Solar kiln

[peterrum]

This is a project I have wanted to complete for a few years now. There is alot of information on line about solar kilns and I am fairly certain I hit most of them in order to plan this. This kiln is 9’x4’ outside dimension. It allows me to put 8’ boards in easily. I had to make this kiln mobile just because of the lay of my property. The plan was to build it so that I could put it at the end of my carport to get the best sun and when needed I could move it aside to get access to the double car garage. I used Sketchup to develop the plans and fiddled with the dimensions using this tool.

Construction started with a 2×4 frame base sandwiched between plywood. Between the 2×4’s I put 2” white styrofoam. This frame has 6×5” caster wheels on it which are rated at 500 lbs. each.The side walls were constructed the same way out of 2×4’s and plywood. The doors were 2×4 framed, plywood on the outside and the styrofoam glued to the plywood on the inside.

The inside of the kiln was then painted with 2 coats of aluminum paint (for moisture), followed by two coats of flat black oil based paint. The outside was then given two coats of a dark green paint.

The clear panels (Palram Suntuf) are about 6’ in length and attached to the frame with roofing screws and silicone caulking. The outside doors have 12”x3” vent doors in them, one at the bottom of the door, one at the top and that is to help with air movement when needed. I went onto Ebay and purchased a small solar fan to move the air around also.
This project cost me about $500 in materials and about 40 hrs. to build.

Today I finished the project and put about 150 bd/ft. in it for my first test run. I’ve got pine and juniper in there which have been air drying for a year and a bunch of flame box elder pen blanks which are fresh cut. The next test will be with some of my bigger slabs. Of course murphy’s law and on my first day its raining.

 

[VA-Sawyer]

I built and ran a solar kiln in VA for 3 years, making a few mods as I learned things.

I noticed a few things about your kiln that you may want to change:

1. It is best not to have the sun shining on the wood. Sunlight can increase degrade due to uneven
drying.

2. You need some sort of baffle setup to force the air to flow through the pile. Simply using a fan
to move the air around inside the kiln isn't enough.

Good luck,
Rick

 

[peterrum]

Two valid points VA.

#1, hard to wrap my head around that and don't recall reading anything in my research about that point. How would you suggest it could be accomplished? Should something be put on top of the wood to cover/protect it from UV.

#2, yes, I recall reading about baffles and should be able to set something up for that easily.

Thanks

 

[RPM]

Shouldn't have to worry about UV with that Suntuf material .... it is 100% UV blocker. I left a piece on my lawn last summer and killed it pronto.

VA Sawyer - How does a solar kiln work if you don't leave it in the sun?

 

[VA-Sawyer]

While the UV may cause some color changes on the exposed side, the real problem is the uneven heating of the exposed wood surfaces vs the unexposed side of the same pieces. #1 cause of cupping.

Fixing problem #1 should also make it easier to fix the airflow issue.

I would suggest a second layer of material that runs parallel to the solar panel and about 6" away. You could use OSB or even thin foam. Paint the topside flat black and position it so air from the bottom front is ducted up along the solar panel and delivered to the top rear of the kiln. This heated air can be forced down and forward through the stack by fan. The added panel will keep the sunlight off the upper wood surfaces so that only the airflow is doing the drying, while at the same time it acts as a duct. Add a vertical wall of thin (1/2") foam from the top of the stack to the bottom of the added panel to act as a baffel and now the airflow is closer to what you need.

Air from the front of the stack flows into the heating chamber and is ducted to the top rear as it is heated. Then it is blown down to the back of the stack and flows forward through the pile of stickered boards.

Rick

 

[MHouse1028]

for the baffle you can use a window blind the kind you pull the string and it goes up or down you can get them at lowes fairly cheap that way after loading the kiln you can lower down to touch the wood and it would be adjustable to any size stack you put in there and it's plastic so it wouldn't stain the wood...just a thought.... i like your design nothin beats home built fabrication

 

[Cody Colston]

VASawyer is correct in his assessment and his solutions for your kiln. You don't want sunlight directly on the wood and air must be forced through the stickered layers in order to evenly dry the wood.

Dr. Eugene Wengert designed the solar kiln at VA Tech, working out the dimensions of the solar collector based on drying Oak...one of the more difficult woods to dry without degrade.

American Woodworker Magazine published an article for a smaller kiln based on Dr. Wengerts design and he also advised on it's construction. I made one similar except that mine loads from the front.

There is a drawing near the bottom of the article in the attached link that illustrates very well how the air flow is forced through the stack by placing the fan in a plenum and using black plastic over the top of the stack.

AW Extra - Solar Kiln - Projects - American Woodworker

 

[VA-Sawyer]

MHouse1028,

That might be a good idea in this case, as I think he could get a single blind the full length of the kiln. In my old 14 footer or my planned 22' kiln, I think multiple blinds would allow too much bypass. I had a hinged panel that sloped from the top of the stack to the inside ceiling. It worked ok, but I sometimes had to fab up something to make the top of the stack even for the full length.
Rick

 

[VA-Sawyer]

Cody,

I used to spend a fair amount of time on WoodWeb reading Dr. Wengert. He really knows a lot about drying wood, but as I got more experience with my Solar kiln, I realized that too much of his advice was geared to large commerical kilns. You can do a lot in things in a Solar kiln that would ruin the wood in a commerical kiln. It isn't effective to try operating a Solar kiln on the basis of throughput, just think of them as a faster version of Air drying that results in a lower final MC.


