Supplies
Needed:
The experiment is designed for teams of 3-4. Each team needs this
lab manual and a solar building kit.
Passive Solar Energy
When you get into a car after it has been sitting in the hot summer
sun with the windows closed, what do you experience? Very hot, right?
Energy from the sun heated the inside of the car, and you didn't
have to do anything to make it happen. No fans, no motors, no pumps,
no nothing. Because you didn’t have to do anything, this is
called passive solar energy.
Passive solar energy can be used in
a number of ways, such as:
- Solar heat can be captured through glass, such as car windows,
atriums in office buildings, and greenhouses. The glass allows
the solar energy to get in, but doesn’t allow the heat
to get back out. "Bubble pack" pool covers also work
this way.
- Some houses in the southwestern US have a thick adobe wall
on the south side of the house, called a trombe wall. During
the day, the sun heats up the outside of the wall. Heat then
moves inward into the wall. During the night, that wall then
warms the inside of the house.
- Using stone floors in areas of buildings that receive direct
sunlight to absorb heat during the sunny hours and warm the
air around it during the non-sunny hours.
- Holding water in black tanks on the roof so that it is already
warmed up by the time it needs to be heated by a conventional
hot water heater.
The National Renewable Energy Laboratory says that buildings built
with passive solar designs use 47% less energy than conventional
new buildings.
Active Solar Energy
One of the problems with passive solar energy is that you might
want to heat something or some area where the sun cannot directly
reach. You might want to heat the water inside your hot water heater,
or the water inside your pool, or the air inside your house. How
can we get the solar energy to move somewhere else?
The answer is to use the sun to warm some fluid (water or air, for
example) and then use some active methods, such as pumps or fans,
to move that fluid to where the heating is really needed.
Certain materials can generate electricity when exposed to the sun.
The photons from the sun "knock" electrons out of the
photovoltaic material. These electrons then flow through a wire,
producing electricity.
In some ways this is hard to imagine, since photons and electrons
are so small. Since we have never seen them, it is hard to visualize
how they really work. One mental image of this process is jumping
into a ball pit at a fast food restaurant. The "photons"
hit the brightly colored "electrons" and give up their
energy to them. The newly energized "electrons" fly up
into the air. If there was a trough about a foot above the surface
of the balls, the energized balls would be caught in it and roll
in the trough on a track that ended up back into the ball pit.
Small photovoltaic panels are used
in many common applications. You see them in uses such as:
- No-battery wristwatches
- Solar calculators
- Power for Help Phones along the freeway
Small photovoltaic panels are simple enough to be used for science
experiments. We will use the one in your kit to understand a few
properties of photovoltaic panels. To make this panel work, you
will need a fairly bright light source. The sun or a bright incandescent
lamp will work. Fluorescent lights and flashlights probably will
not work.
Experiment #1: Angle
Goal: In this experiment, we will see how closely the panel needs
to be pointing at the light source and what happens if it is not.
Supplies needed: solar panel and motor from kit, light source
Procedure:
- Connect the panel to the motor.
- Aim the panel directly at the sun or the light. The light
shining on the panel is producing an electron flow in the wire
that is then turning the motor. Listen to how fast the motor
spins.
- Now, slowly rotate the panel so it is no longer pointing directly
at the light. What happens to the speed of the motor?
- What is happening? When the panel is rotated, the same amount
of light energy falls on the panel as before, but it is spread
out over more area. So, the energy per square inch goes down.
(You can see the same affect if you shine
a flashlight directly at a wall and then rotate it so it is no
longer shining directly at the wall. The spot of light that the
flashlight makes spreads out so the light intensity goes down.)
Scientists who study this know that the light intensity, and therefore
the electrical energy, goes down by something called the cosine
of the angle. In ordinary terms, it produces a graph like this.
When the panel is pointing perfectly at the light, the electrical
voltage produced is at a maximum. When it is rotated either way,
the voltage drops off.
This is why it helps to be closer to the equator. Farther away
from the equator, the sun is lower in the sky and is harder to
point a solar panel directly at.
Experiment #2: Clouds and Dust
Goal: In this experiment, we will see the effect of clouds and dust
on photovoltaic energy.
