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ContentslistsavailableatScienceDirect

Transportation

Engineering

journalhomepage:www.elsevier.com/locate/treng

HeliRail:

A

railway-tube

transportation

system

concept

D.P.

Connolly

,

P.K.

Woodward

Institute for High Speed Rail and System Integration, University of Leeds, United Kingdom

a

r

t

i

c

l

e

i

n

f

o

Keywords: High speed railway Hyperloop Heliox railroad Vacuum transport

Tube guideway transportation

a

b

s

t

r

a

c

t

Helirailisanenergyefficientmasstransittransportationsystemconcept,whichcombinesdevelopmentsin low-pressuretubetransportwithexistinghigh-speedrailwayinfrastructure.Itaddressestheproblemthat,currently atlowspeeds,steelwheelrailwaysareanenergyefficienttransportmode,howeverathighspeeds,>80%of energyisusedovercomingdrag.Thismeansminimisingtheseresistancespresentsahigh-impactopportunity forreducingrailwayenergyconsumption.Toreduceresistance,HeliRailconsistsofanairtighttube-track struc-turethatallowsexistingsteel-wheeltrainstotravelonexistingrailwaycorridorswhereslab-trackissuitable, withminimaldrag.Therunningenvironmentislow-densityhelioxgas,heldinsidelightweighttubes,slightly belowatmosphericpressuretominimisespeciestransport.HeliRailcapturesthisenergysavingasanoperational reduction,thusimprovingtheenergyefficiencyofhighspeedrailby60%.Onahighcapacityroute,annually thiscouldsaveenoughenergytopower140,000homes.DeployingHelirailonanexistinglinedoesnotincrease traincruisingspeeds,howeverasecondarybenefitisjourneytimereduction,achievedusingasmallpartofthe energysavingforimprovedtrainacceleration.Unlikepreviousevacuatedtubetransportationembodiments,the systemisinteroperablewithtraditionalraillines/trainsmeaningvehiclescanpassthroughHeliRailsectionsand ontotraditionalsteel-railnetworks.Thisalsoreducesland-purchaserequirements.Furtherbenefitsinclude im-provedsafetycomparedtovacuumtransportationandfewerservicedisruptionscomparedtorail.Capitalcostis lowcomparedtoanewrailorpressurisedtransportationline,andisrecoveredafteraperiodcompetitivewith renewableenergytechnologies.

1. Introduction

Approximately20%oftheworld’senergyisusedfortransport[1] , andthisispredictedtoincreaseby31%by2050[2] .Further, consid-eringEuropeasanexample,transportationisthesinglelargestemitter ofgreenhousegases(30.8%)andtheonlysectortohaveexperienceda growthinemissionssince2007[3] .Whenconsideringapproachesto re-ducetransportenergyconsumptionandemissions,railisattractive com-paredtoroadandairbecausetherollingfrictionbetweensteelwheels andrailsislow.Thisistruewhenmovingatlowtrainspeeds,however, astrainspeedsincrease,airresistanceanddragincreaserapidly. There-forerailwaytransportbecomessignificantlylessenergyefficient,even atmoderatespeeds,asshowninFig. 1 ,whichisplottedusingEq. (1) . Thisisparticularlyimportantconsideringcurrentplanstoexpandthe globalhigh-speedrail(HSR)network.

Aerodynamicresistanceisawell-knownchallengeinaviation,and toovercomeit,airplanesclimbhighwheretheairdensityislow,thus reducingairresistanceandimprovingenergyefficiency.Thisconcept ofreducingtheairresistanceonvehiclesismorechallengingforland transport,howeverthedesireforfastertravelcontinuestoinspire re-searchinthisarea.

Correspondingauthor.

E-mailaddress:d.connolly@leeds.ac.uk (D.P.Connolly).

Forexample,oneofthefirstconceptsforpressurisedland transporta-tionwasproposedbyMedhurstin1799.Itwasexpandeduponbya vari-etyofengineers,includingbyBrunelwhobuiltan‘atmosphericrailway’ in1847.Theunderlyingconceptwastouseadifferentialairpressureto poweravehicle.Later,Goddard[4 ,5] ,proposedanalternative,where vacuumpumpsreducedpressureinatunnelguideway,whilethevehicle wasmagneticallylevitated.

Theseimplementationswereultimatelycommerciallyunsuccessful, howeverrecentadvancesinvacuumpumptechnologyhaveledto re-newedvigourintopressurisedtransportationsystems.These included conceptsfrom[6–8] ,whichlikeGoddard,proposedusingreduced run-ningenvironmentpressurestominimiseresistance,ratherthan differ-entialair pressuresaspropulsion.Thus,research effortshaveshifted towardsconstructingnew,dedicatedtransportinfrastructurenetworks thatmovemagneticallylevitated,batterypoweredpodsinsidevacuum tubesatspeeds≈1000km/h.Theattractivenessofthiscomesfromthe speedincreaseandenergysavingsprimarilyaffordedbyminimisingair resistance.

Thismode oftransportisoftencolloquiallyreferredtoas ‘Hyper-loop’ andthus,hereafter thetermis usedloosely tosimplyrefer to thegeneralconceptofvacuumtransportation.Significantresearch

ef-https://doi.org/10.1016/j.treng.2020.100004

Received25April2020;Receivedinrevisedform28April2020;Accepted29April2020

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Fig. 1. Typicalresistanceonahighspeedtrain(blackline=320km/h).

fortshave beenplaced on developingHyperloopvehicles,for exam-ple[9 ,10] .Additionally,researchershaveinvestigatedbridge dynam-ics [11] , ground dynamics [12] , aerodynamic design [10 , 13] and earthquakes[14] .

