Let s go green! build a solar powered parabolic

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Let's go green! Build a Solar Powered Parabolic Cooker!
by Weissensteinburg on July 17, 2007
Table of Contents
intro: Let's go green! Build a Solar Powered Parabolic Cooker! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
step 1: How it works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
step 2: What you'll need . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
File Downloads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
step 3: Cut some Shapes! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
step 4: A bit of assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
step 5: Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
step 6: Cooking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
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Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
/>intro: Let's go green! Build a Solar Powered Parabolic Cooker!
Let's face it, energy is expensive. Gas, electricity, whatever. So why pay to cook your food? The challenge I gave myself, was to cook a hot dog, without spending any
money at all. No electricity, no new materials, nothing. And, there are no negative side effects on the environment, resulting from my hot dog.
What I came up with was the parabola. So by using the properties of parabolas, we're going to cook a hot dog. Essentially, we're using math to cook hot dogs :D
I believe I've just proven that I'm a nerd? Oh well, I'm in good company here.
step 1: How it works
"A parabola can also be defined as locus of points in a plane which are equidistant from a given point (the focus) and a given line (the directrix)."
The way it applies to us, is that the light that hits the parabola, will reflect back to one intersection point. That intersection point is called the point of focus. By placing the
hot dog in the point of focus, all of the sun's rays that hit anywhere in the cooker, will reflect onto the hot dog thus cooking it.
Click here to see how the light reflects
By the way the shape of the entire cooker is a parabolic trough.
step 2: What you'll need
After rummaging around my house, this is what I used to build a cooker:
Poster board
Foam board

Tin foil
Plastic wrap (optional)
two blocks (can be anything shoes, wood rolls of tape)
Skewer
Various cutting supplies
Packing tape, and scotch tape.
The attached stencil
A nice sunny day.
/>File Downloads
Stencil.zip (16 KB)
[NOTE: When saving, if you see .tmp as the file ext, rename it to 'Stencil.zip']
step 3: Cut some Shapes!
First, assemble the stencil. There are instructions in the zip file on doing this, but it's pretty self explanatory. Then, trace it on to the foam board twice. Cut those out.
Now you need to cut a rectangle from the poster board. The short side of a standard piece of poster board is just the right length to wrap around the parabola. It should
be about a foot wide.
Next, wrap one side of each cut out piece with the shiny side of tin foil. Make sure not to wrinkle the tin foil. Some other suggestions for what to wrap with (provided by
other members) are:
Plastic mirror
The material that chip bags are made of (use the inside, reflective side)
Draw a dot onto the foam board through the stencil where the point of focus is then poke a hole through that dot.
Image Notes
1. Stecncil
2. Parabolic foam board
/>Image Notes
1. The short side of a standard piece of poster board fits nearly perfectly.
Image Notes
1. This is already the right length
2. Cut about a foot into the long side.
Image Notes
1. Point of focus.

2. All three of these need to be wrapped with reflective material.
Image Notes
1. My original you want it to be as smooth as possible this one was done badly.
step 4: A bit of assembly
Assemble the parts so it resembles a trough with the foil on the inside. Tape along edges to keep everything together.
/>step 5: Alignment
The two blocks are for keeping the cooker at the right angle. You put one on either side of the curve, and adjust everything to be just right. What is just right? That's what
we're about to find out.
Tape two squares of scrap foam board onto the cooker. Using something sharp, poke a hole through the first one, and about halfway into the other. (while they're stacked
on top of each other) Tape the one with a hole all the way through it to the top, middle of the cooker, and the other to the bottom middle they should be right above one
another. Now, when you put the cooker out in the sun, there will be a shadow, with a dot on the ground. You just need to align the dot with the hole on the other square,
and you will know that it's all good to cook!
Note: It's helpful to tape a small piece of board over the whole on this side. You need the hole for the skewer to rest on, but you don't want it to stick out and obstruct your
alignment dot. Not necessary, but helpful.
Image Notes
1. It was kind of shady when I took this the dot will be more prominent in prime
cooking weather.
Image Notes
1. Aligned
/>step 6: Cooking
Woo, it's time to cook your hot dog! Stick the dog on the skewer, and put it through the holes in the cooker. I like to cover the top of it with plastic wrap, so no bugs bother
it. Now, use the blocks to align the cooker, and leave it to cook. Every few minutes, go out and re align it (the sun does move). Once the hot dog is finished, take it out,
and enjoy!
Yes, it does work, and no, it doesn't take an hour to cook. Sure, it takes a bit longer than a grill, but I would suggest that when you try it, after a few minutes, touch the hot
dog. You'll see just how hot it gets
WARNING: Wear sun glasses when you go to realign it, or you won't be able to see well for a minute or so once you go back inside. This thing gets bright
Side note - If you wanted to, you could make a parabolic trough frame out of knex, and then line the inside with something reflective.
Let me know if there are any problems.
Image Notes
1. Scrumdidilyumptious

