A short while ago I decided to build the Parvati parabolic solar cooker, which I first heard about at Homegrown Evolution. Made from cardboard, glue, aluminum foil, and some twist-ties, it cost me nothing more than a few hours of my time and pennies' worth of materials. You can find a pattern for making a parabolic solar cooker here.
Apparently, tall and narrow pots are best with parabolic solar cookers like the Parvati. This is just the opposite of what is recommended for solar box cookers, which work best with short, squat pots and pans. In both cases, black metallic cooking vessels are recommended. Fortunately, just about the time I finished my Parvati cooker, I was given the chance to look over a box of kitchen equipment that a family friend was getting rid of. In it I found an old fashioned coffee percolator pot, which was higher than it was wide, and not too tall. I spray painted it black.
I didn't have much to go on when it came to the actual cooking. I was waiting on a solar cookbook from the inter-library loan program, but I had finished the cooker and the coffee pot was ready to go. I decided to make an experiment on a mostly sunny day. The experiment would consist of taking a shot in the dark with 2 cups of basmati rice. I can't say the cooking was a resounding success, but I think it's worth making a report to illustrate what went well, and where the obvious mistakes were made.
I put two cups of rice in the coffee pot and rinsed it with a few changes of water. I poured off most of the water and added back three cups of water. I put the lid on the pot, and put the pot in the solar cooker, with it aimed at the sun.
The temperature outside was about 80 degrees when the rice began to cook at 11:30 am. I decided to take regular measurements of the temperature with my cooking thermometer. After half an hour the temperature of the water inside the pot was 108. At 12:20 it was 135, at 12:50 - 156 degrees, and at 1:45 it was 170 degrees. Then I had to go run an errand. I got back at 3:20 and the thermometer registered 192 degrees. Just 20 degrees shy of boiling in an unsealed solar cooker on a partly cloudy day! I had shifted the cooker slightly a couple of times to track the sun across the sky.
By the third temperature check, the delicious popcorn-like aroma of basmati rice was wafting out of the cooker. Flies buzzed around, drawn by the scent. But there was no way they were going to even think about landing on the coffee pot; it was far too hot. Instead, they buzzed around ineffectually, perching sometimes on the rim of the cooker. I imagine they were pretty well blinded by the concentrated rays of the sun anytime they got near the inside of the cooker.
After four hours in the solar cooker, the temperature of my rice was 192 degrees F. Using a hotpad, I picked up the coffee pot and carried it inside, where I eagerly removed the lid. Let's just say I'm going to need to do some further experimentation. The rice was indeed cooked. But I probably should have stirred it once or twice during the cooking process, or perhaps allowed the rice a 20-minute soak. The rice on top was fluffy to the point of being waterlogged, while underneath were small grains that hadn't absorbed enough liquid.
But I certainly learned that the parabolic cooker works well with the spray painted coffee pot, even on a partly cloudy day. The heat built quickly at first and then rose steadily. Getting the temperature up to 140 degrees quickly is important, because at that temperature, most food borne pathogens are knocked out of commission. While a few bacteria can survive at that temperature, they certainly won't be thriving or multiplying. I also learned that burning food is not going to happen with a solar cooker. This may not be a fast or fool-proof cooking method. It may not work for all kinds of cooking, and it may not work in my area during all parts of the year. But as a low-effort, low-input source of cooked food, I think it's worth a little more experimentation.
In the meantime, my inter-library loan request has come in. I'll be reading up on solar cooking advice from someone who purports to know. We have overcast weather predicted for the next few days. But the next time the weather's clear, I'm giving this solar cooking thing another shot. Stay tuned for further reports.