System Design and Process Layout for a SOFC
µCHP-Unit with Reduced Operating Temperatures
Thomas Pfeifer, Laura Nousch, Wieland Beckert
Fraunhofer IKTS, Dresden, Germany
European Fuel Cell 2001 – Piero Lunghi Conference & Exhibition
Rome, December 14-16, 2011
© Fraunhofer IKTS
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© Fraunhofer IKTS
Hermsdorf (since 01/2010)
-2-
Fuel Cell System Development Projects
at the Fraunhofer IKTS
1W
10 W
100 W
1 kW
10 kW
H2
PEFC
Butane
SOFC
LPG
SOFC
Natural Gas
SOFC
Biogas
SOFC
Integrated
Stack-Module
Integrated
HotBox-Modules
eneramic
â
by Fraunhofer
Ceramic
Multilayer
© Fraunhofer IKTS
Bundled
Microtubes (ASC)
Planar
Mini-Stack (ESC)
-3-
Multi-Level Simulation Supported System Development
 Core Modules:
sofc.dll
prop.dll
equi.dll
 Development Tools:
MS Excel, VBA, C++,
Matlab / Simulink,
Modelica / SimulationX
 FEA:
COMSOL Multiphysics,
FlexPDE, ANSYS
IKTS contributions
to LOTUS
Level 1
Basic System Concept
Process Layout
Chem. Equilibrium Studies,
Stack Performance Maps,
Idealized Energy Balances
System Specification (SRD)
Process Flow Diagram (PFD)
Process Design Parameters
System Design Loop
Component Design Loop
Component Layout,
Single Device Testing
Level 2A
System Modeling
Matlab / Simulink,
Modelica / Dymola,
SimulationX, C++
Level 2B
Multiphysics & CFD
FlexPDE, COMSOL,
Ansys, Fluent,
DiffPack
Sub-Modules Assembly,
Successive System Setup
Level 3
Controller Design
 CFD:
Fluent, Ansys CFX
© Fraunhofer IKTS
Simulink / Stateflow,
Modelica.StateGraph
Statechart Designer
-4-
Level 4
System Operation
Data Mining,
Model Validation
Preliminary LOTUS Design Studies
0-D Stack-Model Parameterization (sofc.dll)

U/I-Measurements at varying temperature and fuel-input provided by SOFCPower.

