Saved By the Sun
follow a seven-step invention process to design, build, and test a solar cooker
that will pasteurize water.
Students will be able to:
use the invention process to design, build, and test a solar cooker.
describe how transmission, absorption, and reflection are used in a solar cooker to heat water.
evaluate what variables contribute to a successful cooker.
- copy of the "Got Sun? Get Cooking!" student handout
- copy of the "Invention Checklist" student handout
- copy of the "Finding the Focal Point" student handout
- copy of the "Temperature Data" student handout
- meter stick
- 150 ml room-temperature water
- thermometer, with remote sensor if possible
- ultraviolet protective sunglasses
- 2 different-colored pens or pencils
Suggested Materials for Class
for platform: assortment of cardboard or clear plastic
containers (shoeboxes, pizza boxes, ice cream containers, clear plastic tennis
ball cans, long potato chip cans with metallic interior, 1- or 2-liter clear
for lining platform: aluminum foil (heavy duty is best), foil wrapping
paper, foil-covered foam insulation, or any other reflective material
for insulation: cotton, wool, newspaper, pine needles, straw
for holding water: glass, plastic, or metal container large enough
to hold 150 milliliters of water
for other construction: file folders, cardboard, different-colored paper
or paint, paintbrushes, clear plastic wrap, clear transparencies, clear sheet
protectors, cooking bags, wood skewers, scissors, glue, string, duct tape,
clear tape, cardboard cutting tools (must be supervised)
additional materials as determined by class
Unlike many of the fossil fuels humans depend
upon, solar energy is a clean and renewable resource. Although Earth only
receives one ten-billionth of the total output of power from the sun (4 x 1026
watts), more energy strikes Earth in a few hours than is consumed by humans in
an entire year.
A solar cooker uses only a small portion of the
total energy released by the sun. In fact, most of the radiation that reaches
Earth's surface is visible light, or long-wave infrared waves (most
short-wave infrared radiation is absorbed by atmospheric water vapor). Most
higher-energy wavelengths—ultraviolet, X-rays, and gamma rays—are
scattered or absorbed by Earth's atmosphere. Radio waves are very weak
and carry almost no energy.
Infrared waves that strike a cooker are absorbed
by objects in the cooker as thermal radiation. Light waves that reach a cooker
are either transmitted, absorbed, or reflected depending upon the object they
strike. An object will either transmit light waves through itself, absorb light
waves and convert a large part of them to heat, or reflect light waves off its
surface. (When light hits most objects, part of it will be transmitted, part
absorbed, and part reflected.)
While many types of cookers exist, all solar
cookers operate on the same principle—concentrate energy from the sun on
an object to be heated. The three main types of cookers are box, panel, and
parabolic. Box cookers, often covered by clear glass or plastic, work by
trapping energy inside the box (the transparent covering allows light waves to
pass into the box but does not easily let infrared waves out). Glass traps heat
better than plastic. A box cooker works best if the interior is painted a dark
color to absorb most of the light waves, if the box is insulated to help curb
heat loss from the box walls, and if some sort of reflective surface is added
outside the box to help focus light waves inside.
Panel cookers work by reflecting light waves down
onto the food to be cooked. They often contain many panels positioned to
reflect light waves down onto the food. Parabolic cookers also use a reflective
surface to focus light; they differ from panel cookers in that they concentrate
all incoming light waves to a specific point (called the focal point). The
focused light waves are converted to heat when they are absorbed by the food.
Parabolic cookers are often the most effective cooker designs because of this
For both panel and parabolic cookers, successful
cookers will have a large area of reflecting surface to focus light waves.
Parabolic cookers are most effective when the food is placed at or near the
cooker's focal point. Regardless of the type of cooker students build,
all cookers are most effective if they directly face the sun (or are at least
at a 45-degree angle to the sun to catch incoming energy), when their
reflective material is as smooth as possible, and when the cooking container is
a dark color. The ideal cooking container will be dark, lightweight, and only
slightly larger than the water it will hold.
In this activity, students use a systematic
approach to design, build, and test a portable passive solar cooker that will
pasteurize water in the shortest amount of time. This activity can be done as a
guided inquiry or as an open-ended design process. As a collaborative team project
it will take seven to ten class periods to complete. To shorten the project
time, limit the challenge to one style of cooker, collect materials in advance
of the activity, and have teams build and test just one cooker each. Use the
results from that test as the basis for a discussion about what makes a solar
Organize students into teams of no more than
three. Set up the engineering challenge for students. Tell them that they have
been hired by the World Health Organization (WHO) to help solve a major world
health crisis: Curb the spread of disease in developing countries by designing
an inexpensive, efficient, portable solar cooker that will pasteurize
contaminated water and make it safe to drink. (According to WHO between 2
million and 5 million people—many of them children— die each year
from diseases resulting from contaminated water.)
