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Discussion points:
Robinson Article
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•
•
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General comments?
What is the strongest argument?
What is the weakest/most suspect?
Did it change anyone’s thinking?
There are lots of other sites that you can find to argue with points in Gore’s movie
e.g. www.cei.org/pdf/ait/AIT-CEIresponse.ppt ,
Figures:
Robinson Article
Figures:
Gore’s version
Figures:
Robinson Article
Wind Energy
T typical availability of a wind farm is
17-38% for land-based plants and 4045% for off-shore plants.
http://www.windpower.org/en/tour/wres/euromap.htm
An extensive site for Wind
Information!!
Summary of wind power
• Power available is roughly:
– P=2.8x10-4 D2 v3 kW (D in m, V in m/s)
• I.e. you get much more power at higher wind
speeds with larger turbines
• 3-blade turbines are more efficient than multiblade, but the latter work at lower wind speeds.
• At higher wind speeds you need to “feather” the
blades to avoid overloading the generator and
gears.
• Typical power turbines can produce 1 -3.5 MW
Types of Windmills/turbines
7% efficiency,
but work at
low wind
speeds
Altogether, there are 150,000
windmills operating in the US
alone (mainly for water
extraction/distribution)
According to wikipedia, as of
2006 installed world-wide
capacity is 74 GW (same
capacity as only 3.5 dams the
size of the three-Gorges
project in China).
Up to 56 % efficiency with
3 blades, do very little at
low wind speeds
Blade diameter:
100m
Wind range:
3.5m/s to 25m/s
Rated wind speed: 11.5 m/s
GE 2.5MW generator
http://www.gepower.com/prod_serv/products/wind_turbines/en/downloads/ge_25mw_brochure.pdf
Basics of Photo-Voltaics
A useful link demonstrating the design of a basic solar cell
may be found at:
http://jas.eng.buffalo.edu/education/pnapp/solarcell/index.html
• There are several different types of solar cells:
– Single crystal Si (NASA): most efficient (up to 30%) and most
expensive (have been $100’s/W, now much lower)
– Amorphous Si: not so efficient (5-10% or so) degrade with use
(but improvements have been made), cheap ($2.5/W)
– Recycled/polycrystalline Si (may be important in the future)
Basics of atoms and materials
Energy
Gap (no available states)
• Isolated atoms have electrons in shells” of
well-defined (and distinct) energies.
• When the atoms come together to form a
solid, they share electrons and the allowed
energies get spread out into “bands”,
sometimes with a “gap” in between
p- and n-type semiconductors
n-type
p-type
Conduction band
Energy
_
_
_
_
_
_
_
_
Gap
Valence band
Position
•Separate p and n-type semiconductors. The lines in the gap represent extra
states introduced by impurities in the material.
• n-type semiconductor: extra states from impurities contain electrons at
energies just below the conduction band
•p-type has extra (empty) states at energies just above the valence band.
p-n junction and solar cells
n-type
p-type
Conduction band
Energy
_
_
_
_
_
_
_
_
Gap
Valence band
Position
•When the junction is formed some electrons from the n-type material
can “fall” down into the empty states in the p-type material, producing a
net negative charge in the p-type and positive charge in the n-type
p-n junction
n-type
p-type
Energy
_
_
_
_
_
_
_
+_
Conduction band
_
Gap
Valence band
Position
•When the junction is formed some electrons from the n-type material
can “fall” down into the empty states in the p-type material, producing a
net negative charge in the p-type and positive charge in the n-type
p-n junction and solar cell action
n-type
p-type
Energy
+
_ _
_
_
_
_
_
Conduction band
_
_
Gap
Valence band
Position
•When a light photon with energy greater than the gap is absorbed it
creates an electron-hole pair (lifting the electron in energy up to the
conduction band, and thereby providing the emf).
•To be effective, you must avoid:
•avoid recombination (electron falling back in to the hole).
•Avoid giving the electron energy too far above the gap
•Minimize resistance in the cell itself
•Maximize absorption
•All these factors amount to minimizing the disorder in the cell
material
Synopsis of Solar Cells
• Need to absorb the light
– Anti-reflective coating + multiple layers
• Need to get the electrons out into the circuit (low
resistance and recombination)
– Low disorder helps, but that is expensive
• Record efficiency of 42.8% was announced in July 2007
(U. Delaware/Dupont).
