Transcript Fuel Cells: At the Brink of Change

Fuel Cells: At the Brink of
Change
The Push Toward Fuel Cells
 Environmental Benefits
– CO2 levels (40% – 60% less than gas turbine)
– Very low SOx, Nox, and particulate production
– Electrical Eff. and possible contribution to heating
loads and selling back to grid (total possible conversion
of 90% for energy contained in fuels)
– Possible CO2 sequestration
 Long-term energy solution possible
– Many diffferent possible fuels
• Hydrocarbons, gasified coal, alcohols, pure hydrogen, and
water (?)
 Others
– Quiet
– Longevity and dependability
– Cogeneration/Distributed Power: transmission losses
Fuel Cell History
 Fuel Cells are not a myth or dream!
 Research interest peaks during oil price climes
 Initial research in 60’s and early 70’s.
– First use by military and NASA
 Current push by DOE and DOD to commercialize
production
– Corporate cooperation with AlliedSignal Aerospace
Company, Analytical Power Corporation (Boston),
Avista Laboratories, ONSI Corporation,
Siemens/Westinghouse,etc.
– Over 30 organizations/universities/companies.
 Current use world-wide about 1000 +/- 500
Fuel Cell Technical Concept
 Similar to battery design
 Fuel Cell is a “continuous-feed battery”
 Most oxidize hydrogen at anode and reduce
oxygen at cathode. Ionic Conductive Electrolyte
separates the poles. Each system consisting of
these components is called a “cell”
– Hydrogen is obtained via reactions involving common
hydrocarbon fuels (CO2 is in fact produced in process)
 To obtain useful energy, cells are arranged in units
called stacks (Bipolar plate concept)
 Fuel Cells, when arranged in stacks, can produce
from several watts to 10 MW or more as in
conventional power plants. Size of stack varies
with the required load.
Fuel Cell Classifications
 low- (25 –100o C), medium- (100 – 500o C),
High- (500 – 1000o C), and very hightemperature (1000+o C)
 Types of fuels
– Direct hydrogen-oxygen, organic-oxygen,
nitrogenous-oxygen, hydrogen-halogen, metaloxygen
– Current research targets H2/O2 fuel cells
 Types of electrolyte: standard classification
– Alkaline (AFC), phosphoric acid (PAFC),
polymer electrolyte (PEMFC), molten
carbonate (MCFC), solid oxide (SOFC), others
– Current research targets these 5 types
Complete Energy Production Process
 Step 1: Hydrocarbons converted to H2 and
CO2
– At operation Temp < 600 C, hydrocarbons need
“reformed” before entering fuel cell to obtain
H2 and CO2 by-product.
• Two-stage reformation.
– Ex: (1) CH4 + H2O CO + 3H2
(2) CO + H2O  CO2 + H2
(higher temp)
(lower temp)
• 2nd stage eliminates CO, which poisons FC catalyst.
• Reformer reactions are endothermic, requiring
excess heat from within fuel cell or from burning
exhaust fuel. Operating temp of 800 – 900o C.
• Reformer is 85 – 98% in fuel conversion Eff.
• Reformer system comprises at least 75% of total
unit cost.
• AFC, PEMFC, and PAFC all need reformers.
 (Step 1 Continued)
– At Temp > 600 C, fuels spontaneously convert
into H2 and CO2.
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Fuels fed directly to fuel cell anode
Reformers no longer necessary
Fuel conversion Eff. around 90%
MCFC and SOFC don’t require a reformer.
 Second Step: Fuel Cell Reactions
– Anode:
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•
•
•
H2 diffuses from gaseous feed to electrolyte
H2 diffuses through electrolyte to catalyst surface
H2 adsorbs to catalyst and associates for ionization
H2 activation energy for disassociation is lowered and H2 is
ionized in presence of electrolyte
– H+ ions travel through electrolyte solution to cathode for
reaction with O2
– Electrons travel through anode and integrate with circuit.