Model parameters identified by least squares fit of area specific cell resistance.
SOFCPow er S-Design Short-Stack Performance
SOFCPow er S-Design Short-Stack Performance
6,0
2,5
No. of cells: 5 á 50 cm²
Fuel: 100 % H2 (sat. @ 25 °C)
Fuel input: 1.5 slm ≈ 270 J/s
Air stoich. ratio: 4.2
650°C (Model)
5,0
700°C (Test)
700°C (Model)
4,5
750°C (Test)
4,0
750°C (Model)
800 °C (Test)
3,5
Cell Area Resistance RAcell / W cm2
Stack Voltage U / V
5,5
No. of cells: 5 á 50 cm²
Fuel: 100 % H2 (sat. @ 25 °C)
Fuel input: 1.5 slm ≈ 270 J/s
Air stoich. ratio: 4.2
650°C (Test)
2,0
65
65
70
1,5
70
75
1,0
75
80
0,5
80
800°C (Model)
0,0
3,0
0
100
200
300
400
500
600
0
700
200
300
400
500
Current Density jel / mA cm-2
Current Density jel / mA cm-2
© Fraunhofer IKTS
100
-5-
600
700
Preliminary LOTUS Design Studies
Stack Performance Estimation at 650 °C
SOFCPower ASC700+20%
enables LOTUS-development
 Available Cell Technology: ASC700
66 x 50 cm², CH4-SR Reformate
0.35
0.45 0.40
39.6 (0.60)
0.9
0.8
0.25
0.30
0.5
0.6
0.7
 Expected Development: ASC700+20%
66 x 80 cm², CH4-SR Reformate
25 A
el = 0.50
ASC700 +20% , S-Design (80 cm²), 66 Cells
Aact = 66 x 80 cm² / T Cell = 650°C
T
= 650°C / T = 550°C
0.4
Fuel
300 Nl/min
42.9 (0.65)
42.9 (0.65)
200 Nl/min
800 W
0.15
15 A
49.5 (0.75)
PDC = 400 W
56.1 (0.85)
I= 5 A
59.4 (0.90)
500
1000
© Fraunhofer IKTS
1500
600 W
VAir,ad =
FU = 0.2 100 Nl/min
ASC700, S-Design (50 cm²), 66 Cells
Aact = 66 x 50 cm² / T Cell = 650°C
TFuel = 650°C / T Air = 550°C
Stack Voltage | Average Cell Voltage [V]
Stack Voltage | Average Cell Voltage [V]
0.3
52.8 (0.80)
3000
2500
Fuel Input [J/s]
3500
4000
45 A
4500
0.7
0.6
500 Nl/min
46.2 (0.70)
35 A
0.55
30 A
1600 W
0.5 400 Nl/min
0.30
1400 W
300 Nl/min
49.5 (0.75)
0.60
1550 WDC @ 70% FU
UCell = 0.7 V
52.8 (0.80)
0.4 0.25
25 A
1200 W
200 Nl/min
20 A
0.3 0.20
1000 W
0.65
VAir,ad =
15 A
FU = 0.2
59.4 (0.90)
500
5000
-6-
700 Nl/min
600 Nl/min
1800 W
0.35
40 A
56.1 (0.85)
Fuel: CH4-SR @ 650 °C, S/C = 1.5,  = 0
7.2% CH 4 / 60.6% H 2 / 14% H 2O
12.4% CO / 5.9% CO 2 / 0% N 2
0.40
50 A 0.8 800 Nl/min
2000 W
2
LHV = 238.98 kJ/mol
2000
el = 0.50
LHV = 238.98 kJ/mol
0.55
650 WDC @ 70% FU
UCell = 0.7
V
10 A
Air
Fuel: CH -SR @ 650 °C, S/C = 1.5,  = 0
4
7.2% CH 4 / 60.6% H 2 / 14% H 2O
12.4% CO / 5.9% CO / 0% N
2
20 A
46.2 (0.70)
0.45
0.9
39.6 (0.60)
0.20
1000 W
I = 10 A
400 W
1000
1500
2000
2500
3000
Fuel Input [J/s]
100 Nl/min
PDC =
800 W
600 W
3500
4000
4500
5000
Preliminary LOTUS Design Studies
Pre-Evaluation of Fuel Reforming Options
 Stack-Internal Reforming (IR)
Fuel
IR-SOFC
H2O
 Pre-Reforming
Steam Reforming (SR)
Fuel
H2O
SR
SOFC
POX
Heat
Autothermal
Reforming (ATR)
Fuel
H2O
800 °C
650 °C
ATR
ATR
ATR
SOFC
POX
SOFC
POX
Air
Partial Oxidation (POX)
Fuel
Air
SR
Heat
© Fraunhofer IKTS
-7-
SR
Preliminary LOTUS Design Studies
Comparison of Basic System Concepts
 No feasible technology
for IR with anode off-gas
recirculation is available.
P
el   Stack
H fuel  Pmech
Steam Reforming (SR) is the best
option for LOTUS-development
loss
 SR shows electr. efficiency
according to LOTUS
development goals.
ηth
 ATR shows higher total
efficiency. RAPH is
beneficial for electrical
efficiency.
ηel
 POX is not an option at
650 °C due to the risk of
reactor overheating at
soot-preventing air ratios.
© Fraunhofer IKTS
Q
th   CHP
H fuel  Pmech
-8-
Boundary Conditions for the LOTUS System Design
 Stack temperature predetermines reforming temperature  650 °C.
 Soot-free reformer operation requires S/C ~ 2 .. 3.
 In practical µCHP-operation a lower system S/C is essential.
 For start-up and shut-down of ASC a reducing atmosphere > 300 °C is required.
 Controlled stack-internal reforming (IR) is beneficial for system efficiency.
 Part load operation and independent control of power to heat ratio is beneficial
for system economics.
LOTUS system design is governed
by the fuel reforming concept
and its process integration.
© Fraunhofer IKTS
-9-
LOTUS System Design
Process Flow Diagram (PFD)
Implementation of
the LOTUS Fuel
Reforming Concept
~
=
SB
Start-up
Burner
 Downscaled steam
reformer (SR)
 SR directly heated
by burner exhaust
(AB or SB)
 Fuel bypass (FBP)
for controlled stackinternal reforming
 Optional use of
oxidative steam
reforming
© Fraunhofer IKTS
Electricity
FBP Fuel Bypass
SR
Steam
Reformer
Fuel
SOFC
Stack
AB
Afterburner
APH
Air Pre-Heater
Air
EVP
Evaporator
Water
CHP-Hx
Heat
Exchanger
Heat
Exhaust
- 10 -
LOTUS System Design
Balance Sheet & Process Layout Calculations
 Interactive Process
Calculation Sheets in
Microsoft Excel
 Added Functionality
through Visual Basic
UDFs and Macros
 Parameterized SOFC
Stack Model: sofc.dll
 Thermophysical
Properties: prop.dll
 Chemical Equilibrium
Calculations: equi.dll
© Fraunhofer IKTS
- 11 -
LOTUS System Design
System Performance Estimation
© Fraunhofer IKTS
- 12 -
LOTUS Parameter Studies
Efficiencies at Varying Fuel Bypass Ratio
n FBP
n Fuel
0.8
0.9
0.8
Parameter variation:
 Bypass Ratio () = 0 .. 1
0.75
 System-S/C = 1.5 .. 5
0.7
Efficiency (η )
 Effect of  : ηSys
 
