My Solar Kilns.

[KJamesJR]

I received an order of processed wood, 4 cord a few months ago for 2019/2020 winter. To my surprise a large majority of it is red and white oak, about 3 cord worth with about a cord of beech and hard maple mixed in. Because this past winter was so long, and wet I knew I would need a solution to get this sub 20%, and within 6 months.

Heeding the advice given here on the forums I decided to tackle some solar kilns. I somewhat copied a design by a forum member, woodsplitter, but with a little more shade tree engineering incorporated.

All materials I used, I already had laying around. Old PT fence posts, some brick pavers, landscape fabric etc... the only purchase I made was the 3mil plastic tops and shrink wrap. Total was about $27 from Lowe’s.

Each row is about 20’ long and roughly 4 - 5’ high and 22” deep. I’d say a little over a cord in each. I have two rows in kilns and two more stacked in a single row, uncovered, as a control row. All rows get full sun from dawn until about 6pm.

I started with a base of black landscaping fabric. The idea is to keep the bottom clean and allow for better airflow by snuffing out the weeds.

I then built the wood rack on top of that. Up off the ground via bricks. I built the racks in such a way the 22” splits would stack easily within the inside dimension, while the outside dimension, between the vertical 2x4s left roughly a 1” air gap at the split ends for air and moisture evacuation. The idea is to get air in from below and travel across the splits wicking out the moisture then out the ends of the row.

I tacked the 3 mil plastic down over a rope (kinda like a ridge on a roof) with some roofing nails I had. I started one layer of shrink wrap underneath the 3mil plastic to shed rain water away from the wood. Then I proceeded to wrap over that normally.

The top ends of the stacks are left open for ventilation.

The moisture of the oak going in was roughly 30%. I’ll check a couple random samples every two weeks and compare it to the uncovered row.

It’s not perfect and I know some of the wood is contacting the plastic. I’m going to monitor the progress and see if that makes a difference between rot or drying wood.

 

[spudman99]

That looks pretty clever, although I have never built one myself. One suggestion is to screw or nail some 2x4 or other scrap lumber vertically along the sides before shrink wrapping. This will provide a standoff from the splits and allow air an easier path to the top to vent. When finished you just unscrew the same scrap.

 

[Sully1515]

I received an order of processed wood, 4 cord a few months ago for 2019/2020 winter. To my surprise a large majority of it is red and white oak, about 3 cord worth with about a cord of beech and hard maple mixed in. Because this past winter was so long, and wet I knew I would need a solution to get this sub 20%, and within 6 months.

Heeding the advice given given here on the forums I decided to tackle some solar kilns. I somewhat copied a design by a forum member, woodsplitter, but with a little more shade tree engineering incorporated.

All materials I used, I already had laying around. Old PT fence posts, some brick pavers, landscape fabric etc... the only purchase I made was the 3mil plastic tops and shrink wrap. Total was about $27 from Lowe’s.

Each row is about 20’ long and roughly 4 - 5’ high and 22” deep. I’d say a little over a cord in each. I have two rows in kilns and two more stacked in a single row, uncovered, as a control row. All rows get full sun from dawn until about 6pm.

I started with a base of black landscaping fabric. The idea is to keep the bottom clean and allow for better airflow by snuffing out the weeds.

I then built the wood rack on top of that. Up off the ground via bricks. I built the racks in such a way the 22” splits would stack easily within the inside dimension, while the outside dimension, between the vertical 2x4s left roughly a 1” air gap at the split ends for air and moisture evacuation. The idea is to get air in from below and travel across the splits wicking out the moisture then out the ends of the row.

I tacked the 3 mil plastic down over a rope (kinda like a ridge on a roof) with some roofing nails I had. I started one layer of shrink wrap underneath the 3mil plastic to shed rain water away from the wood. Then I proceeded to wrap over that normally.

The top ends of the stacks are left open for ventilation.

The moisture of the oak going in was roughly 30%. I’ll check a couple random samples every two weeks and compare it to the uncovered row.

It’s not perfect and I know some of the wood is contacting the plastic. I’m going to monitor the progress and see if that makes a difference between rot or drying wood.

Lookin' good! Keep up the good work! Just a head's up, you may want to give yourself more than 6 months for the wood to properly season before burning. Red oak, especially this year, seem to be taking a lot longer to season than normal. Overall, keep up the good work and keep us updated on your project!

