Transcript AME 436 Energy and Propulsion

Micro-SOFCs for portable
power generation
Paul D. Ronney
Department of Aerospace and
Mechanical Engineering
University of Southern California, Los
Angeles, CA 90089 USA
Presented at the Institute for Nuclear
Energy Research, Jhong-Li, Taiwan
October 4, 2005
University of Southern California
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Paul Ronney
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B.S. Mechanical Engineering, UC Berkeley
M.S. Aeronautics, Caltech
Ph.D. in Aeronautics & Astronautics, MIT
Postdocs: NASA Glenn, Cleveland; US Naval Research Lab,
Washington DC
 Assistant Professor, Princeton University
 Associate/Full Professor, USC
 Research interests
 Microscale combustion and power generation
(10/4, INER; 10/5 NCKU)
 Microgravity combustion and fluid mechanics (10/4, NCU)
 Turbulent combustion (10/7, NTHU)
 Internal combustion engines
 Ignition, flammability, extinction limits of flames (10/3, NCU)
 Flame spread over solid fuel beds
 Biophysics and biofilms (10/6, NCKU)
Paul Ronney
Micro-scale power generation - Why?
 Energy storage density of hydrocarbon fuels (e.g. propane,
46.4 MJ/kg) >> batteries (≈ 0.5 MJ/kg for Li-ion)
 Mesoscale or microscale fuel  electrical power conversion
device would provide much higher energy/weight than
batteries for low power applications, even with very low
efficiency
 Problems at micro-scales
 Heat losses to walls - quenching, efficiency loss
 Friction losses in devices with moving parts
 Precision manufacturing and assembly difficult
Why solid oxide fuel cells ?
 Advantages
 Uses hydrocarbons (Propane: 12.9 kWh/kg (other HCs similar);
methanol 2.3x lower; formic acid 8.4x lower )
 No CO poisoning
 High power (≈ 400 mW/cm2 vs ≈ 100 mW/cm2 for DMFCs)
 Disadvantages
 Not thought to be suitable for micropower generation because
of high temperature needed (thermal management difficult)
 Sealing / thermal cycling problems
 Coking
 Need to pump & meter 2 separate streams (fuel & air)
Conventional dual chamber SOFC
fuel
CH4 + 4O=
 CO2 + 2H2O +8e-
oxidant
1/2 O2 + 2e-  O=
seals
Solution to thermal management
 Transfer heat from exhaust to incoming gases in “Swiss roll” to
minimize heat losses and quenching
 React in center of spiral counter-current “Swiss roll” heat exchanger
 Operates effectively over wide range of Re and equivalence ratio
 Reduces heat losses, sustain high core temperatures with low surface
& exhaust temperatures, even at small scales
Reaction zone
Reaction zone
600
500
250
150
Products
Reactants
600
400
150
1D counterflow heat
exchanger and reactor
50
Products
Reactants
Linear device rolled up into
2D “Swiss roll” reactor
(Weinberg, 1970’s)
Solution to thermal cycling & coking
 Single chamber solid oxide fuel cell - Hibino et al. Science (2000)
 Fuel & oxidant mixed - no sealing issues, no coking problems
 “Reforming” done directly on anode
 Highly selective anode & cathode catalysts essential since fuel &
oxidant exposed to both anode & cathode
H2O + CO2
CxHy + O2
O2
anode
e-
electrolyte
cathode
O=
CH4 + .5 O2  CO + 2H2
H2 + O=  H2O + 2eCO + O=  CO2 + 2e-
e.5 O2 + 2e-  O=
Objectives
 Assess the feasibility of using a single chamber solid oxide
fuel cell in a Swiss roll heat exchanger for power generation
at small scales
 Test using scaled-up devices operated at low to moderate Re
Swiss roll designs
 Baseline: titanium
(low thermal
expansion &
conductivity), EDMcut & welded
 Also: DuPont Vespel
SP-1 polyimide (25x
lower thermal
conductivity), CNC
milling (world’s first
all polymer
combustor?)
5.5 cm
Single-Chamber Fuel Cell development
Component
Material
Electrolyte
Sm-CeO2 [SDC]
Anode
SDC-NiO
[SDC-Ni]
Cathode
Many types
Anode supported
 Both anode-supported (Caltech) &
cathode supported (LBL) fuel cells
examined; anode-supported
somewhat better, probably due to
increased area for reforming
Dual dry
press
SDC
NiO + SDC
Sinter,
1350oC 5h
NiO+SDC
cathode
electrolyte
anode
Calcine, 950oC
5h, inert gas
Spray
cathode
600oC 5h,
15%H2
Porous
anode
Self-sustaining SOFCs in Swiss-roll reactors
1.3 cm
7 cm
0.71 cm2
Implementation of experiments
NI-DAQ board
PC with LabView
Thermocouples
Fuelcell
Mass Flow
Controllers
Flashback
arrestor
Incoming reactants
PC with LabView
NI-DAQ board
V
Air
Fuel
A

