OK, I may have missed a couple of things here but this system looks no different from any other glycol (proplyene, not ethylene) solar system. Every system with the proper internal pressure (2 bar minimum) and 40% glycol will convert the WATER in the panels to steam at 170C+. This is a very small amount of real liquid changed to steam and the upshot is that the rest of the liquid will be pushed into the expansion tank (which must be big).
There is no issue with this process. Look up Tyfocor LS heat transfer fluid, made in Germany. I have used lots of it over the years and is the standard in Europe. It is non toxic and recyclable. Some companies in Canada also use USP (food grade) grade propylene glycol and water and it lasts 10 + years. It is all in the design of the system.
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