 

[jackatc1]

I personally would not attempt a solar kiln for fear of too much mold.
But I always thought using black plastic would create more heat to speedup drying.

https://www.lowes.com/search?searchTerm=black+plastic+sheeting

 

[KJamesJR]

Lookin' good! Keep up the good work! Just a head's up, you may want to give yourself more than 6 months for the wood to properly season before burning. Red oak, especially this year, seem to be taking a lot longer to season than normal. Overall, keep up the good work and keep us updated on your project!

Weather forecasts only 3 days of partial sun over the next week. It’s been wet, rainy and mostly cold here in the north east since February. I checked the moisture of the oak when it was delivered back in April. It’s only dropped 4% in moisture since then.

I’ll be keeping an eye out for mold. I’m hoping I have enough ventilation through the stacks to keep it at bay when the weather gets wet and cool (like today) If these kilns are able to heat up every other day or so, the emerging mold spores should be rendered sterile, maybe around 130f or so before getting a foothold. At least now they’re out of the rain.

 

[Kevin Weis]

How is the moisture getting out or am I missing something on the pictures?

 

[KJamesJR]

How is the moisture getting out or am I missing something on the pictures?

The stacks are vented from the top (sides). The second picture from the bottom shows the venting hole at the top looking in the length of the stack.

 

[ben94122]

Thanks for posting! I like the control row and the every 2 weeks datapoints--will follow with interest. Of course, you will check freshly split faces with your moisture meter...

 

[brenndatomu]

I thought the idea on these solar kilns was that they were unvented, so they get really hot, and then the water droplets condense on the plastic and then run down the side and out the bottom...

 

[KJamesJR]

I thought the idea on these solar kilns was that they were unvented, so they get really hot, and then the water droplets condense on the plastic and then run down the side and out the bottom...

From what I’ve seen online, they’ve all been vented. I’d be too afraid to do otherwise.

Ideally you’d construct these with more rigid materials that can reliably shed condensation, unlike mil plastic. With the low spots and wind interference, A lot of that condensation would just drip back on to the stack I would imagine. A steep A-frame with corrugated plastic I think would work well.

 

[Kevin Weis]

Okay, I see the venting now. That should work well.

 

[Woodsplitter67]

I thought the idea on these solar kilns was that they were unvented, so they get really hot, and then the water droplets condense on the plastic and then run down the side and out the bottom...

The kiln needs to be vented. You want to feel hot moist air going out the top vents.there will.be no mold as the moisture will be moving out the kiln so no worries about that

 

[Woodsplitter67]

From what I’ve seen online, they’ve all been vented. I’d be too afraid to do otherwise.

Ideally you’d construct these with more rigid materials that can reliably shed condensation, unlike mil plastic. With the low spots and wind interference, A lot of that condensation would just drip back on to the stack I would imagine. A steep A-frame with corrugated plastic I think would work well.

Dont worrie anout the recent rain and cool weather. Theres plenty of summer ahead. Your wood should be sub 20% by august so i would check it then. Dont over kiln the wood getting it to 15 to 18% will be optimal.
I would out an air prob in the kiln on an 80 degree day and see how warm the kiln gets.. you will know then and only then if its vented property.

 

[Dolphus Raymond]

Solar kilns work great. Since I do not have the space to get 3years ahead, I utilize the solar kiln principle and have had great results. How I do it is let my wood air dry for a month then put the plastic on.

 

[KJamesJR]

Dont worrie anout the recent rain and cool weather. Theres plenty of summer ahead. Your wood should be sub 20% by august so i would check it then. Dont over kiln the wood getting it to 15 to 18% will be optimal.
I would out an air prob in the kiln on an 80 degree day and see how warm the kiln gets.. you will know then and only then if its vented property.

I think what you’re saying is it can get too hot? What kind of temps should I be looking for?

 

[EODMSgt]

I think what you’re saying is it can get too hot? What kind of temps should I be looking for?

I don't think Woodsplitter67 was saying your stacks would get too hot in the kiln, more that you can over-dry the wood. I doubt that would happen with oak splits in one summer but beech and birch can get pretty dry. I think this is in line what he is referring to:

Can Firewood Be Too Dry?
Yes, although it is not a common problem
Properly seasoned firewood still has a fair amount of water in it, say 15 to 20 percent of its weight. That water regulates the combustion process along with a few other factors like piece size, load configuration and combustion air supply.

