How to Make and Use the Solar Funnel Cooker

Posted by starstuff on Monday August 11, 2008 3:16pm,UTC
Thanks to

The Solar Funnel Cooker

How to Make and Use The BYU Solar Cooker/Cooler

by Steven E. Jones,Professor of Physics at Brigham Young University (BYU), with
Colter Paulson, Jason Chesley, Jacob Fugal, Derek Hullinger, Jamie Winterton, Jeannette Lawler, and Seth, David, Nathan, and Danelle Jones.


A few years ago, I woke up to the fact that half of the world's peoples must
burn wood or dried dung in order to cook their food. It came as quite a shock to
me, especially as I learned of the illnesses caused by breathing smoke day in
and day out, and the environmental impacts of deforestation -not to mention the
time spent by people (mostly women) gathering sticks and dung to cook their
food. And yet, many of these billions of people live near the equator, where
sunshine is abundant and free. Ergo...

As a University Professor of Physics with a background in energy usage, I set
out to develop a means of cooking food and sterilizing water using the free
energy of the sun. First, I looked at existing methods.

The parabolic cooker involves a reflective dish that concentrates sunlight to
a point where the food is cooked. This approach is very dangerous since the
sun's energy is focused to a point which is very hot, but which cannot be seen.
(BYU students and I built one which will set paper on fire in about 3 seconds!)
I learned that an altruistic group had offered reflecting parabolas to the
people living at the Altiplano in Bolivia. But more than once the parabolas had
been stored next to a shed -- and the passing sun set the sheds on fire! The
people did not want these dangerous, expensive devices, even though the
Altiplano region has been stripped of fuel wood.

The box cooker: Basically an insulated box with a glass or plastic lid, often
with a reflecting lid to reflect sunlight into the box. Light enters through the top glass (or plastic), to slowly heat up the box. Problems: energy
enters only through the top, while heat is escaping through all the other sides, which have a tendency to draw heat away from the food. When the box
is opened to put food in or take it out, some of the heat escapes and is lost.
Also, effective box cookers tend to be more complicated to build than the funnel

While studying this problem, I thought again and again of the great need for
a safe, inexpensive yet effective solar cooker. It finally
came to me at Christmastime a few years ago, a sort of hybrid between the
parabola and a box cooker. It looks like a large, deep funnel, and incorporates
what I believe are the best features of the parabolic cooker and the box cooker.

The first reflector was made at my home out of aluminum foil glued onto
cardboard, then this was curved to form a reflective funnel. My children and I
figured out a way to make a large card-board funnel easily. (I'll tell you
exactly how to do this later on.)

The Solar Funnel Cooker is safe and low cost, easy to make, yet very
effective in capturing the sun's energy for cooking and pasteurizing water ->

Later, I did extensive tests with students (including reflectivity tests) and
found that aluminized Mylar was good too, but relatively expensive and rather
hard to come by in large sheets. Besides, cardboard is found throughout the
world and is inexpensive, and aluminum foil is also easy to come by. And
individuals can make their own solar cookers easily, or start a cottage-industry
to manufacture them for others.

Prototypes of the Solar Funnel Cooker were tested in Bolivia, and
outperformed an expensive solar box cooker and a "Solar Cookit" -
while costing much less. Brigham Young University submitted a patent
application, mainly to insure that no company would prevent wide distribution of
the Solar Funnel Cooker. BYU makes no profit from the invention. ( I later
learned that a few people had had a similar idea, but with methods differing
from those developed and shown here.) So now I'm trying to get the word out so
that the invention can be used to capture the free energy of the sun - for
camping and for emergencies, yes, but also for every day cooking where
electricity is not available and even fuel wood is getting scarce.

How it Works

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 10º C 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.)

How to Build Your Own Solar Funnel Cooker

What You will Need for the Funnel Cooker:

  1. A piece of flat cardboard, about 2 feet wide by 4 feet long. (The
    length should be just twice the width. The bigger, the better.)

  2. Ordinary aluminum foil.

  3. A glue such as white glue (like Elmer's glue), and water to mix with it 50-50.
    Also, a brush to apply the glue to the cardboard (or a cloth or paper towel will
    do). Or, some may wish to use a cheap "spray adhesive" available in
    spray cans. You can also use wheat paste.

  4. Three wire brads-- or small nuts and bolts, or string to hold the funnel together.

  5. For a cooking vessel, I recommend a canning jar ("Ball" wide-mouth
    quart jars work fine for me; the rubber ring on the lid is less likely to melt
    than for other jars I've found. A two-quart canning jar is available and works
    fine for larger quantities of food, although the cooking is somewhat

  6. The cooking jar (or vessel) should be spray-painted black on the outside. I find
    that a cheap flat-black spray paint works just fine. Scrape off a vertical
    stripe so that you have a clear glass "window" to look into the
    vessel, to check the food or water for boiling.

