A solar box cooker is an insulated transparent topped box with a reflective lid. It is designed to capture solar power and keep the inside 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, in order to lower radiative heat loss. The inside walls should be relective to reduce radiative heat loss and bounce the light towards the food and bottom. The cooking containers and inside bottom should be black.
The inside insulator for the solar box cooker has to be able to take temperatures up to one hundred and fifty degrees Celsius. Cardboard and aluminium are commonly used insulators. Aerated concrete and vermiculite concrete could 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 aluminium sheet or plate and occasionally a glass mirror. The black absorber is either a paint like flat-black spray paint (that says "non-toxic when dry" on the can) or black tempera paint; or better yet, a black drip pan.
The solar box cooker typically reaches a temperature of ninety degrees; not as high as in a standard oven; but, still enough to warm food over a long period of say an hour. Because it doesn't reach as high a temperature; food can be left there safely 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 water.
Solar cookers are usually made out 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 Swiss naturalist experimenting as early as 1767; they've only gained popularity since the 1970s. They're a surprisingly useful technology that are mostly seen in poor places or remote locations.
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