Webb Mealy's Solar Cooker Project, February-March 2008*
*Note: all the materials on this webpage excepting the photo immediately below are copyright 2008 J. Webb Mealy.
It all started when I saw this photo of a solar cooker being used by a Bhutanese refugee in Nepal (see http://solarcooking.org/newsletters/scrnov06.htm):
I knew that the Nepalese and others in the region desperately need non-hydrocarbon-based ways of cooking, because they are running out of wood. But when I saw this cooker which I am sure works great, I thought, It's got a number of drawbacks: it's heavy, bulky, expensive, non-portable, and non-windworthy. I set out to design an alternative that was light, relatively compact and stowable, inexpensive, portable and windworthy. What you see below is the results of my quest.
This photo shows the methodology. I've bought secondhand some stiff professional grade foam core board (1/2" thick, with a hard plastic laminated top), and cut it into two 3.5 foot by 3.5 foot rectangles. I then hot-glued 56 half-tennis balls on each one at 5 1/4" intervals. That is, I laid them out in two 7 x 8 arrays, one on each section. Then I bought, on the internet, a remnant roll of ShineRite adhesive-back vinyl mirror material, cut it up into 5" x 5" squares, and applied it to 112 half-CD cases. CD cases were selected because I was looking for a material that was cheap, abundant, stiff enough for the application, and capable of flexing (rather than breaking or permanently deforming) if nudged.
I used a laser pointer angled at an arbitrary "typical angle of the sun", to model the light from the sun. (The sun is more or less a point light source infinitely far away, so all of its light comes at exactly the same angle.) In my case, living in the San Francisco Bay Area, I selected 50 degrees from horizontal, 40 degrees from vertical, to orient my mirrors. See SunCalcs spreadsheet for the methodology.
I selected a focus point about 32 inches above the floor and slightly outside the footprint of my platforms. That way I could have an independent stand for my cooking surface, and food spills wouldn't go onto my mirrors.
All the materials for this project except for the mirror material and the hot glue can be acquired secondhand and it can be built for about $50. It would make a great project for a science class.
I had my two 1.25m x 1.25m platforms linked together and mounted on an adjustible telescope mount so that you could make the whole array of 112 mirrors follow the sun by adjusting a knob. See the darkish spot four mirrors up from the missing mirror on the bottom right--that is a knob that can turn the whole platform by fractions of a degree. I don't think it's worth the trouble and expense, though. You can just put things under your platform to set the angle right, and then just move it around a little to follow the sun. The telescope mounting is not needed.
My cookware was selected for having heavy bottoms, because I correctly predicted that my mirror arrays would create some random hotspots, so my pot needed to even out the temperature by having a thick bottom. Also, cookware tends to be shiny on the bottom and often silvery. Thanks to a generous donation of a sample from Flame Control, LLC (http://www.flamecontrol.com), I was able to coat the bottom of my cookware with high-temperature-resistant flat black coating "450C", which can withstand up to 450 degrees Celsius--in other words, 842 degrees Farenheit. That's a temperature far beyond the point at which wood spontaneously bursts into flames.
As my cooking surface I used a sturdy African drum stand with a little grill that I found; I would recommend using a cart with some kind of horizontal grill mountable off the side of it with velcro or some other adjustable system. That way you can bring your food, utensils, cookware, plates, and so on out to where you're cooking.You're going to have to use wire wrapping or welding in any situation where the heat of the sun is going to hit your fastenings--otherwise your fixings may melt or degrade in the heat, with dangerous results. Depending on the angle of the sun, you're going to have to be able to move your cooking surface up or down in a range of two feet or so, to keep it in the focus as the sun goes up and down through the day and through the seasons.
Below are three demos, the last of which explains how each of the mirrors was oriented and fixed onto its platform. If you have any questions about the project or how you might be able to do something similar, feel free to email me at webb-at-selftest-dot-net. By the way, the first demo exaggerates. The water in the pot is on a gentle boil, not a rolling boil. As I say in one of these films, this solar stove probably generates the equivalent of a medium burner, depending on the conditions.