Another good source for understanding the drying process is 'The Kiln Operators Handbook' from the US Forest Products Lab. It is also geared towards commerical kilns, but they do explain a lot of the hows and whys of the drying process. If you understand those, then you can design your own drying schedules to fit a Solar kiln.
Rick

 

[peterrum]

Thanks for the tips guys, the blind idea is ideal. For now I have covered the pile in the kiln with landscaping fabric. Its black, allows moisture and air to flow through it and will protect the wood from UV. I'll see if that works. It will be interesting to get this thing going, day 2 with rain and its supposed to rain for the next 6 days so I'll have to wait and fine tune in the process.

 

[Cody Colston]

It isn't effective to try operating a Solar kiln on the basis of throughput, just think of them as a faster version of Air drying that results in a lower final MC.

I agree with the caveat that one has more control over the drying process using a solar kiln than with air drying.

A correctly built solar kiln is pretty much a load-it-and-forget-it operation. If using Dr. Wengert's formula of 10 sq ft of solar collector per 100 bft of lumber, it's hard to mess up the lumber. That formula is optimum for drying Oak which has a very low safe drying rate. Also, the beauty of a solar kiln is that it conditions the stack each night when the heat drops and the humidity rises so that any drying stresses are relieved.

My kiln is sized for 400 bft in 10 ft. lengths and has four vents, 12" x 4", two upper and two lower. The single fan is rated at 1400 cfm. Per the advice from the AW article, I crack the vents when the wood is first loaded. After a couple of weeks, I open them up about 1/3 and the final week, they are closed in order to get the heat as high as possible for, hopefully, sterilization. So far it's worked well although I've only dried Walnut and Cedar in it which are both pretty forgiving.

I just finished stickering 450 bft of White Oak that I'll put inside the kiln after a couple of weeks of air drying. That will be the ultimate test of how well it works.

 

[VA-Sawyer]

The final version of my kiln was ventless. The front wall was 6' high and made of the same corragated clear plastic as the roof. It had a 1/2" panel of insulation 5" behind it that stopped about 10" off the inside floor. The wet air coming from the stack flowed up through the front wall before entering the Solar collector, As the inside surface of the verticle plastic was close to the outside air temp, a lot of the moisture would condense on the surface and drain down and out of the kiln. The heat released by the condensing moisture warmed the air around it, which tended to keep the amount of condensing water under control. I didn't loose too much moisture during the early stages, but it kept the drying rate up pretty good through the later stages. Temp and humidity recording devices showed levels running close to desired numbers. I had a 4" section of insulation near the top of the inside wall of the 'condensing wall' that could be opened to allow air to bypass the 'condenser' if it was drying too fast. The big advantage was not having to heat/humidify incoming outside air to replace vent air leaving the kiln. I could have used white painted metal for the front panel instead of the plastic and maybe gotten even more effecient drying.

Rick

 

[mikeb1079]

been following this thread with interest, i started a solar kiln build last fall. looking to get it going this spring. another option for the baffle that i've heard works well is a cheap tarp. just drape it from the back wall over the top of the stack (and sides) and it forces the fan to push air thru the stack to vent.

 

[RPM]

Thanks for the explanations and insight!

 

[VA-Sawyer]

Shouldn't have to worry about UV with that Suntuf material .... it is 100% UV blocker. I left a piece on my lawn last summer and killed it pronto.

VA Sawyer - How does a solar kiln work if you don't leave it in the sun?

Actually, I think you will find the clear plastic passes the UV pretty good. It being UV resistant, means that UV radiation doesn't decomp the plastic very fast. The flat black surface inside the Solar collector absorbs the solar energy and that increases the surface temp. The warmer surface begins to radiate long wave infra-red energy. The plastic appears opaque to the long wave IR trapping it inside the collector. I suspect the grass died because it was cooked.

As for the second question...... Kiln in sun = Good
Wood in Sunlight = Not so good.

Rick

 

[peterrum]

It being UV resistant, means that UV radiation doesn't decomp the plastic very fast.
Rick

Thats what I was thinking as well, the plastic is designed so that the UV won't break it down as fast as some other plastics left in the sun but the UV will still pass through to the wood. No worries now, I have it covered.

 

[ROPECLIMBER]

Won't the plastic over the top drying board "sweat' the top board ?
Paul

 

[Ted J]

You can go to the Virginia Tech website (Design and Operation of a Solar-Heated Dry Kiln - Home - Virginia Cooperative Extension) and there are plans you can download (Building Diagrams - Home - Virginia Cooperative Extension).

Seems you may need to add the board (solar collector) that sits over your wood stack that forces the airflow down through the front of your stickered stack and up the rear of the kiln and back through the fan. You should be able to attach the baffle to it to help seal it and get the circulation flowing better.

Ted
Build a solar kiln is on my short list....... wel it's been on that list for 3 years now....

 

[TPA]

I want to bring this thread back because I have a question.

In the above post design, how do you introduce new air? Seems like you would want to include outside air to some degree in order to remove excess moisture. Is that the case?

 

[Ted J]

I want to bring this thread back because I have a question.

In the above post design, how do you introduce new air? Seems like you would want to include outside air to some degree in order to remove excess moisture. Is that the case?

From what I read on the site is that the solar kiln I linked to was designed for drying red oak, but you still have to monitor the drying process regardless.

Look in the section "Monitoring the Process" on the site link.

Here is another link that helped with questions for me.
http://homepower.com/view/?file=HP63_pg50_Scanlin