Solar dry kiln construction

[GeneWengert-WoodDoc]

Do not forget that a solar kiln is a piece of equipment, so it is a business deduction.

[caveman]

Thanks, Doc.  It rode out another hurricane/tropical storm last night and continued to dry wood today.  I need to clean the poly panels and try to get the heat up.  We have some cypress that needs to get dry, pronto.  

[GeneWengert-WoodDoc]

Glad you survived again.  Let's hope this was the last one this year.

Have you started building an ark yet?

[caveman]

This most recent storm was not too bad.  It did not even blow any of the metal off of our drying stacks.  We have started using mule tape across the tops to tie the metal down.  We have some standing water near the mill but I hauled in a little fill dirt since the summer and the water will soak in within a few days (it'll stand long enough for a bumper crop of mosquitoes to hatch).

Thanks to all who have contributed to this thread.

[Digger Don]

Thanks for the input and help Doc Wengert.  yes I respect the reduce, recycle and reuse concepts going on.  If we could build the perfect Kiln for no cost, our goal would be realized! :D  My favorite work saying is "we have done so much, with so little, for so long, I now think we can do almost anything with nothing!"  

First, I'd like to thank everyone who has contributed to this thread. I just stumbled onto it a few days ago. A friend and I are in the planning process of some sort of kiln, and I've gained a lot of knowledge here. Our biggest issue, at this point, is no electricity to run the dehumidifier.
But, what I wanted to contribute, is that Doc's saying did in fact originate in the military. At least, that is where I first heard it, back in the late '70s. However, he left out a line, or two. Here's how I heard it: "We the willing, led by the unknowing, are doing the impossible, for the ungrateful. We have done so much,  with so little, for so long, we are now qualified to do anything, with nothing." 

[caveman]

Lately, in our solar kiln, we have not been running the dehumidifier.  The past several loads have been partial loads, so we have only been running the fans for about three hours a day.  The wood seems to be drying flatter under these gentler conditions.

Last week when I took a load of sweetgum slabs to a local retail wood store the proprietor commented several times about how flat the sweetgum slabs were.  We have also started sanding a patch on most of the slabs to expose the color and grain before they go to be sold which is what I was doing this particular evening.


 

[Digger Don]

I have read that a good angle for the collector is equal to the latitude of the location. Some say to add ten degrees to that. Here, that would be about 38 degrees. I haven't been able to determine if that is measured from vertical, or horizontal. My first instinct is that it would be 38 degrees above horizontal, but that seems too flat. So, the more I think about it, I think it's 38 degrees below vertical. Which line of thought is correct?

[doc henderson]

It changes winter and summer.  so, 45° most places in the US is fine if you do winter and summer.  I think to max. in winter.  Remember what the good olé Doc @GeneWengert-WoodDoc once told me.  "It is just a solar kiln!"  I guess i had a lot of questions.

[rusticretreater]

The calc is from horizontal as stated in the second paragraph.  +10 degrees is better for northern latitudes with longer winters.  

The difficult explanation: Horizontal(0 degrees latitude) does not mean horizontal to the sun.  It means horizontal to the equator. As the earth is tilted on its axis, at different times of the year the equator will be a maximum of plus or minus 23 degrees with our center line with the sun.  For each latitude on earth, at only two points during our year is the equator exactly horizontal with the sun and that time is different for each latitude.

From the VA Tech guide to Solar Kilns:

Collector Orientation
A surface will receive the most energy if it points directly toward the sun (i.e., is perpendicular to the sun), following the sun as it rises until it sets. As such equipment to accurately (usually within 5 degrees) track the sun is expensive A simpler arrangement is to adjust the flat collecting surface so that it faces due South in the Northern hemisphere (or vice versa for the Southern) and is slanted at an angle so that at solar noon the surface is pointed directly toward the sun. Such an arrangement would have several adjustments ranging from + to – 23 degrees (winter to summer) around the base angle. The base angle is measured from the horizontal and is equal to the latitude of the collector's location. For example, a collector located at 18 degrees North would be tilted southward 41 degrees on December 21, 18 degrees southward on March 21 and September 21, and 5 degrees northward on June 21. Intermediate days would have corresponding intermediate angles.

Collectors that are not movable or adjustable can be set, for best year-round performance, at an angle (from the horizontal) equal to the collector's latitude. An angle of (latitude +10) degrees (that is, steeper) is suggested for best winter performance; an angle of (latitude –10) degrees (that is, flatter) is suggested for best summer performance.
In all collectors, the area of the top of the collector which is perpendicular to the sun (i.e., the area of the shadow made) is the critical area. It is not the area of the absorber underneath the top of the collector.

