new solar kiln

[MattJ]

OK, so I waited to post anything on this project until it was essentially done since it seems nothing is more controversial here than solar kiln designs and I'm not very good at receiving mid-project feedback, just ask my wife!

Here is my attempt at a personal solar kiln with the following goals:

- Can dry ~300bf at a time - we'll see how it works
- Can be moved by 2 people if needed - accomplished
- Costs under $500 - just made it under this with paint
- Most importantly, doesn't get veto'd by my wife as it sits in my backyard in the neighborhood! - so far so good although she said the color I picked looks like gulf war surplus.

I made this from Foamular and pine for the top.  The top can lift off or tilt back and weighs 80-100lbs total.  The heat collector is galvanized roofing painted black and it hinged in the back, and also bends down to lay on the stack and perform the function of the baffle.

The bottom is insulated via a layer of foamular on top and is pressure treated pine and ply.  The length is 10' to handle the lumber I saw to 9'.  The fan is solar powered by a 30W panel.

My goal was to finish drying lumber already air dried for at least a few months.  I only get 1/2 day sun in my yard so it will be slow but steady, and is also the reason I just started with one large fan.  The back vents are adjustable top and bottom.  I don't like the tilt way of loading it right now so I will likely add a side door fairly soon for when I need to load it myself.

I just got it finished before I had to travel for work so I'll load it sometime when I get back.  In the full sun it seemed to heat up quick and the fan was really pushing some air.  We'll see how it performs.

Thanks for looking

Matt



  

  

  

  

  

  

 

[GeneWengert-WoodDoc]

Looks super indeed

[Den Socling]

Nice job on everything!

[MattJ]

Thanks guys, and Gene I have to thank you for great discussions on the forum and all the great literature you've made available, it really helped with the design ideas in my mind.  Also, please don't think I can't take feedback, it just messes me up mid-project sometimes to the point where I get stuck and can't figure out what direction I want to head in.  Therefore I waited until I finished the first version before posting.

I started with one fan given the size is relatively small and I was trying to hit my $500 price tag with this initial effort and the solar fan ate almost half that budget.  I may try to add more later if it needs it or if the drying is exceptionally slow and if this fan seems reasonably durable.  I also wonder if I will need to add more/bigger vents.  I guess I'll just start it up and find out.

Also, one idea I was trying to minimize with this design was thermal bridging.  That's the effect of 2x4's etc short circuiting insulation efforts in insulated walls.  In my design I have only R=5 walls, but there are no frame members that penetrate through and its 100% foamular on the walls and floor.  I may add more insulation and another layer of plexiglass later if needed but I do have a fairly high collector to lumber drying ratio, should be around 50ft2 of collector for 300-360 bf of lumber. 

[GeneWengert-WoodDoc]

You would benefit from large vents for wood species that dry fast, but you might use the door or even open the roof for that extra venting.

[MattJ]

Thanks for the ideas.  When I add the door I'll also drop in a few larger vents as well.  That is a good idea for if I ever drop in some green lumber.  I'll be starting with just finishing off some of the air dried stuff I have pilling up.

[MattJ]

Well I tested out how it works unloaded this weekend. I don't have time right now to get it fully loaded and have a chance to monitor it so that will have to wait a few weeks. The empty kiln with the vents closed hit around 170F within an hour of having the sun on it. I opened the vents when it hit that temp to avoid causing anything to start melting. The outside temperature was 85F so thats a pretty good rise!

I milled some oak and have some maple to cut up so hopefully later this month I'll get something loaded in it.

[MattJ]

So I am ridiculously slow on getting this kiln actually drying something but it finally happened!  My son (7 years old) said to me the other day "Dad, why don't you finish one project before you start another?"  I responded nice idea but ain't going to happen!

Well, here it is loaded.  I made a few mods first.  I didn't like tilting or having to lift the kiln off so I added a side door.  I also added a plywood sheet to the floor above the foamular with stickers screwed onto the top.  I loaded it by sliding a board in, then lifting the board up at the end and sliding the stickers in with a handy shuffleboard like stick I fab'ed up.  Sounds slow, but for my use I was able to load the kiln pretty quick once I got the wood over near it.  I'm not aiming for production, just to dry a few hundred bf at a time for my own use and for friends.

Empty with vents closed the kiln hits 160F+ pretty quick once the sun hits it.  Loaded with the wood that has air dried 3-4 months (air dried under cover but the summer here has been super wet) it hit around 100F the first day with the vents half open, and was getting around 120-130F a few days later on the weekend.  As the evaporative cooling slows down I expect the temp to continue to rise, and when its near dry I'll close the vents off all together to finish it. 

The wood loaded is mixed red oak and thick yellow poplar.  Not my best lumber as a lot was sawed through and through so its a good experimental load.  We'll see how it does!  As an fyi the small pieces of wood on top were just extra stickers hanging out for the time being.

Matt



  

 

[MattJ]

So I went and checked on the lumber drying as it's been ten days. With the outside temp at 85 the inside in full sun with the vents all open was 105. I checked the moisture content of several boards at 7-8 % moisture content per my meter. The lumber per the same meter started at 15% as it was air dried from the early spring under cover.

I closed the vents down to see if I can push the temp up higher to aim to hopefully cook any bugs. Empty I could get it to 160 or more so we can see what we can hit.

Matt

[scsmith42]

Very cool project Matt!

[MattJ]

Thanks Scott.  How's the timber frame coming along?

[scsmith42]

Haven't made much progress on it since the class.  Sometimes work happens... :D

[Ljohnsaw]

Yes, this is an old topic, but I have a question.  You stated you used Foamular for the walls.  Are you saying you just have framing (1x4 or 2x4), a ply bottom and just Foamular for the walls?  That would certainly make it light!  So how is it holding up and performing?  I'm curious about the Plexiglas, why the overhangs?
Thanks

[MattJ]

The pictures at the beginning with the pink foam showing is pretty much the wall structure.  It is just framed on the corners and mid points with 1x pine.  It is very light.  The base is 2x8's joists and ply with a layer of foamular sandwiched in. 

The only changes I have done over time have been to replace the plexi with double walled polycarb.  Its much better and fairly cheap with 2'x6' panels joined together.  Somewhere here I have a post with the comparison.  Its much more rigid, heats and holds heat faster, and I think was cheaper.  The kiln has held up well and the solar fan seems to do a good job moving air and I like not having it plugged in. 

The overhangs were to keep rain from hitting the walls as much as possible to avoid rainwater running into the kiln and also because my 1x material was not pressure treated (the joists and ply were).

Hope this helps

Matt