Solar Energy: Photovoltaics
Title: Solar Energy: Photovoltaics
Author: Eric Christensen
Basic Idea:
Using photovoltaic cells, energy from the sun can be converted into electricity
that we can use every day. Silicon is one of many materials that can be used in a
photovoltaic cell to convert the sun’s energy into electricity. As sunlight strikes a silicon
solar cell the electricity generated can be used to power a motor. A lens can be used to
collect a large amount of light and concentrate it to hit a smaller solar cell.
Applications:
Rooftop solar panels
Solar farms for utility power
Portable power
Solar battery chargers
Extended Discussion and Demonstration:
The sun is a great natural resource of energy. On a sunny day, the sun provides
approximately 1000 watts of power per square meter (W/m2). This energy from the sun
can be used in many ways—from passive solar heating to day-lighting; from heating
water for steam turbines to directly generating electricity through photovoltaic devices.
This paper focuses on solar energy by means of photovoltaic devices.
The solar spectrum extends from the ultraviolet into the infrared (~300nm –
2500nm). Photons (packets of light energy) whose energy is greater than that of the
bandgap of the photovoltaic device can be converted, by the device, into electrons, thus
generating a current. The following diagram shows a plot of the solar spectrum,
indicating the UV, visible, and IR portions of the spectrum, as well as showing where the
bandgap of silicon is located. The more light that hits the device, the more current will
be generated. The power provided by the solar cell depends both on this current that is
generated and the voltage that is produced on the device from the light hitting it.
Direct Solar Spectrum
1.6
Above
Below
silicon’s silicon’s
bandgap bandgap
Spectral Irradiance (W/m^2/nm)
1.4
1.2
1
0.8
0.6
0.4
Visible
0.2
IR
UV
0
0
500
1000
1500
2000
Wavelength (nm )
In this activity, we will see how a silicon solar cell can be used to power a small
motor. We will look at how much of the chip needs to be illuminated in order to make
the motor work. We will also explore the concept of concentrator photovoltaics and
discuss some of the advantages and disadvantages of such systems.
NOTE: This activity is best demonstrated outside using the actual sun, but if necessary it
can be done indoors using a bright lamp or spotlight as the “sun.” If this is done, make
sure to test it out ahead of time to make sure that the concepts you want to demonstrate
will work appropriately with the lamp you are using.
Safety: Do not look directly at the sun and do not burn things while using the lens with
the sunlight.
Equipment:
Solar cell w/ motor
You can do one of the following:
1. Buy silicon solar cell connected to motor—can be bought as a kit at Radio
Shack (and perhaps other places).
2. If inclined to do so, buy silicon solar cell, wire, and motor and put it together
yourself.
cardboard or other surface to attach the solar cell and motor to
lens
paper
2500
sun (or lamp)
Flat-Plate Silicon Solar Cell
1. With glue or tape, mount the solar cell and motor to the cardboard (or other surface) to
make handling easier.
2. Aim the solar cell towards the sun and see the motor spin. Aim the solar cell well
away from the sun to show that it will not work (note: the silicon chip is able to use the
light from the blue sky and other ambient sources, but there is not enough power in those
light sources to power the motor).
3. Put a piece of paper in front of the solar cell and slowly move it across, shadowing the
cell more and more as you move it. At some point, too much of the solar cell will be
shadowed such that it is not receiving enough light to power the motor.
4. Point the solar cell directly at the sun. Rotate the cell until the motor stops. Rotating
the cell so that it is not pointed directly at the sun reduces the amount of sunlight that hits
the cell. Also, the light can not enter the solar cell as efficiently when it is coming in at a
large angle. After rotating the cell enough, it will not receive enough sunlight to power
the motor.
Application: One use for flat-plate silicon solar cells are for solar panels on rooftops.
The solar panels are most effective when they are pointed directly at the sun. When on a
rooftop, however, the solar panels remain in a fixed position, typically pointing south (in
the northern hemisphere) and tilted at an angle equal to latitude of the earth at that spot.
In this configuration, the solar cells work most effectively around noon when the sun is
located due south and is at its highest point in the sky. As the sun moves across the sky,
it is like rotating the solar cell, causing less energy to be generated. Any shadowing of
the direct sunlight by trees or other buildings will also decrease the amount of energy
generated.
Concentrator Photovoltaics
1. Cut a hole in a piece of paper just small enough that when it is over the solar cell the
sunlight hitting it directly will not cause the motor to spin (or cover part of the solar cell
with a piece of paper without a hole in it just enough that when it is pointed at the sun the
motor will not spin).
2. Hold a lens (that is well bigger than the exposed portion of the solar cell) in front of
the cell and aim both the solar cell and the lens directly at the sun. Hold the lens a
distance away from the solar cell that causes the focused light to slightly under fill the
exposed part of the cell without creating a sharp focused spot on the solar cell. (be
careful not to hold the lens in a position that causes the bright focus of the light to hit the
paper, your skin, grass, etc.—it could cause things to burn and/or hurt). Notice that the
motor now spins.
3. Keeping the solar cell pointed directly at the sun, tilt the lens slightly away from the
sun. Notice that the focused light moves. Tilt the lens more so that the light is
completely off of the exposed portion of the solar cell (again, be careful not to create a
sharp focus on that will cause things to burn). Notice that the motor will stop moving at
some point as you tilt the lens.
4. Try to re-aim the lens so that the light falls onto the solar cell and the motor spins
again.
Application: Concentrator photovoltaic systems can be used to decrease the amount of
photovoltaic material used in a solar cell while still collecting a large amount of sunlight
and turning it into electricity. Some photovoltaic devices are more efficient than silicon
but are also much more expensive, causing flat-plate systems with these materials to not
be a viable option. By concentrating the light with optics, such as a lens, you can use less
of the photovoltaic material but still collect the same amount of light. This will decrease
the cost of the system. However, as was seen in the demonstration, if the lens is not
pointed at the sun the light will not hit the photovoltaic device. This means that to use a
concentrator photovoltaic system throughout the day, it needs to be built on a tracking
system that can move the solar cell to keep it pointed at the sun. This adds other costs
and complexity that does not exist with flat-plate silicon. There are both pros and cons to
using concentrator photovoltaic systems.
References
The following is a list of some online references for this document and resources about
solar energy. There are many others, as well, so look around and see what you can find.
http://www.sunwindsolar.com/
http://pvcdrom.pveducation.org/index.html
http://www.nrel.gov/