nov. 20, 2007 solid oxide fuel cells (sofc)ndjilali/mech549/lecture9_10.pdf · carbon monoxide (co)...
TRANSCRIPT
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Nov. 20, 2007
Mech 549
Fuel Cell Technology
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Solid Oxide Fuel Cells (SOFC)
High T Fuel Cells
Operating principles
Overview of Reforming
SOFC Brief History
Components and Materials
SOFC Challenges
Architectures: Tubular; Planar
Performance
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Low, Medium, & High Temp. FCs
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
High T Fuel Cells
- Expanded Fuelling Options-
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Efficiency…
• The Ideal Reversible Potential of a Fuel
Cell decreases with increasing T
h
g
r
r=
sThg rrr =
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
-183.1-0.0561-248.8900
-194.2-0.0549-247.6700
-205.0-0.0533-246.2500
-215.4-0.0507-244.5300
-225.2-0.0466-242.6100
rgrsrhT [oC]
sThg rrr =2.5%Change
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
The Efficiency Issue
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Why Use High T, then ?
• Noble Metal catalyst
not needed
• Better reaction Kinetics higher io
• Fuel Flexibility
• ‘Waste Heat’ can be useful– Reforming
– CHP, CCP
• Bottoming Cycles
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Operating Principle
For H2-O2 operation
Anode: H2 + O2- H2O + 2e
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Cathode: O2 + 2e- O2- O
Overall reaction: H2 + O2 H2O
Carbon monoxide (CO) and hydrocarbons such as methane (CH4) can be
used directly as fuels in SOFCs. In the high temperature environment of
SOFCs steam reforming and water gas shift can take place:
CH4 + H2O 3H2 + CO
CO + H2O H2 + CO2
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Direct oxidation of CO contained in reformed hydrogen is possible.
• Anode: H2 + O2- H2O + 2e
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CO + O2- CO2 + 2e-
____________________________________________________________
aH2 + bCO +(a+b)O2- aH2O + bCO2 + 2(a+b)e
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• Cathode: (a+b)O2 + 2(a+b)e- (a+b)O2-
• Overall cell reaction: (a+b)O2 +aH2 + bCO aH2O + bCO2
e-
e-
~
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Applications
Source: Subhas Singhal, 2003
Boston University Talk
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Reforming Technology
A
EC
O2 AirDepleted Air
CH4, H2O
H2, H2O,CO2,CO
IIR
Indirect Internal
Reforming
A
EC
Fuel
800°C
H2, CO
H2O, CO2
AirDepletedAir
O2 O2
ER
External Reforming
A
EC
CH4,
H2O
H2,H2O,CO2,CO
AirAir
DIR
Direct Internal
Reforming
Technological Evolution
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Steam Reforming
• CH4 + H20 CO + 3H2 H = 206 kJ/mol
• CnHm + nH20 nCO + (m/2+n)H2
Water Gas Shift
• CO + H2O CO2 + H2 H = -41 kJ/mol
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Steam Reforming Cont:
• One mole of steam is needed for each mole of
carbon
– More and more steam is needed as higher hydrocarbons
are reformed
• Carbon Dioxide is produced by these reactions.
