Matt at feral farm shows a "down draft solar dehydrator." The solar heat enters at the top and then goes down. Because as it gathers moisture, the solar heated air gets heavier. He has nettles in there.
Mark Vander Meer, of wildland conservation service in Missoula, Montana shows off his solar food dehydrator still loaded with dried plums. Those plums have been in there all fall, winter and most of the spring. He talks about trying to dry fruit with electric food dehydrators and how expensive that was. This solar dehydrator also uses the down draft technique. He says plums take three days and apples take a day and a half.
These are all passive systems. There are no fans.