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Solar kilns are typically used by very small companies or by hobbyists. The capacity varies, but the biggest one I’ve seen could hold about six thousand board feet of lumber. The capital investment required to erect one is typically a few thousand dollars or less, but they might dry only one or two loads of lumber per year. The throughput depends on the size of the kiln, the moisture content of the lumber going into the kiln and the final moisture content desired. The drying rate is affected by the kiln design, orientation and location. The amount of sunlight and the ambient temperature are critical; drying essentially ceases from October through April each year from the Kentucky–Tennessee state line and northward. I have seen some people add supplemental heat from a wood furnace to successfully extend the drying season. There are a number of good designs for solar kilns available online from Virginia Tech, WoodWeb, Oregon State University and other institutions. A good place to start is BuildItSolar.com, which has a number of plans referenced on its website: http://www.builditsolar.com/Projects/WoodDrying/wood_kiln.htm. Relative humidity (RH) and temperature conditions both affect the lumber drying rate, but these are minimally controllable in solar kilns compared to other types of kilns; usually the only control mechanisms are manually- operated vents. Without good environmental controls the kiln can get too hot for green lumber, resulting in drying defects. Here’s an example: the temperatures in one solar kiln (without fans) I looked at in Kentucky in late spring ranged from 190°F near the peak to 105°F close to the floor. (This kiln probably has a larger area for solar collection than most.) When a couple of fans were added to this kiln the temperatures near the roof peak dropped to 140°F; that’s still too hot for green lumber. This particular installation cleverly used a solar panel to run DC- powered fans for an off-the-grid location. Lumber which is already air-dried can finish drying in a solar kiln more successfully, but temperature variability will affect how uniformly the lumber dries in the kiln. Fans might help to even out kiln temperatures, but they can’t fix relative humidity control issues. Evaporated water must be vented if the lumber is to dry, but vents are typically opened or closed based on the kiln temperature. That’s not the best way to control the RH inside the kiln. Solar kilns do have their advocates. Lumber dried in a solar kiln may have less drying stress (known as casehardening) than lumber dried in conventional kilns because cooler night time temperatures temporarily increase the relative humidity inside the kiln. As the temperature warms up the relative humidity decreases again, inducing further drying. It seems that the daily cyclic changes in relative humidity inside the kiln decrease the amount of drying stress in the lumber, though the tradeoff is that these kilns dry more slowly than other kilns even when the daytime conditions are favorable. 14PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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