Micro-SOFCs for portable Micro-SOFCs for portable power generationpower generation

Paul D. RonneyPaul D. RonneyDepartment of Aerospace and Department of Aerospace and

Mechanical EngineeringMechanical EngineeringUniversity of Southern California, Los University of Southern California, Los

Angeles, CA 90089 USAAngeles, CA 90089 USAPresented at the Institute for Nuclear Presented at the Institute for Nuclear Energy Research, Jhong-Li, TaiwanEnergy Research, Jhong-Li, Taiwan

October 4, 2005October 4, 2005

University of Southern CaliforniaUniversity of Southern California

Established 125 years ago Established 125 years ago this week!this week! ……jointly by a Catholic, a Protestant and a Jew - USC has always jointly by a Catholic, a Protestant and a Jew - USC has always

been a multi-ethnic, multi-cultural, coeducational universitybeen a multi-ethnic, multi-cultural, coeducational university Today: 32,000 students, 3000 facultyToday: 32,000 students, 3000 faculty 2 main campuses: University Park and Health Sciences2 main campuses: University Park and Health Sciences USC Trojans football team ranked #1 in USA last 2 yearsUSC Trojans football team ranked #1 in USA last 2 years

USC Viterbi School of EngineeringUSC Viterbi School of Engineering

Naming gift by Andrew & Erma ViterbiNaming gift by Andrew & Erma Viterbi Andrew Viterbi: co-founder of Qualcomm, co-inventor of CDMAAndrew Viterbi: co-founder of Qualcomm, co-inventor of CDMA 1900 undergraduates, 3300 graduate students, 165 faculty, 30 1900 undergraduates, 3300 graduate students, 165 faculty, 30

degree optionsdegree options $135 million external research funding$135 million external research funding Distance Education Network (DEN): 900 students in 28 M.S. degree Distance Education Network (DEN): 900 students in 28 M.S. degree

programs; programs; 1171 MS degrees awarded in 200571 MS degrees awarded in 2005 More info: More info: http://viterbi.usc.eduhttp://viterbi.usc.edu

Paul RonneyPaul Ronney

B.S. Mechanical Engineering, UC BerkeleyB.S. Mechanical Engineering, UC Berkeley M.S. Aeronautics, CaltechM.S. Aeronautics, Caltech Ph.D. in Aeronautics & Astronautics, MITPh.D. in Aeronautics & Astronautics, MIT Postdocs: NASA Glenn, Cleveland; US Naval Research Lab, Postdocs: NASA Glenn, Cleveland; US Naval Research Lab,

Washington DCWashington DC Assistant Professor, Princeton UniversityAssistant Professor, Princeton University Associate/Full Professor, USCAssociate/Full Professor, USC Research interestsResearch interests

Microscale combustion and power generation Microscale combustion and power generation (10/4, INER; 10/5 NCKU)(10/4, INER; 10/5 NCKU)

Microgravity combustion and fluid mechanics Microgravity combustion and fluid mechanics (10/4, NCU)(10/4, NCU) Turbulent combustion Turbulent combustion (10/7, NTHU)(10/7, NTHU) Internal combustion enginesInternal combustion engines Ignition, flammability, extinction limits of flames Ignition, flammability, extinction limits of flames (10/3, NCU)(10/3, NCU) Flame spread over solid fuel bedsFlame spread over solid fuel beds Biophysics and biofilms Biophysics and biofilms (10/6, NCKU)(10/6, NCKU)

Paul RonneyPaul Ronney

Swiss roll

Energy storage density of hydrocarbon fuels (e.g. propane, Energy storage density of hydrocarbon fuels (e.g. propane, 46.4 MJ/kg) >> batteries (≈ 0.5 MJ/kg for Li-ion)46.4 MJ/kg) >> batteries (≈ 0.5 MJ/kg for Li-ion)

Mesoscale or microscale fuel Mesoscale or microscale fuel electrical power conversion electrical power conversion device would provide much higher energy/weight than device would provide much higher energy/weight than batteries for low power applications, even with very low batteries for low power applications, even with very low efficiencyefficiency

Problems at micro-scalesProblems at micro-scales Heat losses to walls - quenching, efficiency lossHeat losses to walls - quenching, efficiency loss Friction losses in devices with moving partsFriction losses in devices with moving parts Precision manufacturing and assembly difficultPrecision manufacturing and assembly difficult

Micro-scale power generation - Why?Micro-scale power generation - Why?

1/2 O2 + 2e- O=

Conventional dual chamber SOFCConventional dual chamber SOFC

fuelfuel oxidantoxidant

CH4 + 4O=

CO2 + 2H2O +8e-

sealsseals

Why solid oxide fuel cells ?Why solid oxide fuel cells ?

