A solar oven is a passive solar heating device that uses neither fuel or electricity. Instead, it cooks food using only the sun’s rays. Solar ovens are safe to use and cannot burn your food. They do not even need to be watched. You can start your meal cooking, walk away from it and return whenever you wish. A solar oven is cheap and easy to build, and it makes a useful addition to your camping or outdoor entertaining equipment. By adding a Fresnel lens to the basic solar oven design, you can make it even more efficient.
Basic Solar Oven
Basically, a solar oven is a box with a glass or plexiglass top. Both glass and plexiglas let light through but not heat. Light enters the box, is absorbed by the food or the box itself, and is re-emitted as heat. The heat, trapped in the box, builds up and cooks the food.
To improve the efficiency of the solar oven, use two boxes, one a little bigger than the other. Line the bottom of the larger box with crumpled newspapers. Place the smaller box into the larger box. Crumple additional newspaper and stuff it between the sides of the boxes. Trapped air is an effective insulator, so this layer of crumpled newspaper will help prevent the heat built up in the smaller box from leaking out into the air. Painting the outside of the larger box black will help it absorb more heat.
Lining the interior of the smaller box with aluminum foil will reflect light away from the walls of the box and onto a pot placed inside. This will help keep the walls themselves from absorbing the heat and wasting it. Placing a mirror above the box will direct additional sunlight into it. You can use a regular mirror or make your own out of aluminum foil. It is important to keep this foil as flat as possible so that the light is not scattered. Use as little glue as possible to prevent lumps and wrinkles. Tape one side of the mirror to the top of the smaller box, so that it forms a lid.
Put the pot in the middle of the box. Place the Fresnel lens on top of the smaller box, right over the glass or Plexiglas top. The light entering the box will be focused by the lens onto the pot. Normally, light entering the smaller box would be evenly spread through it. The foil lining would keep some of it inside the box, but some of it would inevitably leak out. The focusing action of the lens keeps the light concentrated on the food itself, helping the oven cook more efficiently.
Fresnel Lens Selection
Some Fresnel lenses are exceptionally large, or specially made to focus wide areas of light down to a point. Avoid these lenses when making your solar cooker. The giant lenses can produce too much heat. The high-focus lenses can cause your food to heat unevenly. Instead, use a lens that focuses into an area about the size of your pot instead of to a point. Try a Fresnel lens from a TV or overhead projector, which are simply meant to make images look bigger rather than focus light to a point.