A Homemade Solar Lumber Kiln

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The Forest Service plans specified that the top—or glass—portions of the panels be tilted in a peak-and-valley, or "sawtoothed" fashion. The advantages of this arrangement had evaded me at first, so I had simply placed my collectors side by side to save time, materials, and work. But—since my first experiment had resulted in improved efficiency—I decided to give the recommendation a try and see what, if any, difference it might make:

I removed the glass covers from two adjacent collectors. Then, on top of the 2 X 4 angled stud located between the pair of test panels, I nailed another 2 X 4 board . . . this one set on edge to provide maximum elevation of the panels. I then replaced the glass and positioned thermometers at the tops of the two units. The panel tilted slightly toward the sun showed a heat gain of 10°F, while the panel facing slightly away from the sun was 10° cooler than it had been. The two collectors measured the same temperature only at noon, and so it appeared that the total amount of heat absorption would be the same with or without the sawtoothed arrangement.

Luckily, I decided to stick with the modification for a few days, and the advantage of angling the glass panels quickly made itself known . . . the east-slanted glazing caught the morning sun sooner, while the west-slanted panel kept Sol's attention longer into the afternoon, and the arrangement effectively added about an hour to the kiln's workday. The gains seemed enough to justify changing all my unit's panels over to the saw-toothed pattern.

Still searching for more efficiency, I began to experiment with closing off—bit by bit—the bottoms of the solar panels. When these intakes were left open, the air would rush through and out the tops. I found that the more I closed off the bottoms of the panels, the more slowly the air traveled through . . . and the warmer it got. After several tries, I settled on leaving the lower ends about half closed.

As a result of all my playing around with the solar panels, the heated air now runs about 140°F . . . a gain of 20° over the original setup.

WHY A SOLAR KILN WORKS

The Forest Service literature did a dandy job of telling me how to build a solar-heated lumber kiln, but it never did explain why or how the darn thing works. The principles involved probably seem pretty basic to those of you who are already familiar with methods of harnessing the sun, but I had to think back to my ninth-grade science class before I really understood the contraption.

To put it simply, the solar kiln functions because air expands in direct proportion to its temperature. In other words, if one cubic foot of 50°F air is heated to 100°F, its volume will double. But the amount of water suspended in that original cubic foot of air will remain the same, and—since we now have two cubic feet of air-—he relative humidity will be reduced by 50%. That warmer, drier air is thirsty, so it will absorb moisture from the damp wood stacked in the kiln.

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