Solar Kiln Build. Any Pointers?

[wewacountryboy]

I'm in the process of gathering materials to build two 8x20 solar kilns. I have the VT plans and I have looked at Pineywoods photos and diagrams. I'm going with Pineywoods plan with the clear over black metal. Do I need to leave a gap at the bottom and top of the black metal so the air can circulate between the clear panel and the black metal? I see that Pineywoods has his fans blowing downward instead of forward. Does this make a difference? For a 8x20 kiln, does anyone know how many fans I might need? I have found some solar fans that look great. They are a little pricey, but if they work like I hope they should pay for them selves. I had plan to spray foam the inside walls and floor, but after reading I don't know if that's a good idea. I am a little worried that the moister will have a problem escaping the kiln. I will take photos as I go. Feel free to jump in and tell me what I'm doing wrong. Here is a youtube link to the solar fans I would like to use. I'm thankful for any feed back!


Rand 15 Watt Solar Gable Attic Fan

[Den Socling]

I don't know anything about building a solar kiln but I won't let that keep my mouth shut.  To get good air flow, I would leave a gap top and bottom then I would have the fans toward the back and blowing downward. Now, with a baffled charge, you would have good air flow through the pack.

[pineywoods]

countryboy, you need to leave a 4 inch gap top and bottom.That way you pick up heat from both sides of the metal. Which way the fan blows don't matter, air flow through the stack is what counts. Be cautious about your fan source, they take a lot of heat. Plastic will melt. We had some plastic blades melt and fall off.  Full width doors on the back side save a lot of work, no carts, no rails, no hand stacking. I load stickered stacks of boards into mine with a fork lift. A small room de-humidifier will cut your drying time in half.  Don't pay any attention to published drying schedules. They do not apply to solar kilns. Experiment. I've tried all kinds of odd-ball things, have yet to really mess up any lumber. I'm told that red oak is some of the trickiest wood to dry, I make red oak flooring  from the tree to installed floors, I'm too dumb to know you can't do that

We like pics......

[Hokiemill]

CountryBoy, I built the VT design kiln.  It uses a black surface directly on the stack of lumber and a tarp hanging vertically as a baffle.  I used two pieces of corrugated roofing painted black for the collector surface, and I also used a tarp that is black on one side as the baffle.  My kiln is located where I have no power supply so I also went with the solar powered attic fans.  I got mine at Home Depot, but you're right, they are a bit pricey.  Mine have been running for two years with no problems yet.  Put the solar panels where you can easily access them.  They collect pollen/dust/etc. and you need to wipe them off once in a while.  The great thing is that they pretty much regulate themselves - whey the sun is out and you have heat in the kiln, then the fans run to circulate it.  If the sun isn't out and there is no substantial heat, then the fans don't run.

If you're going big with your kiln build, take a look at timbergreen forestry for their shed-like solar kilns.  They have room to air dry a charge or two, then slide a door over to start the kiln drying.  Pretty neat.

Check out www.solarkilninfo.com - I documented the building of my kiln here.

[WDH]

Hokie, thanks for the link.

[Hokiemill]

You're welcome.  I put the site together specifically to help those who are planning to build their own.  Anyone who has built their own kiln is more than welcome to send me some pictures and a description/story and I'll post them in the "your kilns" section.

[ronwood]

Hokiemill,

Nice job on the website and content!!

Ron

[pineywoods]

Hokiemill, very nice web site, thanks for taking the time. I have been running a solar kiln for several years. Unfortunately, I built mine (actually 2 of them) before I found out about the forestry forum, and I never took pictures during the construction. Mine is almost identical to yours, with one exception. I fastened the black metal collector to the bottom of the rafters. This gives more area and collects more heat due to being oriented to match the sun angle. Temps have exceeded 160 degrees. I don't vent out the humid air. Instead I put a small cheap room de-humidifier inside and run it at night to take the moisture out. Found that works much better than venting. Works good, cheap, every sawyer ought to have one.

[ronwood]

pinewoods,

Did you use cut all your wood to build the kilns?

Ron

[WDH]

Hokie,

What are the maximum temps that you see in your kiln with the collector directly on top of the lumber stack versus attached to the roof rafters like with Pineywood's kiln?  Does it get above 130 degrees for enough hours to kill powderpost beetles (the little scourges, may they all die a horrible death  )?

At the end of the drying cycle, I assume that you could close up the vents and let the kiln heat up to bug killing temperature?

[pineywoods]

pinewoods,

Did you use cut all your wood to build the kilns?

Ron

Every scrap of material for both kilns came off my woodmizer. 4X6 white oak timbers for the foundation, 2X4 pine for framing and 1X10 pine siding.

[Hokiemill]

I have registered temps in the low 150s.  I went back and looked at some numbers from a data logger I had and during June, it was pretty regular to get 4 hours a day above 130F.  As a side note, I once asked Gene Wengert why put the collector down on the wood as opposed to directly below the rafters and his response was that the collector surface was further from the glass/plexi so there would be less loss of heat back out.  Not sure I buy into that.  I went with the standard VT design anyway for simplicity's sake.  Both designs obviously work so I guess it becomes a personal preference.

I wish I could go with the dehumidifier at night, but lack of electricity at the kiln kills that.  If electricity is available, I'm starting to think that it would be simpler and more controllable to go with a straight dehumidification kiln like the one they built at Wisconsin by UW and VT (http://owic.oregonstate.edu/pubs/dhkilns.pdf)

[ronwood]

pineywoods,

Did you use 1x 10 pine for the floor and the inside walls?

Ron

[pineywoods]

pineywoods,

Did you use 1x 10 pine for the floor and the inside walls?

Ron

Floor is 2 inch thick pine. On one of the kilns, the inside walls are construction blackboard, the other is part corrugated metal, part plastic sheet. Insulation is 4 inch fiberglass batt in the walls and 1 inch foam sheet on the doors.

[ronwood]

pinewood,

Would it help to put insulation under the tin on collector so that you don't heat at night.

Ron

[WDH]

Hokie,

So you remove the collector surface from the top of the lumber stack everytime you load/unload the kiln, then put it back on the top stickers when you reload? 

Could it not be put in permantly just above the top of the heighest height of the lumber charge?

[WDH]

One other question.  The VT site says to size the clear polycarbonate area that captures the light so that there is 1 square foot of surface for every 10 board feet of lumber for drying oak.  This designs the kiln for a safer moisture loss/day for oak , one of the harder species to dry. 

Have you had any experience with the sun gathering surface area versus the board feet in the kiln, i.e. the 1:10 ratio?

[Hokiemill]

Yeah, each load/unload I pull the two sheets out.  I have a couple spring clamps I keep in the kiln - I roll up the tarp baffle and clip it to the fan deck then I just pull out the sheets and set them aside.  They rest directly on the last set of stickers above the top level of boards.  No big deal.  Only takes a minute or two.  I could definitely build another structure but I'm all about simple.  Oh, and I do have a couple other folded tarps directly under the collector surface so that they drape down the sides of the lumber stack.  That's my effort at getting all air flow directly through the stack.

I've thought about the collector ratio before.  Yes, they designed the kiln specifically for oak as it is about the most limiting wood that most will dry.  However, I dry a bit of cherry which can handle much faster drying rates and it would be great to push that faster rate.  My thought was to put together a couple auxiliary collectors of just polycarbonate and a metal collector in a shallow box and connect them directly to the main kiln.  Just a thought.

[Backwater]

Hey Wewa.

Looks like our locations ain't far apart. Are you in PC or Wewahitchka? I'd be interested in seeing this kiln project sometime.

[WDH]

Hokie,

How do you rate the VT solar kiln short course?