Fuel Cell Systems: an Introduction for the Engineer (and...

Department of Chemical and Environmental Engineering

Illinois Institute of Technology

Fuel Cell Systems: an Introduction

for the Engineer (and others)

Professor Donald J. Chmielewski Center for Electrochemical Science and Engineering

Illinois Institute of Technology

Presented to the

E3 Class

March 16th, 2010


Illinois Institute of Technology 2

What is a Fuel Cell?

???



Stationary (200 kW) Mobile (50 kW)

Applications

Toyota International Fuel Cells



What is a Fuel Cell?

Fuel Cell

H2

Electric Power

Air

H2O

Answer:

An electrochemical

device that converts

a fuel directly to

electrical power



Where Does the Energy Come From?

Fuel Cell

H2

Electric Power

Air

H2O



Where Does the Energy Come From?

Fuel Cell

H2

Electric Power

Air

H2O

Answer:

The enthalpy released

by the reaction:

H2 + ½ O2 H2O

(H ~ 58 kcal/mole H2)



The Fuel Cell Reactor?

Fuel Cell

H2

???

Air

H2O



The Fuel Cell Reactor?

Fuel Cell

H2

Heat

Air

H2O

Problem:

Heat is

released but

not electric

power



The Fuel Cell Reactor

Fuel Cell

H2 Air

H2O

Solution:

Two reactors

separated by

an electrolyte

membrane.



The Fuel Cell Reactor

Fuel Cell

H2 Air

H2O

Ecell

Voltage * Current

= Electric Power

Current

Solution:

Two reactors

separated by

an electrolyte

membrane.

This allows for

manipulation

of electrons



Electrolyte Types

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

H+

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

O2-

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

H+

H+

H+

O2-

O2-

O2-

Polymer Membrane: Solid Oxide Membrane:



Polymer Electrolyte Membrane

Fuel Cell (PEMFC)

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

H+

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

H+

H+

H+

Electrolyte conducts

ions (H+), but not

electrons (e-).

Electrodes (Anode

and Cathode) conduct

electrons (e-), but not

ions (H+).



Solid Oxide Fuel Cell (SOFC)

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

Electrolyte conducts

ions (O=), but not

electrons (e-).

Electrodes (Anode

and Cathode) conduct

electrons (e-), but not

ions (O=).

O2-

O2-

O2-

O2-



Where Does the Water Go?

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

H+

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

O2-

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

H+

H+

H+

O2-

O2-

O2-




N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

H+

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

N2

N2

N2

H2

H2

H2

H2

H2

H2

O2

O2

O2

O2-

e- e-

Anode

Electrolyte

Cathode

O2 N2

N2

O2

O2

H+

H+

H+

O2-

O2-

O2- H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O H2O

H2O

H2O

Where Does the Water Go?




Where Do the Electrons Go?



Interconnect/Bipolar plate:

(La,Sr)CrO3 or High Temp Alloy

Anode: Ni - (Zr,Y)O2- cermet

Electrolyte: (Zr,Y)O2-

Cathode: (La,Sr)MnO3

Fuel H2

Air

SOFC Configuration:

Where Do the Electrons Go?



Fuel Cell Stack



Current Collectors

ElectrolyteAnode

CathodeO=

O2

H2OH

2

e-

e-

e-

e-

SOFC:



The Electrode Electrolyte Assembly

Anode

Grains

ElectrolyteAnode

CathodeO=

O2

H2OH

2

e-

e-

e-

e- e-

H2H2O

O=Electrolyte



The Three Phase Region

Anode

Grains

Ni

H2O

e-

H2H2O

O=Electrolyte

Ni

NiNiNi

YSZNiYSZ

YSZ

YSZ

YSZ

Ni

Ni

H2O H

2

YSZ

O= e-

Ni YSZ

NiNi



Design Issues

ElectrolyteAnode

CathodeO=

O2

H2OH

2

e-

e-

e-

e-

SOFC:



How Much Fuel Does a Fuel Cell Use?

Fuel Cell

H2 Air

H2O

Ecell

Voltage * Current

= Electric Power

Current




Fuel Cell

H2 Air

H2O

Ecell

Voltage * Current

= Electric Power

Current

Reaction rate is

proportional to

current density

F

2 n

jrH

AreajCurrent *



How Do We Calculate Current Density?