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Comments
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emdarcher says:
Oct 17, 2008. 11:37 AM REPLY
thanks, I used this for a school science prodject and I was able to make one wide enough to cook two hot dogs at once and it cooked them fully in 27 min.
and 27 seconds
cstrike pwner says:
Sep 23, 2007. 3:21 PM REPLY
black paper heats up to 311F while alumunum foil heats up to 110F
Michael Fletcher says:
Aug 31, 2008. 3:06 PM REPLY
Wow you just gave me a great idea! I recently purchased a pack of paper window shades from Lowes. "RediShade". The pack contained 3 peel and stick
window shades for about $6.00 and the shades are simply multi-folded. Mine are 36"x72" but I think they came in other sizes and can be sized with
scissors. I'm going to try spray painting them black and see if they work as effective passive heat collectors. I also thought to use spray on glue and stick
some alum.mylar on one to see if it helped for summer cooling. If I use one of each in the same window I can simply keep the correct one lowered for
each season and the other pulled up and out of site. Do you know if the heat causes the paper to degrade very fast?
cstrike pwner says:
Sep 20, 2008. 12:04 PM REPLY
I have no clue what you just said. There is just to much information there.
Weissensteinburg says:

Sep 23, 2007. 3:26 PM REPLY
It's not about the heat, the aluminum foil reflects the light onto the hot dog.
cstrike pwner says:
Sep 23, 2007. 3:33 PM REPLY
heat or not alumunum foil works good but if u cover it in glad wrapive cooked eggs in black paper but alumunum can't i use mirrors now!!!!!!!!!!!!! but it
dosen't matter they both work.
Weissensteinburg says:
Sep 23, 2007. 3:37 PM REPLY
From what I can tell, you're describing a different cooker one in which the parabola wouldn't do much for.
cstrike pwner says:
Sep 23, 2007. 4:59 PM REPLY
Jes that's a point but u can mod it, u use curved mirors and angle them so there beam hase a concave point magnifing it cooking a hotdog in
minets .iv done it before point it at the foil it'll cook in minets.
Weissensteinburg says:
Sep 23, 2007. 5:01 PM REPLY
No offense, but your grammar is impeding me from understanding exactly what you mean.
cstrike pwner says:
Sep 24, 2007. 3:59 PM REPLY
none taken, grammar slowes me down. Now i don't know if im say'n any thing right. Which annoys me badly
/> I_am_Canadian says:
Aug 22, 2008. 12:03 PM REPLY
I toasted marshmallows in a large mixing bowl lined with tin foil. Fun!
roughrider says:
Feb 4, 2008. 1:08 PM REPLY
Real men just chomp down on the dog without it being cooked. Only sissies would use a cooker. :(
leebryuk says:
Jun 8, 2008. 12:00 AM REPLY
Hate to break it to you, but hot dogs are already cooked from the factory.
kippergirl says:
May 31, 2008. 11:11 AM REPLY

i have decided to use this project for my annual science fair! I hope it works!( its a great idea)
alli-gurl455 says:
May 30, 2008. 10:15 AM REPLY
does it really work??
Weissensteinburg says:
May 30, 2008. 12:07 PM REPLY
Yeah very well, actually.
dreamin0fyoohx says:
May 19, 2008. 10:28 PM REPLY
how do you find the foci of the parabola used in this thing?
Weissensteinburg says:
May 20, 2008. 5:31 PM REPLY
It's marked on the stencil.
dansolis says:
Apr 20, 2008. 9:42 PM REPLY
How do I find out the equation for the parabola in this experiment?
Weissensteinburg says:
Jul 18, 2007. 1:21 PM REPLY
Thanks for pointing that out guys. I'll either amend this instructable to say so, or make it a real parabola. (or both).
Of using other materials: As I said in the intro, a main goal of this was to only use stuff I had laying around. But I will add your ideas as suggestions for what
you could use instead.
Thanks again!
CameronSS says:
Jul 18, 2007. 8:34 PM REPLY
Have you already made a parabolic one? if not, two simple options: 1) use a yardstick to draw a graph on the sides and graph a parabola, or 2) shine a
flashlight sideways and trace the beam. I love the power of the sun. We once had (possibly still do) two reflectors from searchlights. It you aimed it at the
sun, a piece of paper instantly burst into flame at the focus. I also singed clean through an oven mitt with it.
Weissensteinburg says:
Jul 19, 2007. 2:06 PM REPLY
I have made one to make it easy, I'm attaching a parabola stencil to the ible, so you can just print it out, and cut.

