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Now You're Cooking! Building a Simple Solar Oven

Difficulty
Time Required Average (6-10 days)
Prerequisites Sunlight, and fairly warm outside temperatures (>10°C, or 50°F).
Material Availability Readily Available
Cost Low ($20 - $50)
Safety Adult supervision recommended. The oven is designed to cook food or boil water, so use proper caution to avoid burning yourself. Also be careful with the utility knife when cutting cardboard.

Abstract

Here is a project that uses direct solar power, gathering the sun's rays for heating/sterilizing water or cooking. It is a low-cost technology that seems to have everything going for it. Does it work? Can you find ways to improve it? Find out with this project.

Objective

Follow simple instructions to build a box solar oven from materials like cardboard boxes and aluminum foil and then improve on the design to build a second, more efficient oven.

Credits

Andrew Olson, Ph.D., and Teisha Rowland, Ph.D., Science Buddies

Sources

Plans for solar cookers are common. The one built first in this project is based on plans from:

Cite This Page

MLA Style

Science Buddies Staff. "Now You're Cooking! Building a Simple Solar Oven" Science Buddies. Science Buddies, 30 July 2014. Web. 26 Oct. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Energy_p018.shtml>

APA Style

Science Buddies Staff. (2014, July 30). Now You're Cooking! Building a Simple Solar Oven. Retrieved October 26, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Energy_p018.shtml

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Last edit date: 2014-07-30

Introduction

Many devices have been developed that use solar energy—light and heat emitted from the sun — including solar panels, artificial photosynthesis, and solar ovens. Solar ovens can cook food, pasteurize water, or even sterilize instruments using only the power of the sun. How does a solar oven work? The simple answer is that it is designed to absorb more heat than it releases.

Figure 1, below, shows a picture of the type of efficient, easy-to-build solar oven that you will be making and testing in this project. The oven is a box within a box. The inner box is covered with a plastic window (made from a heavy plastic cooking bag available at most grocery stores). The plastic window works like a greenhouse roof, allowing direct and reflected sunlight to pass into the inner box, while retaining radiated heat.

An easy-to-build solar oven made from cardboard boxes, foil, and a plastic cooking bag.
Figure 1. This box-type solar oven is both easy-to-build and very inexpensive! (Image credits: Solar Cookers International Network, 2006)

At the bottom of the inner box, there is a foil-covered shelf, painted black. The shelf serves two purposes. First, it holds the cooking pot. Second (and more importantly) it acts as a "heat sink." The shelf absorbs direct and reflected sunlight, which warms it. The shelf then radiates the heat, and this radiant energy stays mostly trapped in the inner box and warms it. The plastic window holds the heat in, as does the insulation of the air space (and newspaper) that is between the inner box and the outer box.

The Procedure section of this project idea gives you step-by-step instructions on building a simple box-type solar oven. To make this into a complete science fair project, you will need to choose some aspect of the solar oven design to improve and test. Your choice should be based on your background research, and on the experience gained from building the first oven. Build a second oven that includes your design improvement, and make measurements to see if you have improved the oven's performance. You can test your oven by: measuring the internal temperature with an oven thermometer, or by timing how long it takes to boil a given amount of water in a cooking pot. Do not worry about the oven catching fire. Paper burns at 233°C (451°F), and your solar oven will not get that hot.

When you are finished, it would be fun to try using your solar cooker to make a meal. The "Solar Cooking Hints" webpage listed in the Bibliography, below, has some suggestions. Generally it takes about twice as long to cook food with a solar oven than in a conventional oven, so you will need to plan ahead. Rice is a good first dish to try.

Terms and Concepts

  • Solar energy
  • Greenhouses
  • Reflected light
  • Heat sink
  • Radiant energy
  • Insulation

Questions

  • What is radiant energy? How is it used in a box-type solar oven to heat it up?
  • How hot does a typical box-type solar oven get?
  • How hot does an oven need to be to cook food?
  • Can a solar cooker work on a cloudy day?
  • Why use a black cooking pot?

