$7 solar oven design wins $75,000 prize

The Kyoto Box is a cardboard box, with an interior that's painted black, aluminum foil-covered top panels, and covered with clear acrylic. It gets hot enough to boil water. Its inventor, Kenyan-based Jon Bohmer, won a £51,000 prize from the Forum for the Future.

The box will be produced in a factory in Nairobi. He told CNN: "This took me about a weekend, and it worked on the first try. It's mind-boggling how simple it is."

Inventor turns cardboard boxes into eco-friendly oven


  1. This works on the same principle as the solar oven from my grammar school science class! It cooked a hot dog quite well if I remember correctly.

  2. Hmmm… Since the part that’s supposed to get hot is the pot in the middle, I wonder if it wouldn’t get hotter if only the pot were black, and all the sides were reflective, instead of only the top flaps being reflective.

    Even better, the inside of the box could be slightly parabolic to help focus the rays onto the pot. That would make it slightly harder to make DIY versions of it, but probably not that much harder.

  3. Awesome that Mr. Bohmer is publicizing the use of these ovens, but how exactly is this invention new? As the article notes, people have been building and promoting these devices through many designs since the 1960’s at least. We built similar designs in Scouts 30 years ago. Seems to me that Mr. Bohmer should donate some of his winnings to a group such as Solar Cookers International.

  4. @Jim: Agreed! That’s the first thing I thought of as well. This “invention” isn’t new. While I’m all for anything that helps cut down on deforestation and water-borne illnesses, Bohmer really ought to either donate his winnings to the original inventors or make a bunch of these solar box cookers with the money and donate all of them to villages in Africa where they are most needed.

  5. It is a rare mind indeed that can render the hitherto non-existent blindingly obvious. The cry ‘I could have thought of that’ is a very popular and misleading one, for the fact is that they didn’t, and a very significant and revealing fact it is too.

    – Dirk Gently

  6. SAMSAM: The black-painted portions capture light – a broad spectrum of light – and re-radiate it as one narrow width of spectrum: IR. That heats the air in the closed box and the pot as well. There’s foil on the outside to reflect the radiated IR back into the box.

    A parabola of foil to reflect light into the box would be very helpful.

  7. Pretty much everything Mr. Bohmer has done is already covered by an enthusiastic international community (link below)
    Mr. Bohmer has spoken enthusiastically about developing a business concept around the cooker. Moreover, he has stated firmly that aid and charity projects do not work, only profit seeking enterprises can solve these problems (link below, but in Norwegian only, sorry) Hence, I think this is more an award to signal the forum’s attitude to third world development than anything about the box itself.


  8. Having worked on cookstove programs in West Africa, I can bet you all the money Mr. Bohmer has that the adoption rates on these things are *terrible*. Just because you distribute solar cookers like this doesn’t mean people use it. Many folks are in subsistence agriculture, and they therefore cook most frequently in the early morning or late afternoon/early evening–times when solar cookers don’t work so well…

  9. I’m surprised that it won. The use is right on, but it’s not the first or even an improvement or even a cheaper version then others. Here’s a link with some others: http://solarcooking.org/plans/
    I made one a while back and it’s actually in my backyard here in Ottawa Canada baking a vegan chocolate cake as I type (it’s almost done baking). Cost me 0$ to make. :) Definitely wouldn’t enter it into any contests to win $75,000. Shows what you can get by trying. Good on you. Cheers

  10. My apologies to everyone who pointed out that this is nothing new, but I never heard of this and desperately want one.

    I live in Yuma, Arizona (pretty much one of the hottest places on the planet, and where they send Marines to get ready for Iraq), and am looking at another long hot summer. Last year we had 100 consecutive days over 100 degrees.

    So I am loathe to do any kind of cooking inside because it heats up the house, and have a nice toasty back patio to try this out.

    I’ll check the links, but has anybody gone through the trial and error and has the most effective and easily executeable plans. I want to purchase/make one ASAP, and would prefer something more permanent than cardboard.

  11. Isn’t a fairly standard big umbrella more or less parabolic? I believe I saw Bill Nye the Science Guy cook a hotdog with an umbrella lined with foil.

  12. Uh… invention? “design”? We built one of these and used it to heat our lunches at the solar architecture office I worked at 15 years ago. I want my $75K!!!

