The BYU Solar Cooker/Cooler | Page 9

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.

XIII. Recipe for wheatpaste
Prepare 1 cup (2.4 dl) of very hot water. Make a thin mixture of 3
tablespoons (45 ml) of white flour and cold water. Pour the cold
mixture slowly into the hot water while stirring constantly. Bring to a
boil. When it thickens, allow to cool. Smear on like any other glue. For
slightly better strength, add 1 tablespoon (15 ml) of sugar after the glue
is thickened. After using a portion, reheat the remaining in a covered

jar or container to sterilize it for storage or keep refrigerated. If wheat
flour is not available, other flours will work.

XIV. Updates [compiled by transcribists]
Rabbit-wire base
In later experiments Steven E. Jones and Christopher McMillan of
Brigham Young University tested different bases for the cooking pot
used in the Solar Funnel, published on the web in the article "Tests of
the Solar Funnel and Bowl Cookers in 2001. According to Jones, the
rabbit-wire stand they tested has proven to be much better than the
wooden block detailed in this article.
"We found immediately that raising the vessel off the bottom of the
cooker using a rabbit-wire stand provided more rapid and even heating
than the wooden block used previously. Placing the jar or pot on a wire
stand allows as much reflected light onto the cooking vessel as possible.
This allows even the bottom of the cooking container to absorb thermal
energy that is reflected off the lower portion of the funnel."
[Image: 11.jpg -- Photo Description: This photo shows two different
sized cylinders, made of rabbit wire -- fencing material for rabbits,
made of crossed wires tacked together, so to make a single sheet. A
black pot is being lowered into