This home built solar cooker is a parabolic solar reflector with a spit rod at the focus. A simple aligner post at the bottom front shows clearly when it is perfectly aimed at the sun. Aiming seems not that critical and the post makes it easy. We checked and moved very slightly only once during 1/2 hour of cooking.
Today was the first time I was able to try it. For my test, I cooked one hot dog to perfection. The test was done again the next day with four hot dogs. I would like to do a non-meat test.
Two things weren't perfect: I'd started cooking in the late afternoon, at about 4pm as the sun was already part way down the sky. Also, the reflective surface is not a mirror finish but shiny aluminum roofing flashing as it comes off the roll which might give I think about 1/2 performance, yet a useful test in itself. In spite of these two factors, I had a steaming hot dog (160F) in about a half an hour. I was hoping to be able to grill somewhat (brown the surface) but that didn't happen, it did not get hot enough. I will use a mirror film shortly and start cooking earlier in the day.
I can heat small objects which skewer onto the thin stainless rod which slides into a slot along the focal line. I used a round stainless rod but a square, flat, oval or triangular cross section might work better since the payload (what's on the spit/skewer) has a tendency to slide around on the round road as it cooks and shrinks.
The characteristics of the surface of what I put on the spit will determine how it heats. I thought that this might be useful for marshmallows but their bright white color makes it almost certain that they will reflect away most of the heat focused perhaps up to some point at least?
The aiming is manual. The small disk with a perpendicular dowel makes a sensitive indicator of the sun's position. If the shadow is small and falls almost directly near the dowel, the aim is good and the sun's concentration achieved is about 18 times.
click any picture to enlarge
It is hottest closest to the spit. It would not be effective, for example to thread the hot dogs onto the spit through their radial center as opposed to lengthwise, through their axial center even though it might seem to increase the capacity of the bbq (more hot dogs cooked at once?).
With the aluminum roll flashing as the reflector, I estimate about 200 watts of cooking power available. This could possibly reach 400 watts with a mirror reflector.
Still, I am encouraged to see that this works. If you only had access to roof flashing and a bit of plywood, some straight angle metal (aluminum, steel, plastic, fiberglass etc), a workable spit bbq can be achieved at very low cost. The cost of the materials as I did it (not including the tripod) is about Cdn$10-$25. Using flexible mirror in place of the roof flashing might double cost but improve performance, probably near double (will cook faster and hotter).
This is an example of what can be done with the gen2 design in a simplified version. The gen2 book is available here in either downloadable or paper version.