Tips on Construction and CookingMaking a Solar Box Cooker is like making a dress: it must fit your needs and be constructed and used according to certain parameters or guidelines. Knowing how parameters affect the efficiency of a solar box cooker, you will be about to cut corners in one area (i.e. conserve a certain material that is in short supply) while still arriving at a design that works.
Oven SizeIf you are a single person cooking for yourself, or if you just want to experiment with solar energy, you can make your oven rather small. If you plan to cook for a family, then you will want to make yours close to the standard size (18" x 24" glazed opening), and you will want to make it more substantial to stand up to everyday use. One way to decide on the depth of your cooker is to measure the pans you will use. Make the cooker about 1" (2.5 cm) deeper than your pots are high (including lid).
ReflectorIn general, it's best to make your reflector as large as the entire lid -- not just the size of the glazed opening. This is true especially in areas far from the equator or when cooking during the winter.
LidBe sure the lid fits snugly. By making the walls level all the way around on top, you provide a flat surface for the lid to seal against.
TrayThe bottom surface inside the oven of course needs to be dark. While darkened cardboard or other non-metallic materials work, a black sheet metal tray works best. In the US we use aluminum flashing (available at hardware stores) painted with flat black paint used to paint barbecues and woodstoves (also available in hardware stores). One important innovation is to elevate the tray slightly with small pieces of cardboard underneath so that heat gathered by the tray is not drained out of the bottom of the oven.
InsulationWhile a simple foiled double wall with a single airspace will cook, your cooker will cook faster and stand up better against cold weather and wind if you insulate it better by adding one or more "baffles" inside the walls. The simplest way to do this is to foil one or more panels of cardboard and insert it/them into each airspace to form two or more air spaces. If you are short of foil, then use some other locally available material, such as feathers, wool, etc.
PotsWhile some cooking can be done in any non-reflective pot, ones made from thin, black metal work best. Cast iron works but its weight can delay cooking or inhibit it completely during marginal conditions. Shallow pans cook better than deep ones.
Cooking StrategyThe single biggest reason for failure in solar cooking is not putting in the food early enough in the day. For this reason, it's best to follow SBCI's advice and "Get it on early, and don't worry about overcooking!"
Foods take longer to cook when there is smog or haze. Since the clarity of the air can make as big a difference as the sun angle, learn to check out the color of the sky and adjust time accordingly. At times the sun may be shining, but the sky is white, indicating that smog or haze are present. A blue sky indicates clear air.
During optimal conditions, when cooking foods that take less than 3-4 hours, it is not necessary to turn the box to follow the sun. However, by turning the box more often you can increase the efficiency of your oven. For foods such as beans, it is essential to turn the box at least once.
Keeping these design considerations in mind when you build and use your solar box cooker will assure success in cooking.
Louise Seeley is a founding member of SBCN.