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Author Topic: Solar kiln build  (Read 470 times)

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Online JoshNZ

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Solar kiln build
« on: August 24, 2020, 07:25:09 PM »
Since I'm already well down this rabbit hole... I think it's time for a solar kiln.

As this isn't my main profession and more of a side thing, it sounds like a solar kiln is the way to go being a load/close and forget type deal which is what it needs to be really.

I was hoping to get an idea of what kind of materials guys have used to keep the budget down, mainly insulation, wall panels, the clear collector material etc.

Is the collector better off being a clear sheet over a black sheet, with a 12" cavity let's say, or are you better just painting everything inside black, the back of the baffle, top cover etc.

I figure I can drive posts into the ground for piles and frame a structure on top of it with milled pine (which I'll have to have kiln dried haha). Im about to run into a bunch of Japanese cedar and cypress - if I wrap the frame in building paper is shiplap cladding sufficient or does it need ply/sheet on both sides of the insulation?

Any other thoughts or comments, lessons learned the hard way etc would be really helpful.

Offline btulloh

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Re: Solar kiln build
« Reply #1 on: August 24, 2020, 07:45:39 PM »
That's a great idea and a big benefit without costing a lot to build or run.  I saw lumber as a side thing too, mostly for my own use.  I added a solar kiln a couple years ago, and it really paid off for me and makes everything flow better.  Just being able to dead-stack lumber for storage, and getting stuff down to 6,7,8% mc to use for furniture and cabinet work was worth the time and expense and I should have done it sooner.

You can control a fair amount of the expense by sawing your own framing lumber and siding.  Insulation and glazing are hard to cheat on, but some people have found ways to scrounge up one or both of those items.  The inside is best to skin with either OSB or exterior plywood.  It would be hard to seal otherwise.  The coating for the inside is important and can be pricey, but 5 or 10 gallons is all you need.

I really recommend following the Virginia Tech design, which is really the standard design for solar kilns and has been proven over a long period of time.  There's room for a bit of interpretation, but the basic principles of solar collection, air flow, and venting are well thought out, effective, and most of all - important.

Here's a link to the plans and write up for the VT Solar Kiln - VT Plans  

You should be able to saw and air dry your framing lumber to a sufficient degree in a reasonable amount of time.  Once the kiln heats up, it's going to dry stuff out pretty good.  You can see a lot of wood movement as you start to use the kiln at temperature.  Sometimes you even have to go back and re-seal the inside.

Pay attention to vapor barrier placement and choice.  It's all covered in the VT document.

Good luck with the project.  It's a really beneficial thing to add to your sawmilling.

Edit/add:  As far as the collector design - just follow the VT design.  Double glazing though - you need the air space / insulation.  Paint the inside black, cover the top of the stack with black something or other (painted plywood, painted tin, whatever.  No need  to re-think the way it's done in the VT plan (even though we all LOVE to rethink all this stuff.)
HM126

Offline btulloh

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Re: Solar kiln build
« Reply #2 on: August 24, 2020, 07:50:13 PM »
BTW - This whole sawing thing is a big, deep, rabbit hole.  But a good one.  Everyone here is pretty happy with their adventure.  Sawmill.  More tools.  Loaders, skid steers, telehandlers.  Build sheds for air drying.  Now build buildings for storing finished lumber.  Bigger workshop.  Another pole building.  And so forth. . .


Perfect.   :D :D
HM126

Offline Old Greenhorn

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Re: Solar kiln build
« Reply #3 on: August 24, 2020, 07:54:15 PM »
Man, it's a darn shame you missed the VT solar Kiln class a week or so ago. It would have answered all your questions and more. I really enjoyed it quite a bit and learned a lot. Can't imagine a better deal for the $10. registration. You can build it with non KD lumber, it will dry as you go and you might have to re-seal a joint or two after the first cycle. The collector material is pretty important, it has to pass a lot of UV energy or whatever they call it. You kind of need the right stuff for that. I checked around here and I can buy it off the shelf at a home center (Lowes). You have some flexibility in design variants, but airflow, venting, stack sizes, are all critical. The kiln is designed to take a certain load size. 
 Anyway, there is a lot of info available on the web and here on the forum with the searches. Keep your eye open in case that class comes around again. You can also ask the WoodDoc if you have any questions. After all he designed the dang thing. ;D
Oscar 328 Band Mill, Husky 450, 372 (Clone), Mule 3010, and too many hand tools. :) Retired and trying to make a living to stay that way. NYLT Certified.
OK, maybe I am the woodcutter now.
I can work with wood, but I am NOT a Woodworker, yet.

Offline btulloh

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Re: Solar kiln build
« Reply #4 on: August 24, 2020, 08:15:08 PM »
Here’s a link to my build. There are many others on here as well. 

https://forestryforum.com/board/index.php?topic=96418.0
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Online JoshNZ

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Re: Solar kiln build
« Reply #5 on: August 24, 2020, 08:15:50 PM »
Just thinking about it actually, I have enough dried timber to start now  headscratch. Uh-oh.

