Cook outside with the sun on a below-freezing day

Submitted by Shawn Reeves

2007-02-03 13:58:20

February 3, 2007, Newton, MA.
This morning, I grabbed some vegetables from my freezer, poured them into a pot, put that pot in a roasting bag, and put that outside in a simple reflector. An hour later, I checked the oven thermometer I put on top of the food, inside the pot, and it read 170 degrees Fahrenheit. Not only were the vegetables thawed, not only were they hot, they were cooked tender as if they had been steamed for a long time.
Allow me to describe the apparatus and a simple recipe, then I’ll share notes about using cookers in the classroom.

1 Pot

1 qt, dark anodized aluminum, glass lid, silicone coated handles, non-stick interior. Aluminum conducts heat well from the black exterior to the food.

1 Stand

A stand for a fondue pot raises the pot above the bottom of the reflector, allowing more sun to hit the pot.

1 Roasting Bag

A clear plastic bag that's clearly made to be used around hot food. Buy a box of three at a supermarket for about $2.

1 Reflector

The easiest design to build is simply three sides of a large box, with aluminum pasted on the inside of those three sides. I use bulldog clips to hold the reflector together. I protect the edges with clear package-tape. See photos.

And today's recipe:

image of two solar cookers in the snow on a sunny day midwinter.

The cooker on the left uses an aluminum can painted black, inside a sealed glass jar, without a stand. It works quickly because it allows no heat to be lost through evaporation. Note the long shadows and the snow on the ground. The air was below freezing; winds gusted up to 40 kph, so I had to put weights in the bases.

image of solar cooker partly folded.

Here you see the three flaps, from a corner of a large box, covered with aluminum foil. An overlapping, fourth flap is left connected to help hold the reflector together, and to make a portfolio-shape when the reflector is folded in half for carrying.

image of solar cooker fully assembled.

Here you see the reflector fully assembled. See how the fourth flap is clipped to its mate, holding the box together temporarily. I usually put the side with the clip on the bottom, since it weighs more. Notice that the aluminum is not perfectly smooth; this is not a problem at all. I used acrylic glue to paste the aluminum to the box, then packing tape to protect the edges.

image of solar cooker folded for carrying.

Easy to carry means that this cooker will be used more often.

Notes for lesson plans

Don’t make this a cookbook lab; allow your students to come up with their own designs and their own questions for research. Although this design is quite practical, there are better designs for specific purposes, such as cooking eggplant or baking.
This exercise can be a lesson about thermodynamics; electromagnetic radiation/light; cooking; technology; hunger; deforestation; energy resources; insolation; aluminum; pasteurization; or materials science. I don’t think this cooker should be presented simplistically as a tool to save the world; nor should it be used to try to teach all those concepts at once. I perceive in my students that once they understand some of the above concepts, they will arrive at useful conclusions about this and other technologies without me preaching to them.
The box should be much larger than the pot. The pot should not be much larger than the food in it. You and your students should examine the geometry and physics informing these points.
If readers show interest, I will publish notes on the thermodynamics involved in a future article.

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