A Solar Kiln to dry wood. Overview, design and build (2019) | Hacker News Reader

A Solar Kiln to dry wood. Overview, design and build (2019)

adrianpreda.com

112 pointsanimal_spirits36 comments

This reminds me of a firewood seasoning method I saw in (IIRC) Dudley Cook's "The Ax Book", except you pile up your split firewood in concentric circles around a stove pipe with holes drilled all around and along it's length. When the wood is stacked to about breast height, you cover it with shingles/plywood/plastic and put a vent that's been painted black on top of the stove pipe. You end up with air constantly circulating past the wood and significantly reducing seasoning time.

(note, if you try this, make sure to put it on top of some branches so your firewood isn't contacting the ground)

In Germany a similar but more traditional thing (bark-y wood pieces usually used for the roof, no stove pipe, instead a loosely/chaotically packed core for airflow) is called a Holz Hausen: [1]

https://www.logsplittersdirect.com/stories/1202-How-to-Age-Y...

[1]: (Literally: 'Round House')

I think 'Holz Hausen' might be an American word [1], as I've never heard it before. It'd also roughly translate to 'wooden house'.

Over here we call it 'Holzmiete' [2], which translates to 'wood pay/wages'.

Fun fact: haystacks follow the same principle! [3]

[1] https://en.wikipedia.org/wiki/Firewood#Storing [2] https://de.wikipedia.org/wiki/Holzmiete [3] https://en.wikipedia.org/wiki/Hay#Stacks

Link is not available.

Doesn't Holz Hausen translate as "House for wood"?

Seems likely! I don't know German at all. I just saw that when I websearched it, I actually saw these in Austria years ago.

I've been contemplating a solar kiln to dry my split logs we use to heat the house. Our stove is extremely efficient and burns cleaner than many traditional furnaces. But this all comes at the trade off of requiring very seasoned -- dry -- wood. My zero tech solution requires chopping wood for two years in advanced. Not a big deal, having two years of fuel on hand is a very nice feeling. But it does mean moving wood around a lot. I'd be nice to just kiln dry my wood and then know that all the logs I have are stove ready.

FYI - if you don't have a lot of time, you can get away with a much simpler solar kiln than the one in this article, which is still a huge improvement over the natural seasoning process. I highly recommend it with a newer-style wood stove that prefers very dry wood.

We built one last year that was essentially only the frame of this design (but less sturdy), and wrapped it with clear plastic sheeting. Super scrappy, built from 2x3s on top of a pallet. No plywood sheathing (although we did put bubble wrap around the walls), not even painted black. I eventually put an old computer fan at the top of it to blow outward, but originally it didn't even have airflow. It took oak splits with ~30% moisture content down to <20% in a matter of weeks. That wood burns hot!

Awesome, yes I would likely go for the "redneck" chic. My nearest neighbor is over a mile away, I've got no one to please but my pragmatism.

I like to say wood stoves heat you 3 times. Once when you spilt the wood, once when you stack it and once when you burn it.

It's my gym. Chopping wood and fixing old tractors/trucks are two of my favorite things. They both leave me feeling like I really put in a good day's worth of work.

A neighbour nearby has pallets on the ground ringed with wire fencing. After he splits the wood, he throws it haphazardly into the cage and then tarps the top. It sits like that for a full year before he stacks it in the queue for winter burning.

Not sure how that compares to your process, but it seems about as efficient as possible in terms of moving wood around and I think it would be a lot less work than a solar kiln.

I use pallets too. They're great, I put the kids to work and they stack it as I split. Then I shuffle the pallets around. They all sit in a shed, but I have to rearrange that them in the shed since the driest is at the back. My thought is if I could make a kiln that'd hold two or three pallets of wood (a year's worth) than every pallet that goes into the shed would be stove-ready and I wouldn't need to shuffle them around.

How hard would it be to put a door in the other side of the shed so you can take wood directly out of the "back" without shuffling?

One year you fill one half and take wood out the other half, the next year you switch.

People build sheds with removable walls for that and better airflow.

Makes sense!

Only issue is that behind the shed is sloped a good deal and the forks (for lifting pallets) won't pick up pallets that aren't on the same grade. I'd have to build a retaining wall to access from the back side.