ηSys
0.65
0.6
0.55
 Effect of System-S/C : ηSys 
0.5
 Independent: ηel ~ constant
0.45
ηel
0.4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Bypass ratio (τ)
eta_el
© Fraunhofer IKTS
- 13 -
S/C = 1.5
2
2.5
3
 Option for controlled stackinternal reforming:
x’CH4 = 8 .. 33 Vol.-%
 Anode inlet temp. decreases
due to mixing and chemical
equilibrium.
 Recommended Fuel Bypass
Ratio:  = 0.5
at S/Ctot = 1.5 (S/Cref = 3)
Stack inlet temperature TAno‘ / °C
FBP-Implications:
n FBP
n Fuel
700
0.35
600
0.3
500
0.25
400
0.2
300
0.15
200
0.1
100
0.05
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Bypass ratio (τ)
t_An_In
© Fraunhofer IKTS
 
- 14 -
S/C = 1.5
2
2.5
3
xCH4 / molCH4 molP-1
LOTUS Parameter Studies
Reformate Quality at Varying Fuel Bypass Ratio
LOTUS Parameter Studies
Process Control Options
↓ SR
0.8
Efficiency-Shift by oxidative
steam reforming:
0.7
 Reduced reformer heat demand
due to partial oxidation of fuel.
 At λREF > 0.325: ATR-point with
steam supply, further increase
of λREF only with liquid H2O.
0.6
Efficiencies / -
 Effect of λREF : ηth , ηSys , ηel 
ATR ↓
0.5
0.4
0.3
0.2
0.1
0.0
0
0.1
0.2
0.3
0.4
0.5
Air ratio / eta_Sys
© Fraunhofer IKTS
- 15 -
eta_el
eta_th
0.6
LOTUS Parameter Studies
Process Control Options
P
s   net
Q
CHP
1.0
s-Control by oxidative steam
reforming:
0.9
0.8
Effects of λREF :
CHP-heat production 

Reformer heat demand  0
0.6
1.5
σ/-

2
0.7
Heat / kW

2.5
σ
0.5
0.4


σ-Shift: 2.2  1
Cell voltage increases due to
changed fuel composition.
1
0.3
0.2
0.5
0.1
0.0
0
0
0.1
0.2
0.3
0.4
0.5
Air ratio / Q_ CHP
© Fraunhofer IKTS
- 16 -
Q_SR
Sigma
0.6
Conclusions & Outlook
Modelling and Simulation Tasks in the LOTUS-Project
Deliv.
Description
D 3.1
System Requirements Document as developed during a joint
SRD-Workshop, hosted by IKTS
finished
06/2011
D 3.2
Prerequisites & Parameter Studies for principal System Design
Decisions, presented and discussed at a joint Workshop (MS4)
finished
09/2011
D 3.3
Steady State Process Layout with Mass Flow & Energy Balance
Sheet (Excel) based on an agreed Process Flow Diagram (PFD)
finished
09/2011
D 3.4
Dynamic Process Model in Modelica / SimulationX, first used for
detailed recalculation of steady state operation at rated conditions
starting
02/1012
D 3.5
Finite State Machine in Modelica StateChart Designer (MiL) for
Control Logic Development and Virtual System Start-up
© Fraunhofer IKTS
Status
- 17 -
t.b.d.
Thanks for your attention!
Thomas Pfeifer
thomas.pfeifer@ikts.fraunhofer.de
Fraunhofer Institute for Ceramic Technologies and Systems IKTS
Winterbergstraße 28, 01277 Dresden, Germany
www.ikts.fraunhofer.de
www.lotus-project.eu
© Fraunhofer IKTS