An exergy-based approach to the joint economic and environmental impact assessment of possible photovoltaic scenarios: A case study at a regional level in Italy

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An

exergy-based

approach

to

the

joint

economic

and

environmental

impact

assessment

of

possible

photovoltaic

scenarios:

A

case

study

at

a

regional

level

in

Italy

Emanuela

Colombo

a

,

Matteo

V. Rocco

a,

*,

Claudia

Toro

b

,

Enrico

Sciubba

b

a _Politecnico_di_Milano,_Department_of_Energy,_via_Lambruschini_4,₂₁₀₅₆_Milan,_Italy

b _Università_di_Roma_{“Sapienza”,}_Department_of_Mechanical_and_Aerospace_Engineering,_via_Eudossiana_18,_Rome,_Italy

1.Introduction:impactassessmentofenergyconversion

systems

Inmodernindustrialsocieties,engineers,systemanalystsand

policymakersarerequiredtodealnotonlywiththeeconomicsof

thedesignandoperationofenergysystems,butalsowithissuesof

naturalresourcescarcityandwiththeneedforanevaluationofthe

global impactof the conversionsystem on humansociety and

environment.

Thehitherto“virtuous”designgoalofreducingtheexploitation

of natural energy resource while maximizing ﬁrst and second

law efﬁciencies on a life-time basis is undergoing a radical

re-evaluation,inresponsetotheacknowledgedinterdependency

oftheenergysectorwithboththeenvironmentandthesocietyat

large(Rocco et al.,2014a). Theimpactof anenergy conversion

system(ECS),evaluatedwithalife-cycleapproach,mustinclude

the effects at local and global scales on the surrounding

environmentandonthesocietyinwhichthesystemiscalledto

operate.Sucha“holistic”designapproachrequiresthat,fromthe

analyst’sperspective,thetotal(embodied)consumptionofnatural

resources(thatincludestheeffectsofnon-energeticexternalities

suchas humanlaborand capital expenses)mustbe takeninto

account.Toachievethis,itisnecessarya)toidentifyanadequate

quantitativemeasureoftherealconsumptionofnaturalresources

onthepartofthesystem,andb)toapplytheselectednumeraireto

* Correspondingauthor.Tel.:+390223993816.

E-mailaddresses:emanuela.colombo@polimi.it(E.Colombo),

matteovincenzo.rocco@polimi.it(M.V. Rocco),claudia.toro@uniroma1.it(C.Toro), enrico.sciubba@uniroma1.it(E.Sciubba).

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ascenariothatincludesthewholesystem’slifecycleandtheeffect

ofthenon-energeticexternalitiesonresourcesconsumption.

1.1.AssessmentoftheeconomicandenvironmentalimpactofanECS

Exergyisthereal“value”ofenergyexchanges,somuchthata

paradigmhasbeenproposedcalledExergyCostTheoryinwhich

“cost”isdeﬁnedastheamountofexergyembodiedinaunitof

materialorimmaterialgoods.Suchaperspectiveconstitutesthe

theoreticalfoundationofThermo-economics(TA) (Valero etal.,

1993), whose scope is to allocate the production cost among

productsinmulti-productchains,costbeingdeﬁnedin abroad

senseas “anyeffortthathastobemadein ordertoobtainthe

consideredusefuleffect”.

Thermoeconomic analysis evolved into twomain categories,

whichdifferforthecostparadigmtheyadopt:

1. economic cost: indicates thecost of any system product, in

termsofsforexergeticJoule;(Tsatsaronis,1993)

principles with traditional cost accounting methods and it is

consideredastandardtoolinbothacademicstudiesandindustrial

applications(Leeetal.,2014;Petrakopoulouetal.,2013).When

usedinthesecondacceptance,TAconstitutesaproperframework

to evaluate the global (i.e., life cycle) consumption of natural

resources: if an exergy cost, rather than a monetary one, is

attributed to all resources involved in the energy system

production,thecostoftheﬁnalproductturnsouttobeevaluated

inthesameunits(Jperkgorperunit).Theresultsoftheliterature

review and a thorough discussion about the standard and

advancedexergybasedtechniquescanbefoundin(Roccoetal.,

2014a).

1.2.TheItalianelectricgenerationsystemandthegrowingroleof

photovoltaics

In thelasttwo decades, theinstalledpowerofphotovoltaic

systemshasincreasedexponentiallyintheItalianelectricsystem

(Association,2010;GSE,2011).Bytheendof2012,theinstalled

photovoltaic peak power in the Italian grid amounted to

16,450MWe, producing 18,862GWh electric energy per year,

about 3% of the national electricity consumption. More than

95%ofthetotalinstalledPVcapacityconsistsofgrid-connected

PVplants(Programe,2010).Fig.1showsthegrowthofinstalled

PV peak powerfrom 2007 up to2012 (solid line). Dueto the

severereductionoftheFeedInTariff(FIT),itisunlikelythatItaly

willmaintainthisrisingtrendinthenearfuture(Barnhametal.,

2012; Meneghello, 2012). However, the current Italian energy

strategyrequiresthattheinstalledcapacitycontinuetogrowby

1GWeperyear(dottedlineinFig.1),reachingalmost25GWeof

installed capacity by 2020 (GSE, 2011), more than 30% of the

2020expectedtotalinstalledcapacity.Acurrentopenissueisthe

quantiﬁcationoftheeconomicandenvironmentalimpactofsuch

a largeintegrationofphotovoltaic peakpowerwiththeItalian

power system. In this perspective, thermoeconomic help in

evaluatingtheeffectsof different futurescenarios onboththe

economicandenvironmentalcostsofthegeneratedelectricity.It

is important to emphasize that the environmental cost is

evaluatedinTEintermsofnaturalresourcesconsumption,and

thatotherpossibleimpactsonenvironment,suchastoxicityand

effectsofpollutantsintermsofglobalwarmingorbiodiversity,

arenottakenintoaccount.

