A solar box cooker is an insulated transparent-topped box with a reflective lid. It is designed to capture solar power and keep its interior warm. The top can be usually be moved aside for cleaning and to handle the food. The box itself is covered on the outside with a reflector; usually aluminium foil, to decrease radiative heat loss. The inside walls should be reflective to reduce radiative heat loss and bounce the light towards the food and bottom. Cooking containers and the inside bottom of the cooker should be black.
The inside insulator for the solar box cooker has to be able to take temperatures up to 150 ° Celsius. Cardboard and aluminium are commonly used insulators. Aerated concrete and vermiculite concrete can also be used, but rarely are, because they're heavier and less available. The transparent top is either glass, which is durable but hard to work with, or an oven bag, which turns brittle after exposure to the sun but is cheap and easy to cut. The reflector is usually aluminium foil, sometimes an aluminium sheet or plate, and occasionally a glass mirror. The black absorber is either a paint like flat-black spray paint (one that is non-toxic when dry) or black tempera paint, or better yet, a black drip pan.
The solar box cooker typically reaches a temperature of 90 °C; not as hot as a standard oven, but still enough to warm food over a long period of, say, an hour. Because it doesn't reach too high a temperature, food can be safely left in it all day without burning. The cooker is often used to make a large pot of food in the morning, then people eat servings or snack from it all day. The cooker is usually used to warm food and drinks but can also be used to pasteurize milk and sanitize water.
Solar cookers are usually made of locally available materials; relatively few are mass produced. They range from small cardboard devices, suitable for cooking a single meal when the sun is shining, to wood and glass boxes built into the sunny side of a house with thermal time constants of a few hours, so they can cook even when the sun is behind a cloud. Although invented in by Horace de Saussure, a Switzerland naturalist, as early as 1767, they've only gained popularity since the 1970s. They're surprisingly useful appliances that are mostly seen in poor places or remote locations.
