20 分Illustrate the principle of a solar oven ( 原理)

A solar oven is a way of using the sun's power to cook. There are mainly two types of solar ovens: 'concentrating' and 'solar box cookers'. Apart from the obvious need for sunlight and the need to aim the solar oven before use, using a solar oven is not substantially different from a regular oven. However since they use no fuel they are free to run, humanitarian organizations are promoting their use worldwide to help slow deforestation and desertification caused by the need for firewood with which to cook.

A solar box cooker is an insulated box with a transparent top and a reflective lid. The top can usually be removed to allow dark pots containing food to be placed inside. The box usually has one or more reflectors with aluminum foil or other reflective material to bounce extra light into the interior of the box. Cooking containers and the inside bottom of the cooker should be dark-colored or black. The inside walls should be reflective to reduce radiative heat loss and bounce the light towards the pots and the dark bottom, which is in contact with the pots.

The inside insulator for the solar box cooker has to be able to withstand temperatures up to 150° C (302 °F) without melting or off-gassing. Crumpled newspapers, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker, but since most of the heat escapes through the top glass or plastic, very little insulation in the walls is necessary. The transparent top is either glass, which is durable but hard to work with, or an oven cooking bag, which is lighter, cheaper, and easier to work with, but less durable. If dark pots and/or bottom trays cannot be located, these can be darkened either with flat-black spray paint (one that is non-toxic when dry) or black tempera paint.

The solar box cooker typically reaches a temperature of 150 °C (302 °F); not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. It should be remembered that food containing moisture cannot get hotter than 100 °C (212 °F) in any case, so it is not necessary to cook at the high temperatures indicated in standard cookbooks. Because the food does not reach too high a temperature, it can be safely left in the cooker all day without burning. It is best to start cooking before noon, though. Depending on the latitude and weather, food can be cooked either early or later in the day. The cooker can be used to warm food and drinks and can also be used to pasteurize water or milk.

Solar box cookers can be made of locally available materials or be manufactured in a factory for sale. 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. Although invented by Horace de Saussure, a Swiss naturalist, as early as 1767, solar box cookers have only gained popularity since the 1970s. These surprisingly simple and useful appliances are seen in growing numbers in almost every country of the world.