ocean tapping.docx

ocean tapping.docx

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oceantapping

Casehistory

Tappingtheoceans

Jun5th2008

FromTheEconomistprintedition

Environmentaltechnology:

Desalinationturnssaltywaterintofreshwater.Asconcernoverwater’sscarcitygrows,canitofferaquicktechnologicalfix?

THEREarevastamountsofwateronearth.Unfortunately,over97%ofitistoosaltyforhumanconsumptionandonlyafractionoftheremainderiseasilyaccessibleinrivers,lakesorgroundwater.Climatechange,droughts,growingpopulationandincreasingindustrialdemandarestrainingtheavailablesuppliesoffreshwater.Morethan1billionpeopleliveinareaswherewaterisscarce,accordingtotheUnitedNations,andthatnumbercouldincreaseto1.8billionby2025.

Onetime-testedbutexpensivewaytoproducedrinkingwaterisdesalination:

removingdissolvedsaltsfromseaandbrackishwater.Itsappealisobvious.Theworld’soceans,inparticular,presentavirtuallylimitlessanddrought-proofsupplyofwater.“Ifwecouldevercompetitively—atacheaprate—getfreshwaterfromsaltwater,”observedPresidentJohnKennedynearly50yearsago,“thatwouldbeinthelong-rangeinterestofhumanity,andwouldreallydwarfanyotherscientificaccomplishment.”

AccordingtothelatestfiguresfromtheInternationalDesalinationAssociation,therearenow13,080desalinationplantsinoperationaroundtheworld.Togethertheyhavethecapacitytoproduceupto55.6mcubicmetresofdrinkablewateraday—amere0.5%ofglobalwateruse.AbouthalfofthecapacityisintheMiddleEast.Becausedesalinationrequireslargeamountsofenergyandcancostseveraltimesasmuchastreatingriverorgroundwater,itsuseinthepastwaslargelyconfinedtowealthyoil-richnations,whereenergyischeapandwaterisscarce.

Butnowthingsarechanging.Asmorepartsoftheworldfaceprolongeddroughtsorwatershortages,desalinationisontherise.InCaliforniaalonesome20seawater-desalinationplantshavebeenproposed,includinga$300mfacilitynearSanDiego.SeveralAustraliancitiesareplanningorconstructinghugedesalinationplants,withthebiggest,nearMelbourne,expectedtocostabout$2.9billion.EvenLondonisbuildingone.AccordingtoprojectionsfromGlobalWaterIntelligence,amarket-researchfirm,worldwidedesalinationcapacitywillnearlydoublebetweennowand2015.

Noteveryoneishappyaboutthis.Someenvironmentalgroupsareconcernedabouttheenergytheplantswilluse,andthegreenhousegasestheywillspewout.Alargedesalinationplantcansuckupenoughelectricityinoneyeartopowermorethan30,000homes.

Thegoodnewsisthatadvancesintechnologyandmanufacturinghavereducedthecostandenergyrequirementsofdesalination.Andmanynewplantsarebeingheldtostrictenvironmentalstandards.OnerecentlybuiltplantinPerth,Australia,runsonrenewableenergyfromanearbywindfarm.Inaddition,itsmodernseawater-intakeandwaste-dischargesystemsminimisetheimpactonlocalmarinelife.JasonAntenucci,deputydirectoroftheCentreforWaterResearchattheUniversityofWesternAustraliainPerth,saysthefacilityhas“setabenchmarkforotherplantsinAustralia.”

Referencestoremovingsaltfromseawatercanbefoundinstoriesandlegendsdatingbacktoancienttimes.Butthefirstconcertedeffortstoproducedrinkingwaterfromseawaterwerenotuntilthe16thcentury,whenEuropeanexplorersonlongseavoyagesbeganinstallingsimpledesaltingequipmentontheirshipsforemergencyuse.Thesedevicestendedtobecrudeandinefficient,andboiledseawateraboveastoveorfurnace.

Animportantadvanceindesalinationcamefromthesugarindustry.Toproducecrystallinesugar,largeamountsoffuelwereneededtoheatthesugarsapandevaporatethewateritcontained.Around1850anAmericanengineernamedNorbertRillieuxwonseveralpatentsforawaytorefinesugarmoreefficiently.Hisideabecamewhatisknowntodayasmultiple-effectdistillation,andconsistsofacascadingsystemofchambers,eachatalowerpressurethantheonebefore.Thismeansthewaterboilsatalowertemperatureineachsuccessivechamber.Heatfromwatervapourinthefirstchambercanthusberecycledtoevaporatewaterinthenextchamber,andsoon.

Nosalt,please

Thisreducedtheenergyconsumptionofsugarrefiningbyupto80%,saysJamesBirkettofWestNeckStrategies,adesalinationconsultancybasedinNobleboro,Maine.Butittookabout50yearsfortheideatomakeitswayfromoneindustrytoanother.Onlyinthelate19thcenturydidmulti-effectevaporatorsfordesalinationbegintoappearonsteamshipsandinaridcountriessuchasYemenandSudan.

