Solar cord wood kiln operation
- Thread starter Poindexter
- Start date
-
Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.
We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.
We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
2fireplacesinSC
Burning Hunk
I am behind on my wood for next year and am going to do some sort of plastic solar kiln. Price is right with shrink wrap. Was yours wrapped tight to the wood? Was thinking of using some Pvc to create some space and leave maybe leave the bottom third of the ends open.
WoodyIsGoody
Minister of Fire
Interesting stuff. Since most of the moisture exits via the end grain, I wonder if the speed of drying could vary depending upon the sharpness of the chain on your chainsaw? In other words, does it make a difference if the microscopic tube ends are cleanly sliced or mashed?
Easy Livin’ 3000
Minister of Fire
It's this kind of passion that leads to real breakthroughs and innovation in our world. I nominate PD for the first Ph.D in cordwood drying from Hearth.com University.
I think it does make a difference. I read somewhere that a firewood processing machine that used a shear rather than a saw meant that the logs seasoned quicker.
The good news is live spruce felled split and stacked in March 2017 was already dry on June 11, 2017.
The bad news is I know this because the wife was cold on June 11 2017 and I am already in to my 17/18 stash.
I am counting it a win. There is a very high probablility that spruce felled on June 10, split on the 11th and stacked on the 12th would be dry before freeze up.
The bad news is I know this because the wife was cold on June 11 2017 and I am already in to my 17/18 stash.
I am counting it a win. There is a very high probablility that spruce felled on June 10, split on the 11th and stacked on the 12th would be dry before freeze up.
2fireplacesinSC
Burning Hunk
WoodyIsGoody
Minister of Fire
Excellent! I would build a solar drying shed except my wood is all at my cabin where the only spot that gets better than tree filtered sunlight is my back deck which gets an average of 2 hours direct sunlight/day. Since my wood has to dry mostly in the shade, it takes 2-3 years! When I split a piece open that is about half-way done, it will read 18-20% moisture near the ends of the split but the middle is still around 30% moisture! This is how most locals burn it but it's far from ideal.
I'm still in burning season at my cabin. I have a cell phone app that let's me check the indoor/outdoor temperatures remotely. Right now it's 59 degrees outside and 60 degrees inside. By the time I arrive in a couple of days it will likely be about 59 inside. I could remotely turn on my mini-split heat pump so it's up to temperature when I arrive but I like building a nice hot but quick clean burning fire when I arrive. That's all it will take (a 2-hour burn) to bring the structure up to comfort this time of year. Then burning season will be over until September. Probably. You never know around here. I've burned in July before.
Freeze up, when the temp drops below freezing and stays there until spring usually happens in early October and runs until late March.
I am hardly a wood burning expert, but i do feel pretty good about operating the one stove i own .
21:49 of sunlight for solstice day this year, not a cloud in the sky, and my wood is already dry.
Hasufel
Feeling the Heat
Wow! I'm light sensitive and have a hard time getting a full night's sleep this time of year, but I'm also much farther south. How do you manage with barely over two hours of darkness?
So some observations. I am still learning to operate this thing I have wrought. Tagging @Woodsplitter67 .
1. The life expectancy of the cheap stuff clear plastic from Lowes-Depot in my application is around 12 months.
What I have done this year is cut off the complete scraps that were (probably) on the shady north side of my stacks last summer and using those as top cover this year. We are coming up on rainy season here, I am about to go buy a bunch of plastic sheet to keep my dry wood dry - and keep blowing snow out of it over the course if the winter.
2. After last years debacle (my wood was too dry) I am keeping the airflow through the kilns pretty high. My stash should be local EMC, about 11%, here in the next month or so. What I am running this year, really, is a super expensive way to keep my wood covered on top while drying, but really easy to cover the sides and keep the rain off here in the next 3-4 weeks when autumn starts.
I only operate the one wood stove, BK Ashford 30.0, and it ran pretty OK last year on wood at 7-8-9%. Put out real good heat, but my burn times were shorter than ideal and my stack plume wasn't as clean as I am accustomed too. At my house on 15' 6" of stack, 12-16% MC fuel is ideal, I have run the stove for extended periods both above and below that sweet spot.
So I am shooting for 12-16% MC again this year.
3. I do have one module running with mixed sized pieces in it, by accident. It is about 2/3s full with my regular sized splits, the last one I was filling. I had some enormous chunks left, big knots in them blah blah. Some of those I noodled down to just epic size with my chainsaw. I won't really know how well it worked until I burn them. I couldn't split them open before, I don't see splitting them open to measure MC before burning later. I'll see how they do; my hunch is accelerated kiln drying works best when all the pieces are about the same size. It took quite a while for the bigger pieces to start visibly shrinking, and my spruce is easier to dry than a lot of the woods common here.
