二氧化碳直接空气捕获装置的技术经济评价（英）VIP专享VIP免费

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Techno-economic assessment of CO

direct air capture plants

Mahdi Fasihi

, Olga Eﬁmova, Christian Breyer

LUT University, Yliopistonkatu 34, 53850, Lappeenranta, Finland

article info

Article history:

Received 14 October 2018

Received in revised form

25 January 2019

Accepted 8 March 2019

Available online 14 March 2019

Keywords:

Carbon dioxide (CO

)

Direct air capture (DAC)

Carbon capture and utilisation (CCU)

Negative emission technology (NET)

Economics

abstract

direct air capture (DAC) has been increasingly discussed as a climate change mitigation option.

Despite technical advances in the past decade, there are still misconceptions about DAC's current and

long-term costs as well as energy, water and area demands. This could undermine DAC's anticipated role

in a neutral or negative greenhouse gas emission energy system, and inﬂuence policy makers. In this

study, a literature review and techno-economic analyses of state-of-the-art DAC technologies are per-

formed, wherein, DAC technologies are categorised as high temperature aqueous solutions (HT DAC) and

low temperature solid sorbent (LT DAC) systems, from an energy system perspective. DAC capital ex-

penditures, energy demands and costs have been estimated under two scenarios for DAC capacities and

ﬁnancial learning rates in the period 2020 to 2050. DAC system costs could be lowered signiﬁcantly with

commercialisation in the 2020s followed by massive implementation in the 2040s and 2050s, making

them cost competitive with point source carbon capture and an affordable climate change mitigation

solution. It is concluded that LT DAC systems are favourable due to lower heat supply costs and the

possibility of using waste heat from other systems. CO

capture costs of LT DAC systems powered by

hybrid PV-Wind-battery systems for Moroccan conditions and based on a conservative scenario, without/

with utilisation of free waste heat are calculated at 222/133, 105/60, 69/40 and 54/32 V/t

CO2

in 2020,

2030, 2040 and 2050, respectively. These new ﬁndings could enhance DAC's role in a successful climate

change mitigation strategy.

(http://creativecommons.org/licenses/by/4.0/).

1. Introduction

The problem of global warming caused by greenhouse gas

(GHG) emissions, mainly carbon dioxide (CO

), has reached

dangerous levels. CO

concentration in the atmosphere has rapidly

increased from 280 ppm in the pre-industrial era to 403 ppm in

2016, with an annual growth rate of 2 ppm (IEA, 2017). The Paris

Agreement aims to mitigate climate change and keep temperature

rise well below 2



C and preferably 1.5



C in comparison to the pre-

industrial age by united efforts of all countries (UNFCCC, 2015). To

achieve this goal, along with sharply cutting anthropogenic GHG

emissions, actions are needed for active CO

removal by imple-

mentation of Negative CO

Emissions Technologies (NETs) (Kriegler

et al., 2017;Rogelj et al., 2018).

A range of options are available for CO

emissions removal. CO

emissions can be captured at point sources such as ﬂue gases from

conventional power plants or non-energetic sectors such as cement

plants. However, some plants are too old and cannot be retroﬁtted.

Moreover, even in plants with CO

removal systems, not all emis-

sions are captured as the average capture rates are in the range of

50e94% (Leeson et al., 2017). On the other hand, it is not possible to

directly capture CO

emissions produced by long-distance aviation

and marine transport. Large amount of small emitters, such as in

the transport sector, which account for 50% of global GHG emis-

sions, are just impossible to neutralise by conventional CO

capture

applications (Seipp et al., 2017). These facts lead to the undeniable

necessity of ﬁnding additional solutions that are capable of

capturing CO

independent of origin and location.

Another approach for climate change mitigation is capturing

directly from the atmosphere. Hitherto, plants have been doing

it naturally to some extent. Nonetheless, they cannot keep up with

the increasing anthropogenic emissions (Goeppert et al., 2012).

Afforestation, bioenergy with carbon capture and storage (BECCS)

and enhanced weathering were introduced to reduce CO

con-

centration in the atmosphere (Williamson, 2016). However, their

commercial feasibility is limited, as all of these measures are

associated with risks. Large-scale BECCS and afforestation threat

biodiversity, water and food security, as both are characterised by

*Corresponding author.

E-mail address: Mahdi.Fasihi@lut.ﬁ(M. Fasihi).

Contents lists available at ScienceDirect

Journal of Cleaner Production

journal homepage: www.elsevier.com/locate/jclepro

https://doi.org/10.1016/j.jclepro.2019.03.086

Journal of Cleaner Production 224 (2019) 957e980

huge land requirements (Smith et al., 2016). Enhanced weathering

provokes rising pH values in rivers and changing the chemistry in

oceans (Kohler et al., 2010). Besides afforestation/reforestation,

BECCS and enhanced weathering, the full portfolio of NETs also

includes biochar, ocean fertilisation and soil carbon sequestration

(Fuss et al., 2018;Minx et al., 2018), which may have to be applied in

a portfolio of NETs for effective climate change mitigation (Bui et al.,

2018).

Direct Air Capture (DAC), besides BECCS, is the other option

for capturing CO

from the atmosphere, diluted gases and distrib-

uted sources of carbon via industrial processes (Broehm et al., 2015;

Goeppert et al., 2012;Lackner, 2009). DAC is a relatively new and

innovative technology in early commercial stages (Nemet et al.,

2018), which in a long term perspective, along with conventional

technologies, can help humankind to control and mitigate climate

change (Keith, 2009;Sanz-P

erez et al., 2016).

In this paper, a techno-economic assessment of the main CO

direct air capture technologies, from an energy system point of

view, has been carried out. The remaining sections of the paper are

as follows: Section 2describes the methodology. In section 3,a

literature review has been carried out. In section 4, available

technologies have been described and the collected techno-

economic data is categorised and summarised in the form of ta-

bles. The ﬁnal model of main technologies in 2020 are introduced.

Later, DAC capital expenditures, energy demands and costs have

been estimated under two scenarios for DAC capacities and ﬁnan-

cial learning rates in the period 2020 to 2050 and sensitivity ana-

lyses for the most valuable parameters are done. Further, DAC's area

and water demands, as well as CO

compression, transport and

storage are presented. In section 5, relevance of DAC with respect to

the Paris Agreement, as well as beneﬁts and challenges of the main

DAC technologies are discussed. Later, more factors on the ﬁnal

costs of large-scale DAC systems are examined and results are

compared to projections from companies or literature. Moreover, a

cost comparison to point source carbon capture (PSCC), as one of

the competing technologies is performed. In addition, the cost

share of CO

DAC in power-to-gas systems has been investigated.

Finally, conclusions are drawn in section 6.

2. Methodology and data

An extensive review has been performed considering literature

published from the early 2000s to the present time that are rele-

vant to this research. Research was conducted in the following

manner: data gathering via such platforms as ScienceDirect, Sco-

pus, Google Scholar, ResearchGate, ofﬁcial websites of companies

and international agencies such as Intergovernmental Panel on

Climate Change (IPCC) and International Energy Agency (IEA). The

following keywords were used: CO

capture plant, CO

capture

methods, CO

scrubbing, CO

separation, direct air capture, cost of

capturing, carbon capture start-up companies and atmospheric

capture.

A database of relevant data has been created from all the

reviewed publications, for further analyses. Recalculation and

aligning of the ﬁndings were conducted. All parameters are pre-

sented on a comparable scale for classiﬁcation of all available

technologies and to deliver the ﬁnal models, including long-term

estimations. A sensitivity analysis of the most valuable variables

is performed.

Cost numbers from different years presented in USD are con-

verted to euros by using a ﬁxed exchange ratio of 1.33 USD/V, as the

long term average exchange rate. As an exception, cost numbers

from Keith et al. (2018) and values in other currencies are converted

to euros based on exchange rates of the corresponding year.

equations (1)e(4) below have been used to calculate the lev-

elised cost of electricity (LCOE), the levelised cost of heat (LCOH)

and the levelised cost of CO

DAC (LCOD). Abbreviations: capital

expenditures, capex, annuity factor, crf, annual operational expen-

ditures, opex,ﬁxed, ﬁx, variable, var, annual CO

production of DAC

plant, Output

CO2

, full load hours per year, FLh, electricity demand of

DAC plant per t

CO2

produced, DAC

el.input

, heat demand of DAC plant

per t

CO2

produced, DAC

heat.input

, fuel costs, fuel,efﬁciency,

, coefﬁ-

cient of performance of heat pumps, COP, weighted average cost of

capital, WACC, lifetime, N.

A WACC of 7% is used for all the calculations in this study.

LCOE ¼Capex,crf þOpex

fix

FLh þOpex

var

þfuel

(1)

LCOH ¼Capex,crf þOpex

fix

FLh þOpex

var

þfuel

þLCOE

COP (2)

Nomenclature

BECCS Bioenergy with Carbon Capture and Storage

capex Capital Expenditures

CCS Carbon Capture and Storage

CCU Carbon Capture and Utilisation

COP Coefﬁcient of Performance

DAC Direct Air Capture

DACCS Direct Air Carbon Capture and Storage

FLh Full Load hours

GHG Greenhouse Gas

HT High Temperature

LCOD Levelised Cost of CO

Direct Air Capture

LT Low Temperature

MOF Metal Organic Frameworks

MSA Moisture Swing Adsorption

NET Negative Emission Technology

opex Operating Expenditures

PSCC Point Source Carbon Capture

PtG Power-to-Gas

PV Photovoltaic

RE Renewable Energy

SNG Synthetic Natural Gas

TSA Temperature Swing Adsorption

TVSA Temperature Vacuum Swing Adsorption

WACC Weighted Average Cost of Capital

Subscripts

el electricity

ﬁxﬁxed

p peak

th thermal

var variable

M. Fasihi et al. / Journal of Cleaner Production 224 (2019) 957e980958

LCOD ¼Capex

DAC

,crf þOpex

fix

Output

þOpex

var

þDAC

el:input

,LCOE

þDAC

th:input

,LCOH

(3)

crf ¼WACC,ð1þWACCÞ

ð1þWACCÞ

1(4)

Maturity level of technologies is also taken into consideration, as

the focus of this research is on pilot and commercial-scale tech-

nologies, while the theoretical and laboratory-scale studies have

been included as well.

Cost and technical trends based on technology evolution over 20

years of active research and development are identiﬁed. As a result,

up to date data is used for the long-term estimation of key pa-

rameters for the time periods 2020 to 2050 in 10-year steps, based

on adapted learning rates.

3. Literature review

The ﬁrst application of capturing CO

from ambient air was

introduced in the 1930s in cryogenic air separation plants and later

it found its application in life support systems of manned closed

systems such as space stations and submarines (House et al., 2011).

The ﬁrst systems dated back to 1965 were not regeneratable (Isobe

et al., 2016). Whereas, modern space shuttles are all equipped with

regeneratable Carbon Dioxide Removal Assembly (CDRA) that helps

to maintain habitable environment for crewmembers (NASA,

2006).

Due to ultra dilute concentration of CO

in the atmosphere,

chemical sorbents with strong binding characteristics became

widely discussed in literature. An aqueous solution of strong bases

is used in conventional PSCC technologies and many researchers

have investigated its applicability to DAC. Keith et al. (2006) ana-

lysed physical and economic limits of BECCS and aqueous solution-

based DAC and concluded the second option to be feasible in the

near term. However, high-grade (900



C) heat demand of aqueous

solution-based DAC could limit the options for heat source and

increase the costs. Baciocchi et al. (2006) tried to optimise the

system based on the same chemical solution and applied two

different calcium carbonate precipitators. Zeman (2007) was one of

the ﬁrst who proposed the same approach on an industrial scale. In

addition, he has benchmarked the system with two previous

studies on thermodynamic levels. Stolaroff et al. (2008) discussed

optimisation of energy demand and possible reduction of ﬁnal

costs by improving the contactor part. The extensive report of

American Physical Society (APS) by Socolow et al. (2011) compared

post-combustion CO

capture methods to DAC systems based on

the work of Baciocchi et al. (2006).Zeman (2014) investigated the

APS report and proposed a reduction in ﬁnal costs of avoided CO

by using low-carbon electricity and minimising plastic packing

materials of the contactor part. Li et al. (2015) investigated the

optimal operation of the system proposed in the early work of

Zeman (2007) by using wind power and battery as the energy in-

puts. All the above mentioned works applied different approaches

to improve the performance of aqueous alkaline solution, in

particular sodium hydroxide; whereas, Nikulshina et al. (2009)

presented a single-cycle system carrying out continuous removal

of CO

via serial CaO-carbonation at higher temperatures (of about

365e400



C) and CaCO

-calcination at 800e876



C, powered by

concentrated solar power (CSP). Mahmoudkhani and Keith (2009)

suggested a novel approach to avoid calcium carbonate in the

loop, by using Sodium Tri-Titanate. The technique requires 50% less

high-grade heat than conventional causticisation and the

maximum temperature required is reduced by at least 50 K, from

900



C to 850



C. Holmes and Keith (2012) and Holmes et al. (2013)

suggested potassium hydroxide (KOH) as a non-toxic solution and

discussed the results of laboratory-scale and prototype tests of

improved contactor parts. Keith et al. (2018) provided a detailed

techno-economic analysis of a 1 Mt

CO2

/a design based on a real

pilot plant for the ﬁrst time.

Another major group of scientiﬁc publications are focused on

systems based on adsorption process. Temperature swing adsorp-

tion (TSA) is the main DAC method in this category, described by

Kulkarni and Sholl (2012) and Sinha et al. (2017). Unlike typical

aqueous solution-based systems, the regeneration in solid sorbent

DAC happens at relatively lower temperatures (80e100



C), which

is cheaper to produce or could be available as waste heat from some

industrial plants, such as combined heat and power plants, power

plants with cooling tower, pulp and paper mills, steel or glass

making plants, or waste heat from exothermic synthetic fuels

production processes. Choi et al. (2011a;2012) examined modiﬁed

sorbents with higher CO

uptake capacity and higher stability in

dry conditions. Roestenberg (2015) introduced a LT DAC design

based on non-amine sorbent and separate adsorption and

desorption units for increasing the plant's utilisation rate, evalu-

ated costs of small-scale and large-scale systems and considered

heat supply from methanol synthesis plant coupled to the DAC unit.

Derevschikov et al. (2014) suggested using composite solid sorbent

for DAC and using renewable energy (RE) to produce methane on

site. Ping et al. (2018a) introduced a system with full cycle of less

than 30 min. Moisture swing adsorption (MSA) is the other method

in this category in which the regeneration happens by moisturising

of CO

-rich sorbent. Lackner (2009) examined the possibility of

MSA CO

capture on amine-based ion-exchange resin at low tem-

peratures (45



C). Later, Goldberg et al. (2013) studied the combi-

nation of this system with wind energy and offshore geological

storage.

Radical methods have been suggested for DAC by some re-

searchers. Eisaman et al. (2009) examined electrochemical CO

capture. Freitas (2015) suggested the use of nanofactory-based

molecular ﬁlters and claimed that these methods are able to

bring the ﬁnal capture costs down to 13.7 V/t

CO2

(18.3 USD/t

CO2

Seipp et al. (2017) introduced a rather novel approach based on

crystallisation of CO

molecules with a guanidine sorbent with low

temperature requirements of 80e120



C. Despite promising pre-

liminary results, deeper investigations and possible pilot plants are

needed for a better evaluation of these approaches.

