book collections email follower instructable user
Picture of The Sundogger
IMG_3296.jpg
IMG_3304.jpg
venn-diagram.jpg
IMG_3398.jpg

The Sundogger is a solar hot dog cooker I built using my X-Carve CNC machine. I decided to make this project to learn a bit more about carving larger pieces of furniture on my CNC machine. However I was also motivated by a lifelong interest in both solar energy and in hotdogs. When I was a high school student, a mentor of mine built a solar hot dog cooker for the Earth Day Celebration at the local community college. I grew up being fascinated with solar energy, so I wanted to build something like this for my backyard for decades. Around the same time, I remember seeing a similar device for sale in the Edmund Scientific Catalog. My version of this cooker is an homage to this now ancient device.

It is important to note that this while this project is named the Sundogger, it is vegan friendly. It will work with any kind of tubular food that can be put on a skewer or can be made into a cylindrical shape. If you end up using other kinds of foods, make sure they are cooked to a safe temperature before eating them.

Originally I tried to build this project out of foam-core instead of plywood. After some initial tests, I found that I couldn't make smooth cuts with this material. The foam core board fragmented too easily even when using the sharpest knife. I considered moving to a hot wire cutter, but opted to move toward a plywood-based project instead.

In my explorations of the local crafts store, I found a great material to use for the reflector - an aluminized piece of poster paper. The reflectivity seemed high enough to make the project work without having to suffer through the inevitable crinkling of aluminum foil that occurs when you try to smooth over a large surface area. If you can't find this material, aluminum foil mounted on cardboard should work well enough. I also considered purchasing a acrylic mirror sheet to improve the efficiency. In the end, I opted for the simple and cheap poster paper solution for this prototype.

For the frame, I decided to try and make the entire project out of a single 1/2 inch "handy panel" of plywood a bought at the local big box hardware store. These 2 ft x 4 ft pieces is just the right size for the parts needed for this project.

The total costs of supplies was about $35, including the plywood, the reflective poster paper, the finish, and the bolts.

Supplies:

  • 1/2 inch plywood panel - interior grade - 2 ft x 4 ft
  • 1 piece of reflective poster paper (available at the craft stores)
  • 6 - 1/4 inch x 1 1/4 bolts #20 bolts, washers, and butterfly nuts
  • 4 furniture bolts and cross dowel nuts (I used the Hillman 5/8 plain steel barrel bolt + 1/4 inch x 50mm bolts)
  • Set of two small cabinet hinges
  • Bamboo or metal skewers - at least 24 inches long (sharpened dowels will work)
  • Wood glue, finishing nails
  • Wood finish - if you like

Equipment

  • CNC machine with usable working area of at least 24 x 28 inches
  • Files, sandpaper, knife and saw for trimming and cleaning the wood after the carve
  • Clamps and perhaps a pneumatic nailer to join the wood together during gluing
  • Drill - to install cross bolts, install the hinges, and clean up the CNC cut holes

Step 1: Solar Hot Dog Physics

Picture of Solar Hot Dog Physics
parabola.png
IMG_3215.jpg
IMG_3213.jpg

To understand how the Sundogger works, it helps to step back at talk about the physics of hot dogs.

At Earth the total energy flux (flux density) from the Sun is called the the Solar Constant. The value of the Solar Constant is approximately 1360 Watts per square meter or 1.951 calories per square cm as measured on a surface that is perpendicular to the incoming sunlight. This number doesn't change since the distance between the Earth and Sun is approximately constant over the yearly orbit.

The Sundogger we are building has a width of approximately 24 inches or about 60cm. If our Sundogger was a full meter long, we would have an area of 0.6 x 1 = 0.6 square meters. However really understand hot dogs cooking in the sun, it turns out it is easer to think about the energy per centimeter of length of the cooker rather than the total energy of the cooker. The parabolic shape of the collector concentrates the energy on a skewer, so the energy for every cm of length is going to be the energy that is concentrated from the local 1 cm of width in the collector. In our case, we have 1.951 calories per square cm per minute x 60 cm (width) = 117 calories per minute of solar energy for every cm in length along the skewer.

Detailed scientific measurements have show that a typical hot dog has a diameter of about 1 inch of about 2.5 cm. This gives us a hot dog radius of about 1.25 cm. (Precision measurements of hot dogs put this number at 2.726 cm in diameter - but your actual hot dog diameter may vary.) The volume of a hot dog - or anything - is its length times its cross sectional area. The cross sectional area is going to be A = Pi times the radius squared. This means that every linear centimeter of the hot dog has a volume of (1.25 x 1.25 x 3.14) = 5ish cubic cm.

The mass of any object is the density times the volume. According to the manufacturer of the hot dogs I used, each dog has a mass of 57 grams. With the length of the hot dog measured at about 12 cm, this gives us a volume of about 4.8 grams per hot dog cm. This estimate puts the typical hot dog density just below 1 grams per cubic cm.

Combining these energy input per centimeter and the mass per centimeter, we find that we have 117 / 4.8 = 24 calories of energy per gram being added to the hot dog every minute. Thus in every second, we get enough energy to raise the hot dog temperature about 24 degrees Celsius every minute when it's internal temperature is about 20C.

