Forums: Index > Help desk > Why doesn't my solar cooker work?

I built this box cooker some months ago and it has never performed well. It barely warms water. This is winter in Northern California (approx 38 degrees N) but since Brit did her solar cooking at 58 degrees N, with snow on the ground, I figure I should be able to do something. Here are some of my theories:

Not enough sunlight gets into the box?

Beths box cooker-1

I noticed that the only box cooker Brit built was the Reflective Open Box cooker, which catches a lot of light even when the sun is low on the horizon.

I tried tipping my box a bit, but it didn't help significantly. I can't tip it a lot without tipping the floor as well, to keep the pot level, and there isn't much vertical room to spare.

I have the box oriented with the short side facing the sun, otherwise half the interior would be in shade. Would it work better if the long side faced the sun so the reflector could catch more light?

Yes, the long side should face the sun, and you should remove one of the reflectors. You can see from the think shadows cast by your reflectors on the ground behind the cooker that almost no sun is hitting the reflectors. Tom Sponheim 17:35, 8 March 2007 (UTC)
Beths box cooker-4
This one change made a big difference. I put the box out in bright sunshine about 10 AM and by 12:30 it was 235 degrees F in the box! This is about 100 degrees hotter than it ever got before. Very exciting. (That was with 4 cups of water in the pot. I got tired of putting out food and having to finish cooking it on the stove so I switched to water for my tests.) By midday there was a thin high cloud cover and the temp started dropping in the box so I gave up for the day. But I've painted the bottom plate a very black black and will reconfigure the cardboard insulation before the next test. Thanks for the tips, both of you. Beth Ogilvie 00:21, 9 March 2007 (UTC)
I succeeded in cooking rice in my cooker this afternoon! Very exciting. The picture shows the new orientation. It seems a little weird having the reflector angled down so far, but that seemed to make it brightest in the box. Beth Ogilvie 00:35, 14 March 2007 (UTC)

Not enough insulation in the box?

Beths box cooker-2

I kind of followed the directions for the Minimum Solar Box Cooker but using a wooden box someone gave me instead of nested cardboard boxes. Is wood not good enough for this? Developing an intuitive feel for the dynamics of solar cooking says our guesses about insulating properties aren't always right...

The sides looked pretty thin, so I supplemented each side with 3 pieces of cardboard wrapped in foil. The cover is glass.

The added cardboard will help a lot. Tom Sponheim 17:36, 8 March 2007 (UTC)
Air spaces between layers can be an important source of thermal resistance. Wood is not a good insulating material although it is much better than metals. Its thermal resistance is good enough that small wood blocks may be used to separate layers, however. Adding a second layer of glazing, plastic film or glass, will help. The gap between the pot and the walls looks small. A 25 mm gap is desirable, I think. The foil that is wrapped around cardboard layers will conduct heat across the layer. It is better to face only the inner surface with foil. It stops air leaks too. Walter Siegmund 18:52, 8 March 2007 (UTC)
Beths box cooker-3

Here is the new box interior.

The bottom is very black, and the foil only faces the inside.

The day I tried 2 panes of glass, it got up to 250°F in the box. I used two thicknesses of cardboard to separate them. What would be optimum? Thanks. Beth Ogilvie 00:35, 14 March 2007 (UTC)

I find recommendations of 10 to 16 mm separation with Google. The argument is that the resistance increases with thickness until convection develops within the gap. Convection in small gaps is suppressed by the viscosity of the gas. Google does not find graphs of relevant equations of Incropera and DeWitt. I'll add them in a few days. In particular, the case of horizonal surfaces is of particular interest for box cookers.--Walter Siegmund 18:14, 14 March 2007 (UTC)

