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An Investigation to Resolve the Interactions among SOFC, Power-Conditioning Systems, and Application Loads
Project Investigators Sudip K. Mazumder, Kaustuva Acharya, and Rajni K. Burra
(University of Illinois) Michael von Spakovsky, Doug Nelson, Diego Rancruel, and Saravanan Subramanian
(Virginia Tech) Comas Haynes and Robert Williams
(Georgia Tech.) Joseph Hartvigsen and S. Elangovan
(Ceramatec Inc.) Dan Herbison and Chuck McKintyre
(Synopsys Inc.)
SECA Core Technology Program Review MeetingSeptember 30 - October 1, 2003
Albany, New York
SOFC Power-Conditioning System Modeling
gPROMS
(Balance-of-Plant System)
FORTRAN
(SOFC)
(Finite-Element Analysis)
PES
DC-DC DC-ACSOFC
BOPS
LumpedVoltageSource DC-DC
SOFC
BOPS
LumpedHarmonic
Load
iSight(System Integration)
SABER DESIGNER
(Power Electronics System)
Lumped Harmonic
Load
Impact of PES on SOFC
Impact of SOFC+BOPS on PES
Multi-Software Simulation Platform
AL
Comprehensive System Model
Reduced-Order Models
Transient SOFC Response to Electrical Stimulus: Modeling Approach
Reactants inlet flow rates and properties are invariant during relatively short transient episode
Fuel stream effects are dominant
Quasi-steady-state electrochemistry
Lagrangian extension of validated steady-state model to track fuel parcels that travel over electroactive area
Fuel Cell
11
Fuel Stream
Oxidant Stream
x
t + ∆t t
x+ ∆x
t t + ∆t v1
v2
ηelement(t+∆t) =ηfield(x+∆x, t +∆t)
Potentiostatic Control (Power Increase)
Current spikes up, yet the fuel supply remains invariant due to the decoupling of the cell
Fuel utilization thus increases; this causes current (and power) to decrease from t*=0+ values, until a new steady state “match” occurs at the new voltage (t*=1)
Attainment of steady state at the time constant T = Lcell/vfuel
Impact of Electrical Stimulus:Galvanostatic Control (Power Increase)
Duality of potential drop seen: polarization curve effect and subsequent fuel depletion effect
Multiple voltage reductions are “seen” by the reactant streams
Transient is thus longer by multiples of the time constant
Larger initial fuel utilizations prolong the relative transient due to enhanced fuel depletion effects
0.60
0.625
0.65
0.675
0.70
0.725
0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500
τ(s)
Fuel Util.= 50%
Fuel Util.= 75%
Fuel Util.= 62.5%
Cel
l Vol
tage
(V)
A 20% rise in the load current
0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500
τ
Fuel Util.= 50%
Fuel Util.= 75%
Fuel Util.= 62.5%
A 20% rise in the load current
0.60
0.625
0.65
0.675
0.70
0.725
0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500
τ(s)
Fuel Util.= 50%
Fuel Util.= 75%
Fuel Util.= 62.5%
Cel
l Vol
tage
(V)
A 20% rise in the load current
0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500
τ
Fuel Util.= 50%
Fuel Util.= 75%
Fuel Util.= 62.5%
A 20% rise in the load current
Leveraging Approach to Planar Cells
- Initial attempt at seed simulation of vertical team developmental cell GE, SECA Annual Mtg., 4/03, 4 3/8” diameter cell
- Initial and final conditions match reported data
- Trends corroborate those of the tubular results
Seed Case Study: Potentiostatic Stimulus (0.8V to 0.6V)
0100200300400500600700800
0 0.2 0.4 0.6 0.8 1
Dimensionless Time
Cur
rent
Den
sity
[m
A/c
m2]
010203040506070
Fuel
Util
izat
ion
[%]
Current Density Fuel Util.
