Introduction: Cardboard. Foil. Glue: the Solar Funnel Cooker

When the temperature hit 100°F this June I knew I had to avoid lighting my propane stove during the day. I have a box-style cardboard solar oven which I've used for years for granola and beans and even a pie now and again. However, it's bulky to haul out just to heat soup for lunch. Research led me to favor building a solar funnel like that designed by BYU's Professor Jones. They're efficient and a lot easier to build than a parabolic reflector. Mine took about two hours to build.

Now if I'd had a roll-up car sunshade (see also wsalazar's Solar Cooker) I would've used it and saved myself a few steps. Since I didn't, I fell back on the tried and true cardboard-and-foil approach, using things I had at home. As it turned out, the stiffness of the cardboard makes this cooker very easy to adjust and secure.


Step 1: Materials and Tools

The funnel:

2'x4' cardboard;

6' 1" of 18" heavy-duty aluminum foil;

Water-soluble glue (~3 oz. Elmer's Glue-All);

3 or more brass paper fasteners;

1 piece ~8" round aluminum sheeting (or a round of cardboard covered with foil);

1 bucket or planter;

6' string;

2 medium-sized binder clips ;

Pot Stand:

~1 yard of 1" to 1-1/2" wide galvanized sheet roofing;

~10" square of 1/4" mesh hardware cloth;

1 1" machine screw with nut;

Tools:

Framing square (handy but optional);

Tape measure or ruler or yardstick;

Sharp knife or box knife or drywall saw;

Pencil;

Something round for a template, or a drafting compass;

Brush or paper towel or rag;

Scissors or nail or icepick;

Tin snips;

Pliers and screwdriver.


Step 2: Cut Cardboard

Cut a 4' x 2' piece of corrugated cardboard.

Figure out how big you want the hole at the bottom of your funnel. No opening at all (so your funnel is actually a cone) could make standing it upright in the bucket difficult, whereas an opening more than six or eight inches across means losing too much reflective area. I used a slightly less than six inch opening, which meant cutting a 12" diameter half-circle out of the cardboard.

Center your half-circle by finding the halfway point of one of the 4 foot sides of your cardboard. Draw the half-circle using a compass or a handy template like a five gallon bucket lid.

Carefully cut out the half-circle. I used a sharp, narrow-bladed pocket knife, but a box knife or drywall saw will also do the job. Avoid cutting yourself if possible, though sometimes I think a little blood adds soul to a project.


Step 3: Apply Foil

Cut your foil to length. One 4' length and one 2'1" length will be more than sufficient if you cut the latter in half lengthwise.

Dilute water-soluble glue is easy to apply and easy to clean up. It lasts pretty well too, assuming you don't leave your funnel out in all weather. I've tried spray auto trim adhesive and found it doesn't last nearly so well.

Mix 1 part glue with 1 part water or less, to make it easier to spread. I used about 3 ounces of Elmer's Glue-All, and perhaps a quarter cup of water. I applied it with a folded paper towel. A brush would be less messy; a rag would work as well. Apply the glue only on the strip of cardboard your current piece of foil will go on.

Using clean hands! apply the foil, shiny side up*, on the glued section. Avoid wrinkles. Avoid getting glue on the face of the foil.

Now remove the glue you got on the surface of the foil in spite of all your care by using a clean, damp paper towel or rag to wipe it away. Clean the area several times.

Trim the extra foil from the edges. If you trim carefully, the extra foil from the semicircle gap can be used to cover a round of scrap cardboard to fill the base of the funnel (see step 6).

*There is some experimental evidence that a matte aluminum surface actually works better. It is therefore possible that using the less shiny side of the foil would be preferable. I went with tradition this time.

Step 4: Shape the Funnel

Make a series of radiating folds in your foil-covered cardboard. Do this by placing the straightedge where you want each crease to be. Hold it down with one hand (or hand and forearm) while sliding your other hand beneath the cardboard and pushing gently upwards until it creases.

