Transcript TECH 57210 Sustainable Energy I

TECH 57210
Sustainable Energy I
Energy Sources and Systems 8
Dr. Darwin L. Boyd
Fall 2011
Fuel Cells
• Fuel Cell – Electrochemical energy conversion device in which
fuel and oxidant react to generate electricity without any
consumption, physically or chemically, of its electrodes or
electrolyte.
Fuel Cell
Storage Cell
_
+
Pb
H2SO4
H2
_ +
Air
(O2)
Pb
H2O
Storage cell: reactants self contained
and electrodes consumed
Lead-Acid Battery Reaction
Pb + PbO2 + H2SO4  2 PbSO4 + 2 H2O
Fuel cell: reactants supplied
continuously and electrodes
invariant
Overall Fuel Cell Reactions:
H2 + O2  H2O + heat + electrons
Fuel Cells
Photographs from FC History
William Jacques'
carbon battery, 1896
William Grove's drawing of an
experimental “gas battery“, 1843
Allis-Chambers PAFC engine, 1965
US Army MCFC, 1966
4
eee-
e-
e-
O-
H+
H2
2H+
+
2e-
O-
Electrolyte
Cathode +
H+
H+
Anode -
Bipolar Plate
H+
O-
5
HYDROGEN
(H2)
PEMFC: Protons formed
OXYGEN
at the anode diffuse through
(O2)
the electrolyte and react with
electrons and oxygen at the
cathode to form water and heat.
Bipolar Plate
e-
½O2 + 2H+ + 2e-
H 2O
WATER (H2O) + HEAT
6
Single cells are arranged into
“stacks” to increase total
voltage and power output
Ballard PEFC Stack
Cathode:
Anode:
O2 + 4H+ + 4e-  2H2O
1.2 V
2H2  4H+ + 4e- - 0 V
Total Cell:
Power = Volts X Amps
2H2 + O2  2H2O
1.2 V per cell
Fuel Cell System
Electric Power
Conditioner
7
Air
Fuel
Air
Fuel
Processor
Fuel Cell
Stack
Spent-Gas
Burner
H2
Thermal & Water Management
Exhaust
Fuel Processor BARRIERS
 Fuel processor start-up/
transient operation
 Durability
 Cost
 Emissions and environmental
issues
 H2 purification/CO cleanup
 Fuel processor system
integration and efficiency
Power
OXYGEN
HYDROGEN
e -
e -
e -
O2
H+
O2
PROX REACTOR
CO+O2CO2
H2-rich
gas
O2
H+
Bipolar Plate
WGS REACTOR
CO+H2OCO2+H2
CO < 10 ppm
Cathode +
REFORMER
CXHY+H2O+O2H2+CO
CO < 0.5%
Electrolyte
CO = 15%
Anode -
FUEL
Bipolar Plate
H+
8
On-Board Fuel Processing
Five major types of fuel cells
Temperature
Fuel Cell Type
Applications
Electrolyte / Ion
Polymer Electrolyte
Membrane
(PEM)
Alkaline
(AFC)
Phosphoric Acid
(PAFC)
Molten Carbonate
(MCFC)
Solid Oxide
(SOFC)
60 - 100° C
Perfluorosulfonic acid / H+
90 – 100° C
KOH / OH175 – 200° C
H3PO4 / H+
600 – 1000° C
(Li,K,Na)2CO3 / CO2600 – 1000° C
(Zr,Y) O2 / O-
Electric utility
Portable power
Transportation
Military
Space
Electric utility
Distributed power
Transportation
Electric utility
Distributed power
Electric utility
Distributed power
APUs
9
Alkaline Fuel Cell (AFC)
Applications
• Space
• Transportation
Features
• High performance
• Very sensitive to CO2
• Expensive Pt electrodes
Status
• “Commercially” available
Equations
Cathode:
Anode:
½O2 + H2O + 2e¯ → 2OH¯
H2 + 2OH¯ → 2H2O + 2e¯
AFCs from Apollo & Spaceshuttle
Spacecrafts-- NASA
10
Phosphoric Acid Fuel Cell
Applications
• Distributed power plants
• Combined heat and power
• Some buses
Features
11
• Some fuel flexibility
• High efficiency in cogeneration (85%)
• Established service record
• Platinum catalyst
Status
• Commercially available but expensive
• Excellent reliability and availability
• Millions of hours logged
Equations
Cathode: ½O2 + 2H+ + 2e¯ → H2O
Anode:
H2
→
2H+ + 2e¯
UTC Fuel Cells 200-kW
Molten Carbonate Fuel Cells
Applications
• Distributed power plants
• Combined heat and power
Features
• Fuel flexibility (internal reforming)
12
• High efficiency
• High temperature good for cogeneration
• Base materials (nickel electrodes)
• Corrosive electrolyte
Status
• Pre-Commercially available but expensive
Equations
Cathode: ½O2 + CO2 + 2e¯ → CO3=
Anode:
H2 + CO3= → 2H2O + CO2 + 2e¯
Fuel Cell Energy MCFC stack
Solid Oxide Fuel Cells
Applications
• Truck APUs
• Distributed power plants
• Combined heat and power
•
•
•
•
•
•
Slow start – subject to thermal shock
High temperature
High power density (watts/liter)
Can use CO and light hydrocarbons directly
“Cheap” components, solid electrolyte
Low-yield manufacture
Status
• Vehicle APUs
Equations
Cathode:
Anode:
O2 + 2e¯ → 2O=
H2 + O= → H2O + 2e¯
13
Features
Polymer Electrolyte Fuel Cells
Applications
• Transportation, Forklifts, etc.