– Cathode:
• O2 diffuses from air feed onto catalyst surface
• O2 adsorbs to catalyst and associates with neighboring
electrolyte-H+ complex.
• Overall reaction, after formation of several intermediates,
involves reduction of O2 and reformation of water (or –OH).
• O2 reduction is kinetically slow, hence, economic and technical
limiting step due to high bond strength present in O2
– Electrolyte ionically travels to anode/cathode sites
through the electrolyte for reaction
 Third Step: (Optional) Power Conditioning
– DC power created by FC must be converted to AC via a
power conditioner.
 Miscellaneous Design Variables
– H2 feed compressor – higher conversion
– Heat transfer units – increase overall efficiency by
withdrawing heat produced by reaction and electric
resistance losses
• Use by building
• Use within system for generation of steam in reforming or for
heat in endothermic hydrocarbon shift to CO and H2
– Recirculation of hot gases within system
• Heat transfer conduits in bipolar plates
– Blowers and fans
– Gas-turbine system in series with fuel cell to capture
energy of exiting flue/reformed gases
Top Left: 250 kW Ballard
Generation Systems’ natural gas fuel
cell at Crane Naval Surface Warfare
Center. Bottom Left: Fuel Cell Inc.
250 W Direct Fuel Cell at
headquarters, powering company
during day and grid during night. Top
Right: H Power unit sold in NE U.S.
for storms/power failures. Couldn’t
keep up with high demand.
Top Left: Siemens-Westinghouse
220 kW SOFC/gas turbine unit at
Univ. of Cal – Irvine. Total
electrical Eff. of 58%. Bottom
Left: Siemens-Westinghouse
SOFC unit in Netherlands providing
110 kW of electricity to grid and 64
kW of heat to district heating
system. 46% electrical Eff.
Bottom Right: Hyrdogenics
portable PEMFC system that can
operate between –50 and 40 Celcius
Phosphoric Acid Fuel Cell (PAFC)
 Fuels: Reformed hydrocarbons, gasified coal.
 180 – 250oC operating temp.
 Indirect Fuel Cell
 Closest FC technology to commercialization.
Possibilities include:
– Dispersed power plants (5 – 20 MW) using
hydrocarbons
– On-site cogeneration plants (50 – 1000 kW)
 600 – 800 mV/cell
 Expected plant lifetime of stack is 40,000
hours (4.5 yrs) with 5–7% efficiency loss.
Reactions
Anode: H2  2H+ + 2eCathode: ½ O2 + 2H+ + 2e-  H2O
Overall: H2 + ½ O2 H2O
Nernst Equation: N.A.
PAFC Cell Components
COMPONENT
MATERIAL
Anode
Electrode Support
PTFE-bonded Pt/C
0.1 mg Pt/cm2
PTFE-bonded Pt/C
0.5 mg Pt/cm2
Carbon paper (graphite)
Electrolyte Support
PTFE-bonded SiC
Electrolyte
~ 100% H3PO4
Cathode
Advantages
 PA is an excellent
Disadvantages
 Slow oxygen reduction
kinetics: noble metal
electrolyte: thermal,
catalysts necessary
chemical &
(major econ. limitation)
electrochemical stability  Double reforming
 Simple construction:
process (steam and shift
reaction)
carbon, PTFE & SiC
 Carbon base of cathode
 Cell Eff = 50%
degrades over lifetime in
 Power plant eff. = 40%
high temp. or over 0.8V
(major econ. limitation)
 Loss of Pt surface from
sintering
Molten Carbonate FC (MCFC)
 Fuels: Hydrocarbons, gasified coal,
methanol, naphta
 650o C optimal operating temp. (lifetime
and conversion to H2)
 Direct Fuel Cell
 Expected commercialization 5 years after
PAFC. Possibilities include:
– Cogeneration
– Coal-fired baseload electric utility plants
 750 – 950 mV/cell
 Degradation of 5 mV/1000 hr.
 Lifetime unconfirmed: 10,000+ hrs.