Horizontal cavity

Heat transport coefficient for the space between glazing

The convection heat transport coefficient (h) allows you to easily calculate the heat loss through a wall or window. It is complicated to calculate, but easy to use. Simply multiply h by the area of the wall or window and by the temperature difference between the inside and outside of the oven to get the loss in watts. For a heat transport coefficient of 1.7 W/m2°C, typical of double oven windows and walls spaced 10 mm, a wall or window with an area 0.4 m2 (24 x 26 inch) and an oven temperature 50°C above ambient, the loss is 34 W per surface for a total of 200 W for the oven. With reflectors, the effective window area could be 0.5 m2 so that 500 W would enter the oven for several hours per day. Some light is reflected by the windows, some excapes and 200 W of heat is lost through the walls, but perhaps 200 W would be available to heat food. That should bring a liter of water to a boil in 30 minutes.

I've plotted the convection heat transport coefficient for a horizontal rectangular cavity with the ambient temperature = 30°C (86°F) and heat flow is upward. This applies to the double-glazed window in the lid of a box cooker. The heat transfer coefficient decreases as the space between the glazing increases until the onset of convection whereupon the coefficient begins to increase. The optimal spacing is about 10 mm but may be less for ovens intended to operate above 100°C. The minimum coefficient is about 3 W/m2°C. The inside surface has a coefficient of about 5 W/m2°C and the outside, about 19 W/m2°C (depending on the wind speed). The total h is about 1.7 W/m2°C.

Vertical cavity

Heat transport coefficient for the space between layers in the oven wall

In gaps between layers in the oven walls, the behavior is similar except the onset of convection has a much weaker effect. However, little is gained from a spacing larger than 10 mm even at low oven temperatures. The total h will be about the same, 1.7 W/m2°C, as for the window. However, it can be increased by adding layers of paper or cardboard separated by 6 to 8 mm, or by adding insulating materials like closed cell foam.

In the oven floor, convection is suppressed by the temperature gradient and will not occur. Consequently, you can make the space between floor layers 25 to 30 mm so that the heat loss through the floor is much smaller than other losses. For a space of 25 mm, h is only 1.1 W/m2°C and the total h is 0.9 W/m2°C, half that of the other surfaces. --Walter Siegmund 06:22, 17 March 2007 (UTC)

The day of my last experiment, it was sunny all day but there was a cool breeze.

Wind may force air through small openings, e.g., between the cover and the box. This can be an important heat loss mechanism. Silicone sealant (caulk) may be helpful, but use petroleum jelly as a mold release on one surface if you want to be able to separate them later. Walter Siegmund 18:52, 8 March 2007 (UTC)

Bottom of box not black enough?

The pot sits on an aluminum sheet blackened not very successfully with wheatpaste and ground up charcoal briquets. I was going to use black paint, but the man in the paint department at the local hardware store, who said he had a certificate in "coatings", advised against it unless it said "safe for toys" (and of course they didn't have anything like that). He said the ones that say "non toxic when dry" are probably not non toxic at elevated temperatures, and he didn't trust the ones designed for barbecues either.

The paints that say "non-toxic when dry" and the barbecue paints all give off no smell when heated to cooking temperatures. Most people "cure" the paint by heating the oven for a time with no food in it to drive off any remaining fumes. Tom Sponheim 17:39, 8 March 2007 (UTC)

Also keep in mind that your food will not be in actual contact with the paint on the bottom plate, and in fact most suncooked foods are cooked covered (notable exceptions being cookies and some breads). I have not noticed any paint fumes in any of my ovens, though I run them empty a bit first, as Tom suggested. If there were fumes that were not easy to detect, the fact that the food is all in jars or pots with close fitting lids would keep the ineteraction minimal. If you're really worried about it but want to try the paint (which sure seems to do the job, especially the cheaper "non-toxic when dry" stuff), cure it empty and then stick to cooking in canning jars for the first few times until you feel confident that there's not a fume problem.SharonID 06:56, 4 June 2007 (UTC)

Feel free to put your comments up under the appropriate section, or add a new section if I missed a whole topic. Thanks! --Beth Ogilvie 00:53, 8 March 2007 (UTC)

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