  13. We used to make these to take on camping trips all the time when I was a girl scout! Even though this is awesome, and I hope it makes a lot of people’s lives easier, I really don’t think it can be said that Jon Bohmer invented it, since it was not an original idea by any means… :-P

  14. I think that the real cure for global warming is in large part in simple solutions like this spread in the third world. Even though they don’t consume that much CO2 at this point, getting them all on solar tech now could reduce the extent that they ever do and increase the standard of living at the same time. Meanwhile we can ween the 1st world off their addiction.

    A charity that set these and solar powered LED lighting systems in the homes of third world families would be something I would love to donate to.

    I would love to see a version that used mirrors to put the stove inside and had an aperture that allowed one to regulate the temperature and turn it off without standing in the hot sun yourself.

  15. Let me be the first to sound the cry of shenanigans. This is not much of an invention (what’s new here – the nonessential Plexiglas?) and the link is all about hype when it should be about getting together a cardboard box, a few feet of aluminum foil and some black paint. The $75k ought to buy about a half million basic kits for those who can supply their own cardboard box and pot.

  16. Wait till mid day for a warm meal as an early breakfast would have to be cold. Still need fire on rainy days. Still need help and solutions.

  17. This design for solar ovens has been in use for at least 30 years. It’s weird that this guy’s work is being thought of as innovative. I’ve built this exact same design before and they’ve been in use by development agencies for years. I have several different solar-related books that contain this exact same design, or something close to it. How is this going to be a viable commercial product when it’s a couple cardboard boxes, black paint and some plexiglass? People are going to see it and make their own.

    You want a really great solar oven? Cover an old satellite dish with reflective material and cook something in the focus of the parabola. In Arcata, California there was a guy who used to make popcorn at the farmer’s market in a contraption like that. He also wore heavily insulated gloves and welding goggles and kept a safe perimeter around the device. I once saw him melt an aluminum can with that thing.

  18. I made a small demonstration model of this oven for a Simple Solar workshop I am doing next week. Full curriculum at http://www.dailykos.com/story/2009/4/11/719039/-Simple-Solar-at-Boston-Skill-Share

    I used two cardboard boxes, packing peanuts for insulation, aluminized mylar from the inside of chip bags for reflectors and to line the cook box, a black plastic mailer for the solar absorber in the cook box, a pot pie tin for the cooking surface, a clear plastic cup cover and a clear plastic top to a supermarket sushi tray for the double glazing. It’s 100% recycled materials and will work just fine.

    Solar thermal is almost as simple as black and white:
    dark gets hot
    light reflects
    clear keeps the wind out

  19. @ Ill Lich #15: Mine too! What year was yours? It looks like somebody owes somebody some royalties.

    Then again, mine never got hot enough to boil water. I blamed the overcast weather that week.

  20. Well, according to the article, one of the designs he beat was ‘a machine that turns wood and other organic material into charcoal’. Being as I can do that with some friendly advice, a spade and a match, I reckon he did good to actually make something…

  21. But why is it so darn expensive?

    *I* probably wouldn’t spend $7 on it. (Especially since my dollar reserves amount to all of three $1 bills … in a frame, with the Euro exchange rates of April 18th since 2007 written on the back.)

    Seriously, you could probably sell it with profit for less than $2.50 or so if assembly was left to the buyers. (What’s left is some tin foil, cardboard boards and a clear plastic bowl, maybe some glue …) Whoever is ready to spend at least 1 hour a day cooking with one of those, probably won’t mind 15 minutes of IKEAbatics.

  22. Plexiglass and the nested second box are surprisingly critical additions here; heating the air of the entire box greatly reduces heat loss from the pot, and the nested second box plus plexiglass provides quite effective insulation to allow this.

    This requires access to:
    -Good tin foil
    -Good, uniform matte-black paint
    -Two sizeable, nesting boxes*
    -A sizeable sheet of regular plexiglass that fits the edges of the box well
    -A metal pot with a fitted lid

    Believe it or not, many places these are non-trivial to acquire. This seems to be escaping some people.

  23. The challenge is not technological; it’s cultural.

    I spent four months last year living way out back of beyond in Southern Kenya, amongst some of the very people this cooker is supposed to help. We were at six thousand feet, a degree or two south of equator, and the sun was formidable. I used passive solar to heat water for showering, and to sterilize it for drinking. I spent some time trying to apply it too cooking as well, using the same materials as the Kyoto box.

    I quit when it became clear that none of the locals were interested in using solar energy for heating food and water. Some of the reasons:

    –people cook indoors, even on the hottest days. It’s a private family activity.

    –the woodsmoke is tough on the lungs, but it is also tough on the flies and other annoying bugs. The smoky interior of the huts provided a welcome respite from insects. Fire serves purposes other than cooking.

