John Jechura – [email protected]: January 4, 2015
Fuel Cell Technology
Energy Markets Are Interconnected
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https://publicaffairs.llnl.gov/news/energy/energy.html
Topics
• Basics & types of fuel cells
• Fuel cells for transportation
• Hydrogen for the fuel cells
• Efficiencies
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Fuel Cell Principals
Chemistry of Fuel Cell (with H+ transfer)
• Anode side:
2H2 4 H+ 2 e‐
• Cathode side:
O2 + 4 H+ 2 e‐ 2 H2O
• Overall:
2H2 + O2 2 H2O
Fuel cell provides a direct current flow of electrons
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Types of Fuel Cells
• Alkaline fuel cell (AFC)
One of the oldest designs
• U.S. space program used them since the 1960s to make power & drinking water
Very susceptible to contamination, requires pure hydrogen & oxygen
Very expensive, unlikely to be commercialized
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http://americanhistory.si.edu/fuelcells/basics.htm
Types of Fuel Cells
• Solid oxide fuel cell (SOFC)
Operates at very high temperatures – 700 to 1,000C
• High temperature makes reliability a problem when cycling on and off repeatedly
• Very stable when in continuous use
High temperature can produce steam to generate more electricity – improves overall efficiency of the system
Best suited for large‐scale stationary power generators
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http://americanhistory.si.edu/fuelcells/basics.htm
Types of Fuel Cells
• Molten‐carbonate fuel cell (MCFC)
Also best suited for large stationary power generators
• Operate at 600C & can generate steam
Lower operating temperature means they don't need such exotic materials
• Design little less expensive
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http://americanhistory.si.edu/fuelcells/basics.htm
Types of Fuel Cells
• Polymer exchange membrane fuel cell (PEMFC)
DOE focusing on PEMFC as most likely candidate for transportation
High power density & relatively low operating temperature (60 to 80C)
• Doesn't take long for the fuel cell to warm up & begin generating electricity
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http://americanhistory.si.edu/fuelcells/basics.htm
Types of Fuel Cells
• Phosphoric‐acid fuel cell (PAFC) Operates at higher temperature than PEMFCs, longer warm‐up time
Potential for use in small stationary power‐generation systems but unsuitable for use in cars
• Direct‐methanol fuel cell (DMFC) Comparable to PEMFC (operating temperature) but not as efficient
Requires relatively large amount of platinum to act as a catalyst – makes these fuel cells expensive
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PEMFC: Polymer Exchange Membrane Fuel Cell
• Anode Conducts the electrons that are freed from the hydrogen molecules Has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst
• Cathode Has channels etched into it that distribute the oxygen to the surface of the catalyst Conducts the electrons back from the external circuit to the catalyst – recombine with the hydrogen ions & oxygen to form water
• Electrolyte is proton exchange membrane Only conducts positively charged ions & blocks electrons Membrane must be hydrated in order to function & remain stable
• Limits how low a temperature the fuel cell can operate• Catalyst facilitates reaction of oxygen & hydrogen Usually made of platinum nanoparticles very thinly coated onto carbon paper or cloth
http://auto.howstuffworks.com/fuel‐efficiency/alternative‐fuels/fuel‐cell2.htm
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Possible Fuel Cell Vehicle
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http://www.fueleconomy.gov/feg/fuelcell.shtml
Large Scale Hydrogen Production
• Steam Reforming
CH4 + H2O CO + 3∙H2 Highly endothermic
• Partial Oxidation
2 CH4 + O2 2 CO + 4 H2 Highly exothermic
If solid feedstock, one possible gasification reaction
• Autothermal Reforming Combines both steam reforming and partial oxidation to achieve an energy‐neutral process
Often uses oxygen rather than air
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Real Process – Steam Methane Reforming & Water Shift
Steam
Natural Gas
Reforming Reactor
High Temperature Shift Reactor
Low Temperature Shift Reactor
Hydrogen Purification
Fuel Gas
Flue Gas
Hydrogen
Methanation Reactor
CO2
• Reforming. Endothermic catalytic reaction, typically 20‐30 atm & 800‐880°C (1470‐1615°F) outlet.
CH4 + H2O CO + 3 H2• Shift conversion. Exothermic fixed‐bed catalytic
reaction, possibly in two steps.
CO + H2O CO2 + H2HTS: 345‐370°C (650 – 700F)LTS: 230°C (450F)
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• Gas Purification. Absorb CO2 (amine) or separate into pure H2 stream (PSA or membrane).
• Methanation. Convert residual CO & CO2 back to methane. Exothermic fixed‐bed catalytic reactions at 370‐425°C (700 – 800F).
CO + 3 H2 CH4 + H2O
CO2 + 4 H2 CH4 + 2 H2O
On‐Board Fuel Reforming
• Liquid fuel would avoid having heavy high‐pressure gas containers Gasoline, alcohols (methanol, ethanol, …)
• Reforming of fuel produces CO2 emissions Will not qualify as zero emissions vehicles (ZEVs) under California's emissions laws
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Current Problems with Reformers Supplying Fuel Cells
• Reforming reaction takes place at high temperatures – slow to start up & requires costly high temperature materials
• Sulfur compounds in the fuel poison certain catalysts
Research into sulfur‐tolerant catalysts
• Low temperature polymer fuel cell membranes can be poisoned by CO produced by the reactor
PEMFC need complex CO‐removal systems
SOFC & MCFC operate at higher temperatures & do not have this problem
• Efficiency of process 70% ‐ 85% (LHV basis)
• Catalyst in low temperature fuel cells is based on platinum & is very expensive
Typical automotive fuel cell stack (100kW) contains 20‐30 g of platinum metal –currently ~$1700 per troy oz ($60 per g)
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Overall Efficiencies
• Gasoline internal combustion – 20 – 25%
• Battery powered vehicle
65% of electricity in
• Batter efficiency – 90%
• Charging efficiency – 90%
• Motor/inverter – 80%
Efficiency of power generation?
• Combustion based 40% – 26% overall
• Hydro electric based – “free” electricity?
• Fuel cells with pure hydrogen
Potentially 80% efficient
Overall efficiency with 80% efficient motor/inverter – 64%
• Fuel cells with reformed fuel
Including reformer efficiency – 45 to 51%
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