On Tuesday, June 27, 2017 at 8:07:35 AM UTC-4, Roy B. wrote:
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. ...

Roy: "holding" the heat is not the issue. To generate a thermal it needs to pass it on to the air, not hold on to it. The sunlight coming in at 1 kilowatt per square meter offers that much enery no matter the structure. Except that some of the light is reflected rather than absorbed. Put dark solar panels (or black plastic bags) over a light-colored surface and you'll get more heat absorbed and therefore transferred to the air (minus 15% or so that gets turned into electricity in the PV panels). Put them over a surface that's already dark and there would be no effect. Also, leafy plants use some of the heat to evaporate water (drawn up by the roots), that part of the energy is "lost" to thermal generation (until condensation occurs). Remove those leafy plants and replace with solar panels (or a parking lot) and you'll avoid the evaporation and get stronger thermals. So it all "depends".