Solar dry kiln construction

[Banjo picker]

Will do WoodDoc.  Banjo

[firefighter ontheside]

Anybody put HDPE black plastic panels as the inside walls of the kiln?Seems like it would be great for moisture and it's already black so no need for painting.

[GeneWengert-WoodDoc]

HDPE has a melting point around 220 F.  When used as part of a solar collector, it can get this hot...I measured 200 F air temperature in the peak of a kiln without the fans running, so the surface would be even hotter.  Also, HDPE is subject to rapid deterioration from UV light, unless an additive to absorb UV is included.

The black should be "flat black" rather than glossy.

[firefighter ontheside]

Noted.  Thank you.

[Brad_bb]

Interesting thread.  I just discovered it, I don't think I saw it originally.  rjwoelk and I have been talking about kilns for awhile and I've come to the conclusion that a solar kiln may be the only one that makes financial sense for me.  

One of my concerns is a windstorm.  I'm wondering how apt it is to blow over especially if it's empty?  If it blew over and was full that would be even worse.  I was thinking of building 2 of them.  I wonder how we'll they'd do with 2.5" or 3" walnut slabs?

[doc henderson]

I plan to attend the seminar in Blacksburg, Va in may.  I had thought of making one 10 x 20, but was told not to go too wide.  so maybe 8 x 20.  I want to put firewood in it when not drying hardwood.  I think if I use big skids to mount floor too, and with most stack only going up 4 feet or so, that the center of gravity will be fairly low.  although a 12 foot tall wall makes a good sail.  Our wind is from the south in summer and north in winter.  The slant will of course face south, so I would be more worried in winter if empty.  It usually is only  a sudden moment of wind gust that tips something so some anchors in the ground and cable should be an added safeguard.  I am from Kansas!  go Toto!  

[firefighter ontheside]

Mine will be 6x12 and built on skids so that I could drag it around if need be.  The space that I have to put a kiln is nowhere near electric, so solar is my only option.  

[doc henderson]

FF that sounds perfect for most hardwoods.  most do not want over 8 let alone 12 foot boards.  too heavy.  I may alter mine as well but already got a great deal (clearance) on corrugated polycarb sheets wide enough total for a 20 foot length.

[firefighter ontheside]

Currently,my mill will only cut 10' 6".  I'm tempted to buy an extension, but I shudder to think about handling longer logs of any size.  I agree on hardwood length.  If I'm bucking logs, I cut them to 8' 6" if I can.  I will only be putting air dried lumber in the kiln, so I don't think I'll have to deal with a whole lot of moisture.

[btulloh]

At night, the rh will go up pretty close to 100% even when you start with lumber that's air dried to 20%ish.  

[btulloh]

FWIW, mine has taken 50mph wind from the north without turning over or moving.  It was empty at the time.

All in all, anchoring makes for a better nights sleep though.  I just haven't gotten around to installing anchors because it's sitting in "temporary" location.

There are a few members that have had their solar kiln blow over.

[caveman]

One of my concerns is a windstorm.  I'm wondering how apt it is to blow over especially if it's empty?  If it blew over and was full that would be even worse.  I was thinking of building 2 of them.  I wonder how we'll they'd do with 2.5" or 3" walnut slabs?

Brad, I was worried about our solar kiln blowing over last year when Hurricane Irma came through a year and a half ago.  John and I ran some ratchet straps over the frame and attached them to mobile home anchors.  During the height of the storm, I waded out to check on it and to my surprise it was still standing.  The most exciting part of that short walk/wade was that I waded through a floating wad of fire ants in the middle of the night.  



 

[YellowHammer]

They can be quite tippy when they are empty, but mine sits real solid when full.  They excel with thick wood, mine is loaded with about 800 bdft of 9/4 walnut slabs right now.  Its my kiln of choice for these types of loads.

[Brad_bb]

I went through some numbers with Bob on what it would take to pay for an idry.  If we started with a 4 year plan to pay for the idry(not including interest), it would be about $1050 per month.  I figure you'd only be able to do 2.5 load per month average if you're doing 5/4 and slabs that are air dried.  You'd have to keep it loaded continuously.  so 1050/2.5 = $420 per load.  I did some calculations and figured loading 5/4 you'd only get 1000-1200 bf in there. I wonder if the 2000BF advertised size is with 3" slabs? I'm using the advertised kiln dimensions and figuring with stickers as I'm assuming you still need them.  With 2.5 inch slabs you'd get about 1350.  So let's just use 1200 as the average.  420/1200=.35.  So Whatever your labor plus energy costs are you need to add .35 to pay for the kiln capital cost. Let's just assume that loading or unloading is $150 each time (stacking and stickering and material moving might take 2 guys an hour and a half?)= 300 per load, which translates to 300/1200=.25/BF.  So now we're up to .60/BF not including running costs.  Let's assume it's .10/BF.  So that's .70/BF.  In my opinion, a 4 year capitol payoff is not great, but .70/BF is not great either when we have a local DH kiln who will dry for .30-.40/BF.  Plus what if you couldn't keep it filled continuously? That is the basis why I thought it would be too expensive for me.  The only way I can see of matching the local kiln in price is the solar.  Yes it will take longer than both, but maybe that will be ok for me, especially if I build 2 or 3 solar kilns?  I wish I could have and idry, but financially it just doesn't seem to make sense for me, especially since I don't plan on making it a business, at least not a full time thing.  I have lumber.  I've got a decent rate on spray foam....Just my thoughts and some rough calcs.