    –sitting around a fire is primal social behavior. Sitting around a black box the sun? Not so much.

    –the dietary staples are maize meal porridge and fried kale/collards (ugali and sukuma wiki). I suppose you could slow-cook the porridge, but the greens are fried, and as it stands the box isn’t going to do that. Long-established dietary habits would have to change to accommodate the box.

    –people drink chai throughout the day, not just the end of the day, when the Kyoto Box is hot.
    And even if the box were used for chai at dusk, is it really a good idea to leave milk warming in the Sun all day?

    –cookers need tending. Curious children and animals can be scalded. And someone (or something) might ‘borrow’ your meal. The box would need a lot more watching than the equivalent wood-fueled cooker.

    –the rains come twice a year, and last for weeks. Solar is not an option.

    –people did not use old-growth wood, at least where I was. It appeared to be fast growing and renewable.

    –Collecting the wood was onerous, but not deadly, as the video suggests. In some ways it was a welcome social activity. Obviously this varies from place to place, just as sustainability does. But what are the actual numbers?

    And so on…. This is not to say that solar is a total bust. My tiny 5W solar panel was highly coveted, at least by those who had small rechargeable devices like flashlights and cell phones. The local medical clinic used rooftop solar panels and arrays of car batteries to power refrigeration and light. Solar had its place, but its not clear that passive solar slow-cookers are going to catch on. Technologies that ignore the human element do so at their peril.

    The prize should be awarded when the Kyoto Box is widely accepted and used daily in place of wood. That is when it will be a proven, viable alternative. Until then, it’s just an age-old idea in search of a home.

  24. @ #31 Praline,

    Yes, the box is for places like Kenya and rural parts of Africa as Bohmer stated, but you seemed to have missed the point.

    As you said, you spent a while living in Kenya, so now you seem to think the whole of Africa is like that. But thankfully you are quite wrong; yes, many parts of rural africa still have primative means of cooking, but not all of them have the same culture as Kenya.
    Ever wonder why Kenya has so many problems? Its because its culture holds it back. Other contries is Africa such as Botswana have a thriving economy and are pushing constantly forwards in terms of technology and power for all.

    And another point; you mentioned that rainfall. As anyone who has studied geography knows, rainfall is not uniform across a contry, let alone a continent as large as Africa. Montains and winds effect this greatly, so your statement about rainfall being an issue is rediculous.
    But I digress.

    The main point im trying to state is that Kenya is a bad example to use as a base starting point for conception and intergration of this cooker into people’s lives.

  25. #26 gmoke

    that sounds like a toxic fume box. you couldn’t pay me to eat anything that has cooked in the fumes of plastic and packing peanuts.

    you might want to rethink some of your materials, “free” and “recycled” are not always the most prudent materials to use when they release toxic fumes when heated.

  26. Yes, it’s nothing new. My guess is the money is not for an invention, but rather it’s an award to the person or group that’s most likely to make the biggest difference, from the money the receive, and the publicity from winning the award.

    I tried to make one of these solar oven last year. It didn’t work. (I think too much hot air was escaping because I failed to seal all the holes properly). I went back to reading the www. Do you know that it takes hours to cook with these things? I flattened my box and put it with all the other paper for recycling.

    Aprovecho has a nice rocket stove that burns wood more efficiently. It was mentioned in boingboing last year. Their website has plenty of videos, on using and constructing a few designs, including a big one called an Institutional Barrel Stove, that takes huge big pots for when you’re cooking for many people at the same time.

  27. @#31 Anonymous,

    Sorry if you got the impression that I believe Africa has a single, uniform culture. I don’t believe that, and my post doesn’t imply it. I don’t even think Kenya has a single culture–and if I did, I would have been quickly disillusioned by the post-election violence. It engendered endless discussion of ‘tribal differences’, both in the press and among my hosts. (I notice that you believe otherwise–that Kenya as a whole has a “culture that holds it back.” Perhaps you’re projecting a little?)

    As to why I mentioned Kenya at all…

    — the ‘inventor’ of the Kyoto box is an “entrepreneur based in Kenya”;
    — the accompanying video is apparently filmed in Kenya;
    — the cooker has “gone into production in a factory in Nairobi, Kenya, that can churn out about 2.5 million boxes a month”, and
    — Solar Cookers International has been promoting the use of solar cookers in Kenya for ten years. (Their executive director, who seems understandably miffed by the prize situation, wryly comments that “It would be a pleasure to work with Mr. Bohmer in Kenya.”)

