Cooking off the grid, part 1: Building a solar cooker

Photo of a solar ovenA little while ago, I was inspired to reduce my energy consumption in the kitchen by Ed Begley, Jr. Mr. Begley is a well-known environmentalist, the frequent butt of jokes, and an actor who played a supporting role (Stan Sitwell) in Arrested Development, one of the funniest sitcoms in TV history, in my opinion. In an interview on KCRW's Good Food program, Mr. Begley talked about two ways that he conserves energy in his kitchen. The first was a bicycle-powered toaster, a gizmo that requires peddling at a full sprint to crisp and darken two slices of bread. I like bicycling, but peddling to make a piece of toast is a little too much for me.

The second device Mr. Begley talked about was a solar oven. That sounded like a lot more fun.

Commercial ovens are available from many retailers, but I thought it would be more interesting (and a lot cheaper) to build my own. I easily found illustrated step-by-step instructions at the public library in Cooking with the Sun (by Lorraine Anderson and Rick Palkovic). Other plans can be found online here, here and here.

For me the solar oven would be a curiosity of sort. In places with poor energy infrastructure, like the rural parts of developing nations or refugee camps, however, solar cooking could lead to dramatically better lives. Much of the cooking in these places is done over wood fires, using twigs and branches, the gathering of which requires more and more time each day as nearby scrub dwindles from overharvesting. In addition, indoor cooking over a wood stove severely degrades air quality in kitchen (something described in great detail by Professor Kirk Smith of UC Berkeley). A solar oven can avoid these problems. Organizations like Solar Cookers International are working to spread knowledge about solar cooking (and solar water pasteurization) around the world.

Building a solar oven

It didn't take much material to build a solar oven: just two boxes (one smaller than the other, enough for a several-inch gap), a few sheets of cardboard, all-purpose glue (you want a glue, like Elmer's white glue, that will not release gases at high temperature), a brush for spreading the glue, aluminum foil, newspaper, a piece of glass the size of the small box (I used one from a picture frame), a coat hanger, silicone sealer, and various tools (a sharp utility knife, scissors, wire cutters). After acquiring all of the necessary parts, I think I spent five or six hours building the oven.

The next section in this post describes how I built the oven, followed by a scientific explanation of how it works.

The photo below shows the two boxes after one of the most time-consuming and potentially frustrating steps: covering the inside of the large box, and the inside and outside of the small box with foil (although aluminum foil boxes have tearing guides, I could never manage to get a good, clean tear). I used an inner box that was 12" x 18" x 9" and an outer box that was 18" x 24" x 11". The aluminum foil is attached to the outside surface of the inner box and the inside of the outer box to help isolate the cooking chamber (and its warm contents) from the outside world.

Building a solar cooker, part 1

After the boxes were coated with foil, I built a platform out of cardboard stacks in the larger box for the small box (left photo below). For insulation, I put crumpled cardboard in the spaces between the cardboard stacks. Next, I glued the small box onto the platform and filled the inter-box gap with crumpled and rolled newspaper (right photo below).

Photos of building a solar oven - middle steps

To provide insulation on the top of the oven, I built a frame to hold a piece of glass (left photo). Next, I covered a large piece of cardboard with foil to be a reflector panel. Finally, I built a mechanism to hold the cover open. There are many possibilities for this — I used two pieces of corrugated cardboard and a coat hanger. One piece of cardboard is glued to the reflector, one piece to the oven's cover. (You can barely see them in the right-hand photo below; they're the two long, thin rectangles on the right side of the box.) The coat hanger, which is bent in a Z-shape, fits into the corrugations on the two cardboard rectangles. It is a decent reflector prop but somewhat unstable in the wind (more on this in part two of this series).

Photos of building a solar oven - final steps

 

How It Works

The fundamental principle that allows two cardboard boxes, aluminum foil, and a piece of glass to be converted into an oven is the same one that allows life to exist on this planet: the greenhouse effect.

Here's how the effect works in a solar oven: energy in the form of sunlight comes through the glass cover and is either directly absorbed by the cooking pot or reflects off the aluminum foil. Some of the reflected energy hits the pot, some of it exits through the window. The glass cover serves two purposes. First, it separates the pot from the ambient environment, thus preventing air current from cooling the pot. Second, the optical properties of the glass keep the heat of the pot inside the cooking chamber.

When an object is at a different temperature than its surroundings, it loses or gains heat by one of three modes: conduction (heat transfer through solid contact, like your hand touching a hot surface), convection (heat transfer through fluid contact, like a cold breeze) or radiation (heat transfer by electromagnetic waves, like sunlight or the heat lamps in outside dining areas). In case of a solar oven, radiation is the main mode of interest. Without getting into all of the details, the essential idea is that the pot absorbs energy at many wavelengths, but only emits long wavelength energy (Planck's law explains this). In fact, almost all of the radiant energy emitted by the pot has a wavelength longer than 3 micrometers, wavelengths for which glass is opaque. Newport Optics has a diagram that shows the spectral transmissivity of various types of glass. Note the steep drop around 3 micrometers. Thus, the glass cover prevents the hot pot from losing heat to the surroundings. If you want to read more about this, I posted a plot and some technical commentary over at my Flickr account.

