I've drawn up a few designs for medium to large volumes of bulk substrate involving old sliding glass doors and straw bales, but always talked myself out of actually building one.
That's because in order to ensure I didn't waste a day's work and a lot of substrate I'd need to incorporate a conventional pasteurization method anyway as a backup, because clouds happen. And once the Sun sets, it begins to cool back down. So if you don't get the heat you need by then and have no other way to heat things up then you're screwed. Solar-thermal type panels will still accumulate heat under cloudy skies (to a point) but not enough to pasteurize a significant volume of mass unless there are a lot of panels in use (or you're using evacuated tubes). Depending on where you're at that might be a significant problem, or not much of one at all.
I guess it all still ultimately depends on how much mass you're talking about pasteurizing and what your budget for solar tech is.
One option worth considering is evacuated-tube solar collectors. They can generate high-pressure dry steam (≥150℃) and are far more efficient under cloudy skies than flat-plate collectors.
Edit: It can work, but only during certain time of year, depending on the weather.
Edited by TVCasualty, 02 May 2021 - 08:33 PM.