RAM Issues of “Real World” Operating Conditions
2 2CO CO C s→ + ( )CO H H O C s+ → +2 2 ( )
Carbon Monoxide Axial Profiles
2.00E-05
2.40E-05
2.80E-05
3.20E-05
3.60E-05
4.00E-05
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Dimensionless Axial Distance
CO
flow
reat
e (m
ol/s
)
0 to 11 to 11 to 21 to 3
Frozen CO electrochemistry promotes coking via heightened presence of CO along the anode.
electrolyte
cathode
anode
A CB
F
D
GE
Leverage of SECA Failure Analysis and Lab efforts into characterization of impact of electrical conditions upon cell electrochemistry
0 5 10 15 20 25 300
100
200
300
400
500
5%20%30%
Fracture Toughness 2(J/m )cG
(J sec)q
Maximum allowable heating rate for cracking vs. fracture toughness
Microcrack Nucleation
BOPS Modeling: Summary
!Development of dynamic heat transfer, thermodynamic, kinetics, and physical models for each component of the BOPS:
"Compressor, expander, heat exchangers, steam generator, reformer, and fuel storage
!Implementation of models in a dynamic-programming environment using state-of-the-art transient numerical solver
!Integration of BOPS component models into a BOPS sub-system model
!Parametric studies (trade-off analyses) of best-practice control strategies for continuous operation and start-up and shut-down
BOPS: PARAMETRIC STUDIES
Pre-reformer Thermal Transient Response for FU Perturbations (SMR = 3.4 & FRR = 0.30 constant)
5
10
15
20
25
1 to 2 1 to 3 1 to 4 1 to 5 5 to 4 5 to 3 5 to 2 5 to 1
Power Changes (KW)
Tim
e (s
ecs)
FU = 0.85 (Steady Temp.) FU = 0.70 (Steady Temp.) FU = 0.90 (Temp.)
Power-demand and system-level parameter perturbations
Total system efficiency and power-demand and system-level parameter perturbations
0.20
0.30
0.40
0.50
1 2 3 4 5
Power Demand (KW)
Syst
em T
herm
al
Effic
ienc
y
FU = 0.85 FU = 0.70 FU = 0.90
Power demand perturbation Power-demand and system-level parameter
perturbations Small changes in power demand with floating
fuel utilization Power demand perturbation with
temperature control Total system efficiency analysis Start-up and shut-down
Outline
Methane Compressor Thermal Transient Response For SMR Perturbations (FU=0.85, FRR=0.3 constant)
0
2
4
6
8
1 to 2 1 to 3 1 to 4 1 to 5 5 to 4 5 to 3 5 to 2 5 to 1
Power Changes
Tim
e (s
ecs)
Thermal SMR=3.4 Thermal SMR =3 Thermal SMR=3.8
Power-demand and system-level parameter perturbations
BOPS: PARAMETRIC STUDIES
System Thermal Efficiency for Pre-reformer Inlet Gas temperature Control
0.27
0.32
0.37
0.42
5 4 3 2 1
Power Changes (KW)
Syst
em T
hal
Ef
ficie
nc
T4 = 1120K T4 = 1150K T4 = 1200K T4 = 1300KT4 = 1400K T4 = 1500K T4 = 1550K T4 = 1570K
Total system efficiency and power-demand perturbation with temperature control
-100
0
100
200
300
400
0 10 20 30 40 50 60 70 80 90 100
Hea
t Flu
x (k
W/m
2)
Steam Generator Segment
Heat Flux at O Sec. Heat Flux at 5O Sec.Heat Flux at 10O Sec. Heat Flux at 20O Sec.Heat Flux at 40O Sec. Heat Flux at 120O Sec.Max Allowed Heat Flux
Steam generator heat flux
300
400
500
600
700
800
900
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200
Tem
pera
ture
(K)
Time
Stop recirculation at 700 K Stop recirculation at 750 KStop recirculation at 800 K Stop recirculation at 900 KStop recirculation at 600 K
Steam generator start-up
Steam-Methane Reformer Dynamic Reasponse for Start-Up (FU=0.85, SMR=3.4, FRR=0.3)
200400600800