Bibliography

You can find a bunch of alternative solar oven plans on this webpage:

You can find other information on how solar ovens work and how to use them on these webpages:

Materials and Equipment

  • Cardboard boxes (4). You will be building two solar ovens, and each oven requires two cardboard boxes. Here are some notes on picking out two cardboard boxes for making a single solar oven:
    • The inner box should have an opening of at least 38 cm × 38 cm, and be large enough to hold the cooking pot that you intend to use. It should only be about 2.5 cm taller than the cooking pot.
    • The outer box should be larger all around, with at least 1.5 cm of airspace between the two boxes on each side. It should also ideally be about 2.5 cm–5.0 cm taller than the inner box.
    • The distance between the two boxes does not have to be equal all the way around.
    • Tip: Keep in mind that it is very easy to adjust the size of a cardboard box by cutting and gluing it.
  • Metric ruler or measuring tape
  • Straightedge, such as a hard ruler
  • Utility knife
  • Large sheets of cardboard (2) for making the lid for each of the two solar ovens. Each sheet must be approximately 8-16 cm larger than the opening of the inner box, when measuring both dimensions.
  • Metal coat hanger. This is to make a prop for the lids.
  • Pair of pliers. This is for cutting and bending coat hanger.
  • Sheets of cardboard (2) for making the shelf/heat sink for each of the two solar ovens. Each sheet must be the same size as the bottom of the inner box.
  • Sheets of newspaper (several)
  • A small roll of aluminum foil
  • Black tempera paint (at least 4 oz.). Make sure it is not "washable" tempera paint. This can be purchased locally at crafts stores or through online suppliers such as Amazon.com.
  • Small paint brush
  • Elmer's white school glue (at least 8 oz.)
  • Reynolds oven cooking bag, "turkey-size", or 47.5 cm × 58.5 cm, or 19" × 23-1/2" (2; you will need one bag for each of the two solar ovens). Notes on the cooking bag:
    • These are available in almost all supermarkets in the U.S.
    • They are rated for 204°C (400°F) so they are perfect for solar cooking.
    • They are not UV-resistant, thus they will become brittle and opaque over time and may need to be replaced periodically.
    • A sheet of glass can also be used, but this is more expensive and fragile, and does not offer that much better cooking except on windy days.
  • For testing your ovens under the same solar conditions, you will need oven thermometers (2 identical ones) or shallow black cooking pots with covers (2 identical ones). Alternatively, instead of oven thermometers you could use an infrared thermometer (only 1) with a laser pointer so you can aim it accurately inside of the solar ovens, such as this one from Amazon.com.

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies does participate in affiliate programs with Amazon.comsciencebuddies, Carolina Biological, and AquaPhoenix Education. Proceeds from the affiliate programs help support Science Buddies, a 501( c ) 3 public charity. If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

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Experimental Procedure

Safety Note: The solar oven you will be building is designed to cook food or boil water. Just like your kitchen oven, temperatures inside the solar cooker will be high enough to cause serious burns. Use oven mitts and proper caution to avoid burning yourself. Also, be careful with the utility knife when cutting cardboard to make the oven.
Note: This engineering project is best described by the engineering design process, as opposed to the scientific method. You might want to ask your teacher whether it's acceptable to follow the engineering design process for your project before you begin. You can learn more about the engineering design process in the Science Buddies Engineering Design Process Guide.

