MODELING AND ANALYSIS OF GRID CONNECTED FUEL CELLS (Fes) AS A DISTRIBUTED ENERGY RESOURCES

9 Pages·01.995 MB·English

by  Dr. Eng. Mohamed EL-Shimy (Ain Shams University)

THEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6)
MODELINGANDANALYSIS OFGRID CONNECTED FUELCELLS (Fes)
ASADISTRIBUTED ENERGYRESOURCES
M.EL-Shimy
AinShamsUniversity-FacultyofEngineering
Cairo-Egypt
Although manyresearchershaveproposedwide
Abstract-MaJorteehnlealissuesrelatedtoIncreased scaleuse ofIXlS in distribution systems as a cost
reliance on distributed generation systems (DGS) In effective approach to meet growing demand,
distribution I)'ltenal Including lack of: appropriate improvesystemreliabilityand limit environmental
dynamic ......,..ellablecontrol approaches, efficient
impacts, etc. Still,·there are numerous technical
dispatch methods, and control strategies to facilitate
issues related to increased reliance on DOS in
the connedlon of distributed generation resources to
distribution systems [25]· including lack of: (1)
distribution networks. Among available types ofDGS,
fuel eelIIshow particular promise as they can opente appropriate dynamic models, (2) reliable control
on multiple ,.... with low emissions, high emdency, approaches, (3) efflcientdispatchmethods, and (4)
and high reliability. This paper presents a simplified control strategies to facilitate the connection of
dynamic model for SOFC. Moreover, constant-power distributed generation resources to distribution
and const8nt-earrent control strategies are modeled networks.
and analyzed through dynamic simulation of Fe-grid
Major environmental-friendly Distributed
interconnedloL Dynamic limits of Fevariables are
Generation (DO) technologies can be classified as:
considered .. presented model and unique electrl~.1
propertiesofPCs.rediscussed. micro-turbines, fuel cells, solar/photovoltaic
systems, windturbines,and energystoragedevices.
AmongsuchDOS,microturbinesandfuelcellsshow
Index TelMJ-Distributed generation, fuel eells, -particularpromiseasthey can operateon multiple
Nernst eq••tI.cyn.I,. eleetreehemleal react!ons, power fuelswithlowemissions, highefficiency, andhigh
conditioni... mlc modeling, converters, dynamic reliability.
simulation,II.allak. Fuel cells [26]-(28] produce power
electrochemicallybypassingahydrogengasoveran
I. INTRODUCTION anode and oxygen from air over a cathode, and
DECENTLY, newadvances in powergeneration introducing an electrolyte in between to enable
.1.'-techno'.andnewenvironmentalregulations exchange of ions. The hydrogen can be supplied
encourage a significant increase of Distributed directly, or indirectly produced by reformerfrom
GenerationSystems(DOS)aroundtheworld.Hence, fuelssuchasnaturalgas,alcohols,orgasoline.Each
nesareexpectedtobecomemoreimportant inthe unit ranges in s!~~ from 1-250kW or largerMW
future generation system. In general,DOS can be size. Fuel cells feature the potential for high
defined as electric power generation within efficiency(35%-60%),loworzeroemission, quite
distribution networks oronthecustomersideofthe operation,andhighreliabilityduetolimitednumber
network[1]. of moving parts. The major disadvantage of fuel
The use of distributedgeneration systems under cellsistheirhighcost.
theSoo kW levelisrapidlyincreasing recentlydue The effectiveness of ion exchange process is
to technol. improvements in small generators, mainly dependent on the electrolyte to create the
powerelectronics,andenergystoragedevices. DOS chemical reactivityneeded.Therefore,fuelcellsare
can beapplied as: standby, standalone [2] - [6], usuallyclassifiedaccordingtotheelectrolytetypeas
grid-interconnected [3], [5], [7]-[11], cogeneration follows: PhosphoricAcid Fuel Cell (pAFC), Solid
['2], etc. Moreover, DOS can have manybenefits Oxide Fuel Cell (SOFC), Molten Carbonate Fuel
[13],[24]suchas:powerqualityimprovement, fuel Cell (MCFC),AlkalineFuel Celt (AFC),Polymer
flexibility, environmental-friendly and modular ElectrolyteFuelCell(PEFC),etc[29].
