Solar Energy The energy density of sunlight 93 million miles

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Solar Energy
The energy density of sunlight 93 million miles
from the Sun is 1.37 kW/m2 (solar
constant) Luminosity of the sun 3.839 x 1026
W Astronomical Unit 1.496 x 1011 m Radius of
earth 6378.1 km Total solar power on earth
1.74x1017 W
93,000,000 miles
Due to atmospheric absorption and scattering, a
significant fraction of the Suns energy density
does not reach the Earths surface. Peak power
at sea level is 1020 W/m2
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http//www.nrel.gov/srrl/history.pdf
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http//www.nrel.gov/srrl/history.pdf
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http//www.nrel.gov/srrl/history.pdf
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The average power of solar radiation is 200 W/m2
At 8 conversion, PV arrays at these locations
would provide 18TW of energy.
http//www.loster.com/ml/solar_land_area/ A
plain English version http//home.iprimus.com.
au/nielsens/solrad.html Wikipedia
http//en.wikipedia.org/wiki/Solar_power
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visible light 700 400 nm 171 299 kJ/mol 1.7
3.1 eV
400 700 nm (visible light) major fraction of
solar radiation convert this to units of energy
E hc / ? E(400 nm) (6.626 x 10-34 Js) x (3 x
108 m/s) x 1/(4 x 10-7 m) 4.97 x 10-19 J
/ photon convert to kJ/mol E(400 nm) (4.97 x
10-19 J/photon) x (6.02 x 1023 photon/mol) x (1
kJ/1000 J) 299 kJ/mol convert to electron
volts (eV) (299 kJ/mol) x (1 eV/96.4853 kJ/mol)
3.1 eV
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In order to capture the energy of a photon, a
substance absorbs the photon with transfer the
energy to (electrons) within that
substance. Remember the atomic orbitals and the
energy levels of electrons? Substances
that are effective absorbers of visible
light Dyes (sensitizers) , semiconductors
ground state
excited state
hv
The electron is promoted by an amount of energy
exactly equal to that of the absorbed photon.
Energy
Reaction Coordinate
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Sensitizers and Semiconductors
empty
LUMO
HOMO
filled
chlorophyll a visible light absorbing molecule
in nature
representation of the band structure of a
semiconductor
Density of States of Silicon
In sensitizers, the energy difference between the
HOMO and LUMO corresponds to the energy of a
quantum of visible light (photon). In
semiconductors, the energy difference between the
Valence Band and Conduction Band (the band gap)
corresponds to the energy of a quantum of visible
light (photon).
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Ok, some photon of solar energy was absorbed.
Now what? In order to do work, electrons need to
be moved around.
The way nature does it electron transport in
photosynthesis (follow a descending ladder of
reduction potentials)
The photon energy is eventually stored in the
form of ATP and a reducing electron in NADPH, and
the electron hole is used in oxygen evolution.
electron transport quinones cytochromes
(porphyrins) plastocyanin ferredoxin flavins
Z-scheme
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p-n junctions for application as solar cell
(photovoltaic device)
Silicon has 4 valence electrons Ne3s23p2
Si electron As Ga
doped semiconductor
an array of Si atoms
n-type semiconductor
p-type semiconductor
Band Structure
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n-type p-type
depletion zone

formation of a depletion zone
- -
p-n junction at equilibrium
Photovoltaic device
The doped semiconductors allows the construction
of a layered device, where an electrical contact
can be established for each discrete layer.


educational link for p-n junctions and how they
work http//www.mtmi.vu.lt/pfk/funkc_dariniai/dio
d/index.html
current (work)
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Photoelectrochemical Cell
John Turner (NREL) Science 1998 280(5362) 425
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