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Posted: 3/13/2011 10:31:07 AM EDT
and creating a solar oven in my back yard.

your turn.

Link Posted: 3/13/2011 10:48:55 AM EDT
They were used in lighthouses for many years, for dissipating light in the special way a lighthouse does, over great distances.

Turn it around and it can be focused light in a small point almost directly behind it. I've had the idea for building a solar water heater for camping and general goofing off purposes but haven't been able to find big enough lenses locally.

A quick internet search reveals that your best bet is ordering multiple lenses online and building an oven/water heater yourself.
Link Posted: 3/13/2011 1:10:51 PM EDT

check this guy out: http://www.youtube.com/user/GREENPOWERSCIENCE

He's a bit goofy sometimes, but has some good stuff. He believes that parabolic mirrors are better for cooking.

Link Posted: 3/13/2011 2:49:28 PM EDT
If you have connections in education or school system/goverment auctions, salvage them from old overhead projectors on the cheap. New technology has been replacing them for years and you might be able to find some for next to nothing.
Link Posted: 3/13/2011 3:01:40 PM EDT
worlds slowest double tap
Link Posted: 3/13/2011 6:17:13 PM EDT
I'll be making a thread as soon as I get the chance with some details on solar ovens. Had a lot of cloud cover on weekends the last few weeks.
Link Posted: 3/14/2011 9:42:18 AM EDT
A Fresnel lens is basically a series of prisms arranged in a way to bend light back on itself in order to concentrate and increase actual light out put.  Known best for their use in light houses (where my experience comes from), they were required prior to the invention of electricity and more powerful light sources.  When first built, most light houses in the US had a rather small whale oil lamp at it's core.  Without the Fresnel lens it would do little more than light a room.  With the Fresnel lens, it could cast a beam that could be seen over the horizon.

As to using one in a solar oven, I would assume the same rule applies.  You would be concentrating sunlight/heat into a focused beam in order to heat up your oven.  The original light house lenses that I have worked with were very complex, made of many hand cut crystals, and cost a small fortune even when they were first made in previous centuries (They are so valuable now that one of my jobs was removing them from automated light houses and shipping them back to the USCG Museum, replacing them with new mass produced light/lens set ups) .  Seems an expensive way to make an oven.  Of course now they use plastic and molded glass to achieve a similar effect, but they do not hold a candle to the originals.  You could go with a simple magnifier lens and get a similar concentration of light and heat.  Maintaining focus might be problematic with the movement of the sun across the sky (I know, it is the Earth that is actually moving).  

It would be fun to experiment with if you have the time.

Link Posted: 3/14/2011 10:00:35 AM EDT
Chef, if you are looking for solar cooking, and not just with a Fresnel lens, then check out http://www.applied-solar.info/solar-cooking/why-solar-cookers-are-important-for-the-environment-human-health-and-safety/....the parabolic cooker, to be specific. May be what Hcook was talking about.
Link Posted: 3/14/2011 10:04:53 AM EDT
All lenses bend light.

What most people think of "regular" lenses come in two flavors;
  1. Double-convex: this is what you see in most magnifying glasses, both sides of the lens curve outwards
  2. Plano-convex:   Essentially this is half of a double convex lens.   One side curves out, the other is flat.

A Fresnel lens is simply a Plano-Convex lens that has been segmented and shortened.  

It performs much like a PC lens, but without the bulk and weight,
Link Posted: 3/14/2011 10:43:50 AM EDT
I understand lenses
I understand the intent of a solar oven and how it physically works

seen the pictures, and wondering...
HOW DO I BUILD ONE? recommendations?

anyone in DFW know where I can see one hands on?

seems simple enough
lens? lenses?
a metal box?
high polish aluminum/mylar sheeting

Link Posted: 3/14/2011 12:33:13 PM EDT
I'll try to find the link later, but I remember seeing on e a guy built using an old c-band dish and a shitpot of polished license plates mounted on it

Had to put the focal point like 8 feet in the air to keep people and pets out of the focal zone. It would get stupid-hot! I'll try to link later if I can find it.
Link Posted: 3/14/2011 1:17:22 PM EDT
a single "letter size" Fresnel lens will give you a heated area roughly between the size of an Altiod's tin and a match box.

focusing multiple lenses will probably give you multiple hot spots. probably not so good for cooking.

sooo a large reflector is probably a better bet for cooking a meal.

you could make a reflector by covering cardboard for temporary use, or more permanent material for long term use with aluminum foil and pitting a black box at the focal point. would do great for heating water or cooking.
Link Posted: 3/14/2011 6:32:53 PM EDT
I understand lenses
I understand the intent of a solar oven and how it physically works

seen the pictures, and wondering...
HOW DO I BUILD ONE? recommendations?

anyone in DFW know where I can see one hands on?

seems simple enough
lens? lenses?
a metal box?
high polish aluminum/mylar sheeting

how big are these things?.....could you fit it on a vacuum form table.....pull down a clear piece of plastic......then fill with clear acrylic resin......one of my projects here
Link Posted: 3/14/2011 7:37:50 PM EDT
Fresnel lenses can be obtained in some old rear projection TVs. Pawn shops and yard sales might be a good place to find them.

If you want to mess around with this stuff, you can probably pick one up with an hour of extraction work and a sore back hauling the TV from whomever gives it to you.

I tend to agree that focusing by reflection will probably get you a larger heating footprint.
Link Posted: 3/14/2011 8:29:33 PM EDT

there ya go chef - all ya need to know and where to buy about fresnel lens
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