When one batch of wood is dry, the doors slide to the next chamber that has already fully air dried, to make that section into the kiln for the next heating cycle.  The air ducts are quickly changed to direct the hot air from the collector through the piles of wood in this room.  One way valves keep the heat in the fully insulated kiln chamber when the fans are not running. 

A bubble of 6 mil UV protected poly film forms the window of the solar collector.  This bubble lays right over the 2X8 rafters (painted black) that make up the south roof of the building.  The double layer of plastic is inflated with outside air by a tiny fan.  The smooth curve of the outer film sheds rain, snow, and wind, also preventing the plastic from flapping in the wind.  Some insulation is provided by the bubble to retain heat, but prolonging the life of the poly tarps may be the greater benefit from inflation.  This poly film bubble is a very simple and inexpensive window.  It can be obtained from any greenhouse supply company and the aluminum hold down strips are well worth the expense.

A layer of tar paper is fastened to the bottom side of the rafters to form the black surface that converts the sunlight into heat.  Air flows up both sides of the black surface, rising to the top of the collector room.  The hot air is taken from the high point of the collector room and blown into the pressure duct running the length of the building, then down into the selected kiln chamber below.  ( Engineers from the USDA Forest Products Laboratory suggested using sheets of aluminum instead of tar paper to get better heat transfer to the air.  Considering the relative costs of aluminum and roofing felt, we recommend the latter.)

After the air has blown through the pile of lumber, it is drawn back up into the 'cool' air duct that also runs the length of the building at the lower edge of the collector.  This duct distributes the air evenly across the bottom of the collector.  The air rises up the black surface and is heated once more, and cycles through the wood piles again....   
The four chamber kiln has 600 square feet of collector surface exposed to the sun.  About  200,000 BTUs per hour (approx. 333 BTUs/hour/sqft) is available during optimum atmospheric conditions.  We estimate our conversion is about 50%, giving us up to 100,000 BTUs per hour input to the kiln.  On a sunny day, the air temperature in the solar collector reaches about 50 degrees Fahrenheit over the outside temperature. 

The angle of the black surface in our collector is 45 degrees here in S. Wisconsin, and should equal the degrees latitude where the kiln is located.  The south door can also be used to collect heat, especially in winter when the angle of the sun is lower and particularly when the ground is snow covered. 

Water is removed from the kiln by venting a small amount of moist air from the chamber after the air has blown through the wood.  The kiln chamber is under pressure, so a small adjustable opening is used to simply vent moisture when the fans are operating.  A similar amount of fresh air is drawn into the cool air duct at the base of the solar collector (the collector room is under negative pressure).  Other solar kilns that use the front poly tarp as a condensing surface at night lose large amounts of heat, and have no insulation from the cold outside.  An adjustable vent also allows for much better control of the kiln's relative humidity than a condensation system.
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