[WDH]

If I was a younger man, I would jump on the idry like a duck on a June bug.  In my area, kiln drying is in significant demand.

[YellowHammer]

Both DH and Vac kiln drink electricity.  They must be full to be making money, so the power bill is significant.  I have been thinking and mulling over for some time that the best approach for me, especially since I need more space for air drying thick live edge, and since they cost almost nothing to run, is to build a crop of solar kilns.  However, I don't want to build that many, but have been talking to local portable garden shed builders, because after all, a solar kiln is nothing more than an insulated, skid mounted, portable garden shed with wider doors, a single sloped roof covered in clear corrugated plastic instead of metal.  So my strategy is to get the guys to build a few with these slight changes, drive up with one on their little flat bed truck, and push it off in the yard.  Then repeat a few times. 



 

[Tom the Sawyer]

When considering capital equipment purchases, don't forget about depreciation.

[doc henderson]

I can see it now, "yella Hamma solar kilns"!Not a bad idea!  Brad do you mind sharing the rate you will pay for closed cell spray in insulation.  @Brad_bb   @YellowHammer

[YellowHammer]

I'd try to make sure I sell a lot of them in Hinton, WV.  I'd paint them all bright red with big white letters that instead of saying "See Rock City" would say "Roll Tide" on them.
  
That way every time @WV Sawmiller looked out the window, he'd think of Bama.

[WV Sawmiller]

   All those red kilns with white letters saying Roll Tide would be very healthy and you could advertise them as guaranteed weight loss devices too because every time I looked at one I'd probably lose my appetite. I think Ruby Falls and Roll Tide go together better anyway. :D :D

[Brad_bb]

So to boil back to the beginning, the point of this thread was pineywoods design using sheetmetal just below the windows to act as the collector, and the assertion that higher temps can be reached by doing this versus the Virginia tech design?  That this is an improvement?  Has anyone talked to anyone connected with Virginia Tech, like the professor who's name is on the design plans, Brian Bond, Assistant professor and Extension specialist?  I ask just to see if they had considered that design?  Is there a reason they wouldn't want the higher temps? Virgina tech's fan placement and direction for airflow also seem to differ from pineywoods design.    I'm not judging either design, just trying to boil down the differences and wondering if there has been any discussion between the parties to see if info was shared?  I know pineywoods has done his own experiments....

[Planman1954]

This is Planman1954. I am the one behind this thread on solar dry kiln construction. The design I built was Pineywoods. I have NEVER stated that temperatures are higher than the VT design. I have only stated that are in excess of 180 degrees f.

[doc henderson]

So doc Gene Wengert was there years ago, and Brian Bond took his place.  Brian Bonds is teaching the class at VT May 16th and 17th.  If you are referring to backing the collector with black metal and creating a plenum, this serves in passive solar to create air movement. and if you wanted a combination kiln, the back side can be insulated, and at night the fans go off, and you can have air gates so you can hold some heat or add external heat with out big losses via the collector.  i believe the heat gain is dependent on the collector surface area and does not matter so much what it shines on as long as it is darker color.  I am learning along with you fellas and do not claim to know it all.  I have a bio,chem, physics background.  @GeneWengert-WoodDoc 

[GeneWengert-WoodDoc]

I developed the VT kiln in 1978.  When I left VT in 1992, Brian Bond followed my position with the kiln.  He revised the kiln document and did a great job.

The same amount of energy goes into a kiln with a black absorber near the clear cover versus an absorber that is the walls, pile cover, etc.  the exact same solar energy enters the kiln. And with black absorbers, essentially all the entering energy is absorbed by the black surface, wherever they are located. The difference is that the collector right under the cover has less area, so there is more solar Btu's per square inch, which means it gets hotter.  The "away" collector is not as hot but has more area.  Both collectors provide the same amount of heated air inside the kiln.

In fact, if you blow air on one side of the absorber near the clear cover it will get much hotter than if you blow air across both sides.  Even so, the same energy will go into the kiln with one side or two side.

However, with less area convection is less...that is, heat released to the air = area of black surface x temperature difference x convection coefficient.  But, still, the heat released is not any more than the energy coming in. So, it makes no difference if there is an absorber right at the clear cover, or away...the black absorber absorbs the same amount of energy in either location...the energy coming in.  Once the energy is heat, a key is keeping the energy in the kiln until it evaporates moisture...so plenty of insulation for floor and walls, and a clear cover with two or even three layers.

[YellowHammer]

I was typing this and Gene beat me to it, so I might as well repeat what he says.  Collector area drives the energy input into the solar kiln, no matter what the general configuration.  

In my experience, the overall performance of the solar kiln is based more upon its placement relative to the hours of solar exposure, the angle of the collector to the sun, and of course, how much solar there is, from season to season.  In my VA Tech clone, when my power went off, I melted the the plastic roof and blackened the insulation.  In addition the overall thermal efficiency is based upon the air leakage around the roof of other areas where the hot air cannot be driven through the stacks, in essence shortcutting it, and exiting without picking up humidity.  I know from my kiln, it is, in my opinion, a very mature, simple and efficient design.  I love improving things, but I just don't see a whole lot in the Va Tech design that isn't already optimized, and I use my continuously.