AdvantagesAdvantages Uses hydrocarbons (Propane: 12.9 kWh/kg (other HCs similar); Uses hydrocarbons (Propane: 12.9 kWh/kg (other HCs similar);

methanol 2.3x lower; formic acid 8.4x lower )methanol 2.3x lower; formic acid 8.4x lower ) No CO poisoningNo CO poisoning High power (≈ 400 mW/cmHigh power (≈ 400 mW/cm22 vs ≈ 100 mW/cm vs ≈ 100 mW/cm22 for DMFCs) for DMFCs)

DisadvantagesDisadvantages Not thought to be suitable for micropower generation because Not thought to be suitable for micropower generation because

of high temperature needed (thermal management difficult)of high temperature needed (thermal management difficult) Sealing / thermal cycling problemsSealing / thermal cycling problems CokingCoking Need to pump & meter 2 separate streams (fuel & air)Need to pump & meter 2 separate streams (fuel & air)

1D counterflow heat exchanger and reactor

Linear device rolled up into 2D “Swiss roll” reactor

(Weinberg, 1970’s)

Reaction zoneReaction zone

ReactantsReactants

ProductsProducts

600600 500500

600600 400400

250250 150150

150150 5050

Reaction zoneReaction zone

ProductsProductsReactantsReactants

Solution to thermal management Solution to thermal management

Transfer heat from exhaust to incoming gases in “Swiss roll” to Transfer heat from exhaust to incoming gases in “Swiss roll” to minimize heat losses and quenchingminimize heat losses and quenching React in center of spiral counter-current “Swiss roll” heat exchangerReact in center of spiral counter-current “Swiss roll” heat exchanger Operates effectively over wide range of Re and equivalence ratioOperates 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

Solution to thermal cycling & cokingSolution to thermal cycling & coking

Single chamber solid oxide fuel cell - Hibino et al. Science (2000)Single chamber solid oxide fuel cell - Hibino et al. Science (2000) Fuel & oxidant mixed - no sealing issues, no coking problemsFuel & oxidant mixed - no sealing issues, no coking problems ““Reforming” done directly on anodeReforming” done directly on anode Highly selective anode & cathode catalysts essential since fuel & Highly selective anode & cathode catalysts essential since fuel &

oxidant exposed to both anode & cathodeoxidant exposed to both anode & cathode

CHCH44 + .5 O + .5 O22 CO + 2H CO + 2H22

HH22 + O + O== H H22O + 2eO + 2e--

CO + OCO + O== CO CO22 + 2e + 2e--

.5 O2 + 2e- O=

anode cathode

O=

CxHy + O2 O2

H2O + CO2

e- e-electrolyte

ObjectivesObjectives

Assess the feasibility of using a single chamber solid oxide Assess the feasibility of using a single chamber solid oxide fuel cell in a Swiss roll heat exchanger for power generation fuel cell in a Swiss roll heat exchanger for power generation at small scalesat small scales

Test using scaled-up devices operated at low to moderate ReTest using scaled-up devices operated at low to moderate Re

Swiss roll designsSwiss roll designs

Baseline: titanium Baseline: titanium (low thermal (low thermal expansion & expansion & conductivity), EDM-conductivity), EDM-cut & weldedcut & welded

Also: DuPont Vespel Also: DuPont Vespel SP-1 polyimide (25x SP-1 polyimide (25x lower thermal lower thermal conductivity), CNC conductivity), CNC milling (world’s first milling (world’s first all polymer all polymer combustor?)combustor?)

5.5 cm

Single-Chamber Fuel Cell developmentSingle-Chamber Fuel Cell development

ComponentComponent MaterialMaterial

ElectrolyteElectrolyte Sm-CeOSm-CeO22 [SDC] [SDC]

AnodeAnode SDC-NiO SDC-NiO [SDC-Ni][SDC-Ni]

CathodeCathode Many typesMany types

Both anode-supported (Caltech) & Both anode-supported (Caltech) & cathode supported (LBL) fuel cells cathode supported (LBL) fuel cells examined; anode-supported examined; anode-supported somewhat better, probably due to somewhat better, probably due to increased area for reformingincreased area for reforming

Spray cathode

NiO + SDC

NiO+SDC

SDC

Dual dry press

Sinter, 1350oC 5h

600oC 5h, 15%H2

Porous anode

Calcine, 950oC 5h, inert gas

cathode

electrolyte

anode

Anode supported

 

Self-sustaining SOFCs in Swiss-roll reactorsSelf-sustaining SOFCs in Swiss-roll reactors

