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ESCI-61
Introduction to Photovoltaic Technology
Solar Radiation
Ridha Hamidi, Ph.D.
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The Sun
• The Sun is a perpetual source of energy
– It has produced energy for about 4.6 billions of years, and it is expected to
exist for another 5 billion years
• Because the Earth is only a tiny sphere in the vastness of
space, it receives only about one billionth of the Sun’s
energy output.
• More energy from sunlight strikes the earth in one hour
than all of the energy currently consumed on the planet in
one year
• The amount of solar energy reaching the surface of the
planet is so vast that in one year it is about twice as much
as will ever be obtained from all of the Earth's nonrenewable resources of coal, oil, natural gas, and mined
uranium combined
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Yearly Solar Fluxes & Human
Energy Consumption
Solar
3,850,000 EJ
Wind
2,250 EJ
Biomass
3,000 EJ
Primary Energy Use (2005)
Electricity (2005)
487 EJ
57 EJ
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Prefixes of the SI
• kilo, mega, giga, tera,
peta, exa, zetta, yotta
• milli, micro, nano, pico,
femto, atto, zepto, yocto
• Example, for temperatures
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F = C + 32
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C = (F − 32)
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The Sun
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The sun is a giant nuclear fusion, or thermonuclear, reactor that runs on
hydrogen fuel.
It radiates energy in all directions as electromagnetic radiation
Because the Earth is only a tiny sphere in the vastness of space, it receives
only about one billionth of the Sun’s energy output.
Most ultraviolet radiation is absorbed by the ozone layer in the lower
atmosphere.
Visible light, heat, and a small amount of ultraviolet radiation reach the
troposphere.
About 34% of this solar energy is reflected back into space.
The un-reflected radiation interacts with the Earth and degrades into heat,
which the atmosphere radiates.
Greenhouse gases slow the radiation of heat from the atmosphere into
space.
These gases help keep the Earth warm by acting somewhat like the glass in
a greenhouse.
Without the natural greenhouse effect, the Earth would be as cold as Mars
and life as we know it would not exist.
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Definitions
• Radiation
– Energy that expands outward from a source in the form of waves or
particles
• Solar Irradiance (Solar Power)
– Intensity of the solar power (W/m2)
• Solar Irradiation (Solar Energy)
– Total amount of solar energy accumulated on an area over time (Wh/m2)
• Insolation
– Solar energy that reaches Earth’s surface over the course of a day
(KWh/m2/day)
• Solar Constant
– Average extraterrestrial solar power (irradiance) at a distance of 1 AU
(93 million miles) from the Sun (1366 W/m2)
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Energy & Power
• Energy is the ability to do work or to cause change
– Common units of energy: Joule, Calorie, BTU, WattHour, Therm
– 1 BTU = 1055 J, 1 Cal = 4.18 J, 1 Wh = 3600 J, 1
Therm = 100,000 BTUs
• Power is the amount of work done or energy
transferred per unit of time
– Power = Energy / Time or Energy = Power x Time
– Common units of power: Watt, Horse-Power
– 1 KW = 1.36 HP
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Electromagnetic Spectrum
• Many different forms of electromagnetic radiation exist, each
having a different wavelength and energy content.
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Electromagnetic Spectrum
• The Sun emits light
primarily in the visible
spectrum, but it also
emits at other
wavelengths
• Organisms vary in their
ability to sense different
parts of the spectrum.
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Atmospheric Effects
• Solar radiation is absorbed, scattered, and
reflected by components of the
atmosphere
– Ozone, carbon dioxide, water vapor, other
gases and particles
– Cloud cover, dust, storms, air pollution,
volcanic eruptions
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5%
46%
49%
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Image Source: http://www.suntricity4life.com/sites/mnedd/_files/Image/energy.jpg
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Solar Radiation
Light from the sky dome
• Direct from the sun
• Everywhere but the sun
• Entire sky
We call it
• Direct (beam)
• Diffuse (sky)
• Global (total)
Global is the sum of direct and diffuse
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Air Mass
• The amount of solar radiation that is absorbed or scattered in the
atmosphere depends on how much atmosphere it passes through
before reaching Earth’s surface.
• Air Mass (AM) is a representation of the relative distance of
atmosphere that solar radiation must travel through to reach Earth’s
surface.
• AM0 : outside Earth’s atmosphere
• AM1.0 : when the sun is directly overhead at sea level
• AM = 1 / cos θ, where θ is the zenith angle
• AM depends upon the time of day, the time of year, and the altitude
and latitude of the specified location
• AM1.5 is considered representative of average terrestrial conditions
in the US and is commonly used as a reference condition in rating
modules and arrays
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Peak Sun
• One third of total solar energy at Earth’s outer
atmosphere is either reflected from clouds back
into space, or scattered and absorbed by the
atmosphere
• Peak sun is an estimate of peak solar irradiance
reaching Earth’s surface
– Generally accepted value is 1,000 W/m2 or 1 KW/m2
• Peak sun hours is the number of hours required
for a day’s total solar irradiation to accumulate at
peak sun condition
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Peak Sun
• For example
– Average irradiance is 600 W/m2 over 8 hours
– Total irradiation = 600 x 8 = 4,800 Wh/m2 =
4.8 peak sun hours
• References
– Solar Radiation Data Manual on NREL site
• http://rredc.nrel.gov/solar/pubs/redbook/
– Solmetric Insolation Lookup Tool
• http://www1.solmetric.com/cgi/insolation_lookup/go.cgi
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Solar Radiation Data Manual
http://rredc.nrel.gov/solar/pubs/redbook/
http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html
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Solar Radiation Data Manual
http://rredc.nrel.gov/solar/pubs/redbook/
http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html
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Peak Sun - Sacramento
Source : http://firstlook.3tiergroup.com/
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Solar Irradiance
18 TWe correspond
to an energy output
of 13,567 Mtoe per
year.
World total primary
energy supply
(TPES) in 2006:
11,741 Mtoe (Mega
Tons of Oil
Equivalent)
Image Source: http://www.ez2c.de/ml/solar_land_area/
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Land Areas
Location / Desert
Africa, Sahara
Desert Size / km2 Area required / km2 (*)
9,064,960
144,231
Australia, Great Sandy
388,500
141,509
China, Takla Makan
271,950
178,571
2,589,910
138,889
South America, Atacama
139,860
136,364
U.S.A., Great Basin
492,100
170,455
Middle-East, Arabian
(*) assuming a conversion efficiency from incident sunlight to electricity of 8 %