OK - back to the subject at hand (solar farms as thermal generators). I'm not an engineer or physicist but I suspect that part of the consideration is the mass of the structure and it's ability to hold enough heat to effectively transfer it to the air around it. So plowed fields, rock faces on mountains, and heavy metal silos do well to form thermals. But the light weight frames that hold the solar panels don't hold the heat well - and they shade the ground below them. And despite what people think they "should' do - they really don't generate thermals very well.
ROY