EE580 – Solar Cells Todd J. Kaiser Lecture 03 Nature of Sunlight 1Montana State University: Solar Cells Lecture 3: Nature of Sunlight

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


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


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


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)


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


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


Solar Radiation


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


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


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


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


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


Sunlight at Earth’s Orbit

year theofday theisn

/365

)2(360cos033.011353 2mW

nH ticalOrbitEarthEllip


Sunlight at Earth’s Surface

• Atmospheric Effects– Absorption– Scattering– Local Variations

• Clouds• Pollution• Weather

• Latitude• Season of Year• Time of Day


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


Water Vapor0-3 km

6% Absorbed



Lower Dust

0-3 km1% Absorbed 1% Scattered to Earth


18% Absorbed

70% Direct toEarth

3% ScatteredTo Space

7% ScatteredTo Earth


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


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


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


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


Polar Plot of Sun Position for Bozeman

Summer

Spring/Fall

Winter


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


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


Solar Cell Land Area Requirements for the World’s Energy with Solar PV

28

6 Boxes at 3.3 TW Each


Documents

EE580 – Solar Cells Todd J. Kaiser Lecture 03 Nature of Sunlight 1Montana State University: Solar Cells Lecture 3: Nature of Sunlight