Building the Solar Oven Base

  1. Fold the top flaps closed on the outer box and set the inner box on top. Trace a line around the base of the inner box onto the top of the outer box, as shown in Figure 2, below.
Drawing lines around one box onto the top of another box.
Figure 2. Set the inner box on top of the (closed) outer box and draw lines around the bottom of inner box onto the top of the outer box.
  1. Remove the inner box and, carefully using the utility knife, cut along this line to form a hole in the top of the outer box, as shown in Figure 3, below.
    1. Set aside the outer box for now—you will use it again in step 6.
Large box with part of the top cut off.
Figure 3. Carefully cut along the line you made in step 1 to make the outer box have an opening that the inner box can fit inside of.
  1. Decide how tall you want your oven (the inner box) to be. We recommend about 2.5 centimeters (cm) taller than your largest pot, and about 2.5 cm – 5.0 cm shorter than the outer box. (Note that you can change the height of the outer box in step 6, below.) This way there will be a space between the bottoms of the boxes once the cooker is assembled.
  2. Carefully use the utility knife to slit the corners of the inner box down to the height you decided on, as shown in Figure 4, below.
Cutting the corner of a box with a utility knife.
Figure 4. Cut the corners of the inner box down to the height you want the box to be (based on step 3).
  1. Cut the new, extended flaps completely off so that you are left with a box (without flaps) that is the height you want your oven to be. To do this, it is easiest to carefully use a straightedge (e.g., a hard ruler) and the utility knife or a pair of scissors.
    1. Set the inner box aside for now — you will not do anything more to it until step 9.
  2. If you need to make the outer box shorter, do it now.
    1. Remember, you want the outer box to be about 2.5 cm – 5 cm taller than the inner box.
    2. If you need to change the height of the outer box, at each corner measure down to the new height you want the box to be and then make a mark there, along the box's edge. Then carefully use a straightedge and the utility knife or a pair of scissors to cut the box between the marked spots on the edges. You should end up cutting the box into two pieces as shown in Figure 5, below. Stack the pieces on top of each other (with one going inside of the other) and tape or glue them back together to make an intact outer box that is the correct height.
A cardboard box cut into two pieces
Figure 5. If you need to make the outer box be shorter, cut it into two pieces (one being the new height you want the box to be) and then glue or tape them back together (sliding one box inside of the other) to make an intact outer box that is the correct height.
  1. Glue aluminum foil to the inside of the outer box, as shown in Figure 6, below.
A box with the inside coated in aluminum foil.
Figure 6. Glue aluminum foil to the inside of the outer box.
  1. Place some wads of crumpled newspaper into the outer box, as shown in Figure 7, below, so that when you set the inner box down inside the hole in the outer box, the cut edges of the inner box are even with the perimeter of the outer box, as shown in Figure 8, below. Also place wads of crumpled newspaper in the side spaces between the inner and outer box — this will help insulate the oven.
A box with some wads of newspaper in it.
Figure 7. Place some newspaper wads into the bottom of the outer box for the inner box to sit on top of.

A cardboard box set within another cardboard box.
Figure 8. When the inner box is placed inside of the outer box, there should be enough newspaper wads to support the inner box so that its edges are even with the perimeter of the outer box.
  1. Glue aluminum foil to the inside of the inner box.
  2. With the inner box placed inside of the outer box, glue the top edge of the inner box to the perimeter of the outer box where they touch, as shown in Figure 9, below.
    1. Tip: You may need to use some tape to help hold the boxes together while the glue hardens.
A solar oven aluminum foil base.
Figure 9. Place the aluminum foil-coated inner box inside the outer box and glue the two boxes together.
  1. Finally, make a shelf/heat sink inside the inner box. Cut a piece of cardboard the same size as the bottom of the inner box. Glue aluminum foil to one side, as shown in Figure 10, below. Paint the foil black using black tempera paint and allow it to dry, as shown in Figure 11, below.
    1. Tip: To coat the aluminum foil well, apply multiple (2 or 3) thin coats of paint, letting each coat dry before adding the next one.
Cardboard sheet covered with aluminum foil.
Figure 10. Make a piece of cardboard be the same size as the bottom of the inner box, and then cover one side of the cardboard with aluminum foil.

Aluminum foil sheet painted black.
Figure 11. Paint the aluminum foil-covered cardboard piece with black tempera paint. You may need to use multiple, thin layers of paint.
  1. Once the paint has dried, put the shelf/heat sink in the oven so that it rests on the bottom of the inner box (black side up). The solar oven base is now finished and should look similar to Figure 12, below.
Solar oven base completed.
Figure 12. The completed solar oven base should look similar to this one.

