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.

Its been a few years, but I spent a summer touring German technical schools. They had a lot of solar projects. I remember one that used alcohol in the heat exchanger and I think it might have been under a vacuum. The main reason was to lower the boiling point because you gain more latent heat transforming to steam. It also requires less btu to superheat steam vs warming liquid, usually half. So in short, consider a low temperature solar boiler for efficiency, I'm sure easier said than done.

I can't say I've read all of the replies here, and it seems that there are some requests to supply us with a diagram. What I could add is that I've learned over some time now that a good, safe coolant could be 'propylene' glycol, versus the automotive coolant 'ethylene' glycol, which tends to kill animals who are won over by its aroma. There! I added something to a post. Hopefully it helps someone?

I used propylen glycol + water (40%) for my solar sysytem. This winter the lowest temp was -22C and everything is ok. But i know - someone who used alcohol 25% (vodka + water)