Regardingcommercialisation,therehasbeenthedevelopmentoftest tubetunnels[15] ,andscaledtestrunsinabsenceofhumanpassengers, tospeedsof457km/h[16] .Further,genericHyperloopguidelinesfor design,operation,andcertificationhavebeenproposed[17] .Thusfar, mostimplementationsfocusonreducingdragvialowpressureair en-vironments,howeverithasalsobeenproposedby[18] tofillatube withlow-pressureheliox(aheliumandairmix[20] ).Theadvantageof heliumisthatithasasignificantlylowerdensitythanair,thus reduc-ingdrag.Thereforeitwasproposedforthepurposeofslightlyrelaxing thelow-pressuretubeenvironment,thusreducingcostsandimproving safety.

Achallengewiththeseapproacheshowever(evenforthepressure rangeproposed in [18] ), isthat itis costlytomaintain avery low-pressureenvironmentoverlongdistances.This isbecausethepumps requiredareatthelimitofcurrenttechnology,makingthemexpensive andenergyintensivetorun.Further,itischallengingtoovercomethe safetyissuesassociatedwithplacingpeopleinanear-vacuum environ-ment,withoutbuildingsignificantadditionalprotectiveinfrastructure. Therefore[19] proposedreplacingthelowpressureairwithinthe evacu-atedtubes[19] ,withhelioxheldatatmosphericpressure.Thisprovides reducedresistancebecausehelioxhasalowerdensitythanairand dis-penseswithmanyofthesafetyconcerns associatedwithoperatingat lowpressures.Further,thecapitalandoperationalcostsassociatedwith highperformancepumpsarenotrequired.

Achallengewithusinghelioxatatmosphericpressurethoughisthat thepistoneffectsduetohigh-speed vehiclesrunninginanarrow di-ametertubeishigh[21] ,thuscancellingmanyoftheenergyefficiency benefitsachievedwhenoperatingatlowpressure.Consideringthe en-ergyconsumptiontrade-off betweenpistoneffectresistancesand main-tainingalow-pressure tube,ithasprovenchallengingtofindan en-vironmentallyfriendlysolutiontoachievegroundtransportatspeeds of≈1000km/h.Instead,fromanenergyrequirementsstandpoint, Hy-perloophasbeenshowntooffersimilarperformancetoexistingHSR [22] ,butpossiblycarryfewerpassengers(forexample,30peopleper pod[23] ).

Afurtherchallengewithmostcurrentembodimentsofvacuum trans-portationis thatthey proposeusingthemagnetic levitation of vehi-cles.Thismakesinteroperabilitybetweenvacuumtransportand exist-ingtransportinfrastructurechallenging.Evenifsteelwheel-technology

guideway technologies such as ‘Tracked Air Cushion Vehicles’ (e.g. [24 ,25] )failedtotrumpincrementaladvancesinsteel-wheel railway technology.

Considering the challengesassociated with thehigh speed trans-portation of people/goods at near-vacuum conditionsover long dis-tances, this paperpresents a conceptsolutionthatcombines the ad-vantages ofHSRandHyperloopwithinasinglesystem.Theconcept useshelioxfilledtubestoreducedragonsteel-wheelhighspeedtrains. Firstthegeneralconceptispresented,includingthekeydetailsofthe proposedguidewayandvehicledesign.Thenthebenefitsofthesystem arediscussed.Finally,practicalapplicationandkeyrisksareexplored. ItshouldbenotedthatthepresentembodimentofHeliRailisnot pre-sentedasafinalisedsystem,butinsteadasaconcepttobebuiltupon andrefinedbyothers.

2. HeliRailtransportsystemdesign

2.1. Generalconcept

HeliRail builds upon previous vacuum transportation related re-search,butinsteadof offeringanalternativetransportmodetoHSR, keytube-transportationconceptsareintegratedwithHSR,thuscreating ahybridsystem.Thissystemconsistsofsealedtubesfilledwithheliox, thatenclosesteel-rail,concreteslabHSRtrackforms,servingtoreduce aerodynamicforcesonrunningtrains.Thisresultsinasignificant re-ductionofenergyusageduringtrainoperation.Thekeycomponentsof HeliRailareshowninFig. 2 ,andthemaintechnicalpointsfor under-standingare:

1. HeliRailcanbe usedforeithernewlinesorforretrofitting exist-ingHSRlines.Whendeployedtoretrofitaconcreteslabtrack, pre-formedtubesarefixedtotheexistingtrackform.Alternatively,when deployedonballasted lines,theballasttracksuperstructureis re-placedwithpre-formedtube-slabintegralunits

2. Tubesareusedtoenclosedouble-trackrailwaysonly,whicharethe mostcommonHSRtracktype.Comparedtoenclosingsingletracks, thisreducestheblockageratiobymaximisingeffectivetubearea, andthusminimisingthepistoneffect

3. Post-construction,aductingsystemextractstheambientairfrom in-sidethetubesandreplacesitwithheliox.Oncecomplete,theheliox isheldpermanentlyatmarginallybelowatmosphericpressure(≈95– 99%)duringtrainrunning.Theductingsystemmaintainspressure andrecycleshelioxtoensureitmeetstherequiredlevelofpurity. Solarpanelspowertheductingsystem.

