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Author Topic: some questions about solar kilns  (Read 992 times)

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Offline Florida boy

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some questions about solar kilns
« on: July 13, 2017, 09:37:50 am »
I'm going to start a solar kiln build here soon. I have been gathering materials for the last few months. what I have gathered so far (all for free) is as follows-
       ~30 4x8x 2" thick yellow foam insulating boards
        6 sheets of 3/8 plywood
         10 sheets of sliding door glass (edge is green)
         2 rolls of roof paper sticky kind
         several tubes of caulk and can foam
     
I have about 50 2x4 from 8 to 16' long
 looks like I will have to buy the floor plywood and also some more sheathing.
    I've heard on here that the glass with the green edge isn't the best. can it still be used or should I just buy some poly carbonate panels? never seen it hail in Northwest florida but it does happen just not often at all.
 also on the design do you waterproof the inside of the chamber ? I've seen mention of letting the wall breath some what so should I put tyvec on the outside or just put up lap siding or sheathing with out it?

I've seen the pineywoods design of putting black metal mounted to the underside of "roof rafters". I was thinking of building a plenum of sorts about 10" under the glass then putting black metal for the collector. stopping it about 8" short of the  low wall.
does this sound like a good idea? I thought it might direct airflow a little better.
I'm still going to be hanging canvas to funnel the air through the the stack.

 one last design feature I'm thinking about incorporating is to make a removable plenum at the tall side wall in between the vertical fan housing and the wall. essentially separating the inflow vents from the out flow vents. that way when I have vents wide open the moist are Is leaving and new air is nor mixing with it. bad idea? unnecessary? yall tell me. it just seemed like a good idea...

please feel free to pick any part of this apart 
the kiln will be 18L' x 7'W the height will be determined by the angle 35* or so. I will insulate all Walls and floor. and will get some lumber treated for the floor framing.
any pointers and tips are greatly appreciated.

Offline btulloh

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Re: some questions about solar kilns
« Reply #1 on: July 13, 2017, 12:10:15 pm »
I like your idea on the removable plenum between the fans and the vents.  Seems like a good thing.  I'll be waiting to see what the experts have to say.

I'm still trying to get a handle on the air circulation in general.  All I've read and seen still leaves me not quite understanding the whole picture.  I'm proceeding on faith that the VT design does what it's supposed to, but I hope to develop a better understanding of air circulation and venting.  What's clear is the fans moving the air over the collector area and through the stack.  It's the next part that I'd like to understand better.  SEems like a lot of moisture to get out of the building, especially with the vents closed. 
HM126

Offline Florida boy

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Re: some questions about solar kilns
« Reply #2 on: July 13, 2017, 01:32:46 pm »
I also do not fully understand when to vent what and how much but I think the basis of a solar kiln makes it very user friendly. especially if the wood is air dried first as I plan to do. hopefully the experts will weight in and set us straight

Online GeneWengert-WoodDoc

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Re: some questions about solar kilns
« Reply #3 on: July 13, 2017, 11:59:53 pm »
The green edge, from iron, is suppose to act as a heat insulator, but about half of the solar energy is infrared.  So, that is why we do not like this glass.  It will work, but other materials will work even better.  Glass is heavy and breakable.

As we know the Virginia Tech design works very well, why not use a design real close to it?

Having a black absorber close to the collector glass is not necessary...any distance is ok..  air flow will be good without it.

The vent barrier would be ok for wet pine or poplar when first starting with green material, but for most woods, we want more heat than a single pass can provide.  So the vents might only exhaust 1 to 5% of the kiln air.  Recirculating means the air can be heated more and more with repeated passes.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline YellowHammer

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Re: some questions about solar kilns
« Reply #4 on: July 14, 2017, 12:06:05 pm »


Solar kilns excel in their simplicity and effectiveness for certain species of wood.   I used the basic Va Tech design and it's pretty foolproof.  It's not like a DH kiln, it's easy to overthink it.   :D

A couple $20 Wal Mart fans, a plastic tarp, and two layers of the cheap plastic clear plastic roofing, and some vents from Home D or Lowes.

Operating technique is basically open the door, if you get hit with a blast of hot moist air Amazon rain forest air, then open the vents more to let it out. If you open the doors and and the air is basically the same temp as the outside, then close some to get it hotter.  I never fully close the vents, and most times they stay cracked.  If you open the door and the air and the wood feels warm, cozy, and dry then you are done.

The most important thing, as with all kilns, is to make sure the floor is 100% flat, and will stay flat, when loaded with wood.  If it's not flat, or sags, the the wood will come out with the same shape. 

I built mine on skids so I can get under it with my forks and move it around. 

The best advice I can give is build one, and use it.  It's that easy. 
Hobby Hardwood Alabama.com
LT40 Diesel Hydraulic, Stihl 028, MS440, MS660, 2 Kilns

Offline Kbeitz

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Re: some questions about solar kilns
« Reply #5 on: July 14, 2017, 12:29:11 pm »
I'm sure hoping it's that easy. I just finished building one and I'm waiting for the
tar coating to really soak in before I load it up. I can't wait to see how it works.
Collector and builder of many things.
I have a
machine shop
Wood work shop
And a Weld shop
And now a saw mill
and a bunch of new forum friends.

Offline btulloh

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Re: some questions about solar kilns
« Reply #6 on: July 14, 2017, 02:43:44 pm »
K - I'm looking forward to seeing your results also.