Building the Solar Oven's Removable Lid

  1. Take one of the large sheets of cardboard (that you will use for a lid) and set the solar oven base on top of it (centered). Trace the outline of the base onto the lid, as shown in Figure 13, below.
    1. Note: Be sure to orient the corrugations of the lid so that they go from left to right as you face the oven so that later the prop may be inserted into the corrugations (see Figure 22, below).
Sheet of cardboard with traced outline on it.
Figure 13. Trace around the bottom of the solar oven base onto one of the large sheets of cardboard. (The traced lines are faint in this picture.)
  1. Carefully use the utility knife and straightedge (i.e., hard ruler) to cut through one (of the two) sides of the cardboard along the lines you drew. Then fold the cardboard down along the cut edges to make short flaps for the lid. Also cut the corner flaps so that the lid's flaps can all fold down neatly.
  2. Glue the lid's corners to the lid's side flaps to make the lid, as shown in Figure 14, below. You may want to use binder clips to hold the corners together while they glue, as shown in Figure 15, below.
    1. Note: Do not glue the lid to the box! You will need to remove it to move pots in and out of the oven.
Sheet of cardboard cut to make a lid.
Figure 14. Glue the lid's corners and flaps together to make the lid look similar to this one.

Binder clip on cardboard corner.
Figure 15. While gluing the lid's corners, you can use binder clips to hold the corners together while the glue hardens.
  1. When the lid is done gluing, make sure it fits on the solar oven base as expected, as shown in Figure 16, below.
Lid on cardboard box
Figure 16. The lid should fit snugly on the solar oven base.
  1. To make the reflector flap, draw a line on the lid, forming a rectangle the same size as the oven opening (inner box size). Cut around three sides and fold the resulting flap up to form the reflector, as shown in Figure 17, below.
    1. As you did in step 2, carefully use the utility knife to cut through one (of the two) cardboard layers where you want the flap to fold. Be sure to cut on the inside of the lid so that the cut makes the lid fold back the correct way (instead of folding inwards).
Cardboard lid cut to have a flap.
Figure 17. Cut the cardboard lid so that it has a flap that is the same size as the oven's (inner box's) opening.
  1. Glue aluminum foil to the inside of the flap you just cut out, as shown in Figure 18, below.
Cardboard lid with foil-coated flap.
Figure 18. Glue the aluminum foil to the lid's flap.
  1. Next, turn the lid upside-down and glue the oven cooking bag in place, covering the flap's opening, as shown in Figure 19, below.
    1. Use the turkey-size oven bag (47.5 cm × 58.5 cm, or 19" × 23-1/2") applied as is, i.e., without opening it up. This makes a double layer of plastic. The two layers tend to separate from each other to form an airspace as the oven cooks. Be sure to glue the bag closed on its open end to stop water vapor from entering the bag and condensing.
    2. Alternatively, you could cut any size oven bag open to form a flat sheet large enough to cover the oven opening.
Cardboard lid with cooking bag.
Figure 19. Glue the cooking bag to the underside of the lid, covering the flap's opening. (Note that aluminum foil-coated flap is on the other side of the cooking bag in this picture.)
  1. Make a lid prop by using a pair of pliers to bend a 30 cm piece of coat hanger wire as shown in Figure 20, below. Tip: You can carefully cut a piece of coat hanger wire to be this length by making a groove on it and then snapping it at the groove. Be careful handling the wire because the cut edges will be sharp!
Bent coat hanger wire to make a lid prop.
Figure 20. Bend a 30 cm piece of coat hanger wire to make a lid prop like this one.
  1. Insert the lid prop into the lid's corrugations as shown in Figure 21, below, to hold the flap up. Your solar oven's lid is now complete!
Completed solar oven lid
Figure 21. Completed solar oven lid, with the prop holding the flap up.
  1. Once the glue dries, your solar oven is complete and ready for cooking. Once the lid is put on the solar oven base, the completed solar oven should look similar to the one in Figure 22, below.
Completed box-type solar oven.
Figure 22. Completed solar oven.

