My Mother's House Part VI
(Page 2 of 5)
July/August 1982
By the Mother Earth News editors
First of all, we wanted to mount the unit in one of the east-end windows of our earth shelter's greenhouse. The tempered glass could, we figured, serve as the outer glazing, and the moderate environment inside the sunspace would allow us to keep the collector in service throughout the below-freezing winter months. Unfortunately for that plan, the sections of greenhouse glass are only 33" wide (though they are 84" high), so our design was seriously constricted from the beginning.
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Then, when we began to look for off-the-shelf tanks, we encountered yet another problem. Because of the glazing width, we couldn't use a vessel larger than 12" in diameter without seriously compromising the device's reflective capability . . . but the only common 12" tank we found (a glass-lined unit used as a ballast in hydronic heating systems) was only 60" long. That meant, in effect, that we'd have to sacrifice almost 24 inches of available glazing height.
As you can see, then, our design was influenced by the rather strict limitations imposed by the collector's location. Consequently, we'd like you to bear in mind—as we explain the techniques that we used—that your circumstances might allow you to make use of materials with different dimensions from the ones shown here.
REFLECTIONS
We decided to work with what's known as an involuted reflecting surface. This curve can be formed by wrapping a string around the cylinder in question, tying a pencil to the end, and tracing the line formed as the string—held taut—is unwrapped. Because the curve's radius increases, and does so at a rate determined by the diameter of the cylinder, light rays that strike the involute tend to bounce in toward the cylinder. Of course, it takes two of these curves-one on each side of the tank—to make an effective reflector, so they're typically paired by starting each one from a point on the very back of the tank and unwinding in opposite directions.
As attractive as the involute shape is, though, we found that it's no easy matter to form one from common materials. So after a couple of unsuccessful attempts, we settled on a shortcut version of the curve that involved, using a series of flat surfaces to approximate chords of the involute. (Because these reflectors are formed as perpendiculars to tangents of the tank's circumference, they still bounce sunlight onto the container.)
However, in order to get the series of faces to reach the sides of the box, we had to modify the involute chord arrangement. The first surface was described by drawing a line out from the cusp (the point on the circle where the involutes begin) 3 inches to intersect a tangent to the circle at a right angle (this can best be accomplished with a carpenter's square). Then we extended the line another inch.
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