  7. A block of wood is used as an insulator under the jar. I use a piece of
    2"X4" board which is cut into a square nominally 4" x 4" by
    about 2" thick. (10cm square x 5cm thick.) One square piece of wood makes a
    great insulator.

  8. A plastic bag is used to go around the cooking-jar and block of wood, to provide
    a green-house effect. Suggestions:

  • Reynolds™ Oven Bag, Regular Size works great: transparent and won't
    melt. (Cost about 25 cents each in U.S. grocery stores.)

  • Any nearly-transparent HDPE bag (High-density Polyethylene). Look for "HDPE"
    stamped on the bag. I've tested HDPE bags which I picked up for free at my
    grocery store, used for holding vegetables and fruits. These are thin, but very
    inexpensive. Tested side-by-side with an oven bag in two solar funnels, the HDPE
    bag worked just as well! (Caution: we have found that some HDPE bags will melt
    should they contact the hot cooking vessel. For this reason, we recommend using
    the oven-safe plastic bag wherever possible.)

  • An idea attributed to Roger Bernard and applied now to the BYU Funnel Cooker:
    place a pot (having a blackened bottom and sides) in a glass bowl, and cover
    with a lid. Try for a tight fit around the bottom to keep hot air trapped
    inside. The metal pot or bowl should be supported around the rim only, with an
    air space all around the bottom (where the sunlight strikes it). Put a blackened
    lid on top of the pot. Then simply place this pot-in-bowl down in the bottom of
    the funnel - no plastic bag is needed! This clever method also allows the cook
    to simply remove the lid to check the food and to stir. I like this idea - it
    makes the solar cooker a lot like cooking over a fire. See Photographs for
    further details.

Construction Steps

a Half-circle out of the Cardboard

a half circle out of the cardboard, along the bottom as shown below. When the
funnel is formed, this becomes a full-circle and should be wide enough to go
around your cooking pot. So for a 7" diameter cooking pot, the
of the half-circle is 7". For a quart canning jar such as I use,
I cut a 5" radius half-circle out of the cardboard.

the Funnel

form the funnel, you will bring side A towards side B, as shown in the figure.
The aluminum foil must go on the INSIDE of the funnel. Do this slowly, helping
the cardboard to the shape of a funnel by using one hand to form creases that
radiate out from the half-circle. Work your way around the funnel, bending it in
stages to form the funnel shape, until the two sides overlap and the half-circle
forms a complete circle. The aluminum foil will go on the INSIDE of funnel. Open
the funnel and lay it flat, "inside up", in preparation for the next

Foil to Cardboard

Apply glue or adhesive to the top (inner) surface of the cardboard, then
quickly apply the aluminum foil on top of the glue, to affix the foil to the
cardboard. Make sure the shiniest side of the foil is on top, since this becomes
your reflective surface in the Funnel. I like to put just enough glue
for one width of foil, so that the glue stays moist while the foil is applied. I
also overlap strips of foil by about 1" ( or 2 cm). Try
to smooth out the aluminum foil as much as you reasonably can, but small wrinkles
won't make much difference. (If even cardboard is not available, one can simply
dig a funnel-shaped hole in the ground and line it with a reflector, to make a
fixed solar cooker for use at mid-day.)

Join side A to side B to keep the funnel together. 

The easiest way to do
this is to punch three holes in the cardboard that line up on side A and side B
(see figure). Then put a metal brad through each hole and fasten by pulling
apart the metal tines. Or you can use a nut-and-bolt to secure the two sides (A
& B) together.

creative here with what you have available. For example, by putting two holes
about a thumb-width apart, you can put a string, twine, small rope, wire or
twist-tie in one hole and out the other, and tie together.

A and B are connected together, you will have a "funnel with two
wings". The wings could be cut off, but these help to gather more sunlight,
so I leave them on.

Tape or glue a piece of aluminum foil across the hole at the bottom of the
funnel, with shiny side in. 