electric generation, load management, increased Fuel cells (FCs) have several uniqueproperties
stability [14],[20],-cost savings[15], [16],voltage from modelingpoint of view [25]. The electrical
regulation improvements [17], increasedreliability response time ·of the power section of FCs is
[18], [19),. (23), power loss reduction [21], generally fast, being mainly associated with the
expansion~ement [22],etc. speedat whichthe chemicalreactionis capableof
restoringthe charge that has been drainedby the
load.Conversely, thechemicalresponsetimeofthe
Mohamed EL·Shimy Mahmoud, PhD,is withAinShams
University, Faculty of Engineering, Cairo, Egypt (emails: reformeris usuallyslow,beingassociated withthe
[email protected]@ecemail.uWBterloo.ca)
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71lEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6j
timefor the fuelcell stackto modifythe chemical fuel cells are listed in the order of approximate
reactionparameters after a change in the flow of opemting tempemture, ranging frOm -80 C for
reactance. PEFC,-IOO CforAFC,-200 CforPAFC,-6S0 C
for MCFC,-800 C for ITSOFC, and 1000C for
II. FUELCELLMODELING TSOFC[29].
Independent on electrolytetype,all types of the
A.FuelCellOperatingTheory fuel cells produce electricity by electrochemical
A fuelcellisdefinedasanelectricalcell,which reaction of hydrogen and oxygen. Oxygengas is
unlikeotherstomgedevicescanbecontinuouslyfed obtainedfromcompressingair whilehydrogengasis
withafuelinorderthattheelectricalpowercanbe indirectly obtained from the reformer using fuels
maintained. The fuel cells convert hydrogen or suchas natumlgas,propane,metb8l'i~1, gasolineor
hydrogen-eontaining fuels, directly into electrical fromtheelectrolysisofwater,Fig.2,[26],[30]-[32].
energy, heat,andwaterthroughtheelectrochemical
reactionofhydrogenandoxygen,Fig.l.
Fig.2.Fesystemconfiguration.
As shownin Fig. 2, a powergeneration FC has
threemainparts:
I. Reformer (Fuel processor): that
convertsfuelssuchasnatumlgas,
propane, methanol, gasoline or
fromthe electrolysis of waterto
hydrogen.
Fig.1.Fuelcell:principalofoperation.
2. Stack (power section): that
genemte, electrochemically,
AsshownintheFig. I, hydrogen fuelentersthe
electricityandheat
anodeandcombines withoxygenionsto formfour
3. Power Conditioning Unit (pcu):
electrons andfuelexhaustwhichismainlysteam(or
that converts the DC power
water). Theseelectrons areforcedthrougha loadas
outputfromtheFCtoappropriate
electricity(power)andenterthecathodetocombine
ACpower.Thisprocessincludes
withoxygen (thatprovidedby air) to producethe
current, voltage and frequency
oxygen ionsthat flowthroughthe electrolyte. The
control.
overallreactionoftheFCtakestheform:
Since SOFC runs at the highest operating
2H (gas)+O (gas)...
2 2 temperature of all FCtypes, it isconsidered inthis
2H20+Energy(Electricity+Heat) (I} paper. it is more suitableto beused in combjned
cyclepowerplants[29].
Theoperatingtemperatureandusefullifeofafuel
cell dictate the physicochemical and B.ModelingofSOFC
thermomechanicalpropertiesofmaterials·usedinthe A basic SOFC model power section dynamic
cellcomponents(i.e.,electrodes,electrolyte,current modelusedforperformance analysis·duringnormal
collector, ete.), The operatingtemperature of fuel opemtionis presentedin [33].Basedon themodel
cells is mainly dependent on the used electrolyte providedin[32],somecontrolstmtegies ofthefuel
type.Therefore, Themostcommonclassification of cellsystem,responsefunctions offuelprocessorand
fuelcells is by the type of electrolyte used in the powersectionare addedto modeltheSOFCpower
cells and includes I) polymerelectrolyte fuel cell
generation system[2S].Dynamic modelsforMCFC
(pEFC),2) alkalinefuelcell(AFC),3) phosphoric
asahightemperatureFCcanbefoundin[34],[3S].