Keep in mind that the point where all these creases should meet is not at the mid-point of the half-circle, but the still theoretical mid-point of the circle you are going to create when you join the sides. (See diagram). In my case this point was about three inches up from my original center point. But approximating this will work fine, so don't sweat it.

Gently persuade the cardboard into the shape of a funnel, overlapping the edges by an inch or two.

Poke three (or more) holes along this seam, using a nail or icepick or scissors blade or whatever's handy. Put a paper fastener (like a small brass cotter pin with a round head) through each hole and fold the tails outward to join the two sides. Using twine or thin wire such as twist-ties might work just as well.


Step 5: Fill the Gap in the Funnel Base

Unless you plan to use a pot stand that sits down inside your bucket and protrudes up through the hole in your funnel--which I don't recommend--you will want to close off the bottom of your funnel with something reflective. I used a round of scrap aluminum siding, which is always getting blown off my neighbors' trailers. To do this:

 Use the base of your funnel as a template to draw a circle of the right size on the aluminum sheet, making sure to leave an inch or so of material around the edges of your circle.

With your tin snips, cut out a circle of aluminum roughly two inches wider in diameter than the funnel opening. This doesn't need to be a nice neat shape. Mine certainly wasn't.

From the edge of your aluminum, make a series of cuts just to the line of the inner circle. Think of this as though you are making petals of a flower with a rather large center. You may want to round off the corners of these petals so they don't stick you.

Bend the petals upwards at roughly a 60° angle, working methodically around the circle so that each overlaps the one before.

Push this down into the funnel. It should sit there quite happily on its own.

If you don't have aluminum sheeting handy, there's no reason why a round of cardboard covered with foil won't work just as well. Just make it big enough that it won't push through the opening. A lightweight aluminum pie plate might also do the trick with no work at all, if your opening is sized to fit it.

Step 6: Build Pot Stand


My stand is just a 1 1/2" strap of metal bolted together at a single point to make an 8" diameter circle. Over this goes a piece of 1/4" mesh hardware cloth folded to fit, trimmed so the edges of the hardware cloth don't poke holes in the foil of your funnel--or your fingers. This stand allows a lot of heat to be reflected up onto the bottom of your cooking pot and envelope.

If you can find an 8" to 9" diameter wire cake cooling rack, that will work very well also.


Step 7: Options: the Pot and Envelope

The cooking vessel most commonly used with a solar funnel seems to be a black-painted Ball jar, which is placed inside a Reynolds oven bag to retain cooking heat. I have a prejudice against both paint and plastic when I'm cooking, so I looked for an alternative. What I ended up with was a clear Pyrex bowl, a clear Pyrex lid which fits pretty well, and a large, dark enamel cup which fits inside the bowl and lid. This has worked quite well for me. I can heat 14 ounces of soup from refrigerator temperature to bubbling in less than an hour on a sunny day.

It is possible that the oven bag envelope would heat up quicker, and you may want to go that route. An ordinary supermarket type 2 vegetable bag will serve in a pinch, as long as you can keep it from touching your cooking vessel, the heat of which will promptly melt holes in it.


Step 8: Cooking: Setup and Adjustments

Set your bucket (or in my case, planter, found in the local dump) in a sunny spot. preferably out of the wind. If it's breezy, use rocks or chunks of wood around the bottom of the bucket. Something heavy inside the bucket may also help.

Place the solar funnel, narrow end down, inside your bucket and aim it at the sun. I find the "horns" of the funnel helpful here. Ideally from one perspective the sun should sit halfway between the horns, and when you move 90° around the funnel, the horns should point a few degrees higher (if it's morning) or lower (if afternoon) than the sun. That way you've got an hour or so before you need to readjust the aim.

As breezes are a regular feature of my local weather, I secure my funnel to the bucket. My planter had two rings, one on either side. I tied an approximately yard-long piece of string to each ring. On the other end of the string I tied a mid-sized binder clip. The binder clip clips to the top edge of the funnel, with the string caught so there is no slack. This holds the aim as well, and is easy to readjust when necessary.

