Now You're Cooking!

Objective

This project is based on a simple box-type solar oven you can build yourself from simple materials like cardboard boxes and aluminum foil. After you've built your oven, you'll come up with ideas for improving the design. Can you build a second oven that will work more efficiently than the first one?

Introduction

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! (SCI, 2006c)

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. It then radiates the heat, warming the inner box. The plastic window holds the heat in, as does the insulating air space between the inner box and the outer box.

The Experimental Procedure section gives you step-by-step instructions on building a simple box-type solar oven (SCI, 2006c). 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.

Don't worry about the oven catching fire. Paper burns at 233°C (451°F), and your solar oven won't get that hot (SCI, 2004).

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

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

heat sink,
radiant heat transfer,
convective heat transfer,
solar energy.

Questions

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 paint the oven shelf black? Why use a black cooking pot?

Bibliography

The Solar Cooking Archive is a great source of information on solar ovens:
SCI, 2006a. "Solar Cooking Archive," Solar Cookers International [accessed July 17, 2006] http://solarcooking.org/default.htm.
You can find a bunch of alternative solar oven plans on this page:
SCI, 2006b. "Solar Cooking Plans," Solar Cooking Archive, Solar Cookers International [accessed July 17, 2006] http://solarcooking.org/plans.htm.
SCI, 2004. "Solar Cooking Frequently-Asked Questions," Solar Cooking Archive, Solar Cookers International [accessed July 17, 2006] http://solarcooking.org/solarcooking-faq.htm.
SCI, date unknown. "Solar Cooking Hints," Solar Cooking Archive, Solar Cookers International [accessed July 17, 2006] http://solarcooking.org/cooking-hints.htm.
Aalfs, M., date unknown. "Principles of Solar Box Cooker Design," Solar Cooking Archive, Solar Cookers International [accessed July 17, 2006] http://solarcooking.org/sbcdes.htm.
Sponheim, T., date unknown. "Developing an Intuitive Feel for the Dynamics of Solar Cooking," Solar Cooking Archive, Solar Cookers International [accessed July 17, 2006] http://solarcooking.org/intuit1.htm.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

two cardboard boxes;
Notes on boxes:
- The inner box should be at least 38 cm × 38 cm (15" × 15"), but bigger is better.
- The inner box definitely has to be large enough to hold the cooking pot that you intend to use, plus a 2.5 cm air gap.
- The outer box should be larger all around, but it doesn't matter how much bigger, as long as there is at least 1.5 cm (about half an inch) or more of airspace between the two boxes.
- The distance between the two boxes does not have to be equal all the way around.
- Finally, keep in mind that it is very easy to adjust the size of a cardboard box by cutting and gluing it.
ruler or measuring tape;
straightedge;
utility knife;
stylus for scribing cardboard (e.g., ballpoint pen with rounded cap—use it with the cap on);
one sheet of cardboard to make the lid (this piece must be approximately 4–8 cm (2–3") larger all the way around than the top of the finished cooker);
short piece of coat hanger wire to make a prop for the lid;
pair of pliers for cutting and bending coat hanger wire;
one sheet of cardboard to make the shelf/heat sink (same size as the bottom of the inner box);
several sheets of newspaper;
one small roll of aluminum foil;
one can of flat-black spray paint (says on can "non-toxic when dry"), or one small jar of black tempera paint;
at least 8 ounces of white glue;
one Reynolds Oven Cooking Bag ("turkey-size", 47.5 cm × 58.5 cm, or 19" × 23-1/2");
Notes on 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 doesn't offer that much better cooking except on windy days.
for testing your ovens under the same solar conditions, use two oven thermometers, or two identical shallow black cooking pots with covers.

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.

Building the Oven Base

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 (Figure 2). Remove the inner box and cut along this line to form a hole in the top of the outer box.
Figure 2. Marking the outer box for cutting. (SCI, 2006c)
Decide how tall you want your oven to be. We recommend about 2.5 cm (1") taller than your largest pot, and about 2.5 cm (1") shorter than the outer box. This way there will be a space between the bottoms of the boxes once the cooker is assembled.
Slit the corners of the inner box with a knife down to that height. Fold each side down, forming extended flaps (Figure ). Folding is smoother if you first scribe a firm line with a stylus (e.g., ballpoint pen with rounded cap on) from the end of one cut to the other where the folds are to go. Use a straightedge as a guide for your scribing tool.
Figure 3. Cutting and folding the inner box to the proper height. (SCI, 2006c)
Glue aluminum foil to the inside of both boxes and also to the inside of the remaining top flaps of the outer box. Don't waste your time being neat on the outer box, since it will never be seen, nor will it experience any wear. The inner box will be visible even after assembly, so if it matters to you, you might want to take more time here. Glue the top flaps closed on the outer box.
Place some wads of crumpled newspaper into the outer box so that when you set the inner box down inside the hole in the outer box, the flaps on the inner box just touch the top of the outer box (Figure 4).
Figure 4. Putting the inner box in place. (SCI, 2006c)
Glue the flaps of the inner box onto the top of the outer box. Trim the excess flap length to be even with the perimeter of the outer box.
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 foil to one side. Paint the foil black and allow it to dry. Put this in the oven so that it rests on the bottom of the inner box (black side up), and place your pots on it when cooking. The base is now finished.

Building the Removable Lid

Take the large sheet of cardboard and set the oven base on top of it (centered). 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 7,below).
Trace the outline of the base onto the lid. It will help to get a good fit if you hold the pencil up against the oven base, as shown in Figure 5 (you may want to have a helper hold the oven base firmly in place as you do this).
Figure 5. Marking the lid using the oven base as a template. (SCI, 2006c)
Use your stylus and straightedge to scribe lines for folding down the edges of the lid. You will also need to cut short flaps at the ends (see Figure 6). Fold each of the four edges along the scribed lines.
Figure 6. Folding and cutting flaps for the lid. (SCI, 2006c)
Fold the corner flaps around and glue to the side lid flaps. (Figure 6, above). Don't glue the lid to the box. You'll need to remove it to move pots in and out of the oven.
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 (Figure 7). Cover this flap with foil on the inside.
Figure 7. Making the reflector flap and prop. (SCI, 2006c)
To make a prop bend a 30 cm (12") piece of coat hanger wire as indicated in Figure 7. This can then be inserted into the corrugations as shown.
Next, turn the lid upside-down and glue the oven bag in place. We have had great success using 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. When using this method, it is important to also glue the bag closed on its open end. This stops water vapor from entering the bag and condensing. Alternatively you can cut any size oven bag open to form a flat sheet large enough to cover the oven opening.
Once the glue dries, your oven is complete and ready for cooking.

Testing Oven Performance

Time how long it takes to boil water. Use the same amount of water, at the same starting temperature for each oven. You should also use the same color and type of pot in each oven. Place the ovens side-by-side so that the test conditions are the same for both ovens. Repeat the measurement at least three times to be sure that your results are consistent.
Alternatively, you could measure the temperature inside each oven. You'll need a separate oven thermometer for each oven, so that you don't have to keep opening the ovens to move the thermometer back and forth (which would cause heat loss). 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. With a thermometer, you can also test to see if the temperature inside the oven is uniform.

Improving Efficiency

The oven you have built should cook fine during most of the solar season. If you would like to improve the efficiency to be able to cook on more marginal days, you can modify your oven in any or all of the following ways:

Make pieces of foiled cardboard the same size as the oven sides and place these in the wall spaces.
Make a new reflector the size of the entire lid (see photo above).
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.

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 Bill?
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 Solar Cooking Archive page (SCI, 2006b). 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).

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

The original plans for the solar cooker used in this project are from: