A kiln, regardless of the type, is a controlled environment where the monitored extraction of moisture can take place. The conditions the wood is exposed to throughout the drying process remain somewhat constant and allow for minimal degrade. Wood used for fine furniture and cabinetry should initially be dried to 6%-8% moisture content (mc), some woodworkers prefer 5%-6%, and only a kiln of some sort can remove that much moisture.

A solar kiln is an affordable method for almost any woodworker to gain the capacity to dry wood in a controlled environment. It enables you to purchase your choice of lumber from a local mill at tremendous savings and to protect your investment while insuring you a steady supply of stock.

When I decided to graduate from air-drying and move on to kiln drying, I spent the better part of 2 years reading everything on solar kilns that I could get my hands on. I viewed plans upon plans as I conceptualized my own. I read several theories and zeroed in on the similarities and discovered the dynamics involved as I designed a solar kiln that would fit my needs.

The constants I discovered in the basic design were that wood takes up x amount of space, and 10 SF of surface area on a solar panel will be sufficient to dry 100 BF of lumber. The variables I found were that the amount of wood you would like to have the capacity to dry is proportionate to the cost of building a solar kiln.

My Design

I will discuss the dynamics in a moment but for now let me say that I designed a solar kiln that would dry 1400 BF of 5/4 lumber in 12' lengths. I calculated the space required for the lumber stack using 3/4" stickers (minimum requirement), and included 18" of space needed on both the north and south sides of the stack so that air can circulate efficiently through the wood.

My kiln cost approx. $1,300. The base measures 8' x 14' and it has a very offset roof that includes 140 SF of solar panels (corrugated iron painted flat black glazed with clear corrugated fiberglass 1 1/2" above the iron). It is set at a 35-degree angle (5 degrees greater than my latitude to gather more heat in the winter). Ceiling, floor and walls are insulated and the solar panels face due south.

Inside, I channeled a fan downward toward the south wall to direct the air throughout the entire length and height of the stack of lumber. In the ceiling behind the fan I placed return vents that allowed circulation inside the dryer.

Then I placed exhaust vents (with baffles) on the south wall. As moisture-laden air is exhausted from the dryer through the exhaust vents the same amount of hot "dry" air is pulled into the drying chamber from the solar panels.

Dynamics

Air can soak up just so much moisture. During the beginning of the drying cycle the relative humidity inside the chamber will remain near 100%, in fact water should bead on the walls of the dryer! That stands to reason since the weight of green lumber might include 50% (or more) water. Still, dry air is being introduced into the chamber through the solar panels and that dry air quickly absorbs the moisture given off by the wood. Moisture is continuously expelled through the exhaust vents and that "flow" is continued until the fan is turned off each evening.

Only the faces and edges of the wood are giving up their water (The ends of the wood are painted with latex). When the fan is turned off (no more dry air is being introduced), the heat remains and is conducive to gaining 100% mc inside the chamber. When 100% mc is achieved the drying process will actually restrict the wood from releasing anymore water to its surrounding environment and then the wood itself will begin to equalize. The drier outer extremities will absorb the water held deeper within until equalization of all forces involved is achieved... This is a gentle process that allows the wood to "breath", greatly reducing stresses that damage wood.

Then the process begins again at sunrise. As the heat inside the chamber remains well after dusk and slowly dissipates, it takes several hours each morning to reintroduce heated air into the chamber. But the process indeed begins again when the fan is turned on each morning! It begins with exhausting 100% moisture laden air captured during the cooling process that night, only to take in whatever air the outside environment has to offer.

The theories and plans that I studied while designing my kiln all indicated that the optimum level of performance for a solar kiln is to slowly achieve no more than 140 degrees (f) during daytime heating. This balance produces the shortest drying time/highest level of acceptable degrade. At this rate, all indications were that 30 days was sufficient to dry each inch of thickness. I preferred more time/less damage.

Continue to Solar Kiln Design - Page 2 - Advantages and Conclusion

written by Paul Koch