7 cm1.3 cm

0.71 cm2

Implementation of experimentsImplementation of experiments

Mass Flow

Controllers

Air

PC with LabView

Fuel

Flashback

arrestor

NI-DAQ board

Thermocouples

Incoming reactants

PC with LabView NI-DAQ board

Fuelcell

V A V A

Keithley 2420 sourcemeter

Operation limits in Swiss rollOperation limits in Swiss roll

Determine parameters providing optimal operating conditions (T, mixture, Determine parameters providing optimal operating conditions (T, mixture, residence time) for SCFCresidence time) for SCFC

NHNH33-conditioned catalyst very beneficial at very low Re-conditioned catalyst very beneficial at very low Re Lean limit can be richer than stoichiometric (!) (catalytic only)Lean limit can be richer than stoichiometric (!) (catalytic only) Near stoichiometric, higher Re: reaction zone not centeredNear stoichiometric, higher Re: reaction zone not centered

0.1

1

10

1 10 100 1000Reynolds Number

Out-of-centerreaction zone

(cat. & gas-phase)

Catalytic combustion

only

Nocombustion

Nocombustion

Catalytic orgas-phasecombustion

NH3

conditioned catalytic

combustiononly

SCFC targetconditions

fuel lean

fuel rich

propane-air mixtures

Re = VD/ V = Velocity D = Channel width = kinematic viscosity

Calculated at burner inletCalculated at burner inlet

Best performance - Best performance - 370 mW/cm370 mW/cm22 (propane fuel) - higher than PEM (propane fuel) - higher than PEM fuel cells using methanol or formic acidfuel cells using methanol or formic acid

Performance similar to stand-alone fuel cell in furnacePerformance similar to stand-alone fuel cell in furnace

SCFC in Swiss roll - performanceSCFC in Swiss roll - performance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200 1400

Current density (mA/cm2)

T = 540˚CO

2:C

3H

8 = 2.07:1

370 mW/cm2

 

Effect of cell temperature and OEffect of cell temperature and O22:fuel ratio:fuel ratio

Performance not to sensitive to temperature - range of T within Performance not to sensitive to temperature - range of T within 20% of max. power ≈ ±50˚C20% of max. power ≈ ±50˚C

Performance sensitive to Performance sensitive to OO22:fuel ratio - best results at lower O:fuel ratio - best results at lower O22:fuel :fuel ratio (more fuel-rich)ratio (more fuel-rich)

150

200

250

300

350

400

450

460 480 500 520 540 560 580 600 620

1.9 2 2.1 2.2 2.3 2.4 2.5

Temperature effect (O2:fuel = 2.07)

O2:fuel effect (T = 550˚C)

Cell temperature (˚C)

Fuel to O2 mole ratio

Butane: slightly higher power density, but more excess fuel Butane: slightly higher power density, but more excess fuel required to obtain higher powerrequired to obtain higher power

SCFC in Swiss roll - butaneSCFC in Swiss roll - butane

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

50

100

150

200

250

0 100 200 300 400 500 600 700

Voltage (V) Power (mW/cm^2)

Current (mA/cm2)

Propane:O2 = 1:2.26

T = 565-670˚C

0

0.1

0.2

0.3

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0.5

0.6

0.7

0

50

100

150

200

250

300

0 200 400 600 800 1000

Voltage (V) Power (mW/cm^2)

Current (mA/cm2)

Butane:O2 = 1:2

T = 555-660˚C

SCFC in Swiss roll - butaneSCFC in Swiss roll - butane

580

600

620

640

660

680

160

180

200

220

240

490 500 510 520 530 540 550 560 570

Fuel cell T Max. power

Gas temperature (˚C)

Butane:O2 = 1:1.9

600

650

700

750

160

180

200

220

240

1.8 2 2.2 2.4 2.6 2.8 3

Fuel cell T Max. power

Fuel to O2 ratio

Butane, gas temperature 545˚C

Best power: ≈ 570˚C, Fuel:OBest power: ≈ 570˚C, Fuel:O22 ≈ 2 (3.5x stoichiometric!) ≈ 2 (3.5x stoichiometric!) Need supplemental air after partial reaction for improved fuel Need supplemental air after partial reaction for improved fuel

utilizationutilization

Effect of cell orientationEffect of cell orientation

0.1

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0.5

0.6

0.7

0.8

0

50

100

150

200

250

0 200 400 600 800 1000Current Density (mA/cm

2)

