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Build a solar hot dog cooker

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This is a parabolic cooker I built a while ago, so unfortunately I can't show step by step photos of it's construction. However, it's relatively easy to see how it was put together.

It makes a great science project. Unlike most, you'll actually get some use out of it afterwards. Some of the principles it demonstrates are:
1. Optics: focusing parallel rays of incident light through the use of a parabolic mirror.
2. Energy transformation: light to heat
3. Renewable energy: solar power

Materials:
2 - 2x4 cut to 17" length (ends)
2 - 1/2" plywood cut to 1ft x 4ft (sides)
1 - 5/16" plywood cut to 18" x 4ft (backing)
1 - sheet of silvered mylar or tin foil cut to 17" x 4ft (mirror)
2 - 1/2" plywood cut to 3/4" x 12" (skewer mounts)
1 - 1/4" wooden dowel with a sharpened point (skewer)
Plus miscellaneous fasteners and spray adhesive

Tools:
Jig saw
Power drill
Drill bits
Screw driver bits
 
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Step 1: Optics 101: Focussing light with a parabolic mirror

Picture of Optics 101: Focussing light with a parabolic mirror
In order to focus parallel rays of incident light on a fixed point you need a parabolic mirror. There are a few different ways to generate a parabolic curve, but the easiest to understand (though not necessarily the easiest in practice) is simply doing the math.

The formula for a parabola is:
Y = X2/(4F)
Where:
X is the horizontal distance from the origin
F is the focal distance (a design choice)
Y is the vertical distance to the curve at any point X

See the image below for a graphical representation.

Step 2: Cut the sides to a parabolic profile

Picture of Cut the sides to a parabolic profile
I designed my parabolic cooker with a focal distance F of 18 inches. Thus my formula was:
Y = X2/72 (where both X and Y are in inches).

I used a spreadsheet to calculate and plot the value of Y for all values of X from -24" to +24".
Y is the same whether X is positive or negative, so I've only shown Y values for X values from 0 to 24 here). Those values are:

X - Y
0 - 0.000
1 - 0.014
2 - 0.056
3 - 0.125
4 - 0.222
5 - 0.347
6 - 0.500
7 - 0.681
8 - 0.889
9 - 1.125
10 - 1.389
11 - 1.681
12 - 2.000
13 - 2.347
14 - 2.722
15 - 3.125
16 - 3.556
17 - 4.014
18 - 4.500
19 - 5.014
20 - 5.556
21 - 6.125
22 - 6.722
23 - 7.347
24 - 8.000

Draw a grid with 1" spacing on one of the 2 sheets of plywood to be used for the sides. similar to that shown below, and plot the curve. Then simply cut along the curve with a jig saw. You can repeat the process on the second sheet or use the first sheet as a template to trace the curve onto the second (or if you are a brave enough, you can attempt to cut them both at the same time... clamp them together well first).

Step 3: Assemble the parts

Attach the sides to the 2x4s that will become the ends. I used 3" #10 screws because they were handy, but 2" screws are probably long enough.

Draw a pencil mark on each side at the origin of the parabolic curve.

Draw a line across the center of the backing (the 18" x 4ft sheet of 5/16" plywood) dividing it into two halves (each 18" x 2ft).

Line up the backing with the sides and put a small screw (I used 3/4" #6 screws) through the center line on the back and into each side at the origin marks you made.

Every 6" or so, put another screw through the backing into each side until you have attached the backing to the sides along the entire length of the sides (bend the backing to match the shape of the sides as you go).

Now glue your mylar or tin foil to the inside surface of the backing. Spray adhesive works well. Try to smooth out the surface as much as possible. Any little wrinkles will reduce the efficiency of the cooker. You can see that I wasn't that careful myself.

Step 4: Attach the skewer mounts

Picture of Attach the skewer mounts
The parabolic mirror you have created has a focal distance of 18", but the sides are only 12" wide. I designed it this way (It could have just as easily been made with sides 18" wide) so it wouldn't be so big. Therefore, you need a way of holding your skewer 6" away from the top edge of each side.

I did this with a couple pieces of 1/2" plywood cut to about 12" x 3/4", but you can use any material you have available. Screw these pieces to the center of each side so they stick out a little over 6" to hold the skewer. If you want you can make them pivot so they can fold out of the way when not in use.

Make a notch in the end of one skewer holder, and drill a hole in the end of the other. The sharpened end of the skewer will stick into the hole. The other end of the skewer will rest in the notch.

Now you are completely assembled and ready to cook.

Step 5: Cook your weiner.

On a sunny day, lean your parabolic cooker up against a fence or deck or whatever you have available (or you can design in an adjustable stand). Adjust it until it is aimed directly at the sun.