The higher the fuel moisture, the slower the wood breaks down when heated because of all the heat energy soaked up in boiling the water out of the wood and raising the temperature of the steam.

Conversely, the dryer the wood, the more quickly it breaks down when heated. By breaking down, I mean the vaporization of the volatile components of the wood; that is to say, it smokes. The dryer the wood, the more dense is the smoke at a given heat input rate.

Since wood smoke is fuel, we want to burn it as completely as possible and that means mixing with adequate oxygen in the combustion air. The problem is that a firebox load of very dry wood produces far more smoke than the air supplies of stoves are designed to provide. Besides, even if you could supply enough air, you would produce an inferno that would howl in the stove and make everyone in the house nervous. Fires that intense can seriously damage the stove's innards. Wood that is very dry produces a fire that is hard to control without making a lot of smoke.

Kiln-dried wood is down around 10 percent moisture. Depending on climate and conditions of storage, normal firewood won't dry down to kiln-dried moisture because of normal outdoor humidity. For example, I've never measured wood below about 14 percent in my firewood supply. But I suppose that firewood could get very dry by natural seasoning in desert conditions. Or firewood stored in old barns, which are like kilns in hot summer weather.

The right band of firewood moisture is between 15 and 20%. When you get much over 20% you start to see symptoms of sluggish ignition and the inability to turn down the air without extinguishing the flames. Towards 30% the wood sizzles and fires are very sluggish and it is hard to get a clean burn until the wood is almost to the charcoal stage. Above 30% water bubbles from the end grain when the wood is heated and it is very hard to burn at all. Species like poplar/aspen, which have very high native moisture content are virtually non-combustible when not adequately seasoned.

The main difference between EPA low-emission certified stoves and conventional stoves is that you can turn down EPA stoves for a long burn without extinguishing the flames. That is, they are better at producing a clean, controlled fire. The EPA test method requires wood with a moisture content between 16 and 20 per cent (19 - 25% dry basis) and when the wood is outside this moisture band, the stove's emission rate goes up. So even the best wood stove's performance will suffer if the wood is not in the right moisture range.

If you have some very dry firewood, like kiln-dried cut offs or old wood stored in a hot place, mix it with regular firewood to raise the moisture content of a full load.

 

[Woodsplitter67]

I think what you’re saying is it can get too hot? What kind of temps should I be looking for?

So with out knowing what the temperature is in the Kiln you do not know if it is running correctly if your veins are too large the Kiln is operating at a lower temperature than what it should be if it is not vented well enough the temperatures get very warm and the moisture does not work its way out the way that it should so that being said. you need to do is put an air probe in the Kiln on a day that is around 80 degrees or a time that is around 80 degrees and then you will be able to adjust your vents properly the vents should not be cut out but you should cut a flap in a u shape and that will allow you to adjust the airflow coming out of the vents there for regulating the temperature within the kiln. When the outside temperature is approximately 80 degrees and the Sun is hitting on the kiln I prefer to run my kiln at about a hundred and fifteen degrees which is a 35 degree difference from the inside of the Kiln to the outside of the kiln on the warmer days the Kiln run warmer and on the cooler days it will run a little bit cooler but at 80 degrees that's kind of where I like to run mine.
There was some advice on This Thread about using black plastic do not use black plastic black plastic will actually have the kiln run cooler.
Also there was some advice on This Thread about the oak needing to need six months in the kiln do not run the oak 6 months in the kiln. One year I left my wood in the Kiln for 4 months and weigh over dried it my Oak was down to 9% and my cherry was down to like two or 3% I did take some logs and put them in the wood stove and they burned hot so what I did was remove the wrap around the Kiln and just put a tarp over it and allowed the wood to get back up to 15 to 18%.

I would open the Kiln towards the end of
August and check it with a moisture meter and you will know exactly where it is at that point really you only need to run the Kiln about 60 days and you should have it at sub 20%. That is if its running good..

