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Environmental Implications of PEM Fuel Cells

CEE 304
by

C Byers

on 20 September 2013

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Transcript of Environmental Implications of PEM Fuel Cells

Ejeong Baik
C. Byers
Angela Jiang
Lucia Wang

Proton Exchange Membrane
Fuel Cell Applications in Airplanes

Analysis of life cycle
emissions
Applications to
in-flight airplane electricity
in Boeing 737-800s
Stage I: Best case scenario
Method of hydrogen extraction: electrolysis
Assumes nuclear energy source
0.0038 kg CO2/MJ

Added 10 new domestic and international routes to its services
Compare use of conventional jet fuel and fuel cells in generating electricity for the passengers on flight
CO2 emitted per year through new routes
Calculations
Generators need to produce 30 GJ of energy per flight
Conventional Jet Fuel
68239 BTU of energy input needed to produce 1 MMBTU of jet fuel
Assume input energy comes from coal fire plant
365 kg of bituminous coal must be combusted, which produces
603 kg of CO2 per flight
Barrel of jet fuel produces 401.9 kg of CO2 combusted
Approximately 5 barrels of jet fuel needed per flight
CO2 emitted through combustion
is

2009.5 kg per flight
Total: 2612.5 kg of CO2 emitted per flight

For the new routes: 19.7 kilotonnes CO2
Stage II: Best case scenario
Case 4
Renewable electricity source
PGM recycling
22 kg CO2-eq/kW of fuel cell capacity
Fuel Cells
What are fuel cells?
Electrochemical devices that convert chemical energy of fuel to electrical energy
Require constant supply of fuel to produce energy
Many applications including space use, transportation, backup power, among others
Proton Exchange Membrane Fuel Cells
Fuel cells defined by type of electrolyte
PEMFCs use proton conducting polymer barriers
How do they
work?
Components:
Anode: oxidize fuel
H2 -> 2e- + 2H+
Electrolyte: barrier
Cathode:
O2 + 4e- + 4H+ -> 2H2O
General Reaction:
2H2 + O2 -> 2H2O
Background
Expensive
Mobile
Applications
High Efficiency
45 - 60%
Zero Emissions
During Use
Applications to Airplanes
Currently: airplanes use generators powered by jet fuel for inflight electricity
Jet fuel emits carbon in both processing and combustion
Particularly concerning because emits CO2 directly into tropopause
Environmental Implications
Energy Flow Through a Fuel Cell
PEMFC has efficiency 60% when used in mobile applications
LHV of hydrogen is 120 MJ/kg
Energy required to deliver hydrogen extracted by electrolysis has an
EROI between 0.5 : 1 and 0.6 : 1 (Bossel 2006)
Total EROI is 0.38
for delivery and conversion of hydrogen fuel to useful energy (using electrolysis and conventional compression)
Total useful energy produced by a fuel cell is
72 MJ/kg hydrogen
Case 1
Case 2
Case 3
Case 4
Conventional
Hydropower
Conventional
Hydropower
No PGM recycling
No PGM recycling
75% PGM recycling
75% PGM recycling
Electricity & Recycling
in Fuel Cell Production
Case 1
Case 2
Case 3
Case 4
61
kg/kWel
41
kg/kWel
43
kg/kWel
22
kg/kWel
CO2-equivalent emissions in fuel cell stack production
Best case scenario results in emissions of
22 kg/kWel
of fuel cell capacity in Case 4
Electricity from hydropower for fuel cell stack production and PGM recycling
Life Cycle Assessment
Stage II: Production of fuel
cell stacks
Stage I: Production of hydrogen fuel
Carbon emissions and water withdrawals vary greatly with the method of hydrogen extraction
United Airlines
December of 2012
Extracting Hydrogen Fuel
Energy is required to extract hydrogen
3 main methods
Steam reformation
Coal gasification
Electrolysis
Extraction can emit pollutants and have significant water withdrawals
Electrolysis
Steam
reformation
Coal
Gasification (3kg)
Water
withdrawals
CO2
9 kg
4.5 kg
9 kg
11 kg
5.5 kg
.45 kg
Electrolysis
Steam reforming of methane from natural gas
Coal
gasification
CO2 emissions in obtaining 1 kg hydrogen
Conclusion
Direct Combustion
May 9th, 2013
0.45kg
if nuclear
5.5kg
11kg
CO2 emitted
per flight
12.5kg
1375kg
1375kg
2750kg
CO2 emitted
for new
routes
0.821
kilotonnes
10
kilotonnes
20
kilotonnes
Storage of Hydrogen
MASS
-350 bar compressed gas
-Tank Mass= 16.19*H2 mass + 0.3862
-
4047.89kg per flight

VOLUME
-1.06 lb of H2/ft3
-
14.72 cubic meters of storage space
-compare to 0.758 cubic meters
for conventional jet fuel
Water Withdrawals
Use does not have carbon and other pollutant emissions
But, LCA analysis shows potentially significant CO2 emissions dependent on H2 extraction method
Water withdrawals might be problematic
Applications in airplanes have potential but currently not feasible due to storage and costs
Electrolysis
-9L water / 1kg H2
-
2250L per flight
Coal Gasification
-9L water / 1kg H2
-
2250L per flight
Steam Reforming
-4.5L water / 1kg H2
-
1125L of water
Processing Jet Fuel
EROI and MJ/kg H
Production of 1 kg H
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