A solar cooker still! I have been thinking about creating one of these to distill water... but you have thought about the opposite end of the same idea, gathering salt! Very cool!
I actually have an annual project for my high school students to see who can build the most efficient solar cooker. Here are some suggestions for you:
Paint the inside of your cooker matte black. The shiny stainless steel is actually reflecting most sunlight back out, and it is this energy that is converted into heat to raise the temperature. Black will absorb the most light and convert much more into heat. If you are getting 150F now while actually losing most of your light, you can probably come close to boiling your water if it is painted black. High heat spray paint, the kind designed for wood stoves, would be perfect. If you want to avoid paints, black paper or cardstock would be the next best thing, but getting black paint on the metal itself would be leagues more efficient at conducting that heat into the metal and raising temperatures higher.
Get rid of the rocks. Although they hold heat, they are not increasing the temperature to promote evaporation. All they are good for would be holding the temperature higher for slightly longer once the sun goes down, acting as a thermal battery. The rocks, being a lighter color, will also block and reflect light from hitting your newly painted black floor, limiting the amount of light turned into heat.
Definitely use glass containers instead of metal, you are on the right track. Otherwise your salt will be contaminated with various oxidized metals.
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