CameronSS says:
Jul 19, 2007. 3:44 PM REPLY
Fancy!
Weissensteinburg says:
Jul 19, 2007. 5:27 PM REPLY
Ok, the instructable has been fixed. Let me know what you think.
/> CameronSS says:
Jul 19, 2007. 6:28 PM REPLY
Nice stencils! Does it cook better with a parabola?
Also, in Step 1, you link to this image. I attached a modified version that better illustrates the reflection. If you want to use it, feel free.
nagutron says:
Dec 13, 2007. 11:36 AM REPLY
Nice. I just added a similar image to the Wikipedia entry on Parabolas. It actually looks like we worked off the same base SVG!
Here's my caption on that image: "Parabolic curve showing arbitrary line (L), focus (F), and vertex (V). L is an arbitrary line
perpendicular to the axis of symmetry and opposite the focus of the parabola from the vertex (i.e. farther from V than from F.) The
length of any line F - Pn - Qn is the same. This is similar to saying that a parabola is an ellipse, but with one focal point at infinity."
RiddleOfSphinx says:
Apr 2, 2008. 4:27 PM REPLY
I can just see "roughrider" 's head exploding now lmao.
gaiatechnician says:
Aug 26, 2007. 6:36 PM REPLY
You do not need any math to construct a parabola!
If you have a vertical post going through point f and a horisontal swing arm attached on top of the post reaching out to point P3 and a
slidable vertical arm coming down from Q3, all you do is attach a piece of string from f to p3! (through a hole3 in the bottom of vertical
slider q3) . Keep the string taut and your slider pottom will touch every point on the parabola as you pull it in from Q3 towards the pole
at the centre!
Thats how I made my parabolic solar cooker. (Currently on utube).
The problem with math is it is scary stuff.
You can make a parabolic cooker (a big one) with no math at all!
Do not be afraid!

dawnmathmom says:
Jun 5, 2008. 7:59 PM REPLY
actually gaia, what you just described is a semicircle, not a parabolic section
by definition a parabola is the collection of points which are equidistant from the focus AND the directrix
gaiatechnician says:
Jun 6, 2008. 12:14 PM REPLY
Nope, you are not correct. I made 2 and they were definitely not semicircles. they were parabolic dishes. I have videos about
the "mechanical mathematian" on utube.
I redesigned a known method for making parabolas on paper (with string and a setsquare) so that it could be used in 3
dimensions.
dawnmathmom says:
Jun 7, 2008. 11:52 AM REPLY
missed the part/misunderstood the part about the vertical slider on th "q" line, but with it to the inside of the curve I think
you would be drawing a "half-ellipse" instead of an actual parabola. Parabolas are all points equidistant from the focus and
the directrix (a line which lies OUTSIDE the curve).
There is a fairly simple way to geometrically construct a parabola. I've used the string method for it, but found it much
easier with grid paper.
/> gaiatechnician says:
Jun 7, 2008. 7:22 PM REPLY
is the setsquare method for drawing a parabola. I modified this
method to make the mechanical mathematician.
I hope this finishes it!
You can make an entire parabolic dish from cob in one go with no math or paper!
(This cuts out several steps in the process).
I have only ever made 2 dishes with this method.
An Austrian NGO (working in Asia) has asked for more info and really my dishes were just proof of concept.
I did not even try to do a perfect job.
So if anyone wants to help alleviate poverty,
get cobbing!
and see how good the parabolic dishes are!

