Solar cord wood kiln operation

[Poindexter]

Distinct from design/ build. I already have a design/ build thread here for the solar cord wood kilns I am running: https://www.hearth.com/talk/threads/experimental-passive-solar-wood-kilns.149388/

What I would like to do in this thread is figure out how to run the fool things with out dragging all the build data back and forth off the hard drive in the hearth.com server.

Here is my passive solar "load one". It is 1.8 cords of dead standing fire killed spruce that meters 22-28%MC per handheld gizmo.



The kiln is my module one and module two end to end, each 42x96", total foot print 42x192". Long axis is almost perfect N-S. I overlapped the plastic sheeting in the middle to create what I hope is one convective unit.



The unit is open at the north end for now (qv) and closed at the south end thus:



Realistically I can expect (without benefit of passive solar) to have this "load one" down to 12-16%MC around June first. I would like to sell it ASAP when it is down under 16%MC, refill with green wood and see how fast I can get that dry with part of the summer already gone. It will be what it will be.

 

[Poindexter]

I have been reading up quite a little bit. The US Forest Service (USFS) Dry Kiln Operator's Guide is free online as 9 .pdf's, one chapter each. I find chapters 1, 2 and 9 to be especially relevant. I anticipate reproducing here the appendix to chapter one that gives Equilibrium Moisture Content (EMC) as a function of dry bulb temp and wet bulb depression.

I don't own a wet bulb thermometer yet, though I have several in my shopping cart on Amazon. I am a little concerned that I am not going to have good wet bulb reading because I won't have enough airflow to get a good wet bulb temp. No room to really run a psychrometer in there either.

For now it doesn't matter, I have about 1 hour of direct sunlight daily on load one, outdoor ambients haven't been above +20dF in over a month.

Also, incorporated herein by reference, Haque, N and Langrish TAG (2004) OPTIMISING DRYING SCHEDULES FOR HARDWOOD TIMBER IN SOLAR KILNS, New Zealand Journal of Forestry Science 34(3): 354–369 (2004).

Entire article here: http://www.scionresearch.com/__data/assets/pdf_file/0004/5377/09_Haque.pdf

Relevant quotations:

The diffusion model used in this study assumes moisture content gradient as a driving force and was applied successfully by Wu (1989), Doe et al. (1994), and Langrish et al. (1997)...The drying run was terminated when the desired final moisture content was reached, which was specified to be 12% (average)...At the start of the experiment maintaining very low wet-bulb depressions proved to be difficult...The long “tail” in the moisture content curve (Fig. 4) arose because of internal resistance to moisture movement, not internal resistance to heat transfer...The simulation program was formulated so that, once the strain reached its maximum, the difference between the dry- and wet-bulb temperatures (the wet-bulb depression) was adjusted to keep the strain within this limit, and so the maximum strain was considered as a constraint...[later when the wood was drier]...The slightly harsher conditions (higher wet-bulb depressions) made little difference to the strain because large moisture content gradients are not possible in timber at these low average moisture contents...For the drying process, it is evident from this analysis that strain is a constraint in the early and middle stages of drying due to moisture content gradients, whereas dry-bulb temperature is a constraint in the final stages of drying, because the moisture content gradient is then very low...[and finally]...A step-wise drying schedule is convenient to apply for kiln operators with simple kiln control systems. Once the actual average moisture contents fall below 35%, then the dry-bulb temperature can be increased to 35°C, with a 5° or 6°C wet-bulb depression (Table 2). The dry-bulb temperature may be increased to 53°C, with a wet-bulb depression of 16°C, for drying below moisture content 22%. A dry-bulb temperature of 60°C may be applied with a 30°C depression for drying below a moisture content of 18%. This may be a sudden change of drying conditions compared with the original schedule but, by this stage, the timber may be able to tolerate such harsh conditions. For this final stage of drying, from moisture content 25% to 12%, the new schedule predicts that in solar kilns 16 days may be necessary for drying if a constraint is put on the dry-bulb temperature (i.e., a maximum of 60°C for a solar kiln).

