Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. To avoid long wait lines for aluminum foil and plastic wrap, cut a 12" x 15" piece of each for each group of students.
2. The pointed tips of the wood skewers are sharp. You may want to blunt the tips by cutting off the tips with scissors or by blunting them with sandpaper.

Safety Precautions

Use caution when cutting with scissors or a craft knife. Always cut away from yourself and others. The point of the wood skewer is sharp; use caution. Food items that may be eaten should not be brought into the lab. If testing the solar oven outdoors, apply full spectrum sunscreen to exposed skin and wear sunglasses. Never look directly at the sun. Please follow all laboratory safety guidelines.

Lab Hints

• Enough materials are provided in this kit for 8 groups of students. All parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The prelaboratory assignment may be completed before coming to lab. Allow time for students to brainstorm, construct and test their improved designs.
• If time is a factor, small chocolate candies are a good food source to test. Depending on the type of chocolate, it will melt at 30–45 °C. Mini chocolate bars, chocolate chips or Hershey’s Kisses^® are good choices. Marshmallows will get soft, but will not turn brown and take longer to melt. The solar ovens may also be tested without food.
• The solar oven may be tested indoors using a 150-W incandescent light bulb about 20 cm away from the window. The oven should get hot enough to melt milk chocolate in 15 minutes.
• Consider giving students a list of constraints for their modified designs (e.g, type or amount of material, cost of material, only material they have on hand [zero additional cost]).

Teacher Tips

• The environmental conditions are a critical factor in successful solar cooking. Ideally, the sky should be clear and one’s shadow should be shorter than one’s height or the UV index in the area should be greater than 7. Visit http://www.epa.gov/sunsafety/uv-index-1 to find the UV index based on zip code. Even under ideal conditions, however, some foods may take several hours to reach the desired internal temperature.
• Compare the students’ results with a commercial solar oven, Flinn Catalog No. AP6388.

Answers to Prelab Questions

1. Which method of heat transfer does not require matter?

   Radiation can travel through the vacuum of space.

2. Briefly explain how a solar oven becomes a heat trap.

   As the light from the Sun passes through the window of the oven, the dark-colored surface absorbs energy and heats up, then re-radiates the energy in the form of infrared radiation. The longer wavelength radiation strikes the window and is absorbed or reflected back into the oven.

Sample Data

Answers to Questions

Design Challenge

1. The solar oven is designed to trap heat. List the ways that heat transfers out of the oven and describe the method of heat transfer that is occurring.

   Heat may transfer by conduction from the black surface on the bottom of the oven, to the cardboard bottom and to the surface the oven is resting on. In the same way, heat may transfer through the sides and cardboard portion of the top to the air outside the oven. Heat may transfer by convection out any open areas between the lid and the bottom of the box.

2. Research the types of materials that are good insulators of heat.

   How can the solar oven be better insulated against heat loss? The inside of the box could be lined with an insulating material (e.g., additional cardboard, crumpled newspaper, foam insulation, resealable bags of water, bubble wrap).

3. Discuss other ways to improve the design of the solar oven to increase the internal temperature in a more efficient or faster way.

   Other improvements may include using double-pane glass instead of plastic for the window, placing the food inside a dark metal covered container, adding reflectors on each side, etc.

4. What other factors may affect the temperature in the solar oven? Which can be controlled and which cannot be controlled?

   The ambient temperature, cloud cover, wind, time of day and season of the year may all affect the temperature of the oven. Outside a classroom setting, the time of day may be controlled, as well as choosing the season of the year.

Post-Lab Questions

1. Consider the temperature inside the oven in Part B.
   1. How much did the temperature increase?

      The temperature inside the oven increased 27 degrees Celsius in 15 minutes.

   2. How much higher was the temperature inside the oven than the outside air temperature?

      The temperature inside the oven was 29 degrees higher than the outside air temperature.

2. Describe and give a reason for each modification your group made to the solar oven.

   The model that was tested had an extra layer of cardboard under the black paper for insulation, reducing heat loss through the bottom of the oven. The food (a mini milk chocolate bar) was placed in a small covered aluminum dish that was painted black with a permanent marker. The metal dish conducted heat to the food and the black surface absorbed more heat energy than a shiny aluminum surface would. Velcro^® was placed around the edges of the bottom and the top to seal the oven closed in order to keep hot air from escaping through the space between the lid and bottom of the box. Two more aluminum foil-covered flaps were added to the sides to reflect the sunlight into the oven. A thicker clear plastic sheet was used to minimize ripples that could scatter the light striking the window.