• These are equilibrium reactions
some CO will always be present in the reformate
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
High Temperature Fuel Cells & Reforming:
Thermodynamics
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Reforming Cont’d
Important points:
– CO is a poison to the noble metal catalysts Fuel for PEM/AFC/PAFC must be treated
– Reforming is an endothermic process, and as such
requires heat input (this heat is less available with a low
temperature system)
– At the higher operating temperatures of SOFC/MCFC,
reforming can be carried out on Nickel catalysts used in
the Anode Internal reforming possible for SOFCs
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Reforming Cont’d
– In both MCFC and SOFC, water is formed at the
Anode
• Benefit needed for steam reforming
• Detriment H2 partial pressure decreases
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
• Recall the Nernst Equation
• Isolating the effect of a change in H2 partial
pressure
Hydrogen Concentration
+=OH
OHo
P
PP
F
RTEE
2
22
2/1
ln2
=1
2ln
2 P
P
F
RTV
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Hydrogen Concentration
• As H2 is consumed, and water/CO2 is produced, the
H2 partial pressure drops, and the V term is
negative (P2 < P1)
– Hydrogen Utilization is an important consideration
– This is worse as T
=1
2ln
2 P
P
F
RTV
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
The Efficiency of a Combined Cycle
Hydrogen
Octane
Methane
Fuel
23843079-141,788
22773108-48,254
22273054-55,496
Flame T,
Air [K]
Flame T,
O2
[K]
HHV
[KJ/kg]
ICE’s Reach ~ 1700 K
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Bottoming Cycles
Heat used to Raise Steam Temperature
which then drives a turbine
The whole system is pressurized, and the
high pressure exhaust drives a gas turbine
Maximum Efficiency gTA/ h
– TA –> Ambient Temperature
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Bottoming Cycles
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
“Thus a high temperature fuel cell combined with, for
example, a steam turbine is a ‘perfect’
thermodynamic engine. The two components of this
perfect engine have the advantage of practically
attainable technologies. The thermodynamic losses
of the high temperature fuel cell are low, and a
thermal engine can be easily designed to operate at
typical heat source temperatures”
– A. J. Appleby (one of the Gurus of FC Technology)
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Combined Heat & Power Generation
• Traditional power generation: 60-70% of fuel energy is in
the form of “waste heat”
• Co-generation recovers heat and can achieve overall
efficiencies of up to 80-90%.
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC: History• In 1897, Nernst made this observation:
“The conductivity of pure oxides rises very slowly with temperatureand remains relatively low, whereas mixtures possess anenormously much greater conductivity, a result in completeagreement with the known behaviour of liquid electrolytes'‘.
• Early 1900s:
Many mixed oxide combinations (at high temperatures) wereidentified, including, 85% zirconia, 15% yttria
• In 1937, Baur & Preis:
reported the operation of first ceramic fuel cell at 1000°C
• 1940s, Davtyan :
Increased conductivity and mechanical strength by adding monazitesand to mix of sodium carbonate, tungsten trioxide, and soda glass
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC: History• 1950s:
Research by Central Technical Institute (Holland), Consolidation Coal
Company (USA), General Electric (USA)
• 1960s:
Solid electrolyte, 100 W fuel cell
power system
(Westinghouse Res. Lab)
– 20 batteries of 20 cells each
– Open circuit voltage = 200 V
– Current = 1.2 A
– H2/O2 system
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Viability of high temperature fuel cells due to advances in materials and
engineering and some key operational advantages:
+ solid state system with no liquid electrolyte with its attendant materialcorrosion and electrolyte management problems and with two-phase (gas-
solid) contacts that reduce corrosion
+ operating temperature of ~1000°C allows internal reforming, promotesrapid kinetics circumventing the need for noble metal electrocatalysts, and
produces high quality byproduct heat for cogeneration
– The free energy of formation is less negative at high temperature,resulting in lower OCV (~0.9 V)
– The high temperature of SOFCs is very demanding on materials. Theidentification and development of suitable low cost materials, low cost
manufacturing of ceramic structures and lowering of operating temperature
are the key challenges being addressed currently.
SOFC Technology: Pro’s and Con’s
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The solid state nature of all SOFC components provides more freedom for the
cell architecture. Developments have taken place around two distinct
configurations: Tubular cell (Siemens Westinghouse Corporation),and
Flat plate design (AlliedSignal, SOF Co. etc.)
Flat Plate
Tubular
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Components
Material constraints
– High temperature (~1000oC)
– Chemical stability in reducing and oxidizing environment
– Contact compatibility
– Conductivity
– Thermomechanical properties capable of sustaining thermal
cycling
Fabrication is currently based on use of thin film concepts where
films of electrode, electrolyte, and interconnect material are deposited
one on another and sintered, forming a cell structure.
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Electrolyte
• Material: Yttria-stabilized Zirconia (8-10 mol%
yttria) or YSZ
• Conductivity: 0.1 S/cm at 1000oC
– Low conductivity at lower temperature
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Solid Oxide Electrolyte Conductivities
Stainless Steel
La (Ca) Cr O3
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Electrodes
Main functions are to
a) promote oxygen reduction/fuel oxidation,
b) serve as a current collector.
Electrode material should be electrochemically and thermomechanically
stable and of course ideally inexpensive!
Specific issues for electrode material:
– stability over wide range of partial pressures of O2
– non-reactive with other cell components (during operation or
fabrication)
– low contact resistance with electrolyte layer
– thermal expansion coef. very similar to that of electrolyte layer
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Anode
• Material: CERMET Composite of ceramic electrolyte and
Nickel (Metal)
• Porosity: 20-40 %
• Thickness: up to 1.0 mm for anode supported cells
• Fabrication Method: Conventional ceramic processing methods
such as tape casting, screening printing
• Issues: Nickel can sinter at high temperatures leading to reduced
surface area
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Cathode
• Material: Lanthanum Strontium Manganate (LSM);
also, composites of or mixtures of electrolyte (YSZ)
and electro-catalyst (LSM)
• Porosity: ??
• Thickness: 20-40 microns
• Fabrication Method: Conventional screening printing, slurry method
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
• Metal cathodes considered in early days. But only noble metals werepractical because of highly oxidizing environment too costly
• Oxide matrix with wire grid metallic conductor: Porous zirconia + Pt (or
Pd)
• Electronically conducing oxide
[Source:Blomen,
Fig. 10.2, p469]
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Micro-Structure of Porous
Cathode(Source: SOFC Research Group Queen’s University)
20μm
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Triple Phase Boundary in
Composite Anodes & Cathodes
O2-
e-
Triple-Phase Boundary
e-
O2-
e-
Triple-Phase Boundary
e-
O2-
Particle of pure
electronic conductivityParticle of
mixed conductivity
O2-
O2-
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Effect of Temperature
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC Component Issues
Source: Subhas Singhal, 2003
Boston University Talk
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
SOFC - Different Cell Geometries
• Seal-less Tubular Design
• Segmented-cell-in-series Design
• Planar Design
• Monolithic Design
Low power density
High power density
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Comparative Performance
Cell Performance of Major SOFC Companies
0
200
400
600
800
1000
1200
600 650 700 750 800 850 900 950 1000
Operating Temperature ( oC)
Po
wer
Den
sit
y (
mW
/cm
2)
Westinghouse
CFCL
AlliedSignal
Siemens
Tokyo Gas Co
Juelich
Global 1stgeneration
Global SC-1
Global SC-2
Humidified H2 fuel
Power at 0.7 V
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Planar Design
Rectangular shape Circular shape
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
F F F
F F F
O
O
O
O
Anode
Cathode
Electrolyte
Interconnect
Bell and Spigot
Configuration
Monolithic
Configuration
Stack Configurations
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Seal-less Tubular Design
Tubular Westinghouse SOFCs
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Tubular Cells
Tubular Cell Manufacturing
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Westinghouse Seal-less Tubular Design
Seal-less Tubular Design
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Single-Cell/ Cell Module/ Stack
Tube Diameter = 2.2 cm
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Siemens Westinghouse SOFC Unit
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Siemens Westinghouse SOFC System
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Hybrid System - Fuel Cell + Gas Turbine
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Demonstration Units
Source: Subhas Singhal, 2003
Boston University Talk
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Solid Oxide Electrolyte Conductivities
Stainless Steel
La (Ca) Cr O3
Planar SOFCs: microstructure
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Single-Cell Manufacturing
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Planar Stack
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Cell Performance
Long-term Performance
T= 750C, 556 mA/cm2; 3000 mlpm air; air utilization = 27%
636 mlp H2, 630 mlpm N2, 3% H2O fuel utilization = 54 %
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Gen.4 Stack Assembly
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Stack Performance
16-cell stack test data (4 Jan 2003)
10 x10 cm2 (81 cm2 active area)
750 C, 0.21 A/cm2, 60% fuel utilization, 25% air utilization)
Cell Voltage = 0.75 – 0.85 V
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Global’s RP2 System
Reformer
20-cell stack
Internal Volume = 1.9 m3
University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
RP2 Stack Performance
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University of Victoria Department ofMechanical Engineering IESVic
Nov. 20, 2007
Tubular vs. Planar Architectures
RequiredNot necessaryHigh Temperature Seals )
LowHighManufacturing Cost ($/kW)
HighLowVolumetric Power (W/cm3)
High (0.6-2.0)Low (0.2-0.25)Specific Power (W/cm2)
Planar
Cells
Tubular
Cells