Solar Kiln: Do they work and best practices?

I've tried to burn freshly cut ash just to see if it's possible, and it burned horribly in my stoves. So I get your point. But I'm not sure that green cottonwood would burn even in an outdoor fire pit on a windy day. Have you ever tried? I'm honestly curious. I tried to burning some recently cut / partially rotted white oak logs in a fire pit a few years back, and I ultimately went through about 10 gallons of diesel and a bunch of kiln dried pallet wood in order to finish the job. My conclusion was that freshly cut white oak simply does not burn on its own.

 

If cooler temperatures mean lower humidity, that might not be a bad thing as long as you get some sunshine.

 

You’d have to know Dennis. You might also know him as the original proponent of the three year plan you may have heard about so often here on fhc. He was joking about the green cottonwood.

 

Let me correct myself. Cracked open at the top 1.5”. The bottom has no flaps. I need to experiment with the venting. I really have not had sunny days lately. After being closed up condensation on the plastic seems to disappear with very little venting.

 

But the bottom is open, so the condensate can freely run out, correct?

 

Yes, I even poked holes at the bottom those walls.

 

I did do a modification yesterday and thought I would share. We spent a few days away just after I finished this. We got a ton of rain and the plastic was not pitched enough so water accumulated on top. See pictures. I used fiberglass fence posts for this. I am not planning to leave this outside over the winter so no concern about snow load. I know you are seeing condensation. I had all the vents closed at that time.

 

The wood on the top layer will dry the fastest. Next to the bottom not so much. Ideal case is to rotate the top and bottom, 2 much work though.

You could pull the top foot of wood off as soon as it's dry and store it somewhere else. Then the next layer will dry.

The goal in a solar kiln is to have it as hot as possible with just enough ventillation to keep the relative humidity down. Heat promotes the movement of moisture through the wood.

Another fancy feature would be a relative humidity sensor that triggered ventillation fans.

From the pics it seems more ventillation is needed. If the RH is high inside then no drying will occur regardless of temperature.

At first the wood is giving off lots of moisture. More ventillation needed. As it dries reduce air flow so that it gets as hot as possible.

I have some small solar kilns to dry firepit wood. Been using them for several years. Wood comes out reading 0% in the interior of a split. And boi does it burn well on a January day!

 

How long does it take you to dry wood and what is your climate like? Here I have not had 1 day that I could call a good sunny day since I started. Friday it will break and I will get mid 70s low 80s for several days. For moisture I am just watching condensation inside right now. Got to have sun to have heat and air movement. I am thinking this is when I can really dial things in. There are bath fan switches that you can set up to turn on when it senses humidity above a certain level. I have thought about running one of those with a box fan to work on non sunny days. I did do some reading on solar kilns for lumber. Some of those guys do vent but are careful
to not do a lot of venting focusing more on heat. He said lots of humidity at first but there was a point where it started to drop pretty fast. He described heating and cooling of the wood as a sort of pumping action. Still not sure I am going to have dry wood this year.

 

(some of) The guys with (portable) lumber mills using them to dry lumber have rather elaborate setups with ventilation/fans.

 

I am in Kentucky. Oak will take 3 months to fully dehydrate in small splits during midsummer. Cherry 1 month.
That is starting with 15% wood going in.

Temp in the kiln mid day will be 110F.

You need enough air movement so that the condensation is not apparent. Maybe something like this


Ideally at first wet wood should be stacked in the open in a windy place until it looses the intercellular moisture.
Then finished in the kiln.

 

It took two weeks to get delivery of the 4 cords. It was stacked as soon as it came so did do some normal air drying. I did see some cracking on the ends of the wood while I was building the frames. This is freshly cut ash BTW.

Well today outside temp did not get past 75 and we never saw the sun. Inside the kiln I saw 90 with 70% humidity. Air should be moving. I am not seeing much of any condensation just a bit on the backside of the stack. Hopefully with full sun I will see some higher temps.

 

Bigkahuna427 first I commend your efforts to obtain dry wood. Can you buy a cord of kiln dried in Maine? To
Blend with your close to get you to next year

 

First day of full sun since I built the kiln. We have had overcast skies and rain for more than a week. A couple of days ago we had a few hours of full sun. I was pretty surprised at how quickly it heated up. This morning I went down to check on it. Inside the kiln it was ambient temp at 62 degrees and 100% humidity. Checked it again this afternoon and inside was 115 degrees with 30% humidity.

 

I have closed up the kiln more now. So, at the moment I have the center lid cracked open. That is a 1.5” opening 8’ wide on a 40’ long pile. My typical venting is to vent 1.5” the whole length of the pile. I have been monitoring temps in three places and humidity at the top. The lowest temps are near the bottom and highest at the top. My strategy for venting is to vent as little as possible without getting condensation. This seems encouraging as my humidity is now at the lowest I have seen so far at 25%. Temp in the top is over 120, 16” down in between pieces of wood is 100 and near the bottom of the stack is 90 plus degrees.

 

You definitely want to raise the temp as much as possible. Rule of thumb is for every 20 degrees that the temp goes up, the relative humidity is cut in half, for every drop of 20 degrees the rh doubles. The lower the rh, the easier it is for the air to pick up the moisture evaporating from your wood and carry it away. Which is why you need to find that sweet spot that holds in the most heat while allowing the moisture to escape the kiln.

 

Measured yesterday at top of the stack and near the bottom. We have been having a nice stretch of dry sunny weather with most days in the 80s.

 

Bigkahuna427 read member Paul bunion post carefully. He is one of very few that has scientific facts behind his data