicationsforthedevelopmentofahydrogeneconomyanddeploymentofahydrogenstorageinfrastructure.Giventhenatureofrisksinvolvedinthehydrogenstorageinfrastructureinvestment,itisverylikelythatproviders,ratherthanusersofstoragefacilities,shouldbeincentivised.Theincentiveneedstobeprovidedthroughviablebusinessmodelswhichrequirepolicyandcommercialinterventions.4.1.RangeofpossiblebusinessmodelsInprinciple,businessmodelsforhydrogenstoragecanbearrangedinmanyways.Infact,theycanbeputonaspectrumbetweentwoextremes,witharangeofpossibleonesinbetween(Figure5).Here,onesideofthespectrumrepresentsapurelycommercial(market-based)approach.Inthismodel,marketparticipantsinvestinhydrogenstoragetobenefitfromthepricedifferenceacrosstime.Themainassumptionbehindthismodelisthatthemarketiswell-developed,andnogovernmentsupportisneeded.Theothersideofthespectrum,inturn,representsacentrallycoordinatedmodel.Inthiscase,42Althoughlarge-scalehydrogenstoragehasexistedinsomeplaces,itwastherenotbecauseH2wasviewedasamajorcomponentindecarbonization.Instead,whilehydrogenstoredinthosefacilitieswastreatedasacommodityofitsown,itwaslaterusedinspecificsectorsandthushadnicheapplication.Forinstance,theChevronPhillipsClemensTerminalinTexashasstoredhydrogensincethe1980sinasaltcavern(Forsberg,2006).TheH2keptthereisusedforthemanufacturingofchemicalproductssynthesizedintheregion(ibid),whichmakesthismodeleconomicallyadvantageousbutneitherrepresentativenorreplicableeverywhereinanet-zerocarboncontext.43Or,forexample,intheabsenceofcurrentlargescaleproduction,thereisariskthathydrogengenerationandhencetheneedforstoragedoesnottakeoffatall.44Forinstance,inJanuary2023,StoragEntzelandGasunieannoucedtheirintentiontojointlydeveloptwocavernsattheEntzelsiteforH2storageinthefuture(Gasunie,2023).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.23thegovernmentisdirectlyinvolvedintheprojecteitherasaninvestor,storageproviderorstorageuser.Thisinvolvementcanbeinafullorpartialmanner.Figure5:RangeofpossiblebusinessmodelsSource:UKDepartmentofEnergyandClimateChange(2014),Bhagwatetal(2017),NSEnergy(2019),InspiredEnergy(2020),NetSuite(2021),PalovicandPoudineh(2022),EuropeanCommission(2023).Abroadrangeofoptionslyinginbetweenrelatetotheregulatedmodel.Here,thegovernmentincentivizesprivateentitiestoprovidehydrogenstorageorbuystorageservicesfromaprovider.Thiscanbedoneinvariousformssuchas:•Contract-basedmodelsinwhichpricesandrevenuesareregulated;•Obligationsthatareimposedoneitherhydrogenproducersoruserstokeepspecificvolumesinstorage;•Accesstohydrogenstorageissubsidisedforendusers.Althoughallthesemodelscouldtheoreticallybeapplied,notallofthemwillbesuitableforaparticularcontextwhenhydrogenstorageneedstobecreated.Thekeyissuewillbetoaddressthetwospecificrisksoutlinedabove–thoseofpriceanddemand.Inthiscase,itisimportanttoidentifywhichbusinessmodeladdressesthepriceriskandwhichoneaddressesthevolumeriskmoreefficiently.4.2.1.AddressingthepriceriskNotsurprisingly,toaddressthepricerisk,focusisonthepriceinacontract-basedscheme.Therearevariouswaystodoso.Forinstance,fixed-pricecontractsrepresentoneofthesimplestandmostbroadly-usedoptions.Inessence,suchcontractsservearolewhichissimilartothatoffeed-intariffs(FiT)forrenewablesandarecontractualagreementswithapredeterminedvaluefortheservicesprovided(here,hydrogenstorage)(InspiredEnergy,2020).Thistypeofcontractprovidescertainty,aclearandpredictablerevenuestructureandreducesoverallriskexposure.However,settingtherightpriceforthesecontractscanbedifficultnotleastbecauseofuncertaintyaboutthecostsaswellasunforeseensituationsthatmayimpactcosts.Anauctioncanbeusedtoidentifyleastcostprovidersbutthesuccessofsuchanapproachdependsonmanyfactorsincludingparticipationofasufficientnumberofnon-colludingbidders.Theexperiencefromtheelectricitysectorshowsthat,fixedpricecontractssuchasFiTsendastrongsignaltoinvestors.However,whentheyaresetadministratively,theycanleadtosignificantinefficienciesintermsofoverpaymentorunderpaymentbecauseofinformationasymmetryandconstantlychangingmarketconditions.Thatiswhy,intheabsenceofauction-basedpricesetting,fixed-pricecontractstendtobebestsuitedforwhenaproject’sscopecanbeclearlydeterminedupfront,andthecostsofstorageprovisionedinthecontract’stermscanbeestimatedwithreasonablecertainty(NetSuite,2021).Alternatively,afixedpremiumcanbepaidontopofthepriceachievedinthemarket.Ingeneral,afixedpremiummeansanoperatingsupportintheformofapremiumpervolumeofstoredhydrogenadditionaltothemarketprice,theamountofwhichisnormallydeterminedbythegrantawardprocedure(EuropeanCommission,2023).Afixedmarketpremiumalsomeansthatitremainsataconstantlevel,evenifthemarketpricesfluctuate(ibid).Usually,whileexposingstorageproviderstosomedegreeofmarketpricerisk,italsostimulatesmarketdevelopmentperse.Atthesametime,incasesofmarketinefficiencies,establishingfixedpremiumsmayleadtoovercompensation.Inordertoavoidthis,asliding(variable)marketpremiumcanbeappliedinstead.WhilesimilartoContractsforDifference(CfD),45suchpremiumswouldnormallypaydevelopersapremiumwhenthe45ContrastsforDifference(CfD)(alsoknownas‘symmetricalmarketpremium’)isasubsidymodelinwhichbothpositiveandnegativedeviationsfromafixedreferencepricearepaidouttothecontractualpartner(NextKraftwerke,2022).ItisthemodelthatiscurrentlybeingusedtoprocurelowcarbongenerationsourcesintheUK(UKDepartmentforBusiness,Energy&IndustrialStrategy,2022).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.24marketpriceisbelowtheagreedone(ibid).Ingeneral,however,aslidingmarketpremiummayvarydependingontheevolutionofmarketpricesandcoverthegapbetweenthecostofhydrogenstorageanditsmarketprice.Apremiseofthissortofmodel,however,istheexistenceofamarketforhydrogenstoragewhichisnotthecaseatanearlystageofhydrogeneconomydevelopment.4.2.2.AddressingthedemandriskSimilartothepricerisk,thedemandriskcanalsobeaddressedthroughvariousbusinessmodels.Forinstance,storageprovidercouldbeofferedavailabilitypayment.Workinginawaysimilartothatofcapacitymarketsintheelectricitysector,thistypeofpaymentwouldbeusedtopayforstorageavailabilitytomeetpeakhydrogendemand.Inprinciple,itcanimprovesupplyadequacyandreduceconsumercostsbutcanalsofallvictimtostorageoversupply,withfinancialconsequencestotheprovideriftheavailability-basedpaymentdoesnotcoverfullcosts(Bhagwatetal,2017).Alternatively,thegovernmentcanbecometheoff-takeroflastresort(OLR).Inthiscase,itwillprovidesomesortofguaranteetothestorageproviderifagivenvolumeofstoragecapacityremainsunsold.Inthisrespect,theaimoftheOLRistoencouragecompetitioninthemarket,reducethecostofinvestmentinstoragefacilities,andlowercoststoconsumers(UKDepartmentofEnergyandClimateChange,2014).Atthesametime,suchamechanismstimulatingthedevelopmentofhydrogenstoragefacilitiesmayalsoresultinafinancialburdentothegovernmentincaseofamassiveinfluxofnewproviders(ACER,2022).Thedemandriskcanalsobeaddressedthrougharegulatedreturnmodelsuchasregulatoryassetbase(RAB)orcostofserviceregulation,whichcanbothbedeliveredwithinthecapandfloorframework.46Here,usuallyrepresentingamodelusedtoincentivizeprivateinvestmentintopublicprojectsbyprovidingasecurepaybackandreturnoninvestmentfordevelopers,RABcanserveasaperceivedunderpinningofinvestorexpectationsagainstretrospective‘asset-taking’andprospectiveasset-stranding(NSEnergy,2019).Atthesametime,opponentsofsuchmodelssuggestthattheyareeffectivelyan‘openchequebook’fordeveloperstospendwhattheylikeandthatcustomerswouldhavetoshouldertheburdenifaprojectgoeswrong(ibid).Also,theymaynotincentivizeefficientutilizationofanasset.Thesenegativeconsequenceshowevercouldpotentiallybemitigatedthroughappropriateregulatoryoversight.474.2.3.ChoosinganoptimumbusinessmodelforhydrogenstorageThechoiceofanoptimalbusinessmodelforhydrogenstorageislikelytobeinfluencedbymanyfactors,includingthosepertainingtothetwokeyrisksmentionedabove–theriskofinsufficientdemandforH2storageandtheriskofthestorageaccesspricenotbeinghighenoughtojustifytheinvestment.Therefore,itislikelythatbusinessmodelsforhydrogenstorageneedtoprovidesomedegreeofcertaintywithrespecttobothtypesofrisks.Ingeneral,therearevariouswaysthatthiscanbedone.Forinstance,thiscanbedonethrougharegulatedrevenuemodelsuchasRAB(withorwithoutcapandfloor)whenthestorageproviderwouldagreeonallowedrevenuewitharegulatoraheadofapricecontrolperiodsothatitisreflectiveofthecostsincurredbytheowneroftheoperatedstoragefacility(Snam,2022).Thestorageproviderthencanrecoveranamountuptotheleveloftheallowedrevenuefromstorageusers,whichwouldbedoneinaccordancewithanagreedchargingmethodology.Thedownsideofthisapproach,however,isthatperformancecannotbesufficientlyguaranteed.ItcanalsobedonethroughaCfDcontractthatcouldbeawardedwithorwithoutanauctionorganizedtodeterminethereferenceprice.Here,areferencepriceisthepricethatastorageproviderconsiderstobesufficienttoinvestinandoperatethehydrogenstorage.Althoughthisoptionseemsreasonable,itislikelythatsettingaspecificreferencepriceordesigninganauctionforthispurposewouldbeacomplexundertakingduetoinformationasymmetry,(Matthäus,2020).Anotherapproachwouldbetoapplyahybridmodel.Inparticular,acapacitypaymentcanbecombinedwithafixedpricecontract.Here,themainadvantageswillbethesimplicityofthismechanismthatwillbecombinedwithitsincentiveprovisionforinfrastructureutilization.Atthesametime,inthiscasejustlikeinthepreviousone,settingtherightpricemaybechallengingduetothesamechallengeofinformationasymmetry(Mühlbacher,Amelung,andJuhnke,2018).Atthisveryearlystageofmarketdevelopmentforhydrogenstorage,itislikelythatnotallmodelswillbeequallyeffective.Infact,someofthemwillbemoreapplicableandefficientlaterwhentheH2storage46ThecapandfloormodeliscurrentlybeingusedforinterconnectionsintheUK(Ofgem,2016).Here,thecapisthemaximumamountofrevenuethatastorageproviderwouldbeallowedtorecover.Excessrevenuewouldbetransferredtowhoeverwasexpectedtosubsidizethefloor(i.e.,minimumamount)revenueifitwasnotreached.47SuchasRIIO(Revenue=Incentives+Innovation+Outputs),theUKincentiveregulationofnetworks(Ofgem,2010).