• Power backup systems
• Consumer electronics with methanol fuel
Features
Quick start
Low temperature
Expensive Pt electrodes
Easy manufacture
Operating window limits
53-67% thermal efficiency
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•
•
•
•
•
•
Status
• Vehicle demonstrations underway
• Stationary/backup power “commercially” available
Equations
Cathode: ½O2 + 2H+ + 2e¯ → H2O
Anode:
H2
→
2H+ + 2e¯
Toyota Fuel Cell Forklift
Direct Methanol Polymer Electrolyte FC (DMFC)
O2 out
O2 in
CH3OH in
CH3OH out
Cathode
Anode
Endplate
Features
• A subset of Polymer Electrolyte
• Modified polymer electrolyte fuel cell
components
• Methanol crossover lowers efficiency
Status
• Pre-Alpha to Beta testing
Equations
Cathode: 1.5 O2 + 6H+ + 6e¯ → 3H2O
Anode:
CH3OH + H2O → CO2 + 6H+ + 6e¯
Bipolar plate
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Applications
• Miniature applications
• Consumer electronics
• Battlefield
Fuel Cells
• Advantages
• Emissions
• Efficiency
• Some have fuel flexibility
• Disadvantages
• Cost – (Pt catalyst)
• Some are high temp – may be slow to start
• Some sensitive to fuel impurities (need pure H2)
Fuel Cells
Hydrogen
•
•
•
•
Hydrogen is a secondary energy source
Very clean energy both in ICE and fuel cells
Poor energy density by volume
Hydrogen Storage
Hydrogen Storage Overview
• Physical storage of H2
•Compressed
•Cryogenically liquified
•Metal Hydride (“sponge”)
•Carbon nanofibers
• Chemical storage of hydrogen
•Sodium borohydride
•Ammonia
•Methanol
•Alkali metal hydrides
• New emerging methods
•Amminex tablets
•DADB (predicted)
•Solar Zinc production
•Alkali metal hydride slurry
Compressed
•Volumetrically and Gravimetrically inefficient, but
the technology is simple, so by far the most common in
small to medium sized applications.
•3500, 5000, 10,000 psi variants.
Liquid (Cryogenic)
•Compressed, chilled, filtered, condensed
•Boils at 22K (-251 C).
•Slow “waste” evaporation
•Gravimetrically and volumetrically efficient
•Kept at 1 atm or just slightly over.
but very costly to compress
Metal Hydrides (sponge)
•Sold by “Interpower” in Germany
•Filled with “HYDRALLOY” E60/0
(TiFeH2)
•Technically a chemical reaction, but
acts like a physical storage method
•Hydrogen is absorbed like in a
sponge.
•Operates at 3-30 atm, much lower
than 200-700 for compressed gas
tanks
•Comparatively very heavy, but with
good volumetric efficiency, good for
small storage, or where weight
doesn’t matter
Carbon Nanofibers
• Complex structure presents
a large surface area for
hydrogen to “dissolve” into
• Early claim set the standard
of 65 kgH2/m2 and 6.5 % by
weight as a “goal to beat”
• The claim turned out not to
be repeatable
• Research continues…
Methanol
•
•
•
•
CH3OH
Broken down by reformer, yields CO, CO2, and H2 gas.
Very common hydrogen transport method
Distribution infrastructure exists – same as gasoline
Ammonia
• Slightly higher volumetric efficiency than methanol
• Must be catalyzed at 800-900 deg. C for hydrogen
release
• Toxic
• Usually transported as a liquid, at 8 atm.
• Some Ammonia remains in the catalyzed hydrogen
stream, forming salts in PEM cells that destroy the
cells
• Many drawbacks, thus Methanol considered to be a
better solution
Alkali Metal Hydrides
• “Powerball” company, makes small
(3 mm) coated NaH spheres.
• “Spheres cut and exposed to water
as needed”
• H2 gas released
• Produces hydroxide solution waste
Sodium Borohydrate
• Sodium Borohydrate is the most popular of many
hydrate solutions
• Solution passed through a catalyst to release H2
• Commonly a one-way process (sodium metaborate
must be returned if recycling is desired.)
• Some alternative hydrates are too expensive or toxic
• The “Millennium Cell” company uses Sodium
Borohydrate technology
Amminex
•Essentially an Ammonia storage method
•Ammonia stored in a salt matrix, very stable
•Ammonia separated & catalyzed for use
•Likely to have non-catalyzed ammonia in hydrogen stream
•Ammonia poisoning contraindicates use with PEM fuel cells,
but compatible with alkaline fuel cells.
Amminex
•High density, but relies on ammonia production for fuel.
•Represents an improvement on ammonia storage,
which still must be catalyzed.
•Ammonia process still problematic.
Diammoniate of Diborane
(DADB)
• So far, just a computer
simulation.
• Compound discovered via
exploration of
Nitrogen/Boron/Hydrogen
compounds (i.e. similar to
Ammonia Borane)
• Thermodynamic properties
point towards spontaneous
hydrogen re-uptake – would
make DADB reusable (vs.
other borohydrates)
Solar Zinc production
• Isreli research effort utilizes
solar furnace to produce
pure Zinc
• Zinc powder can be easily
transported
• Zinc can be combined with
water to produce H2
• Alternatively could be made
into Zinc-Air batteries (at
higher energy efficiency)
Alkaline metal hydride slurry
• SafeHydrogen, LLC
• Concept proven with
Lithium Hydride, now
working on magnesium
hydride slurry
• Like a “PowerBall” slurry
• Hydroxide slurry to be recollected to be “recycled”
• Competitive efficiency to
Liquid H2
Storage Method Comparison
Sodium Hydride slurry
.9
1.0
Must reclaim used slurry
DADB
.1 - .2
.09-.1
(numbers for plain “diborane”and
sodium borohydride, should be similar)
Amminex
9.1
.081
Zinc powder
US DOE goal
unsure
9.0
.081