Will the cooker work in winter (in the United States)?
As the sun moves closer to the southern horizon in the winter,
the solar cooker is naturally less effective. A good measure of
the solar intensity is the "UV index" which is often reported
with the weather. When the ultraviolet or UV index is 7 or
above--common in summer months--the solar cooker works very well.
In Salt Lake City in October, the UV index was reported to be
3.5 on a sunny day. We were able to boil water in the Solar
Funnel Cooker during this time, but we had to suspend the
black jar in the funnel so that sunlight struck all sides.
(We ran a fishing line under the screw-on lid, and looped the
fishing line over a rod above the funnel. As usual, a plastic
bag was placed around the jar, and this was closed at the top
to let the fishing line out for suspending the jar.)
The solar "minimum" for the northern hemisphere occurs on winter
solstice, about December 21st each year. The solar "maximum"
occurs six months later, June 21st. Solar cooking works best
from about March 20 to October 1 in the north. If people try to
cook with the sun for the first time outside of this time window,
they should not be discouraged. Try again when the sun is more
directly overhead. (One may also suspend the jar in the funnel,
which will make cooking faster any time of the year.)
It is interesting to note that most developing countries
are located near the equator where the sun is nearly directly
overhead all the time. Solar Cookers will then serve year-round,
as long as the sun is shining, for these fortunate people.
They may be the first to apply fusion energy (of the sun) on a
large scale! And they may accomplish this without the expensive
infrastructure of electrical power grids that we take for granted
in America.
How do you cook bread in a jar?
I have cooked bread by simply putting dough in the bottom of the
jar and placing it in the funnel in the usual way. Rising and
baking took place inside the jar in about an hour (during summer).
One should put vegetable oil inside the jar before cooking to
make removal of the bread easier. I would also suggest that
using a 2-quart wide-mouth canning jar instead of a 1-quart jar
would make baking a loaf of bread easier.
What is the optimum "opening angle" for the funnel cooker?
A graduate student at Brigham Young University did a calculus
calculation over two years ago to assess the best shape or
opening angle for the Solar Funnel. Jeannette Lawler assumed
that the best operation would occur when the sun's rays bounced
no more than once before hitting the cooking jar, while keeping
the opening angle as large as possible to admit more sunlight.
(Some sunlight is lost each time the light reflects from the
shiny surface. If the sunlight misses on the first bounce,
it can bounce again and again until being absorbed by the black
bottle.) She set up an approximate equation for this situation,
took the calculus- derivative with respect to the opening angle
and set the derivative equal to zero. Optimizing in this way,
she found that the optimum opening angle is about 45 degrees,
when the funnel is pointed directly towards the sun.
But we don't want to have to "track the sun" by turning the funnel
every few minutes. The sun moves (apparently) 360 degrees in
24 hours, or about 15 degrees per hour. So we finally chose
a 60-degree opening angle so that the cooker is effective for
about 1.2 hours. This turned out to be long enough to cook most
vegetables, breads, boil water, etc. with the Solar Funnel Cooker.
We also used a laser pointer to simulate sun rays entering the
funnel at different angles, and found that the 60-degree cone
was quite effective in concentrating the rays at the bottom of
the funnel where the cooking jar sits.