The easiest way to tell when it is aimed correctly is to look at the shadows cast by the two skewer mounts. When you can see a shadow cast on the inside surface of each side, then the cooker is aimed in the right direction. When both shadows pass through the origin of your parabola then the cooker is tilted to the right height. You will have to adjust the cooker from time to time as the sun moves (or rather as the Earth rotates).

Shove your skewer through up to 3 hot dogs, stick it on your skewer mounts and wait.

Be patient. My experience with solar cooking is that it looks like nothing is happening (you don't hear anything or see anything) until suddenly, your hot dog starts blistering and/or your skewer catches on fire. It takes a bit of practice to learn when you should rotate your skewer.

I find it takes about 6 minutes to cook ordinary weiners, and 8-10 minutes to cook the large smokies you've seen in these images. I try to rotate the skewer about once every couple minutes.

For more ways you can reduce your environmental impact and save money, visit my website www.IWillTry.org. Thanks for reading.
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How much did it cost you to make this project, because I want to do this for my solar cooker project and I would like to make a solar cooker 1/2 this size?
emma10mint3 years ago
how long dus it take
MoritzB3 years ago
How often do you have to move it, so the sun shines directly into the Mirror?
rogueleadr4 years ago
 I like the house heater.... Fire Hazard? Can you explain more how it is built? Thanks

iwilltry (author)  rogueleadr4 years ago
You can find an explanation of its construction at my website, www.iwilltry.org. Yes, it is a potential fire hazard, depending how you build it, what you aim it at, and how many of them you point at the same target ;-)
Tetrafish5 years ago
http://en.wikipedia.org/wiki/File:Parabola_with_focus_and_directrix.svg

I've always had trouble with the math... Okay, maybe I haven't even tried. But when I seen the pic in the link above - making parabolic curves immediately became clear to me. The distance to the focal point to the bottom of the curve needs to be mirrored for the flat/base line below, then finding every other point along the curve is a matter of finding the right intersecting radii. It's a bit more complicated than that - but it can be figured out.

To scale it up - you can keep doubling the distances of the points on the different angles.

Then there is using wax paper. Put a point where the focus will be, then a line twice the distance of where the base of the curve (or trough) will be. Then fold the paper over to where the line meets the point, and keep folding along the line to get the full curve.
gwendelynn5 years ago
i need to cook an egg for my science project is there anything i can change so it cooks eggs instead? please help
Karat2065 years ago
we built a smaller version, but are having a hard time getting it to come up to temperature with the snow on the ground. The breeze kicked up a bit, do you have any recommendations to help bring a dog up to temp in the winter?
iwilltry (author)  Karat2065 years ago
Depending on how much smaller you made it, you may be out of luck. I think the size I built mine is just about right for cooking a hot dog within a reasonable time in the summer. Much smaller and you won't be collecting enough solar power. Solar radiation at the Earth's surface is around 1000 Watts per square meter. The area of sunlight captured per hot dog in my design is about 0.2 square meters (about 20cm wide of hot dog times 1m high of collector). So I capture about 200 Watts per hot dog. Given the geometry is not perfect and the mylar is not 100% reflective, probably less than 100 Watts reaches the hot dog. If your collector is half the height, then you could expect less than 50 Watts to reach your hot dog which is probably not enough to cook on a cold (especially breezy) day.

