Solar Kiln Design

Solar Kiln

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.


Advantages

A solar kiln will give you the opportunity to "get to know your wood" better than you have ever known it before! The average "wood weight" of 1 BF (1" x 12" x 12") of red oak I have purchased in Southeast Texas has been 4.1 pounds (#). The average "water weight" has been 3.075# (or 75% mc). To achieve a 7% mc in a 1,000 BF load, the dryer must exhaust 2,738 pounds of water from the wood I have used as an example here. During a 60 day drying cycle that means 45.6# of water per day (1.67% of the moisture content) needs to be exhausted from the dryer. That is well within the tolerances that red oak can stand without inviting degrade.
If you don't quite understand how to measure the moisture content of wood here is a short explanation. Do the math and you will find the green board in the above example weighs 7 1/8 pounds per board foot. All that means is that a 10' long board is heavy. Mc is determined by comparing the weight of water in the wood to the absolute dry weight of the wood itself.

  • If the absolute dry weight = 4.1 pounds per board foot then...
  • 3.075 pounds of water = 75% of the absolute dry weight of the wood = 75% mc.
  • One board foot @ 7% mc = 4.387 pounds.

It is during the first 1/3 of the drying cycle that the dryer will restrict the release of water from the wood due to 100% saturation of the air. As the mc of the wood drops, the dryer will begin to 'extract' the water from the middle portions of the wood. This is because there is less water (from the wood) available to readily saturate the air inside the dryer. Thus more dry air remains available to soak up the remaining water during the last 2/3 of the cycle.
A solar kiln is an enclosed 'natural environment' that allows you to control of the amount of heat (dry air) that is introduced. Do that by adjusting the exhaust baffles during the drying cycle. The rest is between the wood itself and the natural environment surrounding your dryer.

  • 1 cubic foot of 70-degree air, at 100% relative humidity, heated to 140 degrees becomes 2 cubic feet of air containing 50% humidity. This means that 50% of the fresh air introduced into the dryer is dry air. (Air expands; water does not).
  • Except in extreme conditions, the temperature inside your kiln will not reach 140 degrees. I would still suggest daily monitoring during the "heat of the day" so adjustments can be made as necessary.
  • The performance of your kiln will depend on your geographic location. Learn the weight of the wood you choose to dry on the first day of the drying cycle and monitor daily. You will gain confidence with each load you dry and use.

Conclusion

Solar kilns are ineffective for high production shops, but they certainly have their place with serious hobbyists and individual furniture and cabinetmakers. You will receive as much gratification designing your solar kiln to fit your geographical location and your own needs as you do working the wood you produce with it.



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