Cook hotdogs with the Sun in minutes.
In this section we will show you how to make a powerful
solar concentrator that can cook four or five hotdogs in
minutes.
The Solar Hotdog Cooker is made out of a thin (1/8 inch thick)
plastic mirror that can be found at plastic shops and glass
stores (although it may have to be special ordered at some
stores). The plastic is bent into the shape of a parabola,
so that the sun's rays are collected over an eight square foot
area, and focused in a thin line. The hotdogs are roasted on
a spit placed at the focus, and turned every once in a while
to prevent them from burning.
Click on picture to see a larger image
Materials
For the solar cooker you will need:
- Two pieces of plywood, 1/2 inch thick, 2 feet wide and 4 feet long.
- Two pieces of lumber, (2x4) 1 1/2 inch thick, 3 1/2 inch wide,
and 8 feet long.
- 16 wood screws, 2 inches long.
- One stiff steel wire, 3 feet long.
- 92 small nails or wooden pegs, about an inch long.
- One plastic mirror, 1/8 inch thick, 2 feet wide, 6 feet long
(although 5 1/2 feet long might work better).
- A drill and a bit that matches the diameter of the 92 small nails
or pegs. A larger bit (over 1 inch wide) is needed for the food
hole.
Assembly
Place the two sheets of plywood together, one on top of the other.
Using a tape measure and a carpenter's square, mark off where the
holes will be drilled for the mirror supports (the 92 small nails
or pegs).
All holes are drilled completely through both sheets
of plywood. The holes are to be drilled according to the following
table:
Inches from left |
Inches from bottom |
0 |
22.16 |
2 |
18.94 |
4 |
16.00 |
6 |
13.34 |
8 |
10.96 |
10 |
8.86 |
12 |
7.04 |
14 |
5.50 |
16 |
4.24 |
18 |
3.26 |
20 |
2.56 |
24 |
2.00 |
28 |
2.56 |
30 |
3.26 |
32 |
4.24 |
34 |
5.50 |
36 |
7.04 |
38 |
8.86 |
40 |
10.96 |
42 |
13.34 |
44 |
16.00 |
46 |
18.94 |
48 |
22.16 |
|
Spacing for drilled holes
A second row of holes is drilled above these, separated
by the thickness of the mirror.
|
Next drill a set of holes above the first set, about a third of an inch
above the first set of holes. The first set of holes will eventually
have 23 of the small nails placed in each side, to hold the mirror up.
The second row will also have 23 small nails pushed in, this time to
hold the mirror in place from above. The exact spacing is not critical,
but you don't want them too close together, or the top nails will hit
the mirror instead of resting on top of the mirror.
Next drill eight holes for the screws that will hold the 2x4 lumber
in place. The holes are 3/4 inch from the edges of the plywood.
On the left and right, a pair are drilled 15 inches from the bottom and 13
inches from the bottom. At the bottom, a pair are drilled 10 and 12 inches
from the left, and the last pair is 36 and 38 inches from the left.
The focus of the parabola is 9.14 inches from the bottom, and 24 inches
from the left. Drill a hole that is the same diameter as the spit wire,
or a little bit larger. This hole should go through both sheets of
plywood.
Just above one of the focus holes, drill a large hole in one plywood
sheet, just touching the hole for the spit. This large hole will
accomodate the food (hotdogs or kebabs), so it should be at least
an inch in diameter, but three or four inches would be better. The
spit with the food on it will be inserted into this hole, and the spit
will then drop into the much smaller hole at the focus, to keep the
spit in exactly the right place.
Cut four pieces from the 2x4 lumber. Each piece should be exactly 2 feet
long.
Using the 2 inch long screws, screw the 2x4 pieces to one of the plywood
sheets, centering each pair of screws in the end of each piece of 2x4.
The result should look something like the legs of a small table.
Attach the second plywood sheet to the other end of the 2x4 pieces.
Click on picture to see a larger image
The photo above shows the back side of the cooker, where the 2x4
spreaders can be seen. Note also the remaining length of 2x4 is used
as a support (more about that later).
Next push 46 of the small nails into the bottom row of holes.
Now set the mirror onto the top of the cooker, and gently push it down
to rest on the nails. Put a pair of nails in the center pair of holes
on top of the mirror, then work your way outwards, placing pairs of nails
to hold the mirror down. (I used cotton tipped wooden swabs in the picture
because they photograph better than nails.)