In addition, several papers have presented an overview of

available technologies. Simon et al. (2011) analysed LCOD of a

generic DAC based on capture devices, energy supplies, footprint,

water use and sequestration costs. Goeppert et al. (2012) discussed

capturing CO

from point sources, raised the question as to why

DAC is needed, summarised and discussed all available technolo-

gies on a technical level and listed active companies. It is concluded

that DAC is indispensable for stabilising climate change. In addition,

it points out that even though CO

concentration in the atmosphere

is about 250e300 times less than concentrated sources, the theo-

retical energy demand by DAC is only 2e4 times higher. However,

the vast range of projected overall costs of CO

DAC can become

clearer only after the construction of pilot plants. The detailed

numerical analyses by Wilcox et al. (2017) conﬁrm the comparison

on minimum work of carbon capture from atmospheric and

concentrated sources by Goeppert et al. (2012), however, they show

M. Fasihi et al. / Journal of Cleaner Production 224 (2019) 957e980 959

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Techno-economicassessmentofCO2directaircaptureplantsMahdiFasihi,OlgaEﬁmova,ChristianBreyerLUTUniversity,Yliopistonkatu34,53850,Lappeenranta,FinlandarticleinfoArticlehistory:Received14October2018Receivedinrevisedform25January2019Accepted8March2019Availableonline14March2019Keywords:Carbondioxide(CO2)Directaircapture(DAC)Carboncaptureandutilisation(CCU)Negativeemissiontechnology(NET)EconomicsabstractCO2directaircapture(DAC)hasbeenincreasinglydiscussedasaclimatechangemitigationoption.Despitetechnicaladvancesinthepastdecade,therearestillmisconceptionsaboutDAC'scurrentandlong-termcostsaswellasenergy,waterandareademands.ThiscouldundermineDAC'santicipatedroleinaneutralornegativegreenhousegasemissionenergysystem,andinﬂuencepolicymakers.Inthisstudy,aliteraturereviewandtechno-economicanalysesofstate-of-the-artDACtechnologiesareper-formed,wherein,DACtechnologiesarecategorisedashightemperatureaqueoussolutions(HTDAC)andlowtemperaturesolidsorbent(LTDAC)systems,fromanenergysystemperspective.DACcapitalex-penditures,energydemandsandcostshavebeenestimatedundertwoscenariosforDACcapacitiesandﬁnanciallearningratesintheperiod2020to2050.DACsystemcostscouldbeloweredsigniﬁcantlywithcommercialisationinthe2020sfollowedbymassiveimplementationinthe2040sand2050s,makingthemcostcompetitivewithpointsourcecarboncaptureandanaffordableclimatechangemitigationsolution.ItisconcludedthatLTDACsystemsarefavourableduetolowerheatsupplycostsandthepossibilityofusingwasteheatfromothersystems.CO2capturecostsofLTDACsystemspoweredbyhybridPV-Wind-batterysystemsforMoroccanconditionsandbasedonaconservativescenario,without/withutilisationoffreewasteheatarecalculatedat222/133,105/60,69/40and54/32V/tCO2in2020,2030,2040and2050,respectively.ThesenewﬁndingscouldenhanceDAC'sroleinasuccessfulclimatechangemitigationstrategy.©2019TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).1.IntroductionTheproblemofglobalwarmingcausedbygreenhousegas(GHG)emissions,mainlycarbondioxide(CO2),hasreacheddangerouslevels.CO2concentrationintheatmospherehasrapidlyincreasedfrom280ppminthepre-industrialerato403ppmin2016,withanannualgrowthrateof2ppm(IEA,2017).TheParisAgreementaimstomitigateclimatechangeandkeeptemperaturerisewellbelow2Candpreferably1.5Cincomparisontothepre-industrialagebyunitedeffortsofallcountries(UNFCCC,2015).Toachievethisgoal,alongwithsharplycuttinganthropogenicGHGemissions,actionsareneededforactiveCO2removalbyimple-mentationofNegativeCO2EmissionsTechnologies(NETs)(Kriegleretal.,2017;Rogeljetal.,2018).ArangeofoptionsareavailableforCO2emissionsremoval.CO2emissionscanbecapturedatpointsourcessuchasﬂuegasesfromconventionalpowerplantsornon-energeticsectorssuchascementplants.However,someplantsaretoooldandcannotberetroﬁtted.Moreover,eveninplantswithCO2removalsystems,notallemis-sionsarecapturedastheaveragecaptureratesareintherangeof50e94%(Leesonetal.,2017).Ontheotherhand,itisnotpossibletodirectlycaptureCO2emissionsproducedbylong-distanceaviationandmarinetransport.Largeamountofsmallemitters,suchasinthetransportsector,whichaccountfor50%ofglobalGHGemis-sions,arejustimpossibletoneutralisebyconventionalCO2captureapplications(Seippetal.,2017).ThesefactsleadtotheundeniablenecessityofﬁndingadditionalsolutionsthatarecapableofcapturingCO2independentoforiginandlocation.AnotherapproachforclimatechangemitigationiscapturingCO2directlyfromtheatmosphere.Hitherto,plantshavebeendoingitnaturallytosomeextent.Nonetheless,theycannotkeepupwiththeincreasinganthropogenicemissions(Goeppertetal.,2012).Afforestation,bioenergywithcarboncaptureandstorage(BECCS)andenhancedweatheringwereintroducedtoreduceCO2con-centrationintheatmosphere(Williamson,2016).However,theircommercialfeasibilityislimited,asallofthesemeasuresareassociatedwithrisks.Large-scaleBECCSandafforestationthreatbiodiversity,waterandfoodsecurity,asbotharecharacterisedbyCorrespondingauthor.E-mailaddress:Mahdi.Fasihi@lut.ﬁ(M.Fasihi).ContentslistsavailableatScienceDirectJournalofCleanerProductionjournalhomepage:www.elsevier.com/locate/jcleprohttps://doi.org/10.1016/j.jclepro.2019.03.0860959-6526/©2019TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).JournalofCleanerProduction224(2019)957e980hugelandrequirements(Smithetal.,2016).EnhancedweatheringprovokesrisingpHvaluesinriversandchangingthechemistryinoceans(Kohleretal.,2010).Besidesafforestation/reforestation,BECCSandenhancedweathering,thefullportfolioofNETsalsoincludesbiochar,oceanfertilisationandsoilcarbonsequestration(Fussetal.,2018;Minxetal.,2018),whichmayhavetobeappliedinaportfolioofNETsforeffectiveclimatechangemitigation(Buietal.,2018).CO2DirectAirCapture(DAC),besidesBECCS,istheotheroptionforcapturingCO2fromtheatmosphere,dilutedgasesanddistrib-utedsourcesofcarbonviaindustrialprocesses(Broehmetal.,2015;Goeppertetal.,2012;Lackner,2009).DACisarelativelynewandinnovativetechnologyinearlycommercialstages(Nemetetal.,2018),whichinalongtermperspective,alongwithconventionaltechnologies,canhelphumankindtocontrolandmitigateclimatechange(Keith,2009;Sanz-Perezetal.,2016).Inthispaper,atechno-economicassessmentofthemainCO2directaircapturetechnologies,fromanenergysystempointofview,hasbeencarriedout.Theremainingsectionsofthepaperareasfollows:Section2describesthemethodology.Insection3,aliteraturereviewhasbeencarriedout.Insection4,availabletechnologieshavebeendescribedandthecollectedtechno-economicdataiscategorisedandsummarisedintheformofta-bles.Theﬁnalmodelofmaintechnologiesin2020areintroduced.Later,DACcapitalexpenditures,energydemandsandcostshavebeenestimatedundertwoscenariosforDACcapacitiesandﬁnan-ciallearningratesintheperiod2020to2050andsensitivityana-lysesforthemostvaluableparametersaredone.Further,DAC'sareaandwaterdemands,aswellasCO2compression,transportandstoragearepresented.Insection5,relevanceofDACwithrespecttotheParisAgreement,aswellasbeneﬁtsandchallengesofthemainDACtechnologiesarediscussed.Later,morefactorsontheﬁnalcostsoflarge-scaleDACsystemsareexaminedandresultsarecomparedtoprojectionsfromcompaniesorliterature.Moreover,acostcomparisontopointsourcecarboncapture(PSCC),asoneofthecompetingtechnologiesisperformed.Inaddition,thecostshareofCO2DACinpower-to-gassystemshasbeeninvestigated.Finally,conclusionsaredrawninsection6.2.MethodologyanddataAnextensivereviewhasbeenperformedconsideringliteraturepublishedfromtheearly2000stothepresenttimethatarerele-vanttothisresearch.Researchwasconductedinthefollowingmanner:datagatheringviasuchplatformsasScienceDirect,Sco-pus,GoogleScholar,ResearchGate,ofﬁcialwebsitesofcompaniesandinternationalagenciessuchasIntergovernmentalPanelonClimateChange(IPCC)andInternationalEnergyAgency(IEA).Thefollowingkeywordswereused:CO2captureplant,CO2capturemethods,CO2scrubbing,CO2separation,directaircapture,costofCO2capturing,carboncapturestart-upcompaniesandatmosphericCO2capture.Adatabaseofrelevantdatahasbeencreatedfromallthereviewedpublications,forfurtheranalyses.Recalculationandaligningoftheﬁndingswereconducted.Allparametersarepre-sentedonacomparablescaleforclassiﬁcationofallavailabletechnologiesandtodelivertheﬁnalmodels,includinglong-termestimations.Asensitivityanalysisofthemostvaluablevariablesisperformed.CostnumbersfromdifferentyearspresentedinUSDarecon-vertedtoeurosbyusingaﬁxedexchangeratioof1.33USD/V,asthelongtermaverageexchangerate.Asanexception,costnumbersfromKeithetal.(2018)andvaluesinothercurrenciesareconvertedtoeurosbasedonexchangeratesofthecorrespondingyear.equations(1)e(4)belowhavebeenusedtocalculatethelev-elisedcostofelectricity(LCOE),thelevelisedcostofheat(LCOH)andthelevelisedcostofCO2DAC(LCOD).Abbreviations:capitalexpenditures,capex,annuityfactor,crf,annualoperationalexpen-ditures,opex,ﬁxed,ﬁx,variable,var,annualCO2productionofDACplant,OutputCO2,fullloadhoursperyear,FLh,electricitydemandofDACplantpertCO2produced,DACel.input,heatdemandofDACplantpertCO2produced,DACheat.input,fuelcosts,fuel,efﬁciency,h,coefﬁ-cientofperformanceofheatpumps,COP,weightedaveragecostofcapital,WACC,lifetime,N.AWACCof7%isusedforallthecalculationsinthisstudy.LCOE¼Capex,crfþOpexfixFLhþOpexvarþfuelh(1)LCOH¼Capex,crfþOpexfixFLhþOpexvarþfuelhþLCOECOP(2)NomenclatureBECCSBioenergywithCarbonCaptureandStoragecapexCapitalExpendituresCCSCarbonCaptureandStorageCCUCarbonCaptureandUtilisationCOPCoefﬁcientofPerformanceDACDirectAirCaptureDACCSDirectAirCarbonCaptureandStorageFLhFullLoadhoursGHGGreenhouseGasHTHighTemperatureLCODLevelisedCostofCO2DirectAirCaptureLTLowTemperatureMOFMetalOrganicFrameworksMSAMoistureSwingAdsorptionNETNegativeEmissionTechnologyopexOperatingExpendituresPSCCPointSourceCarbonCapturePtGPower-to-GasPVPhotovoltaicRERenewableEnergySNGSyntheticNaturalGasTSATemperatureSwingAdsorptionTVSATemperatureVacuumSwingAdsorptionWACCWeightedAverageCostofCapitalSubscriptselelectricityﬁxﬁxedppeakththermalvarvariableM.Fasihietal./JournalofCleanerProduction224(2019)957e980958LCOD¼CapexDAC,crfþOpexfixOutputCO2þOpexvarþDACel:input,LCOEþDACth:input,LCOH(3)crf¼WACC,ð1þWACCÞNð1þWACCÞNÀ1(4)Maturityleveloftechnologiesisalsotakenintoconsideration,asthefocusofthisresearchisonpilotandcommercial-scaletech-nologies,whilethetheoreticalandlaboratory-scalestudieshavebeenincludedaswell.Costandtechnicaltrendsbasedontechnologyevolutionover20yearsofactiveresearchanddevelopmentareidentiﬁed.Asaresult,uptodatedataisusedforthelong-termestimationofkeypa-rametersforthetimeperiods2020to2050in10-yearsteps,basedonadaptedlearningrates.3.LiteraturereviewTheﬁrstapplicationofcapturingCO2fromambientairwasintroducedinthe1930sincryogenicairseparationplantsandlateritfounditsapplicationinlifesupportsystemsofmannedclosedsystemssuchasspacestationsandsubmarines(Houseetal.,2011).Theﬁrstsystemsdatedbackto1965werenotregeneratable(Isobeetal.,2016).Whereas,modernspaceshuttlesareallequippedwithregeneratableCarbonDioxideRemovalAssembly(CDRA)thathelpstomaintainhabitableenvironmentforcrewmembers(NASA,2006).DuetoultradiluteconcentrationofCO2intheatmosphere,chemicalsorbentswithstrongbindingcharacteristicsbecamewidelydiscussedinliterature.AnaqueoussolutionofstrongbasesisusedinconventionalPSCCtechnologiesandmanyresearchershaveinvestigateditsapplicabilitytoDAC.Keithetal.(2006)ana-lysedphysicalandeconomiclimitsofBECCSandaqueoussolution-basedDACandconcludedthesecondoptiontobefeasibleinthenearterm.However,high-grade(900C)heatdemandofaqueoussolution-basedDACcouldlimittheoptionsforheatsourceandincreasethecosts.Baciocchietal.(2006)triedtooptimisethesystembasedonthesamechemicalsolutionandappliedtwodifferentcalciumcarbonateprecipitators.Zeman(2007)wasoneoftheﬁrstwhoproposedthesameapproachonanindustrialscale.Inaddition,hehasbenchmarkedthesystemwithtwopreviousstudiesonthermodynamiclevels.Stolaroffetal.(2008)discussedoptimisationofenergydemandandpossiblereductionofﬁnalcostsbyimprovingthecontactorpart.TheextensivereportofAmericanPhysicalSociety(APS)bySocolowetal.(2011)comparedpost-combustionCO2capturemethodstoDACsystemsbasedontheworkofBaciocchietal.(2006).Zeman(2014)investigatedtheAPSreportandproposedareductioninﬁnalcostsofavoidedCO2byusinglow-carbonelectricityandminimisingplasticpackingmaterialsofthecontactorpart.Lietal.(2015)investigatedtheoptimaloperationofthesystemproposedintheearlyworkofZeman(2007)byusingwindpowerandbatteryastheenergyin-puts.Alltheabovementionedworksapplieddifferentapproachestoimprovetheperformanceofaqueousalkalinesolution,inparticularsodiumhydroxide;whereas,Nikulshinaetal.(2009)presentedasingle-cyclesystemcarryingoutcontinuousremovalofCO2viaserialCaO-carbonationathighertemperatures(ofabout365e400C)andCaCO3-calcinationat800e876C,poweredbyconcentratedsolarpower(CSP).MahmoudkhaniandKeith(2009)suggestedanovelapproachtoavoidcalciumcarbonateintheloop,byusingSodiumTri-Titanate.Thetechniquerequires50%lesshigh-gradeheatthanconventionalcausticisationandthemaximumtemperaturerequiredisreducedbyatleast50K,from900Cto850C.HolmesandKeith(2012)andHolmesetal.(2013)suggestedpotassiumhydroxide(KOH)asanon-toxicsolutionanddiscussedtheresultsoflaboratory-scaleandprototypetestsofimprovedcontactorparts.Keithetal.(2018)providedadetailedtechno-economicanalysisofa1MtCO2/adesignbasedonarealpilotplantfortheﬁrsttime.Anothermajorgroupofscientiﬁcpublicationsarefocusedonsystemsbasedonadsorptionprocess.Temperatureswingadsorp-tion(TSA)isthemainDACmethodinthiscategory,describedbyKulkarniandSholl(2012)andSinhaetal.(2017).Unliketypicalaqueoussolution-basedsystems,theregenerationinsolidsorbentDAChappensatrelativelylowertemperatures(80e100C),whichischeapertoproduceorcouldbeavailableaswasteheatfromsomeindustrialplants,suchascombinedheatandpowerplants,powerplantswithcoolingtower,pulpandpapermills,steelorglassmakingplants,orwasteheatfromexothermicsyntheticfuelsproductionprocesses.Choietal.