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Saved By the Sun

Classroom Activity

Learning Objective | Materials | Background | Procedure | Activity Answer | Links & Books | Standards  Print this Teacher's Guide (PDF, 12 pages)

Activity Summary
Students follow a seven-step invention process to design, build, and test a solar cooker that will pasteurize water.

Learning Objectives
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 (PDF or HTML)
• copy of the "Invention Checklist" student handout (PDF or HTML)
• copy of the "Finding the Focal Point" student handout (PDF or HTML)
• copy of the "Temperature Data" student handout (PDF or HTML)
• meter stick
• 150 ml room-temperature water
• thermometer, with remote sensor if possible
• timer
• 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 soda bottles)

• 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

Background
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 feature.

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 cooker work.

1. 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.)

2. 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.)

3. 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.

4. 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.

5. 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.

6. 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 design.

7. Review the safety rules with students prior to the testing phase of the activity.

8. 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

   solardat.uoregon.edu/SunChartProgram.html

9. 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).

10. 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).

11. 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.

12. 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 why.

13. 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?

14. Discuss with 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 plants.)

15. 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.)

16. 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

1. Based on your data, was your first model or your redesigned model more efficient at heating the water? Answers will vary.

2. What factors do you believe account for the differences in the efficiency of the two solar cooker designs? Answers will vary.

3. 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:

   1. size of solar cooker
   2. area of reflecting surface
   3. whether cooker is covered with a transparent material
   4. whether cooker is insulated
   5. color of cooker's interior
   6. position of water being heated in relation to focal point of cooker (parabolic cookers)
   7. size and shape of water container
   8. position of cooker in relation to position of sun
   9. cloud cover
   10. time of day water is heated
   11. season of year

4. 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.

5. 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.

6. 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.

Sample Cookers

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.

The 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.

Temperature Data

Web Sites

NOVA—Saved By the Sun
www.pbs.org/nova/solar/
Provides 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 board.

Building a Solar Oven
www.re-energy.ca/t-i_solarheatbuild-1.shtml
Offers a blueprint for building a solar oven, a list of safety precautions, and a set of comprehension questions.

Energy Quest: Energy Story
www.energyquest.ca.gov/story/chapter17.html
Details different forms of energy, how they work, and the pros and cons of each.

How to Build a Solar Cooker
solarcooking.org/plans
Lists plans for several types of solar cookers with instructions.

National Energy Policy Overview: Germany and United States
energytrends.pnl.gov/germany/ge004.htm
energytrends.pnl.gov/usa/us004.htm
Provides energy consumption statistics for Germany and the United States.

Solar Cookers
pbskids.org/zoom/activities/sci/solarcookers.html
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
www.worldofenergy.com.au/fact_sheets.html
Includes fact sheets that include the advantages and disadvantages of various types of energy.

World Resources Institute: Energy and Resources Searchable Database
earthtrends.wri.org/searchable_db/index.php?theme=6
Provides a searchable database of energy consumption by fuel type and country.

Books

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).

Grades 5-8
Physical Science
Transfer of energy

Science and Technology
Abilities of technical design
Understandings about science and technology

Mathematics Standards
Algebra
Data Analysis and Probability

Grades 9-12
Physical Science
Interactions of energy and matter

Science and Technology
Abilities of technical design

Mathematics Standard
Algebra

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.