acidfuelcell(pAFC),4)moltencarbonatefuelcell
Herein, based on the models provided in [33],
(MCFC), S) intermediate tempemture solid oxide
[2S], constant-power and constant-eurrent control
fuelcell(ITSOFC), and6) tubularsolidoxidefuel
stmtegies ofthefuelcellsystem, responsefunctions
cell(TSOFC).These
of fuel processor and power section, and grid-
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71lEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6j
connection are added in this paper to model the
SOFCpowergenerationsystem. (4)
Fig.3 showsa detailedblockdiagramof theFC
system..For modelingsimplification purposes, the
whereVanisvolumeoftheanodechannel;Risideal
followingassumptionsareconsidered:
gasconstant(= 8.314J/mollK); Tistemperature(K);
1. Thegasesareideal.
nH1ismolesofhydrogenintheanodechannel.
2. Thefuelcellisfedwithhydrogen
andair.
Takingthelittittle-derivativeof(4)resultsin:
3. The electrodechannels are small
enough that the pressure drop
acrossthemisnegligible.
4. Theratioofpressuresbetweenthe
insideandoutsideoftheelectrode
channelsislargeenoughtoasswne
chokedflow. The hydrogen flow can be separated to the
5. Thefuelcelltemperatureisstable. followingthreeparts:
6. TheNemstequationapplies.
7. Ohmiclossesareonlyconsidered. (6)
qii:
where ishydrogenmolarflowoutoftheanode
channel.
Substituting(6)in(5) weget:
Fig.3.DetailedblockdiagramofFCsystem
(7)
Fuelutilization factor(U; is defmedasthe ratio
of the. amount of hydrogen (molar flow rate,
Basedontheelectrochemicalrelationships,the
kmollsec) thatreactswiththe oxygenionsoverthe
amountofhydrogenthatreactscanbecalculatedby:
amountofhydrogenenteringtheanode(molarflow
rate,kmollsec).
N
q' =_0I=2K I (8)
2F
H2 r
(2)
whereN"illnumberofcellsinthestackseries;Fis
Faraday's constant (= 96487 C/mol); I is stack
where q~2 ishydrogen molar flowinto the anode current;andK,ismodelingconstant(=NDI(4F).
qH
channel; ishydrogen molarflowthatreactsin Based on the assumption that the electrode
2 channels are small enough that the pressure drop
theanodechannel.
acrossthemisnegligible,then
Byconsideringthatthemolarflowrate(q)ofany
gas through thevalve is proportional to its partial (9)
pressure(P),thefollowingequationsarederived:
Using (9), (3), and (8) equation (7) can be
q
writtenas:
-=K (3)
p
whereKisvalvemolarconstantsforthegas. (10)
Basedontheassumptionofidealgasses,the idea1
gas lawisusedto find the partialpressuresofthe
gassesflowingthroughtheelectrodes.Therefore,for
hydrogenwehave:
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THEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6)
V (o~ed fuel). the cells may suffer from fuel
where1" = an is the response time for starvationandbepermanentlydamaged.
82 RTK
8 To meetthe aforementioned usagerequirements,
2 an
hydrogenflow. the"basictargetoftheFCcontrolleristomaintain
optimalhydrogen utilization, Uopt arowd 8S%[29],
Partialpressures ofwater(steam) andoxygencan [25], [31]. Fuel processor controller controls the
befound in similarwayasthatdoneforhydrogen, amountofhydrogeninputtothefuelcellforoptimal
andaregivenby: . fuel utilization, for simplicity its dynamics canbe
representedbythefollowingequation:
(11)
dq:z =_1(2KzI _q: )
(14)
dt 'rJ Uopt 2
.where1j isthefuelprocessorresponsetime.
c.
FuelCellCurrentControl
Based on (8), it is shownthat the reacting fuel
where rH20 ,t"~ areresponse timesforwaterand quantity, tfH1, is directlyproportional totheoutput
oxygen respectively; rHO - Ratio pf hydrogen to current,I. Hence, the fuel utilization is translated
oxygen. intoacorrespondingoutputcurrentdemand:
U ,
=
Thestackoutputvoltagecanbe describedbythe I.mond 2K qHz (IS)
Nemst equation (~9], [36] including stack ohmic r
losses which are due to the resistance of the
electrodes andtheresistance oftheflow of0.2ions Thelimits ofthe utilization factor aretypically
throughtheelectrolyte. from 0.8 to 0.9 Le, Ujin <UI <Uj' [29],
[2S], [37]. Therefore, for an acceptable current
controlschemeofSOFCthefuelutilizationdynamic
limitsshouldnotbeviolated.