Testing the Solar Oven's Performance

You will now be testing the solar oven's performance. You can do this by testing how long it takes to boil water or by measuring the temperature inside the oven after letting it heat up. When you test the solar oven, be sure you test it on a sunny day when it is fairly warm outside (>10°C, or >50°F) with the solar oven facing the direction of the sun.

  1. Testing how long it takes to boil water: Pour a specific amount of water (such as 2 cups), into a shallow, black cooking pot (you will need two identical ones later for testing two solar ovens at once). Time how long it takes the solar oven to boil the water. In your lab notebook, be sure to record how much water you used and how quickly the water boiled. You may want to create a data table to record this information.
  2. Measuring the temperature inside the solar oven: Either use an oven thermometer (you will need two identical ones later for testing two solar ovens at once) or an infrared thermometer to measure the temperature of the solar oven after you have let it warm for a certain amount of time, such as 45 minutes. To use the oven thermometer, keep the thermometer in the oven and then quickly open the oven and read the temperature (if you are unable to read the temperature through the oven bag). To read the infrared thermometer, quickly open the oven and use the thermometer to find the temperature of the black shelf/heat sink.
    1. In your lab notebook, be sure to record the temperature of the solar oven and what method you used to take the temperature.
  3. How well did your solar oven perform? Are you surprised by your results? Do you think you could make it work even better?

Improving Efficiency

The solar oven you have built should cook fine during most of the solar season. To improve the efficiency to be able to cook on more marginal days, or make the solar oven be more efficient overall, modify your solar oven's design. Think about how you want to modify it and then build a second solar oven using the modified design. Test the original solar oven and your modified solar oven next to each other and see if your modified solar oven is more efficient than the original

  1. Here are some ideas for modifying the solar oven's design to make it more efficient:
    1. Make pieces of foiled cardboard the same size as the oven sides and place these in the wall spaces.
    2. Make a new reflector the size of the entire lid.
    3. Make the shelf/heat sink using sheet metal, such as aluminum flashing. Paint this black and elevate this off the bottom of the oven slightly with small cardboard strips. Note that you will want to make the inner box a bit taller to accommodate the elevated shelf.
    4. For additional help on thinking about how to improve the basic solar oven design, you may want to check out the Science Buddies' resource on The Engineering Design Process.
  2. After you have built your second solar oven using the modified design, test both solar ovens together by repeating the "Testing the Solar Oven's Performance" section of the Procedure with both ovens at the same time.
    1. Note that you will need two identical, shallow, black cooking pots if you are testing how long it takes water to boil in the ovens. Likewise, you will need two identical oven thermometers or one infrared thermometer if you are testing the temperature inside the ovens.
      1. If you are using two identical oven thermometers, check first to make sure that both thermometers give the same reading using your kitchen oven. If the readings are different, make sure that the difference is consistent, and then use the difference to correct one of the readings so that the measurements can be compared.
    2. Place the solar ovens side-by-side when testing them so that the conditions are the same for both ovens. Be sure that both solar ovens are receiving the same amount of light (i.e., one should not be partly shaded).
  3. Repeat step 2 at least two more times so that you have compared the two solar ovens in at least three trials.
  4. Compare your results for each solar oven. Did the new design perform better than the original design? Are your results what you expected them to be? Do you think you could improve the solar oven design to make it even more efficient?

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Variations

Here are some of the many possible experiments you can try with your solar ovens. You can probably think of others yourself.

  • Test with and without a reflector.
  • Try different types of heat-absorbing materials for the oven shelf/heat sink.
  • Try different types of insulation between the inner and outer boxes.
  • Why is it necessary to paint the shelf black and to use black cooking pots? See for yourself! Try black vs. shiny shelf and cooking pots. See the Science Buddies project Can the Color of Your House Reduce Your Energy Footprint?
  • Try re-orienting the oven towards the sun once or twice an hour, vs. leaving the oven stationary.
  • For a more advanced project, study the plans at the Build a Solar Cooker webpage. Choose two or three different types of solar ovens to build, and see which design is most efficient at heating a standard volume water (e.g., 1 L).

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Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

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