When I read this article I picture a rimmed cookie sheet on the kitchen counter next to the sink, brimming with water. I mean over the top, relying on the surface tension of water for it not to spill, put one more drop of water in there and you get a tablespoon of water on the counter.

As a masters thesis in forestry, they figured out how fast they could tip the cookie sheet without spilling a drop. They started slow, and then went faster and faster. It makes perfectly good intuitive sense to me.

It seems to me what they did was create and maintain a smooth diffusion gradient between the surface and center of each board. If they ran their dry bulb temp up too high too fast the surface of the board would dry out faster than the middle, creating a step or notch in the diffusion gradient that is only going to be overcome by applying more heat than was absolutely needed. Waste not want not and so on.

So I am going to start here and see how it works. From green wood down to fiber saturation point (FSP) I am going to try to keep the dry bulb (regular) temperature under +95dF with really really good airflow to keep the wet bulb depression to a minimum. Once I get to FSP, I'll let the dry bulb stick near and slightly exceed +95dF and let the wet bulb depression reach 25-30 degrees F. Once the stack is under 22% MC I can really put the screws to it, let the dry bulb reach +140dF if I can get there, with a wet bulb depression around 55dF (so +95dF indicated wet bulb temp).. and expect to be at 12%MC in a matter of a few days. If I get good weather.

Clear as mud?

 

[Poindexter]

reserved also, running out of steam tonight.

 

[KBCraig]

Why LANS, instead of E/W?

I would think that one long side getting full sun the entire day would provide more solar gain than the glancing blows in morning/evening, and very small frontal area noon-ish, in an E/W layout.

 

[Poindexter]

Why LANS, instead of E/W?

That's a fair question I feel up for. I am building 8 modules total, I anticipate dividing those into three separate loads. Schematic:



Modules 1,2,4 and 5 are built. 1 and 2 are loaded and rolling as load one. 3 & 4 are going to hold my left over wood that I had going into winter of 2015/2016. 4 is already loaded, 3 is not built. 5 is built but empty, 6,7 and 8 are not built.

I anticipate filling 5-8 with less expensive green healthy standing timber, compared to the more expensive dead standing I used to fill modules 1 and 2.

I am on a tiny suburban lot at 64 degrees north latitude. On summer solstice the sun will rise 10 degrees east of north, and set 20 hours later about 15 degrees west of north. Notice "north" is upside down to standard lower 48 perspective, everything is south from here.

At this juncture I should tag @solarguy2003 . He is one of several people I was planning to tag in post three, but not tonight. solarguy2003 is running a solar firewood kiln I think in Indiana. Besides firewood he is also keeping fruit trees, I wanna say fig trees, in his green house. He is operating a legit solar firewood kiln and I hope he is willing to share his observations about his unit on his property here too. When google users get to here I hope they are rewarded with observations from multiple people at multiple latitudes.

 

[Poindexter]

Load two is up. It is modules 3 &4 as platted above.

Filled with wood already seasoned one year in the open and likely to be left over at the end of this season. Mix of birch and spruce at 18-20% MC, but with a bunch of mold and mildew and fungus growing on it. My top covered wood pile got hammered on the sides with summer rain, I have the micro-organism ranch to prove it.

I also loaded, near the bottom, all the 16" splits I could find that have done duty holding down the plastic top covering on stacks in previous years. Those have a bunch of crusty snow on them. MC unknown.

Short term I am going to keep the sides rolled up on this one. The load is getting about an hour of direct sunlight daily, forecast highs are in the +20s dF. I don't want the crusty snow turning into water vapor and condensing on the dry wood above it; hopefully with the sides open I can minimize greenhouse effect and give any liberated vapor a chance to blow away on the wind.

Long axis of load two is E-W.

