The Solar Kiln (II). First run and results

This is the batch I have loaded into the kiln for the first time. Two bottom rows of walnut, nine of cherry and one more of walnut, this last lower quality, sawn close to a 1 inch, 4/4, or 24-25 mm thick. They sit outside about 3.5 months for air drying prior to kiln and their moisture content reached around 12%.

Temperature sensor, not shown in pic, is placed inside the stack in the middle, and the relative Humidity sensor on top (this last one has also temperature measuring, but it can go up to 59C, then shows “HI”).

I took around 6-7 moisture content measurements on the cherry, all where in the range of 11.7-12.1% and just one was 14.3% (that shown in the video), so for a comparison during drying I choose to measure these two values, 14 and 12%, to observe their evolution.

Temperature, Relative Humidity and Moisture Content Probes

If you seen the kiln build video I’ve shown there how I did the MC measurements from outside the kiln using a pin moisture meter. This type of meters measures the electrical resistance between the pins when inserted into the wood then translates this to a moisture level, and for best accuracy they need to go like 1/3 deep into the wood thickness. So in order to make external measurements I have screwed and hammered some M2.5 screws (2.5 mm same as the pins diameter) into the holes/marks that the meter made to keep the same distance, hooked some wires outside the kiln, and place the meter on them whenever I measure. I’ve seen no considerable difference on display, maybe 0.1-0.2%, when inserting the meter with its own pins about 8 mm deep, or when measuring on top of the screws, or when measuring at the end of the wires. The resistance of the screws and wires is negligible, maybe under 1 ohm, wood with less than 20% MC has hundreds or thousands of megohms at that pin distance. When connecting the meter on the wires don’t touch both the pins with your hands, just one or none, otherwise your body resistance will alter the readings.

Kiln operation. Let’s start from late morning, when the sun starts to hit the roof. My kiln is oriented to the South but a few degrees more to West, so at around 11am it starts to be visible for the sun. At this moment the vents are closed and the fan starts running and stays like this the whole afternoon. The heat is produced up in the kiln then pushed down by the fan towards the stack, heating the wood to release the moisture. After the sun goes away and the kiln is in almost full shadow I turn the fan off and open for about 15-20 minutes the upper and bottom vents. Naturally, on the upper ones hot and humid air will get out and on the bottom vents fresh cold air will come in, replacing the humid one. The upper jet is so choking and hot that you can barely stay couple of seconds with your face 2 feet away :) After these minutes the vents are closed, leaving the wood to rest through the night. During this time the moisture will try equalize, for two reasons: (1) the outside part of the wood is more dry, because it was heated more, so the moisture will try to come out away from the middle towards the exterior, and (2) the wood will still be warm, it will release a few more moisture until it will be cooled, and this will probably set again on it during the night. Then, in the morning vents are opened again for a few minutes to exchange the air, mine didn’t do any effect, will talk about that later, then at 11am the cycle continues, fan is on and so on.

Upper vent, air coming out naturally
(kiln is empty here)

Lower vent, air coming in

Here below is a graph of the kiln operation on a 24 hour period. and some interpretations I made (note that the time line on the graph is not linear, some intervals are more compressed)
Weather conditions of those days were 35-40C (95-105F) average daytime, and RH 30/60% day/night.

Temperature inside the kiln, sensor placed on the boards

• at 11am before fan starts, kiln temp is above 40C (104F), outside is 32C (90F);

• it starts to increase as the sun sets more on the kiln, at around 14pm I have him perpendicular on the roof;

• between 3pm-5pm it reaches the highest point, 70C (158F), max inside the kiln, between upper vents and fan baffle was 95C (200F);

• from 5pm it starts to slowly decrease, no considerable drop when vents are opened at 7pm, maybe 2-3 degrees in 20 minutes;

• lowest point in the morning at 7am, around 30C (85F), few degrees more than outside, then starting to increase as the outside temp rises .

Humidity inside the kiln chamber

• at 11am it is usually the same as the outside RH, 40%, probably because the kiln is not perfectly air sealed;

• when fan starts it goes down 5% less, for about two hours;

• then it starts to go up as the wood is heated and releasing moisture, max I got 60% in the first days, then down to 50% at the end of the process;

• vents are opened at 7pm and quickly drops in a mater of minutes to 20%;

• increases again during night because of the warm wood that is still releasing some moisture;

• then again in the morning goes close to the outside humidity level;

• no significant change in rH when vents are opened in the morning, probably same because is not perfectly sealed, and gets the outside humidity in early morning when is cooled.

Now let’s look on the whole evolution during the eight days of drying.

Here are shown the temperatures, humidity and the two moisture content levels on the bottom. Same, time intervals are not linear. The color fills shows the fan running during sun time, in yellow, vents opening, in white, and nighttime, in grey.

• Day 1, because I have finished loading the kiln around noon, the temperature did not went more than 55C (131F);

• Day 4 was a cloudy day, although very hot as well, you can see a drop in temperature compared to other days;

• Last four days were very hot, then I got the maximum temperatures inside the kiln;

• Observe the humidity evolution, in red, day by day its peaks are decreasing;

• Moisture content, green and yellow, are going down as well, but check out those peaks during kiln heating. Because is an electrical resistance measurement, it is influenced by the temperature, once the wood is heated it will alter the correct value.

Here below is a chart with daily moisture measurements taken in the morning, during 9-11 am, when the wood is cooled enough.

Moisture level drop throughout the cycle:

Moisture levels daily recordings at 11am:

I took the wood out and stack it inside to rest for a while then start making something from it. After one week I made a few more moisture measurements to check if everything is ok, all readings were in this interval 7.5-8.5%, so it seems it worked well. It was indeed quite dried from outside before putted into kiln, I wished I had a more wet one to make a better, comprehensive test, but this is what I had, hope by next year to get more trees, different species to experiment.

As a final disclaimer, I am no professional in wood drying, this was my first experiment with this, so take all this information as such. Some are basic physics, some I have documented online, made my own interpretations etc There are plenty resources to look over on web if you wish to learn more about solar kiln.