The last step is to place a few screws in the remaining long piece
of 2x4, leaving the head of the screws sticking an inch or two out
of the wood. These will act as supports to hold the cooker so it
is tilted toward the sun.
The spit is formed from the 3 foot piece of wire. A coat hanger
can be used, but wires that thin tend to sag in the middle when
burdened by a few hotdogs. A thicker, stiffer wire is better.
To make it easier to turn the food, a crank is formed by bending
the wire at one end as shown in the labeled photo.
Cooking with the sun
Carefully poke the 3 foot wire spit through the hotdogs or kebabs.
Try to center the food on the spit, so the food will rotate when
you rotate the spit, instead of slipping to keep the heavy part down.
Insert the spit through the food hole, and insert the far end of the
wire into the small focus hole in the far plywood sheet.
Rest the near end of the spit in the small focus hole at the bottom of the
food hole.
Align the solar cooker with the sun. Start with the cooker flat on the
ground, then turn it until it is parallel with your shadow. The sun
will just barely graze both of the plywood sheets when the cooker is aligned
properly (this can be seen in most of the photos on this page).
Next tip one end of the cooker up until the shadow of the spit falls directly
on the center nail at the bottom of the parabola. This can be clearly seen
in the labeled photo.
Hold the remaining scrap of 2x4 up against the back side of the cooker, and
mark where a screw should be placed to hold the cooker at the right elevation.
Screw the screw into the 2x4, leaving an inch or two sticking out to hold
the top 2x4 spreader. If you like, the screw can be placed a little
higher up, and the cooker can be adjusted to the exact angle by tilting
the support backwards.
Click on picture to see a larger image
When the cooker is adjusted properly, the sun will be focused on the food,
making bright lines across it (sunglasses are recommended at this phase).
You can see the shadows of the nails on the walls of the cooker. These
shadows should all cross at the focus, where the hotdogs are.
The hotdogs can be seen in the mirror, highly magnified. The shadow of
the hotdogs can be seen being cast by the mirror onto the back side of the
hotdogs in the photo above. In the photo below, the shadows of the nails
can be clearly seen, crossing at the focus of the parabola.
Click on picture to see a larger image
The hotdogs will start steaming in less than a minute. The spit should
be turned every couple minutes to prevent black lines from being burned
into the food (unless you like your hotdogs with black stripes). The hotdogs
will be quite hot in about 10 minutes, or burned black all over in about
20 minutes.
Click on picture to see a larger image
Things to notice in the above photo:
-
The shadows of the mirror supports seem to meet at the focus.
-
The shadow of the hotdogs is projected onto
the enlarged reflection of the hotdogs.
-
The enlargement of the hotdogs
only occurs in their width, not their length, because the mirror is only
curved in one dimension.
-
The poor hotdogs have been burned to a crisp
(oops...).
How does it do that?
A parabola is a shape with some interesting properties that make it
perfect for cooking hotdogs.
The sun is bigger than the earth, and very far away. This means that
the sunlight that hits the earth appears to be in parallel rays.
If we had thousands of tiny mirrors, connected by hinges in a line,
and we tilted each mirror so it would reflect these parallel rays
onto one spot, the mirrors would line up in a parabola.
Mathematically, a parabola is defined as a set of points that are
the same distance from both a point (called the focus) and a straight
line (called the directrix).
The formula for the parabola used in the solar cooker is
y = 0.035x2+2
I chose this formula so the parabola would be deeply curved, and would fit
into the 2 foot by 4 foot plywood sheets. We want the focus to be close
to the mirror, so that as the sun moves, the focus does not move very
much.
Having the focus close to the mirror is like having the fulcrum
of a lever close to one end. The sun end of our lever can move a lot,
while the hotdog end of our lever hardly moves at all. This means that
we don't have to raise or lower the cooker very often as the sun moves.
The +2 part of the equation says that the bottom of the parabola will
be 2 inches from the bottom of the plywood. This gives us room for
the 2x4 spreaders, and room to drill the bottom hole for the support
nails.
The bottom of the parabola is called the vertex. The vertex is always
halfway between the focus and the directrix. The distance from the
vertex to the focus is
1
0.035
4
or about 7.14 inches.
A square meter of the earth's surface gets about 1000 watts of power
from sunlight. Our mirror intercepts about 8 square feet of sunlight,
or about three quarters of a square meter. This means that our cooker
is the rough equivalent of a 750 watt electric stove.
Next:
Exploring invisible light
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Simon Quellen Field
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