I had some materials left over after building my solar tracking heliostat for lighting and heating my home, so I decided to make a miniature version for cooking hot dogs. This is fairly simple project (it only took me a couple hours to build after working out the proper dimensions) and it works great.
Tools and Materials
- Jig saw
- Power drill
- Drill bits
- Screw driver bits
- 2 – 2×4 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
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Formula for a Parabola
In order to focus parallel rays of incident light like those from the sun 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:
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.
Making the parabolic profile
I designed my parabolic cooker with a focal distance F of 18 inches. Thus my formula was:
(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).
Assembling 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.
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Attaching 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.
Cooking 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 see in these images. I try to rotate the skewer about once every couple minutes.
The image below shows the finished product. The sides and back are constructed of plywood, with a 2×4 at the top and bottom. The mirror is made from silvered mylar that I purchased to test on my larger mirror array. It turned out to be a poor choice for that project, but works great for this one. Aluminum foil would likely work too. I used a wooden dowel for a skewer. The particular hot dogs I’m cooking have built-in cheese which drips pretty badly, thus the improvised drip tray hanging from the skewer.
The image below is a side view. Note the much larger tracking mirror array in the background reflecting sunlight onto the sliding glass doors to the right. At noon on a clear day, the tracking mirror array puts about 2 kW of free heat into the house, and lights up the room just as if it was a south facing window. It’s quite surreal.
The image below is a closeup of my lunch. Note the charred dowel. Solar cooking (on a cooker this size) does not progress very quickly, but it does progress faster than it appears to be. The process is relatively uneventful until something suddenly catches fire so don’t be fooled if it looks like nothing’s happening, and don’t turn your back for too long. These large hot dogs took about 8 minutes to cook, rotating 1/4 turn every 2 minutes or so. The design could be improved by mounting a smaller flat mirror (maybe 3″ wide) opposite the hot dogs from the parabolic mirror, thus cooking from both sides at the same time.
That’s all. Have fun.