Reactions
Reforming at Anode:
(1) CH4 + H2O  3H2 + CO
(2) 3H2 + 2CO32-  3H2O + 3CO2 + 6e(3) CO + CO32-  2CO2 + 2eOverall: CH4 + CO32-  2H2O + 5CO2 + 2e-
Anode: (1) H2 + CO32-  H2O + CO2 + 2e(2) CO + CO32-  2CO2 + 2eShift:
CO + H2O  H2 + CO2
Cathode: ½ O2 + CO2 + 2e-  CO32Nernst Equation:
(minor)
MCFC Cell Components
Component
Material
Anode
Ni – 10 wt % Cr
0.1 – 1 m2/g
Lithiated NiO
0.5 m2/g
 – LiAlO2
0.1-12 m2/g
Al2O3 fibers
62 LiCO3 – 38 K2CO3
50 LiCO3 – 50 Na2CO3
50 LiCO3 - 50 K2CO3
(~ 50 % wt) tape cast
CaCO3, SrCO3, BaCO3
Cathode
Electrolyte Support
Electrolyte
(mol %)
Additives
Advantages
Disadvantages
 Internal fuel reforming
 Noble metal catalysts
 High temp. elec.
at electrodes (major
economic limitation)
 Reforming of fuel gas
for CO removal
 Heat leakage
 Corrosion  lifetime
 Slow O2 reduction
 Ni dissolution causing
cathode reduction and
anode deposition
 Carbon deposition on
anode
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Efficiency & waste heat
transfer
Rapid kinetics
CO2 recycle
Greater fuel flexability
and overall Eff. over
PAFC
100% fuel reformation
highest possible cell
voltage
Cooling load eliminated
by internal reforming
Solid Oxide FC (SOFC)
 Fuels: hydrocarbons, alcohols, gasified
coal, diesel oil, naphta, coal gas.
 1000oC operating temperature
 Direct Fuel Cell
 Potential Uses:
– Coal gasification plants, industrial & utility
power, commercial buildings
 35,000 hours (4.0 yrs) of run-time achieved
 3 types of cell design: tubular
(Westinghouse), flat-plate (Ztec
Ceramatec), or monolithic (Allied Signal)
Reactions
Anode: (1) H2(g) + O2-  H2O(g) + 2e(2) CO(g) + O2-  CO2(g) + 2eCombined Anode:
aH2(g) + bCO(g) + (a+b)O2-  aH2O(g) +bCO2(g) + 2(a+b)e-
Overall Cathode:
½(a + b)O2 (g) + 2(a + b)e-  (a+b)O2Overall Cell Reaction:
½(a+b)O2(g) + aH2(g) + bCO(g)  aH2O(g) + bCO2(g)
SOFC Cell Components
Component
Material
Anode
Ni – Y2O3 stabilized ZrO2
Cathode
Sr – doped lanthanum
manganite
Y2O3 – stabilized ZrO2
Electrolyte
Cell Interconnect
(bipolar plate)
Support Tube
Mg – dobed lanthanum
cromit
Calcia – stabilized ZrO2
Advantages
 Elec. Eff. Over 80%
 High-grade heat
available
 High tolerance to fuel
impurities (sulfur)
 Simplfication: No
CO2 recycle necessary
 No catalysts: quick
oxidation/reduction
 96% of theoretical
voltage maintained
 Solid oxide electrolyte
very durable/reliable
Disadvantages
 6% less efficient than
MCFC in terms of
maximum voltage
output (equilibrium
conversion)
 Needs 5 – 10 year
lifespan for
commercialization:
corrosion effects
Environmental Analysis
 Emissions
– Fuels with highest H to C ratios are best for low
CO2 emissions (petroleum, methanol, natural
gas)
– Because fuel cells demand clean fuel for correct
operation, their emissions are very low by
default
– Very low NOx emissions from PAFC because
tail gas is burned for heat for fuel processor
– Very low NOx emissions in MCFC because
anode gas passed by cathode, which effectively
scrubs NOx compounds
– Overall economic benefit in preventing capital
costs of product gas cleansing equipment
Comparison of Power Plant
Emissions
Plant Type
SOx
(kg/kW*h)
NOx
(kg/kW*h)
Particulates
(kg/kW*h)
Gas-fired
----
0.89
0.45
Oil-fired
3.35
1.25
0.42
Coal-fired
4.95
2.89
0.41
FCG-1 FC
0.000046
0.031
0.0000046
EPA
LIMITS
1.24
0.464
0.155
Other Green Aspects
 Acoustic Emissions: Noise level at 30 m from a
PAFC unit is 55 dB
– Equivalent to a household air conditioner
 Low amounts of waste heat (as opposed to
the Carnot cycle
– Heat not used for cogeneration can be dumped into air
 Prior two facts make the fuel cell ideal for use in
urban, residential, and isolated areas
 Dismantling of cells and reclamation of parts after
end of lifespan will definitely occur because of the
economic value of catalysts (particularly Pt),
electrolytes, electrodes, and support materials
(ceramics)
Current FC Economics
 Positive Economics/Performance
– Strong DOE funding and support
– Some authors estimate cost of air pollutants to be 13%
of GNP
– Carbon Dioxide tax?
• Proposed in Germany
• Collection of CO2 by reformers
– Twice as efficient as conventional plants even without
cogeneration
– Peak oil production expected between 2010 and 2020:
• Definitely not 100 years of oil remaining
• fuel cells will provide significant energy for U.S.
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Variety of fuels capable of reformation or use in IRFCs
Modularity in mass production and efficiency
Partial load efficiency
No voltage spikes/current oscillations w/ cogeneration
 Difficulties in economics
– Gasoline Infrastructure difficult to reverse.
– Consequently, methanol and methane
infrastructure difficult to establish
 Most important current costs to overcome
– Capital cost
– Stack replacement costs and O&M
– FC fuel costs/kW compared to gas turbine fuel
costs/kW
Current Performance
 20 – 40% energy service cost savings over
conventional energy service in large buildings
– 36% of U.S. energy consumption by building sector
 Residential units available for $3000-$5000/6 kW
– Small-Scale FC Commercialization Group:
• $0.07/kW for methane and $0.11/kW for propane in residential
fuel cells
• Compared to $0.03/kW to $0.15/kW throughout U.S.
– Conclusion: best economic performance in areas of high
electricity costs and low methane costs (California!)
 Power plant PAFC from ONSI Corp: $4000/kW
 Economics improved by several adjustments
– Cogeneration with water and space heating
– Electricity sold back to the grid
– Production of hydrogen during off-peak hours and
energy storage for peak loads
Anticipated Performance
 2nd generation (2003) FC performance
targets
– $1000 - $1500/kW total cost
– 50 – 60% efficiencies
 21st century FC performance targets
– Stack cost of $100/kW
– Total system cost of $400/kW
– 70 – 80% efficiencies
– Near-zero emissions
The Future
 Water Fuel Cells
– This is a myth (currently)
– no catalysts known can decompose water at a
sufficient rate
• Current method of splitting water is electrolysis:
obvious problem
– No mechanism even found in research
• Possibly H2O  H2 + 1/2O2 (energetically
unfavorable at standard conditions)
Current Research and Production
FC TYPE
USES
Organization
Internat’l Fuel Cells
Alkaline (AFC)
Molten Carbonate
(MCFC)
space
Phosporic Acid
(PAFC)
Proton Exchange
(PEM)
Stationary
power/vehicles
Specialty
power/vehicles
Solid Oxide
(SOFC)
Allied Signal Aerospace
Stationary
power/vehicles Institute of Gas Technology
M-C Power (IL)
Energy Research
Corporation (CT)
International Fuel Cells
Fuel Corp. of America (PA)
Dow Chemical (MI, AR)
Electrochem (MA)
Energy Partners (FL)
H-Power (NJ, CA)
Internat’l Fuel Cells (CT)
SOFCo (UT)
Technology Management
Westinghouse (PA)
Ztek (MA)