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.

Comments

[dave367]

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... )

[NinjaJay]

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.

[dave367]

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!

[KoffeeKommando]

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.

[KoffeeKommando]

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.

[leeroper]

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!

[doctorbigdaddy]

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~!

[thepelton]

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

[fenris]

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.

[doctorbigdaddy]

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!

[thepelton]

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.

[Waren-Neutron]

it is great?

[pattyaitch]

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.

[iamunique127]

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

[gwylan]

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.

[velacreations]

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.

[katz]

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....

[gwylan]

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!

[johnny3h]

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!!!!!

[velacreations]

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.

[gwylan]

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

[Wildrat]

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

[sugarandfat]

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.

[arkie]

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.

[Javin007]

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.

[pepelepew]

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)...

[gwylan]

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.

[dmlandrum]

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.

[gwylan]

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.

[omnibot]

It's beautiful.

[rimar2000]

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