55 gallon Ugly drum Smoker for scale, module 5 (part of future load three) is left edge of photo. Proceeding left to right are modules 4 and 3 (load two) and then a 90 degree bend to modules 2 and 1 that make up load one as detailed above.

The east (left) end of module 4 is covered just like the south end of module 2 as picd in post one this thread to keep mold and mildew spores out of module 5/ load three.

At the west end of #3 I put another vertical sheet of membrane, but left an air gap at the top.



At the end of the burn season all my remaining moldy wood, even the chunks and uglies on the pallet are going into load two and i am going to cook it as hard as I can. Might even seal up the air vent, dunno. By the end of summer I want dry cord wood with non-viable organic debris smeared on it.

 

[Poindexter]

So riddle me this.

A the north end of load one, total shade, ambient temps only, I have a growing layer of kosher salt grain sized ice crystals. I am not super worried about them, but I see the accumulation get thicker on a week to week basis. The black grains are charcoal off the fire killed spruce stacked in this module.


Ambient temps have been pretty much -10dF to +10df for about a month, relative humidity is running 70-95%, but at temps this low we are talking about precious little water vapor in the air.

North end looks like this, no accumulation on the vertical surfaces, just a slowly growing pile on the floor.



But on the east end of module #5 I am getting about 1-2 hours of direct sunlight daily and I can't explain this.

Top of ice accumulation on vertical 2x4, 3-4 feet above the "grade" level of hard packed snow:



Same 2x4 lower down, say 6-24" above "grade" level of nearby hard packed snow:



Why is that? Clearly the available sunlight on the east end is doing some "work" that isn't happen on the fully shaded north end...

I got my infrared tire pyrometer out, I'll see what I can find for temperature differentials tomorrow.

 

[paul bunion]

On the north side there was no sunlight, so no energy, which means very little help with evaporation/sublimation. With little evaporation/sublimation on that side there is little condensation/deposition so you all you have is a small pile of ice crystals.

On the sunny side your stack caught some sun giving it more energy on that side to make more water vapor out of the frozen water. Then it hit the cold plastic, sublimated and you see the ice crystals forming.

Yes, 10 degree air has a low absolute moisture content but RH is what really counts. It hit 100% on the inner surface of your tent at your 2x4s.

 

[Longstreet]

Sounds like you are having what I've heard as "accidental dehumidification" in building science. Warm air with high relative humidity hits a cold surface and makes dew. Just happens that it's so cold in AK that the dew freezes.

 

[Poindexter]

"Riddle me this", well crud. I came up with 8 data points, been wandering around the lawn every four hours with my IR thermometer for the last 24 hours to try to learn something about water vapor moving around in below freezing temperatures.

Well and truly above freezing today. I got this (woo hoo!)



I did find a physicist at CalTech with "The Physics of Crystal Growth and Pattern Formation in Ice. " as his second research interest. Primary is gravity waves, I am guessing he isn't answering a lot of snowflake emails right now anyway. I didn't write him since I understand pretty good how water vapor moves around in above freezing temperatures.

Dr. Libbrecht does have an illustrated page of frost types here: http://www.its.caltech.edu/~atomic/snowcrystals/frost/frost.htm . I suspect the ice crystals forming on my 2x4 and cord wood splits above are simple hoarfrost, though the mechanism of vapor transport is still mysterious to me.

The battle du jour at my house was the 'window frost' on the inside of all my plastic membrane was melting and dripping onto my cordwood.

From reading several of Dr. Libbrecht's pages I think what has been happening is when it cools off at night, the water vapor inside each kiln module freezes to the inside of the plastic membrane, plain old regular window frost. The next day, as the air inside the kiln warms up it convects out, fresh water vapor laden air comes in behind, it freezes again and my window frost layer gets thicker.

I got most of the window frost off today, but it will be something to avoid this fall and next spring and next fall and next next spring. Besides getting water out of the cord wood in hot weather I am going to have to minimize water getting into each unit during shoulder weather when outdoor temps are below freezing but temps inside the kilns are above freezing.

