After I built the solar oven, it was time to test it. Being an engineer, I wanted to start out with some measurements.
My first two tests were as simple as it gets: heating water over the course of a sunny afternoon. To monitor the performance of the oven, I inserted a thermocouple (a device which measures temperature) into the water and recorded its output in an electronic datalogger. Before I describe the results, I'll do another thing that engineers like to do: attach caveats to test results. These results are for my home-made oven, not for all solar ovens. The book I consulted ("Cooking with the Sun") and an article in the San Francisco Chronicle Food section say that solar ovens are capable of far higher temperatures than mine produced (some solar ovens can even bake cookies or bread). That said, I think the results are pretty amazing, considering that I started with cardboard boxes, foil, glue and glass.
Let's take a look....
The figure below shows temperature traces for the water in the pot and the air in my corner of Berkeley (air temperature data are from a Weather Underground station near the Claremont Hotel). The water got pretty hot — 180°F (82°C) — but not to the boiling point. There are a few possible explanations for this: the gap between pot and lid resulted in evaporative loss, the pot was too heavy, my oven is poorly built, or the reflector is too small. (Lightweight pots are actually preferred for solar cooking so that the solar energy goes to heating the food instead of the pot. The heating is so gentle that you never need to worry about burning the food.)
The first dish I cooked was lentil soup. The recipe was preposterously simple:
Combine washed brown lentils, chopped carrot, chopped onion, a bit of Mexican oregano, and water in a pot. Place the pot in the solar oven. Let it cook for 3 hours, occasionally turning the oven to follow the sun (about every 30 minutes). Just before serving, add salt, pepper and chopped fresh tomatoes to taste.
I started cooking at 2 PM, with a temperature of 75°F (24°C). At 3:30 p.m. the mixture was 137 °F (58 °C). At 5 p.m., 196°F (91°C). Again, it didn't reach the boiling point, which might set off food-safety alarms in some people's heads. Pathogens such as salmonella or E. coli are killed at temperatures below water's boiling point, provided that the mixture is held at a high enough temperature for a suitable period of time. The Answers.com entry on pasteurization has a few datapoints, including this one for milk, from the Sci-Tech Encyclopedia: "145°F (63°C) for 30 min; 161°F (72°C) for 15 seconds; 191°F (89°C) for 1 second; 194°F (90°C) for 0.5 second; 201°F (94°C) for 0.1 second; 204°F (96°C) for 0.05 second; or 212°F (100°C) for 0.01 second."
The ease of cooking was matched by the quality of the results. The slow, low temperature cooking preserved the integrity of each lentil, resulting in some of the finest lentils I can remember. (I hope to try cooking the sublime Rancho Gordo heirloom beans someday)
In subsequent weeks I cooked brown rice, basmati rice, and Indian lentils (Kali dal) with good success. I also tried drying slices of tomato and slices of plum, with less success. It is not easy to have sufficient air circulation in my oven (even with the lid propped open slightly), so it was more about heating than drying. Perhaps I'll try again one of these days with some modifications (e.g., place the fruit on a rack a few inches below the glass, with the cover propped open slightly).
One day I even tried something that solar oven enthusiasts often tout: cooking dinner while you're at the office. In the morning, I put a pot of ingredients for lentil soup and a pot of rice and water into the oven and went off to work. When I got home, I expected to have my dinner ready to eat. But no. A gust of wind during the day blew the cover closed, causing my solar oven to become a "no-lar no-ven."