4. Highspeedtrainsaresealedbydesign,howeverthepressure differ-entialbetweentrainandtubeminimiseshelioxcontaminationvia airpassingacross vehiclecar bodyseals.Also, althoughhelioxis breathable,thepressuredifferentialminimiseshelioxcontactwith passengers

5. Duetotheminimaltube-vehiclepressuredifferential,comparedto evacuatedtubetransportation,onlylightweighttubeinfrastructure andsealsarerequired.Thereforearangeofmaterials(e.g. transpar-entplastics)areviablecandidatesforthetubestructure.Lightweight materialshelpreducecapitalcosts,whilereducedleakagerateshelp reduceoperationalcosts

6. TrainscantransitionfromHeliRailto/fromexistingsteel-rail net-worksseamlessly.Fast-deployingair-locksallowtrainstoenter/exit HeliRailsectionswithoutalossofheliox,thusmaintainingpressure anddensity.AnexampleHeliRailjourneyisshowninFig. 3

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Fig. 2. KeyHeliRailcomponents(nottoscale).

Fig. 3. AnexampleHeliRailvehiclejourney(top=start,bottom=end).

Consideringpredictedchangestoourclimateandtheneedto pre-serveresources,HeliRailshiftsthefocusofpressurisedland transporta-tionfromincreased speeds,toenergyefficiency. Thesystemis com-patiblewithexistingrailcorridors,transportingpassengersatthesame speedasexistingHighSpeedRail(HSR)technology,howeverwith60% lessenergythanthatusedbyeitherHSRorHyperloop.Thisfacilitates: high interoperability, high line availability, low capital/operational costsandminimalmaintenancerequirements.

2.2. Guidewaystructuredesign

Atlowtrainspeeds,themajorityofatrain’senergyisusedto over-comerollingresistance(Fig. 1 ).However,aerodynamicresistancehas asquaredmathematicalrelationshipwithspeed(e.g.Eq. (1) ),meaning athighspeeds,muchgreaterenergyisrequiredtomovethetrain.For example,baseduponEq. (1) ,at320km/h,84%ofenergyisusedto over-comeaerodynamicresistance.Alternatively,iftheguidewayprovidesa

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vacuumconditionwithinwhichthetraincanmove,thisaerodynamic dragisclosetozero,thusallowingthetraintomoveusingminimal energy.

Vacuum conditions however are very challenging to achieve/ maintaininpractise,soalternativelyairresistancecanbe minimised viaeitherplacingthetubeinastateoflowpressure,loweringthegas densitywithinthetube,oracombinationofboth.Aconsequenceofthis howevercanbethecreationofthe‘pistoneffect’,whichoccurswhenan objectmovesinaconfinedgaseousspace.Thepistoneffectcreates addi-tionalresistance,potentiallycounteractingtheenergybenefitsachieved byrunningatraininsidealowpressureenvironment.

Toovercomethis,HeliRailwillbedeployedsolelyondualtrack ar-rangementsratherthansingletracks,andthusthemaintubewillhavea largecross-sectionalarea(Figs. 2 and4 ).Thiswillreducetheblockage ratiocomparedtoindividualsmallertubes,asiscommonlyproposedfor evacuatedtubetransportation,thusminimisingthepistoneffect[21] . Achallengewithdualtrackshoweveristhatvehiclestypicallytravelin oppositedirections,thuschangingtheeffectivecross-sectionalareaand possiblycreatingahighlyturbulentzone.Therefore,whentrainspass eachotherinsidethesametubebutinoppositedirections,apressure reliefarrangementtominimisetrain-traininteractionwillbe used.It willoperateautomaticallyduringcarefullytimedvehiclepassages.

Thetubewillbeheldatslightlylowerthanatmosphericpressureto minimisehelioxseepingintoHeliRailvehicles,ratherthanfortheaimof reducingdrag.Toachievethis,onlyasmallreductioninpressureis re-quired,estimatedtobeintherange95–99%.Ifthepressuredifferential istoohigh,thengasseepagefromthevehiclesintothehelioxtubeswill accelerate,thuscontaminatingthehelioxmixwithnitrogen.Toachieve thedifferential,fantechnologywillbeusedratherthancompressor tech-nologybecauseitissuitableformarginalpressurereductions,yethasa lowercost.

TrainsmovingwithinHeliRailtubeswillinevitablyinducegaseous compressions.Althoughthehelioxwillbeheldatareducedpressure tominimiseaircontamination,compressionswillincreasethe probabil-ityofthetransportofspeciesacrossthetubeandvehicleseals.Thisis particularlytrueconsideringthesmallsizeofheliumatoms,meaning thatovertime,thehelioxrunningenvironmentwillbecomelesspure. Tomanagethis,thefantechnologyusedtomaintainthepressure dif-ferentialwillalsobeconnectedtoaductingsystemthatwillmaintain contaminationatapercentagethatbalancestheneedforlowdensity withthecostofpurification.

Thetubestructuremustbedurable,cost-effectivetomanufacture, able to handle high-speed pressure forces, air-tight and resistantto weathering.AlthoughsteelhasbeenproposedforHyperloopduring

ini-tialtrials[16] ,ithaschallengeswithimpactloads,corrosion, repairabil-ity, thermal expansioncharacteristicsandcost. Thereforealternative materialswillbeusedforHeliRail.

