My Solar Kilns.

KJamesJR Posted By KJamesJR, Jun 12, 2019 at 7:32 PM

  1. KJamesJR

    KJamesJR
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    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.
     

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  2. spudman99

    spudman99
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    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.
     
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  3. Sully1515

    Sully1515
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    Jun 5, 2017
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    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!
     
  4. jackatc1

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  5. KJamesJR

    KJamesJR
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    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.
     
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  6. Kevin Weis

    Kevin Weis
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    How is the moisture getting out or am I missing something on the pictures?
     
  7. KJamesJR

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    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.
     
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  8. ben94122

    ben94122
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    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...
     
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  9. brenndatomu

    brenndatomu
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    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...
     
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  10. KJamesJR

    KJamesJR
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    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.
     
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  11. Kevin Weis

    Kevin Weis
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    Okay, I see the venting now. That should work well.
     
  12. Woodsplitter67

    Woodsplitter67
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    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
     
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  13. Woodsplitter67

    Woodsplitter67
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    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.
     
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  14. Dolphus Raymond

    Dolphus Raymond
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    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.
     
  15. KJamesJR

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

    EODMSgt
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    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.
     
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  17. Woodsplitter67

    Woodsplitter67
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    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..
     
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  18. Woodsplitter67

    Woodsplitter67
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    Yo.. good read .. thanks for posting this.. I'm sure this will definitely help someone out..
     
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