Experimental passive solar wood kilns

Kristen said:

renewablejohn posted in another thread that he has a multi-span polytunnel for drying wood, and that is erected on a hillside, so the hot air naturally rises during the day and draws the moist air out of the "top" end. Sounds ideal to me

https://www.hearth.com/talk/threads...od-working-awesome.129149/page-2#post-1744430

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Yup yup.

renewablejohn said:

Our commercial solar kilns are 8mtr x 25mtr with no fans. Instead it is designed 90 degrees to the prevailing wind open at each end but the important part is the ridge is not horizontal but has a 1 mtr slope over the 25mtr length. This is important as it can give a a 10C to 15C difference in temperature from one end of the tunnel to the other creating a strong convection current taking the moisture out of the tunnel.

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You guys are getting ahead of me ;-)

I found a picture of such a commercial kiln, with 1 meter rise to the ridgepole along a 25 meter span. Without the context, they are kinda funny looking, and the WAF is low. Clearly a brilliant idea, didn't bookmark the file to add it here.

I am going to pontificate a little bit. I have been trying to stay away from acres of text to let the pictures do the talking.

I do appreciate the input along the way, two heads are better than one.

Some parameters I am working with local to me that may not apply at your address.

#1. I have no trouble getting my wood dry. Everything I have split and stacked on May 15th, covered on top, open on the sides is going to be very burnable, low to mid teens MC by July 4th. You probably don't have 20 hours of sunlight day after day that time of year.... So I know from experience I can fill all these modules with freshly split green as possible wood the night of May 14th, roll up the Roman curtains I was fooling with the other night so all my stacks are open at the sides, and go into July 5th with burnable wood.

#2. I do have enormous trouble keeping my wood dry. Some time between July 5th and August 10th the clouds roll in, the rain starts, and then in late September the rain turns to snow. I think I am good for keeping rain and snow off just by rolling the side curtains down. Keeping water vapor out during Jul-Aug-Sep is something I'll have to figure out when I get there.

Then we get the winter time melts, usually 3-5 days around Thanksgiving and in 2015 a couple days around Christmas of daytime highs above freezing. I think I am good on those.

Then there is breakup, when the ice on the rivers "breaks up" and flows out to sea, just before spring. Enormous amounts of water vapor in the air, and puddles of liquid snow collecting in all the low places. Break up will start in mid March, but I'll be running the stove into mid-May. I think I can use some batten strip between the PVC poles at the bottom to connect the plastic roof sheet directly to the floor framing to minimize water vapor intrusion then.

I do not have WAF for a sloped ridge pole, but I do have wife approval to not level every single module to each other. Given the contour of my lawn, some ridge poles are going to be higher than others, I will be putting in some ridge vents.

I am going to learn how to use wet bulb thermometers this coming summer. While I don't "need" to speed the drying process, I am interested to see what I can do with a green cord split and stacked on July 5th.

If WAF was not a factor in this project I would probably go with a much wider flatter Gothic arch, use knowledge like this to utilize schedule 40 PVC for the hoops : http://www.pvcworkshop.com/bendpipe.htm , and definitely utilize a sloped ridge pole.

@Kristen and @Knots and @solarguy2003 are all three kinda stabbing at how water vapor moves around during sunlight hours and how condensate behaves on the overnights when convection slows.

I don't know how this is going to work. That's why the title is "experimental" rather than "proven."

The step forward I am taking from solarguy2003's work is dispensing with fans. I am going to try to make this work just by changing the size and location of vents while monitoring wetbulb thermometers scattered around the interior.

If I have to get fans I will at least go with solar panels and 12vdc fan units, but I am going to put that off as long as possible. If I have to go to fans I anticipate placing them at the ridge vents with the shortest possible wiring runs and then manually adjusting the air intakes from ground level. I'll orient the solar panels so the fans kick on and run hardest as late in the solar day as possible when humidity inside the units should be at maximum, and right before the cooling/condensing phase starts.