Closed symbols: cathode hotterOpen symbols: anode hotter

T = 480˚C, C3H8 : O2 = 1 : 2, and Re = 65

Better performance with cathode side facing the inner (hotter) wallBetter performance with cathode side facing the inner (hotter) wall Cathode function:Cathode function:

Electrochemically react OElectrochemically react O22 with with

ee-- to make O to make O== ions (faster at ions (faster at higher temps)higher temps)

Anode function:Anode function: Prefer lower temps to obtain partial Prefer lower temps to obtain partial

but not complete oxidation of fuelbut not complete oxidation of fuel

SCFC in Swiss roll - effects of temperatureSCFC in Swiss roll - effects of temperature

600

620

640

660

680

700

0

50

100

150

200

500 510 520 530 540 550 560 570 580

Fuel cell T Max. power

Gas temperature (˚C)

Butane:O2 = 1:2.8

620

630

640

650

660

670

680

690

700

0

50

100

150

200

250

510 520 530 540 550 560 570 580

Fuel cell T Max. power

Gas temperature (˚C)

Propane:O2 = 1:2.1

Effect of temperature similar in propane & butaneEffect of temperature similar in propane & butane Fuel cell temperature ≈ 100˚C higher than gas (small T rise Fuel cell temperature ≈ 100˚C higher than gas (small T rise

compared to complete oxidation, ≈ 1500˚C)compared to complete oxidation, ≈ 1500˚C)

SCFC Operation on MethaneSCFC Operation on Methane

Ni + SDC | SDC (20 Ni + SDC | SDC (20 m) | SDC + Bam) | SDC + Ba0.50.5SrSr0.50.5CoCo0.80.8FeFe0.20.2 O O33 (BSCF) (BSCF) Haile et al., Nature, Sept. 9, 2004Haile et al., Nature, Sept. 9, 2004 Monotonic increase in power output with temperatureMonotonic increase in power output with temperature Higher power outputs than with propane (less fuel decomposition at cathode, higher “Octane number”)Higher power outputs than with propane (less fuel decomposition at cathode, higher “Octane number”)

0 500 1000 1500 2000 2500 3000 3500 40000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

100

200

300

400

500

600

700

800

Power density (mW/cm

2)

650 625 600 575 550 525 500

Voltage (Volts)

Current density (mA/cm2)

CH4:87sccm

O2:80sccm

He:320sccm

730 mW/cm2

H

H

H C

2, 2, 4 trimethylpentane(iso-octane)

C

C

C

H

H

C

H

C

C

H

H

C H

H HH

HH H H HH

Higher (liquid) hydrocarbonsHigher (liquid) hydrocarbons

Iso-octane (2, 2, 4 Iso-octane (2, 2, 4 trimethylpentane) used as a trimethylpentane) used as a surrogate for various hydrocarbon surrogate for various hydrocarbon fuels including gasoline, diesel & fuels including gasoline, diesel & JP-8JP-8

““1.5 chamber” fuel cell1.5 chamber” fuel cell Cathode: Ni-SDC, reactant airCathode: Ni-SDC, reactant air Anode: LSCF-GDC, reactant fuel-Anode: LSCF-GDC, reactant fuel-

rich (7% iso-octane in air) mixturerich (7% iso-octane in air) mixture Electrolyte SDCElectrolyte SDC

Enabling technology: “special Enabling technology: “special catalyst layer” on anode (Barnett et catalyst layer” on anode (Barnett et al., Nature 2005)al., Nature 2005)

Iso-octane / air SOFCIso-octane / air SOFC

Power density ≈ 550 mW/cmPower density ≈ 550 mW/cm22 at 600˚C at 600˚C Power density ≈ 250 mW/cm at 450˚C (temperature limit for polymer Power density ≈ 250 mW/cm at 450˚C (temperature limit for polymer

Swiss rolls)Swiss rolls) Iso-octane power comparable to hydrogenIso-octane power comparable to hydrogen Cell stable over 60 hr test, no coking observedCell stable over 60 hr test, no coking observed Needs to be tested in single-chamber cellsNeeds to be tested in single-chamber cells Results should transfer well to other hydrocarbons…Results should transfer well to other hydrocarbons…

Iso-octane / air SOFCIso-octane / air SOFC

Catalyst layer greatly increases longevityCatalyst layer greatly increases longevity

Automotive gasoline / air SOFCAutomotive gasoline / air SOFC

Catalyst/Ni-YSZ/YSZ/LSCF-GDC cellCatalyst/Ni-YSZ/YSZ/LSCF-GDC cell Power density ≈ 900 mW/cmPower density ≈ 900 mW/cm22 at 800˚C at 800˚C No coking except at T < 650˚CNo coking except at T < 650˚C SEM-EDX measurements showed sulfur on the catalyst layer is SEM-EDX measurements showed sulfur on the catalyst layer is

responsible for degradation over timeresponsible for degradation over time

 