 

[English BoB]

Distinct from design/ build. I already have a design/ build thread here for the solar cord wood kilns I am running: https://www.hearth.com/talk/threads/experimental-passive-solar-wood-kilns.149388/

What I would like to do in this thread is figure out how to run the fool things with out dragging all the build data back and forth off the hard drive in the hearth.com server.

Here is my passive solar "load one". It is 1.8 cords of dead standing fire killed spruce that meters 22-28%MC per handheld gizmo.

View attachment 173424


The kiln is my module one and module two end to end, each 42x96", total foot print 42x192". Long axis is almost perfect N-S. I overlapped the plastic sheeting in the middle to create what I hope is one convective unit.

View attachment 173425

The unit is open at the north end for now (qv) and closed at the south end thus:

View attachment 173426

Realistically I can expect (without benefit of passive solar) to have this "load one" down to 12-16%MC around June first. I would like to sell it ASAP when it is down under 16%MC, refill with green wood and see how fast I can get that dry with part of the summer already gone. It will be what it will be.

Great job man, hope this works for you. Keep us updated.

bob

 

[Firefighter938]

Looks like they are working so far. I wouldn't be too concerned with the ice forming. As long as it has a way to get out when temps are warmer.

 

[St. Coemgen]

Interesting. But complicated.

After drying wood for more than 30 years, and building all sorts of drying systems, I finally found the KISS principle works well for me (keep it simple stupid, or simply applying Occam's razor). And that is, and again this is my experience, this has converged on the "Holzmiete" method (requires nearly zero extra construction, maintenance or post stacking effort):

http://www.holzmiete.de/instruction.php

Yes, that is the method being used in my avatar image.

For cooler climates where wood dries slower, just buy and stack three years in advance.

 

[Poindexter]

Hey @solarguy2003 , you remember when you first built that thing and put some wood in it and waited like a month, and then stuck a meter in some fresh splits "just to see"?

Were you speechless? Like no way can this be right? And you went had a beer and came back an hour later and split some more open and finally just had to sit down and cogitate a little while?

My load one from post one above is metering 18-20%, and burning in my stove right now like the moisture meter is working just fine. I went from cold stove to engaged combustor in 19 minutes. My IR thermometer is telling me I got dry wood in there. My stack plume is clean.

I was not looking for this wood to be dry until June 1.

I'll wait and dig through the ash bed in the morning looking for tell tale chunks before I get to ululating.

 

[kennyp2339]

"Holzmiete" method

I done did gots one of them in my yard.

 

[Poindexter]

No tell-tale clumps in the ash bed this AM. I am switching to burning out of load one full time.

I'll give it a week or two before I really really believe it, but "working awesome" does seem to be a fine description of a woodpile with a vapor impermeable layer under it to keep groundwater out.

Gobsmacked I am.

 

[Poindexter]

Too late for me to edit post #6 above. "Load Two" was sold in early March at 19% average MC. I put an ad up on CL 'just to see' at a bit below market rate because of the mold on it. It sold in two hours, the buyer came and picked it up same day, brought cash, and paid the going rate for delivered dry wood with no mold on it.

On the one hand I was kinda interested to know if I could achieve dry bulb temps high enough to kill micro-organisms. I can still look for peak dry bulb temp values, but without 'needing' to achieve them.

 

[Poindexter]

Load three (winter cut live spruce, probably felled around valentine's day 2016) was up on March 17. The north side is pretty open, with the membrane rolled all the way down on the south side:



Most of the bottom clips on the South side are 36-40" off the ground. I get enough breeze in the twilights to pull the clips off if I locate them lower for now.

Around 4 or 5 oclock in the PM, before the breeze starts blowing I am seeing ambients round +50dF, and +50 to +52dF at the bottom of the load on the shady side. At the top of the load on the shady side I am seeing +65dF on sunny days and +60ish dF on cloudy days.

Pragmatically it is early April outside the kilns and "early May" inside the kilns.

