How to Build a Solar Cooker

Learn the latest breakthrough in solar cooker design from an article which originally appeared in the June 1923 issue of Science and Invention.


| May/June 1977



045-088-02-solar-cooker_01

Here is the solar cooker built By Dr. C. G. Abbot, assistant secretary to the Smithsonian Institute. Polished aluminum sheets serve as the reflectors.


ILLUSTRATION: MOTHER EARTH NEWS STAFF

A hot oven without a fire, a kitchen as cool as the parlor, these are the luxuries that come with a solar cooker. There are two ways of making one. The simpler kind is like that which Mr. W. Adams of Bombay made nearly a half century ago and described in the Scientific American of June 5, 1878. He built of wood an eight-sided cone which he lined inside with mirror glass and hinged upon a board so it could face the sun. The dish to be cooked was enclosed in a blackened, tightly covered pail hung at the center. A glass cover enclosed the pail to keep away wind. If, now, the cone was kept pointed towards the sun, a lot of its rays would be roughly focused upon the blackened pail, and its contents to be cooked could quickly be brought to boiling or, if no water was present, even hotter. Mr. Adams states that the rations of seven soldiers, consisting of meat and vegetables, were thoroughly cooked by it in a couple of hours, in January, the coldest month of the year in Bombay; and that the men declared the food to be cooked much better than in the ordinary manner. The dish is stewed or baked according as the steam is retained or allowed to escape.

But suppose the sun goes behind a cloud just as one wants to get dinner, and the clouds hangs provokingly in front of the sun for an hour though the rest of the sky remains clear. Such a catastrophe, a few times repeated, would be apt to make the men of the house kick the sun-cooker out into the brush.

To avoid this unreliability in solar cooking a new scheme designed to keep the food cooking during nights and cloudy weather has been tried on Mount Wilson, California, at the solar observatory of the Smithsonian Institution. Instead of hanging the dish to be cooked directly in focus of the sun-rays, we provided a hot reservoir of oil above the mirror in which were ovens for the cooking. In order to carry heat from the focus of the mirror to the ovens, there was a circuit of oil pipe running up from the mirror focus to the top of the reservoir and back from the bottom of the reservoir underneath the mirror to complete the circuit. This plan is exactly like the gravity water circulation used with cook stoves and furnaces to heat a reservoir of water for the house, only that gas engine oil, instead of water, had to be used for the solar cooker, so that baking temperatures much higher than the boiling point of water would be possible.

In the contrivance as shown in the illustration, the sun-rays are focused by a great concave parabolic sheet of shiny metal upon a blackened brass tube containing the oil. The tube must lie so as to point at the polar star, and the metal mirror must rotate around it to follow the sun from east towards west every day.

In order that the solar heat may not be wasted, or the mirror be dimmed by dust, the whole top of it, about 10 feet by 7 feet in area, is covered by 10 sheets of flat window glass, laid on a steel frame work. This glass may be easily cleaned occasionally, so that the sun-rays can pass through it freely and it keeps out the wind currents which would tend to cool the focus-tube. To still further prevent loss of heat, the focus tube, which is about 1 1/2, inches in diameter, is surrounded by a glass tube about twice the diameter. Notwithstanding its two glass protectors, more heat is lost from the focus tube than from all other parts of the apparatus. All the other parts, including the reservoir and connecting tubes, are covered to a thickness of about six inches with layers of asbestos and cotton-batting enclosed by bright steel metal to keep out the rain.

The iron reservoir, 40-by-40-by-120 centimeters. (16-by-16-inches-by-4 feet) outside, has approximately 20,000 centimeters squared (21 1/2 square feet) area. It is protected from wind by its galvanized iron case, and from heat conduction by 10 centimeters. (4 inches) of boxwood and 10 centimeters. (4 inches) of cotton wool, moderately compressed, on every side.





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