[Digger Don]

Thanks for the clarifications.

I'm thinking we will either maximize for winter (38 degrees plus 10), or just go with the simpler 45 degrees. If my math is correct, 45 degrees would pretty near maximize the winter performance. Gaining heat around these parts in summer should not even be a consideration! Most any building, with no collector at all, should be plenty hot!

[Okrafarmer]

We've had ours up for a year or so now. Just loaded it again today. We are still having trouble getting enough heat differential out of it.  It's been around 120° in there lately, but a temp diff of probably not more than about 50° from ambient temps. it is drying wood, though. We've been using it a lot for firewood, in steel baskets. It dries down split firewood in about 2-3 weeks, no problem. We also have done some lumber, with decent results.

[aromeza]

Hello you all.
First of all,I wanna thank you for this tremendous Forum,
I have build an hybrid between the VT and Pineywoods and it works like a champ.
at this location RH average 70% and temp about 27°C.
the lat is 20, so we set an angle of 30 or so.
For the ceiling we have use glass in one of the kilns and polycarbonate for the other.



 

We add one LG inverter minisplit a/C wich has the super dehumidifier function and runs above 23°C



 

 
on the bottom vent we add a 10" fan, which runs once the temp is about 7°C higher than the ambient, and stops venting 5°C higher.


 
the vent on top, opens when the H its 75% or so, depending of the month, and closes when its about 60% (it depends on the month)
by doing this, we allow the DH to work properly, and not to saturate the air, so it keep drying D&Nite.
the EMC Average during the year is 14% and we mostly work Parota. (guanacastle, guanacaste monkey ear tree)
I have had add some pictures, of the system.
im not an expert but it works faster, than a VT Solar Kiln, and a little bit slower than an nyle DH kiln, (we run a 250 model and a 450 aswell) but waaaay cheaper; upfront, operational, and maintenience.

Once again, thank you. and here are my 2 cents.

[beenthere]

aromeza
Welcome to the Forestry Forum 
Looks like you are getting the job done. 
Link your pics to your post (you can do that by modifying your post and then click the "Click here to add Photos to post" button). 

Take a moment to read the forum rules at the bottom of every page. That should help out. Some explanation of what is shown in each pic would be great.

Shout out if you have a question. 

[Okrafarmer]

Welcome, Aromeza! It looks like you have a nice setup there!

[wkf94025]

Welcome @Aromeza.  Your setup is a lot like mine.  I don't have a Nyle or pure solar to benchmark against, so glad to hear you do, and can attest to the cost and efficiency of the hybrid kiln.

[Planman1954]

I took the time to document the original building of the solar dry kiln, along with a cross section that I've put on a blog post. I will not post a link, due to rules here though. I DID post a link to it on the Planman1954 personal website a few days ago...just search for it. Just look for the solar powered dry kiln post.

[teakwood]

Interesting thread. Do I need to insulate the kiln being in the tropics, it's never under 20 celcius (68 Fahrenheit)here

[Ianab]

Interesting thread. Do I need to insulate the kiln being in the tropics, it's never under 20 celcius (68 Fahrenheit)here

Don't really know. I've seen some Australian solar kiln designs that weren't big on the insulation. But they also have a drier climate in most areas...  Main thing in the tropics would still be to get the kiln temp up above the ambient, so the RH in there drops below 100%, and the wood starts to dry. Insulation probably isn't AS important because things are naturally warm with temps up in the better drying range already. But then it's also the increased temp that lowers the RH and speeds the drying. ??? :P 
Thinking out loud, drying down to US MC levels is probably going to be a problem without a DH style kiln? I haven't done the maths, but if your ambient is 25C @95% RH, how much do you need to heat that air to achieve an 8% EMC? And if you don't control the moisture when it comes out of the kiln, it's soon going to bounce back up to 15+%

[teakwood]

Average temp is 27, at least.

I don't need 8% EMC, our wood stays within 12-16% between dry and rain season. so if i get under 18% i'm already good for outside decks and maybe 14-15% for furniture wood. teak is extremely easy to dry, with some wind and a little dry air it air dries under 25% in no time.

I have some insulation laying around, can use it on the walls, the roof isn't insulated anyways with the plastic sheets on it. does the flor needs insulation?

I will go with the enclosed design where you put a dehumidifier inside, so that thing will suck all the humidity out of the wet air and wood.