Firstly, considering structural design, HeliRail tube pressure will onlybemarginallydifferentfromatmospheric,meaningthestatic pres-sureforcesonthetubeliningwillbemuchlowercomparedtoan evac-uatedtube.Instead,dynamicforcesontheliningarelikelytobemore dominant, arisingduetothefluid-structureinteractions duringtrain passagecausedbygaseouscompressions.However,vehiclespeedswill besignificantlylowerthanHyperloop,meaningthatthedynamicforces willalsobemuchlower.Thereforealternative,morelightweightand costeffectivematerials,constructedusingmorerelaxedtolerancesare viable.

Inparticular,transparentplasticsareaviablechoiceforHeliRail. Theywillprovideanenhancedpassengerexperiencecomparedtoan opaque/translucentstructure.Further,passengersaretypicallysatisfied witha50%fieldofvisionfromwithintrains.Thereforethetubecan beconstructedfromacombinationofmaterials,includingthese plas-tics.Forexample,transparentplasticwindowscanbecombinedwith novelprecastconcretes(e.g.carbon-fibretextilereinforcedconcreteas proposedby[26] andintermittentsteelbracing).Anexampleofthisis showninFig. 5 .Usingahybridstructurewillallowfortheoptimisation ofparameterssuchasstrength,costandcarbonfootprint.

Perhapsmoreimportantlythough,ahybridtubestructurewill al-lowforgreaterdesignflexibilityforexpansionjointsandseals.These interfaceswillbemorestraightforwardtodesignincomparisontoan evacuatedtubebecausethetubepressure(staticanddynamicpressures) aremuchlessonerous.Thenumberandlocationofsealsdependson whethertheinstallisaretrofitornew-build,becausenew-buildswill bepre-casttrack-tubeunitsmeaningexpansionjointswillbethemain locationofseals.

Forthecase of newlines,trackslabswill be assembledtogether withthetubetocreateasinglestandaloneintegralunit.This allows forhigherprecisionsealingcomparedtoretrofittingexistinglines.On existinglines,themethodforfixingthetubetotheslabdependsupon thetracktype,howeverauniversalmethodcouldbedeveloped.A he-lioximpermeablesealantwillbeappliedatalljointsasasafeguardto minimiseseepage.

2.3. Vehicledesign

ThetrainsusedonHeliRailaresteel-wheeledandoperableboth in-sidetubesectionsandonstandardrailrouteswithouttubes.This max-imisesintegrationwithexistingnetworks,yetcanbeachievedvia

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mod-Fig. 5. Semi-transparentHeliRailside-viewshowinganexampletubewindowsolution.

ificationstoexisting train technology.Compared todeveloping new trainsandnewtechnologies,thissavesthesignificanttimeandcost asso-ciatedwithbuildingandcommissioningnewrollingstock.HeliRail op-eratesusingsteel-wheelvehiclesatsimilarspeedstoexistinglines mean-ingmodificationsarenotrequiredtoaltertrackalignments,cant, wheel-railcontactcharacteristics,vehicledynamics…etc.Untilbatterytrain technologybecomesmorecommonplace,currentcollectionisachieved viaanoverheadrigidcatenaryelectricalsupply,ascommonlyusedin tunnels,andisagaincompatiblewithmostexistingtraintechnology.For existingnetworksoperatingsolelyusingbatterytrains,HeliRailwillnot requirecatenaryequipment.

Adifference in requirementsbetween a train runningin air and withinhelioxistheneedtosealcabinsfromthetransport ofspecies betweentubeandtrain.Althoughsealedtrains(e.g.non-opening win-dows)havebecometheHSRindustrynorm,theyarenotcurrently de-signedfortheHeliRailtubeenvironment.Regardingspeciestransport fromtubeintovehicle,thesmalldifferenceinpressureanddensity be-tweenvehicleandtubepreventstheseepageofhelioxintovehicles.In theoppositedirection,theatomicsizeofairislargerthanheliox mean-ingthatsmallimprovementstoexistingtrainsealswillprevent signif-icanttransportofairintothehelioxtube,particularlyduringgaseous compressions.Regardless,itisrecommendedthatanoxygenmonitoring system(andregulator)isinstalledinthevehicletodetectandcorrect leakage.Finally,trainshaveawiderangeofmechanicalandelectrical components(e.g.brakesandairconditioning)thatarealsonotdesigned forHeliRailrunning.Thisincludeselectricalconnectionsthatfor rail-wayapplications,typicallyoperateatSafetyIntegrityLevel4.However, itisunlikelythatasustained,yetminorchangeinpressurewillhavea significantnegativeperformanceimpact.