In part 2, I'll show how my oven performed and describe some of my early tests.

 

16 Responsesto “Cooking off the grid, part 1: Building a solar cooker”

  1. Wow, I've been wanting to make one since I found out about them a couple of years ago, but without an outdoor space to call my own, it doesn't seem to make sense for me at the moment. I'm looking forward to hearing more about your cooking results!

  2. I grew up eating out of solar box cookers (much like the one you made), but now am a huge fan of the Solar CooKit's (see http://solarcooking.org for free plans). They only cook one pot at a time, but they cook faster and are very cheap and compact ovens to own. When I do demonstrations I often carry 3 or 4 cookers in a backpack on my bike. These are also the models used by Solar Cookers International for their passive solar water pasteurization projects in Africa and South America, mainly because they are the lowest tech and therefore cheapest way to use the sun.

    The food out of solar ovens is fantastic, the early motto of Solar Cookers International being "get it on early, don't worry about overcooking", it works much like a slow cooker. Meat especially is exceptionally good. Before I went to college I had more meals out of one of these than I did from a conventional stove or oven.

    Nice post, the actual energy involved in cooking is often overlooked as a critical step in food preparation around the world, I am very glad you are drawing attention to it and showing how simple it is to make a very usable and low-tech oven.

  3. Sarah C. says:

    Haha. This is SO funny to see here. Sorry, its just that we did this back when I was a girl scout about 20 years ago. I had no idea it was food blog worthy. Hilarious! Thanks for bringing back memories...

  4. thm says:

    Like Thermochronic, I also grew up with solar box cookers.

    After reading Cook's Illustrated's completely botched explanation of how Solar Cookers work (it's behind their paywall), I'm very glad to see your clear and correct explanation.

    If you open up an old cookbook, you'll often see baking directions given in terms of the oven "speed" which is a sort of proxy for temperature, before ovens had thermostats. I bring this up because I think the best way to approach a solar box cooker is as a combination of slow oven and slow cooker.

    Foods that are slowly roasted, braised, baked, and simmered do very well. Most of the moisture of foods is retained. You don't get much browning. A baked chicken will be fall-apart tender but the skin won't crisp. Baked lasagna (without pre-boiling the noodles) does well. Foods that are grilled, fried, or broiled will not turn out well. Hamburgers, for example, turn into a grey soggy mess.

    The energy used to cook food is an interesting question. Boiling 4 quarts of water takes about 1100 BTU--suppose it takes 5000 BTU to cook dinner. Gasoline has roughly 124,000 BTU/gallon; 1 gallon has enough energy to cook for 25 days. Or one cooking of dinner uses the same energy as driving for one mile, if your car gets 25mpg.

  5. Jack says:

    This isn't made by Viking.

  6. cookiecrumb says:

    Cool, cool, cool!
    Er, well... Hot!

  7. Mirko Junge says:

    You should try glas mirrors or at least mirror finished stainless stead instead of the aluminum foil: The reflective characteristics are much better. We acutally did some measurements on the reflective characteristics of aluminum foil coated cardboard which confirmed the poor material properties. The solar cookers used in Tibet are (partial) parabolic dishes which are taped with 3" wide self-sticking aluminum foil, though...

  8. Thanks for all of your comments on this post.

    thm hits one of the energy nails on the head, one that I mentioned in my first post about rice: driving a car uses a heck of a lot of energy. And on the subject of cooking, slow cooked foods are what I've been trying, generally with good success.

    Mirko's idea of a mirror is a good one, so I'll keep my eyes open for a surplus or used mirror. I might also try some side pieces to direct more solar energy into the cooking zone.

  9. Tim says:

    Would you consider posting a link to my solar grill kit page?

    http://www.mealwormfarm.com/hqsolar/hqsolargrillonline.html

    It's a super nice grill, and I have endeavored to make it as simple to build and easy to use as possible. I have been cooking on mine for over a year now, and offer solar cooking tips with my plans. The kit comes with plans, solar cooking guide and reflective material.

    Tim Norton
    Cocoa, FL

    Here's some additional info if you want to look it over.

    The kit was designed around a Channel Master (part#3040865) satellite dish.

