The reflector is shaped like a giant funnel, and lined with
aluminum foil. (Easy to follow instructions will be given soon.)
This funnel is rather like the parabolic cooker, except that
the sunlight is concentrated along a line (not a point) at the
bottom of the funnel. You can put your hand up the bottom of
the funnel and feel the sun's heat, but it will not burn you.
Next, we paint a jar black on the outside, to collect heat,
and place this at the bottom of the funnel. Or one can use
a black pot, with a lid. The black vessel gets hot, fast.
But not quite hot enough to cook with... We need some way to
build up the heat without letting the air cool it. So, I put
a cheap plastic bag around the jar--voila, the solar funnel
cooker was born! The plastic bag, available in grocery stores
as a "poultry bag", replaces the cumbersome and expensive box
and glass lid of the solar box ovens. You can use the plastic
bags used in American stores to put groceries in, as long as
they let a lot of sunlight pass. (Dark-colored bags will not do.)
I recently tested a bag used for fruits and vegetables, nearly
transparent and available free at American grocery stores,
that works great. This is stamped "HDPE" for high-density
polyethylene on the bag (ordinary polyethylene melts too easily).
A block of wood is placed under the jar to help hold the heat in.
(Any insulator, such as a hot pad or rope or even sticks, will
also work.)
A friend of mine who is also a Physics Professor did not believe
I could actually boil water with the thing. So I showed him
that with this new "solar funnel cooker," I was able to boil
water in Utah in the middle of winter! I laid the funnel on
its side since it was winter and pointed a large funnel towards
the sun to the south. I also had to suspend the black cooking
vessel--rather than placing it on a wooden block. This allows
the weaker sun rays to strike the entire surface of the vessel.
Of course, the Solar Funnel works much better outside of winter
days (when the UV index is 7 or greater). Most other solar
cookers will not cook in the winter in northern areas (or south
of about 35 degrees, either).
I thought that a pressure cooker would be great. But the
prices in stores were way too high for me. Wait, how about
a canning jar? These little beauties are designed to relieve
pressure through the lid--a nice pressure cooker. And cooking
time is cut in half for each 10 degrees Celsius we raise the
temperature (Professor Lee Hansen, private communication).
I used one of my wife's wide-mouth canning jars, spray-painted
(flat) black on the outside, and it worked great. Food cooks
faster when you use a simple canning jar as a pressure cooker.
However, you can also put a black pot in the plastic bag instead
if you want. But don't use a sealed container with no pressure
release like a mayonnaise jar--it can break as the steam builds
up! (I've done it.)