Author Topic: Fresnel Lens for Solar Oven  (Read 54590 times)

Offline Ann

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Fresnel Lens for Solar Oven
« on: November 11, 2009, 10:36:57 AM »
Not sure if this is the right place to post...mods, please assist.

Came across the Fresnel Lens on ebay while searching for "solar oven" (just to look, I know I can build my own).

Has anybody actually used one of these to either 1. start a fire, or 2. boil water?

I assume by the descriptions I have seen that they are a type of magnifying tool that comes in a wide variety of sizes.

Just checking to see IF anybody has any experience with them and what their opinions are.  I'm THINKING about getting a couple of small ones as a backup for fire starting OR keeping a larger one around the house to assist with boiling water without fire.

Offline Roswell

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Re: Fresnel Lens for Solar Oven
« Reply #1 on: November 11, 2009, 10:47:29 AM »
moved

sarahluker

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Re: Fresnel Lens for Solar Oven
« Reply #2 on: November 11, 2009, 10:53:21 AM »
I saw a youtube video about them.  They are very powerful and I wouldn't leave it out where just anyone could get to it...oops there goes the house!   Having said that I would like to get one for the purposes you stated.

Offline idelphic

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Re: Fresnel Lens for Solar Oven
« Reply #3 on: November 11, 2009, 11:42:29 AM »
A full sheet (8.5 x 11) lens might be good to boil water with,..  I would think it would burn food easy.

Offline Ann

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Re: Fresnel Lens for Solar Oven
« Reply #4 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.

Offline creuzerm

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Re: Fresnel Lens for Solar Oven
« Reply #5 on: November 18, 2009, 04:35:07 PM »
My office has an old rear projection TV that recently died. When it gets tossed, I am going to raid the dumpster for the Fresnel lens in it. 2x3 feet? lots of solar power concentrated down.

Solar oven, a lens for a solar panel so it gets more light, etc. Lots of uses.

I wonder if you curve it a little if they will focus into a horizontal band instead of a point. You don't want them to actually come to focus onto a pot, not a big one at least, as it can get hot enough to melt metal. But a out of focus bright band that covers the pot set up so that as the sun moves, the band will keep on the pot.  A back reflector to reflect the light that missed the pot back onto it, you could probably get a pot quite hot for a good period of time.

Offline Steve W

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Re: Fresnel Lens for Solar Oven
« Reply #6 on: November 19, 2009, 04:46:22 AM »
Acquired a nice size Spot version (28 in x 40 in) to experiment with.

These lenses have several different configurations, varying focus & focal lengths, and varying materials.

Will be fun to experiment.  We have a small sterling engine to run up wit this lens once I construct stands for all the components.


Offline HelenWheels

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Re: Fresnel Lens for Solar Oven
« Reply #7 on: November 19, 2009, 09:55:51 AM »
http://www.youtube.com/user/GREENPOWERSCIENCE

This guy does a LOT of things with Fresnel lenses. There are a couple of youtube videos where they're using the Fresnel aid in cooking.

Offline Gadget

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Re: Fresnel Lens for Solar Oven
« Reply #8 on: December 04, 2009, 07:38:56 PM »
On a similar note, I once investigated a fire at a house where a magnifying makeup mirror caught sunlight from a skylight in a bathroom and burned a hole in the vanity countertop. We had a time trying to figure out what happened until I saw the skylight.

So you might try a makeup mirror also for boiling water. If you try it on food it will likely burn a spot in it.

Offline Truik

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Re: Fresnel Lens for Solar Oven
« Reply #9 on: December 04, 2009, 08:04:51 PM »
The 49-inch circular spot fresnel lens will melt 1/4-inch brown glass in 15 seconds and melt concrete in 30 seconds just by concentrating the sunlight.

That is powerful.


Offline Truik

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Re: Fresnel Lens for Solar Oven
« Reply #10 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.

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?


Offline creuzerm

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Re: Fresnel Lens for Solar Oven
« Reply #11 on: December 08, 2009, 06:26:28 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.

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.

Offline d0j0w0

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Re: Fresnel Lens for Solar Oven
« Reply #12 on: November 19, 2010, 09:21:23 PM »
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.

