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Author Topic: Underground (almost) solar kiln  (Read 595 times)

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Offline 50 Acre Jim

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Underground (almost) solar kiln
« on: July 17, 2017, 07:40:36 AM »
Iím thinking of taking a slightly different approach to my solar kiln build and felt I would open myself up to some constructive criticism. (This is probably gonna hurt.  :D)    I plan on keeping the conventional shape (straight back wall, sloped front, etc) and build it 18í wide and 10í deep and about 12í high.  Iíll use Locust for the frame and corrugated steel (painted black) for the outside construction.   The sloped front will be removable to allow easy stacking of wood. 

Now imagine cutting a 20íX12í rectangle out of the sloped mountain behind the saw mill and shoving the solar kiln into that cutout.  Nothing except the sloped metal front will be visible and to someone who didnít know what it was it would look like some corrugated metal roofing laying against the mountain.  But when you open it up you find an 18íX10í room built into the side of the mountain.

The kiln is insulated on 3 sides by the earth around it with only the front exposed, and that exposed part is where the sun beats down and heats the steel box.  Iíll build supports inside that allow for the first layer of wood to be about 5í off the ground so that plenty of hot air can accumulate under the stack and move the moisture out the top vent via natural convection.  Adjustable vents at the bottom front of the kiln will allow air in and provide flow control.   Glorified outdoor drying if you will.

The kiln will get direct sun for about 7 hours a day during the summer and I would expect temps to be quite high during those times.  Probably wonít be much good during the winter other than a place to store wood. 

Suggestions?
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Offline Kbeitz

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Re: Underground (almost) solar kiln
« Reply #1 on: July 17, 2017, 08:16:00 AM »
Maybe you will come up with something new...
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Offline samandothers

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Re: Underground (almost) solar kiln
« Reply #2 on: July 17, 2017, 11:23:09 AM »
I wonder if you will lose heat trying to warm the earth.  A lot of mass there at a lower temp.

Offline 50 Acre Jim

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Re: Underground (almost) solar kiln
« Reply #3 on: July 17, 2017, 04:29:22 PM »
I wonder if you will lose heat trying to warm the earth.  A lot of mass there at a lower temp.
I will still insulate the walls but I would think that the earth around the kiln would help hold the temp better.  Possibly not but I guess we'll have to give it a try and see. 
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Offline Gearbox

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Re: Underground (almost) solar kiln
« Reply #4 on: July 17, 2017, 04:47:15 PM »
Earth is an R 6 FYI .
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Offline GeneWengert-WoodDoc

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Re: Underground (almost) solar kiln
« Reply #5 on: July 18, 2017, 01:10:49 AM »
Wet or damp soil is both cool and a poor insulator.  You would need at least 8" of insulation for three side and the floor.  The earth will be cooler than the outside air during the active solar drying time.  The earth will be warmer in the winter, but there is so little solar that active drying is minimal.

Do not forget that solar kilns need vents.  We want to vent the air after it has gone through the lumber pile, and bring in outside air before the air is heated in the collector.

The black collector needs to have a clear plastic or glass cover.  There was a good article about solar kiln basics a few months ago in Sawmill & Woodlot magazine.  Included was info on the collector.  Two clear layers are also essential.

The popular VT kiln uses the collector and the roof as one.  But, there is no need in your case to combine them.  So why not make an insulated box for the kiln and then a separate collector on the hillside?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more


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