The
“Cardboard” Solar Box Cooker
A
solar oven you can build quickly from two cardboard boxes
·
Insulation material is not essential in the walls—a foiled airspace
is all that is necessary.
·
Aluminum foil can be reduced to just one layer (though a layer on the
inside of each box makes a hotter oven).
·
The airspace between the walls can be very small.
·
Almost any size oven will cook. In general, larger ovens get hotter and can
cook more food, but the limiting factor is still the ratio between the mass of
the food and the size of the oven. In general it is best to make an oven as
large as is conveniently possible so that it will perform adequately even under
marginal conditions.
·
Our experience shows that a double layer of plastic film (such as Reynolds
Oven Cooking Bags®) works at least as well as a single sheet of glass.
·
Shallower ovens cook better since they have less wall area through which to
lose heat. It’s best for the inside of the oven to be just slightly taller
than the biggest pot you plan to use.
A
New Simpler Design
Taking these factors
into account, we are able to take our best shot at describing the cardboard
solar box cooker—one that can be built by anyone with access to cardboard,
foil, glue, and plastic or glass.
What
You Will Need
·
One sheet of cardboard to make the lid. This piece must be approximately
7.5 cm larger all the way around than the top of the finished cooker.
·
One roll of aluminum foil.
·
One can of flat-black spray paint (says on can “non-toxic when dry”).
·
At least 8 ounces of white glue or wheat paste.
·
One sheet of glass or one Reynolds Oven Cooking Bag®. These are
available in almost all supermarkets in the
Building the
Base
Fold the top flaps
closed on the outer box and set the inner box on top and trace a line around it
onto the top of the outer box, Remove the inner box and cut along this line to
form a hole in the top of the outer box (Figure 1).
Decide how deep you
want your oven to be (about 2.5cm bigger than your largest pot and at least
2.5cm shorter than the outer box) and slit the corners of the inner box down to
that height. Fold each side down forming extended flaps (Figure 2). Folding is
smoother if you first draw a firm line from the end of one cut to the other
where the folds are to go.
Place some wads of crumpled
newspaper into the outer box so that when you set the inner box down inside the
hole in the outer box, the flaps on the inner box just touch the top of the
outer box (Figure 3). Glue these flaps onto the top of the outer box. Trim the
excess flap length to be even with the perimeter of the outer box. The base is
now finished.
Building
the Removable Lid
Take the large sheet of
cardboard and lay it on top of the base. Trace its outline and then cut and fold
down the edges to form a lip of about 7.5cm. Fold the corner flaps around and
glue to the side lid flaps. (Figure 4). Orient the corrugations so that they go
from left to right as you face the oven so that later the prop may be inserted
into the corrugations (Figure 6). One trick you can use to make the lid fit well
is to lay the pencil or pen against the side of the box when marking (Figure 5).
To make the reflector flap, draw a line on the lid, forming a rectangle the same size as the oven opening. Cut around three sides and fold the resulting flap up forming the reflector (Figure 6). Foil this flap on the inside.
To make a prop bend a
30-cm piece of hanger wire as indicated in Figure 6. This can then be inserted
into the corrugations as shown.
Next, turn the lid
upside-down and glue the oven bag (or other glazing material) in place. We have
had great success using the turkey size oven bag (47.5cm x 58.5cm) 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.
When using this method, it is important to also glue the bag closed on its open
end. This stops water vapor from entering the bag and condensing. Alternately
you can cut any size oven bag open to form a flat sheet large enough to cover
the oven opening.
Finally, to make the
drip pan, cut a piece of cardboard, the same size as the bottom of the interior
of the oven and apply foil to one side. Paint this foiled side black and allow
it to dry. Put this in the oven (black side up) and place your pots on it when
cooking.
Improving
Efficiency
The oven you have built
should cook fine during most of the solar season. If you would like to improve
the efficiency to be able to cook on more marginal days, you can modify your
oven in any or all of the following ways:
·
Make pieces of foiled cardboard the same size as the oven sides and place
these in the wall spaces.
·
Make a new reflector the size of the entire lid (see photo).
This document is
published on Solar Cooking Archive at http://solarcooking.org/minimum.htm