Itshouldalsobenotedthatratherthanmodifyinganexisting vehi-cle,afuturealternativeistodevelopanewvehiclethatisbetter op-timisedforHeliRailnetworks.Thisvehiclewouldhavefastdeploying carbodysealsandanoptimisedtrainnoseshapetomaximise aerody-namicperformance.Thismayincludeareducedcross-sectionalareato eitherminimisepistoneffectsorallowfortheconstructionofsmall di-ameterHeliRailtubes.Further,ondedicatedHeliRaillines,afont-end compressorcouldevenbedesignedtofurtherreducepistoneffects.The vehiclemightalsousebi-modalbattery-electricpower,meaningitcan runwithorwithoutcurrentcollection.Thiswoulddispensewiththe needforoverheadconductorrailsinHeliRailtubes,butalsoallowfor chargingontheopennetworkintheabsenceof tubes.This technol-ogyisalreadyprovenonStadler,‘FlirtAkku’trainsetsandtheJapanese EV-E301series[27] ,whichcanrunonintermittentlyelectrifiedlines. 3. HeliRailbenefits

3.1. Energysavings

Theresistance(R)tomovementof aShinkansenSeries 200high speedtrainisapproximatedinkNusingtheDavisequation[28] : 𝑅=8.202+0.10656𝑣+0.01193𝑣2 (1) wherev=speed(m/s).ThisrelationshipisplottedinFig. 1 andshows thatat320kmh,aerodynamicsaccountfor84%ofthetotalresistance.

Fig. 6. ResistanceforcecomparisonbetweenHeliRailandhigh-speedrail.

Helioxreduces thetube gas density toapproximately 25%of atmo-spheric,however,todeterminetheeffectofalowerdensitygasontotal resistance,factorsincludingthepistoneffectandMachnumberneedto be considered.Firstly,theopposingpistoneffectresistanceislimited becausethechoiceofasinglelargediametertubestructure,ratherthan twintubes,ensuresthetubecross-sectionalareaismuchgreaterthan thatofthetrain[29] .Thiscanalsobeimprovedbyoptimisingvehicle aerodynamics.AlsotheMachnumberforatrainrunninginHelioxis notahugeconcern.Therefore,forthepurposesofillustratingthe con-cept,thepistoneffectisconsiderednegligibleandtheflowconsidered asincompressible.

Therewillbesmallyetinevitableleakageofthemarginallyhigher pressureairfromvehiclesintothetube,howeveraductingsystemwill maintainhelioxpurityatathresholdlevel.Assumingthislevelresults inthetruehelioxmixhavingadensityof28%comparedtoair,then theaerodynamic resistancereductionis 72%.Thisequatestoatotal resistancesavingof60%(Fig. 6 ).Tomaintainthemarginalpressure differential,low-costandlow-energyconsumerfansareused,powered bysolarpanelsattachedtotheoutertubeshell(Fig. 2 ).Air-locksat sta-tionsmaintainthelowdensity/pressureenvironmentwhentrains en-ter/exitthesystem,meaningminimalenergyisrequiredforregaining pressure.

Finally,thepowerrequirementforatypicaltrain at320km/his approximately10MW(Fig. 1 ),meaningHeliRailwillsave6MW. Con-sideringa280kmrailjourneyat320kph,with20kmofboth acceler-ationanddeceleration,thejourneytimewillbeapproximately1hIn thisscenario,HeliRailwillofferanenergysavingof6MWh. Consider-ingatypicalhomeuses4MWhofelectricityperyear,theneachtrain journeywillsavetheequivalentofelectricityfor1.5homesperyear. Forahighcapacitylinewith8trainsperhour,running16hperdayin bothdirections,HeliRailwillsaveenoughenergytoprovideelectricity for140,000homeseveryyear.

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ofhumanexposure.Secondly,thehelioxmixis≈10–15%oxygen,which althoughcancauseimpairedcoordination,isabovethethresholdfor hu-mansurvival(≈6%).Therefore,althoughair-masksarekepton-board HeliRailvehicles,ifthecar-bodyruptures,thenegativeeffectsof passen-gerexposuretohelioxwillbelimited.Similarly,iftherewasarupture inthetube,theonlyeffectwouldbeincreaseddragonthetraincaused byairingress,duetotheincreaseddensityoftherunningenvironment. Further,HeliRailtubeshavealargediametercomparedto Hyper-loop,thusmakingevacuationproceduresmorestraightforward.Itisalso saferthanHSRtunnelsfromafireperspectivebecauseheliumisinert andoxygencontentwillbebelowthethresholdforburning(≈16%). ThereforeHeliRailofferselevatedfireprotection.Toprotectagainstthe gradualseepageofairintothetube(e.g.viavehicleortubecracks)and increasingoxygenlevelsabovethethresholdlevel,theductsystemwill maintainathresholdlevelofhelioxpurity.

HeliRail’senclosedenvironmentalsoincreasessafety.Itprevents un-desirablemodificationstothewheel-railcontactpatchfromtreeleaves, iceandotherrelatedcontaminates.Further,regardingtrainstrikes, Heli-Rail’senclosednaturepreventspublic/animaltrespassontheline, there-forereducingaccidents.

3.3. Economics

HeliRail capital costs vary depending upon whether the line is a ballastupgrade, slab upgrade, or a newline. Regardless, some of the larger physical item costs are: the tube structure, helium gas, pumps/fans andduct system,seals, photovoltaiccells andconductor rail.Additionalcostsmayincludeconcrete slabtrackifupgradinga ballastedlineorplanninganewline.Forcaseswhereexistingrolling stocktechnologyisused,additionalvehiclecostsarelow,howeverfor caseswherenewtraintechnologyrequiresdevelopment,extracostis likely.