(2011a;2012)examinedmodiﬁedsorbentswithhigherCO2uptakecapacityandhigherstabilityindryconditions.Roestenberg(2015)introducedaLTDACdesignbasedonnon-aminesorbentandseparateadsorptionanddesorptionunitsforincreasingtheplant'sutilisationrate,evalu-atedcostsofsmall-scaleandlarge-scalesystemsandconsideredheatsupplyfrommethanolsynthesisplantcoupledtotheDACunit.Derevschikovetal.(2014)suggestedusingcompositesolidsorbentforDACandusingrenewableenergy(RE)toproducemethaneonsite.Pingetal.(2018a)introducedasystemwithfullcycleoflessthan30min.Moistureswingadsorption(MSA)istheothermethodinthiscategoryinwhichtheregenerationhappensbymoisturisingofCO2-richsorbent.Lackner(2009)examinedthepossibilityofMSACO2captureonamine-basedion-exchangeresinatlowtem-peratures(45C).Later,Goldbergetal.(2013)studiedthecombi-nationofthissystemwithwindenergyandoffshoregeologicalstorage.RadicalmethodshavebeensuggestedforDACbysomere-searchers.Eisamanetal.(2009)examinedelectrochemicalCO2capture.Freitas(2015)suggestedtheuseofnanofactory-basedmolecularﬁltersandclaimedthatthesemethodsareabletobringtheﬁnalcapturecostsdownto13.7V/tCO2(18.3USD/tCO2).Seippetal.(2017)introducedarathernovelapproachbasedoncrystallisationofCO2moleculeswithaguanidinesorbentwithlowtemperaturerequirementsof80e120C.Despitepromisingpre-liminaryresults,deeperinvestigationsandpossiblepilotplantsareneededforabetterevaluationoftheseapproaches.Inaddition,severalpapershavepresentedanoverviewofavailabletechnologies.Simonetal.(2011)analysedLCODofagenericDACbasedoncapturedevices,energysupplies,footprint,wateruseandsequestrationcosts.Goeppertetal.(2012)discussedcapturingCO2frompointsources,raisedthequestionastowhyDACisneeded,summarisedanddiscussedallavailabletechnolo-giesonatechnicallevelandlistedactivecompanies.ItisconcludedthatDACisindispensableforstabilisingclimatechange.Inaddition,itpointsoutthateventhoughCO2concentrationintheatmosphereisabout250e300timeslessthanconcentratedsources,thetheo-reticalenergydemandbyDACisonly2e4timeshigher.However,thevastrangeofprojectedoverallcostsofCO2DACcanbecomecleareronlyaftertheconstructionofpilotplants.ThedetailednumericalanalysesbyWilcoxetal.(2017)conﬁrmthecomparisononminimumworkofcarboncapturefromatmosphericandconcentratedsourcesbyGoeppertetal.(2012),however,theyshowM.Fasihietal./JournalofCleanerProduction224(2019)957e980959thattheratiooftherealworkdemandofcarboncapturefromat-mospheretoconcentratedsourcescouldbehigher.Ontheotherhand,itindicatesthateventhoughtheminimumworkofsepara-tionformambientairslightlyincreasesbyaimingforhigherCO2capturerates,therealworkdemandsigniﬁcantlydecreasesathighercapturerates.Broehmetal.(2015)dividedallavailabletechnologiesintothreegroups(aqueoussolutionsofstrongbases,amineadsorptionsandinorganicsolidsorbents),comparedthembasedoncriticalcriteriasuchasenergydemandandeconomicestimation,addressedlimitingfactorssuchaslandandpotentiallocationoptions,associatedemissionsandwaterlosses.Inordertoprovidemoredetailsofthetechnology,Broehmetal.(2015)closelyanalysedtwocasestudies,onebasedonSocolowetal.(2011)technologyandtheotherbasedontheresultsachievedinprivatecommercialcompanies.HepointedoutthatsuccessofDACdoesnotonlydependonthetechnicalandeconomicperformance,butalsodependsonexternalfactorssuchasmarketdemandforCO2,developmentofsyntheticfuelsandsupportingtechnologiessuchasstorage.AbroadcomparisonofalltechniquescapturingCO2fromambientairwasdonebyWilliamson(2016),wherestrengthsandlimitationsofallpossibleapplicationswerepointedout.SeveralcompaniesareactiveintheﬁeldofCO2DAC,whichareshowninFig.1.CarbonEngineering,establishedin2009byKeithinSquamish,Canada(CarbonEngineering,2018a),istheonlydetec-tedcompanyactiveinhightemperature(HT)aqueoussolution-basedDAC.ThecompanyispartlyfundedbyBillGates(CarbonEngineering,2018a).The1tCO2/daydemonstrationplantwasintroducedinOctober2015andthecurrentgoalofthecompanyistoestablishbroadcommercialdeploymentofsyntheticfuelspro-ductionbasedontheirDACtechnology(CarbonEngineering,2018b).Atalargescale,thecompanyexpectstoachievecostsof75e113V/tCO2captured,puriﬁed,andcompressedto150bar(CarbonEngineering,2018c).Climeworks,foundedbyGebaldandWurzbacherin2009inZurich,Switzerland(Climeworks,2018a),isthemostwell-knownlowtemperature(LT)solidsorbent-basedDACcompany.In2014,inapartnershipwithAudiandSunﬁre,thecompanylaunchedapilotplantinDresdenthatcaptures80%ofCO2moleculesfromairpassingthroughthesystemandconvertsthemintosyntheticdiesel(Audi,2015).In2017,thecompanycommissionedanothercommercialscaleDACplantinSwitzerlandthatprovidesCO2foranearby-locatedgreenhouse.Inthesameyear,anotherDACunithasbeeninstalledinIcelandtopermanentlyﬁxaircapturedCO2inamineralisationprocess700munderground(Climeworks,2017).Thisistheworld'sﬁrstdirectaircarboncap-tureandstorage(DACCS)systemcoupledtoenhancedweathering,whichmayevolvetobeamajorNEToption(Fussetal.,2018;Minxetal.,2018).Thecompanyistargetingproductioncostsofabout75V/tCO2forlarge-scaleplants(Climeworks,2018b).GlobalThermo-stat,formedin2010byEisenbergerinNewYork,USA,istheotherLTDACcompany,withitsmultifunctionaltechnologycapableofcapturingCO2fromboththeatmosphereaswellaspointsourceemissions(GlobalThermostat,2018a).Majortechnologicalknow-how,particularlyintheﬁeldofcatalystsislicensedfromGeorgiaInstituteofTechnology(GlobalThermostat,2018b;Sanz-Perezetal.,2016).Thecompanyalreadyhaspilotandcommercialdemonstartionplantsoperatingsince2010atSRIInternationalinMenloPark,California(Pingetal.,2018a).Themodularunitscanutilisewasteheatat85e95CforCO2regenerationandhaveacapacityof40000tCO2/a.ThecompanyhasannouncedambitiousplanstodeliverCO2atacostof11e38V/tCO2(Kintisch,2014).Antecy,foundedin2010byO'ConnorinHoevelaken,Netherlands,istheotherEuropeanLTDACcompany(Antecy,2018)thatrequiresmoderatetemperaturesof80e100CforCO2regeneration.Afterlaboratorytestsandcompletingcommercial-scaledesigns,incooperationwithShell,thecompanyisreadyfortheimple-mentationofapilotplant(Roestenberg,2015).OyHydrocellLtdisaFinnishcompanyfoundedin1993thathasprovidedaDACsystemtoVTTTechnicalResearchCenterofFinland(Hydrocell,2018;Elfvingetal.,2017).The1.387tCO2/asystemispackedinastandardshippingcontainerandisfullyportable.Byusingtemperaturevacuumswingadsorption(TVSA),at70e80C,ithasthelowestregenerationtemperatureamongdetectedtechnologies,whichwidenstheoptionsforapplicablewasteheatsources(Bajamundi,2015;Bajamundietal.,2018).OtherDACcompaniesareSkytreeandInﬁnitree,howevertheirdisclosedinformationisverylimited.Skytree,foundedin2008andlocatedinAmsterdam,Netherlands,commercialisesaCO2capturingtechnologybasedonelectrostaticabsorptionandmoisturisingdesorption,asaspin-offoftheEuro-peanSpaceAgency(Ishimotoetal.,2017;Skytree,2018).Inﬁnitree,foundedin2014andlocatedinHuntington,NewYork,utilisesanionexchangesorbentmaterialinamoistureswingprocess(Inﬁnitree,2018).EarlynichemarketsforSkytreeandInﬁnitreeareurbanfarmingprojectsforwhichtheyprovideCO2forfastergrowthofplants.4.Results4.1.DescriptionoftechnologiesBasicaircapturemodelsconsistofcontactingarea,solventorsorbentandregenerationmodule.Contactingareaexposessorbenttoambientairandfacilitatesairﬂowthroughthemodel,increasingFig.1.CompaniesactiveintheﬁeldofCO2DAC.Abbreviations:hightemperature,HT,lowtemperature,LT,moistureswingadsorption,MSA,temperatureswingadsorption,TSA.M.Fasihietal./JournalofCleanerProduction224(2019)957e980960theabsorptionoradsorptionofCO2molecules.Solventorsorbentmustbeeasytohandle,resistanttocontaminationandshouldnotvanishduringtheprocess,asitspropertiesdeterminethewholeprocess.ThemainDACsystemsaredescribedbelow.4.1.1.Hightemperature(HT)aqueoussolutionAqueoussolutionconsistsoftwocyclesthatcanhappensimultaneously.ThebasicexampleoftheapproachisillustratedinFig.2.Intheﬁrstcycle,knownasabsorption,ambientairisbroughtintocontactwithsprayedsodiumhydroxide(NaOH)asthesolventintheabsorptioncolumn,withtheaidoffansornaturalairﬂow.CO2moleculesreactwithNaOHandformasolu-tionofsodiumcarbonate(Na2CO3)(Eq.(5)).Theabsorptionhap-pensatroomtemperatureandambientpressure.ThissolutionistransportedtotheregenerationcycleandCO2depletedairleavesthecolumn.Inthesecondcycle,knownasregeneration,Na2CO3ismixedwithcalciumhydroxide(Ca(OH)2)inthecausticiserunit,wheresolidcalciumcarbonate(CaCO3)isformedandNaOHisregenerated(Eq.(6)).NaOHissentbacktothecontactorandreadytostartanotherabsorptioncycle.Meanwhile,inthemostenergyintensivestep,CaCO3isheateduptoaround900Cinthekiln(calcinerunit)toreleaseCO2.AsshowninTable1,accordingtoliteratureandbasedonthelevelofheatintegration,theoverallheatdemandisintherangeof1420e2250kWhthpertonCO2.Theoutputsofthisreactionarecalciumoxide(CaO)andapurestreamofCO2(Eq.(7)).CO2iscollectedandCaOismixedwithwaterintheslakerunitforCa(OH)2regeneration(Eq.(8)).contactor2NaOHþCO2/Na2CO3þH2O(5)causticiserNa2CO3þCaðOHÞ2/2NaOHþCaCO3(6)calcinerCaCO3þheat/CaOþCO2(7)slakerCaOþH2O/CaðOHÞ2(8)Besidesheat,thesystemalsoneedselectricalpowerforblowingairthroughthecontactor,sprayingtheaqueousandmovingsolu-tionsfromoneunittoanother.Inliterature,thiselectricalpowerispresentedtobeintherangeof366e764kWhelpertonCO2(Table1).ThisalsoincludestheenergydemandforCO2compression,tothementionedpressuresasinTable1,priortotransportorstorage.AscanbeseeninTable1,inearlierliterature,naturalgashasbeenmainlysuggestedforthesupplyofhigh-gradeheatdemand.However,thiswouldnotbeasustainablesolution.Providing2000kWhthhigh-gradeheatbyoxy-fuelcombustionofnaturalgaswith90%efﬁciencyforcapturing1tonofatmosphericCO2,wouldrelease0.44tonofdirectnaturalgasbasedCO2emissions,withouttakingintoaccountitslifecycleemissions.OneofCarbonEngineering(2018c)DACtechnologiesfullypoweredbynaturalgaswouldrelease0.5tonofCO2pertonofatmosphericCO2captured.EventhoughthisCO2canbecapturedandutilisedasfeedstockforotherpurposes,itwillﬁnallyendupintheatmo-sphereaftersomecyclesofutilisation.Inaddition,thisimpactwoulddramaticallyincreasethecostofthenet-capturedCO2,asthereportedcostsinliteraturearemainlybasedonatmosphericortotalcapturedCO2.Theuseofcarbon-neutralrenewablesyntheticnaturalgas(RE-SNG)mightbeasolutiontothisproblem.However,evenwitha100%closedcycleofSNG-basedCO2andnoextraen-ergydemandforCO2recycling,convertingthat0.5tonoffuel-basedCO2tosyntheticnaturalgas(SNG)wouldneedabout4400kWhelforgenerationoftherequiredhydrogen,utilising2030electrolysertechnology(Fasihietal.,2017a).ThisisahugeincreaseinprimaryenergydemandandproductioncostsduetothehighcostsofSNGproduction.Thus,asustainableandaffordablesystemshouldbefullyelectriﬁed,whichhasbeendiscussedinrelativelynewerstudies.ContentoftheCarbonEngineeringwebsite(2018c)inMarch2018includedafullyelectriﬁedsystemwithatotalof1500kWheldemandforbothpowerandheating,inordertodeliver1tonofatmosphericCO2at150bar.Thus,afullyelectriﬁedHTDACispracticallypossibleandhasbeenchosenastheﬁnalmodelforaqueoussolutiontechnologyinourstudy.Inourstudy,tohaveacommongroundforcomparisonbetweendifferenttechnologies,theCO2compressionstepisavoided.ThelatestpublicationfromtheCarbonEngineeringgroup(Keithetal.,2018)showssomeimprovementintotalenergydemandandpresents3differentscenarios.Intheﬁrstscenario,alltheheatandpowerdemandisprovidedbynaturalgasoxy-fuelcombustion,gasturbineandsteamturbine.Inthesecondscenario,gasturbinehasbeenremovedandtherespectivepowerissuppliedfromthegrid.Thisalsodecreasestheratiooffuel-based/atmosphericcapturedCO2from0.48to0.3,whichresultsindownsizingofseveralprocessFig.2.ExampleofCO2directaircapturebasedonaqueoussolutionofsodiumhydroxide(NaOH)andpotassiumhydroxide(KOH)asanalternative.ReproducedandmodiﬁedbasedonaprocessdiagrambyKeithetal.(2018).M.Fasihietal./JournalofCleanerProduction224(2019)957e980961unitsaswell.Forbothscenarios,allthecapturedCO2iscompressedto150bar.Inathirdscenario,heatingisstilldonebynaturalgascombustion,whileCO2isnotcompressedanditisassumedthatO2isavailableforfree,thuspowerdemandandcostsofCO2compressorandairseparationunithavebeenavoided.Thefuelandelectricitydemandinallthe3scenariosarepresentedinTable1.Inafullyelectriﬁedsystem,thetotalcapturedCO2wouldbeloweredtothelevelofatmosphericCO2captured.Thiswoulddecreasethesizeandenergydemandofseveralparts.Theexactelectricityde-mandofafullyelectriﬁedmodelcouldbecalculatedviathesamesimulationsoftware.Inaconservativeapproach,assumingthesameenergydemandasthethirdscenariowouldresultin1535kWhel/tCO2forafullyelectriﬁedsystembasedontheCarbonEn-gineeringtechnology.Inthistechnology,NaOHhasbeenalsosubstitutedbypotassiumhydroxide(KOH)(CarbonEngineering,2018c;Keithetal.,2018).4.1.2.Lowtemperature(LT)solidsorbentMainly,technologiesinthisgrouphaveasingleunitwithsolidsorbent,whereadsorptionanddesorption(regeneration)happenoneafteranother.AsillustratedinFig.3,intheﬁrststepthesystemisopen,ambientairgoesthroughnaturallyorwiththehelpoffans.Atambienttemperature,CO2chemicallybindstotheﬁlterandCO2depletedairleavesthesystem.ThisstepiscompletedwhenthesorbentisfullysaturatedwithCO2.Inthenextstep,thefansareswitchedoff,theinletvalveisclosedandtheremainingairisoptionallysweptoutthroughapressuredropbyvacuumingorinsertingsteamintothesystem.Then,regenerationhappensbyheatingthesystemtoacertaintem-perature,dependingonthesorbent.ReleasedCO2iscollectedandtransportedoutofthesystemforpuriﬁcation,compressionorutilisation.Inordertostartanothercycle,thesystemshouldbecooleddowntoambientconditions.Thesorbentdeterminesthespeciﬁcconditionsofthecycles.Severaldifferentsorbentswereproposedinliterature,whichhavebeendescribedsubsequently.AminesareknownfortheirselectiveabilitytoabsorbCO2moleculesfromdilutedconcentrations.Climeworksusesaﬁltermadeofspecialcelluloseﬁberthatissupportedbyaminesinasolidform,whichbindsCO2moleculesalongwithairmoisture,thustheplantprovidesenoughwaterforitsownuse(Climeworks,2018b;Vogel,2017).InordertoreleaseCO2,pressureisreducedandthesystemisheatedto100C.Thesystemrequires200e300kWhel/tCO2mainlyforthefansandcontrolsystems.Italsoneeds1500e2000kWhth/tCO2forregeneration,whichcanbesuppliedbylow-gradeorwasteheat,asdemonstratedintherecentrespectivepilotplant(Climeworks,2018b).Afullcycleofthesystemtakes4e6hwithanoutputof99.9%purestreamofCO2.GlobalThermostat'sproprietaryamino-polymeradsorbentde-creasesthesystem'sfullcycletimetowellbelow30min,wheretheregenerationoccursinlessthan100sattemperaturesof85e95C.Toachievesuchafastprocess,saturatedsteamatsub-atmosphericpressureisusedasadirectheattransferﬂuidandasasweepgas.50%oftheregenerationheatisrecoveredanddependingontheplant'ssize,locationanddesiredCO2purity(>98.5%),theoverallFig.3.ExampleofalowtemperaturesolutionDACsystem.