EE580 – Solar Cells
Todd J. Kaiser
• Lecture 03
• Nature of Sunlight
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
1
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
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
2
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 ( l) are inversely related:
f = 1/ l
– High Energy High Frequency  Short Wavelength
– Low Energy Low Frequency  Long Wavelength
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
3
Electromagnetic Spectrum
Color, wavelength and
frequency are
interrelated
Wavelength
(meters)
Roy G. BIV
0.77 microns
Red
Orange
0.6 microns
Blue
10-1
109
10-2
10-4
1012
1013
Visible
Ultraviolet
1015
1016
1017
X Rays
10-10
1018
10-11
1019
10-12
(Hz)
1014
10-7
0.4 microns
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
Infrared
Frequency
1010
1011
10-9
0.38 microns
Microwaves
10-3
10-8
Indigo
Violet
108
10-6
0.5 microns
Radio Waves
107
1
10-5
Yellow
Green
10
Gamma Rays
1020
4
Photon Energy
E  hf 
hc
l
 E eV  
1.24
l  
High energy photon for blue light
Low energy photon for red light
Very low energy photon for infrared
(invisible)
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
5
Photon Flux & Energy Density
Number of Photons
Photon Flux 

second Area 
W 
 hc






Energy Density : Photon Flux Energy per Photon : H  2   
J 
m 
l

For the same intensity of light shorter wavelengths require fewer photons,
since the energy content of each individual photon is greater
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
6
Radiant Power Density
• The total power density emitted from a light source
N

H   F l dl   F li li
0
i 0
F(V)
F(I)
F(B)
F(G)
F(Y)
F(O)
F(R)
• The spectral irradiance is multiplied by the wavelength
range for which is was measured and summed over the
measurement range
l1 l2 l3 l4 l5 l6 l7
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
7
Spectral Irradiance
Visible
Xenon – (left axis)
Sun - (right axis)
Halogen – (left axis)
Mercury – (left axis)
Power Density at a particular wavelength
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
8
Solar Radiation
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
9
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
H T
•
4
This function has a maximum given by Wien’s Displacement Law
2900
l peak   
T K 
•
The spectral irradiance of a blackbody radiator is given by Planck’s Law
FBB l  
2hc 2
l5
1
e
hc
klT
1
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
10
Plot of Planck’s Law
2900
l peak   
T K 
30
25
'T=3500'
'T=4000'
'T=4500'
'T=5000'
'T=5500'
'T=6000'
x10
12
20
15
10
5
0
0.5
1.0
Wavelength (microns)
FBB l  
2hc 2
l5
1.5
2.0
1
e
hc
klT
1
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
11
Sun’s maximum at visible spectrum
15
10
13
10
11
10
9
10
Visible
7
10
5
10
3
10
1
10
2
3
4
0.1
5
6
7
8
9
1
Wavelength (microns)
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
12
Sun
Fusion reaction at
20,000,000 degrees
Converts H to He
Convection
heat transfer
Photosphere (sun’s
surface) at 5760±50 K
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
13
Sunlight in Space
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
2
2
4 Rsun
Rsun
2
H D  
H

H

H

1353
W
/
m
0
0
EarthOrbit
4 D 2
D2

Montana State University: Solar Cells
Lecture 3: Nature of Sunlight

14
Sunlight at Earth’s Orbit

 360(n  2) 
2
H EarthEllipticalOrbit  13531  0.033 cos
 W / m
 365 

n is the day of the year

Montana State University: Solar Cells
Lecture 3: Nature of Sunlight

15
Sunlight at Earth’s Surface
• Atmospheric Effects
– Absorption
– Scattering
– Local Variations
• Clouds
• Pollution
• Weather
• Latitude
• Season of Year
• Time of Day
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
16
100 % input
Clear Sky Absorption & Scattering
2%
Absorbed
1%
Absorbed
18% Absorbed
8%
Absorbed
BOR0606C
Ozone
20-40 km
3% Scattered
To Space
Upper
Dust Layer
15-35 km
0.5% Scattered to Space
Air
Molecules
0-30 km
1% Scattered to Space
1% Scattered to Earth
4% Scattered to Earth
0.5% Scattered to Space
6%
Absorbed
Water
Vapor
0-3 km
0.5% Scattered to Space
1%
Absorbed
Lower
Dust
0-3 km
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
70% Direct to
Earth
1% Scattered to Earth
1% Scattered to Earth
7% Scattered
To Earth
17
Air Mass
• The path length that light takes through the atmosphere
normalized to the shortest possible path length
x 1

h h
1
h  AM 
cos q
cos q 
q
x
h
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
18
Measuring the Air Mass
q
cos q 
L

h
1
h
AM 
 
cos q L
S 2  L2
S
   1
L
L
2
h
q
Object of
height (L)
Neglects the curvature of the
earth’s atmosphere
Shadow from
object (S)
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
19
EAST
SOUTH
NORTH
WEST
March 22: Spring Equinox
What time of year is this image true?
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
Sept 23: Fall Equinox
20
Declination Angle
Mar 23
Declination (d) = 0º
Jun 23
Dec 23
Declination (d) = 23.45º
Declination (d) = -23.45º
 360
d  81
d  23.45 sin 
 365


Sep 23
Declination (d) = 0º
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
21
Declination Angle of Sun
Summer Solstice
d = 23.45º
North
Pole
d
Fall Equinox
d = 0º
d
Winter Solstice
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
d = -23.45º
22
Elevation Angle
The elevation angle is the angular height the sun
makes with the horizontal
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
23
Polar Plot of Sun Position for Bozeman
Summer
Spring/Fall
Winter
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
24
Passive Solar Heating
Summer Sun
Winter 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
25
Insolation
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
26
Solar Cell Land Area Requirements for
the USA’s Energy with Solar PV
• 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.
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
27
Solar Cell Land Area Requirements for
the World’s Energy with Solar PV
6 Boxes at 3.3 TW Each
Montana State University: Solar Cells
Lecture 3: Nature of Sunlight
28