Assuming you don't have handy rings, wrap a piece of wire around your bucket under the lip, with a loop on either side to which you can tie your string.

(This, by the way, is one advantage of using cardboard over a sunshade, which isn't stiff enough to clip securely into place.)

 Start cooking!


Comments

author
dave367 made it!(author)2010-07-08

How hard would it be for a Green company (maybe someone who ships a lot of stuff to developing countries?) to order all of its cardboard shipping boxes aluminum-coated on the inside. A few microns would be sufficient, and eventually ALL scrap cardboard would be silvered on one side, and this kind of stuff could be built with 100% found materials? If they wanted to really be Green, they could print directions and patterns on the inside of the boxes, too. (Hey, I had the same idea for the giant parachutes the Army uses to drop humanitarian materials with--print "How to turn this parachute into a tent for 50" right on the nylon. I never got a call back... )

author
NinjaJay made it!(author)2012-05-20

Hey Dave, those sound like really great ideas.

As a way of feeling your silence, I once contacted Bono's charity for Africa to suggest that they immediately get solar cooking going, as a way of protecting the women of Sudan who were being raped while searching for firewood.

Maybe there is a DIY way that you can get the information you already seem to know into the hands of people. A young stranger woman once put into my hands a folded photocopied booklet to show how women can effectively deal with physical threats. There was a lot of information crammed into that single sheet of paper. I was so inspired by that act that I made copies to hand out also.

Maybe there are some people you could find to write to who would appreciate the info and be able to pass it on locally. For the cost of a few international stamps and some photocopies, it could be really satisfying.

To petition our government to add printing to the inside of the parachutes sounds like something to open up to the public, maybe using Petition.org.

author
dave367 made it!(author)2012-05-21

Good thoughts, Ninja. Thanks. I'm more of a tech guy than a salesman, but good thoughts, nevertheless.

A follow-on thought (on tech): Solar ovens do much better with some form of "glazing" to keep the heat inside. This can be as simple as a "Bake in a bag" plastic bag from the supermarket. It isn't stretched over the mouth or anything--you just put the food inside then puff up/seal up the bag, then place it at the focus of the over. It only requires a form of plastic which can take the heat and not out-gas anything yucky.

I've been told that ordinary plastic bags like the ones grocery stores use by the bazillion, will serve in this capacity. Can anyone confirm or deny? Thanks!

author
KoffeeKommando made it!(author)2011-08-22

Oh, I get 225+ degrees F inside my glass 2 gal cookie jar (from WalMart).

But I'm in New Mexico too ;)

Watch my videos, you get to see the temps on an oven thermometer.
The first video shows how to build.

I hooked mine to a "spaceframe" chair (like Wal Mart types).
This way you never worry about wind tipping it over (if you spike it down clever)

But my new cart makes the chair deal obsolete. And I can turn much easier.

author
KoffeeKommando made it!(author)2011-08-22

I built a BYU Funnel too.
Video:

Part 1:
http://youtu.be/ZjC1BlLg6YE

Part 2:
http://youtu.be/0-5M6quwxPk

I just built a clever wooden cart to mount it on. It easily turns to follow the sun all day.

Use coroplast to make the funnel instead of cardboard. Also, use chrome adhesive vinyl (4-5 year rating!) as the reflective surface.

You can get the chrome vinyl at a signmaking shop:
http://www.hhsignsupply.com/productcart/pc/Chrome-c463.htm

Make sure to buy 4-5 year rating.

author
leeroper made it!(author)2011-05-12

Very good idea, simple concept and quite easy to-do but not many would think of it. I wasn’t exactly sure how it would look once all together, it looks good!

author
doctorbigdaddy made it!(author)2010-07-08

What a great idea. I have been thinking of a way to make solar stills for use in areas with poor quality water( most of the underdeveloped world) . this design ,coupled with a type of distilling column would produce several liters of water a day.The still could consist of two glass bottles or even copper pipe with a coil of copper tubing inside.Does anyone have an idea for a still that could be produced by people with very limited resources and tools. . This solar oven is exactly the thing that could power the still; if only some cleaver "maker" could come up with an equally simple and elegant still~!