Provide a guided inquiry about what might make
a solar cooker work. Show students a shoebox and invite discussion about how it
might be used to make a solar cooker. (Some students might mention painting
it black to absorb heat, others might suggest using a lens to focus the
sun's rays or lining it with a reflective surface.)
Discuss what variables are important to
consider when designing a solar cooker, including transmission, absorption,
reflection, insulation, and sun position in relation to the cooker. Review
these concepts with students and discuss what role they can play in the cooker.
Explain to students that light rays from the sun are converted to heat when
they strike an object.
Distribute the student handouts to each team
and review the instructions and criteria for success listed on the handouts.
Allow students time to research types of solar cookers and to brainstorm materials
for their cooker. Once students have completed their research, review the
suggested materials listed in this activity and, as a class, determine a final
list for the challenge (students may come up with additional suggestions). Note
that masking tape does not hold well, and that a magnifying glass should not be
used as it could lead to temperatures hot enough to ignite flames. Depending on
temperatures, clear plastic wrap may melt.
To help students who are building parabolic
cookers understand how the curvature affects where the focal point is, have
those students use the information on the "Finding the Focal Point"
handout to find an online demonstration of how focal point changes with depth
of cooker or to derive this point themselves.
Allow students time to design and build their
prototype. Only allow one team to use the cardboard cutting tools at a time
and supervise their use. Instruct
students to record their invention process in a team journal, including what
materials they used and why, and a drawing of the cooker and its dimensions. If
possible, have students photograph their solar cookers with a digital camera
and insert the images into their journals to supply a visual record of their
Review the safety rules
with students prior to the testing phase of the activity.
Choose a sunny, calm day for testing (cookers
are best tested near solar noon). Students can obtain sun path information for
their location and time of year from the University of Oregon Solar Radiation
Monitoring Laboratory at
Include a control in the experiment, such as a
bottle with a thermometer in it that sits in the sun (the thermometer should
only show a rise of a degree or so).
Have students measure the temperature of their
water (in centigrade) at the start of the experiment and then every five
minutes. Pasteurization, which kills off most harmful microbes, occurs when
water is heated for a short period of time at 65° C. (If you live in a
particularly sunny southern latitude you may want to amend the challenge to
have students attempt to get the water to its boiling point—100° C.
Tell students that although pasteurization occurs at 65° C, many people in
developing countries don't have access to a thermometer and instead use
the boiling point to ensure that the water has been pasteurized).
After the first testing session, create a
class chart for the temperature data. Have each team first plot and graph its
own temperature data on the team's student handout and then present its
designed cooker to the class and add its temperature data to a class chart
(each team should use a different-colored pen). Discuss with students what
worked and did not work on their initial designs.
Next, have students choose a variable (or
variables) on their designs to improve. Have students redesign their models,
noting on their original drawings what they changed and listing the reasons
When teams have built their second models,
conduct another round of testing. After the second testing session, have each
team graph its temperature data on its original student handout graph in a
different color. As a class, discuss the strengths and weaknesses of the final
solar cooker designs. How did the first data set compare with the second? What
design factors seemed to be the most important? Was the most efficient cooker
(the one that heated water the fastest) also the least expensive in terms of
materials cost? The most portable? Which of the designs appeared to best meet
all three criteria?
students the advantages of using the sun as an energy source. What advantages
does the sun have over fossil fuels like coal and oil, which are the currently
most-used energy sources? (The sun is a clean, renewable energy source;
fossil fuels increase atmospheric carbon dioxide and are nonrenewable.) What are some ways that the sun can be collected and
used as an energy source? (Some ways include solar cookers, solar
photovoltaic panels on houses, solar home water heaters, and solar power
To help students understand how a solar cooker uses the sun's energy differently than photovoltaic (or PV) solar panels found on homes, show the video clip at right (2:51) that explains how these panels work. (QuickTime, RealVideo or Windows Media plug-in required.)
After students have viewed the video, ask them to
explain the differences between the two methods of using the sun's
energy. (A solar cooker primarily uses absorbed and reflected
electromagnetic radiation from the sun to cook food; photovoltaic panels
convert the sun's photons into electricity by knocking free
negatively-charged particles from silicon atoms.)
As an extension, have students determine which
locations on Earth would be the best for using a solar cooker and why,
including whether their city or town would be a good location.
