Scientific Discussion Energy for the Future

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Scientific Discussion Energy for the Future
Augustin J. McEVOY Madrid, Spain 13th. March
2006
augustin.mcevoy_at_epfl.ch
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• For fuel cells, durability and cost are the most
• difficult goals, and for hydrogen storage, the
• most difficult are size, weight, and cost. In
  most
• instances, solutions depend on yet-to-be-conceived
  
• or -proven component and
• manufacturing technology rather than
• incremental improvement.
• - Review of the Research Program of the
• FreedomCAR and Fuel Partnership, first report
  2005,
• National Academies of Science, Washington, D.C.
• www.nap.edu/books/0309097304/html/

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A hydrogen-fuelled vehicle
WEIGHT Empty 78,100 lbs Propellant
1,585,379 lbs Propellent Weight Liquid
oxygen 1,359,142 lbs Liquid hydrogen
226,237 lbs Propellent Volume Liquid oxygen
tank 143,060 gallons Liquid hydrogen tank
383,066 gallons
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Shuttle tank section
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European energy policy by 2020, replace 20 of
petroleum fuels with 2 hydrogen 5 - 8
biofuels 10 natural gas Source European
natural gas vehicle association,ENGVA
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Volumetric and gravimetric energy density -
hydrogen storage
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Scale of the problem
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Chemistry of carbon and hydrogen -Hydrogen
supply from fossil resources.

• CH4 H2O ? 3H2 CO Steam reforming
• CH4 CO2 ? 2H2 2CO Carbon dioxide reforming
• 2CH4 O2 ? 4H2 2CO Partial oxidation
• CO H2O ? H2 CO2 Water - gas shift.
• CH4 ? 2H2 C Hydrocarbon pyrolysis
• C H2O ? H2 CO Steam decoking.
• 7. 2CO ? C CO2 Boudouard coking

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Electrochemistry of water -electrolysis and fuel
cell systems
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Effect of overpotential on fuel cell/electrolysis
efficiency.
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Energy - electric and thermal - demandfor water
and steam electrolysis.
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Principle of high-temperature electrolysis of
steam (reverse reaction of SOFC).
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Structural drawing of electrolysis tube with12
cells (tubular type cell).
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Low temperature electrolysers
A well established industrial product. Above two
atmospheric electrolysers, each at 200 Nm3/h
(photo Norsk Hydro), having an efficiency of
over 80 (higher heating value). Efficiency is an
important factor in electrolysis because the use
of energy (4.5 kWh/Nm3 H2). In these alkaline
electrolysers a liquid electrolyte is used
typically a 25 potassium hydroxide solution.
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More recent technology - polymer electrolyte
systems
The PEM electrolyser system operates at a nominal
electrolysis voltage of 48 V and the maximum
operating power of 1.75 kW corresponds to a
hydrogen production of 390 Nl/h H2.The operating
pressure and temperature set points are 15 bar
and 75C respectively. The water-cooled PEM
electro- lyser stack consists of 26 cells with an
active area of 57 cm² each. (RE Hydrogen Systems).
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High pressure electrolyser - direct production of
compressed gas.
Claind, Italy.
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Thermochemical hydrogen production -reaction
scheme of the IS process.
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Thermochemical process - sulfur/iodine.
(1) SO2(g) I2(l) 2H2O ? 2HI H2SO4(aq,
inf), exothermic reaction -165kJ (2)
2HI(aq, inf) ? H2(g) I2(g), endothermic
reaction 173kJ (3) H2SO4(aq, inf) ? H2O (g)
SO2(g) 0.5 O2(g), high temperature endothermic
reaction 371kJ A high temperature nuclear
reactor was indicated as the heat source
however a heliostat-type solar thermal System
should be considered.
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Solar siting advantages
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Hydrogen generation by solar photolysis of water

• The three options
• The brute force approach silicon PV cells in
  series and coupled (with Maximum Power Point
  Tracker electronic control) to a water
  electrolyzer. Advantage - proven technology
  disadvantage - cost.
• The direct water decomposition by
  photoelectrochemical cells remains the prime
  target of research in photoelectrochemistry
• 3. The integrated solid state tandem cell
  approach

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Photoelectrochemical hydrogen productionat
illuminated semiconductor electrodes
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Photoelectrochemical energetics of semiconductors.
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Dye-sensitised photovoltaic cell
Oraganometallic dye molecular structure
M. Graetzel, Nature, 2001, 414, 338.
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Photovoltaic characteristics of mesoscopic film
of TiO2 sensitised by N-719 dye.
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Nanocrystalline oxide photoanode for oxygen
production
Advantage of nanocrystalline oxides electrode
1) translucent electrode - avoids light
scattering losses 2) Small size is within
minority carrier diffusion length, the
valence band holes reach the surface before
they recombine.
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Z connection for 2-photon photolysis
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Tandem cell - optical and electrical series
assembly
I-V curve of a tandem cell using a DSC as top
electrode. The device size is 1.6 cm2. Values
for isc, Voc, ff and overall efficiency are 10.16
mA/cm2, 2.01 V, 0.737 and 15.05
In collaboration with Dr. A. Tiwari ETH Zurich
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Laboratory validation of tandem cell concept
The tandem cell electrodes are mesoporous WO3 and
dye sensitized TiO2.
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With thanks for your attention and for this
opportunity to visit Spain!