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.25marketbecomesmoremature.Forinstance,CfDcontractsmaycurrentlynotbesuitableduetothelackofamarkettodeterminethespread(Simhauser,2019).RAB,inturn,willbetheoreticallyimplementablebutwillentailalikelihoodofstrategicbehaviourbecauseoftheincentiveitprovidesforovercapitalization(OECD,2015).Inthesecircumstances,itislikelythatahybridmodelwhentheproviderofH2storagereceivesbothcapacitypaymentandafixedpricecontractwillbetterbalanceinvestmentincentivesandmarketdevelopmentdespitechallengesassociatedwithsettinganappropriateprice.4.3.FurtherchallengesandquestionsAlthoughdemand-andprice-relatedconcernsappeartobethekeyonesfortheprocessofcreatingaviablebusinessmodelforhydrogenstorage,theywillmostlikelybeaccompaniedbyotherchallenges.Forinstance,thecapacityofhydrogenstoragethatwouldbeneededmaynotbeeasytopredictnotjustbecauseofthesupply-demanduncertaintybutalsoduetothelackofclaritywithrespecttotheultimateuseofH2,whichwilldefineitstypeandspecifics.Itislikelythattherequirementforstoragewillbedifferentifhydrogenisusedintheheating,transport,orpowersector,thanifitisdedicatedforalong-distanceshipmentrightafterthestoragephase(TÜVSÜD,2022).Furthermore,aH2energysystemcanbedesignedtominimizetheneedforhydrogenstorage.Forexample,ifthereisgoingtobeawell-developedH2network,theamountofhydrogenstorageneededislikelytobelowerbecauseitispossibletoaggregatedifferentstoragefacilities.Inaddition,ifhydrogenproductionisco-locatedinindustrialclustersorclosetothem(i.e.,thesituationresemblesthatofmostH2generationtoday),theamountofstoragethatwouldbeneededmaydecline(WorldEconomicForum,2020).48Identifyingthemostsuitablelocationforhydrogenstorage,inprinciple,isaveryrelevanttaskinthisrespect,sinceplacingitclosetoproducersratherthanconsumerswillincreasethecostofitsdeliveryandviceversa(PatoniaandPoudineh,2022).49Inthiscontext,choosingthe‘right’typeofH2storagewillalsobecrucial.Alarge-scaleundergroundtypeofstoragehasgeologicalandthusgeospatiallimitationswhereasabovegroundsolutionsarenotlimitedbythisfactor(seePart3.2.).Thepreferredoptionsfortypesofhydrogenstoragemaychangeintimeasamatureandwell-designedhydrogennetworkisdeveloped.Anotheraspecttoconsiderwhiledesigningabusinessmodelforstimulatinghydrogenstorageistheissueofownership.Specifically,inprinciple,storagefacilitiescouldbeownedandoperatedbyvariousactors,forexample,H2producers,consumers,networkoperators,shippersorindependententities.Here,themostimportanttaskinrelationtoallocatingownershiprightsforanefficientorganizationofstoragewouldbetomakesurethattheentityinchargeofastoragefacilityisnotincentivizedtoengageinstrategicbehaviourthatwouldhaveanegativeimpactontheentiresystemsuchaswithholdingstoragecapacitytomaximizetheprofit(Ofgem,2011).Thistaskalsorelatestoabroaderneedtoensurethatthepartyreceivingsubsidiesforhydrogenstoragemakesefficientinvestmentdecisions.Ingeneral,itisextremelyimportanttoensurethatstorageinfrastructurewouldbeusedinpractice.Whilesomebusinessmodelssuchascapacitypaymentmayincentivisetheconstructionofastoragefacility,theymayprovidelittleornoincentivefortheuseofthisfacility(Sioshansi,2020).Asaresult,althoughaH2storagefacilitycanbeconstructedthroughbusinessmodelsthatfocusoncapitalcosts,ifthereisinsufficientincentiveforitsuse,itmaybeunderutilized.Thegovernmentwouldalsoneedtospecifyhowtorecoverthecostofsubsidiesforhydrogenstorage.Althoughthereareseveralwaystodothat,eachofthemhasitsownchallenges.Forinstance,while,inprinciple,subsidiescouldberecoveredfromtheusersofhydrogen,theH2sectoritselfmaynothavealargeenoughuserbase.Thiscostcanalsoberecoveredfromnaturalgasconsumersvianetworkchargesbut,inthecurrentconditionsofhighpricesfornaturalgas,suchadecisionislikelytocausesocialdiscontent.Thesameislikelytohappenifthiswillbedonethroughgeneraltaxation.Theroleofcompetitionindeliveringanefficientbusinessmodelforhydrogenstorageshouldalsobediscussedinthiscontext.Whilesomebusinessmodelsaremoreeasilycombinedwithacompetitivemechanism(e.g.,anauction),othersaremoredifficultinthisrespect(Baumgarte,Glenk,andRieger,48However,eveninthiscase,ifthehydrogenproductionisfromintermittentrenewables,thestorageaspectwillstillbeimportant.49Forinstance,thehydrogenstoragecavernsinEntzelthatarecurrentlybeingdevelopedbyGasunieandStoragEntzelarelocatedwithaperfectconnectiontotheDutchandGermanhydrogenmarket,nearthefutureGasuniehydrogennetworkHyPerLinkandtheEnergy-HubPortofWilhelmshaven(Gasunie,2023).Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.262020).Thisbringsupthequestionofwhetherthegovernmentshouldprioritizethosemodelsthatcanbedeliveredcompetitively.Anotherrelevantquestioniswhetherthebusinessmodelitselfshouldonlyfocusonriskmitigationorshouldincludeadditionalfeaturesrelatedtothedevelopmentofhydrogenmarket.Here,oneofthekeytrade-offswillbeasfollows.Specifically,addingextrafeaturestoabusinessmodeltomeetotherobjectives(e.g.,hydrogenmarketdevelopment)mayincreasevalueformoneybut,atthesametime,mayincreasecomplexityofthebusinessmodelitselftotheextentthattheoriginalobjective–thedeploymentofinfrastructure–isnotachievedefficientlyornotachievedatall.Finally,thegovernmentwouldneedtoconsidertheexitstrategy.Thisisbecause,inprinciple,hydrogenstorageshouldbepaidforbyitsuserseventuallyandnotthegovernment.Thatiswhy,havingexercisedsupportinthebeginning,thegovernmentwilleventuallyhavetoexit.5.ConclusionInthispaperwehaveanalysedsixmajorhydrogenstoragetypesthatarecurrentlybeingconsideredbyresearchersandindustry,highlightedkeybarrierstotheirinvestmentanddiscussedthespecificationofaviablebusinessmodeltomitigateinvestmentrisks.Thestorageoptionsanalysedarepurehydrogenstorage,synthetichydrocarbons,chemicalhydrides,LOHCs,metalhydrides,andporousstructures.Althougheachoftheseoptionshasitsadvantagesanddisadvantagesandcouldbeusedfordifferentapplicationsandindifferentconditions,noneofthemisidealandcouldaddressallthechallengesofH2preservation.Besides,theirtechnologicalreadinessvariessignificantly,whichmeansthatsomeofthemcannotbeusedforthispurposeinthecurrentconditionsbutareexpectedtobereadyinyearstocome.Beingthemostmatureintermsoftechnology,market,andcommercialreadiness,thepurehydrogenstorageoptioncouldberealizedwhenH2iseithercompressedorliquefied.Whilebothcompressionandliquefactionwillsignificantlyimproveitsvolumetricdensity,bothprocessesareextremelyenergyintensiveandthuswillincurfurtherexpense.Besides,storingbothcompressedandliquefiedhydrogenwouldbeassociatedwithsignificantsafetyrisks.Synthetichydrocarbonsareoftenconsideredtobeanoptionthatwoulddramaticallysimplifyhydrogenstorage.Thisisbecause,beinginessencean‘artificialalternative