Of course, this estimate is doesn't include several important factors:

  1. The 1360 Watts per square meter figure is actually the amount of sunlight hitting the outer atmosphere of Earth. The actual amount that reaches the surface is typically about 1000 Watts. The actual number depends how far the Sun is from overhead. (Thanks to the user redrok for pointing this out.)
  2. The energy from the collector to the hot dog is not transferred at 100% efficiency. The reflective paper we are using probably reflects less about 50% the light directly to the focal point.
  3. The hot dog itself also reflects some of this light rather than absorbing it. The hot dogs albedo (the net reflectivity) is perhaps about 0.2 to 0.4, suggesting that 20% to 40% of the visible light reflects off its surface.
  4. Much of the remaining energy is converted into thermal radiation that escapes from the hot dog's surface rather than cooking it. The exterior heats up, and then Planck's law causes much of the energy to escape back into the air rather than thermally diffusing into the interior. This thermal radiation effect probably changes as the hot dog''s internal and surface temperature rise.
  5. The energy need to raise the hot dog's internal temperature depends the hot dog's current internal temperature, surface temperature, and the thermal conductivity it has.

I briefly considered doing a simulation of the hot dog thermodynamics to understand these effects a bit better, but decided it was outside the scope of this Instructable. Instead, we are going to just kind of eyeball these effects to make a prediction about the cooking time. Assuming the actual net efficiency of the cooker is probably about 20%, the temperature increase of the hot dog should be about 5 degrees Celsius per minute in bright sunlight. To get the temperature up to a safe temperature of 80C from a starting temperature of 20C, we should probably let them cook for about 15 minutes if our efficiency estimates are accurate.

I should note that the hot dogs used in this experiment were pre-cooked, so they technically do not need to be cooked at all. However, the USDA recommends a cooking temperature of 145F for pork. It seems best to use as a guideline for this project.

Finally - a note about terminology. The flux density is defined as actually a measure of power per square meter per second. Watts is a measurement of power - the amount of energy change per unit time. Calories or Joules are a measurements of energy. Calories is almost always related to the thermal energy or heat in a system. Temperature, on the other hand, is a measure of the average thermal motion of molecules inside an object. Temperature is related to the heat, but is a bit different. The properties (specific heat, mass, etc) of a body determine how temperature and heat are related to each other. I was a little loose on this terminology when I first wrote this up.

sgeorge56017 days ago
Especially love the Venn diagram.
JohnW539 (author)  sgeorge56016 days ago
This project has a niche market. :)
KMoffett20 days ago
Did you experience drippings from the hot dogs falling on the reflector? I did on mine. :(

Note the nail on the center of the top right edge. This is used to align the panel. When the shadow of the head is concentric with the bottom of the nail, you are at the best alignment with the sun.

S-Hotdog.jpg
JohnW539 (author)  KMoffett20 days ago
Yes! It did get a bit drippy. I used some 409 and a paper towel to clean it up. I also used the skewers for alignment. I think using cup hook might be helpful to set it up.
shandafin20 days ago
Hilarious! Loved your write up - entertaining and really informative. Nice work.
JohnW539 (author)  shandafin20 days ago
Thanks. I had fun working on the project and the writeup. I wasn't sure about including the hot dog physics, but felt like it would be fun to do some back of the envelope calculations to provide some context. I also enjoyed doing the experiments, although it was heartbreaking to watch the hot dogs cool down when the clouds came.
tbell8320 days ago
This is incredible.

v2 should include a solar power rotisserie.
JohnW539 (author)  tbell8320 days ago
The problem then becomes the thermometer cord for the hot dog core temperature. I guess going with servos would work.... hmmm.
redrok20 days ago
No JohnW539 you don't get 1360 Watts of Energy.
1. Firstly, it's not energy, it's Power.
2. You only get about 1360 Watts per square meter if you are outside the Earth atmosphere and near Earths orbit.
3. The rule of thumb for us on Earth is it's about 1000 W/m^2 for full sky radiation. The other 360 W/m^2 is lost to the atmosphere as heat.
4. Lastly, the rule of thumb for solar concentrators is you only get about 800 W/m^2 for radiation directly from the Sun. The other 200 W/m^2 is wasted because it wouldn't hit the hotdogs.
redrok
redrok@redrok.com
JohnW539 (author)  redrok20 days ago
Technically we are talking about the flux density of the incoming radiation, but sure - energy per unit time is power. The other effects you mentioned are lumped into the category of efficiency in the writeup. Atmospheric losses, reflectivity of the collector, the reflectivity of the hot dog, and thermal losses all drop the amount of energy that actually goes to cooking. I did simplify this discussion a lot intentionally. For a short while, I thought about doing full simulation on hot dog cooking but decided it was going into way way too much detail for a CNC project.
Tony Rimmer21 days ago
Nice work.
Alex in NZ22 days ago
Beautiful piece of work. Thank you for sharing it :-)
sclausson22 days ago
I am going to share this in many solar cooker Facebook groups. I hope you join us in solar cooking discussions around the world. I have a CNC and will try this.
JohnW539 (author)  sclausson22 days ago
Thanks! Let me know how it work out.