Solar Oven

I've been interested in solar power for a while, and not just in using the sun to generate electricity via solar panels, but also in the use of solar heat (e.g. Stirling Engines, etc).  I read about solar box ovens as a cooking solution for places where resources are scarce, and was intrigued by how easy they seemed to build.

These ovens work using the greenhouse effect.  Sunlight enters an enclosed space and the light is converted into heat and trapped in the oven, cooking the food.  Two boxes are used to create insulation and minimize heat-loss.  Tinfoil reflects the light/heat and cooking is done in dark containers which will absorb rather than reflect the heat.  Glass or high temperature plastic (e.g. baking bags) is used to create the window through which the sunlight enters.  The temperature inside gets high enough to cook food but not burn wood or paper.

Two weeks ago I looked up the instructions, and since the kids were otherwise occupied and I had two appropriately-sized cardboard boxes at my disposal, I decided to give it a shot.

The first thing you need is some decent sized cardboard boxes, where the smaller box is about 1 or 2" smaller in all dimensions.  The instructions recommended they be at least 15x15".  I used an old speaker box and a paper box.

You'll also want a roll of tinfoil, a large piece of cardboard (3" bigger than the footprint of your big box on all sides), newspaper or a few cardboard tubes, a bottle of non-toxic white glue, a box cutter, a pencil, and a turkey oven bag. Clamps are also helpful.

The first thing to do is to close up the large box, and center the smaller box on top, then trace the outline (see below).  Then cut through the lid and flaps of the large box so the smaller one can fit snuggly into the hole.

Next, line the inside of both boxes with tinfoil, shiny facing towards the center of the box.  Use lots of non-toxic white glue.  I assume white glue is used because it won't melt or give off fumes. I used about half a bottle.  You want the entire inside of the box (bottom, sides, remainder of the inside-top of the outer box to be lined with foil.

Here's the finished inner box.  Note that in my case, there were no top flaps. Normally you'd use the top flaps to help connect the inner box & outer box.  Since I couldn't do that, I left lots of spare tinfoil sticking out the top.

The top of the inner box should be flush the top of the outer box, so you have to put some kind of spacers between the two.  I had a really strong cardboard tube in a kind of curvy v-shape, so I measured the height difference and cut it into a number of pieces with a saw.  I then glued them in place in the bottom of the outer box (below).  You can also use newspaper or cardboard tubes cut to size.

Now you can slide the inner box into the outer box.  If your inner box has flaps, fold them out and glue them down.  Since mine didn't, I folded the extra foil over and glued it down.  The results are below.  Note that I did not put any additional "insulation" between the boxes, but I think it would work better if you do (e.g. crumpled newspaper).

The next step is to make the lid. For this you can either use the top to your outer box if you have one, or you can make your own lid with a large sheet of cardboard, which is what I had to do.  Turn the oven upside down onto the center of the cardboard and trace it.  You'll need 3" or so of overhang around the whole thing.   Cut tabs so that you can fold the cardboard into a lid, and glue the edges together.  It helps to use clamps here.  Next, cut a flap the size of the inner box into the lid.  This will be a reflector, so only cut it on three sides.

Line the inside of the flap with foil, since the goal is to reflect light into the opening.   You can use a piece of coat hanger to hold the flap open.   This is visible in some of the pictures below.

The last part of the solar oven is the window.  You can use a sheet of glass, but it's much safer and lighter to use a turkey oven bag.  The large sized Reynolds oven bag was perfect for my lid.  Glue the bag closed (run a line of glue along the inside edge, and then press down with something heavy like a stack of National Geos. Try to make sure the bag is sealed with no air inside, and flat. Wrap the edge in wax paper so if glue leaks out the bag isn't stuck to your furniture workbench (no honey, I definitely did not do this on our buffet).

When that is dry, glue the bag to the inside of the lid, sealing it all the way around.  Here's what it looked like when dry.

You can optionally make a "drip pan" by cutting a piece of cardboard that fits into the oven, gluing foil to one side, and then painting the foil black with non-toxic paint.

Now we're ready to cook!

There's no better way to break in an oven than with brownies.  The first thing I did was "preheat" the oven by leaving it in the sun for an hour with a few bricks inside.  The bricks help keep the oven hot after you open the lid to put the brownies in.  They're resting on some black construction paper that was on my "drip pan."

We used regular brownie mix and put it into a dark cupcake tin.  Dark pans work best because they'll absorb rather than reflect the light and heat.  The cupcake tins also mean more surface area vs a flat pan.  Once the oven was hot, we quickly opened the oven and lowered them in.  Remember to use oven mitts!

Here's the oven with the brownies cooking.  When the sun moved, I noticed that the front (left, in the picture) side of the pan was falling into the shade, and the brownies on that side were not cooking as fast.  To fix this you can quickly remove and rotate the lid 180 degrees and turn the oven to face the sun again.  The result is like rotating the pan, but this way you don't have to remove the pan from the oven.

The finished product!  Of course, since the oven doesn't get as hot as a real oven, you have to let things cook longer.  Ours cooked for about 90 minutes, and we checked them using the "clean toothpick" method.  They could have cooked for another 15-30 minutes since the bottoms were not quite done, but then the "mushy part" was the kids' favorite.