author
thepelton made it!(author)2011-01-29

Another source of reflective items that could be turned into a Solar oven mirror is old music and computer disks.

author
fenris made it!(author)2010-07-17

I think you could make a solar drip still. Go up the page where you see the glass container with the glass lid. Imagine that glass lid upside down, with the knob downward. Imagine further that there is black paint on the outside of the glass container, and a small amount of water in it. Imagine a cup, centered in the larger container so as to catch the condensing steam which will flow down the curved glass lid from all directions to the knob, and will drip into the cup. Try it on your stove just to prove the concept. Put some cheap wine in the larger pot, apply heat very carefully, and by trial and error you will find that the alcohol will make steam before water does, and you can get some decent brandy from wine so bad as to be almost undrinkable. You will want a very low setting on the burner, lower than "simmer", and you will need to be patient - one drop at a time takes a while. Of course, in the house with access to ice cubes to put in the inverted top, you can make the thing work faster, and at a slightly higher temperature - but in the case of alcohol distillation remember to keep the overall temperature well below the boiling temperature of water (100C=212F). I have done this lots of times (don't worry, I no longer drink, for years now...), and if you want more information just let me know.

author
doctorbigdaddy made it!(author)2010-07-21

Thats a great idea.I wish there was a way to scale the concept up so that liters of water could be produced per day.Thjank you for your idea!

author
thepelton made it!(author)2010-12-18

I saw a solar oven made up of a number of car mirrors, each focused on a single spot. I also imagine that those suspended mirror balls put in dance halls could be cannibalized for a project like this.

author
Waren-Neutron made it!(author)2010-11-23

it is great?

author
pattyaitch made it!(author)2010-07-10

My first solar cooker was made from a styrofoam cooler box. We cut the front edge on a 30 degree angle~~we were in California at the time. The top was a sheet of double strength window glass cut to fit the opening. Favorite recipe was the old strata of Ortega chili peppers with bread and cheese. In my home state of Montana, I'd probably cut the cooler more on a 45 degree angle. Never had it blow over, come to think of it. We were set up to make a solar cooker from a large Pizza box for 5th grade granddaughter's Science fair. Fair was canceled by by the new teacher (one room school w/5 students) as too much work. She was planning to make Nachos, had the Fair happened.

author
iamunique127 made it!(author)2010-07-09

Great Instructable- simple, clear and funny. Thanks for including alteratives - they got me thinking of my own mods. I'll soon be spending quite a bit of time outdoors while on vacation and I'll be needing to eat. I've always got the mylar blanket and sunshade in the truck anyway so I'll have most of the supplies already. A few found items and I'm good to go. This project will be lots of fun to amaze the wife. Thanks

author
gwylan made it!(author)2010-07-09

The sunshade version is a great mobile option. And on days when you spend a lot of time driving you can always cook on your engine block.

author
velacreations made it!(author)2010-07-06

I've used this style and the sunshield style cookers for several years, now. They are great, but I do have a few issues with them. They are unstable in wind. If you are not around, and a wind picks up, it throws your almost-cooked food on the ground! They are small. This is my biggest complaint. I wish they were just a bit bigger to cook thing a little faster.

author
gwylan made it!(author)2010-07-06

So far (fingers crossed!) mine hasn't blown over. I've had that happen a couple of times with my solar oven, which generally means not only do I lose the food, but I have get a new piece of glass as well. But the size issue could be dealt with. Just get a bigger piece of cardboard. I gather that as long as you keep the ratio the same, it should give you the same results. Of course bigger means a juicier target for the wind!

author
velacreations made it!(author)2010-07-07

yeah, I've tried scaling up before, and it works to a point, but then the cardboard is too floppy. So, really, to scale up, you need a better materials, maybe wood or metal would work.

author
gwylan made it!(author)2010-07-09

How big did you go? Metal would probably work. You could suit the gauge to the size of the funnel. Might get expensive.

