SUNSHINE POWER
(Page 3 of 5)
May/June 1971
By the Mother Earth News editors
Energy from the sun is usually measured in terms of 'horizontal incidence'—that is, the amount of energy falling on a horizontal surface. For 40 degrees latitude, the yearly average is 500 BTU per square foot per day. You can improve on this by tilting the collector (s) of your system up from the horizontal toward the south (and the sun). An angle equal to the latitude (i.e. 40 for 40) decreases summer insulation by 18% but increases the mid-winter intake by a whopping 130% . . . an annual overall improvement of 25%.
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Some further juggling is possible. Increasing the angle by 23 degrees over the latitude (63 at 40) favors the winter solstice; a lesser amount of tilt favors the center of the cold season, which comes about one month after the solstice.
A good workable average seems to be latitude plus 15 degrees (55 degrees angle for latitude 40).
A vertical collector (like a billboard) might seem easier to build but it gives away 10-15% to tilted collectors during December, January and February . . . and even more during the rest of the year.
So, a tilted collector it must be, and ideally, tilted at the angle of latitude plus 15 degrees.
In the past, however, such ideals have been compromised. To understand the part compromise has played in collector design, look at Whillier's comments on the design of the 1953 MIT house. The home was planned for 1100-1200 square feet and it's design cost was $12,000-$18,000 EXCLUSIVE of heating system. Hardly a bargain, even in 1953. And—for appearance—the architect was allowed the range of 45 to 75 degrees collect—or angle. I suppose we should be thankful that, at least, the collector was to face south . . . some solar houses in suburbs have been aligned to the streets rather than the sun! No wonder there are now only about twenty solar-heated homes (in spite of a "reliable" forecast of 13 million units in the United States by 1975!)
One useful thing (to you) to come out of the MIT study was the general shape of a solar house. Four designs were considered, three of which are shown here (See fig. 2). The fourth included a curved wall, rejected on a cost basis.
The final package arrived at was TYPE A. Type A is a two-story house with the collector oriented to the south. The eastwest dimension of Type A is about 20% greater than the north-south dimension. These figures are close to the optimum of minimum cost and heating load for the floor area and the shape is well worth considering for the homesteader. Here the collector serves as a major portion of the roof (which saves material) and allows room beneath it for south-facing windows.
A house of this pattern is theoretically impractical for total solar heat in a climate as harsh as upper New England and a 600 square foot collector would require auxiliary heat (of about 20% of total). A collector of twice the area would still require some additional heat. On the other hand, the MIT house used the southfacing windows . . . and on sunny days in winter the extra heat trapped required excessive ventilation. So you'll have to work out a balance. The lower efficiency of a home-brewed unit should preclude the danger of having much excess heat to throw away, anyhow.
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