Nomenclature

A area(m2₎

c costperexergyunit(s/J–J/J)

_C costofaﬂow(s/S–J/S)

CExC cumulativeexergyconsumption(J/J)

EE extendedexergy(J)

eec speciﬁcextendedexergycost(J/J)

eeK exergyequivalentofthemonetaryunit(J/s)

eeL exergyequivalentoflabor(J/h)

_Ex exergy(W)

LCOE levelizedcostofenergy

M massﬂowrate(kg/s)

M2 totalmonetarycirculationwithinasociety(s)

n rotatingspeed(rpm)

N number,amountof

Nwh cumulative number of work-hours generated by a

society

P pressure(kPa)

P power(W)

T temperature(K)

t time(s)

tec speciﬁcThermo-EcologicalCost(J/J)

TEC Thermo-EcologicalCost(J)

_Z costofasystem(s/S–J/S)

a

;

b

1stand2ndeconometricfactor

Subscript/Superscript CO2 carbondioxide D destruction eco economic Env environment ex exergetic fuel fuel gen generated I loss

i,j,l,k -thmaterialorenergyﬂux

in in_ﬂow

K monetarycapitals

L labor

om operating&maintenance

P product

R resource

tot total

Fig.1.Cumulativeinstalledphotovoltaicpeakpowerfrom1995to2020.IEAdata (Programe,2010).

2.exergeticcost:indicatesthecostastheamountofenergyand

materials (expressed by means of their exergy equivalents)

involved in theproduction of anysystem productsand it is

expressedinJex/Jex.

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2. Proposedapproach

Thenovelapproachproposedhereprovidesamoreconsistent

evaluation of both themonetary and primary resource impact

of the integration of renewable peak power capacity with a

pre-existingenergyconversionsystem.WithreferencetoFig.2,

foreverygivenscenariothemainstepsoftheproposedapproach

are:

1 data collection and thermodynamic modeling of the energy

conversionsystem;

2 thermoeconomicanalysis:

aeconomiccostanalysis:investmentandoperativecosts,FeedIn

Tariff,effectsofoff-designoperation,etc;

b total environmental cost analysis: calculation of the total

embodiedexergycostsofthesystemproductconsideringboth

renewable and non-renewable contributions. These costs

include direct resource consumption, plant equipment

production, operating and maintenance and externalities,

accordingtotheextendedexergycost(EE)paradigm;

c partialenvironmentalcostanalysis:calculationoftheembodied

exergycostsofthekWhrelatedtothenon-renewableprimary

naturalresourcesaccordingtotheThermo-EcologicalCost(TE)

paradigm;

3 determinationofthemaincostindexes:

amonetarycostofsystemproducts;

b embodiedexergycostsofsystemproducts,exergypaybacktime

(ExPBT),exergyreturnfactor(ExRF);

4 comparisonofeachscenariobasedonthecalculatedindicators;

Considering a system operating atsteady state asshown in

Fig. 2, once its energy and material ﬂow diagrams have been

properlyquantiﬁed,the general thermoeconomicmathematical

systemcanbewrittenasfollows:

X i _ExR;i¼ X j _ExP;jþ X l

_ExI;1þ _ExD;tot (1)

X i _CR;iþ _Ztot¼ X j _CP;jþ X l _CI;1 (2) _Ck¼ck _Exk (3)

Fig.2.ThedifferentSystemboundariesforthecalculationoftheeconomic,thermo-ecological,andextendedexergycost.

Eq. (1) is the exergy budget, written according to the

resource/product/waste (R/P/I)criterion and expressedin terms

ofexergyrates(J/s)assumingthesystemoperatesatsteadystate:a

genericsystem hasiresourceinputs,jproductoutputs,lwaste

streams and it is affected by a total amount of irreversibility

representedbytheexergydestructionrateterm.Eq.(2)isthecost

balanceofthesystem:itisalsowrittenaccordingtoR/P/Icriterion

intermsofcost(Jex/s,s/s),anditincludestheﬁxedcoststerm Z_ tot,

which representsallcoststhatarenotdirectlyassociatedtoan

exergy ﬂow, such as the costs of the system operation and

maintenance,oftheexternalities,etc.Theconceptuallinkbetween

the exergybudget (1) and thecost balance(2) is given bythe

constitutiverelation(3),thatexpressesthecostofagenericﬂowk

astheproductofitsassociatedexergyrate _Exk(J/s)timesitscost

per exergy unit ck (J/J, s/J). The application of this generic

thermoeconomicmathematicalformulationtoacomplexenergy

system with different components and multiple inputs and

outputsrequirestheintroductionofanumberofauxiliarycosting

relations. A detailed mathematical treatment of the numerical

approach tothermoeconomicsforindustrialprocesses,basedon

input-outputtechniquesandcanbefoundin(Queroletal.,2012;

Usónetal.,2012).

Table1

Referencegasturbineoperativeparameters.

