The Solar Funnel Oven
How to Make and Use The BYU Solar Oven/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
What You will Need to Make Your Solar Oven:
- 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.)
- Ordinary aluminum foil.
- 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.
- Three wire brads-- or small nuts and bolts, or string to hold the funnel together.
- 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 slower.).
- 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.
- 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.
- 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.
Cut a Half-circle out of the Cardboard
Cut 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 radius 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.
Form the Funnel
To 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 step.
Glue 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 and B) together.
Be 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.
When 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
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 oven.
For 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.
At 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
Place 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
Next, 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.
Place the entire bag and its contents inside the funnel near the bottom as shown
in the Photographs.
Place the Solar Funnel Cooker so that it Faces the Sun
Remember: 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.
Put 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.
It 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.
Remember 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!
Open 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.
- 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.
- 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.
- 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.
- 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).
- 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.
- 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.
Water and Milk Pasteurization
Contaminated 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
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.
Think of all the lives that can be saved simply by pasteurizing water using a simple
Safety 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.
How to Use the Solar Funnel as a Refrigerator/Cooler
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.
The Solar Funnel Cooker is set-up just as you would during sun-light hours, with
- 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.
During 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").
As 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
In 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.
I 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 Cookers
The 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.
Try that without electricity or fuels!
Americans 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!
Here's 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!