Keithley 2420
sourcemeter
Operation limits in Swiss roll
 Determine parameters providing optimal operating conditions (T, mixture,
residence time) for SCFC
 NH3-conditioned catalyst very beneficial at very low Re
 Lean limit can be richer than stoichiometric (!) (catalytic only)
 Near stoichiometric, higher Re: reaction zone not centered
propane-air mixtures
Equivalence Ratio
Re = VD/n
No
combustion
fuel rich

SCFC target
conditions
10
Catalytic
combustion
only
1
NH3
conditioned
catalytic
combustion
only
fuel lean

Catalytic or
gas-phase
combustion
Out-of-center
reaction zone
(cat. & gas-phase)
No
combustion
0.1
1
10
100
Reynolds Number
1000

V = Velocity
D = Channel width
n = kinematic viscosity
Calculated at burner inlet
SCFC in Swiss roll - performance
 Best performance - 370 mW/cm2 (propane fuel) - higher than PEM
fuel cells using methanol or formic acid
 Performance similar to stand-alone fuel cell in furnace
0.8
400
0.7
350
0.6
300
0.5
250
0.4
200
0.3
150
2
Power (mW/cm )
Voltage (V)
370 mW/cm2
0.2
100
T = 540ûC
O :C H = 2.07:1
0.1
2
3
50
8
0
0
200
400
600
800
1000
2
Current density (mA/cm )
1200
0
1400
Effect of cell temperature and O2:fuel ratio
 Performance not to sensitive to temperature - range of T within
20% of max. power ≈ ±50˚C
 Performance sensitive to O2:fuel ratio - best results at lower O2:fuel
ratio (more fuel-rich)
Fuel to O mole ratio
2
1.9
450
2
2.1
2.2
2.3
2.4
2.5
2
Maximum power (mW/cm )
Temperature ef fect (O:f uel = 2.07)
2
O :f uel ef fect (T = 550ûC)
400
2
350
300
250
200
150
460
480
500
520
540
560
580
Cell temperature (ûC)
600
620
SCFC in Swiss roll - butane
 Butane: slightly higher power density, but more excess fuel
required to obtain higher power
Voltage (V)
Pow e r (m W/cm ^2)
0.7
Voltage (V)
250
Voltage (V)
150
0.4
0.3
100
Pr opane :O = 1:2.26
2
0
0
100
200
300
400
0.6
250
0.5
200
0.4
150
0.3
100
0.2
Butane :O = 1:2
50
T = 565-670ûC
0.1
300
2
T = 555-660ûC
0.1
500
2
Current (mA/cm )
600
0
700
0
0
200
400
50
600
2
Current (mA/cm )
800
0
1000
2
2
0.2
0.7
Power (mW/cm )
0.5
Power (mW/cm )
200
Voltage (V)
0.6
Pow e r (m W/cm ^2)
SCFC in Swiss roll - butane
 Best power: ≈ 570˚C, Fuel:O2 ≈ 2 (3.5x stoichiometric!)
 Need supplemental air after partial reaction for improved fuel
utilization
Fue l ce ll T
Max. pow e r
Fue l ce ll T
640
200
620
Butane :O = 1:1.9
2
580
490
500
510
520
530
540
550
Gas temperature (ûC)
160
560
570
220
700
200
650
180
Butane , gas te m pe r atur e 545ûC
600
1.8
2
2.2
2.4
Fuel to O
2
2.6
ratio
160
2.8
3
2
600
2
180
Fuel cell temperature (ûC)
220
240
Power (mW/cm )
660
Max. pow e r
750
240
Power (mW/cm )
Fuel cell temperature (ûC)
680
Effect of cell orientation
 Better performance with cathode side facing the inner (hotter) wall
 Anode function:
 Cathode function:
 Electrochemically react O2 with
e- to make O= ions (faster at
higher temps)
0.8
250
Closed symbols: cathode hotter
Open symbols: anode hotter
200
Voltage (V)
0.6
0.5
150
0.4
100
0.3
50
0.1
2
0.2
Power Density (mW/cm )
0.7
 Prefer lower temps to obtain
partial but not complete
oxidation of fuel
0
800
600
400
200
2
Current Density (mA/cm )
0
1000
T = 480˚C, C3H8 : O2 = 1 : 2, and Re = 65
SCFC in Swiss roll - effects of temperature
 Effect of temperature similar in propane & butane
 Fuel cell temperature ≈ 100˚C higher than gas (small T rise
compared to complete oxidation, ≈ 1500˚C)
Fue l ce ll T
690
150
660
100
650
Pr opane :O = 1:2.1
2
630
620
510
50
520
530
540
550
560
Gas temperature (ûC)
570
0
580
680
150
660
100
640
2
2
640
200
Power (mW/cm )
670
Power (mW/cm )
200
680
Max. pow e r
700
250
Fuel cell temperature (ûC)
700
Fuel cell temperature (ûC)
Fue l ce ll T
Max. pow e r
50
620
Butane :O = 1:2.8
2
600
500
510
520
530
540
550
560
Gas temperature (ûC)
570
0
580
SCFC Operation on Methane
 Ni + SDC | SDC (20 mm) | SDC + Ba0.5Sr0.5Co0.8Fe0.2 O3 (BSCF)
