This instructable is compacted "in a nutshell" version of "director's cut" tutorial. If you look for mor more details and some experimentations overviw, here's the original instructable.
This idea originates from Roger Bernard's Solar Panel Cooker design.
Basically The Solar Panel Cooker's design involves a dark colored metal pot enclusured within bigger transparent glass pot with reflective panels enhancing the amount of sunlight provided to them. While inner pot absorbs heat and transfers it to food within and surrounding air, the outer one incapsuletes the air inside, thus creating greenhouse effect which allows to achieve reasonably high temperatures.
One thing I'm implementing here, while taking Delaney's design as a basis is substituting inner pot with magnetite rocks, which gives some advantages and disadvantages over the initial design.
This tutorial will show you how to make a solar driven cooking device that... is... good.
So, first thing, you'll going to need is some magnetite rocks.
Magnetite is an iron oxide mineral. It was widely used for centuries as black pigment in paints, but even now magnetite has a bunch of uses in industry. It also is known as being a food additive (colorant) E172.
You can buy magnetite online or find in nature where it's readily available. You can often find it in a beach sand or in a rubble washed by river.
To identify magnetite you have to look for black heavy rock with rather glossy surface. But the main distinguishing feature of magnetite is that it is one of few minerals that are attracted by magnet.
Since magnetite is naturally black it suits perfectly for the porpouse of this project. Although you can use any black coloured stone, magnetite is preferable choice, since it has high heat capacity accomplished with high density.
When you have your magnetite gathered, you want to wash it before using it for cooking. Rince it first to wash away all the sand or soil. Then brush each stone with some dish washing liquid.
You'll also going to need some stand for cooking. You can make the simplest one with three metal pipes (aluminium in my case) and a metal ring. It doesn't has to e tightened: the structure interlocks itself and can be aesily adjusted.
With this technique you can cook even meat. To demonstrate this I'm going to bake some chicken legs. The essential thing here is to get the temperature in the pot higher than 60*C. At this point all the bacteria are going to die, and the cooking process will take place. Cooking at such temperatures will take more than few hours making the technique identical to cooking with slow cooker.
In the "director's cut" version of instructable, I'm making a test run and going in deeper details on measuring temperetures within a pot while the cooking process. But, anyway, I recommend to buy yourself a suitable thermometer and use it to keep everything under control.
To collect sunlight and dirrect it to your pot you'll need some reflector.
In this particular case I used some old parabolic dish antenna. I've made it's surface reflective by gluing strips of aluminum ahesive tape to it.
The idea here is to collect as much of sunlight as posible on the cooking pot as possible. It doesn't have to be placed strictly in a focal point of the dish, and all the way other variations of reflector can be used as well, as it'll be showed in next step.
The advantage of magnetite cooking technique is that you can use any transparent container to cook your food. Like here I'm using 3L glass jar to bake some potato.
Alternative version of reflector presented here was made out of a piece of cardboard with aluminium foil glued to it. Some reflective tape was used to reinforce the structure.
It took me abot five hours at a outside temperature of ~26*C to cook those chicken leggs.
Since the cardboard reflector is less efficient than parabolic dish antenna, it'll take a bit longer to cook stuff with it, as it was with my potatoes, but it's still reasonable option.
At some point while cooking my chicken and potatoes, I've had run out of solar light due to the shadows from the house and trees. And this is where magnetite qualities come to play, since you can use storred in the stones heat to continue cooking (or atleast keeping the food hot for a while) your food for some time even after the solar energy is no longer applied.
Just wrap your pot in few warm blankets, and at least for an hour the temperature within will stay high enough to continue cooking your dish. For two more hours (approximately) it'll stay hot enouch for comfortable consumption.
As you can see I was wrapping food bits in aluminium foil for the cooking. If you're sure enuough that your magnetite is all the way around food grade, you may not use the foil. But despite the wrapping I still had some chicken gravy leaking out of the "packages" (I used it spice up the potatoes later). Since you want to reuse your magnetire stones, you have clean them each time after cooking. To do so I just rinced them with warm water with some dish cleaning liquid involved.
In the result of all mine magnetite cooking I've got some tasty supper today. The main disavantage of the magnetite stone method of cooking is that you can not make soups or similar dishes with it. I'm touching this issue as well in expanded instructable. The main advantage, on the other hand, is that you can use any transparent vessel to cook with. I have my mind still working on this idea, and I already have n concept of a survival-ish version of solar-magnetite cooker.
So, this is it for now. Thanks for your attention, and go cook something.