MOTHER'S Solar Wood-Drying Kiln: Part Two

Here's a report on the initial test of our solar-powered wood drying kiln.

| November/December 1984

Those of you who are following our solar wood-drying kiln project (A Look at MOTHERs Solar Wood-Drying Kiln: Part One) are probably looking forward to further data concerning the success of our experiment, which entails using the sun's energy to reduce moisture levels in freshly harvested lumber to a point of equilibrium with its environment.

Well, in this second installment, we're prepared to offer reliable figures. However, we still can't promise that our kiln design will actually achieve its intended goal, simply because our two test batches (500 board feet each of oak and yellow pine) haven't yet completely given up their available moisture.

As you might imagine, the economic savings realized in solar drying is offset to some degree by the sheer unpredictability of Mother Nature; if there's less sunlight available over a period than was anticipated, the only option is to wait it out. Then again, as we explained last time, seasoning wood isn't all that simple a task because the rate of moisture removal — be it fast or slow — directly affects the quality and serviceability of the finished product. In short, since we know that at this writing each of our sample charges should still give up about 8 percent more moisture (based on ambient temperature, relative humidity, and particular species), it's simply too early to determine with any accuracy whether or not our boards will have a successful seasoning.

Rain, Rain, Go Away

Traditionally, the Blue Ridge Mountains of western North Carolina (where our kiln is located) experience a moderate climate. Cloud cover is the exception rather than the rule, but last summer our region endured an unusually stubborn rainy spell, which happened to coincide with our first instrumentated test, initiated in mid-June.

This extended circumstance affected our project in two ways: First, the relative humidity (a measurement of the amount of water vapor in the air compared to the maximum amount of moisture that the air could contain at a given temperature) was increased — even during showerless periods — by the fact that reduced ambient temperatures lessened the ability of the air to hold moisture.

Second, since our air distribution system is photovoltaically powered, the marked lack of direct sunlight effectively shut down the conditioning airflow that's critical to successful drying. (This was both a help and a hindrance: Though we didn't want moisture-laden air passing through the wood stacks, stagnant air near its dew point was even less desirable because it promotes the formation of mold, a condition we had to contend with in our freshly cut pine pile.)

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