    It's 39 1/2" wide X 29 1/2" tall, with a focal length of approximately 18" and an effective aperture of 33"
    Any satellite dish with similar aperture and focal length should work as well.
    At full sun, with a solar insolation of 1,000 watts per square meter, the lens aperture receives about 550 watts of power.
    The lens is coated with a layer of reflective Mylar and a polyurethane protective layer.
    The grill uses standard pots and pans with a black bottom.
    The grill works with any standard fuels, by removing the lens and using the tripod grill alone.
    The grill produces recorded cooking container temperatures of over 450 degrees f and can even smoke foods by focusing light onto a black aluminum drip pan containing wood chips, twigs, leaves, etc. Juices drip into the pan and the flavor is wonderful. Texture is great, as high heat produces a nice crispy brown. By adjusting the grill surface height, the grill can produce temperatures ranging from barely warm to very hot.
    High winds are not a problem as the grill has a wide base and low center of gravity. I live in Florida, and mine stays outside in the yard. It has never tipped over.

  10. Brian White says:

    Please check out the "mechanical mathematician" for making parabolic bowls with simple tools at my website or elsewhere.
    It needs a few people to try it before it will be widely adopted for making parabolic reflectors.
    I used it in september to make a large parabolic reflector from cob and it was a good sucess.
    Thank you Brian white

  11. Ken Carman says:

    When solar ovens are mentioned, most people picture camping . . . or humanitarian efforts in third world countries.

    I am posting this article to tell you what I believe are good reasons why to consider incorporating a solar oven in your conventional modern home.

    Now this will not be for everyone. Certainly you have to have the right conditions, which ideally would include a kitchen with an unshaded south facing wall.

    Yes, it would be less expensive to incorporate this "feature" when building a new home. Still, for those with the ability to do so, I believe there is ample reason to consider knocking a hole in your wall and adding one to an existing home. Here is why.

    The obvious benefits of using a solar oven are 1. that it does not use up non renewable fuel resources and 2. that it does not create or cause to be created any kind of pollution.
    Another benefit that you would come to appreciate more when you use it is the way a solar oven cooks food, slowly and without burning, much like in a crock pot.

    But perhaps the most important benefit that you would gain in addition to these others listed is that the "heat" of your oven can be kept outside the house and insulated from the interior. When you turn on a conventional oven, virtually all of that heat is working directly against the energy you spend to cool your home in the summer. How many times did your mom serve soup and sandwiches because she didn't want to heat up the house with the oven?

    If it isn't obvious, the reason I suggest building the solar oven into your south facing wall is so that accessing the food is as easy as if it were cooking in your conventional oven, and not requiring a trip outside to the back yard.

    In conclusion, any one of these benefits of using a solar oven may not be enough by themselves for considering such a bold move, but collectively, I think they give a substantial argument in support of my idea.

  12. Shirley B says:

    Great advice. I am going to build one too. But i want to make mine out of wood. And i love the one on building a solar oven into the south facing wall of your home.

  13. Maria G. says:

    This is the time to get busy and build one of these. If only to keep the kitchen cool, but mostly to save money. Thanks for posting these instructions!

  14. Ida Know says:

    COOL Totally I am going to build a solar oven with some other people our's will be a lot like your's

  15. Great article you posted which shows just how easy building a solar cooker can be. What kind of temperatures did you manage to obtain?
    The box design is the easiest design to build of all solar cookers, followed by the trough, cone, and the most difficult of them all, the parabolic solar cooker.
    Here is a link to a post I wrote detailing how to build a parabolic solar cooker.
    How to build a parabolic solar cooker
    I'd love to hear how people get on with their build.
    Regards
    SolReka
     
     

  16. C says:

    I just bought a Sun Oven last week, and I love it! I also have a wood stove. When the sun is shining well, the sun oven is great and has gotten up to 350F for me even in Nov. When it's cloudy, it's colder in the house, so I fire up my wood stove and can cook on that. I live in a forested area, so I have a seemingly endless supply of wood from all the trees that fall over. So, in effect, with all the trees falling over in the winter during the stormy rainy season, I can be without power for cooking! Plus, I live in earthquake country, which could potentially knock out power for long periods of time. So now I have two ways to do cook.

    And I want a way to pasteurize water, too, if needed. The sun oven website (http://www.sunoven.com/order.asp) has a water pasteurizer indicator, which I ordered.

    A few things I realized later. In the winter, you need sun from the south. You have to tip the unit using the telescoping leg in the back to capture the sun. I have that, but some people might get full sun only in the summer when the sun moves overhead instead of down to the south. If you want to bake bread, you have to move the unit every 20 mins to keep the temp up, and it must be a clear day with no chance of clouding. Most days are like that for me, except maybe in the winter. You can use the sun oven as a slow cooker, and it will even shed rain. You just put a dish in there in the morning, like maybe some frozen food, and it will be ready by dinner. Although, mine got pretty hot, so I wonder about the "no burn" claim. I browned some eggs pretty good! Although, I think it does take more effort to burn things in it. The Sun Oven not only sheds rain, but it's harder for animals to get into, and the homemade version doesn't appear to have those features.

    I love the Ed Begley show. It's so inspiring. But the wife seems very dissatisfied. I hope it's a joke... :(

    I keep an air purifier by my wood stove. In my house, I have a large HEPA purifier, and an ozone purifier. Of course, that wouldn't work when the power's out, but oh well!