FC current control strategies can be either
constant-power control or constant-current control.
The current dynamic equation for constant-power
where VisfuelcelloutputDCvoltage; Eo is cell
controltakestheform:
ideal standard DC potential; and r is ohmic
resistanceofthestack.
dI (Pre! -I)
=_1 (16)
.baTsehdeonidGeailbbstsafnrdeearedneprogtyen[2ti9a]l,E[D36o]fis1H.222/092VFfCosr dt t"e V
liquidwater productand 1.18V for gaseouswater and The current dynamic equation for constant
product. currentcontroltakesthefonn:
Fuelprocessing Is-defined astheconversion ofa
commerciallyavailablegas,liquid,orsolidfuel(raw (11)
fuel)toafuelgasrefonnatesuitableforthefuelcell
anode reaction. Fuel processing encompasses the
cleaning andremoval ofharmful speciesintheraw whereYo istheinitialFCDCvoltage; andre isthe
fuel,theconversion ofthe rawfuelto the fuelgas electricalresponsetime.
refonnate, and downstream processing to alter the
fuel gas refonnate accordingto specific fuel cell In either control strategies the fuel utilization
requirements.
dynamicslimitsshouldnotbeviolated.Basedon(8)
An important operating variable of FCs is the
thecurrentdynamic equations incaseofdemanded
reactantutilization factor, U] The utilization factor
VIthat exceed UImaXimum or minimum dynamic
placesoperational constraints on the FC system. If
limitsaregivenby:'
the fuel utilization drops below a certain limit
(underosedfuel), thecellvoltagewillriserapidly.If
thefuelutilization increases beyonda certainvalue
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71lEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6)
purpose of dynamic simulation it is sufficiently
accurate to reprd the network dming
electromechanical transients as dynamic-free [39].
The converterAC outputvoltageandconsequently
reactivepowerflowcanbe controlledbytheULTC
01.FUELCELLGRIDINTERCONNECTION transformer and by adjustment of the converter
Thegoalofcommercialfuelcellpowerplantisto amplitude modulation index m. The process is
deliverusableACpowertoanelectricaldistn"bution describedbythefollowingtransferfunction:
.system. This goal is accomplished through a
subsystem that hasthe capabilityto deliverthereal
m = K",
power(WBtt$)andreactivepower(VARS)to a load (l9}
V,.j-V" l+st'",
in standalone installation or to a utility's grid. The
power conditioning equipment of a fuel cell
installationhastwomainpUIpO$eS [29]: Suchthat:
(20)
1. Adaptthe fuelcelloutputto suit
the electric:al requirements at the
pointofpowerdelivery. .whereK,. isgainofthe voltagecontrolloop; ~,. is
2. Provide power to the fuel cell time constant of the voltage control loop; V, is
systemauxiliariesandcontrols. networkvoltage;and Yrrf isre~ voltagetothe
voltagecontroller.
In the iDitial phase of systems analysis, the
aspect
important of power conditioning is the TheconverteroutputvoltageV,isgivenby:
efficiencyoftilepowerconversionandlncorporstion
ofthe small power loss into the cycle efficiency. ~ =0.6124mV (21)
jc
Powerconditioning efficiencies typicallyareon the
orderof94to981.'....
Based on phasor diagram of Fig. 5, the phase
When a fuelcellpowerplantis usedforelectric
angle of the converter output voltage, ~ can be
utility applications, the inverter is the interface
writtenas:
equipment betweenthefuel cell and the electrical
network. The inverter acts as the voltage and
frequency adjusterto the final load. The interface 8. =8. +sin-I IfcX' ) (22)
conditions require the following capabilities: (
'" 0.6124mV"
synebronization to the network, output voltage
regulation typically ± 2%, output frequency
regulation typically ± 0.5%, protection against
system faults, suppression ofharmonics sothatthe
I
power quality is within the IEEE-S19 harmonic I
I
limits requirements, stable operation, VAR control
I
mustalsobeaddressed,etc. pi
-LXi
Typically, the fuelcell systemis interfaced with
theACgridofthemediumtensiondistributionlevel ~
I
via a converter/transformer unit Fig. 4 shOWI a I
I
blockdiagramforFC-gridconnectionwithavoltage ~ I
sourcesin-PWMconverter[38]. .L..-.L.-...;.....__---4~_---:X _..J
l:; ~ t
Fig.5.Phasor-diagnun.