Student Handout Questions
Based on your data, was your first model or
your redesigned model more efficient at heating the water? Answers will
What factors do you believe account for the
differences in the efficiency of the two solar cooker designs? Answers will
List three variables that affect the time it
takes to heat water in a solar cooker. Students may identify the following
variables or come up with others:
- size of solar cooker
- area of reflecting surface
- whether cooker is covered with a
- whether cooker is insulated
- color of cooker's interior
- position of water being heated
in relation to focal point of cooker (parabolic cookers)
- size and shape of water
- position of cooker in relation
to position of sun
- cloud cover
- time of day water is heated
- season of year
Draw a diagram to show how transmission,
reflection, absorption, and insulation played a part in your solar cooker
(depending on your type of cooker not all of these concepts may apply). Diagrams
will vary based on the type of cooker students built. Students with box cookers
should note that light is transmitted through the clear covering on the box and
absorbed by the dark materials inside. Insulating materials inside the box will
help retain the heat. Reflecting materiale outside the box will reflect some of
the light waves in. Panel and parabolic cookers reflect the light waves toward
the container of water where they are absorbed. These types of cookers may or
may not have a covering or insulation.
If you had to change one design element in your
cooker to improve it for a subsequent test, what would it be? Why would you
make the change? Answers will vary.
Compare the use of the sun as an energy source
to one of the following types of energy sources: wood, fossil fuels such as
coal or oil, nuclear power, wind, or water. Compared to wood—sun is
inexhaustible, burns clean (wood creates smoke that can be harmful to human
health); compared to fossil fuels—sun burns clean and is inexhaustible
(fossil fuels create CO2 that contributes to greenhouse gases);
compared to nuclear power—sun has no hazardous waste byproducts (nuclear
power creates radioactive plutonium and other high-level nuclear waste, nuclear
power plants can pose risk of a nuclear accident); compared to wind and
water—sun is similar to wind and water in that it is a clean, renewable
energy source that gives off no harmful byproducts.
In classroom testing of the activity using 200
milliliters of water, the following cookers best met the criteria outlined for
students. The cookers, which were all first designs, were tested in
Brownsville, Texas (25.54 degrees N latitude) on March 5. The day was sunny
with no clouds. The first class tested at about 9 a.m. and the last class
finishing at 3 p.m. No classes collected data between 11:30 a.m. and 1 p.m.
two best-performing cookers both used large reflecting panels, a dark container
at the base for water, and a cover to retain heat. Both were lightweight and
thus relatively portable.
NOVA—Saved By the Sun
information on the latest solar technologies, shows how to convert a home to
solar power, details how a photovoltaic cell works, and offers a discussion
Building a Solar Oven
Offers a blueprint for building a solar oven, a
list of safety precautions, and a set of comprehension questions.
Energy Quest: Energy Story
Details different forms of energy, how they work,
and the pros and cons of each.
How to Build a Solar Cooker
Lists plans for several types of solar cookers
National Energy Policy Overview: Germany and
Provides energy consumption statistics for Germany
and the United States.
Details how to create your own solar cooker step-by-step
and presents testimonials from kids who have tried it themselves.
World of Energy: Energy Information
Includes fact sheets that include the advantages
and disadvantages of various types of energy.
World Resources Institute: Energy and Resources
Provides a searchable database of energy
consumption by fuel type and country.
Cooking with Sunshine: The Complete Guide to
Solar Cuisine with 150 Easy Sun-Cooked Recipes
by Lorraine Anderson and Rick Palkovic. Marlowe, 2006.
Describes how a solar cooker works, gives a short
history of solar cooking, answers commonly asked questions about solar cooking,
and provides instructions for how to build a cooker with cardboard and foil.
Going Solar: Understanding and Using the Warmth in Sunlight
by Tomm Stanley. Chelsea Green, 2004.
Provides information on the history and basic principles
of solar design, the science of sunlight, the nature of materials, and ideas
for practical applications. Includes information on solar ovens.
Got Sun? Go Solar
by Rex A. Ewing and Doug Pratt. PixyJack Press, 2005.
Includes explanations of electricity and solar cells, provides
reasons for installing alternate energy systems, and profiles different systems
that can be installed in a house.
The "Got Sun? Get Cooking!" activity aligns with the following National
Science Education Standards (see
books.nap.edu/html/nses) and the Principles and Standards
for School Mathematics (see standards.nctm.org/document/index.htm).
Transfer of energy
Science and Technology
Abilities of technical design
Understandings about science and
Data Analysis and Probability
Interactions of energy and matter
Science and Technology
Abilities of technical design
Classroom Activity Author
Jeff Lockwood taught high school astronomy,
physics, and Earth science for 28 years. He has authored numerous curriculum
projects and has provided instruction on curriculum development and science
teaching methods for more than a decade.