Now You're Cooking! Building a Simple Solar Oven

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.

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.

2. 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.

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.

3. 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.
4. 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.

Figure 4. Cut the corners of the inner box down to the height you want the box to be (based on step 3).

5. 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.
6. 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.

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.

7. Glue aluminum foil to the inside of the outer box, as shown in Figure 6, below.

Figure 6. Glue aluminum foil to the inside of the outer box.

8. 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.

Figure 7. Place some newspaper wads into the bottom of the outer box for the inner box to sit on top of.

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.

9. Glue aluminum foil to the inside of the inner box.
10. 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.

Figure 9. Place the aluminum foil-coated inner box inside the outer box and glue the two boxes together.

11. 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.

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.

Figure 11. Paint the aluminum foil-covered cardboard piece with black tempera paint. You may need to use multiple, thin layers of paint.

12. 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.

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).

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.)

2. 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.
3. 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.

Figure 14. Glue the lid's corners and flaps together to make the lid look similar to this one.

Figure 15. While gluing the lid's corners, you can use binder clips to hold the corners together while the glue hardens.

4. When the lid is done gluing, make sure it fits on the solar oven base as expected, as shown in Figure 16, below.

Figure 16. The lid should fit snugly on the solar oven base.

5. 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).

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.

6. Glue aluminum foil to the inside of the flap you just cut out, as shown in Figure 18, below.

Figure 18. Glue the aluminum foil to the lid's flap.

7. 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.

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.)

8. 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!

Figure 20. Bend a 30 cm piece of coat hanger wire to make a lid prop like this one.

9. 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!

Figure 21. Completed solar oven lid, with the prop holding the flap up.

10. 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.

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 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?