10001200
0 500 1000 1500 2000 2500 3000
Time (sec)
Tem
pera
tue(
K)
0.E+00
4.E-05
8.E-050 10 20 30 40 50
Time (Sec)
Ref
orm
ate
Fel
(K
mol
/s)
Reformate Exit Temperature Reformer Wall TemperatureReformate Gases Mass Flow
Steam methane reformer start-up
0.47
erm yu
PES Modeling and SOFC PCS System Interactions: Summary
! Development of nonlinear switching models of PES! Integration of SOFC, PES, and BOPS models to develop a
comprehensive SOFC PCS model! Development of reduced-order models for fast and convergent
simulations on a PC! Investigated the impact of PES low- and high-frequency current
ripples and the effects of load-transients on SOFC performance! Analyzed the impact of SOFC-output-voltage variations on the
dynamics and stability of PES using bifurcation analysis! Investigated effects of PES control and modulation strategies on
SOFC performance! Conducted a preliminary trade-off study to determine the optimal size
of a energy-storage device (comprising a battery and a pressurized hydrogen fuel tank) to cost-effectively improve PCS transient response
! Designed a (low-cost) novel zero-ripple, energy-efficient, reduced-voltage-stress, and direct energy-conversion PES
PES Topological Models
Self-Commutated Voltage-Source InverterLine-Commutated Current-Source Inverter
°°°°°
n
Load3Φ
DC/ACBoost
°
Filter
Vin
Electric Utility
Zig-ZagTransformer
Electric Utility
SCR InverterDC Link
Vin
n
LOAD3Φ
°°°°°°
Filter
High-Frequency Transformer-Isolated Topology
n
n
Load3Φ
AC/ACBoost HF Inverter
°°°°°°
Filter ElectricUtility
Vin
BATTERY
SOFC PCS Steady-State Interaction Analyses
Hydrogen utilization increases with the increase in load
Current ripple has effect on the hydrogen utilization athigh load conditions
Low-frequency ripples do not necessarily lead to increased fuel utilization unless their magnitude is high
For high loads, rise in the temperature observed at low frequencies (high temperatures can cause interaction between SOFC electrolyte and electrodes leading to formation of high resistivity material and high microcrack densities)
Effect of Ripple Factor and Current Amplitude
Effect of Frequency and Current Amplitude
Novel SOFC PES
AC/AC HF Inverter
Zero Ripple Boost
Control
Power supply
FEATURES: Direct power conversion Reduced device voltage stress High energy efficiency Minimize SOFC output-current ripple Filter size and weight 50% less than conventional filter
Cost effective
FuelCell
Zero
-Rip
ple
Boo
st C
onve
rter
Bat
tery
HF
Inve
rter
AC
-AC
Filte
r3-ΦLoad
To U
tility
Grid
PES
SOFC Output Current
Novel SOFC PES
SOFC Output Current
0.8
0.84
0.88
0.92
0.96
1
0 0.1 0.2 0.3 0.4 0.5
Load (x 10000) (W)
Effic
ienc
y (x
100
) (%
)
Boost
HF Inverter
AC-AC
Overall0.8
0.84
0.88
0.92
0.96
1
0 0.1 0.2 0.3 0.4 0.5
Load (x 10000) (W)
Effic
ienc
y (x
100
) (%
)
Boost
HF Inverter
AC-AC
Overall
3 Phase 1 Phase
0.8
0.84
0.88
0.92
0.96
1
0 0.1 0.2 0.3 0.4 0.5
Load (x 10000) (W)
Effic
ienc
y (x
100
) (%
)
Boost
HF Inverter
AC-AC
Overall0.8
0.84
0.88
0.92
0.96
1
0 0.1 0.2 0.3 0.4 0.5
Load (x 10000) (W)
Effic
ienc
y (x
100
) (%
)
Boost
HF Inverter
AC-AC
Overall
3 Phase 1 Phase
SOFC PCS Load-Transient Interaction Analyses
0.5
0.6
0.7
0.8
0.9
1.0
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Time (sec)
Voltage
0.5
0.6
0.7
0.8
0.9
1.0
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Time (sec)
Voltage
Vol
tage
(V)
Time (sec)
1073
1074
1075
1076
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Time (sec)
Tem
pera
ture
(K)
125 A
100 A
75 A
50 A1073
1074
1075
1076
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Time (sec)
Tem
pera
ture
(K)
125 A
100 A
75 A
50 A
1070
1080
1090
1100
1120
1130
1140
1150
1160
1170
0 60 120 180 240 300 360 420 480 540 600
Time (sec)
Tem
pera
ture
(K)
1110
125 A100 A
75 A50 A
1070
1080
1090
1100
1120
1130
1140
1150
1160
1170
0 60 120 180 240 300 360 420 480 540 600
Time (sec)
Tem
pera
ture
(K)
1110
125 A100 A
75 A50 A
Temperature
(Cell)
(Stack)
TemperatureTemperature
(Cell)
(Stack)
Load-Transient Mitigation Techniques
Inverter Modulation Strategies
z (mm) x (mm) z (mm) x (mm)
Hyd
roge
n M
olar
Flo
w R
ate
Cur
rent
Den
sity
(A/s
q.m
m)
Res
pons
e Ti
me
(sec
)
Battery Size (Ah)Hydrogen Flow Rate (moles/sec)
Battery vs. Pressurized Hydrogen Tank
Time (sec)
Cur
rent
(A)
! Pressurized Hydrogen Fuel Tank and Battery
# Instantaneous supply of additional energy requirement from SOFC stack
! Inverter Modulation Strategies# Space-vector modulation (SVM) vs sine-wave
PWM (SPWM) used for the inverter # Battery acts as a stiff voltage source, providing
additional energy requirements during transients# Slower boost converter voltage-controller
response to prevent immediate change in SOFC energy demands
Nonlinear Hybrid Controller for DC-DC Boost ConverterLo
ad R
esis
tanc
e (Ω
)
Inpu
t Vol
tage
(V)
Out
put V
olta
ge (V
)
Inpu
t Cur
rent
(A)
Time (sec) Time (sec)
Time (sec)Time (sec)
! FEATURES# Hybrid control concept
based on combining integral-variable-structure control (IVSC) scheme and multiple-sliding-surface control (MSSC)
# Excellent steady-state and transient responses even under parametric variations and under perturbations of SOFC stack voltage and load
# Controller eliminates the bus-voltage error with a reduced control effort
# Control scheme can reduce the impact of very high-frequency dynamics due to parasitics on an experimental closed-loop system
Phase-II Objectives
! Develop and enhance fully transient nonlinear and temporal models for a variety of PES and BOPS components and for planar SOFC stacks
! Experimental validations of interaction-analyses results
! Develop capabilities for analyzing long-term performance and durability of SOFC planar and tubular stacks due to their system interactions with the PESsand application loads and the BOPSs
! Develop cost-effective optimal PES designs and design guidelines for (i) mitigation of electrical feedbacks on SOFC stack and (ii) technology transfer to SECA industry team
! Develop transient PES and PCS models and load profiles for vehicular APUs for performance and reliability analyses
! Develop optimal control and modulation strategies for robust PES and BOPS
! Develop decomposition techniques for optimizing PCS with respect to cost, reliability, size (power density), and response time
Acknowledgement
Department of Energy
Don Collins and Randall Gemmen (NETL)
Moe Khaleel (PNNL)
Daniel A. Norrick, Brad K. Palmer, Charles Vesely, and Todd Romine
(CUMMINS)
Tony Campbell and Nguyen Minh (GENERAL ELECTRIC)
Sean M. Kelly, John G. Noetzel, and K. S. Rajashekara (DELPHI)
Shailesh D. Vora (SIEMENS WESTINGHOUSE)
John Stannard (FUEL CELL TECHNOLOGY)
Jeff Bond (ENGINEOUS SOFTWARE INC.)
Jonathan Felton (PSE, UK)