Logs to Lumber #1: Building a Solar Kiln

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Blog entry by JohnMcClure posted 01-07-2018 09:40 PM 585 reads 1 time favorited 1 comment Add to Favorites Watch
no previous part Part 1 of Logs to Lumber series Part 2: Milling a Cherry Tree (and a video) »

The project started with a big aluminum box. It’s a jobsite toolbox in a size my employer no longer requires. The box is roughly 6ft long, 3ft deep and 32” high.

First I painted it with a dark flat metal primer.

Then I built an angled upper portion from OSB.
Good to have a helper!

The angle of the roof should be equal to your N lattitude, and facing south (if you’re north of the equator). 30 in my case. Trigonometry is very helpful here.

And made a frame from 2×2s for a roof/lid of clear plastic.The 2×2s fit reasonably tightly over the box – it isn’t airtight, but hopefully close enough. The roofing panels are held to the 2×2s with those molded strips, then sealed with spray foam.

At this point, the greenhouse effect will heat the inside of the box above the surrounding air temperature – an advantage over air drying; but being enclosed, there is no real air circulation – a major disadvantage. Also, the box is not insulated, so I don’t know how hot it would get and don’t think it would stay hot without direct sun.

So I bought 1” thick foam board insulation and used spray adhesive to insulate the back and sides. I ran out before I could do the front, but since that gets sunshine on it I’m not to worried.

And I bought a car radiator fan on Amazon for $25 – a 12V, 80W fan – and a 100W solar panel. (The kiln is not located in easy reach of power).
I mounted the solar panel directly behind the kiln, once again at 30 degrees.

To establish air circulation, I built an airbox (plenum) in the back of the kiln. The fan is mounted to suck air out of the plenum and into the roof area. The plenum will suck air through the stacks of boards. A picture is worth a thousand words, so here’s the air circulation diagram: :

This is from the PW/American Woodworker solar kiln plans located here:

Note on running radiator fan from solar panel: It’s awesome! In full direct sun – a few hours a day – it runs like a leaf-blower. Fan is rated at 1750CFM and I have to assume performing near that level. During the rest of the daylight hours, it seems to blow at a very reduced capacity, but anything is better than nothing. I would have been happy with the slight air movement I get just from indirect sun.

Of course you could couple this with a battery, timer, and charge controller and have whatever blowing schedule you wanted… I’m shooting for cheap and simple, so the panel wires directly to the fan.

One more thing – vents. I didn’t put any vents in yet, and I’m not sure that I will. Opening the vents decreases the temperature dramatically but also releases the humidity drawn from the wood. My theory (and hope) is that the non-air-tight roof will release enough humidity and it won’t be a problem.

At this stage, I just need to paint everything flat black and load it with green wood!

-- I'd rather be a hammer than a nail

1 comment so far

View shipwright's profile


7936 posts in 2728 days

#1 posted 01-08-2018 03:57 AM

I had one for a few years, very similar to yours except mine had no bottom. It sat on the ground. At night when the condensation ran down the collector and box sides, the earth absorbed it. I ran it closed to about 17% (inter-cellular water gone) and then cracked the top a little for better removal of humidity.
It worked amazingly well. The only problem I had was that I didn’t originally understand that the collector size needed to relate directly to the amont of wood in the kiln. One small batch got over cooked. I could have covered part of the collector and this wouldn’t have happened.
Overall a great way to dry wood at home.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

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