This completes assembly of your solar funnel cooker.

stability, place the Funnel inside a cardboard or other box to provide support.
For long-term applications, one may wish to dig a hole in the ground to hold the
Funnel against strong winds.


this stage, you are ready to put food items or water into the cooking vessel or
jar, and put the lid on securely. (See instructions on food cooking times, to

a wooden block in the INSIDE bottom of the cooking bag. I use a piece of 2X4
board which is cut into a square nominally 4"X4" by about 2"
thick. Then place the cooking vessel containing the food or water on top of the
wooden block, inside the bag.

gather the top of the bag in your fingers and blow air into the bag, to
inflate it
. This will form a small "greenhouse" around the cooking
vessel, to trap much of the heat inside. Close off the bag with a tight twist
tie or wire. Important: the bag should not touch the sides or lid of the cooking
vessel. The bag may be called a "convection shield," slowing
convection-cooling due to air currents.

the entire bag and its contents inside the funnel near the bottom as shown in
the Photographs.

the Solar Funnel Cooker so that it Faces the Sun

Sunlight can hurt the eyes: Please wear sunglasses when using a Solar Cooker!
The Funnel Cooker is designed so that the hot region is deep down inside the
funnel, out of harm's way.

the Solar Funnel Cooker in the sun pointing towards the sun, so that it captures
as much sunlight as possible. The design of the funnel allows it to collect
solar energy for about an hour without needing to be re-positioned. For longer
cooking times, readjust the position of the funnel to follow the sun's path.

helps to put the Solar Funnel Cooker in front of a south-facing wall or window
(in the Northern Hemisphere) to reflect additional sunlight into the funnel. A
reflective wall is most important in locations farther from the equator and in
winter. In the Southern Hemisphere, put the Solar Funnel Cooker in front of a
North-facing wall or window to reflect additional sunlight into your cooker.


that the cooking vessel will be very hot: Use cooking pads or gloves when
handling! If you are heating water in a canning jar, you may notice that the
water is boiling when the lid is first removed - it gets very hot!

the plastic cooking bag by removing the twist-tie. Using gloves or a thick
cloth, lift the vessel
out of the bag and place it on the ground or table. Carefully open the vessel
and check the food, to make sure it has finished cooking. Let the hot food cool
before eating.


  1. Avoid leaving fingerprints and smudges on the inside surface of the cooker. Keep
    the inner surface clean and shiny by wiping occasionally with a wet towel. This
    will keep the Solar Funnel Cooker working at its best.

  2. If your funnel gets out-of-round, it can be put back into a circular shape by
    attaching a rope or string between opposite sides which need to be brought
    closer together.

  3. For long-term applications, a hole in the ground will hold the Funnel Cooker
    securely against winds. Bring the funnel inside or cover it during rain storms.

  4. The lids can be used over and over. We have had some trouble with the rubber on
    some new canning-jar lids becoming soft and "sticky." "Ball
    canning lids" do not usually have this problem. Running new lids through
    very hot water before the first use seems to help. The lids can be used over and
    over if they are not bent too badly when opened (pry off lid carefully).

  5. The jar can be suspended near the bottom of the funnel using fishing line or
    string (etc.), instead of placing the jar on a block of wood. A plastic bag is
    placed around the jar with air puffed inside, as usual, to trap the heat. The
    suspension method allows sunlight to strike all surfaces of the jar, all around,
    so that heats faster and more evenly. This suspension method is crucial for use
    in winter months.

  6. Adjust the funnel to put as much sunlight onto the cooking jar as
    possible. Look at the jar to check where the sunlight is hitting, and to be
    sure the bottom is not in the shadows. For long cooking times (over about an
    hour), readjust the position of the funnel to follow the sun's path. During
    winter months, when the sun is low on the horizon (e.g., in North America),
    it is helpful to lay the funnel on its side, facing the sun.

Tests in Utah

have personally used the Solar Funnel Cooker to cook lunches over many weeks. My
favorite foods to cook are potatoes (cut into logs or slices) and carrot slices.
Vegetables cook slowly in their own juices and taste delicious. I also make
rice, melted cheese sandwiches, and even bread in the Solar Funnel Cooker. I
usually put the food out around 11:30 and let it cook until 12:45 or 1 pm, just
to be sure that it has time to cook. I've never had any food burn in this

have also cooked food in the mountains, at an altitude of around 8,300 feet. If
anything, the food cooked faster there - the sunlight filters through less
atmosphere at high altitudes.

find that people are surprised that the sun alone can actually cook food. And
they are further pleasantly surprised at the rich flavors in the foods which
cook slowly in the sun. This inexpensive device does it!

at Brigham Young University have performed numerous tests on the Solar Funnel
Cooker along with other cookers. We have consistently found much faster cooking
using the Solar Funnel Cooker. The efficiency/cost ratio is higher than any
other solar cooking device we have found to date. Mr. Hullinger also performed
studies of transmissivity, reflectivity and absorptivity of alternate materials
which could be used in the Solar Funnel Cooker. While there are better materials
(such as solar-selective absorbers), our goal has been to keep the cost of the
Solar Cooker as low as possible, while maintaining safety as a first priority.

