Box Style Solar Oven

Solar oven
Box type solar oven

A basic box style solar oven is quite easy to make. The power supply is already there and the only connection problems are called ‘clouds’.

The principle is simple too. Simply allow the sun to shine on an object and capture the heat that falls on it. Not much to it, really.

What do we need?

The basic requirements are also simple. You need a box and transparent cover, a pot and something to hold the pot with when you take it out. Then you can modify it…that’s where the fun starts.

The box

The box should be able to easily hold your pot and be made of an insulating material to help keep the heat in. I chose wood because I had an old lightbox handy. Cardboard is another cheap option. Metal isn’t such a great option as the oven will lose heat quickly once the sunlight stops shining. I think its better to keep a constant, high, temperature than to have lots of spikes and drops.

The box should be as airtight as possible to stop heat escaping and the place where the cover sits on the box should be as air tight as possible. Gaps here are where most heat can escape. I have a thin film of roofing silicon on both the glass and the wood that pressed together well when the lid is closed.

The cover

The cover can be clear perspex or plastic, I prefer glass and conveniently, I had a piece that fitted with a little fiddling with the box. The cover in the picture above is sloped at around 20 degrees. That makes it able to increase the sunlight hitting its surface and passing through. 35 degrees would have been perfect for where we live and will probably be different where you are too.

The lid doesn’t have to be attached either, it can just sit on the top of the box.

The inside of the box
I found that the size of the box was too big, so made these panels to fit the corners to reduce the overall internal size.

To reflect or to insulate?

There are two schools of thought about the inside of the box. The one I subscribe to likes a lot of reflective material to reflect the sunlight and heat t onto the pot. The other school likes dark, heat absorbing materials that will capture heat and radiate it so that the cooking pot will be heated.

You can also add reflector panels made from any reflective surface to bounce more light and heat into the box. Reflective insulation such as those car windscreen sun shades can help improve the ability of the box to retain heat too. I added one to the bottom of our cooker.

This one’s even fan forced.

As an experiment, at the end of last summer, I fitted a small fan inside the box. This recirculates the air inside of the box and, I hope, contributes to more even heating of the pot. I added a raised wire tray to allow air flow underneath (a part of the pot that doesn’t get directly heated in some designs). Its only recently that I’ve started using the cooker this Summer, so haven’t got any comparison figures yet.

The fan and insulation
A fan circulates the air in ours and the base is lined with a reflective car window shade

The fan is powered by an old solar panel of the type that sits on car dashboards to keep the battery charged. It is directly wired to the fan so that the fan only blows when the sun is shining and blows more as the sun gets brighter.

A stand

A stand isn’t necessary, the cooker can, quite happily, sit on a table or the ground. Our cooker gets around on the base of an old swivel chair to make it easier to move.

Something to cook in

Now for the cooking vessel, or pot. Obviously, we need something that can absorb heat and transfer it to the food. Thick glass, stone-wear or metal will do the job nicely. A darker coloured material is also more effective than a shiny one.  The vessel should have a lid too, to keep the heat in. I like those really old heavy enamel camping pots that you get from camping stores. Cast iron works too, but it has to be really well prepared.

So, how does it really work?

Sunlight is light and heat. Some of the heat is absorbed through the glass and conducted insided the box, but most of the heating comes from the light.  Photons which have very short wavelengths pass easily through the glass. Once through, they either strike the cooking vessel or are reflectes from the walls onto the cooking vessel.

Once they strike and are absorbed by the cooking vessel, they lose a lot of their energy which is radiated as heat. Now, here’s the good bit…

Heat has a much longer wavelength than light and won’t pass through the glass (well, a little will be conducted, but we’re generalising here) and are reflected back to be picked up by the cooking vessel and passed to the food inside of it.

As the cooking vessel, food, and the air inside the box heat up, the insulating properties of the box help to keep that heat right around the cooking vessel, heating it further or at least, not letting it lose heat.

The result – slow cooked food!

What temperatures can we expect?

My first test this year was on a clear day with an air temperature of 21 degrees. The pot got to 60 degrees and stayed there. Obviously, the hotter the day, the hotter the inside temperature will be. The fan didn’t seem to make a difference to the temperature but probably distributes the heat more evenly.

What can we cook in a Box Style Solar Oven?

These ovens are best for things that benefit from being cooked slowly. Anything that you can roast or bake. I like doing potatoes and carrots in our cooker. Peas, beans, all that good stuff. Rice and beans do well, just add the same amount of water to them that you would do when cooking, then half a cup more.

These cookers are also excellent for defrosting food as they provide a constant heat that doesn’t get too high.

Will there be any problems?

A couple of minor things. Watch for things that will block the sun. People, washing, anything that gets in between the oven and the sun.

As the day progresses, the amount of heat in the sunlight varies due to differences in the angle it comes to us through the atmosphere. This means that in the morning, you will have less heat than in the middle of the day because the sunlight is coming to you through more atmosphere which absorbs some of the heat.

Also, though its not really a problem, just worth noting, the sun moves across the sky during the day. That means it won’t always be shining directly on the oven. All you need to do to compensate for this is move the oven to several times during the day so that the face of it is directly in as much sun as possible.

In the late afternoon, you will notice a drop in temperature as the same thing happens.

My last recommendation is to keep oven mitts handy. Those pots get hot!

Soon, I’ll be posting even simpler designs from even simpler materials on our Low Technology page

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