• Crystalline Si: highest efficiency (typically 15-25%), poorer
coverage, bulk material but only the surface contributes,
expensive (NASA uses them).
• Amorphous Si: lower efficiency (5-13%)
Solar Cell Costs
http://www.nrel.gov/ncpv/pv_manufacturing/cost_capacity.html
Essentials of PV design
Engineering work-around # 2:
Martin Green’s record cell. The grid deflects light into
a light trapping structure
Power characteristics (Si)
100 cm2 silicon
Cell under different
Illumination conidtions
Material
Level of
efficiency in
% Lab
Level of efficiency in %
Production
Monocrystalline
Silicon
approx. 24
14 to17
Polycrystalline
Silicon
Amorphous
Silicon
approx. 18
approx. 13
http://www.solarserver.de/wissen/photovoltaik-e.html
13 to15
5 to7
Advanced designs-multilayers
http://www.nrel.gov/highperformancepv/
Typical products
Flood light system for
$390 (LED’s plus xtal.
cells)
40W systems for
$250, 15 W for $120
Typical pattern for crystalline
cells
Typical patterns for amorphous
cells
http://www.siliconsolar.com/
Battery charges (flexible
Amorphous cells)
Review for Thursday
• Solar Cells
• Need to get the electrons out into the circuit (low
resistance and recombination)
– Low disorder helps with both (hence crystal is more
efficient than amorphous)
• Crystalline Si: highest efficiency (typically 1525%), poorer coverage, bulk material but only
the surface contributes, expensive (e.g. NASA).
• Amorphous Si: lower efficiency (5-13%), less
stable (can degrade when exposed to sunlight).
Fuel Cells- sample schematics
http://www.iit.edu/~smart/garrear/fuelcells.htm
For more details on these and other types, see also:
http://www.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html
Ballard Power Systems (PEM)
•85kW basic module power
(scalable from 10 to 300kW
They say) for passenger cars.
•212 lb (97 kg)
•284 V 300 A
•Volume 75 liters
•Operates at 80oC
•H2 as the fuel (needs a
reformer to make use of
Methanol etc.)
•300kW used for buses
Fuel Cell Energy (“Direct Fuel Cell”)
•Appears to be a molten
carbonate systme based on
their description
•Standard line includes units
of 0.3,1.5 and 3 MW
•Fuel is CH4 (no need for
external reformer) can also
use “coal gas”, biogas and
methanol
•Marketed for high-quality power
applications (fixed location)
This is a nominal 300kW unit (typically delivers
250kW according to their press releases). Most
of the units installed to date are of this size.
http://www.netl.doe.gov/publications/proceedings/03/dcfcw/dcfcw03.html
http://www.netl.doe.gov/publications/proceedings/03/dcfcw/Cooper%202.pdf
The Hydrogen Hype
•H2 burns with 02 to make water
•H2 comes from the oceans (lots of it)
•Fuel cells can “burn” it efficiently/cleanly
The Realities
•Can’t mine it, it is NOT an energy source
–Why not just use electricity directly?
•Even as a liquid, energy density is low
–Storage and transport are difficult issues
•More dangerous (explosive) than CH4
• No existing infrastructure
Hydrogen Economy
•Hydrogen seems to be an attractive
alternative to fossil fuels, but it cannot be
mined. You need to treat it more like
electricity than gasoline (i.e. as a carrier
of energy, not as a primary source).
•Need lots of research in areas such as:
–Production
–Transmission/storage
–Distribution/end use
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/4_science_stevens_04.pdf
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/4_science_stevens_04.pdf
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/4_science_stevens_04.pdf
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/4_science_stevens_04.pdf
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/4_science_stevens_04.pdf
Storage Possabilities
Physisorbtion
Chemical Reaction
Chemisorbtion
Encapsulation
Weak binding energy -> Low T required
Carbon nanotubes
Porous materials
Zeolites
Reversible Hydrides
PdH, LiH, …
Large energy input to release H2
Slow Dynamics
H
Al
Very large energy input to release H2
Not technologically feasible
H2 trapped in cages or pores
Variation of physical properties
(T or P) to trap/release H2
4 H molecules
in 51264 cage
DOE report from 2004
is available at:
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/4_science_stevens_04.pdf
Nature and Physics Today articles:
Nature Vol. 414, p353-358 (2001)
Physics Today, vol 57(12) p39-44 (2004)
MIT web site on photo-production:
http://web.mit.edu/chemistry/dgn/www/research/e_conversion.html