Parameter u.m. Value

Nominalpower MW 70

Gasturbineadiabaticefﬁciency % 87.7

Compressoradiabaticefﬁciency % 88.0

Pressureratio – 12.7:1

Thermalefﬁciency % 32.7

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Inthefollowingparagraphs,thecostbalance(2)isrewrittento

accountrespectivelyfortheeconomicandtheembodiedexergy

costsofthesystemproduct.

2.1.Economiccostevaluation:standardthermoeconomicanalysis

In standard TA, the cost balance (2) and the constitutive

relations (3) are writtenreplacing the generic costswith their

respectivemonetarycosts:

X i _C_s;R;iþ _Zs;tot¼X j _C_s;P;jþX l _C_s;I;l (4) _Cs;k¼cs;k _Exk (5)

where _Csrepresentsthecostofthegenericexergyﬂow(s/s),csis

thespeciﬁccostoftheconsideredexergyﬂow(s/J),and _Zecoisthe

costnotdirectlyassociatedtoanexergyﬂow(s/s),suchasﬁxed

investments,operatingandmaintenance,feedintariff,taxes,etc.

cs;P¼_C_{_Ex}s;P

p

(6)

Theobjectiveoftheanalysisistocalculatethemonetarycostofthe

systemproduct,asexpressedby(6),foreachgivenscenario.As

expected,itwillbeshownbelowthatinthecasestudydiscussed

here,wherethereisonlyoneproduct,namelyelectricity,theTA

costof thesystemproduct isthesame asthelevelized costof

electricity(LCOE).

2.2.Embodiedexergycostevaluation:EEAandTECanalyses

Thecostbalance(2)andtheconstitutiverelations(3)arehere

rewritten by replacing the generic costs with their respective

exergeticcosts: X i _Cex;R;iþ _Zex;tot¼X j _Cex;P;jþ X l _Cex;I;l (7)

_Cex;k¼cex;k _Exk (8)

where _Cex represents theembodied exergy cost of the generic

stream(J/s),cexitsspeciﬁccounterpart(J/J)and _Zexistheembodied

exergycostnotdirectlyassociatedtoanexergyﬂow(J/s).Inthis

study,twodifferentcostparadigmswerecompared:theextended

exergy(EE)(Sciubba,2013;ChenandChen,2009;Daietal.,2012)

andtheThermo-EcologicalCost(TEC)(SzargutandStanek,2007;

Szargut et al., 2002). Without entering details, embodied

exergycosts arecomputed onthebasis ofvalidated numerical

procedures and databases developed in the ﬁeld of Industrial

Ecology(Suh2009;UNI,2001).

2.2.1.Theextendedexergycost

Extendedexergy(EE)isdeﬁnedasthetotalamountofequivalent

primaryexergy(expressedinJ)neededto generateaproduct.It

measures the primary resource consumption by means of a

systematicandreproduciblethermodynamicprocedurethatallows

for the formulation of an exergy budget for the so-called

externalities,i.e.,labor,capital,andenvironmentalremediationcost.

TheapplicationoftheEEcostingtechniquesiscalledextended

exergyaccounting(EEA),anditsdetailsaredescribedin(Daietal.,

2014;Roccoetal.,2014b;Sciubba,2013).WithreferencetoFig.2,

theEEofaproductiscomputedasthesumoftheprimaryexergy

consumptionofeachoneofthecostfactorsincommonuseamong

industrial economists: materials, energy (all forms), capital,

human labor, environmental remediation. The cumulative

consumptionofprimaryexergyinvolvedintheproductionofan

energyormaterialﬂuxisknownasCumulativeExergy

Consump-tion(CExC)(Böschetal.,2007;Fengetal.,2004).Fortheremaining

threecostfactors,EEproposesaspeci_ﬁcparadigm,seebelow.

Therefore,theEEofageneric“product”p(beitamaterialor

immaterialcommodity,aprocessortheentireproductionchain)

resultsfromthelife-cycleintegral:

EEp¼ Z LC CExCpþEEL;pþEEK;pþEEEnv;p dt (9)

Fromitsvery deﬁnition,it followsthat the“controlvolume”

considered by an EE analysis must be ideally expanded to

Parameter u.m. Value

Nominalpower kW 250

Moduleefﬁciency % 15.37

MaximumpowervoltageVmpr V 30.02

MaximumpowercurrentImpr A 8.33

TemperaturecoefﬁcientofPmax %/C 0.46

Modulesize mm 164099240

Fig.3. Monthlydistributionsoftheelectricenergyandofthepeakpowerdemands. Table3

Scenario’smainparameters.

Symbol Units 0(REF) 1(GTopt) 2(PV10) 3(PV30) 4(PV60)

PTG MWe 70 52.4 70 70 70

PPV MWe 0 0 10 30 60

APV m2 – – 58406 175219 350438

Nmd,pv n – – 40000 120000 240000

Table2

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encompassthemines,wells,quarries,etc.,ontheoneend,andthe

recyclingplantorthelandﬁllontheoppositeend.