author
johnny3h made it!(author)2010-07-08

For strengthening sheet corrogated cardboard I've simply thickened it by laminating a second or third layer. This has been so successful that I've even made benches, coffee tables, a pigeon-hole shoe and boot racks for my closet, and even some furniture. For the laminating I've used Elmer's White Glue applied with a brush, and 3M aerosol Office Contact Cement. Altough it's more epensive, I like the contact cement better as the water based glue tends to shrivle and wrinkle the cardboard. In use, cardboard has to be protected from water [especially liquid, but also humidity] so after completion, I totally "seal" all my projects with Urethane [either colored or clear]. As an experiment, I also made some construction materials [2x4,6,8,10, and 2x12s. in lengths up to 16 feet, and I also made some "timbers" - 4x6, 4x10, and 4x12], all of which in trial construction of various projects, including a lawn storage building and a couple of dog houses, worked great. Again, the major critical factors in using cardboard for any project requiring rigidity and strength are 100% glue contact in the laminating process, and TOTAL protection from moisture!!!!!

author
katz made it!(author)2010-07-09

Maybe it could be solved by tying strings to the top edges of the funnel and placing weights or sand bags on the bottom of these strings - then they could act as ancors against the wind. Also - the whole construction could be placed into something that reaches the top of the funnel - since only the inside of the funnel is used for cooking - the outer side doesn't need to be exposed. Maybe a cage or a wooden box or something....

author
Wildrat made it!(author)2010-07-08

Did this in the scouts back in the 70's. Solar heating has been around for millions of years.

author
SolarFlower_org made it!(author)2010-07-08

Emergency thermal blankets (aka space blankets) are a common and cheap source of reflective mylar, which is about 80-95% reflective compared to foil's 60-70. Double sided flooring tape is definitely the easiest and stickiest way to get two things together. Not the cheapest stuff in the world but generally worth it.

author
arkie made it!(author)2010-07-08

gwylan, great writing! Using clean hands! apply the foil, shiny side up*, on the glued section. Avoid wrinkles. Avoid getting glue on the face of the foil. Now remove the glue you got on the surface of the foil in spite of all your care Your humour grabbed me and the cooker fascinated. Nice work.

author
Javin007 made it!(author)2010-07-07

According to a chart in one of the links in the comments here, these things will peak out at just under the boiling point in about an hour and a half. Seems it'd be a great way to slow cook ribs or something.

author
pepelepew made it!(author)2010-07-06

Is the aluminum foil necessary? When i was an art student I've sometime used some sort of mirror paper, probably just regular waxed paper covered with some sort of reflecting plastic, but it did reflect everything like a real mirror, well almost like the real thing (there was a limit to it, as it couldn't perfectly reflect distant object beyond 4 or 5 meters away, but it did reflect perfectly sunlight). This special paper would be much easier to manipulate (no $#|@% folds like aluminum foil)...

author
gwylan made it!(author)2010-07-06

Aluminum foil was what I had, so that's what I used. That being said, a number of other things have been used by others, such as mylar and polished aluminum sheeting. If you have the paper, give it a try. Sounds like it would do the job.

author
dmlandrum made it!(author)2010-07-05

Have you been able to measure what kind of temperatures you're able to achieve at the focus? Good job on this build. I've been interested in making a solar cooker, and this looks like a good design.

author
gwylan made it!(author)2010-07-06

So far I haven't tried. To be honest, I'm more interested in how fast it will heat my lunch. There's a variety of test results on the solar funnel at http://solarcooking.org/plans/funneltests01.htm. They might have a figure for you. So far I've been quite happy with mine. I'm still learning the tricks of it.

author
omnibot made it!(author)2010-07-05

It's beautiful.

author
rimar2000 made it!(author)2010-07-05

That solar cooker is awesome, I did one 6 or 7 years ago, with great success. Good instructable!

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Bio: I live on a small homestead in western New Mexico, in a small light-straw-clay house I built with much help from friends. My spare time ... More »
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