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Author Topic: Which to build? Solar or DH Kiln  (Read 4627 times)

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Offline giant splinter

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Re: Which to build? Solar or DH Kiln
« Reply #20 on: August 22, 2014, 12:18:46 PM »
 http://www.americanwoodworker.com/blogs/projects/archive/2009/09/29/solar-kiln.aspx
Glenn that link is very helpful and for the most part just about covers the solar kiln, the other links and photos are also very helpful so I suspect this will be a good winter project for many of us and thanks goes out to all the others for their input also is very important.
Great Posts and an informative effort on everyones part ..... Thank You Glenn for starting this one off in the right direction. ;D
roll with it

Offline Bill Gaiche

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Re: Which to build? Solar or DH Kiln
« Reply #21 on: August 22, 2014, 01:22:18 PM »

Offline GeneWengert-WoodDoc

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Re: Which to build? Solar or DH Kiln
« Reply #22 on: August 22, 2014, 03:43:12 PM »
Casehardening develops when the lumber is over 50% MC...early in the drying process.  Actually the outside is not harder than the inside, so the name is not very descriptive.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Glenn1

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Re: Which to build? Solar or DH Kiln
« Reply #23 on: August 22, 2014, 03:56:58 PM »
You have all given me great information and I think that I can now build a viable solar kiln.  One more dilemma.  I live on 7 acres that is virtually all trees.  There is one location near my shop that can get direct sun from approximately 11am to 2 pm in the summer.  In the winter, it should not be a problem.  Is that enough sun to make this cook?  There are tall trees to the east and also the west and my wife loves her trees just as they are.    I tried to download photos but they are in jpeg format so I guess they are not accepted that way.  Any suggestions?
Vacutherm IDry, Nyle 53 Kiln, New Holland Skid Steer, Kaufman Gooseneck Trailer, Whitney 32A Planer

Offline GeneWengert-WoodDoc

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Re: Which to build? Solar or DH Kiln
« Reply #24 on: August 22, 2014, 04:12:26 PM »
The black absorber is more effective if it is far from the collector' sclera cover.  So, in the VT design, the interior walls and the fan baffle are black as well as the top of the lumber pile.  Plywood is fine, but plastic is ok too.  Avoid metal if possible as its heat properties are not as good as wood.

A collector cover should transmit as much solar energy as possible.  So, it should be clear.  On the other hand, if it is clear but breaks, tears or degrades quickly, it is not so good.  Hence many people do not use glass, but use corrugated fiberglass.  It transmits about 85% of the solar.  The loss of 10% over glass means that drying will take 10% longer! but what is three or four more days in a solar kiln?

Once the solar is in the kiln, it must be converted to heat.  That is what the black collector is for. 

Once you have heat, the collector will lose energy by conduction (hotter on the inside and cooler on the outside) and by infrared radiation.  To reduce the heat on the immediate inside of the cover, move the collector further away.  Second, use two layers which forms an insulating effect.  With three layers the insulating is even greater, but the solar transmission is further reduced.

The ideal cover for controlling the transmission of infrared through the cover to the outside will reflect the infrared heat.  (To reduce emission of infrared heat in glass windows, the glass can be coated to make "low e" glass.). Some materials like glass reflect a lot already.  Some films also do this, although some so-called solar films do not do it well, but these products are stabilized to avoid uv degradation and that is why they are used for solar.  Interestingly, glass that has a green hue on the edge does not reflect as well as non- green on the edge glass.  Again, with low e glass being expensive and the risk of glass breakage and the weight of glass, fiberglass and plastic films that are uv stabilized are used.

Overall, It is a compromise between the best energy product and the cost and the durability and ease of installation.  For some people, the best might be some old thermo pane windows.  Other might even use some old sliding patio doors which gives two layers and an insulated space...thermo pane.  For others, fiberglass corrugated on the outside and a solar film on the inside to make two layers.