EEA has been successfully applied to analyze and optimize

individualcomponentsas electrical wires(Rocco et al., 2014b),

energy systems as traditional gas turbines and biogas plants

(Dai etal.,2014;Sciubba, 2004),complexprocesses andsupply

chains(TalensPeiró et al., 2010), societalsectors, and complex

systems(Chenetal.,2014;ChenandChen,2009;Daietal.,2012;

Sciubbaetal.,2008).Itsobjectiveistodetermineforeachgiven

scenariothespeciﬁcextendedexergycostofthesystemproduct,as

expressedby(10),where _Cee;P isthetotalEEcostinJ/sandcee;P

speciﬁcEEcostofthesystemproduct,bothinJ/J.

cee;P¼_C_{_Ex}ee;P

P

(10)

The EEA method is based on two fundamental postulates,

neededtocomputetheextendedexergyequivalentsoflaborand

monetarycapital.

-1st postulate:inanysociety,aportionoftheglobalinﬂuxof

exergyresourcesExinis“used”todirectlyorindirectlysustain

theworkerswhogeneratelabor.Inexergyterms:

ExL¼aExin (11)

-2ndpostulate:theexergyﬂuxneededtogeneratethemonetary

circulationwithinasocietyisproportionaltotheloaborexergy.

Inexergyterms:

Exk¼

b

ExL (12)

whereboth

a

and

b

arenumericalfactorsthatdependonthetype

of societalorganization, thehistorical period, thetechnological

level,andthegeographiclocationofthesociety.Theirvalueisnot

assignedbythetheory,andmustbecalculatedfromeconometric

data:this“anchors”anEEanalysistothelocationandtheinstantof

Fig.4. MonthlydistributionofelectricalenergyandpowerforScenario2.

Fig.5. MonthlydistributionofelectricalenergyandpowerforScenario3.

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timeinwhichitwasperformed,andalsoconstitutesitsexplicit

validation,becauseif

a

and

b

assumevaluessuchthatthecost

balancesinwhichEqs.(11)and(12)areuseddonotcloseorleadto

unphysicalresults,thiswouldfalsifythetheory.

Theprimaryexergyequivalentofonework-hourisobtainedby

dividing the net total exergy ﬂux that goes into labor by the

cumulativenumberofwork-hoursgeneratedinagivenperiodof

time:

eeL¼

a

Exin

Nwh ½J=workhour

(13)

Similarly, to computethe primary exergy equivalent of one

monetaryunit, thetotal exergyﬂux allocatedtocapital bythe

secondpostulateisdividedbythecumulativemonetary

circula-tionmaintainedforthatperiodoftime(expressedbythemonetary

aggregateM2):

eeK¼

a

b

Exin

M2 ½J=s (14)

Adetaileddiscussionabouttheseexergyequivalentsisprovided

in(Sciubba,2011),whiletheirvalues calculatedfortheItaliansociety

in2010canbefoundin(Roccoetal.,2014b;Sciubba,2011).

Finally,theEEAevaluatestheenvironmentalexternalitywitha

remediation approach:theexergyequivalent amountof allthe

primaryresourcesneededtoeliminatetheeffects ofallsystem

ef_ﬂuentsontheenvironment,denotedEEEnv,iscalculatedasthe

additionalEEofthe(realorvirtual)processesneededtotreatthe

efﬂuentssothattheexergyofeachoneofthemisequalto0atthe

dischargesite.

2.2.2.TheThermo-EcologicalCost

AccordingtoSzargut(Szargut,2005;Szargutetal.,1988),the

future existence of the human species is endangered by our

excessive consumption of non-renewable natural resources;

therefore,aminimization ofsuchexploitmentwouldconstitute

anessentialsteptowardtheconservationofboththesocietyand

thenaturalenvironment.TheThermo-EcologicalCost(TEC)isthe

cumulative consumption of non-renewable primary natural

resourcesoverthelifecycleoftheconsideredsystemorprocess.

Inthisperspective,withreferencetoFig.2,TECtakesintoaccount

onlya subsetof theEEcostfactors,namelythenon-renewable

Fig.7.GTexergyefﬁciencyforeveryscenarios.

Table4

Valuesofthesystemoperativeparametersassumedforthesimulations.

Symbol Units Value Description

T0 K 298.15 Referenceenvironmenttemperature

p0 kPa 101.3 Referenceenvironmentpressure

hPV % 14 PVmodulenominalefﬁciency

hinv % 90 Inverterefﬁciency

hs % 90 PVAuxiliaresefﬁciency

Table5

Maindataforeconomiccostanalysis.

Symbol Units GTplant PVplant Description

Cequity % 20 Shareonequity Shareonequity

Cdebt % 80 Shareondebt Shareondebt

i % 6 Interestrateondebt Interestrateondebt

r – 0.01 EscalationofO&Mcosts EscalationofO&Mcosts

oplife,GT y 25 EconomicoperativelifeGT EconomicoperativelifeGT

oplife,PV y 25 EconomicoperativelifePV EconomicoperativelifePV

Zinv s/kWe 200 1.000 Investmentcost

Zom,ﬁxed s/(kW*y) 6 35 Fixedo&mcost

Zom,variable s/MWh 9 – Variableo&mcost

cfuel s/Nm3 0.3 – Fuelcost

cCO2 s/t 6 – CostofCO2emissions

TO s/MWh 113 Tariffaomnicomprensiva

eeMP s/MWh 60 Electricenergyaverageprice

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part.InawaycompletelyanalogoustoEE,theTECresultsfroma

doubleintegration(inspaceandtime)oftheabovementionedcost

factors.

Theideaunderlyingthepresentstudyistocomparetheresults

oftheapplicationofEEandTEcoststothesamesystem,tryingto

gain someadditionalinsight aboutthequantitative assessment

of the appropriateness of the integration of renewables into a

fossil-fuelledelectricitygenerationnetwork.WhileEEAreliesonly

onits“extendedexergycost”cEEasameasuringstick,TECproposes

twoindicators.