Tests in Bolivia

BYU Benson Institute organized tests between the Solar Funnel Cooker and the
"old-fashioned" solar box oven. The solar box oven cost about $70 and
was made mostly of cardboard. It took nearly two hours just to reach water
pasteurization temperature. The Bolivian report notes that "food gets cold
every time the pots are taken from and into the oven." The solar box oven
failed even to cook boiled eggs. (More expensive box cookers would hopefully
work better.)

The Funnel Cooker can also be made from Mylar.
aluminized-mylar Solar Funnel Cooker was also tested in Bolivia, during the Bolivian
winter. Water pasteurization temperature was reached in 50 minutes, boiled eggs
cooked in 70 minutes, and rice cooked in 75 minutes. The Bolivian people were
pleased by the performance. So were we! (La Paz, Bolivia, August, 1996)

I also donated two dozen solar funnel cookers for people in Guatemala. These
were taken there by a group of doctors going there for humanitarian service. The
people there also liked the idea of cooking with the sun's free energy! For an
aluminized-Mylar Solar Funnel Cooker kit, please contact CRM (licensed
manufacturer) at +1 (801) 292-9210.

Water and Milk

drinking water or milk kills thousands of people each day, especially children.
WHO reports that 80% of illnesses in the world are spread through contaminated
water. Studies show that heating water to about 65º - 70º C (150º F) is sufficient
to kill coliform bacteria, rotaviruses, enteroviruses and even Giardia. This is
called pasteurization.

depends on how hot and how long water is heated. But how do you know if the
water got hot enough? You could use a thermometer, but this would add to the
cost, of course. When steam leaves the canning jar (with lid on tight) and forms
"dew" on the inside of the cooking bag, then the water is probably
pasteurized to drink. (The goal is to heat to 160º Fahrenheit for at
least six minutes.) With a stripe of black paint scraped off the jar, one can
look through the bag and into the jar and see when the water is boiling - then
it is safe for sure.

of all the lives that can be saved simply by pasteurizing water using a simple
Solar Cooker! (See also Recent Advances in Solar Water


was my first concern in designing the Solar Funnel Cooker, then came low cost
and effectiveness. But any time you have heat you need to take some precautions.

  • The cooking vessel (jar) is going to get hot, else the food inside won't cook.
    Let the jar cool a bit before opening. Handle only with gloves or tongs.

  • Always wear dark glasses to protect from the sun's rays. We naturally squint,
    but sunglasses are important.

  • Keep the plastic bag away from children and away from nose and mouth to avoid
    any possibility of suffocation.

Cooking with the Solar Funnel

do you cook in a crock pot or moderate-temperature oven? The same foods will
cook about the same in the Solar Funnel Cooker -- without burning. The charts
below give approximate summer cooking times.

solar cooker works best when the UV index is 7 or higher. (Sun high overhead,
few clouds.)

times are approximate. Increase cooking times for partly-cloudy days, sun not
overhead (e.g., wintertime) or for more than about 3 cups of food in the cooking

is not necessary for most foods. Food generally will not burn in the solar

carrots, squash, beets, asparagus, etc.)

Preparation: No
need to add water if fresh. Cut into slices or "logs" to ensure
uniform cooking. Corn will cook fine with or without the cob.

1.5 hours

and Grains
wheat, barley, oats, millet, etc.)

Preparation: Mix
2 parts water to every 1 part grain. Amount may vary according to individual
taste. Let soak for a few hours for faster cooking. To ensure uniform cooking,
shake jar after 50 minutes. CAUTION: Jar will be hot. Use gloves or
cooking pads.


and Dehydrated Soups

heat water to near boiling (50-70 minutes). Then add the pasta or soup mix. Stir
or shake, and cook 15 additional minutes.



tough or dry beans soak overnight. Place in cooking jar with water.



need to add water. Note: If cooked too long, egg whites may darken, but
taste remains the same.

hours, depending on desired yolk firmness.

beef, and fish)

Preparation: No
need to add water. Longer cooking makes the meat more tender.

Chicken: 1.5
hours cut up or 2.5
hours whole; Beef: 1.5
hours cut up or 2.5-3
hours for larger cuts; Fish: 1-1.5 hours


vary based on amount of dough.

1-1.5 hours; Biscuits:
1-1.5 hours; Cookies: 1 hour

almonds, pumpkin seed, etc.)

Preparation: Place
in jar. A little vegetable oil may be added if desired.

1.5 hours

and prepackaged foods

foods in dark containers, simply place the container in the cooking bag in place
of the black cooking jar.

time varies with the amount of food and darkness of package.