HSRlinesareconstructed tooffera step-changeinthe transport linksbetween strategiclocations,compared toexistinginfrastructure (e.g.traditionalrail).Theyaretypicallydeployedtoeitherconnect lo-cationswherethereisnoexistingraillink,orasanadditionalroute connectinglocations.Whereanolder/slowerraillinecurrentlyexists,it isuncommontoupgradethisforsignificantlyhigherspeeds.Thisis be-causeengineering(e.g.alignmentcurvature)andpassengerexperience (e.g.delays)factorsmakethisoptionchallenging.Thereforeadditional landpurchaseisrequired.Hyperloophasbeenmootedasasuccessorto HSR,andthereforethepossibilityofconstructingHyperloopasan addi-tionalrouteconnectingcitieswhicharealreadyservedbyHSR,hasbeen considered.ForsimilarreasoningtoHSRdeployment,suchHyperloop linesarelikelytorequirelandpurchase.

Alternatively,forcaseswhereHeliRailisusedtoupgradeexisting HSRlines,thelargecostsassociatedwithlandacquisitionneededfor analternativevacuumtuberoutearenotrequired.Instead,HeliRailis retrofittedtotheexistinginfrastructure.Thisisalsoattractivefroma constructiontimelineviewpoint,becauselarge-scalelandpurchase of-teninvolvesprotractedlegalnegotiations,whichcantakeyearsto nav-igatedependinguponnationallegislation.

Further,minimalgroundworkscostsarerequiredduringupgrade becausethetubestructureprovidesadditionalbendingstiffnesstothe concreteslabtrack.Further,fornewlines,thiselevatedtrackstiffness willalsoreducegroundworkscostsbecauselessremedialworksare requiredtoaddresspoorgroundconditions.Also,Hyperloopoperates usingmagneticlevitationwhichatveryhighspeed,isatriskofdynamic instabilityiftherearesmallchangesinthedistancebetweenvehicleand

track.Incomparison,HeliRailissteel-wheelandoperatesatthetrack designspeed(i.e.lowerthanHyperloop),meaningitismoretolerantto lateral/verticalcurvesandtemperatureexpansion.Thereforeguideway geometryisrelaxed,meaningconstructionandmaintenancecostsare reduced.

HeliRailtubesarepreformed/preassembledunitsthusmaximising construction quality and precision. Forballasted upgrades andnew lines,HeliRailtubesandtheconcreteslabtrackaredesignedand con-structredtogether,resultingincombinedtrack-tubeunitsthatare as-sembledon-siteinastraightforwardmanner.Wherepossible,ancillary equipment(e.g.conductorrail)isinstalledinsidetheHeliRailunitsprior toarrivingon-site,meaningon-siteconstructiontimesareminimised, qualityismaximisedandworking-at-heightrisksareeliminated. Fur-ther,thelessonerous pressuresinsideHeliRailtubes meansthat the seals,expansionjoints,air-locksandpumpinghardwareareless expen-sivetodesignandconstruct.

Heliumisnottradedonglobalmarketsanditspricehasbeenvolatile inrecentyears.Itspricevariesaroundtheworld,dependingupon prox-imitytosource,supplychainsandwhetheritissuppliedasaliquidor gas.ThereforetheinitialfillingofHeliRailtubesisanimportant Heli-Railconstructioncost,andsubjecttofluctuation.Althoughthepricefor wholesalepurchasesuggestedby[19] is≈$2/m3USD,toaccountfor thisuncertainty,thepriceassumedhereis$20/m3.

Assumingdesigncostsof10%,consideringthecostitems and im-plicationsoutlinedabove,andaslabtrackcostof$2.2M/km,thetotal HeliRailcapitalcostsareapproximatedinTable 1 .Notethatthecost fornew-buildlineshasnotbeenshownbecausetheselinesvaryvastly dependinguponarangeofcase-specificfactors,includinglandpurchase costs.Comparingthetypicalconstructioncostofahighspeedrailline (≈$40M/km[30] ),HeliRailofferssignificantbenefitsforamodest ad-ditionalcost(18%forballastand12%forslab).

Comparedtoexistingevacuatedtubesolutions(e.g.[18] ),the oper-ationalcostsofmaintainingtherunningenvironmentarereduced be-causeonlyminimallossesof helioxoccurduringoperation,meaning thecostofreplacingheliumislow.Regardingpassengerridership, Hy-perlooppodsaredesignedtotransportsmallnumbersofpeopleathigh speed(≈30 passengers accordingto[23] ).Incontrast,because Heli-Railusesexistingtraintechnology,rolling-stockissignificantlylarger andlongerthanHyperlooppods,withthecapacityformanyhundreds ofpeople.ThereforeHeliRailtransportsgreatervolumesofpassengers compared toHyperloop.However,considering HeliRailusesexisting HSRcorridors,itisanticipatedthatridership willbebroadlysimilar toHSR,sotheinfluenceofincreasedfaregenerationoneconomicsis disregardedforthepurposesofthisconceptpaper.

It should also be noted that HeliRail tubes areinstalled using a phasedconstructionapproachwithoutpressurisationpriorto commis-sioning. This means it is possible to install HeliRail tubes on exist-inglineswithminimaldisruptiontocurrentservices.Therefore punc-tuality metrics and passenger revenue are less impacted compared to replacing the track with an evacuated tube. This reduces oper-ational costsincurred by theexisting railway administrationduring construction.