(1)Conditional(dependsonthesystem).Table1HTaqueoussolutionDACspeciﬁcations.type1stcyclesorbent2ndcyclesorbentCO2con.absorptiondesorptionenergydemandoutletpressureCO2purityreferenceppmT(C)T(C)kWhel/tkWhth/tbybar%2-cycleNaOHCa(OH)2-ambient900--NG100-Keithetal.(2006)NaOHCa(OH)2500ambient9004401678NG58-Baciocchietal.(2006)NaOHCa(OH)2380ambient9007641420NG/coal--Zeman(2007)NaOHCa(OH)2--9001199-2461el,tha---Stolaroffetal.(2008)NaOHCa(OH)2500-9004942250NG100-Socolowetal.(2011)NaOHCa(OH)2-ambient9002790-wind+batteryb--Lietal.(2015)cKOHCa(OH)2--900-2780NGd150-CarbonEngineering(2018c)KOHCa(OH)2--9001500-el.150-KOHCa(OH)2400ambient900-2450NG15097.1Keithetal.(2018)(CarbonEngineering)KOHCa(OH)2400ambient9003661458NG+el.15097.1KOHCa(OH)2400ambient90077e1458NG+el.197.1NaOHNa2O.3TiO2-ambient850-f-15gpureMahmoudkhaniandKeith(2009)1-cycle-CaO500365-400800-875--CSP-99.9Nikulshinaetal.(2009)2-cycleKOHCa(OH)2400ambient9001535-el.1>97ﬁnalmodel(thisstudy)aBasedondifferentcontactorsbBasedonZeman(2007),withoutheatrecycling.cTheheatgenerationmethodnotavailable.dHeatandelectricitygenerationrationotavailable.eAirseparationunitandCO2compressorexcluded.f50%lesshigh-gradeheatthanconventionalcausticisation.gCO2separationat15barandthencompressionto100bar.M.Fasihietal./JournalofCleanerProduction224(2019)957e980962electricityandheatdemandare150e260kWhel/tCO2and1170e1410kWhth/tCO2,respectively(Pingetal.,2018a).ThesystemproposedbyKulkarniandSholl(2012)isdifferentinthewaythatdesorptionofthesorbentsilica(TRI-PE-MCM-41)occurs,bytheintroductionofsteamat110C.Theoutputofthissystemis88%CO2and12%N2andwatertogether.Sinhaetal.(2017)hasstudiedthesametemperatureswingsystemandanalysedtwoamino-modiﬁedmetalorganicframeworks(MOF),MIL-101(Cr)-PEI-800andmmen-Mg2(dobpdc).Thissystemhasthesamecy-cles,butduetohighpossibilitiesofMOFsoxidisationathighertemperatures,vacuumisnecessarybeforeheating.Coolingisach-ievedbywaterevaporationfromthesurface.HeconcludesthatamongtwoMOFoptions,theonebasedonmagnesium(Mg)ismorefavourableduetolowerelectricityandheatdemand,whichis997kWh/tCO2(Sinhaetal.,2017).InAntecy'ssystem,CO2isadsorbedbycompositesorbentbasedonpotassiumcarbonate(K2CO3)atambientconditions.Beforeregeneration,airneedstobeevacuatedbywater,thenpressureisreducedandthesorbentisheatedupto80e100Cbylow-gradeheat(Roestenberg,2015).Thisslightlylowerregener-ationtemperatureincomparisontoClimworksisachievedbyduetothemoisture-aidedprocess.Derevschikovetal.(2014)intro-ducedaDACsystembasedonK2CO3/Y2O3sorbentpoweredbywindenergythatregeneratesattemperaturesof150e250C.Thesorbentisrathersensitivetohightemperaturesandcanbeeasilydestroyed.Table2summarisesthemaintechnicalcharacteristicsgatheredfromliterature.AlthoughAntecyclaimstobeneﬁtfrombothcheapermaterialandlowerenergydemandthanotherDACtech-nologies(Antecy,2018),thereportedenergydemandofClime-worksappearstobelower,followedbyanevenlowerenergydemandbyGlobalThermostat(Pingetal.,2018a).Thus,asameanvalue,Climeworks'averageenergydemandhasbeenselectedastheenergydemandfortheLTDACtechnologyinthisstudy.AimingforagenericLTDACsystemfromanenergysystemperspective,noﬁnalsorbenthasbeenselected.Inaconservativeapproach,adesorptiontemperatureof100Chasbeenchosen,asthehighestrequiredtemperaturebyreviewedLTDACcompanies.Thecom-parisonsofchosenelectricity,heatanddesorptiontemperaturevaluestotheavailablerangeofdatahavebeenvisualisedinFig.4.Antecy'sCO2purityisunknowntotheauthors,howeverAntecy'splanforsyntheticfuelsproductioncouldonlybeachievedwithaCO2purityofabout99%.Thus,aCO2purityofmorethan99%hasbeenassumedfortheﬁnalmodelasanaverageofCO2purityfromClimeworks,GlobalThermostatandAntecy.4.1.3.OthertechnologiesInadditiontothedescribedmajormodels,newapproacheshaveTable2LTsolidsorbentDACspeciﬁcations.sorbentCO2con.adsorptiondesorptionenergydemandcoolingCO2purityreferenceppmT(C)T(C)P(bar)kWhel/tkWhth/tbyT(C)by%amine-based400ambient1000.2200e3001500e2000wasteheat15air/water99.9Climeworks(2018b);Vogel(2017)amino-polymer400ambient85e950.5e0.9150e2601170e1410steamambientwaterevaporation>98.5Pingetal.(2018b)(GlobalThermostat)TRI-PE-MCM-41400ambient1101.42181656steamee88KulkarniandSholl(2012)MOF(Cr)400ambient135e48011420HTsteameeeSinhaetal.(2017)MOF(MG)400ambient135e4801997HTsteameeeK2CO3/Y2O3400ambient150e250eeeel.heatereeeDerevschikovetal.(2014)K2CO3eambient80e100e6942083wasteheatambientairﬂoweRoestenberg(2015);Antecy(2018)-400ambient100e2501750heatpump/wasteheatee>99ﬁnalmodel(thisstudy)Fig.4.Thecomparisonsofelectricity,heatanddesorptiontemperaturevaluesoftheﬁnalLTDACmodelin2020totheavailabledata.M.Fasihietal./JournalofCleanerProduction224(2019)957e980963beensuggestedinliterature.Duetolackofpubliclyavailabletechnicalandﬁnancialinformationorpilot-scaleimplementationofthesetechnologies,theyhavenotbeenfurtherdiscussedinthispaper.ElectrochemicalCO2captureandmodiﬁedfuelcellapproachesatambienttemperatureweresuggestedbyEisamanetal.(2009).However,nocostassumptionshavebeenpresented.Ion-exchangeresincancaptureCO2byMSAapproach.Thinresinsheetsareexposedtoambientairtofacilitatefreeﬂowoftheairthroughthematerial.Whenloadingisﬁnished,thesheetsaremovedtoaclosedsystem.Insidethesystem,airisremovedandmoistureisadded.TheresinreleasesCO2bycontactingwithwater.CO2iscollected,driedandcanbecompressedifneeded.Aftergasisremoved,thesystemisheatedupto45Ctospeedupthedryingprocess(Lackner,2009;Goldbergetal.,2013).Lackner(2009)claimsthatthesystemwithnaturalairﬂowwouldonlyrequireelectricalenergyintheamountof316kWhel/tCO2,includingcompressionforliquefaction,butusingfanwilladd10kWhel/tCO2.Thesystemutilisesheatreleasedfromcompressionaswell.Goldbergetal.(2013)hasproposedacomplexDACsystemwhereCO2,afterbeingcaptured,iscooleduntilitprecipitatesasdryiceandafterwarming,itturnsintoapressurisedliquidforseques-tration.Thissystemispoweredbywindenergyandrequires423kWhel/tCO2,excludingfreezingand631kWhel/tCO2includingit.Asmentionedinsection3,MSAtechnologyisalsousedbythecom-paniesInﬁnitreeandSkytree.Freitas(2015)hasproposedaconceptualdesignofnanofactorybasedmolecularﬁltersthatareabletocaptureCO2fromtheairpoweredbysolarenergy.Thesystemrequiresonly333kWhel/tCO2ofelectricityanddeliverspureCO2streamatapressureof100barwiththeﬁnalproductioncostofabout14V/tCO2.Ifthisapproachmakesitatacommercial-scale,itcouldbearevolutionforDACtechnologies.Seippetal.(2017)hassuggestedanewtwo-cycleapproachbasedonNa2CO3andPyBIG(2.6-Pyridine-bis(iminoguanidine)).Inthismethod,regenerationcanhappenattemperaturesof80e120C,avoidinghigh-gradeheatdemandbyconventionalaqueoussolution-basedDACplants.Itisclaimedthatthiscrystallisationapproachcouldoffertheprospectsforlow-costDACtechnologies,however,noﬁnancialdatahasbeenprovided.Availabletechnicalparametersfromliteratureforthetechnol-ogiesinthisgrouparepresentedinTable3.4.2.EconomicsofCO2DACMostarticlesregardingDACarefocusedontechnicalparametersandonlyafewhaveconductedeconomicestimations.AllreviewedeconomicspeciﬁcationsandtherecalculatedcostsaresummarisedinTable4.TheﬁrstoriginallyreportedcostsassociatedwithHTaqueoussolutionDACreviewedinthisstudywas376V/tCO2byKeithetal.(2006).LaterHolmesandKeith(2012)changedthecontactdesignofthepreviousmodelfundamentally,whichreducedthecostto258V/tCO2.Socolowetal.(2011)presentabenchmarkDACsystemwithrelativelymoredetailsforbothenergybalanceandeconomicaspects.Consideringtheequipmentinvestmentcosts,thestudyintroducesanoptimisticandrealisticscenario.Fortheoptimisticscenario,aninstallationmultiplyingfactorof4.5(sameasPSCC)isusedtoconvertequipment'spurchasepricetothetotalplant'sinstallationcost.ConsideringthenoveltyofDACtechnology,aninstallationmultiplyingfactorof6hasbeenusedfortheinstallationcostofthesystemintherealisticscenario.ThishasincreasedthetotalreportedcostsofcapturedCO2from309to395V/tCO2.ThebenchmarksystemdescribedbySocolowetal.(2011)wasfurtherinvestigatedbyMazzottietal.(2013),wherenewpackingmaterialsweresuggestedfortheoptimisationofaircon-tactingunitandtheﬁnalestimatedcostswerereducedto283e300V/tCO2,dependingonthecostsandtheenergyconsumptionofthethreedifferentproposedpackingmaterials.Zeman(2014)alsomodiﬁedandrecalculatedthecostsandenergydemandoftheSocolowetal.(2011)modelandconcludedthattheequipmentinvestmentcostscouldbeloweredby2.4%andtheannualopexcouldbereducedfrom4%to3%.Keithetal.(2018)fromCarbonEngineeringprovidedthetechno-economicassessmentsoftheirHTDACsystems,wherebothcapexandﬁnalcostsoftheHTDACsystemshavebeensigniﬁcantlyloweredincomparisontoAmer-icanPhysicalSocietymodel(Socolowetal.,2011),throughanewdesignandchoiceofmaterial.CarbonEngineeringhasalsoprac-ticallyachievedaCO2capturerateof74.5%,incomparisonto50%capturerateintheAPSmodel.Forthebaseconﬁguration(poweredbynaturalgasandoutletCO2at150bar),itprovidesthecapexoftheﬁrstplantat1132V/tCO2(1146USD/tCO2)andtheNthplantisexpectedtobe31%cheaperat714V/tCO2$a,duetoimprovementofconstructabilityandbuiltsupplychainrelationships.Inaddition,thecapexoftheNthplantbasedonthesecondconﬁguration(usinggridelectricityinsteadofgasturbine)andthethirdconﬁguration(avoidingCO2compressionandassumingavailabilityoffreeO2)arereportedat625V/tCO2$aand549V/tCO2$a,respectively.TheCO2capturecostsofallconﬁgurations,basedon5.6%and11.7%WACC(7.5%and12.5%annuityfactor)and27e54V/MWhforgridelec-tricity,areprovidedinTable4.ThethirdconﬁgurationbyKeithetal.(2018)istheclosesttothedesiredfullyelectriﬁedsystemexplainedinsection4.1.1.,withCO2atambientpressureinordertohaveacommongroundforcom-parisonwithLTsolidsorbentDACtechnology.Inaddition,suchasystemwouldnotneedO2duetosubstitutionofnaturalgaswithdirectelectriﬁcation.Moreover,anydirectfuel-basedCO2isavoi-dedinsuchasystem,whichcoulddownscalesomesubunitsandthecostsoffuelcombustionandheatingsystemcouldbepossiblyloweredaswell.Withaconservativeapproach,inourmodel,suchcostreductionshavenotbeenincludedandtheprojectcostshavebeenrecalculatedfortheﬁrstplantratherthantheNthone,whichincreasesthecapexfrom549to815V/tCO2$a.TheannualopexhasTable3TechnicalspeciﬁcationsofotherDACtechnologies.sorbentCO2con.adsorptiondesorptionenergydemandcoolingCO2purityreferenceppmT(C)T(C)P(bar)kWhel/tkWhth/tbyT(C)by%ion-exchangeresin400ambient,driedresinbymoisturising-316-self-heating45drying-Lackner(2009)ion-exchangeresin400ambient,driedresinbymoisturising-423-631-windpower45drying-Goldbergetal.(2013)K2CO3a400-2510-1002209-----Eisamanetal.(2009)Na2CO3&PyBIG400-80-120-------Seippetal.(2017)Nanofactory-basedb400---333-solarpower--100Freitas(2015)aElectrodialysis-basedCO2capturesystem.bMolecularﬁlters.M.Fasihietal./JournalofCleanerProduction224(2019)957e980964beensetto3.7%,accordingly.Thelifetimein2020issetto25yearsaccordingtoKeithetal.(2018).TheeconomicdataofLTsystemsbasedonsolidsorbentsaremorelimited.Climeworkshasclaimedatargetcostoflessthan75V/tCO2forlarge-scaleplants(Climeworks,2018b);however,noelectricitypriceorﬁnancialassumptionhavebeenprovided.GlobalThermostatexpectsCO2capturingcostsbelow113V/tCO2(150USD/tCO2)fortheirﬁrstcommercial-scaleplant(Pingetal.,2018b),whileKintisch(2014)hasreportedatargetcostof11e38V/tCO2,dependingonthelifetimeofaminesurfaces.Thetimeorscaleforreachingthiscostlevelisunknowntotheauthors.AlthoughCli-meworksistheforerunnerincommercialisingofsolidsorbentDACtechnologies,Antecy'scapexestimationof730V/tCO2$aistheonlyvalidpublicdatafound,asexplainedinsection4.1.2.Inaconser-vativeapproach,thelowerreportedlifetimeofClimeworks(20years)and4%annualopexhavebeenassumedfortheﬁnalLTDACmodelinthisstudy,for2020.Lackner(2009)hasproposedaverypromisingLCODof144V/tCO2formoistureswingtechnologyasoftoday,whichisduetorelativelylowercapexof421V/tCO2$a,amountofresinrequiredandassumedcostofelectricity.However,intheabsenceofapilotplant,ithasnotbeenconsideredforfurtheranalysisinthisstudy.Withaskepticalapproach,Houseetal.(2011)investigatedtheenergeticandcapitalcostsofexistingDACsystemsinanempiricalanalysis,andconcludedthattheﬁnalcostsofthesystemareunderestimatedandcouldbeatthelevelof750V/tCO2.Themainargumentisthatat500ppmCO2concentrationinambientair,theworkrequirementand,toalargerscale,thecapitalcostsofCO2DACwouldbemorethanthoseproposedinliterature.Inaddition,carbon-freeelectricitywithcostsof75e150V/MWhinaforesee-ablefuturehavebeenconsideredastheonlysourceofenergy.IthasbeenstatedthattheaircaptureofCO2wouldlikely,requiremorethermodynamicworkthanNOxremovalfromﬂuegasat500kJ/molNOx,equalto3156kWhel/tCO2.However,withanoperationalplant,CarbonEngineeringhasalreadyproventheseenergyde-mandandcapexassumptionstobetoohigh.Inaddition,withtheTable4EconomicsofDACasreportedandrecalculated.technologycapacitycapexopexlifetimeel.demandel.priceheat/fueldemandindicatedtimeofcostcostreportedcostrecalculatedtypeofsourceareferencetCO2/a€/tCO2$a%yearskWhel/t€/MWhelkWhth/tyear€/tCO2€/tCO2HTaqueoussolution280000--20---2005376-OKeithetal.(2006)1000000----60--258-OHolmesandKeith(2012)10000001583b4d204945322502011309b314OSocolowetal.(2011)10000002086c4d204945322502011395c3881000000----5318402013283-300e-OMazzottietal.(2013)----1500-0large-scale75-113f-OCarbonEngineering(2018c)10000001032k3.7250-24502018151,209j200OKeithetal.(2018)(CarbonEngineering)1000000714k,l3.8250-24502018114,153j1581000000625k,l3.72536627-5414602018110-112,137-147j1391000000549k,l3.8257727-541460201885-87,115-117j1151000000815k3.72515355002020-186ﬁnalmodel(thisstudy)LTsolidsorbent36001220-25694-20832015-244,203gORoestenberg(2015)(Antecy)360000730-25694-20832015-177,135g----150-260-1170-1410ﬁrstplant<113-OKintisch(2014);Pingetal.