-Exergy pay back time (ExPBT) (Font de Mora et al., 2012),

measuredinyears,isdeﬁnedastheratioofthetotalTECofthe

producttotheyearlyprimaryexergysavingsduetoelectricity

generatedbyrenewables:

ExPBT¼Exinput

Exgen½years

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-Exergy return factor (ExRF): in complete analogy with the

energyreturnfactor(ERF)proposedin(Alsema,1997),anExRF

canbeformulatedastheratioofthetotalexergygeneratedby

therenewablesystemdividedthetotalprimarynon-renewable

exergyembodiedinitslifecycle(i.e.,itsTEC):

ExRFi¼

Exgen;i;tot

TECi ½J=J

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3. Descriptionofthecasestudy

Thesystem takenasa casestudyin thispaperconsistsofa

simplecyclegasturbinepowerplantoperatingundervariableload

conditions.Severalscenarioshavebeenanalyzedthatdifferfrom

each otherwithrespecttothegenerationmix:tomore loosely

re_ﬂecttheactualstateofaffairs,eachscenarioconsidersadifferent

PV installed power while the GT plant keeps the same ﬁxed

installednominalpoweroperatingasapeakingpowerplant.The

GE7EA(GE,2009)heavydutysimple-cyclegasturbineoperating

data have been used, and are shown in Table 1. The PV array

parameters (ET-P66250) used for the study are summarized in

Table2(ET-Solar,2011).ThecityofTrapani(Italy)wastakenasthe

designateduser:itsglobalannualenergydemand(125GWh/y)is

consideredasthereferenceinallscenarios,whilethemonthlyload

distributionandthecorrespondingelectricpowerdemandwere

Table6

Totaleconomiccostofthesystemproduct,inMs/y.

Symbol Units 0REF 1GTopt 2

PV10 3 PV30 4 PV60 Description

Cinv,GT Ms 14.00 10.48 14.00 14.00 14.00 Investmentcost,GT

Com,GT Ms/y 1.55 1.44 1.43 1.21 0.86 o&mcost,GT

Cfuel,GT Ms/y 13.06 11.88 11.95 9.66 5.91 Fuelcost,GT

,GT Ms/y 0.51 0.47 0.47 0.38 0.23 CO2emissionscost,GT

CP,GT Ms/y 16.39 14.76 15.10 12.47 8.19 Totalcostofelectricity,GT

Cinv,PV Ms – – 10.00 30.00 60.00 Investmentcost,PV

Com,PV Ms/y – – 0.35 1.05 2.10 o&mcost,PV

FIT Ms/y – – 0.66 1.98 3.96 Feedintariff,PV

CP,PV Ms/y – – 1.34 4.01 8.01 Totalcost,PV

CP,PV,FIT Ms/y – – 0.67 2.02 4.05 TotalcostwithFIT,PV

CP,GT+PV Ms/y 16.39 14.76 16.44 16.47 16.20 Totalcost,GT+PV

CP,GT+PV,FIT Ms/y 16.39 14.76 15.78 14.49 12.24 TotalcostwFIT,GT+PV

Fig.8.LevelizedcostofenergyforthesingleGTandPVplants.

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calculatedconsidering2393h/yofoperationthatcorrespondtoa

plantfactorofabout0.27(Fig.3).

Thisenergydemandissatisﬁedconsideringthefollowingﬁve

scenarios:

-scenario0:REF70MWGTpowerplant;

-scenario1:GTopt52MWGTpowerplant;

-scenario2:PV1070MWGTpowerand10MWPVpowerplant;

Scenario0isthecurrentsituation(referencescenario),whereas

Scenario 1 represents the optimized GT size for the required

energy demand, whose nominal power has been calculated

deﬁning theGTsizein ordertosatisfythedemandatfullload,

withthe currently available average plant efﬁciency. Scenarios

2_–4modeltheeffectsofanincreasingintegrationofthePVpower

plant. Table 3 summarizes the main parameters of the ﬁve

scenarios.

The resulting distribution of the electric powerand energy

production between PV and GT for each scenario is shown in

Figs.4–6.

Underthestrongsimplifyingassumptionthatboththehourly

distributionandtheintensityoftheelectricaldemandremainsthe

sameasin thereferencescenario ofFig.3,it is apparentfrom

Fig.4–6thateveryincreaseinthePVinstalledpowerforcestheGT

planttooperatefartherfromitsnominalcondition,thusreducing

itsefﬁciency.

Sincetheaimofthisstudyistocalculatetheeffectofthe

off-designonthemonetary,extendedexergyandThermo-Ecological

Costofelectricity,theﬁvescenarioshavebeensimulatedandthe

respectiveinstallationandoperationcostshavebeenseparately

calculated for each scenario. Simulationshave been performed

by the CAMEL-ProTM ₍_Sapienza, ₂₀₀₈_), _a _C#, _{object-oriented,}

modularprocesssimulatorequippedwithauser-friendlygraphical

interface.Thesystemisrepresentedasanetworkofcomponents

connected bymaterial and energystreams;each componentis

characterized byitsown (local)setof equationsdescribing the

thermodynamicchangesimposedonthestreams.TheCAMEL-Pro

GTcomponentsmodelsusedinoursimulationsarediscussedin

AppendixA,detailsofPVcomponentinCAMEL-Proarepresented

in(Espostoetal.,2010).

