Solar kiln idea

[Panovak]

I am wanting to dry my own lumber now, particularly larger slabs of oak, walnut, hickory.
I don't have the funds to buy an expensive DH kiln nor do I really want that electric bill.

My option, it seems, is a solar kiln. I like this idea,  however I live in Minnesota so winter drying will be extremely limited.  (I'm around 44 degrees latitude. )

My idea is to build a solar kiln where the collection chamber of the heat is separated from the area where the wood is drying by a very well insulated wall. The portion of the kiln that houses the lumber will also be very well insulated.
The solar collection portion could be filled with mass, such as black stones or black concrete, or it could be a simple trombe wall. Between them, on the wall, will have thermally controlled baffles. One up high and one down low. At night when Temps drop the baffles close. It also would give me the option to add a bit of supplemental heat without burning a hole in my pocket.

Has this idea been done or explored?
It seems like a good idea but I may be missing something. My wife tells me I usually am.

[doc henderson]

yes! is the short answer.  but with all that comes expense and complexity.  a 20° elevation relative to outside temps, will dry lumber.  so even in the cold, you just need that differential to drop the RH.  Advise given to me when I tried to improve on a simple design by Dr. Wengert was "it is only a solar kiln".  i.e. keep it simple.  He was a pioneer in the research and design of the solar kiln, with a PhD in wood products.  there are tons of threads on the forum on solar kiln design, and we all have had input.  so, you might start with a search and then ask questions as they arise.

[doc henderson]

one of the nice things about your design is you could easily make a glaze/collector surface area much bigger than the wood compartment, to overcome some of the low temp difficulties you may encounter.  A solar kiln should be great for the hardwoods you describe.

[Panovak]

I've looked pretty hard through the forum and others for something similar. Maybe it hasn't been done because the ideas no good.

[doc henderson]

you may have to read the threads, as i know we have talked about it.  you are correct, if you want to heat at night or make headway with supplemental heat, you have to have a well-insulated compartment for the wood and be able to seal off the collector.  It gains heat during the day but loses it easily after dark.  there are plans online that used foam board to make a baffle and "gate valve" of sorts to stop heat loss after dark.  it used airflow to open it when the fans were on, and they were solar, so it made sense.  I will look tonight and see what I can find here and elsewhere.  again, you can also increase the collector to board feet capacity of the compartment and do well.  If I cannot find what I was thinking of, I can sketch some ideas for you.

[doc henderson]

you can build a traditional solar kiln and cover the glaze at night with an insulator, like moving blankets.  you could also get a forced air wood furnace and do it all with wood heat, in a well-insulated building.  The complexity begins with trying to do both.  letting things cool down at night is not all bad for stress release in the wood, but it will take longer.

[doc henderson]

I have to go to work soon but did the search and it turned up 21 threads with reference to "supplemental heat in a solar kiln".  search all topics.

[btulloh]

Don't give up on it quite yet. It's all about collecting energy plus a desire to store some in the thermal mass. You need a certain number of btus per unit of wood to dry it. Be aware that rate of dryi ng safely varies by species the current moisture content as the wood dries. This data is available.


You are wanting to collect more energy during periods when the sun is supplying less so you need a larger collector area to receive the same amount of solar energy. Then you want to store it and use the btus later.  That's where you rocks come in. You should probably have a way to reduce heat loss through the glazing after sundown because that is where a lot of loss occurs.


All these things can be quantified based on published info.  The right collector size, storage medium, and kiln heat transfer method should all be worked to get good results. All this may be the reason it hasn't been done (it probably has) but the complexity would scare many people. It could be viable if you like to take on design challenges and build complexity, but there needs to be some well defined factors for collection, storage, and heat transfer or the results will be less than desired.


Dig in and good luck. Let this thread evolve for a while and there are some clever people on this forum that will have some good contributions.

[doc henderson]

 

[Ianab]

Thinking about Doc's diagram with the separate collector chamber, would it be practical to build a conventional solar kiln with the collector on the roof, AND an extra one as a lean-to.

Now the clever part of the "standard" solar kiln design is that things are sized so you basically can't dry your wood too fast and mess it up, even in the height of Summer. This means you don't need fancy measurements and controls to maintain a safe drying schedule. The night time cool off also reconditions the wood each night, so even if your peak drying was a little too fast, it's only for a few hours. Then things cool off and equalise, so you shouldn't get case hardening or surface checking. Solar kilns are always slower because they deliberately keep below a safe drying rate.