How Do We Calculate Current Density?

A fuel cell looks

like a battery to

the electrical

world.

Current output

depends on the

load.

Ecell

Eo

Rint

Load

DC

Fuel Cell I=j*Acell



Review of Circuits 101

Eload

Eo

Rint

DC

Battery I

Rload




Eload

Eo

Rint

DC

Battery I

Rload

Equation #1:

Eload = Eo - I*Rint

Equation #2:

Eload = I*Rload




Eload

Eo

-Rint

I

Eload

= Eo - R

int*I

Equation #1:

Eload = Eo - I*Rint




Eload

Eo

-Rint

I

Rload

Eload

= Eo - R

int*I

Eload

= Rload

*I

Equation #1:

Eload = Eo - I*Rint

Equation #2:

Eload = I*Rload




F

/2 n

AIrH

Eload

Eo

-Rint

I

Rload

Eload

= Eo - R

int*I

Eload

= Rload

*I

2

2

sec m

Hofmoles



Fuel Used is Proportional to Current

F

/2 n

AIr cell

H

2

2

sec m

Hofmoles

Fuel Cell

H2 Air

H2O

Ecell

Voltage * Current

= Electric Power

Current



Changing the Reaction Rate

Eload

Eo

-Rint

I

Eload

= Eo - R

int*I

Eload

= Rload

*I

Eload

Eo

Rint

DC

Battery I

Rload

F

/2 n

AIrH



Circuit Perspective of the SOFC

Ecell

Eo

Rint

Load

DC

Solid Oxide Fuel Cell I=j*Acell




Ecell

Eo(P

H2,P

O2, P

H2O)

Rint

(Tcell

)

Load

DC




Resistance in the SOFC

Zirconia Electrolyte Cathode (~30 μm)

Electrolyte (10-200 μm)

Anode ( up to 1 mm)

Bipolar Plate (3-10 mm)

Air

Fuel

Rint= r (T ) * ( thickness / Area )



Eload

Eo

I

Rload

Eload

= Eo - R

int*I

Lower T





Ecell

Eo(P

H2,P

O2, P

H2O)

Rint

(Tcell

)

Load

DC




Equilibrium Voltage

OH

OHmo

P

PPRTgE

2

22

2/1

log22 FF

Fuel Cell

H2 Air

H2O

Eo



Eload

Eo

I

Rload

Eload

= Eo - R

int*I

Lower P




Circuit Perspective of the PEMFC

Ecell

Eo(P

H2,P

O2, P

H2O, j )

Rint

(Tcell

, j )

Load

DC

PEM Fuel Cell I=j*Acell



0 2000

PEMFC Polarization Curve



PEMFC Polarization Curve

0 2000



How Much Heat Does a FC Generate?

eOHOHfgen PrHQ 22

)( ,




0 200 400 600 800 10000

0.2

0.4

0.6

0.8

1

1.2

1.4

Current Density (mA/cm2)

Cel

l V

olt

age

(V)

0 200 400 600 800 10000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Po

wer

Den

sity

(w

atts

/cm

2)

E

eP

cell

eOHOHfgen PrHQ 22

)( ,

celle VjP *

F

2 n

jrH




0 200 400 600 800 10000

0.2

0.4

0.6

0.8

1

1.2

1.4

Current Density (mA/cm2)

Cel

l V

olt

age

(V)

0 200 400 600 800 10000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Po

wer

Den

sity

(w

atts

/cm

2)

E

eP

cell

2/25.0 cmwPe

F

/4.0

2

2 n

cmArH

2

, /49.022

cmwrH OHOHf

2

,

/24.0

22

cmw

PrHQ eOHOHfgen



Stationary (200 kW) Mobile (50 kW)

Applications

Toyota International Fuel Cells



The Fuel Cell System

Fuel

Processor Fuel Cell

Stack

Spent-Fuel

Burner

Thermal & Water Management

Air

Air

Fuel

H2

Exhaust

H2O

CO2

Electric Power

Conditioner



Stationary Applications

International Fuel Cells



Flat-Plate Hydrogen Fed SOFC

Fuel Cell Stack

Anode

Flow

Cathode

Flow

Cathode

2 O=

V

-

+

Solid

Electrolyte

Anode

2H2

2H2O

O2+ 4 e- 2 O=

2H2+ 2 O= 2 H

2O +4 e-

O2

4 e-

4 e-

HEATRELEASED



Plug Flow Reactor Analogy

Feed Exhaust

Reaction

Rate

Conventional Design



Figure taken from Selimovic Dissertation, Lund University, (2002).

