A Solar Kiln to dry wood. Overview, design and build (I)
We had to cut down last winter some trees from the garden, two 40 yo cherry and walnut, as they were endangering the house when there was storm, wind, ice etc. So I have managed to save few usable trunks and large branches and went to a local sawmill to make some 25 mm (1”) thick lumber and ended up with circa 0.4 m3 (170 bf) of cherry and a few boards of walnut. I would have get more wood from those trees, but the base trunks were full of nails, wires, even a damn horseshoe was inside that cheery :)
Having no kiln in my area to dry this wood properly I decided to build one myself, mostly for experimentation and the satisfaction to make my own workable wood from the beginning of laying down the tree and having it properly cut and dried in your workshop ready to make a project from it.
The solar kiln is based on the well know Virginia Tech design, adapted for my own needs and materials I could find easily on local hardware stores. Mine is a small one, with a gross storage space inside of about 1.5 m in length, 0.85 m wide and 0.65 high (~5’ x 2.75’ x 2’ ). Why this small, first, I do mainly small projects, so 1.5 m (5’) long boards are more than enough for me, second, at this length I can easily transport with a small car some trunks to the mill, take some trees from peps who cut down theirs from the gardens etc, and maybe third reason, storage space after drying, once you have it dried to 7-9% you must keep it somewhere inside, or use it, otherwise leaving it outside the MC (moisture content) will return to 12-14%.
How this kiln works briefly. The entire build should be an insulated enclosure with a clear roof on top inclined at ~ your latitude (45 deg my case) and oriented to the South so you get the most out of the sun the entire day. During daytime, the sun will heat up the inside, an electrical fan will push the heat towards the wood stack heating them so they will release moisture, then recirculating the air trough the back, up again through the roof and so on until the evening when the vents needs to be opened for some minutes so the HOT/HUMID air formed inside will come out through the upper vents, and COLD/DRY air gets in through the bottom ones. Throughout the night the wood will rest, the moisture from the middle of the wood board will go towards the outsides, where it is more dried because it was heated whole day long, until they get even sort of. This in fact is one of the main advantage of a solar kiln compared to an industrial one, with permanent heating, the wood will have time to rest, no fast drying so will be prone to cracking and defects and an even moisture content.
(take this as a brief non professional explanation, there are more small details to be considered based on specific conditions. I am a newbie in this and more I will describe in the First run and results post.
The build. As I said earlier, the design is also based on the materials and sizes I could find in local hardware stores, similar in all Europe I suppose, and probably NA too.
The foundation consists of six ~30 cm (12”) tall pylons made from paving bricks placed on cement bases I made before. The floor frame is made from some reclaimed tongue and groove pallet wood I had, I removed those joints and ended up with an approximately 2 by 4 board, ~ 95 x 36 mm. An outer mitered frame I added where I have left one of those rebates so that the OSB floor will sit hidden inside. The walls where made from 46 x 46 mm ( ~2 x 2”) construction lumber, sandwiched with OSB on both sides and 50 mm (2 “) wool inside, and all connected with corner poles made from 80 x 80 mm (3 x 3”) beams. The inside is painted black to absorb heat. Two upper vents, approximately 12 x 12 cm (5 x 5”) opening, and other same two on the bottom placed on the doors. The roof is made from 3.5 mm (1/8”) clear poly-carbonate siting on a foam tape to minimize air/heat leakage. A cheap desk fan to push the air; this is probably not a proper solution, you need a more sturdy fan to handle the heat built inside, up can go to 95-100 C (200 F), but this is what I could find easily then, I will be looking for a better one to upgrade it. This one did handled well all this summer, but I was afraid that it will burn at any moment :). Also wood paneling on the front faces for a nice look. Cost of all these was about 250$ more or less.
First I had in mind to make a closed back, without doors, and retractable/hinged roof but switched to this design mainly because is more easy and practical to fill the kiln from behind, and also the roof sealing had to be more complex. I have in plan for the next year when I will upgrade the kiln, to make the doors smaller by placing the bottom vents fixed, then the doors on top of them, make a steel frame on the floor to stack the boards on a more flat and sturdy area and put straps to tighten them so they will not twist (now I haven’t bothered with to much weights on the stack, the cherry was sitting for 4 months outside on air drying, did twist a bit, so it wasn’t so much to be done at this stage as it was dried), and also to insulate the floor, now is just an OSB board, maybe is ok in the summer, the heat loss there probably is not so meaningful, but I will add either a layer of wool or polystyrene sheet to get more efficiency if I will be using in spring and fall too.
This summer I have managed to dry two batches of that 25 mm (1”) thick cheery wood. It was already sitting about four months outside, stacked in a cool and no direct sun place, so it had about 12-14% MC. In eight days was able to bring the MC down to around 8%, I was lucky to catch some very hot days in August, outside temperature rising to about 36-38 C (100 F) and daytime RH to 25-30%, so in the kiln I could bring up the wood temperature at a maximum of 70C (158F) during the peak time at around 3-5 pm. More is described in the next post, charts, hourly measurements, details etc.