I have a stack of 6x2 native stuff that has been airdrying for 2 years, none of it is long enough to span the width I was imagining but I remember now there are a handful of 5m+ 12x2s in the bottom shed.

I have come across the VT plans before, I'll have a good read through and come back to you guys. Thanks!

And yes this rabbit hole is a good one I agree. I seem to be the type who is prone to landing in the bottom of rabbit holes so it's good when they are fruitful.

Offline GeneWengert-WoodDoc

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Re: Solar kiln build
« Reply #6 on: August 25, 2020, 08:04:54 AM »
The amount of energy coming into the kiln is the same if the interior absorber is a black sheet under the collector or everything inside the kiln that the sun "sees" is painted black.   However, if you have the sheet, it will get quite hot At that location and then will have more heat loss.  So, there is a slight advantage to having everything black. Do not use a plastic sheet, as plastic gets brittle and will then break.  Perhaps use a black painted metal or wood absorber.

The VT design is indeed well proven, very simple, and still very effective, although it was originally designed in 1978.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline doc henderson

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Re: Solar kiln build
« Reply #7 on: August 25, 2020, 11:16:03 AM »
I graduated HS in 1978... good year.  I think the only advantage to putting a back on the collector is if you were going to try and insulate it at night and shut off flow through the collector.  you would insulate the back of the collector and all the heat would be between the glazing and the back.  this is only needed if you were going to try and make a hybrid kiln, that you could add heat for sterilization later in the cycle.  it has been discussed that it may be better to have a separate "hot box" to sterilize and set pitch at higher temps, by adding heat.  adding ideas is fun, but add to complexity and cost usually.  As @GeneWengert-WoodDoc had told me, " it is just a solar kiln"  or in other words keep it simple.  You really want the vapor barrier on the inside of the insulation so it is not ruined by the humidity.  it is fine to have a water shedding outer siding, that has an air space so humidity can get out of the insulation.  so lap of board and batten siding is fine.  
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Offline Crusarius

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Re: Solar kiln build
« Reply #8 on: August 25, 2020, 12:32:15 PM »
This has been on my list to. definitely want a hybrid though since I need to sterilize the wood.

Offline farmfromkansas

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Re: Solar kiln build
« Reply #9 on: August 25, 2020, 09:17:28 PM »
I have a neighbor who is good with welding and has oil well pipe, so had him make me a skid to start with.  It will be able to be moved, and the pipe will keep the wood off the ground, and make a stiff base that will not twist.  Then I built my floor, put boards on the bottom, then rolled it onto the skid, bolted it on then put the insulation, plastic sheeting and then the floor boards. Had to use a few shims to make the thing straight, as a few joints in the pipe stuck up and made the floor less than perfect.  The shims made it right, then I screwed them under the floor so they would not fall out. The pipes are doubled to add strength to the length.
Most everything I enjoy doing turns out to be work

Online JoshNZ

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Re: Solar kiln build
« Reply #10 on: August 29, 2020, 01:23:44 AM »
When you guys say "sterilize" your timber are you killing boring bugs? What sort of temp for how long? That would be a handy feature, I've already milled one log and then seen dust rings all over the stack

Offline WDH

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Re: Solar kiln build
« Reply #11 on: August 29, 2020, 07:14:52 AM »
For hardwoods, yes, to kill beetles, their larvae, and any unhatched eggs.  Critical internal wood temp is 133° held at that temp for a critical time, maybe a bit less than an hour.  Hard to know the precise internal temp of the wood, so many of us heat the kiln to 150° and hold that temp for 24 hours to assure 133° internal temp for enough time to do the job. 

For softwood, the high sterilization also helps to "set the pith" so that pitch does not ooze out of the wood at the in-use temperature. 
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Online JoshNZ

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Re: Solar kiln build
« Reply #12 on: August 29, 2020, 05:22:12 PM »
Can one not slip an oil fin heater in there and set it on max for a day in the sun?

Also I have another question I've been meaning to ask, when you're pricking the wood to test moisture content are you supposed to do it in the end grain of a fresh cut? And if you drive a pair of pins into a board to sample the MC frequently, do the pins provide a reliable reading of the overall board or do the pins having damaged cells around them/conducting heat into their small area etc, skew the reading?

Offline doc henderson

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Re: Solar kiln build
« Reply #13 on: August 29, 2020, 05:51:43 PM »
I use the pin-less but I think the answer is side or face along the grain.  If you go to the national hardwood lumber association, you can watch the webinars presented by Dr. Gene Wengert @GeneWengert-WoodDoc .  it is free, you have to register but not join the NHLA,.  even your name and phone ect cannot have spaces.  there are about 6   2 hour long webinars of different topics, most are about sawing and drying hardwoods.  the pin depth if insulated also matters, if you want to know the surface, average, or core MC,  there are limits to the high and low MC.
timberking B 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor 12 volt tarp motor


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