The following developments make this design possible:
- 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 minimum solar box cooker -- one that can
be built by anyone with access to cardboard, foil, glue, and plastic
or glass.
What You Will Need
- Two cardboard boxes. We would suggest that you use an inner
box that is at least 15" x 15" (38cm x 38cm), but bigger
is better. The outer box should be larger all around, but it doesn't
matter how much bigger, as long as there is a half inch (1.5cm)
or more of an airspace between the two boxes. Also note that the
distance between the two boxes does not have to be equal all the
way around. Also, keep in mind that it is very easy to adjust
the size of a cardboard box by cutting and gluing it.
- One sheet of cardboard to make the lid. This piece must be approximately
2" - 3" (4 - 8cm) larger all the way around than the
top of the finished cooker.
- One small roll of aluminum foil.
- One can of flat-black spray paint (says on can "non-toxic
when dry") or one small jar of black tempera paint. Some
people have reported making their own paint out of soot mixed
with wheat paste.
- At least 8 ounces of white glue or wheat paste.
- One Reynolds Oven Cooking Bag®. These are available in almost
all supermarkets in the U.S. and they can be mail-ordered from
Solar Cookers International. They are rated for 400° F (204.4°
C) so they are perfect for solar cooking. They are not UV-resistant;
thus they will become more brittle and opaque over time and may
need to be replaced periodically. A sheet of glass can also be
used, but this is more expensive and fragile, and doesn't offer
that much better cooking except on windy days.
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 1" or 2.5cm
bigger than your largest pot and at least 1" shorter than
the outer box) and slit the corners of the inner box with a knife
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.
Glue aluminum foil to the inside of both boxes and also to the
inside of the remaining top flaps of the outer box. Don't waste
your time being neat on the outer box, since it will never be seen,
nor will it experience any wear. The inner box will be visible even
after assembly, so if it matters to you, you might want to take
more time here. Glue the top flaps closed on the outer box.
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.
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. 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 3" (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). Don't glue
this lid to the box; you'll need to remove it to move pots in and
out of the oven.
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 12" (30cm) 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 (19" x 23 1/2", 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.
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).
- Make the drip pan using sheet metal, such as aluminum flashing.
Paint this black and elevate this off the bottom of the oven slightly
with small cardboard strips.
Experiments in Seattle and Arizona have proven
that solar box cookers can be built more simply than even
the simple method we have been using. These discoveries have
paved the way for a simpler construction method that allows
a cooker to be built in a few hours for very little money.