CERC Green Solar Oven
Intro: CERC Green Solar Oven
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.
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
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
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
STEP 2: The Science Behind It
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.
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.
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
We are about to begin. Safety should be the first thought! We will be using razors to cut most of the cardboard. You may use scissors as well but most people find it more difficult. Always remember to ask for help when you need it. Always cut away from your body or hands so that if the blade slips, it will not strike you (or anyone around you). Also, again remember that this is a real oven which within minutes reaches temperatures that can cause significant burns. Treat it like a real oven when opening to recover cooked items.
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.
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
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 . Easy!
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
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.
STEP 6: Install the Glazing and Reinforce
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.
STEP 7: Assemble the Cooking Chamber - Heat Collector
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.
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
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. Continue on for detailed instructions on the base.
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
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.
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.
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
Now that your oven is ready, here are some final thoughts. 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. 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. Enjoy and thank you for reading.
-University of South Florida, Clean Energy Research Center Team
design and writeup by: mec 2010
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
28 Comments
Chrystalkay 12 years ago
Or wouldn't a black ceramic lidded cooking pot work inside the chamber without the glass lid?
I am thinking of camping and such where you wouldn't want to cary the glass around, as well as weekend yard cooking.
SolarPoweredGardener 13 years ago
I'm now building a big box type with sun tracker... Am doing a instructable for it so keep your eyes open... Should have it finished by xmas 2010....
hyperfocused72 13 years ago
flio191 13 years ago
3point2 13 years ago
Sarah Stephens 14 years ago
gaiatechnician 14 years ago
That way, no glass at all! You might get less energy in because it is all reflected and none direct but you could have the top super well insulated and there would be much less convection losses from under the oven.
A guy called David Delaney thought of that years ago but I do not think he ever tried it.
3point2 13 years ago
sclausson 14 years ago
CapnChkn 14 years ago
The glass used should be TEMPERED GLASS. I know that will make it hard to cut, but I built one of these using quarter inch (6 mm) plate I picked up somewhere. After adding the reflectors, I saw using an oven thermometer, the temperature had reached 350 degrees F (175 C), grabbing my camera, I leaned over to take a picture, letting my shadow fall across the glass. When I leaned away again I heard a "CLINK!?!"
Looking back, the glass had broken. It didn't shatter, it cracked in a wavy pattern from one side of the glass to the other, perpendicular to the surface of the glass. Though that sounds anti-climactic, the edges were still "sharp as glass."
sdallesasse 14 years ago
redplanet 14 years ago
However, if you have a piece of tempered glass to recycle, e.g. from a refrigerator shelf or whatever, you could design the oven around it.
johnny3h 14 years ago
IF the edges of one's multiple glass panels are properly fitted, the millimeter, or so, air gap will not be significant in the escape of either heated air, OR reflected infrared energy.
So, to make a larger opening, use multiple pieces of Tempered glass windows from whatever source.
gaiatechnician 14 years ago
So just like concrete roads need expansion joints, it seems that glass needs them too in solar ovens.
johnny3h 14 years ago
For years, all across Texas, tempered automobile back glass windows have shattered during the summer months due to being parked in the hot sun with all the other windows closed. It has happend to me once, and I have known several people over the years who have experienced the same problem.
Therefore, Tempered glass is no improvement, AND you CANNOT CUT Tempered glass AFTER it has been tempered. It HAS to be cut/shaped, and edges finished BEFORE TEMPERING. I got this info from a Tempered glass fabricating company years ago.
scraptopower 14 years ago
3point2 14 years ago
pattyaitch 14 years ago
My first solar oven was one we made from a foam cooler box. We cut the front 33°, as that seemed to be the best angle for the sun in S.Cal. The glass we had cut from double strength~~never even thought of tempered way back then. It was lined with foil, with a black cloth on the bottom (over the foil) and the reflector was made of three asphalt floor tiles covered in foil. Favorite thing to cook was Jalapeno/cheese strata.
Sonoffar 14 years ago
lieuwe 14 years ago