’tonaturallydepositedfossilfuels,thesechemicalswouldalreadyhavewell-developedstorageinfrastructure.Moreimportantly,mostofthemwillnotbeassociatedwithadditionalcostsrelatedtotheirpreservation.Ontheotherhand,producingsyntheticfuelsinacarbon-freeway(e.g.viathepower-to-Xpathway)aswellasdehydrogenatingthemforfurtheruseofH2islikelytobeextremelyenergy-intensiveandthuscostly.Thisis,perhaps,oneofthekeyreasonswhythetechnologyreadinesslevelofe-fuelsiscurrentlyatalowlevel.Chemicalhydridesinaformofammonia,methanol,formicacidorisopropanolarealsooftenviewedasadvantageoustopurehydrogenstorage,astheyareeitherliquidunderambientconditionsorcanbecomeliquidwithouttheneedforsignificantenergy(ammoniaisliquefiedunder-33oC).Asaresult,theyareeasierandcheapertopreserveandalreadyhaveextensivestorageinfrastructure.Ontheotherhand,itisstillnotcompletelyclearhowcompetitivetheirproductionanddehydrogenationinacarbon-freewaywillbeincomparisontootherhydrogenstorageoptions.Liquidorganichydrogencarriers(LOHCs),beingmostlyby-productsofoilrefining,representanothercategoryofsubstancesviewedaspotentiallysuitableoptionsforstoringH2.Althoughtheyarealsoliquidandwouldbeeasiertostoreinvesselsandfacilitiessuitableforhydrocarbons,theirhydrogencontentislowerthaninmanyotherhydrogenderivatives(e.g.,ammonia,methanol,ande-fuels)thustheymaynotnecessarilyalwaysbeviewedasatop-tierstorageoption.Inaddition,producinganddehydrogenatingLOHCsinacarbon-freewaymightnotmakeeconomicsenseunderallconditions.Metalhydridesrepresentoneofthefewoptionsthatwouldallowhydrogentobestoredinasolidandthusmoreconcentratedform.Whiletheirvolumetrichydrogendensityindicatorsareoftenimpressive,theirgravimetrichydrogendensityonesarelessso.Asaresult,despiteenablingthestorageofH2inlesservolumesthanmostotherstoragealternatives,metalhydridesareusuallytheheaviestsubstancesforstorage.This,combinedwithrelativelyslowhydrogenation/dehydrogenationspeedaswellasthenascentnatureofthesetechnologiesmayresultintheirslowermarketpenetrationcomparedtootheroptions.Finally,porousmaterialsrepresentedbymetalorganicframeworksandcarbon-basedsystemssuchascarbonfibres,nanotubes,templatedandactivatedcarbonaswellasgrapheneareanothertypeofThecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.27hydrogenstoragealternativesthatwouldallowforsafeandstableH2preservation.However,becauseoftheirlowtechnologicalreadiness,thescaleoftheirapplicationatthemomentisquitelow.Soisthevolumeofhydrogenthattheycancurrentlystore.Sinceeachoftheseoptionscouldpotentiallyofferauniquecombinationofbenefitsthatmaybehardtobeatinspecificconditionsofhydrogenapplication,itislikelythateachofthemwouldhaveprospectsforbeingdevelopedinthefuture.Forinstance,whilemetalhydridesandporousmaterialsseemtobelesscompetitiveatthemoment,theyappeartobetheonlyoptionsofferinganon-toxic,non-corrosive,andnon-flammablestoragemode.Thisfeatureislikelytobeofextremeimportancewhenthehighestsafetyofhydrogenpreservationneedstobeprioritized.Alternatively,syntheticfuelsandchemicalhydridesmaybechosenwhencost-efficienthydrogenstorageneedstobecoupledwithtransportation.LOHCs,inturn,couldbechosenbyentitieshavingasubstantialstakeinoilrefiningandthechemicalindustry.Finally,hydrogenstorageinpureformmaybepreferredinsituationswhenhighH2purity,quickdischargeorextremelyhighstoragevolumeisneeded.Theveryexistenceofthisextremelylarge-scalehydrogenstorageisoftenseenasakeyprerequisiteforthedevelopmentofaviablehydrogenvaluechainandthusfortheprogressofahydrogeneconomy.ThisissobecauseH2storagehassignificantvalueperse,sinceitcanhelptoaddresstheimbalanceofdemandandsupply.Atthesametime,despitethishighpotentialvalue,investmentinhydrogenstoragehassofarbeenlimited.Thiscouldbeexplainedthroughthekeyexistingrisksthat,incombination,createrevenueuncertainty.TheserisksaretheriskoflowdemandforH2storageaswellastheriskoflowerthanbreak-evenpriceforutilizingahydrogenstoragefacility.Toaddressrisksandmakehydrogenstoragemoreattractiveforinvestors,variousbusinessmodelscouldbeapplied.Inessence,aviablebusinessmodelentailspolicyandcommercialinterventionstoallocatetherisksbetweenthegovernmentandaprivatepartyinanefficientmanner.Whiletherearevariouswaystoarrangeriskallocationefficiently,possiblebusinessmodelsforhydrogenstoragecouldbebroadlygroupedintothreemaincategorieswiththemarket-basedonebeingthemostliberalisedtypewheremarketparticipantsmakeinvestmentsinanticipationofprofitandwithoutgovernmentsupport.Thecentrally-coordinatedcategoryrepresentstheotherextremewheregovernmenteitherdirectlyinvestsinhydrogenstorage,createsapublicprivatepartnershiporrepresentstheoff-takeroflastresort.Theregulatedtype,inturn,iscomprisedofvariousmodelsandbroadlydescribescaseswhenthegovernmentincentivizesprivateentitiestoprovidehydrogenstorageorbuysthestorageservicefromaprovider.Althoughnotallthebusinessmodelsareusefulinthecurrentcontext,someofthemcouldbeappliedtoaddressthepriceriskandotherstoaddressthevolumerisk.Inparticular,fixedpricecontractsaswellasfixedandvariablepremiumcontractsrelatetothemodelsdealingwiththepricerisk.Allocatingavailabilitypayments,organisingtheactivityviaaregulatedreturnmodelsuchasRABormakinggovernmenttheoff-takeroflastresortwillhelptoaddressthedemandrisk.Atthesametime,itislikelythatanoptimumbusinessmodelwillhavetoprovidesomedegreeofcertaintywithrespecttobothpriceanddemandrisks.Inprinciple,thiscanbedoneinanumberofways.Forinstance,througharegulatedrevenuemodeloraCfDcontract,thoughbothoptionshavesignificantchallenges.Alternatively,ahybridmodelcouldbeapplied:forexample,acapacitypaymentcoupledwithafixedpricecontract.Whilethisislikelytomaximisethebenefitsandminimisethedrawbacks,eventhishybridapproachwilldemonstrateitsdeficienciesandposechallengesinsomeconditions.Finally,apartfromthenecessitytoaddresspriceanddemandrisks,thedesignersofviablebusinessmodelsforhydrogenstoragewillhavetofaceanumberofotherimportantcomplexities.Thesechallengeswillinclude,butwillnotbelimitedto,choosingthetypeofstoragefacilitiesthatshouldreceivesupport,specifyingtherelativelocationofstoragefacilitieswithrespecttoproductionanddemandcentres,determiningtheownershipmodelofstorage,definingthecharacteristicsofpartieseligibleforgovernmentsupport,decidingwhetherthebusinessmodelshouldaimforobjectivesbeyondinvestmentincentiveandriskmitigation,formulatinganapproachtorecoverthecostsofsubsidiesandfinallyprovidingapathforgovernmenttoeventuallyexitsubsidiesoncetheindustryismature.Althoughtheanswertoeachofthesequestionsislikelytodifferdependingonspecificcircumstances(suchasthecharacteristicsofstorageandend-usedemand,thespecificmarketsthatcountriesconsidersuitableforhydrogen,andtheextentofsupply-demandimbalances),designersofbusinessmodelsforhydrogenstoragewillhavetoconsideralltheseaspectsinadditiontomakingappropriatechoiceswithrespecttomitigatingthetwocrucialrisks.InpracticethiswillmeanthatfindingthemostThecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.28suitablecombinationoftheanswerstoeachofthesequestionswillultimatelydeterminetheapproachthatwillbeusedineachparticularcase.Theseanswers,choices,andapproacheswillhelptorevealthegreatestvalueofH2storage,whichiscrucialforthedevelopmentofasoundandresilienthydrogenvaluechainandthecreationofahydrogeneconomy.Thecontentsofthispaperaretheauthors’soleresponsibility.TheydonotnecessarilyrepresenttheviewsoftheOxfordInstituteforEnergyStudiesoranyofitsMembers.29ReferencesACER(2022)ACER’sfinalassessmentoftheEUwholesaleelectricitymarketdesign[Online].Availablefrom:https://acer.europa.eu/Official_documents/Acts_of_the_Agency/Publication/ACER's%2520Final%2520Assessment%2520of%2520the%2520EU%2520Wholesale%2520Electricity%2520Market%2520Design.pdf(Accessed:07February2023).Ahmed,A.,etal(2019)‘Exceptionalhydrogenstorageachievedbyscreeningnearlyhalfamillionmetal-organicframeworks’,NatureCommunications,10(1568),pp.1-9.AirLiquide(2022)Hydrogenapplications[Online].Availablefrom:https://energies.airliquide.com/resources-planet-hydrogen/uses-hydrogen(Accessed:12December2022).Alberico,E.andNielsen,M.(2015)‘Towardsamethanoleconomybasedonhomogenouscatalysts:methanoltoH2andCO2tomethanol’,ChemicalCommunications,51(1),pp.6714-6725.AMF(2010)Dieselandgasoline[Online].Availablefrom:https://www.iea-amf.org/content/fuel_information/diesel_gasoline(Accessed:07September2022).Alyea,E.andKeane,M.A.(1993)‘Theoxidativedehydrogenationofcycloxehaneandcyclohexeneoverunsupportedandsupportedmolybdenacatalystspreparedbymetaloxidevapourdeposition’,JournalofCatalysis,164(1),pp.28-35.Andersson,J.andGrönkvist,S.(2019)‘Large-scalestorageofhydrogen’,InternationalJournalofHydrogenEnergy,44(2019),pp.11901-11919.Armijo,J.andPhilibert,C.(2020)‘Flexibleproductionofgreenhydrogenandammoniafromvariablesolarandwindenergy:CasestudyofChileandArgentina’,InternationalJournalofHydrogenEnergy,45(3),pp.1541-1558.Asif,F.,etal(2021)‘Performanceanalysisoftheperhydro-dibenzyl-toluenedehydrogenationsystem–Asimulationstudy’,Sustainability,13(11),pp.1-14.Audi(2018)Pushfortheenergyrevolution[Online].Availablefrom:https://www.audi.com/en/innovation/alternative-drive-systems/energy-revolution.html(Accessed:18December2022).Aziz,M.,Wijayanta,A.T.,andNandiyanto,A.B.D.(2020)‘Ammoniaaseffectivehydrogenstorage:Areviewofproduction,storageandutilisation’,Energies,13(3062),pp.1-25.Azzaro-Pantel,C.(2018)Hydrogensupplychains.Cambridge,MA:AcademicPress.Balakhonov,S.V.,Vatsadze,S.Z.,andChuragulov,B.R.(2015)‘Effectofsupercriticaldryingparametersonthephasecompositionandmorphologyofaerogelsbasedonvanadiumoxide’,RussianJournalofInorganicChemistry,60(1),pp.9-15.Banerjee,S.andTyagi,A.K.(2011)Functionalmaterials:Preparation,processingandapplications.Amsterdam,theNetherlands.Barnes,A.(2023)TheEUHydrogenandGasDecarbonisationPackage:helpofhindranceforthedevelopmentofaEuropeanhydrogenmarker?Oxford,UK:OxfordInstituteforEnergyStudies.Baumgarte,F.,Glenk,G.,andRieger,A.(2020)‘Businessmodelsandprofitabilityofene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