Powerproduction ofPV plants, andthus theireffectsonGT

operation, have been computed on a monthly based average,

neglectingtheeffects of thedailyvariation in solarirradiation.

Since themainpurposeofthepaperis toapplytheevaluation

framework ratherthan toperform GTand PV systemsdetailed

simulations, the simpliﬁcation made above can be considered

morethanacceptable.

Allsimulationswereperformedassumingthattheprescribed

energydemandwastobesatisﬁedonlybythetwoplantsworking

inanideallyisolatedenvironment.Suchanassumptionisnottoo

farfromreality,becauseinactualcasesthePVisoperatedatits

maximumcapacityduringpeakinsolationhours(from10a.m.to

2p.m.)whiletheGTcoverstheremainingportionofthedemand

(Barnhametal.,2012).

WhatexplainedaboveisevidentlyshowninFig.7,whereit

appearsclearhowtheincreaseoftheinstalledPVpoweraffects

negativelyGTefﬁciency.

In Table 4 themain operative parametersfor the simulated

scenariosarereported.Inallsimulationsofthepresentworkthe

PVmoduleselectedasabenchmarkisthe_“ETSolarET-P660220

220Wp” [17]and ayearlyperformance lossof0.2% intermsof

producedenergy.Theclimatedata,suchasglobalirradiance(in

Symbol Units 0REF 1GTopt 2

PV10 3 PV30 4 PV60 Description LCOEGT s/MWh 130.7 117.7 134.0 142.8 166.6 LCOE,GT LCOEPV s/MWh – – 105.1 105.1 105.1 LCOE,PV

LCOEPV,FIT s/MWh – – 53.1 53.1 53.1 LCOEwithFIT,PV

LCOEGT+PV s/MWh 130.7 117.7 131.1 131.4 129.2 LCOE,GT+PV

LCOEGT+PV,FIT s/MWh 130.7 117.7 125.9 115.6 97.6 LCOEwithFIT,GT+PV

Table8

Maindataforembodiedexergyanalysis.

Symbol Units EE TEC Description

Naturalgas MJex/MJex 1.07 1.07 Naturalgas,highpressure,atconsumer/RERU

GTplant MJex/MWe 608.000 518.000 Gasturbine,10MWe,atproductionplant/RER/IU

PVpanel MJex/m2 4.990 3.360 Photovoltaicpanel,single-Si,atplant/RER/IU

BoS MJex/m2 1.470 1.001 Opengroundconstruction,onground

Inverter MJ/kWe 5.260 5.020 Inverter,500W,atplant/p/RER/I

eeL MJ/h 85.33 – Exergyequivalentoflabor

eeK MJex/s 4.58 – Exergyequivalentofthemonetaryunit

Table9

ResultsoftheEEanalysis.

Symbol Units 0 REF 1GTopt 2 PV10 3 PV30 4 PV60 Description

Cee,GT GWh/y 0.47 0.35 0.47 0.47 0.47 Plantcost,GT

Cee,PV GWh/y – – 19.30 57.91 115.82 Plantcost,PV

Cee,NG GWh/y 480.58 436.96 439.62 355.21 217.39 Naturalgascost,GT

Cee,S GWh/y – – 103.03 309.09 618.18 Solarradiation,PV

Cee,K GWh/y 2.07 1.88 2.74 4.05 5.98 Capitalscosts,GT+ PV

Cee,L GWh/y 0.51 0.51 0.68 0.68 0.68 Laborcosts,GT+PV

Cee,P GWh/y 483.63 439.71 565.85 727.42 958.53 Productcost,GT+PV

cee,P – 3.86 3.51 4.51 5.80 7.64 sp.productcost,GT+PV

Table7

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kW/m2_), _ambient _temperature _and _wind _speed, _for _the _city _of

Trapaniareobtainedfrom(UNI,1994).Inthefollowingparagraphs

theimpactoftheintegrationofPVandGTandtherelateddecrease

ofGTefﬁciencyonelectricityeconomicandexergeticcostwillbe

discussedindetails.

4. Economicandembodiedexergycostanalysis

Inthisparagraphtheeconomicandembodiedexergycostsofthe

exergyunitproducedbythewholesystemiscomputed,accordingto

thethermodynamicmodelsdevelopedforeverysinglescenario.

4.1.Evaluationofthelevelized(economic)costofenergy(LCOE)

The LCOEis an evaluation of the life-cycle energy cost and

life-cycleenergyproduction:it isexpressedintermsofs/MWh

andprovidesacommonwaytocomparethecostofenergyacross

technologiesbecauseit takesinto accounttheinstalled system

price and associated costs such as _ﬁnancing, land, insurance,

transmission, operation and maintenance and depreciation,

among otherexpenses. Other factorsthat affects the costslike

carbonemissiontax,FeedInTarifforsolarpanelefﬁciencycanalso

betakenintoaccount(Campbelletal.,2008).Notethatthemethod

is not suitable for determiningthe cost efﬁciency of a speciﬁc

powerplant:inordertodothat,aﬁnancingcalculationmustbe

completedtakingintoaccountallrevenuesandexpenditureson

thebasisofacash-ﬂowmodel.

LCOE allows alternative technologiesto be compared when

differentscalesofoperation,investmentoroperatingtimeperiods

exist: in this paper, it is adopted to compare the economic

performancesof differentintegrated GTand PVpowersystems.

