I put together a little solar kiln in the beginning of April. I put it on the north side of my yard where I get the most sun. The best sun that I can get is about 5 hours of sun up till about 1:30 due to the oak trees on the back line. I've been seeing some pretty impressive drying rates, especially on the topmost pieces that get the full benefit of the sun. So now mid June comes along, the sun is higher in the sky and the crown of the maple that I put it under is just big enough to shade it for all but about 25 minutes. Guess I'm going to have till a week or two into July as the sun starts getting lower in the sky. (The shadow at noon was at the foot of the plastic. )
I remember when you posted that you built this. Despite the shade, im still interested in the final results.
Im interested in your build of that. What did you use for the plastic wrap, how is it holding up and what kind of results are/were you seeing with it in full/partial sun?
I tried something simular back in 2008, trying to dry some oak in short time. My arguement came when using black plastic or clear. I thought the black would produce more heat. But was told clear was preferred.
The plastic on the sides is a drop cloth. I think the choices were 6 and 8 mil and I remember choosing the heavier one. The top is a clear garbage bag. I put a piece of tar paper on the pallet in the front where the sun hits it. Under the wood it is open. The plastic has held up so far. It has been through some moderate storms, I would think a strong wind could rip it apart. The piece of wood whose weight that I have tracked the most often has gone from 6811 grams (15 lbs) on April 1 to 4997 grams (11 lbs) today. It is white oak that came to me green on Oct 31 of last year, was partially split and left out uncovered all winter. The rate of weight loss has slowed down quite a bit. The first 1000 grams were gone by April 25 and it has lost only 800 since. Just going by the pieces heft you can feel that is is getting dry. I do not know its % moisture content but if we assume it is at 20% today then it was at 63.5% on April 1. The 13 grams that it lost today would equate to a loss of .31% I'll see how much more water comes out if it and see how reasonable my assumption is. It definitely is tracking to make burnable oak in one spring/summer. I weighed most of the other pieces in it, but have not been tracking their progress closely. The pieces lower down have not dried as fast as the pieces on to that get the direct sun on top of them. If I get motivated next weekend I'll take them out and check their progress.
I also have some uncovered wood adjacent to the kiln to compare it to. The closest in weight would be a piece of red oak of the same vintage and treatment as the white oak. The red oak has gone from 7158 gm to 6096 gm today for a loss of 1060 gm, 740 grams less than the kilned wood. The biggest eye opener for me while tracking the uncovered wood was to see the effect of rain on it. It can soak in 300+ grams of water on a rainy day. And then it takes several days to lose it, such that, so far this month the red oak has weighed in at less than its previous low on only 8 days. That is red oak with sapwood, pieces of heartwood doesn't seem to be as porous. Another interesting thing that I observed was that the white oak inside the kiln weighed in more than the previous day the last time it rained. It went up by two grams. Humidity and lack of sun definitely has an effect.
We had my moms wood covered with an old solar cover for a pool. Last week I left the cover out on the lawn for 3 hours to dry and than cleaned it up. The lawn was completely burned in that area. I wonder how well that would work if the wood was wrapped in it with a little slit for air.
I have been thinking about building a small solar kiln like yours, but never could find a decent way to vent it. How well will your kiln work without any type of air circulation? I was always under the impression that you had to vent the unit to let out moist air. Am I wrong about that?
I'd say give it a try. I think making sure the blanket didn't touch the wood and there was some kind of air movement from bottom to top it may just do the trick
University of Fairbanks did a seasoning firewood study. link: http://www.uaf.edu/files/olli/CCHRCFirewoodStudies.pdf Their best ( & most efficient) was : 1. Split & stack the wood 2. thru the summer, for sun & warmer temperatures 3. off the ground, so air can move thru the bottom, 4. in an open area for sun & wind, 5. good air circulation , loose stacked with space between rows 6. top covered to prevent re-wetting Gosh Where have I heard that before, ? ? pretty sure it wasn't from a "college boy" A passive kiln helps, but not a lot & is tough to build one to do 5 or 6 cords Now a solar kiln with fans would be a way to speed it up but, requires energy, & the size problem again. Solar panel to run the fan. ?
Graphical results so far: Here we have graphed the progress of one piece of white oak (green line) that has been inside the solar kiln. The other two lines are red oak and have been adjacent to the kiln. Weights are in grams and the vertical axis is the total weight loss of the particular piece. Since they started out at different weights I would expect them to lose different amounts, so the graph illustrates trends more than an absolute. All three pieces are on top of the pile, thus get the most direct sun. As I said, my yard at best can give 5 hours of sun and it became much less than that when the sun got higher in the sky as the kiln was shaded by the canopy above it. I wasn't weighing or documenting weights that regularly in the beginning, I started keeping better track in mid June. So what I see is the uncovered pieces started out at a faster pace. Argument that covering/wrapping your wood traps moisture? Maybe but not by much. If anything, covering might be a little detrimental in the early stages of drying when there is a lot of easy water to be lost. In mid April it started to rain and continued to. That is shown quite clearly by the saw tooth graph. I had no idea how much water wood (or red oak in my case) can reabsorb from a thunderstorm and how long it took to catch back up under the conditions I can offer. The piece represented by the blue line is heavier today that it was on July 1. A couple of days ago after a good storm it was back to mid April's level. This offers clear argument to me to keep my wood covered at all times. Top cover is essential in my yard. The covered wood did pick up a few grams of moisture on some days. This happened on rainy or very high humidity days with little or no sun. Humidity is a factor. The piece of white oak started at 6811 grams and is now 4791 grams. If it is at 20% today it was 70.5% when I started with it. (4791/120=39.935, 6811/39.925=170.5) I think it has at least 330 grams (or a 12 oz beer) to lose as I have a smaller piece from the same round that has lost proportionally more weight. If it does lose 330 gm that works out to 11% if the original moisture was 170.5 So I think the piece was a bit over 70% to begin with and isn't quite 20% yet. At any rate I conclude that letting rain get on your wood is really really bad and that it is easy to dry oak in less than a year under the right conditions. I have some other pieces in the kiln that will tell more of a story when I pull it all apart. I'm pretty sure that they will not be as dry as the top pieces as the lower pieces certainly are not getting the benefit of direct solar energy that the top pieces are getting.
whitey I googled this and saw it among several designs. Maybe go from here? TurboDiesel, they speak of venting in this a lot, what Im worried about is not the water from the wood to condense on the top of wood and cause mold but the condensation dripping back onto the wood in other places that may be more likely to create a pocket of moisture that isnt able to move around... since solar can work a fan I don't see why airflow can't be used. One thing that this may create difficulties is when the air is the same humidity all the time and its heavy, then the kiln is just fighting to stay stable in relative humidity from within.
Every day the cycle starts over. Last night's pocket of moisture is vaporized again today and has another chance to condensate on the cold plastic tonight.
FatBoy85 Venting moisture out also vents heat out. You could use a solar collector and push heated air in if you wanted to vent moist air out. But that takes energy to run and added expenses for materials. Of course, you could just get on the three year plan and then your wood would always be dry...