Were I running a bigger operation on more acreage I might just elevate and top cover all my regular sized splits and then build a single kiln (or two) for the oversize chunks and uglies. I can't imagine two years in a passive kiln wouldn't be enough to dry them all the way through.
4. Having been able to operate the stove pretty normally on June 11, I have a pretty good shot at running these kilns to season two crops or harvests or rotations each summer. To do that, I need a holding shed large enough to hold at least four seasoned cords, and a teenager to come over in early June to move 4 cords from kilns to shed.. Then I can ditch 4 of my 8 modules to get some yard space back. To put the holding shed under my deck I first need to move the fire exit window in a downstairs bedroom around the corner of the house onto the adjacent wall... after the wife and I get home from the Paris trip she wants and I get a new boat. But it should work, at least with soft woods.
FWIW on June 11th 2017 the sun rose @ 0306 (am) 17 degrees east of north, set at 0034 (am) on the 12th 18 degrees west of north; 21:29 of daylight, and we had some civil twilight in there before the sun came up again. YMMV with shorter days.
1. The life expectancy of the cheap stuff clear plastic from Lowes-Depot in my application is around 12 months.
What I have done this year is cut off the complete scraps that were (probably) on the shady north side of my stacks last summer and using those as top cover this year. We are coming up on rainy season here, I am about to go buy a bunch of plastic sheet to keep my dry wood dry - and keep blowing snow out of it over the course if the winter.
2. After last years debacle (my wood was too dry) I am keeping the airflow through the kilns pretty high. My stash should be local EMC, about 11%, here in the next month or so. What I am running this year, really, is a super expensive way to keep my wood covered on top while drying, but really easy to cover the sides and keep the rain off here in the next 3-4 weeks when autumn starts.
I only operate the one wood stove, BK Ashford 30.0, and it ran pretty OK last year on wood at 7-8-9%. Put out real good heat, but my burn times were shorter than ideal and my stack plume wasn't as clean as I am accustomed too. At my house on 15' 6" of stack, 12-16% MC fuel is ideal, I have run the stove for extended periods both above and below that sweet spot.
So I am shooting for 12-16% MC again this year.
3. I do have one module running with mixed sized pieces in it, by accident. It is about 2/3s full with my regular sized splits, the last one I was filling. I had some enormous chunks left, big knots in them blah blah. Some of those I noodled down to just epic size with my chainsaw. I won't really know how well it worked until I burn them. I couldn't split them open before, I don't see splitting them open to measure MC before burning later. I'll see how they do; my hunch is accelerated kiln drying works best when all the pieces are about the same size. It took quite a while for the bigger pieces to start visibly shrinking, and my spruce is easier to dry than a lot of the woods common here.
Were I running a bigger operation on more acreage I might just elevate and top cover all my regular sized splits and then build a single kiln (or two) for the oversize chunks and uglies. I can't imagine two years in a passive kiln wouldn't be enough to dry them all the way through.
4. Having been able to operate the stove pretty normally on June 11, I have a pretty good shot at running these kilns to season two crops or harvests or rotations each summer. To do that, I need a holding shed large enough to hold at least four seasoned cords, and a teenager to come over in early June to move 4 cords from kilns to shed.. Then I can ditch 4 of my 8 modules to get some yard space back. To put the holding shed under my deck I first need to move the fire exit window in a downstairs bedroom around the corner of the house onto the adjacent wall... after the wife and I get home from the Paris trip she wants and I get a new boat. But it should work, at least with soft woods.
FWIW on June 11th 2017 the sun rose @ 0306 (am) 17 degrees east of north, set at 0034 (am) on the 12th 18 degrees west of north; 21:29 of daylight, and we had some civil twilight in there before the sun came up again. YMMV with shorter days.
Woodsplitter67
Minister of Fire
So i decided to put one together real quick. So with in the first hour the inside was 110 inside and the outside temp was 85. Later in the day it was 86 outside and 124 on the inside. I would say that this would spead up the process of drying. I have some thick uglies and odd pieces with some large oak and cherry
I did a mid season check last week on the MC so i will see how this works on the hard woods.
I did a mid season check last week on the MC so i will see how this works on the hard woods.
Last edited by a moderator:
It doesnt take long running one of these to see why "they" say to never leave dogs and babies in the car with the windows cracked, eh?
WoodyIsGoody
Minister of Fire
Is there any air movement in that thing? If the water can't get out, the humidity in the tent will be 100%.
