Solar Oven Design: insulation

A solar oven needs a clear window on top, reflectors to increase the sunlight and therefore the energy going into the oven, a black surface to turn that light into heat, and insulation, so that the heat is retained, rather than lost. But a couple of problems arise on the subject of insulation. First, the window that lets in light is difficult to insulate. Second, the insulation on the sides and bottom has to stand up to heat.

On the first point, you are making a trade-off between insulation (R-value) and transmittance, which is how much light makes it through the window. Take a look at this page of twin-wall greenhouse glazing. They tell you the transmittance and R-value (always choose the clear version of the glazing).

transmittance / R-value
82 / 1.59
81 / 1.67
80 / 1.79
76 / 2.50
60 / 3.57

You might think that the higher R-value would be worth the decrease in light entering the oven. So I thought until I ran the calculations, taking into account heat gain, heat loss, the size of the reflectors and the window, and of course R-value and transmittance. With reflectors, the amount of light entering (transmittance) becomes much more important than the R-value of the window. Without reflectors, the top three pairs of values in the chart above are about equal; for small solar ovens, it’s a wash. But a solar oven without reflectors is not going to get very hot. (I’ll be testing my solar oven design with and without reflectors later on.) Large reflectors can easily triple the amount of energy going into the oven.

So this is what you want in your solar oven design: windows with high transmittance, even if the R-value is low, and large reflectors.

Other than greenhouse glazing with a known R-value and transmittance, what type of window is best? The thinnest glazing will have the highest transmittance. For a solar oven with relatively low-temperatures (which is what this set of posts is about), plastic will be sufficient. A very hot solar oven would need specialized glass to stand up to the heat and resist breaking.

As for R-value, you simply need a double layer of that thin plastic glazing with an air space in the middle. A space of about one-half to one inch seems to be sufficient. I would guesstimate the values of this type of window to be in the range of 80 or so transmittance and between 1.5 and 2.0 R-value. I will do a test of home-made glazing versus commercial glazing later to get a better feel for this. (The greenhouse glazing should arrive by the end of this month.)

How much insulation does a solar oven need, on the sides and bottom, in order to be effective? Not as much as you might think. The window has to be fairly lightly insulated, in order to let in as much energy as possible, so insulating the sides and bottom are of limited benefit. As far as I can tell at this point, looking at the numbers on paper, the R-value only needs to be in the 8 to 10 range. You get a big jump in efficiency from an oven with an R-value of 1 or 2 (essentially not insulated except for inner and outer walls) up to 3 or 4. But you reach a point of diminishing returns at about 8 to 10 R-value.

Insulation materials for a solar oven are problematic. Do NOT use fiberglass insulation; it contains “Urea, polymer with formaldehyde and phenol” that may produce toxic fumes if heated. I’ll be testing some recycled denim/cotton insulation (UltraTouch by Bonded Logic), which contains no fiberglass or formaldehyde. However, that material only safe up to about 250 degrees Fahrenheit. Since the oven will be used to heat water (hopefully to about 165 degrees F) or to distill water well below the boiling point, it should be safe.

I will also be testing cardboard as insulation. The R-value is said (by a couple of online sources) to be about 2.25 per inch. At four inches, plus the inner and outer walls of the oven, that should put us in the 8 to 10 R-value. Real world R-values are generally less than the ideal values for the material itself. There can be air leaks and thermal bridging in the structure as a whole that reduces the actual value. But we should be able to get inside of 8 to 10 with 4 inches of insulation, even if it is cardboard.

Note: if you decide to build a solar oven, you do so at your own risk. My articles on this topic only describe my own efforts and thoughts. I am not asserting that any of these materials or building designs are recommended or safe for you to use.

More on this topic as the design and building of the solar oven unfolds.

– Thoreau

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