Afewmulti-effectdistillationplantswerebuiltinthefirsthalfofthe20thcentury,butaflawinthesystemhampereditswidespreadadoption.Mineraldepositstendedtobuilduponheat-exchangesurfaces,andthisinhibitedthetransferofenergy.Inthe1950sanewtypeofthermal-desalinationprocess,calledmulti-stageflash,reducedthisproblem.Inthis,seawaterisheatedunderhighpressureandthenpassedthroughaseriesofchambers,eachatalowerpressurethantheonebefore,causingsomeofthewatertoevaporateor“flash”ateachstep.Concentratedseawaterisleftatthebottomofthechambers,andfreshwatervapourcondensesabove.Becauseevaporationdoesnothappenontheheat-exchangesurfaces,fewermineralsaredeposited.

CountriesintheMiddleEastwithalotofoilandalittlewatersoonadoptedmulti-stageflash.Becauseitneedshotsteam,manydesalinationfacilitieswereputnexttopowerstations,whichgenerateexcessheat.Foratime,thecogenerationofelectricityandwaterdominatedthedesalinationindustry.

Researchintonewwaystoremovesaltfromwaterpickedupinthe1950s.TheAmericangovernmentsetuptheOfficeofSalineWatertosupportthesearchfordesalinationtechnology.AndscientistsattheUniversityofFloridaandtheUniversityofCalifornia,LosAngeles（UCLA）begantoinvestigatemembranesthatarepermeabletowater,butrestrictthepassageofdissolvedsalts.

Suchmembranesarecommoninnature.Whenthereisasaltysolutionononesideofasemi-permeablemembrane（suchasacellwall）,andalesssaltysolutionontheother,waterdiffusesthroughthemembranefromthelessconcentratedsidetothemoreconcentratedside.Thisprocess,whichtendstoequalisethesaltinessofthetwosolutions,iscalledosmosis.Researcherswonderedwhetherosmosiscouldbereversedbyapplyingpressuretothemoreconcentratedsolution,causingwatermoleculestodiffusethroughthemembraneandleavebehindevenmorehighlyconcentratedbrine.

Initialeffortsshowedonlylimitedsuccess,producingtinyamountsoffreshwater.Thatchangedin1960,whenSidneyLoebandSrinivasaSourirajanofUCLAhand-casttheirownmembranesfromcelluloseacetate,apolymerusedinphotographicfilm.Theirnewmembranesboastedadramaticallyimprovedflux（therateatwhichwatermoleculesdiffusethroughamembraneofagivensize）leading,in1965,toasmall“reverseosmosis”plantfordesaltingbrackishwaterinCoalinga,California.

Theenergyrequirementsforthermaldesalinationdonotmuchdependonthesaltinessofthesourcewater,buttheenergyneededforreverseosmosisisdirectlyrelatedtotheconcentrationofdissolvedsalts.Thesaltierthewater,thehigherthepressureittakes（andhencethemoreenergyyouneed）topushwaterthroughamembraneinordertoleavebehindthesalt.Seawatergenerallycontains33-37gramsofdissolvedsolidsperlitre.Toturnitintodrinkingwater,nearly99%ofthesesaltsmustberemoved.Becausebrackishwatercontainslesssaltthanseawater,itislessenergy-intensive,andthuslessexpensive,toprocess.Asaresult,reverseosmosisfirstbecameestablishedasawaytotreatbrackishwater.

Anotherimportantdistinctionisthatreverseosmosis,unlikethermaldesalination,callsforextensivepre-treatmentofthefeedwater.Reverse-osmosisplantsusefiltersandchemicalstoremoveparticlesthatcouldclogupthemembranes,andthemembranesmustalsobewashedperiodicallytoreducescalingandfouling.

Gettingbetterallthetime

Inthelate1970sJohnCadotteofAmerica’sMidwestResearchInstituteandtheFilmTecCorporationcreatedamuch-improvedmembranebyusingaspecialcross-linkingreactionbetweentwochemicalsatopaporousbackingmaterial.Hiscompositemembraneconsistedofaverythinlayerofpolyamide,toperformtheseparation,andasturdysupportbeneathit.Thankstothemembrane’simprovedwaterflux,anditsabilitytotoleratepHandtemperaturevariations,itwentontodominatetheindustry.Ataroundthesametime,thefirstreverse-osmosisplantsforseawaterbegantoappear.Theseearlyplantsneededalotofenergy.Thefirstbigmunicipalseawaterplant,whichbeganoperatinginJeddah,SaudiArabia,in1980,requiredmorethan8kilowatthours（kWh）toproduceonecubicmetreofdrinkingwater.

Theenergyconsumptionofsuchplantshassincefallendramatically,thanksinlargeparttoenergy-recoverydevices.High-pressurepumpsforceseawateragainstamembrane,whichistypicallyarrangedinaspiralinsideatube,toincreasethesurfaceareaexposedtotheincomingwaterandoptimisethefluxthroughthemembrane.Abouthalfofthewateremergesasfreshwaterontheotherside.Theremainingliquid,whichcontainstheleftoversalts,shootsoutofthesystemathighpressure.Ifthathigh-pressurewastestreamisrunthroughaturbineorrotor,energycanberecoveredandusedtopressurisetheincomingseawater.

Theenergy-recoverydevicesinthe1980swereonlyabout75%efficient,butneweronescanrecoverabout96%oftheenergyfromthewastestream.Asaresult,theenergy