Supplies needed: solar panel and motor from kit, light source, wax
paper, 2 pieces of clear plastic, colored plastic.
Procedure:
- Again, point the panel directly at the light.
- Listen to how fast the motor spins.
- Now place various materials between the panel and the light.
Try:
- Clear plastic
- Two pieces of clear plastic
- Colored plastic
- Wax paper
Try anything else you can find that is somewhat clear. Listen to
how fast the motor spins with each material. Which materials are
best at letting solar energy through? Which are worst?
Experiment #3: Other Lights
Try aiming the panel at other light sources such as different types
of lamps. Listen to how fast the motor spins with each. What lamps
are intense enough to produce photovoltaic energy? What lamps are
not?
Conclusions:
Photovoltaic systems are a way that solar energy can augment other
energy sources, but they cannot be used everywhere. Photovoltaic
systems are very sensitive to their angle with respect to the sun
and sensitive to having clouds and dust block the sun. Also, as
shown in the photograph, photovoltaic panels must be much larger
than your panel in order to produce enough electrical power to be
useful.
Can Everyone Use Solar Energy?
Every one of us takes advantage of solar energy in some form. Otherwise
we would not be alive. But, in order for solar energy to be really
practical as a major contributor to our daily energy needs, you
need to live in a part of the world that:
- Is close enough to the equator that the sun is intense enough
- Has very few clouds or dust in the air
In the diagram below, Bahm and Associates have created a map showing
where the best parts of the United States are for solar energy.
In this map, red is bad, yellow is good, and green is better. This
is why most of the serious solar energy effort in the United States
is in New Mexico, Arizona, Nevada, and California.
Benefits of Solar Energy
Solar energy has three major benefits: financial, energy conservation,
and cleanliness:
The sun shines for free and will do so for many generations to come.
If we can figure out how to tap its energy for our own needs, we
will have to spend less money on energy.
The sun is considered to be an unlimited energy source. It will
never burn out, at least not for many thousands of years. However,
other energy sources, such as oil and natural gas, are limited.
Relying on more solar power will help us stretch our limited reserves
of fossil fuels.
The sun is a clean energy source. Unlike oil and natural gas, using
it produces no pollutants.
So, Why Isn’t Everyone Using Solar Energy?
There are two major reasons, cost and capacity:
At the moment, active and photovoltaic solar energy is expensive.
Per kilowatt, it is more expensive than other common forms of
energy production.
Solar energy is not yet efficient enough to produce the massive
amounts of energy necessary to support entire cities and countries.
Solar energy researchers are working to correct these problems,
but for the foreseeable future, solar will continue to be used
in spot applications.
But There's Hope – Check This Out!
Pacific Park on the Santa Monica Pier has just created the world’s
first solar-powered Ferris Wheel! The Wheel is 9 stories tall
and can hold up to 120 people. It is powered by a $360,000 photovoltaic
system. To design it, engineers figured out what motors were needed,
how much electricity they would use, how many revolutions the
Wheel would make in a year, and how much sun Santa Monica could
expect to get each year. From that, they determined that the photovoltaic
system would need to provide 50 kilowatts of electricity. This
translated into 7 large panels of photovoltaic cells, one of which
is shown above in the photograph.
Web Connections
Raymond J. Bahm and Associates is an excellent resource for information
on solar energy. Their web site also contains pointers to other
sites of interest. http://www.ima.kth.se/im/envsite/renew.htm
Mr. Solar is a company specializing in alternative energy solutions.
Through this page you can find papers on solar energy and a catalog
of solar energy products. http://www.mrsolar.com/
The American Solar Energy Society (ASES) is a national organization
dedicated to advancing the use of solar energy for the benefit
of U.S. citizens and the global environment. ASES promotes the
widespread near-term and long-term use of solar energy. http://www.ases.org/
Using the Solar K’Nex Kits
The Solar K’Nex kits come with detailed building plans.
For younger students, the recommended model is the solar flyer
shown on page 10 of the 10 Model set building guide. Extra copies
of the plans are contained in the kit. Older students should be
encouraged to try some of the more complicated models.
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