Finally,ignoringallotheroperationalbenefitsandcosts,and assum-inganaverageenergypriceof$0.14perkWh[31] ,theannualenergy savingfortheprevious280kmlineis$78M.Thisequatestoa break-evenperiodforaHeliRailinvestmentof25yearsforballastedlinesand 18 yearsforslablines.Theseperiodscomparefavourablywithother formsofrenewables,particularlyconsideringthepotentiallongevityof

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HeliRailtubestructures,andnon-monetised(andthusnot accounted for)secondarybenefits.

3.4. Interoperability

AkeyfeatureofHeliRailisthatitallowstrainstopassthroughthe tubesystemandontoexistingballast/slabrailnetworkswithout pas-sengersdisembarking.Toachievethislevelofinteroperability,HeliRail operatesusing steel-wheelvehiclesatsimilar speedstoexistinglines andcurrentcollectionisachievedviaexistingoverheadrigidcatenary technologycommonlyusedinrailtunnels.Therefore themechanical runningofthevehicleis identicalinside andoutsideHeliRailtubes. Further,thecombinationofapressureanddensitydifferentialbetween tubeandvehicle,coupledwithstandardairtighttraincarbody technol-ogy,willallowtrainstooperatesafelyatbothHeliRailandatmospheric pressures.

3.5. Journeytimes

The energy savingsprovided by HeliRail are primarily captured asanenvironmental benefit, however,afraction can be used to in-crease the initial vehicle acceleration phase of each journey. Com-paredtoalternativeformsoftransport(e.g.airplanes),high-speedtrains are slow toreach top speed and therate of acceleration is signifi-cantlybelowthelimitsofpassengercomfort.ThereforeHeliRailuses asmallfractionoftheenergysavingtoincreasetherateof accelera-tion.Thisreducesjourneytimeswithoutnegativelyimpactingpassenger comfort.

AsshowninFig. 1 ,aerodynamicsstarttoaffecttrainpoweratspeeds above≈90 km/h.Therefore trainaccelerationreducesastrainspeed increases.Assuming anaverage accelerationrateof 0.17m/s2 [32] , thenfortheroutescenariooutlinedabove,thetimerequiredtoreach 320km/hfrom100km/h is6min.AssumingHeliRailimprovesthe accelerationrateto0.34m/s2 then3minaresavedperjourney.For the280kmroutecaseconsideredabove,includingdecelerationtime, journeytimesarereducedby4%.Itshouldbenotedthatjourneytime improvementsareintendedsolelyasasecondaryHeliRailbenefitand theprimaryfocusisenergysaving.

Regardingembarking/disembarkingtimes,theseareonlymarginally longerthancurrent HSRdurations.Thisis becauseHeliRailoperates at ≈95–99% of atmospheric pressure, meaning airlocks can be de-signedtobelightweight,andtomeetsafetyrequirementswithout re-lyingon complexmechanicalsafety systems.Thismakesthemfaster tooperateincomparisontoavacuumsystem,whichneedstoprotect againstthehighpressuredifferentialbetweenthetubeenvironmentand atmospheric.

Regarding maximum cruising speed, railway alignments are de-signedforarangeofvariables,includingtheexpectedtraintype(e.g. tiltingornottilting),andthehighestpossibleexpectedoperationaltrain speed.Iffastertrainsarerequired,thentheroutealignmentneeds mod-ification,forexampletoincreasetheradiusofverticalandhorizontal curves.Therefore,tomaximisesustainabilityandtoreducethecostof newroutes,HeliRailaimstorunonexistinglines,andprovideonlya minimaltrainspeedincreasewithrespecttoHSR.Thisapproach max-imisesinteroperabilityandenergyefficiency.

3.6. Additionaladvantages 3.6.1. Lineavailability

HeliRailtubesensurethetrackisheldwithinamorehighlyregulated environmentcomparedtoHSR.Thisprotectsagainstleavesontheline, blownsand(i.e.importantforlinesnear deserts)andotherweather relateddelays.Thereforedelaysandcancellationsarereduced,creating valueforexistingusersandcreatingademanduplift(includingfrom modalshift),aswellasleadingtooperationalcostsavings.

3.6.2. Maintenance

HeliRailtubesprovideanenvironmentsuitableforextensiveremote conditionmonitoring(e.g.lasersandcameras).Althoughthephysical maintenanceoftrackswillbemorechallengingduetoaccess restric-tions,therailindustryisactively movingawayfromthisformof in-spection.Further,itisrequiredlessfrequentlyincomparisontoHSR becauseoftheshieldingprovidedbythetubestructure.However,when required,smallsectionsofthetubeswillbede-pressurisedusingairlocks andalocalisedductingsystem.Thisminimisesthelossofhelioxgas. 3.6.3. Reducedtimetomarketforvacuumtransportation

Vacuumtransportationsolutions suchasHyperlooparesubjectto ongoingresearcheffort,witharangeoftechnical,financialandhuman factorscurrentlybeinginvestigated. HeliRailisahybridrailway sys-temwhichbuildsuponexistinginfrastructure,meaningthereare poten-tiallyfewerresearchchallengestosolvecomparedtoavacuum-based solution.ThereforerealisingHeliRailintheshort-termwillbenefitthe futureofHyperloopbyprovidingvaluableinsightsforHyperloop de-signers.Theseinsightsarenumerousandincludethepractical opera-tionoftube-basedtransport,publicperception,physicaltestsamplesto performfull-scaletesting…etc.Theseadvantagesmayservetominimise thetime-to-marketforvacuumtransport.