Thermal Stresses in the Literature

Peters et al., state that

“ Large temperature gradients in either direction can cause damage to one or more of the components or interfaces due to thermal stresses”

Yakabe et al., state that

“ … the internal stress would cause cracks or destruction of the electrolytes”



Fuel Cell Stack

Air Channel

Fuel Channel

InterconnectCathode

Anode

Electrolyte

Internal Reforming SOFC

CH4

Air Flow

H2OCO

2H2 H

2O

O=

Fuel Flow

O2



Internal Reforming

44224 3 CHrefCH

kCkrHCOOHCH ref ++

+ +

eq

COH

COOHfshiftCO

k

K

CCCCkrHCOOHCO fshift 22

22

,

,222

is very large Endothermic

is also large Exothermic fshiftk ,

refk

2

1222

2 OHOH Hr+ Exothermic



Plug Flow Reactor Analogy (Internal Reforming)

ReformingReaction Rate

Reforming HeatGeneration

ElectrochemicalReaction Rate

ElectrochemicalHeat Generation

Combined HeatGeneration



Figure taken from Selimovic Dissertation, Lund University, (2002).

Impact of Internal Reforming



Effective Structure of IR SOFC

• Heat and steam produced not used by reforming.

• Pre-heating steam is expensive.

• Steam in the feed lowers hydrogen utilization (reaction rate is a function of hydrogen to steam ratio).

ElectrochemicalSection

ReformingSection

Pre-Heater

222

224 3

COHOHCO

COHOHCH

++

++

OHOH 22 +

Methane

Steam



Exhaust

Feed

Feed

Distributed Feed Plug Flow Reactors

Distributed Feed Design

• Makes PFR act like a CSTR.

• Improves Yield and Selectivity.

• Improves Thermal Management.



Hydrogen Fed Simulations

Solid Temperature Profile



Simulation of the

Internal Reforming Case



2-D Distributed Feed Design

Side FeedChannels

z1 z

2z

3z

4z

5

Active Area WallInactive Area

zx

Section of a stack layer



Fuel Cell Stack

Air Channel

Fuel Channel

InterconnectCathode

Anode

Electrolyte

Internal Reforming SOFC

CH4

Air Flow

H2OCO

2H2 H

2O

O=

Fuel Flow

O2



Mobile Applications

Toyota




Fuel

Processor Fuel Cell

Stack

Spent-Fuel

Burner


Air

Air

Fuel

H2

Exhaust

H2O

CO2

Electric Power

Conditioner



Hydration Model for MEA

MEA

Anode

In

(H2, H

2O) H

2

Cathode

Air in

Cathode

Exhaust

O2

H2O

N2

Solid Material Current Collector

H+

H+

H+

H+

H+

H+

H+

H+

Anode

Exhaust

H2O



Water Transport in the Membrane

ELECTRO-OSMOTIC DRAG

DIFFUSION



Concentration Profiles

GDL Membrane GDLAnode Gas Cathode Gas

δm δ cδ a

2

( )an

H OC

( )ˆ mem

oC

( )( )

20mem

H OC

2

( ) ( )mem

H OC z

( )ˆm

memC2

( )ca

H OC

2

( ) ( )mem

H O mC




Fuel

Processor Fuel Cell

Stack

Spent-Fuel

Burner


Air

Air

Fuel

H2

Exhaust

H2O

CO2

Electric Power

Conditioner



Why On Board Fuel Processing?

