Search | Up to SCIENCE HOBBYIST
INFINITELY LARGE SOLAR FURNACE
(c)1996 William Beaty
For those who dream of giant solar collectors which can generate
temperatures high enough to melt steel, below 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" 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 as 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.
The trick is simple: Make or buy a large number of mirror chips of glass
or plastic, each one about 2cm or 3cm in size. 12in glass mirror tiles
can be had from the larger hardware stores, and these can be scored and
broken into 2cm chips with a glass cutting tool. Or, plexiglas
mirror-plastic can be cut up with a power saw. The smaller your mirrors,
the hotter the focus.
Obtain a sheet of plywood and a big bag of screws which are about 1/4in
longer than the thickness of the wood (if the wood is 1/2in thick, buy
screws that are 3/4in. or slightly longer.) I used 6-32 machine screws,
but most any screw will probably work. If your mirrors are larger than 1
in., use longer screws. If your furnace is going to be large, use thicker
plywood to prevent warping. Lay out and drill the holes as shown,
choosing a drill bit which is only slightly smaller than the screw
diameter. The screws should turn easily with a screw driver. If your
furnace is going to be large, consider making four separate regions where
the mirrors in each region tilt towards the center. If not done this way,
then the toothpick spacer mentioned below might not make the glue-globule
tall enough, and the aim of some mirrors might be unable to hit the focus.
(Or, just start aligning your array using a focus that is appropriately
To make "hinges", cut a number of round wooden toothpicks into 1/8in
segments, place a small dollop of silicone sealant at the corner position
of a mirror, lay the toothpick in the silicone as a spacer, then lay down
the mirror on the correct spot on the wood. The silicone lightly attaches
mirror to the wood, while the toothpick tilts the mirror and gives it a
greater adjustment range. The spacer also keeps the mirror from smashing
the silicone into a too-thin layer. Carefully perform this operation for
all your mirrors. The toothpick tilts the mirror one way, and when you
later add the screws, the screws will lift the mirror as desired.
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
Cut up some Post-it(tm) notes, and stick a square of paper to each mirror
to block the light. LEAVE ONE MIRROR NEAR THE CENTER OF THE ARRAY
UNCOVERED. This one mirror will act as a reference for adjusting all the
others. (As the position of the sun changes, the position of this spot
will also change, so you shouldn't need to move the whole device to track
the sun while adjusting mirrors.) Take your mirror array out in the sun,
and position it a few
feet from a convenient target. The distance between the mirrors and this
target will become the focal length of the 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, then romove
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, the moving sun might
unexpectedly illuminate it and start a fire!
Once 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 LENGTH
The 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, it
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 image
of the mirror-chip shapes. Each little square of light will develop a
blurry edge, and only the center of the square image will have "full sun."
To compensate for this blurring effect, use larger mirrors. The rule:
the focal length must be 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 give a blurry
hotspot if F.L. is longer than a few feet, and at 9.5ft the blurred region
swallows the hot center of the hotspot. This "blur" is an image of the
sun. If you want to burn objects from 100ft away, you'll have to build a
furnace with much bigger mirrors. The size of the sun-disk is the cause.
(If our sun was tiny but just as bright, you could form its light into an
intense parallel beam like a laser!)
|MIRROR CHIP||MAX F.L. ||BRIGHTNESS|
|1 in.||9 ft.||576|
|4 in.||38 ft.||36|
Rather 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.
DeathMeK@aol.com 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
Date: Sat, 27 Nov 1999 12:31:50 -0700
Subject: Solar Furnace
My name is Robert Duncan and I live in the Phoenix, Arizona metropolitan
area. While searching for solar energy-related subjects, I found your
website through Infoseek. After reading your 1996 article on the
Infinitely Large Solar Furnace, I became interested in your technique and
constructed my own. The finished product is a 48 inch by 48 inch solar
furnace with 1440 mirrors. While this is hardly infinite, I was able to
transform an aluminum beverage can into smoke.
The materials and method of construction differed only slightly from the
information posted on your website. I used drywall screws instead of
machine screws because they were cheap. I pre-adjusted all the drywall
screws for a thirty inch focal length prior to gluing the mirrors in
place. I also omitted the toothpick spacers since my final adjusments
would be very small. The programming was done by placing an unfrosted
lightbulb at the focal point, allowing each mirror to reflect the light
onto a grid pattern drawn on a piece of plywood placed along the focal
plane. Each mirror was adjusted until its reflection was aligned with it's
corresponding grid sqaure on the focal plane.
The result was approximately 1000 watts of solar influx concentrated on an
area the size of a silver dollar. Wood ignited with an audible "pop" the
instant it entered the focal point. Toast burns instantly. Aluminum melts
after 15 seconds. Half inch copper tubing deforms under it's own weight
after 20 seconds. Steel glows red in about the same time. The temptation
for onlookers to stick thier hand in front of the furnace is strong, so I
keep a cardboard box handy to demostrate what will happen to them if they
I appreciate the information and I wanted to pass along this success
story. All this was accomplished with less than a $100 investment in
materials. My next step is to build a heliostat and attenuator to create
an apparatus similar to the ones at the National Solar Thermal Test
facility in New Mexico.
Want books? Try searching
Help Support the Science Club,
use the above form to order books.
(We make a few $$ on any books ordered via these links.)
Created and maintained by Bill Beaty. Mail me at: firstname.lastname@example.org.
If you are using Lynx, type "c" to email.