[doc henderson]

you are trying to decrease the surrounding (inside the Kiln) RH so you need the interior to be hotter than outside.  As mentioned, this will drop the RH.  the insulation will allow for a bigger temp difference by holding the heat inside the kiln longer achieving a higher temp and lower RH.  In theory.  the goal is to speed up the process, so a higher temp gradient and or lower RH is what you want.  Adding the DH is not a bad idea since venting to the outside and entraining air from the outside at 95% humidity will slow the process.  Or do the venting during the day when RH is lower, however the water content is prob. the same and lower RH is lower just due to increased daytime temp. outside.

[YellowHammer]

Dehumidifiers do not remove remove moisture from the wood, they remove the moisture from the air.

High local temperatures and high airflow velocities or 600 fpm our above across the lumber stack remove the moisture from the wood, until the air gets saturated.

It's a two step process.  Moisture from wood goes into the air, then the moisture is removed form the air.  Or the air is removed form the control volume and replaced with dry outside air.

The efficient way to remove moisture from air is not through a dehumidifier, it's by removing the air itself through venting.  That's why DH kilns like Nyle that are rated to dry 4,000 bdft of hardwood (slow moisture removal rate) can only dry 1,000 bdft of pine (high moisture removal rate), the dehumidifier gets overwhelmed and is the limiting factor.  In those cases it's more efficient to just turn the DH off and simply open the vents and let the air out.  Instant dry air.     

For highly moisture laden air, the best alternative is to vent the air outside.  That's why a solar kiln is very effective but also very slow unless there is enough solar energy to reheat the incoming air quickly.  For highly moisture emitting wood, with lots of air transfer, the solar collector needs to be resized to be much greater than the conventional Va Tech 10:1 ratio.  Big building, small load size = inefficent.  So increasing the solar collector size allows and increase in airflow and an increase in venting.

At the moisture levels you are talking, you need high heat input, high airflow velocities and increased venting, non of which happen in a solar kiln or even a DH kiln.  Since you are not going down very far in moisture, a lumber pre dryer would be more appropriate than a conventional kiln.  

Solar kilns only dry during the day.  So 50% of the time, they are non functional. 

In order to optimize the drying rate and cycle, the maximum allowable moisture removal rate of the wood must be reached but not exceeded.  Solar kilns are optimized for slow drying wood, so are DH kilns. 

[teakwood]

OK...back to work on the kiln, after a long hot summer. Sorry for the long delay, but had some other things to attend to. The Kiln is now completed, and ready for a test run.

After attaching the top plastic panels and completing the wiring for the fans and dehumidifier plug, I ran some blocking between the front beam and rafters and made a framed box for the two fans to rest:





After the fans were set in place, I began installing the metal panels under the rafters. Remember, the metal pieces are centered between the top and bottom of the rafters, creating about an 8" space at the top and bottom for air to enter and exit. You can see the first few pieces in place behind the fan above. I used dry wall screws to hold them in place. Of course, the black painted side is facing up. Here's a photo showing the metal installed around the fans:





After reaching this point, my buddy Jimmy came by and took a look and suggested that I seal off the area between the fans to create a plenum, or a mini attic inside to direct the air flow. This shows a picture of the boards filled in between the fans with insulation above:





And then a photo with my camera held really high to show the insulation laid on top of the boards:





After these photos were taken, I filled in the end areas.

I have some questions now that you have lots of experience  @Planman1954 .
the space between plastic roof and black metal is just 4"? would that be more efficient with 6 or 8"?
How do the normal store fans hold up? can they take the heat? the space between wood pile and north and south wall is 1' , 30cm?
thanks 

[teakwood]

anybody can help me out real quick with the questions above? i'm drawing some plans for the kiln and need these measurements. thanks @doc henderson 

[doc henderson]

teak there are plans for the one developed Virginia tech.  usually, the fans go above the wood and have a tarp to create a dam, so the air goes through the pile.  final dimensions can be modified to fit your space needs ect.  

Design and Operation of a Solar-Heated Dry Kiln | VCE Publications | Virginia Tech

@GeneWengert-WoodDoc

fairly detailed plans are at the end of the document.  You could just go all dehumidifier kiln as well.  I think in your unique environment/weather the combination makes sense.  I would set it up with a bucket to tally the daily water output.  to save on electricity you can then limit the DH use when the water drops off.  like run it at night when the kiln RH is higher.

[teakwood]

Thanks doc, I will go with the design used in this topic, the one pinewood and planman built. I think the black roofing sheets underneath the plastic panels will heat up to a extreme and should work pretty good. What I can't see anywhere is how much needs the distance between plastic sheet and black metal sheets be? You have the rafter between so what's best, 4,5 or 6" ?

Where do I get a good DH? I only found modern household dehumidifier.