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. 

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

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. 

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. 

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!

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. 

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. 

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. 

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

Comments

[Chrystalkay]

Question... Wouldn't a glas topped pyrex type baking pot be an acceptable heat absorber? Does the whole top have to be glass? Or would there have to be extended reflector material around the inside of the box too to make something like this work?
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]

I made an open type cooker to see if they worked, the type you put the pot in an oven bag... It worked well.... The bags kepy breahing though so what I found was big plastic domes... You can get them in gardening shops... Thay are cloches used to cover young plants to protect them from the frost... They work great and it's easier to remove of you need to stir whats cooking...
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]

You can not use styrofoam for insulation because they give off toxic fumes when heated. Read the introductory manual at www.solarcooking.org for safer alternatives.

[flio191]

To add to the efficiency of the heating, it would be cool too see an insulated window used in place of the glass, as air, or a vaccum would be the best insulator for convection heat.

[3point2]

Yes, double glazing would be more efficient.  Keep in mind, though, that the additional panes of glazing due cause losses of their own so you have to weigh your options.  An evacuated (vacuum) glazing is the most ideal since there is absolutely no convective losses.  That said, this was supposed to be a project anyone could do using stuff they found around the house that would end up in the trash.  It is very unlikely someone has the setup required to create an evacuated double pane glazing for their oven.  ... although... that does sound pretty cool now that I say it!  8^)

[Sarah Stephens]

I've heard of making a solar oven before using transparent oven bags in place of glass.  Can anyone tell me if this is a good idea?

[gaiatechnician]

Why not a thin black piece of metal  in a hollow under the oven and reflect the light to it?
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]

Yeah, this larger black absorber box underneath might work but would probably cause extreme losses in heat.  Consider this, there is a very good reason your oven (or even toaster oven for that matter) has a door.  It's not that you couldn't heat the food without it but you would loose most of the heat through convection.  Actually, very little light is reflected off the surface of the glass if the recommended angles are observed.  Given the Fresnel equations I solved for, the losses are only about 8% in the worst case.  A way better scenario than the losses due to an open air cooker. 

[sclausson]

When I made solar box ovens I used the large oven bags in place of the glass. To hold the bag, cut down both sides and open up. I made a frame from a screen repair kit. It sounds like a drum when you thump it. When bag wears out simply remove and put a new one in. Make a foil hinge to hold it.

[CapnChkn]

Excellent Instructable, just one caveat.

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]

I would think that laminated glass or wire reinforced glass would be the two best choices considering this information.  They allow the least fragmentation to be released.

[johnny3h]

I do hate to have to inject a negative here, BUT... thermal imbalances on Tempered glasss WILL also cause shattering.  The only difference being that instead of a single crack as described, the Tempered glass will have thousands of smaller cracks following the crystal pattern in the temper.

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.

[redplanet]

Tempered glass cannot be cut. I speak as a former professional glazier. Any attempt to cut tempered glass will shatter it into tiny crumbles.

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]

In addition to redplanet's point, what difference does it make IF one's glass is not one piece??

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]

There is a guy on youtube who made his oven from scraps from construction sites.  He just uses 2 pieces of glass, he has them loosekly put in  and accepts the little loss between them.  I think this cracking (which is a big feature of amateur build solar ovens) is mostly because people have the glass in tight. The expansion widthways and lengthways (with no room to move)  when it heats up and cools down is what is causing so many breaks.
So just like concrete roads need expansion joints, it seems that glass needs them too in solar ovens.

[scraptopower]

Could you use the glass from an old microwave or oven ? That would be easy to get hold of and temperature resistant ?

[3point2]

Good Point.  I wrote this with a broad audience in mind so I was hoping anyone much under 18 would choose acrylic with the risk of it sagging under the high temperature that can be reached.  Indeed, the sudden heat change from a cloud can cause the temperature to drop and thus risk cracking the glass.  One possible option is to place several black or black painted stones inside to act as heat reservoirs.  Not only will this keep the temperature more steady but will hopefully put less stress on the glazing.  The downside is that it will not heat up nearly as fast from a cold start. 

[pattyaitch]

I love it.
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]

A very good instruct-able. I think even I could follow your guide and end up with a functioning solar cooker. I assume this was a student design assignment and I am curious if any one has done a cradle to grave cost assessment on this sort of "resource friendly" item. By that I mean:  What does it cost to produce and or manufacture all of the components utilized? Which components could be reused or recycled? Which components would be 'trashed'? And of course is there any cost or environmental savings achieved with this sort of project?

[lieuwe]

 awesomecakes, about the flaps needing to be 35 degrees tough, that's only valid if your flaps are as long as your glass, the angle is approximately 26.4735*tanh(2.33776*x) with x being the glass-to-flap ratio, eg if your glass is 13 cm and your flaps are 26cm, x is 0.5, which would result in an angle of 22 degrees. but don't rely on this heavily, i could have made a huge mistake somewhere in that formula. anyhow, nice ' ible, just built my own, gotta test it tomorrow tho, it's already dark here(and it's cloudy)...

[lieuwe]

 that formula is too inaccurate, i made a huge mistake, making a new formula, should be done in a couple of hours 

[3point2]

Though I don't think I mentioned it, the simple multiplication guide to get the ideal angle takes into consideration the refractive index of the glazing.  If the light comes in at a super low angle, it will be almost totally reflected due to the higher refractive index of the glazing.  (At low angles, windows can act like mirrors.)  While those are not the idea numbers, they are very close.  If someone was looking to improve dramatically, they could use real scrap or broken mirrors as the reflectors and that would probably give good performance enhancement. 

[lieuwe]

 making it round ought to help some too, now the sun hitting the edges off the reflectors needs to be reflected twice, but it works pretty good already

[peaceamidst]

    Nice, reminds me of when me and my brother used to make small tin foil ovens. We didn't make a hole in the top or anything though. We would just put like a piece of bread in it and set it outside. But we used to make little drawers and stuff. It was funny.

[kelseymh]

Extremely well written!  There are a few typos ("Lets" instead of "Let's"), but nothing substantial.  Good explanations of the "why" behind the design, as well as practical construction details.

Rated and featured.

[Kiteman]

(I don't see any check-marks in the feature settings?)

[kelseymh]

What?  You're right.  Let me try again...

[Kiteman]

I see them now.