How to Build Your Own Solar Funnel Cooker/Refrigerator

[Dutchess]

This was created and tested by BYU

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 slower.).
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.
-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.
Construction Steps
1. 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.
2. 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.
3. 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.)
4. 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. 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.
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 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. 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.
Final Steps
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 follow.)
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 bag.
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 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.

[Dutchess]

Helpful Hints
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 it 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.
Water and Milk Pasteurization
Contaminated drinking water or milk kills thousands of people each day, especially children. The Word
Health Organization (WHO) reports that 80 percent of illnesses in the world are spread through contaminated
water. Studies show that heating water to about 65 degrees to 70 degrees Celsius (150 degrees Fahrenheit) is
sufficient to kill coliform bacteria, rotaviruses, enteroviruses and even Giardia. This is called pasteurization.
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 160 degrees 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.
Cooking with the Solar Funnel Cooker
What 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.
The solar cooker works best when the UV index is 7 or higher. (Sun high overhead, few clouds.)
Cooking 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 jar.
Stirring is not necessary for most foods. Food generally will not burn in the solar cooker.
VEGETABLES (Potatoes, 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. Cooking
Time: About 1.5 hours
CEREALS AND GRAINS (Rice, 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.
Cooking Time: 1.5-2 hours
PASTA AND DEHYDRATED SOUPS Preparation: First heat water to near boiling (50-70 minutes). Then
add the pasta or soup mix. Stir or shake, and cook 15 additional minutes. Cooking Time: 65-85 minutes
BEANS Preparation: Let tough or dry beans soak overnight. Place in cooking jar with water. Cooking Time:
2-3 hours
EGGS Preparation: No need to add water. Note: If cooked too long, egg whites may darken, but taste remains
the same. Cooking Time: 1-1.5 hours, depending on desired yolk firmness.
MEATS (Chicken, beef, and fish) Preparation: No need to add water. Longer cooking makes the meat more
tender. Cooking Time: 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
BAKING Preparation: Times vary based on amount of dough. Cooking Times: Breads: 1-1.5 hours; Biscuits:
1-1.5 hours; Cookies: 1 hour
ROASTED NUTS (Peanuts, almonds, pumpkin seed, etc.) Preparation: Place in jar. A little vegetable oil may
be added if desired. Cooking Time: About 1.5 hours
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.
Hot-and-cold running water.
By placing a large funnel around a water barrel, it will be solar-heated during the day, or cooled at night.Thus, having two barrels each with a funnel-like reflector around it, and covering one when not sunny and the
other whenever it is sunny, one can have hot and cold water. These barrels could be placed on a platform or
roof to provide hot-and-cold running water!
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 two exceptions:
1. 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.).
2. 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 29
degrees Fahrenheit (10 degrees Celsius) below ambient air temperature using this remarkably simple scheme.
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 78 degrees Fahrenheit. 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.

[crashdive123]

Dutchess - we merged your two posts so that they are in the same thread. It'll make it easier for others to follow.