One thing you could try that *may* help is placing a transparent shell around the hot dog to help insulate it (the air inside is what actually insulates it. the shell just hold the air in place). Cut the tops off two 1 liter pop bottles, drill a hold in the bottom of each to fit over your skewer, and slide the two pieces over each end of your hot dog so they meet in the middle. Might help, but probably you just need a bigger reflector. Also, if it gets too hot that you smell plastic fumes, you may not want to eat the results ;-).
Derin iwilltry5 years ago
20W,20cm*1m=0.02m2
Karat206 Derin5 years ago
placed a lucite shield over the top drilled a hole for the thermometer, got the dog to an internal temperature of 146 on a 34 degree day. Tried the soda bottle but was as effective.
iwilltry (author)  Derin5 years ago
Huh? I think you may be mixing up your cm and mm.
20cm = 0.2m.
0.2m * 1m = 0.2m2
0.2m2 * 1000W/m2 = 200W
justin1h65 years ago
dose it acutely work
iwilltry (author)  justin1h65 years ago
Yes. It actually works.
awang85 years ago
wWhy do the smokies have black lines that look like grill marks on them?
iwilltry (author)  awang85 years ago
I'm not sure what black lines you are talking about. There are a series of diagonal slices cut in the smokies with a knife. This makes them cook better and prevents blistering. I do that whether I'm using a BBQ or solar cooker.
reio365 years ago
this is GENUS!
jermah31 reio365 years ago
yEAHits reaLly GENUS!!!
fireclown5 years ago
Parabolic hot dog cookers have been done before , but the big one in the background is really, really interesting. Could you give us one on that one, including the metrics by which you came up with the 2KW of heat? That is daggne interesting!
iwilltry (author)  fireclown5 years ago
I've been considering posting that to instructables but it will be quite a chore to post the construction in detail. I have some details of the design including power calculations on my website www.iwilltry.org. Thanks for the comment.
Pumpkin$5 years ago
popcorn?
tony stark6 years ago
You could probably turn the grease to biodiesel and get money off it and save the world! I am so building this!
tony stark6 years ago
I did this in kindergarten! I can't believe I remember that. Man, I wish I was still in kindergarten!
Why do they have the grill lines?
blodefood6 years ago
Note: This works for vegetarian hot dogs too! You might shorten the time a little.
samchic6 years ago
hahaha... we did this as an experiment in pairs when i was in primary school..
Sure would like to see the information you learned from building your "house cooker". Would you consider emailing me your file for tracking the sun with your computer? I'd like to build something like that for my house. I need it to bounce the sun in my west facing glass door. Actually I'd like to see you make an instructable on the project. Thanks Jim
bonzabuy6 years ago
Thanks for the post - Solar Cooking is amazing - I have been so impressed with the Global Sun Oven I even started importing them into Australia from the USA!
I am looking at designing a simply tracker for them and a base for the leg keep a look out on http://www.bonzabuy.com.au
Also might be keen on linking to this for people who want to try & build a parabolic for themselves - thanks again!
so... why not use a metal rod instead of a dowel? the thermal conductivity of the metal would help the food cook, would it not? And besides, it allows a smaller skewer, which leads to less damage to the food. BUT, good job! I like it! Now just to find some silvered mylar...
iwilltry (author)  thecheatscalc6 years ago
Oh... a good source of silvered mylar is those "emergency blankets" you see in the camping section of hardware stores, or in emergency roadside kits. You can pick up a 4ftx6ft sheet for about $3 I think. Aluminum foil will work quite well though the silvered mylar has slightly higher reflectivity.
I got some blanket in a dollar store for a couple of bux BUT it seems to let some light through (not fully reflective). Is that mylar or is it an immitation? I always used kitchen foil before and it seems to work ok. My parabola is a concave bowl. Will the mylar stretch to the bowl? That would be awesome because foil will not stretch and there will be some wrinkles.
iwilltry (author)  gaiatechnician6 years ago
Your blanket may or may not be mylar. I think mylar itself is transparent and there is a silver coating that is applied for a mirror finish. You may have mylar and just the coating is not very good. I got my mylar from http://www.mirrorsheeting.com/. Mylar will not stretch much. If you want to shape it to a parabolic bowl, you would need to cut it into appropriately shaped pie slices and glue it down.
iwilltry (author)  thecheatscalc6 years ago
Metal has higher conductivity, but it needs a heat source to conduct the heat from. Bare metal would reflect most of the radiation that hits it, so the rod would not heat up as much as the weiner. It would instead conduct heat out of the weiner and dissipate it to the air by natural convection (the same way a cup of coffee cools down faster with a spoon sticking out of it). You could paint a metal rod black but I'd rather not stick a painted rod through my food. Also, when fully loaded with 3 weiners, there is no part of the rod exposed to sunlight anyway.
I am in the process of making a parabolic cooker. First part is on utube. It is a parabolic mirror focusing to a point. I made it of cob, which means I must let it harden for a few days before the next step.. I used a far easier (mechanical) way of calculating the parabolic curve. I am not much at doing instructions but I would welcome someone here trying it and making the instructions. Anyone interested? My name there is same as here if you want to look. Basically when it is done it will be a sandwich board with the parabola pointed at the sun. A hole in the board somewhere to have an aimer to line it up and perhaps wheels on the sandwich board to turn it away from the sun when not in use. focus is about 16 inches above the centre in my first prototype. Brian
acer736 years ago
Its not original, you just beefed up Weissensteinburg's instructable.
or weissenstein made this one smaller :/
iwilltry (author)  acer736 years ago
That sounds rather accusatory. I actually built this before I ever heard of instructables, but whether you believe that or not is a moot point since people have been building parabolic cookers for probably as long as mirrors have existed. I even referenced two books that describe similar cookers. I never claimed to have invented the idea. Neither did Weissensteinburg, last time I checked.
acer73 iwilltry6 years ago
Still, he already posted a cooker in the contest way before you.
iwilltry (author)  acer736 years ago
I can't argue there. I didn't notice any rules against entering a project similar to an existing one. I'm new to instructables, thought, so let me know if I'm breaking any taboos. I thought all projects were welcome, and showing different ways of doing similar things is encouraged.
canida iwilltry6 years ago
You're quite right. Thanks for posting!
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