1. Micro-SOFCs for portable power generation
Paul D. Ronney
Department of Aerospace and Mechanical Engineering
University of Southern California, Los Angeles, CA USA
Presented at the Institute for Nuclear Energy Research, Jhong-Li, Taiwan
October 4, 2005

2. University of Southern California
Established 125 years ago this week!
…jointly by a Catholic, a Protestant and a Jew - USC has always been a multi-ethnic, multi-cultural, coeducational university
Today: 32,000 students, 3000 faculty
2 main campuses: University Park and Health Sciences
USC Trojans football team ranked #1 in USA last 2 years

3. USC Viterbi School of Engineering
Naming gift by Andrew & Erma Viterbi
Andrew Viterbi: co-founder of Qualcomm, co-inventor of CDMA
1900 undergraduates, 3300 graduate students, 165 faculty, 30 degree options
$135 million external research funding
Distance Education Network (DEN): 900 students in 28 M.S. degree programs; 171 MS degrees awarded in 2005
More info:

4. Paul Ronney 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)

5. Paul Ronney

6. 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
Swiss roll

7. 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)
1/2 O2 + 2e-  O=
Conventional dual chamber SOFC
fuel
oxidant
CH4 + 4O=
 CO2 + 2H2O +8e-
seals

8. 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
Products
Reactants
Reaction zone
600
500
250
150
Products
Reactants
600
400
150 50
1D counterflow heat
exchanger and reactor
Linear device rolled up into
2D “Swiss roll” reactor
(Weinberg, 1970’s)

9. 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
cathode O=
electrolyte e- e-
CH O2  CO + 2H2
H2 + O=  H2O + 2e-
CO + O=  CO2 + 2e-
.5 O2 + 2e-  O=

10. 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

11. Swiss roll designs
Baseline: titanium (low thermal expansion & conductivity), EDM-cut & welded
Also: DuPont Vespel SP-1 polyimide (25x lower thermal conductivity), CNC milling (world’s first all polymer combustor?)
5.5 cm

12. Single-Chamber Fuel Cell development
Both anode-supported (Caltech) & cathode supported (LBL) fuel cells examined; anode-supported somewhat better, probably due to increased area for reforming
Component
Material
Electrolyte
Sm-CeO2 [SDC]
Anode
SDC-NiO [SDC-Ni]
Cathode
Many types
Anode supported
Sinter, 1350oC 5h
Dual dry press
NiO + SDC
SDC
Well-suited to electrode studies because one can make many bilayers with the same general morphology, and then try out different electrodes for the third layer.
NiO+SDC
600oC 5h, 15%H2
cathode
electrolyte
anode
Calcine, 950oC 5h, inert gas
Spray cathode
Porous anode

13. Self-sustaining SOFCs in Swiss-roll reactors
1.3 cm
7 cm
0.71 cm2

14. 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 A 
Keithley 2420 sourcemeter
Air
Fuel

15. 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
SCFC target conditions
fuel lean
fuel rich
propane-air mixtures
Re = VD/n
V = Velocity
D = Channel width
n = kinematic viscosity
Calculated at burner inlet
Update this figure

16. SCFC in Swiss roll - performance
Best performance mW/cm2 (propane fuel) - higher than PEM fuel cells using methanol or formic acid
Performance similar to stand-alone fuel cell in furnace
370 mW/cm2

17. 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)

18. SCFC in Swiss roll - butane
Butane: slightly higher power density, but more excess fuel required to obtain higher power

19. 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

20. Effect of cell orientation
Better performance with cathode side facing the inner (hotter) wall
Cathode function:
Electrochemically react O2 with e- to make O= ions (faster at higher temps)
Anode function:
Prefer lower temps to obtain partial but not complete oxidation of fuel
T = 480˚C, C3H8 : O2 = 1 : 2, and Re = 65

21. 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)

22. 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”)
730 mW/cm2
Excellent performance due to low activity of cathode for methane oxidation. At higher temperatures, oxygen electroreduction at cathode improves faster than methane oxidation. OCV goes down with temperature, probably due to increasing electronic conductivity of ceria.
To my knowledge, we now hold the record for SCFC power densities on methane. We’re not exactly sure why SDC + BSCF as the cathode works better than just BSCF for SCFCs, but it may have something to do with SDC suppressing the negative impact of CO2 on the performance of BSCF.

23. 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 fuel-rich (7% iso-octane in air) mixture
Electrolyte SDC
Enabling technology: “special catalyst layer” on anode (Barnett et al., Nature 2005)

24. 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…

25. Iso-octane / air SOFC
Catalyst layer greatly increases longevity

26. Automotive gasoline / air SOFC
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

27. 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)

28. 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

29. Polymer combustors
Experimental & theoretical studies show importance of wall thermal conductivity on combustor performance (counterintuitive: lower is better)
Polymer Swiss rolls???
Low k ( 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

30. Results - extinction limits
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

31. 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?

32. Thanks to… Institute of Nuclear Energy Research Prof. Shenqyang Shy
Combustion Institute (Bernard Lewis Lectureship)
DARPA, USAF (funding for this research)