It turns out there's a guy who spent many years developing an ideal dehydrator which uses nothing but passive solar power.

There's 1 page with graphics that are too small to read.

https://www.motherearthnews.com/diy/solar-food-dehydrator-plans-zm0z14jjzmar/

There's a higher resolution graphic on

https://www.shiftfrequency.com/build-solar-food-dehydrator/

The general idea is to not expose the food to UV light.  Most of it is a permeable, heat absorbing panel bisecting an air passage.  The resulting updraft of hot air blows through the food area & out the top.  

Of course, there's no assurance the incoming air is at a lower dewpoint than the exiting air & it's not air tight.  At night, wetter air moves down & the food reabsorbs a lot of water.  There is a user closeable vent on top to manually regulate the airflow.  He faces the same problem of how much heat to trade for fresh air.  Where lions add fresh air whenever inside dewpoint is higher than outside dewpoint, he recommends less fresh air & more heat when the food is wetter.

There could be a duct leading air from the collection box to the top of the drying chamber, down from the top to the bottom of the collecting chamber, & back out through the top.  That would passively seal it against colder air but not against a higher dewpoint.

Not having a brain creates inefficiency, no matter what.  Based on how passive dehydrators work, an electronic brain combined with solar heating is always going to have an edge over a purely passive solar dehydrator.  A purely solar powered active system would take less space than the purely solar powered passive system.  The pump, fan & sensors just don't use enough juice to justify the space for a solar panel.

Day 6 continued the downward trend in dew points at night & less extraction of water by day.  

Anyways, there's a way to clean up the dew point by using floating point instead of fixed point  & using a more expensive microcontroller.