YH - that's probably the best and most useful advice I've heard.  A little seat-of-the-pants with some Kentucky windage thrown in for good measure.

My build is taking too long, although it looks like it's picking up steam now.  It's given me time to over think and over analyze all this, which is about normal for me.  Thanks for the dose of common sense.

I see you painted yours black on the outside.  I guess it can all contribute to solar gain.  I hadn't thought of that. 

I have four loads of SYP at about 20% mc to run first, so that should be a pretty easy way to start.  Then I may run a couple loads of WO that's also around 20% or I may saw some SYP and run that next.  It seems like the maybe running the green SYP with fans on and doors or vents open for a few days might be a good way to go.    (There I go, thinkin' again.)

Offline YellowHammer

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Re: some questions about solar kilns
« Reply #7 on: July 14, 2017, 10:38:21 pm »
The quickest bang for the buck, or most efficient use of the solar kiln is in the later stage of drying which benefits from heat input.  So any of the air dried loads you mention would work fine.  The classic VaTech kiln was sized, if I remember correctly, for 750 or more bdft of hardwood, but that's a guess.  So the load size should be reduced in softwood to match the water removal rate.  For example, in a DH kiln, the load size is typaiclly reduced to 1\3 when using wet green softwood, pine for example, instead of oak.  When the wood gets more air the load sizes of the softwood can increase. 
Best thing is try an air dried, or pre dried load, and watch it to make sure the it  doesn't stain from too much humidity.

Remember, these kilns will get hot in the summer, and here's what happened to mine when we had a power outage.  It got so hot inside it charred the framing under the clear plastic in multiple places, and even melted a hole in one area.
Lesson learned.....
Hobby Hardwood Alabama.com
LT40 Diesel Hydraulic, Stihl 028, MS440, MS660, 2 Kilns

Online GeneWengert-WoodDoc

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Re: some questions about solar kilns
« Reply #8 on: July 14, 2017, 11:06:35 pm »
The original VT design was for 1500 bf of 4/4 oak.

With pine at 20% MC, we need heat to finish it---final 10 to 12% MC---which will take two or three sunny days.

It is indeed best with green pine to run with the vents wide open.

In all cases, avoid running the fans when the heat in the kiln, compared to outside, is minimal---6pm to 9am perhaps.  Running fans 24 hours will mean 12% MC but no drier.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline YellowHammer

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Re: some questions about solar kilns
« Reply #9 on: July 15, 2017, 07:56:21 am »
I built their 800-1,000 bd ft design of (10 foot lumber), they also have a larger 1,500-2,000 bdt design.  I've only been able to cram about 750 bdft of 8 foot long or so into mine, and that's with me hitting the plastic every now and then with the forks. 

Here's a real good pub from Va Tech on the design and fab details.
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/420/420-030/420-030_pdf.pdf

Originally, I only used one layer of the clear plastic, but I couldn't get the heat up.  So I later added a second layer and it made a considerable difference.

I'm thinking of building a second one, I'd like it to be significantly larger, maybe in the 4,000 - 6,000 bdft range. 

I really like the simplicity of these kilns, and I call mine the "Easy Bake Oven."
Hobby Hardwood Alabama.com
LT40 Diesel Hydraulic, Stihl 028, MS440, MS660, 2 Kilns

Offline btulloh

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Re: some questions about solar kilns
« Reply #10 on: July 15, 2017, 02:13:08 pm »
Good tip on the double layer on the roof.  I'd be tempted to skip that at first and add later, but now I know better.

What did you use for spacers between the two layers?  I see the fillers they make to go between the purlins and the plastic.

By the way - I made mine taller than the VT plans because it looked like a good idea to get some vertical clearance.  Besides, I use mostly 32" wide pallets because of the forks I'm using.
HM126

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Re: some questions about solar kilns
« Reply #11 on: July 15, 2017, 09:51:16 pm »
I didn't add any spacers, I just screwed it to the underside of the rafters so they are spaced 3 1/2" apart.  Then I used some expanded foam to seal it and create a dead air space between the two, but that didn't work overly well so then I used silicone to seal it.
 
Hobby Hardwood Alabama.com
LT40 Diesel Hydraulic, Stihl 028, MS440, MS660, 2 Kilns

Offline btulloh

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Re: some questions about solar kilns
« Reply #12 on: July 15, 2017, 10:02:27 pm »
Aha!  Seems obvious now that you mention it. Thanks.

You come up with a lot of good solutions. The FF keeps me humble.
HM126

Online GeneWengert-WoodDoc

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Re: some questions about solar kilns
« Reply #13 on: July 15, 2017, 10:19:10 pm »
It is best to leave a few weep holes between the layers at the bottom to drain any liquid water.  The wood might let a little moisture in the space and so we need to have an easy way out.

When making larger solar kilns, it is best to make them longer and not much wider.  A wider load means that the risk of uneven moisture in the interior increases.  Always try to keep 10 bf feet per 1 square foot of roof.

Overall, three adjacent solar kilns are better than one large one.  The three allow more versatility in species, thickness, incoming moisture.

Remember from a tax point of view, a kiln is a piece of equipment and so should likely be depreciated over 7 years or so.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline btulloh

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Re: some questions about solar kilns
« Reply #14 on: July 23, 2017, 02:51:32 pm »
Florida boy, don't go building that pole barn without keeping all of us up to date.  It's not official if you don't post pictures.  :D
HM126