Transportation

Applications

PEMFCFuel

Processors

Liquid Fuel

Storage Tank

CmHn

H2

CO

H2O

CO2

PEMFCHydrogen

Storage Tank

H2



CO Poisoning



High Temperature Membranes

Humidity with Increases ,

ty,Conductivi Electrical

)(TP

PxRH

satw

xw = 0.35



Fuel Processing Reactors

PEMFCPreferential

Oxidation

(PrOx)

Water-

Gas

Shift

(WGS)

Reformer

Hydrocarbon Feed

Large Hydrocarbons Cracked:

H2 / CO ratio ~2 Most CO converted to CO2: ~ 1% CO remaining

CO levels down to ~ 10 ppm



Reforming Reactors

Steam Reforming

Partial Oxidation

Autothermal Reforming



Steam Reforming

22 )2/( HnmmCOOmHHC nm +++

222 HCOOHCO ++

Fuel

Steam

Heat



Catalytic Partial Oxidation (CPOX)


222 HCOOHCO ++Fuel

Air OHnmCOOnmHC nm 222 2/)2/( +++



Autothermal Reforming (ATR)


222 HCOOHCO ++Fuel

Air OHnmCOOnmHC nm 222 2/)2/( +++

Steam



Start-up and Regulation of an ATR

ATR

TT

TC

Air Flow In

Fuel Flow In

Water Flow In

Reformat Flow Out



The Effect of Water Injection



Closed-loop Water Injection



Slower Water Injection Rate



Fuel Processing Reactors

PEMFCPreferential

Oxidation

(PrOx)

Water-

Gas

Shift

(WGS)

Reformer

Hydrocarbon Feed

Large Hydrocarbons Cracked:

H2 / CO ratio ~2 Most CO converted to CO2: ~ 1% CO remaining

CO levels down to ~ 10 ppm



Water Gas Shift Reactors

( )eq

s

CO

s

H

s

OH

s

CO Kyyyykr )(

2

)(

2

)(

2

)(

33

222 HCOOHCO ++

High

Temp

WGS

Medium

Temp

WGS

Low

Temp

WGS



Preferential Oxidation Reactors

222

1COOCO +

PrOx

OHOH 2222

1+

Reformat

Air




222

1COOCO + OHOH 222

2

1+

Reformate

Air

100oC 100oC

Intercooler Intercooler

Prox

Stage 1

Prox

Stage 2

Prox

Stage 3

Air Air




0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5 3 3.5

Inlet CO Concentration (%)

Hyd

rog

en

Co

nve

rete

d (

%)

1-Stage3-Stage

2-Stage




Fuel

Processor Fuel Cell

Stack

Spent-Fuel

Burner


Air

Air

Fuel

H2

Exhaust

H2O

CO2

Electric Power

Conditioner



Hybrid Fuel Cell Vehicle

FC Kfc

iafcifc

Vfc

iab

ib

Vb

Rb

Eb

ia

Ra

La

VaKb



Separation of Time-Scales

FC Kfc

iafcifc

Vfc

iab

ib

Vb

Rb

Eb

ia

Ra

La

VaKb

5 10 15 20 25 30 35

-200

0

200

400

600

800

1000

Power Profles [W]

time, sec

Pload

(sp)

Battery

Fuel Cell

Armature

Vehicle

kbatPmot

+-

kfc

x

Pfc

+-

x

Pbat

Pmot(sp)

Pbat(sp)

VfcFUEL CELL

VOLTAGE

CONTROLLER

Vfc(sp)

PI

PI



Hybrid Fuel Cell Vehicle (Double Storage Configuration)

iascapiscap

Rscap

Escap

iarm

Rarm

Larm

DC-DC

Converter

iabatibat

Rbat

Ebat

DC-DC

Converter

iafcifc

EfcDC-DC

Converter

Fuel

Cell

Power Bus

warm

Earm

kfc kbat kscap



Supervisory Control

Vehicle

Power

System

+ -Pscap

(sp) Pscap

kscap

Supervisory

Controller

Pmotor

+ -Pbat

(sp) Pbat

kbat

+ -Pfc

(sp) Pfc

kfc

PI

PI

PI




Fuel

Processor Fuel Cell

Stack

Spent-Fuel

Burner


Air

Air

Fuel

H2

Exhaust

H2O

CO2

Electric Power

Conditioner



Acknowledgements

• IIT Collaborators: Said Al-Hallaj J. Robert Selman

Ali Emadi Satish Parulekar Herek Clack Jai Prakash

• Argonne National Laboratory: Shabbir Ahmed Dennis Papadias Rajesh Ahluwalia Qizhi Zhang • Students: Kevin Lauzze Ayman Al-Qattan • Funding: Kuwait Institute for Scientific Research Graduate College and Armour College, IIT Argonne National Laboratory

Fuel Cell Systems: an Introduction for the Engineer (and...

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