So far Ive read several threads on the subject of solar kilns. The jury seems to be out on their effectiveness. Some say they work while others say they dont. Well this year I am way behind in my wood. So I figured Id do my take on a kiln. Its not fancy, just used vapour barrier to wrap the pallets like Uncle Augie did as in these threads: http://firewoodhoardersclub.com/forums/index.php?threads/solar-kiln-firewood-drying.6041/ http://firewoodhoardersclub.com/for...on-or-redrying-wood-over-and-over-again.6816/ A firewood shed is likely in the cards next year so I hope to be able to work towards having a stockpile of dry, but right now Im trying to get this years wood as dry as fast as possible. What Ive seen so far is that there is no doubt that plastic wrap makes for a significant difference in pile temperature. Im seeing about 3-8*C (37- 46*F) difference in surface temps between the covered and uncovered pile. A lot of difference depends on how intense the sun is obviously. Uncovered (control) pile: Solar Kiln I have a moisture metre, but I can only do end testing, which is highly inaccurate. In end testing, there is basically no reduction in moisture that is in the Kiln. No way this wood is 15%. Tearing appart the piles to do a fresh split and test probably isnt going to happen. There does appear to be a lot of water coming out of the wood. The question is should I ventilate to get rid of it and if so, how to do it solar and cost effectively if possible.
You want air flow coming in from the bottom to help ventilate and get rid of the water. From everything Ive read these things work pretty good I had every intention of building some this year to help get my wood up to speed but life hit me with a fastball right in the face and I havent had time to think about my solar kilns oh well maybe next year. I hope it works for you and gets your wood at least a little better for this winter.
I would also recommend that you raise the plastic up off of the wood so that its not touching it I think that would help a lot too.
No ventilation, don't worry about the plastic touching, but make sure it is completely wrapped up, I saw that one end had an opening in your pictures, did you place the plastic under the wood as well? If not I do not think you will have the results you are looking for. You DO NOT need airflow. Just some holes in the plastic on the bottom to let condensed water drain.
How much did you say the plastic cost? I wonder how much it would be to wrap 3 or 4 cords. I'm interested to see how this turns out. A fascinating thread.
$13.89 for enough to wrap 2 cords up. Since I am scrounging my wood I'm guessing I'm up to a total cost of $25 per cord not including my time. That is for a kiln dried cord. I leave them in there over the summer and they end up getting below 10% moisture content.
I think it needs some air flow. What happens if you take a shower with the heat on and the fan not on? It is going to steam up in the bathroom and there is going to be moisture all over everything. I think that is what is happening with the solar kiln.
Except Augie's design is a closed (or nearly) system. Once water drains out the weep holes it is gone, like water down the shower drain. There is no new moisture being introduced (from the shower head). Yes the humidity will rise in the plastic envelope, as you theorize, from moisture coming out of the wood. But the heat gain by wrapping it is much more important. That heat accelerates the migration of moisture out of he wood. What is going on isn't much different than your's or anybody else's stacks. Once the RH rises in the evening they are sucking water back in, and continue to do so all night long, wrapped in plastic or not. Just the extremes offered by his system are a bit different.
I have done some reading on the solar kiln idea. Some people say they work others don't. I think it will be interesting to see what kind of moisture readings Scooby gets from his control pile and the kiln.
No one who has built one correctly thinks they don't work. Only skeptical people, who haven't built one correctly, are suggesting that it will not work.
Some good discussion.. I see the ventilation discussion continues. When I did this, their is VB on the pallets underneath the wood. The bottom VB has lots of slashes in it to hopefully let the water out. Just like Augies. There are 2x4's down the "spine" of the pallets between the two lenghtwise piles of wood. The idea was to form a peak in the centre so that water would run out to the sides and down the plastic. I left the space between the two rows of wood mostly open to allow air to flow internally, hopefully evening out the temps between the front side where the sun hits it and the back side where it is in shade. Usually Id throw the uglies down in there between the two rows. Yep, the end closest to the camera is not closed off yet. Its only temporary. I still have lots more wood to add to the rows. When I get all the wood in, Ill close up the open end tightly.
A quick test today in direct afternoon sun. Solar Kiln (sun side) 28*C Solar Kiln (Shade Side) 13*C Control, (sun side) 10.5*C Air Temp 8.5*C
I believe enough in the idea to try it next summer. I am eagerly awaiting the end results in another 7 weeks.
Quick Chemistry lesson to explain a little better how this works. A split is a matrix of tubes that when the tree is alive carry water and nutrients to the leaves from the roots. This matrix of tubes contains water in a vapor, as well as a liquid state and air. Think little bubbles all along the length of the tubes. At 10.5C the vapor pressure of water is 1262.8481 kpa(kilopascals) think of this as pressure pushing the water out the end of the tube from the middle. but normal air pressure is 101.32500kpa+whatever the humidity is. at 8.5C is 1103.3102kpa if you had 100% humidity, lets say you have 60% relative humidity or 661.98612kpa vapor pressure + 101.325kpa or 763.31112kpa pushing into the wood. So assume the air bubbles are at 100% humidity the net pressure pressing the water out of the wood is 1103.3102kpa-763.31112kpa or 542.64908kpa of pressure pushing the water out of the wood. At 28*C the vapor pressure of water is 3880.8292kpa. Using the above calculations we get 1653.65668kpa of pressure pressing the water out of the wood. Ahhhh but wait, remember water is always condensing on the plastic in the kiln because the Delta from the inside to the outside is 20C. So we also have to take into account the rate of condensation of the water inside the kiln as well removing water from the air lowering the relative humidity and increasing the delta of the pressure inside the wood and the pressure outside the wood. So in effect we have a 3x difference in pressure using the above calculations. If we take into account the condensation factor, which appears to add an additional 2-4x effect giving us a total of 6-12 x speed in drying wood, the effect I am seeing. Regardless it works, to suggest otherwise is just incorrect.
I am hoping it does work. I have read some studies on drying that say it does not work. I have also looked at plans for building a solar kiln and they have fans.
http://firewoodhoardersclub.com/forums/threads/solar-kiln-firewood-drying.6041/ This is my second year doing this, I can assure you it works.