Once load three is visibly shifting and settling, clearly having reached and dropping below FSP (Fiber Saturation Point, ~30% here); then I will roll down the membrane on the north side to trap some more heat in there.

I am feeling somewhat optimistic that green wood up on March 17 can be ready to go into a storage shed at 12-16% measured around June 1. That _ought_ to give me enough time to season a second load of green wood each season, but there are many variables.

 

[Poindexter]

Everything is down under 30%MC, I can tell because the stacks are shifting and visibly shrinking. Even the spruce winter cut in early March that I got stacked in mid March has already shrunk visibly and unequivocally. The stacks are moving and shrinking.

At +50dF ambient I am seeing +15dF solar gain on sunny days and +10dF solar gain on cloudy days, and this with the plastic rolled up on the north side.

I rolled all the plastic down today so it looks like posts one and six above, not like post 16 above, only with the snow all melted.

Some of my grass is starting to green up, my delphiniums are sprouting back, and the birches are leafing out all over town. The Arctic geese got here about ten days ago and left last Friday, five days ago. They stop in Fairbanks to rest on their way to the north slope.

I feel pretty good that I should be able to pull dry spruce out of all these units around June first ( from March 17 start) and maybe maybe not get a second crop of cord wood dried to 16%MC before freeze up.

For the next few days I'll keep a careful eye on the temps. Per Haque and Langrish (above in post 2) I am planning to keep wet bulb depression to a minimum and hold dry bulb temps to about +95dF max until I am down to 20% average MC. Once I have a good feeling for how much solar gain I am getting with the plastic rolled down I can hopefully kinda keep an eye on the weather forecast to know when I might hit +95dF dry bulb with the plastic rolled down all the way around.

No pics. We have our big suburban house listed, an offer in on a small cabin on acres and my camera is in a box "open after showing" with some other stuff I don't want laying around the house with strangers coming through.

EDIT: Now that the plastic is rolled down all the way round I have the bottom clips about 12" above the floors. I was catching a LOT of wind with the plastic up on the north side and down on the south side, hopefully there is less for the wind to get hold of with the plastic down on both sides.

 

[paul bunion]

Why are you so keen on trying to us a drying schedule that is designed to minimize defects in lumber? I don't think that warping and checking are concerns with firewood. Go to their last step. You should be trying to get it as warm as possible. The heat results in a depressed RH level, remember every 20 degrees F rise in temp cuts the RH in half. Then figure out how much or little ventilation is needed to maintain your heat while allowing the water vapor to get out. Condensation and direct draining of moisture also plays in.

 

[Poindexter]

Why are you so keen on trying to us a drying schedule that is designed to minimize defects in lumber? I don't think that warping and checking are concerns with firewood.

I agree that warping and checking are not concerns in cord wood. What I want to avoid is a dry shell surrounding wet wood in the middle of each split.

There was a fair amount of discussion about this in the design/build thread I was running over the winter. The outcome is admittedly buried in a six page thread, if I may kindly direct attention to posts 109 and 110 in that thread:

https://www.hearth.com/talk/threads/experimental-passive-solar-wood-kilns.149388/page-5#post-2037109

The A#1 finding is get the water out of the tubules via capillary action so the remainder is down to FSP before heating it up.

Go to their last step. You should be trying to get it as warm as possible.

This is exactly what I don't want to do, for a couple reasons.

One, I don't want a "burrito" made of a dry wood tortilla with a wet wood filling.

Two, while I am very familiar with local weather patterns, I don't "know" how many days of how bright of sunshine with how much wind I need to pull a completed dry load out of a module so I can reload the module with fresh green wet wood. I need all of the wood in there now for this coming winter, I don't have anything left over from last winter. Once I have this coming winter's wood in the bag, yes I will be willing to experiment with accelerating the drying process some more.

