I need someone to explain this to me.

DIY Fresnel reflector

Just like destroying an ant colony with a magnifying glass, there’s nothing like cooking hot dogs and roasting marshmallows with a nice parabolic reflector. Of course covering an old satellite dish with mylar or aluminum tape doesn’t take much skill, however cool it is. [Uwe] came up with a much more technical means of building a Fresnel reflector that will cook your hot dogs in seconds, but only on sunny days.

[Uwe] channeled a little bit of [Apollonius] when he realized that a flat cardboard ring with a section removed could be joined together into a conic section. The resulting section looks just like one concentric ring in a Fresnel reflector. [Uwe] wrote a small program in Visual Basic to calculate the necessary diameter and angle of his conic sections.

A bit of cardboard was cut out and pieced together with some very reflective aluminum tape. The resulting Fresnel reflector concentrates 117 times the normal solar radiation onto a small point. It’s more than enough to burn holes in construction paper, but we’ll be using a microwave for our lunch today.

Comments

  1. Hirudinea says:

    Very clever, and with a little tweeking of the program you could use this technique on other frequencies, like a cardboard an tape WIFI booster. (A really good one.)

  2. Laura Harris says:

    We used to take the lids from tin cans and cut them into a flat spiral, then twist the track just right to focus the sun well enough to start kindling. This is a nice modern variation!

  3. medix says:

    “The resulting Fresnel reflector concentrates 117 times the normal solar radiation onto a small point.”

    How about some actual numbers about (final) focused spot size?

    • Panikos says:

      go read the primary source, its one click away, a news article doesn’t have to contain all the information.

      if its not in the primary source then they couldn’t have included it. either way your comment is about as pointless as the one i am currently writing

      • medix says:

        Read the source. No information is given there in terms of focusing efficiency either. A metric such as ‘burns paper’, while ‘neat’, is pretty much useless.

        Other than that, good design on the lens.

        … and don’t tell me that this guy “did this because he could” since that is the same reason I asked this question: because I’m curious and want to know! Getting defensive is not the best (nor should it be the first) approach.

    • Laura Harris says:

      Well, the reflector has a certain size- it is intercepting an amount of sunlight equal to its area. So, if the concentration factor is 117x, the spot is 117th the size of the reflector.

      • monster says:

        i think he’s curious about the focal size of the beam, is it a 117x golf ball-sized sphere or a softball? that could be the difference between cooking a hotdog or cutting a cold one in half

    • DRoseDARs says:

      “Building the Fresnel Reflector

      In my example there are six rings that provide a 59.9% reflective coverage. If built exactly, the light at the focus will be concentrated 117 times (area of the rings as seen from directly above divided by 1/2 the surface area of the sphere).”

      It’s after the section with all the maths.

  4. MrTaco says:

    Wonder if you could rig up some kind of automated adjustment system that widens or narrows the reflectors as needed to change the focus point…

  5. bob says:

    maybe this concept can make use of a 3d printer like reprap, to print 3d fresnel lenses, what for i do not know, maybe lots of these 3.d. fresnels, coated with aluminium foil could be used as solar concentrators for heating, could be i for these could be used to make an oogoo mould to make multiple frenels on the cheap, who knows?

  6. Adric says:

    I keep wondering why a wound spiral cut sheet of metal/shiny plastic… would not work well. ive been able to find one patent.

    http://www.google.com/patents?vid=USPAT4350412

    one article about a guy with one who lit a torch at burning man. and a article in popular science back in 1981.

  7. Johan G says:

    This could, in a slightly larger size, be neat for a solar cooking oven.

  8. Dale Whitworth says:

    There was a college kid at Georgia Tech back in the 70′s who built a solar concentrator from a flat sheet that worked very well. He was shown on TV burning a 2×4 with it! The focus could be made in front of the material (silvered aluminum in his case, I think) or behind it – like the x-ray focusing done on the space telescopes. Still a great idea!

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