Parabolic reflectors for solar applications are nice stuff, and making your own is a great project in itself. One of the easiest ways we have seen is that of [GREENPOWERSCIENCE], who uses nothing more than a trash can lid, mylar film, and tape. You need a way to make a partial vacuum though.
The idea is so simple that it´s almost like cheating. Cut a circle of mylar slightly larger than the lid, and tape it all around, taking care of stretching the mylar in the process. After you´re done with this, you end up with a nice flat mirror. Here´s where the vacuum is needed to force the film into parabolic shape. Extract the air from a little hole in the lid that was previously drilled, and tape it to prevent the loss of the vacuum. The atmospheric pressure on the mylar film will take care of the job, and magically you get a nearly-parabolic reflector ready for work.
In this other video, you can see the reflector in action burning stuff. One obvious problem with this technique is the loss of the vacuum after some time, about an hour according to the author. Here´s another way to make a more durable mirror also with mylar as the reflecting element, however the quality of the resulting mirror is not as good.
Parabolic reflectors are pretty handy devices. Whether you’re building a microwave antenna or a long-distance directional microphone, suitable commercial dishes aren’t that hard to come by. But a big, shiny mirror for your solar death-ray needs is another matter, which is where this pressure-formed space blanket mirror might come in handy.
Pressure-forming was a great choice for [NighthawkInLight]’s mirror. We’ve covered pressure-formed plastic domes before, and this process is similar. A sheet of PVC with a recessed air fitting forms the platen. The metallized Mylar space blanket, stretched across a wooden frame to pull out the wrinkles and folds, is applied to a circle of epoxy on the platen. After curing, a few puffs with a bicycle tire pump forms the curve and stretches the film even smoother. [NighthawkInLight]’s first attempt at supporting the film with spray foam insulation was a bust, but the later attempt with fiberglass mesh worked great. A little edge support for the resulting shiny taco shell and the mirror was capable of the required degree of destructive potential.
We doubt this process can be optimized enough to produce astronomy-grade mirrors for visible light, but it still has a lot of potential applications. Maybe a fiberglass radio astronomy dish could be pressure-formed directly with a rig like this?
For a university project [Adam Libert] decided to make his very own parabolic hot dog cooker. Now, we must say, this is a project that could probably be cobbled together in a weekend from scraps, but since it was for a lab, [Adam] decided to go all out — complete with a perfect laser cut frame.
The objective of the lab was to design a project that can use solar radiation to accomplish a task, and being partial to hot dogs, the hot dog cooker was a natural choice. He designed the parabolic mirror to focus 1/5th of a square meter of sunlight directly at a hot dog. To do this, he laser cut the frame out of MDF, and using tinfoil, toothpicks, and poster paper, assembled the mirror. The whole thing cost less than $5 (ignoring laser time) and can be setup in a matter of minutes.
He determined the heat output of the cooker to be around 10W at the hot dog, which means he was able to bring the hot dog to 150°F in about 10 minutes — which was surprisingly close to his original calculations, because let’s face it, tin foil is hardly an ideal mirror.
Interested in other solar cookers? Why not cover a satellite dish in foil tape? Or if you want a quicker-cooked-hot-dog, why not plug it directly into the wall?
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.