Three, I am trying to do this as cheaply as possible. For 8 empty modules I am currently in for about $2400, never mind assembly time and the cords of splits in there drying. I do have an Arduino system (still in the box from Amazon) that I hope to install in the kilns 'someday'. I got the deluxe starter kit and a 5 pack of water proof temperature sensors and a PCB, a shield I think is the correct term, so the Arduino unit in the back yard can connect to my PC via my wireless router and send me five data points of temperatures at whatever sampling interval I program. Once I get to there Is should be able to push the data to my smartphone using the SMS text message format without too much trouble.

While I do want to know what is going on in there, I am specifically trying to not spend a bunch more money on controlling what does happen in there. I could spend thousands of dollars on a live steam system, install a bunch of insulation and pull beautiful splits, cord after cord of them every couple weeks or so, but the break even point will be decades even if oil prices head back up tomorrow.

The heat results in a depressed RH level, remember every 20 degrees F rise in temp cuts the RH in half. Then figure out how much or little ventilation is needed to maintain your heat while allowing the water vapor to get out. Condensation and direct draining of moisture also plays in.

All true. Alternatively, if my inlet air is at +50dF and 100%RH at one axe handle of water per bushel of air, I could heat the one bushel of air to +70dF, find the RH is now 50% with the one axe handle of water in it, pull another axe handle of water out of the drying cord wood to take the RH up to 100%, and then pump it out.

I am trying to not buy fans, and power to run the fans. Just letting nature take its course if you will.

Having said all that, I too will be out of town at the first of the month, my youngest is graduating college. I pulled some splits and measured some MCs today. In the winter cut spruce (cut about 02-14-2016, split, and then stacked on 03-17-2016, I find today 27%MC inside the split and 25%MC outside the split. I am maintaining a gentle diffusion gradient from the center to surface of each split - so I will not later in the drying cycle have to expend extra energy pulling water from the inside of each split over a notch in the diffusion gradient to the surfaces.

So far Haque and Langrish are working for me. I am seeing about +15dF solar gain on sunny day with ambients in the +50 to +60dF range. At +80dF ambient (ish) I will expect to see about +95dF dry bulb inside the kilns. I don't have a LOT of data to predict on, it could be I will see +95dF dry bulb inside the kilns at +75dF or so. I expect and hope this happy circumstance will occur after the centers of all my splits are down to +/- 20%MC. We are having odd weather patterns this spring, but it should be weeks before I regularly see temps over +75dF.

I am still on track to have all of this winter's wood ready on June first. If I am wrong, it will likely be ready earlier. When I get to working on green wood for Sep 2017 in June 2016 I'll see about speeding up the process.

Great questions, and timely too. Thanks for contributing.

 

[Poindexter]

In the last couple days I have found load one (post one) to be down to 13-18%MC per handheld meter. Load three (post 18) is still in the mid 20s.

I sealed up the north end of load one this afternoon:



Leaving a tiny window at the top to let hot wet air dribble out and a temp probe from one of my BBQ thermometers in:



With out door ambients in the mid +50s dF I am seeing +93dF at the top of the kiln. I went ahead and started pinging my local network, at 13-18% this wood is already very desirable for many local burners, I am looking to trade stick for stick, you bring me green spruce, I give you solar seasoned spruce. The entire 1.8 cords is going on Craigslist Monday night if I don't get any takers.

EDIT: 5-8-2016, with ambients at +69dF I found +104dF at the top of load number one. A small thing, but I don't know that ambients of +104dF have ever been recorded here. This stuff is drying out, in mid-May faster than it ever will just top covered and open on the sides.

 

[maple1]

You want to trade 'even up', kiln dried wood for green wood? After going through all this? What did I miss?

 

[kennyp2339]

After going through all this? What did I miss?

I was thinking the same exact thing

 

[Poindexter]

I was thinking the same exact thing

Right now I just want data. I want data real bad. Giving away 2 cords seasoned in trade for 2 cords green and data (how fast can I dry wood starting with green on say May 15th?) seems like a good deal to me.