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EE580 – Solar CellsTodd J. Kaiser
• Lecture 03
• Nature of Sunlight
1Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Wave-Particle Duality
• Light acts as– Waves: electromagnetic radiation
• Refraction - bending of light through a prism• Diffraction - bending of light around an edge• Interference – adding of light waves
– Particles: photons – individual packets of energy
• Photoelectric Effect• Blackbody Radiation
2Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Properties of Light
• Sunlight contains photons of many different frequencies ( we see as different colors)
• Visible Light– 0.38 microns – 0.77 microns (10-6 meters)– Blue - Red
• Frequency (f) and Wavelength ( ) are inversely related: f = 1/– High Energy High Frequency Short Wavelength– Low Energy Low Frequency Long Wavelength
3Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Electromagnetic Spectrum107
108
109
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
10
1
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
Gamma Rays
X Rays
Ultraviolet
Infrared
Microwaves
Radio Waves
Visible
Frequency
(Hz)
Wavelength
(meters)
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Roy G. BIV
0.6 microns
0.4 microns
0.5 microns
0.38 microns
0.77 microns
Color, wavelength and frequency are
interrelated
4Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Photon Energy
24.1
eVEhc
hfE
High energy photon for blue light
Low energy photon for red light
Very low energy photon for infrared(invisible)
5Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Photon Flux & Energy Density
Areasecond
Photons ofNumber FluxPhoton
For the same intensity of light shorter wavelengths require fewer photons, since the energy content of each individual photon is greater
Jhc
m
WH
2:Photonper Energy FluxPhoton :DensityEnergy
6Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Radiant Power Density
• The total power density emitted from a light source
• The spectral irradiance is multiplied by the wavelength range for which is was measured and summed over the measurement range
i
N
iiFdFH
0
0
1 2 3 4 5 6 7
F(R
)
F(O
)
F(Y
)
F(G
)
F(B
)
F(I
)
F(V
)7Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
Spectral Irradiance
Xenon – (left axis) Sun - (right axis)
Halogen – (left axis)
Mercury – (left axis) Power Density at a particular wavelength
Visible
8Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Solar Radiation
9Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Blackbody Radiation
• A body in thermal equilibrium emits and absorbs radiation at the same rate
• A body that absorbs all the radiation incident on it is an ideal blackbody
• The power per area radiated is given by the Stefan-Boltzmann Law
4TH
• This function has a maximum given by Wien’s Displacement Law
• The spectral irradiance of a blackbody radiator is given by Planck’s Law
KTpeak
2900
1
125
2
Tk
hcBB
e
hcF
10Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Plot of Planck’s Law
1
125
2
Tk
hcBB
e
hcF
30
25
20
15
10
5
0
x101
2
2.01.51.00.5Wavelength (microns)
'T=3500' 'T=4000' 'T=4500' 'T=5000' 'T=5500' 'T=6000'
KTpeak
2900
11Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Sun’s maximum at visible spectrum
101
103
105
107
109
1011
1013
1015
0.12 3 4 5 6 7 8 9
1Wavelength (microns)
Visible
12Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Sun
Fusion reaction at 20,000,000 degrees
Converts H to He
Fusion reaction at 20,000,000 degrees
Converts H to He
Convection heat transfer
Photosphere (sun’s surface) at 5760±50 K
13Montana State University: Solar Cells Lecture 3: Nature of Sunlight
The energy is emitted from the surface of the sun as a sphere of light
The size of the sphere grows as the light moves farther from the sun causing the power density to reduce
Sunlight in Space
202
2
02
2
/13534
4mWHH
D
RH
D
RDH EarthOrbit
sunsun
14Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Sunlight at Earth’s Orbit
year theofday theisn
/365
)2(360cos033.011353 2mW
nH ticalOrbitEarthEllip
15Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Sunlight at Earth’s Surface
• Atmospheric Effects– Absorption– Scattering– Local Variations
• Clouds• Pollution• Weather
• Latitude• Season of Year• Time of Day
16Montana State University: Solar Cells Lecture 3: Nature of Sunlight
100 % inputClear Sky Absorption & Scattering
BOR0606C
Ozone20-40 km
2% Absorbed
Upper Dust Layer15-35 km
1% Absorbed
0.5% Scattered to Space
1% Scattered to Earth
Air Molecules0-30 km
8% Absorbed
1% Scattered to Space
4% Scattered to Earth
Water Vapor0-3 km
6% Absorbed
0.5% Scattered to Space
1% Scattered to Earth
Lower Dust
0-3 km1% Absorbed 1% Scattered to Earth
0.5% Scattered to Space
18% Absorbed
70% Direct toEarth
3% ScatteredTo Space
7% ScatteredTo Earth
17Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Air Mass• The path length that light takes through the atmosphere
normalized to the shortest possible path length
xh
cos
1
1cos
AMh
hh
x
18Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Measuring the Air Mass
1cos
1
cos
222
L
S
L
LS
L
hAM
h
L
Object of height (L)
Shadow from object (S)
h
Neglects the curvature of the earth’s atmosphere
19Montana State University: Solar Cells Lecture 3: Nature of Sunlight
EAST
SOUTH
WESTNORTH
What time of year is this image true?March 22: Spring Equinox
Sept 23: Fall Equinox20Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
Declination Angle
Sep 23
Declination () = 0º
Mar 23
Declination () = 0º
Jun 23
Declination () = 23.45º
Dec 23
Declination () = -23.45º
81365
360sin45.23 d
21Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Declination Angle of Sun
North Pole
Summer Solstice
= 23.45º
Fall Equinox
= 0º
Winter Solstice
= -23.45º 22Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Elevation Angle
The elevation angle is the angular height the sun makes with the horizontal
23Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Polar Plot of Sun Position for Bozeman
Summer
Spring/Fall
Winter
24Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Passive Solar Heating
25
Winter Sun
Summer Sun
Overhang designed to remove direct sunlight from window during the
heat of summer and allow the sunlight in during cooler winters
Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Insolation
26Montana State University: Solar Cells Lecture 3: Nature of Sunlight
27
• 100 miles by 100 miles in Nevada would
provide the equivalent of the entire US
electrical demand
• Distributed (to sites with less sun) it would
take less than 25% of the area covered by US
roads.
Solar Cell Land Area Requirements for the USA’s Energy with Solar PV
Montana State University: Solar Cells Lecture 3: Nature of Sunlight
Solar Cell Land Area Requirements for the World’s Energy with Solar PV
28
6 Boxes at 3.3 TW Each
Montana State University: Solar Cells Lecture 3: Nature of Sunlight