Of course the down side of this is that less sun and cooler ambient temp for 1/2 the year means even slower drying. So my thought is adding a 2nd collector, that can be blanked off in the hot weather (and vented to stop it melting?) so as not to overheat things in the hot weather. When the weather cools, take out the baffles, turn on the fans from the aux chamber, and have 2X the solar collector.  You could even angle the main collector for best Summer sun, and a steeper angle on the Winter panel to make the most of what lower angle sun there is.

I understand the idea of adding thermal mass, but here the problem is that you just don't have enough heat going into the system, so trying to store that heat isn't going to help much. What you need is more heat collected.

[Stephen1]

In the conventional solar collector, it uses the wood as the thermal mass? LL you are about right on in that you will need a bigger collector system to store heat in, let's say rocks. If you are going to do that why not just build 2 solar collectors.

[Panovak]

Doc, that image is nearly exactly what I had in mind. I think I'm going to start the design phase around that general idea and take it from there. If it turns out the two chamber idea is no good I'll make it so it's easy enough to remove the partition wall.
Everyone around me right now wants white oak slabs and the cost to have the local idry guy to dry is astronomical. (Around 2bd ft). I've run through my stash of 3 year old slabs that I air dried.  While this isn't my main source of income now I'd love to get to a place where it will be. I'll continue to ask for guidance here and will certainly provide updates and pictures. Thank you

[doc henderson]

If you think you will need more collector surface area, then the solar collector could be twice as tall and be the roof for the main building.  i looked and did not find the baffle/valve, but it was made from foam board, so the fan pressure could open it and gravity would close it when the fans were off.  Oak dries slow, so you may not need more than the usual 1 sq. foot per 10 board feet.  and the thicker the slab, the slower. so, a more traditional size may be fine. 

[doc henderson]

working now but here is another idea to get more surface and have circuits to close at night.  internal fan to circ with aux. heat on, and outer circuit to move hot air from solar.



 

[Panovak]

Lots of great discussion here..
So I've let the idea marinate a bit and here's my current thought/plan. Many of these ideas come from VT design.
1. Glazing will be two layers of double wall polycarbonate.  One on either side of 2x4.  Thankfully I found some dirt cheap that was used but in really good shape.

2. Increase angle to 55-60 degrees.

3. I will increase square foot of glazing from 1:10 to maybe around 1.2:10.

Square feet to board feet.

This increase works out nicer too as it'll increase my depth of kiln that I'll lose with the steeper angle.

4. Potentially down the road I can add a blanket system to the underside of glazing which could be on a pV sensor.  These systems already exist for  greenhouses.

[doc henderson]

If the glazing is cloudy at all (does not sound like it) it will block or reflect UV light and heat.  the double glaze will also reduce losses after the sun goes down.  I like the angle change.  You could if you like, go way up on the surface area and then cover if needed in the summer.  But, also good to make it mate up with the sizes and lengths of material you have.  Nice score on the glaze.  any details? 

[doc henderson]

I also went back and read your original post.  Drying oak needs to go slow, and the thicker the slab, the slower.  The drying early is from circulation of relatively dry air.  a temp differential of 20° will provide a benefit over air drying.  so a 1:10 would work fine, and just be slower, but is what you want with thick oak.  Thanks for a fun exercise.

[Panovak]

Glazing is 4'x 11' x 10mm double wall polycarbonate. I have 9 pieces.

Plan is to keep the same length of kiln as VT design.  Lower knee wall I'll also keep at 32". At 55 degrees the upper wall is 32"+ 108  if I keep the 11' length of the panels.

This is 146 sq ft of glazing vs 120sq ft in the VT design.  Not drastically more but I also don't want to make drastic changes to a proven design.

I'll make sure to make easy change of the lower panel should I need to in the future.

I think the increased angle will accomplish a couple things for me in northern climates besides the obvious of a more advantageous angle in winter.

1. It'll allow me to increase my surface area in winter

2. It'll reduce heat load in summer that comes with the larger area.

[doc henderson]

I agree with all that.  you are on your way!

[Panovak]

Thanks for input and discussion doc.  I'll definitely post pics and ask questions along the way. You a real doc, doc?

Edit, nevermind I just read in your profile that indeed you are!  I'm an icu nurse. Ive Been thinking about moving to the local small town ED.

[doc henderson]

Guys like you are needed everywhere.  good luck!