One thing I have noticed is the temperature of the plastic membranes tracks ambient air temp within 10-30 minutes. When I bring a split from -20dF outdoors to +55dF in the garage it takes two days for the split to warm up all the way through. So roughly a split is at least two orders of magnitude a better heat sink than the plastic membrane, potentially three orders of magnitude.

If I am starting with green wood stacked in say February and seal it up, any water vapor I get during sunlight hours will condense on the wood at sunset, unless I can get it pumped out. In July, once the wood is warmed through, at sunset the membrane should cool faster than the surface of the wood so the vapor should preferentially condense on the plastic- and hopefully drip on the lawn rather than my splits because of my roof pitch.

This year I am going to keep all the sides open on my green stuff until the stacks start moving, indicating capillary flow is completed and I have reached FSP. Then I'll see about cooking some of them.

@DoubleB @renewablejohn @BlueRidgeMark re: casehardening, I found a link thingy:

http://www.wood-database.com/wood-articles/drying-wood-at-home/

specifically:

wood database dot com said:

Drying defects
But kiln drying may also introduce internal stresses into the wood—particularly if an improper kiln schedule is used, or if corrective measures are not employed—resulting in a condition known as case-hardening. This defect is caused when the outer shell begins to dry faster than the core: the shell tries to shrink, but is inhibited by the still-wet core. If the moisture difference between the core and the shell is too great, the shell can dry in a stretched condition. Later, as the core eventually begins to dry and shrink, the condition is reversed, and the stretched shell prohibits the core from completely shrinking. In extreme instances of case-hardening, the core can split and check in an irreversible condition called honeycombing.

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I think what is happening to folks in the UK and there was a recent thread from a guy in New Hampshire of Vermont too, is firewood is getting pulled from at least some kilns after the split is casehardened, but before the wet inner core finishes kiln drying. So they are paying good money for wood that is dry on the outside but still over 30% MC on the inside.

Two possible solutions. One, longer kiln time for the water in the still wet core to diffuse through the dry outer shell. Two, air dry the wood to FSP first, then kiln it.

Given my very brief summer, I will be going from standing green to FSP top covered (takes about two weeks locally with the birch and spruce we have to choose from), then close up the kilns and crank up the heat. If I had a long enough summer sunshine season I would be tempted to just crank up the heat in closed up kilns from the get go. But I don't have room for two years of wood on my tiny suburban lot. If I don't get this stuff dry this year I am heating with oil next winter.

The most recent American example is in central NH: https://www.hearth.com/talk/threads/kiln-dried-wood-reading-30-moisture.151737/

nhcwb said:

I ordered a cord of kiln dried wood ...split a big chunk and tested the fresh face, only to get a reading of 37%...[seller] stated it sounded fine to him and that the reason his wood burns well is that it's bone dry on the outside but still wet on the inside which makes it burn longer.

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This is what I want to avoid and might be incorrectly referring to as a case hardened split.

Also FionaD from Scotland in this thread: https://www.hearth.com/talk/threads/seriously-thinking-about-giving-up-on-burning-wood.149234/

FionaD said:

I've measured some 'kiln dried' splits up to 45% at their core!

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Whatever the correct name is for splits in this condition, case hardened or other, I can't afford to make nine cords of them this summer.

First build of the mark 3 vertical framing went up yesterday. I dispensed with the central vertical wall and then put in an upper bunk style storage space specifically to maximize the height of the double thick stack below.



Here is another view from standing inside the unit, the headache bar on the right is a carryover from Ur to keep the roof shapes the same, on the left is part of the floor for the upper layer:



And sunlight!! WooHoo! The sun cleared the shrubbery between the houses of my two neighbors to the south on the 18th of Jan, I am getting about an hour of direct sunlight every day now.

I loaded the top hamper or upper level or what have you without a ladder, just standing on the empty lower deck. Only got 0.136 cords up there, but I am calling it a win. When the branch I cut off the outgoing Xmas tree turns brown it will be a clue to get out the moisture meter.