Get wood in early spring at most, felled in winter, if you get it chopped while it’s still cold it’ll be sub 20% moisture by fall by just sitting in the sun and wind. The trick is that it starts relatively dry.

What this guy said. You drop a tree before the sap starts running it'll be mostly dry.

This is great advice, thanks. I'll give this a shot this year.

The previous owner of my house (in rural Wisconsin) built some simple-but-effective wood sheds. They're old corn crib roofs, held up on 4x4 posts. Since they're not walled in you can access wood from any side (no need to move stacks), there's plenty of airflow, and the roofs keep out the rain and snow. I may put up some chicken wire fencing between some of the posts to keep dry leaves from blowing in.

Of course, their appearance is... rustic.

Well "rustic" is probably some of my best work! We run a farm as a hobby/side hustle/retire-early dream and I always enjoy doing things the "old" way. Lots of rough hewn lumber and timber. I got an ox even ha!

Cant waste heat from the furnace be used to dry the next batches of wood?

Be sure to check out the second part, with data: https://www.adrianpreda.com/blog/solar-kiln-run

Also it make me wonder if a Peltier unit could provide enough power for the fan here.

Of course you can slap a solar panel there (even those camping ones should work, I think) but IMO it's a bit cbeating.

I saw the finished solar kiln photo and thought "holy crap, they did an amazing job, that looks so much better than if I did it!" then I navigated around the site and realized they are a very skilled woodworker. They have a nice YouTube channel:

https://youtube.com/@AdrianPreda

Another idea would be to pipe your wood stove's chimney straight out the wall, and right behind the house have the chimney go in a ring, and stack the wood around the ring. That way the wood is being dried by your chimney exhaust all winter. Of course if you're gonna do that you might instead wanna use the exhaust to heat the floorboards or something.

In terms of a solar kiln type setup, I would make a "greenhouse" sans the green. Hard to get hotter than sunlight going straight through all the walls and then hot air that can't escape.

That would be a hypocaust.

https://en.wikipedia.org/wiki/Hypocaust

The comment on the cyclic nature of the solar kiln being better vs cracking than industrial process of just heating until done was interesting.

I didn't get why he would only pull in fresh air once per day. Once the humidity gets too high even the high temperatures will lead to less and less evaporation, right?

I would have expected that it would be best to bring in a light constant flow of dry air while maintaining a certain temperature.

That last bit is the devil in the details, you won't get anywhere near the temperatures shown in the followup post if you keep even a little bit of airflow going.

Counterflow?

How well would this work in a hot, humid environment? He mentions that having a vent at the bottom is essential for getting cooler air in which makes sense, but I live in a place where it's also very humid.

Humidity affects the final equilibrium moisture content [0] of wood, but even at 100% relative humidity it’ll eventually stabilize at around 25-30% MC (green wood is usually >60% MC, for reference). Humidity averages around 65% in the areas of the US where most of the timber trees grow, at that humidity EMC will be around 12-15%.

[0] https://en.m.wikipedia.org/wiki/Equilibrium_moisture_content

Wood fires emit NO and pm25 particulate soot. Kiln dried is safer but overall, this is pretty heat with massive downside.

The solar kiln illustrated is for plank wood, woodwork grade milled lumber.

If you want to dry your wood for a fire, it's not a bad approach. Dry firewood has lower risk. But, think about asthma and longterm health risks to yourself as well.

https://www.theguardian.com/environment/2023/feb/02/how-to-g...

Do re-burning wood stoves have safe levels of particle emissions? From what I've read, they generate more heat from a given amount of wood as well.

People that use firewood for heating usually live in places where the outside air is much cleaner than the cities engulfed with smog. I wouldn't be surprised if it balances on average better for them.

Perhaps in the US but the guardian article discusses UK urban situations. Also, draft proofing in rural contexts too, would mean indoor CO/CO2/NO and pm25 would exceed safe levels.

Well designed stoves are of course critical for women and child health worldwide. I don't think a solar kiln is high on the list yet for people burning wood which demands a 10km walk to fetch it, and burn in a mud stove. This topic is aimed at folk who own a jotl.

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