Building a Solar Cooker

At a recent gathering of family and friends, the topic of conversation started with the current hot conditions. From there, it naturally progressed to solar energy and cooling. From there to the possibility of a solar cooker.

In places like Florida, where part of the family lives, the idea has natural appeal. By cooking inside the home you pay for the energy twice... first, the energy to heat the oven and second, the energy to remove the heat from your house with the air conditioner. And, besides heating by electric resistance, as most ovens do, is an inefficient application.

This got me thinking it might be worth discussing. Building a solar oven is relatively easy to do. The website, http://solarcooking.org/plans/ has a great selection of plans for building solar ovens. I particularly like the "Minimum" Solar Box cooker. Obviously, the appeal of the design is their use of simple materials like cardboard and foil.

But, for more practical, everyday use, while keeping it simple, a few changes might be worthwhile. Also, it might be worthwhile to dig into the theory so we can understand the process and obtain more customized results.

First, practicality... if the cardboard box is left out during a typical Florida afternoon thundershower you would soon have a pile of ruined cardboard. So, I'd suggest using a sheet of Polyisocyanurate covered on both sides with aluminum foil. This material is relatively weather resistant and rigid. It can be obtained at pretty much any building supply store for about $10 per 4'X8' sheet and has good insulating properties, R value of approximately 4 per half inch.

That leads us to some theory...the authors don't say what temperatures can be attained with the simple ovens, but with a little understanding of the theory, it is possible to estimate temperatures and see how adjustments can effect it.

For any space, the equation "Heat in = Heat out" represents the equilibrium, or steady condition. This allows us to estimate the temperatures which can be obtained and to make adjustments to obtain the desired results.

"Heat in" is a function of the solar rays entering the box. It is generally accepted that for most subtropical locations the radiant heat of the sun is somewhat above 2oo BTU/sq ft/hr.

"Heat out" is a function of the insulation around the space, represented by the equation

Heat out = UxAxdT/R
Where:
U= heat transfer coefficent. This depends of the surfaces and the fluid on each side, but generally for thin, smooth surfaces with air on both sides is about 1.5 BTU/sq ft/degree F.
A= area in sq ft
dT= difference in temperature, or (T inside - T outside)
R = Resistance to heat transfer of insulation, generally referred as to R value.

So, let's build an oven and estimate the temperature which can be obtained. Assume the box is a 1 foot cube, built of 1/2" Polyisocyanurate board, with the inside painted flat black, so absorption is close to 100%. Let's have a 1 foot clear film on the top to allow entrance of the sun. And let's have a somewhat oversize reflector on the back side to reflect more sunlight into the clear film area. Assume we can get 1.5 sq ft of sunlight into the box. We would probably want to raise the pot off the floor of the oven with a canning ring or other pedestal so it is heated from the bottom as well. I picked this general design because the discussion was around a slow cooking "Crock Pot Type" cooking style where food could be put on in the morning and ready to eat for dinner with minimum attendance.

Then,

Heat in = 200 x 1.5 = 300 BTU/hr

Heat out is equal to the heat escaping through the 5 insulated walls with an R value of about 4, plus the heat escaping through the clear film, with an R-1. Therefore, heat out is represented by:

Heat out = (1.5 x 5 x dT/4) + (1.5 x 1 x dT/1) BTU/hr = (1.9 x dT + 1.5 x dT) BTU/hr = 3.4 x dt BTU/hr.

Therefore:

300 BTU/hr = 3.4 x dT BTU/hr

or:

dT = 300/3.4 = 88 degrees temperature difference

So, this oven would obtain a temperature difference with the outside air of about 88 degrees. Assuming 90 degrees outside, the inside temperature would be about 178 degrees.

Disappointing, you say? Well, use the above theory to build a better oven! Since we have plenty of material left over from our sheet of foam board, let's make a slightly larger box to put the first box inside of, with 1.5" of wadded newspaper in the space between the boxes. Overall the walls and floor now have an R value of 10. Also, I like the "Simple" box cooker idea of using a turkey cooking bag so you have double film over the opening, doubling the insulating value of the film. Also, let's design a reflector which increases the area of sun into the box to 2 sq ft. Now,

2 x 200 = (1.5 x 5 x dT/10) + (1.5 x 1 x dT/2) = .75 dT + .75 dt = 1.5 dt

dt = 400/1.5 = 267 degree difference

Again, assuming outside temperature of 90 degrees, your oven temperature would approach 357 degrees. Oops, better start thinking about the melting and ignition temperatures of the foam or some type of insulating liner!

Keep in mind, these are the equilibrium temperatures, which the empty oven could be expected to approach pretty quickly assuming a tight enclosure and good sun. Any reflective pot would decrease the heat captured, and the mass of pot and food, plus the energy absorption of moisture would substantially slow the approach to these temperatures.

So, there you have it. For less than $15, you can cook outside for free, rather than endure the double whammy to your utility bill of cooking in the kitchen in the summer.