3. Compare the temperature data from Part B to the data from Part C.
   1. Did the modifications to the design achieve the desired results?

      Even though the chocolate kept its shape, it melted in the same amount of time; and the increase in temperature in 15 minutes was nearly the same—26 degrees in Part C compared to a 27-degree increase in Part B. However, the temperature inside the oven did not get as hot as in the first trial. In the first trial, the temperature inside the oven was 29 degrees warmer than the outside temperature. In the second trial, the inside temperature was only 24 degrees warmer than outside.

   2. Why do you think this was so?

      The weather conditions were not the same from one test to the next. During the test of the improved design, the sky was partly cloudy, and often clouds blocked the sun. The outside temperature was 6 degrees cooler than on the day of the prototype test.

4. What other improvements might make the solar oven more efficient?

   Possible answers may include using glass instead of plastic for the window, doubling the cardboard walls all around, adding more insulating material on the bottom and sides of the oven, etc.

5. List the advantages and disadvantages of cooking with solar energy.

   Advantages of solar cooking include using a renewable source of energy, no fuel needed, no pollution produced, inexpensive materials. Disadvantages include lack of control over the environmental conditions (sensitivity to temperature fluctuations), longer cooking time, may need to rotate the oven over time, and difficulty maintaining a safe internal temperature of the food over a long cooking period.

References

Solar Cookers. Cook’s Illustrated. https://www.cooksillustrated.com/equipment_reviews/1078-solar-cookers. Accessed December 2015.

Introduction

The amount of energy the sun radiates in a single day is more than the amount of energy the entire world uses in a whole year. Cooking with solar energy does not require fuel or pollute the air with smoke. Make your own solar oven and see how well it works!

Concepts

• Infrared radiation
• Heat transfer
• Solar energy

Background

All matter has heat energy, also called thermal energy. Thermal energy is the energy a substance has due to the continuous motion of the atoms or molecules that make up the substance. Thermal energy always flows from a region of higher temperature to a region of lower temperature. This flow of heat is known as heat transfer. Heat can be transferred in three ways—by conduction, convection and radiation. Conduction occurs when two objects of different temperatures are in contact with each other, such as a pan on a hot stove burner. Convection occurs with the movement of fluids, such as convection currents in the Earth’s atmosphere and ocean currents. Radiation does not require matter to transfer heat—it travels through the vacuum of space. The energy from the Sun, or solar energy, that we can see (visible light) is about 43% of the total radiant energy the Sun emits. Most of the thermal energy from the Sun (49%) is in the region of the electromagnetic spectrum called infrared radiation, much of which is absorbed by water vapor in the Earth’s atmosphere. Infrared radiation cannot be seen by the naked eye, but its effects can be felt as heat.

When light hits a boundary between two different substances, three phenomena can occur—reflection, absorption and transmission. Reflection occurs when a light ray bounces off a surface, such as when light strikes a mirror. Absorption occurs when light energy is retained in a substance and changed into another type of energy such as heat. A classic example of this is a black surface becoming extremely hot on a sunny summer day. The black surface absorbs most of the light energy, which is then converted into heat energy. Transmission occurs when light strikes a new substance and then continues to travel through that substance, such as light that travels in air and then passes through a window.

Solar ovens can vary in style, but most work on the same principles as a greenhouse. In a solar oven, visible light (medium wavelength) and ultraviolet light (shorter wavelength) from the Sun pass through a transparent window while infrared radiation (longer wavelength) is absorbed or reflected (see Figures 1 and 2).

Since the angle of the Sun’s rays striking the Earth is dependent upon the time of day and latitude, most solar ovens have one or more reflective panels to direct the light through the window into the oven. The visible and UV light that pass through the window are absorbed by a dark-colored surface inside the oven. This dark-colored surface absorbs the light energy and heats up, and then re-radiates energy from the surface. The re-radiated energy, however, is infrared (long wavelength) radiation and not the shorter wavelengths like those that entered the oven. The longer wavelength radiation strikes the underside of the window and is absorbed or reflected back into the oven. Thus, the original short wave light rays have been transformed and “trapped” inside the oven. The entire structure becomes a “heat trap” (see Figure 2).