 

[Woodsplitter67]

I don't think Woodsplitter67 was saying your stacks would get too hot in the kiln, more that you can over-dry the wood. I doubt that would happen with oak splits in one summer but beech and birch can get pretty dry. I think this is in line what he is referring to:

Can Firewood Be Too Dry?
Yes, although it is not a common problem
Properly seasoned firewood still has a fair amount of water in it, say 15 to 20 percent of its weight. That water regulates the combustion process along with a few other factors like piece size, load configuration and combustion air supply.

The higher the fuel moisture, the slower the wood breaks down when heated because of all the heat energy soaked up in boiling the water out of the wood and raising the temperature of the steam.

Conversely, the dryer the wood, the more quickly it breaks down when heated. By breaking down, I mean the vaporization of the volatile components of the wood; that is to say, it smokes. The dryer the wood, the more dense is the smoke at a given heat input rate.

Since wood smoke is fuel, we want to burn it as completely as possible and that means mixing with adequate oxygen in the combustion air. The problem is that a firebox load of very dry wood produces far more smoke than the air supplies of stoves are designed to provide. Besides, even if you could supply enough air, you would produce an inferno that would howl in the stove and make everyone in the house nervous. Fires that intense can seriously damage the stove's innards. Wood that is very dry produces a fire that is hard to control without making a lot of smoke.

Kiln-dried wood is down around 10 percent moisture. Depending on climate and conditions of storage, normal firewood won't dry down to kiln-dried moisture because of normal outdoor humidity. For example, I've never measured wood below about 14 percent in my firewood supply. But I suppose that firewood could get very dry by natural seasoning in desert conditions. Or firewood stored in old barns, which are like kilns in hot summer weather.

The right band of firewood moisture is between 15 and 20%. When you get much over 20% you start to see symptoms of sluggish ignition and the inability to turn down the air without extinguishing the flames. Towards 30% the wood sizzles and fires are very sluggish and it is hard to get a clean burn until the wood is almost to the charcoal stage. Above 30% water bubbles from the end grain when the wood is heated and it is very hard to burn at all. Species like poplar/aspen, which have very high native moisture content are virtually non-combustible when not adequately seasoned.

The main difference between EPA low-emission certified stoves and conventional stoves is that you can turn down EPA stoves for a long burn without extinguishing the flames. That is, they are better at producing a clean, controlled fire. The EPA test method requires wood with a moisture content between 16 and 20 per cent (19 - 25% dry basis) and when the wood is outside this moisture band, the stove's emission rate goes up. So even the best wood stove's performance will suffer if the wood is not in the right moisture range.

If you have some very dry firewood, like kiln-dried cut offs or old wood stored in a hot place, mix it with regular firewood to raise the moisture content of a full load.

Yo.. good read .. thanks for posting this.. I'm sure this will definitely help someone out..

 

[James02]

I'm going to ask the question, what if somebody used the black landscaping fabric as the kiln material.. I'm thinking of doing it this weekend.. Sorry if I missed it..
Tks..

 

[barnaclebob]

I'm going to ask the question, what if somebody used the black landscaping fabric as the kiln material.. I'm thinking of doing it this weekend.. Sorry if I missed it..
Tks..

I think the main heating benefit of clear plastic comes from the sun heating up the wood itself while the clear plastic prevents infrared heat and the warmer air from convecting the heat away too quickly. With black plastic you'd get a lot of heat generated on the plastic itself but then that would need to be radiated to the wood and conducted/convected to the air inside. Black landscape fabric might be even worse because it would let a lot of air pass through it. You'd essentially be providing shade for your wood.

 

[KJamesJR]

One other option is to put the black plastic behind the stack so it absorbs any residual sunlight and heat the stack from behind. Otherwise the black fabric would just heat up and shade the wood.

 

[James02]

Thanks @barnaclebob & @KJamesJR .. that's why I asked, wanted another set of minds thinking it through..

 

[Woodsplitter67]

I'm going to ask the question, what if somebody used the black landscaping fabric as the kiln material.. I'm thinking of doing it this weekend.. Sorry if I missed it..
Tks..

