Tuesday, March 19, 2013

the best DIY sun oven design

Last week I got to teach a class at the Chinati Foundation in Marfa, Texas on DIY sun oven building for kids. Since Julian and I have happily used the fancy "Sun Oven Brand" factory model for many years (we have two and use them all day long, nearly every sunny day of the year - please see Alyce's Solar Kitchen for recipes and lots more info), we haven't had occasion to try out other designs, although we've always been curious to know how well they work. The opportunity to teach a class gave us the perfect excuse.

We got most of our ideas from solarcooking.org. We based our test models on the Fun-Panel, the Reflective Open Box Cooker, the Minimum Solar Box Cooker, and the Solar Funnel Cooker.

We were quite surprised by the results! The Reflective Open Box Cooker - the two very simple models on the left-hand side - got hotter faster than any of the other designs, including the more complex Fun-Panel (the one in the back). The Fun-Panel worked well, but it is much more difficult to build, requiring exact measuring, scoring, and cutting. The Reflective Box is nothing more than a 90 degree angle covered in foil with a foil-covered surface, and a single adjustable foil-covered flap.

Above we are demonstrating the oven's ability to boil water in a black enamel cup covered with a glass plate. Prior to adding the water we'd measured the temp in the cup with an oven thermometer: 250F.

Above we are using a slightly smaller oven of the same design to cook cornbread in a cast-iron pan with a glass plate on top. It took a long time for the cast iron to heat up - the bread required nearly two hours to cook, but cook it did, and it was delicious!

I also used this model to cook applesauce in a larger black enamel pot with black lid sealed inside a turkey roasting bag (available at grocery stores). The turkey roasting bag held the heat in well and could be fine for use in emergencies, but for everyday use we prefer to use only metal and glass implements if possible (black pot with glass lid, or black pan or jar inside a clear glass pot...always the black to attract heat, covered by glass as an insulator).

For the kids' class we used hi-temp black paint to color enough squat mason jars with metal lids to go around - each student could cook a single muffin inside his or her own tiny "pot". again, the black jars must be placed inside a clear glass pot, underneath an upside-down glass bowl, or inside a plastic bottle or bag to work efficiently.

The kids (aged 9 - 13) were easily able to grasp the solar oven concept, and were invited to create their own experiments inspired, if they wished, by any of the designs we had on hand. Several of the students chose to build something based on the simple "Open Box" model, and were delighted with their results.

On the day of the workshop the wind was light and the sun very intense - the fancy "Sun Oven Brand" models got up to 400F easily, and the "Open Box" and "Fun Panel" designs we built prior to the class got up to 350F.

The following crazy experiment also worked surprisingly well...it is made of two picture frames acquired from the thrift store. A slightly larger frame has two panels with foil under the glass. A slightly smaller frame with three panels (again, foil under the glass) rests on the larger frame, one panel of which can be adjusted to focus the light into the glass pot with a painted mason jar inside:

I wrote the following text to be included in a hand-out which contained some drawings of basic designs. Please feel free to take/use this text in the development of your own workshop! Hope you'll enjoy building and sharing DIY sun ovens...for mere pennies compared to the cost of a commercial model!

COSMIC COOKING! solar ovens 101

Concentrating and focusing sunlight is the basic premise of solar cooking. You only need to understand a few basic principles in order to make your own simple oven that uses absolutely no fossil fuel, produces no emissions, and costs nothing to run! 

Solar cooking works best in places like here in the high desert, where there are few cloudy days and lots of direct, intense sunlight. Sun ovens work great even in winter – the outside temperature doesn’t make a difference, since you are directing the light into an insulated container. Aside from clouds, the only thing that can make using a sun oven tricky is too much WIND, since most solar cookers have reflective flaps that can blow around and make the oven unstable.

Before you begin, stand or sit outside for a few minutes and observe the weather conditions carefully. Where is the sun in the sky? Depending on the season, the sun will be higher (summer) or lower (winter) in the sky. Where is the sun in its daily trajectory? Are there any clouds? Wind?

You want to pick a spot to do your cooking that is flat and protected from the wind. A few clouds are okay, but if there’s too much wind, it might be best to wait ‘til a calmer day to cook in your sun oven.

If you’ve ever gotten into a car that’s been parked in the sun, you’ll know that darker colors absorb heat, and lighter colors reflect. The basic elements of a sun oven are: a black container inside a bigger glass (or plastic) container, and shiny surfaces that direct the heat into the box. All of the ovens we will be experimenting with are variations on this theme. Some cookers are simpler than others – most can be made with materials you probably have around the house already!

Fun, simple things to cook in a sun oven: 

Nachos (chips & cheese) – 15 minutes (quick and fun to watch the cheese melt!)
Applesauce (chopped apples with a little cinnamon) – 40 minutes
Cornbread (if you don’t have your own recipe, follow the instructions on a bag of Bob’s Red Mill mix) – depending on the size of your muffin cups, 1 – 2 hrs

(Lots more recipes at Alyce's Solar Kitchen!)