Solar Kiln Designs Compared

Thoughts on roof angle, dimensions, and other considerations for solar drying kiln construction. January 8, 2010

I need to build a solar kiln and I need to be able to increase the turnaround time on lumber, kill the mesquite and pecan borer’s, and be able to dry small cedar logs (mostly just the bark). I would like to find someone in the central Texas area that has built a solar kiln to know just how well it works for them.

I already have an abundance of stickers that are 1 ½” thick and 32” long, so I am kind of planning the kiln around them (should I?). I am also figuring on accommodating 10 foot lumber. With my latitude at 32°, my roof angle should be 32 ° correct? Therefore, in order to get a charge of approximately 1250 bd ft (5’t x 64”w x 10’l ) into it, it would need to be 15’ 8” wide by 10’ 4” tall (north wall) by 12’ long. With the roof essentially being the south wall and ending at the ground. This would give me a total of approximately 1.5 ft²/10 bd ft if I ever did use 1” stickers, and 1.3 ft²/10 bd ft using the 1 ½” stickers. All of this is assuming a very tightly stacked charge though. If the layers are not a full 32” wide due to gaps in the lumber, then the total charge is going to be less giving a slightly higher ratio of ft²/bd ft.

1. Can I use the thicker, 1 ½” stickers, or do I need to use 1”?

2. Am I shooting for a ratio of roof area to charge that is just too high?

3. Do I need to increase the roof angle to say 38° or maybe 42° to increase the winter performance, and possibly decrease the summer-time performance? Stretches of 100° days are common here.

Thanks in advance for all of the help. Any “recommended reading” would be greatly appreciated as well. I have already reviewed “Drying Hardwood Lumber”, “Drying and Control of Moisture Content and Dimensional Changes”, “Processing Trees to Lumber for the Hobbyist and Small Business”, and “The Appalachian Solar Lumber Kiln”(which is the basic design that I am considering) plus many other smaller posts and such on the web.

Forum Responses
(Sawing and Drying Forum)
From contributor O:
Wood-Mizer sells a kit complete with detailed plans, and the roof is at 45. I think the exact angle is important with solar panels but with a kiln the light just has to pass inside to be absorbed by the black interior and the angle isn't as critical as just being open and unobstructed. Be sure you’re facing south. I use 1" stickers. 1.5 isn't necessary and would waste a lot of kiln space in my opinion. Even with 100 degree days you will probably need supplemental heat to kill borers.

From contributor J:

I've never built one, but I believe that when they say the roof angle should be about equal to your latitude, they're talking about the angle between the roof and a horizontal plane, not between the roof and a vertical plane. In other words, the collector would be essentially level at the equator, and absolutely vertical at the poles. Your idea that the roof would essentially be the south wall suggests you might be getting this backwards.

From the original questioner:
This is how I had it figured, which I am pretty sure is what you just said (I think). This of course is a view from the east or west wall of it.

Click here for higher quality, full size image

From Gene Wengert, forum technical advisor:
I do encourage you to use 3/4" thick stickers. 1, 3 to 10 is a bit high. 38 would be better for wintertime performance indeed.

From contributor P:
Has anyone on this forum built their own Solar Cycle kiln?

From contributor O:
Yes I have built a solar kiln about eight years back and am within weeks of building a couple more. I patterned mine after the Wood-Mizer design. I looked theirs over while having my saw serviced.

From contributor I:
I built a Wood-Mizer design years ago and was not happy with it. I then built a Nyle kiln and it works great. I am now looking at building the Solar Cycle kiln to cut operation costs. I like the Solar Cycle kiln because you stack your green lumber in one area and slide the doors over that area when it is ready to dry. This way I do not have to handle the wood twice before I can dry it.

From contributor P:
We are on the same page. I was in hopes that someone that reads this forum had built a Solar Cycle kiln. I have got as far as the frame and concrete pad. I need to move forward on the roof trusses.

From Gene Wengert, forum technical advisor:
The Virginia Tech design will dry as well as this other design. The VA Tech design is likely less expensive. Certainly, using a shed for pre-drying is a great idea, but the simple VA Tech design will dry green from the saw quickly and with equal quality. Or one could have a shed and a kiln, with the double handling issues mentioned. It is my opinion that both options, if run properly, will provide excellent quality and efficient, low costs. I do believe that the VA Tech design has the ability to achieve lower final MCs.

Of course, all solar designs require solar input, so on cloudy days or in the wintertime when daylight is short and outside temperatures are low (what little heat you get from solar is used to heat the kiln rather than do any drying), there is a strong case for using a DH kiln or similar to provide dry lumber year-round. Remember that in all common drying systems (solar, air, DH, steam, etc except vacuum) the wood dries because of the heat, humidity and velocity. So, if there is a good combination of heat, RH and velocity that can provide superior quality, then a DH or other kiln can be operated to provide the same conditions to achieve the same drying results. The lumber does not know the source of the heat, RH or air flow.

Of course, sometimes the equipment is not operated in a good manner and then quality loss can occur. It is well to remember that prior to the recent decline in wood manufacturing we were drying about 5 billion BF of oak and other furniture grade hardwoods every year and making excellent furniture. The quality loss with proper operation was minimal and the wood processed without major problems (except for inherent , non-drying issues such as crossgrain, bacterial infections, etc.).

Be careful of claims made with any system. Make sure that they have the same species, thickness, MC, etc. and that you see it yourself. Remember, some people claim that a Dodge Ram is better than a Ford F-350; it is not true of course; those who claim so are biased and refuse to admit that they made a mistake.