ConclusionsConclusions

(Probably) world’s smallest thermally self-sustaining solid oxide (Probably) world’s smallest thermally self-sustaining solid oxide fuel cellfuel cell

Maximum power density ≈ 420 mW/cmMaximum power density ≈ 420 mW/cm22 at T ≈ 550 ˚C at T ≈ 550 ˚C Superior performance was obtained when the cathode side Superior performance was obtained when the cathode side

facing the hotter inner wallfacing the hotter inner wall Fuel cell performance is dependent on both temperature and Fuel cell performance is dependent on both temperature and

mixture composition, but > 50% of peak performance is obtained mixture composition, but > 50% of peak performance is obtained over over T ≈ 200 ˚C (≈ 400 ˚C to 600 ˚C) and T ≈ 200 ˚C (≈ 400 ˚C to 600 ˚C) and ≈ 2 ( ≈ 2 ( ≈ 1.5 to 3.5) ≈ 1.5 to 3.5)

 

Future workFuture work

Potential complete micropower systemPotential complete micropower system Polymer 3D Swiss rollPolymer 3D Swiss roll Hydrocarbon fuelHydrocarbon fuel Single-chamber solid oxide fuel cell for power generationSingle-chamber solid oxide fuel cell for power generation - -

direct utilization of hydrocarbonsdirect utilization of hydrocarbons Thermal transpiration pumping of fuel/air mixtureThermal transpiration pumping of fuel/air mixture - no moving - no moving

parts, uses thermal energy, not electrical energyparts, uses thermal energy, not electrical energy

Polymer combustorsPolymer combustors

Experimental & theoretical studies show importance of wall Experimental & theoretical studies show importance of wall thermal conductivity on combustor performance thermal conductivity on combustor performance (counterintuitive: lower is better) (counterintuitive: lower is better)

Polymer Swiss rolls???Polymer Swiss rolls??? Low k (0.2 - 0.4 W/m˚C)Low k (0.2 - 0.4 W/m˚C) Polyimides, polyetheretherketones, etc., rated to T > 400˚C, even in Polyimides, polyetheretherketones, etc., rated to T > 400˚C, even in

oxidizing atmosphere, suggesting SCFC operation possibleoxidizing atmosphere, suggesting SCFC operation possible Inexpensive, durable, many fabrication optionsInexpensive, durable, many fabrication options

Key issuesKey issues SurvivabilitySurvivability Control of temperature, mixture & residence time for SCFCControl of temperature, mixture & residence time for SCFC

Results - extinction limitsResults - extinction limits

Sustained combustion as low as Sustained combustion as low as 2.9 W2.9 W thermal (candle ≈ 50 W) thermal (candle ≈ 50 W) Extinction limit behavior similar to macroscale at Re > 20Extinction limit behavior similar to macroscale at Re > 20 Improved “lean” limit performance compared to inconel macroscale burner Improved “lean” limit performance compared to inconel macroscale burner

at 2.5 < Re < 20at 2.5 < Re < 20 Good performance under target conditions for SCFCGood performance under target conditions for SCFC Sudden, as yet unexplained cutoff at Re ≈ 2.5 in polymer burnerSudden, as yet unexplained cutoff at Re ≈ 2.5 in polymer burner

1

10

1 10Reynolds Number

Polymer burner,NH

3-conditioned Pt catalyst

Inconel burner,NH

3 conditioned

Pt catalyst

Target conditionsfor SCFCs

Results - temperaturesResults - temperatures

Prolonged exposure at > Prolonged exposure at > 400˚C400˚C (high enough for single chamber SOFCs)(high enough for single chamber SOFCs) with no apparent damage with no apparent damage

Sustained combustion at TSustained combustion at Tmaxmax = = 72˚C72˚C (lowest T ever self-sustaining (lowest T ever self-sustaining hydrocarbon combustion?)hydrocarbon combustion?)

If combustion can be If combustion can be sustainedsustained at 72˚C, with further improved thermal at 72˚C, with further improved thermal management could room temp. management could room temp. ignitionignition be possible? be possible?

 

Thanks to…Thanks to…

Institute of Nuclear Energy ResearchInstitute of Nuclear Energy Research Prof. Shenqyang ShyProf. Shenqyang Shy Combustion Institute (Bernard Lewis Lectureship)Combustion Institute (Bernard Lewis Lectureship) DARPA, USAF (funding for this research)DARPA, USAF (funding for this research)