(2018a;2018b)(GlobalThermostat)----150-260-1170-1410n/a11-38-300--20200-300-1500-20002014--OClimeworks(2018b)-------large-scale75-360000730420250-17502020-155,120g-ﬁnalmodel(thisstudy)moistureswingsolidsorbent365421-1030638-200914499OLackner(2009)-41-----long-term23-generic4004701.530-low-2011900h-OSimonetal.(2011)4009402.530-medium-mid-term220h-40023503.530-high-long-term75h-----315675-150-2011750-RHouseetal.(2011)----315675-150-2050225-500000330450---202945-ONemetandBrandt(2012)500000330450---205023-500000330450---210014--------long-term30,71,105i-RBroehmetal.(2015)a(O)originalsourceand(R)reviewarticle.bOptimistic.cPessimistic.dAdditional2.88V/tCO2asopexvariable.eBasedondifferentpackingmaterial.fCompressedto150bar.gBasedonfreewasteheat.hWindpower,waterconsumptionandcarbonsequestrationcostincluded.iOptimistic,realisticandpessimisticassumptions.jWACC:5.6%,11.7%.kBasedonV/USDexchangeratein2016:1.11.lNthplant.M.Fasihietal./JournalofCleanerProduction224(2019)957e980965ongoingsharpdeclineinthecostsofrenewableelectricity(Lazard,2017;Liebreich,2017),theassumedLCOEistoohighaswell.Thecostsfromliteraturearenotcomparable,duetolackoftransparencywithtechnologydescriptions,differentoutputconditions(e.g.pressureandCO2purity)andcostassumptionsforinputenergy(heatandelectricity)orWACC.Thus,agenericstandardisedcostevaluationhasbeenperformedfortheﬁnalmodels,basedonthefollowingassumptions:WACC7%,elec-tricitycostof50V/MWhel,low-temperatureheatcostof20V/MWhth(<100C),high-temperatureheatcostof25V/MWhth(900C)andFLhof8000h.Itisimportanttoemphasisethatsuchcostsarecomparablewithtoday'sNG-basedelectricityandheatgenerationsystems,whichinduceCO2emissions.Foratrulysustainablesystem,renewableenergyshouldbeapplied,whichhasbeenlaterstudiedinsection4.3.2.,fortheyears2020e2050.Incaseoflackofdata,alifetimeof30yearsandanopexof4%ofthecapexhavebeenassumed.Theserecalculatedcostsarepre-sentedinTable4.ThecostrecalculationwasonlypossibleforthesystemsofSocolowetal.(2011),Keithetal.(2018),Roestenberg(2015)andLackner(2009),asthecrucialdata,suchasinputenergyorcapexismissingfromtheothermodels'speciﬁcations.Accordingtotheresults,itcanbeseenthattheﬁnalcostsre-portedbySocolowetal.(2011)arerecalculatedto314e388V/tCO2,whichmatchesthereportedcosts.At200V/tCO2,therecalculatedcostofthebasescenarioofKeithetal.(2018)is36%cheaperthanAPS'soptimisticmodelat314V/tCO2.Despiteofhighercostassumptionsforelectricitythanhightemperatureheat,at186V/tCO2,therecalculatedLCODoftheﬁnalHTDACmodel(fullyelectriﬁed,1stplant,1baroutletpressure)islowerthanthebasescenarioofKeithetal.(2018),duetolowercapexandenergydemandofthesystem,consideringthereliefofavoidedairseparationunit,CO2compressionto150baranddownscalingoffuel-basedCO2handlingsubunits.TheLCODoftheﬁnalLTDACmodelisslightlylowerthantherecalculatedcostsofAntecy'scommercial-sizemodel,whichisduetolowerenergydemandofClimeworkstechnology;however,Antecy'soriginallyreportedcostsareunknowntotheauthors.ThecostofLackner'ssystemisloweredfrom144V/tCO2to99V/tCO2.SuchadifferencecouldbepossiblyrelatedtoahigherrateofopexforMSAsystems.Intotal,therecalculatedLTDACsystemcostsarelowerthanHTDACsystem.Currently,DACisinanearlystageofdevelopment.However,itisassumedthatthemaintenancecostswillbereducedalongwithequipmentcapexduetomassproduction,alongwithloweren-ergyconsumptionduetotechnicaladvancesinthelongterm(Lackner,2009;Simonetal.,2011).Keithetal.(2006)consideredafactorofthree,astheaccuracyrangeforanyestimationofDACplants.Whilesuggesting376V/tCO2asanachievablecostwithtoday'stechnology,thisstudyexpectsacostdeclinetothesamerangeasconventionalmitigationtechnologies,duetoindepen-dencyandstrongereconomiesofscale.Socolowetal.(2011)emphasisedthesigniﬁcantamountofuncertainty,whichmakesithardtopredicttheperformanceofaplantcommissionedinthefuture.WhilethecurrentCO2DACcostsforLTsolidsorbenttech-nologiesareratherunrevealed,Climeworkshassetagoalof75V/tCO2forlarge-scaleplants(Climeworks,2018b).Inaddition,moistureswingsolidsorbentsarealsopredictedtodevelopsigniﬁcantly.Lackner(2009)expectsthatthesorbentmaterial(asthemostexpensivepartofthistechnology)willbeimprovedsigniﬁcantlywith10timeshighersurfaceareaanduptakeca-pacityperkgofsorbent.Thiswouldalsodecreasethevolumeoftheﬁlterboxby10times,increasingCO2capturecapacitypervolumeofthedeviceaswell.Itisprojectedthatthis,togetherwiththeeconomiesofscaleanddecreaseinthecostsofothermaterials,woulddecreasethecapexfrom421to41V/tCO2$aandtheﬁnalcostcouldbeloweredto23V/tCO2,takingintoaccountlow-costelectricityalongwithreductionofoperationalexpenditures.InordertoestimatepotentialcostsofDAC,Simonetal.(2011)conductedresearchwhereagenericDACtechnologywasexam-inedbasedonassumptionssuchaselectricity,heat,landandwateruse.ThestudyclaimsthatitispossibletocaptureCO2for220V/tCO2,however,itpointsoutthatsubstantialresearchintokineticsandthermodynamicsofcapturechemistryisneededtoproveit.Inadditiontothereferencescenario,basedonpessimistic(achievabletoday)andoptimisticscenarios,acostrangeof75V/tCO2tomorethan800V/tCO2havebeenprovided.Asmentionedintheprevioussection,Houseetal.(2011)skepticallypointedoutthatthecurrentcostsofCO2capturefromambientairhavebeenunderestimatedandcouldbearound750V/tCO2.However,thestudysuggeststhattechnologicalbreak-throughcandramaticallyimproveDACtechnology,whichcouldmakeitpossibletoreduceproductioncoststoamoderatelevelof225V/tCO2.Nevertheless,mostofthepapersagreeonthelongtermimprovementofDACtechnologies.ItwasconcludedbyBroehmetal.(2015)that,amongalldifferentapproaches,aqueoussolu-tionisthemostdevelopedDACsystemandhasshownsigniﬁcanttechnologicalimprovementoverthepastyearsandwillcontinuetofollowthepatternwhichinthelongtermwillbringcapitalandoperationalexpendituresdown.ForagenericDACsysteminthelongterm,Broehmetal.(2015)expectthecostsforcapturedCO2togodownto30,71and105V/tCO2foroptimistic,realistic,andpessimisticassumptions,respectively.ThesameopinionissharedbyNemetandBrandt(2012).Theyperformedasensitivityanalysisoftheappropriatetechno-economicenvironmentforDACimplementationonalargescale,estimatedcompetitivecostsofDACandtheeffectsitwillhaveonconventionaltypeofliquidfuels.TheypointedoutthataftercommercialisationofDAC,whichislikelyinthenearterm,thecostswillgodownrapidlyduetoeconomiesofscaleandlearningbydoing.Theyconsiderlearningratesof0.101forcapitalcosts,0.135forenergycostsand0.135foroperationalandmaintenancecostsfrompreviousresearchesperformedbyRubinetal.(2007)andvandenBroeketal.(2009),dedicatedtoPSCC(theclosesttechnol-ogytoDAC).NemetandBrandt(2012)concludethatundertheseassumptions,by2029,DACwillreachtheﬂoorcostof45V/tCO2,withpossiblefurtherreductionto23V/tCO2and14V/tCO2in2050and2100respectively.Inaddition,NemetandBrandt(2012)suggestalifetimeof50yearsforagenericDACsystem,whichexceedsanyotherlifetimeassumptionsinliteratureby20years.4.3.EstimatesforDACdevelopmentintheperiod2020to20504.3.1.PotentialcumulativeDACcapacitydemandandthelearningcurveimpactoncapexThestandardlearningcurveapproachisappliedforestimatingtheDACcapexdevelopment,accordingtoCalderaandBreyer(2017),assummarisedinEquations(9-11)forthelog-linearlearningcurveconcept.Abbreviationsarecapitalexpenditures,capex,progressratio,PR,binaryexponentialexpressionoftheprogressration,b,learningrate,LR,appliedforthehistoriccumu-lativeproductionforspeciﬁcyears,prod:capexnew¼capexinitial,prodnewprodinitialÀb(9)M.Fasihietal./JournalofCleanerProduction224(2019)957e980966PR¼ð2ÞÀb(10)LR¼1ÀPR(11)ThreefundamentalinputdataarerequiredforestimatingfutureDACsystemcapex:(1)TheinitialcapexaretakenfromTable4;(2)ThehistoriccumulativeDACcapacitydemandisderivedinthefollowingmanner;(3)ThelearningratesforDACsystemsaredis-cussedbasedonrespectiveDACliteratureandexperiencefromcomparabletechnologies.Article2oftheParisAgreementsetsthetargetoflimitingglobaltemperatureincreasetowellbelow2Candpreferably1.5Cabovepre-industriallevels.InArticle4,reachinganetzeroGHGemissionsysteminthesecondhalfofthecenturyissuggestedasameansofachievingthetargetsofArticle2.However,scientistsreportthatwearealreadyontheedgeofexploitingthecarbonbudgetfor1.5CscenarioandnetnegativeGHGemissionsystemsarenecessarytoachievethetargetsoftheParisAgreement(Kriegleretal.,2017;Rogeljetal.,2018).InthefollowingitisassumedthatthetargetsoftheParisAgreementshallbeachievedbythemidofthiscentury.Weconsidera2050scenariowithhighratesofdirectRE-basedelec-triﬁcation,substitutionofremainingfossilfuelsdemandbyRE-basedsyntheticfuelsandchemicals,aswellasdirectcarbonremovalintheenergysystem.Assuch,theremainingCO2sources,demandsandsinksarepresentedaccordingtothelistedbulletpointsandtheequivalentDACcapacities,asthemainpotentialCO2suppliers,areestimated.SeawaterCO2extractionisnotconsideredduetoitsearlystageofdevelopment.TheannualDACcapacitydemands,assummarisedinTable5,areestimatedfortheperiod2020to2050,inthesectorspower(power-to-gas,waste-to-energy,sewageplants),transport(road,rail,marine,aviation),industry(chemicalindustry,pulpandpaper,cementmills,others)andinfuturesectorofCO2removal.Inthepowersector,carboncaptureandutilisation(CCU)isnotappliedtoSNG-basedgasturbinesduetotheirbalancingrolewithlowFLhinaRE-basedpowersystem.ThefossilenergybasedCCUislimitedtoalmostunavoidablelimestonerelatedCO2emissionsfromcementmills.Afossil-basedtransportandchemicalindustrycouldreducetheirrespectivesyntheticfuelsand,consequently,CO2demand.How-ever,approximatelythesameamountofCO2wouldadduptothedirectCO2removalsectiontokeepthesystem'sCO2atthesamelevel.Since,PSCCcannotbeappliedtotransportandmostremainingnon-energeticusesoffossilfuelsinchemicalindustry.Moreover,thiswouldcauseadditionalCO2emissionsfromfossil-basedhydrogenproduction(mainlysteammethanereforming)insteadofwaterelectrolysis.Theironandsteelsectorisnotlistedduetotheassumptionthattheleastcostpathwayforthisindustrywouldleadtohydrogen-baseddirectreductionofiron(H2-DRI)andlatertoelectricityreducediron,asdiscussedbyOttoetal.(2017)andFischedicketal.(2014).TheDACcapacitydemandisfor:power-to-gastakenfromBreyeretal.(2018)andRametal.(2017);waste-to-energynegativeduetoitsCCUpotentialandbasedonwasteresourcepotentialtakenfromBreyeretal.(2018)andRametal.(2017),aCCUimplementationrateassumedtogrowfrom2%(2025)to50%(2050),acarboncaptureefﬁciencyof87%(EC,2014)andaCO2contentinwastetobe0.37MtCO2/TWhth,waste(IPCC,2003);transportmodes'demandistakenfromBreyeretal.(2019a)forthesyntheticFischer-Tropsch-fueldemand,whereas,therela-tivemixofdieselandkerosenemaychangethroughoutthetransitionperiod,andaspeciﬁcCO2DACcapacitydemandof0.36MtCO2/aperTWhth,fuelfor8000FLhbasedonFasihietal.(2017a;2017b);chemicalindustrybasedonthechemicalindustry'senergyfeedstockandﬁnalprocessenergydemandgrowth,excludingelectricity,from10280TWhth(2015)to19200TWhth(IEA,2009;RembrandtandMatt,2016),ademandcoveragegrowthbynaphthaastheby-productofFischer-Tropschfuelsproduc-tionfrom2%(2030)to16%(2050)accordingtoBreyeretal.(2019a),anestimated80%energyshareofcarbon-basedchemicalsandaspeciﬁcCO2DACcapacitydemandof0.25MtCO2/aperTWhth,feedstockfor8000FLhbasedonFasihietal.(2017a)foranaverageofcarbon-basedfeedstockchemicals;pulpandpapernegativeduetoitsCCUpotentialandbasedonthe2015pulpproductionandextrapolatedtill2050withtheTable5GlobalannualCO2DAC(orequivalent)capacitydemandbysector.sectorunit2020203020402050powerpower-to-gasMtCO2/a37142363waste-to-energyMtCO2/a0À17À99À165sewageplantMtCO2/a0n/an/an/atransportroad(cars/bus/trucks)MtCO2/a021813091101railMtCO2/a076682marineMtCO2/a0569621667aviationMtCO2/a0549641543industrychemicalindustryMtCO2/a022411573255pulpandpaperMtCO2/a0À8À52À95cementmills(limestone)MtCO2/a0À69À425À607othersMtCO2/a0n/an/an/aCO2DAC,energysystemMtCO2/a3.047340257144CO2removalMtCO2,captured/a00100010000thereofotherNegativeEmissionTechnologiesMtCO2,captured/a003002500thereofCO2DAC,CO2removalMtCO2/a007678213CO2DAC,totalMtCO2/a3473479115356M.Fasihietal./JournalofCleanerProduction224(2019)957e980967compoundannualgrowthratebetween2000and2015takenfromKuparinenetal.(2018),aCCUimplementationrateassumedtogrowfrom2%(2025)to50%(2050),acarboncaptureefﬁciencyof87%(EC,2014);cementmillsnegativeduetoitsCCUpotentialandbasedoncementproductionestimatestakenfromFarfanetal.(2019),aCCUimplementationrateassumedtogrowfrom5%(2030)to50%(2050),anefﬁciencyincreaseofoverallcapturedCO2from60%(2030)to80%(2050);CO2removaldemandbasedonKriegleretal.(2017),butwithademandfor10GtCO2/aremovalfor2050insteadof2055forahigherlevelofsustainabilityandthereofa300and2500MtCO2/aremovalsharebyafforestationin2040and2050,respectively,andtheremainingforCO2DACsystemswith8000FLh.TheestimatedannualCO2DACcapacitydemandgrowsfrom3MtCO2/a(2020)toabout15360MtCO2/a(2050),thereofabout8200MtCO2/afromCO2removal(2050).TheestimatesofannualCO2DACcapacitydemandsinTable5andrespectiveDAClifetimesdeﬁnethehistoriccumulativeCO2DACcapacitydemandsinTable6,whicharetakenasinputfortheDACcapexestimates,accordingtothelearningcurveapproachforaconservativeandabasecasescenario.ThetwoscenariosforDACcapexdevelopmentaredeﬁnedasfollows:Theconservativescenarioassumesonly50%realisationofthecumulativeDACcapacitydemandduetodelayedexecutionoftheParisAgreementandaDAClearningrateof10%,asassumedbyNemetandBrandt(2012)andbasedonRubinetal.(2007),vandenBroeketal.(2009)andRubinetal.