For now I am caught up splitting and stacking. I used 7 offcuts of 1/2" PVC to make a bottom vent on this one. Intuitively I think it is likely useless, but it keeps those pieces out of the landfill for a couple years. Plenty of extra membrane from the module on the right to overlap the roofing and make these two adjacent modules function as one convective unit.



I have another cord of "dead standing fire killed light it with a match" spruce coming Monday, I expect it will measure 20-25% just like what is already stacked, same forest fire, different cutter.

Once I have the current module filled up, left over fire killed spruce will go in the upper bunks of the higher numbered units yet to be built to the east. Like this one, #5, the last unit built with Mark II vertical framing and henceforth known as "that SOB #5". The third time I put a ridgepole on it the plastic went on. I look forward to taking a chainsaw to that SOB someday.



Finally I found someone cutting green spruce on Craigslist. Once the incoming firekilled spruce is stacked and covered I plan to bring in three cords of green spruce to finish filling units 5-6-7 (and 8) to see what this plan can do with live sap up trees.

And there is a new hope. My inexpensive membrane from Home-Depot seems to be good enough. I started seeing these replacements for Jack Frost's merry S curves on January 18 when the sun was high enough in the sky to clear the shrubbery of my neighbor to the south. Day time highs, Jan 18 to 23 inclusive were (Farenheit) 7.2, 11.1, 0.1, 4.1, 5.2 and -2.7.

Minimum I am getting 20.9 degrees F of solar gain at +11.1dF:



All those wee bits of ice are on the inside of the plastic membrane. The round ones make my heart pitter patter, those aren't from Jack Frost, those are from Mr. Sun.

Not that I think the MC of my stacks has changed at all yet, what with a seven day high temp of +11dF, but the potential, my gosh, with a month at +70dF to warm the wood through and a week of highs in the +80dF range I should be seeing dry bulb temps inside the kilns firmly up into three digits.

Jees, if I hit 120dF drybulb and 80dF wetbulb consistently the week between solstice and July 4th, it boggles my little brain, 3.3% MC equilibrium point, assuming I don't lose any ground during the four hours of twilight we get every night then.

Thank you for publishing that table USFS. http://www.fpl.fs.fed.us/documnts/usda/ah188/chapter01.pdf , page 26.

Yes, three point three. @BKVP (channeling Yoda) will have a cow. "Dryer is better I said." "Typical homeowners struggle to get to 20% MC I said."

EDIT: 20.9 degrees F gain, and the latent heat of crystallization. Shazam!

Here is the Mark III vertical framing almost full.



And I got 1.005 cords in that sucker. Finally. It has been an unspoken goal all along to fit at least 1.0 cords on the 42x96" footprint.



So Mark III vertical framing (or better) on the remaining four units to build and I'll be built out. I got figger out how to put a floating roof on one of these...

therdlesstravld said:

The way you frame things interests/scares me. Pretty when all filled up though!

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Firefighter938 said:

Yep, that is some unique framing.

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Well, lets talk about framing. It took me a week to swallow my pride, but the point of this thread is to get help from people who know what they are talking about.

So framing. Cheapest possible build. I started with two pallets (no side cut outs for forks) and six cinder blocks already on the dirt.



You can kinda see bottom front right I already got 1" pipehangers on the corners, those get tricky to install once the floors are on the cinder block.

Bare framing from the long edge:



You can see there is four verticals at each end. There is no screw or bolt or nail or glue or anything else holdign the 2x4 down to the pallets, just gravity.

Here is an end view of a different unit, same design, showing the ends of all seven of the 2x4s running left to right in the pic above:



Tracking?

Then I put in diagonals.




And started filling with cord wood.



I got .135 cords up top. If it was fresh green birch at 4000#/ cord we're talking 540 pounds.

The plastic on the floor is plenty loose and 2 layers. I don't expect it to last multiple seasons, but it ought to last one.