Woodsplitter67
Minister of Fire
Its vented at the top with 2 vents. The vents are small to keep the warmth up, but large enough to alow fresh air in. I can feel the warm air coming out the vents. Definitely fresh air is going in.
WoodyIsGoody
Minister of Fire
That's good. Although I would recommend a small slit along the entire length to insure all parts of the pile are getting even air movement. Faster airflow will pull more moisture off the surface of the wood than higher temperatures and slower flow.
Also, you want the pile to cool off fully each night. The day/night temperature differential is what will cause the wood to "breath" more which will result in faster drying.
Though relatively tightly wrapped, i suspect splitter has good enough intake air at base around the entire perimeter.
Three things to track.
1. Capillary action.
2. RH calc
3. EMC
Ill come back to this after church...
Three things to track.
1. Capillary action.
2. RH calc
3. EMC
Ill come back to this after church...
Ashful
Minister of Fire
Poindexter, although it might not give you the same level of control as shifting cords from kilns to shed, when ideal MC% is achieved, have you considered just putting a transparent roof on your shed? You could kiln in-place, then, and not need that teenager to move wood around for you.
The Hypotheses that work just fine for me so far.
I am going for gracious here. I recognize this thread is text dense. If you disagree with my methods I invite you to build your own kilns, run them your way and let us know how it works out for you.
1. Capillary action In a live tree there is sap in the tubules. In above freezing temps with thoroughly thawed wood that isn't frozen on the inside capillary action and good air flow (with dry bulb temps less than +95dF which I'll get to shortly) takes about two weeks.
Two weeks, all the water in the sap is out of the tubules, the splits are at FSP or fiber saturation point and the stacks start to move as the individual pieces change size. It's very simple. If the stacks are moving you are below FSP. If the stacks haven't started moving yet there is still wet sap in the tubules. NO discussion, it's a definition.
If you run the temperature inside the kiln up above ~+95dF while the splits are above FSP you run the risk of shrinking the ends of the tubules shut and trapping wet sap inside the split. Every year, without fail, we get a couple or three new threads in the wood stove section "I bought kiln dried wood, it lights good but doesn't burn all the way through. I finally split one open and it meters 35% MC on the inside. Did i get ripped off? What's wrong?" And within a day or two someone with similar experience will say they 'kiln drying' that happened to the wood was just to kill parasites so the wood could cross jurisdictional lines for retail sale, esp common in the NE US.
If you want to make little burritos out of all your splits, a wet wood filling with a dry wrapped wooden tortilla, you go right ahead. I ain't got no time for that mess.
You may look up the FSP of whatever species you are wondering about here: https://www.google.com/search?q=fib...0j69i57j0l4.4036j0j7&sourceid=chrome&ie=UTF-8
2. Relative Humidity Calculation To figure out how dry the wood in the kiln is, or is going to be, the operator needs to know both the temperature inside the kiln, and the relative humidity inside the kiln. Having BOTH pieces of data will allow anyone to look up the Equilibrium Moisture Content the splits are tending towards, EMC is point three.
One way to accurately measure RH is with a wet bulb thermometer. I don't have room inside mine to do that. Plus it is hot and humid in there and spinning the thermometer in there for minutes at a time every day is not going to be fun.
Another option is to buy a hygrometer. The cheap ones are calibrated for typical household ranges. They don't work well, and don't work very long in rain forest like environs.
The third option, and one that works "good enough" on my lot is to recognize, with a broad brush, the vapor carrying capacity of air doubles with each 20 dF rise in air temperature. Thus a volume of air at say +50dF and 100%RH, inside a tank, or a baloon or what have you, will, when heated to +70dF and no vapor exchange with the rest of the atmosphere will have an RH of 50%. Same volume heated to +90dF will now have a RH of 25%.
It is the same effect as letting cold "dry" winter air into your heated home. Say your house is +70dF and you are maintaining maybe 30% RH with a humidifier on your forced air furnace system. Outside is +10dF and the RH of the outside air is 70%. Your kid opens the door to watch the mail man work the door on the mail box. You get a big slug of 10/70 air in, and the open door lets a big slug of 70/30 out.
When that cold air gets heated from +10 dF to +70dF it's vapor carrying capacity doubles on the +10 to +30 jump, and on the +30 to +50 jump, and doubles _again_ on the +50 to +70 jump. Doubles three times. it was 70% RH when it rolled in cold, gets cut in half to 35%, gets cut in half again to 17.5% and gets cut in half a third time to 9.25% RH when it finishes warming up. So it feels pretty dry.