(2004).ThebasecasescenarioassumesaneffectiveexecutionoftheParisAgreementwithoutdelay,leadingtonetzeroGHGemis-sionsfromtheenergysystemandalreadystartedCO2removal.TheDAClearningrateisassumedtobe15%,whichmatchesbetterthetechnologyspeciﬁccharacteristicsofCO2DACsys-temsthantheeffectivelyassumedsulphurremovalsystemsoflarge-scalecentralisedcoal-ﬁredpowerplantsthatarethebasisfortheassumed10%learningrate(NemetandBrandt,2012;Rubinetal.,2007,2004).Highlymodularenergytechnologiesexhibitlearningratesaround15%,asdocumentedforwaterelectrolyserswith18%(Schmidtetal.,2017),seawaterreverseosmosisdesalinationwith15%(CalderaandBreyer,2017),lithium-ionbatterysystemswith12%e17%(Kittneretal.,2017;Schmidtetal.,2017),whichareﬁnallyaconsequenceofmorecomprehensiveinternationalproductstandardisationandsub-stantialeconomiesofscale.ResultsofthelearningcurveapproachforestimatingtheDACsystemcapexaresummarisedinTable6andvisualisedinFig.5.TheDACsystemcapexareassumedtobe730V/tCO2$a(LT)and815V/tCO2$a(HT)in2020(Table4).ThecapexcandeclineforLTDACsystemsto199V/tCO2$a(conservative)and84V/tCO2$a(basecase)andforHTDACsystemsto222V/tCO2$a(conservative)and93V/tCO2$a(basecase)in2050,respectively.TheconsideredinitialDACcapacityin2020(1.5MtCO2/a)usedincapexdevelopmentcalculationsarewellabovethereportedcapacitiesofreferenceunitsofLTDAC(0.36MtCO2/a)andHTDAC(1MtCO2/a),whichemphasisestheroomforfurthercapexreduction.TheestimatedDACcapexprojectionsareusedinthefollowingasinputforthecostscenariosinlevelisedcostofCO2DACforspeciﬁcsites.4.3.2.LevelisedcostofCO2DAC(LCOD)intheperiod2020to2050HTaqueoussolutionandLTsolidsorbentarethetwomaintechnologies,whicharereadyforcommercialscaleimple-mentation.TheﬁnalmodelsofHTaqueoussolutionandLTsolidsorbentDACtechnologiesin2020,presentedinsections4.1.and4.2.,havebeenfurtherstudiedbasedonassumptionsforlongtermdevelopmentofthemainspeciﬁcationsbasedonthecon-servativescenariodescribedinsection4.3.1.,andasshowninTable7.Thecapexassumptionsforbothtechnologiesarebasedonthecumulativeinstalledcapacitiesandlearningrates,applyingtheconservativescenario,describedinsection4.3.1.ThelifetimeofLTDACtechnologyin2020is20years(Climeworks,2018b),whichhasbeenexpandedto25and30yearsin2030andbeyond,respec-tively,accordingtothelong-termestimationsforgenericDACplantsinliterature.ThelifetimeofHTDACissetto25yearsin2020accordingtoKeithetal.(2018),andlaterhasbeenextendedto30Table6ConservativeandbasecasescenariosforLTandHTDACcapexreduction.parameterunit2020203020402050CO2DAC,totalMtCO2/a3.0473479115356thereof50%,conservativescenarioMtCO2/a1.523723967678thereof100%,basecasescenarioMtCO2/a1.5473479115356historiccumulativecapacity(conservative/basecase)MtCO2/a1.5/1.5237/4732397/47937679/15357doublingsbetweenperiods(conservative/basecase)-07.3/8.33.4/3.41.7/1.7capexCO2DACLT(conservative/basecase)V/tCO2$a730338/189237/110199/84capexCO2DACHT(conservative/basecase)V/tCO2$a815378/211265/122222/93Fig.5.CO2DACcapexdevelopmentforLTandHTsystemsbasedonthelearningcurveapproachandtheappliedconservative(CS)andbasecasescenarios(BS).TheDACcumulativecapacityisbasedontheﬁndingsinTables5and6andtherespectiveDACcapexarebasedonEqs.(9)-(11).M.Fasihietal./JournalofCleanerProduction224(2019)957e980968yearsfortheyears2030andbeyond,asSimonetal.(2011)andNemetandBrandt(2012)consideralifetimeof30and50years,respectively.Inaddition,althoughNemetandBrandt(2012)sug-gestahigherlearningrateforopexofDACsystems,ithasbeenkeptconstantat3.7%and3%ofcapexfrom2020to2050forLTandHTDACtechnologies,respectively.InapersonalcommunicationwithClimeworks(Kronenberg,2015),theaverageelectricityandLTheatdemandfor2030wereestimatedtobe10%and14.3%lessthanthecurrentnumbers.ConsideringtheminimumachievedelectricityandheatdemandbyotherLTcompanies(Fig.4),thesamedemandreductionrateshavebeenassumedforeach10-yearstepuntil2050.Anelectricityde-mandreductionrateof5%hasbeenappliedtoafullyelectriﬁedHTDACsystemfrom2020onwards,consideringthelimitsbythetheoreticalheatdemandofthecalcinerunitinthecurrentlyknownsetup.MoroccowaschosenasapotentialsiteforalargescaleDACplantimplementationwith2400FLhforsingle-axistrackingsolarphotovoltaic(PV)systemand3500FLhforwindpowertechnology(Fasihietal.,2017a).DACplantsandheatpumpsarecapexinten-sive,thusitisimportanttorunthemonhighFLh,whichwoulddemandhighavailabilityofelectricity.Batteriesareneededtoin-creasetheavailabilityofrenewableelectricity,especiallyforaPV-basedsystem.Fig.6illustratestheimpactofDACFLhonthenetLCOEandLCOD.Ascanbeseen,increasingFLhfrom3000to8000hwouldincreasethenetLCOE.However,thenegativeimpactofhigherLCOEonLCODhasbeenoffsetbyhigherDACFLhwhichprovidesthechanceforfurtherreductionofLCOD.ThehighestimpactisobservedforincreasingFLhofLTDACfrom3000to6000handfromthereto8000FLh,thedecreaseinLCODisverylow.ForthecaseofHTDAC,LCODstaysmoreorlessstableat5000to7000FLh,withaslightincreaseat8000FLh,whichisduetobiggerimpactofenergycostonHTDAC.TheLCOEnetfor4000and8000FLhhavebeencalculatedbasedonthespeciﬁcationsofpowersectorcomponentsasinTable8.TheratioofinstalledcapacityofPVtowindgraduallyincreasesfrom7in2030to10in2050,duetofasterdeclineinPVLCOEandcostdeclineofsupportivebatterysystems.Batteryshareis11%and56%for4000FLhand8000FLh,respectively.ElectricalcompressionheatpumpshavebeenusedforLTheatgeneration,wheretheCo-efﬁcientofPerformance(COP)graduallyincreasesfrom3.0in2020to3.5in2050(DEA,2016).LCOE,LCODandLCOHwerecalculatedfor4000and8000FLhconditionsandbasedontheconservativescenariowiththeabovedescribedassumptions.TheresultsarepresentedinTable7.Fig.7illustratestheﬁnalcontributionsforLCODoftheLTandHTDACsystemsat8000FLhin2040fortheconservativescenario.TheLCODoftheLTDACsystemreaches69V/tCO2,wherethehighestsharesbelongtoheatdemandat43%andtoDACcapitalexpendi-turesat30%.Incaseofaccesstofreewasteheat,theLCODcouldbeloweredtoabout40V/tCO2.Ontheotherhand,at91V/tCO2,theLCODoftheHTDACsystemin2040ishigherthanLTDAC,wheretheelectricitycostdominatesthetotalcostsat62%andtheshareofcapitalexpendituresisabout26%.Thus,itisrathercrucialforbothDACsystemstohavetheDACplantslocatedatsitesofabundantandverylow-costrenewableelectricityinordertobringtheﬁnalCO2productioncostsdown.Inthecaseofaccesstoverylow-costorfreewasteheatforLTsystem,itsdependencyonverylow-costelectricityisrelativelylower.4.3.3.SensitivityanalysisDuetouncertaintiesaboutliterature-basedDACsystemmodels’speciﬁcationsandtheirdevelopmentsinthelongterm,asensitivityanalysisiscrucialinthisstudy.Inaddition,inputvaluescanvarybasedontheselectedlocationoftheDACplantandoveralleco-nomicenvironment.Thus,asensitivityanalysiswasconductedfor±10%changesineconomic,energeticandgeographicalfactorsforLTandHTDACsystemswith8000FLhin2040,forwhichtheresultsarepresentedinFig.8.AsillustratedinFig.8a,a10%changeinWACCorDACcapexhasa4e5%impactonLCOD,followedby2e3%and1%impactfrombatteryandPVcapex,respectively.Althoughwindhasahighercapex,itsimpactontheresultsarenegligible,astheinstalledcapacityofwindissettooneninthofPVcapacityin2040.AsshowninFig.8b,theimpactofa10%changeinopexofPV,windorbatteryonLCODisnegligible,whiletheDACopeximpactisabout1.5e2%.However,thebiggestoperationalcostimpactsareFig.6.TheimpactofDACFLhonnetLCOE,LTLCODandHTLCOD(conservativescenario).M.Fasihietal./JournalofCleanerProduction224(2019)957e980969associatedwithenergyconsumptionoftheplant.FortheLTDACsystem,witha5%changeinLCOD,heatdemandhasthebiggestimpactasthemajorityofenergydemandissuppliedbyheat,whileat7%,electricitydemandofafullyelectriﬁedHTDACwouldhavethesameimpactonLCOD.Fig.8cemphasisesthebigroleofDACFLhonLCOD,however,unlikeFig.6,hereitisassumedthatincreasingDACFLhhasnoimpactonLCOE.Italsoshowsthataregionwith10%morePVFLhcoulddecreasetheLCODbyabout2%.Fig.8dillustratesthata10%lowerbatteryorLTDAClifetimecouldincreasethecostbyabout1%.4.4.AreademandandriskoflocalCO2depletionOneofthemostcommonconcernsaboutwidescaleDACplantsimplementationislocalCO2depletion,asitmayaffecttheenvi-ronmentandvegetation.Inaddition,aCO2-poorenvironmentwoulddecreasetheefﬁciencyofDACsystemsandincreaseﬁnalCO2capturecosts.Thus,itisimportanttoevaluatetherecoverytimeandtheminimumdistancebetweenDACunitstoavoidsuchproblems.Inaddition,footprintandrespectivelandusagemaybeakeyissue,assubstantiallandrequirementmightbeabarrierfortheTable8Powerandheatsectorkeyspeciﬁcations.unit2020203020402050referencePVsingle-axistrackingplantETIP-PV(2017);Bolingeretal.(2017)capexV/kWp638429330271opexﬁxV/(kWp$a)15.012.010.08.0opexvarV/kWhel0000lifetimeYear30354040WindpowerplantNeij(2008);Breyeretal.(2018)capexV/kWp11501000940900opexﬁx%ofcapexp.a.2.0%2.0%2.0%2.0%opexvarV/kWhel0000lifetimeYear25252525PV/Windcapacityratioe78910thisstudyBatteryBreyeretal.(2018)capexV/kWhel(energy)30015010075opexﬁx%ofcapexp.a.2.5%2.5%2.5%2.5%opexvarV/kWhel0.00020.00020.00020.0002lifetimeYear20202020cycleeff.%91939595energytopowerratioe6666ElectricalCompressionHeatPumpDEA(2016)capexV/kWth660590554530opexﬁxV/(kWth$a)2222opexvarV/kWhth0.001800.001700.001630.00160lifetimeyear25252525COPe33.263.413.51Table7Long-termspeciﬁcationsofDACandgenericcosts(conservativescenario).unit2020203020402050LTDACcapexV/tCO2$a730338237199opex%ofcapexp.a.4%4%4%4%lifetimea20253030el.demandkWhel/tCO2250225203182LTheatdemandkWhth/tCO21750150012861102HTDAC(electriﬁed)capexV/tCO2$a815378265222opex%ofcapexp.a.3.7%3.7%3.7%3.7%lifetimea25303030el.demandkWhel/tCO215351458138513164000FLhLCOEnetV/MWhel44282118LCOH-LT(byheatpump)V/MWhth36272422LCOD-LTV/tCO22891419780LCOD-LT(freewasteheat)V/tCO2226996756LCOD-HTV/tCO228613898808000FLhLCOEnetV/MWhel103584132LCOH-LT(byheatpump)V/MWhth51302320LCOD-LTV/tCO22221056954LCOD-LT(freewasteheat)V/tCO2133604032LCOD-HTV/tCO22681339171M.Fasihietal./JournalofCleanerProduction224(2019)957e980970largescaleimplementationofthetechnology.ThestudybyJohnstonetal.(2003)hasanalysedthecarboncapturepotentialofanengineeredﬂatsinkwith4⁰Â5⁰latitude/longituderesolutionanditslongtermlocalCO2depletionimpactin5differentregionsoftheworld,withathreedimensionalchemicaltransportmodel.Itconcludesthatthelocaldepletioncouldbeintherangeofnaturaldaily,seasonalCO2ﬂections,thusnotanissuefortheimple-mentationofDACsystems.Inaddition,itnotesthattheCO2uptakedependsontheCO2velocityand,atatypicalvelocityof1m/s,anareaof75000km2wouldbeenoughtocapture3GtCO2/a,repre-sentingafootprintof25km2/MtCO2.Ithasbeenemphasisedthatthesameuptakecapacitiescanbeexpectedfromverticallyalignedsystems,whiledecreasingthedirectlanduseatthesameactivearea.Ontheotherhand,byspreadingthisverylarge-scalesysteminsmallerunitsacrossdifferentregionstoavoidmeetingeachother'sCO2shadow,theactiveareademandcouldbereducedbymorethananorderofmagnitude.Socolowetal.(2011)claimsthatforaHTaqueousbasedDACsystemwith1MtCO2/acapacity,thetotalareademandwouldbe1.5km2thatleadstoafootprintof1.5km2/MtCO2,whichisbasedonthefollowingassumptions:Fivecontactingfacilitieswithalengthof1kmandwidthof1marelocated250mapartfromeachother,whichistheminimumalloweddistancetopreventdualdepletedairintake.Thisindicatesthat,likewindfarms,themajorarede-mandofDACsystemsisreservedforthefreespacebetweenDACunits.Inaddition,awarehouseforchemicalstorageandaregen-erationunitisincludedinthefootprint.Climeworks(2018b)cap-tureplanthas18unitslocatedin3rowsontopofeachotherandiscurrentlythemaximumverticalexpansionforClimeworks.How-ever,theoverallfootprintoftheirsystemforcapturing8GtCO2peryearis3300km2,whichisequalto0.4km2/MtCO2annually.Although,noneofthesetwosourceshavespeciﬁedhowthetotallanddemandortheminimumalloweddistancebetweenunitswereestimated,theiroverallfootprintisinlinewithJohnstonetal.(2003),whichconductedlanduseestimationforrelativelysmall-scaleDACsystems.Keithetal.(2006)alsodiscussedDACsystemslandrequirementsinlesserdetail,whereithasbeenconcludedthatthedirectareademandofpotentialDACplantscanberathersmall,asthelandbetweentheunitscanbefreelyusedforotherpurposes.4.5.WaterdemandThewaterdemandofDACunitsistheotherfactorwhichshouldbeconsideredforthelargescaleimplementationofthetechnology.ThewaterlossinHTaqueoussolutionDACsystemscouldbebe-tween0and50tonspertonCO2captured,dependingonthetemperatureandhumidityoftheambientairandconcentrationofthesolution(Keithetal.,2006;Stolaroffetal.,2008;Smithetal.,2016;Zeman,2007).ThenewCarbonEngineeringdesignneeds4.7tonsofwaterpertonCO2captured,atambientconditionsof64%relativehumidityand20C(Keithetal.,2018).Incaseofwaterstressintheregion,waterdesalinationandtransportationcouldcost0.6to1.6V/m3in2030(Calderaetal.,2016),whichwouldaddabout3e8V/tCO2,accordingtowatercostimpactinKeithetal.(2018).ThiscouldlimitthelocationalﬂexibilityofDACplants,particularlyindryandremotedesertregionswherebothwaterdemandanditstransportationcostcouldbesigniﬁcantlyhigher.Ontheotherhand,someLTDACsystemscancapturewaterasaby-product.Forexample,Climeworkstechnologycancapture2e5molofwaterpermoleCO2captured,equalto0.8e2tonwaterpertCO2.Generally,fromanenergypointofview,itistheirgoaltocaptureaslittlewateraspossible.However,at2molwaterpermoleCO2theenergydemandwouldbeinthelowerendofenergyconsumptionrangeofClimeworkstechnology(Kronenberg,2015).AccordingtoBajamundietal.(2018),Hydrocell'sDACsystemoperatedintheFinnishclimatehasalsoproduced4.6molofwaterpermoleofcapturedCO2,equalto1.9tonwaterpertCO2.Thus,waterdemandwouldnotbeaconstraintforLTDACsystems,quitetothecontraryDACsystemscouldprovidewaterneededforsubsequentwaterelectrolysisprocesses,asrequiredforpower-to-fuelandpower-to-chemicalconversion(Fasihietal.,2017a,2017b).4.6.CO2compression,transportandstorageThecapturedCO2couldbestoredorutilisedasfeedstockforotherapplications.Forthesematters,additionalstepssuchaspu-riﬁcation,compressionandtransportation(ingaseousorliquidphase)maybeneeded,whichcouldbeenergyandcostintensive(AspelundandJordal,2007;Johnsenetal.,2011;Knoopeetal.,2014).CO2couldbeliqueﬁedbycompressiontoacriticalpressureof73.8barandthencanbepressurisedfurtherbypumps(McCollumandOgden,2006).WhencompressingCO2,recoverableheatisgeneratedandcanbeutilisedinotherpartsofthesystem(Lackner,2009).InPSCC,priortocompression,CO2needstobecleanedfromawiderangeofimpuritiesassociatedwithﬂuegases.Thus,theFig.7.LCODcostbreakdownforthefullyelectriﬁedHTDACsystem(left)andLTDACsystem(right)for8000FLhandconditionsinMoroccoin2040.M.Fasihietal./JournalofCleanerProduction224(2019)957e980971Fig.8.SensitivityanalysisofLCODfortheLTDAC(left)andHTDAC(right)systemsbasedoninputdatafor(a)investment,(b)operational,(c)FLhand(d)lifetimeassumptionsfor8000FLhin2040.M.Fasihietal./JournalofCleanerProduction224(2019)957e980972compressionstationiscombinedwiththepuriﬁcationunit(Skaugenetal.,2016).Simonetal.