 Haile et al., Nature, Sept. 9, 2004
 Monotonic increase in power output with temperature
 Higher power outputs than with propane (less fuel decomposition at
cathode, higher “Octane number”)
650
625
600
575
550
525
500
0.7
Voltage (Volts)
0.6
0.5
0.4
0.3
800
2
730 mW/cm2
700
600
500
400
300
CH4:87sccm
0.2
200
O2:80sccm
0.1
100
He:320sccm
0.0
0
500
0
1000 1500 2000 2500 3000 3500 4000
2
Current density (mA/cm )
Power density (mW/cm )
0.8
Higher (liquid) hydrocarbons
 Iso-octane (2, 2, 4
trimethylpentane) used as a
surrogate for various hydrocarbon
fuels including gasoline, diesel &
JP-8
 “1.5 chamber” fuel cell
 Cathode: Ni-SDC, reactant air
 Anode: LSCF-GDC, reactant fuelrich (7% iso-octane in air) mixture
 Electrolyte SDC
 Enabling technology: “special
catalyst layer” on anode (Barnett et
al., Nature 2005)
H H H
H
H
C
H
H
H
C
C
C
C
C
H
C
H
C
H
H H H H H H
2, 2, 4 trimethylpentane
(iso-octane)
H
Iso-octane / air SOFC
 Power density ≈ 550 mW/cm2 at 600˚C
 Power density ≈ 250 mW/cm at 450˚C (temperature limit for polymer
Swiss rolls)
 Iso-octane power comparable to hydrogen
 Cell stable over 60 hr test, no coking observed
 Needs to be tested in single-chamber cells
 Results should transfer well to other hydrocarbons…
Iso-octane / air SOFC
 Catalyst layer greatly increases longevity
Automotive gasoline / air SOFC
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Catalyst/Ni-YSZ/YSZ/LSCF-GDC cell
Power density ≈ 900 mW/cm2 at 800˚C
No coking except at T < 650˚C
SEM-EDX measurements showed sulfur on the catalyst layer is
responsible for degradation over time
Conclusions
 (Probably) world’s smallest thermally self-sustaining solid oxide
fuel cell
 Maximum power density ≈ 420 mW/cm2 at T ≈ 550 ˚C
 Superior performance was obtained when the cathode side facing
the hotter inner wall
 Fuel cell performance is dependent on both temperature and
mixture composition, but > 50% of peak performance is obtained
over T ≈ 200 ˚C (≈ 400 ˚C to 600 ˚C) and  ≈ 2 ( ≈ 1.5 to 3.5)
Future work
 Potential complete micropower system
 Polymer 3D Swiss roll
 Hydrocarbon fuel
 Single-chamber solid oxide fuel cell for power generation direct utilization of hydrocarbons
 Thermal transpiration pumping of fuel/air mixture - no moving
parts, uses thermal energy, not electrical energy
Polymer combustors
 Experimental & theoretical studies show importance of wall
thermal conductivity on combustor performance
(counterintuitive: lower is better)
 Polymer Swiss rolls???
 Low k (0.2 - 0.4 W/m˚C)
 Polyimides, polyetheretherketones, etc., rated to T > 400˚C, even in
oxidizing atmosphere, suggesting SCFC operation possible
 Inexpensive, durable, many fabrication options
 Key issues
 Survivability
 Control of temperature, mixture & residence time for SCFC
Results - extinction limits
Equivalence ratio at extinction limit
 Sustained combustion as low as 2.9 W thermal (candle ≈ 50 W)
 Extinction limit behavior similar to macroscale at Re > 20
 Improved “lean” limit performance compared to inconel macroscale burner
at 2.5 < Re < 20
 Good performance under target conditions for SCFC
 Sudden, as yet unexplained cutoff at Re ≈ 2.5 in polymer burner
10
Inconel burner,
NH conditioned
3
Pt catalyst
Target conditions
for SCFCs
1
Polymer burner,
NH -conditioned Pt catalyst
3
10
1
Reynolds Number
Results - temperatures
 Prolonged exposure at > 400˚C (high enough for single chamber SOFCs)
with no apparent damage
 Sustained combustion at Tmax = 72˚C (lowest T ever self-sustaining
hydrocarbon combustion?)
 If combustion can be sustained at 72˚C, with further improved thermal
management could room temp. ignition be possible?
Thanks to…
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Institute of Nuclear Energy Research
Prof. Shenqyang Shy
Combustion Institute (Bernard Lewis Lectureship)
DARPA, USAF (funding for this research)