ThecalculationoftheLCOEforagenericpowersystemconsistsin

theratiobetweenthetotaleconomicexpendituresdividedbythe

total energy produced in the operative life of the plant. For a

genericenergysystemwithaneconomicoperationallifetimeof

Nyears,LCOEcanbecomputedaccordingtothefollowingrelation

(Brankeretal.,2011): LCOE¼I0þ

S

N t¼1At=ð1þ iÞtDS

S

N t¼1Et;el=ð1þ iÞt (17)

whereI0istheinitialinvestmentexpenditureins,Et;elisthetotal

amountofenergyproducedinMWh,iistherealinterestratein%

and At isthetotal costsattheyeart-th,computedasthesum

of ﬁxed and variable costs for the operation of the plant,

maintenance, service, repairs, taxes, and insurance payments.

Forsakeofsimplicity,thedepreciationtaxshieldDandthesavage

valueofanyphysicalassetsattheendoftheoperativelifeSare

hereneglected.Relation(17)isadoptedheretocomputetheLCOE

ofboththeGTandthePVplant;theglobalLCOEofthecompound

systemresultsbyaweightedaveragewithrespecttotheenergy

producedbyeachplant.TheLCOEhasbeencomputedalsotaking

intoaccounttheeffectofthecurrentItalianFeedInTariff:forthe

consideredPVplant,theFITresultsasthedifferencebetweenthe

so-called Tariffa Onnicomprensiva of 113s/MWh and the local

electricenergy marketprice.InTable 5themaindataforLCOE

evaluationareresumed.

Fig.10.EmbodiedexergycostsforthesingleGTandPVplants.

Fig.11.EmbodiedexergycostscomparisonfortheintegratedGT+PVsystem.

Table10

ResultsoftheTECanalysis.

Symbol Units 0 REF 1GTopt 2 PV10 3 PV30 4 PV60 Description

Ctec,GT MWh/y 0.40 0.30 0.40 0.40 0.40 Plantcost,GT

Ctec,PV MWh/y 0.00 0.00 4.94 14.82 29.64 Plantcost,PV

Ctec,NG MWh/y 480.58 436.96 439.62 355.21 217.39 Naturalgascost,GT

Ctec,P MWh/y 480.98 437.26 444.97 370.43 247.44 Productcost,GT+PV

ctec,P – 3.84 3.49 3.55 2.95 1.97 sp.productcost,GT+PV

ExPBT Y – – 2.49 2.39 2.20 Exergypaybacktime

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AswecanappreciatefromTables5and6fromFigs.8and9

LCOEGT+PVdoesn’tundergotosubstantialvariationsbecausethe

reductionofthefuelcost(Cfuel,GT)isbalancedbythegrowingPV

investment cost (Cinv,PV). Nevertheless, the lowest LCOEGT+PV

correspondstotheScenario1(GTopt).

Analyzing theLCOEwiththe effectof theFIT trend, named

LCOEGT+PV,FIT, we spota differentbehavior and its minimum is

located in correspondence of Scenario 4: the effects of FIT

overcome the ones related to increase of the global PV+GT

investmentcost.

Note that LCOE for theconsidered PV technology results as

0053_s/kWh,asshown inFig. 8and Table 7. Thisvalueresults

lower with respect to the average LCOE of PV technology in

Europeancontextof0.06–0.08s/kWhandthisismainlyduetothe

effectoftheItalianFIT.(PVPS,2012)

4.2.ExtendedexergyandThermo-EcologicalCostofelectricity

Primaryandsecondarydataandassumptionsonmaterialsand

processes needed toevaluate theembodied exergy costs were

collected from recent published studies, manufacturer’s data

sheetsontherelevantprocessesinvolvedintheproductionofthe

systemsandfromtheecoinventdatabasepublishedbytheSwiss

Center for Life Cycle Inventories (http://www.ecoinvent.org)

(Battisti and Corrado, 2005;Doneset al.,2007; Halasah et al., 2012;Jungbluth,2005;Jungbluthetal.,2012;Raugeietal.,2007; Stoppato,2008).

Alltheelectricenergyinputsintheprocessesinvolvedinthe

ﬁnal electricity production were converted to primaryenergy

units based on the average European energy production mix

(withanUCTEaverageefﬁciencyvalueforelectricityproduction

of 32% (Dones et al., 2007)). The exergetic cost of ecological

remediationfortheextendedexergyanalysishasbeencalculated

convertingCO2emissionscostinitsequivalentexergybymeans

ofexergyequivalentofcapitalsinEq.(14).Table8presentsallthe

main aggregated processes and input data adopted for the

embodied exergy analysis; Appendix B reports all the details

aboutthebalanceof system(BoS)for thephotovoltaicsystem.

On the other hand, the Thermo-Ecological Cost analysis

reportedinTable10showsanoppositetrend:thescenariowith

thelowestThermo-EcologicalCost istheone withthehighest

PVintegration,thatresultsalsointhelowestExPBTintermsof

years.

Theresultsobtainedbytheextendedexergyanalysisfortheﬁve

scenariosarecollectedinTable9andtheglobalresultisshownin

Fig.10.Theexergeticcapitalcost,Cee,Kincludesonlythosecosts

whicharenotobtainedfromtheecoinventdatabase,suchasthe

portionofcostsrelatedtotaxesandincentivesconvertedinexergy

considering the eeK coefﬁcient reported in Table 8. The Cee,P

stronglyincreaseswiththeintegrationofPVmainlybecauseofthe

highexergy contentof solarirradiation.Embodiedexergycosts

comparison for the integrated GT+PV system is presented in

Fig.11.