4. Practicalapplication

4.1. Linetypes

HeliRailispredominantlyintended forsteelwheelhigh-speedrail linesandhas3mainapplications.Allthreeuseastandardconcrete slab-trackwithsteelrailsatthegaugerelevanttotherelevantlocal/national network,meaningtrackacceptanceproceduresareminimised:

1.ExistingHSRballastedlines:HeliRailtubescannotberetrofitted directlytoballastedtracksduetotheporousnatureofballast.Instead, theballasttrackstructureisremovedandreplacedbyacombined He-liRailtubeandconcreteslab-tracksystem.TheHeliRailsystemhasa higherstiffnessthanaballastedtrackmeaningremedialgroundworks areminimal.

2.ExistingHSRslablines:HeliRailtubesareretrofittedtopre-cast concreteslabtracks.There areawiderangeof commercialslab sys-tems,somewithbettersuitabilitythanothers(e.g.permeability consid-erationsduetodrainagechannelsandshearkeyarrangements). There-forefixationsystemsmayneedtobebespokefordifferentslabtypesto minimisepermeability.Castin-situslabs,suchasRheda2000,present greaterchallengesduetothelowerconstructionprecisionusedduring theiroriginalinstallation.

3.NewHSRlines:Newlinesareconstructedusingacombined Heli-Railpre-casttube-slabsystem,resultinginahigheroveralltrack stiff-ness comparedtoHSR tracks.Vehicle speedsarelowerthan Hyper-loopmeaningthelinealignmentandearthworksrequirementsaremore relaxedcomparedtobuildinganewvacuum-tubeline,thusreducing route-relatedcosts.

4.2. Interfacingwithexistinginfrastructure

WhenretrofittingHeliRailtechnologytoexistinglines,current in-frastructuremayneedtobeconsideredduringdesign.Forexample:

HeliRail willuse air-locks toenter/exittheexisting rail network whiletrainsarestationary.Whenappliedonexistinglines,air-locks maybelocatedeitherinsidestationsorontheapproachspur, de-pendinguponstationturnoutcomplexity.Thisisbecauseswitches andcrossingsarelikelytoposeachallengetoHeliRail,howeveron highspeedlinesthesearemostcommonlyfoundincloseproximity tostations.

HeliRailtubeswillblockaccessacrossat-gradelevelcrossings. How-ever,thisscenarioisunlikelytooccurinpractisebecauselevel cross-ingsarerarelyusedonhighspeedlines.Further,therailindustryis

(8)

curonhighspeedlines,comparedtotraditionallines.Regardless,it canbeovercomebymodifyingthelocalcivilinfrastructure,orusing dedicatedHeliRailvehicles,perhapswithfront-endcompressors(as proposedin[33] )thatautomaticallycommenceoperationontrack sectionswithreduceddiametertubes.Thisapproachwouldlikely onlybeconsideredonaroutewithmanyexistingHeliRailbranches, afterHeliRailtechnologyhadsignificantlymatured.

5. Risks

SomeoftheriskstorealisingHeliRailinclude:

Systemintegration:Railwaysystemscommonlycomprisealarge num-berofcomponentswhichhavecomplexinteractions.Further,they canconsistofamixofnewandlegacyinfrastructure.Thisneeds tobeconsideredwhendesigningHeliRailforbothexistingandnew lines.ItwouldbeaparticularconcernifHeliRailwasintendedfor deploymenton slowercommuterlines,however itsbenefits arise fromoperatingathigherspeeds,meaninglowerspeedlinesare un-likelytobeconsideredforHeliRaildeployment.Higherspeedlines tendtobemoremodernandthusarelesslikelytorelyonlegacy systems.

Heliumavailability:Thepriceofheliumisvolatileduetoglobal sup-plychainchallenges,andisdependantupontherequiredlocationof deployment.Regardless,pricefluctuationsmightaffectbothcapital andoperationalcosts.Ensuringsustainableusagewillhelpmanage risk.

6. Conclusions

Thispaperpresentsaconcepttransportationsystem,knownas Heli-Rail.Itisanenergyefficientmasstransittransportsystem,which com-binesnewdevelopmentsinvacuumtransportwithexistingrailway in-frastructure.Itimprovestheenergyefficiencyofhighspeedrailby60%, andonasingleroute,cansaveenoughenergytopower140,000homes everyyear.Traincruisingspeedsdon’tincrease,howeverasecondary benefitisthereductionofjourneytimes,achievedusingasmallpartof theenergysavingforimprovedtrainacceleration.Unlikecurrent evacu-atedtubetransportsystems,itisinteroperablewithtraditionalrailways meaningrollingstockcanpassthroughHeliRailsectionsandonto al-mostanyothersteel-railpartofthenetwork.Thisalsomeansthat addi-tionalland-purchaseisnotrequired.Furtherbenefitsincludeimproved safetycomparedtovacuumtransportandfewerservicedisruptions com-paredtohighspeedrail.Capitalexpenditureislowcomparedtoanew railorevacuated-tubeline,andisrecoveredafteraperiodcompetitive withcurrentrenewableenergytechnologies.Itshouldbenotedthatthe presentembodimentofHeliRailisnotpresentedasafinalisedsystem, butinsteadasaconcepttobebuiltuponandrefinedbyothers. DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingfinancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinfluence theworkreportedinthispaper.

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