(2011)reported62.5kWhel/tCO2astheminimumenergyrequirementforCO2compressionto138barafterDAC,equalto104kWhel/tCO2ofpracticalenergyde-mandbasedonacompressionefﬁciencyof60%.Kolsteretal.(2017)reportedanenergyrequirementof96e103kWhel/tCO2forCO2dehydrationandcompressionto120bar,forPSCC.Keithetal.(2018)reportedanenergyrequirementof132kWhel/tCO2forCO2compressionto150barfromDAC,wherethecompressorstandsforabout3%ofthetotaldirectﬁeldcost.CO2transportationcanbedonebypipelines,ships,railways,trucks,tankcontainersoracombinationofthem.Transportationtypestronglydependsontheterrain,distanceandcapacity.Pipe-lineisawell-regulated,safeandmatureoption(IEA,2016)thatisfavourableforbigvolumesofCO2withannualtransportationca-pacityof1e5milliontonanddistancesintherangeof100e500km(IEA,2010).Overlongdistances(>2400km),shiptransportismorecost-effective(IEA,2016).Italsohasadvantagesoverpipelinenetworkintermsofﬂexibilityandscalability.Ontheotherside,shipsrequirewell-developedhubsandterminals.CO2istrans-portedonlyintheliqueﬁedformbyships,thusanadditionalpressurisationstationisneededattheharbour.Karjunenetal.(2017)hasanalyseddifferentsitesattheterrainwhereshipsandsufﬁcientinfrastructureofpipelinesdonotexistandconcludedthatthepriceofCO2transportationbytrucks,trainsandpipelinesforshortdistances(100e400km)willbeintherangeof4.4e14V/tCO2.CostparametersassociatedwithallmentionedmeansoftransportationaresummarisedandpresentedinTable9.TraditionaloptionsforCO2sequestration(permanentstorage)arelimitedtodeepsalineformations(1000e~10000GtCO2),depletedoilandgasﬁelds(675e900GtCO2),coalseams(3e200GtCO2),basalts,shales,saltcavernsandabandonedmines(IEA,2016;Svenssonetal.,2004).ChenandTavoni(2013)havere-portedaCO2storagecostofabout10V/tCO2forthebestsitesintheworld,withacumulativecapacityofabout700GtCO2.Trans-portationdistanceandassociatedcostscouldincreaseforfutureprojects.Amoresustainablelong-termCO2storagesolutionwouldconvertCO2intoachemicallyinertcompoundwithahighcom-bustionpointsothatlateremissionriskscanbereducedtoanabsoluteminimum.Also,CO2fromDACtechnologiescouldbeusedforsyntheticfuelproductioninaclosedcarbonloop(Vazquezetal.,2018).Forcouplingthesesectors,dependingontheoperationtimingandcapacitiesofDACsystemsandsyntheticunits,inter-mediateandseasonalstoragewithhighcapacitiesmightbeneeded.However,mostplantswouldbeoperatedclosetobaseload,whichreducestheneedforseasonalstorage.GastankscanbeusedforintermediatestorageofCO2.Karjunenetal.(2017)hasstatedthatcostsofintermediatestorageincylindricaltankscanbeabout10V/tCO2.5.Discussion5.1.RelevanceofDACwithrespecttotheParisAgreementTheParisAgreementsymbolisesacommonunderstandingoftheextremesituationandactionsneededtobetaken.Toreversethistrend,adeepandfastdefossilisationofthepower,heat,transportationandindustrysectorsthroughhigherutilisationofREby2050isneeded(Breyeretal.,2018;Jacobsonetal.,2017;Mathiesenetal.,2015;Rametal.,2017;Sgouridisetal.,2016).However,a100%directelectriﬁcationofthesesectorsisnotpossible,especiallyforhightemperatureindustrialheating,long-rangeaviationandmarinetransportation,wherefossilfuelsandlatersyntheticfuelsareexpectedtobeplayaroleby2050.Somebalancinggaspowerplantswouldbeneededforthepowersectorduringthetransitionperiod,whichcouldswitchduringthetran-sitionperiodtosyntheticnaturalgasorothersyntheticfuels(Rametal.,2017).Inaddition,apartofnon-energeticuseoffossilfuelsinthechemicalindustrycouldﬁnallyturntoCO2emissionsintheatmosphere,suchasburningofplasticwastesinincinerators.TheunavoidablefossilCO2emissionsfromcementindustrywouldalsostillcontributetoclimatechange(Farfanetal.,2019).PointsourceTable9CO2transportationcost.transp.typecapacitydistancecostreferenceMtCO2/akmV/tCO2truck15e20Â10À6>10013Freitas(2015)train1.465987.3Gaoetal.(2011)onshorepipeline0.731006.8McCollumandOgden(2006)0.7350043.6McCollumandOgden(2006)2.51805.4ZEP(2011)7.31001.5McCollumandOgden(2006)7.35009.8McCollumandOgden(2006)201801.5ZEP(2011)207505.3ZEP(2011)offshorepipeline2.51809.3ZEP(2011)2.5150051.7ZEP(2011)201803.4ZEP(2011)20150016.3ZEP(2011)shipping275011.1aAspelundetal.(2006)2.518013.5aZEP(2011)2.5150019.8aZEP(2011)3195011.8bKujanp€a€aetal.(2011)2018011.1aZEP(2011)20150016.1aZEP(2011)aLiquefactioncostincluded.bLiquefactioncostnotincluded.M.Fasihietal./JournalofCleanerProduction224(2019)957e980973carboncaptureandstorage(CCS)couldonlydecreasetheGHGemissionsfromthesesectors,andnotfullyremovethem(Leesonetal.,2017).Inaddition,pointsourceCCScouldnotbeappliedtoships,planesandsmallerpollutersThus,afossil-basedsystem,evenwithCCSorCCU,wouldstillbeanet-polluter(SAPEA,2018).DACwillﬁnallyallowtoclosethecarboncycleinaworldwhereitisnotpossibletoeliminateGHGemissionsproducedbyaviationandmarinesectors,alongwithuntappedCO2fromunavoidablepointsources(cementandwaste-to-energyincinerators)andfromlanduseandagriculture.TargetsoftheParisAgreementaremostlikelynotachievablebypointsourceCCS,asnotasingleproposedtechnologycancaptureallemittedCO2,whereasitcanbecollectedbyDACplants.Inaddition,asmentionedinsection4.3.1.,DACca-pacitiescoupledwithCO2storageareneededasanegativeemis-siontechnologytoreverseclimatechangeimpacts.IntegratedAssessmentModels(IAMs)havepreferredBECCSasNETinthepast,ascriticisedbyCreutzigetal.(2019),whereasDACsystemsaresuperiorinmostcriteria.ButthereportedcostsofDACarestillregardedasthemainobstacleforabroaderconsiderationofDACsystemsasanimpactfulNET.Breyeretal.(2018)foundforthecaseoftheMaghrebregionthatDACsystemscanbeaverycost-effectiveNETin2050.Creutzigetal.(2019)alsopointoutthattheenergysystemintegrationofDACCScanbeexpectedtobesuperiortoBECCSforrenewablesbasedenergysystems,whichisconﬁrmedbyBreyeretal.(2019b)forDACCScoupledtoa100%renewableenergysystemintheMaghrebregion.AccordingtoWilcoxetal.(2017),CO2puritiesofabout50%aresuitableformineralcarbonationasapermanentCO2storagesolution.Atsuchpuritylevels,theenergydemandandconsequentlythecostofDACsystemsarerelativelylowerthanDACsystemswithmorethan99%puritydiscussedinthiswork.ThiscouldpotentiallyleadtoacheaperDACCSprocess,assumingnosigniﬁcantnegativeimpactbyimpuritiesduringpost-captureprocesses.5.2.BeneﬁtsandchallengesofthemainDACtechnologiesLTsolidsorbet-basedandHTaqueoussolution-basedDACsys-temshavebeenreviewedinthisstudy.TheHTtechnologyisadoptedfromPSCCwithprovenabsorbentmaterials.Nevertheless,inmostHTDACmodelsfossilfuelsareusedtoprovidetherequiredhigh-gradeheat.ThiswouldbeanunsustainablesystemforCCU,asthefossilCO2partwouldﬁnallyendupintheatmosphere.ACCSchainbasedonthissystemcouldenableonlypartiallynegativeemissions,whichwouldincreasethenetLCODofavoidedCO2.Theuseofsyntheticfuelswouldalsodramaticallyincreasetheprimaryenergydemandandthecostofthesystem.However,afullyelec-triﬁedHTDACtechnologyprovidesthechancetofullyrunthesystemonRE.Ontheotherhand,LTDACsystemshavemoreop-tionsforprovidingheat,suchasheatpumps,whicharemoreen-ergyefﬁcientandcanbedirectlypoweredbyRE.Thewasteheatfromindustrycouldbeseenasasourceoffreeorcheapenergy,whichcouldreducetheLCODsigniﬁcantly(Fig.6).ForsomeCCUprocesses,andinparticularsyntheticfuelsproductionintegratedwithLTDACunits,thewasteheatfromfuelproductionprocessescouldberecycledandusedintheLTDACunits,reducingtheoverallcostsofﬁnaloutput(Fasihietal.,2017b;Rametal.,2018).ThisisaclearadvantageofLTDACsystems.Moreover,whilethewaterde-mandofHTDACsystemsistypicallyseenasanegativefactor,someLTDACtechnologiesarecapableofcapturingmoisturefromtheatmosphereasaby-product,whichcouldbeusedforhydrogenproductionastheﬁrststepinsyntheticfuelsandchemicalspro-duction.Thiscouldpartiallyorfullyavoidthedependencyofsuchsystemsonexternalwater(Fasihietal.,2017b).ThemoistureaidedLTDACtechnologies,suchastheoneappliedbyAntecy(Roestenberg,2015),couldfurtherdecreasethetemperatureandtheamountofheatdemand,openingtheroomformorevarietyofwasteheatsourcesandhigherenergyefﬁciencyofsuchDACsys-tems.Ontheotherhand,totheknowledgeoftheauthors,sofarLTDACsystemshavebeenoperatedinmoderateclimates.OperatingthesesystemsinsunnyandhotregionssuchasMoroccomaydecreasetheheatingdemand,howeverthiswouldincreasethecoolingdemandofthesystem,asthenaturalaircoolingwouldnotbeenoughanymore.Theimpactofsuchachangeontheoverallenergydemandofthesystemisunknowntotheauthors,sameasthechangeinmoistureintheatmosphereofdryregions.HTDACsystemsconsistofseparatecarboncaptureandregen-erationunits,whichmaketheconstantcarbonabsorptionandregenerationpossible.Ontheotherhand,fortheavailableLTDACtechnologies,bothadsorptionanddesorptionhappenstepwiseinoneunit,whichlimitstheoperatinghoursofeachstep.ThismaycauseadditionalcoststothesystemfortimemanagementandstorageofREandRE-basedheat.Antecy(2018)andGlobalTher-mostat(Pingetal.,2018b)haveintroducedLTDACsystemswithseparateunitsexclusivelydesignedforadsorptionordesorption,whichhaveahigherefﬁciencyandoperatingtimethatcoulddecreasetheﬁnalsystemcosts.Fromaneconomicperspective,judgingbypubliclyavailableinformationandpromisesforfurthercostreductions,LTDACseemstobethecheaperoptionwhenfreewasteheatisavailable.ItisimportanttoemphasisethatevenlowercostsareforeseenforLTDACbyGlobalThermostat(Table4).However,asnoﬁnancialdetailsarepubliclyavailable,ithasbeenexcludedfromthiscomparison.5.3.Finalcostoflarge-scaleDACAsdiscussedinsection4.3.2,LCODofHT/LTDACsystemswith8000FLh,undertheconservativescenariowith50%implementa-tionofneededDACsystemsand10%learningrateofDACcapex,isprojectedtobeabout268/222,133/105,91/69and71/54V/tCO2in2020,2030,2040and2050,respectively.However,asillustratedinFig.9,underbasecasescenarioassumptionswith100%imple-mentationofneededDACsystemsand15%learningrate,thecostsforHT/LTDACsystemscouldgodownto268/222,111/84,72/53and54/38V/tCO2in2020,2030,2040and2050,respectively.At111e133V/tCO2,theresultsforHTDACin2030arehigherthantheprojectedcostsofCarbonEngineering(thirdscenario),at85e87V/tCO2,fortheNthgas-basedplantwithawellregulatedFig.9.LCODforLTandHTDACsystemswith8000FLhand7%WACCforthecon-servativescenario(CS)andbasecasescenario(BS)assumptions.M.Fasihietal./JournalofCleanerProduction224(2019)957e980974constructionandsupplychainrelationship.Inthisyear,thehistoriccumulativeDACinthesetscenarioshavewellpassedthematuritylevelandthecapexisbelowtheNthplant'scapex.However,theimpactofclose-to-baseloadrenewableelectricity(58V/MWhel)ontheﬁnalcostsissigniﬁcantlybiggerthantheassumedcheapgaspriceof11V/MWhthbyKeithetal.(2018),whereinthecostsoffossil-CO2emissionshavenotbeentakenintoaccount.Basedonthis,thelong-termcostsrecalculatedinthisresearchof54e71V/tCO2forHTDACin2050appearsachievable.NoﬁnalnumberisprovidedinliteratureforthecurrentcostsofLTDAC,however,asthedeﬁnedgenericLTDACinthisresearchisfullybasedonspec-iﬁcationsofavailableplantsorcommercialdesigns.Thecalculatedcostof222V/tCO2in2020isexpectedtobereliable.Forthe2040e2050period,LTDACcostshavebeencalculatedtobe38e69V/tCO2,whichisinlinewithprojectedcostsofClimeworks(75V/tCO2)andGlobalThermostat(11e38V/tCO2)forlong-termorlarge-scaledeploymentofDACtechnologies.Besidestheimpactofmoreenergyefﬁcientsystemsandaccessibilitytocheaperenergyinthefuture,thecapexdevelop-mentandrelativecostreductionfrom2020to2050emphasisesthebigroleoftheimplementationrateandlearningcurve.Neverthe-less,therelativelyhighcostofDACin2020couldremainasachallengeforattractinginvestmentsneededforintroducingthesystemtothemarketandscalingup.Inthisstudy,aCOPof3hasbeenusedasaglobalaverageforheatpumpsin2020,whichgraduallyincreasesto3.51in2050(DEA,2016).However,thiscouldbearatherconservativeassumptionforthewarmclimateofMorocco.Ontheotherhand,thenatureofthemarketin2020e2030couldberelativelydifferentfromthosein2040e2050,asDACcoupledwithpermanentstorageasanegativeemissiontechnologyisprojectedtobeimplementedfrom2040onwards.AlltheprojectedDACcapacityin2020e2030isassociatedwithsyntheticfuelsandchemicalsproductionwithpossiblyavailableexcessheatasabyproduct,fromwhichtheLTDACsystemcouldbeneﬁttofurtherdecreasetheproductioncostto133,60,40and32V/tCO2in2020,2030,2040and2050,respec-tively,whichcouldmakemarketentryeasier.Ontheotherhand,forthe2020e2030period,theglobalenergysystemwouldstillincludehighsharesoffossil-basedpointsourceCO2frompowerandindustrialsectors,whichcouldbeacheapersourceofCO2throughpointsourceCCUandcouldbeacompetitortoDAC.Whetherregulatorsallowitremainsunclear.However,lookingatthebigpicture,therewouldnotbeenoughpointsourceCO2tomeetalltheCO2demandinafullysustainableenergysystemin2050(Table5).Thus,DACwouldﬁnallyhavethelargermarketshareand,tomakeitcheapbythen,theimplementationneedstostarttoday.Allcostanalysesinthisresearcharebasedon7%WACC,how-ever,abusinesscasewith5%WACCmaybepossibleinsomere-gionsoftheworld.AccesstocheapREinsuchregionscouldfurtherreducetheLCODbyabout12%.NichemarketscouldalsohelptoincreasethemarketshareofDAC.Asanexample,theawarenessanddemandforsustainableCO2orCO2-basedproductssuchassyntheticfuelsorchemicalsisgrowing,whichmakesDAConeoftheveryfewavailableoptions.Finally,CO2emissionscostsarethelastmeansofregulatingthemarket,whichcouldhaveasmallorbigimpactoncostcompetitivenessofDACsystems,dependingontheregulationsdevelopedbypolicy-makers.5.4.CO2DACvs.pointsourcecarboncapture,todayandinthefutureDACisusuallycomparedtoPSCCbasedoncosts,whichisnotfullycorrect.Asexplainedinsection5.1.,pointsourceCCUandCCScanonlyreducethepaceofCO2emissionsincreaseduetoimplementationrateandcarboncaptureefﬁciencylosses,whileDACcoupledwithCO2storage(DACCS)cantrulyactasanetNET(Choietal.,2011b).Inaddition,asshowninsection4.3.1,inasustainableenergysystemby2050,therewouldnotbeenoughpointsourceCO2asfeedstockforotherapplications.Thus,thetwotechnologiescouldcomplementeachotherfordifferentapplica-tions.However,therecouldbesomemarketsegmentswherebothtechnologiescouldbeapplied,buttheﬁnalchoicecouldbederivedbyﬁnancialfactors.Underequalpreconditionssuchasenergydemandandcosts,itseemslogicaltoexpectlowercarboncapturecostsfrompointsourcesduetothehigherconcentrationofCO2.Fig.10illustratesthecostrangeofPSCCin2020andprojectedcostsin2050,basedonthescenarioexplainedbyLeesonetal.