The kiln is like a greenhouse. Putting black plastic on it will actually shade the kiln and make it run cooler. There is no need to add any fabric to the inside at all. Its just a waste of time. If the kiln is in a sunny location it will not have any issues on getting warm inside. If you have ever noticed a greenhouse it is always clear plastic to allow all of the sunlight to filter in. The plastic does not heat the kiln nor does it add heat. The heating comes from the mass inside the plastic. The object of the plastic is to keep the heat in. The best advice is to try the kiln as originally posted. It works great as is with out modifications. I will be setting up mine as well as my neighbors in july and will be done the end of august. A 60 day run will be plenty and will have sub 20% mc in my splits

 

[Mike.O]

Made my first solar kiln this April. Neatly stacked about 5 cord of wood and utilized the stacks as the sides. 11'x20' with lots space between each row.

Wood was split for 6 months before covering, but was uncovered and was VERY wet. Once wrapped I had tons of condensation. I had to basically cut the gables clean open to vent it. Today or tomorrow I'll cover up the gables and only leave a little vent hole. It gets very hot with the gables opened up. I can only imagine how hot it will get once I close it up. I'm in CT, and it has been crappy weather this year, cool and wet. Hoping a few hot months will have me set up nicely for winter.

Oak went in around 40% and a test yesterday has it down to sub 30 already. Test piece was a piece easily accessible on the top front though. I'm curious how the pieces on the middle bottom are in comparison.

Just figured I'd share and recommend using the stacks themselves as support for a roof.

 

[peakbagger]

If you looks at industrial kilns, they are controlled by humidity and temperature. Note that we are not talking about lumber kilns which have to worry about drying the wood too fast and checking and cracking it. A firewood kiln just wants to dry it as fast as possible and checks and cracks are good as they increase the surface area of the wood. Long before computers, kilns had mechanical process controls with chart recorders. There is tradeoff between temperature of the interior humidity and air flow which a control system would try to optimize. Air at given temperature can only hold and carry a given amount of water. Relative humidity is a measure of how much moisture is in the air compared to how much moisture it can hold when fully saturated. Water vapor in the wood flows from a point of high humidity to low humidity so the goal is to keep the air below the moisture content of the wood. Note I said water vapor not water. Unless the wood came out of pond, liquid water will drain on the ground so the only way out is as a vapor. The warmer the wood, the more vapor is formed in the wood that eventually wants to flow to an area of lower moisture. The suns radiant heat in theory goes through the plastic and physically heats the wood and the air inside the container. Vapor is created in the wood and it needs to flow out to drier air. The typical kiln pulls outdoor air in at the bottom through leakage or deliberate design and that has a lower relative humidity and that carries the moisture out of the wood. The air rises and then needs to have someplace to go. If you see condensation on the insides of the plastic that implies that the air is saturated and is not doing any more drying and all its doing is hopefully draining somewhere outside the kiln as it takes heat to turn it back into vapor again. Ideally the vents should be controlled to some relative humidity setpoint so that water drops do not form, that's going to vary with outdoor air temps and sun intensity. Note some designs are counterflow where air is brought in from the top above the wood pile and sucked out the bottom with fans, they can dry wood quicker as the heat is pulled down through the pile whereas a top vented kiln tends to vent the heat from the sun right out the vent so the bottom of the stack doesn't see the heat. Top vent increases natural circulation and doesn't need a fan so expect most folks stick with top vent.

Ideally the way to go is preheat the incoming bottom vent air. If you take cold drier outdoor air and heat it with a flat plate absorber in the sun, the relative humidity drops as the air heat ups. Introduce that in the bottom of the stack of wood and it will dry the wood even quicker especially at the base of the stack. The other source of warm dry air is the vent off the kiln. The warm damp air leaving the top of the kiln has heat in it that could be used to preheat the incoming air. as long as the exchanger is vapor tight. Heating dry outdoor air lowers its relative humidity and makes it more effective at absorbing more moisture from the firewood. A heat exchanger could be made to take the hot damp air and preheat the incoming vent air. The problem with this heat exchanger is it needs quite a bit of heat exchanger surface to work efficiently and it has to be designed to deal with the water that will condense on the cold surfaces. Dehumidification type dryers use the approach that they circulate the same heated air around continuously and just remove dry the air out before they reintroduce it in the bottom of the pile. Its a lot more efficient than a steam heated dryer but both are outside the realm of a home brew unit.

As the day time temps drop as the sun sets water vapor will begin to form on the plastic. The goal should be to open the vents full and flush out any damp air before the cycle starts again up in the AM.