Solar mini kiln #3: Results are in…

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Blog entry by Sycamoray posted 01-25-2022 04:07 AM 655 reads 0 times favorited 6 comments Add to Favorites Watch
« Part 2: Mistakes and Marginal Improvements Part 3 of Solar mini kiln series Part 4: Second Run »

The first run of my mini solar kiln has ended.

Some numbers and a graph:
  • The load was one piñon stem, cut into 7 pieces measuring roughly 1.5” thick, 8-14” wide, 50” long. This totaled approximately 36 board feet.
  • It ran for 50 days, of which two were snowed out with no solar gain or even data.
  • Average daily solar irradiance, extrapolated from the PV array (not directly measured in the manner suggested by Splintergroup), was 1.5w. I’m not sure that’s the right unit of measurement, but it’s for comparison against itself so the units don’t truly matter. Like golf scores.
  • The estimated cumulative energy requirement was achieved yesterday, but I forgot that 49 days = seven weeks and wanted to let it hit the round number today.
  • Internal humidity ranged from 85% at the beginning of the run, down to 6.6% on the most effective days.
  • Wood moisture of the tree at cutting exceeded the measuring device.
  • After pulling the boards, I stabbed each at least once and sacrificed the ugliest by cutting it up to take many measurements, for a total of 16.
  • Wood moisture spanned 6-12%, with an average of 8.8125%, standard deviation of 1.905, and median of 9%. Nominally good results.

Looking over the monitoring data, I’ve cross checked with the simplest of statistics all the variables I could identify: internal humidity (high and low), external humidity (high and low), external temperature, energy input, etc. Most of these variables don’t matter. Even plotting certain trend lines don’t illuminate the situation, though I want them to mean something.

There are two relationships which matter – energy & internal low humidity, and internal low humidity vs external low humidity. The former is only a moderately strong relationship, so weak that the confidence interval is useless for research standards. The internal versus external humidity measurements started out heavily negative, and gradually realigned until they achieved perfect positive correlation in the last week.

Qualitative results of the boards indicate some flaws. Each board had some checking around the knots, but I don’t think that’s avoidable with piñon so I won’t be too hard on myself. I also cut two tuning forks from the board which I sacrificed for testing. One was fine, the other showed a 1/16” deviation over a 8” length. The only real issue was that the two bottom boards showed a spot of mold each. These were evident at the end which was opposite the exhaust outlet.

I’m going to install an second circulation fan and institute a slightly more aggressive exhaust schedule at the beginning of the run. Informed by the relationship over time between internal and external humidity, I’m inclined to set it so the exhaust fan rarely comes on at the end of the run.

On a related note, I’m not going to upgrade the exhaust fan yet. I like to avoid incremental changes in equipment, having learned my lesson in buying chisels. Better to see if there’s an administrative control which can achieve the desired effect.

6 comments so far

View Dave Polaschek's profile

Dave Polaschek

10281 posts in 2040 days

#1 posted 01-25-2022 01:13 PM

Sounds like a successful first run! Not perfect, but good, and with a few things you can improve. Good job!

-- Dave - Santa Fe

View HokieKen's profile


21695 posts in 2596 days

#2 posted 01-25-2022 01:49 PM

Excellent write up on this kiln! Thanks for such detailed information :-) Std. deviations and confidence intervals get me all excited ;-)

-- I collect hobbies. There is no sense in limiting yourself (Don W) - - - - - - - - Kenny in SW VA

View splintergroup's profile


6945 posts in 2680 days

#3 posted 01-25-2022 06:43 PM

I like your analysis methods!

If I understand the legend correctly, In/Out humidity is in the box and the outside air right?

“Remaining” is what?

I certainly see the drying trend with the boxes drop in humidity as the wood gives up it’s moisture, Seems to have flatlined about 2/3rds of the way through indicating the fibers have (almost) reached equilibrium. Any sap will take longer which may be showing in the gentle slope down over the last 1/3rd.

A comparison with summer operation would be interesting of course. Winter humidity is typically low due to the cold, but you’d get a heat boost during the summer. Vent timing would become interesting as the afternoon monsoons kick the RH up in the afternoons.

View Ocelot's profile


3808 posts in 4096 days

#4 posted 01-25-2022 08:21 PM

You got a picture of the actual kiln and the actual wood? (For us simpletons) The graphs look very impressive (as graphs are wont to do). :-)

-- I intended to be a woodworker, but turned into a tool and lumber collector.

View Mosquito's profile


11757 posts in 3750 days

#5 posted 01-25-2022 08:55 PM

You got a picture of the actual kiln and the actual wood? (For us simpletons) The graphs look very impressive (as graphs are wont to do). :-)

- Ocelot

Part one of the blog series had some

-- Mos - Twin Cities, MN - -

View Sycamoray's profile


133 posts in 698 days

#6 posted 01-26-2022 01:46 AM

Thanks, folks.

That’s right – the internal and external, low and high humidity measurements for the day. Remaining is the estimated percentage of energy which still needed to be input to achieve dry wood. I ran it as percentage so it would graph well with RH. The sap would take forever, as the kiln doesn’t sustain high enough temps to crystallize it. I’m intending to seal projects with shellac to fight the weeping. I hope to do 5 more runs this year; the monsoons will hopefully mess with the July/August one. We don’t have enough snow this winter, so having moisture would be better than dry lumber.

That’s the point of graphs, they distract from the absence of substance. :) Here are two representative photos of results. You can see the checking and uneven saw work:

Piñon has a goofy branching habit, and the trees on my property spent their first 50+ years growing however they wanted after the overstory of ponderosa and Douglas were clear cut in the mid20th century. Lots of confused grain makes for challenging lumber.

This shows one of the patches of mold, and some evidence of my experimenting with milling techniques:

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