Ambjörn Naeve discovered that if you put 2 parabolic trough concentrators in series at the right distance appart, you can get a burning point of light. Professor Naeve has given me permission to use his images here to show his device.
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He showed that by bending 2 pieces of reflective sheet material you can mimic a parabolic trough. It is much easier to bend 2 pieces than the bend and cut and bend and cut many times to make a parabolic dish. Here is the proof
Proof, click to enlarge
Here is Naeve using the device to melt copper!
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Here is Naeves diagram of a Solar wheelbarrow
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I believe that Ambjörn Naeve made a great breakthrough back in the 1970"s when he discovered and worked on these concepts. http://kmr.nada.kth.se/wiki/Main/PointFocus is his story of the discovery and how he tried to spread the knowelege.
For me, I think the naeve cross holds the potential for people to design a combined trough solar cooker with a 2 hour cook time (or a 3 hour cook time)
(Which would be excellent in its own right) and then "reverse engineer" a dish from it. This combined trough or dish with a 2 or 3 hour UNATTENDED cooktime would be a wonderful breakthrough for solar cooks all over the world. Brian White