Adventures in solar cooking

Yesterday in Sunday school, two groups of kids started making solar ovens. While they were working, we had solar s’mores cooking in the solar oven I made on Saturday. However, it took a long time for the solar s’mores to cook. First problem: the morning clouds didn’t begin to clear until halfway through Sunday school. Second problem: thin clouds persisted most of the morning, and even the thinnest of clouds caused the temperature to drop at least ten degrees inside the oven. We started cooking the s’mores at about 10:00, and they weren’t really done until just before noon — after most of the kids had already gone home.

The clouds finally cleared away completely, and I left the solar oven outside my office for several hours in the early afternoon. The inside temperature rose to over 200 degrees Fahrenheit (200 degrees is as high as the meat thermometer goes), with outside air temperature in the high 70s. I heated up a mug of water, to over 170 degrees, and made a nice cup of tea. While I was making tea, Fred Z., from the Green Sanctuary Committee, stopped by and suggested trying cast iron cookware in the oven — it’s dark and absorbs heat well, plus it provides a good thermal mass to even out cooking temperature.

So this morning I dug out a small cast iron frying pan, and decided to try cooking a fried egg in the solar oven. The air temperature was about 65 degrees, but in spite of clear skies I couldn’t get the inside temperature over 190 degrees — which suggests I need better insulation in the oven. I cooked a fried egg, over easy:


It took about twenty minutes, and was really more of an egg baked in butter than it was a fried egg (it tasted good, though); obviously there is a lot more to be done to improve the efficiency of the oven.

Solar oven prototype

Tomorrow, the middle school ecojustice class in Sunday school is going to make solar ovens. So of course I had to make a prototype:


I started with a basic design made out of carboard boxes, a design that is sometimes called the “Minimum Solar Box Cooker.” But instead of just nesting one smaller box inside another box, I took the smaller box, cut out the ends, and turned it 45 degrees:


While this reduces the amount of cooking space inside the oven, it also reduces the amount of air that has to be heated. And then, too, it’s easy to run a couple of dowels through the inner box to make a support for a cooking pot.

In preliminary tests, the oven worked reasonably well. I set the oven out at 2:45 p.m., stuck a meat thermometer in one end of the oven, and within twenty minutes, the thermometer was reading between 190 and 200 degrees F. (the thermometer only goes up to 200). At about 3:10, I put in a cup of water in a glass container. By 3:40, the water temperature was 155 degrees F., and the glass container was more like 190 degrees F. (Air temperature is 75 degrees F. this afternoon.)

Tomorrow comes the real test: we’ll set the oven out at the beginning of Sunday school and see how quickly we can make solar s’mores.

Update, one year on: This solar oven prototype proved to be only marginally effective. After using it fairly extensively, it has one big problem: when you open the lid, much of the hot air escapes; there is very little thermal mass, aside from the heated air. At the very least, I need to provide a significant thermal mass (preferably black in color, to better absorb heat). In addition, it would make sense to place the door low on one side of the oven, to minimize the loss of heated air.