Survivalism & Self Sufficiency Topics > Food Preps

Fresnel Lens for Solar Oven

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Truik:
So, If I were to use a fresnel lens in a solar oven design, it would seem as though focusing it on the food or the food container itself would cause uneven cooking and overheated/dried-out food.

Perhaps, if I got a spot-focused fresnel lens and focused it on a large chunk of rock or a solid iron object in the bottom of the oven, the radiant heat from it could cook the food, like a roaster.

Any thoughts on how to effectively do this?

creuzerm:

--- Quote from: Truik on December 06, 2009, 07:35:18 AM ---So, If I were to use a fresnel lens in a solar oven design, it would seem as though focusing it on the food or the food container itself would cause uneven cooking and overheated/dried-out food.

--- End quote ---

Don't focus it to a pin-point focus point. Leave it out of focus to a basketball size or however big your pot is. You can do this by focusing it in front of, or behind your pot.

If you focus it in front of your pot, if you forget to remove the fresnel lens, and reach it to check the pot, you could put your arm in just the spot where the focus is and have a NASTY burn.

If you focus it behind the pot, if the sun moves enough to slide the focus point off the pot (or off the behind the pot technically), it could focus on your solar oven and melt/burn/start it on fire.

d0j0w0:
There is a video link on the forum that covers making a solar still with a fresnel lens.  I think a piece of clear glass or plexi glass maybe the way to go with the solar oven.  I have a small lens in my wallet 2X3 or so, it makes a great fire starter back-up.

Nicodemus:

--- Quote from: Ann on November 12, 2009, 09:08:33 PM ---They did have rather large ones, which I was very curious about.  However, it would seem VERY irresponsible to leave them outside...anywhere.  But they would be AWESOME for boiling water in larger quantities.  Esp since they are not expensive at all.

--- End quote ---

This really isn't much of a problem. All you have to do is make sure the focal length is far enough away from anything that will burn so as not to start an accidental fire.

Also, what a lot of people don't think about is that the lens needs to be directed toward the sun to focus enough light to burn something quickly at the focal point. When I did some experiments with a fresnel lens water distiller, the lens needed to be moved fairly often to track the sun and direct enough intense sunlight to the boiler. Ambient, reflected and refracted light didn't produce much heat at the focal point and beyond. For clarity, while I wasn't focusing light at the boiler, the lens was left perpendicular to the ground. I didn't specifically test the minimal amount of light or the minimal angle necessary to start a fire at the focal point and beyond.

I am by no means a scientist, these are just observations I made while experimenting. In other words, don't blame me if you accidentally burn down your house by leaving all sorts of magnifying optics laying round.  ;D

Nicodemus:
On a similar note, a long parabolic reflective trough with a focal point directed at a black tube filled with water might prove to be an easier method to heat water for purification. If the trough is parallel to the path of the sun it might collect enough light all day long without any need to refocus for light intensity. I don't know if this would produce enough constant heat to purify water, but this is how some solar hot water heaters work.

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