Assmnmglose-lessconverter/transfonner,thereal
powerinjectedto the networkis equalsto the FC
output DC power. Therefore FC-Gridpower flow
equationsare:
Fig.4.FC-Gridconnectionblockdiagram.
(23)
Generally, convertershaveveryfastresponseand
can be considered inertia-less equipment For the
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TIlEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6)
Q =V;V" COS(O _e)_~2 (24) 0.15 ..j I-PH2 --P02 -..-- PH2.P021
"X X
t"
t t
G
Based on (21),(23) and the phasor diagramof 0.1
Fig.5,thereactivepowerflow(24) canbewrittenas
{O.o5
a function of the converter amplitude modulation
indexasfollows: I
0
+--"""T""-........
·0.05 ---,--r---.---i
o 20 40 110 80 100 120
lIml••to
Fig.6-c.ResponseofPH" POz, andPHrP
01
0.0009-r--------------,
IV.SIMULATIONSTUDIESANDRESULTS
The overall Fe-Grid interconnection model of
Fig. 4is built on SIMULINK~. The SOFCstack G0.0008
parameterscanbefoundin{25]. Ratedoutputinitial
~
conditionsoftheFCareconsidered.Thetransformer
... 0.0007
reactance is takenas 0.1 p.u,the systemvoltageis
takenas 1.0p.u,withzerophaseangle,O.=0.0rad.
Moreover, Km and Tm aretakenas 100p.u, and 10 O.COM +---.--""T""--....-........---.---i
secrespectively. The reference voltageis taken !'s o 10 20 30 80
l1mI,seo
1.035p.u,
Fig.6-d.ResponseofQH1,
A. Constant-PowerControl I·.... --O.,pul
II
A step increase in the reference power P~ is 0.20 ---l
~'fl~
simulatedandtheresultsareshowninFig.6.
0.24
1-··.. I
Praf,PIl--=-Pdo,pi! -P.,pu 0.19
1.25
0.14
Zl I.Ul 0.09 _ . •r-
~ o 10 20 30
lime.sec
a. •~.~. Fig. :~.r,:$ponseore(rad)andQ•.
1.i6
o.~ +----....---~--"""T""--_i
o 10 20 30 1.74
tim...seo
1.13
Fig.6-a.Response ofPde(FCOUtput Depower)and
p. 1.72
e
1.11
--...-- Ifo_demand.p~lfo_mod.pu 1.7
----- Uf_dem311d -Uf_mod 1.69
1.68
0 20 40 110 80 100
linl.SIO
Fig.6-f.Responseofm.
10 2D 30 40
tirr,!.,ec
Fig.6-b.ResponseoflIeandVI
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71lEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6j
B. Corutant-C"".elflControl
--\e,pu•••••,",0,pu
With the same parameters as in part A,
c:onstant-c:urrent control is implemented and the
systemissimulatedfora20%stepincreaseinthe
P,.,.
referencepower Theresults areshowninFig.
7.
I····· I
I'ref,pu-No,pu-Ps,pu
Uti
OM +---""T"""----.-----.---........j
o 10 20 30
tirM.NO
Fig.7-a.Responseofp.andPI
It is shown in Fig. 7, that also with constant
currentcontrolofthe Fesystem, alltheoperational
requirements andconstraints aremet.BasedonFig.
6-a and Fig. 7-8, constant-power control strategy
havemoreac:curacy thanconstant-eurrentcontrolin
achieving the desired power output with smaller
steady-stateerror.
I-pta pta.
--P02 ••••• P021
D.lt1
I
0.1
D~
(
I
0
-OM4--......- __- __- .......-~--1
o 20 41 11meoe._ 80 100 120
Fig.7-e.ResponseofP/UI Po" andPHrP01
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THEELEVENTHINTERNATIONALMIDDLEEASTPOWERSYSTEMSCONFERENCE (MEPCON'2oo6)
1--\t,PIl-n--,""o.pul
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