And report your results, good or bad.
Brian
dawnmathmom says:
Jun 9, 2008. 9:02 PM REPLY
Since an ellipse also contains foci one could easily take a section of it's graph (say the end third) and lay it over a
section of a parabola and have it match up pretty closely. For the small section you are constructing it
approximates a parabola, but is actually an elliptic section.
You just gave me a great idea for an extra credit project for my 10th graders ;-)
gaiatechnician says:
Jun 9, 2008. 10:37 PM REPLY
An ellipse has 2 focal points not 1
dawnmathmom says:
Jun 10, 2008. 3:01 PM REPLY
yes, gaia - an ellipse does contain two foci but you are not drawing an entire ellipse which is why your
conic section only contains one - which is why you seem to think it is a parabola.
btw, constructing a hyperbolic section (half or less) would also contain one foci and approximate a parabola,
even though an actual/complete hyperbola contains two foci.
For the purpose of building a solar cooker it seems any of the three would work (since all contain foci), but
just because it contains a single foci does not make it a true parabola - you have constructed a section of a
(non-parabolic) conic which approximates a parabola.
gaiatechnician says:
Jun 10, 2008. 9:32 PM REPLY
By all means do not take my word for it, but please take somebody's. It is a parabolic dish and here is
an applet explaining the parabola with a setsquare
a parabola
has 1 focal point Brian
CameronSS says:
Jun 7, 2008. 1:42 PM REPLY
If you could get a hold of a math teacher's TI-83 with the display cable, and just project it onto a wall to trace. Or any
other graphing program, for that matter. I'd probably draw out a grid and graph it just for the heck of it.

Yeah, I think it is part of an ellipse.
Weissensteinburg says:
Aug 26, 2007. 6:53 PM REPLY
Who was afraid?
craz meanman says:
Apr 2, 2008. 5:48 PM REPLY
ummmm, i was. those parabolas scare me. just kidding. I don't think anybody was scared of the math, but that is a good idea
for a perfect curve no matter what.
gaiatechnician says:
Aug 26, 2007. 7:09 PM REPLY
If parabola math was easy, lots of home made parabola solar cookers would be about. I do not see many.
Thats why I use the mechanical alternative.
Weissensteinburg says:
Aug 26, 2007. 7:26 PM REPLY
Actually, parabola cookers are quite common as fun middle school science/home projects.
What other way is there to draw a true parabola (without a computer) than the mechanical method?
/> snoyes says:
Jun 16, 2008. 6:26 AM REPLY
I've seen instructions on using just a square, without the string:
- Mark the focal point
- Draw a baseline some distance from the focal point
- Set a square so one leg goes through the focal point and the 90 degree angle lies on the baseline. Draw the other
leg. This line will be tangent to the curve.
- Repeat for many positions, drawing enough tangents that the curve is defined.
- See />I don't know if the resulting shape is really a parabola or some other curve.
nagutron says:
Dec 13, 2007. 11:28 AM REPLY
Any string of consistent composition along its length will hang in a parabolic shape. This is a really fast shortcut: just
hang a string, right in front of a piece of paper tacked to a wall, then trace the shape.
CameronSS says:

Dec 13, 2007. 4:00 PM REPLY
Not a true parabola. It is a very quick way to get an approximation of a parabola, but that actually forms a catenary.
Where a parabola is a graph of the nice, easy little equation y=x
2
, a catenary is the obnoxious little equation
y=a*cosh(x/a), or y=a/2(e
x/a
+e
-x/a
).
I don't blame you for the error, Galileo made the same mistake.
nagutron says:
Dec 13, 2007. 4:30 PM REPLY
I stand corrected! Thanks.
Weissensteinburg says:
Jul 19, 2007. 7:00 PM REPLY
Thanks =D It takes under 15 minutes to cook a hot dog with it.
I just didn't want to do anything to it, or post the image directly, because it wasn't mine, and I hadn't asked permission from it's owner.
As a photographer, I'm pretty sensitive to respecting copyrights. Thanks though.
Weissensteinburg says:
Jul 18, 2007. 4:27 PM REPLY
Update: I made a new one, and took new pictures. So I will amend this a.s.a.p.
demon93 says:
Apr 29, 2008. 8:24 AM REPLY
how soon
Weissensteinburg says:
Apr 29, 2008. 12:01 PM REPLY
Already did.
gaiatechnician says:
Aug 26, 2007. 5:54 PM REPLY

I am making a sandwich board parabolic solar cooker from cob. The parabola is going to be about 4 ft by 40 inches. Should be good for cooking a few
hotdogs!
Making a big parabola is surprisingly easy!
Brian
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