Note that when the sun is not shining, the collector becomes 100% a loser of heat.   With the huge heat losses in the cool or cold winter when even on a sunny day the solar input is small (sun angle low and sunny hours few), a totally insulated collector is best, which points to 100% DH or 100% hot water kiln.

Note that the amount of solar energy into a kiln of any design is controlled or limited by the size of the collector at noon perpendicular to the sun.  Another way to say this, is that it is the shadow area that the collector makes at noon perpendicular to the sun.  For this reason, most good solar kilns use the 1 sq ft to 10 BF ratio.  That gives a good amount of energy for drying reasonably fast, but not too fast.

Questions?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline GeneWengert-WoodDoc

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Re: Which to build? Solar or DH Kiln
« Reply #25 on: August 22, 2014, 04:14:49 PM »
Let's say your wooded location gives you 2/3 of the open location.  So, instead of 45 days to dry green oak, it will take you 67...that is, 50% longer.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Glenn1

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Re: Which to build? Solar or DH Kiln
« Reply #26 on: August 25, 2014, 07:16:51 PM »
Here is a hypothetical sizing question.  If I build a kiln that is 8 feet high in the back by 8 feet deep and 12 feet in length, I come up with approximately 132 sq feet for the collector.  Based on the 10 bf of wood to 1 sq ft of collector, I should be able to dry upwards to 1300 bf at a time.(I think)   Now what would happen to the wood if I only put 300 bf into the kiln either as green wood or air dried wood?  Would the heat from the kiln  overpower the 300bf of wood and cause all kinds of problems or would it work fine for any amount smaller than the maximum?
Vacutherm IDry, Nyle 53 Kiln, New Holland Skid Steer, Kaufman Gooseneck Trailer, Whitney 32A Planer

Offline WDH

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Re: Which to build? Solar or DH Kiln
« Reply #27 on: August 25, 2014, 08:48:25 PM »
For many species drying from green, the answer is Yes.

With very fast drying species like most softwoods and some soft hardwoods, you might get away with it.  With green oak or other slower drying species, that would wreck it. 

Air dried wood would tolerate it better. 
Woodmizer LT40HDD35, John Deere 2155, Kubota M5640SU, Nyle L53 Dehumidification Kiln, and a passion for all things with leafs, twigs, and bark.  hamsleyhardwood.com

Offline GeneWengert-WoodDoc

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Re: Which to build? Solar or DH Kiln
« Reply #28 on: August 26, 2014, 12:44:26 AM »
With thicker wood, slower drying is required.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Ianab

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Re: Which to build? Solar or DH Kiln
« Reply #29 on: August 26, 2014, 01:52:52 AM »
You can always rig up a removable cover for part of the collector. Couple of sheets of tin and some studs and wing nuts to hold it in place. Cover 2/3 of the collector area and your ratio is sensible again. Of course if you are drying softwoods, then just let it cook.

Ian
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Offline Glenn1

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Re: Which to build? Solar or DH Kiln
« Reply #30 on: August 26, 2014, 07:43:30 AM »
You can always rig up a removable cover for part of the collector. Couple of sheets of tin and some studs and wing nuts to hold it in place. Cover 2/3 of the collector area and your ratio is sensible again. Of course if you are drying softwoods, then just let it cook.

Ian


Hello Ian,
So you are saying  to put the tin cover on the outside of the clear collection plate to block the sun? Possibly, I can find the same corrugated sizing so the tin cover fits right on top of the clear top.  Since the cubic footage of the larger kiln will be considerably larger than the stack of lumber, I am guessing that it will be a trial and error scenario adding coverage until I get the correct temperature.    Would that be a correct assessment?
Vacutherm IDry, Nyle 53 Kiln, New Holland Skid Steer, Kaufman Gooseneck Trailer, Whitney 32A Planer

Offline Ianab

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Re: Which to build? Solar or DH Kiln
« Reply #31 on: August 26, 2014, 04:12:03 PM »
Yeah, it's not an exact thing. But the collector sizing doesn't need to be exact, you have quite a margin between too fast and too slow. Covering part of the collector is just a way of "throttling" the solar power.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)


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