h a l f b a k e r y
Assume a hemispherical cow.
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A solar cooker (solar oven) made of an overturned umbrella with the material inside being shiny-mirror silver color, probably mylar is good enough. This umbrella is set upside down creating a parabolla "sattelite dish" shape, which concentrates the solar rays, to heat up the center.
A folding "pot
holder" would be located in correct position to put pot on, which would balance the whole thing, and another folding contraption would hold a thin nylon cover, completely enclosing the pot, letting the light and radiation in, while keeping the heat in. Another addition may be an additional "external tripod" for changing the position of the upturned umbrella as needed to focus the rays with the changing position of the sun.
This would fold away just like an umbrella, and, when turned back upwards (without the pot of food of course), can double to serve either as a sun-shade or as an umbrella for the rain.
How to make one:
This is not collapsible. Aluminum struts are added to the handle to form a tripod. [Amos Kito, Jun 10 2008]
A bunch of oven designs:
[Amos Kito, Jun 10 2008]
It's all umbrellas & mirrors...
Prior art? [Canuck, Jun 10 2008]
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||I thought this sounded familiar. (see link)
||[+] so why are umbrellas "naturally parabolic"(according to one of the links). Note that all the links are for doing something half-assed to an umbrella instead of making one from reflective mylar.
||Presumably a solar oven doesn't actually have to be accurately parabolic.
||Why bother bringing the sunlight to a sharp focus? All you need to do is get most of it to hit your central cooking vessel.
||Any deviation from a true parabola will blur the focus, but since your pot is larger than a point, this might not matter.
||An umbrella only needs to be "close enough", not truly parabolic.