It is generally assumed that a solar cooker should have some minimal capacity in order
to work properly. For instance, in the booklet Your Own Solar Box, Solar Cookers
International (SCI) recommends an inner box at least 45 cm X 55 cm (18" x 22").
The result is a rather large box, well suited to family use, but which can prove
unnecessarily cumbersome in some cases

Smaller cookers would be appreciated by the following:

people living or traveling alone,

people living with their family but needing a special diet,

elderly people who feel reluctant to carry a heavy box,

teenagers wishing to build and experiment their own first cooker.

If you belong to one of these categories, here is how you can build a cheap and fairly
efficient small cooker.

Cut the flaps off the box. Then cut the seams along FG and GC. Do the same on the other
side along EH and HD. The carton folds out to a flat assembly of five rectangles as shown
in figure 2 (Letters appearing twice on this figure indicate two points which were the
same point before cutting). If the cardboard is thin, reinforce the rectangle CDHG by
gluing another rectangular piece of cardboard onto it to better insulate the bottom of the
pot. Then glue aluminum foil to one side of the five rectangles (the inside of the
original box).

Now, keeping the rectangle CDHG horizontal on a table or on level ground, position the
other rectangles as shown in figure 3. The front "mirror" EFGH is tilted about
30 degrees above the horizontal plane (Put a rock or other object under it). The
"wing mirrors" BFGC and AEHD are vertical, the angles GCG and HDH being about 45
degrees. A few rocks as shown in figure 4 will be helpful, especially in windy weather.

The black cooking pot is put on the horizontal base CDHG and covered with a colorless
glass salad-bowl [or oven cooking bag, see next article, ed.] replacing the glass window
of a classical box cooker. To avoid convective heat losses, the diameter of the salad-bowl
should not exceed the width CD.

A more convenient way of keeping the reflective system in good shape is to mount the
panels on a wooden board in which you will drive a few nails on each side of GC and HD to
maintain the wing mirrors in their correct position (see figure 5).

Although this cooker uses (slightly) concentrated sunlight, it is not necessary to
worry about a constant tracking of the sun. A big vertical nail at the front of the board
can act as an "orientation indicator". Its shadow should be seen on the white
triangular piece of paper glued on the board (figure ) and whose 30 degree angle roughly
corresponded to 2 hours of absentee cooking in my experiments. Most of the following
results have been obtained without any readjustment of the cooker orientation.

All cooking was done in an aluminum pot painted black. Scrounged glass jars may be
used, even without the salad-bowl, but cooking times are increased.

The latitude of Paris is about 45 degrees. When cooking at lower latitudes the vertical
reflectors become less effective; it remains to be tested whether this design will work as
well there. On the other hand, people living at latitudes above 40 degrees could find it
more efficient than a simple, one-reflector box cooker. I'm interested in knowing the
results obtained by readers who live in other parts of the world.

Barbara Kerr Tests The Solar Panel Cooker

I am really excited by the opportunities opened up by Roger Bernard's panel cooker
design. We have known that multiple reflectors can be used to concentrate solar radiation,
but until I watched my lentil stew bubbling under the glass salad bowl, I did not see this
as a serious cooker. Suddenly, I realized that an oven cooking bag could be used in place
of the salad bowl. This would provide a very abbreviated solar "box." All we
have learned over the years with box-type cookers could be helpful in utilizing Solar
Panel Cookers (SPCs). Now we may have both our "oven" and our "hot
plate."

By limiting ourselves to flat foil-covered panels, the danger of eye damage is greatly
reduced but remains a problem. Retinal damage, which can occur when sunlight shines into
the eyes, is not painful. You cannot tell it is happening, but a retinal burn produces
permanent damage and can result in blindness. Be extremely careful if using anything that
concentrates the light or reflects the sunlight directly into your eyes.

The first cooker I made based on Roger Bernard's specifications would not fold neatly.
The illustrations show a slight modification of Roger's design. Since the panels are all
the same size, they fold to form a flat packet which is so small and light-weight that it
can be used by backpackers and others who do not have space for much storage. The extra
cuts and folds also provide areas where rocks can be placed to anchor the panels without
blocking any sunshine. Our winds are fierce and unpredictable! I have found that a
cardboard SPC in this configuration tolerates the wind very well.

Place the cooking pot in an oven cooking bag with the opening at the top so you can
open the bag, check the food and seal it again without disturbing the cooking. And that
part of the bag is usually dry. This is important because food in a panel cooker usually
does need to be stirred and checked, since the heat is not as even.

First I closed the baking bags with clothes pins--too heavy and bulky. Then with paper
clips tore up the baking bag which otherwise was good for many uses. Then with a thin
piece of wire worked well but the twisted wire broke after several uses. Now I am using
wire but simply wrapping it tightly around the baking bag top, without twisting. Since
there is no pressure, it works fine and the twist ties last a long time.

Too often I have found jars or pots have vigorously boiled over, spilling juice and
making a mess. Got to remember to allow more space at the top to contain a boil, at least
until I get a handle on when this is going to happen. A delightful problem. Food does not
have the delicately enhanced flavor of SBC cooked food, probably because of the higher
temperatures.

Then I noticed a cold spot on the foiled reflector directly under a pot and remembered
the advantage the University of Washington engineers found through elevating the SBC tray
off the cardboard bottom. I looked around the kitchen for an "elevator" and
seized on canning rings. It is clear that pots heat faster when sitting on a canning ring
than when sitting on the foiled cardboard. Darkened canning rings work better than shiny,
of course. But the center is dark and I wondered if it would help to get light to shine
under the pot. I put three little pebbles, dark and oiled, under a pot. That seems to work
even better. I like that . . . three little pebbles used in memory of the historic three
stone fire that has served humanity for thousands of years. Women, nostalgic for the wood
fire where there is no more wood, might even take tiny pieces of wood and form a little
"fire" within the pebbles under the pot. It would keep us from feeling so torn
away from roots.

Solar cooking continues to get simpler. I have put major attention on simple solar
cooker designs for 20 years, working to have them easier and more accessible to everyone.
Today, I held a Solar Panel Cooker and realized our 20 year mountain of work had truly
brought forth a mouse. A mighty mouse! Simplicity is so difficult...difficult to see, not
difficult to do, once the idea forms. I think that box-style cookers will remain part of
the solar kitchen where time, material and circumstances dictate, but the SPC has opened
up a new level of simplicity.

We are doing pretty well with reducing the materials needed to solar cook. If only we
could eliminate the need for oven cooking bags of heat-resistant nylon. I used regular
kitchen plastic bags and they seemed to hold up under the heat for several cooking times.
But January 12 was brighter, the air was warmer, and three different kinds of regular
plastic bags melted. I guess we will have to stick with baking bags . . . too bad. But the
baking bags I started with two months ago are still in good shape, having cooked many
dishes and been washed and dried many times. Perhaps they can be obtained wholesale and
distributed one or two at a time where they are not available in stores. We should be able
to get them wholesale if someone puts a little effort into it.

It seems that "open-box" cookers will now be an integral part of serious
solar cooking. It will just take finding out specifically how to use them.

Please send your findings and comments to me as well as to Roger Bernard. We'll keep
everyone else up-to-date through Solar Box Journal.