Same thing in the kiln, except we are starting with "cold dry" air in the back yard at maybe +80dF and 80% humidity, but using the kiln to jack up the temperature and drop the RH.
https://en.wikipedia.org/wiki/Relative_humidity
https://en.wikipedia.org/wiki/Psychrometrics
Equilibrium Moisture Content Once we know the temperature and RH of the air inside the kiln we can easily look up the EMC all the splits in the kiln are tending towards via diffusion. The bigger the gap between EMC and actual MC, the faster the wood dries. As it gets closer and closer to EMC the drying goes more and more slowly.
I posted a lovely table less than a year ago, it is on page two in this thread, post #30.
https://www.hearth.com/talk/threads/solar-cord-wood-kiln-operation.152699/page-2#post-2105773
I f we knew the temperature and RH in splitter's backyard we could easily calculate the RH inside his kiln, knowing the temperature inside the kiln is +110dF. Today in Woolwich, NJ it is +82dF and 38%RH. So looking at the table that you just opened in a new tab you can see that if it was just top covered the wood in his stacks is tending towards 7.6% MC, because that is the EMC of wood for +80dF and 40%RH.
But with solar gain the inside of his kiln is +110dF. And he isn't pumping water vapor in, other than what is coming out of the wood, so if we look at the EMC for 110dF at 20%RH were going to get a high number. If we look at the EMC for 110dF at 10% RH we're going to get a low number, but the range is 2.2 to 3% MC for the wood to be at equilibrium with the air in the kiln.
That wider diffusion gradient he has created by lowering the RH and raising the temperature around his stack is going to make the stack dry faster than it would if it was just covered on top.
This seat of the pants rule of thumb SWAG is good enough. Does the kiln need "some" air turnover, yes. Does it need a lot of air turnover? No. A cord and a half, two cords like he has pictured above are going to do just fine with a couple holes at the roof peak, one at each end, each about big enough for a cantaloupe to pass, maybe 6-10" diameter.
I am going for gracious here. I recognize this thread is text dense. If you disagree with my methods I invite you to build your own kilns, run them your way and let us know how it works out for you.
1. Capillary action In a live tree there is sap in the tubules. In above freezing temps with thoroughly thawed wood that isn't frozen on the inside capillary action and good air flow (with dry bulb temps less than +95dF which I'll get to shortly) takes about two weeks.
Two weeks, all the water in the sap is out of the tubules, the splits are at FSP or fiber saturation point and the stacks start to move as the individual pieces change size. It's very simple. If the stacks are moving you are below FSP. If the stacks haven't started moving yet there is still wet sap in the tubules. NO discussion, it's a definition.
If you run the temperature inside the kiln up above ~+95dF while the splits are above FSP you run the risk of shrinking the ends of the tubules shut and trapping wet sap inside the split. Every year, without fail, we get a couple or three new threads in the wood stove section "I bought kiln dried wood, it lights good but doesn't burn all the way through. I finally split one open and it meters 35% MC on the inside. Did i get ripped off? What's wrong?" And within a day or two someone with similar experience will say they 'kiln drying' that happened to the wood was just to kill parasites so the wood could cross jurisdictional lines for retail sale, esp common in the NE US.
If you want to make little burritos out of all your splits, a wet wood filling with a dry wrapped wooden tortilla, you go right ahead. I ain't got no time for that mess.
You may look up the FSP of whatever species you are wondering about here: https://www.google.com/search?q=fib...0j69i57j0l4.4036j0j7&sourceid=chrome&ie=UTF-8
2. Relative Humidity Calculation To figure out how dry the wood in the kiln is, or is going to be, the operator needs to know both the temperature inside the kiln, and the relative humidity inside the kiln. Having BOTH pieces of data will allow anyone to look up the Equilibrium Moisture Content the splits are tending towards, EMC is point three.
One way to accurately measure RH is with a wet bulb thermometer. I don't have room inside mine to do that. Plus it is hot and humid in there and spinning the thermometer in there for minutes at a time every day is not going to be fun.
Another option is to buy a hygrometer. The cheap ones are calibrated for typical household ranges. They don't work well, and don't work very long in rain forest like environs.
The third option, and one that works "good enough" on my lot is to recognize, with a broad brush, the vapor carrying capacity of air doubles with each 20 dF rise in air temperature. Thus a volume of air at say +50dF and 100%RH, inside a tank, or a baloon or what have you, will, when heated to +70dF and no vapor exchange with the rest of the atmosphere will have an RH of 50%. Same volume heated to +90dF will now have a RH of 25%.
