The functioning principles of a solar oven are simple: concentrate, convert, contain. Sunlight — or visible light — is concentrated by several reflective surfaces to pass through a glass lid into an insulated box. A pot of food you put inside the box will absorb the light and convert it into longer-wavelength infrared energy, or heat. The insulation will inhibit the heat from escaping, and the wavelengths will be too long to pass back through the glass lid. So, they’ll bounce around and heat up your food.
How to Build a Solar Oven
This solar cooker requires only cheap materials and the design is so simple: square, with an inner box (metal) and an outer box (wood) separated by 1 or 2 inches of insulation (heat resistant), and a glass cover on the inner box to let in sunlight. Additionally, four reflectors are arrayed at obtuse angles to the glass to focus more light into the box and raise its internal temperature enough to make cooking possible.
Cover reflectors with basic aluminum foil, Mylar tape, acrylic mirror or other highly reflective products. The top of your solar cooker should be inclined, either by design or by tilting the box on blocks. The best angle is based on your latitude and the declination of the sun, but in the Northern Hemisphere it should be about 30 degrees during summer and 60 degrees during winter. To increase your solar oven’s ability to convert light to heat, paint the bottom of its interior with black, high-heat paint. To improve heat-holding ability, add some thermal mass, such as a large rock or brick.
How to Use a Solar Oven
Time. Cooking times are longer than in a conventional oven because average temperatures fluctuate during the day. As long as the oven temperature remains above 93°C, your food will still cook.
For best results, preheat the oven for at least 30 minutes. Adjust the cooker now and then. The reflectors will angle sunlight into the box for about two hours of the sun’s path across the sky, after which you should rotate the cooker to follow the sun. With this method, solar cooking will take about twice as long as cooking with electricity or gas, but cooking time will decrease dramatically if you rotate the oven every 30 minutes. On days with strong sun and no cloud cover, two to three hours is enough to cook almost anything, from a pot of rice to a loaf of bread, without adjusting the cooker.
Cooking with Solar Energy
When cooking grains or beans, you only need a little more than half as much water as on a stovetop. Cooking veggies in the slow, even heat results in incredibly savory dishes. Even bread, pastries and meat will do well, although you’ll need to maintain a high average temperature, so plan in advance, and cook on days forecasted to be completely clear and sunny. Frying is difficult as the temperature typically isn’t high enough and you’ll lose heat every time you open the cooker to stir the pan — but you could experiment with quick-cooking foods, such as eggs.
Dark-colored cookware works best — dark objects convert sunlight into heat energy more easily than light-colored or reflective objects. Cast iron, black enamel and dark ceramic are good options, and lids help hold in heat. But, when cooking colorful veggies, don’t use a clear lid. The concentrated sunlight will bleach the vegetables’ color.
I painted the outside of a couple of wide-mouth, quart-sized Mason jars flat black with high-heat paint (including the outsides of the lids), and these became my rice- and bean-cooking jars. They heat up quickly, are space-efficient, and double as storage jars for leftovers. Never fill the jar more than half-full, including water. A cup of brown rice needs 11⁄2 cups of water and is usually done after 70 minutes. Don’t tighten the lid all the way when cooking in a jar or you’ll risk an explosion.
Model du Pont de Vie
- Cooking space; box made of sheet metal painted with heat resistant paint inside
- Glass panel and access hatch
- Insulation cut out of a roxul (stone whool) insulation panel
- 1/2″ plywood outside box pieces
- Cover lid with mirror surface to add sunlight
The dimensions are determined by the size of the oven you need.