How to Use the Solar Funnel as a

university student (Jamie Winterton) and I were the first to demonstrate that
the BYU Solar Funnel Cooker can be used - at night - as a refrigerator. Here is
how this is done.

Solar Funnel Cooker is set-up just as you would during sun-light hours, with two

The funnel is directed at the dark night sky. It should not "see" any
buildings or even trees. (The thermal radiation from walls, trees, or even
clouds will diminish the cooling effect.).

It helps to place 2 (two) bags around the jar instead of just one, with air
spaces between the bags and between the inner bag and the jar. HDPE and ordinary
polyethylene bags work well, since polyethylene is nearly transparent to
infrared radiation, allowing it to escape into the "heat sink" of the
dark sky.

the day, the sun's rays are reflected onto the cooking vessel which becomes hot
quickly. At night, heat from the vessel is radiated outward, towards
empty space, which is very cold indeed (a "heat sink").

a result, the cooking vessel now becomes a small refrigerator. We routinely
achieve cooling of about 20º F (10º C) below ambient air
temperature using this remarkably simple scheme.

September 1999, we placed two funnels out in the evening, with double-bagged
jars inside. One jar was on a block of wood and the other was suspended in the
funnel using fishing line. The temperature that evening (in Provo, Utah) was 78º F. Using a Radio Shack indoor/outdoor thermometer, a BYU student (Colter
Paulson) measured the temperature inside the funnel and outside in the open air.
He found that the temperature of the air inside the funnel dropped quickly by
about 15 degrees, as its heat was radiated upwards in the clear sky. That night,
the minimum outdoor air temperature measured was 47.5 degrees - but the water in
both jars had ICE. I invite others to try this, and please let me know if you
get ice at 55 or even 60 degrees outside air temperature (minimum at night). A
black PVC container may work even better than a black-painted jar, since PVC is
a good infrared radiator - these matters are still being studied.

would like to see the "Funnel Refrigerator" tried in desert climates,
especially where freezing temperatures are rarely reached. It should be possible
in this way to cheaply make ice for Hutus in Rwanda and for aborigines in
Australia, without using any electricity or other modern "tricks." We
are in effect bringing some of the cold of space to a little corner on earth.
Please let me know how this works for you.

Conclusion: Why We Need Solar

BYU Funnel Cooker/Cooler can:

  • Cook
    food without the need for electricity or wood or petroleum or other fuels.

  • Pasteurize
    water for safe drinking, preventing many diseases.

  • Save
    trees and other resources.

  • Avoid
    air pollution and breathing smoke while cooking.

  • Use
    the sun's free energy. A renewable energy source.

  • Cook food with little or no stirring, without burning.

  • Kill
    insects in grains.

  • Dehydrate
    fruits, etc.

  • Serve
    as a refrigerator at night, to cool even freeze water.

that without electricity or fuels! See also Balancing the

burden for gathering the fuel wood and cooking falls mainly on women and
children. Joseph Kiai reports from Dadaab, Kenya: "Women who can't afford
to buy wood start at 4 am to go collecting and return about noon... They do this
twice a week to get fuel for cooking... The rapes are averaging one per
week." From Belize: "Many times the women have to go into the forest
dragging their small children when they go to look for wood. It is a special
hardship for pregnant and nursing mothers to chop and drag trees back to the
village... they are exposed to venomous snakes and clouds of mosquitoes."
(Anna K.) (Quoted in newsletters by
Cookers International

the forests are dwindling in many areas. Edwin Dobbs noted in Audubon Magazine, Nov. 1992, "The world can choose sunlight or further
deforestation, solar cooking or widespread starvation..."

should be prepared for emergencies, incident to power failures. A Mormon pioneer
noted in her journal: "We were now following in their trail traveling up
the Platte River. Timber was sometimes very scarce and hard to get. We managed
to do our cooking with what little we could gather up..." (Eliza R. Snow)
Now there's someone who needed a light-weight Solar Cooker!

another reason to use a solar cooker. Many people in developing countries look
to see what's being done in America. I'm told that if Americans are using
something, then they will want to try it, too. The more people there are cooking
with the sun, the more others will want to join in. A good way to spread this
technology is to encourage small local industries or families to make these
simple yet reliable solar cookers for others at low cost. I've used this cooker
for three summers and I enjoy it.
Cooking and making ice with the funnel cooker/cooler will permit a significant
change in lifestyle. If you think about it, this could help a lot of people. The
BYU Solar Funnel Cooker uses the glorious sunshine -- and the energy of the sun
is a free gift from God for all to use!

Answers to commonly-asked questions

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 - 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.

See also: Tests of the Solar Funnel and Bowl
Cookers in 2001

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