It is the same effect as letting cold "dry" winter air into your heated home. Say your house is +70dF and you are maintaining maybe 30% RH with a humidifier on your forced air furnace system. Outside is +10dF and the RH of the outside air is 70%. Your kid opens the door to watch the mail man work the door on the mail box. You get a big slug of 10/70 air in, and the open door lets a big slug of 70/30 out.
When that cold air gets heated from +10 dF to +70dF it's vapor carrying capacity doubles on the +10 to +30 jump, and on the +30 to +50 jump, and doubles _again_ on the +50 to +70 jump. Doubles three times. it was 70% RH when it rolled in cold, gets cut in half to 35%, gets cut in half again to 17.5% and gets cut in half a third time to 9.25% RH when it finishes warming up. So it feels pretty dry.
Same thing in the kiln, except we are starting with "cold dry" air in the back yard at maybe +80dF and 80% humidity, but using the kiln to jack up the temperature and drop the RH.
https://en.wikipedia.org/wiki/Relative_humidity
https://en.wikipedia.org/wiki/Psychrometrics
Equilibrium Moisture Content Once we know the temperature and RH of the air inside the kiln we can easily look up the EMC all the splits in the kiln are tending towards via diffusion. The bigger the gap between EMC and actual MC, the faster the wood dries. As it gets closer and closer to EMC the drying goes more and more slowly.
I posted a lovely table less than a year ago, it is on page two in this thread, post #30.
https://www.hearth.com/talk/threads/solar-cord-wood-kiln-operation.152699/page-2#post-2105773
I f we knew the temperature and RH in splitter's backyard we could easily calculate the RH inside his kiln, knowing the temperature inside the kiln is +110dF. Today in Woolwich, NJ it is +82dF and 38%RH. So looking at the table that you just opened in a new tab you can see that if it was just top covered the wood in his stacks is tending towards 7.6% MC, because that is the EMC of wood for +80dF and 40%RH.
But with solar gain the inside of his kiln is +110dF. And he isn't pumping water vapor in, other than what is coming out of the wood, so if we look at the EMC for 110dF at 20%RH were going to get a high number. If we look at the EMC for 110dF at 10% RH we're going to get a low number, but the range is 2.2 to 3% MC for the wood to be at equilibrium with the air in the kiln.
That wider diffusion gradient he has created by lowering the RH and raising the temperature around his stack is going to make the stack dry faster than it would if it was just covered on top.
This seat of the pants rule of thumb SWAG is good enough. Does the kiln need "some" air turnover, yes. Does it need a lot of air turnover? No. A cord and a half, two cords like he has pictured above are going to do just fine with a couple holes at the roof peak, one at each end, each about big enough for a cantaloupe to pass, maybe 6-10" diameter.
One other thing I guess I might oughta explicate. Visible light passes through clear membranes like kiln covers, bounces off the cord wood inside, passes back through the kiln cover again unchanged and you can see the wood in there.
UV light passes through a clear membrane unchanged, then hits the cordwood inside and becomes heat, infra red radiation. What you don't want to do is pay $$ for clear corrugated panels with UV blockers added in the mfr process.
UV blockers are a good thing if you are trying to grow orchids, not so good if you are trying to heat the place up.
UV light passes through a clear membrane unchanged, then hits the cordwood inside and becomes heat, infra red radiation. What you don't want to do is pay $$ for clear corrugated panels with UV blockers added in the mfr process.
UV blockers are a good thing if you are trying to grow orchids, not so good if you are trying to heat the place up.
Kinda sorta. If I had more space I could see building basically a greenhouse with no UV blockers in the glass, and then rigging up some kind of opaque curtains inside to keep the sunlight off the wood after it was dry enough.
The shed I am planning on is gonna be shaded by the deck, opaque walls, as vapor impermeable as I can build it, expensive door with a good weather seal system, and one little tiny vent, probably 4" stove pipe ducting from inside the vapor barrier to the driest air I can get to under the deck.
Couple screens in the stove pipe to keep squirrels out.
I though about collecting old freezers and refrigerators from the dump, but I would need a pretty big pile of them to hermetically seal four cords of wood for long term storage through the rainy season.
kennyp2339
Minister of Fire
Woodsplitter67
Minister of Fire
Poindexter
Thanks for the good reads and the time and effort you have put into your explanations. The hard woods that i have will be a good test to this experiment. I will post back my findings if you dont mind. Thanks for posting. I think its a great idea.
Thanks for the good reads and the time and effort you have put into your explanations. The hard woods that i have will be a good test to this experiment. I will post back my findings if you dont mind. Thanks for posting. I think its a great idea.
Last edited:
Similar threads
