## Introduction: 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

- 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.

## 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.

## 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.

## 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.

where do I find the stencils?

Just first try /draft

This may sound silly, bit I have to ask. If the sides dont provide any reflection, what is the main purpose to covering them in reflective material ?

@Blofish - Sides might not reflect much light BUT they will reflect heat. And keep a barrier from outside temps.

Awesome project. Thanks for sharing. :)

how long did it take you to cook the hotdog

u should ry or my guess 30 min

subscribed.

I made this, and cooked a wiener, i also took a old meat thermometer and put it at the focus point of the oven, which i accidentally made sideways, so its like a big, curvy rectangle, and i got to 175 degrees Fahrenheit, when i cooked the wiener, it only got to 135 about unfortunately

Exellent. Subscription time.

this idea is stupid but if you had dismantled a calculator and took the solar panel out of it and used that to make a circuit of the solar panel and something that provides heat, and then attatched that to the cooker, will it cook faster? (i think that the solar panel won't give out enough electricity to power the heat source).

the solar panels in a calculator dont put out anywhere near enought voltage to cook with or let alone put out any noticable heat. besides, the panel would have to be connected to a battery, then to the heat source. solar panels slowlycharge batteries, they dont usually directly power things

a solar panel acts as a power source. not as a battery charger

yeah, but they use solar panels to power a battery charger to charge a battery and the end user (the machine or electronic gadget) gets the power from the batteries, not the solar panels.

I wonder what would happen if a battery powered a solar panel in the dark (with no diodes or anything to keep the flow in one direction). That would be the dumbest thing ever.

Thanks for your work and insight!

HELL YEAH FOR QUADRATICS AND CONICS!!

Great Instructable - very innovative use of simple materials.

What would I need to do to make it on a larger scale?

Could you modify this to follow the sun? It is possilble...

If you attached it to a clock's motor somehow :D

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!

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.

What size were your reflectors and were they parabola's or concave? I had an 8 ft. carbon arc search light I built with an 8 ft. satelite dish I covered with mylar. It was 9 sections bolted together, which made it easy to disassemble it and cover it with spray on glue and add the mylar reflector to it, then reassemble it. It was pretty intense light. I first used a 5 kW generator to power it up. Later I found a bigger generator at 50 kW. Sold it to some advertisement agency later on.Parabolas

areconcave.There are two types of parabola's I am aware of, concave, and compound which is deeper, and tracks the sun up to 3-4 hours without movement. Compound's are typically used in flashlights. Some modern searchlights now use compound parabola's where the older ones used concave dishes with long focal lengths. That is why I was asking about the parablola or concave shape.Parabola's are usually deeper than plain concave reflectors, like satelite dishes are just concave parabola's, with longer focal lengths. There is a difference. I call satelite dishes, old search lights, concave shapes, and parabola's have two different shapes, deeper than concave dish reflectors with very long focal lengths, where the parablola has a short focal length.

I'm not disputing it is concave or not, I hope you can see the point I am trying to convey in my question. My satelite dish search light and older search lights that used dish reflectors, were concaves with very long focal lengths compared to true parabola's that have very short focal lengths like flashlights, headlights, modern search lights... I'm writing you right now using a parabola dish with a short focal length concentrating my wifi signal to my antenna just an inch from the center of the reflector, I made from a link on here at Instructables. Thus keeping my focal point very short. See what I am trying to say? Pretty soon I will add a satelite dish reflecting my signal to the parabola to gain even more signal strength to continue running the wifi at full potential at 48 Mbps speeds. Right now I am only getting enough signal to hit 18 Mbps speeds. I don't dispute parabola's are concave, simply asking what kind of parabola are you using in your search light?

I see, I thought you were implying that if it's concave, it's not a parabola.

BTW, I am 2 blocks from my wifi free source. Before I got the larger antenna and made the parabolic dish, I was only getting about an average of 43% signal strength, when weather conditions allowed it. Now I am getting 79% signal strength, and it is always on now, not tempermental to weather. A storm came through last night and I never lost signal. My cable modem has been taken out by lightning before, so I disconnect it and run wifi, especially when I want to use higher download and upload speeds compared to my cable at only 1.5 Mbps speed. At minimum, my speed tests have doubled compared to my cable. They have gone much faster too.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.

Fancy!

Ok, the instructable has been fixed. Let me know what you think.

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.

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!

Who was afraid?

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.

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?

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 http://www.usajohnsons.com/cool_energy_stuff/experiments/akidheater.html

I don't know if the resulting shape is really a parabola or some other curve.

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.

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.

I stand corrected! Thanks.

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

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.

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.

http://home.germany.net/100-441770/amsi-model.html 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

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

approximatesa parabola, but is actually an elliptic section.You just gave me a great idea for an extra credit project for my 10th graders ;-)

An ellipse has 2 focal points not 1

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

sectionof a (non-parabolic) conic whichapproximatesa parabola.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

http://www.sciences.univ-nantes.fr/physique/perso/gtulloue/conics/drawing/para_string.html a parabola has 1 focal point Brian

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.

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."

I can just see "roughrider" 's head exploding now...lmao.

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.