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Scott with a building of that size you can extend the capacity of your proposed kilns by just air drying ahead of the kiln without even going the solar route. Of course just orienting the building properly, providing some translucent panels on the side facing the sun and providing some air flow will be a plus. Seems like the benefit of enclosing another chamber and heating it somewhat would give a small return over just air drying.
There is a little problem with you theory. 1. A solar kiln works on the concept that the sun will heat the lumber up to around 135F or so during the day, with fans circulating the air in the chamber so it absorbs moisture from the lumber while lowering the RH% in the chamber as the air heats. This is not quite true, as with green lumber and good air flow, the moisture evaporation will use the heat energy nearly as fast as it comes into the kiln. If you achieve 135 F and a low humidity, something is not right as the lumber should be using the heat energy, evaporating moisture, cooling the air, and increasing the humidity if the vents are closed. In fact, it will take only about 1/8% moisture loss from the lumber to fully saturate the air. So, we need to vent, and this also will cool the dryReally.2. During the day a small amount of the chamber's air is exchanged with outside air, thus releasing some of the absorbed moisture. The truth is that it is more than a small amount. So, you cannot close the vents during the day.3. The heat during the day will not dry much lumber. You need low RH as well. The heat in air at 135 F is rather small, so the amount of drying is small. That is, it takes very little heat, compared to the energy for evaporating, to heat 70 F air to 135 F.So, overall, go one way or the other, but not mixed.
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