Many people believe that these ovens are novelties and can not be used for anything. The truth is this oven heats up very fast and care should be taken when handing it or any contents when at operating temperature. Under spring weather conditions and when pointed properly at the sun on a clear day, it will pass 150°C (300°F) within 10 minutes.
Step 1: Supplies and Materials
Several Different Sized Boxes (some very large ones are useful)
Clear Packing Tape
Styrofoam Packing Peanuts or Shredded Paper
Black Spray Paint (enamel) or Black Enamel Paint in a can (you should not use latex paint)
Aluminum Foil (heavier and shinier the better)
A piece of Glass or Acrylic
Glass cutter (if glass is used), a Saw (if acrylic is used)
Can to cook food in
Short pencils or dowels which no longer have a use
A ruler (yard or meter stick would be best)
Pen or Pencils
Thermometer Probe to monitor temperature
Step 2: The Science Behind It
This oven works by concentrating the sun's energy into a smaller area and absorbing it with high efficiency. Parabolic or trough solar cookers work by concentrating the sun's rays. They are more efficient than this linear reflector type but generally only heat one side at a time and can only generate heat in a small point. This oven uses a piece of glass, known in the industry as "glazing", to trap heat in a small chamber which has been painted black. The reason the cooking chamber is painted black is because this color absorbs all wavelengths of the sun's light. Black appears black to our eyes because none of the light rays are able to reflect back. Because of this, the energy in the light gets converted into heat which in turn is trapped in the pocket of air enclosed by the glazing. Neat huh?
But how do those reflectors work? Well, the trick is the angle that they are pitched. It might seem logical to set the angle of the reflector paddles at 45 degrees (as I have seen many suggest in their instructions) but this is absolutely wrong. If you set the angle at 45 degrees you will create what is known as a retroreflector. It is the same thing used in traffic signs which seem to "glow" at night when your headlights hit them. All the sun's energy which hit reflectors at this angle will be directed right back at the sun. See the illustration for an explanation.
The angle you actually want is about 32 degrees from vertical. You can find this value yourself by assuming the sunlight is hitting the collector area in a parallel fashion. From there, we take the half-angle and get the final spot where the light beam lands. It is important to not make the reflector too big because with a straight reflector it is only possible to get a certain concentration factor. For this oven, it is 5:1.
Step 3: The First Cut
USF and CERC present this as a guideline by which to build your own oven. We do not assume any responsibility or and assume no liability (expressly stated or implied) as to the performance or safety of this project. Please exercise caution when attempting to build and use this oven.
Now, lets get started.
Step 4: Create the Reflector Paddles
The reflector paddle is a simple trapezoid with the following special formula. The small upper part of the trapezoid is the same as the width of your cooking chamber box. The wider size of the trapezoid is 2.25 x [cook chamber width] and the height is 1.25 x [cook chamber width. Using these simple multiplications, the reflector will scale up to whatever size you desire. Additionally, it will effortlessly create the desired 32 degree angle talked about in the theory section.
Get some foil and tape it at the back to the cardboard with the SHINY SIDE FACING OUT. Try not to put too much tape on the surface of the reflector side. You will need to repeat this 3 more times . Easy!
Step 5: Assemble the Reflector Paddles
Step 6: Install the Glazing and Reinforce
Step 7: Assemble the Cooking Chamber - Heat Collector
You now have to paint the collector black. You must use a paint which can handle higher temperatures. Latex paint is not acceptable. Try to find a matte or flat finish black enamel paint. You can use a spray paint but be sure to get one without CFC's which can harm the ozone layer. Also, spray paints will require many thin coats but can produce and easier to apply and more even coating. Brushes work just as well because we are actually NOT looking for a smooth finish. In the end, be sure the paint is VERY black since there are many times it can look tinted and will reduce efficiency greatly.
After the paint is dry, fashion some clips to hold everything together. Grab your rubber bands and paper clips and connect them in a paperclip-rubber band-paperclip segment. Depending on the size of your oven, you may need one, two, or three clips per side. Below, we show an example of one and two clips per size. On the Cooking chamber, push one end of the paperclip through the cardboard to anchor it. Place the reflector on the cooking chamber and stretch the rubber and to clip the other paperclip to the edge of the reflector. Repeat for all sides.
Step 8: Assemble the Cooking Chamber - Insulation Layer
The last picture shows one of the two optional stands you can build. If you find a box just slightly large than the insulation box, you can build a very simple but somewhat flimsy base. Continue on for detailed instructions on the base.
Step 9: Building the Stand
NEVER STARE DIRECTLY AT THE SUN TO DETERMINE WHERE TO POINT!!!!
One option is to build a simple stand out of a box slightly larger than the insulation holding box.
Because this design was not as stable, we decided to show an option to a more complicated but far steadier box frame design.
The first two pictures show the easy way of building a stand out of a single box and putting pencils through as pivots. Alternatively, you can create a box frame by finding long rectangular pieces of cardboard and scoring the board with a knife creating 4 equally spaced panels. Tape this together to make a square-tube of sorts. You can fill it with more cardboard to stiffen if desired. Tape them inside a larger box and push the pencils through. This design is a little more elaborate and is customized for every build. Because of this, we can not really go into greater detail about how to build it.
Step 10: Test it out
So there you have it: an oven that can be built out of things normally destined for a landfill. Be sure to post your best temperatures in the comments below. Enjoy and thank you for reading.
-University of South Florida, Clean Energy Research Center Team
design and writeup by: mec 2010