    The interesting question is why the Kyoto Box is presented (by CNN and others) as a novel “solution to one of the world’s biggest problems.” As lots of previous BB posts point out, passive solar cookers of various design have been around a while. The Peace Corps was using them in the sixties. If they actually are a solution to water sterilization, deforestation, back injuries, and so forth, then why haven’t they worked?

    I suspect there are significant social obstacles to the adoption of this technology, and my post listed a few reasons that “some” of the people the box purports to help might be less than enthused by it. I made it clear that these observations apply to the “locals”, where I was staying.

    Do all these issues apply to all of Africa (or the world)? Obviously not. But are there likely to be other obstacles, similar or not, to the adoption of the Kyoto box? That question is worth asking; and I don’t see it being addressed, whether by the ‘inventor’, the foundation awarding the prize, or the media reporting on it.

  28. @ #7 Bardfinn:

    The black-painted portions capture light – a broad spectrum of light – and re-radiate it as one narrow width of spectrum: IR. That heats the air in the closed box and the pot as well. There’s foil on the outside to reflect the radiated IR back into the box.

    Just to clear up a quick misconception: IR is not “heat.” There is just as much energy in the original stream of light as there would be if it were all IR. In fact, there is a lot more energy in the original stream than what is being emitted as IR. That’s because the black sides are absorbing all of the energy, and then re-radiating it less efficiently, in all directions.

    Reflective sides, if they reflected the light towards the center where the black cooking pot is, would concentrate a lot more energy onto the pot, meaning the pot would heat up much faster.

    The problem, I assume, is getting sides that reflect towards the center (i.e. are parabolic) cheaply and easily.

  29. The box was invented some 240 years ago, look up Horace Bénédict de Saussure to learn more. Designs using two cardboard boxes are quite old as well.

    Bohmer may of course have worked out his design alone, but I’d expect that he would have noticed earlier designs and the presence of a solar cooking organisation in Nairobi when doing basic market research. Bohmer used to be an Internet entrepreneur and is likely to know how to google for information. ;-)

    It’s worth noting that about 15 years ago some of the people promoting solar cooking contraptions improved the solar box cooker design, turning it into a reflector cooker. The result of this development is the CooKit. It will be interresting to see a test that compares the Kyoto Box and the CooKit.

    CNN ran a story featuring the CooKit in 2007: Solar lifeline saves Darfur women.

    Regarding the question whether this is charity or business, Bohmer has stated repeatedly that it’s business. The Forum for the Future writes the following:

    The trials will generate data to back an application for carbon credits, the crucial element which will make the project scalable, [Bohmer] explains. He expects each stove to make a yearly profit of 20-30 euros, which will more than cover the manufacturing cost. The surplus will fund production of a suite of other products which offer solar-powered solutions for villagers in the developing world: a torch; a plastic bag which heats and cleans water; and a smokeless cooker which burns biomass.

    Maybe it was the business idea that was innovative?

  30. Hi there,
    Concerning the solar oven, could it melt #5,6, and 7 plastics? I’d like to melt them down into bricks so as to store them until we have a more extensive recycle program. Currently our neck of the woods only takes #1 & #2 plastics. I dislike tossing all the other plastics. Surely our system will be upgraded soon, but until then, I’d like to store the bricks. Any comments on the subject would be very welcome, as I just thought of this last night and probably haven’t thought it through. Negative aspects as well are welcome. For example, the toxins release when plastics are heated; do I pollute the air to lessen a landfill site? Any thoughts! Thanks, kabug

  31. a lot of design is drawn from current or past similar usage. Saying this has been done is fine, but really its like saying a lamp that won a design award for lamps has been done before- don’t you think? Small changes in design can make something that was ignored before widely adopted and with consideration to energy solutions this is what we should award. ingenuity and innovation come in more forms than concept alone.

  32. This is one of the easiest way to help saving the planet, I mean, using solar power is a great advantage and we should all consider it, no matter where we live.

    Of course, Mr Jon Bohmer lives in Kenya and had all the more reasons to invest time into researching solar ovens, in that area things are a bit different. His efforts were now paid in full, $75k as a prize, man, that’s a good prize, no matter you live in Kenya or in US. Personally I run a blog on solar cookers and I know a lot of people invest time in making solar cookers better and more efficient. I hope they all receive one way or the other money to continue their research in this direction.
    ANyway, congratulations to Mr Jon Bohmer!

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