Cooking with the Sun
You would have expected to run into an ocassional solar cooker, here in the tropics, where the sun shines bright and hot, most of the year. If you look up a site like http://solarcooking.wikia.com/, you’ll find plans to make a box cooker for a few hundred rupees, about $5 or so. But I don’t remember ever seeing a solar cooker, except perhaps in some exhibition, long time back.
About a year back, I decided to make one. Mostly because there was some granite tiles left over, from a renovation work. I got some aluminium angles,
and a piece of glass, and made a box with granite sides, and a glass top. Granite is relatively a good conductor of heat, had to insulate the sides, and bottom with newspaper.
One thing, during these days of hectic construction, where every available space, is filled with concrete boxes, or paved with concrete, anything related to construction is cheap. With a hacksaw, drill, C-clamps, and some idle time, you can build a few things.
Anyway, the first version of the cooker, didn’t work at all. With a black clay pot, and some rice, nothing happened. After a couple of attempts, on one occasion, a grain of rice got cooked. Just one grain, enough to inspire another attempt to make a better cooker.
I didn’t realize at this time, that rice is relatively hard to cook. This first cooker, was used to dry things. If you are in a city (smart or otherwise), keep anything outside for just a day, you’ll have a layer of dust, and smoke. Not something you want on your food.
Anyway, after almost a year, I built a new one, with granite, and better insulation. I also added two mirrors, on the longer sides. The mirrors, equal in area to the top glass surface, are at an angle of 60 degrees to the horizontal. So each mirror redirects only half the amount of light (cosine of 60 degrees is 0.5). With two mirrors, the amount of light is doubled (not tripled), and that too, only when the sun is directly overhead. Mornings and evenings, a good deal of light falling on the mirrors, don’t get directed toward the top glass.
The inside surface of the granite tiles were coated with charcoal from coconut shells.
We tried a little rice, on a black clay pot. Regular clay is porous, and lots of transpiration takes place, which cools it down. The one we used was from Manipur, black longpi pot. This turned out to be a poor conductor of heat,
and didn’t work. The next day, I tried just plain water, in an uncovered pot, but water didn’t get to boiling. Just a few bubbles formed at the bottom of the pot. But the steel pot holder (coaster) got unbearably hot.
For the next trial, I decided to use cardboard on the inside, with black paint -coconut shell charcoal, with coconut oil. Coconut oil is not a great binder, but it suited our purpose.
Since cardboard is a great insulator, and granite really bad, this change was a crucial one.
Also we used a heavy duty copper-clad pan, with horse tail (kuthuravali, குதிரைவாலி) millet. By noon, the millet got cooked.
Over several days, we tried moong dal (green gram), rice and toor dal (pigeon pea). Moong dal took 3 hours, in the evening sun, but rice and toor dal failed. So we decided to try whole day cooking, for rice, and finally managed to cook it, after 6 hours in the sun.
The cooker cost about 2000 rupees, but with a little optimization could be made for 1000 or less. At this point, I’m guessing that with two cookers, and some planning, we can do 30% of our cooking with the sun. 20 kg commercial gas cylinders cost about 1300 ( it even went to 2000), and a family of four/five will use 6 to 8 cylinders. That makes it a year, to recover costs. If one uses 15 kg domestic, subsidized cylinder which costs 450, then it will take 2 years. Of course with a different set of materials, one could make a cooker, lot cheaper. Also, there are things, which may be a lot efficient on a solar cooker, say roasting peanuts. A few more experiments are required.