Then i filled up the bottom...



I don't think I am Bob Villa, but I think this is strong enough. It is a temporary structure on cinder block. The framing is supporting up to 540# of cord wood, 10 poles of 1/2" PVC, one sheet of 6 mil plastic 10x20 feet, 8 binder clips from OfficeMax.

I don't know what the possible retained snow load on the plastic membrane is, haven't gotten much snow lately. In summer I would be hard pressed to store 1 pint of water on the plastic roof of a single unit overnight.

The 540# of wood up top can be represented as a point load pushing down in the middle, that load is going to want to push the feet apart. There is more lateral loading from the wood on the lower deck also tending to push the feet apart. So far my Spanish windlasses are holding.

The empty unit i could pick up and carry away from the pallets. Loaded, the lateral loading from the wood on the bottom deck seems to give me enough torsional resistance that I am not thinking about upgrading.

This would undoubtedly be a stiffer stronger unit If I tacked three sheets of plywood to it, but it would seriously cut down on air flow. All I really have here is a PVC and 6mil plastic tent that holds 540# five feet up off the ground.

I think my most likely problem is going to be end splitting on the horizontal 2x4s as the moisture content varies dramatically over the course of a year. With a week of thinking about it (and the advantage of walking around the physical structures with a sledgehammer just making sure) the only thing I have thought of that _might_ make a difference is to paint the ends of all my 2x4 with some latex paint so that all the moisture moving out of the framing is radial and tangential with a minimal longitudinal displacement.

I think. Please do bring on the informed critique.

Not insulted. I tried it "that way". this would be I guess Mark IV module framing. This is module #8, so far I have six of eight built and five of those six filled up for next winter.

I figger I'll build the remaining two this way also (module numbers 6 and 7). I juggled the 2x4 lengths a little bit to fit a bit more than .135 cords up top.

Built out. I got ribs and membrane on 6-7-8 today. I was rushing to get the membrane up since the forecast called for rain today. In Alaska. In February.





I think I might have an avatar worthy image here when I take the time to pull the wrinkles out of the membrane.

Couple notes. On the earlier units I used galvanized pipe hangers to hold the PVC 45s down to the ridge pole - I was cementing 2 45 onto a a short piece of 1" diameter pipe, 3 7/8" worked good for me.

Zinc is a cat poison, copper is not, I switched to copper pipe hangers to not poison the cat because I have a right lot of "window frost" collecting on the inside of the membrane in shoulder weather. I discussed this a little bit in the operation thread, with a link to a CalTech physicist's hobby ice crystal page.

My yard- my latitude - my solar exposure the window frost - the ice accumulating on the inside surface of the membrane- runs all the way down to the lawn on the inside surface of the membrane without dripping off the ceiling onto the wood very much - but it only takes a little tiny bit of zinc to kill a combustor, so I switched to copper.

FWIW this last iteration of framing, Mark IV is tall enough that I am using full length ( ten foot) PVC poles for the ribs and not having to cut any length off them before they go on. I am cramming every scrap of wood up top I can. These last few modules will hold slightly more than one cord, but not 1.2 cords I don't think. After giving up 18% shrinkage in seasoning I do not think I will be getting a full seasoned cord out of any single 8' unit.

Out of 8 units I have 5 different designs in use, but this last one that I made three of is the way to go I think. I can make them cheaper, but some of those less expensive ones that I saved 30-50 bucks on two months ago are and will continue to be a pain in my neck until I upgrade them to Mark IV. I have pretty well maximized internal volume and storage capacity, and maximized ease of use without resorting to special order non standard materials.

I plan to post up working drawings of the Mark IV design so anyone can build however many they want - or start there and build a better one. Once that is up I -intend- to ask the mods to close this thread so we can all get on with our lives.

Solar wood kiln operation thread is here: https://www.hearth.com/talk/threads/solar-cord-wood-kiln-operation.152699/