For those who dream of giant solar collectors which can generate
temperatures high enough to melt steel, here is a method for building
your own Solar Furnace of any size you desire.|
These diagrams show a tiny 6 X 6 mirror-array as an example. Also,
I used the silicone-glue "hinges" for this version so my array could be
reprogrammable. If you only want a fixed-focus furnace, you can use
screws to adjust the mirror chips, then glue them permanently into
position. Obviously you can increase the size of the array to large as
you wish. My dream is to build an array the size of a 4ft X 8ft sheet of
plywood. With 1 inch mirror chips, it should create a focused spot which
is 4000 times brighter than normal sunlight.
Let the mirrors harden for 24hrs. When the silicone has cured, screw all your screws into their holes just enough to let them touch the mirrors.
Programming the array as a solar furnace
First use your screwdriver to adjust the single bare mirror ship so it is
no longer tilted by the silicone glob. Bounce some sunlight from this
small mirror towards your target, and move the whole device so the spot of
light hits the target. Now remove the Post-it(tm) note from one other
mirror chip, and use the screwdriver to adjust this other mirror so the
spots of sunlight from the two mirrors combine together. Cover this
newly-adjusted mirror, and uncover the next one. Repeat the adjustment
process, then cover it and go to the next. Do not of course change the
setting of your central "reference" mirror. When each mirror has been
adjusted to combine with the "reference" mirror, peel all the paper from
the mirrors and see what you've accomplished. All the bright spots should
now shine on the same place. Tweak any stragglers to put them in line.
If you wish, go through and glue down each mirror permanently and remove
all the screws.
A 6X6 array is pretty safe for experimenting. It won't set anything on
fire, but in the summer sun it will heat a black garbage bag almost to the
melting point. Once you have the whole process learned, try making a 16 X
16 array (256 mirror chips), or even larger. The above process lets you
slowly "coat" any flat wooden surface with solar-furnace arrays. With
thoughtful planning you could even cover a non-flat surface with a solar
furnace array. If you had enough time, you could build one of ANY size
and temperature. ***WARNING*** if you build a big one, keep it covered
when not in use. If you leave it around the house, moving sunbeams might
unexpectedly bounce off it and start a fire!
WOODBURNEROnce I had a brainstorm, and when I tried it out, it actually worked: send intense light into one end of a fiber-optic cable, and use the other end as a woodburner. I used an expensive glass-fiber cable 1cm in diameter which was about 1/2m long, and I placed one end of the cable at the focus of a 12-in fresnel lens in summer sunlight. The other end of the cable could char a wood surface, but just barely. A bigger solar furnace might have made it impressive. Sign your name as charcoal! Note: if you try this stunt, realize that you probably will damage your fiber optic cable, so don't try it with one that you can't afford to lose! Plastic opto fibers might melt, so use glass if anything.
MIRROR SIZE AND LONG FOCAL LENGTHThe smaller your mirror chips, the smaller and hotter the focus. After all, the hotspot is approximately the size of a single mirror. An array of 1in. mirrors a foot across will make 144 beams, but if you use 2in. mirrors for your 1ft furnace, the hotspot only receives 36 beams.
However, if you use small mirror chips and adjust your solar furnace for a
very long focal length, you'll find that the hotspot grows larger,
fuzzier, and cooler. This occurs because the sun is not a tiny point,
is a disk, and the mirror-facets act as the pinholes of a "pinhole
camera." Small mirror-chips form an image of the sun, rather than an
of the mirror-chip shapes. Each little square of light will develop a
blurry edge, and only the center of each square image will get "full
To compensate for this blurring effect, use larger mirrors. Here's the
choose a focal length which is lots shorter than 120 times the width of a
mirror-chip. (This 120 comes from 1/tan(.5deg), the sun being about 1/2
degree in angular size.) For example, the 1in. mirrors would give a
hotspot if F.L. was longer than a few feet, and at 9.5ft the blurred
swallow the hot center of the hotspot. This "blur" is an image of the
sun. If you want to burn objects from 120ft away, you'll have to build a
furnace using mirrors which are wider than 1ft each.
The size of the sun-disk is the cause.
(If our sun was tiny, but still just as bright, then this blur would be
gone, and you could form its light into an intense parallel beam like a
For a 2-ft solar furnace:
MIRROR ARTRather than "programming" your adjustable array to form a solar furnace, you could instead use it to form any desired pattern of light spots. For example, make it project your initials. First adjust all the mirrors so they are flat. In the sun, the array should reflect a square grid of light spots. Now choose one-half of the mirrors and adjust them so they form one of your initials. Do the same with the other half, for your second initial. Now you have a "magic mirror" which looks like a bunch of little squares; like an almost-flat mirror, but when exposed to sunlight it creates a giant projection of the initials of your name.
Chinese "magic mirror"Some science museums have ancient Chinese "magic mirrors" on display. These look just like a flat mirror, but if you bounce the sun off them, you'll see a picture in the projected sunbeam. The surface of these mirrors contain tiny wobbles. We can program the solar furnace to do something similar. First program your solar furnace so it's perfectly flat; so it projects a square array, and so it acts like a big flat mirror when you use it to look at your own face. Then go out in the sun, aim the beam at a far distant wall, then program the mirror chips to make a tiny pattern (such as a number, or your initials). Now use the furnace as a mirror to look at your own face. It acts like a flat mirror, no? The deflections used to produce the pattern are tiny, so the mirror is ALMOST flat, just like the Magic Mirrors.
MISCDeathMeK says to use old CDROMS (or free ones from junkmail) as zero-cost mirrors. Unfortunately the silicone glue used as a "hinge" just makes the thin mirrors bend, so you'll have to attach them some other way. Or perhaps use silicon glue, but only use an incredibly tiny dot of the stuff.
Date: Sat, 27 Nov 1999 12:31:50 -0700 From: rwduncan