Solar Ovens in the Garden
Carolyn Wason, FoodCorps service member in Liberty (Walker Elementary is August’s School Garden of the Month!)
As a one-time activity (i.e. at a summer camp):
Time: 1.5 hours to make the ovens, 2(ish) hours to cook
Ages: mixed kindergarten and up (pair younger students with older ones)
- Introduction: Discuss with students how greenhouses work—Why do we use them? Why is it so hot inside? How do they trap heat? Ask students if they think it’s hot enough in the greenhouse to cook something. Propose the idea of a solar oven: a way to cook food with heat from the sun.
- The Challenge: Divide students into small groups. Present the challenge: with limited supplies, they have to design and build a solar oven that can get as hot as possible in as little time as possible. This will be tested by placing a thermometer in the oven, as well as getting to cook something in it. I showed them a few pictures of solar ovens—if you do this, make sure to present multiple styles, and discuss the merits of all of them. Otherwise you’ll end up with them all looking exactly the same.
Ideas for supplies: cardboard boxes (printer paper, pizza, etc); plastic wrap and clear plastic bags; aluminum foil (set a limit on this as it’s a non-renewable resource); tape, black paper/paint
- The Build: Give students time to plan their oven, then build it—this probably takes about an hour, depending on the group. Supplies like tape, plastic wrap, and foil should be kept by the teacher, and students must ask (this prevents fighting, as well as reducing waste). Ask questions to improve their designs—What angle is the sun at? Will it stay at that angle? What color holds heat? Why would you want to reflect sunlight? What natural materials hold heat? (rocks? sand? woodchips? leaves?) How could you incorporate more natural materials into your design? Should your oven have holes in it? How will it open and close?
- Cooking, Waiting, Wishing: When all ovens are complete (a timed challenge with definitive limits is helpful, and makes it fun for the students when they have to make a final push to finish the oven in less than a minute), prepare the food to be cooked. S’mores are easy and don’t take too long. We also like making quesadillas with garden veggies. Leafy greens tend to wilt instead of getting crispy. Place food in ovens, as well as a thermometer in each, then let students select a location for their oven (you might compare ovens inside the greenhouse versus outside in the garden). Have students check on their food every hour until they decide it’s ready. Enjoy your solar-cooked treats together!
As a classroom unit:
Time: 4 sessions, each 45 minutes to an hour
Ages: grades 2 and up (I did it with second graders)
- We did this unit following a growing experiment comparing plants grown outside, inside, and inside PLUS within homemade mini-greenhouses. Our conclusion was that the greenhouses (plastic bottles, plastic wrap, etc) helped the plants grow because they kept it warmer.
- Introduction Lesson (45 minutes): Discuss how greenhouses trap heat. Where does that heat come from? (The sun, of course!). Explain that you can trap the sun’s heat in another way, and use it to cook food: solar ovens. Present the challenge. In small groups, students will design and build a solar oven that can successfully cook [snack of your choosing]. They must complete a rigorous multi-step process, like any professional engineer. Present a few examples of solar ovens (discuss the pros and cons of each design as a class). Make it clear that their design, while it may borrow from and be inspired by the pictures you showed them, must be uniquely theirs and not a copy (would a real designer or inventor get away with taking someone else’s idea?). Ask them to think about how each of those designs could be improved. Present the supplies they have to work with. Then:
- Divide class into groups
- Have them brainstorm ideas, jotting down supplies they might need, etc
3. Begin drawing blueprints
- Design Lesson (45 minutes): In groups, students draw clear, well-labeled designs for their ovens. Have them write out (in complete sentences): a list of supplies they will need, a clear explanation of their design and why they think it will work. Students may not proceed to the next step until their blueprint has been approved by their supervisor (you!).
- The Build (1 hour, possibly more): Assign each group a workspace. Present the materials (for an added challenge, you might leave out one of their requested materials, to see if they can improvise). Make it clear that their actual oven must look exactly like their design. They cannot change the design without a written request to the supervisor.
- Concluding Lesson: Prepare your treats, and place them in the oven with a thermometer. Have groups select a location for their oven. Leave them for a few hours, then come back and check later in the day. See which oven, and which location, reached the highest temperatures. Then eat those snacks!
For high school students: This could be presented as a water-purification challenge. Students must then build an oven that can get water to 140F (at which most bacteria are killed) in the shortest amount of time.