Experiment Overview

The purpose of this activity is to build a solar oven out of the materials provided. The maximum internal temperature of the oven in a given amount of time will be measured. Improvements will then be made to the oven to increase its efficiency.

Materials

Prelab Questions

1. Which method of heat transfer does not require matter?
2. Briefly explain how a solar oven becomes a heat trap.

Safety Precautions

Use caution when cutting with scissors or a craft knife. Always cut away from yourself and others. Rotate the cardboard box if necessary for better control while cutting. The point of the wood skewer is sharp; use caution. Food items that may be eaten should not be brought into the lab. If testing the solar oven outdoors, apply full spectrum sunscreen to exposed skin and wear sunglasses. Never look directly at the sun. Please follow all laboratory safety guidelines.

Procedure

Part A. Build a Solar Oven

1. Assemble the cardboard box by folding in the sides and securing with the side flaps.
2. Mark a 3-cm border around the top of the box (see Figure 3).
   {14055_Procedure_Figure_3_Top view}
3. Carefully cut the sides and the front edge along the border. Do not cut the back edge.
4. Slightly score (scratch a line without cutting through) the back edge of the border and then create a flap by gently folding the top back along the scored border (see Figure 4).
   {14055_Procedure_Figure_4_Flap folded back}
5. Measure and cut a piece of aluminum foil the same size as the flap.
6. Smooth out the piece of foil and glue it to the inside of the flap, shiny side out.
7. Measure and cut a piece of black construction paper to fit the inside bottom of the box.
8. Place the black paper in the bottom of the box, making sure it lays flat.
9. Cut a piece of plastic wrap to fit over the opening in the top of the box, making sure the plastic extends beyond the edges of the opening.
10. Using masking tape, secure the plastic wrap to the underside of the opening, pulling it tight so it is smooth and completely seals the opening (see Figure 5).
    {14055_Procedure_Figure_5}
11. Using a paper clip, make a hole in the middle of one side border of the box top (see Figure 6).
    {14055_Procedure_Figure_6_Top view}
12. Cut a 10 cm x 10 cm square of aluminum foil and place in the bottom center of the solar oven. The foil will be a “plate” for the food item.

Part B. Testing the Oven

1. Place the solar oven on a flat surface in a sunny location.
2. Record the outside temperature on the Solar Oven Worksheet under Testing the Oven.
3. Open the box and place a food item on the aluminum foil square.
4. Lay the thermometer on the bottom of the solar oven next to the food item so the Celsius markings can be seen through the plasticcovered window.
5. Close the box lid and open the flap.
6. Orient the solar oven so the sun reflects off the aluminum-covered flap. Adjust the angle of the flap so the reflected sunlight is directed onto the food item. Caution: Never look directly at the sun or reflected sunlight, even when wearing sunglasses.
7. Insert the tapered end of the wood skewer into the hole in the top of the oven and tape the blunt end of the skewer to the flap to keep the flap at the correct angle.
8. Record the temperature inside the oven in degrees Celsius in the Testing the Oven data table on the worksheet and start the timer.
9. Record the temperature inside the oven every minute until the food item is cooked or melted. Stop recording if the temperature does not change over a 3-minute period or after a maximum of 15 minutes.
10. Open the oven and remove the 10 cm x 10 cm aluminum foil square with the food item. Use caution as the item may be very hot.

Part C. Design Challenge

Form a working group with other students and discuss the following questions.

1. The solar oven is designed to trap heat. List the ways that heat transfers out of the oven and describe the method of heat transfer that is occurring.
2. Research the types of materials that are good insulators of heat. How can the solar oven be better insulated against heat loss?
3. Discuss other ways to improve the design of the solar oven to increase the internal temperature in a more efficient or faster way.
4. What other factors may affect the temperature in the solar oven? Which variables can be controlled and which cannot be controlled?
5. Write and carry out a plan for an improved solar oven design that will reach a higher temperature than in Part B in the same amount of time or that will reach the same temperature as the prototype in less time. Include a list of materials, a step-by-step procedure for how to build the oven, safety precautions that must be followed, and how the oven will be tested.

Student Worksheet PDF

14055_Student1.pdf