Basics of Solar Energy
The Sun Always there; lots of Energy
How many photons (energy) reach the surface of the
Earth on Average?
The energy balance in the atmosphere is shown here:
The main components in this diagram are the following:
What Happens to the 69% of the incoming radiation that doesn't
get reflected back:
- Short wavelength (optical wavelengths) radiation from
the Sun reaches the top of the atmosphere.
- Clouds reflect 17% back into space. If the earth gets more
cloudy, as some climate models predict, more radiation will be
reflected back and less will reach the surface
- 8% is scattered backwards by air molecules:
- 6% is actually directly reflected off the surface back into
- So the total reflectivity of the earth is 31%. This is
technically known as an Albedo . Note
that during Ice Ages, the Albedo of the earth increases as
more of its surface is reflective. This, of course, exacerbates
- 19% gets absorbed directly by dust, ozone and water
vapor in the upper atmosphere. This region is called the stratosphere
and its heated by this absorbed radiation. Loss of stratospheric
ozone is causing the stratosphere to cool with time
this has caused some of use stratospheric cooling as an argument against
the occurence of global warming. The two are not connected
- 4% gets absorbed by clouds located in the troposphere. This
is the lower part of the earth's atmosphere where weather happens.
This part of the equilibrium cycle is being changed as the troposphere,
particularly at tropical latitudes, is getting cloudier.
- The remaining 47% of the sunlight that is incident on top of the
earth's atmosphere reaches the surface. This is not a real
significant energy loss Therefore it makes
absolutely no sense to put solar panels in orbit and then "beam"
the energy back to the surface.
How much energy from the sun reaches the surface of the
Earth on Average?
Note that we measure energy in units of Watt-hours. A watt
is not a unit of energy, it is a measure of power.
ENERGY = POWER x TIME
1 Kilowatt Hour = 1KWH = 1000 watts used in one hour =
10 100 watt light bulbs left on for an hour
Incident Solar Energy on the ground:
So over this 8 hour day one receives:
But to go from energy received to energy generated requires
conversion of solar energy into other forms (heat, electricity)
at some reduced level of efficiency.
- 8 hours x 600 watts per sq. m = 4800 watt-hours per sq. m which
equals 4.8 kilowatt hours per sq. m
- This is equivalent to 0.13 gallons of gasoline
- For 1000 square feet of horizontal area (typical roof area) this
is equivalent to 12 gallons of gas or about 450 KWH
We will talk more about PV cells in detail later. For now the
only point to retain is that they are quite low in efficieny!
Collection of Solar Energy
Amount of captured solar energy depends critically on orientation of
collector with respect to the angle of the Sun.
A typical household Winter energy use is around 2000-3000 KWHs
per month or roughly 70-100 KWH per day.
- Under optimum conditions, one can achieve fluxes as high as
2000 Watts per sq. meter
- In the Winter, for a location at 40 degrees latitude, the sun is
lower in the sky and the average flux received is about 300
Watts per sq. meter
Assume our roof top area is 100 square meters (about 1100 square
In the winter on a sunny day at this latitude (40o) the roof will
receive about 6 hours of illumination.
So energy generated over this 6 hour period is:
300 watts per square meter x 100 square meters x 6 hours
= 180 KWH (per day) more than you need.
But remember the efficiency problem:
- 5% efficiency 9 KWH per day
- 10% efficiency 18 KWH per day
- 20% efficiency 36 KWH per day
At best, this represents
1/3 of the typical daily Winter energy usage and it assumes the
sun shines on the rooftop for 6 hours that day.
With sensible energy conservation and insulation
and south facing windows, its possible to lower your daily use
of energy by about a factor of 2. In this case, if solar shingles
become 20% efficient, then they can provide 50-75 % of your
Another example calculation for Solar Energy which
shows that relative inefficiency
can be compensated for with collecting area.
A site in Eastern Oregon receives 600 watts per square
meter of solar radiation in July. Asuume that the solar
panels are 10% efficient and that the are illuminated
for 8 hours.
How many square meters would be required to generate
5000 KWH of electricity?
each square meter gives you 600 x.1 = 60 watts
in 8 hours you would gt 8x60 = 480
watt-hours or about .5 KWH per square meter
you want 5000 KWH
you therefore need 5000/.5 = 10,000
square meters of collecting area
Solar Energy: Collection, Energy
Generation and Heat Transfer
How Solar Energy is Used:
- Heat Water into Steam
- Turn photons into electrons
Solar Thermal Power:
Photons into Electrons: PhotoVoltaic Devices
- Focus sunlight on a bucket of water
This requires about 2000 heliostats
Maintenance, initial costs make energy
expensive (25-50 cents per KWH).
- Direct conversion into electricity
Photovoltaics; conversion of solar photons into electrons that
flow down a semi-conductor.
Main problem is low efficiency (about 10%).
When photons strike a metal, their energy is used to liberate loosely
bound electrons and therefore induce a current.
Efficiency of this process depends upon the material
This is the principle behind Digital Cameras (all college graduates should know how
a digital camera works -- its a literacy test).
To make use of the photoelectric effect, we need material that is a
good conductor of electricity and which can be manufactured in bulk
at reasonable cost. This conditions strongly constrain the available
choices. For most practical aspects, Silicon is the material of choice.
Schematic structure of energy bands in Silicon:
- is abundant on the earth and readily found in the crust.
It is a direct product of fusion inside stars. It can be easily
recovered from the crust and mass-produced.
are cheap because silicon works well for circuit boards and is
an easily recoverable material from the earth's crust.
The result is a world-wide economy centered around semi-conductor technology.
- Has four outer (valence) electrons to bond silicon atoms together
in a crystal
- Under normal circumstances, there are no free electrons available
in silicon to conduct electricity. All the electrons are used to
bind the atoms in place to form the crystal.
- The conduction band is empty and therefore no current can be
carried by the material.
Hence, if a silicon atom receives at least 1.11 Electron
Volts from some source, a valence electron will move
to the conduction band. Once an electron is in the conduction
band, the material can carry a current and the material is
now a conductor.
So much energy is 1.11 Electron Volts?
- 1.11 eV corresponds to the energy that a photon of
wavelength 1.12 microns has.
- 77% of the energy from the sun is carried in photons
with wavelength less than this and therefore can move
a valence electron in silicon into the conducting band.