# Solar kiln design



## jessesnowden (Mar 15, 2015)

So if any of you saw my post about milling a couple oak trees 2 weeks ago, this is the next step. Bought an old frame of a camper for 150 dollars and am working on a portable solar kiln. I live in an apartment complex so no land to build something stationary.

The usual design of a 45° angled roof is what I started building straight away. But I have a unique situation. I was lucky enough to get 60 sheets of 3/4 CDX plywood from my jobsite that was heading for the dumpster. So with all that free material I figured it would be fun to experiment!

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Got to this point and decided to try something different!

http://i1175.photobucket.com/albums...obile Uploads/20160405_133446_zps6yupuvea.jpg

Instead of a rigid clear roof, gonna try using clear string reinforced poly on PVC hoops like a green house. If it works, it will be much easier to tear down. If not, I'll build it the virginia tech way. I'll update once its up and running, hopefully by weeks end.


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## jessesnowden (Mar 15, 2015)

Inside dimensions are 4'6"W × 10'6"L x 6'6"H

Gonna see how this setup holds up. So far I'm $400 dollars in. 150 for the trailer, 150 for a 100' roll of plastic, and assuming roughly 100 dollars for all the PVC, screws and little odds and ends. Some of which was already in the my garage. Cross your fingers people! Haha




http://i1175.photobucket.com/albums...obile Uploads/20160406_195236_zpssp1vwiut.jpg

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Parts yet to be installed are the fan with baffle, front and back walls and exhaust/intake ports.


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## Tony B (Jul 30, 2008)

Why not putting photos on here rather than photobucket?


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## jessesnowden (Mar 15, 2015)

Doesn't work on my phone and I don't have Internet in my apartment...


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## jessesnowden (Mar 15, 2015)

Got it all assembled and loaded the first stack in the trailer today! Until I figure out and order all the solar components this floor fan will be plugged in. Wish me luck!



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## jessesnowden (Mar 15, 2015)

Here are the first 2 days of monitoring. The little meter shows the highs and lows for that 24hr period. Tennessee tim, do you think this is an acceptable range for drying or should I make it more air tight to turn up the heat?


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## Tennessee Tim (Dec 15, 2010)

I'm not familiar enough with/how the venting is used in a solar kiln....for temp or humidity control. 

Your monitor... does it keep a log every so many hours (the 2 I have log every 3 hrs and saves I think 2 weeks) or just a daily 24 hr high/low.

It would be better to see a higher/closer temp range, but your bigger concern is it dropping the MC any in 24 hrs. The lower temp just makes it slower drying. 

Ever who you got your solar plans from did you get their drying theory??? VERY important...different kilns dry and operate differently.

I'll reread above posts later but do you have circulating fans??? A correct passive solar layout ???(this design supposedly makes the air roll/circulate without elect BUT it has to be design correct)


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## jessesnowden (Mar 15, 2015)

This monitor operates in 24hr cycles. Simply recording the high, low and current humidity and temp. Resets every day. 

From what I've experienced the venting directly controls the temp. More outside air circulating in will keep it cooler. And with that I'm assuming a lower temp means less moisture being drawn from the wood. The fan is set on a timer to run until 9pm to make sure any hot moist air is cycled out. Then it sits all night and the fan kicks on at 9am once the sun has come up and started to heat the chamber. 

I guess the most important factor is the rate that moisture is leaving the wood. Do you use a moisture meter that is perminantly inserted in the stack?


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## Tennessee Tim (Dec 15, 2010)

I cheat.....I use a weather station in both of mine. One logs and views all on it's base (I personally like it best due to I don't carry a laptop/computer with me) and the other I can view BUT I can't scan the past log without a computer. I set the RH monitor/reader inside kiln area (I basically center mine IF possible but in your case keep out of direct sun, it will effect readings) and put the base in a area close enough to read monitor. My kiln with insulated foam and double metal (int & ext) walls reduces my distance so I'm limited on distance.

The fans circulating on and off along with heat up and down is probably the big MC range swings. You don't want to lose too fast BUT you can't hold moisture either. I would check with Dr Gene's solar kiln.....I'm guessing you want around a minimum of 1.5% MC loss. When you shut down in the evenings IF your monitor will show MC only at what it is, then I'd read prior to it refiring up for the day. That would give you a more balanced MC....This may be your high reading already logged. As long as it's dropping your definitely in better shape than staying constant.


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## bmarshall9686 (Jan 17, 2016)

From what I know about venting solar kilns is primarily important in the beginning. When the humidity level is high because the wood is still at its highest moisture content you want to vent more to get the moisture out. As this happens you open the vents less and less, closing them at night to maintain the heat over night. The fan should also stop circulating at night at this point. 

