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Build a Working Solar Oven with Cardboard and Tin Foil and Tape—240 Degrees in Minutes

Posted by Univ. of Florida, Guest Column

Dec 24, 2015 8:27:27 AM

At almost no cost, why not spend this holiday season making a low-cost, sustainable sun oven?solar-oven-green-builder-diy.jpg

A solar oven is a great way to conserve resources and tap into the wealth of energy that hits the Earth every day.  In this Instructable, we will show you how to construct a high efficiency solar oven out of mostly waste or scrap parts you probably have lying around your house. The intention here is to use these materials that would normally take up space in landfills for harnessing the power of the sun.  In this way, we are not only re-purposing and reducing waste, we are conserving resources. 

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.  

For this project you will need to find:

Several Different Sized Boxes (some very large ones are useful)
Clear Packing Tape
Styrofoam Packing Peanuts or Shredded Paper
Razor blade
Black spray paint (enamel) or black enamel Paint in a can (you should not use latex paint)
Scissors
Paper clips
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)
Rubber bands
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
Calculator
Optional:
Thermometer Probe to monitor temperature.

The Science Behind Solar Cooking

Before you begin building your oven, you may be curious as to how one works.  You may also wonder why prior ovens you have built did not work very well. Today, we are here to help with a quick explanation. 

solar-oven-materials-green-builder-diy.jpgsolar-oven-boxed-green-builder-diy.jpgThis 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. 

Safe Cutting

We will be using razors to cut most of the cardboard. You may use scissors as well, but most people find it more difficult. Use safe cutting techniques and don't rush. For example, always cut away from your body or hands so that if the blade slips, it will not strike you (or anyone around you). Also, once you start cooking with your new oven, remember that within minutes, it reaches temperatures that can cause significant burns. Treat it like a real oven when opening to recover cooked items. 

DISCLAIMER: USF, CERC and Green Builder Media 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. 

solar-oven-reflector2-green-builder-diy.jpg
Now, lets get started. 

Your first goal is to find a (somewhat small) to make the cooking chamber. To make the math simple here, we am going to assume you will find a box with identical side lengths.  What we mean by this is that where the box opens up, the length is equal to the width.  The depth should ideally be no more than the length or width. 

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.

After you have 4 reflector trapezoids, start by placing two adjacent with the shiny side facing in and tape along outside the diagonal side as shown in the pictures. Continue this for all 4 pieces and you should end up with what looks like a pyramid with the top missing. If you did this right, the reflective side will be on the inside of this pyramid. solar-oven-glazing-green-builder-diy.jpg

Now comes the dangerous part. You will need to find either glass or acrylic and cut it.  If you have glass on hand, you need to borrow or purchase a glass cutter which will allow you to score and snap the glass to the desired size to fit perfectly into the small square of the pyramid as shown. If you have never worked with glass, it is probably a lot safer for you to work with acrylic. Polycarbonate can be used as well but there will be a 2.2% loss in oven efficiency.

Cut the acrylic to the desired size and tape in to the reflector. At this time, add some tape around the edges to strengthen the easy to tear foil in preparation for the next step. It is okay to have a layer of tape on the reflective side near the edges.

Lets focus back on the cooking chamber. Grab some more foil and gently crumple it to obtain a textured surface. You should not crumple it to the point that holes start to develop.  When you straighten it back out, hold it up to the light to verify that it is not full of holes. Repeat this step as many times necessary to cover the inside of the box. From here, tape the protruding foil down. 

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. 

Find a box about 2-3 inches (5 - 8cm) larger on all sides than the cooking chamber. Fill the bottom with a layer of foam packing peanuts (or shredded paper) about 3 inches (8cm) thick. Place the cooking chamber on the peanuts and center in the box.  Proceed to fill the box with packing peanuts and tape secure. 

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. 

Build a Base

Now that the oven is ready to go, you need a base stand so that you can easily and accurately point it at the sun. Do not stare directly at the sun while finding the right focus. This may go without saying, but I've seen people do it.
solar-oven-stand3-green-builder-diy.jpg
solar-oven-stand1-green-builder-diy.jpgOne 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. 

Cooking Instructions

Now that your oven is ready, here are some final thoughts on using it: 

Cooking containers should also be black, so consider finding a can and painting it black as well. Because the paint may need some time to cure and may release extra solvents (read: bad smell), it is probably wise to run the oven a full temperature a few times without food in it to clear any smell. 

Find a place with a clear, unobstructed view of the sky. The time of day does not matter as much as one would expect.  We easy got past 115°C (240°F) in minutes at 6pm. Align to the sun by pointing in the general direction and tilting/turning until no shadow from the sidewalls could be seen in the cooking chamber. solar-oven-stand2-green-builder-diy.jpg

Remember that you need to adjust this every so often for optimum performance. Use a meat thermometer or temperature probe to ensure the position is perfect and the hottest temperatures are being achieved.  Lastly, if your food needs to be kept flat and the tilting might cause a spill, you can push a wooden dowel, piece of metal, clothes hanger rod, just about anything through the middle of the chamber to be able to hang a gondola for food. This will allow you to boil water even when significantly tilted. 

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.  

Original Project: University of South Florida, Clean Energy Research Center Team. Originally printed at Instructables.com


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