Offline Nicodemus

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Re: Fresnel Lens for Solar Oven
« Reply #13 on: November 20, 2010, 12:54:56 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.

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
« Last Edit: November 20, 2010, 01:11:52 PM by Nicodemus »

Offline Nicodemus

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Re: Fresnel Lens for Solar Oven
« Reply #14 on: November 20, 2010, 01:27:30 PM »
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.

Offline JAG2955

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Re: Fresnel Lens for Solar Oven
« Reply #15 on: November 22, 2010, 03:52:10 PM »
My dad had a 12" square one from when he was a science teacher that I would play with a lot.  I do believe that I tried to heat a pot of water with it once, without much luck.  It would melt plastic army men like a champ, though.  Wear your welder's goggles, the focal point is extremely bright.

Offline mangyhyena

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Re: Fresnel Lens for Solar Oven
« Reply #16 on: November 29, 2010, 02:35:08 PM »
Fresnel lenses have several uses for self sufficiency.  At a web site called Green Power Science, large Fresnel lenses are sold.  They can be bought in a tilting frame, much like a full length mirror frame.  If you want one for free, you can look on craigslist for old big screen TV's people are giving away.  Those TV's have a Fresnel lens in them.  If you're willing to haul the TV away, you can get your big lens for free and frame it yourself.

You can purchase one here. http://greenpowerscience.com/SHOPFRESNELHOME.html

These lenses are extremely hands-on, meaning you can't just set it and forget it; you have to be there to operate it to ensure that nothing catches fire and to move it as the sun crosses the sky.  But, the sun is free and if you're willing to spend the time using one, there are a lot of things you can accomplish with it.

As mentioned already, you can cook with them.  I watched a video of a woman who used one to cook scrambled eggs in a matter of seconds, as well as another where a pasta dinner was cooked using only a Fresnel lens.  They are excellent fire starters, of course, capable of lighting a 2X4 in a matter of seconds.  They can easily distill water quickly by focusing the beam on a heat exchanger.
There are YouTube videos of people doing these things.  Green Power Science has quite a few of them up for viewing and I'd suggest viewing those.

A few things that I haven't seen done yet, but that should be possible, are:
Using a Fresnel lens as the heat source in a wood or coal gasifier.
Using a Fresnel lens as the heat source for a TEG, (thermal electric generator)
Using a Fresnel lens as the heat source to flash steam water, the resulting steam used to power a micro turbine to produce electricity or do other useful work.
Using a Fresnel lens as the heat source for distilling fuel alcohol.
Using a Fresnel lens to heat rocks or clay pots full of sand during the day, then bringing those hot rocks or clay pots in at night for a radiant heat source.


I think survivalists and preppers need to really look into the uses of a Fresnel lens for emergency purposes.  Who knows, with enough information and experimentation, maybe we can figure out how to use Fresnel lenses in a more set-it-and-forget-it way.  Seems like using a solar tracker is a first step toward that.  And perhaps a metal shield that sits much closer to the lens than the focal point, stopping the beam from focusing too hot, might make these things safer, in terms of accidental fires starting.

Offline Nicodemus

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Re: Fresnel Lens for Solar Oven
« Reply #17 on: December 01, 2010, 05:59:11 PM »
While not a fresnel lens, but a solar collector, here is the power of two square meters of sunlight when focused.

Take that you stupid rock!  :D

http://www.youtube.com/watch?v=z0_nuvPKIi8#

Offline mangyhyena

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Re: Fresnel Lens for Solar Oven
« Reply #18 on: March 23, 2011, 12:39:39 AM »
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.

You're right, for heating a lot of water, a parabolic trough is the way to go.  Length of trough, how reflective the reflective coating is, and flow rate of water will determine how hot the water gets.  A parabolic trough gets plenty hot for water purification. A parabolic trough makes a focal line that runs all the way along a collector tube, rather than focusing down to a little pinpoint.  That focal line running along every inch of the collector is the reason a trough can heat a large volum of water.

The beam from a Fresnel lens can be bounced off a mirror. I saw a video where the beam was reflected back up and onto the underside of a pan, cooking eggs really fast.  It can also go through CLEAR glass, so maybe it could jump start or enhance the performance of a standard solar oven.  If the glass isn't perfectly clear the beam will shatter it, so be careful.

Offline Falling_blue

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Re: Fresnel Lens for Solar Oven
« Reply #19 on: May 21, 2011, 05:12:27 PM »
Just got a free non-working rear projection tv off of cragslist, built a frame and stand for the fresnel lens and am currently cooking cornbread in a dutch oven.  Got a temp ~300 in the dutch oven before I started so it should turn out well.  I plan on finding a mirror to bounce the spot up to a grill for pans but couldn't wait to cook something with it.

Offline mangyhyena

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Re: Fresnel Lens for Solar Oven
« Reply #20 on: May 31, 2011, 03:43:27 PM »
Just got a free non-working rear projection tv off of cragslist, built a frame and stand for the fresnel lens and am currently cooking cornbread in a dutch oven.  Got a temp ~300 in the dutch oven before I started so it should turn out well.  I plan on finding a mirror to bounce the spot up to a grill for pans but couldn't wait to cook something with it.

I think that's great.  Way to be innovative.  Any chance you could post pictures or a vid?

Offline KYdoomer

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Re: Fresnel Lens for Solar Oven
« Reply #21 on: May 31, 2011, 08:14:20 PM »
So what I read is the best way to use a fresnel for cooking is to heat a cook surface up.  I'd love to try this but what cook surface could stand up.

My parabolic cooker will light a 2x4 in about 10 seconds.  Strong enough to boil water and cheap at only 30 bucks.

J

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Re: Fresnel Lens for Solar Oven
« Reply #22 on: June 02, 2011, 10:23:12 PM »
I think that's great.  Way to be innovative.  Any chance you could post pictures or a vid?

Haven't built the frame for the mirror and grill.  Still finding out the best way to make it adjustable to set optimal angles during the day.  But here is pics of the progress.   Oh and now I have three free lenses and also took the trapezoid mirrors out and will be building another solar box oven.

Top one is just a test run.  Got 325 degrees in the dutch oven.
Bottom one was cooking cornbread.





Offline mangyhyena

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Re: Fresnel Lens for Solar Oven
« Reply #23 on: June 03, 2011, 10:45:19 AM »
That is awesome.  Looks like a great backup cooking method.

Offline Nicodemus

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Re: Fresnel Lens for Solar Oven
« Reply #24 on: June 03, 2011, 03:32:03 PM »
That's a great looking set up, falling_blue!

Offline d0j0w0

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Re: Fresnel Lens for Solar Oven
« Reply #25 on: June 04, 2011, 07:20:54 PM »
Really nice set up.  Do you need to baby sit it, or do you set it and forget it?

Offline Hurricane

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Re: Fresnel Lens for Solar Oven
« Reply #26 on: June 04, 2011, 07:37:27 PM »
For those who live along the coasts, you can see serious glass Fresnels at your local lighthouse, or museums dedicated to them.

Offline Falling_blue

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Re: Fresnel Lens for Solar Oven
« Reply #27 on: June 04, 2011, 10:24:47 PM »
Really nice set up.  Do you need to baby sit it, or do you set it and forget it?

Still have to babysit, just because that pesky sun moves.  But I could leave it for about 30 min without worry.

Offline NickJ

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Re: Fresnel Lens for Solar Oven
« Reply #28 on: December 16, 2011, 01:49:56 PM »
Damn! I trashed a rear projection TV about a year ago. I kept the clear sheet of plastic but that funny looking one did not seem so useful.

My county dump has a section for electronics and I have seen some rear projection TVs there. That could be a source if they would let you take it off. If I remember correctly it was just part of the front screen and did not take much to remove.

NickJ

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Re: Fresnel Lens for Solar Oven
« Reply #29 on: December 16, 2011, 03:04:18 PM »
Damn! I trashed a rear projection TV about a year ago. I kept the clear sheet of plastic but that funny looking one did not seem so useful.

My county dump has a section for electronics and I have seen some rear projection TVs there. That could be a source if they would let you take it off. If I remember correctly it was just part of the front screen and did not take much to remove.

NickJ

watch your local craigslist , i see them there quite often