When I build mine I am going to use a humidity controlled duct fan from northern tool. It will be mounted on a dryer flap that opens when the fan pushes. This fan will help keep the moisture out of the circulating air. This fan is also separate from the main fan. 

Some add heaters or light bulbs to help keep the heat, however the first few stages should be able to utilize the sun and make you kiln as sealed as possible.


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## aardvark (Sep 29, 2011)

Ok, so the weather station and monitoring have added to the $500.00 price.
Where are you now $$ wise??
Not being critical, but I do similar with a space hearer/tarp/thermometer for under $75.00.
Mines not solar but is only used a few times a year and then broken down easily and put in a small box on a shelf.


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## jessesnowden (Mar 15, 2015)

Aard, you have a very valid point! There is definitely a portion of this that is just guys nerding out because we like it. Haha my costs are:
Trailer: $150
100' plastic roll: $150
100' of schedule 40 pvc: $30
10 2x4's $20

The rest was free/salvaged materials. 

Not too terribly expensive considering I can use the trailer for anything else when not drying wood. The pvc hoops just pull out. And the walls are drop in pieces with pockets. So it's a flat bed if needed. But still more expensive than your method which I'm sure delivers a decent product or you wouldn't use it. There is something to be learned from that, we have a tendency to over build and over think things. Sometimes the hardest thing to do is simplifying a process to its basic parts, like you do! Haha well done sir.


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## bmarshall9686 (Jan 17, 2016)

You have that right if you're not moving Lumber you really don't need to do anything more.


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## aardvark (Sep 29, 2011)

Not disrespecting or questioning your call on how you use your kiln or the expenses (which are still minor, congrats).
I assume you use a kiln far more often than I do.
Also far different than Tenn Tim does, since his kiln is almost always in use and he turns out massive numbers in board feet.

Here I don't have the space or constant sun for a larger or a solar kiln. We technically are in a rainforest so cloud days are more common than sun days. I'm a stickler for maintaining a 90 degree temp 24 hours a day and then a day or two of 130+ bug kill temps. I need a power/heat source for that. In what I do with the tarp, I can size the kiln to the amount of wood to be dried. That's important since my heat source is electricity, and too much unused area costs heating dollars. 
With solar you don't have those concerns unless your sun source is spotty.


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## jessesnowden (Mar 15, 2015)

Are you in my neck of the woods Aard? I'm in seattle area. And I'm hoping to use the kiln a lot but I may end up throwing and extra layer of plastic come winter time or even put a heater in there. We'll see! I'm in the same boat though, I live in an apartment complex. No room at all! Which is where the whole idea of having a trailer came from. Some day when I've got a fullsized shop I'll probably try your method since it's definitely easier.


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## aardvark (Sep 29, 2011)

No, I'm on the opposite end of the country. East coast. In the Smoky Mountains of North Carolina. Sitting at an elevation of 3450'. In the low clouds. Plus the mountains tend to form some clouds. Classified as a "temperate rainforest". 

Solar (even though many try it here in the name of being "green") doesn't work here and is grossly inefficient and cost ineffective. Certain areas of the country it makes efficiency and cost effective sense, but not here.
I watch people buy expensive electrical/solar setups here for their homes and they're proud of it.
Problem is their payback time to where the equipment is paid off vs their electric bill savings is over 20 years, and by that time their equipment is outdated, in bad shape and needing replaced.
But that is all a different story.


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## jessesnowden (Mar 15, 2015)

Here is the data from the solar kiln. Each peak is one day. I have 2 thermometers in there. one that reads current temp and humidity as well as highs and lows for that day. The other is strictly temp but it takes a reading every fifteen minutes, that's where this graph comes from. I stuck the data logging one deep inside the stack of wood. This past week it has been low 60's to mid 70's, partly cloudy or sunny. On all of these days it got up to 120° up in the plenum, but it only reaches roughly 75-80 inside the stack. Which I think is for the best. Another important point, which I think is really key to this working is the moisture content in the kiln. By 11am it gets up to between 70-90% inside. But once the sun gets low it drops to roughly 20%. The fan runs until 9pm, Giving all that hot wet air time to cycle out before shutting down for the night. There are a lot of surface cracks all over the slabs even at this temp. I've been told garry oak tends to check and crack like a mofo no matter how you dry it. Which is why I cut these slabs 3" thick. so they could be planed with a router sled later to roughly 2".

Just wanted to update on the cheap-o solar kiln with some real numbers!

http://i1175.photobucket.com/albums...obile Uploads/20160507_194107_zps9wibmcac.jpg


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