As showed in Table 10, exergy return factor (ExRF) of the

considered photovoltaic plant results in a constant value of

2.57forscenarios2,3, and4.Thisresultis inlinewithresults

obtained by Weißbach et al. in (Weißbach et al., 2013) and

resultingin 1.6 (small differences between energyand exergy

values for primary resources are not considered here for

simplicity). Weißbach et al. computed such value assuming

thatPVoperatesin Germanywithsameaveragecharacteristics

oftheanalyzedphotovoltaiccell,andsuggestthatforlocations

insouthEurope,theEROIcouldbeabout1.7timeshigherdueto

thehighersolarirradiation.

5. Conclusions

Thispaperpresentsacomparativeevaluationoftheimpactof

theprogressiveintegrationofa PVplantwithagas-fuelled gas

turbinepowerplantintermsofeconomiccostandglobalresource

consumption.

Theresultsare–notsurprisingly–completelydifferentforthe

adoptedcost functions:a monetaryapproach that includesthe

CO2-savingincentives indicatesScenario4(high- PVgeneration

mix) astheoptimum.Anextendedexergyanalysis,thatseemsto

represent the best approach from a “resource-saving” point of

view,identi_ﬁesScenario1(optimaldesignofGTsystemwithno

PV contribution) as the optimum, while the minimum

Thermo-EcologicalCost(thatprivilegesthelowestconsumption

of non-renewable primary resources) indicates Scenario 4

(maximumPVcontribution)astheoptimalone.

Itisinterestingtonoticetheinnovativeapproachofextended

exergyaccountinginincludingtheeffectsofexternalitiesintermsof

primarynaturalresourcesconsumption,anditwouldbeinteresting

to include non-renewable resources consumption due to such

externalitiesalsointhetraditionalTECapproachinafuturework.

AppendixA.

Theoff-designgasturbinemodel

InCAMEL-ProTM_the_simple-cycle_GT_group_could_be_simulated

assemblingitsmaincomponents:anaircompressor,agasturbine,

acombustionchamberand apowersplitter. Themodelofeach

component is based on energy and mass balances coupled

with appropriate expressions for the heat transfer coefﬁcients

calculation, thermodynamicconstants, and material properties.

Thebalanceequationsarewrittenasmacroscopicbalances,inthe

formofﬁniteequations.Inordertoproperlyanalyzethedifferent

scenariositwasnecessarytoconsidertheoff-designbehaviorof

gasturbineandcompressorthatallowustocalculatetheeffective

efﬁciencyoftheglobalGTpowerplant.

The characteristic equations describing the gas turbine

behavior at part load included in CAMEL-ProTM _have _been

adapted from publicdomain literature sources,and are based

onsemi-empirical calculationmethods(Zhangand Cai,2002).

Thefollowingperformanceformulasforcompressorandturbine

areproposed. Compressor:

b

_¼_c 1M2þc2Mþc3þwith

b

¼

_b

b

des

h

_¼_½1_c 4ð1kÞ2 k Mð2 k MÞwith

h

_¼

h

ad;off

h

ad;des

whereM¼m=Gdeswithm¼mi

ffiffiffiffiffi Ti p =piand Gdes¼mdes ffiffiffiffiffiffiffiffiffi Tdes p =pdesk¼n= ffiffiffiffiffi Ti p ndes= ffiffiffiffiffiffiffiffiffi Tdes p

c1,c2,andc3arecoefﬁcientsdependingoncompressorreduced

velocitykanditsgeometry.

Turbine: g Gdes¼

a

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1=

b

2 1 1=

b

des1 s and

h

½10:3ð1kÞ2 _g_=Gk des 2_g_=Gk des where:g¼mi ffiffiffiffiffi Ti p =pi,Gdes¼mdes ffiffiffiffiffiffiffiffiffi Tdes p =Pdes,

a

¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1:40:4k p andk¼ n ffiffiffiffiffiTi p ndes ffiffiffiffiffiffiffiffiffi Tdes p AppendixB. SeeTableB.1

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TableB.1

BalanceofSystem(BOS)for1m2_of_PV._Data_from₍_Jungbluth_et_al.,₂₀₁₂_).

Balanceofsystems(opengroundconstruction,onground) 1 m2

Materials

Packaging,corrugatedboard,mixedﬁbre,singlewall,atplant/RERU 0.0846 kg

Aluminium,productionmix,wroughtalloy,atplant/RERU 3.98 kg

Polyethylene,HDPE,granulate,atplant/RERU 0.000909 kg

Polystyrene,highimpact,HIPS,atplant/RERU 0.00455 kg

Chromiumsteel18/8,atplant/RERU 0.247 kg

Reinforcingsteel,atplant/RERU 7.21 kg

Concrete,normal,atplant/CHU 0.000537 m3

Processes

Sectionbarextrusion,aluminium/RERU 3.98 kg

Sectionbarrolling,steel/RERU 6.15 kg

Wiredrawing,steel/RERU 1.06 kg

Zinccoating,pieces/RERU 1.56 m2

Zinccoating,coils/RERU 1.09 m2

Transport,lorry>16t,ﬂeetaverage/RERU 0.217 tkm

Transport,freight,rail/RERU 5.14 tkm