(2017)andDACcostsin2040and2050,basedontheconserva-tive(upperlimit)andbasescenarios(lowerlimit)presentedinthisstudy.Literature-basedindustrialPSCCcostrangeforeachFig.10.CostdistributionrangeofPSCCfordifferentindustries(reproducedafterLeesonetal.(2017))andDAC.Abbreviations:naturalgasproduction,NGP,steammethanereforming,SMR.M.Fasihietal./JournalofCleanerProduction224(2019)957e980975sectorin2020representdifferentstudiesunderdifferenttech-nologies,carboncaptureefﬁcienciesandcostassumptions.ThecostsinLeesonetal.(2017)havebeenconvertedfromUSD/tCO2,avoidedtoV/tCO2,capturedbytheﬁxedUSD/Vconversionrateof1.33andtheavoidedCO2issetto78%ofcapturedCO2basedoncoal-poweredPCSSdescribedinSocolowetal.(2011).Themin-imumPSCCcostfromironandsteelindustryisrelatedtousageofsteelslagforcarbonationwithonly8%efﬁciency,whichcannotfulﬁlltheinitialgoalofPSCC.Inaddition,thesustainablepathwayforironandsteelindustrycouldbeCO2-free(Fischedicketal.,2014;Ottoetal.,2017).PSCCcostrangefromreﬁneriesandcementin2020isabout30e100V/tCO2,whilethecostrangefrompulpandpaperisbetween29and41V/tCO2,originatedfromreviewingtwopublications.Theprojectedcostsofindus-trialPSCCin2050havebeencalculatedbyLeesonetal.(2017)basedonthemeanvaluesforthecostsofbestcandidatePSCCtechnologiesforeachsectorin2020andappliedinstalledca-pacitiesby2050.Underthisscenario,PSCCcostsfromcementreach12V/tCO2,however,stillusingfossilfuel,whichmaybepricedveryhighforavoidablefossilfuelsourcesin2050.Inaddition,highpurity(>95%)sourcesofindustrialCO2alreadyhavethePSCCpotentialwithcostswellbelow20V/tCO2,assuchCO2streamsonlyrequirecompression,anddonotneedexpen-sivecarboncapturedevices.Incasefossilfuelshadbeentheinputfeedstock,onemayhavetopayinadditionaCO2pricefortheﬁnalemissions,whichcoulddrasticallyincreasethecosts,comparedtothesustainableDACroute.Naturalgasprocessing(NGP),ammoniaproduction,ethyleneoxideproductionandsteam-methanereformingforhydrogenproductionaresomeoftheprocesseswithfossil-basedfeedstock,whichaccountforabout7%ofindustrialemissionsLeesonetal.(2017).ThesearethesocalledlowhangingfruitsforPSCC,however,someoftheseprocessescouldbesubstitutedwithadecarbonisedtechnologyinthefutureenergysystem,decreasingtheirCO2productionpo-tential.Forinstance,withfurthercostdeclineofREandelec-trolysers,hydrogencouldbeproducedbywaterelectrolysiswithoxygenastheonlyby-product,andfurtherupgradedtohighervaluedproductswithadditionalRE-basedpower-to-chemicalsprocesses.AsshowninFig.10,HTDAC,LTandfree-heatLTDACin2040starttobecostcompetitivewithupperrange,middlerangeandlowerrangePSCCcostsin2020,respectively.ThefurthercostreductionofDACsystemsin2050couldmakethemevenmorecostcompetitive,dependingonwhethertheprojectedPSCCcostsin2050areachievedornot.However,thePSCCcostsarenotthefullcostsforcapturedCO2,ifthesourcehadbeenfossilfuels.Because,notallthereleasedCO2couldbecapturedandalsolateremissionstotheatmospherehavetobeconsideredforalmostallroutes(e.g.syntheticfuelsusedinaviationormarinesectors)andtherespectiveCO2pricehastobetakenintoaccountasanadditionalcost.ThisisnotthecaseforRE-basedDAC.Therefore,DACmaybecostcompetitiveearlierasindicatedbyFig.10.HighermodularityandlocationalﬂexibilityofDACsystemscouldmakethemevenmorecostcompetitiveinthewholeCCUorCCSchain.PSCCislimitedtolocationswithhighcapacitiesofCO2stream,whichcouldalsolimittheaccesstocheapenergyforPSCC.Aprobablelongdistancetoutilisationorstoragesitescouldincreasethetransportationcostsaswell.Ontheotherhand,DACsystemscouldbelocatedincost-optimalsites,takingintoaccountaccesstolowcostenergyandutilisationorstoragesites.5.5.ThecostshareofCO2inpower-to-gasAsmentionedinsection5.3.,CO2fromDACisnotonlyaproductforstorage,butalsoafeedstockforproductionofsyntheticfuelsandchemicals.Inthatcase,costsoftheﬁnalproductisofhigherimportancethantheLCOD,aslongasthecostshareofCO2isnotsigniﬁcantoriftheﬁnalproductcostsareinanattractiverangeregardlessofLCOD.Asanexample,theproductioncostsofsyntheticnaturalgas(SNG),integratedwithCO2DACinMoroccohavebeeninvestigated.Theelectrolyserandmethanationplants’speciﬁcationsaretakenfromRametal.(2017).AccordingtoFasihietal.(2017a),thewasteheatfromwaterelectrolyserandmethanationunitscansupplyalltheLTheatdemandofLTDACsystemsin2030,whichwouldbethecaseforyearsafterwards,astheenergydemandoftheDACsystemdecreases.However,in2020,theenergydemandofLTDACsystemscouldexceedtheavailablewasteheatby10%.Forthesakeofsimplicity,thisextrademandhasnotbeentakenintoaccountandisassumedthattheheatintegrationsuppliesalltherequiredheat.TominimisetheLCOD,LTDACFLhshouldbeﬁxedat8000FLh,asshowninFig.6.However,forconsistency,inthissectionDACFLhfollowspower-to-gas(PtG)FLhandthecostsofSNGunderdifferentPtGFLhhavebeenillustratedinFig.11.Forsuchaconﬁguration,theLCOGofacoupledDAC-PtGsystemwith4000FLhwouldbe148,84,63and52V/MWhth,HHVin2020,2030,2040and2050,respectively.Ascanbeseen,unlikeDACunits,byincreasingtheFLhofPtGunits,thelevelisedcostofgas(LCOG)increases.ThisisbecausePtGisrelativelylesscapexintensivethanDACandthehigherLCOEnetforhigherFLhhasabiggerimpactontheﬁnalLCOG.Itshouldbenotedthatinthissimpliﬁedsystem,DAC,electrolyserandmethanationunitshavebeencoupledandnoCO2orhydrogenstoragehasbeenincluded.ThecostshareofLTDAC-basedCO2(conservativescenario)ofLCOGfor4000FLhunderrespectiveLCOEisillustratedinFig.12.Ascanbeseen,CO2costsharewouldbeabout27%in2020,whichwoulddropto21%,19%and19%in2030,2040and2050,respectively.ItisimportanttoemphasisethatadecoupledsystemwithhigherDACFLhwouldhaveasigniﬁcantlylowerLCOD,LCOGandconsequentlylowerCO2costshare.Forexample,adecoupledsys-temwith8000DACFLhand4000PtGFLhwouldresultinlowerLCODandLCOGof133,60,40and32V/tCO2and132,76,58and48V/MWhth,HHV,respectively.Inaddition,theabsolutecostofCO2anditsrespectivesharewouldbeevenlowerintheDACbasecasescenario.Therefore,thecostsofCO2DACmaynothaveadecisiveFig.11.TheimpactofPtGFLhonLCOG.M.Fasihietal./JournalofCleanerProduction224(2019)957e980976impactontheintroductionofsyntheticnaturalgastothemarket.Thissimpliﬁedmodelanditsresultsdonotrepresenttheconditionforallclimatesorsyntheticproducts.6.ConclusionsLarge-scaleCO2DACsystemsareneededtomeettheParisAgreementtargetsbymid-21stcentury,eveninaworldwithhighlevelsofdefossilisationandPSCCimplementation.Itisestimatedthat3,470,4798and15402MtCO2/aDACcapacitiesareneededby2020,2030,2040and2050,respectively.AliteraturereviewonDACtechnologiesisperformedandtheavailabletechnologiesarecat-egorisedfromanenergysystemperspective.Hightemperatureaqueoussolution-baseddirectaircapture(HTDAC)andlowtem-peraturesolidsorbent-baseddirectaircapture(LTDAC)arethetwomaincategoriesofcommerciallyavailabletechnologieswhicharefurtheranalysedfortheperiod2020to2050,technicallyandeconomically.AlthoughtheenergydemandofaLTDACsystemishigher,forhighFLh,itstotalenergydemandcouldbemetataboutthesamecost,asthemajorshareofenergydemandcouldbesuppliedbyrelativelycheaperlow-gradeheatsuppliedbyheatpumps.Inaddition,althoughthecapitalexpenditureofbothtechnologiesisatthesamelevel,theLTDACtechnologyisthemorefavourableoptiontodayandinfuture,duetoitspotentialforextensivecostreductionbyutilisationofwasteheatfromothersources.Moreover,theLTDACsystemshowsahighmodularity,andhasnodemandforexternalwater.TheLCODdevelopmentinthedecadestocome,mainlydependsonthelearningcurveofcapitalexpenditures,itsenergydemandandthecostdevelopmentofrenewableelectricity.Inaconserva-tivescenariowith10%learningrateofcapexandtherealisationofhalftherequiredDACcapacitiesateachtimestep,thecapexofHT/LTDACsystemsarecalculatedtobe815/730,378/338,265/237and222/199V/tCO2$ain2020,2030,2040and2050,respectively.Whilethebasecasescenariohasbeendeﬁnedas100%implementationoftherequiredcapacitieswith15%learningcurveofcapex,inlinewiththeexperiencefromcomparabletechnologies.Underthisscenario,thecapexofHT/LTDACsystemswouldshrinkto815/730,211/189,119/106and89/79V/tCO2$ain2020,2030,2040and2050,respectively.Inaddition,a5/10%electricitydemandreductionforHT/LTDACand14.3%low-gradeheatdemandreductionisforeseenateach10-yeartimestep.Asacasestudy,theCO2capturecostsinMorocco,suppliedbyhybridPV-Wind-batteryplantsandheatpumpshavebeeninves-tigated.Theresultsshowthat,despitehigherelectricitycosts,DACsystemswithhigherFLhwouldhavelowerLCOD.Intheconser-vativescenario,theLCODofHT/LTDACsystemswith8000FLharecalculatedtobe268/222,133/105,91/69and71/54V/tCO2in2020,2030,2040and2050,respectively.Whileinthebasecasescenario,thecostswouldbereducedto268/222,111/84,72/53and54/38V/tCO2,respectively.Basedonthediscussioninsection5.3.,itcanbeenconcludedthatsuchresultsareinlinewithresultsofmajorpublicationsandcompanies’targetsasoftodayandinthelongterm.However,suchcostreductionscouldbeexpectedonlyifthetechnologyimplementationstartsin2020accordingtothedeﬁnedscenarios.Inaddition,accesstofreewasteheatcouldfurtherdecreasetheLCODofLTDACby40e57%,dependingontheyearandappliedscenario.Atsuchcosts,DACiscompetitivetoPSCCwithlessrestrictionsoncapacityandlocation.Whereas,fossilfuelbasedCO2mayinducefurthercoststhanonlythecapturingcosts,sincelateremissionstotheatmospherearemostlikelytobeincluded,inparticularforanetzeroemissionssystem,sothatthecapturingcostsmaybetheminorcostfractionforPSCC.SuchfreedomcouldfurtherincreasethecompetitivenessofDACinprojectsassociatedwithCO2storageorutilisation,byminimisingthetransportationcosts.AcknowledgementsTheauthorsgratefullyacknowledgethepublicﬁnancingofTekes,theFinnishFundingAgencyforInnovation,forthe‘Neo-CarbonEnergy’projectunderthenumber40101/14.WealsothankCyrilJoseE.BajamundiforthevaluablecommentsandManishRamforproofreading.ReferencesAntecy,2018.Aboutus.Hoevelaken,Netherlands.Availableat:http://www.antecy.com/about-us/.(Accessed5February2018).Aspelund,A.,Jordal,K.,2007.Gasconditioning-theinterfacebetweenCO2captureandtransport.InternationalJournalofGreenhouseGasControl1(3),343e354.Aspelund,A.,Mølnvik,M.J.,DeKoeijer,G.,2006.ShiptransportofCO2technicalsolutionsandanalysisofcosts,energyutilization,exergyefﬁciencyandCO2emissions.Chem.Eng.Res.Des.84(A9),847e855.Audi,2015.CorporateResponsibilityReport2014.AudiAG,Ingolstadt,Germany.Availableat:https://www.audi.com/content/dam/com/corporate-responsibility/nachhaltigkeit_pdfs/Audi_CR-Report%202014_English_Printversion.pdf.(Accessed24May2018).Baciocchi,R.,Storti,G.,Mazzotti,M.,2006.Processdesignandenergyrequirementsforthecaptureofcarbondioxidefromair.Chem.Eng.Process:ProcessInten-siﬁcation45(12),1047e1058.Bajamundi,C.,2015.ProgressPresentationonCO2CaptureDeviceAcquisition.NeoCarbonEnergy2ndReseachers'Semniar,Lappeenranta.March.Availableat:http://www.neocarbonenergy.ﬁ/wp-content/uploads/2016/02/12_Bajamundi.pdf.(Accessed24May2018).Bajamundi,C.,Elfving,J.,Kauppinen,J.,2018.Assessmentoftheperformanceofabenchscaledirectaircapturedeviceoperatedatoutdoorenvironment.In:InternationalConferenceonNegativeCO2Emissions,Gothenburg,May22-24.Bolinger,M.,Seel,J.,HamachiLaCommare,K.,2017.Utility-ScaleSolar2016-AnEmpiricalAnalysisofProjectCost,PerformanceandPricingTrendsintheUnitedStates.LawrenceBerkeleyNationalLaboratory,Berkeley,September.Availableat:https://utilityscalesolar.lbl.gov.(Accessed24May2018).Breyer,C.,Bogdanov,D.,Aghahosseini,A.,Gulagi,A.,Child,M.,Oyewo,A.,Farfan,J.,Sadovskaia,K.,Vainikka,P.,2018.Solarphotovoltaicsdemandfortheglobalenergytransitioninthepowersector.Prog.PhotovoltaicsRes.Appl.26,505e523.Breyer,C,Khalili,S.,Bogdanov,D.,2019a.SolarphotovoltaiccapacitydemandforasustainabletransportationsectortofulﬁltheParisagreementby2050.Prog.PhotovoltaicsRes.Appl.1e12.https://doi.org/10.1002/pip.3114.Breyer,C.,Fasihi,M.,Aghahosseini,A.,2019b.Carbondioxidedirectaircaptureforeffectiveclimatechangemitigationbasedonrenewableelectricity:anewtypeofenergysystemsectorcoupling.MitigAdaptStrategGlobChange.https://doi.org/10.1007/s11027-019-9847-y.Broehm,M.,Streﬂer,J.,Bauer,N.,2015.Techno-economicreviewofdirectaircap-turesystemsforlargescalemitigationofatmosphericCO2.SSRNElectronicJournal.Availableat:https://papers.ssrn.com/sol3/papers.cfm?abstract_Fig.12.CO2costshareofLCOGcostdevelopmentforDACandPtGwith4000FLh.M.Fasihietal./JournalofCleanerProduction224(2019)957e980977id¼2665702.(Accessed24May2018).Bui,M.,Adjiman,C.S.,Bardow,A.,Anthony,E.J.,Boston,A.,Brown,S.,Fennell,P.S.,Fuss,S.,Galindo,A.,etal.,2018.Carboncaptureandstorage(CCS):thewayforward.EnergyEnviron.Sci.11,1062.Caldera,U.,Breyer,C.,2017.Learningcurveforseawaterreverseosmosisdesali-nationplants:capitalcosttrendofthepast,presentandfuture.WaterResour.Res.53,10523e10538.Caldera,U.,Bogdanov,D.,Breyer,C.,2016.LocalcostofseawaterROdesalinationbasedonsolarPVandwindenergy:aglobalestimate.Desalination385,207e216.CarbonEngineering,2018a.Teamandboard.Squamish,Canada.Availableat:http://carbonengineering.com/company-proﬁle/.(Accessed15January2018).CarbonEngineering,2018b.CEDemonstrationPlanteaYearinReview.Squamish,Canada.Availableat:http://carbonengineering.com/ce-demonstration-plant-a-year-in-review/.(Accessed5February2018).CarbonEngineering,2018c.DirectAirCapture.Squamish,Canada.Availablefrom:http://carbonengineering.com/about-dac/.(Accessed25January2018).Chen,C.,Tavoni,M.,2013.DirectaircaptureofCO2andclimatestabilization:amodelbasedassessment.Clim.Change118,59e72.Choi,S.,Gray,M.,Jones,C.,2011a.Amine-tetheredsolidadsorbentscouplinghighadsorptioncapacityandregenerabilityforCO2capturefromambientair.ChemSusChem4(5),628e635.Choi,S.,Drese,J.H.,Eisenberger,P.M.,Jones,C.W.,2011b.Applicationofamine-tetheredsolidsorbentsfordirectCO2capturefromtheambientair.Environ.Sci.Technol.45,2420e2427.Choi,S.,Watanabe,T.,Bae,T.,Sholl,D.S.,Jones,C.W.,2012.ModiﬁcationoftheMg/DOBDCMOFwithaminestoenhanceCO2adsorptionfromultradilutegases.J.Phys.Chem.Lett.3,1136e1141.Climeworks,2017.ClimeworksStartsPlantinIcelandan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