A Look at MOTHER's Solar Wood-Drying Kiln - Part One

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Understandably, some daylight is needed inside, but the wood shouldn't be exposed directly to the sun's rays. By the same token, solar energy was needed in the form of heat, so we harnessed it in both ways by covering the back surface of the joist framing with black-painted aluminum press plate, from the collar ties down to a point halfway to the stub wall. A barrier consisting of I" foil-faced polyisocyanurate was extended from the base of that wall to a horizontal sill strip 68" below the aluminum plate, and the open space between was covered with more press plate and 16"-wide sheets of fiberglass-reinforced plastic glazing placed at eye level.

Essentially, this arrangement permits an airflow from the intake grilles to pass through the cavity between the stub wall and the foil-covered insulation board and up through the channels between the joists, gathering warmth as it moves. At the same time, it blocks most of the sunlight while letting the absorber plate heat up. (In fact, the aluminum sometimes has a tendency to transfer too much heat into the structure by radiation ...a problem that could be solved by insulating the rear surface of the plates, thus forcing the thermal energy to remain in the channels.)

To deliver the tempered air to the stacks of wood, we constructed a 13" X 15" horizontal plenum of insulation board at the upper terminus of the roof-joist channels, and routed four sections of 6" insulated duct tube between it and a second vertical air shaft built onto the rear wall. Five lengths of capped 6" perforated ABS drainpipe were likewise used to distribute the air from the base of this box to the stack piers ...which were simply a series of concrete blocks.

Because the kiln isn't located near any power source, we decided to let the sun drive the air as well as temper it. One 12-volt, 35-watt ARCO photovoltaic panel provides the energy needed to operate a 12" fan within the vertical plenum. Since the PV panel is wired directly to the fan motor, the air delivery late peaks out in fall sunlight ...at 350 cubic feet per minute. (One panel can handle about 1,000 board feet of wood ...so to increase the kiln's drying capacity, we'd have to add the appropriate number of panels.) Unfortunately, the perodic stagnation that occurs on overcast or inclement days is conducive to the development of mold in especially moist, fresh-cut lumber ...and if that turns out to be a chronic problem, we'll alleviate it by installing a storage battery and charge controller in the circuit.

PREPARATION IS IMPERATIVE

The success of each seasoning depends, in part, on the manner in which the green wood is prepared and stacked. Ideally, the raw material should be end-sealed immediately after being cut. This is done by applying a latex paint to the exposed end grain and covering that, when dry, with an oil-base coat. If the wood is to be stored prior to drying, it must be kept in a ventilated shed, and the boards must be stacked on sturdy supports—with air space and stickers between each one—to allow proper circulation. If this air-drying stage is eliminated, the lumber can be loaded right into the kiln after sealing ...but it's still imperative that the pieces be properly ticked. We place each board 2" from the next, and separate the layers with 1X1 pine stickers. Finally, each stack of material (a full charge consists of two stacks) is wrapped in poly at the sides, with the top left open. This allows the tempered drying air to work its way completely through each pile, with a minimum of leakage. The roof vents can then be adjusted to regulate the flow. (Our kiln differs from many in this feature ...the stacks have their own microenvironment within the rest of the structure.)

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